Functional role of phosphatidylcholine-specific phospholipase C in regulating CD16 membrane expression in natural killer cells.

CD16, the low-affinity FcIgG receptor (FcgammaRIIIA), is predominantly expressed in human NK cells. Our recent findings indicate that CD16 expression on the outer membrane surface of NK cells is correlated with the membrane expression of phosphatidylcholine-specific phospholipase C (PC-PLC). In the present study we analyzed the trafficking of CD16 from the plasma membrane to cytoplasmic regions, after stimulation with specific mAb. The CD16 receptor is internalized, likely degraded and newly synthesized; its endocytosis is independent of ATP, but requires an integral and functional actin cytoskeleton. Antibody-mediated CD16 cross-linking results in an approximately twofold increase in PC-PLC enzymatic activity within 10 min. Analysis of PC-PLC and CD16 distribution in NK cell plasma membrane demonstrates that the proteins are physically associated and partially accumulated in lipid rafts. Pre-incubation of NK cells with a PC-PLC inhibitor, D609, causes a dramatic decrease both in CD16 receptor and PC-PLC enzyme expression on the plasma membrane. Interestingly, among phenotype PBL markers, only CD16 is strongly down-modulated by D609 treatment. CD16-mediated cytotoxicity is also reduced after D609 incubation. Taken together, these data suggest that the PC-PLC enzyme could play an important role in regulating CD16 membrane expression, the CD16-mediated cytolytic mechanism and CD16-triggered signal transduction.