NAADP activates two-pore channels on T cell cytolytic granules to stimulate exocytosis and killing

A cytotoxic T lymphocyte (CTL) kills an infected or tumorigenic cell by Ca(2+)-dependent exocytosis of cytolytic granules at the immunological synapse formed between the two cells. Although inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release from the endoplasmic reticulum activates the store-operated Ca(2+)-influx pathway that is necessary for exocytosis, it is not a sufficient stimulus. Here we identify the Ca(2+)-mobilizing messenger nicotinic acid adenine dinucleotide phosphate (NAADP) and its recently identified molecular target, two-pore channels (TPCs), as being important for T cell receptor signaling in CTLs. We demonstrate that cytolytic granules are not only reservoirs of cytolytic proteins but are also the acidic Ca(2+) stores mobilized by NAADP via TPC channels on the granules themselves, so that TPCs migrate to the immunological synapse upon CTL activation. Moreover, NAADP activates TPCs to drive exocytosis in a way that is not mimicked by global Ca(2+) signals induced by IP(3) or ionomycin, suggesting that critical, local Ca(2+) nanodomains around TPCs stimulate granule exocytosis. Hence, by virtue of the NAADP/TPC pathway, cytolytic granules generate Ca(2+) signals that lead to their own exocytosis and to cell killing. This study highlights a selective role for NAADP in stimulating exocytosis crucial for immune cell function and may impact on stimulus-secretion coupling in wider cellular contexts.