Search

A polymorphism in the receptor for the Fc region of IgG, Fc γ-receptor IIIa (FcγRIIIa, FCGR3A rs396991), has been inconsistently shown in the literature to have an effect on response to monoclonal antibody therapy in several indications. The rs396991 (T/G) polymorphism leads to an F176V substitution and increased affinity for IgG. This variant has proven difficult to genotype accurately, primarily because of extensive homology between the FCGR3A and FCGR3B genes. We have shown that rs396991 can be genotyped by PCR amplification, followed by direct Sanger sequencing of the product, without coamplification of FCGR3B, and that the rs396991 TaqMan assay (C__25815666_10) agrees with Sanger sequencing results in 100% of European and Asian samples tested, but it has a small error rate in African and American populations. C__25815666_10 is therefore suitable to interrogate rs396991 in studies involving Europeans and Asians; however for other populations, the default genotyping method should be PCR followed by Sanger sequencing.

Recent work has emphasised the marked genetic variability that exists in the Fc receptor locus. This variation can contribute to the risk of autoimmune disease in both mice and humans, but can also have a profound impact on defence against infection. Using FcγRIIB and FcγRIIIB as examples, we demonstrate that variations associated with increased susceptibility to autoimmunity may be maintained in populations for their beneficial effect against infection. We examine the KIR locus from the same perspective and highlight similarities between the two loci. Intense selection pressure by pathogens presumably accounts for the marked variability within both regions and leads to susceptibility to autoimmunity for some alleles., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The response of a leukocyte to immune complexes (ICs) is modulated by receptors for the Fc region of IgG (FcgammaRs), and alterations in their affinity or function have been associated with risk of autoimmune diseases, including systemic lupus erythematosus (SLE). The low-affinity FcgammaR genomic locus is complex, containing regions of copy number variation (CNV) which can alter receptor expression and leukocyte responses to IgG. Combined paralogue ratio tests (PRTs) were used to distinguish three intervals within the FCGR locus which undergo CNV, and to determine FCGR gene copy number (CN). There were significant differences in FCGR3B and FCGR3A CNV profiles between Caucasian, East Asian and Kenyan populations. A previously noted association of low FCGR3B CN with SLE in Caucasians was supported [OR = 1.57 (1.08-2.27), P = 0.018], and replicated in Chinese [OR = 1.65 (1.25-2.18), P = 4 x 10(-4)]. There was no association of FCGR3B CNV with vasculitis, nor with malarial or bacterial infection. Linkage disequilibrium (LD) between multi-allelic FCGR3B CNV and SLE-associated SNPs in the FCGR locus was defined for the first time. Despite LD between FCGR3B CNV and a variant in FcgammaRIIB (I232T) which abolishes inhibitory function, both reduced CN of FCGR3B and homozygosity of the FcgammaRIIB-232T allele were individually strongly associated with SLE risk. Thus CN of FCGR3B, which controls IC responses and uptake by neutrophils, and variations in FCGR2B, which controls factors such as antibody production and macrophage activation, are important in SLE pathogenesis. Further interpretations of contributions to pathogenesis by FcgammaRs must be made in the context of LD involving CNV regions.

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease more prevalent in people of African and Asian origin than Caucasian origin. FcgammaRIIb is an inhibitory Fc receptor with a critical role in immune regulation. Mouse data suggest that FcgammaRIIb deficiency increases susceptibility to autoimmune disease but protects against infection. We show that a SNP in human FCGR2B that abrogates receptor function is strongly associated with susceptibility to SLE in both Caucasians and Southeast Asians. The minor allele of this SNP is more common in Southeast Asians and Africans, populations from areas where malaria is endemic, than in Caucasians. We show that homozygosity for the minor allele is associated with substantial protection against severe malaria in an East African population (odds ratio = 0.56; P = 7.1 x 10(-5)). This protective effect against malaria may contribute to the higher frequency of this SNP and hence, SLE in Africans and Southeast Asians.

The autoimmune disease systemic lupus erythematosus (SLE) is characterized by the deposition of immune complexes in organs such as the kidney. This occurs as a result of multiple immunological abnormalities, including the production of high levels of autoantibody and dysregulated handling of immune complexes. Receptors for the Fc portion of IgG are critically involved in immune complex handling and clearance and in the regulation of B-cell activation. Polymorphisms in the low-affinity Fcgamma receptors have been associated with susceptibility to a number of autoimmune diseases, including SLE. We review the role of two such receptors in the pathogenesis of lupus-the inhibitory receptor FcgammaRIIB and the glycosylphosphatidylinositol-linked activatory receptor FcgammaRIIIB. Recent work has enhanced our understanding of the mechanism of action of the FcgammaRIIB I232T polymorphism and the overall role of this receptor in SLE. The human neutrophil antigen-1 allotypes of FcgammaRIIIB and the role of the receptor in SLE are discussed with regard to the recent determination of copy number variation in FCGR3B and the association of low copy number with SLE.

FcgammaRIIb is an inhibitory Fc receptor expressed on B cells and myeloid cells. It is important in controlling responses to infection, and reduced expression or function predisposes to autoimmunity. To determine if increased expression of FcgammaRIIb can modulate these processes, we created transgenic mice overexpressing FcgammaRIIb on B cells or macrophages. Overexpression of FcgammaRIIb on B cells reduced the immunoglobulin G component of T-dependent immune responses, led to early resolution of collagen-induced arthritis (CIA), and reduced spontaneous systemic lupus erythematosus (SLE). In contrast, overexpression on macrophages had no effect on immune responses, CIA, or SLE but increased mortality after Streptococcus pneumoniae infection. These results help define the role of FcgammaRIIb in immune responses, demonstrate the contrasting roles played by FcgammaRIIb on B cells and macrophages in the control of infection and autoimmunity, and emphasize the therapeutic potential for modulation of FcgammaRIIb expression on B cells in inflammatory and autoimmune disease.