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In this study, we present evidence that interaction of Fc gamma R(CD16) with ligands (immune complexes or anti-CD16 antibodies) induces a rapid rise in Ca2+i and fast production of both inositol 1,4,5 triphosphate (IP3) and IP4 in homogeneous NK cell preparations. Part of the initial Ca2+i rise observed upon stimulation of NK cells with either anti-CD16 antibodies alone or after their crosslinking at the cell membrane depends on Ca2+ mobilization from intracellular stores, but sustained Ca2+i levels are maintained, after the initial spike, through influx of extracellular Ca2+. The Ca2+i rise is mediated, at least in part, by increases in IP3 after receptor-induced hydrolysis of membrane polyphosphoinositides (PPI). The role of extracellular Ca2+ in Fc gamma R(CD16)-dependent induction of lymphokine gene expression has been tested by evaluating production, mRNA accumulation and transcription of IFN-gamma and TNF in NK cells stimulated with Fc gamma R(CD16) ligands and/or rIL-2 in the presence of EGTA. Under these conditions, accumulation and transcription of both IFN-gamma and TNF mRNA induced by CD16 ligands, but not that induced by rIL-2, is completely abolished and neither cytokine can be detected at significant levels in the supernatant fluids of cells so treated. These data confirm that NK cell activation by specific ligands occurs through mechanisms distinct from those induced by IL-2, and indicate that extracellular Ca2+ represents a stringent requirement for cytokine production induced in NK cells through specific (Fc gamma R) stimulation. Our data also indicate that the Ca2+i rise induced upon Fc gamma R(CD16) crosslinking, though necessary, is not sufficient per se to induce activation of lymphokine genes, compatible with the hypothesis that Fc gamma R(CD16) crosslinking generates additional transducing signals that synergize with IL-2 to maximally activate NK cells.
Cassatella et al. (Wed,) studied this question.