Human inborn errors of immunity to herpes viruses

Abstract

Infections with any of the nine human herpes viruses (HHV) can be asymptomatic or life-threatening. The study of patients with severe diseases caused by HHVs, in the absence of overt acquired immunodeficiency, has led to the discovery or diagnosis of various inborn errors of immunity. The related inborn errors of adaptive immunity disrupt α/β T-cell rather than B-cell immunity. Affected patients typically develop HHV infections in the context of other infectious diseases. However, this is not always the case, as illustrated by inborn errors of SAP-dependent T-cell immunity to EBV-infected B cells. The related inborn errors of innate immunity disrupt leukocytes other than T and B cells, non-hematopoietic cells, or both. Patients typically develop only a single type of infection due to HHV, although, again, this is not always the case, as illustrated by inborn errors of TLR3 immunity resulting in HSV-1 encephalitis in some patients and influenza pneumonitis in others. Most severe HHV infections in otherwise healthy patients remains unexplained. The forward human genetic dissection of isolated and syndromic HHV-driven illnesses will establish the molecular and cellular basis of protective immunity to HHVs, paving the way for novel diagnosis and management strategies.

Introduction

Nine human herpes viruses (HHVs) have been identified, all with strict human tropism and a capacity to alternate latent and lytic phases (reactivation). The first isolation and characterization of a herpes virus was performed in 1919, and two serotypes (HSV1 and HSV2) were distinguished in 1962 (1). Other herpes viruses were subsequently isolated. Varicella zoster virus (VZV or HHV3) was isolated in 1953 from human embryonic tissue infected with the chickenpox virus (2, 3). Human cytomegalovirus (HCMV or HHV5) was isolated in 1956 from the salivary gland of an infected child (4). Epstein-Barr virus (EBV or HHV4) was isolated in 1964 from cultured tumor cells derived from an African case of Burkitt’s lymphoma (5); this virus was the first human oncogenic virus to be discovered (6). HHV6 was isolated in 1986 from leukocytes of patients with lymphoproliferative disorders, including lymphoma (7). Two variants of HHV6, 6A and 6B, have been defined since 2014; these variants are closely related but have different molecular and epidemiological characteristics (8). In 1990, HHV7 was isolated from activated T cells from a healthy individual (9), and, finally, in 1994, HHV8 was isolated from a Kaposi sarcoma lesion from a patient with HIV-induced acquired immune deficiency (AIDS) (10).

The classification of HHVs into three subfamilies is based on multiples features, including DNA sequence similarity, site of latent infection, cytopathologic effect, and replicative cycle (short or long). HSV1, HSV2, and VZV belong to the α-herpes subfamily, HCMV, HHV6 and HHV7 belong to the β-herpes subfamily, and EBV and HHV8 constitute the oncogenic γ-herpes subfamily. During the latent phase of infection, the cellular reservoir is sensory ganglia neurons for α-herpesviruses (11), bone marrow progenitors, including those of the myeloid lineage, for HCMV, HHV6A and HHV6B, CD4 T cells for HHV7 (12, 13), and B lymphocytes for γ-herpesviruses (14). In all cases, the viral genome is maintained as an episome (15). These viruses display human-to-human transmission, through saliva (HSV1, EBV, HCMV, HHV6, HHV7 and HHV8), skin lesions (HSV1, VZV), respiratory droplets (VZV, HCMV), sexual contact (HSV2 and HHV8), or from infected mother to child in utero (HCMV) or during delivery (HSV1 and 2, VZV and HCMV). Breast milk transmission has been documented for HCMV (16). HHV6 is the only herpes virus for which chromosomal integration has been observed as a mechanism of congenital infection (17). With the exception of HSV2 and HHV8, HHVs are almost ubiquitous in the general population, with seroprevalence rates exceeding 80–90% by the age of 20 years for HSV1, VZV, and EBV, and reaching 100% by the age of 10 years for HHV6 and HHV7, and more than 50% by the age of 45 years for HCMV (18).

The vast majority of herpesvirus infections are asymptomatic. Furthermore, efficient antiviral drugs, such as acyclovir and ganciclovir, have improved the treatment of symptomatic HHV infections since their first clinical use in 1977 (19, 20). A very small proportion of infected individuals develop life-threatening disease. These individuals are frequently patients with acquired immune deficiencies, due to AIDS or following organ or hematopoietic stem cell transplantation (HSCT) (21–31). EBV-induced lymphoproliferative diseases and HHV8-associated Kaposi sarcoma can affect individuals with acquired immunodeficiencies or otherwise healthy people, but its frequency is much higher among those with acquired immunodeficiencies. The clinical manifestations of infection with other HHVs tend to differ between individuals with acquired immunodeficiencies and the general population. HHV infection may cause only benign manifestations (e.g. labial herpes) or no manifestations at all in the general population. However, in patients with acquired immunodeficiencies, infection frequently leads to the development of invasive diseases potentially affecting multiple tissues, such as encephalitis, hepatitis, retinitis, pneumonitis or cancers. In rare cases, life-threatening diseases are reported in otherwise healthy individuals, in which case the clinical course may resemble that in patients with acquired immunodeficiency. Some otherwise healthy patients present a unique clinical course, such as HSV1 encephalitis (HSE) not associated with acquired immunodeficiency in children. Here, we review the inherited genetic defects associated with severe HHV infections, either isolated or syndromic (i.e. in combination with other infections), and discuss how the study of primary immunodeficiencies (PIDs) has defined the requirements of adaptive, innate and tissue-intrinsic immune responses to HHV.

