Roles of Fc receptors in autoimmunity

[Fcγ receptor and systemic autoimmune disease]

The systemic autoimmune disease such as systemic lupus erythematosus (SLE) is characterized by the deposition of immune complexes in multiple organs. Fcγ receptors (FcγR) recognize the Fc portion of IgG and are important in determining the response of leukocytes to deposited immune complexes. FcγR also provide positive and negative regulation of immune cell responses. The activatory FcγR including the FcR common γ chain take balance with Fcγ RIIB, the only inhibitory FcγR. Development of lupus-like autoimmune disease as well as monocytosis in BXSB mice is dependent on the activatory and inhibitory FcγR. In human SLE, dysregulated expression of FcγRIIB on memory B cells is reported and numbers of associations with genetic polymorphism are also reported. The cell-specific modulation of these activatory or inhibitory FcγRs are expected for the new therapeutic strategy in autoimmune diseases.

TNF receptor II fusion protein with tandemly repeated Fc domains

The extracellular domain of tumour necrosis factor (TNF) receptor II fused with the human IgG1 Fc region (TNFRII-Fc), as well as antibodies against TNF, has been used to treat rheumatoid arthritis. However, TNFRII-Fc is less effective than these antibodies in terms of antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) against cells bearing TNF on the cell surface. We hypothesized that these activities could be increased by fusing TNFRII with tandemly repeated Fc (TNFRII-Fc-Fc). The affinities of TNFRII-Fc-Fc for soluble TNF-α and transmembrane TNF-α and the TNF-α cytotoxicity-inhibitory activity were as potent as those of TNFRII-Fc. TNFRII-Fc-Fc showed much higher binding avidity for Fcγ receptors than TNFRII-Fc and was more potent in terms of both ADCC and CDC against cells expressing transmembrane TNF-α. TNFRII-Fc-Fc of 80 kDa, as well as TNFRII-Fc-Fc of 200 kDa, was detected. TNFRII-Fc-Fc (80 kDa) was as potent as TNFRII-Fc in terms of both ADCC and CDC. These results suggest that Fc multimerization of receptor-Fc fusion proteins can augment effector functions such as ADCC and CDC, and thereby have the potential to provide a superior therapeutic effect. This may be the case not only for TNFRII-Fc but also for other receptor-Fc fusion proteins.

High-dose dexamethasone shifts the balance of stimulatory and inhibitory Fcgamma receptors on monocytes in patients with primary immune thrombocytopenia

The human Fcγ receptor (FcγR) system is composed of 2 opposing families, the activating FcγRs (FcγRI, FcγRIIa, and FcγRIII) and the inhibitory FcγR (FcγRIIb). The disturbed balance of the activating and inhibitory FcγRs has been implicated in the pathogenesis of many autoimmune diseases. In this study, the expression of FcγRs on monocytes was determined in 23 patients with primary immune thrombocytopenia (ITP) before and after high-dose dexamethasone (HD-DXM) treatment. The FcγRI expression was significantly higher in ITP patients and decreased after HD-DXM treatment. The ratio of FcγRIIa/IIb mRNA expression on monocytes was significantly higher in untreated patients than in healthy controls. After HD-DXM therapy, the ratio decreased and the increased expression of FcγRIIb mRNA and protein coincided with a remarkable decrease in the expression of FcγRIIa, FcγRI, and monocyte phagocytic capacity. There was no significant difference in FcγRIII expression on monocytes between patients and controls. In vitro cell-culture experiments showed that DXM could induce FcγRIIa and FcγRIIb expression in monocytes from ITP patients, with FcγRIIb at higher amplitudes. These findings suggested that the disturbed FcγR balance might play a role in the pathogenesis of ITP, and that HD-DXM therapy could shift monocyte FcγR balance toward the inhibitory FcγRIIb in patients with ITP.

Specific Btk inhibition suppresses B cell- and myeloid cell-mediated arthritis

Bruton's tyrosine kinase (Btk) is a therapeutic target for rheumatoid arthritis, but the cellular and molecular mechanisms by which Btk mediates inflammation are poorly understood. Here we describe the discovery of CGI1746, a small-molecule Btk inhibitor chemotype with a new binding mode that stabilizes an inactive nonphosphorylated enzyme conformation. CGI1746 has exquisite selectivity for Btk and inhibits both auto- and transphosphorylation steps necessary for enzyme activation. Using CGI1746, we demonstrate that Btk regulates inflammatory arthritis by two distinct mechanisms. CGI1746 blocks B cell receptor-dependent B cell proliferation and in prophylactic regimens reduces autoantibody levels in collagen-induced arthritis. In macrophages, Btk inhibition abolishes FcγRIII-induced TNFα, IL-1β and IL-6 production. Accordingly, in myeloid- and FcγR-dependent autoantibody-induced arthritis, CGI1746 decreases cytokine levels within joints and ameliorates disease. These results provide new understanding of the function of Btk in both B cell- or myeloid cell-driven disease processes and provide a compelling rationale for targeting Btk in rheumatoid arthritis.

Role of periodontal pathogenic bacteria in RANKL-mediated bone destruction in periodontal disease

Accumulated lines of evidence suggest that hyperimmune responses to periodontal bacteria result in the destruction of periodontal connective tissue and alveolar bone. The etiological roles of periodontal bacteria in the onset and progression of periodontal disease (PD) are well documented. However, the mechanism underlying the engagement of periodontal bacteria in RANKL-mediated alveolar bone resorption remains unclear. Therefore, this review article addresses three critical subjects. First, we discuss earlier studies of immune intervention, ultimately leading to the identification of bacteria-reactive lymphocytes as the cellular source of osteoclast-induction factor lymphokine (now called RANKL) in the context of periodontal bone resorption. Next, we consider (1) the effects of periodontal bacteria on RANKL production from a variety of adaptive immune effector cells, as well as fibroblasts, in inflamed periodontal tissue and (2) the bifunctional roles (upregulation vs. downregulation) of LPS produced from periodontal bacteria in a RANKL-induced osteoclast-signal pathway. Future studies in these two areas could lead to new therapeutic approaches for the management of PD by down-modulating RANKL production and/or RANKL-mediated osteoclastogenesis in the context of host immune responses against periodontal pathogenic bacteria.

