Role of the Fc gamma R subclasses Fc gamma RII and Fc gamma RIII in the activation of human neutrophils by low and high valency immune complexes.

Two Fc gamma receptor (Fc gamma R) subclasses on human neutrophils, Fc gamma RII and Fc gamma RIII, activate different cellular functions. To examine the involvement of each receptor subtype in polymorphonuclear leukocyte activation, Fab and F(ab')2 fragments of subclass-specific monoclonal antibodies ([mAbs] mAb IV.3 against Fc gamma RII and mAb 3G8 against Fc gamma RIII, respectively) were used to block the binding of low valency immune complexes (LICs) and high valency immune complexes (HICs). Flow cytometry then permitted the simultaneous quantitation of antibody and ligand binding, the elicited intracellular Ca2+ concentration (delta[Ca2+]int), initiation of the oxidative burst, and/or the phospholipase A activation in the same cell. We have previously demonstrated that subsaturating dosages of HIC bind uniformly to all the cells but elicit an "all-or-none" (i.e., dose independent) maximal delta[Ca2+]int in a dose-dependent subpopulation of the cells. In contrast, both the proportion of cells responding and the magnitude of the delta[Ca2+]int transient depend on the subsaturating dose of LIC, even though it too binds uniformly to all the cells, nonresponding as well as responding. These earlier findings have here been extended by single cell flow cytometric analysis to demonstrate that F(ab')2 Fc gamma RIII is the major Fc gamma R involved in HIC binding (and [Ca2+]int mobilization), as well as in oxidative burst and phospholipase A activation. In contrast, both receptor subclasses must be available for LIC-elicited delta[Ca2+]int, as blockage by either of the mAb Fab or F(ab')2 fragments abrogates this response, even though LIC binding to the receptors is not decreased. Furthermore, LIC elicited little oxidative burst activity and failed to activate phospholipase A but cross-linking to achieve multivalency, previously shown to induce [Ca2+]int and oxidative burst responses, elicited phospholipase A activity via Fc gamma RIII. Fc gamma RII's role appears to be modulation of the small, late Ca2+ influx observed at > 1 min, whereas Fc gamma RIII modulates all the earlier larger events. Thus, simultaneous observation of receptor identity, receptor occupancy, and consequent activation parameters in the same cell by flow cytometry permits use to demonstrate that Fc gamma RII is necessary for the small signal transduction elicited by LIC; it plays a relatively small role in polymorphonuclear leukocyte stimulation by HIC. Fc gamma RIII is the main receptor responsible for immune complex-elicited polymorphonuclear leukocyte responses; its efficacy is greatly enhanced when the receptors are cross-linked, either by preequilibrated multivalent complexes or by in situ cross-linking of bound LIC with excess antibody.