Human Parvoviruses

SUMMARY

Parvovirus B19 (B19V) and human bocavirus 1 (HBoV1), members of the large Parvoviridae family, are human pathogens responsible for a variety of diseases. For B19V in particular, host features determine disease manifestations. These viruses are prevalent worldwide and are culturable in vitro, and serological and molecular assays are available but require careful interpretation of results. Additional human parvoviruses, including HBoV2 to -4, human parvovirus 4 (PARV4), and human bufavirus (BuV) are also reviewed. The full spectrum of parvovirus disease in humans has yet to be established. Candidate recombinant B19V vaccines have been developed but may not be commercially feasible. We review relevant features of the molecular and cellular biology of these viruses, and the human immune response that they elicit, which have allowed a deep understanding of pathophysiology.

INTRODUCTION

Parvovirus, a word derived from the Latin word “parvus,” meaning small, is the name for a family of small (∼25-nm), nonenveloped viruses. Parvoviruses have a linear and single-stranded DNA (ssDNA) genome of 5 to 6 kb, which is flanked by two terminal hairpin structures (1, 2). The first parvoviruses identified in humans were adeno-associated viruses (AAVs), which are nonpathogenic (1, 3). Later, two pathogenic parvoviruses were identified, human parvovirus B19 (B19V) and human bocavirus 1 (HBoV1). B19V was discovered in 1975 by Cossart and colleagues during screening for hepatitis B virus. The serum sample, which contained parvovirus-like particles, was coded as panel B and number 19 and hence named “parvovirus B19” (4). B19V is highly infectious and causes a wide range of pathological conditions: fifth disease in children, persistent anemia in immunocompromised patients, transient aplastic crises, hydrops fetalis in pregnant women, and arthropathy (5–7) (see “Diseases Caused by B19V Infection,” below). It should be emphasized that many B19V infections are likely asymptomatic without apparent illness after seroconversion (8). HBoV1 was first identified in respiratory nasopharyngeal aspirates of children with lower respiratory tract infections (9). HBoV1 is an important cause of acute respiratory tract infections, with wheezing being the most common symptom (10) (see “Diseases Associated with HBoV Infection,” below). Several other parvoviruses, including HBoV2 (11), HBoV3 (12), HBoV4 (13), parvovirus 4 (PARV4) (14), and human bufavirus (BuV) (15), are emerging viruses associated with human diseases of unclear clinical significance.

VIRUS TAXONOMY

Based on the type of infected host, the family Parvoviridae is divided into two subfamilies, Parvovirinae and Densovirinae, which infect vertebrates and invertebrates, respectively. A revised taxonomy of the family Parvoviridae was proposed by the International Committee on Taxonomy of Viruses (ICTV) in 2014 (16). In the revised taxonomy, parvoviruses are classified based on phylogenetic analysis of the amino acid sequence of the large nonstructural protein NS1 (16). Notably, data from sequence analyses of core capsid proteins are overall in conformity with the NS1-based classification. All the viruses in a given genus should be monophyletic, with >30% of the amino acid sequences of the NS1 proteins being identical to each other or <30% of the amino acid sequences being identical to those of the NS1 proteins of parvoviruses in other genera. Within a given species, >85% identity of the NS1 proteins is required. Based on this principle, the Parvovirinae subfamily has been divided into 8 genera: Protoparvovirus, Amdoparvovirus, Aveparvovirus, Bocaparvovirus, Dependoparvovirus, Erythroparvovirus, Copiparvovirus, and Tetraparvovirus. Parvoviruses that infect humans, discussed in this review, are B19V, HBoVs, BuV, and PARV4, which belong to the Erythroparvovirus, Bocaparvovirus, Protoparvovirus, and Tetraparvovirus genera, respectively (Table 1).

TABLE 1.

Human parvoviruses of the family Parvoviridae, subfamily Parvovirinae, discussed in this review

Genus	Species	Member(s)
Bocaparvovirus	Primate bocaparvovirus 1	HBoV1, HBoV3
	Primate bocaparvovirus 2	HBoV2, HBoV4
Erythroparvovirus	Primate erythroparvovirus 1	B19V
	Primate erythroparvovirus 2	SPV
Protoparvovirus	Primate protoparvovirus 1	BuV
Tetraparvovirus	Primate tetraparvovirus 1	PARV4

BASIC VIROLOGY

Virus Structure

The parvovirus capsid comprises 60 copies of the capsid (VP) proteins that assemble into a T=1 icosahedral symmetry, with the larger protein, VP1, as a minor constituent (see “B19V entry and the role of B19V VP1u in virus entry,” below). The 3-dimensional structures of recombinant B19V VP2 and HBoV1 VP3 capsids as well as B19V native virions have been resolved by X-ray crystallography and cryoreconstruction to 3.5- to 8-Å resolutions, and they are quite similar (17–21).

The cores of the capsids are structurally similar among all parvoviruses, formed by subunits of an α-helix and an eight-stranded antiparallel β-barrel motif (21). Large insertions between the β-strands form long loops shaping the surface of the capsid. Contrary to the highly conserved core, the surface is highly variable among parvovirus species. The capsid surface is involved in many functions in the virus life cycle: specific binding to cellular receptors, intracellular trafficking with its phospholipase A2 (PLA2) activity, nuclear entry and exit, genome encapsidation, and recognition and avoidance of the host immune response (21–24).

At the capsid 5-fold axis, five β-barrels arrange to form a cylindrical structure with a wide surrounding canyon, which is less pronounced in HBoV1 (20). In HBoV1, as in many other parvoviruses, this cylinder creates an open channel that has been proposed to be a portal for genome packaging and VP1-unique region (VP1u) externalization, but this portal seems to be closed in B19V (17–20, 23), instead possibly presenting a flexible cap that may be opened upon receptor attachment (18, 25). There is a preserved depression at the 2-fold axis among the parvoviruses, whereas the 3-fold structure varies widely. Many parvoviruses contain prominent 3-fold protrusions, which in HBoV1 are much less pronounced and in B19V are depressed centrally to render smooth capsid surface topologies (18–21, 23). The B19V capsid recognizes its cellular receptor, globoside, in the region of the 3-fold depression (23), which, along with VP1u, is a major immunodominant area. Four antigenic epitopes have been mapped in the HBoV1 capsid: three HBoV1-specific epitopes have been mapped to the 3-fold protrusions and the wall between the 2- and 5-fold axes, and one HBoV1- to HBoV-4-cross-reacting epitope has been mapped to the 5-fold axis (26).

The N terminus of B19V VP1 remains unresolved, reflecting unordered differential conformations and small amounts of VP1u in the capsid. Nevertheless, from measuring the binding of neutralizing monoclonal B19V VP1u antibodies and the VP1u-associated PLA2 activity of native virions, VP1u alters its conformation after receptor attachment to become more exposed on the capsid surface (25), although this has not been structurally confirmed (17). The very tip of the VP2 N terminus is localized on the capsid surface (17, 27). The N terminus of HBoV1 VP1 has not been visualized, but contrary to VP1 of B19V, it does not contain immunodominant epitopes (28).

B19V Genome Organization and Expression

B19V genome and infectious clones.

B19V contains a linear ssDNA genome that is 5,596 nucleotides (nt) long (J35 strain [GenBank accession no. AY386330]) (29). The central coding region is flanked on both sides by identical inverted terminal repeats (ITRs) (Fig. 1A) (30). B19V is a homotelomeric parvovirus capable of packaging an equal number of minus and plus strands of the ssDNA genome in separate virus particles. The ITR is 401 nt long with an imperfect palindromic sequence that folds into a hairpin-like structure (31–33). The ITR exists in two equal configurations named “flip” and “flop,” with one being the inverted complement of the other. ITRs carry the origin of replication (Ori) and form active replication origins in double-stranded (replicative-form [RF]) DNA during viral DNA replication (34). The B19V RF DNA genome harbors a single promoter at map unit 6 (P6), with a transcription start site at nt 531 (Fig. 1B) (35, 36). A number of enhancer elements (nt 180 to 490) upstream of the P6 promoter bind the cellular transcription factors CREBP, C-Ets, GATA, YY1, and Oct-1, which strengthen P6 promoter activity (37–40). B19V NS1 binds NS1-binding elements (NSBEs) (5′-CCGGCGGC-3′) at nt 337 to 354 (41), which are located within the ITR and transactivate the P6 promoter.

FIG 1.

B19V transcription map. (A) B19V packages a linear ssDNA genome of either positive or negative polarity. The ssDNA genome of B19V is shown in negative polarity. Two ITRs (nt 1 to 383 and nt 5214 to 5596) are diagrammed at two ends of the genome with unpaired or mismatched bases in the palindromes represented by “bulges” or “bubbles,” respectively. (B) Schematic diagram of the duplex replicative form (RF) of the B19V genome. It is capable of expressing viral genes, replicating, and producing progeny virions. The P6 promoter, the RNA initiation site, splice donor sites (D1 and D2), splice acceptor sites (A1-1, A1-2, A2-1, and A2-2), and proximal/distal polyadenylation sites [(pA)p/(pA)d] are indicated, along with the nine major mRNAs (R1 to R9) that are polyadenylated at nt 2840 of the major (pA)p site and three minor mRNAs (R1′ to R3′) that are polyadenylated at nt 3141 of the minor (pA)p site. The numbering of nucleotides is according to the numbering for the B19V J35 isolate (GenBank accession no. AY386330). Proteins encoded by each mRNA are shown on the right with detected molecular masses in kilodaltons. The ITRs and the NS1- and VP1-encoding regions are not to scale. The coding capabilities of R3 and R3′ mRNAs are unknown. The size of each mRNA is shown in nucleotides without inclusion of the poly(A) tail. Different ORFs are depicted in different-colored boxes.

A number of B19V variants, which have >11% genome sequence divergence from previously characterized B19V isolates, have been reported (42–46). B19V is now classified into three distinct genotypes, genotypes 1, 2, and 3 (see “Genotypes and Molecular Epidemiology,” below). Biological properties, at least in vitro, of the three B19V genotypes are similar (47, 48). However, genotype 2 (based on the A6 isolate) has two unique features: it uses only one splice acceptor, A1-1, to remove the first intron (Fig. 1B), and the prototype B19V ITR did not support replication of the A6 genome (49). The NS1 protein has a divergence of ∼6% between genotype 1 and genotypes 2 and 3 (48). Genotype 2 also has variations in the ITR (50). However, at present, no ITR sequence of genotype 3 has been reported. The clinical spectrum associated with genotype 2 or 3 infection is similar to that observed for genotype 1 B19V infection (42). The NS1 proteins of both genotypes 2 and 3 are potent inducers of apoptosis in B19V-permissive cells (49).

The first molecular clone of B19V was constructed without the two ITRs (51). A full-length B19V genome (J35 isolate) named plasmid pB19-M20 has been successfully cloned with the two ITRs (29). pB19-M20 replicated and produced infectious virions in B19V-semipermissive UT7/Epo-S1 cells (29) as well as in human embryonic kidney 293 (HEK293) cells when an adenoviral helper plasmid that expresses the adenoviral E2, E4orf6, and VA genes was provided (52, 53). The production of infectious progeny virions from pB19-M20-transfected UT7/Epo-S1 cells was significantly improved when cells were cultured under hypoxia (1% O₂) (54), as hypoxia has been shown to enhance B19V replication (55, 56). Several full-length genomes of B19V have been sequenced (57, 58), confirming the sequence of the ITR. In addition, two full-length B19V genome clones, pB19-FL (NAN isolate) and pB19-HG1 (HV isolate), were constructed and could replicate in UT7/Epo-S1 cells (57). However, a point mutation in the VP1 PLA2 motif of pB19-FL inhibits the production of infectious progeny virions, even though the mutation was present in the wild-type virus in viremic blood.

B19V transcription.

The B19V genome consists of the large nonstructural protein (NS1) and the capsid protein (VP1/2) genes at the left and right sides, respectively. In addition, the B19V genome contains two genes that encode small nonstructural proteins, a 7.5-kDa protein in the middle and a 11-kDa protein at the right end. B19V transcription uses the single promoter P6 to transcribe a single precursor mRNA (pre-mRNA). In total, 12 mature mRNA transcripts are generated from alternative splicing and polyadenylation of the single pre-mRNA (59–61) (Fig. 1). There are two polyadenylation sites, proximal and distal [(pA)p and (pA)d, respectively]. (pA)p consists of the (pA)p1 and (pA)p2 sites, which account for internal polyadenylation of 90% and 10%, respectively (Fig. 1B, large and small arrowheads) (60). B19V pre-mRNA harbors two introns with alternative splice acceptor sites. Both the unspliced mRNAs and the mRNA transcripts that are spliced only at the A1-1 acceptor, which are polyadenylated at (pA)p, encode the large nonstructural protein NS1 and a small nonstructural protein of 7.5 kDa, respectively (62) (R1/R1′ and R2/R2′) (Fig. 1B). The first intron is spliced out from all the mRNA transcripts that are polyadenylated at (pA)d. The mRNAs that are polyadenylated at (pA)d and splice out the first intron encode capsid protein VP1 (R4 and R5) (Fig. 1B). The mRNAs that are polyadenylated at (pA)d and excise both the first intron and the second small intron (D2 to A2-1) encode VP2 (R6 and R7) (Fig. 1B), and the mRNAs that are polyadenylated at (pA)d and excise both the first and the second large introns (D2 to A2-2) encode the 11-kDa protein (R8 and R9) (Fig. 1B) (63). The coding capability and function of the small viral mRNAs spliced at the D1-to-A1-2 intron are unknown (R3/R3′) (Fig. 1B). Notably, the majority of viral mRNAs generated during B19V infection comprise the small 0.6- to 1.2-kb mRNAs (R2/2′, R3/R3′, R8, and R9) (Fig. 1B) (59, 64).

B19V gene expression regulation.

B19V RNA transcripts are alternatively spliced from a single pre-mRNA transcript and alternatively polyadenylated (59, 60) (Fig. 1), and therefore, the relative abundance of processed mRNA transcripts varies considerably and depends on the efficiency of splicing and polyadenylation.

(i) Internal polyadenylation controls production of VP- and 11-kDa protein-encoding mRNAs.

Differential expression of VP- and NS1-encoding RNAs has been observed in B19V-permissive and -nonpermissive cells. In nonpermissive cells, all the mRNAs are polyadenylated at (pA)p, and thus, mRNAs encoding capsid proteins are limited. However, in permissive cells, most B19V mRNAs read through the (pA)p sites and produce VP- and 11-kDa protein-encoding mRNAs (65, 66). Early blockade of the production of full-length B19V mRNA transcripts has been identified as a mechanism of B19V tropism (65). In permissive cells, a block in the production of the full-length mRNA transcripts is overcome by the replication of the viral genome (66). A careful quantitation of different species of viral mRNAs in B19V-infected primary CD36⁺ human erythroid progenitor cells (EPCs) revealed two distinct patterns in the viral mRNA profile with regulation using mRNA processing signals: at an early phase of infection, a block at (pA)p leads to relatively higher-level production of NS1-encoding mRNAs, and at a late phase, readthrough of (pA)p is more efficient, which leads to the abundant generation of VP- and 11-kDa protein-encoding mRNAs (67). How precisely viral DNA replication overcomes the transcription block in B19V-permissive cells is not known.

(ii) Multiple splicing enhancers function to define the central exon of B19V pre-mRNA splicing.

B19V pre-mRNA has two splice donor sites (D1 and D2) and four acceptor sites (A1-1, A1-2, A2-1, and A2-2) (59). Alternative splicing is tightly regulated in B19V-infected cells to maintain appropriate levels of virus-encoded proteins. All the D2-spliced (pA)p readthrough transcripts, which are VP1- and 11-kDa protein-encoding mRNAs, contain a 55-nt-long leader sequence and a central exon (exon 2) spanning from the A1-1/A1-2 site to the D2 site (R4-9) (Fig. 1B). Serine-arginine (SR) protein-binding GAA motifs have been found in exon 2 (68). The GAA motif in the region between A1-1 and A1-2, also called exon-splicing enhancer 1 (ESE1), facilitates splicing at the A1-1 acceptor site and is also required to define exon 2. The 5′ end of exon 2 serves as ESE2 and facilitates splicing at the A1-2 acceptor site, while ESE3 at the 3′ end of exon 2 plays a role in recognizing the D2 donor site. The G/GU-rich region adjacent to the D2 site acts as an intron-splicing enhancer (ISE2). The definition of exon 2 is a consequence of the weak splice donor site D2, ISE1/2, and ESE1/2/3 (68).

Alternative splicing coordinates alternate polyadenylation to generate VP- and 11-kDa protein-encoding B19V mRNA transcripts (69). Efficient splicing of the pre-mRNA within the first intron (D1-A1) stimulates polyadenylation at the (pA)p site, and splicing of the second intron (D2-A2) promotes polyadenylation at the (pA)d site. Splicing of the second intron competes with polyadenylation at the (pA)p site. U1 small nuclear RNA, which binds to the 5′ splice donor site of the second intron, inhibits polyadenylation at the (pA)p site (69).

B19V proteins.

The large B19V nonstructural protein NS1, of 671 amino acids (aa), has a molecular mass of ∼78 kDa (59, 70, 71). NS1 localizes predominantly to the nucleus (71) and contains a nuclear localization signal (NLS) at aa 177 to 180 (KKPR) (72, 73). NS1 is essential for viral DNA replication (74) and has an origin DNA-binding/endonuclease domain at the N terminus (41), an ATPase- and nucleoside triphosphate (NTP)-binding motif of 160 aa in the central region (75), and a putative transactivation domain (TAD) at the C terminus (76). NS1, with the help of the transcription factor Sp1/Sp3, binds the P6 promoter of the virus to regulate viral gene expression (77, 78). Apart from acting on the P6 promoter, NS1 has been shown to transactivate several other host genes, including tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and p21 (79–81). NS1 induces apoptosis, which involves caspase-3 and -9, in B19V-semipermissive erythroid-lineage K562 and UT7/Epo cells (82) and nonpermissive HepG2 cells (83, 84). The NTP-binding motif at aa 328 to 335 (Walker box A) of NS1 has been implicated as a key motif in NS1-induced apoptosis (75, 82, 83). NS1 also induces cell cycle arrest (73, 85, 86) and the DNA damage response (DDR) (76, 87), which are discussed below (see “Cellular Response to Productive B19V Infection”).

