Syntaxin11 serves as a t‐SNARE for the fusion of lytic granules in human cytotoxic T lymphocytes

CTL s kill target cells via fusion of lytic granules ( LG s) at the immunological synapse ( IS ). Soluble N‐ethylmaleimide‐sensitive factor attachment protein receptors ( SNARE s) function as executors of exocytosis. The importance of SNARE s in CTL function is evident in the form of familial hemophagocytic lymphohistiocytosis type 4 that is caused by mutations in S yntaxin11 ( S tx11), a Q a‐ SNARE protein. Here, we investigate the molecular mechanism of S tx11 function in primary human effector CTL s with high temporal and spatial resolution. Downregulation of endogenous S tx11 resulted in a complete inhibition of LG fusion that was paralleled by a reduction in LG dwell time at the IS . Dual color evanescent wave imaging suggested a sequential process, in which first S tx11 is transported to the IS through a subpopulation of recycling endosomes. The resulting S tx11 clusters at the IS then serve as a platform to mediate fusion of arriving LG s. We conclude that S tx11 functions as a t‐ SNARE for the final fusion of LG at the IS , explaining the severe phenotype of familial hemophagocytic lymphohistiocytosis type 4 on a molecular level.