Genomic organization of classical human low-affinity Fcgamma receptor genes

Fc gamma receptors: Their evolution, genomic architecture, genetic variation, and impact on human disease

Fc gamma receptors (FcγRs) are a family of receptors that bind IgG antibodies and interface at the junction of humoral and innate immunity. Precise regulation of receptor expression provides the necessary balance to achieve healthy immune homeostasis by establishing an appropriate immune threshold to limit autoimmunity but respond effectively to infection. The underlying genetics of the FCGR gene family are central to achieving this immune threshold by regulating affinity for IgG, signaling efficacy, and receptor expression. The FCGR gene locus was duplicated during evolution, retaining very high homology and resulting in a genomic region that is technically difficult to study. Here, we review the recent evolution of the gene family in mammals, its complexity and variation through copy number variation and single-nucleotide polymorphism, and impact of these on disease incidence, resolution, and therapeutic antibody efficacy. We also discuss the progress and limitations of current approaches to study the region and emphasize how new genomics technologies will likely resolve much of the current confusion in the field. This will lead to definitive conclusions on the impact of genetic variation within the FCGR gene locus on immune function and disease.

Association between functional FCGR3A F158V and FCGR2A R131H polymorphisms and responsiveness to rituximab in patients with autoimmune diseases: a meta-analysis

OBJECTIVES

To investigate the association between the functional Fc gamma receptor 3 A (FCGR3A) V158F and FCGR2A R131H polymorphisms and rituximab therapy in patients with autoimmune diseases.

METHODS

We searched the Medline, Embase, and Cochrane databases for relevant articles. We conducted a meta-analysis of the association between FCGR3A V158F and FCGR2A R131H polymorphisms and responsiveness to rituximab in patients with autoimmune diseases.

RESULTS

Eleven studies, consisting of 661 responders and 267 non-responders for FCGR3A V158F polymorphism and 156 responders and 89 non-responders for FCGR2A R131H polymorphism, were included. The meta-analysis revealed a significant association between the FCGR3A V allele and responsiveness to rituximab (odds ratio [OR] = 1.600, 95% confidence interval [CI] = 1.268-2.018, P < 0.001). Furthermore, associations were found using the dominant and homozygous contrast models. Subgroup analysis showed an association between the FCGR3A V allele and responsiveness to rituximab in European, RA, ITP, small (<50) and large (≥50) groups, and short- (≤6 months) and long-term follow-up periods (≥6 months). These associations were also found in recessive, dominant or homozygous contrast models. Meta-analysis revealed no association between the FCGR2A R allele and responsiveness to rituximab (OR = 1.243, 95% CI = 0.825-1.873, P = 0.229).

CONCLUSIONS

We demonstrated that the FCGR3A F158V polymorphism is associated with better responsiveness to rituximab therapy in patients with autoimmune diseases, indicating that individuals carrying the FCGR3A V allele will likely respond better to rituximab. However, FCGR2A R131H polymorphism was not associated with better response to rituximab.

Association between the functional FCGR3A F158V and FCGR2A R131H polymorphisms and responsiveness to biologics in rheumatoid arthritis patients: A meta-analysis

OBJECTIVES

This study aimed to investigate the association between functional Fc gamma receptor 3A (FCGR3A) V158F and FCGR2A R131H polymorphisms and biologic therapy in rheumatoid arthritis (RA) patients.

METHODS

We searched Medline, Embase, and Cochran databases for available articles. This study is a meta-analysis of the association between the FCGR3A V158F and FCGR2A R131H polymorphisms and their responsiveness to biologics in RA patients.

RESULTS

Seventeen studies involving RA patients with FCGR3A V158F (n = 1884) and FCGR2A R131H (n = 1118) polymorphisms were considered. This meta-analysis showed that the FCGR3A V allele was associated with responsiveness to rituximab (odds ratio [OR] = 1.431, 95% CI = 1.081-1.894, P = 0.012), but not with tumor necrosis factor (TNF) blockers, tocilizumab, or abatacept. A significant association was also found between the FCGR3A V158F polymorphism and responsiveness to biologics using the dominant-recessive model. Additionally, the FCGR3A V158F polymorphism was associated with responsiveness to TNF blockers in the homozygous contrast model. Meta-analysis revealed an association between the FCGR2A RR + RH genotype and responsiveness to biologics (OR = 1.385, 95% CI = 1.007-1.904, P = 0.045).

CONCLUSIONS

This meta-analysis demonstrates that FCGR3A V allele carriers show better responsiveness to rituximab, and FCGR2A R allele carriers may show a better response to biologics in RA treatment. Genotyping of these polymorphisms could be a useful tool to find associations with the responsiveness of personalized medicine to biologics.

Comparative Analysis of FCGR Gene Polymorphism in Pulmonary Sarcoidosis and Tuberculosis

The clinical outcome of sarcoidosis (SA) is very similar to tuberculosis (TB); however, they are treated differently and should not be confused. In search for their biomarkers, we have previously revealed changes in the phagocytic activity of monocytes in sarcoidosis and tuberculosis. On these monocytes we found a higher expression of receptors for the Fc fragment of immunoglobulin G (FcγR) in SA and TB patients vs. healthy controls. FcγRs are responsible for the binding of immune complexes (ICs) to initiate an (auto)immune response and for ICs clearance. Surprisingly, our SA patients had a high blood level of ICs, despite the abundant presence of FcγRs. It pointed to FcγR disfunction, presumably caused by the polymorphism of their (FCGR) genes. Therefore, we present here an analysis of the occurrence of FCGR2A, FCGR2B, FCGR2C, FCGR3A and FCGR3B variants in Caucasian SA and TB patients, and healthy individuals with the use of polymerase chain reaction (PCR) and real-time PCR. The presented data point to a possibility of supporting the differential diagnosis of SA and TB by analyzing FCGR2C, FCGR3A and FCGR3B polymorphism, while for severe stages of SA also by studying FCGR2A variants. Additionally, the genotyping of FCGR2A and FCGR3B might serve as a marker of SA progression.

Role of Fcγ receptors in HER2-targeted breast cancer therapy

Several therapeutic monoclonal antibodies (mAbs), including those targeting epidermal growth factor receptor, human epidermal growth factor receptor 2 (HER2), and CD20, mediate fragment crystallizable gamma receptor (FcγR)–dependent activities as part of their mechanism of action. These activities include induction of antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP), which are innate immune mechanisms of cancer cell elimination. FcγRs are distinguished by their affinity for the Fc fragment, cell distribution, and type of immune response they induce. Activating FcγRIIIa (CD16A) on natural killer cells plays a crucial role in mediating ADCC, and activating FcγRIIa (CD32A) and FcγRIIIa on macrophages are important for mediating ADCP. Polymorphisms in FcγRIIIa and FcγRIIa generate variants that bind to the Fc portion of antibodies with different affinities. This results in differential FcγR-mediated activities associated with differential therapeutic outcomes across multiple clinical settings, from early stage to metastatic disease, in patients with HER2+ breast cancer treated with the anti-HER2 mAb trastuzumab. Trastuzumab has, nonetheless, revolutionized HER2+ breast cancer treatment, and several HER2-directed mAbs have been developed using Fc glyco-engineering or Fc protein-engineering to enhance FcγR-mediated functions. An example of an approved anti-HER2 Fc-engineered chimeric mAb is margetuximab, which targets the same epitope as trastuzumab, but features five amino acid substitutions in the IgG 1 Fc domain that were deliberately introduced to increase binding to activating FcγRIIIa and decrease binding to inhibitory FcγRIIb (CD32B). Margetuximab enhances Fc-dependent ADCC in vitro more potently than the combination of pertuzumab (another approved mAb directed against an alternate HER2 epitope) and trastuzumab. Margetuximab administration also enhances HER2-specific B cell and T cell–mediated responses ex vivo in samples from patients treated with prior lines of HER2 antibody-based therapies. Stemming from these observations, a worthwhile future goal in the treatment of HER2+ breast cancer is to promote combinatorial approaches that better eradicate HER2+ cancer cells via enhanced immunological mechanisms.