The minor capsid protein VP1, of 781 aa, has a molecular mass of 84 kDa. The major capsid protein VP2, of 554 aa, has a molecular mass of 58 kDa (71). VP1 is poorly translated from the VP1-encoded mRNA that has multiple AUG codons upstream of the VP1 initiation site (88) and has an additional 227 aa at the N terminus, known as VP1u, compared with VP2 (59, 71). VP1 and VP2, together at a ratio of ∼1:20 (71), assemble to form a B19V capsid of a T=1 icosahedral structure, which consists of 60 proteins, with 3 proteins on each face of the capsid (17). VP2 alone can assemble virus-like particles (VLPs) (89), which resemble the icosahedral structure of the B19V capsid (17–19). VP2 harbors a nonconventional NLS motif (KLGPRKATGRW) at the C terminus (90), which localizes VP1 and VP2 proteins to the nucleus to assemble into empty capsids (Fig. 2, steps 12a and 13). The N terminus (aa 1 to 100) of VP1u plays a crucial role in virion binding and internalization during B19V entry into cells (91), whereas the central portion of VP1u (aa 128 to 160) contains a motif with PLA2 activity (22, 92, 93). The PLA2 motif is possibly utilized during intracellular trafficking to escape late endosomes for nuclear entry (Fig. 2, step 4), as for other parvoviruses (94, 95).

FIG 2.

B19V infection of human erythroid progenitor cells (B19V life cycle). When human erythroid progenitor cells are infected with B19V, the virus initially interacts with P antigen (globoside) (step 1), the primary low-affinity attachment sugar molecule on the cell surface. This interaction confers a conformational change of VP1, extruding itself on the surface of the virion instead of remaining embedded inside the virion (step 2). The VP1u region on the virion then binds a putative cellular surface receptor (VP1u-interacting protein), which mediates internalization of the virion inside the cells, presumably by endocytosis (step 3). Virions undergo several steps of intracellular trafficking in the endosome, are eventually released from the endosome through a function of the PLA2 motif of VP1u, and enter the nucleus (step 4). In the nucleus, the virion is uncoated and releases either a positive-sense ssDNA or a negative-sense ssDNA (−ssDNA) genome (step 5). The ssDNA genome, shown as negative-sense ssDNA, is first converted into dsDNA that is primed by the 3′ OH of the left-hand ITR (step 6), a process that requires cellular DNA polymerase (DNA Pol) and other DNA replication factors. Phosphorylated STAT5 (p-STAT5), which is activated through the Epo/Epo-R/Jak2/STAT5 pathway, is required for viral DNA replication. After dsDNA conversion, NS1 binds NSBEs and nicks one of the strands at the trs (step 7a). This event creates a new 3′ OH to lead DNA synthesis following melting of the hairpinned ITR, which subsequently repairs the ITR and results in an open-ended duplex replicative intermediate (step 7b). The repaired ITR is then denatured (step 7c), which likely requires the helicase activity of NS1, and is reannealed, in a process termed reinitiation, to form a double-hairpinned intermediate, which creates a new 3′ primer (3′ OH) to initiate a round of strand displacement synthesis (step 7d). The RF DNA is capable of transcription of viral mRNA by cellular RNA polymerase II (RNA Pol II). Various B19V mRNAs are generated through alternative processing of the pre-mRNA (step 9) and are exported to the cytoplasm (step 10). Capsid proteins; VP1 and VP2; and the NS1, 11-kDa, and 7.5-kDa nonstructural proteins are translated in the cytoplasm (step 11). VP1 and VP2 are assembled as trimers (capsid precursors), which are transported into the nucleus (step 12a) to assemble empty capsids (step 13). A viral ssDNA genome is presumably produced through a process called strand displacement (step 7e), when an empty capsid is available, through a putative NS1-mediated packaging mechanism (step 14). NS1 is transported to the nucleus (step 12b) and is essential for viral DNA replication. NS1 also induces cell cycle arrest at G₂ phase. Finally, apoptosis is induced, in which both NS1 and the 11-kDa protein play important roles (step 16). Apoptosis releases the matured virion from infected cells through the broken nuclear membrane (step 15). As noted, steps 6, 7, and 12 to 14 are partially hypothetical.

In addition, B19V expresses two small nonstructural proteins of 11 kDa and 7.5 kDa (62, 96). The 11-kDa protein is abundantly expressed in B19V-infected CD36⁺ EPCs (97). It is localized more in the cytoplasm than in the nucleus, and its protein expression level in the cytoplasm of these cells is at least 100 times higher than that of nuclear NS1 (63, 97). The 11-kDa protein contains three proline-rich regions and binds in vitro to the SH3 domain-containing protein Grb2 (98). The 11-kDa protein is a more potent inducer of apoptosis, as it is abundantly expressed during infection, which involves caspase-10 in B19V-infected CD36⁺ EPCs (97). A role for the 11-kDa protein in VP2 production and cellular distribution has also been suggested (74). However, the 11-kDa and 7.5-kDa proteins are not required for DNA replication of the infectious clone pB19-M20 in UT7/Epo-S1 cells (74). Currently, nothing is known about the function of the 7.5-kDa protein during B19V infection.

An open reading frame (ORF) in the VP1-unique region is predicted to encode a third small nonstructural protein (X protein) of 9 kDa (72). An X protein knockout B19V infectious clone did not show any differences between the wild type and the knockout mutant with respect to viral DNA replication (74). Furthermore, it has not been demonstrated to be expressed during either transfection of a B19V clone or B19V infection.

B19V Tropism and Entry

B19V cell culture.

In patients, productive B19V infection is highly restricted to erythroid progenitor cells of the bone marrow (99). B19V was first demonstrated to infect cultured erythroid progenitor cells isolated from human bone marrow cells (100). More primitive erythroid progenitors, at stages of burst-forming unit–erythroid (BFU-E) and CFU-erythroid (CFU-E), were permissive to B19V infection (100, 101). Various sources, including human bone marrow (100–103), umbilical cord blood (104, 105), peripheral blood (106, 107), and fetal liver (108, 109), were used to propagate erythroid progenitor cells for in vitro infection by B19V. Target cells of B19V infection are in various stages of erythroid differentiation, from BFU-E to proerythroblasts, with susceptibility to the virus increasing with differentiation (110). A pure population of CD36⁺ EPCs, which are expanded ex vivo and derived from hematopoietic stem cells (HSCs) isolated from either human bone marrow or peripheral blood mononuclear cells (PBMCs), are permissive to B19V (111), and they are widely used for B19V infection and neutralization antibody tests (54, 73, 112–114). Hypoxic conditions, about 1% O₂, significantly increase B19V infectivity in CD36⁺ EPCs (54). Although CD36⁺ EPCs and hypoxia facilitate B19V infection, the production of infectious progeny virions may be limited due to a failure of genome encapsidation (115).

Megakaryocyte-erythroid lineage cell lines have been tested for B19V infection. MB-02, UT7/Epo, and UT7/Epo-S1 cells are megakaryoblastoid cell lines (116–119) prone to B19V infection. Two erythroid leukemia cell lines, JK-1 and KU812Ep6, have also been documented to support B19V infection (120, 121). Based on the expression of the viral NS1 protein and viral DNA replication, UT7/Epo-S1 cells appear to be most permissive, but they are not as efficient as CD36⁺ EPCs for virus propagation, even under hypoxia (54, 85).

B19V receptor and coreceptors.

Globoside or P antigen is the primary cell surface receptor for B19V infection (122). Both the purified soluble P antigen and a monoclonal antibody to P antigen prevent B19V infection of human erythroid progenitors (122). B19V VP1- and VP2-containing VLPs also bind to P antigen in vitro (123), confirming the role of globoside as a receptor for B19V. P antigen is expressed largely on the cell surface of human erythroid progenitors (111, 112). However, not all P-antigen-expressing cells are permissive to infection by recombinant B19V, indicating that P antigen is necessary for but not sufficient in mediating recombinant B19V infection (124). Therefore, individuals who lack P antigen are resistant to B19V infection (125). Mature human red blood cells (RBCs), despite expressing P antigen, are not permissive to virus entry (126); viral particles remain attached to the surface of human RBCs during the course of virus infection, with P antigen aiding in systemic dissemination (126). Two potential coreceptors for B19V, integrin α5β1 (127) and Ku80 (128), have been proposed. However, the expression of Ku80 on the surface of CD36⁺ EPCs does not correlate with high infectivity of B19V (112). As B19V VP1u plays a key role in the binding and internalization of B19V virions, a VP1u-interacting protein, which has not yet been identified, has been hypothesized to function as a coreceptor (91, 129).

In nonerythroid cells such as endothelial cells, despite similar expression levels of P antigen, Ku80, and α5β1 on the cell surface, internalization of B19V is inefficient (130). An alternative route for B19V internalization in endothelial cells might be mediated by the C1q receptor CD93 and B19V-antibody complexes (130, 131).

B19V entry and the role of B19V VP1u in virus entry.

VP1 has an unique N-terminal VP1u of 227 aa in comparison to VP2 (70, 71). VP1u displays PLA2 activity during the transport of virus to the nucleus via the endosomal pathway (132–135). In many parvoviruses, VP1u is hidden inside the capsid and not accessible during virus binding to cells, while in some other parvoviruses, it is exposed on the surface. Despite its low proportion in the virion, B19V VP1u represents a dominant antigenic target for neutralizing antibodies (89, 136, 137), implying that it must be exposed to the extracellular milieu prior to B19V internalization (138–140). B19V VP1u becomes accessible to neutralizing antibodies upon the interaction of the capsid with the P antigen on the cell surface (25, 126, 141) (Fig. 2, step 1). During B19V uptake, the VP2 capsid predominantly attaches to P antigen of target cells (126), which in turn induces structural changes in the capsid that lead to the exposure of VP1u (141). The N-terminal 100 aa of the exposed VP1u then binds on the cell surface, leading to the internalization of the capsid (91, 141) (Fig. 2, step 2). It is hypothesized that the interaction of the B19V virion with host cells may require a VP1u-interacting protein on the cell surface to accomplish the binding and internalization of B19V virions. Further study has shown that N-terminal aa 5 to 80 of VP1u are necessary and sufficient for cellular binding and internalization, representing the VP1u-interacting protein-binding domain required for B19V uptake (142). Little is known about B19V intracellular trafficking. One study has shown that B19V was internalized by clathrin-dependent endocytosis and traffics rapidly throughout the endosomal compartment to the lysosomal compartment (143). The virus is supposed to escape the late endosome; otherwise, it may be degraded in the lysosome, as observed for other parvoviruses (132).

B19V Replication

Elements involved in B19V DNA replication both in cis and in trans.

The B19V minimum Ori is located at nt 5214 to 5280 (67 nt) and contains 4 repeats of the NSBE. NSBE1 and NSBE2 are 8-bp-long identical motifs separated by 2 bp, while NSBE3 and NSBE4 are degenerate sequences (41). NSBE1 to -3 are essential for viral DNA replication, and NSBE4 further enhances replication (52). The Ori also harbors a terminal resolution site (trs), where NS1 presumably nicks ssDNA to generate a free 3′-OH end that primes the DNA extension (Fig. 2, step 7b) (34). NS1 specifically binds NSBE1 and -2 in vitro (41).

Cellular control of B19V DNA replication.

The remarkable erythroid tropism of B19V partly depends on the expression of the virus receptor and coreceptors on erythroid progenitor cells (122, 127, 128); however, it is also dependent on erythroid-lineage-specific host factors. B19V has been shown to alter various cell signaling pathways (e.g., erythropoietin [Epo] signaling, the DDR, and cell cycle arrest) for efficient viral DNA replication (54, 76, 85, 112).

(i) S-phase-dependent viral DNA replication.

B19V, without a viral DNA polymerase (Pol), relies solely on the host DNA replication machinery. B19V induces cell cycle arrest of infected cells in “G₂” phase with a 4N DNA content, which is assessed only by 4′,6-diamidino-2-phenylindole (DAPI) staining for DNA content (118, 144). When assessed by both BrdU (5′-bromo-2-deoxyuridine) incorporation and DAPI staining, B19V infection induces cell cycle arrest in late S phase with both BrdU incorporation and a 4N DNA content (85). Several S-phase factors, such as DNA Pol δ, proliferating cell nuclear antigen (PCNA), replication factor complex 1 (RFC1), cyclin A, and minichromosome maintenance complex (MCM), colocalize in B19V replication centers (85). B19V exploits a prolonged S phase and utilizes S-phase cellular factors for viral DNA replication.

(ii) Epo-dependent B19V DNA replication.

Epo is essential for the differentiation of erythroid progenitor cells, and both CD36⁺ EPCs and B19V-semipermissive cell lines, e.g., UT7/Epo-S1, depend on Epo for cell proliferation and survival. Epo, a hormone produced by human renal interstitial fibroblasts in response to local partial oxygen pressure, precisely regulates erythropoiesis. The earlier stages of differentiation to BFU-E are Epo independent but rely on stem cell factor (SCF), IL-6, and IL-3. The later stage of differentiation from BFU-E to CFU-E requires Epo. CFU-E progenitors and proerythroblasts are highly susceptible to B19V infection (110, 112). The permissivity of these cells to B19V infection depends on Epo: the Epo/Epo receptor (Epo-R)/Jak2 signaling pathway plays a direct role in B19V replication (112). CD36⁺ EPCs differentiated from CD34⁺ HSCs in the absence of Epo are not permissive to B19V infection, and the B19V genome replicates in CD36⁺ EPCs only in the presence of Epo. The activation of Epo-R activates three major pathways, MEK/extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase (PI3K), and JAK2-STAT5A, but only the phosphorylation of STAT5A is essential for B19V replication, the MEK/ERK pathway has a negative effect, and the PI3K pathway is dispensable for B19V replication (54).

(iii) Hypoxia-facilitated B19V replication.

Propagation of B19V in ex vivo-expanded CD36⁺ EPCs requires a multiplicity of infection (MOI) of >1,000 viral genome copies (vgc)/cell and produces infectious progeny virions at a low level, even under hypoxia (54, 115). The plasma of B19V-infected patients may contain virions at levels as high as 10¹³ to 10¹⁴ vgc/ml (145, 146). Thus, ex vivo propagation of B19V is not as efficient as that in human bone marrow of B19V-infected patients. Areas of bone marrow are at low O₂ tension (1.3%) (147), and lower oxygen pressure favors erythroid cell development in culture (148). CD36⁺ EPCs under hypoxia (1%) have enhanced B19V gene expression, viral replication, and virus production (55). Hypoxia also enables B19V-infected KU812Ep6 and UT7/Epo-S1 cells to yield a higher level of progeny virions than under normoxia (54, 149). Hypoxia regulates the Epo/Epo-R signaling pathway through the upregulation of STAT5A activation and the downregulation of MEK activation, enhancing B19V DNA replication in both B19V-infected CD36⁺ EPCs and pB19-M20-transfected UT7/Epo-S1 cells (54).

HBoV1 Biology

HBoV1 genome and infectious clone.

Only one full-length HBoV1 genome of 5,543 nt has been sequenced and cloned (150); the source of viral DNA was a nasopharyngeal aspirate from a child with community-acquired pneumonia in Salvador, Brazil, who had acute viral infection (seroconversion, viremia, and >10⁸ vgc per ml of aspirate) (151). The sequence of the Salvador isolate has been deposited in GenBank (accession no. JQ923422). Ninety-five percent of bocaparvoviruses contain a negative-sense genome, and only 5% of bocaparvoviruses have a positive-sense genome (152, 153). The HBoV1 negative-sense genome has an imperfect “rabbit-ear-type” palindromic hairpin structure of 140 nt at the left-end hairpin (LEH) and a perfect palindromic structure of 190 nt at the right-end hairpin (REH) of the genome (150). A plasmid DNA clone of pIHBoV1, which contains the full-length HBoV1 genome of the Salvador isolate, replicates and produces progeny virions in HEK293 cells. HBoV1 virions generated from this production system exhibit a typical icosahedral structure of ∼26 nm in diameter and are capable of productively infecting polarized primary human airway epithelial cells cultured at the air-liquid interface (HAE-ALI cultures) (150).

The HBoV1 genome has heterogeneous terminal repeats, as is characteristic of the parvoviruses of the genus Protoparvovirus. The REH contains the terminal resolution site, which plays a role in the replication of viral replicative-form DNA, whereas the LEH is critical for junction resolution, which generates the ssDNA genome from the replicative-form DNA for encapsidation into capsids (34, 154). The HBoV1 LEH and REH do not share conserved NS1-binding sequences, and the REH is a perfectly paired palindromic sequence (150, 154). HBoV1 ssDNA genome replication in HAE-ALI cultures generates intermediates of double-replicative forms (dRFs) and monoreplicative forms (mRFs) (155).

HBoV1 gene transcription and regulation.

HBoV1 transcription uses one promoter, P5, at nt 282, to transcribe a single pre-mRNA, which is both alternatively spliced and polyadenylated at (pA)p and (pA)d, respectively, to generate at least 12 mature mRNA transcripts for encoding viral NS and structural proteins (156–159) (Fig. 3). The HBoV1 genome consists of the nonstructural protein (NS1 to -4), NP1, and capsid protein (VP1 to -3) genes at the left, middle, and the right sides, respectively. NS1 is encoded by R1 mRNA transcripts that are spliced at the internal small intron (D3-A3) and are alternatively polyadenylated (R1) (Fig. 3). A small NS1 protein, NS1-70, is expressed from an unspliced R1 mRNA. Additionally, three small NS proteins, NS2, NS3, and NS4, are expressed from R2, R3, and R4 mRNA transcripts, respectively, which are alternatively spliced at the D1-A1′, D1′-A1, or both introns (R3-4) (Fig. 3). Unique to bocaparvoviruses is an ORF located in the middle of the viral genome. A small NS protein, NP1, is encoded by R5 mRNA transcripts that are spliced at the D1-A1 and D2-A2 introns (R5) (Fig. 3). The R6 mRNA transcript, which is spliced at all three major introns (D1-A1, D2-A2, and D3-A3) and polyadenylated at the (pA)p site, encodes the capsid proteins VP1, VP2, and VP3 (Fig. 3).