FcγRIV deletion reveals its central role for IgG2a and IgG2b activity in vivo

Cellular Fcγ receptors are essential for IgG-dependent effector functions in vivo. There is convincing evidence that selective activating Fcγ receptors are responsible for the activity of individual IgG subclasses. Thus, IgG1 activity is absent in FcγRIII-deficient mice, and several studies suggest that the activity of the most potent IgG subclasses, IgG2a and IgG2b, might be dependent on either individual or a combination of activating FcγRs. To study the role of individual activating FcγRs for IgG subclass activity, we generated an FcγRIV-deficient mouse and showed that a variety of IgG2a- and IgG2b-dependent effector functions are impaired in the absence of this activating Fc receptor in models of autoimmunity and antibody-dependent cellular cytotoxicity.

Leukotriene B4 augments and restores Fc gammaRs-dependent phagocytosis in macrophages

Phagocytosis by macrophages is essential for host defense, i.e. preventing invasion of pathogens and foreign materials. Macrophages engulf immunoglobulin G (IgG)-opsonized particles through the action of the receptors for the Fc of IgG (FcγRs). Leukotriene B(4) (LTB(4)) is a classical lipid chemoattractant derived from arachidonic acid. Leukotriene B(4) receptor 1 (BLT1), a high affinity LTB(4) receptor, is expressed in a variety of immune cells such as neutrophils, macrophages, and dendritic cells. Although LTB(4) has been shown to enhance macrophage phagocytosis, few studies have investigated the intracellular mechanisms involved in this in detail. Furthermore, there have been no reports of the direct cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling. Here, we show that FcγRs-dependent phagocytosis was attenuated in BLT1-deficient macrophages as compared with wild-type (WT) cells. Moreover, cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling was identified at the level of phosphatidylinositol 3-OH kinase (PI3K) and Rac, downstream of Syk. In addition, the trimeric G(i) protein (G(i)) was found to be essential for BLT1-dependent phagocytosis. Surprisingly, we found that LTB(4)-BLT1 signaling restores phagocytosis in the absence of FcγRs signaling. These data indicate that LTB(4)-BLT1 signaling plays a pivotal role in macrophage phagocytosis and innate immunity.

Effect of immunoglobulin G (IgG) interchain disulfide bond cleavage on efficacy of intravenous immunoglobulin for immune thrombocytopenic purpura (ITP)

Intravenous immunoglobulin (IVIG) has been used widely to treat immune thrombocytopenic purpura (ITP), but the mechanisms of its action remain unclear. We investigated the affinity for Fcγ receptors (FcγRs) and the thrombocytopenia-ameliorating effect of S-sulfonated gammaglobulin (SGG) and S-alkylated gammaglobulin (AGG), in comparison with unmodified gammaglobulin (GG), in a mouse ITP model. Cleavage of immunoglobulin (Ig)G interchain disulfide bonds by either S-sulfonation or S-alkylation did not decrease the affinity for FcγRIIA (CD32A) and FcγRIIB (CD32B), but did decrease the affinity for FcγRIA (CD64A) and FcγRIIIA (CD16A), presumably because of changes in H-chain configuration. The interchain disulfide bond cleavage decreased the affinity much more for mouse FcγRIV than for mouse FcγRIIB. The ability of AGG to ameliorate ITP was greatly diminished, while SGG, whose disulfide bonds are reconstituted in vivo, was as effective as GG. These results suggest that the interchain disulfide bonds are important for therapeutic effect. It is also suggested that the interaction of IVIG with the inhibitory receptor FcγRIIB is insufficient for effective amelioration of ITP and that, at least in this model, direct binding of IVIG to FcγRIIIA is also required.

Role of PIR-B in autoimmune glomerulonephritis

PIR-B, an inhibitory receptor expressed on murine B cells and myeloid cells, regulates humoral and cellular immune responses via its constitutive binding to the ligand, MHC class I molecules, on the same cells (cis) or on different cells (trans). Although it has been speculated that PIR-B is important for maintaining peripheral tolerance, PIR-B single deficiency does not cause overt autoimmune diseases. Recently, however, the combination of its deficiency with the Fas lpr mutation was found to result in augmented production of autoantibodies such as IgG rheumatoid factor and anti-DNA IgG, leading to glomerulonephritis in mice. Although the precise molecular mechanism for the overall scenario is unclear, PIR-B was found to suppress TLR9-mediated production of naturally autoreactive antibodies by innate B cells or B-1 cells by inhibiting the activation of Bruton's tyrosine kinase. Thus, PIR-B is an important regulator of innate immunity mediated by TLR9 in B-1 cells, which can otherwise provoke autoimmunity when overactivated.

The inhibitory effects of intravenous administration of rabbit immunoglobulin G on airway inflammation are dependent upon Fcγ receptor IIb on CD11c(+) dendritic cells in a murine model

Immunoglobulins (Igs) play important immunomodulatory effects on allergic asthma. Among these, IgG has been reported to regulate allergic inflammation in previous studies about immunotherapy and intravenous immunoglobulin therapy. In this study, to examine the immunomodulatory mechanisms of IgG and FcRs we evaluated the effects of intravenous (i.v.) rabbit IgG administration (IVIgG) on allergic airway inflammation and lung antigen-presenting cells (APCs) in a murine model of ovalbumin (OVA) sensitization and challenge. In OVA-challenged mice, IVIgG attenuated airway eosinophilia, airway hyperresponsiveness and goblet cell hyperplasia and also inhibited the local T helper type (Th) 2 cytokine levels. Additionally, IVIgG attenuated the proliferation of OVA-specific CD4(+) T cells transplanted into OVA-challenged mice. Ex vivo co-culture with OVA-specific CD4(+) cells and lung CD11c(+) APCs from mice with IVIgG revealed the attenuated transcription level of Th2 cytokines, suggesting an inhibitory effect of IVIgG on CD11c(+) APCs to induce Th2 response. Next, to analyse the effects on Fcγ receptor IIb and dendritic cells (DCs), asthmatic features in Fcγ receptor IIb-deficient mice were analysed. IVIgG failed to attenuate airway eosinophilia, airway inflammation and goblet cell hyperplasia. However, the lacking effects of IVIgG on airway eosinophilia in Fcγ receptor IIb deficiency were restored by i.v. transplantation of wild-type bone marrow-derived CD11c(+) DCs. These results demonstrate that IVIgG attenuates asthmatic features and the function of lung CD11c(+) DCs via Fcγ receptor IIb in allergic airway inflammation. Targeting Fc portions of IgG and Fcγ receptor IIb on CD11c(+) DCs in allergic asthma is a promising therapeutic strategy.

