Fcγ receptors in autoimmunity and end-organ damage

Introduction

Antibodies are potent inducers of inflammation. A consistent feature of systemic lupus erythematosus (SLE) is the presence of autoantibodies and complement-fixing immune complexes (ICs) resulting in inflammatory lesions in multiple organ systems. Fc receptors, the receptors for IgG antibodies, are a group of transmembrane glycoproteins which contribute to host defense against pathogens but, not surprisingly, can also inflict significant tissue damage when antibodies are directed against host antigens as in SLE. Fc receptors are largely expressed in hematopoietic cells and mediate a wide array of immune functions such as the recruitment and activation of inflammatory cells, degranulation, antibody-dependent cell-mediated cytotoxicity (ADCC), phagocytosis, enhancement of antigen presentation, regulation of B cell antibody production, and IC clearance. Fc receptors exist for all the different immunoglobulin classes (IgA, IgD, IgE, IgG, IgM). Although recent publications showed roles for FcαRI and FcɛRIα in lupus nephritis, our discussion will focus on the Fc receptors for IgG-the Fcγ receptors (FcγRs)-as studies in mouse models suggest primary roles of FcγRs in development of SLE and functional polymorphisms in these receptors are associated with disease susceptibility in humans.

FcγRs structure

The extracellular region of FcγRs contains two or three extracellular immunoglobulin-like domains, which is a common feature of the immunoglobulin superfamily. In humans, three different classes of FcγRs exist-FcγRI, FcγRII, and FcγRIII-that are defined by their structural differences, signaling capacity, and variable affinity for different IgG subclasses. Based on their role in inhibition or exacerbation of immune functions, the FcγRs are also divided into inhibitory receptors (FcγRIIB) or activating receptors (FcγRI, FcγRIIA, FcγRIIC, and FcγRIIIB). Humans also express additional FcγRs, FcRn, FcRL5 and TRIM21, which bind IgG once internalized. The FcγRI (CD64) and FcγRIII (CD16) are expressed as oligomeric complexes with γ (monocytes and macrophages), or ζ or β (uniquely for human FcγRIIIA in NK cells and mast cells, respectively) chain. The adaptor chains contain the cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM), which links FcγRs to protein tyrosine-based intracellular signaling. Partnering with the γ chain is also required for the surface expression of both FcγRI and FcγRIII through amino acid interactions between charged residues present in the transmembrane domain of the adaptor and FcγR. On the other hand, members of the FcγRII class (CD32) exist as single polypeptide transmembrane receptors containing either an activating (FcγRIIA) or inhibitory (FcγRIIB) signaling motif within their own cytoplasmic tail. A more recently recognized FcγRIIC is a cross-over between FcγRIIA and IIB that contains an intracellular ITAM containing tail related to FcγRIIA and an extracellular domain homologous to FcγRIIB. Genetic deletion of FcγRs in mice has revealed primary roles for these receptors in a number of autoimmune diseases including SLE. However, the repertoire of FcγRs between human and mice differ in some important aspects, which should be considered when extrapolating data from mice models to human biology. Both humans and mice share a structurally common FcγRI, FcγRIIB, and FcγRIII but they differ in their repertoire of other FcγRs. Mice express the species-specific activating FcγRIV and lack the uniquely human activating FcγRIIIB and FcγRIIA ( Fig. 22.1 ). The mouse FcγRIV and FcγRIII are considered orthologs of the human FcγRIIIA and FcγRIIA respectively, based on sequence similarity in their extracellular domain. The repertoire of FcγRs expressed on specific cell types can also differ between humans and mice ( Fig. 22.1 ) . For example, human monocytes/macrophages and neutrophils express FcγRIIA, while these cell types in mice express the species-specific FcγRIV. Moreover, human platelets express FcγRIIA, while mouse platelets do not express any FcγRs. FcγRs are defined as high or low-affinity receptors based on their affinity for monomeric IgG versus avidity for oligomeric ICs. Affinity values for low affinity FcγRs varies considerably between studies as interaction with monomeric IgG has very rapid “on” and “off” rates. Nonetheless, a study conducted to assess a range of affinities of FcγRs for IgG shows that the affinity of FcγRI for IgG1-4 is in the nanomolar range compared to the micromolar range of the low-affinity receptors. Notably, both high and low-affinity receptors bind ICs with high avidity. In general, FcγRIIB has the lowest affinity for all IgG classes compared to all other FcγRs, which may be biologically important as the ITIM-driven inhibitory signals through FcγRIIB maybe most needed only when IC concentrations increase during a normal immune response.

IgG and FcγR interactions

Mirroring the complex organization of FcγRs and IgG are also subdivided into four classes that exhibit different patterns of affinity for their receptors. This topic was revisited in a study where the affinity of different IgG isotypes for all four FcγRs and known allelic variants were compared in parallel in vitro. The distinct affinity pattern of the different IgG subclasses for FcγRs may account for their variable pathogenicity both in mice and humans. In human SLE, IgG3 and IgG1 are the dominant isotypes involved in the anti-DNA response and preferentially engage FcγRIIA (ortholog of FcγRIV) and FcγRIIIA. Anti-dsDNA IgG3 antibody appears to be more specifically involved in SLE among other connective tissue diseases, and in human lupus nephritis, IgG3 and IgG1 renal deposition is most commonly observed. However, IgG2 deposition is also frequently observed in lupus nephritis and interestingly intense IgG2 deposition in the kidney was associated with the FcγRIIA R131 allele, a polymorphism shown to be associated with susceptibility to SLE. The FcγRIIA R131 polymorphism exhibits a decrease in affinity for IgG2 that may result in impaired clearance of IgG2 containing IC clearance, a subsequent increase in IC deposition, and the observed increase in lupus nephritis (refer to section “FcγR Polymorphisms and Copy Number Variation in Lupus”).