Polymorphisms in Fc Gamma Receptors and Susceptibility to Malaria in an Endemic Population

Repeated infections by Plasmodium falciparum result in a humoral response that could reduce disease symptoms and prevent the development of clinical malaria. The principal mechanism underlying this humoral response is that immunoglobulin G (IgG) binds directly to the parasites, thus causing their neutralization. However, the action of antibodies alone is not always sufficient to eliminate pathogens from an organism. One key element involved in the recognition of IgG that plays a crucial role in the destruction of the parasites responsible for spreading malaria is the family of Fc gamma receptors. These receptors are expressed on the surface of immune cells. Several polymorphisms have been detected in the genes encoding these receptors, associated with susceptibility or resistance to malaria in different populations. In this review, we describe identified polymorphisms within the family of Fc gamma receptors and the impact of these variations on the response of a host to infection as well as provide new perspectives for the design of an effective vaccine for malaria.

Genetic Variation in Low-To-Medium-Affinity Fcγ Receptors: Functional Consequences, Disease Associations, and Opportunities for Personalized Medicine

Fc-gamma receptors (FcγR) are the cellular receptors for Immunoglobulin G (IgG). Upon binding of complexed IgG, FcγRs can trigger various cellular immune effector functions, thereby linking the adaptive and innate immune systems. In humans, six classic FcγRs are known: one high-affinity receptor (FcγRI) and five low-to-medium-affinity FcγRs (FcγRIIA, -B and -C, FcγRIIIA and -B). In this review we describe the five genes encoding the low-to-medium -affinity FcγRs (FCGR2A, FCGR2B, FCGR2C, FCGR3A, and FCGR3B), including well-characterized functionally relevant single nucleotide polymorphisms (SNPs), haplotypes as well as copy number variants (CNVs), which occur in distinct copy number regions across the locus. The evolution of the locus is also discussed. Importantly, we recommend a consistent nomenclature of genetic variants in the FCGR2/3 locus. Next, we focus on the relevance of genetic variation in the FCGR2/3 locus in auto-immune and auto-inflammatory diseases, highlighting pathophysiological insights that are informed by genetic association studies. Finally, we illustrate how specific FcγR variants relate to variation in treatment responses and prognosis amongst autoimmune diseases, cancer and transplant immunology, suggesting novel opportunities for personalized medicine.

Evolutionary Story of the Low/Medium-Affinity IgG Fc Receptor Gene Cluster

Low/intermediate affinity Fc-gamma receptors (FcγR) are crucial for the recognition of immune complexes and IgG-sensitized microorganisms by phagocytic and cytotoxic effector cells. In all mammalian species studied so far, their genes are clustered in a single locus. However, this locus differs between humans and mice, both in the number of genes and the structure/function of the encoded receptors. We show that murine fcgr3 evolved through several steps into FCGR2A, its ortholog, which is specific to primates. One of these steps was the insertion of a retroviral element bringing a new intracellular exon comprising a non-canonical ITAM motif. We also show that the fcgr3-hspa6-fcgr4-fcgr2b module in mammals that has evolved in a FCGR2A-HSPA6-FCGR4-FCGR2B module in primates, was subsequently duplicated in apes through a Non-Allelic Homologous Recombination (NAHR), giving birth to FCGR2C, a hybrid gene between FCGR2B and FCGR2A. The FCGR4 duplication, which occurred simultaneously, eventually resulted in the emergence of FCGR3B, while FCGR3A remained the true FCGR4 ortholog. FCGR2C and FCGR3B, markers of this NAHR, are present in gorillas and chimpanzees, whereas they are absent in orangutans and more distant primates, such as gibbons and macaques. These data need to be taken into account when testing IgG-based therapies in animal species.

The porcine translational research database: a manually curated, genomics and proteomics-based research resource

BACKGROUND

The use of swine in biomedical research has increased dramatically in the last decade. Diverse genomic- and proteomic databases have been developed to facilitate research using human and rodent models. Current porcine gene databases, however, lack the robust annotation to study pig models that are relevant to human studies and for comparative evaluation with rodent models. Furthermore, they contain a significant number of errors due to their primary reliance on machine-based annotation. To address these deficiencies, a comprehensive literature-based survey was conducted to identify certain selected genes that have demonstrated function in humans, mice or pigs.

RESULTS

The process identified 13,054 candidate human, bovine, mouse or rat genes/proteins used to select potential porcine homologs by searching multiple online sources of porcine gene information. The data in the Porcine Translational Research Database (( http://www.ars.usda.gov/Services/docs.htm?docid=6065 ) is supported by >5800 references, and contains 65 data fields for each entry, including >9700 full length (5' and 3') unambiguous pig sequences, >2400 real time PCR assays and reactivity information on >1700 antibodies. It also contains gene and/or protein expression data for >2200 genes and identifies and corrects 8187 errors (gene duplications artifacts, mis-assemblies, mis-annotations, and incorrect species assignments) for 5337 porcine genes.

CONCLUSIONS

This database is the largest manually curated database for any single veterinary species and is unique among porcine gene databases in regard to linking gene expression to gene function, identifying related gene pathways, and connecting data with other porcine gene databases. This database provides the first comprehensive description of three major Super-families or functionally related groups of proteins (Cluster of Differentiation (CD) Marker genes, Solute Carrier Superfamily, ATP binding Cassette Superfamily), and a comparative description of porcine microRNAs.

Anti-retroviral antibody FcγR-mediated effector functions

The antiviral activity of antibodies reflects the bifunctional properties of these molecules. While the Fab domains mediate highly specific antigenic recognition to block virus entry, the Fc domain interacts with diverse types of Fcγ receptors (FcγRs) expressed on the surface of effector leukocytes to induce the activation of distinct immunomodulatory pathways. Fc-FcγR interactions are tightly regulated to control IgG-mediated inflammation and immunity and are largely determined by the structural heterogeneity of the IgG Fc domain, stemming from differences in the primary amino acid sequence of the various subclasses, as well as the structure and composition of the Fc-associated N-linked glycan. Engagement of specific FcγR types on effector leukocytes has diverse consequences that affect several aspects of innate and adaptive immunity. In this review, we provide an overview of the complexity of FcγR-mediated pathways, discussing their role in the in vivo protective activity of anti-HIV-1 antibodies. We focus on recent studies on broadly neutralizing anti-HIV-1 antibodies that revealed that Fc-FcγR interactions are required to achieve full therapeutic activity through clearance of IgG-opsonized virions and elimination of HIV-infected cells. Manipulation of Fc-FcγR interactions to specifically activate distinct FcγR-mediated pathways has the potential to affect downstream effector responses, influencing thereby the in vivo protective activity of anti-HIV-1 antibodies; a strategy that has already been successfully applied to other IgG-based therapeutics, substantially improving their clinical efficacy.

The Role and Function of Fcγ Receptors on Myeloid Cells

A key determinant for the survival of organisms is their capacity to recognize and respond efficiently to foreign antigens. This is largely accomplished by the orchestrated activity of the innate and adaptive branches of the immune system. Antibodies are specifically generated in response to foreign antigens, facilitating thereby the specific recognition of antigens of almost infinite diversity. Receptors specific for the Fc domain of antibodies, Fc receptors, are expressed on the surface of the various myeloid leukocyte populations and mediate the binding and recognition of antibodies by innate leukocytes. By directly linking the innate and the adaptive components of immunity, Fc receptors play a central role in host defense and the maintenance of tissue homeostasis through the induction of diverse proinflammatory, anti-inflammatory, and immunomodulatory processes that are initiated upon engagement by the Fc domain. In this chapter, we discuss the mechanisms that regulate Fc domain binding to the various types of Fc receptors and provide an overview of the astonishing diversity of effector functions that are mediated through Fc-FcR interactions on myeloid cells. Lastly, we discuss the impact of FcR-mediated interactions in the context of IgG-mediated inflammation, autoimmunity, susceptibility to infection, and responsiveness to antibody-based therapeutics.