HSV1/HSV2 immunity

The genome sequences of HSV1 and HSV2 are 70% identical (32, 33). The seroprevalence of HSV1 in the general population reaches about 80% by adolescence, and that of HSV2 reaches about 30% in young adults. Both these viruses cause diverse diseases, including cutaneous herpes, keratitis, retinitis, cold sores, genital herpes, hepatitis, meningitis, and encephalitis. HSV1 is usually associated with orofacial infections and encephalitis, whereas HSV2 usually causes genital infections (33, 34). However, the clinical syndromes of HSV1 and HSV2 overlap, and infection severity (i.e. cutaneous infection vs. encephalitis) has not been associated with particular HSV strains. Recurrent or severe HSV infection is relatively rare among infected individuals and prognosis remains poor, particularly for encephalitis, due to neurological sequelae (35–37). The causal link to the virus is clear, but the pathogenesis of the various HSV diseases otherwise remains largely unknown. The observation of patients with PIDs and severe HSV infection (mostly due to HSV1), whether syndromic or isolated, has shed light on host protective immune responses to HSV.

Patients with various inborn errors of immunity affecting the development and/or function of various leukocytes frequently present severe/recurrent cutaneous/mucous HSV1 infection (Table 1). These conditions include, but are not limited to, autosomal recessive (AR) deficiencies of MCM4 (38, 39), GINS1 (40), DOCK8 (41–43), and STK4 (MST1) (44), autosomal dominant (AD) GATA2 deficiencies (45–49), gain-of-function (GOF) CXCR4 mutations (50), and X-linked recessive (XR) Wiskott-Aldrich syndrome (51, 52). The most common immunological abnormality in these PIDs is numerical and/or functional deficiency of natural killer (NK) and/or T cells. These observations suggested a potentially important role of NK cells and αβ T cells in HSV1 control in the skin. Severe/recurrent cutaneous/mucous HSV infection has also been reported in patients with PIDs affecting both adaptive and innate immunity, including IFNγ-R deficiencies (due to mutations of IFNGR1 or IFNGR2), AR TYK2 deficiency, XR NEMO deficiency (IKBKG mutations), and immune deficiency due to AD GOF STAT1 mutations (53–59). These observations suggested an involvement of immune responses mediated by IFNγ-R and/or TYK2/STAT1 signaling pathways in skin-resident cells, leukocytes or both, in the progression of HSV1 skin infection. Interestingly, STAT1 GOF mutations impair NK cell proliferation and function, in addition to Th17 differentiation (60). Several patients with these PIDs can develop life-threatening clinical forms of HSV1 infection, manifesting as systemic HSV infection in a patient with AD GATA2 deficiency characterized by monocytopenia together with B, NK and dendritic cell (DC) lymphopenia (48), or HSE in one patient with XR NEMO deficiency and one patient with AR complete STAT1 deficiency (61, 62). Further molecular and cellular characterization of these PIDs in the context of HSV1 infection will clarify the contributions of T, NK and tissue-resident cells in host defense against HSV1. Interestingly, patients with leukocyte deficiencies, including severe combined immunodeficiencies (SCID), do not develop childhood HSE, even if they develop severe/recurrent cutaneous/mucous HSV1 infections or meningitis (63). These observations raise questions about the host specific immune factors protecting against HSV infection, either systemically or in a specific tissue (i.e. the skin) or organ (i.e. the central nervous system).

Table 1.

