Fcγ Receptors: Structure, Function and Role as Genetic Risk Factors in SLE

Abstract

Over 30 years ago, receptors for the Fc region of IgG (FcγR) were implicated in the pathogenesis of SLE. Since those pioneering studies, our knowledge of the structure and function of these FcγRs has increased dramatically. We now know that FcγR contribute to regulation of acquired immunity and to regulation of innate immune responses where FcγRs act as specific receptors for innate opsonins (CRP and SAP). Our understanding of the genomic architecture of the genes encoding the FcγR has also witnessed remarkable advances. Numerous functionally relevant SNP variants and copy number (CN) variants have been characterized in the FcγR genes. Many of these variants have also been shown to associate with risk to development of SLE and some have been associated with disease progression. This review will provide an overview of the FcγR in relation to SLE including consideration of the role of genetic variants in FcγR in SLE pathogenesis. The difficulties in assessing genetic variation in these genes will be discussed. To enhance our understanding of the functional roles of these receptors in SLE, future research will need to integrate our knowledge of SNP variants, CN variants and the functional diversity of these receptors.

Introduction

SLE is a prototypic autoimmune disease characterized by the presence of autoreactive B cells and the formation and deposition of antibody-antigen immune complexes consistent with the diathesis, which involves multiple organ systems. Evidence from familial aggregation studies^1-5 together with a high concordance among monozygotic twins^6-8 suggest a genetic contribution. However, a single gene with a clearly causal Mendelian effect has not been identified underscoring a multigenic mode of inheritance. Among the quantitative trait loci identified from candidate gene-association studies of murine lupus models and diverse human populations and the recently published genome-wide scan, consistent linkage to 1q21.1-24, a region that includes a functionally and structurally diverse group of receptors that recognize the constant (Fc) portion of specific immunologlobulin (Ig) isotypes has been demonstrated^9-15.

Consistent with a role for receptors for the Fc region of IgG (FcγR), the pathogenesis of SLE involves the production of autoantibodies resulting in immune complex (IC) formation. Altered or delayed clearance of these autoantibody containing IC results in deposition of these IC in various tissues, eliciting inflammation and damage by engaging IgG Fc receptors (FcγRs) and complement. Even from decades ago it was well known that FcγR expression levels and function were altered in SLE. The classic EA clearance studies^16,17 demonstrated delayed in vivo FcγR mediated clearance in patients with SLE and ex vivo studies demonstrated altered FcγR functions such as phagocytosis¹⁸. These alterations in FcγR function correlate with disease activity suggesting that disease activity may alter receptor function. However, extensive studies of FcγRs in both animal models and human population studies have since been performed^19,20 and several functional genetic polymorphisms have been identified and associated with SLE in several ethnic populations. Thus FcγR function in patients with SLE is influenced by both inherited genetic variation and by acquired differences in function attributable to disease activity.

Fc receptors are a heterogeneous group of hematopoeitic cell surface glycoproteins that facilitate the efficiency of antibody-antigen interactions with effector cells of the immune system. These receptors regulate a variety of humoral and cellular immune responses including phagocytosis, degranulation, antibody-dependent cellular cytotoxicity (ADCC), transcriptional regulation of cytokine and chemokine expression, B cell activation and immune complex clearance. The cellular distribution and Ig isotype (IgA, IgD, IgE, IgG and IgM) specificity influence the regulatory roles of Fc receptors. In addition, common variation in the genes that encode Fc receptors, capable of altering the efficiency of the mononuclear phagocyte system to clear Ig immune complexes, provides a mechanism for the heritable differences observed in the susceptibility to SLE and subsequent immune complex-mediated tissue injury^{21, 22}.

The Fc receptor complex is noteworthy for its high degree of sequence homology, pattern of linkage disequilibrium (LD) and common copy number variants (CNV). This group of structurally and functionally diverse receptor genes are co-localized to a region on chromosome 1q21.1-24 that includes the classical Fc receptors for IgG (Fcγ) and non-classical Fc-like receptors (FCR1-FCRL6L), Fc receptors for IgE (FcεRI) and IgA/IgM (Fcα/μR). The consistency of genotype-phenotype findings coupled with the topology of the Fcγ receptor gene cluster underscores the importance of this locus as containing susceptibility alleles with potentially deleterious functional consequences.