Association of Fcgamma receptor type 2A and 3A genotypes with rheumatoid arthritis in Chinese population

AIM

SNPs of FcγRs were implicated in pathogenesis of rheumatoid arthritis (RA) and treatment efficacy of TNF inhibitors (TNFi). This study aims to investigate the associations of FcγRIIa and FcγRIIIa genotypes with autoantibody production and treatment response to TNFi in Chinese patients with RA.

PATIENTS & METHODS

FcγRIIa and FcγRIIIa polymorphisms were genotyped in 158 RA patients. Response to TNFi was evaluated in 18 patients at 3 and 6 months after treatment.

RESULTS

FcγRIIa-131H allele was significantly increased in autoantibody-negative RA patients. FcγRIIa-131H/H+H/R was closely associated with differences in 28-joint disease activity score in patients at months 3 and 6 of TNFi treatment.

CONCLUSION

FcγRIIa-131H allele may have a protective role in autoantibody production and might be a biomarker for predicting good response to TNFi in Chinese RA patients.

Associations between functional FCGR2A R131H and FCGR3A F158V polymorphisms and responsiveness to TNF blockers in spondyloarthropathy, psoriasis and Crohn's disease: a meta-analysis

Aim: The aim of the current study was to investigate whether FCGR polymorphisms are associated with responsiveness to anti-TNF-α therapy in patients with spondyloarthropathy, psoriasis, and Crohn's disease. Materials & methods: We conducted a meta-analysis to evaluate the association between the functional FCGR3A F158V and FCGR2A R131H polymorphisms and responsiveness to TNF blockers. Results: The meta-analysis indicated that responsiveness to TNF blockers was associated with the FCGR3A V allele (odds ratio: 3.308; 95% CI: 1.053–10.39; p = 0.040) and the FCGR2A RR + RH genotype (odds ratio: 3.904; p = 0.027) in patients with a follow-up time of ≥6 months. Conclusion: FCGR3A V and FCGR2A R allele carriers show better responsiveness to anti-TNF-α therapy in patients with follow-up times ≥6 months.

IgG4 Characteristics and Functions in Cancer Immunity

IgG4 is the least abundant subclass of IgG in normal human serum, but elevated IgG4 levels are triggered in response to a chronic antigenic stimulus and inflammation. Since the immune system is exposed to tumor-associated antigens over a relatively long period of time, and tumors notoriously promote inflammation, it is unsurprising that IgG4 has been implicated in certain tumor types. Despite differing from other IgG subclasses by only a few amino acids, IgG4 possesses unique structural characteristics that may be responsible for its poor effector function potency and immunomodulatory properties. We describe the unique attributes of IgG4 that may be responsible for these regulatory functions, particularly in the cancer context. We discuss the inflammatory conditions in tumors that support IgG4, the emerging and proposed mechanisms by which IgG4 may contribute to tumor-associated escape from immune surveillance and implications for cancer immunotherapy.

Associations between PTPRC rs10919563 A/G and FCGR2A R131H polymorphisms and responsiveness to TNF blockers in rheumatoid arthritis: a meta-analysis

We aimed to investigate whether the PTPRC rs10919563 A/G and Fc gamma receptor 2A (FCGR2A) R131H polymorphisms can predict the response to anti-TNF therapy in rheumatoid arthritis (RA) patients. We conducted a meta-analysis of studies on the association between the PTPRC rs10919563 A/G or the FCGR2A R131H polymorphism and responsiveness to anti-TNF therapy in RA patients. Eighteen studies (twelve on PTPRC and six on FCGR2A) from eight articles involving 3058 patients were considered in this meta-analysis. The meta-analysis showed a significant association between the PTPRC rs10919563 A allele and response to TNF-α blockers in RA. The OR of the PTPRC A allele was significantly lower in responders (OR = 0.584, 95 % CI = 0.409-0.835, P = 0.003). Meta-analysis revealed no association between the FCGR2A HH + HR genotype and responsiveness to TNF blockers in all study subjects (OR = 0.762, 95 % CI = 0.543-1.068, P = 0.115). However, stratification by TNF inhibitor type showed that the FCGR2A HH + HR genotype was associated with responsiveness to adalimumab (OR = 0.591, 95 % CI = 0.369-0.947, P = 0.029), but not infliximab and etanercept (OR = 0.929, 95 % CI = 0.354-2.440, P = 0.881; OR = 0.804, 95 % CI = 0.293-2.207, P = 0.673). The PTPRC rs10919563 A allele shows a poor response to anti-TNF therapy, and the FCGR2A HH + HR genotype shows a poor response to adalimumab for RA. Genotyping for these polymorphisms may be useful for predicting the response to TNF-α blockers with respect to personalized medicine.

Polymorphic Regions in Fc Gamma Receptor and Tumor Necrosis Factor-α Genes and Susceptibility to Chronic Periodontitis in a Cohort From South India

BACKGROUND

Polymorphisms in the immunoglobulin G Fc receptor II (FcGR) and tumor necrosis factor-α (TNFA) genes are known to influence pathogenesis and severity of several inflammatory conditions. Association of FcGR and TNFA gene polymorphisms with chronic periodontitis (CP) susceptibility has been found to be diverse among different ethnic populations. Objectives of the present study are to determine association of functional single nucleotide polymorphisms (SNPs) in FcGR and TNF-α genes with CP susceptibility in a cohort from South India.

METHODS

Polymorphisms of: 1) FCGR2A 131His/Arg (rs1801274); 2) FCGR2B 232Ile/Thr (rs1050501); 3) TNFA -1031T/C (rs1799964); and 4) TNFA -863C/A (rs1800630) were analyzed among patients with healthy gingiva (n = 176) and patients with CP (n = 177). Genotyping was performed using allele-specific real-time polymerase chain reaction assay. Association between CP and SNPs was examined by multivariable logistic regression analysis with adjustment for: 1) age; 2) sex; and 3) oral hygiene index (OHI). Epistatic interaction between FcGR polymorphisms and interleukin 1B (IL1B) +3954C/T (rs1143634) was assessed using multifactorial dimensionality reduction analysis.

RESULTS

Among four SNPs analyzed, only FCGR2A 131His/Arg showed significant association with CP in a dominant model (odds ratio: 1.6; 95% confidence interval: 1.028 to 2.530). This significance disappeared after correcting for multiple comparisons using Bonferroni analysis, or after adjusting for age, sex, and OHI. A significant redundant interaction between IL1B +3954 C/T and FCGR2A 131His/Arg was observed.

CONCLUSION

Study results suggest the variant form of the SNP in FCGR2A 131His/Arg, FCGR2B 232Ile/Thr, TNFA -1031T/C, and TNFA -863C/A are not associated with CP susceptibility in the selected cohort from South India.

Genotyping Single Nucleotide Polymorphisms and Copy Number Variability of the FCGRs Expressed on NK Cells

Natural killer (NK) cells are one of the main effector immune cells involved in antibody-dependent cell-mediated cytotoxicity (ADCC). Upon recognition of cell-bound IgG antibodies, which occurs through Fc gamma receptors (FCGRs) expressed on the cell surface of NK cells, NK cells become activated and lyse target tumor or infected cells. The FCGRs, FCGR3A and FCGR2C, expressed on the surface of NK cells have single nucleotide polymorphisms (SNPs) that result in differential activity of NK cells. In addition to SNP genetic variation within each of these genes, the FCGRs are subject to copy number variation (CNV), which leads to variable protein expression levels on the cell surface. Studies have found that FCGR genotype for FCGR3A and FCGR2C is associated with variation in the response to immunotherapy.Due to high sequence homology within FCGR3 and FCGR2 families, there are difficulties associated with genotyping these specific receptors related to cross-amplification of non-targeted FCGRs. To improve specificity for both FCGR3A and FCGR2C, Rnase-H (RH) primers were designed to amplify specifically FCGR3A (while not co-amplifying FCGR3B) and FCGR2C (while not co-amplifying FCGR2B). In addition, fluorescently labeled locked nucleic acid (LNA) probes provide additional precision for determination of the SNPs within both FCGR3A and FCGR2C. For CNV determination, separate fluorescently labeled probes for FCGR3A, and for FCGR2C, can be used with the same RH primers for each gene. These probes can be combined in the same well with control primers/probe for a known diploid gene and used to calculate the copy number of both FCGR3A and FCGR2C. Here we provide new detailed methodology that allows for the specific amplification of these FCGRs in a single PCR reaction, allowing for genotyping of both the SNPs and CNVs using real-time PCR.

