Convergence of Fcγ Receptor IIA and Fcγ Receptor IIIB Signaling Pathways in Human Neutrophils

Abstract

Human neutrophils (PMNs) express two receptors for the Fc domain of IgG: the transmembrane FcgammaRIIA, whose cytosolic sequence contains an immunoreceptor tyrosine-based activation motif, and the GPI-anchored FcgammaRIIIB. Cross-linking of FcgammaRIIIB induces cell activation, but the mechanism is still uncertain. We have used mAbs to cross-link selectively each of the two receptors and to assess their signaling phenotypes and functional relation. Cross-linking of FcgammaRIIIB induces intracellular Ca2+ release and receptor capping. The Ca2+ response is blocked by wortmannin and by N,N-dimethylsphingosine, inhibitors of phosphatidylinositol 3-kinase and sphingosine kinase, respectively. Identical dose-response curves are obtained for the Ca2+ release stimulated by cross-linking FcgammaRIIA, implicating these two enzymes in a common signaling pathway. Wortmannin also inhibits capping of both receptors, but not receptor endocytosis. Fluorescence microscopy in double-labeled PMNs demonstrates that FcgammaRIIA colocalizes with cross-linked FcgammaRIIIB. The signaling phenotypes of the two receptors diverge only under frustrated phagocytosis conditions, where FcgammaRIIIB bound to substrate-immobilized Ab does not elicit cell spreading. We propose that FcgammaRIIIB signaling is conducted by molecules of FcgammaRIIA that are recruited to protein/lipid domains induced by clustered FcgammaRIIIB and, thus, are brought into juxtaposition for immunoreceptor tyrosine-based activation motif phosphorylation and activation of PMNs.