Human Fcγ Receptor Structure and Function

FcγRs bind to the Fc portion of IgG, and serve as a crucial link between humoral and cell mediated immune responses. In addition, more recent data demonstrated that FcγR also function as receptors for innate immune opsonins (CRP and SAP) and provide a link between innate and acquired immunity. In human, the classical FcγR family is divided into three receptor families (FcγRI (CD64), FcγRII (CD32) and FcγRIII (CD16)) based on structural homology, difference in affinity and differences in specificity for IgG subclasses. These FcγRs are also defined as either activating receptors (FcγRI, FcγRIIA/C, FcγRIII) or inhibitory receptor (FcγRIIB) as they elicit or inhibit immune functions such as phagocytosis, cytotoxicity, degranulation, antigen presentation and cytokine production via immune tyrosine activating or inhibitory motifs (ITAM or ITIM). These signaling motifs are either on the same ligand binding α-chain as for FcγRII, or on the associated homodimer accessory chains such as the common Fc receptor γ-chain (for FcγRI and FcγRIII, Figure 1) that is also shared with FcεRI and FcαRI¹⁹. On NK cells, FcγRIIIa associates with a homodimer of the TCR ζ-chain. The ligand binding α-chain of most FcγRs consists of two or three immunoglobulin (Ig)-like domains in extracellular region, a transmembrane domain and an intracellular cytoplasmic domain.

Figure 1.

Structure of human classical Fcγ Receptors and location of functionally characterized SNP variants.

FcγRI

Located in chromosome 1q21, FcγRI is the only high affinity Fcγ receptor which can strongly bind monomeric IgG. This receptor is also unique from other FcγRs in having three extracellular Ig-like domains (see Figure 1). The FcγRI family has three genes (FCGRIA, FCGRIB and FCGRIC) but only the FcγRIa-product of FCGRIA has been identified as a full length cell surface receptor²³. FcγRI has been found on surface of monocytes, dendritic cells (DC), macrophages and also activated neutrophils²⁴. Stable expression of FcγRIa requires the presence of the common FcR γ-chain homodimer resulting in the expression of a complex of FcγRIa (the α-chain)/γ-γ on the cell surface. FcγRIa can be expressed in the absence of this γ-chain homodimer but expression is transient.

FcγRII

The FcγRII subclass of receptors on human chromosome 1q23 is composed of three genes (FCGR2A, FCGR2B and FCGR2C) which encode the FcγRIIa, FcγRIIb and FcγRIIc proteins. Expressed on monocytes, certain dendritic cells, neutrophils, B cells, platelets and NK cells, FcγRII (CD32) is the most widely distributed FcγR with low binding affinity for IgG²⁵. With two extracellular Ig-like domains, FcγRII has low binding affinity for monomeric IgG, but binds IgG aggregates and IC readily. As mentioned earlier, unlike other FcγRs, the FcγRII proteins bear signaling motifs directly in their intracellular cytoplasmic domains and don't require the common FcR γ-chain for stable expression or function. While the FcγRIIa and FcγRIIc proteins contain the immunoreceptor tyrosine activation motif (ITAM), the FcγRIIb protein is the only inhibitory Fcγ receptor containing an immunoreceptor tyrosine inhibitory motif (ITIM) in its cytoplasmic domain. An additional level of complexity in the FCGR2B locus is that three alternatively spliced transcripts can be expressed: b1 and b2 differ by an insert of 19 amino acids in the FcγRIIb cytoplasmic domain while the b3 form lacks part of the signal sequence. FcγRIIb1 is expressed on B cells as the only currently recognized Fcγ receptor on B cells, while FcγRIIb2 is found on myeloid cells together with FcγRIIa. The FCGR2C gene has a stop codon (STP)/glutamine (Q) polymorphism at amino acid position 13 in the first extracellular domain and has been described as an expressed protein on NK cells only when the 13Q allele is present^26,27. The FCGR2C gene encodes an extracellular domain that is highly homologous to FcγRIIB and an intracellular cytoplasmic domain that is nearly identical to the cytoplasmic domain of FCGR2A. Characterization of the FCGR2C locus has suggested that the FCGR2C gene is the result of an unequal crossover event between FCGR2A and FCGR2B.