Biliary apotopes and anti-mitochondrial antibodies activate innate immune responses in primary biliary cirrhosis

Our understanding of primary biliary cirrhosis (PBC) has been significantly enhanced by the rigorous dissection of the multilineage T and B cell response against the immunodominant mitochondrial autoantigen, the E2 component of the pyruvate dehydrogenase complex (PDC-E2). PDC-E2 is a ubiquitous protein present in mitochondria of nucleated cells. However, the damage of PBC is confined to small biliary epithelial cells (BECs). We have previously demonstrated that BECs translocate immunologically intact PDC-E2 to apoptotic bodies and create an apotope. To define the significance of this observation, we have studied the ability of biliary or control epithelial apotopes to induce cytokine secretion from mature monocyte-derived macrophages (MDMphis) from either patients with PBC or controls in the presence or absence of anti-mitochondrial antibodies (AMAs). We demonstrate that there is intense inflammatory cytokine production in the presence of the unique triad of BEC apotopes, macrophages from patients with PBC, and AMAs. The cytokine secretion is inhibited by anti-CD16 and is not due to differences in apotope uptake. Moreover, MDMphis from PBC patients cultured with BEC apoptotic bodies in the presence of AMAs markedly increase tumor necrosis factor-related apoptosis-inducing ligand expression.

CONCLUSION

These results provide a mechanism for the biliary specificity of PBC, the recurrence of disease after liver transplantation, and the success of ursodiol in treatment. They further emphasize the critical role of the innate immune system in the perpetuation of this autoimmune disease.

Mechanisms of action of intravenous immunoglobulins

Intravenous immunoglobulin (IVIg) has been used for nearly three decades as an efficient anti-inflammatory therapeutic regimen in a growing number of autoimmune diseases. Despite this their success in clinical application, the mechanism of action of IVIg therapy remains elusive. During the last few years, several mechanisms dependent on either the IgG variable or constant fragment have been proposed to explain the potent immunomodulatory activity of IVIg. This review will discuss which molecular and cellular pathways might be involved in the anti-inflammatory activity of IVIg and for which types of autoimmune diseases they might be relevant.

Divergent intracellular sorting of Fc{gamma}RIIA and Fc{gamma}RIIB2

The human low affinity FcγRII family includes both the activating receptor FcγRIIA and the inhibitory receptor FcγRIIB2. These receptors have opposing signaling functions but are both capable of internalizing IgG-containing immune complexes through clathrin-mediated endocytosis. We demonstrate that upon engagement by multivalent aggregated human IgG, FcγRIIA expressed in ts20 Chinese hamster fibroblasts is delivered along with its ligand to lysosomal compartments for degradation, while FcγRIIB2 dissociates from the ligand and is routed separately into the recycling pathway. FcγRIIA sorting to lysosomes requires receptor multimerization, but does not require either Src family kinase activity or ubiquitylation of receptor lysine residues. The sorting of FcγRIIB2 away from a degradative fate is not due to its lower affinity for IgG and occurs even upon persistent receptor aggregation. Upon co-engagement of FcγRIIA and FcγRIIB2, the receptors are sorted independently to distinct final fates after dissociation of co-clustering ligand. These results reveal fundamental differences in the trafficking behavior of different Fcγ receptors.

Could the expression of CD86 and FcγRIIB on B cells be functionally related and involved in driving rheumatoid arthritis?

Aberrant immune responses play a pivotal role in the processes that cause inflammation and joint damage in patients with rheumatoid arthritis (RA). Polyclonal B cell activation and the production of autoantibodies are immunological hallmarks of the disease. However, controversy surrounds the pathogenicity of autoantibodies, mainly because not all patients are seropositive (10% of RA patients are seronegative), suggesting that they could be markers rather than makers of disease. Catalán and collaborators report that patients with RA display reduced expression of FcγRIIB on memory B cells and plasma cells, which inversely correlates with autoantibody levels. Considering that FcγRIIB stimulation down-regulates antibody production, this work strengthens the link between autoantibodies and pathogenicity.

Maternally transferred anti-factor VIII IgG reduce the anti-factor VIII humoral immune response in factor VIII-deficient mice

Replacement therapy with exogenous factor VIII (FVIII) to treat haemorrhages or used in prophylaxis induces inhibitory anti-FVIII immunoglobulin G (IgG) in some patients with haemophilia A. Therapeutic strategies to prevent the onset of the deleterious anti-FVIII immune response are still lacking. Maternal IgG is transferred to the offspring during fetal and neonatal life. While protecting the offspring from bacterial and viral infections, maternal IgG may alter the repertoires of T and B lymphocytes, and may impair vaccination in early infancy. Using haemophilic mice, we demonstrate that the transfer of maternal anti-FVIII IgG modulates the onset of anti-FVIII inhibitory IgG in early adulthood. The protective effect is reproduced upon reconstitution of naive mice with anti-FVIII IgG, suggesting that the reduced ability to mount an anti-FVIII immune response is the result of an interference between circulating anti-FVIII IgG and the administered FVIII rather than to a profound remodelling of lymphocyte repertoires occurring during the ontogeny of the immune system.

Immunoglobulin IgG Fc-receptor polymorphisms and HLA class II molecules in Guillain-Barré syndrome

OBJECTIVE

To analyze host genetic factors immunoglobulin G Fc receptors (FcgammaRs) and human leukocyte antigen (HLA) class II in GBS patients.

METHODS

FcgammaRIIA, IIIA and IIIB polymorphisms were studied in 80 each GBS patients and healthy controls by allele specific PCR. HLA class II DRbeta1 and DQbeta1 typing was performed at the two-digit level by PCR in randomly selected 54 GBS patients and 202 controls.

