Divergent Phosphotyrosine Signaling via FcγRIIIA on Human NK Cells

Previous studies have indicated that interaction of FcγRIIIA on natural killer (NK) cells with various immunoglobulin ligands or monoclonal antibodies (mAbs) can have either stimulatory or inhibitory effects on cytotoxic activity, but the basis for such divergent functional effects has been unclear. We report here that stimulation of NK cells via FcγRIIIA by monoclonal anti-human CD16 (3G8), monomeric IgG (mIgG), or dimeric IgG (dIgG), used either alone or cross-linked by secondary Ab (goat anti-mouse IgG or goat anti-human IgG), resulted in different phosphotyrosine protein patterns. These results suggest that distinct substrates are involved in signaling pathways activated via various agonists of the same triggering surface molecule. Three protein tyrosine kinases, i.e., LCK, LYN, and SYK, were activated by occupancy of the FcγRIIIA, and only LCK activity showed a divergence in effects induced by the various ligands, with strong autophosphorylation induced by mIgG upon cross-linking. We observed no ligand-induced activation of p59fyn, p60c-src, or p62c-yes, src-related protein tyrosine kinases which are expressed in NK cells. Activity of phosphatidylinositol 3-kinase (PI 3-kinase) induced by receptor-specific antibodies or IgG ligands had different kinetics while the level of cytoplasmic free calcium was greatest upon 3G8-induced stimulation. Although the changes in kinase activities associated with FcγRIIIA-mediated regulation of NK cells are complex, it appears that the patterns induced varied with the nature of the ligand and the direction of the regulation of NK activity.