FcγRIII

Two genes, FCGR3A and FCGR3B encode the two receptors of the FcγRIII family (FcγRIIIa and FcγRIIIb). FcγRIII is also considered low affinity; however, FcγRIIIA can bind monomeric IgG with an intermediate affinity and both FcγRIIIA and FcγRIIIB bind multimeric IgG and IC efficiently. FcγRIIIa ligand binding is further influenced by receptor glycosylation. Different glycoforms of FcγRIIIa are expressed on human NK cells and monocytes/macrophages and these glycoforms have different abilities to bind IgG ligand.

The FcγRIIIa protein is expressed as a transmembrane protein on monocytes, tissue specific macrophages, dendritic cells, δ/γT cells, and natural killer cells¹⁹. On these cells γ-chain (or the TCR ζ-chain on NK cells) is necessary for both stable expression of the protein on the cell surface and for signal transduction through the FcγRIIIa receptor complex. Interestingly, in mast cells, the FcγRIIIa receptor complex can also incorporate the β-chain from the IgE receptor resulting in an α/β/γ-γ complex on the cell surface.

The FcγRIIIB gene is unique amongst the FcγR genes in that it encodes a GPI-anchored receptor, FcγRIIIb. This receptor is expressed on the surface of neutrophils and basophils²⁸. Of note, on the surface of neutrophils, FcγRIIIb is highly expressed with 150,000 to 200,000 receptors expressed per cell. Biochemical studies have suggested that when FcγRIIIb is expressed on the surface of neutrophils, it interacts with the β2 integrin CD11b/CD18. This is of interest given the recent demonstration of strong genetic association between a variant in the CD11b/ITGAM locus and SLE.

As mentioned above, each FcγR protein has differential ligand binding preferences with respect to IgG subclasses and differing affinities for IgG subclasses. These preferences are summarized in Table 1 and have been recently reviewed in great detail²⁹. Of note, the binding of IgG2 is generally limited to the R131 allele of FcγRIIa while binding of IgG4 binding is limited to the 159V allele of FcγRIIIa (see below).

Table 1.

General features of human Fcγ Receptors.

Name	Chromosome Location	Isoforms	IgG subclass specificity	Distribution*
FcγRI(CD64)	1q21		G1=G3>G4≫G2	Mon/Mac/PMN/DC
FcγRII(CD32)	1q23		G3≥G1≫G4>G2
		FcγRIIA		Mon/Mac/DC/PMN/Platelet
		FcγRIIB		B/Mon/Mac/DC
		FcγRIIC		NK
FcγRIII(CD16)	1q23		G1=G3≫G2>G4
		FcγRIIIA		Mon/Mac/NK/δγT
		FcγRIIIB		PMN

^*Mon: monocyte; Mac: macrophage; PMN: neutrophil, DC: dendritic cell; NK: natural killer cell.