Primary immunodeficiencies associated with human herpes viruses infection

PID	Defective cells	HHV infection						References
		HSV1/HSV2	VZV	EBV	CMV	HHV6 HHV7	HHV8
Inborn errors of adaptive humoral and cellular immunity
SCID genes1	All lymphocytes	Recurrent skin infection; Meningitis	Disseminated infection	Viremia Meningitis	Viremia; Pneumonia; Retinitis; Meningitis	Hemophagocytic syndrome		(63, 111, 139, 168)
GATA2 (10–20%)	B, NK cells, DCs and monocytes	Disseminated skin infection; HLH2; Meningitis	Pneumonitis	Lympho-proliferation after HSCT	Disseminated infection			(45–49)
DOCK8	T, NK and B cells	Recurrent skin infection; Keratitis; Pneumonia; Conjunctivitis	Disseminated skin infection; CNS vasculopathy	Pneumonia	Viremia; Pneumonia; Meningitis; Retinitis			(41–43, 95, 192, 193)
DOCK2	T and NK cells		Disseminated infection due to vaccine strain VZV; Encephalitis		Viremia			(100, 145, 146)
CORO1A	All lymphocytes		Pneumonia; Meningitis; Disseminated infection	Lympho-proliferation				(96, 194)
WAS	T, B, cells, DCs, monocytes, macrophages, granulocytes, thrombocytes	Recurrent keratitis	Severe recurrent zoster	Lympho-proliferation; B-cell lymphoma	Viremia; Hemophagocytosis; Encephalitis; Hepatitis; Pneumonia		KS	(51, 52, 186, 195, 196, 197 )
MHC class-II deficiency3	Impaired CD4+ T cell activation	Recurrent gingivostomatitis; Meningitis	Varicella		Viremia; Pneumonia; Meningitis; Retinitis			(151, 195, 198, 199)
CD40L	Impaired CD4+ T cell activation, B cells	Stomatitis; Pneumonia			Pneumonia; Encephalitis; Disseminated infection			(200, 201)
CARMIL2	T and B cells	Skin infection	Recurrent varicella	Viremia; Disseminated infection	Viremia			(114, 144, 202, 203)
IL2RB	T cells			Viremia	Viremia with hepatitis; Pneumonia; Disseminated infection			(142, 143)
MAGT1	T and NK cells	Recurrent skin infection	Severe varicella; recurrent zoster	Lympho-proliferation; B-cell lymphoma				(204–208)
ORAI1	T cells			B-cell lymphoma	Pneumonia			(209, 210)
STIM1	T cells	Recurrent stomatitis	Severe varicella; Cellulitis	Chronic viremia	Chronic viremia		KS	(187, 211–214)
ICOS	T and B cells	Recurrent labialitis; Keratitis			Colitis; Vulvovaginitis; Disseminated infection	Colitis; Hepatitis		(147, 169)
STK4 (MST1)	T and B cells	Recurrent oral infection	Recurrent zoster	Viremia; Lympho-proliferation; B-cell lymphoma				(44, 215–218)
CXCR4	B and T cells neutrophils	Recurrent oral, genital infection	Recurrent zoster	Lympho-proliferation; B-cell lymphoma				(50, 219, 220)
OX40	CD4+ T cells						KS	(188)
CARD11 GOF	T, B, NK, NKT cells			Viremia; B-cell lymphoproliferation				(117, 118)
CARD11 LOF	T and B cells	Skin infection	Persistent skin infection		Disseminated infection			(141, 221)
SH2D1A (SAP)	CD8+ T and NK cells			Chronic active infection; HLH		Hemophagocytic syndrome		(120–122, 222)
ITK	CD4+ T, CD8+ T and iNKT cells			Lympho-proliferation; HLH, HL; B-cell lymphoma				(124)
CD27	CD8+ T cells			Lympho-proliferation; B-cell lymphoma				(124)
CD70	CD8+ T cells		Severe varicella	B-cell lymphoma				(129, 130)
CTPS1	T cells		Recurrent pneumonia; gastritis	Viremia; Severe mononucleosis; Lympho-proliferation (CNS)				(127)
TNFRSF9 (4–1BB)	CD8+ T cells			Lympho-proliferation; B-cell lymphoma				(123, 125)
Inborn errors of adaptive and innate immunity
STAT5B	CD4+, CD8+ T cells, NK cells		Hemorrhagic varicella; recurrent zoster; keratitis	EBV-T cell lymphoma				(97, 98, 195)
IFNGR1	All cells	Gingivo-stomatitis; Esophagitis; Recurrent skin infection	Varicella; Pneumonia; CNS infection		Disseminated infection		KS4	(53, 55, 184)
IFNGR2	All cells				Disseminated infection			(223)
IL12RB	T cells			Viremia	Colitis; Lymphoid hyperplasia of the esophagus			(143)
TYK2	All cells	Recurrent skin, mucosal infection	Recurrent skin infection					(56)
NEMO	All cells	Recurrent skin infection; HSE						(57, 62)
STAT1 GOF	All cells	Recurrent skin infection						(58–60)
STAT1 AR LOF	All cells	Recurrent skin infection; HSE	Disseminated varicella	Fulminant EBV infection (during HSCT)	Pneumonia			(61, 224)
POL III5	Leukocytes?		CNS infection; Pneumonia					(103)
POL D1	Leukocytes	Recurrent labial and skin infections	Severe and recurrent chickenpox Encepahlitis					(225)
XIAP	Lack of NKT cells			Chronic active infection; HLH				(119)
Inborn errors of innate and tissue-intrinsic immunity
MCM4	Decrease in NK cells	Recurrent skin infection	Severe varicella	Lympho-proliferation				(38, 39)
GINS1	Decrease in NK cells	Recurrent skin infection	Severe varicella		Pneumonia			(40, 158)
IRF8	Decrease and impaired function in NK cells, decrease in dendritic cells			Severe mononucleosis				(115)
UNC-93B	CNS neurons and oligodendrocytes	Forebrain HSE						(68)
TLR3	CNS neurons and oligodendrocytes	Forebrain HSE	Recurrent zoster ophthalmicus					(69, 73, 74, 102)
TRAF3	CNS neurons and oligodendrocytes	Forebrain HSE						(71)
TRIF	CNS neurons and oligodendrocytes	Forebrain HSE						(72)
TBK1	CNS neurons and oligodendrocytes	Forebrain HSE						(70)
IRF3	CNS neurons and oligodendrocytes	Forebrain HSE						(77)
snoRNA31	CNS neurons	Forebrain HSE						(83)
DBR1	Brainstem cells?	Brainstem HSE						(84)

