Neutrophil functional responses depend on immune complex valency

Abstract

Ligand-induced cross-linking of Fc gamma receptors (Fc gamma R) on neutrophils plays a significant role in their stimulation, shown here by contrasting the responses induced by low valency immune complexes (LICs) and high valency immune complexes (HICs) and by cross-linking LICs in situ (L/Ab) after their addition to the cells. Multiparameter flow cytometry was used to measure immune complex (IC)-elicited changes in cytoplasmic Ca2+ concentration and initiation of the oxidative burst simultaneously in the same cell and to correlate these with Fc gamma R occupancy. We have previously shown that subpopulations of neutrophils respond maximally to subsaturating concentrations of HIC; saturating dosages stimulate the entire population. This discrepancy was not due to differences in receptor occupancy. The magnitude of the transient Ca2+ increase was independent of the dose of HIC but depended on the dose when an LIC was used. As shown here, L/Ab cross-linking elicited Ca2+ responses similar to those observed in HIC-stimulated cells. In contrast, LIC elicited only minimal intracellular delta pH and no oxidative burst or membrane potential changes at all unless Fc gamma R was cross-linked, accomplished by HIC or by L/Ab. However, azurophilic degranulation, as determined by elastase release, was not observed in cells stimulated by the in situ cross-linking method, whereas the HIC preparation triggered azurophilic degranulation. Thus, some Fc gamma R-mediated neutrophil effector functions such as azurophilic degranulation and oxidative burst initiation have an absolute requirement for Fc gamma R cross-linking, whereas signaling functions such as changes in membrane potential, intracellular pH, and intracellular Ca2+ concentration can occur, albeit more slowly and to a lesser extent, if single Fc gamma R are occupied.