FcγR function

Regulation of cell signaling

Upon cross-linking by IC, activating FcγRs (FcγR associated with the common FcR γ-chain or the TCR ζ-chain or FcγRIIa/c) initiate a signaling cascade via the ITAM, which normally starts with sequential activation of protein tyrosine kinases of the Src-family followed by activation of the Syk tyrosine kinase^30,31. This results in the tyrosine phosphorylation of the ITAM itself with recruitment of various downstream targets including other kinases, adaptor molecules and other signaling intermediates (see ³² for a review). In many cell types, activation of PI3K results in the recruitment of phospholipase Cγ (PLCγ) that in turn induces a PLCγ mediated calcium influx triggering different effector functions^33,34. In antigen presenting cells (APC), such as macrophages and dendritic cells, activating FcγRs mediate cell functions such as phagocytosis, respiratory burst and cytokine production (TNF-α, IL-6); in neutrophils and NK cells, they trigger antibody-dependent cellular cytotoxicity (ADCC) and degranulation; and in mast cells, they induce degranulation ^35,36. Of note is the finding that these so-called activating receptors are not merely overlapping in functional properties but in fact have specialized functions that are due in part to the differences in the cytoplasmic domains of the ligand binding α-chains. For example, recent work has shown the unique ability of FcγRIa to facilitate antigen presentation. Even more surprising is our recent finding that FcγRIa is unique amongst FcγR, and amongst other γ-chain associated Fc receptors, in its ability to regulate the cell surface expression of TNFSF13b (also called BLyS or BAFF) upon cross-linking with either IgG ligand or with the innate immune opsonin CRP³⁷. Other differences in receptor function are the result of differences in cell surface expression. FcγRIIa is unique on its expression on platelets and cross-linking of FcγRIIa can directly activate platelets resulting in platelet aggregation. While all of these functions are dependent on the ITAM, it is likely that the sequences in the associated α-chains are also contributing to net receptor function. Indeed, we have shown that the cytoplasmic domain of FcRIa, devoid of any known signaling motifs, modifies γ-chain based signaling and other recent work has demonstrate that these cytoplasmic domains can recruit different molecules to the receptor complex. Future work will be required to fully delineate the signaling elements engaged by the ITAM expressing FcγR and this work promises to provide additional insights into the functions of each of these different FcγRs.

A key function of these activating FcγR is the removal or clearance of immune complexes. As noted above, this function is known to be altered in patients with SLE. Of all FcγRs, FcγRIIIa is thought to play an important role in immune complex clearance. The intermediate affinity of this receptor on macrophages (fixed tissue macrophages) makes it ideally suited as a capture receptor to facilitate clearance. Of course, this does not preclude a role for other FcγR in IC handling and it is likely that IC clearance represents the integrated function of multiple FcγR working in concert with complement receptors.

As the sole Fcγ receptor bearing an ITIM, FcγRIIb acts as a suppresser of IC-mediated cell activation and when co-cross-linked with surface Ig FcγRIIb functions as a suppressor of B cell activation. When FcγRIIb is cross-linked with other activating receptors, the ITIM is phosphorylated by a Src family kinase such as Lyn which leads to the recruitment of Src homology 2 domain-containing protein tyrosine phosphatase (SHP-1), SHP-2 and/or SH2-domain-containing inositol polyphosphate 5′ phosphatase (SHIP). In B cells, SHIP can inhibit activation pathways mediated by pleckstrin homology (PH)-domain-containing kinases, such as PLCγ and Bruton's tyrosine kinase (BTK), therefore suppressing downstream events such as the Ig-induced calcium flux and diminish multiple cellular functions induced by ITAM-containing receptors as mentioned above³⁸. A key role for FcγRIIb in regulating autoimmunity has been demonstrated in both murine models and in patients with SLE. Certain mouse strains that are deficient in FcγRIIb develop a spontaneous lupus-like disease resulting in glomerulonephritis and premature mortality³⁹. Conversely, increases in FcγRIIb expression in these mice can restore tolerance and ameliorate the spontaneous autoimmune phenotype⁴⁰. In patients with SLE, alterations in FcγRIIb expression have also been noted. However, disease is associated with increased expression of the FcγRIIb protein on the surface of B cells. Clearly, additional work is needed to more fully understand the critical role of this receptor in regulation of immune tolerance and autoimmunity.