Fcγ receptor pathways during active and passive immunization

IgG antibodies are actively produced in response to antigenic challenge or passively administered as an effective form of immunotherapy to confer immunity against foreign antigens. Their protective activity is mediated through their bifunctional nature: a variable Fab domain mediates antigen-binding specificity, whereas the constant Fc domain engages Fcγ receptors (FcγRs) expressed on the surface of leukocytes to mediate effector functions. While traditionally considered the invariant domain of an IgG molecule, the Fc domain displays remarkable structural heterogeneity determined primarily by differences in the amino acid sequence of the various IgG subclasses and by the composition of the complex, Fc-associated biantennary N-linked glycan. These structural determinants regulate the conformational flexibility of the IgG Fc domain and affect its capacity to interact with distinct types of FcγRs (type I or type II FcγRs). FcγR engagement activates diverse downstream immunomodulatory pathways with pleiotropic functional consequences including cytotoxicity and phagocytosis of IgG-coated targets, differentiation and activation of antigen presenting cells, modulation of T-cell activation, plasma cell survival, and regulation of antibody responses. These functions highlight the importance of FcγR-mediated pathways in the modulation of adaptive immune responses and suggest a central role for IgG-FcγR interactions during active and passive immunization.

Association between FCGR3B copy number variations and susceptibility to autoimmune diseases: a meta-analysis

OBJECTIVE

This study determined whether FCGR3B copy number variations (CNVs) were associated with susceptibility to autoimmune diseases.

METHODS

A meta-analysis was conducted to determine the association between FCGR3B CNVs and susceptibility to autoimmune diseases by comparing low FCGR3B CN (<2 to ≥2) and high FCGR3B CN (>2 to ≤2).

RESULTS

In all, 28 comparative studies from 15 reports involving 12,160 patients and 11,103 controls were included in this meta-analysis. The meta-analysis showed a significant association between low FCGR3B CN and autoimmune diseases (OR=1.496, 95% CI=1.301-1.716, p=1.0×10(-9)). Subgroup analysis according to ethnicity indicated an association between low FCGR3B CN and autoimmune diseases in Caucasians (OR=1.482, 95% CI=1.219-1.801, p=7.7×10(-6)) and Asians (OR=1.498, 95% CI=1.306-1.717, p=1.0×10(-9)). Meta-analysis according to the type of autoimmune disease indicated a significant association of low FCGR3B CN with systemic lupus erythematosus (SLE; OR=1.797, 95% CI=1.562-2.068, p<1.0×10(-9)), primary Sjogren's syndrome (pSS; OR=2.263, 95% CI=1.316-3.892, p=0.003), and Wegener's granulomatosis (WG; OR=1.973, 95% CI=1.178-3.302, p=0.010), but not with rheumatoid arthritis (RA; OR=1.333, 95% CI=0.947-1.877, p=0.099). However, the meta-analysis showed no association between high FCGR3B CN and SLE, RA, pSS, and WG.

CONCLUSIONS

Thus, the results of this meta-analysis indicated that low FCGR3B CN increased susceptibility to autoimmune diseases, especially SLE, pSS, and WG.

Association of Fc gamma-receptors IIa, IIIa, and IIIb genetic polymorphism with susceptibility to chronic periodontitis in South Indian population

BACKGROUND AND OBJECTIVE

Fc gamma receptors (FcγRs) are the members of the immunoglobulin superfamily and may play a role in the pathogenesis of periodontitis. Genetic variation in these receptors and its link with various forms of periodontitis is being studied in different populations. The aim of the present study is to determine whether specific FcγRIIa, FcγRIIIa, and FcγRIIIb alleles and/or genotypes are associated with risk for susceptibility to generalized chronic periodontitis (GCP) in South Indian population.

MATERIALS AND METHODS

The study population consisted of 120 South Indian subjects; 60 with GCP and 60 periodontally healthy. Deoxyribonucleic acid (DNA) was extracted from samples collected by scrapping buccal epithelium. FcγRIIa and FcγRIIIa genotyping were performed by polymerase chain reaction (PCR) amplification of DNA with allele-specific primers followed by allele-specific restriction digestion of the products. However, FcγRIIIb genotyping was done by allele-specific PCR.

RESULTS

No significant difference in the distribution of FcγRIIa H/R and FcγRIIIa NA1/NA2 genotypes or their respective alleles was observed in GCP patients and healthy subjects. For FcγRIIIa F/V genetic polymorphism, the homozygous V/V genotype and V allele were significantly overrepresented in GCP patients while F/F genotype and F allele in controls.

CONCLUSION

The present study demonstrates that FcγRIIIa V/V genotype, as well as V allele, could be a possible risk factor for chronic periodontitis in South Indian population.

Nonallelic homologous recombination of the FCGR2/3 locus results in copy number variation and novel chimeric FCGR2 genes with aberrant functional expression

The human FCGR2/3 locus, containing five highly homologous genes encoding the major IgG receptors, shows extensive copy number variation (CNV) associated with susceptibility to autoimmune diseases. Having genotyped >4000 individuals, we show that all CNV at this locus can be explained by nonallelic homologous recombination (NAHR) of the two paralogous repeats that constitute the majority of the locus, and describe four distinct CNV regions (CNRs) with a highly variable prevalence in the population. Apart from CNV, NAHR events also created several hitherto unidentified chimeric FCGR2 genes. These include an FCGR2A/2C chimeric gene that causes a decreased expression of FcγRIIa on phagocytes, resulting in a decreased production of reactive oxygen species in response to immune complexes, compared with wild-type FCGR2A. Conversely, FCGR2C/2A chimeric genes were identified to lead to an increased expression of FCGR2C. Finally, a rare FCGR2B null-variant allele was found, in which a polymorphic stop codon of FCGR2C is introduced into one FCGR2B gene, resulting in a 50% reduction in protein expression. Our study on CNRs and the chimeric genes is essential for the correct interpretation of association studies on FCGR genes as a determinant for disease susceptibility, and may explain some as yet unidentified extreme phenotypes of immune-mediated disease.

Polymorphism of FCGR2A, FCGR2C, and FCGR3B Genes in the Pathogenesis of Sarcoidosis

We have previously presented evidence that the polymorphism of the FCGR3A gene, encoding the receptor for Fc fragment of immunoglobulin G IIIa (FcγRIIIa) plays a role in the enhancement of circulating immune complexes (CIs) with the occurrence of Mycobacterium tuberculosis heat shock proteins in patients with sarcoidosis (SA). The immunocomplexemia might be caused by decreased affinity of CIs to Fcγ receptors, with the subsequently decreased receptor clearance by immune cells. In the present study we examined whether the polymorphisms of other related genes (FCGR2A, FCGR2C, FCGR3B) encoding other activatory Fcγ receptors, could have a similar effect. To this end, we genotyped 124 patients with sarcoidosis and 148 healthy volunteers using polymerase chain reaction with sequence-specific primers. We revealed a significant decrease in the percentage of the FCGR2A and FCGR2C variants that ensure effective CIs clearance, with a concomitant increase of less functional variants of these genes in Stages I/II, compared with Stages III/IV of SA. There was no aberration in FCGR3B allele/genotype frequencies. We conclude that the FCGR2A and FCGR2C polymorphisms may also contribute to immunocomplexemia present in SA. The assessment of FCGR genes could become a tool in presaging a clinical course of sarcoidosis and in its personalized therapy.