RESULTS

FcgammaRIIA-H/H (56% vs 9%; P < 0.0001) and FcgammaRIIIA-V/V (40% vs 13%; P < 0.0001) genotypes, H131 allele frequencies (0.73 vs 0.26, P < 0.0001) and HLA DQbeta1*060x (OR, 1.96; 95% CI, 1.26-3.04; P < 0.01) were significantly increased in GBS than controls. DRbeta1*0701 alone (OR, 10; 95% CI, 45.90-2.25; P < 0.001) and together with FcgammaRIIA-H/H (OR, 11.03; 95% CI, 2.63-46.20; P < 0.001) was significantly associated with GBS patients having microbiological evidence of recent infection.

CONCLUSIONS

The study indicates that homozygous FcgammaRIIA and FcgammaRIIIA genotypes and FcgammaRIIA H131 allele are associated with GBS. HLA class II molecule DRbeta1*0701 is identified as novel genetic risk factor for development of GBS in patients with preceding infection.

Fcgamma receptor-mediated antigen uptake by lung DC contributes to allergic airway hyper-responsiveness and inflammation

During asthma, lung DC capture and process antigens to initiate and maintain allergic Th2 cell responses to inhaled allergens. The aim of the study was to investigate whether allergen-specific IgG, generated during sensitization, can potentiate the acute airway inflammation through Fcgamma receptor (FcgammaR)-mediated antigen uptake and enhance antigen presentation resulting in augmented T-cell proliferation. We examined the impact of antigen presentation and T-cell stimulation on allergic airway hyperresponsiveness and inflammation using transgenic and gene-deficient mice. Both airway inflammation and eosinophilia in bronchoalveolar lavage fluid were markedly reduced in sensitized and challenged FcgammaR-deficient mice. Lung DC of WT, but not FcgammaR-deficient mice, induced increased antigen-specific CD4+ T-cell proliferation when pulsed with anti-OVA IgG immune complexes. Intranasal application of anti-OVA IgG immune complexes resulted in enhanced airway inflammation, eosinophilia and Th2 cytokine release, mediated through enhanced antigen-specific T-cell proliferation in vivo. Finally, antigen-specific IgG in the serum of sensitized mice led to a significant increase of antigen-specific CD4+ T-cell proliferation induced by WT, but not FcgammaR-deficient, lung DC. We conclude that FcgammaR-mediated enhanced antigen presentation and T-cell stimulation by lung DC has a significant impact on inflammatory responses following allergen challenge in asthma.

African-derived genetic polymorphisms in TNFAIP3 mediate risk for autoimmunity

The TNF alpha-induced protein 3 (TNFAIP3) is an ubiquitin-modifying enzyme and an essential negative regulator of inflammation. Genome-wide association studies have implicated the TNFAIP3 locus in susceptibility to autoimmune disorders in European cohorts, including rheumatoid arthritis, coronary artery disease, psoriasis, celiac disease, type 1 diabetes, inflammatory bowel disease, and systemic lupus erythematosus (SLE). There are two nonsynonymous coding polymorphisms in the deubiquitinating (DUB) domain of TNFAIP3: F127C, which is in high-linkage disequilibrium with reported SLE-risk variants, and A125V, which has not been previously studied. We conducted a case-control study in African-American SLE patients using these coding variants, along with tagging polymorphisms in TNFAIP3, and identified a novel African-derived risk haplotype that is distinct from previously reported risk variants (odds ratio=1.6, p=0.006). In addition, a rare protective haplotype was defined by A125V (odds ratio=0.31, p=0.027). Although A125V was associated with protection from SLE, surprisingly the same allele was associated with increased risk of inflammatory bowel disease. We tested the functional activity of nonsynonymous coding polymorphisms within TNFAIP3, and found that the A125V coding-change variant alters the DUB activity of the protein. Finally, we used computer modeling to depict how the A125V amino acid change in TNFAIP3 may affect the three-dimensional structure of the DUB domain to a greater extent than F127C. This is the first report of an association between TNFAIP3 polymorphisms and autoimmunity in African-Americans.

Fc receptor-mediated antiviral antibodies

PURPOSE OF REVIEW

We summarize current information on Fc receptor-mediated antiviral activities of antibodies. These activities include Fcgamma receptor-mediated inhibition and neutralization of HIV on antigen-presenting cells, antibody-dependent cellular cytotoxicity, and antibody-dependent cell-mediated virus inhibition (ADCVI).

RECENT FINDINGS

An Fcgamma receptor-mediated mechanism that results in augmented neutralization and may render nonneutralizing antibodies inhibitory has been demonstrated in antigen-presenting cell. Antibody-dependent cellular cytotoxicity antibody activity correlates inversely with HIV disease progression in humans, and higher vaccine-induced antibody-dependent cellular cytotoxicity antibody responses are associated with lower acute simian immunodeficiency virus viremia levels in macaques. Following vaccination with rgp120, ADCVI antibody levels are higher among those with a lower rate of sexually acquired HIV infection. Nonneutralizing simian immunodeficiency virus immune serum that prevents infection of newborn macaques after oral challenge has potent ADCVI antibody activity. Abrogating the ability of the Fc segment of the broadly neutralizing mAb b12 to bind to Fcgamma receptors and to mediate ADCVI substantially reduces b12's protective effect in a simian/human immunodeficiency virus vaginal challenge model.

SUMMARY

Fc-FcgammaR interactions play a critical role in the biological function of antibody and are likely to be instrumental in preventing or modulating lentiviral infection. Exploiting antibody responses that depend on Fc-FcgammaR interactions may help widen the breadth and increase the potency of vaccine-induced antibody. Although the importance of generating optimal Fab-antigen interactions cannot be overestimated, improving Fc-FcgammaR interactions through adjuvants or other strategies provides another option for improving HIV vaccines and immunotherapies.

Fcgamma receptors modulate pulmonary inflammation by activating innate immune cells in murine hypersensitivity pneumonitis

Background

Hypersensitivity pneumonitis (HP) is an interstitial lung disease that develops following repeated exposure to inhaled particulate antigens. The family of Fcγ receptors (FcγRs) has emerged as central regulators for modulating both pro-and anti-inflammatory responses. However, the role of FcγRs in the development of HP has not been investigated yet.

Methods

To explore the functional roles of FcγRs in HP, FcγR^-/- and B6 mice were challenged with Saccharopolyspora rectivirgula (SR) antigen intranasally, and compared these mice in terms of the histological change, infiltrated immune cells in BALF and in vitro immune responses.

