The overarching goal of this R01 is to develop non immunosuppressive strategies to prevent and treat gastrointestinal (GI) GVHD, which remains a major cause of morbidity and mortality from allogeneic hematopoietic cell transplantation (HCT). We have uncovered an exciting, previously unknown role for metabolic aberrations in allo-reactive T cell target tissues, the intestinal epithelial cells (IECs) in regulating the severity of GI GVHD. Our preliminary data identified a profound defect in mitochondrial complex II (MC II), specifically reduction of its component, succinate dehydrogenase A (SDHA), in IECs that contributed to severity of GI GVHD. These studies have identified a key role for SDHA in bulk IECs, which is made up of a number of distinct subsets. Emerging data in the recent years suggest that quantitative loss of only a few key IEC subsets, specifically Lgr5+ intestinal stem cell (ISCs) is a critical features of severe GI GVHD. But the ISC intrinsic pathways that are critical for their function and maintenance remain unknown. Our new preliminary data demonstrate that the mitochondrial metabolic defect, reduction of SDHA component of the mitochondrial complex II (MC II), in the bulk IECs, is observed in the ISC subsets. In this project, we will build on these exciting and seminal preliminary observations and explore the role of mitochondrial metabolic and bioenergetics functions in the biology of ISCs and its contribution to mitigating the severity of GI GVHD. Specifically, we will test the central hypothesis that host Lgr5+ ISCs cell autonomous deficiency of mitochondrial complex II component, SDHA amplifies GI GVHD. The specific aims are: Specific Aim (SA) 1: To determine the functional relevance of mitochondrial complex II component (SDHA) in host Lgr5+ ISCs to GI GVHD. SA 2: Determine the impact of mitochondrial complex II SDHA deficiency on the biology of Lgr5+ ISC. If successful, our proposal will provide seminal insights into fundamental biology of ISCs, provide novel targets to mitigate T cell mediated target organ damage without adding more immune suppression and thus have potential implications not only for allo-HCT but also for autoimmunity, solid organ transplantation and T cell mediated therapies.