The incidence of autoimmune diseases, many of which lack effective treatments, is rapidly increasing in the developed world. Mendelian diseases allow the study of autoimmunity in humans, enabling new insights into the underlying pathology. In this study, I have identified a patient cohort suffering from a novel recessive Mendelian disease of immune dysregulation characterized by severe lymphopenia, splenomegaly, thrombocytopenia and liver failure. Whole exome sequencing revealed mutations in GIMAP5, a small GTPase primarily expressed in T, NK and endothelial cells. The missense mutations in these patients destabilize the protein in vitro and lead to a near complete loss of protein in patient cells. Animal models lacking GIMAP5 develop a disease remarkably similar to that observed in the human patients; however, the molecular role of this gene in the immune system remains obscure. To address this, I defined the interactome of GIMAP5 via immunoprecipitation and high-throughput mass spectrometry. This revealed a robust interaction with all seven members of the Ragulator complex which I went on to confirm via endogenous co-immunoprecipitation and proximity ligation assays. This complex has recently been described as a key regulator of mTORC1, Erk signaling and lysosome positioning. In order to study the functional relevance of this interaction I utilized an in vitro CRISPR mediated approach to knockout Gimap5 in murine primary T cells. I observed a very rapid and robust induction of apoptosis accompanied by significant increases in ceramide levels following the loss of GIMAP5 which is consistent with the lymphopenic phenotype. Future studies will relate the increased ceramide to the Ragulator complex and induction of apoptosis in GIMAP5-deficient T cells and leverage these findings to develop novel treatments for GIMAP5 deficiency and other autoimmune diseases.