Neutrophil functional responses depend on immune complex valency

Ligand-induced cross-linking of Fey re- ceptors (FcyR) 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 UCs in situ ((lAb) after their addition to the cells. Multiparameter flow cytometry was used to measure immune complex (IC)-elicited changes in cytoplasmic Ca2'� concentration and initia- tion of the oxidative burst simultaneously in the same cell and to correlate these with FcyR occupancy. We have previously shown that subpopulations of neutro- phils respond maximally to su.bsaturating concentra- tions of HIC; saturating dosages stimulate the entire population. This discrepancy was not due to differ- ences in receptor occupancy. The magnitude of the transient Ca2+ increase was independent of the dose of HIC but depended on the dose when an UC was used. As shown here, (JAb cross-linking elicited Ca2+ responses similar to those observed in HIC-stimulated cells. In contrast, UC elicited only minimal mtracel- lular EpH and no oxidative burst or membrane po- tential changes at all unless FcyR was cross-linked, accomplished by HIC or by L/Ab. However, azuro- philic degranulation, as determined by elastase re- lease, was not observed in cells stimulated by the in situ cross-linking method, whereas the HIC prepara- tion triggered azurophilic degranulation. Thus, some Fc'yR-mediated neutrophil effector functions such as azurophilic degranulation and oxidative burst initia- tion have an absolute requirement for FcyR cross- linking, whereas signaling functions such as changes in membrane potential, intracellular pH, and intra- cellular Ca2+ concentration can occur, albeit more slowly and to a lesser extent, if single Fc'yR are occupied. J. Leukoc. Biol. 58: 403-414; 1995.