The pediatric immunodeficiency syndromes familial haemophagocytic lymphohistiocytosis types 3 (FHL3) and 4 (FHL4), and Griscelli syndrome type 2 (GS2) are associated with loss-of-function of Munc13-4, syntaxin 11 (Stx11) and Rab27a, respectively. These proteins are members of families of membrane fusion-regulating proteins, but the precise molecular role of these proteins in granule release is incompletely understood. All of these mutations result in a loss of cytotoxic function by NK cells and cytotoxic CD8+ T lymphocytes, resulting in a life-threatening sepsis-like condition. This project proposes to define the precise molecular role of these known regulators of NK cell granule exocytosis. In addition, other proteins will be screened for a role in granule release. These additional regulators may have mutations in the many patients with immunodeficiencies of no known genetic basis. In order to define the role of individual proteins in the granule release process, a detailed map of the sub-cellular events leading to granule exocytosis will be defined. The movement of proteins and organelles prior to fusion of granules with the cell surface is more complex than previously thought, and remains controversial. Thus, this project aims to increase understanding of the molecular mechanisms of lymphocyte cytotoxicity, in addition to broadening the spectrum of immunodeficiency syndromes and improving the clinical diagnosis and treatment of immunodeficiencies.