Heterotypic FcγR clusters evoke a synergistic Ca2+ response in human neutrophils

Abstract

Both Fcγ receptors on human neutrophils (FcγRIIa and FcγRIIIb) are capable of initiating signal transduction after multivalent cross-linking. However, immune complexes most likely activate neutrophils by a combined homotypic and heterotypic cross-linking of FcγRs. We have investigated the effect of homotypic and heterotypic FcγR cluster formation on changes in the intracellular free Ca2+ concentration. Combined heterotypic and homotypic cluster formation resulted in a Ca2+ response that was strongly enhanced as compared to the sum of both individual FcγR responses. This synergistic response was caused by the formation of heterotypic clusters of FcγRs and not by the simultaneous formation of homotypic clusters. This conclusion was supported by experiments with a bispecific antibody binding to both FcγRIIa and FcγRIIIb. The heterotypic FcγR cross-linking results in efficient activation of Ca influx, probably caused by a more pronounced depletion of intracellular Ca2+ stores. Stimulation with immune complexes also induced Ca2+ influx in normal neutrophils, but not in FcγRIIIb-deficient neutrophils. The synergism between both FcγRs was also apparent in other responses of neutrophils, such as the activation of the respiratory burst. This study shows that the two different FcγRs on neutrophils complement each other in mediating an important cellular response.