The Fc fragment consists of the carboxy-terminal constant domains of the IgG heavy chains, which contain an N-glycan at Asn 297 that is essential for binding to FcγRs (reviewed in Ref. [29]). The differences in the degree of galactosylation, fucosylation, and sialylation can dictate selectivity in binding to particular classes of Fc receptors. Nuclear magnetic resonance spectroscopy studies have shown that glycan branches of an IgG are highly mobile and that their mobility allows for enzyme modification of the glycan termini.

Sugars at the N-glycan termini can influence the Fc structure and in turn the FcγR binding. Thus, these N-glycan modifications can give rise to a range of conformations with different affinities for the FcγRs. This may be an important regulatory mechanism in order to avoid high-affinity conformations, which may be undesirable. Notably, IgGs with altered glycosylation states (often reduced) are detected in patients with SLE and rheumatoid arthritis, and also in mouse models of autoimmunity. Altered glycosylation, specifically higher exposure of fucosyl residues by immobilized IgG complexes, correlated with SLE disease activity. In contrast, engineered in vivo IgG sialylation decreases kidney and joint injury in mouse models of immune complex-mediated glomerulonephritis (nephrotoxic nephritis model) and arthritis (KBx/N model), respectively, by enhancing IgG affinity to the inhibitory FcγRIIB and thereby attenuating autoantibody-mediated inflammation. It could also potentially reflect a reduction in the interaction of activating FcγRs with renal deposited sialylated IgG and subsequent leukocyte recruitment and activation albeit this has to be formally investigated. This study opens a very interesting therapeutic avenue in autoimmune diseases.

FcγRs and complement

The complement system is a major effector mechanism in innate immune responses. Complement can be activated via the classical, alternative, or mannose-binding lectin pathways, each of which converges on a central C3 component. Deficiency in components of the complement cascade and in particular the classical pathway has been strongly associated with autoimmunity and SLE. Genetic variants in factor H, a regulatory protein of the alternative pathway, have also been associated with SLE susceptibility. In the classical complement pathway, the first component of the cascade, C1q, binds to and is activated by the Fc-portion of immunoglobulins in ICs. C1q has both protective and pro-inflammatory roles that have been shown to potentially contribute to the development of SLE, as discussed in detail elsewhere. Several of C1q’s pro-inflammatory effects are manifested directly or indirectly through FcγRs. For example, C1q may aid in the deposition of immune complexes within the vasculature, which in turn triggers FcγR-mediated immune cell recruitment. Complement C1q triggers the production of C3b, which subsequently catalyzes C5 to its active form C5a and these components have been shown to directly modulate FcγR functions. For instance, C5a binding to C5aR on macrophages increases the transcriptional expression of the activating FcγRIII and downregulates the inhibitory FcγRIIB. In turn, FcγRs induce the production of C5a, as shown in vitro following cross-linking on macrophages, and in vivo in a model of autoimmune hemolytic anemia. It is notable that IgG can also activate the alternate pathway of complement via binding C3b, which can be amplified by C5a-dependent neutrophil activation. This is of interest as in mouse models of K/BxN arthritis, FcγRIII and C5aR deficient mice are equally protected from disease development whereas C1q deficient mice are not. A role for the alternative pathway is also suggested by the finding that Factor B deficient mice are protected in this model (reviewed in Ref. [55]). In models that require C5aR but not complement C3, C5a may also be generated by a proteolytic pathway independent of C4b. For example, C5a can be generated by thrombin, a serine protease in the coagulation pathway. Thus, FcγRs and C5aRs may play codominant roles in IC-induced inflammatory responses. Clarification of the mechanisms underlying their collaboration may thereby uncover critical steps involved in autoimmune-mediated end-organ damage. Nevertheless, the utility of complement blockade therapy in SLE remains controversial. Despite interesting results in lupus mice models (reviewed in Ref. [57]), eculizumab, the only commercially available anti-C5a antibody to date, was only used as a rescue treatment in refractory SLE patients with thrombotic microangiopathic (TMA) features and showed some benefits. Only one case-report describes a patient with refractory class IV lupus nephritis, without TMA, who received eculizumab with an improvement in kidney function. On the other hand, a recent Phase II clinical trial demonstrated that a novel oral C5a receptor (CD88) inhibitor (avacopan, formerly CCX168) allowed a reduction or complete withdrawal of steroids from induction protocols in patients with Anti-Neutrophil Cytoplasmic Antibody (ANCA)-associated vasculitis. It would be of interest to determine whether this inhibitor affects FcγR-mediated leukocyte functions in preclinical models of lupus.