More recent data from a number of different receptor systems has challenged the conventional ITAM/ITIM concept^41,42. Extensive cross-talk, or co-modulation, between receptor systems has now been described and is likely to be pervasive across many different receptor systems. For example, the IgA Fc receptor, FcαRI, can not only activate cells but can also cross-inhibit other receptor including the classical FcγR. Similarly, engagement of receptors containing ITIMs can result in induction of functional responses such as antigen presentation via FcγRIIb on dendritic cells. The regulation and determination of the activating and inhibitory potential of FcγR is likely influenced through the complex interplay of the degree of receptor cross-linking, the nature of interaction between receptor subunits and the interactions with other cell surface receptors (such as CD11b/CD18).

FcγR Polymorphisms and Association with SLE

Sequencing of the genes in the Fcγ receptor cluster has resulted in the identification of numerous genetic variants including functionally relevant variants. The importance of FcγR variants as SLE susceptibility factors and as factors influencing disease outcomes is highlighted by the consistent association of many of these genetic variants with disease. Table 2 highlights some of the consistent associations observed between FcγR variants and SLE. However, limiting the ability to examine the role of genetic variants in these genes is the high degree of sequence homology between genes (Figure 2). Indeed, this sequence homology precludes assessment of most SNPs in these genes on the currently available genome wide SNP arrays. In the first two published genome wide association studies (GWAS)^43,44, the arrays contained only 11 and 19 SNPs across the entire low affinity FcγR cluster precluding a detailed analysis of association between these genes and SLE. We will summarize the current knowledge regarding associations between genetic variants in each of the low affinity classical FcγR and SLE.

Table 2.

Functional polymorphisms of classical Fcγ Receptors and association with SLE

Name	Alleles	Association with SLE	Mechanism
FcγRII (CD32)
FcγRIIA	H/R131 (rs1801274)	R131(multiple ethnic populations)	Altering ligand binding (R: lower affinity, no binding to G2)
FcγRIIB	I/T187*(rs1050501)	T187 (Asian)	Altering signaling
	2B.1/2B.4*	2B.4 (Cauc.)	Altering transcription factor binding
FcγRIIC	STP/Q13	unknown
FcγRIII (CD16)
FcγRIIA	V/F158 (rs396991)	F158 (multiple ethnic populations)	Altering ligand binding (F: lower affinity)
FcγRIIB	NA1/NA2	NA2 (Japanese)	Altering ligand binding (NA2: lower affinity)
	CNV	Low CN (Cauc.)	Altering protein expression level

^*Promoter haplotype. 2B.1: -120G-386T; 2B.4: -120C-386A.

Figure 2.

Schematic genomic structure of the classical low affinity Fcγ receptor cluster on human chromosome 1q23. Identical colors represent regions of high sequence homology between genes.

FCGR2A

A non-synonymous G to A variant in the FCGR2A gene (rs1801274) results in a single amino acid difference at position 131 (R131 and H131) in the second extracellular Ig like domain of the FcγRIIa protein. This allelic difference alters recognition of ligand. The FcγRIIA-H131 (Histidine residue at position 131) allele is able to bind IgG2 effectively while the R131 (Arginine residue at position 131) allele does not bind IgG2^21,45. This affinity variation leads to functional difference: phagocytes from homozygous H131 individuals are much more effective than those from homozygous R131 donors in terms of phagocytosis of IgG2 opsonized particles⁴⁶. Several independent studies in multiple ethnic groups have reported the association of R131 with the susceptibility to SLE and/or lupus nephritis in Dutch Caucasians, European-Americans African-Americans and Koreans^47-49 while other studies have presented inconsistent results^50-54. Karassa et al. did a meta-analysis of 17 published studies and inferred that the R131 allele was 1.3-fold higher risk for SLE susceptibility compared to the H131 allele⁵⁵. Besides contributing to susceptibility, the FcγRIIa-R131 allele was also shown to be related to SLE severity⁵⁶. Haseley et al.⁵⁶ found that the presence of 131R/R was associated with renal involvement in patients with IgG2 anti-C1q autoantibody positive nephritis. Inconsistencies between studies might be attributable to ethnic differences, disease heterogeneity (i.e. subclass of autoantibodies), or to genotyping error.

