? PROJECT 1 Gastrointestinal (GI) microbiome is implicated in maintenance of heath, and in disease outcomes, including graft versus host disease (GVHD), the most significant complication of allogeneic hematopoietic cell transplantation (allo-HCT). Current understanding of the role of microbiome and its effects on GVHD is limited to correlations. The biology of GVHD is complex, and mostly understood and therapeutically targeted, from the perspective of the donor and host derived immune cells. The cell intrinsic role of the epithelial targets of GVHD, such as the host intestinal epithelial cells (IECs) is not well understood. An emerging concept in biology is that rewiring of cellular metabolism is crucial for their resilience to stress. The host IECs are in a constant state of stress from donor T cells and inflammation during GVHD. But whether the metabolic responses of IECs are altered and are critical for GVHD severity has never been explored. The microbiome plays a critical role in nourishing the IECs through generation of metabolites, including the short chain fatty acids, butyrate, as a function of host diet. Their role in the metabolism of IECs after allogeneic HCT, and whether it regulates GVHD severity is unclear. This proposal aims to fill the above knowledge gaps and test the central premise that the cross-talk between butyrate and the mitochondria of host IECs corrects its cellular metabolic defect and mitigates GVHD. The proposal will build on the exciting and seminal preliminary data, which show that following allo-HCT the host IECs demonstrate profound metabolic defect characterized by deficiency of mitochondrial complex II, and butyrate mitigates the defect and attenuates GVHD. We will thus probe the heretofore unexplored nexus between microbiome derived metabolites and the host metabolic pathways in regulation of GVHD. If successful, it could lead to strategies to rationally modify microbiome to target non-immune host cells and mitigate GVHD without causing global immuno-suppression.