Human receptors for immunoglobulin G: Key elements in the pathogenesis of rheumatic disease

Receptors for immunoglobulin constitute the link between humoral and cellular aspects of the immune cascade and play an integral part in the process by which opsonized material, both foreign and endogenous, is identified and destroyed. Recent advances in our understanding of the physiology of these cell-based binding sites for antibody, termed Fc receptors (FcR) because they interact with the constant region (Fc portion) of the immunoglobulin heavy chain, have highlighted their importance in rheumatic disease. While specific receptors recognize each antibody class, this review will focus on human receptors for IgG, FcγR. FcγR, a diverse group of receptors expressed as hematopoetic cell surface molecules, often present as stimulatory and inhibitory pairs. Triggering stimulatory FcγR initiates phagocytosis, antibody-dependent cell-mediated cytotoxicity (ADCC), and the release of inflammatory mediators such as cytokines, reactive oxidants, and proteases by phagocytes. Experiments in mice deficient in stimulatory FcγR have shown that these receptors play an essential role in the initiation of type II and type III hypersensitivity reactions. In vivo murine studies have also demonstrated that stimulatory FcγR are both necessary and sufficient to trigger the Arthus reaction, autoimmune glomerulonephritis and cytopenia, and antigen-induced arthritis (1). Given such protean and potent reactivity, how is cell activation triggered by FcγR modulated to respond appropriately to variations in environmental stimuli? Recent work has suggested that inhibitory FcγR, which modulate thresholds for activation and terminate stimulating signals, are a key element in the regulation of effector function (2, 3). When coaggregated with stimulatory receptors on the cell surface, inhibitory FcγR can abolish cellular signaling, while when self aggregated, they do not trigger effector functions. Inhibitory FcγR play a central role in afferent and efferent immune responses as negative regulators of both antibody production and immune complex–triggered activation. Mice deficient in inhibitory FcγR demonstrate enhanced antibody responses, a proclivity toward autoimmunity, and increased inflammation in all 3 types of antibody-mediated hypersensitivity reactions (2, 3). Given that inhibitory and stimulatory FcγR are often coexpressed, the effector response to a specific stimulus in a particular cell represents the balance between stimulatory and inhibitory signals. FcγR are encoded by members of the immunoglobulin superfamily of genes. In humans, 8 genes for FcγR are clustered on the long arm of chromosome 1 (1q21–23). Extensive structural diversity among FcγR family members leads to differences in binding capacity, distinct signal transduction pathways, and cell type–specific expression patterns (for review, see refs. 1,4-6). Such diversity allows IgG complexes to activate a broad program of cell functions relevant to autoimmunity, inflammation, and host defense against microbes and cancer. In addition, allelic variation within genes provides an inherited basis for predisposition to inflammatory diseases. This review describes structure–function relationships in human FcγR and provides a framework for understanding how FcγR may contribute to susceptibility, pathogenesis, and therapeutic interventions in immune complex–mediated diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and vasculitis. FcγR capable of triggering cellular activation possess intracellular activation motifs, termed immunoreceptor tyrosine-based activation motifs (ITAMs), similar to those of B cell receptors (BCR) and T cell receptors (TCR) (7-9). Stimulatory FcγR are typically multichain receptors composed of a ligand-binding α subunit, which confers ligand specificity and affinity, and associated signaling subunits with ITAMs in the cytoplasmic domains (Figure 1). Schematic representation of the human Fcγ receptor (FcγR) family members. FcγR α chains contain 2 or 3 disulfide-linked immunoglobulin-like extracellular domains (ellipses) that mediate binding to IgG. The cytoplasmic domains of FcγR or their associated subunits are responsible for signal transduction. FcγRIIIb is the only FcγR that lacks a cytoplasmic tail. FcγRI and FcγRIIIa are multichain receptors that associate with immunoreceptor tyrosine-based activation motif (ITAM; black cylinders)–bearing γ- or ζ-chain dimers to mediate positive signaling. FcγRIIa and FcγRIIc are single-chain stimulatory receptors containing ITAMs in their cytoplasmic tails. FcγRIIb1 and FcγRIIb2 are single-chain inhibitory receptors containing immunoreceptor tyrosine-based inhibitory motifs (ITIMs; white cylinders) in their cytoplasmic tails. TM = transmembrane; GPI = glycosyl phosphatidylinositol. FcγR α chains are predominantly present on myeloid cells. The α subunits, encoded for by 7 genes, are transmembrane molecules that share the structural motif of 2 or 3 extracellular immunoglobulin-like domains, but vary in their affinity for IgG and in their preferences for binding different IgG subclasses (IgG1, IgG2, IgG3, and IgG4). There are also allelic variations in the ligand-binding region of specific FcγR that influence the ability to bind certain IgG subclasses and that dramatically alter the responses of phagocytes to IgG-opsonized antigens (10-13). The transmembrane domains of the α subunits contain a basic residue to mediate the physical interaction with associated signaling chains required for efficient expression and signal transduction. The two multichain FcγR isoforms are termed FcγRI, a high-affinity receptor for IgG that binds monomeric IgG, and FcγRIIIa, an intermediate-affinity receptor that binds only multivalent IgG. Homodimeric γ chains are transducing modules for FcγRI and FcγRIIIa (Figure 1). Heterodimers of γ–ζ chains or ζ–ζ chain homodimers can also transduce signals through FcγRIIIa in human natural killer (NK) cells. In addition to multichain receptors, there are 3 other types of activating FcγR, all unique to humans. FcγRIIa and FcγRIIc are single-chain, low-affinity receptors that include an extracellular ligand-binding domain and ITAMs in the cytoplasmic domain (Figure 1). Both single-chain receptors, FcγRIIa and FcγRIIc, are low-affinity receptors. The other isoform, FcγRIIIb, has neither an ITAM nor a transmembrane domain, but is maintained in the plasma membrane outer leaflet by a glycosyl phosphatidylinositol anchor (Figure 1). Biologic responses triggered by stimulatory FcγR appear to depend more on the cells that express the receptor than on the receptor itself (Figure 2). For example, macrophages, the most efficient professional phagocytes, express FcγRI, FcγRIIa, and FcγRIIIa, all of which initiate the same responses in these cells, namely, internalization, respiratory burst, and secretion of inflammatory cytokines, proteases, and prostaglandins. Conversely, when expressed on different cells, a particular receptor isoform triggers cell type–specific function. FcγRIIa trigger respiratory burst and internalization by neutrophils, while on platelets they trigger aggregation and degranulation. On dendritic cells, ITAM-bearing FcγR mediate endocytic transport of antigen–antibody complexes and induce efficient processing and presentation of antigen (14) (Figure 2). Activating FcγR on mast cells trigger the secretion of tumor necrosis factor α (TNFα), which has been shown to contribute to the pathogenesis of autoantibody-mediated vasculitis (15). Cellular distribution and functions of FcγR. Shown are hematopoetic cells expressing one or more isoforms of FcγR: polymorphonuclear leukocytes (PMN), monocytes and macrophages (MØ), dendritic cells (DC), mast cells (MC), B lymphocytes (B), platelets, and natural killer (NK) cells. The effects of FcγR engagement by immune complexes are cell specific. ADCC = antibody-dependent cell-mediated cytotoxicity; TNF-α = tumor necrosis factor α; IL-1 = interleukin-1; Ag = antigen; IFN-γ = interferon-γ; Ab = antibody (see Figure 1 for other definitions). Inhibitory FcγR, FcγRIIb, are single-chain, low-affinity receptors with extracellular domains highly homologous to their activating counterparts and cytoplasmic domains containing an immunoreceptor tyrosine-based inhibitory motif (ITIM) (16) (Figure 1). They are encoded for by a single gene on chromosome 1q23–24. Alternative splicing generates 2 isoforms, FcγRIIb1 and FcγRIIb2, which differ only in their intracytoplasmic regions (17). FcγRIIb1 contains an insertion of 19 amino acids that significantly alters receptor function. FcγRIIb are widely expressed on hematopoetic cells: FcγRIIb1 on B lymphocytes and FcγRIIb2 on myeloid cells (Figure 2). Neither isoform can trigger cell activation. Instead, both isoforms of FcγRIIb, when coaggregated with ITAM-bearing receptors, are negative regulators of activation. In addition, FcγRIIb2 participates in endocytosis of multivalent ligands by phagocytes and antigen-presenting cells, while the intracytoplasmic insertion in FcγRIIb1 inhibits internalization (18). FcγRIIb can modulate cell activation by stimulatory FcγR as well as responses triggered by BCR, TCR, and FcR for IgE (19). However, in order to inhibit cell activation, FcγRIIb must be coaggregated with ITAM-expressing receptors by a multivalent ligand, and cell activation must be triggered by the receptors that are coaggregated with FcγRIIb (20). For example, FcγRIIb coaggregation with FcγRIIa by IgG-opsonized particles blocks phagocytosis, and FcγRIIb coligation to BCR by antibody–antigen complexes inhibits B cell proliferation and antibody production (21, 22). Thus, FcγRIIb-mediated negative regulation of ITAM-dependent cell activation endows IgG-containing immune complexes with the capacity to regulate B cells and inflammatory cells. The balance between stimulatory and inhibitory input determines cellular response. Stimulatory FcγR. Effector cell activation is initiated when FcγR are clustered at the cell surface by multivalent antigen–antibody complexes. Monovalent ligand binding is insufficient to generate a signal. Stimulatory FcγR have no intrinsic enzymatic activity, but are associated with membrane-anchored Src family kinases. The presence of 2 YxxL