FCGR2B

A non-synonymous T/C SNP in the FCGR2B gene has been identified that results in the change of an isoleucine (I) to threonine (T) substitution at position 187 in the transmembrane domain of the FcγRIIb protein^57,58. This single amino acid substitution has been shown to affect the inhibitory function of FcγRIIb on B cells. The 187T allele is excluded from lipid rafts and has a decreased inhibitory potential towards BCR signaling^59,60. Likewise, the 187T allele has been reported to be associated with SLE in Asian populations including Japanese, Chinese and Thais,^58,61,62 but no association has been found in African-American or Caucasian in the US or Europe^57,63. A meta-analysis of association of this variant in FCGR2B with SLE in three independent studies of Asian populations demonstrated an odds ratio of 2.45 for 187T/T vs 187I/I for SLE susceptibility⁵⁹.

In 2004, Su et al. identified a promoter haplotype that alters FcγRIIb promoter activity⁶⁴. The less frequent promoter haplotype (–386C-120A) showed increased promoter activity and drove higher receptor expression in both transfected cell lines and on cells ex vivo from genotyped donors than the more frequent haplotype (–386G-120T)^64,65. The less frequent and more active promoter haplotype was associated with SLE in a Caucasian population with an odds ratio of 1.6⁶⁴. Confirmation of this association has been published and demonstrated that the same -386G/C SNP associates with SLE susceptibility⁶⁶. The analysis of these promoter variants in FCGR2B are hampered by complete sequence identity with the proximal promoter of FCGR2C⁶⁴. Studies examining these SNPs need to take care to ensure gene specificity in any analysis of this genomic region.

FCGR3A

As noted above, FcγRIIIa is thought to play a critical role in immune complex clearance. In addition, this receptor is an important mediator of antibody-dependent cell-mediated cytotoxicity (ADCC) functions on NK cells⁶⁷. A non-synonymous SNP (rs396991) that encodes a T to G change at nt559 that results in a phenylalanine (F) to valine (V) amino acide change at position 159 in the second extracellular domain of the FcγRIIIa protein. These alleles of FcγRIIIA have differing binding affinity for IgG. The FcγRIIIA-158V allele (also known as 176V/F when the leader sequence is included) binds IgG1, IgG3 and IgG4 with higher affinity relative to the 158F (176F) allele. In addition, the increased binding capacity of the 158V allele results in more robust downstream functional effects. Using pericpheral blood NK cells from genotyped donors, individuals homozygous for the 159V allele demonstrate higher calcium transients, higher induction of CD25 expression and more rapid apoptosis when compared to donors homozygous for 159F^68,69. Consistent with a critical role for FcγRIIIa in IC clearance, the lower IgG binding allele 158F is associated with SLE susceptibility in several case-control studies in multiple ethnic groups (Caucasian and African-American).

A role for FcγRIIIa in disease severity has also been demonstrated. Within patients with renal disease, the higher biding 159V allele is associated with development of ESRD⁷⁰. This result, in conjunction with the SLE susceptibility studies of FCGR3A, demonstrates that this gene can influence not only disease susceptibility but also disease progression. In patients with renal disease, progression to ESRD is associated with the higher binding allele of FcγRIIIa consistent with a role for this receptor in promoting more vigorous local inflammatory responses in the kidney.

FCGR3B

Three different allotypic variants of FcγRIIIb, NA1, NA2 and SH, have been identified through serological studies. The sequence bases for these FCGR3B variants are now known (Figure 3). The six SNP differences underlying these three serologic allotypes include 5 non-synonymous SNPs and 1 synonymous SNP. The five amino acids changes are all in the first extracellular domain of FcγRIIIb (marked as black in Figure 3) with the amino acid 65 change resulting in a loss of a glycosylation site in the NA2 allele^22,71. The enhanced functional capacity of the NA1 allele is firmly established. Some studies have suggested differing biding affinity for IgG1 and IgG3 between the FcγRIIIB-NA1 and FcγRIIIb-NA2 alleles with the NA1 allele demonstrating higher binding. Alternatively, the NA1 and NA2 alleles may interact differently with other cell surface receptors such as the β2 integrin CD11b/CD18. Interactions with other cell surface receptors may be critical or essential to FcγRIIIb function. As noted above, this receptor lacks a transmembrane and cytoplasmic domain and is anchored to the cell surface through a GPI-linkage. In the absence of these domains, such inter-molecular interactions may be necessary for FcγRIIIb to function and glycosylation change between the NA1 and NA2 alleles may alter such interactions.

