Arf6:: a new player in FcγRIIIA lymphocyte-mediated cytotoxicity

The activation of phosphoinositide metabolism represents a critical step in the signaling pathways leading to the activation of cytolytic machinery, but its regulation is partially understood. We report here that the stimulation of the low-affinity receptor for immunoglobulin G (IgG) (Fc gamma RIIIA, CD16) on primary human natural killer (NK) cells induces a phosphatidylinositol 3-kinase (PI3K)dependent activation of the small G protein Arf6. We first demonstrate a functional role for Arf6-dependent signals in the activation of the antibody-dependent cellular cytotoxicity (ADCC) attributable to the control of secretion of lytic granule content. We also show that Arf6 couples CD16 to the lipid-modifying enzymes phosphatidlylinositol4phosphate 5-kinase type I alpha (PI5KI alpha) and phospholipase D (PLD) that are involved in the control of granule secretion; Arf6, but not Rho family small G proteins RhoA and Rac1, is required for receptor-induced PI5KI alpha membrane targeting as well as for PI5KI alpha and PLD activation. Our findings suggest that Arf6 plays a crucial role in the generation of a phosphatidylinosito14,5-bisphosphate (PIP2) plasma membrane pool required for cytolytic granule-mediated target cell killing.