^1.SCID genes: IL2RG, RAG1, RAG2, ADA, JAK3, IL7R

^2.HLH: hemophagocytic lymphohistiocytosis

^3.MCH class II genes: CIITA, RFXANK, RFX5, RFXAP

^4.CD4 lymphopenia at the time of KS

^5.POL III genes: POLR3A, POLR3C

Since 2006, HSE has been studied as an example of inborn errors of immunity conferring a predisposition to isolated severe viral infection affecting a specific organ in otherwise healthy children. In about 1 in 10,000 infected individuals, HSV1 invades the central nervous system (CNS) via the olfactory bulb, causing forebrain HSE (~95% of cases) or, more rarely, via the trigeminal nerves, causing brainstem HSE (~5% of cases)(37, 64–66). HSE typically strikes otherwise healthy individuals, who are not particularly susceptible to other clinical forms of HSV infection, and no viral dissemination has been reported during the course of HSE (35, 37). Genetic studies of isolated HSE of the forebrain led to the discovery of single-gene inborn errors of the Toll-like receptor 3 (TLR3)/IFN-α/β and -λ pathway, due to mono- or biallelic mutations of six TLR3 pathway genes (TLR3, UNC93B1, TRIF, TRAF3, TBK1 or IRF3) (67–77) (Table 1). These findings, together with the previous observation of HSE in patients with either XR NEMO deficiency (impairing IFN-α/β and -λ production upon viral infection)(78) and AR complete STAT1 deficiency (61) (impairing IFN-α/β and -λ responses), suggested that TLR3-dependent IFN-α/β and/or -λ immunity is crucial for host defense against HSV1 in the CNS. It has been suggested that other mutations of these and other TLR3 or IFN pathway genes probably underlie HSE in adults or children (79–81). It has been further shown in a series of experiments that TLR3 pathway-deficient fibroblasts (68–74) and induced pluripotent stem cell (iPSC)-derived cortical neurons and oligodendrocytes (75) are much more susceptible to HSV1 infection than control cells, probably due to impaired TLR3-dependent IFN-β and IFN-λ production. By contrast, TLR3-mediated antiviral immunity is redundant in most other TLR3-expressing cell types, including leukocytes, iPSC-derived peripheral nervous system trigeminal neurons and CNS neural stem cells and astrocytes (75, 82). These data suggested that TLR3-dependent, IFN-mediated cortical neuron- and oligodendrocyte-autonomous anti-HSV1 immunity is critical for host defense against HSV1 infection of the human forebrain. Other, distinctive molecular mechanisms may also underlie forebrain HSE, as suggested by the more recent discovery of AD snoRNA31 deficiency, which impairs the control of HSV1 infection in iPSC-derived cortical neurons despite normal responses via TLR3 and the IFN-α/β receptor (83).

Human genetic studies of forebrain HSE paved the way for investigations of inborn errors of immunity conferring a predisposition to other types of isolated viral encephalitis (VE). In this context, AR partial DBR1 deficiency - due to biallelic hypomorphic mutations of DBR1 - has recently been reported in five otherwise healthy children with brainstem VE (BVE) due to various viruses, including HSV1 in two patients (84) (Table 1). In most patients with devastating BVE, the brainstem is the only region of the CNS affected, suggesting that, if there is an inborn error of immunity underlying BVE, it may affect brainstem-specific immunity. DBR1 encodes the only known RNA lariat-debranching enzyme (84, 85). DBR1 protein levels are highest in the brainstem and spinal cord, strongly suggesting that DBR1 deficiency disrupts immunity in brainstem-resident cells (84). DBR1-deficient fibroblasts from the patients, whose TLR3 and IFN-α/β responsive pathways were intact, were found to contain higher RNA lariat levels than control cells, this difference becoming even more marked during HSV1 infection. Moreover, DBR1-deficient fibroblasts were highly susceptible to HSV1 and vesicular stomatitis virus (VSV), like TLR3- and STAT1-deficient fibroblasts (84). Inherited DBR1 deficiency probably underlies viral infection of the brainstem through the disruption of brainstem-specific and cell-intrinsic immunity to viruses, including HSV1. Human genetic studies of HSE have thus provided proof-of-principle that TLR3 governs cell-intrinsic immunity to HSV1 in the forebrain, whereas DBR1 governs cell-intrinsic immunity to various viruses, including HSV1, in the brainstem, paving the way for further investigations of CNS tissue-specific cell-intrinsic, as opposed to hematopoietic cell-mediated, immunity to HSV1 in humans.

VZV immunity

In the general population, the seroprevalence of VZV reaches about 80% by adulthood (86–88). VZV causes two different diseases: chickenpox (varicella) upon primary infection and shingles (herpes zoster) following reactivation. In rare cases, primary VZV infection or a reactivation of VZV infection may be complicated by severe clinical outcomes, including hemorrhagic varicella, pneumonitis, gastroenterological infection (ulcers, pancreatitis, hepatitis), eye infection (retinitis, keratitis), CNS infection (encephalitis, cerebellitis, meningitis, cranial nerve palsies, and vasculopathy), or systemic multiorgan infection (86, 89). In developed countries, ~5 in 1,000 people with varicella are hospitalized, and 2–3 per 100,000 patients die (86–88). However, the precise immunological correlates of human protective immunity to VZV are incompletely understood. Antibody-mediated humoral immunity is not generally thought to make a major contribution, as suggested by the absence of VZV infection as a prominent clinical phenomenon in individuals with antibody deficiencies (90). In this setting, as in other HHV infections, severe or recurrent VZV infection has been observed in some patients with various PIDs, often in combination with other infections.