Results

FcγR^-/- mice exhibited attenuation of HP in terms of histological alterations, and reduced numbers of neutrophils and macrophages in and the increased CD4:CD8 ratio of bronchoalveolar lavage fluid. The lungs of FcγR^-/- mice showed high production of Th2 cytokine such as IL-4 and slightly low production of Th1 cytokine, INF-γ compared to those of B6 mice. However, SR-specific adaptive immune responses of FcγR^-/- mice were similar to those of B6 mice.

Conclusion

These results demonstrate that activating Fcγ receptors play an important role in activating neutrophils and macrophages in pulmonary inflammation and inducing Th1 differentiation by regulating cytokine expression in SR-induced HP.

Differential recruitment of activating and inhibitory Fc gamma RII during phagocytosis

Human myeloid cells express both activating and inhibitory receptors of the FcgammaRII family. FcgammaRIIA mediates processes associated with cell activation, including phagocytosis of IgG-opsonized particles, whereas coengagement of the inhibitory FcgammaRIIB downregulates such signaling. We analyzed the relative recruitment of these two receptors during phagocytosis of IgG-coated particles by ts20 Chinese hamster fibroblast cells cotransfected with both receptors carrying distinguishable fluorescent protein tags. We found that FcgammaRIIA is substantially enriched at sites of particle binding relative to its inhibitory counterpart, with a greater than 2-fold increase in the local ratio of activating to inhibitory receptor compared with that for the plasma membrane as a whole. Experiments with chimeric receptors revealed that the preferential enrichment of FcgammaRIIA results from differences between the extracellular domains of the receptors, and indicated that the lesser recruitment of FcgammaRIIB limits its ability to effectively inhibit FcgammaRIIA-mediated phagocytosis. Mutagenesis studies indicated that FcgammaRIIA residues leucine 132 and phenylalanine 160, which lie in IgG-binding regions of FcgammaRIIA and which differ in FcgammaRIIB, both contribute to the local relative enrichment of FcgammaRIIA by increasing its affinity for IgG1 relative to that of FcgammaRIIB. In human monocytes, engagement of approximately equal amounts of FcgammaRIIB was required to substantially inhibit FcgammaRIIA-mediated phagocytosis. These results demonstrate that differences in affinity for IgG between activating and inhibitory FcgammaR can result in substantial local changes in their relative concentrations during phagocytosis, with important functional consequences.

Transgenic mice expressing human FcgammaRIIa have enhanced sensitivity to induced autoimmune arthritis as well as elevated Th17 cells

The major human Fc receptor, huFcgammaRIIa, is implicated in the development of autoimmune arthritis in humans but until recently has not been studied in mouse models. We evaluated potential roles of FcgammaRIIa by using transgenic mice expressing the receptor. We examined two models of induced autoimmune arthritis pristane-induced arthritis (PIA) and collagen-induced arthritis (CIA) as well as the anti-collagen-II antibody-induced arthritis (CAIA) model. In the induced arthritis models PIA and CIA, the transgenic mice developed a more severe arthritis than the other arthritis-prone SJL or DBA1 mice. Interestingly, anti-collagen-II antibodies were elevated in PIA in the susceptible mice. In the CIA model, the highly susceptible transgenic mouse had IgG subclass levels equivalent to the unaffected and disease resistant C57BL/6 mouse strain implying that the FcgammaRIIa lowers the threshold of IgG dependent leukocyte activation. This is consistent with the greatly enhanced sensitivity of the FcgammaRIIa transgenic mice to CAIA which clearly indicates a role for the receptor at least at the inflammatory effector cell level. Other roles for huFcgammaRIIa or other gene products in the development of autoimmunity cannot be ruled out however, especially as the mice exhibited elevated Th1 or Th17 CD4 T cells in the draining lymph nodes.

Both FcgammaRIV and FcgammaRIII are essential receptors mediating type II and type III autoimmune responses via FcRgamma-LAT-dependent generation of C5a

FcgammaRIV is a relatively new IgG Fc receptor (FcgammaR) that is reported to contribute to the pathogenesis of autoimmune diseases, although its specific role in relation to FcgammaRIII, complement and IgG2 subclasses remains uncertain. Here we define FcgammaRIV on macrophages as a receptor for soluble IgG2a/b complexes but not for cellular bound IgG2a and show that simultaneous activation of FcgammaRIV and FcgammaRIII is critical to mediate certain type II/III autoimmune responses. FcgammaRIII-deficient mice display compensatory enhanced FcgammaRIV expression, are protected from lung inflammation after deposition of IgG complexes, and show reduced sensitivity to IgG2a/b-mediated hemolytic anemia, indicating that increased FcgammaRIV alone is not sufficient to trigger these diseases in the absence of FcgammaRIII. Importantly, however, blockade of FcgammaRIV is also effective in inhibiting phagocytosis and cytokine production in IgG2b-induced anemia and acute lung injury, processes that display a further dependence on C5a anaphylatoxin receptor. Using gene deletion and functional inhibition studies, we found that FcgammaRIII and FcgammaRIV are each essential to trigger an FcRgamma-linker for activation of T-cell-dependent signal that drives C5a production in the Arthus reaction. Together, the results demonstrate a combined requirement for FcgammaRIII and FcgammaRIV in autoimmune injury, and identify the linker for activation of T cells adaptor as an integral component of linked FcgammaR and C5a anaphylatoxin receptor activation to generate inflammation.

[Enhancement of antibody-dependent cellular cytotoxicity by tandem Fc multimerization]

Monoclonal antibodies are being used as therapeutics for a number of cancers, such as leukemia, breast and colon cancers, and a lot of monoclonal antibodies specific for tumor-related antigens have been on clinical trials. Antibody-dependent cellular cytotoxicity (ADCC) is one of the major mechanisms by which antibodies exert anti-tumor effects. ADCC occurs through interaction between the Fc domains of IgG antibodies bound to target cells and Fcgamma receptors on the surface of effector cells. In our study, a chimeric antibody, designated M-Ab, was constructed with the V regions from mouse anti-CD20 mAb 1F5 and the C regions from human IgG1 and kappa chain. Two or three Fc domains were tandemly repeated downstream of the C-terminus of the M-Ab to give D0-Ab (Fc dimer Ab without a linker), T0-Ab (Fc trimer Ab without a linker), and T3-Ab (Fc trimer Ab with a (GGGGS)(3) linker in front of the second and third hinge regions). Here, we show that Fc tandem repeat antibodies bind to all the low-affinity Fcgamma receptors with very potent avidities and have greatly enhanced ADCC activity. T3-Ab is about 100 times more potent than the parental 1F5 chimeric antibody in terms of both Fcgamma receptor binding and exerted ADCC activity at a 50-100 times less concentration as compared with the parental antibody. Thus, Fc tandem repeat antibodies are anticipated to be candidates for anti-tumor therapeutics and useful tools to elucidate the biological roles of Fcgamma receptors.