motifs separated by 7 variable residues (or 12 in the case of FcγRIIa and FcγRIIc) in the signaling subunit of activating FcγR is necessary for docking the protein tyrosine kinase syk and for initiation of positive signaling. Tyrosine kinases phosphorylate many intracellular substrates, including phospholipid kinases, phospholipases, adapter molecules, and cytoskeletal proteins. Activation of phospholipase C and phosphatidylinositol-3 kinase by syk leads to the production of phosphoinositol messengers and a sustained increase in cytoplasmic Ca2+ (23) (Figure 3). Recruitment of the adapter protein shc allows signals triggered by FcγR to reach the nucleus via the Ras pathway, leading to phosphorylation of mitogen-activated protein kinase, activation of transcription factors, and induction of gene expression (24). These critical signaling molecules are potential targets for pharmacologic inhibition of immune complex–initiated inflammation. Signaling pathways triggered by coaggregation of positive- and negative-signaling FcγR. Crosslinking of stimulatory FcγR leads to 1) phosphorylation (P) of the tyrosines (Y) in the ITAM by Src kinases, 2) recruitment of the SH2-domain tyrosine kinase Syk, and 3) activation of phosphatidylinositol-3 kinase (PI3K). This leads to a series of downstream events that lead to an influx of calcium from intracellular and extracellular sources as well as cytoskeletal changes, release of inflammatory mediators, and activation of transcription factors (left panel). Coaggregation of inhibitory FcγRIIb with ITAM-bearing FcγR leads to 1) phosphorylation of tyrosine in the ITIM, 2) recruitment of SH2-containing inositol polyphosphate 5′-phosphatase (SHIP), and 3) hydrolysis of phosphatidylinositol (3,4,5)-diphosphate (PIP3) into phosphatidylinositol (4,5)-triphosphate (PIP2). This results in abrogation of Ca2+ influx and inhibition of the downstream events of the activation cascade (right panel). IC = immune complex (see Figure 1 for other definitions and explanations). Inhibitory FcγR. FcγRIIb isoforms are important negative regulators of ITAM-dependent activation and establish the threshold for effector cell activation. when self aggregated, inhibitory FcγRIIb abolish cellular signals when with stimulatory receptors. The in a present in the intracytoplasmic domain of both FcγRIIb1 and FcγRIIb2, is essential for the negative of FcγRIIb and other inhibitory receptors (for review, see refs. are by protein tyrosine kinases and SH2-containing cytoplasmic Inhibitory function the recruitment of to the the protein tyrosine and bind to SH2-containing inositol polyphosphate 5′-phosphatase has been shown to be to FcγRIIb and to play the role in FcγRIIb-mediated inhibition by Ca2+ influx the of the membrane of kinases required to Ca2+ and the influx of extracellular calcium The importance of in the transduction of inhibitory signaling by FcγRIIb isoforms has been most in B cells and mast cells, but that is expressed in phagocytes, is to mediate inhibition by FcγRIIb2 in monocytes and Coaggregation of different FcγR. most cells express more than one FcγR isoform, is that antigen–antibody complexes more than one type of of stimulatory FcγR represents a by which different FcγR to signals and more efficient activation of effector cells. FcγR that each leading to activation of tyrosine kinases and downstream and initiation of intracellular Ca2+ with cytoskeletal and activation (Figure 3). In the regulation of ITAM-dependent cell activation by inhibitory FcγR is a for negative of inhibitory FcγRIIb with ITAM-bearing FcγR the influx of extracellular Ca2+ and effector cell activation (Figure 3) protein tyrosine kinases to phosphorylate the of FcγRIIb, stimulatory FcγR play a role in their In cells that express both stimulatory and inhibitory receptors for IgG, the of these 2 types of receptors the of cell activation interaction with immune complexes. in mice deficient in activating or FcγR have the role these receptors play in immune complex–triggered inflammation. Mice with of the γ subunit of FcγR do not express the activating FcγR, FcγRI and macrophages are of These mice and antigen–antibody immune complex Arthus and ADCC against tumor cells FcR mice are from glomerulonephritis by of membrane and from in antigen-induced arthritis In with such as FcR mice also have the presence of immune and of IgG These murine the importance of activating FcγR as of at sites of immune complex of by gene has the is mice have humoral II and type hypersensitivity responses and responses In studies of of inflammatory these mice have enhanced immune complex–mediated and glomerulonephritis Mice deficient in also in afferent immune They immunoglobulin in response to and antigens and demonstrate a proclivity toward For example, mice with complex II genes associated with to arthritis are to autoimmune arthritis when is In specific mice and autoimmune The autoimmune has been shown to be associated with B cells, that of FcγRIIb-mediated inhibitory signals in B cells leads to autoimmune in a These a role for in immune and autoimmune disease. 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