Cellular immunity, exerted by T and NK cells, clearly plays an important role in host defense against VZV (90). In addition to severe infections caused by other pathogens, several PIDs confer a predisposition to severe disseminated primary varicella and frequent and extensive zoster or varicella pneumonia (Table 1). The most classic example is SCID, in which severe disseminated VZV infection is frequently documented (91). Severe VZV infection, including VZV-induced hemophagocytic lymphohistiocytosis (HLH), has also been reported in patients with AD GATA2 deficiency (92–94). Moreover, disseminated cutaneous VZV infection has been observed in other PIDs, including, but not restricted to, CORONIN1A, DOCK8, MCM4 and GINS1 deficiencies (38–40, 95, 96). Hemorrhagic or invasive VZV infection (including CNS infection) has been reported in patients with STAT5B (97–99) or DOCK2 deficiencies (100), respectively. Impaired activation of αβ T and NK cells due to STAT5B, DOCK8 or DOCK2 deficiency probably underlies severe cutaneous VZV infection in these patients. DOCK2 deficiency also compromises IFN-α/β and -λ production by peripheral blood cells upon viral infection, and cell-intrinsic antiviral immunity in fibroblasts. Thus, the severe invasive VZV infection observed in DOCK2-deficient children may result from defective adaptive (T cells) and/or innate (NK cells, IFN immunity in non-hematopoetic cells) immunity (100). Like cutaneous HSV infection, other PIDs affecting both adaptive and innate immunity (AD/AR IFNγ-R, AR TYK2 deficiencies) confer a predisposition to severe/recurrent cutaneous VZV infection in combination with mycobacterial diseases (56, 101). IFNγ-R and TYK2 deficiencies interfere with macrophage function and defense against intracellular pathogens, and with IFNγ-R/TYK2 signaling, suggesting a role of IFNs in restricting VZV replication and spread, as also suggested by some in vitro studies of VZV infection (86, 89, 90).

Further investigations of molecular and cellular mechanisms are required to confirm or clarify the role of IFN-mediated immunity in VZV infection. Recurrent zoster ophthalmicus has recently been reported in a patient with AD TLR3 deficiency, providing additional support for this hypothesis (102). Moreover, RNA polymerase III (POL III) deficiency has been added to the list of PIDs conferring predisposition to isolated severe VZV infection of specific organs, following the identification and characterization of an AD POL III defect in four otherwise healthy children with severe CNS or pulmonary VZV infection (103). POL III is a protein complex comprising 17 subunits organized into different subcomplexes involved in DNA/RNA binding, the initiation of transcription, enzymatic activity, and the termination of transcription (104). POL III is a cytosolic DNA sensor recognizing and transcribing AT-rich DNA to RNA, and then triggering IFN induction through the RIG-I pathway (105, 106). The four affected children were found to have rare heterozygous missense mutations of POLR3A, POLR3C, or both. Leukocytes from all four patients displayed poor induction of IFN-α/β and -λ in response to synthetic or VZV-derived AT-rich DNA. Moreover, leukocytes from three patients displayed defective IFN-α/β and -λ production upon VZV infection and poor control of VZV replication (103). This work suggested an important contribution of innate immunity to antiviral defenses against VZV through the recognition of the AT-rich VZV genome, and demonstrated a major role of type I and III IFNs. AD POLR3F mutations were subsequently reported in monozygotic adult twins experiencing repeated CNS vasculitis presenting in a stroke-like manner with hemiparesis, sensory deficits, and headache, clinically diagnosed as being caused by recurrent VZV reactivation (107). Collectively, these recent findings suggested POL III may be important not only in protection against VZV CNS and pulmonary disease during primary infection, but also during reactivation, by controlling VZV latency in sensory ganglia (104, 107, 108). Future studies will clarify whether POL III is a VZV-specific antiviral immune sensor.

EBV immunity

EBV seroprevalence has been estimated at more than 90% in the general adult population (31, 109). The vast majority (>90%) of EBV-exposed individuals remain asymptomatic, or experience only mild self-limited disease. EBV causes infectious mononucleosis, which mostly affects adolescents. EBV was also oncogenic, causing several malignancies of lymphoid (non-Hodgkin lymphoma, Hodgkin’s lymphoma, NK/T-cell lymphoma) and epithelial (nasopharyngeal carcinoma) origin (31, 110). However, in the setting of acquired immunodeficiencies, EBV infection can have dire consequences. For example, post-transplant lymphoproliferative disease due to the EBV-induced transformation of donor B cells is a frequent adverse event in immunocompromised patients undergoing organ transplantation or HSCT for various conditions (23, 31). Similarly, AIDS patients frequently develop EBV-related diseases, including EBV encephalitis (31). Many inborn errors of immunity predisposing affected individuals to EBV diseases, such as viremia, encephalitis, meningitis, and pneumonia, or EBV-induced diseases, such as lymphoproliferation, HLH, and B-cell lymphoma, have been identified. Severe EBV infection has frequently been reported in patients with PIDs that affect the development and/or function of various leukocytes (Table 1), including SCID (i.e. RAG2 deficiency (111)), CID (i.e. DOCK8 deficiency (112, 113)), syndromic CID (i.e. CARMIL2 deficiency (114)), AR IRF8 deficiency (115, 116), and GOF mutations in CARD11 (117, 118). These clinical observations revealed a strict balance between host and pathogen, as alterations to this homeostatic relationship were sufficient to cause disease.