FcγRs in health and disease

Genetic defects affecting the humoral immune response and especially the production of antibodies of the immunoglobulin G (IgG) isotype result in a heightened susceptibility to infections. Studies over the last years have demonstrated the crucial role of Fc-receptors for IgG (FcγRs) widely expressed on innate immune effector cells in mediating the protective function of IgG. During the last years, additional ligands interacting with FcγRs as well as additional receptors binding to IgG glycosylation variants have been identified. In this review, we discuss how the interaction of these different ligands with classical and novel Fcγ-receptors influences the immune response and which strategies microorganisms have developed to prevent them.

The other side of immunoglobulin G: suppressor of inflammation

Immunoglobulin G (IgG) molecules can have two completely opposite functions. On one hand, they induce proinflammatory responses and recruit innate immune effector cells during infection with pathogenic microorganisms or autoimmune disease. On the other hand, intravenous infusion of high doses of pooled IgG molecules from thousands of donors [intravenous IG (IVIG) therapy] represents an efficient anti-inflammatory treatment for many autoimmune diseases. Whereas our understanding of the mechanism of the proinflammatory activity of IgG is quite advanced, we are only at the very beginning to comprehend how the anti-inflammatory activity comes about and what cellular and molecular players are involved in this activity. This review will summarize our current knowledge and focus upon the two major models of either IVIG-mediated competition for IgG-triggered effector functions or IVIG-mediated adjustment of cellular activation thresholds used to explain the mechanism of the anti-inflammatory activity.

Antibody-Mediated Fcγ Receptor-Based Mechanisms of HIV Inhibition: Recent Findings and New Vaccination Strategies

The HIV/AIDS pandemic is one of the most devastating pandemics worldwide. Today, the major route of infection by HIV is sexual transmission. One of the most promising strategies for vaccination against HIV sexual infection is the development of a mucosal vaccine, which should be able to induce strong local and systemic protective immunity. It is believed that both humoral and cellular immune responses are needed for inducing a sterilizing protection against HIV. Recently, passive administration of monoclonal neutralizing antibodies in macaques infected by vaginal challenge demonstrated a crucial role of FcγRs in the protection afforded by these antibodies. This questioned about the role of innate and adaptive immune functions, including ADCC, ADCVI, phagocytosis of opsonized HIV particles and the production of inflammatory cytokines and chemokines, in the mechanism of HIV inhibition in vivo. Other monoclonal antibodies - non-neutralizing inhibitory antibodies - which recognize immunogenic epitopes, have been shown to display potent FcγRs-dependent inhibition of HIV replication in vitro. The potential role of these antibodies in protection against sexual transmission of HIV and their biological relevance for the development of an HIV vaccine therefore need to be determined. This review highlights the potential role of FcγRs-mediated innate and adaptive immune functions in the mechanism of HIV protection.

CD16/32-specific biotinylated 2.4G2 single-chain Fv complexed with avidin-FITC enhances FITC-specific humoral immune response in vivo in a CD16-dependent manner

Fcgamma receptors (FcgammaRs) play an essential role in the regulation of immune response due to their ability to bind immune complexes. Activating FcgammaRs may facilitate antigen presentation and dendritic-cell maturation, while in the late phase of the immune response, the inhibitory FcgammaRIIb may down-regulate B-cell activation upon cross-linking with activating receptors. In this study, we investigated the in vivo role of FcgammaRs on the modulation of humoral immune response. In order to get well-defined immune complexes that can bind to both the activating and the inhibitory FcgammaRs, we designed a mono-biotinylated single-chain fragment variable construct from the rat anti-mouse CD16/32 clone 2.4G2, linked to avidin-FITC, and tested its effect on the FITC-hapten-specific T-independent type 2 (TI-2) and T-dependent (TD) immune response. When injected intravenously in mice, the complex bound to a small portion of B220+, CD11b(high) and CD11c(high) cells and was localized in the spleen on marginal zone macrophages 15 min after treatment. When applied as a booster following primary immunization with TI-2 (FITC-dextran) or TD (FITC-keyhole limpet haemocyanin) antigens, the complex elevated the number of hapten-specific IgM/IgG-producing B cells. This effect was diminished in CD16KO mice, suggesting that the activating-type FcgammaRIII might be a key mediator of this mechanism.

FCGR3B copy number variation is not associated with lupus nephritis in a Chinese population

Copy number variation (CNV) is common in genomic regions encoding immune-related genes and can impact polygenic autoimmunity. FCGR3B CNV is associated with susceptibility to systemic autoimmunity in Caucasian populations. In this study, we examined FCGR3B CNV in patients with the autoimmune disease lupus nephritis (LN) in a Chinese population. The study includes 202 patients with histologically verified LN and 146 geographically matched healthy controls. To identify CNV at the FCGRB locus, quantitative polymerase chain reaction (PCR) was done with TaqMan( TM) probes and relative copy number was estimated with relative quantitative 2(-DeltaDeltaCt) method. The distribution of FCGR3B relative copy number did not differ significantly between the LN patients and the controls (1.17 +/- 0.42 for LN; 1.15 +/- 0.37 for controls; p = 0.627). The difference was still not significant when the data were stratified by gender. There was no significant difference when the LN patients were divided by pathological phenotype (proliferative LN compared with non-proliferative p = 0.511; AI < 12 compared with AI > or = 12, p = 0.401; and chronicity index (CI) < 4 compared with CI > or = 4, p = 0.058) or lupus disease activity index (SLEDAI < or = 10 compared with SLEDAI > 10, p = 0.996). The data suggest that FCGR3B CNV was not associated with LN development or progression in this Chinese population. These results were surprising given the strong in a Caucasian population.

