PROJECT SUMMARY Graft versus host disease (GVHD) is the major complication associated with allogeneic hematopoietic stem cell transplantation (HSCT). During the acute phase of this disease, a restricted set of organs is affected of which the gastrointestinal (GI) tract is the most clinically significant. Proinflammatory cytokines play a critical role in the pathophysiology of intestinal GVHD, in part, by activating donor T cell populations which subsequently induce tissue damage. In pre-clinical murine studies, we have identified interleukin 23 (IL-23) as a key inflammatory cytokine that mediates pathological damage in the GI tract during GVHD. The overall goal of this proposal is therefore to define the mechanistic pathways by which IL-23 induces inflammation in the GI tract, and to directly translate these findings into the clinic to examine whether blockade of this pathway reduces the severity of GVHD in human allogeneic HSCT recipients. