PROJECT SUMMARY Dr. Weiner is a transplant surgeon and immunology researcher. His long-term career goal is to become a leading surgeon-scientist in the practice of intestinal transplantation (ITx) and the understudied discipline of intestinal immunobiology with the unique expertise and immersion in both to translate novel scientific findings into major clinical advances. To that end, his short-term career goals are to 1) gain clinical expertise in intestinal transplantation, 2) build on current understanding of the pathophysiology of rejection after ITx, 3) augment his personal understanding of microbiology, innate immune defenses, and advanced microbial and T cell analytic methods, and 4) develop an independent research career that is funded by R01 support. He has organized a mentoring team with the expertise and experience to facilitate career development in this field, and he has the commitment of resources and protected research time from his department chair and division chief. The significance of his project is that ITx is the only therapeutic option for patients with intestinal failure once parenteral nutrition fails. However, its use is limited by high rates of rejection, which necessitates increased immunosuppression levels, predisposing patients to morbid side effects. This proposal investigates how the relationship between the gut microbiome and host mucosal defenses correlates with rejection after ITx. Prior work demonstrates that rejection is associated with both modulations in the gut microbiome and dysfunction of host defenses that maintain homeostasis with flora. It is also known that rejection is associated with decreased ratios of graft-versus-host (GVH)-reactive donor T cells relative to host-versus-graft (HVG)-reactive recipient T cells within the graft. Based on these concurrent findings, Dr. Weiner hypothesizes that homeostasis between the gut microbiome and mucosal defenses plays an essential role in maintaining the balance of GVH:HVG- reactive T cells in the graft, thus regulating rejection. If true, rejection could be targeted with novel therapies, such as fecal transplantation. Thus far, no longitudinal studies in human ITx patients have investigated microbiome modulations in relationship to clinical course, T cell chimerism, GVH:HVG-reactive T cell ratios in the graft, and rejection. Therefore, this work will characterize how changes in the microbiome in the gut after ITx correlate with decreased chimerism and GVH:HVG ratios in the blood and graft, which are associated with higher rates of rejection.