A genome-wide association study identifies three new susceptibility loci for ulcerative colitis in the Japanese population

Ulcerative colitis is one of the principal forms of inflammatory bowel disease with complex manifestations. Although previous studies have indicated that there is a genetic contribution to the pathogenesis of ulcerative colitis, the genes influencing susceptibility to the disease have not been fully determined. To identify genetic factors conferring risk of ulcerative colitis, here we conducted a two-stage genome-wide association study and subsequent replication study using 1,384 Japanese individuals with ulcerative colitis and 3,057 control subjects. In addition to the expected strong association with the major histocompatibility complex (MHC) region, we identified three new susceptibility loci: the immunoglobulin receptor gene FCGR2A (rs1801274, P = 1.56 x 10(-12)), a locus on chromosome 13q12 (rs17085007, P = 6.64 x 10(-8)) and the glycoprotein gene SLC26A3 (rs2108225, P = 9.50 x 10(-8)). rs1801274 is a nonsynonymous SNP of FCGR2A that is reported to have a critical effect on receptor binding affinity for IgG and to be associated with other autoimmune diseases. Our findings provide insight into the molecular pathogenesis of ulcerative colitis.

Role of Fc Receptors as a therapeutic target

It has been forty years since the discovery of Fc Receptors and their function. Fc Receptors include the IgG receptors (FcgammaR), high-affinity IgE receptor (FcepsilonRI), IgA and IgA/IgM receptors, and neonatal Fc receptor for IgG (FcRn). In particular, the FcgammaRs have been well known to play an important role in many biologic processes including those associated with the response to infection and cancer as well as in the pathogenesis of immune-mediated diseases. Both positive and negative regulatory function has ascribed to Fc receptors and FcgammaRs in particular which serve to establish a threshold for immune cell activation. In other cases, Fc receptors such as FcRn possess a novel structure and function by playing a major role in the transport of IgG across polarized epithelial barriers at mucosal surfaces and in the regulation of IgG half-life. These diverse functions highlight the potential effectiveness of targeting Fc receptors for therapeutic purposes. This review summarizes new information available in the therapeutic applications of this biology.

Autoimmunity triggers in the NOD mouse: a role for natural auto-antibody reactivities in type 1 diabetes

The nonobese diabetic mouse (NOD) is widely used as a model to study human type 1 diabetes (T1D). In the NOD mouse T1D is a T cell-mediated autoimmune disease of complex etiology in which B cells play an essential role. One of the major unresolved issues in T1D is the genetic and/or environmental factors that trigger the autoimmune reaction. In the NOD mouse, as in humans, auto-antibodies to pancreatic islets are present at early ages and are highly correlated to diabetes progression, but their etiological role has long been disputed. NOD auto-antibodies have the characteristics of a natural repertoire, and B1 cells, the main natural antibody producers, exhibit functional differences in this strain that could have consequences for disease determination. Using a genetic approach, we propose to test if the NOD natural auto-antibody repertoire includes innate reactivities that participate in diabetes pathogenesis by promoting insulitis initiation.

Association of copy number variation in the FCGR3B gene with risk of autoimmune diseases

Copy number variation (CNV) in the human genome is an important determinant of susceptibility to autoimmune diseases. Many autoimmune diseases share similar clinical and pathogenic features. Thus, CNVs of genes involved in immunity may serve as shared determinants of multiple autoimmune diseases. Here, we determined the association between CNV in the gene encoding FCGR3B with the risk of developing autoimmune diseases and whether the observed associations are modified by the CNV in CCL3L1 (CC chemokine ligand 3-like 1), a gene encoding a potent chemokine. In a cross-sectional study of 774 subjects, we estimated FCGR3B and CCL3L1 gene copy number in 146, 158 and 61 subjects with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and primary Sjögren's syndrome (SS), respectively, and 409 healthy controls. The median gene dose of FCGR3B in the study population was two. FCGR3B copy number < or >2 was associated with an increased risk of SLE and primary SS but not RA. This association was mostly evident in subjects who also had two copies of CCL3L1. Thus, our data suggest that epistatic interactions between CNV of FCGR3B and CCL3L1, two immune response genes, may influence phenotypically related autoimmune diseases.

Susceptibility for ischemic stroke in Korean population is associated with polymorphisms of the Fc gamma receptor IIA

Fc gamma receptor IIA could influence atherogenic processes through the production of superoxide anions, cytokines, and proteolytic enzymes as well as by oxidation of lipoproteins and enhancement of foam cell formation. In this study, we performed an interaction analysis between FCGR2A polymorphisms and ischemic stroke using direct DNA sequencing after the selection of Fc gamma receptor IIA gene based on genome-wide association study. Four of the FCGR2A polymorphisms, rs7511868 [odds ratio (OR) = 3.21; P = 0.027], rs6427595 (OR = 3.12; P = 0.008), rs7512140 (OR = 5.71; P = 0.002), and rs6696854 (OR = 3.65; P = 0.004) were significantly associated with ischemic stroke. These four polymorphisms still showed significant association after stratification analysis using the Mantel-Haenszel method. In the multivariate logistic regression, the adjusted OR estimates for rs6427595, rs7512140, and rs6696854 were 3.04 (P = 0.016), 4.84 (P = 0.015), and 3.80 (P = 0.006), respectively. The diplotype consisting of two homozygous haplotypes (H2 = AAAC) was significantly associated with ischemic stroke (OR = 17.39; P < 0.001). These results suggest that FCGR2A polymorphisms may be associated with a genetic susceptibility to ischemic stroke in a Korean population.

FCGR2B gene polymorphism rather than FCGR2A, FCGR3A and FCGR3B is associated with anti-GBM disease in Chinese

BACKGROUND

The Fcgamma receptors play important roles in anti-glomerular basement membrane antibody disease (anti-GBM disease) in animal models, and FCGR gene polymorphisms have been reported to be associated with numerous human autoimmune diseases. We aimed to clarify the genetic association of FCGR gene polymorphisms with anti-GBM disease in Chinese patients.

METHODS

A total of 48 patients with anti-GBM disease and 225 geographically and ethnically matched healthy controls were involved. Genotyping of the previously identified polymorphisms FCGR2A131H/R (rs1801274), FCGR2B 232I/T (rs1050501) and FCGR3A176F/V (rs396991) were detected by the TaqMan genotyping assay and FCGR3B NA1/2 by the PCR-sequence specific primer (SSP). Allele type, genotype and haplotype of identified polymorphisms were analysed between patients and controls.