FIG 3.

HBoV1 transcription map. The ssDNA genome of HBoV1 is shown in negative polarity. The transcription and posttranscriptional units are depicted to scale, including the P5 promoter, 5′ splice donor sites (D1, D1′, D2, and D3), 3′ splice acceptor sites (A1, A1′, A2, and A3), the internal proximal polyadenylation site [(pA)p], and the distal polyadenylation site [(pA)d], which are functional when the ssDNA genome is converted to a dsDNA form. The left- and right-end hairpin structures of the genome (LEH and REH, respectively) are diagrammed. Six groups of HBoV1 mRNA transcripts detected during infection, which have either a long form of mRNA (Rx_L) that reads through the (pA)p site or a short form of mRNA (Rx_S) that is polyadenylated at the (pA)p site, are shown below the diagrammed genome. R1 mRNA has a minor species (R1m) that is unspliced at the central small intron (D3-A3). Major ORFs are depicted as colored boxes with the nucleotide and amino acid of the start codon and the stop codon indicated. Proteins expressed from each mRNA species are indicated beside their respective mRNAs with molecular masses in kilodaltons detected during infection.

The HBoV1 nonstructural proteins NS1, of 781 aa; NS2, of 472 aa; NS3, of 507 aa; and NS4, of 199 aa, have detected molecular masses of ∼100, ∼66, ∼69, and ∼34 kDa, respectively. They share a C terminus of aa 639 to 781 of the NS1 protein (Fig. 3, red) (158). NS1, which has a putative DNA origin-binding/endonuclease domain (OBD), a helicase activity domain, and a TAD at the N terminus, middle, and C terminus, respectively, is essential for viral DNA replication (150). The OBD structure is canonical for the histidine-hydrophobic histidine superfamily of nucleases, combining two distinct DNA-binding sites: (i) a positively charged region mediated by a surface hairpin (aa 190 to 198) that is responsible for the recognition of the viral DNA Ori and (ii) the endonuclease active site that performs strand-specific cleavage at the Ori (160). NS2 contains the entire OBD and TAD of NS1, while NS3 contains the helicase domain and TAD of NS1, and NS4 has only the TAD. NS2 to -4 are not required for viral DNA replication of the pIHBoV1 infectious clone in HEK293 cells; NS2 plays an important role in virus replication in HAE-ALI cultures (158). The functions of NS3 and NS4 are currently unknown.

HBoV1 NP1, of 219 aa, has a molecular mass of 25 kDa. NP1, which is unique to bocaparvoviruses, plays an important role not only in viral DNA replication (150, 152) but also in viral pre-mRNA processing (161). It is required for viral mRNA splicing at the A3 splice site and readthrough of the viral mRNA from the (pA)p site (159). Therefore, NP1 is essential for generating VP-encoding mRNA (R6) (Fig. 3) and for the production of viral capsid proteins. Of note, HBoV1 NP1 colocalizes with autonomous parvovirus-associated replication (APAR) bodies and complements some functions of minute virus of mice (MVM) NS2 during early-phase infection (162).

Unlike B19V, HBoV1 expresses three capsid proteins, VP1, VP2, and VP3, during HBoV1 infection, at a ratio of ∼1:1:10 (158, 159), similar to that of AAV (3). Like AAV VP1 (3), HBoV1 VP1 has a VP1u of 90 aa, which is shorter than that of B19V (227 aa). A motif of aa 11 to 66 of VP1u exhibits PLA2 activity (163). VP2 is translated from a noncanonical GUG translation initiation codon at nt 3422 of the HBoV1 genome (159, 164) (Fig. 3). VP3, the major capsid protein, assembles into VLPs with a typical T=1 parvovirus icosahedral structure (20, 164–166). The VLP capsid formed by HBoV1 VP3 contains putative epitopes of neutralization/receptor binding, which are recognized by anti-HBoV1 monoclonal antibodies 4C2, 9G12, and 12C1 and anti-HBoV1, -2, and -4 monoclonal antibody 15C6 (26).

HBoV1 cell culture.

HBoV1 infects only well-differentiated or polarized primary human airway epithelial cells (150, 156, 167, 168). An HAE culture is generated by growing isolated human airway (tracheobronchial) epithelial cells on collagen-coated, semipermeable membrane inserts, and cells differentiate at the ALI for 3 to 4 weeks (150, 155). HBoV1 infects primary HAE-ALI cultures efficiently (167). Immortalized human airway epithelial cells of the CuFi-8 cell line, which were originally derived from a cystic fibrosis patient (169), have been polarized successfully to produce ALI cultures, which support HBoV1 infection but at a 1-log-lower level of apical virus release than that of infected primary HAE-ALI cultures (150). Although two commercially available primary HAE-ALI cultures, EpiAirway and MucilAir HAE-ALI, can be infected with HBoV1, infectivity was much poorer than that in in-house-made (primary and CuFi-8) HAE-ALI cultures (168). However, the infection was also persistent, releasing virions for as long as 50 days (168). HBoV1 infects HAE-ALI cultures from both the basolateral and apical sides of the ALI, and infected HAE-ALI cultures release virions from both sides. The amount of virus released from the apical side is 1 to 2 logs larger than that released from the basolateral side (150, 167).

Monolayer-cultured primary airway epithelial cells or airway epithelial cell lines do not support HBoV1 infection or replication of infectious DNA (pIHBoV1) (150). The steps in virus infection or viral DNA replication that are blocked during HBoV1 infection of monolayer-cultured cells is not known; the limiting step is likely at the stage of viral DNA replication.

Cellular control of HBoV1 DNA replication.

In contrast to S-phase-dependent B19V DNA replication, HBoV1 infects and replicates in terminally differentiated or nondividing airway epithelial cells of HAE-ALI cultures. Therefore, HBoV1 is independent of the cell cycle (155). HBoV1 infection of nondividing epithelial cells employs the cellular DNA damage and repair machinery in order to amplify the viral genome. HBoV1 infection activates all three PI3K-related kinases, ataxia telangiectasia mutated kinase (ATM), ATM- and rad3-related kinase (ATR), and DNA-dependent protein kinase (DNA-PKcs), at serine 1981 on ATM, threonine 1989 on ATR, and serine 2056 on DNA-PKcs, which are functionally required to transduce DDR signaling. The Y-family DNA polymerases Pol η and Pol κ function in HBoV1 genome amplification. Thus, HBoV1 replication is cell cycle independent, and the DNA repair process recruits cellular DNA repair DNA polymerases in viral DNA replication centers (155).

HOST CELL RESPONSE AND PATHOGENESIS

Cellular Response to Productive B19V Infection

B19V infection-induced DNA damage response.

B19V infection induces a DDR by activating the ATR, ATM, and DNA-PKcs kinases of the PI3Ks (87). Activation of ATR and DNA-PKcs is essentially required for efficient B19V DNA replication, and DDR effectors (e.g., Chk1 and Ku70/80) associate with replicating viral DNA (76, 87). None of the virus-encoded proteins (NS1, VP1, VP2, 11-kDa, or 7.5-kDa protein) are responsible for the phosphorylation of RPA32 and H2AX, typical of the DDR, in CD36⁺ EPCs (76). The B19V infectious clone pB19-M20, but not its replication-defective mutant, induces a DDR in transfected UT7/Epo-S1 cells, implying that viral DNA replication, and not merely the expression of the viral genome, is required to induce a DDR (76). NS1 is essential for DNA replication and is required for inducing the DDR during B19V infection. Viral DNA replication is a prerequisite for a B19V-induced DDR, and DNA replication intermediates could be potential inducers of the DDR. How the DDR facilitates the replication of the B19V genome is unclear; probably, the linear DNA genome initially recruits the DDR machinery for repair and exploits it to accomplish second-strand DNA synthesis prior to viral DNA replication (Fig. 2, step 6).

B19V infection-induced cell cycle arrest.

B19V infection of CD36⁺ EPCs and UT7/Epo-S1 cells induces arrest in G₂ phase, a cell cycle status with 4N DNA content (118, 144). Upon G₂ arrest with 4N DNA content, there is also incorporation of BrdU, so infection-induced arrest occurs in late S phase (85). In these studies, there was a gradual switch of the arrested CD36⁺ EPCs from late S phase (50% of infected cells) during early infection to G₂ phase (60% of infected cells) later. However, the expression of only B19V NS1 induces true G₂-phase arrest, with the majority of the cells having 4N DNA content and no BrdU uptake (85). B19V infection of UT7/Epo-S1 cells also induced 4N cell cycle arrest, and prevention of the nuclear import of the activated cdc2/cyclin B1 complex was observed (118). In CD36⁺ EPCs, NS1 induces stable G₂ arrest by interacting with repressive E2F transcription factors (E2F4 or E2F5) and facilitating their nuclear import (73). The predicted transactivation domain 2 (TAD2) at the C terminus of NS1 is required for NS1-induced G₂-phase arrest (76). Nevertheless, the mechanism underlying NS1-induced G₂-phase arrest requires further validation. In addition to G₂-phase arrest, B19V infection of UT7/Epo-S1 cells has been reported to induce G₁-phase arrest, and G₁-phase arrest was confirmed to be induced by NS1 in NS1-transfected UT7/Epo-S1 cells (86). However, G₁-phase arrest has not been observed in either B19V-infected or NS1-expressing CD36⁺ EPCs (73, 76, 85).

Of note, a 5′-GTTTTGT-3′ sequence in the P6 promoter, a CpG oligodeoxynucleotide-2006 (containing the CpG motif 5′-GTCGTT-3′) analog that is a ligand of Toll-like receptor 9 (TLR9), was shown to inhibit the growth of BFU-E progenitors by arresting cells at the S and G₂/M phases (113). Thus, the viral genome is also capable of inducing S- and G₂/M-phase arrest.

Taken together, these findings show that the viral genome and/or its replication is capable of inducing S-phase arrest, while NS1 per se induces G₂-phase arrest. Therefore, B19V infection-induced late-S-phase arrest is a compromised outcome of genome replication-induced S-phase arrest and NS1-induced G₂-phase arrest (85). While S-phase arrest enriches S-phase factors that favor viral DNA replication, G₂ arrest halts erythropoiesis of erythroid progenitors and eventually kills the cells.

B19V infection-induced erythroid cell death.

B19V infection of human erythroid progenitors in bone marrow and fetal tissues ultimately leads to cell death, which results in transient aplastic crisis (99). B19V specifically infects BFU-E and CFU-E progenitors, thereby arresting erythropoiesis (101, 102, 170). The mechanism of B19V-induced cell death of infected erythroid progenitors was apoptotic (171): hydrops fetalis tissue infected with B19V had characteristics of apoptosis (172), and fetal erythroid progenitors infected by B19V revealed ultrastructural features of apoptotic cell death (109).

Examination of B19V-mediated cytotoxicity in CD36⁺ EPCs and UT7/Epo-S1 cells revealed that both B19V infection and NS1 transfection induced apoptotic cell death, which involved caspase-3, -6, and -8 activation and DNA fragmentation (82, 144). B19V induced extrinsic apoptosis pathway activation, which involved the TNF-α pathway, in both infected CD36⁺ EPCs and NS1-expressing UT7/Epo cells (144). Furthermore, it was found that the virus-encoded 11-kDa protein played a role in B19V-induced apoptosis of CD36⁺ EPCs (97). The 11-kDa protein was shown to be a more potent inducer of apoptosis, due to its high expression level (∼100 times higher than that of NS1) and localization (cytoplasmic), and involved the activation of caspase-10 (97). In conclusion, upon B19V infection of erythroid progenitor cells, viral NS1 and 11-kDa proteins, possibly with other unidentified viral factors, synergistically act to induce the apoptosis of erythroid progenitor cells upon B19V infection.

B19V infection of CD36⁺ EPCs has been reported to coincide with the downregulation of thyroid, retinoid, and estrogen hormone receptors (173), which is of unknown consequence.

Cellular Response to Unproductive B19V Infection

The presence of B19V, as detected by viral DNA or viral proteins, has been associated with clinical diseases such as acute and chronic inflammatory cardiomyopathies (174–177), rheumatoid arthritis (178–181), vasculitis (182, 183), meningoencephalitis (184, 185), hepatitis (186, 187), and thyroid diseases (173, 188–190) (see “Diseases Caused by B19V Infection,” below). However, it has not been established that nonerythroid cells/tissues support productive viral DNA replication and the release of progeny virions (191).

B19V infection of endothelial cells.

B19V DNA is highly prevalent in endothelial cells of the myocardium during acute and chronic inflammatory cardiomyopathies (192). In one study, B19V DNA was frequently detected in patients with normal coronary anatomy that clinically mimicked acute myocardial infarction (193). However, another study also showed that B19V DNA was highly prevalent in myocardial autopsy specimens from subjects without myocarditis or dilative cardiomyopathy (194). B19V was reported to infect fetal capillary endothelia in placental villi and to express viral proteins (195). These findings suggest the potential role of B19V in cardiomyopathies.

Although primary endothelial cells from the pulmonary artery, umbilical vein, and aorta express the B19V receptor/coreceptors and bind virus similarly to UT7/Epo-S1 cells (130), B19V uses an alternative route, antibody-mediated endocytosis, to enter endothelial cells. The B19V-antibody complex could interact with the complement factor C1q and use the C1q receptor (CD93) for cell entry via endocytosis (130). Antibody-dependent virus entry might explain the frequent prevalence of B19V in various endothelial cells. B19V was also reported to infect U937 cells by exploiting antibody-dependent enhancement of entry, although this infection was abortive (131). NS1 expression in transfected and immobilized human endothelial cells (HMEC-1) activates STAT3/PIAS3 signaling, which upregulates immune response genes (IFNAR1 and IL-2) and downregulates genes associated with antiviral defense (OAS1 and TYK2) (196). In mice, anti-B19 VP1u IgG aggravates cardiac injury by the induction of inflammation (197). Recent studies of B19V infection of bone marrow-derived circulating angiogenic cells (CACs) and CD34⁺ KDR⁺ endothelial progenitor cells from patients who had chronic B19V-associated cardiomyopathy highlight the potential for B19V to be a pathogen in microvascular disease and cardiomyopathy (198, 199). B19V DNA replicative intermediates and mRNA transcripts were detected in nearly one-half of patients as well as in B19V-infected epithelial progenitor cells in vitro (198). VP1 was identified as a novel inducer of apoptosis with the activation of caspase-8 and caspase-10, through the activation of death receptor signaling. B19V causally impaired endothelial regeneration and spread in epithelial progenitor cell-xenografted SCID Beige mice (198). These observations are evidence that B19V infection can damage CACs and results in dysfunctional endogenous vascular repair.

Nevertheless, no studies reported to date show progeny virion production during B19V infection of endothelial cells or CACs in vitro. Possibly, B19V enters such cells and undergoes only one step of double-stranded DNA (dsDNA) conversion, expresses NS1 and VP proteins, and induces apoptosis or an inflammatory immune response in host cells and tissues.

B19V infection in other tissues.

The presence of viral DNA in a wide range of tissues in both healthy and diseased subjects reveals a lifelong persistence of B19V infection (200, 201). Persistence of viral DNA has been detected in up to 50% of biopsy specimens of the spleen, lymph nodes, tonsils, liver, heart, synovial tissues, skin, brain, and testes, for decades after infection (58, 178, 191, 200–204). Persistence may be partly maintained by silencing of viral gene expression by CpG DNA methylation (205). Increased NF-κB, COX2, and IL-6 expression levels in thyroid, colon, synoviocytes, and lymphoid tissues was correlated with the expression of B19V capsid proteins (206–209). The PLA2 activity of VP1u is responsible for the inflammatory response in synoviocytes (208). NS1 ectopic expression in the hepatocyte cell line HepG2 induces apoptosis, involving caspase-3 and -9 but not caspase-8 activation (83, 210).

In summary, B19V may enter and persist in various non-erythroid-lineage tissues, but there is no clear evidence that infection is productive, and it does not seem to cause disease.

Airway Epithelium Damage Caused by HBoV1 Infection

From in vitro modeling of HBoV1 infection of HAE-ALI cultures, HBoV1 infection appears to disrupt the epithelial barrier and shows hallmarks of lung airway tract injury, including disruption of the tight-junction barrier, loss of cilia, and epithelial cell hypertrophy (150, 167, 168). Infected HAE-ALI cultures manifest a clear dissociation of the tight junctions as the transepithelial electrical resistance of infected HAE-ALI drops significantly after infection. HBoV1 infection abolishes cilia on the apical side of the airway epithelium. Infected HAE cultures have a discernibly thinner epithelium and show nuclear enlargement at late stages of infection. Infected HAE cultures undergo gradual thinning of the epithelium and loss of epithelial cells (150, 167), suggesting that HBoV1 infection eventually kills HAE cells. The infected airway epithelium is regenerated by epithelial cells produced from airway stem cells, which maintains both its integrity and apical virus release for at least 50 days postinfection in vitro (168). HBoV1 infection causes a DDR with the hallmark phosphorylation of H2AX and RPA32 (155). The DDR could induce programmed cell death, for example, through reactive oxygen species (ROS) (211, 212). However, the nature of HBoV1 infection-induced airway epithelial cell death and maintenance of the integrity of infected epithelia is not known.

In bronchoalveolar lavage fluids of HBoV1-infected individuals, the cytokines epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), CCL-17, TNF-α, TNF-β, and TIMP-1 are upregulated. These cytokines were also detected in apical wash specimens of HBoV1-infected CuFi HAE-ALI cultures (213). HBoV1-induced airway damage might be mediated by HBoV1-induced cytokine expression.

