Inhibitory ITAM signaling by Fc alpha RI-FcR gamma chain controls multiple activating responses and prevents renal inflammation

Inhibitory signaling is an emerging function of ITAM-bearing immunoreceptors in the maintenance of homeostasis. Monovalent targeting of the IgA Fc receptor (FcalphaRI or CD89) by anti-FcalphaRI Fab triggers potent inhibitory ITAM (ITAM(i)) signaling through the associated FcRgamma chain (FcalphaRI-FcRgamma ITAM(i)) that prevents IgG phagocytosis and IgE-mediated asthma. It is not known whether FcalphaRI-FcRgamma ITAM(i) signaling controls receptors that do not function through an ITAM and whether this inhibition requires Src homology protein 1 phosphatase. We show in this study that FcalphaRI-Fcgamma ITAM(i) signals depend on Src homology protein 1 phosphatase to target multiple non-ITAM-bearing receptors such as chemotactic receptors, cytokine receptors, and TLRs. We found that anti-FcalphaRI Fab treatment in vivo reduced kidney inflammation in models of immune-mediated glomerulonephritis and nonimmune obstructive nephropathy by a mechanism that involved decreased inflammatory cell infiltration and fibrosis development. This treatment also prevented ex vivo LPS activation of monocytes from patients with lupus nephritis or vasculitis, as well as receptor activation through serum IgA complexes from IgA nephropathy patients. These findings point to a crucial role of FcalphaRI-FcRgamma ITAM(i) signaling in the control of multiple heterologous or autologous inflammatory responses. They also identify anti-FcalphaRI Fab as a new potential therapeutic tool for preventing progression of renal inflammatory diseases.

IgA Fc receptor I signals apoptosis through the FcRγ ITAM and affects tumor growth

The IgA Fc receptor (FcαRI) has dual proinflammatory and anti-inflammatory functions that are transmitted through the immunoreceptor tyrosine-based activation motifs (ITAMs) of the associated FcRγ subunit. Whereas the involvement of FcαRI in inflammation is well documented, little is known of its anti-inflammatory mechanisms. Here we show that monomeric targeting of FcαRI by anti-FcαRI Fab or serum IgA triggers apoptosis in human monocytes, monocytic cell lines, and FcαRI+ transfectants. However, the physiologic ligand IgA induced apoptosis only when cells were cultured in low serum conditions, indicating differences with induction of anti-inflammatory signaling. Apoptosis signaling required the FcRγ ITAM, as cells transfected with FcαRI or with a chimeric FcαRI-FcRγ responded to death-activating signals, whereas cells expressing a mutated FcαRIR209L unable to associate with FcRγ, or an ITAM-mutated chimeric FcαRI-FcRγ, did not respond. FcαRI-mediated apoptosis signals were blocked by treatment with the pan-caspase inhibitor zVAD-fmk, involved proteolysis of procaspase-3, and correlated negatively with SHP-1 concentration. Anti-FcαRI Fab treatment of nude mice injected subcutaneously with FcαRI+ mast-cell transfectants prevented tumor development and halted the growth of established tumors. These findings demonstrate that, on monomeric targeting, FcαRI functions as an FcRγ ITAM-dependent apoptotic module that may be fundamental for controlling inflammation and tumor growth.

Identification of FcalphaRI as an inhibitory receptor that controls inflammation: dual role of FcRgamma ITAM

Serum IgA is considered a discrete housekeeper of the immune system with multiple anti-inflammatory functions, whereas IgA-immune complexes mediate inflammatory responses. Here, we identify FcalphaRI as a molecular device that determines the nature of IgA responses. In the absence of sustained aggregation, receptor targeting by serum IgA or anti-FcalphaRI Fab inhibits activating responses of heterologous FcgammaR or FcepsilonRI. The inhibitory mechanism involves recruitment of tyrosine phosphatase SHP-1 to FcalphaRI and impairment of Syk, LAT, and ERK phosphorylation induced by FcepsilonRI engagement. SHP-1 recruitment is dependent on ERK. Conversely, sustained aggregation of FcalphaRI by multimeric ligands stimulates cell activation by recruiting high amounts of Syk and aborting SHP-1 binding. Both types of signals require the FcRgamma-ITAM motif. Anti-FcalphaRI Fab treatment suppresses manifestations of allergic asthma in FcalphaRI transgenic mice. These findings redefine FcalphaRI as a bifunctional inhibitory/activating receptor of the immune system that mediates both anti- and proinflammatory functions of IgA.