FcgammaRII and multi-system autoimmune disease

Impacts of FcγRIIB and FcγRIIIA gene polymorphisms on systemic lupus erythematous disease activity index

OBJECTIVE

Systemic lupus erythematous (SLE) disease is a chronic autoimmune disease with unknown etiology that can involve different organs. Polymorphisms in Fcγ receptors have been identified as genetic factors in susceptibility to SLE. This study was aimed to investigate effects of two single nucleotide polymorphisms (SNPs) within FcγRIIB and FcγRIIIA genes on systemic lupus erythematous disease activity index (SLEDAI) in an Iranian population.

RESULTS

Our findings indicated TT and GG genotypes were the common genotypes of FcγRIIB and FcγRIIIA SNPs in SLE patients, respectively. There were no significant differences in genotype and allele frequencies of FcγRIIB and FcγRIIIA SNPs in SLE and healthy subjects. However, the frequencies of genotypes and alleles of FcγRIIB and FcγRIIIA SNPs were significantly associated with some clinical manifestations used to determine SLEDAI (P < 0.001-0.5).

Crystal structure of Fcγ receptor I and its implication in high affinity γ-immunoglobulin binding

Fcγ receptors (FcγRs) play critical roles in humoral and cellular immune responses through interactions with the Fc region of immunoglobulin G (IgG). Among them, FcγRI is the only high affinity receptor for IgG and thus is a potential target for immunotherapy. Here we report the first crystal structure of an FcγRI with all three extracellular Ig-like domains (designated as D1, D2, and D3). The structure shows that, first, FcγRI has an acute D1-D2 hinge angle similar to that of FcεRI but much smaller than those observed in the low affinity Fcγ receptors. Second, the D3 domain of FcγRI is positioned away from the putative IgG binding site on the receptor and is thus unlikely to make direct contacts with Fc. Third, the replacement of FcγRIII FG-loop ((171)LVGSKNV(177)) with that of FcγRI ((171)MGKHRY(176)) resulted in a 15-fold increase in IgG(1) binding affinity, whereas a valine insertion in the FcγRI FG-loop ((171)MVGKHRY(177)) abolished the affinity enhancement. Thus, the FcγRI FG-loop with its conserved one-residue deletion is critical to the high affinity IgG binding. The structural results support FcγRI binding to IgG in a similar mode as its low affinity counterparts. Taken together, our study suggests a molecular mechanism for the high affinity IgG recognition by FcγRI and provides a structural basis for understanding its physiological function and its therapeutic implication in treating autoimmune diseases.

Is FCGR2A a susceptibility gene to systemic lupus erythematosus in Chinese?

Recent genome-wide association scans and replication studies reinforce that FCGR2A is a susceptibility gene in systemic lupus erythematosus (SLE) in Caucasians. However, previous case control studies denied such conclusions in Chinese people. Besides genetic heterogeneity among different ethnicities, copy number variation (CNV), non-homogenous phenotypes and insufficient power may be confounders. We performed a case control study with 1066 Chinese (589 SLE patients and 477 healthy controls) and a meta-analysis based on 2328 SLE patients and 2313 healthy controls. FCGR2A CNV and FCGR2A131H/R [rs1801274] were detected by TaqMan assays. No variation of copy numbers of FCGR2A gene was found in Chinese. A further case control study suggested a dose-response character for FCGR2A131H/R and it affected disease activity, severity and prognosis. Finally, meta-analysis indicated FCGR2A that was a susceptibility gene to SLE in Chinese with an odds ratio of 1.094 and population attributable risk proportion of (PARP) 0.031. By an integrative strategy, we validate that FCGR2A bears no population-specific CNV. FCGR2A131H/R contributes to SLE susceptibility in Chinese, and affects disease activity, severity and prognosis. The undetected association in Chinese derives from under-power rather than any methodological obstacle due to CNV or population-specific genetic effect.

Inhibition of destructive autoimmune arthritis in FcgammaRIIa transgenic mice by small chemical entities

The interaction of immune complexes with the human Fc receptor, FcgammaRIIa, initiates the release of inflammatory mediators and is implicated in the pathogenesis of human autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus, so this FcR is a potential target for therapy. We have used the three-dimensional structure of an FcgammaRIIa dimer to design small molecule inhibitors, modeled on a distinct groove and pocket created by receptor dimerization, adjacent to the ligand-binding sites. These small chemical entities (SCEs) blocked immune complex-induced platelet activation and aggregation and tumor necrosis factor secretion from macrophages in a human cell line and transgenic mouse macrophages. The SCE appeared specific for FcgammaRIIa, as they inhibited only immune complex-induced responses and had no effect on responses to stimuli unrelated to FcR, for example platelet stimulation with arachidonic acid. In vivo testing of the SCE in FcgammaRIIa transgenic mice showed that they inhibited the development and stopped the progression of collagen-induced arthritis (CIA). The SCEs were more potent than methotrexate and anti-CD3 in sustained suppression of CIA. Thus, in vitro and in vivo activity of these SCE FcgammaRIIa receptor antagonists demonstrated their potential as anti-inflammatory agents for autoimmune diseases involving immune complexes.

The dendritic cell subtype-restricted C-type lectin Clec9A is a target for vaccine enhancement

A novel dendritic cell (DC)-restricted molecule, Clec9A, was identified by gene expression profiling of mouse DC subtypes. Based on sequence similarity, a human ortholog was identified. Clec9A encodes a type II membrane protein with a single extracellular C-type lectin domain. Both the mouse Clec9A and human CLEC9A were cloned and expressed, and monoclonal antibodies (mAbs) against each were generated. Surface staining revealed that Clec9A was selective for mouse DCs and was restricted to the CD8(+) conventional DC and plasmacytoid DC subtypes. A subset of human blood DCs also expressed CLEC9A. A single injection of mice with a mAb against Clec9A, which targets antigens (Ags) to the DCs, produced a striking enhancement of antibody responses in the absence of added adjuvants or danger signals, even in mice lacking Toll-like receptor signaling pathways. Such targeting also enhanced CD4 and CD8 T-cell responses. Thus, Clec9A serves as a new marker to distinguish subtypes of both mouse and human DCs. Furthermore, targeting Ags to DCs with antibodies to Clec9A is a promising strategy to enhance the efficiency of vaccines, even in the absence of adjuvants.