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

Abstract

The IgA Fc receptor (FcalphaRI) has dual proinflammatory and anti-inflammatory functions that are transmitted through the immunoreceptor tyrosine-based activation motifs (ITAMs) of the associated FcRgamma subunit. Whereas the involvement of FcalphaRI in inflammation is well documented, little is known of its anti-inflammatory mechanisms. Here we show that monomeric targeting of FcalphaRI by anti-FcalphaRI Fab or serum IgA triggers apoptosis in human monocytes, monocytic cell lines, and FcalphaRI+ 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 FcRgamma ITAM, as cells transfected with FcalphaRI or with a chimeric FcalphaRI-FcRgamma responded to death-activating signals, whereas cells expressing a mutated FcalphaRI(R209L) unable to associate with FcRgamma, or an ITAM-mutated chimeric FcalphaRI-FcRgamma, did not respond. FcalphaRI-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-FcalphaRI Fab treatment of nude mice injected subcutaneously with FcalphaRI+ mast-cell transfectants prevented tumor development and halted the growth of established tumors. These findings demonstrate that, on monomeric targeting, FcalphaRI functions as an FcRgamma ITAM-dependent apoptotic module that may be fundamental for controlling inflammation and tumor growth.