RESULTS

Our results revealed that FCGR2A131H/R, FCGR3A176F/V and FCGR3B NA1/2 were not associated with anti-GBM disease. The frequency of the FCGR2B 232T allele (30.2% versus 15.6%, corrected P = 0.00028, 95% CI: 1.42-3.89) and genotypes of I232T (60.4% versus 31.1%, corrected P = 0.0004, 95% CI: 1.78-6.43) was significantly increased in patients compared with controls.

CONCLUSION

The present study demonstrates the genetic association of polymorphism of FCGR2B (I232T) with susceptibility to anti-GBM disease in Chinese.

[Molecular mechanisms of B lymphocyte depletion by CD20 immunotherapy]

Anti-CD20 antibody immunotherapy effectively treats non-Hodgkin's lymphoma and autoimmune disease. However, the cellular and molecular pathways for B cell depletion remain undefined and the in vivo effect of immunotherapy on tissue B cells and their subsets is generally unknown. To identify the mechanisms for B cell depletion in vivo, a new mouse model for anti-CD20 immunotherapy was developed using a panel of twelve mouse anti-mouse CD20 monoclonal antibodies. Anti-CD20 antibodies rapidly depleted the vast majority of circulating and tissue B cells in an isotype-restricted manner that was completely dependent on effector cell Fc receptor expression. B cell depletion utilized FcgammaRI-, FcgammaRIII- and FcgammaRIV-dependent pathways, while B cells were not eliminated in FcR common gamma chain-deficient mice. Monocytes were the dominant effector cells for B cell depletion, with no demonstrable role for T or NK cells. Although most anti-CD20 antibodies activated complement in vitro, B cell depletion was completely effective in mice with genetic deficiencies in C3 complement components. The considerable factors that determine the effectiveness of anti-CD20 immunotherapy are following: the expression level of CD20 on B cell surface; the dosage of anti-CD20 mAb; the association of Fcgamma receptor with the isotype of the antibies; B cell subpopulations within different tissues. These findings have important clinical implications for anti-CD20 and other antibody-based therapies.

Anti-ribosomal phosphoprotein autoantibody triggers interleukin-10 overproduction via phosphatidylinositol 3-kinase-dependent signalling pathways in lipopolysaccharide-activated macrophages

Anti-ribosomal phosphoprotein autoantibodies have been shown to be significantly associated with multiple manifestations of systemic lupus erythematosus (SLE). High levels of interleukin-10 (IL-10) have been demonstrated to contribute to lupus susceptibility and severity. In this study, we investigated the molecular mechanisms of anti-ribosomal phosphoprotein monoclonal antibody (anti-P mAb)-induced autoimmune responses. Anti-P mAb promoted IL-10 overproduction in a dose- and time-dependent manner in both lipopolysaccharide (LPS)-activated RAW 264.7 cells and primary human macrophages. Anti-P mAb enhanced phosphorylation of Akt (PKB; protein kinase B), extracellular signal regulated kinase 1/2 (ERK1/2) and c-Jun NH2-terminal kinase 1/2 (JNK1/2), while phosphorylation of p38 remained unaltered. Furthermore, anti-P mAb decreased glycogen synthase kinase 3 (GSK3) activity and reduced the phosphorylation of I kappaB alpha in LPS-activated macrophages. The Syk, phosphatidylinositol 3-kinase (PI3K), protein kinase C (PKC), JNK and ERK signalling pathways involved in anti-P mAb-triggered IL-10 secretion were also confirmed using various pharmacological inhibitors. In addition, nuclear factor (NF)-kappaB had negative regulatory effects on anti-P mAb-triggered IL-10 secretion. Using reporter plasmids containing the nuclear factor binding sites of NF-kappaB, cAMP-enhanced activation protein 1 (AP-1), serum response element (SRE) or cyclic AMP response element (CRE), treatment of anti-P mAb led to activation of the corresponding factors that bind to the AP-1 site, SRE and CRE in the LPS-activated macrophages. Furthermore, by transfection with reporter plasmids bearing various lengths of the IL-10 promoter, the AP-1 binding site, SRE and CRE were shown to be required for anti-P mAb-induced effects. Collectively, our results provide a molecular model for anti-P mAb-induced IL-10 overproduction in LPS-activated macrophages, which may play a role in the pathogenesis of SLE.

The role of the anaphylatoxins in health and disease

The anaphylatoxin (AT) C3a, C5a and C5a-desArg are generally considered pro-inflammatory polypeptides generated after proteolytic cleavage of C3 and C5 in response to complement activation. Their well-appreciated effector functions include chemotaxis and activation of granulocytes, mast cells and macrophages. Recent evidence suggests that ATs are also generated locally within tissues by pathogen-, cell-, or contact system-derived proteases. This local generation of ATs is important for their pleiotropic biologic effects beyond inflammation. The ATs exert most of the biologic activities through ligation of three cognate receptors, i.e. the C3a receptor, the C5a receptor and the C5a receptor-like, C5L2. Here, we will discuss recent findings suggesting that ATs regulate cell apoptosis, lipid metabolism as well as innate and adaptive immune responses through their impact on antigen-presenting cells and T cells. As we will outline, such regulatory functions of ATs and their receptors play important roles in the pathogenesis of allergy, autoimmunity, neurodegenerative diseases, cancer and infections with intracellular pathogens.

Fcgamma receptors: structure, function and role as genetic risk factors in SLE

Over 30 years ago, receptors for the Fc region of IgG (FcgammaR) were implicated in the pathogenesis of systemic lupus erythematosus (SLE). Since those pioneering studies, our knowledge of the structure and function of these FcgammaRs has increased dramatically. We now know that FcgammaR contributes to the regulation of acquired immunity and to the regulation of innate immune responses where FcgammaRs act as specific receptors for innate opsonins (CRP and SAP). Our understanding of the genomic architecture of the genes encoding the FcgammaR has also witnessed remarkable advances. Numerous functionally relevant single-nucleotide polymorphism (SNP) variants and copy number (CN) variants have been characterized in the FcgammaR 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 FcgammaR in relation to SLE, including consideration of the role of genetic variants in FcgammaR 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.