Bridging autoantibodies and arthritis: the role of Fc receptors

Autoantibodies represent a hallmark of Rheumatoid arthritis (RA), which is a chronic inflammatory autoimmune disease characterized by inflammation and damage in the joints. Anti-Citrullinated Protein Antibodies (ACPA) are the most prominent autoantibodies present in RA patients. These autoantibodies have been intensively investigated during the last 20 years due to their diagnostic and predictive value. Furthermore, they are believed to be involved in mediating the damage associated with RA. Antibodies of the IgG isotype interact with the immune system via Fcγ receptors expressed on immune cells as well as nonimmune cells. These receptors, therefore, form the bridge between Fcγ receptor-positive cells and antibodies complexed to antigen allowing the modulation and activation of cellular immune responses that are involved in immune defense against invading microorganisms. However, in case triggered by antibodies against self-antigens, they can also play a pivotal role in the induction and perpetuation of autoimmune diseases such as RA. Mouse models have been indispensably important for understanding the role of Fcγ receptors in the development of arthritis. Here we discuss the contribution of autoantibodies to the pathogenesis of arthritis in preclinical animal models, as well as RA, in relation to their interaction with the different (immune inhibitory and activating) Fcγ receptors.

Clearance of human IgG1-sensitised red blood cells in vivo in humans relates to the in vitro properties of antibodies from alternative cell lines

We previously produced a recombinant version of the human anti-RhD antibody Fog-1 in the rat myeloma cell line, YB2/0. When human, autologous RhD-positive red blood cells (RBC) were sensitised with this IgG1 antibody and re-injected, they were cleared much more rapidly from the circulation than had been seen earlier with the original human-mouse heterohybridoma-produced Fog-1. Since the IgG have the same amino acid sequence, this disparity is likely to be due to alternative glycosylation that results from the rat and mouse cell lines. By comparing the in vitro properties of YB2/0-produced Fog-1 IgG1 and the same antibody produced in the mouse myeloma cell line NS0, we now have a unique opportunity to pinpoint the cause of the difference in ability to clear RBC in vivo. Using transfected cell lines that express single human FcγR, we showed that IgG1 made in YB2/0 and NS0 cell lines bound equally well to receptors of the FcγRI and FcγRII classes but that the YB2/0 antibody was superior in FcγRIII binding. When measuring complexed IgG binding, the difference was 45-fold for FcγRIIIa 158F, 20-fold for FcγRIIIa 158V and approximately 40-fold for FcγRIIIb. The dissimilarity was greater at 100-fold in monomeric IgG binding assays with FcγRIIIa. When used to sensitise RBC, the YB2/0 IgG1 generated 100-fold greater human NK cell antibody-dependent cell-mediated cytotoxicity and had a 10³-fold advantage over the NS0 antibody in activating NK cells, as detected by CD54 levels. In assays of monocyte activation and macrophage adherence/phagocytosis, where FcγRI plays major roles, RBC sensitised with the two antibodies produced much more similar results. Thus, the alternative glycosylation profiles of the Fog-1 antibodies affect only FcγRIII binding and FcγRIII-mediated functions. Relating this to the in vivo studies confirms the importance of FcγRIII in RBC clearance.

Associations between FCGR2A rs1801274, FCGR3A rs396991, FCGR3B NA1/NA2 polymorphisms and periodontitis: a meta-analysis

The aim of this study was to determine whether the Fcγ receptors (FCGRs) polymorphisms confer susceptibility to periodontitis in ethnically different populations. We did a literature search using PubMed and Embase, and conducted a meta-analysis on the associations between the FCGR2A H131R (rs1801274), FCGR3A F158V (rs396991), and FCGR3B NA1/NA2 polymorphisms and periodontitis using allele contrast, the recessive model, the dominant model, and the homozygote contrast. A total of 17 separate comparisons with 1,421 patients with periodontitis and 1,454 controls, involving six Caucasian, six East Asian, two African and one South Asian population were considered in the meta-analysis. Meta-analysis of the FCGR2A H131R polymorphism showed no association between periodontitis and the FCGR2A R allele (OR=0.987, 95% CI=0.881-1.107, p=0.827). Stratification by ethnicity revealed an association between the RR+RH genotype with periodontitis in Caucasian population (OR=0.624, 95% CI=0.479-0.813, p=4.7×10(-5)), but not in East Asian, and African populations. Meta-analysis of the FCGR3A F158V polymorphism revealed an association between the FCGR3A V allele and periodontitis is in Caucasians (OR=1.457, 95% CI=1.014-2.092, p=0.042), but not in East Asians and Africans. In addition, analysis using the dominant model and homozygote contrast showed the same pattern for the FCGR3A V allele. Meta-analysis of the FCGR3B NA1/NA2 polymorphism using the recessive model revealed a significant association between the NA2/NA2 genotype and periodontitis in aggressive periodontitis (OR=2.853, 95% CI=1.673-4.863, 1.1×10(-5)). This meta-analysis demonstrates that the FCGR2A, and FCGR3A polymorphisms may confer susceptibility to periodontitis in Caucasians, and that the FCGR3B polymorphism may be associated with susceptibility to aggressive periodontitis.

A novel splice variant of FcγRIIa: a risk factor for anaphylaxis in patients with hypogammaglobulinemia

BACKGROUND

Our index case was a patient with common variable immunodeficiency (CVID). She had anaphylactoid reactions on administration of intravenous immunoglobulin (IVIg) associated with the presence of IgG antibodies against IgA.

OBJECTIVE

We sought to determine the role of Fcγ receptor (FcγR) IIa in IVIg-induced anaphylactoid reactions.

METHODS

Neutrophils and PBMCs were isolated from healthy subjects and IVIg-treated patients. FcγRIIa mRNA and DNA were analyzed by using real-time PCR and sequencing. IgG-mediated elastase release and intracellular Ca(2+) mobilization were determined in neutrophils and transfected cell lines, respectively.

RESULTS

A novel splice variant of FcγRIIa containing an expressed cryptic exon 6* (FcγRIIa(exon6∗)) was identified in our index patient. This exon is normally spliced out of all FcγRII isoforms, except the inhibitory FcγRIIb1. Compared with healthy control subjects, the heterozygous FCGR2A(c.742+871A>G) mutation was more frequent in patients with CVID (n = 53, P < .013). Expression in patients with CVID was associated with anaphylaxis on IVIg infusion (P = .002). On screening of additional IVIg-treated patient cohorts, we identified 6 FCGR2A(c.742+871A>G) allele-positive patients with Kawasaki disease (n = 208) and 1 patient with idiopathic thrombocytopenia (n = 93). None had adverse reactions to IVIg. Moreover, FcγRIIa(exon6∗) was also demonstrated in asymptomatic family members. Functional studies in primary cells and transfected murine cells demonstrated enhanced cellular activation by FcγRIIa(exon6∗) compared with its native form, as shown by increased elastase release and intracellular calcium mobilization.

CONCLUSION

A novel splice variant, FcγRIIa(exon6∗), was characterized as a low-frequency allele, coding for a gain-of-function receptor for IgG. In the presence of immune complexes, FcγRIIa(exon6∗) can contribute to anaphylaxis in patients with CVID.

