The Ca2+ Dependence of Human Fcγ Receptor-initiated Phagocytosis

Abstract

Differing roles for [Ca2+]i transients in Fc gamma R-mediated phagocytosis have been suggested based on the observations that antibody-opsonized erythrocyte phagocytosis by human neutrophils shows a [Ca2+]i dependence, while that by murine macrophages appears [Ca2+]i-independent. To explore whether this difference might reflect different receptor isoforms or different cell types, we studied the [Ca2+]i dependence of receptor-initiated phagocytosis by human Fc gamma RIIa and a panel of Fc gamma RIIa cytoplasmic domain mutants expressed in murine P388D1 cells and by human Fc gamma R endogenously expressed on human neutrophils and monocytes. Wild-type and point mutants of huFc gamma RIIa stably transfected into murine P388D1 cells have different capacities to initiate a [Ca2+]i transient, which are closely correlated with quantitative phagocytosis (r = 0.94, p < 0.0001). Phagocytosis both by huFc gamma RIIa in P388D1 cells and by huFc gamma RIIa endogenously expressed on neutrophils and blood monocytes shows [Ca2+]i dependence. Phagocytosis of antibody-opsonized erythrocytes by neutrophils demonstrated greater susceptibility to [Ca2+]i quenching compared with Fc gamma RIIa-specific internalization with E-IV.3, suggesting that the phagocytosis activating property of Fc gamma RIIIb in neutrophils also engages a [Ca2+]i-dependent element. In contrast, phagocytosis by human Fc gamma RIa, endogenously expressed on blood monocytes, is [Ca2+]i-independent. Despite the importance of a consensus tyrosine activation motif for both receptors, Fc gamma RIa and Fc gamma RIIa engage at least some distinct signaling elements to initiate phagocytosis. The recognition that both of the phagocytic receptors on murine macrophages and human Fc gamma RIa associate with the Fc epsilon RI gamma-chain, which contains a tyrosine activation motif distinct from that in the Fc gamma RIIa cytoplasmic domain, suggests that [Ca2+]i-independent phagocytosis is a property associated with the utilization of gamma-chains by Fc gamma R.