A role for endobrevin/VAMP8 in CTL lytic granule exocytosis

CTL clear virus-infected cells and tumorigenic cells by releasing potent cytotoxic enzymes stored in preformed lytic granules. The exocytosis process includes polarization of lytic granules toward the immunological synapse, tethering of lytic granules to the plasma membrane and finally fusion of lytic granules with the plasma membrane to release cytotoxic enzymes. Although much is known about the molecular machineries necessary for the earlier steps in lytic granule exocytosis, the molecular machinery governing the final step in the fusion process has not been identified. Here, we show using control and VAMP8 KO mice that VAMP8 is localized to the CTL lytic granules. While the immunological synapse and granule polarization appears normal in both VAMP8 KO and control CTL, CTL-mediated killing was reduced for the Vamp8(-/-) CTL. Analysis of lytic enzyme secretion demonstrated that granzyme A and granzyme B secretion is significantly compromised in VAMP8(-/-) CTL, while the levels of the lytic enzymes in the cells are unaffected. Our results clearly show that VAMP8 is one of the v-SNARE that regulate the lytic ability of CTL by influencing the ability of the lytic granules to fuse with the plasma membrane and release its contents.