Genomic pathology of SLE-associated copy-number variation at the FCGR2C/FCGR3B/FCGR2B locus

Reduced FCGR3B copy number is associated with increased risk of systemic lupus erythematosus (SLE). The five FCGR2/FCGR3 genes are arranged across two highly paralogous genomic segments on chromosome 1q23. Previous studies have suggested mechanisms for structural rearrangements at the FCGR2/FCGR3 locus and have proposed mechanisms whereby altered FCGR3B copy number predisposes to autoimmunity, but the high degree of sequence similarity between paralogous segments has prevented precise definition of the molecular events and their functional consequences. To pursue the genomic pathology associated with FCGR3B copy-number variation, we integrated sequencing data from fosmid and bacterial artificial chromosome clones and sequence-captured DNA from FCGR3B-deleted genomes to establish a detailed map of allelic and paralogous sequence variation across the FCGR2/FCGR3 locus. This analysis identified two highly paralogous 24.5 kb blocks within the FCGR2C/FCGR3B/FCGR2B locus that are devoid of nonpolymorphic paralogous sequence variations and that define the limits of the genomic regions in which nonallelic homologous recombination leads to FCGR2C/FCGR3B copy-number variation. Further, the data showed evidence of swapping of haplotype blocks between these highly paralogous blocks that most likely arose from sequential ancestral recombination events across the region. Functionally, we found by flow cytometry, immunoblotting and cDNA sequencing that individuals with FCGR3B-deleted alleles show ectopic presence of FcγRIIb on natural killer (NK) cells. We conclude that FCGR3B deletion juxtaposes the 5'-regulatory sequences of FCGR2C with the coding sequence of FCGR2B, creating a chimeric gene that results in an ectopic accumulation of FcγRIIb on NK cells and provides an explanation for SLE risk associated with reduced FCGR3B gene copy number.

Genetic variation of the Fc gamma receptor 3B gene and association with rheumatoid arthritis

Background

Fc gamma receptors (FcγRs) play a crucial role in immunity by linking IgG antibody-mediated responses with cellular effector and regulatory functions. Genetic variants in these receptors have been previously identified as risk factors for several chronic inflammatory conditions. The present study aimed to investigate the presence of copy number variations (CNVs) in the FCGR3B gene and its potential association with the autoimmune disease rheumatoid arthritis (RA).

Methodology/Principal Findings

CNV of the FCGR3B gene was studied using Multiplex Ligation Dependent Probe Amplification (MLPA) in 518 Dutch RA patients and 304 healthy controls. Surprisingly, three independent MLPA probes targeting the FCGR3B promoter measured different CNV frequencies, with probe#1 and #2 measuring 0 to 5 gene copies and probe#3 showing little evidence of CNV. Quantitative-PCR correlated with the copy number results from MLPA probe#2, which detected low copy number (1 copy) in 6.7% and high copy number (≥3 copies) in 9.4% of the control population. No significant difference was observed between RA patients and the healthy controls, neither in the low copy nor the high copy number groups (p-values = 0.36 and 0.71, respectively). Sequencing of the FCGR3B promoter region revealed an insertion/deletion (indel) that explained the disparate CNV results of MLPA probe#1. Finally, a non-significant trend was found between the novel -256A>TG indel and RA (40.7% in healthy controls versus 35.9% in RA patients; P = 0.08).

Conclusions/Significance

The current study highlights the complexity and poor characterization of the FCGR3B gene sequence, indicating that the design and interpretation of genotyping assays based on specific probe sequences must be performed with caution. Nonetheless, we confirmed the presence of CNV and identified novel polymorphisms in the FCGR3B gene in the Dutch population. Although no association was found between RA and FCGR3B CNV, the possible protective effect of the -256A>TG indel polymorphism must be addressed in larger studies.

Fcgamma receptor IIB and IIIB polymorphisms and susceptibility to systemic lupus erythematosus and lupus nephritis: a meta-analysis

The aim of this study was to explore whether polymorphisms of the Fcgamma receptors (FcgammaRs) IIB T/I232 and FcgammaRIIIB NA1/NA2, confer susceptibility to systemic lupus erythematosus (SLE) and lupus nephritis (LN). The authors conducted a meta-analysis on associations between the FcgammaRIIB T/I232 and FcgammaRIIIB NA1/NA2 polymorphisms and SLE and LN susceptibility as determined using 1) allele contrast, 2) recessive, 3) dominant models and 4) contrast of homozygotes. A total of 16 separate comparisons were considered, consisting of 2887 SLE patients and 3105 controls. Meta-analysis of the FcgammaRIIB T/I232 polymorphism showed a significant association between the FcgammaRIIB T allele and the risk of developing SLE compared with the FcgammaRIIB I allele (OR = 1.207, 95% CI = 1.061-1.373, P = 0.004). In subjects of Asian descent, a significant association was observed between the FcgammaRIIB T allele and SLE (OR = 1.332, 95% CI 1.138-1.558, P < 0.001). However, in Europeans no such association was found. In contrast, no association was found between SLE or LN and the FcgammaRIIIB NA1/NA2 polymorphism in all subjects, or in European and Asian populations. This meta-analysis shows that the FcgammaRIIB T/I232 polymorphism confers susceptibility to SLE, especially in Asian-derived populations.

Specificity and affinity of human Fcγ receptors and their polymorphic variants for human IgG subclasses

Distinct genes encode 6 human receptors for IgG (hFcγRs), 3 of which have 2 or 3 polymorphic variants. The specificity and affinity of individual hFcγRs for the 4 human IgG subclasses is unknown. This information is critical for antibody-based immunotherapy which has been increasingly used in the clinics. We investigated the binding of polyclonal and monoclonal IgG1, IgG2, IgG3, and IgG4 to FcγRI; FcγRIIA, IIB, and IIC; FcγRIIIA and IIIB; and all known polymorphic variants. Wild-type and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. We found that (1) IgG1 and IgG3 bind to all hFcγRs; (2) IgG2 bind not only to FcγRIIAH131, but also, with a lower affinity, to FcγRIIAR131 and FcγRIIIAV158; (3) IgG4 bind to FcγRI, FcγRIIA, IIB and IIC and FcγRIIIAV158; and (4) the inhibitory receptor FcγRIIB has a lower affinity for IgG1, IgG2, and IgG3 than all other hFcγRs. We also identified parameters that determine the specificity and affinity of hFcγRs for IgG subclasses. These results document how hFcγR specificity and affinity may account for the biological activities of antibodies. They therefore highlight the role of specific hFcγRs in the therapeutic and pathogenic effects of antibodies in disease.

Copy number variation in the human genome and its implication in autoimmunity

The causes of autoimmune disease remain poorly defined. However, it is known that genetic factors contribute to disease susceptibility. Hitherto, studies have focused upon single nucleotide polymorphisms as both tools for mapping and as probable causal variants. Recent studies, using genome-wide analytical techniques, have revealed that, in the genome, segments of DNA ranging in size from kilobases to megabases can vary in copy number. These changes of DNA copy number represent an important element of genomic polymorphism in humans and in other species and may therefore make a substantial contribution to phenotypic variation and population differentiation. Furthermore, copy number variation (CNV) in genomic regions harbouring dosage-sensitive genes may cause or predispose to a variety of human genetic diseases. Several recent studies have reported an association between CNV and autoimmunity in humans such as systemic lupus, psoriasis, Crohn's disease, rheumatoid arthritis and type 1 diabetes. The use of novel analytical techniques facilitates the study of complex human genomic structures such as CNV, and allows new susceptibility loci for autoimmunity to be found that are not readily mappable by single nucleotide polymorphism-based association analyses alone.

Fc receptor genes and the systemic lupus erythematosus diathesis

Fc receptors represent a distinct group of hematopoeitic cell surface glycoproteins that have a characterized role in affecting the efficiency of the mononuclear phagocyte system to clear IgG immune complexes. Functional genetic variations in this family of receptors have been identified as heritable susceptibility factors for SLE and lupus nephritis across diverse populations. In this review, we describe the roles of the classical Fc receptors for IgG (Fc gamma) and non-classical Fc-like receptors (FCR1-FCRL6L), Fc receptors for IgE (Fc epsilon RI) and IgA and IgM (Fc alpha/mu R) in SLE diathesis. The combined effects of these genes on SLE pathogenesis, either via linkage disequilibrium or epistasis with additional genetic or environmental factors, provide a challenge for future investigations. The pursuit of a polygenic SLE-profile that includes longitudinal evaluations of SLE and markers involved in the protean clinical manifestations associated with SLE will facilitate our understanding of the cascade of inflammatory events associated with the diathesis.