In acute respiratory tract infections due to HBoV1, wheezing is a common symptom, and HBoV1 pathogenesis is at least partially due to airway epithelium damage, as seen in in vitro-infected HAE-ALI cultures.

EPIDEMIOLOGY

Virus Prevalence and Transmission

B19V.

B19V infection is common worldwide, showing regional epidemiological differences, with generally over one-half of the adult population having been exposed. The prevalence of B19V-specific antibodies in the population is age dependent, increasing from 2 to 20% in children <5 years old to 15 to 40% in children 5 to 18 years old and to 40 to 80% in the adult population, depending on both the assays used and the population (214–225). Seroprevalence, however, is much lower in some isolated areas, such as the Rodriguez Islands and among some Brazilian tribes, with adult seroprevalences of only 2 to 10% (226, 227). Prevalence can also be much higher, such as the 85% seroprevalence reported for 9-year-olds in Papua New Guinea (228). The typical age when an individual contracts B19V infection is 5 to 15 years, but susceptible adults may also be infected. Infection induces an immune response, which confers lifelong protection against reinfections. Neutralizing IgG is formed about 2 weeks after infection and is very effective in eradicating the virus from the bloodstream.

B19V is transmitted mainly by the respiratory route, but prodromal symptoms are fever, malaise, headache, and myalgia rather than respiratory symptoms (229). It is currently unknown how B19V overcomes the airway epithelium barrier to eventually reach bone marrow for infection. The virus can also be transmitted via blood or pooled-blood products, from a pregnant mother to her fetus, and possibly even from tattooing (230). Higher seroprevalences than those among controls have been detected among patients receiving blood products and women having experienced abortions but not in people with tattoos (218, 231, 232).

Droplet transmission was evident after intranasal inoculation of volunteers, as B19V was shown to be able to infect subjects and cause disease (229, 233). Furthermore, during the prodrome, viral DNA can be detected in the upper airways (229, 234–236). Detectable DNA also coincides with a transient high-titer viremia of >10¹⁰ vgc/ml, which rapidly declines to a low level that can persist for many months or even years (236–242). The viral load in the acute phase, however, does not correlate with disease severity (229). In patients with different chronic pathological backgrounds, B19V DNA has also been detected at a low frequency in the lower respiratory tract (243).

Due to the relative ease of spread of the virus, outbreaks of B19V-induced childhood rash (erythema infectiosum) are most common in schools and day care centers, affecting up to one-half of schoolchildren and one-fifth of susceptible staff (244–246). B19V outbreaks occur mostly in the winter and spring, with major epidemics occurring every few years. The high-risk period for spread is early in the acute phase of infection, before rash or arthralgia appears, when the viral loads are at their highest. A convalescent child, even with recurring episodes of rash, is no longer infectious and may attend school. In patients with an underlying hemolytic disorder who suffer from B19V-induced aplastic crisis, titers as high as 10¹⁴ vgc/ml can be observed (146). In contrast to erythema infectiosum patients, these patients are at the time of disease extremely contagious, so to hinder nosocomial spread, aplastic crisis patients should be isolated. Among both hospital staff and patients, the risk of nosocomial spread of the disease, acquired from close contact or environmental surfaces, is quite high, with reported attack rates of 50% (247–249). Control measures such as handwashing, closure of the ward, utilization of B19V-immune staff, and B19V education likely are crucial to contain transmission. To avoid contagion, standard and droplet precautions and isolation should be implemented (250).

The timing of viremia before rash symptoms, high viral load, persistence, and resistance of this nonenveloped virus to most virus inactivation procedures used in the manufacturing of blood products create a risk of transmission through blood or blood products such as plasma, blood cells, and clotting factors (251–264) as well as through bone marrow and solid-organ transplantations (265–270). Comparisons of subjects with and those without blood transfusions have revealed a significantly higher seropositivity rate in individuals who have received blood transfusions (218, 231). Even if symptomatic transfusion-transmitted B19V infections are generally rare (259, 271), among eight patients with transfusion-transmitted B19V infection, five became ill with anemia, pure red blood cell aplasia (PRCA), or pancytopenia, all of whom had an underlying hematological disorder, whereas recipients without such disorders exhibited only moderate symptoms (264). Among solid-organ transplant recipients, most seronegative pediatric kidney transplant recipients of B19V DNA-positive organs became infected within 1 month (with four exhibiting anemia) (265). Patients at high risk of severe complications due to B19V infection from contaminated blood products are immunocompromised individuals (AIDS patients, patients with congenital immunodeficiencies, transplant recipients, and other immunosuppressed patients), individuals who are hematopoietically deficient, and pregnant women. In 2004, the U.S. Food and Drug Administration (FDA) implemented the regulation that B19V DNA levels in plasma pools used for manufacturing of blood products must not exceed 10⁴ IU/ml (see http://www.fda.gov/BiologicsBloodVaccines/GuidanceComplianceRegulatoryInformation/Guidances/default.htm). Similar regulations apply in Europe (European Pharmacopoeia Commission, Council of Europe, European Directorate for the Quality of Medicines). This DNA limit is intended to ensure the safety of blood products (259, 261). Most probably, the existence of neutralizing antibodies in the donor is protective for the recipient if the viral load is low and the recipient has no underlying hemolytic diseases (238).

B19V can also be transmitted from an infected pregnant mother to the fetus. Although the normal outcome of intrauterine infection is delivery of a healthy baby, miscarriage and fetal death can also result if the mother is infected before her 20th week of pregnancy (272–275) (see “Erythema infectiosum,” below). The rate of transmission from mother to fetus has been estimated to be 25 to 50%, and the incidence of fetal loss in B19V-infected mothers has been estimated to be 1.7 to 12.5% (276–280). Seroprevalence has been shown to be higher in pregnant women who have experienced abortion than in those who have not (232). The risk of B19V infection during pregnancy is greatest among susceptible day care workers and schoolteachers (281), but as the risk is also high for pregnant women who are not in these professions, it has been debated whether a policy of excluding women from high-risk workplaces should be recommended (246, 277, 281–284). The increased risks of B19V infection among day care employees compared with those of socioeconomically similar health care professionals was recently estimated by using proportional-hazard regression. The relative risks were estimated to be 2.63 (95% confidence interval [CI], 1.27 to 5.46) among all women and, eliminating the effect of a woman's own children, 5.59 (95% CI, 1.40 to 22.4) among nulliparous subjects (281).

HBoV.

The other human-pathogenic parvovirus, HBoV1, is most likely also transmitted by the respiratory route; it causes respiratory illness, and it can be detected in very high loads in the airways during the acute phase, after which it may persist at low viral loads for months (10, 285–292). The general lack of the other HBoVs, HBoV2 to -4, in airway samples and their presence in feces suggest that these viruses are transmitted by the fecal-oral route (11–13, 293–301).

The HBoVs have not been shown to be transmitted by blood products or vertically (302), nor have they been shown even to be present in blood products (146, 303, 304), with the exception of one recent study from China (305). The most common method of detection of bocavirus infections is PCR, by which HBoV1 has been found globally and throughout the year in about 2 to 20% of airway samples, mainly from children aged 6 months to 5 years with upper or lower respiratory tract illness (306–310). In adults and the elderly, detection is infrequent (310–314). In stool, the most prevalent bocavirus is HBoV2 followed by HBoV1, HBoV3, and HBoV4. Besides airway and stool samples, HBoVs have also been detected worldwide by PCR in serum, tonsils, saliva, urine, gut, and cerebrospinal fluid (CSF) (28, 291, 315–327) as well as in river and sewage water (328–331).

Like B19V, HBoVs also cause systemic infections leading to viremia and an immune response (9, 28, 322, 323, 332–334). However, viremias seem to be more rare and short-lived and/or of lower titers in infections by the enteric HBoVs than in infections by HBoV1 (334, 335). Likewise, the corresponding IgG responses are generally weaker and more prone to waning (335, 336). In a follow-up study of children from birth to adolescence, the median age for HBoV1 infection was 2 years, whereas the median age for both HBoV2 and -3 infections was slightly lower (335). HBoV1 is the most common HBoV in the population, with a seroprevalence of 80% in 6-year-olds, while the seroprevalences of the enteric HBoVs for the same age group are 50% for HBoV2 and 10% for HBoV3 (335). HBoV4 is too rare to make any conclusions regarding transmission or seroprevalence. Due to serological cross-reactivity leading to overestimation and the immunological phenomenon of “original antigenic sin” (337) leading to underestimation (see “HBoV Laboratory Diagnosis,” below), the true frequency of exposure to these closely related viruses is difficult to determine (334, 335, 338).

In a study of saliva samples from 87 infants monitored from birth to 18 months, 76% had a primary HBoV1 infection. Based on the detection of single-nucleotide polymorphisms (SNPs), 12 of these infants had HBoV1 DNA demonstrating multiple variants over time, suggesting reinfection with different HBoV1 strains (291). However, high mutation rates of a persistent virus or contamination from other infants was not excluded, and secondary infections were not confirmed by, e.g., detected increases in IgG levels (336). Moreover, another follow-up study based on identical virus sequences suggested that prolonged DNA positivity could be the result of reactivation of a latent virus (292); if reactivations were to occur, the rate of HBoV1 detection would be expected to be high in elderly individuals, which is not the case (314).

Genotypes and Molecular Epidemiology

B19V.

For many years, the sequence divergence of B19V isolates was considered minimal, <2% in the whole genome (339–342). However, in the late 1990s, a new isolate, V9, was identified and shown to diverge by >11% from the prototypical B19V isolates (44, 45). Later, in 2002, yet another B19V variant emerged, represented by isolates LaLi and A6 (42, 43, 46). The three variants were named genotype 1, for the prototypical B19V isolates; genotype 2, for LaLi- and A6-like isolates; and genotype 3, for the V9- and D91.1-like isolates (42). Of the three B19V genotypes, by far the most common genotype currently circulating in the population worldwide is genotype 1 (343). Since the 1960s, genotype 2 has been seldom found in acute infections or in blood (58, 258, 343–348). However, due to the ability of the B19V genome to persist in tissues for decades after infection (58, 200–204) (see “B19V infection in other tissues,” above), it can frequently be detected within various soft tissues of subjects born before 1972 (58). Recently, B19V DNA was detected in 45% of 106 old bones of Finnish victims from World War II, and all sequences were of genotype 2, except for two, which surprisingly were of genotype 3 (349). Genotype 3 is currently circulating endemically, mainly in some geographical regions such as Ghana, but it has sporadically been encountered in Europe, Brazil, India, and South Africa (42, 343, 344, 350–355). All three genotypes appear to have similar biological, pathogenic, and antigenic properties and make up a single serotype (42, 48, 356, 357). Although rare, it is possible for more than one genotype to infect the same host (354, 358). Recombination between B19V genotypes has also been documented (354, 359). Of the three B19V genotypes, genotype 3 seems to be the most diverse, which might indicate that it has a longer evolutionary history than the other two genotypes (343, 357).

Considering that B19V is a DNA virus and the level of sequence divergence among the prototypical B19V isolates is low, the evolutionary speed is strikingly high, with substitution rates of up to 4 × 10⁻⁴ substitutions per site per year, which is within the range for RNA viruses (357, 360–362). Such high evolutionary rates have also been observed for canine parvovirus and other ssDNA viruses that use the host cellular DNA polymerase(s) for their replication (360, 362, 363).

HBoV.

Four differing HBoV variants, HBoV1 to -4, have been identified (9, 11–13). Of these, HBoV1 is the only respiratory virus; the others are more often detected in stool and therefore seem to be enteric. HBoVs have been shown to have a close evolutionary relationship to bocaviruses found in great apes (364–367). HBoV1 and -3 as well as gorilla and chimpanzee bocaviruses are, based on the NS1 protein sequence, members of the same species, Primate bocaparvovirus 1, whereas HBoV2 and -4 belong to the Primate bocaparvovirus 2 species (Table 1) (16, 367). It has been postulated that HBoV1 diverged from the ancestor common to chimpanzee bocavirus ∼60 to 80 years ago, whereas HBoV4 separated from great ape bocaviruses ∼200 to 300 years ago (366). More data are needed to confirm this theory. Furthermore, it seems that extensive inter- and intraspecies recombination has occurred among primate bocaparvoviruses (364, 365, 367–370).

IMMUNE RESPONSE

Adaptive Immune Response

Humoral immune response to B19V infection.

Following either natural (371) or experimental (229, 233) infection by B19V, a strong humoral response is elicited (372). IgM antibody is initially produced at 8 to 12 days postinfection, clears viremia, and lasts for 3 to 6 months (215, 229). The production of IgG antibody follows IgM a few days later. During the following weeks and months, IgM antibody wanes to an undetectable level, whereas IgG prevails. IgA antibody has also been detected and probably protects nasopharyngeal mucosa (373).

The IgM response, as well as the IgG response, is directed against both the VP1 and VP2 proteins (372, 374–376). Several epitopes have been identified on VP2 (377–380) and VP1 (93, 140). Most of the neutralizing epitopes of VP1 are localized in the VP1u or the VP1-VP2 junction region, eliciting a stronger response than VP2 epitopes (136, 139). Neutralization epitopes on the VP1u region are mainly linear and have been mapped to the N-terminal 80 aa (139), which also contain two epitopes for two neutralizing monoclonal antibodies generated from human peripheral blood mononuclear cells (381). B19V-specific B cell memory has been shown to be well established and maintained against conformational epitopes of VP2 and linear epitopes of VP1 but not linear epitopes of VP2 (382, 383).

During both acute and persistent B19V infections, the presence of antibodies against viral nonstructural proteins has also been documented. Anti-NS1 IgG has been proposed to be associated with persistent and complicated infection (384), while anti-NS1 IgM may also appear in acute infections (385) (see “B19V Serology,” below). Detection of antibodies against the two nonstructural 11-kDa and 7.5-kDa proteins has not been reported, although the 11-kDa protein was expressed at a level 100 times higher than that of NS1 during infection (97).

Cellular immunity to B19V infection.

In PBMCs from healthy individuals naturally infected with B19V, B19V-specific T cell-mediated responses are directed against the VP1 and VP2 proteins and presented to CD4⁺ T cells by HLA class II molecules (386). B19V-specific T helper cell proliferation can be detected in infected patients, and B cells, which recognize the VP1/2 capsids, receive class II-restricted help from CD4⁺ T cells (387).

Striking CD8⁺ T cell responses can be observed in patients acutely infected with B19V, which are sustained over a period of months, even after viremia clears (388). Ex vivo measurement of B19V-specific CD8⁺ T cell responses confirmed that the HLA-B35-restricted peptide derived from the NS1 protein is highly immunogenic in B19V-seropositive donors (389). In contrast, persistently infected individuals show more cellular immune responses to VP1 and VP2 than to NS1 (390). Both the VP1/2 and VP2-only capsids stimulate T helper cells to release gamma interferon (IFN-γ) and IL-10, suggesting that VP2 provides the major target for B19V-specific T helper cells years after virus infection (387). In disagreement, a study reported that the VP1/2 capsid did not promote positive responses for the production of TNF-α and IL-1α from a human monocytic cell line, THP-1, exposed to the B19V capsid (391). The VP1u-specific IFN-γ response is predominant in recently infected subjects, while VP1u-specific IFN-γ and IL-10 responses are absent in remotely infected patients despite the presence of B cell immunity against VP1u (392). Examination of cytokine responses to B19V infection shows that they are of the Th1 type, with IL-2, IL-12, and IL-15 being detected in acutely infected patients, correlating with the sustained CD8⁺ T cell response (393). There is no imbalance of cytokine patterns in persistent infection, except for an elevated IFN-γ response.

Overall, B19V-specific cellular immunity develops, which is directed against not only the capsid proteins VP1 and VP2 but also the nonstructural protein NS1. The CD8⁺ T cell response may play a prominent role in the control of acute B19V infection.

Humoral and cellular immune response to HBoV1 infection.

(i) Humoral immune response.

HBoV1 has been shown to induce a strong and long-lasting, albeit often fluctuating, antibody response (336). In contrast to systemic HBoV1 infections, infections by the enteric HBoVs have been hypothesized to be more local and result in weak immune responses (334). However, HBoV2 and -3 also cause systemic infections, including both viremia and antibodies; nevertheless, the IgG responses are generally weaker and more prone to waning than those to HBoV1 (335).

HBoV1 to -4 are structurally similar (26), differing within the major capsid protein VP3 by only 10 to 20%. The high similarity in virion capsid structures and capsid protein sequences among HBoV1 to -4 results in considerable cross-reactivity of IgG antibodies in VLP-based enzyme immunoassays (EIAs) (334, 394). Serological cross-reactivity partially accounts for the high HBoV1 seroprevalences reported previously (164–166). Exclusion of cross-reactivity, by competition with heterotypic VLPs, is a prerequisite for the detection of IgG toward specific epitopes of HBoV1, especially in past immunity (334, 335, 394). In addition to cross-reactivity, it has been shown that interactions between consecutive diverse HBoV infections affect HBoV immunity via a phenomenon called original antigenic sin (337). This was detected by observing that preexisting HBoV2 immunity in a subsequent HBoV1 infection resulted in low-level or nonexistent HBoV1-specific antibody responses (335). Instead, a vigorous recall response against the first HBoV2 strain appeared. Noncompetition HBoV1 and HBoV2 EIAs, however, showed the IgG responses to both consecutive virus types. This original antigenic sin was further characterized in a more controlled noninfectious setting in 10 sequentially VLP-inoculated rabbit pairs, 5 of which exhibited immune responses of various degrees, in line with this phenomenon (338). Based on the newly established HBoV competition EIA, the median age of HBoV1 IgG seroconversion is 1.9 years (range, 0.5 to 8.0 years). The HBoV1-specific IgG seroprevalence in children aged 6 years is 80% (335).

(ii) Cellular immune response.