X-linked lymphoproliferative disease (XLP) is the textbook example of a PID with a selective inability to generate effective immunity against EBV infection in otherwise healthy individuals. XLP1 and XLP2 are caused by inactivating mutations of SH2D1A, encoding SAP (SLAM-associating protein) and XIAP, encoding XIAP (X-linked inhibitor of apoptosis), respectively (119, 120). SAP is a cytoplasmic adaptor protein that links SLAM-family surface receptors to intracellular signaling pathways, to regulate T and NK cell function. In the absence of SAP, cytotoxicity of CD8⁺ T and NK cells induced by the engagement of the SLAM-family receptors CD244 (2B4) and NTB-A is abolished. However, the cytotoxic effects of these cells following signaling through other activating receptors remain intact, potentially accounting for specific susceptibility to EBV but not to other HHVs (120, 121). Analyses of responses of SAP-deficient and SAP-sufficient CD8⁺ T cells specific for the same antigenic peptide presented on different APCs confirmed this hypothesis. The killing of Ag-presenting DCs, monocytes or fibroblasts is unaffected by SAP deficiency, but responses of Ag-specific CD8⁺ T cells to Ag-presenting B cells are strictly SAP-dependent (122). CD244 and NTB-A therefore play non-redundant roles in SLAM/SAP-mediated costimulation of Ag-specific CD8⁺ T cells. Consistently, SLAM family ligands (CD48, NTB-A) are strongly expressed on B cells, but not fibroblasts or DCs (122). Other receptor-ligand pairs would therefore serve this costimulatory function in the setting of Ag presentation by non-B cells. EBV is B-cell tropic, whereas many other viruses infect more than one type of host cell. These findings therefore account for the unique inability of XLP patients to control EBV infection, rendering them specifically vulnerable to EBV-induced disease.

Biallelic deleterious mutations of ITK (IL-2 inducible T cell kinase), CD27, CD70, CTPS1 (CTP synthase 1) or TNFRSF9 (4–1BB), and hemizygous mutations of MAGT1 (encoding a magnesium transporter) all result in PIDs characterized by susceptibility to EBV diseases (viremia) or EBV-induced diseases (lymphoproliferation, lymphoma)(121, 123–127). Patients with these mutations also suffer from infections with other viral and bacterial pathogens, but investigations of their immune cell defects have shed more light on the molecular requirements for host defense against EBV. MAGT1 plays an important role in regulating TCR signaling, and maintaining expression of the activating receptor NKG2D on cytotoxic cells. Low levels of NKG2D on T and NK cells of MAGT1-deficient individuals is sufficient to compromise the ability of these cells to respond to EBV-transformed B cells (128). Remarkably, CD70-deficient memory CD8⁺ T cells have abnormally low levels of expression for both NKG2D and 2B4, and are also unable to undergo activation by and to respond to autologous EBV-B cell targets (129). As CD70 is the counterpart structure of CD27, similar defects would be predicted to occur in CD27 deficiency. CD70 expression by B cells is required for the engagement of CD27 on responding CD8⁺ T cells to induce activation and subsequent effector function (130). Given the important roles of 2B4 and NKG2D in CD8⁺ T cell-mediated anti-EBV immunity, deduced from studies of XLP1 and MAGT1 deficiency, it has been suggested that impaired expression of these activating receptors underlies the inability of CD70-deficient (and CD27-deficient) CD8⁺ T cells to kill EBV-infected B cells (121, 129). The recent discovery of TNFRSF9 mutations in eight individuals with EBV viremia, lymphoproliferation and malignancy revealed that 4–1BB-deficient CD8⁺ T cells had impairments of IFNγ and perforin production and of the killing of MHC class I-matched EBV-transformed B cells (123, 125, 131). As 4–1BB appears to have similar functions to CD27 (132), it will be important to determine whether the expression of CD244, NKG2D and even CD27 are affected by 4–1BB deficiency.

Thus, different genetic etiologies underlie these specific PIDs (Table 1), but the mechanisms resulting in susceptibility to EBV disease or EBV-induced disease all affect the expression or function of key receptors providing EBV-specific T cells with costimulatory signals in the context of Ag presentation by B cells. These observations, together with the preferential expression of ligands for CD244 (CD48), NTB-A (NTB-A), NKG2D (MIC-A/B), CD27 (CD70) and 4–1BB (4–1BBL) by B cells – the host cell for EBV – highlight the differences in molecular interactions between cognate CD8⁺ T cells and APCs as a function of the nature of the invading pathogen and the APC itself, and the selective impairment of immunity to EBV infection in this group of specific PIDs.

HCMV immunity

The seroprevalence of HCMV ranges between 45 and 100% of the population (133, 134). Acute infection is usually asymptomatic in healthy individuals, or is associated with a mononucleosis syndrome (135). HCMV infection has a profound impact on the immune repertoire in healthy individuals, influencing >50% of measured immune parameters in a large cohort of serodiscordant monozygotic twins (136). HCMV infection in pregnant women can lead to congenital infection, causing stillbirth or severe sequelae in the newborn, such as sensorineural hearing loss in particular (137, 138). HCMV infection or reactivation is also a major health problem in immunocompromised patients, such as AIDS patients, and patients on immunosuppression for HSCT or solid organ transplantation (22). Severe clinical manifestations, including encephalitis, colitis, hepatitis, retinitis, pneumonitis and vasculitis, have been reported in these patients (22, 27). The pathogenesis of these various HCMV diseases remains largely unclear. The same HCMV diseases have also been observed in some patients with various PIDs, with and without other infections (Table 1).