Copy number variation of the activating FCGR2C gene predisposes to idiopathic thrombocytopenic purpura

Gene copy number variation (CNV) and single nucleotide polymorphisms (SNPs) count as important sources for interindividual differences, including differential responsiveness to infection or predisposition to autoimmune disease as a result of unbalanced immunity. By developing an FCGR-specific multiplex ligation-dependent probe amplification assay, we were able to study a notoriously complex and highly homologous region in the human genome and demonstrate extensive variation in the FCGR2 and FCGR3 gene clusters, including previously unrecognized CNV. As indicated by the prevalence of an open reading frame of FCGR2C, Fcγ receptor (FcγR) type IIc is expressed in 18% of healthy individuals and is strongly associated with the hematological autoimmune disease idiopathic thrombocytopenic purpura (ITP) (present in 34.4% of ITP patients; OR 2.4 (1.3-4.5), P < .009). FcγRIIc acts as an activating IgG receptor that exerts antibody-mediated cellular cytotoxicity by immune cells. Therefore, we propose that the activating FCGR2C-ORF genotype predisposes to ITP by altering the balance of activating and inhibitory FcγR on immune cells.

Simple genotyping of functional polymorphisms of the human immunoglobulin G receptors CD16A and CD32A: a reference cell panel

The Fcgamma receptors CD16A and CD32A connect the innate and the adaptive immune responses by transmitting activating signals to natural killer lymphocytes and myeloid cells upon recognition of antigen-immunoglobulin G (IgG) complexes. Two allelic dimorphisms of these receptors, valine/phenylalanine-158 of CD16A and histidine/arginine-131 of CD32A, modulate their affinity for certain human IgG subclasses. Furthermore, these polymorphisms are clinically relevant because they modify the susceptibility, the clinical course and the response to therapy of several human diseases. Genotyping of CD16A and CD32A alleles, encoded by FCGR3A and FCGR2A, respectively, is complicated by the fact that they both belong to families of highly homologous genes. In this study, we present an original method for genotyping the FCGR3A and FCGR2A dimorphisms based on the technique of polymerase chain reaction with confronting two-pair primers. The new method simplifies the analysis of FCGR3A and FCGR2A because the two alleles of each gene are detected simultaneously in a single reaction and separated, with no further manipulations, by their different electrophoretic mobilities in regular agarose gels. We also present the CD16A and CD32A genotypes of cells from the Tenth International Histocompatibility Workshop, which can serve as a reference cell panel for investigating the influence of CD16A and CD32A polymorphisms on human health.

Expression profile of FcgammaRIIb on leukocytes and its dysregulation in systemic lupus erythematosus

FcgammaRIIb (CD32B, Online Mendelian Inheritance in Man 604590), an IgG FcR with a tyrosine-based inhibitory motif, plays a critical role in the balance of tolerance and autoimmunity in murine models. However, the high degree of homology between FcgammaRIIb and FcgammaRIIa in humans and the lack of specific Abs to differentiate them have hampered study of the normal expression profile of FcgammaRIIb and its potential dysregulation in autoimmune diseases such as systemic lupus erythematosus (SLE). Using our newly developed anti-FcgammaRIIb mAb 4F5 which does not react with FcgammaRIIa, we found that FcgammaRIIb is expressed on the cell surface of circulating B lymphocytes, monocytes, neutrophils, myeloid dendritic cells (DCs), and at very low levels on plasmacytoid DCs from some donors. Normal donors with the less frequent 2B.4 promoter haplotype have higher FcgammaRIIb expression on monocytes, neutrophils, and myeloid DCs similar to that reported for B lymphocytes, indicating that FcgammaRIIb expression on both myeloid and lymphoid cells is regulated by the naturally occurring regulatory single nucleotide polymorphisms in the FCGR2B promoter. FcgammaRIIb expression in normal controls is up-regulated on memory B lymphocytes compared with naive B lymphocytes. In contrast, in active SLE, FcgammaRIIb is significantly down-regulated on both memory and plasma B lymphocytes compared with naive and memory/plasma B lymphocytes from normals. Similar down-regulation of FcgammaRIIb on myeloid-lineage cells in SLE was not seen. Our studies demonstrate the constitutive regulation of FcgammaRIIb by natural gene polymorphisms and the acquired dysregulation in SLE autoimmunity, which may identify opportunities for using this receptor as a therapeutic target.

Association of FCGR2A and FCGR2A-FCGR3A haplotypes with susceptibility to giant cell arteritis

The Fc gamma receptors have been shown to play important roles in the initiation and regulation of many immunological and inflammatory processes and to amplify and refine the immune response to an infection. We have investigated the hypothesis that polymorphism within the FCGR genetic locus is associated with giant cell arteritis (GCA). Biallelic polymorphisms in FCGR2A, FCGR3A, FCGR3B and FCGR2B were examined for association with biopsy-proven GCA (n = 85) and healthy ethnically matched controls (n = 132) in a well-characterised cohort from Lugo, Spain. Haplotype frequencies and linkage disequilibrium (D') were estimated across the FCGR locus and a model-free analysis performed to determine association with GCA. There was a significant association between FCGR2A-131RR homozygosity (odds ratio (OR) 2.10, 95% confidence interval (CI) 1.12 to 3.77, P = 0.02, compared with all others) and carriage of FCGR3A-158F (OR 3.09, 95% CI 1.10 to 8.64, P = 0.03, compared with non-carriers) with susceptibility to GCA. FCGR haplotypes were examined to refine the extent of the association. The haplotype showing the strongest association with GCA susceptibility was the FCGR2A-FCGR3A 131R-158F haplotype (OR 2.84, P = 0.01 for homozygotes compared with all others). There was evidence of a multiplicative joint effect between homozygosity for FCGR2A-131R and HLA-DRB1*04 positivity, consistent with both of these two genetic factors contributing to the risk of disease. The risk of GCA in HLA-DRB1*04 positive individuals homozygous for the FCGR2A-131R allele is increased almost six-fold compared with those with other FCGR2A genotypes who are HLA-DRB1*04 negative. We have demonstrated that FCGR2A may contribute to the 'susceptibility' of GCA in this Spanish population. The increased association observed with a FCGR2A-FCGR3A haplotype suggests the presence of additional genetic polymorphisms in linkage disequilibrium with this haplotype that may contribute to disease susceptibility. These findings may ultimately provide new insights into disease pathogenesis.

Analysis of Fcgamma receptor haplotypes in rheumatoid arthritis: FCGR3A remains a major susceptibility gene at this locus, with an additional contribution from FCGR3B

The Fcγ receptors play important roles in the initiation and regulation of many immunological and inflammatory processes, and genetic variants (FCGR) have been associated with numerous autoimmune and infectious diseases. The data in rheumatoid arthritis (RA) are conflicting and we previously demonstrated an association between FCGR3A and RA. In view of the close molecular proximity with FCGR2A, FCGR2B and FCGR3B, additional polymorphisms within these genes and FCGR haplotypes were examined to refine the extent of association with RA. Biallelic polymorphisms in FCGR2A, FCGR2B and FCGR3B were examined for association with RA in two well characterized UK Caucasian and North Indian/Pakistani cohorts, in which FCGR3A genotyping had previously been undertaken. Haplotype frequencies and linkage disequilibrium were estimated across the FCGR locus and a model-free analysis was performed to determine association with RA. This was followed by regression analysis, allowing for phase uncertainty, to identify the particular haplotype(s) that influences disease risk. Our results reveal that FCGR2A, FCGR2B and FCGR3B were not associated with RA. The haplotype with the strongest association with RA susceptibility was the FCGR3A–FCGR3B 158V-NA2 haplotype (odds ratio 3.18, 95% confidence interval 1.13–8.92 [P = 0.03] for homozygotes compared with all genotypes). The association was stronger in the presence of nodules (odds ratio 5.03, 95% confidence interval 1.44–17.56; P = 0.01). This haplotype was also more common in North Indian/Pakistani RA patients than in control individuals, but not significantly so. Logistic regression analyses suggested that FCGR3A remained the most significant gene at this locus. The increased association with an FCGR3A–FCGR3B haplotype suggests that other polymorphic variants within FCGR3A or FCGR3B, or in linkage disequilibrium with this haplotype, may additionally contribute to disease pathogenesis.