Figure 3.

Genomic structure of the FCGR3B locus. The sequence basis and corresponding amino acids for the NA1, NA2 and SH alleles are shown.

In a Japanese population, the NA2 allele has been reported as a susceptibility factor for SLE⁷². However, this observation has not been replicated. More recently, a re-examination of the importance of variation-copy number variation (CNV) at the FCGR3B locus has suggested a role for decreased copy number of FCGR3B in SLE susceptibility (see below).

Genome Wide Association Studies

A number of GWAS studies in SLE have now been reported. As noted above, SNP coverage in the low affinity Fcγ receptor is poor with some genes (FCGR2C and FCGR3B) receiving no coverage. Nonetheless, statistically significant association at the non-synonymous H131/R131 variant (rs1801274) was noted in the recent GWAS performed by the International Consortium on Systemic Lupus Erythematosus⁴⁴. The SNPs in other FCGR genes previously associated with SLE were not represented on any of the arrays used in the recent GWAS studies. The presence of high sequence homology between the FCGR genes with the presence of a segmental duplication in this region (Figure 2) preclude any GWAS based conclusions regarding association at the FCGR3A or FGR2B loci with SLE. Additionally, association between SNP variants and SLE in this genomic region may also be confounded by the presence of known copy number variants.

CNV in the Fcγ Receptor Cluster

FCGR3B CNV and SLE

CNV of FCGR3B in SLE has been known since the 1990s, although it was not reported as such⁷³. Although FCGR3B is almost identical at the sequence level to FCGR3A, unique alleles in the 3′-most exon have been used to detect FCGR3B CNV. Deficiency (i.e. copy number of zero) of FCGR3B was first observed as a lack of all three alleles in two healthy individuals in 1990^74,75. Increased copy number (CN) was later observed via the presence of all three alleles in three individuals⁷⁶.

Association of FCGR3B CNV with SLE has been determined using a real-time PCR assay developed by Aitman et al.⁷⁷; Relative copy number of <2 at the FCGR3B locus was associated with risk for SLE (with and without nephritis). Relative copy number of >2 FCGR3B was not significantly associated with protection against SLE but was significantly associated with protection against Addison's disease and Wegener's granulomatosis in a French population⁷⁸. The same assay has been used to show an association of FCGR3B CN with risk for glomerulonephritis and lupus nephritis: increased copy number was protective and decreased copy number was a risk factor⁷⁷. There is evidence that CNV of FCGR3B is associated with alterations of surface expression on neutrophils, neutrophil adhesion, and immune complex uptake by neutrophils⁷⁹. Deficiencies in these neutrophil functions could cause or contribute to the characteristic buildup of immune complexes seen in SLE.

CNV of the other FCGR genes in the 1q23 cluster

CNV has been reported for other FCGR genes, but no association with SLE has been reported yet. CNV of FCGR2C and FCGR3B have been reported to be linked⁸⁰. CNV of FCGR2C, assessed by multiplex ligation-dependent probe amplification (MLPA), was determined to be associated with idiopathic thrombocytopenic purpura²⁷. In the same report, CNV of FCGR3A was also reported. CNV of the two remaining receptors in the chromosome 1q23 FCGR cluster, FCGR2A and FCGR2B, has yet to be reported. Although CNV of the other FCGR genes has yet to be associated with SLE, if reported at all, the clear significance of FCGR3B CNV in SLE, the evidence of more extensive CNV in the cluster, and the logical connection between FCGRs and the pathology of SLE suggest that further study of CNV in the 1q23 FCGR cluster is warranted.