HCMV infection can be fatal in patients with SCID, who completely lack T cells (e.g. IL2RG, JAK3, RAG, and ADA mutations)(139). Similarly, patients with CID due to LOF recessive mutations broadly impairing T-cell immunity (e.g. CARD11, IL2RB, IL12RB, ORAI1, STIM1) are vulnerable to HCMV infection (140–143). However, some CID patients may have CMV viremia without clinical manifestations, as observed in CARMIL2 (144), DOCK2 (100, 145, 146) and ICOS deficiencies (147). In addition, inborn errors of immunity leading to an isolated absence of B cells (e.g. BTK deficiency)(148), complement deficiency (e.g. C3 deficiency)(149) or phagocyte deficiency (e.g. CYBA, NCF1 or NCF4 deficiencies)(150) are not associated with a specific risk of HCMV disease. Inborn errors of immunity reveal the crucial role of T cells in defense against HCMV. CD4⁺ T cells have been shown to have a non-redundant role, based on the high susceptibility to HCMV of patients with MHC class II and CD40L deficiencies, which impair CD4⁺ T-cell activation (151, 152). This finding is consistent with susceptibility to this virus of AIDS patients with low CD4⁺ T-cell counts (153, 154). The lack of inborn errors resulting in isolated complete CD8⁺ T-cell or NK cell deficiency makes it impossible to draw firm conclusion about the absolute requirements for these lymphocyte subsets in anti-HCMV immunity. MHC class I and CD8A deficiencies, which specifically impair, but do not abolish, CD8⁺ T-cell and/or NK cell function, do not result in susceptibility to HCMV (155–157). However, GINS1 deficiency, which results in very low NK cell counts, neutropenia and a modest decrease in T-cell counts, is associated with fatal HCMV infection in one patient (40, 158).

Isolated susceptibility to HCMV is exceedingly rare, but there have been many reports of otherwise healthy patients with severe or life-threatening HCMV infection (159–162). Infection is usually restricted to a single organ in these patients. To date, only patients with hypomorphic RAG1 mutations have been reported to suffer from isolated multivisceral and recurrent HCMV infection and autoimmunity (163). The immunological phenotype of these patients is characterized by low levels of αβ T lymphocytes, normal levels of NK cells and a clonal expansion of the γδ T-cell population (163). Analyses of patients with isolated HCMV infection will undoubtedly refine our understanding of anti-HCMV immunity. Interestingly, HCMV colitis or enteritis has mostly been reported in elderly individuals, although some teenagers or young adults have also been reported to be affected, in the absence of known morbidity cofactors (162, 164). Finally, fetal HCMV infection is common, with a prevalence of 0.7% worldwide, but its pathogenesis is unclear. It is diagnosed on the basis of clinical and/or laboratory abnormalities. The symptoms are diverse, and include hearing loss, microcephaly and hepatitis. Fetal morbidity is not correlated with primary or secondary infection of the mother, suggesting that genetic factors may be involved (165). Based on restricted organ involvement, it seems likely that, as for HSE or severe influenza virus pneumonitis (75, 166), these patients are likely to have inherited cell-intrinsic immune defects with incomplete penetrance.

Immunity to HHV6 and HHV7

More than 90% of adults are seropositive for HHV6A, HHV6B and HHV7, with primary infection occurring between the ages of one and five years, depending on the virus (18). HHV6A and HHV6B have genomes that are 90% identical and about 50% identical to that of HHV7 (167). HHV6A and 6B can infect many different cell types, including CD4⁺ and CD8⁺ T lymphocytes, myeloid cells, some epithelial cells and neuronal cells, such as microglial cells, oligodendrocytes and astrocytes. By contrast, HHV7 has a narrow tropism, restricted to CD4⁺ T cells, with CD4 playing a crucial role in virus entry (167). Primary infection with these three viruses is usually asymptomatic. The main clinical expression of HHV6B primary infection is exanthema subitum (ES), or roseola infantum, a benign childhood infection. Clinical expression following primary infection with HHV6A and HHV7 is less well documented, but some cases of ES have been reported.

Severe manifestations are extremely rare. Cases of encephalitis and hepatitis due to reactivation have been reported in patients with acquired immune deficiency, either due to AIDS or immunosuppression for HSCT or solid organ transplantation (26, 29, 30). HHV6 reactivation and detectable viremia are common, with a frequency of 40 to 60% after HSC or solid organ transplantation, but only a few patients develop encephalitis (28). Diagnosis is based on electroencephalography and the detection of replicative HHV6 by immunochemistry or PCR in brain tissue and/or peripheral blood, together with antibody detection in CSF (29). HHV7 reactivation associated with encephalitis has been observed in patients post-HSCT (24, 29). One patient with X-linked SCID developed HLH due to activation of the integrated HHV6 genome (168). Finally, one ICOS-deficient patient with colitis and mild chronic hepatitis was found to be HHV6-positive by PCR on colon, duodenum and liver biopsy specimens. Her hepatitis symptoms improved, but those of colitis did not, following intravenous ganciclovir and oral valganciclovir treatments (169). However, some cases of encephalitis associated with HHV6 or HHV7 (170–174), and fulminant hepatitis due to HHV6 (175–178) have been reported in otherwise healthy individuals based on serology tests and/or the detection of viral DNA in CSF and/or whole blood. Patients who developed meningoencephalitis following HHV6 infection may also have ES manifestations (179, 180). Interestingly, no known inherited immunodeficiency has yet been associated with isolated severe HHV6 or HHV7 infections causing encephalitis or fulminant hepatitis. Novel inborn errors of immunity might explain these phenotypes. The identification of genetic etiologies responsible for severe clinical manifestations will facilitate the dissection of anti-HHV6 and anti-HHV7 immunity.