The presence of HBoV1-specific CD4⁺ T cell immune responses in adults is strong and age dependent (395). In cultures of PBMCs isolated from healthy adults with HBoV1-specific IgG, CD4⁺ T helper cell responses specifically against HBoV1 VP3 VLPs include the release of IFN-γ, IL-10, and IL-13 (395, 396), and there is no cross-reactivity with responses against B19V VP2 VLPs (396). In HBoV1-infected individuals, levels of the cytokines TNF-α, IL-2, IL-5, and IL-8 are increased in sera (397). In nasopharyngeal aspirates of HBoV1-infected patients, levels of Th1/2 cytokines, especially IFN-γ, IL-2, and IL-4, are increased in children with HBoV1-related bronchiolitis (398).

Innate Immunity

Innate immunity to B19V infection.

The innate immune response during B19V infection has not been well studied. TLR9 can recognize the CpG oligodeoxynucleotide-2006 analog (5′-GTTTTGT-3′) with a phosphodiester backbone, localized in the P6 promoter region of the B19V genome (113). CpG oligonucleotide-2006 selectively inhibited the growth of BFU-E progenitors in a sequence-specific manner and stalled cells in S and G₂/M phases (113). Transfection of B19V NS1 and VP2 in nonpermissive COS7 cells significantly increased the expression levels of defensins and also regulated the expression of TLR4, -5, -7, and -9 (399). These findings need to be validated in B19V-permissive cells and during the natural course of B19V infection. SNPs associated with acute symptomatic B19V infection are present in the SKIP, MACF1, SPAG7, FLOT1, c6orf48, and RASSF5 genes, and these genes and their products might have a role in parvovirus infections (400).

Innate immunity to HBoV1 infection.

HBoV1 VP3 has been reported to modulate the IFN pathway by targeting ring finger protein 125 (RNF125), a negative regulator of type I IFN signaling. RNF125 conjugated Lys48-linked ubiquitination to RIG-I and subsequently led to the proteasome-dependent degradation of RIG-I. VP3 not only upregulated IFN-β promoter activity but also enhanced IFN-β production at both the mRNA and protein levels. VP3 interacted with RNF125, which resulted in the reduction of RNF125-mediated ubiquitination and proteasome-dependent degradation of RIG-I (401). In contrast, NP1 blocked IFN-β activation. NP1 interacted with IFN regulatory factor 3 (IRF-3) through the DNA-binding domain of IRF-3, preventing associations between IRF-3 and the IFN-β promoter (402). These studies were performed by using Sendai virus-infected HEK293 cells, which were transfected with HBoV1 VP3 or NP1. Whether or not HBoV1 modulates the innate immune response through the IFN-β pathway during virus infection of human airway epithelia awaits further investigation.

CLINICAL MANIFESTATIONS

Diseases Caused by B19V Infection

In several diverse diseases, B19V is the etiological agent (Table 2). Their pathophysiology has been established by studies of B19V outbreaks, series of well-defined cases, and intensive investigations of small numbers of patients. Furthermore, intranasal inoculation of B19V into healthy volunteers produced symptoms, signs, and laboratory findings later identified in ill patients, particularly cutaneous eruption, arthralgia, and depression of blood counts (229) with associated pathognomonic bone marrow morphology (233). Fifth disease and transient aplastic crisis are typical features of B19V infection in normal and hematologically stressed individuals; hydrops fetalis can follow in utero infection, and B19V can persist in marrow, causing pure red cell aplasia (PRCA) in an immunocompromised host. Many of the historical complications of clinical fifth disease have been observed in acute B19V infection and can be assumed to be unusual presentations of infection. In other syndromes, the evidence is less firm, often based on single, invalidated, marginally positive, or misinterpreted test results. As a result, some of the literature is in conflict, and initial reports may not be confirmed by more rigorous and better-controlled studies. Hepatitis, myocarditis, autoimmunity, and chronic fatigue syndrome have been linked to B19V infection, and there are mechanisms to explain these peculiar manifestations of the virus in each disease. It should be stressed that most B19V infections are likely asymptomatic (8): seroconversion occurs without apparent illness.

TABLE 2.

Major diseases caused by B19V infection

Disease(s)	Progression	Host(s)
Erythema infectiosum	Acute	Healthy children
Arthropathy	Acute or chronic	Healthy adults
Hydrops fetalis/fetal loss	Acute or chronic	Fetus
Transient aplastic crisis	Acute	Patients with a high rate of red blood cell turnover, e.g., sickle cell disease patients
Persistent infection and pure red cell aplasia	Chronic	Immunocompromised patients, e.g., HIV/AIDS or postchemotherapy patients

Erythema infectiosum.

Erythema infectiosum or fifth disease is more accurately designated acute B19V infection. “Slapped cheek” disease was categorized in the 19th century as fifth among a series of childhood rash illnesses and, as their etiologies were uncovered, fifth disease also was suspected of having a viral origin, perhaps as a rubella variant. Shortly following the discovery of B19V, this virus was established as the etiological agent of fifth disease.

Historically, fifth disease was described as occurring in seasonal outbreaks in patterns similar to those of rubella, being most prevalent in winter and spring and in 3-year cycles, with a brief period of incubation, a high infectivity rate, and likely droplet spread (403–406). Clinically, prodromal symptoms did not usually precede the rash, which appeared suddenly in otherwise well children. The rash appeared in stages, first as facial erythema (the “slapped cheek”) and then as erythematous maculopapular eruptions over the trunk and proximal extremities, followed by fluctuating and evanescent exanthematous eruptions lasting for weeks or longer. Arthralgia and myalgia were seen in more affected adults. Arthritis and encephalitis (407–409) were noted as rare complications of fifth disease.

Following the discovery of B19V, its candidacy as a disease agent was rapidly established by investigations of epidemics of fifth disease: B19V-specific IgM and seroconversion of IgG appeared in the blood of affected children (234). Numerous confirmatory studies of fifth-disease epidemics throughout the world have been reported (214, 410, 411). Variable presentations of infection, even within a family (412); a high proportion of asymptomatic seroconversion (8); and the prevalence of rheumatic symptoms in adults, especially women (8, 413), were corollary results. B19V was detected in skin biopsy specimens (414, 415). When parvovirus is epidemic in a community, it causes a mild rash illness in the normal pediatric population and a severe but purely hematological syndrome in susceptible sickle cell disease patients (235) (see below).

Variants in the stereotypical pattern of cutaneous eruption (416–419) have attracted the attention of dermatologists, usually as “glove-and-socks” syndromes (420, 421). The characteristic distribution of glove and socks can be seen with papular, petechial, pustular, and bullous morphologies (422–428), even desquamation (429); concurrently in family members (430); and with oral lesions (431), lymphangitis (432), and vasculitic features (see below). Lymphadenopathy is present in some children with fifth disease (433–435), and presentation with enlarged lymph nodes and hepatosplenomegaly can mimic mononucleosis (436, 437).

B19V arthropathy.

About one-third of adults with fifth disease have acute joint symptoms (8), more frequently than in children (438, 439), as predicted from earlier observations of fifth disease (406). However, chronic prolonged rheumatic symptoms can occur in children as well as adults (440, 441). Arthralgia and arthritis can persist for months or years after parvovirus infection and may be debilitating, but there is no joint erosion (442). The arthropathy resembles that of rheumatoid arthritis, but B19V is not the etiological agent of that disease. The relationship of parvovirus infection to other rheumatic syndromes is controversial (180, 443–449) as discussed below.

Arthropathy following B19V infection was described soon after the virus was linked to fifth disease (450, 451). In patients with serological confirmation of infection, many will have recently experienced a rash (450–457). Arthritis—swelling and redness—as well as pain mimic rheumatoid arthritis in joint distribution, but in most cases, resolution occurs within a few weeks of symptoms and signs (451), and upon long-term follow-up, patients are without symptoms (458). Of patients identified in a chronic arthropathy clinic, about 15% of adults had had a recent parvovirus infection (450); in rheumatic diseases of childhood, 20 to 35% of patients may show serological evidence of recent B19V infection (459, 460). In a German series, about one-quarter of synovial biopsy specimens from arthritis cases contained B19V DNA by PCR, and these cases were typically diagnosed as undifferentiated mono- and oligoarthritis (461). In more recent surveys, serological evidence of recent B19V infection was present in only 9/813 patients with persistent joint swelling and possible rheumatoid arthritis (462).

Rheumatoid factor (409) and other autoantibodies (463–468) may be detected, transiently, in B19V arthropathy, as in viral arthropathy in general (469). There are many reports of B19V DNA and occasionally of capsid protein (suggesting viral replication) in joints (446, 461, 470–475). However, in other studies, B19V infection has been infrequently determined to precede the onset of arthritis (476, 477). Conversely, B19V DNA also appears in control samples, either in joint fluid from patients without inflammation (473, 476, 478) or in healthy individuals who have undergone arthrocentesis for trauma (202, 476). That synovial tissue may be positive for multiple virus types suggests that inflammatory sites may harbor cells containing viruses (475).

The treatment of patients suffering from chronic parvovirus arthropathy is unclear. Most patients respond to nonsteroidal anti-inflammatory drugs or short courses of corticosteroids. Eventually, symptoms should resolve without residual joint disease (with exceedingly rare exceptions) (479). There are sporadic, unique reports of immunological abnormalities of uncertain clinical significance: histocompatibility antigen overrepresentation and low complement levels (480), low levels of proinflammatory cytokines and chemokines (481), and anti-NS1 antibodies (482). As B19V does not circulate in arthropathy patients, and the significance of detection of viral DNA in joint fluid or tissue is unclear, there is no obvious justification for immunoglobulin therapy, and the benefit of its occasional use (483, 484) is uncertain due to polypharmacy and/or natural resolution of the inflammatory process.

Hydrops fetalis.

Maternal B19V infection is a serious complication of pregnancy; it can cause both miscarriage (death before week 22) and intrauterine fetal death (IUFD) thereafter. Hydrops fetalis, massive edema in the fetus associated with death in utero or at birth, is best characterized, and B19V is important in the consideration of the differential diagnosis of nonimmune hydrops. Hydrops follows most commonly midtrimester infection of the mother but may occasionally occur with earlier- and later-trimester transmission (485–487). B19V exposure in early pregnancy may increase the rate of spontaneous abortion. Fetal demise was recognized early and has been extensively reviewed (488–498). From seroepidemiology, an annual loss of 120 fetuses was calculated for Japan (499), and about twice this number might be extrapolated for the United States. Congenital malformations due to B19V are rare, but occasionally neonates who survive in utero infection may have hematological abnormalities similar to those of persistent B19V infection (see below). Hydrops late in pregnancy and spontaneous fetal loss are the typical clinical sequelae of B19V, but other reported complications of infection have been isolated fetal effusions (rather than generalized edema) (500), mirror syndrome (edema concurrently in mother and fetus) (501), and preeclampsia and eclampsia (502).

(i) Pregnancy risks.

About 50% of women of childbearing age are seronegative for antibodies to B19V, but the proportion may be higher in developing countries (503–505). Seroconversion rates are variable during nonepidemic and epidemic periods, over which they may range 10-fold, from 1.5 to 13% (282). In early studies from the Public Health Laboratory in the United Kingdom (277) and the U.S. Centers for Disease Control and Prevention (506, 507), fetal transmission occurred in about 30% of women who became infected with B19V. The excess risk of fetal death is 3% to 11% when maternal infection occurs before 20 weeks of gestation, but the risk is very low thereafter (487, 508). In a smaller series of 39 B19V-infected pregnant women, no cases of hydrops were detected, but the overall fetal loss rate was 5% (278). In a long-term retrospective survey from Pittsburgh, PA, numbers of cases of B19V infection in pregnancy appeared to vary from year to year, usually without typical fifth-disease symptoms; hydrops occurred in 12% and intrauterine death occurred in 16% of 25 pregnancies (509). In another study of 43 intrauterine B19V infections (determined by fetal/infant PCR or IgM), none of the cases developed hydrops or died; the incidence of intrauterine B19V infection was 48% in the first half and 56% in the second half of pregnancy (280). In one prospective study of third-trimester fetal death, 7.5% of placentas contained B19V DNA (510). The variability in transmission from mother to fetus is illustrated by instances of infection and death of only a single twin (511, 512).

Fetal loss early in pregnancy has been associated with first-trimester B19V infections. In a population-based Danish study, the presence of anti-B19V IgM almost doubled the risk of fetal loss, but only 0.1% of the losses were attributable to the virus (513). In a cross-sectional study of fixed tissue from fetal loss, 6% contained B19V DNA, all in first-trimester spontaneous abortions (514). Some (515, 516) but not all (517) other cross-sectional studies have been confirmatory in implicating B19V in spontaneous abortion in early pregnancy.

Retrospectively, B19V was found in some series of tissues from nonimmune hydrops in 8 to 17.5% of cases, perhaps due to sampling during epidemics (518–521), but others have not shown risks of fetal loss and hydrops associated with serological evidence of recent B19V infection (522). One meta-analysis yielded a real but relatively low risk of fetal demise with maternal B19V infection, 10% overall and 12.5% for infection during the first 20 weeks of pregnancy (523). In a large British cohort study, the fetal risk was similar, 9%, and confined to the first 20 weeks of pregnancy (508). Risks of both infection and a poor fetal outcome have been sufficiently low that some experts have argued against the exclusion of women from occupations with exposure to children and monitoring during pregnancy (524, 525).

Children are the likely source of infection for most adults, and thus, multiparous women (with children at home) as well as teachers and health care workers may be at special risk of infection during pregnancy (246, 281, 526, 527) (see “Cellular response to productive B19V infection,” above).

It has been argued that lack of surveillance and virological investigations led to inaccurate estimates of fetal loss and associated risk factors such as maternal environmental contacts (528, 529). Of note, B19V infection may be without symptoms in an expectant mother, and fetal loss secondary to B19V might not be suspected except by the interested specialist (487, 530, 531).

(ii) Pathogenesis.

Hydrops is characterized by gross and global edema of fetus or newborn, often with visceral organomegaly. Severe anemia is typical, often with leukoerythroblastic reactions in the blood and evidence of inflammation, especially in the liver, where erythropoiesis is located. While anemia is the dominant hematological feature of B19V hydrops, fetal thrombocytopenia occurs and may be common (532, 533). Cytopathic effects of B19V infection include typical eosinophilic intranuclear inclusions and “balloon” cells, especially in fetal liver. Many postmortem pathological studies have documented B19V infection in fetal tissues by a variety of methods, including PCR, in situ hybridization, electron microscopy, and histochemistry (274, 489, 534–547). A few brief reports focus on myocardial infection and its consequences, such as heart failure and myocardial necrosis (548–550).

Maternal levels of antibody to B19V are higher in the mother than in the fetus, while viral DNA levels have been higher in fetal blood than in maternal blood (551).

(iii) Diagnosis.

Hydrops in utero is detected by ultrasound (552). Other diagnostic tests include specific evidence of cardiac failure in the fetus (553), maternal serum alpha-fetoprotein (554, 555), and detection of virus by amniocentesis (556) and cordocentesis (557). The presence of anti-NS1 antibodies may signal recent infection of a pregnant woman (558, 559).

(iv) Treatment.

There are many case reports of successful treatment of hydrops by intrauterine red blood cell transfusion (560–567), but this procedure may fail to rescue the fetus (568). The mortality rate in one series of even aggressively transfused fetuses was still high, with a survival rate of 70% overall (569). In 10 South Korean pregnancies, transfusion was associated with better outcomes, but the overall survival rate was only 60% for the entire cohort (570). Other therapeutic strategies have included immunoglobulin and digitalis (571). The efficacy of interventions is difficult to estimate, not only because transfusion is not completely effective but also because hydrops due to B19V can resolve spontaneously (572–576).

(v) Congenital infection.

Infants usually survive without any clinical consequence of uterine exposure to B19V. IgM and even on occasion viral DNA can be detected in infants months after birth (577); however, in other follow-up studies, evidence of chronic infection among exposed infants was lacking (578). Surveys of infants with congenital malformations have failed to implicate B19V (508, 579). In a large Danish cohort of >1,000 children whose mothers tested positive for B19V infection during pregnancy, there was no increase in rates of mortality and morbidity for children monitored for a median of 9 years after birth (580).

Pure red cell aplasia is the best-described and perhaps only congenital lesion following fetal infection. Anemia with a deficiency of erythroid precursors is indistinguishable from Diamond-Blackfan syndrome. In one small series from Denmark, B19V was considered the etiological agent in 3/11 children with Diamond-Blackfan syndrome, in all of whom anemia resolved (581). Anemia due to infection with B19V in utero may be fatal or resolve as the infant produces appropriate anti-B19V antibodies (582–584). Immunoglobulin infusions for persistent infection diminished viremia and led to apparent remission (585, 586) but in other instances have not been effective (529, 587, 588). Other infants resolved anemia and infection with blood transfusion support alone (584).

Congenital anemia may be accompanied by other hematological abnormalities such as thrombocytopenia (589) and transient leukoerythroblastosis (590). A few instances of fetal liver disease with B19V hydrops have been described (548, 591). Neurological symptoms, mainly developmental delays but also seizures and hydrocephalus, may be direct sequelae of viral infection or indirectly related to severe anemia in utero and at birth (587, 592, 593). Other diverse associations include ascites (594–596), bone lesions (597), and corneal opacification (598). Embryonic malformations have been seen in incomplete embryos after uterine infection (599). However, infants who survive hydrops do not have other congenital abnormalities, nor is B19V considered a major etiological agent of congenital malformations.

Transient aplastic crisis (TAC).

When parvovirus is epidemic in the general population, some patients in hematology clinics will coincidentally suffer a specific complication of B19V, transient aplastic crisis (235). Transient aplastic crisis was recognized as a life-threatening acute event in sickle cell disease long before its viral etiology was known (600–602). Its sudden appearance in families, apparently brief incubation period, and transiency were suggestive of an infection (603); early findings in marrow giant pronormoblasts are now recognized as a viral signature of infection. The target cell of B19V is the erythroid progenitor, always infected during parvovirus illness. As demonstrated in healthy volunteers infected with B19V, blood counts fall and there is an absence of reticulocytes (the young circulating erythrocytes) (229). Anemia does not develop in healthy volunteers and in otherwise healthy individuals, due to the brief cessation of erythropoiesis and the long life span of erythrocytes in the circulation. Under conditions of hematopoietic stress, usually in patients with sickle cell disease, in which there is increased red cell destruction due to chronic hemolytic anemia, B19V infection leads to a precipitous worsening of anemia. Red blood cell transfusion is adequate to treat transient aplastic crisis, but anemia can be severe and fatal if transfusions are not available or not administered urgently.

