The project aims at analysing other immune functions regulated by Syntaxin11 (STX11), beside degranulation of lytic granules, and how they impact HLH pathogenesis. The goal is to unravel the molecular mechanism(s) behind the dysfunctional CD4 T cells in Stx11-deficient mice to define the role of STX11 in T-B cooperation. Furthermore, the impact of the dysfunctional CD4 T cells on the disease-inducing CD8 T cells will be analyzed. The hypothesis is that CD4 T cells are so far unrecognized modifiers of HLH pathogenesis. Accordingly, the experiments will also include blood samples of FHL4 patients to elucidate a human correlate of this phenotype.