HHV8 immunity

The seroprevalence of HHV8 is variable worldwide, ranging from <20% in non‐endemic areas, (eg North America, Northern Europe, most of Asia), to > 40% in parts of Africa and South America (181). HHV8 causes Kaposi sarcoma (KS) or B-cell lymphoproliferative disorders, such as multicentric Castleman’s disease and primary effusion lymphoma. The vast majority of individuals exposed to HHV8 remain asymptomatic, or experience mild self-limited disease. KS has been classified into four epidemiological forms: epidemic and iatrogenic KS, both related to acquired immune deficiencies, due to HIV and following immunosuppression for solid organ transplantation, respectively; classic KS, which is observed in elderly men from Mediterranean countries and Eastern Europe; and endemic KS in sub-Saharan Africa. The histological features of the four forms of KS are identical, with spindle-shaped cells, inflammation, and angioproliferation with erythrocyte extravasation, suggesting a similar pathogenic mechanism. With the exception of epidemic KS, KS is very rare in children, suggesting that affected patients may suffer from an inherited immunodeficiency (182, 183).

Two inherited PIDs have been associated with KS in combination with other infectious diseases: AR IFNGR1 and XR WASP mutations. Complete AR IFNγR1 deficiency affects both adaptive and innate immunity, and is associated with life-threatening mycobacterial infections (53). In addition to such recurrent infections, one IFNγR1-deficient patient developed disseminated cutaneous and systemic KS. This patient presented a low CD4⁺ T-cell count, probably due to recurrent infections with mycobacteria, HHV8, or both. He died at 12 years of age (184). XR WAS affects cells involved in both innate and adaptive immunity, including myeloid cells, NK cells, B and T cells. Two cases of XR WAS with KS have been reported: one in a child and one in an adult with concomitant T-cell lymphoma (185, 186). The adult case was diagnosed with WAS in early childhood, but had a mild clinical phenotype. The child developed many infections during his two years of life. He received systemic steroid treatment in the month before onset of KS, which might have contributed to the development of KS. KS remission was complete after non-T cell-depleted allogeneic HSCT (186). The observation of KS in patients with acquired immunodeficiencies and these two PIDs suggested a role for T cells, probably CD4⁺ T cells, in anti-HHV8 immunity.

AR complete deficiencies of STIM1 and OX40 have recently been shown to be associated with isolated KS (187, 188). For both deficiencies, patients have aggressive, disseminated, cutaneous and systemic KS. The patient with STIM1 deficiency died at two years of age from pulmonary KS lesions, and would probably have developed the many infections seen in other STIM1-deficient patients had she lived long enough (182, 187). By contrast, complete remission was observed in the patient with OX40 deficiency after drug treatment. With the exception of leishmaniasis at the age of nine years, she has not suffered from other unusual infections, and is now 25 years old (188). Interestingly, in both these PIDs, T-cell immunity was impaired, but T-cell counts were normal. STIM1 is involved in the Ca²⁺ release-activated Ca²⁺ channel, which is essential for Ca²⁺ influx in T lymphocytes following TCR stimulation, leading to proliferation and production of the cytokines required for T-cell effector function and differentiation (189). Similarly, OX40 is highly expressed on activated T cells and functions as a costimulatory molecule for inducing proliferation, survival and cytokine production by T cells (190). OX40 deficiency is associated with impaired CD4⁺ T-cell responses to recall antigens in terms of proliferation and IFNγ production (188). Studies of acquired and inherited immunodeficiencies have, thus, highlighted the crucial role of OX40-dependent CD4⁺ T-cell immunity in the control of HHV8 infection (Table 1). Further studies are required to define the immune responses to HHV8 crucial for preventing KS, multicentric Castleman’s disease and primary effusion lymphoma.

Conclusions

The first herpes virus was identified 100 years ago, and the last 25 years ago. Due to high seroprevalence, the lack of an effective vaccine, and the emergence of antiviral drug-resistant strains, HHV infections continue to be a public health problem (191). Despite a century of virological studies, our understanding of human immunity to HHVs remains partial. HHVs are associated with diverse clinical manifestations, involving different cells and tissues. Interestingly, PIDs of leukocytes conferring a predisposition to HHV infection, including even the most severe forms of SCID, although are frequently associated with severe HHV infections of peripheral tissues (i.e. skin, lung), may, only in rarer cases, be associated with invasive infections of internal organs (i.e. brain, liver). The discovery of the molecular basis of severe HHV infections in some patients has shed light on the contribution of leukocytes and other cells to protective immunity to HHVs. Some HHV infections can be attributed, at least partly, to abnormally small numbers or impaired functions of leukocyte subsets, such as CD4⁺ T, CD8⁺ T, and NK cells (CD8⁺ T cells in anti-EBV immunity, CD4⁺ T cells in anti-HCMV and HHV8 immunity, NK cells in anti-HSV, VZV and possibly EBV immunity). IFN-α/β or -λ-mediated immunity provided by leukocytes, including DCs in particular, may be crucial in some infections, as suggested by the observation of severe VZV infection in POL III-deficient or DOCK2-deficient patients (whose leukocyte IFN-α/β and -λ responses to VZV-related stimulations are impaired) and in GATA2-deficient patients (whose leukocyte IFN immunity is impaired due to DC lymphopenia). Other HHV infections can be attributed to deficiencies of cells other than leukocytes, as illustrated by the role of forebrain neuron and oligodendrocyte cell-intrinsic immunity in forebrain defense against HSV1 infection, and brainstem cell-intrinsic immunity at this anatomical site. These studies highlight the importance of studying the human genetic and immunological basis of severe HHV disease restricted to a specific cell, tissue, or organ, in otherwise healthy individuals.