Transient aplastic crisis was the first human illness associated with B19V, and it was discovered during routine immunoelectrophoresis for testing for antibodies to B19V and antigen in young children (604). Antigenemia, signifying viremia, is common in cases of aplastic crisis but almost never observed in fifth disease; in healthy volunteers, marrow suppression occurs with viremia and fifth-disease symptoms with antibody production and immune complex formation (229) (occasional case reports document this sequence [605]). Levels of circulating virus in cases of aplastic crisis can be extraordinary, 10⁸ to 10¹² vgc/ml (146, 371). IgM is usually detectable in serum (371, 606, 607).

Transient aplastic crisis due to B19V infection has been well characterized by studies of outbreaks (371, 608, 609) and many large case series (606, 607, 610–615). B19V explains virtually all community-acquired cases of transient aplastic crisis. Parvovirus infection is the presumptive diagnosis when anemia worsens acutely in sickle cell disease, especially if the usually elevated reticulocyte count is low. Reticulocytopenia is diagnostic, and bone marrow examination is not required. Aplastic crisis in cases of sickle cell disease is serious and life-threatening. Patients are acutely ill, febrile, and often in pain; not only is the anemia usually extremely severe, but there also may be accompanying infarction events, acute chest syndrome, and splenic sequestration. Recovery within a week or two is typical, associated with the appearance of IgM and IgG antibodies specific to B19V; rarely, anemia may be prolonged (616). Particularly severe B19V aplastic crisis in cases of hemolytic anemia can produce marrow necrosis (617–619) and infarction (620), hemophagocytosis (621, 622), fat embolism (623, 624), and splenic sequestration (625, 626). Conversely, and as in the general population, B19V infection may be subclinical in sickle cell disease, inferred from seroconversion without a history of aplastic crisis (610, 627).

Siblings and other family members are affected at a high rate, consistent with the known rate of transmission of B19V, and the incubation period is brief, usually just a few days. Seroconversion occurs early in childhood and continues throughout adult life, as in the general population; in a well-studied cohort in Philadelphia, PA, the incidence was estimated to be 11/100 patient years (611). About one-half of sickle cell patients enter adulthood seronegative and thus at risk of infection. Transient aplastic crisis appears to be a unique event in the course of sickle cell disease (611, 613): from lack of recurrence, lifelong immunity to B19V has been inferred.

Transient aplastic crisis occurs with other hematological syndromes (628). It can be the first evidence of hereditary spherocytosis, in which hemolysis is well compensated and baseline anemia is mild (606, 607, 629–636). B19V aplastic crisis has been documented in cases of hereditary somatocytosis (637), thalassemia (638, 639), hemoglobin C disease (640), red cell enzyme deficiencies (641–645), iron deficiency anemia (646), acquired immune hemolytic anemias (621, 647–649), Diamond-Blackfan anemia (650), hereditary erythrocyte multinuclearity (651), human immunodeficiency syndrome (652), myelofibrosis (653) and complicating or mimicking myelodysplastic syndrome (628, 654–657), and acute (658) and chronic (659) lymphocytic leukemia.

Transient cessation of erythropoiesis likely occurs in all individuals infected with B19V, as first observed among inoculated normal volunteers (229) and later in laboratory personnel who were accidently infected (659). Erythroblastopenia immediately precedes fifth disease when early blood counts are obtained (660). Acute anemia with parvovirus infection in apparently normal children has been reported occasionally (651, 660–664), but B19V infection is not the cause of transient erythroblastopenia of childhood (665–667), although recent B19V infection has been reported in a few instances (661, 663, 668).

Persistent infection and pure red blood cell aplasia.

B19V infection can persist in individuals with compromised immune systems. As infection causes acute anemia when the patient's bone marrow is susceptible because of underlying erythroid stress, anemia is chronic when the immune system fails to mount a neutralizing antibody response. The marrow morphologies in acute and chronic B19 infections are the same, with a lack of erythroid precursors and with the presence of scattered giant pronormoblasts and anemia accompanied by a virtual absence of reticulocytes. Other blood counts are normal. As with transient aplastic crisis, in pure red cell aplasia due to the persistence of B19V, there is little clinical evidence of viral infections. These symptoms and signs—fever, malaise, cutaneous eruption, and joint pain—are mediated by immune complexes, which do not form in the absence of antibodies to the virus. Because persistent infection does not manifest as a typical viral illness and lacks the typical features of fifth disease, the diagnosis can be obscure. Recognition of B19V persistence is important because antibody therapy is effective in ameliorating or curing disease manifestations. However, an inaccurate diagnosis subjects the patient to unnecessary toxicity from immunoglobulin infusions, which are costly (669).

Typically, in cases of red cell aplasia due to persistent infection, titers of B19V in the circulation are extremely elevated. Viral DNA can be measured readily by direct hybridization methods. Low levels of B19V, detectable by gene amplification, may be present in the blood of normal persons for months or longer after an acute infection, unassociated with symptoms or blood count changes (373, 670–672). The virus may persist in many visceral tissues as well, complicating attempts to associate B19V infection with disease (see below).

In the first described case (137), the patient had had a diagnosis of pure red cell aplasia for a decade; his brother had developed the same syndrome at the same time and succumbed to it. The presence of giant pronormoblasts in marrow was the clue to a viral etiology, and B19V DNA was found at very high concentrations in the living patient and in his and his brother's pathological specimens. The proband was ultimately diagnosed with Nezelof's immunodeficiency, and as a result, he lacked reactivity to the VP1-unique region of the B19V capsid protein and neutralizing antibodies to the virus. In general, for persistent infection, serological testing is uninformative: either antibodies are not produced or there is weak reactivity of anti-B19V IgG on capture assays or enzyme-liked immunosorbent assays (ELISAs). DNA testing, preferably by direct hybridization and quantitative PCR (qPCR), should show abundant viral genetic material. The initial patient was treated with normal donor immunoglobulins to restore antibody activity, with prompt reticulocytosis and complete correction of his long-standing transfusion-dependent anemia. Identification of persistent parvovirus infection causing anemia is important, as effective therapy is available and easily applied; also, persistently infected patients remain highly infectious to others.

B19V persistence occurs with immunodeficiencies of different origins: congenitally (673–675); secondary to chemotherapy and with immunosuppression, especially posttransplantation; and in patients infected with human immunodeficiency virus (HIV). Children are particularly susceptible because of a lack of prior immunity to B19V. In a survey of cases of pediatric red cell aplasia, 7/33 children had persistent B19V infection (676). B19V is a particular complication in cases of acute lymphocytic leukemia, the major leukemia of childhood (137, 677–685), and other pediatric cancers (620, 686, 687). Adults with acute and chronic lymphocytic leukemia and lymphoproliferative disease can be affected; they have often been treated with lymphocyte-depleting drugs and monoclonal antibodies (620, 688–690). Iatrogenic immunosuppression is required after organ replacement, and B19V DNA, like DNAs of other viruses, may be found in the circulation in this context, without clinical manifestations (691–693). B19V anemia should be accompanied by reticulocytopenia and viremia (267). PRCA due to B19V occurs in post-hematopoietic stem cell transplant (692, 694–702) and post-solid-organ transplant (703–722) patients. In post-hematopoietic stem cell transplant patients, persistent B19V infection was associated with severe lymphopenia (723). B19V persistence can cause PRCA in HIV-infected patients, sometimes as the presenting syndrome and prior to the development of AIDS (652, 724–734). B19V is not prevalent in HIV-infected individuals, however, and it is likely that a very-high-titer infection is required for the development of PRCA (735, 736). B19V has been detected in marrow specimens from AIDS patients with PRCA, with accompanying giant pronormoblasts (733, 737).

Persistent B19V infection causing PRCA has been described rarely for immunologically normal persons (738–740) and patients without an apparent immunological defect (741, 742). Nevertheless, about 30% of patients admitted to the hospital with chronic anemia in one serological screen showed DNA or IgM evidence of B19V infection (628).

Anemia due to chronic B19V infection is responsive to immunoglobulin administration, but repeated courses may be necessary (724, 743, 744). HIV patients also clear B19V with immune reconstitution upon highly active antiretroviral therapy (744–746).

B19V infection and malaria.

Of relevance to potential vaccine development, B19V occurs in areas where malaria is endemic (747), and serological surveys have suggested worse chronic anemia in children who are seropositive for B19V (IgG) or have evidence of recent infection (IgM and viral DNA) (748–751). Acute B19V infection may occur concurrently with malaria (752, 753). Among pregnant Sudanese women, seropositivity for IgG to B19V was associated with lower hemoglobin levels (754).

Other hematological syndromes linked to B19V.

B19V has been linked to other blood disorders by serological evidence of recent infection and sometimes detection of the virus in blood and marrow, but an etiological role of B19V in these diseases is less defined than for transient aplastic crisis. Coincidental infection usually cannot be excluded, and the causative role suggested in individual case reports has often not been validated when sera from larger numbers of patients have been systematically examined (6, 755).

Hemophagocytosis is generalized, usually severe, and frequently fatal marrow failure and pancytopenia, with accompanying fever and a marked elevation of levels of inflammatory markers and cytokines. Hemophagocytosis, especially in children, is triggered by viral infections, most often herpesviruses and less commonly B19V (621, 729, 756–762). Some cases are fatal. Hemophagocytosis may be the manifestation of B19V infection during pregnancy (763). Hemophagocytosis can be observed in marrow of patients with transient aplastic crisis (764, 765).

Although anemia is the hallmark of symptomatic hematological disease due to B19V infection, thrombocytopenia, usually moderate, can accompany transient aplastic crisis (605, 663, 766), as may neutropenia (767–769), or both, producing transient pancytopenia (770). Mild decreases in blood counts have been noted rarely in cases of fifth disease, without underlying hematological disorders (771), but they may be more frequent if actively ascertained (772). Cytopenias without manifest fifth disease but with serological evidence of acute B19V infection may occur (773). B19V protein has been detected in granulocytes of an affected patient (702).

Thrombocytopenia occurs alone in B19V infection, which may lead to a diagnosis of idiopathic thrombocytopenic purpura (ITP) (in which immune destruction of platelets is the pathophysiology) (774–776) and occasionally megakaryocytic thrombocytopenic purpura (in which marrow does not produce platelets) (777). For ITP, systematic surveys suggest that a minority of otherwise typical cases might have a B19V origin (701, 778). The absence of megakaryocytes upon bone marrow examination in a few cases (663) implies a direct effect of B19V on platelet production.

Neutropenia may also result from immune-mediated peripheral destruction of granulocytes or a failure of the marrow to produce white blood cells, termed agranulocytosis. As described above, a modest decrease in neutrophils can be observed with B19V infection. Rare instances of isolated neutropenia have been blamed on B19V in children (779) and adults (780, 781). A claim of B19V as the etiological agent of chronic neutropenia in childhood (782) could not be confirmed (783). In a single convincing case, recurrent agranulocytosis was associated with persistent B19V infection (784). In a newborn with apparent congenital neutropenia, or Schwachman-Diamond syndrome, delivery had been proceeded by maternal-fetal B19V transmission and infection (785).

Aplastic anemia, pancytopenia with marrow hypocellularity, is not due to B19V in general; rare instances with associated seroconversion at presentation may be coincident with B19V infection rather than evidence of causation (786–793).

B19V infection can occur in patients with leukemia, especially during or after chemotherapy, producing the expected reticulocytopenia (794–797). Its significance in other respects, especially for pathogenesis, is unclear (6, 798). Viral infection preceding presentation with acute lymphoblastic leukemia has been observed (799, 800). Giant pronormoblasts can mimic the appearance of myelo- and lymphoblasts and therefore complicate the interpretation of blood and marrow histology (415).

Encephalitis and other neurological syndromes.

Fifth-disease patients were occasionally recognized to suffer from neurological symptoms, and diverse neurological complications have been associated with parvovirus infection, including encephalitis, meningitis, stroke, and neuropathy (407, 408, 801–804).

Neurological complications of B19V may be more frequent in immunocompromised hosts who have underlying diseases, in patients undergoing chemotherapy or immunosuppression, and in the setting of hemolytic anemia (805), but they can occur in immunocompetent, apparently normal children and adults and in the context of acute or chronic parvovirus infection. Pediatric cases include diagnoses of encephalitis (184, 805–810), meningitis (811), status epilepticus (812) and seizures (813), stroke (814, 815), amyotrophy (816), Guillain-Barré syndrome (817), ocular disease (813, 818), transverse myelitis (819), peripheral neuropathy (820), cerebellar ataxia (821), and others. Chorea and ataxia may be features of B19V encephalitis (184, 805, 822, 823). Fetal brain infection has occurred after in utero infection in hydropic births (824, 825). B19V DNA has been detected by gene amplification in brains at autopsy of patients with psychiatric disorders (826, 827), likely due to either laboratory contamination or a persistence of low levels of viral DNA in these tissues (828, 829), as in many others (see below). B19V DNA was detected by gene amplification in about 5% of cerebrospinal fluid samples from patients with encephalitis (184, 830).

A specific genotype of B19V in the blood of encephalitis cases was described (831). Associations with histocompatibility antigens and serum cytokines have been inferred to support an immune pathophysiology (822).

Intravenous immunoglobulin (IVIG) has been employed therapeutically for neurological syndromes linked to B19V (805, 808, 824), but efficacy is difficult to judge, as many patients improve spontaneously.

Myocarditis and other heart diseases.

B19V infection can be followed by serious cardiac disease, especially pediatric myocarditis. Other viruses are more frequently associated clinically or found by molecular methods in endocardial biopsy specimens (832). The clinical course of B19V myocarditis can be severe, with significant morbidity and mortality, including the need for a transplant.

B19V infection, detected most often by serological evidence of recent infection and sometimes by the detection of viral genomes in cardiac tissue, has been reported in myocarditis in the fetus (see the section on hydrops below), in infancy, during childhood, and among young adults (174, 176, 832–840). In one center's series of 19 pediatric patients, the presenting symptoms were upper respiratory and gastrointestinal symptoms; all infected children had ventricular dysfunction, 2 died, and 3 required heart transplantation (176). B19V infection has been cited as a cause of cardiac inflammation after heart transplantation (175, 841–845) and as a possible etiological agent of adverse events after coronary artery stenting (846). In some series, B19V myocarditis was distinguished clinically by presentation with infarction-type pain but with a better prognosis for recovery (847). In children with acute heart failure, the presence of B19V DNA has been associated with improvement of cardiac function over time and a better long-term prognosis (848). Dilated cardiomyopathy in a few Chinese systemic lupus patients was associated with serological evidence of B19V infection and immune cytokine levels (849).

As with other tissues, the prevalence of B19V DNA in the hearts of individuals not suspected of having a B19V syndrome (essentially, controls) is high (193, 203, 850, 851). In an autopsy study, B19V was detected by gene amplification in one-third of postmortem hearts (viral DNA was also detected in about 10% of livers, 20 to 30% of marrow, and 10% of kidneys, not correlated with serological status) (852). Therefore, it is not surprising that investigators have detected B19V DNA in various percentages of patients with myocarditis (192, 853, 854), pediatric heart transplant recipients (855), and cases of dilated cardiomyopathy (853, 856). The presence of replicative intermediate B19V DNA forms (857–859) and B19V capsid protein in biopsy specimens has been reported (860), but B19V viral loads have not correlated with outcomes (861). Myositis affecting muscles other than the heart is rarely associated with B19V (798).

A virus-triggered immune pathophysiology has been inferred from NS1-specific cytotoxic lymphocytes in cases of inflammatory cardiomyopathy (862). T cell infiltrates in the heart and elevated circulating concentrations of cytokines may accompany putative viral infection (174, 863) and immunomodulatory therapies applied to eliminate the virus (864).

Hepatitis and other liver diseases.

B19V has been reported to be associated with a range of liver findings (865), from a mild elevation of levels of hepatic transaminases (866, 867) to viral-like acute hepatitis (186, 793, 868–876) and acute or fulminant liver failure (765, 877–882). As in other tissues, livers at autopsy or sampled by biopsy can show persistence of B19V DNA (883, 884), confounding the interpretation of data from case reports. Systematic studies of sera from patients with non-A, non-B, non-C hepatitis have not shown B19V infection (885–888).

B19V and autoimmune/immune-mediated diseases.

Similarities between B19V arthropathy—specific clinical and serological features—suggested that parvovirus infection might be etiological or associated with rheumatoid arthritis, especially as an infectious etiology for rheumatoid arthritis for many years, and earlier work even suggested a candidate parvovirus, RA-1 (889) (likely a cell culture contaminant, in retrospect). Viruses have been suspected of triggering or causing a wide variety of immune-mediated and autoimmune diseases, with elegant mechanisms and models proposed for B19V as well (179, 890–893).

Some investigators claimed evidence of a strong link between B19V infection, most often serological but including detection of DNA in joint tissue, and classic rheumatoid arthritis (181, 894–897), including possible interactions with histocompatibility antigen (898). However, these data have not been confirmed (446, 899–901). The relationship of B19V to juvenile rheumatoid arthritis (Still's disease) followed a similar pattern of enthusiastic claims of causation (902–905) but had a lack of reproducibility of results (202, 449, 906). A higher prevalence of antibodies to NS1 was present in children with a variety of rheumatic diseases, interpreted as evidence of persistent infection (907).

Facial (malar) rash and joint pain and swelling, even angioedema, are features of systemic lupus erythematosus (SLE), and B19V may mimic the clinical presentation of SLE (908–912). In a few cases, B19V infection seemed to be followed by the development of SLE (913), although misdiagnosis of B19V arthropathy with the more serious autoimmune disease seems possible in retrospect (824, 913–915). Interesting speculation has hypothesized a mechanistic link of B19V to SLE (891, 916, 917). However, B19V could not be serologically linked to SLE (918). B19V DNA was first detected in a high proportion of marrow specimens and later in skin (920) from cases of systemic sclerosis, a severe multiorgan fibrotic disease (919); the same group found viral DNA localized to endothelial and perivascular inflammatory cells in skin biopsy specimens by in situ hybridization, with negative controls (921). However, the virus has also been detected in skin samples from controls in other studies (922).