Role of B cell inhibitory receptor polymorphisms in systemic lupus erythematosus: a negative times a negative makes a positive

B lymphocytes play a pivotal role in the pathogenesis of systemic lupus erythematosus (SLE). Here, we will review our studies on the role of polymorphisms of two genes coding for B cell inhibitory receptors, FCGR2B and CD72. In FCGR2B, a single nucleotide polymorphism leading to a nonsynonymous substitution, Ile232Thr, within the transmembrane domain was identified, and a significant association of the 232Thr/Thr genotype with SLE was observed in Japanese, Thai and Chinese populations, while this allele was found to be rare in Caucasians. On the other hand, the association of FCGR2B promoter polymorphism with SLE in Caucasians has been reported by two independent groups, but this allele was not found to be present in Japanese. These observations demonstrate that the association of FCGR2B polymorphisms with SLE is common to multiple populations, but the alleles associated with SLE depend upon the genetic background of each population. Functional analyses using a human B cell line lacking endogenous FcgammaRIIb revealed that SLE-associated 232Thr allele product was partially excluded from membrane lipid rafts under resting conditions and after coligation with B cell receptor, and was significantly less potent at inhibiting B cell activation. Two haplotypes were identified in CD72, one of which was associated with increased production of an alternative splicing isoform that substantially alters the extracellular region of CD72. Interestingly, the presence of the haplotype significantly decreased the risk of SLE conferred by FCGR2B-232Thr in an epistatic manner. These observations emphasize the need to understand human immune system diversity if we are to improve our understanding of the pathogenesis of autoimmune diseases.

Identification and Characterization of a FcR Homolog in an Ectothermic Vertebrate, the Channel Catfish (Ictalurus punctatus)

An FcR homolog (IpFcRI), representing the first such receptor from an ectothermic vertebrate, has been identified in the channel catfish (Ictalurus punctatus). Mining of the catfish expressed sequence tag databases using mammalian FcR sequences for CD16, CD32, and CD64 resulted in the identification of a teleost Ig-binding receptor. IpFcRI is encoded by a single-copy gene containing three Ig C2-like domains, but lacking a transmembrane segment and cytoplasmic tail. The encoded Ig domains of IpFcRI are phylogenetically and structurally related to mammalian FcR and the presence of a putative Fc-binding region appears to be conserved. IpFcRI-related genomic sequences are also present in both pufferfish and rainbow trout, indicating the likely presence of a soluble FcR in other fish species. Northern blot and qualitative PCR analyses demonstrated that IpFcRI is primarily expressed in IgM-negative leukocytes derived from the lymphoid kidney tissues and PBL. Significantly lower levels of IpFcRI expression were detected in catfish clonal leukocyte cell lines. Using the native leader, IpFcRI was secreted when transfected into insect cells and importantly the native IpFcRI glycoprotein was detected in catfish plasma using a polyclonal Ab. Recombinant IpFcRI binds catfish IgM as assessed by both coimmunoprecipation and cell transfection studies and it is presumed that it functions as a secreted FcR akin to the soluble FcR found in mammals. The identification of an FcR homolog in an ectothermic vertebrate is an important first step toward understanding the evolutionary history and functional importance of vertebrate Ig-binding receptors.

Role of Fc gamma receptor IIb polymorphism in the genetic background of systemic lupus erythematosus: insights from Asia

FCGR2B codes for an inhibitory receptor expressed in B cells and monocytes. Polymorphisms of Fcgr2b in mice have been shown to be associated with autoimmune diseases including systemic lupus erythematosus (SLE) and targeted disruption of Fcgr2b renders mice susceptible to induced or spontaneous autoimmunity, depending on the genetic background. Polymorphism screening of FCGR2B has been hampered by the complexity and extreme homology among FCGR family members. We established a specific genotyping system, detected a SNP that changes position 232 amino acid in the transmembrane region from Ile to Thr and found a significant association of 232Thr with SLE in the Japanese, Thai and Chinese populations. In contrast, promoter polymorphism of FCGR2B, but not Ile232Thr, was shown to be associated with SLE in Caucasians. Linkage disequilibrium was observed among FCGR2A, 2B, 3A and 3B genes with varying degrees, but in the Asian populations, each of FCGR2B, 3A and 3B genes was suggested to contribute to the susceptibility to SLE. These results indicate that FCGR2B is a susceptibility gene to SLE in the context of a genetic background, both in humans and mice.

Polymorphism of the Fcgamma receptor IIA and malaria morbidity

Fc receptors (FcRs) are expressed on the surface of all types of cells of the immune system. They bind the Fc portion of immunoglobulin (Ig), thereby bridging specific antigen recognition by antibodies with cellular effector mechanisms. FcγRIIA, one of the three receptors for human IgG, is a low-affinity receptor for monomeric IgG, but binds IgG immune complexes efficiently. FcγRIIA is believed to play a major role in eliciting monocyte- and macrophage-mediated effector responses against blood-stage malaria parasites. A G → A single nucleotide polymorphism, which causes an arginine (R) to be replaced with histidine (H) at position 131, defines two allotypes which difer in their avidity for complexed human IgG₂ and IgG₃. Because FcγRIIA-H131 is the only FcγR allotype which interacts efficiently with human IgG_2, this polymorphism may determine whether parasite-specific IgG₂ may or may not elicit cooperation with cellular imune responses during blood-stage malaria infection. Here, we review data from four published case-control studies describing associations between FcγRIIA R/H131 polymorphism and malaria-related outcomes and discuss possible reasons for some incongruities found in these available results.

Genetics of lupus nephritis

Susceptibility to lupus nephritis is the end-result of complex interactions between polymorphic genetic factors involved in the regulation of immune responses. In humans, genome-wide screens and candidate-gene analyses led to the identification of several loci containing potential targets (FcgammaRIIa, PTPN22, PD-1, IL-10) for physiopathological research and therapeutic interventions. In mice, the generation of congenic mice, bearing in a normal genetic background one single disease-associated locus, greatly improved our understanding of the mechanisms mediating the genetic contribution to the disease. In the future, the identification of disease-associated genes will open new perspectives for the development of more targeted therapies of lupus nephritis.

Fc gamma R expression on NK cells influences disease severity in rheumatoid arthritis

Rheumatoid arthritis (RA) is associated with autoantibodies, the best known of which is rheumatoid factor (RF). RF/IgG complexes interact with FcgammaR on the surface of neutrophils, NK cells and monocyte/macrophages. We have analyzed the expression pattern and allelic polymorphisms of three FcgammaR genes (FcgammaRIIA, FcgammaRIIC and FcgammaRIIIA) in a large sample of RA patients and normal donors. We have found that the level of FcgammaR (CD16 and CD32) expression on NK cells is lower in RA patients than in normal individuals. Genotypic analysis demonstrated that the CD32 isoform expressed by the majority of RA patients was not the activating FcgammaRIIc1 isoform, commonly seen in normal individuals, but rather the inhibitory FcgammaRIIb isoform. The combination of the FcgammaRIIIA-176F allele with a lack of CD32 expression in NK cells appeared to be characteristic of RA subjects with aggressive disease. Since FcgammaRII and FcgammaRIIIA are predominantly expressed by NK cells, these data further suggest that FcgammaR-mediated activation of NK cells could be a disease-determining factor in RA patients.