Future Directions

Efforts to characterize the joint contribution of Fc receptor loci with additional genotypic and environmental factors will expedite our understanding of SLE pathogenesis and importantly, its natural history.

Heritable indices of SLE severity and progression

Recent advances in human genome sequencing and marker density together with estimating unknown phase and accounting for population stratification in large case-control association studies have become key elements for identifying heritable risk factors for lupus susceptibility. However, longitudinal studies will be essential to facilitate our understanding of the genetic markers involved in influencing disease severity and progression, which is characterized by flare, chronically active disease and quiescence. Such characterization would facilitate an evaluation of the effectiveness of a prognostic index for SLE.

Fc receptor co-expression and function

Immune surveillance is redundant and is reflected by the co-expression of multiple ITAM- or ITIM-containing Fc receptors on the same cell. The co-expression of multiple stimulatory ITAM-dependent Fc receptors provides a mechanism whereby the joint effect conceivably results in a synergistic activation of effector cells and enhanced efficiency for immune complex clearance. Alternatively, in cells that co-express inhibitory ITIM- and ITAM-dependent Fc receptors, the magnitude of the effector cell response upon IgG isotype cross-linkage is attenuated reflecting an antagonistic joint effect. Thus, the SLE diathesis may reflect the relative difference in the magnitude of ITAM- and ITIM-dependent effects on the activation of effector cells, which in turn may be influenced by the genetic heterogeneity observed in the genes that encode for the Fc receptors. The delineation of individual stimulatory and inhibitory effects of Fc receptor gene loci or haplotypes may advance our understanding of the combined effects these receptors on the risk of, or time to, SLE and related sequelae or clinical manifestations.

Gene by gene interactions

In addition to the Fcγ receptors, strong and consistent gene associations were shown across diverse populations for HLA antigens^69,71,81,82, components of the classical complement activation system belonging to the major histocompatibility complex class III region^69,71,82,83, and more recently, the type I interferon regulatory factor-5 (IRF5)^68,84,85,86, the protein tyrosine phosphatase, PTPN22^{68,74,77,87,88},.⁸⁹, ITGAM, PXK and KIAA1542⁴⁴. Despite strong associations with SLE, these susceptibility loci do not sufficiently contribute to the risk of this complex phenotype alone. The joint effects of deleterious variation in these susceptibility loci and Fc receptor genes, which offer fundamental biologically plausible mechanisms for pathologic processes, may contribute to our understanding of SLE pathogenesis and natural history of SLE.

Gene by environment interactions

Tantamount to the exploitation of gene-gene interactions are evaluations of micro-environment factors that alter Fc receptor expression and function. Several cytokines, chemotactic factors and growth factors have been shown to alter Fcγ receptor expression on diverse cell types in an autocrine and paracrine manner. Fcγ receptor effector functions may also be altered by reactive oxygen species and serine proteases, which promote differences in receptor phagocytotic and binding capacity, respectively. It is conceivable that additional micro-environmental factors including infectious agents, sex hormones, C-reactive protein and mannose binding lectin that correlate with the SLE diathesis influence Fc receptor function. Such modifying effects may underscore the variation in Fcγ receptor gene associations observed with disease and lupus nephritis. This is particularly noteworthy because the pleiotropic effects of these inflammatory molecules act locally to perturb Fcγ receptor-mediated immune complex clearance leading to chronic inflammation of the glomeruli, fibrionoid necrosis and reduced renal capacity, consistent with lupus nephritis.

Concluding Remarks

The study of Fcγ receptors in SLE has a rich heritage dating back over 30 years to the initial Fcγ receptor mediated clearance studies. Our knowledge of the diversity of Fcγ receptor structure and function has developed substantially since that time. Based on this increase in knowledge about these receptors, many pivotal mechanistic and genetic studies have reinforced the functional importance of these receptors in the pathogenesis of SLE. As we develop additional tools to assess genomic variants in this region, we will undoubtedly be surprised by the diversity of variation amongst these genes and gain increased insight into the role of Fcγ receptors in SLE.