Necrosis of skin lesions and sometimes visceral organ involvement suggestive of vasculitis have been reported in cases of fifth disease (923–929). As a group, various vasculitis syndromes have been linked to B19V infection, usually by single case reports or small case series, including leukocytoclastic vasculitis (930), Kawasaki disease (931), Henoch-Schonlein purpura (932, 933), polyarteritis nodosa (183), Wegener's granulomatosis (183), and giant cell arthritis (934). In some of these reports, documentation of acute infection and characteristic vasculitis pathology is convincing, and patients may even have responded to immunoglobulin therapy after failing immunosuppression (183, 935), suggesting an etiological rather than coincidental role for parvovirus. Anti-NS1 antibodies were prevalent in one series of Henoch-Schonlein disease cases (936), as in cases of childhood arthritis (see above). However, systematic attempts to relate B19V infection to a range of vasculitis syndromes have often yielded negative results (356, 937–941).

B19V and kidney disease.

B19V infection has been linked to kidney diseases (942). The most common disease is red cell aplasia in immunosuppressed allograft recipients (943). There are case reports of B19V and glomerulonephritis (944–946), nephrotic syndrome (947), thrombotic microangiopathy (948, 949), and other post-renal-transplant complications. B19V DNA may be present in tissue obtained at biopsy but, as with other organs, may represent only viral persistence long after infection.

Other associations.

Chronic fatigue syndrome may be a debilitating consequence of apparent viral infection. B19V has been reported to be the trigger in individual cases (950–952) and case series (952). Psychological stress has been suggested to worsen symptomatology (953). Viremia has been present in some series (952) and absent in others (954). A few patients appeared to have improved after immunoglobulin administration (955), but in one Italian patient, immunoglobulin infusion worsened symptoms and was associated with the appearance of circulating viral DNA (956). Fibromyalgia has also followed B19V infection (957).

Pneumonia, usually in the setting of other apparent organ involvements, has been observed in B19V-infected patients (436, 870, 958–962).

Other syndromes for which a causative role for B19V is unclear include generalized edema (963–966), myositis (967), the onset of diabetes (968) and inflammatory bowel disease (969), uveitis (970), retinal detachment (971), retinal pigment epitheliopathy (972), Rosai-Dorfman histiocytic proliferation (973), laryngitis (974), inner ear disease (975), and testicular tumors (976). B19V DNA persistence in the thyroid after previous infection may account for associations with thyroid cancer (191) and autoimmune thyroiditis (977).

Diseases Associated with HBoV Infection

Respiratory tract infections.

Although Koch's postulates cannot be fulfilled for HBoV1 by using animal models, there is increasing evidence suggesting that HBoV1 is indeed an etiological pathogen, rather than an innocent bystander, in both upper and lower respiratory tract infections, especially in children under 5 years of age.

HBoV1 DNA has been found in nasopharyngeal samples worldwide in ∼2 to 20% of patients with upper or lower respiratory disease (10). In early studies of respiratory tract infections, and unfortunately also in current laboratory diagnostics, nonquantitative PCR detection of HBoV1 DNA is overwhelmingly used for the detection of HBoV1 infections (9, 307, 308, 310, 311, 317, 978–996) (see “HBoV Laboratory Diagnosis,” below). A high coinfection rate (up to 83%) is generally detected in respiratory specimens, and the virus has also been detected in asymptomatic children. The etiological role of HBoV1 in respiratory diseases has consequently been questioned. However, the ubiquitous presence of HBoV1 is actually due to the persistence of the virus in the nasopharynx for several weeks to up to a year after infection (285–289, 291). To determine the causative role of HBoV1, other diagnostic markers are needed beyond mere PCR positivity in airway samples. These markers include high viral loads, mRNA or antigen detection in respiratory samples, viremia, serodiagnosis, and monoinfection (28, 316, 322, 323, 332, 333, 997–1001). Careful clinical studies applying more accurate markers have been done, providing accumulating evidence that HBoV1 is an important respiratory pathogen in children (28, 151, 285, 286, 288, 290, 310, 316, 322, 323, 332, 333, 1001–1010). Pneumonia, bronchiolitis, acute otitis media, the common cold, and exacerbations of asthma are the most common clinical manifestations of HBoV1 respiratory tract infections, with symptoms of cough, fever, rhinitis, wheezing, and diarrhea (151, 287, 306–308, 310, 311, 316, 317, 322, 979, 990, 1002, 1005–1007, 1011–1020). In one study, seven children with severe acute respiratory tract illness in a pediatric intensive care unit had HBoV1 as the sole pathogen, as verified by next-generation sequencing (NGS) (1010).

A low virus load of less than 10⁴ to 10⁶ vgc/ml, depending on the study, detected by quantitative PCR in respiratory specimens should not be used as a diagnostic marker for acute HBoV1 infection, as prolonged/persistent infection/reinfection by HBoV1 in the respiratory tract also results in low viral loads (285, 287, 288, 291, 316, 1012, 1021–1025). Considering viral loads below this threshold as being diagnostic leads to high rates of detection in asymptomatic subjects or patients coinfected with other respiratory pathogens.

HBoV1 monodetection, high viral loads of >10⁴ to 10⁸ vgc/ml in respiratory specimens (151, 285, 290, 316, 322, 323, 999, 1009, 1026–1031) and detection of HBoV1-specific IgM or a 4-fold increase in the levels of HBoV1-specific IgG antibodies (28, 151, 332, 1006) have been clearly linked to acute respiratory infections. In some studies, high viral loads during acute infection have further been shown to correlate with increased symptom severity or duration (290, 1026, 1029), while other studies found no such differences (285, 1028). Lower respiratory tract infection has been correlated with the detection of HBoV1 spliced mRNA, an indirect marker of active HBoV1 replication, in nasopharyngeal aspirates (997, 1001, 1032) and the detection of viral DNA in serum (viremia) (1009, 1033). In a clinical assessment of HBoV1 infection among 258 wheezing children (332), 96% of the children with HBoV1 loads of >10⁴ vgc/ml in nasopharyngeal aspirates also had a serodiagnosis (IgM and often also IgG conversion or a ≥4-fold increase in the IgG level), compared with only 38% of the children with low viral DNA loads. Of 49 children with HBoV1 viremia, 92% exhibited a serodiagnosis. Furthermore, among 39 HBoV1 PCR-positive children with codetection of other respiratory viruses, 64% had true serologically proven acute HBoV1 infections, i.e., had IgM and often also seroconversion or a ≥4-fold increase in IgG levels.

Life-threatening HBoV1 infections in pediatric patients, detected by high virus loads or diagnostic HBoV1-specific antibodies, have been reported (1007–1009, 1030, 1034). In a case of severe airway constriction, a 4-year-old girl who had a history of wheezing had bronchiolitis, leading to severe respiratory failure and the need for extracorporeal membrane oxygenation (1009). The patient was negative for both bacterial and viral respiratory pathogens but positive for HBoV1 infection, with virus loads of up to 10⁹ vgc/ml in nasopharyngeal aspirates and of up to 10⁴ vgc/ml in serum, and she was serum IgM positive. In another case, a 20-month-old child had acute bronchiolitis that developed into a severe airway constriction course characterized by pneumothorax, pneumomediastinum, and respiratory failure with air leak syndrome. HBoV1 infection was verified by electron microscopy and HBoV1 genome monitoring in respiratory secretions and plasma (1013). In addition, an immunosuppressed adult patient who died of respiratory complications was diagnosed with HBoV1 infection with very high virus loads in respiratory secretions (>4.3 × 10⁹ vgc/ml) and in serum (1.5 × 10⁴ vgc/ml) but without HBoV1-specific IgG/IgM. No other pathogens were detected at high titers, suggesting that HBoV1 may indeed have been the causative agent of the respiratory complications that led to the death of this patient (1031). Besides being a respiratory pathogen, HBoV1 has also been detected as the sole pathogen in cerebrospinal fluid samples of five patients with encephalitis, one of whom, a malnourished child from Bangladesh, died (326, 327).

In conclusion, primary acute HBoV1 infection should be diagnosed only when high virus loads are detected, active virus is found replicating in respiratory secretions, or the patient is IgM positive and/or has a seroconversion or a ≥4-fold increase in IgG titers or viremia. Primary acute HBoV1 infection causes both upper and lower respiratory tract infections, with manifestations ranging from the mild symptoms of the common cold to severe pneumonia and bronchiolitis, which may be life-threatening.

Diseases associated with HBoV2 to HBoV4.

HBoV2 to -4 are detected primarily in stool; however, the role of HBoV2 to -4 in gastrointestinal infections is unclear. HBoV2 was present in 20% of symptomatic and asymptomatic patients, whereas lower prevalence rates have been found for HBoV3 and HBoV4 (295, 298, 300, 1035–1037). Although some reports show a difference in the prevalences of HBoV2 in stool specimens between patients with gastroenteritis and healthy or nondiarrheal controls (11, 1036), other reports have found no such associations. Furthermore, HBoV2 has often been codetected with other viral pathogens, especially rotavirus and norovirus (300, 1036, 1038). In addition, HBoV2 infection does not appear to exacerbate clinical symptoms of gastroenteritis (1036). In most studies, however, there were no associations of HBoVs with gastroenteritis (300, 1035, 1038, 1039). Consequently, there is no clear evidence yet to support a causative role of HBoV2 in gastroenteritis. HBoV2 has been involved in two fatal cases, one of myocarditis and one of encephalitis, as the sole finding in a range of tissues, blood, or CSF (326, 1040). HBoV3 and -4 are too rare for clinical associations, although HBoV3 has been detected as the sole agent in the CSF of a child with encephalitis (327).

LABORATORY DIAGNOSTICS

Antibody detection in serum is the cornerstone for the diagnosis of B19V infection, while nucleic acid tests give further valuable aid (1041, 1042). Antigen detection is infrequently used, and B19V culture is not included in the routine diagnostic laboratory. Among immunocompromised patients, detection of B19V DNA may be required due to a deficiency in antibody production. For pregnant women, it is also recommended that levels of both antibodies and DNA be measured in serum. Past infection is determined by the detection of IgG antibodies, while the time of acute infection is possible to deduce by determining the presence of IgM and the quality of IgG or an increase of the IgG quantity, as discussed below.

Detection of HBoVs has been achieved primarily with PCR-based assays (1043). However, to accurately diagnose acute infections, other means such as serology, qPCR, antigen detection, or viral mRNA detection by reverse transcription (RT)-PCR are needed to overcome the problem of viral DNA persistence in both the respiratory tract and stool.

B19V Serology

Detection of IgM and a 4-fold increase or seroconversion of IgG in paired serum samples are the most reliable markers for acute B19V infection. IgG persists for life and is thus a marker of past infection. Furthermore, low epitope-type specificity (ETS); low IgG avidity; and the presence of IgG3, IgA, or IgE as well as NS1 antibodies can be helpful for a definitive diagnosis, as discussed below.

The first antibody assays for B19V used native virus as an antigen. The assays were in the formats of immunoelectron microscopy (IEM), IgM antibody capture radioimmunoassays (MACRIAs), IgG antibody capture radioimmunoassays (GACRIAs), enzyme immunoassays (EIAs), immunofluorescence assays (IFAs), and hemadherence tests (HATs), followed by chemiluminescent immunoassays (CLIAs) and Luminex-based singleplex and multiplex microsphere suspension immunoassays (SIAs) applying recombinant structural antigens (215, 1044–1057).

Recombinant empty capsids (VP2- or both VP1- and VP2-assembled VLPs) expressed in insect or mammalian cells are suitable sources of antigen for serological assays (89, 373, 1053, 1055, 1058–1063). Insect cell-produced VP1 or prokaryotically produced VP1 or VP2 has also been used successfully (375, 1061, 1064–1069), in addition to synthetic peptides (1070–1072). There are many in-house as well as commercial antibody detection assays of variable quality (1056, 1073–1078).

VLPs are antigenically analogous to the native viral capsid and, if treated and immobilized gently, exhibit conformational epitopes, whereas prokaryotically expressed proteins are denatured, exhibiting linear epitopes. Especially for VP2 as the antigen, it is of the utmost importance to maintain conformational epitopes (383, 1079). IgG antibodies to linear VP2 epitopes are detected mostly during acute infections and early convalescence, whereafter they disappear. In contrast, IgG antibodies to conformational epitopes persist for life (383). The kinetic difference between or the ratio of the antibody responses of linear and conformational epitopes has been exploited to develop unique serological assays, named ETS EIAs, to differentiate past from recent B19V infections (383, 1072, 1080). In ETS EIAs, either VP1 versus VP2, linear denatured VP2 versus conformational VP2, or a linear acute-phase-specific peptide versus conformational VP2 may be employed. Another means of distinguishing between past and recent infections is by measuring IgG avidity or functional affinity, the binding force between the antigen and the antibody (1081). The avidity of specific IgG antibodies is initially low after primary antigenic challenge but, upon clonal selection of B cells in germinal centers, increases during subsequent weeks and up to 6 months (1082, 1083). The avidity of IgG is measured by comparing the antibody titration curves from a normal EIA with those from a protein-denaturing EIA, where weakly binding antibodies are detached and washed away by a denaturing agent, whereas high-affinity antibodies remain antigen bound. For B19V, the antigen of the IgG avidity EIA should be VP1, because VP2 is not recognized by past-immunity IgG after denaturing treatment (376, 383). Both ETS and IgG avidity EIAs can be reliably performed on a single IgG-positive serum sample and are therefore important for confirmation of occasional unreliable IgM results. While this ETS phenomenon seems to be specific for parvoviruses, IgG avidity measurements have been used successfully for the detection of a wide variety of microbial infections (1081, 1084), including those by B19V and HBoV1 (374, 376, 1085). ETS and IgG avidity EIAs have been shown in different settings to increase the accuracy of the diagnosis of B19V infections when applied together with an IgM EIA (1042, 1080, 1086, 1087).

The levels and response kinetics of B19V-specific IgG subclasses to the VP1 and VP2 proteins have also been elucidated (1088). For both antigens, the predominant IgG subclass at all times is IgG1, whereas IgG2 levels remain very low and IgG3 is associated with the acute phase of infection. In contrast, IgG4 is detected exclusively against VP1 and only after 5 to 6 months postinfection, suggesting long-term immune stimulation. The different kinetics of IgG3 and IgG4 were further utilized for a diagnostic test for recent B19V infection, with good sensitivity and specificity (1088). The past-immunity differential responses of IgG1 to -3 to B19V were later confirmed and further shown to be similar to those for mycoplasmal but different from those for chlamydial protein antigens (1089).

Besides the more common IgM and IgG measurements, B19V-specific IgA has also been detected in human serum, but the specificity in cases of acute infections does not seem to be high enough for diagnostic purposes (373, 1061). B19V-specific IgE is present in human sera, but its potential for diagnostic use has not been evaluated (1090, 1091). B19V-specific IgE has also been detected by line blot assays of breast milk and infant sera, perhaps providing further antiviral protection in nursing children (1092). Besides breast milk, B19V antibodies have also been detected in saliva and thumb prick blood samples, suggesting that these samples may be convenient alternatives to serum for the serodiagnosis of B19V infection (1093–1095).

Although the structural proteins VP1 and VP2 are the most important antigens, the diagnostic value of the nonstructural protein NS1 has been assessed (384, 385, 559, 1096–1102). B19V NS1-specific IgG has been proposed to be a serological marker for patients with severe arthritis and chronic infections but not for those without complications, suggesting a potential involvement of NS1 or anti-NS1 antibodies in pathogenesis (384, 1096, 1099, 1102). However, this association has been disputed (1098, 1100).

B19V Nucleic Acid Testing

B19V DNA is detected in the respiratory tract and blood a week after infection, and high-titer viremia, detectable by dot blot hybridization, is present for a few days to a week (1103), whereafter viremia persists at lower levels (236–240). Nucleic acid detection in blood, serum, or plasma is important for both B19V diagnostics and screening of blood products. B19V PCR may have diagnostic utility in a very early phase of infection, before the appearance of antibody, and also in the late convalescence phase (1041). PCR may be the only diagnostic method for immunodeficiency and is of valuable help for the detection of B19V infection in pregnant women and fetuses. However, due to virus persistence, detection of B19V DNA in tissue specimens does not indicate acute infection (203). The possible persistence of B19V in blood (viremia) also among immunocompetent subjects greatly complicates the interpretation of PCR positivity. A threshold of 10⁴ vgc/ml has been suggested as a diagnostic criterion (1042).

The first nucleic acid tests for B19V were based on dot blot hybridization (1103). They were later replaced by more sensitive qualitative PCRs (542, 1104, 1104–1106) and qPCRs (1107, 1108). There are many in-house and commercial qPCR assays for genotype-specific as well as pan-B19V amplification for diagnostic and blood screening purposes (242, 1042, 1109–1114). Many PCR methods show poor sensitivity or fail to detect all three genotypes. For validation of B19V PCR-based assays, a reference panel for B19V DNAs of different genotypes is available from the World Health Organization Expert Committee on Biological Standardization (ECBS) (1115). In addition to PCR, in situ hybridization is used to detect B19V DNA in cells and tissues (1116, 1117).

B19V Antigen Assays

B19V antigen in serum can be detected with monoclonal antibodies in EIAs, radioimmunoassays, or immunoblot assays, but these methods are relatively insensitive and not reliable for the detection of acute infections, except perhaps for patients with aplastic crisis (215, 535, 1046, 1118, 1119). Antigen detection by an EIA has been used to screen blood donors (1120). Antigen detection by immunohistology or detection of virus particles by electron microscopy (543, 728, 860, 1121–1123) can be useful for localizing the virus in individual host cells in tissue sections. DNA hybridization or amplification assays are preferred for more sensitivity of virus detection.

HBoV Laboratory Diagnosis

The diagnosis of respiratory infections traditionally has included immunofluorescence antigen detection and virus culture but increasingly is based on PCR with respiratory samples (1124). Several commercial and in-house multiplex respiratory virus panel PCR assays have been developed, but many do not include HBoV1, and some are not very sensitive for HBoV1 (1125–1128). It is also possible to apply a metagenomics approach for the detection of respiratory pathogens, with fairly good sensitivity (1129). However, the long persistence of HBoV1 in the nasopharynx complicates the interpretation of positive DNA amplification test results (285–292), and the clinical significance of low viral loads is doubtful. PCR positivity in the airways is therefore not a diagnostic marker of primary infection (28, 332, 335). The same consideration applies to enteric HBoVs in tissues and stool (325, 335). Detection of anti-HBoV1 antibodies or viral DNA in serum or of HBoV1 spliced mRNA, high copies of viral DNA, or antigen in airway samples is a more reliable tool to detect primary infection (28, 316, 322, 323, 332, 333, 997–1001, 1032).

Probe-based qPCR assays for HBoV2 (1130, 1131), HBoV3, and HBoV4 (295, 298), in a singleplex or multiplex format, have been developed for the detection of HBoV2 to -4 genomes in clinical specimens. These qPCR assays are highly specific for each type of HBoV and have a limit of detection of ∼10 vgc per reaction (298, 1131).

HBoV1 capsid proteins produced in Escherichia coli, baculovirus-infected insect cells, or yeast have been used as antigens in serological assays, mostly as VLPs (28, 165, 166, 322, 333, 394, 1132, 1133). In contrast to B19V serology, VP1u is not a good antigen for HBoV serology (28). Serological markers for HBoV diagnosis are the presence of IgM, seroconversion or a ≥4-fold increase in the IgG titer in paired sera, and low IgG avidity, the latter of which can stage a primary infection by a single serum sample (1085). Serodiagnosis of HBoV1 infection is not simple due to the existence of HBoV2 to -4 complicating the immune response. The HBoVs are closely related and structurally very similar (26), with VP3 protein sequence divergences of 20% between HBoV1 and the enteric viruses and only 10% among the enteric HBoVs (13). This similarity generates both serological cross-reactivity and an immunological phenomenon called original antigenic sin, familiar in dengue and influenza virus research (337). Such factors need to be taken into consideration in the design of and interpretation of results from serological assays (334, 335, 338, 394). For the detection of HBoV1-specific antibodies, a competition EIA has been developed, in which the immobilized VLP antigen of HBoV1 is competed with soluble VLPs of the other heterologous HBoVs for patient antibodies, thus blocking the antibodies against the shared epitopes and leaving only those that react with the unique epitopes of the HBoV1 VLP antigen (334, 394). In a recent study of constitutionally healthy children who were monitored by serology for many years, several PCR-confirmed heterotypic secondary HBoV infections with inefficient or no specific IgG responses to the unique epitopes of the second virus type were observed. Instead, a strong recall response against the first virus type appeared (335). Because of original antigenic sin and/or cross protection, HBoV1 infections in individuals with preexisting HBoV2 or -3 IgG may be difficult to detect serologically. HBoV1 serological assays are not yet commercially available.

Recently, HBoV1 was added to the platform of an innovative multiplex point-of-care antigen test for respiratory tract infections, which is based on separation-free two-photon excitation fluorometry (1000, 1134, 1135). The HBoV1 antigen test was subsequently used to estimate the period of active HBoV1 infection to about 1 week, which coincided with the decrease in the severity of clinical symptoms (1000).

TREATMENT AND PREVENTION

IVIG is effective in treating certain disease conditions triggered by B19V infection. High-dose IVIG is currently administered to patients with B19V-associated chronic anemia and PRCA (137, 1136, 1137). Symptoms can recur when IVIG treatment is interrupted (1138–1141). The administered IVIG may be contaminated with B19V or other viruses (e.g., hepatitis A virus) (700, 1138). There are no known effective antiviral drugs for the treatment of diseases caused by B19V, especially for transient aplastic crisis due to B19V.

In certain B19V-infected patients, e.g., pregnant women and sickle cell disease patients, B19V vaccination would provide substantial benefit by preventing fetal loss, aplastic crisis, and blood dyscrasias. A B19V vaccine might also prevent other B19V-associated diseases, such as myocarditis.

An early study showed that antisera resulting from immunization with baculovirus-derived B19V VP1-containing VLP capsids neutralized B19V infection of human erythroid progenitor cells, highlighting that VP1-containing VLPs are applicable as a human vaccine for preventing B19V infection (89, 1142). A B19V vaccine candidate, which was produced as B19V VLPs from insect cells expressing wild-type VP1 and VP2, has been tested in phase I clinical trials using the MF59 adjuvant (1143, 1144). Insect cells were coinfected with two baculoviruses at different MOIs at a VP1-to-VP2 ratio of ∼1:3. After receiving at least two doses of the vaccine intramuscularly, all vaccinated volunteers seroconverted to become positive for anti-B19V antibody, as determined by EIAs and neutralization assays (1143, 1144). B19V-neutralizing antibodies were sustained for at least 6 months after the third dose (1143). In the first clinical trial, safety evaluations revealed mostly injection site reactions that were mild to moderate (1143). However, the second clinical trial was halted because skin rashes near the infection site occurred (1144). Insect cell contaminants in the vaccine preparations and/or the PLA2 activity of the VLPs might cause reactogenicity.

For the production of the B19V VLP vaccine, the relative amounts of VP1 and VP2 expressed were adjusted by manipulating the MOIs of the two baculoviruses, which created process reproducibility and logistical challenges. B19V PLA2-mutated VLPs have been produced by expressing PLA2-mutated VP1 and VP2 in a consistent ratio of 1:5 from Saccharomyces cerevisiae transfected with a bicistronic plasmid (1145). Immunization of BALB/c mice demonstrated that mouse PLA2-mutated VLPs (mPLA2-VLPs) did not exhibit PLA2 activity and elicited a neutralizing response in the presence of the MF59 adjuvant as strong as that of the wild-type VLP counterpart.

The MF59-adjuvanted vaccine candidates elicited B19V-neutralizing antibody levels similar to those following infection in humans (1143, 1144). However, it is unknown whether vaccine-induced neutralizing antibody titers are sufficient for protection against B19V-associated diseases. High-risk groups with diseases caused by B19V are children with sickle cell disease, who have significant morbidity following B19V infection, and pregnant women who have not previously been infected with B19V. Targeted immunization of these groups will potentially reduce the risk of life-threatening B19V-associated diseases. In addition to the prevention of B19V-associated diseases, the demonstration of an effective B19V vaccine might attract development and application utilizing B19V VLPs as antigen carriers for the presentation of antigenic determinants of other infectious agents, such as dengue 2 virus and anthrax (1146, 1147).

OTHER EMERGING HUMAN PARVOVIRUSES

Human Parvovirus 4

Human parvovirus 4 (PARV4) was first discovered in 2005 in a hepatitis B virus-infected injection drug user (14). The virus has been detected in plasma samples worldwide (1148). PARV4 and PARV4-like viruses have been classified as members of a new genus, Tetraparvovirus, in the family Parvoviridae (Table 1) (16). The full-length genome of PARV4 has not been sequenced, lacking information on the ITR (14).

PARV4 genomes have been detected in human plasma pools (14, 1149), in the livers of both hepatitis C virus (HCV)-positive individuals (1150) and healthy individuals (1150), and in the bone marrow of HIV-positive individuals (828, 1151). PARV4 DNA was also found in cerebrospinal fluid of two children with encephalitis of an unknown etiology (1152). A few cases of acute PARV4 infections have been demonstrated but with no clear clinical manifestations (1153, 1154). There are currently three known genotypes of PARV4 (1155), of which genotypes 1 and 2 are widespread in Europe, Asia, and North America and, as suggested by the high prevalence of PARV4 in hemophiliacs, injection drug users, and HIV- or HCV-infected subjects, seem to be transmitted parenterally by contaminated needles or other blood contact (828, 1156, 1157). Genotype 3, in contrast, has been detected in non-drug users in sub-Saharan Africa, where viral DNA has been found in nasal and stool samples of children, indicating foodborne, respiratory, or contact spread (1158–1161). PARV4 DNA has been detected in plasma during acute infection but often with a low viral load (<3 × 10⁴ vgc/ml) (1153, 1162, 1163). The highest PARV4 load reported for acute viremia so far is 10¹⁰ vgc/ml (1153). An 8.6% prevalence of asymptomatic viremia due to PARV4 in infants in Africa has been found (1159). Viremia can last for various time periods, from 30 days to 7 months (1153, 1164). In most cases, PARV4 viremia appears to be self-limiting and asymptomatic (1159), and PARV4 infection does not seem to increase the severity of disease in coinfections with other blood-borne viruses (1165). Overall, the clinical significance of PARV4 infection remains unclear. Nevertheless, due to its blood-borne nature, PARV4 infection may manifest clinical symptoms, particularly in immunocompromised patients.

Little is known about the biology of PARV4. PARV4 has not been cultured in vitro. It is predicted that PARV4 has two identical termini at the ends. The gene expression profile of PARV4 has been studied only by transfection of an incomplete PARV4 genome (1166) (Fig. 4). Two promoters, P6 and P38, are used to transcribe NS-encoding and VP-encoding mRNAs from the left and right sides of the genome, respectively. A spliced form of R2 mRNA, which is transcribed from the P6 promoter and spliced in the NS1-encoding region, likely encodes a small NS2 protein, although it was not detected by transfection (1166). During transfection, the R4 mRNA, which encodes VP2, was abundantly expressed, ∼80% in total viral mRNA. However, the R1M mRNA, which encodes NS1, was expressed at a level of ∼10% in total. Other species of viral mRNAs were minimally expressed, at <5%. NS1 and VP2 were clearly detected at molecular masses of ∼80 and ∼65 kDa, respectively. However, which start codon VP1 actually uses to translate mRNA is not clear (Fig. 4). PARV4 VP1u should be long, at least 261 aa, based on the location of a putative PLA2 motif, although VP1u did not exhibit PLA2 activity in vitro (1166). NS1 exhibited cytopathic activity in ex vivo-generated HSCs (1166).

FIG 4.

Transcription map of PARV4. The incomplete PARV4 ssDNA genome (GenBank accession no. NC_007018) is shown to scale in negative sense with predicted inverted terminal repeats (ITRs). Transcription units, including promoters, the polyadenylation site [poly(A)], and splice donor (D) and acceptor (A) sites, are indicated with their respective nucleotide positions. Major detected mRNA species are diagrammed to display their identities and respective sizes in the absence of the poly(A) tail. The encoding ORFs are diagrammed in colored boxes, and the predicted molecular masses (kilodaltons) of translated proteins and detected molecular masses upon transfection are indicated at the right. Potential VP1 start sites are speculated to be located at nt 2378 for a VP1 protein of 914 aa and at nt 2678 for a VP1 protein of 814 aa, as shown. ND, not detected.

For PARV4 DNA detection, there is a qPCR with a probe targeting the NS-encoding region with a limit of detection of 50 vgc/reaction (1167). Multiplex PCR can identify and quantify levels of PARV4 genotypes 1 and 2 (303) and genotype 3 (1159). A two-step qPCR assay for quantitation and genotyping of all three PARV4 genotypes has been further established (1163); it first applies a single panprobe for screening for and quantitation of the levels of PARV4 and then, for positive samples, employs multiple genotype-specific probes for genotyping. This novel PARV4 genotyping qPCR has high sensitivity and specificity.

PARV4 VP2-generated VLPs have been used to establish an IgG EIA and a μ-capture-format IgM EIA for serodiagnosis (325, 1154, 1157). In the general population, the seroprevalence of PARV4 varies by genotype and geographically, from 20 to 37% in sub-Saharan Africa (1168) to 9.4% for a low-risk population in Lithuania (1169), 4.76% for blood donors in the United Kingdom (1170), and 0% in healthy subjects in Nordic countries (1154, 1171). Additionally, PARV4 infection induced strong, broad, and persistent T cell responses (1164).

Human Bufavirus

BuV was first identified in 2012 in feces from children <5 years of age with acute diarrhea by metagenomics analysis (15). A prevalence of 4% among the rotavirus antigen-negative cases of childhood diarrhea was found (15). The NS1 protein of BuV shares 38% identity with those of the other protoparvoviruses, barely including it in this genus (15). BuV has three genotypes (15, 1172), belongs to a new species called Primate protoparvovirus 1 (Table 1), and is the first human parvovirus in the genus Protoparvovirus (16). NS1 shares >95% identity among all bufaviruses, but capsid proteins share only 72% identity (15). The longest genome of BuV1 strain BF.7 (GenBank accession no. JX027295) reported so far has a size of 4,822 nt, which includes NS1, a middle ORF protein, and capsid protein genes, from the left, middle, and right sides of the genome, respectively. It does not contain the left- and right-end hairpins and therefore is an incomplete genome.

BuV DNA has been detected worldwide in feces of children and adult patients with gastroenteritis but with a low detection rate (<1.4%) and a low viral load (10³ to 10⁴ vgc/ml of the supernatant of feces) (1172–1177). BuV DNA has not been detected in stool samples of healthy individuals (1174, 1176). These studies suggest that bufavirus may, albeit infrequently, cause gastroenteritis, but further studies are needed to confirm causality. Assays for serodiagnosis are being developed (M. Söderlund-Venermo, unpublished observations).

Another protoparvovirus, tusavirus, has been detected in the stool of one Tunisian child with unexplained diarrhea, but whether this rodent parvovirus-like virus is a true human virus is still unconfirmed (1178).

FUTURE DIRECTIONS

Disease Validation and Animal Models

An animal model of B19V infection would greatly contribute to our understanding of the pathogenesis of infection in humans and to test antiviral drugs. The remarkable similarities between simian parvovirus (SPV) and B19V suggest that experimentally SPV-infected cynomolgus monkeys may serve as a useful animal model of B19V infection (1179). However, SPV has not been cultured in vitro and is difficult to obtain in large quantities from sick animals. B19V replicates in cynomolgus bone marrow (1180), and thus, cynomolgus monkeys may be a suitable animal model for pathogenesis studies of B19V infection (1181).

HBoV1 infection is highly restricted to human airway epithelia. No animal models of HBoV1 infection have been reported. An open-ended human bronchial xenograft nude mouse model (1182–1184) could be used as an alternative in vivo model. This model, which develops a fully differentiated mucociliary epithelium and mimics human airways, supports productive HBoV1 infection (J. Qiu, unpublished observations). In addition, animal viruses of the same genus as HBoVs or BuV may give further information on diseases and pathogenesis, some of which may also apply to humans (1185–1187).

Antiviral Drug Development

No specific antiviral drugs have been developed for the treatment of B19V infection. High-dose IVIG is administered to patients with B19V chronic anemia due to PRCA (137, 1136, 1137), but symptoms can recur when IVIG treatment is halted (1138–1141). Repeated applications of IVIG and maintenance therapy (744, 1136, 1188–1193) may be cost-prohibitive. Anti-B19V drugs to prevent virus entry or to inhibit B19V replication would be an effective approach to the treatment of bone marrow failure due to B19V infection. Cidofovir (CDV), an acyclic nucleoside phosphonate, has shown activities against five families of human dsDNA viruses (1194) and relevant anti-B19V activity in UT7/Epo-S1 cells as well as in ex vivo-expanded CD36⁺ EPCs (1195, 1196).

The neutralization activity of anti-B19V antibody is directed to linear epitopes of VP1u (136, 139). An human monoclonal antibody against a peptide (aa 30 to 42) of VP1u showed strong neutralization of B19V infection in vitro (381). An anti-VP1u human monoclonal antibody would be ideal to treat B19V infection of human bone marrow.

An inhibitor of PI3K kinases significantly decreased HBoV1 replication in HAE-ALI culture (155); these inhibitors are in development for cancer treatments and might be repurposed to treat HBoV1-associated diseases.

Biographies

Jianming Qiu obtained his Veterinary Medicine (B.S.) and Biochemistry (M.S.) degrees from Zhejiang University, China, and his Ph.D. in Virology from the National Institute for Viral Disease Control and Prevention, China CDC, Beijing, China. He acquired postdoctoral training in parvovirology at the Hematology Branch, National Institutes of Health, Bethesda, MD, and at the Department of Molecular Microbiology and Immunology, University of Missouri—Columbia, Columbia, MO. He is currently Professor of the Department of Microbiology, Molecular Genetics and Immunology, University of Kansas Medical Center, Kansas City, KS. His current research interests have focused on the molecular biology and pathogenesis of human parvoviruses B19 and HBoV and the use of parvoviruses as vectors for human gene therapy. He is a member of the ICTV Parvoviridae Study Group.

Maria Söderlund-Venermo obtained her M.Sc. at the University of Helsinki, Department of General Microbiology, and her Ph.D. at the Department of Virology, Helsinki, Finland. She was a visiting scientist at the University College London, London, UK, and did her postdoctoral training at the Departments of Molecular Microbiology and Immunology and Veterinary Pathobiology, University of Missouri—Columbia, Columbia, MO, USA. She is a docent, specialist in clinical microbiology, and senior lecturer at the University of Helsinki Medical Faculty, Department of Virology. Her research interests include both clinical and molecular virology, with the main focus on B19V, HBoV, and the emerging human parvoviruses. She is furthermore a member of the ICTV Parvoviridae Study Group.

Neal S. Young received an A.B. from Harvard College and an M.D. from the Johns Hopkins School of Medicine. His internal medicine residency was at Massachusetts General Hospital. He completed a clinical fellowship in the Hematology-Oncology Division at Barnes Hospital, Washington University, St. Louis, MO. His entire career has been in the Intramural Research Program of the National Institutes of Health in Bethesda, MD. He is currently Chief of the Hematology Branch of the National Heart, Lung, and Blood Institute. He is primarily known for work in the pathophysiology and treatment of aplastic anemia and is also known for his contributions to the pathophysiology of B19V infection.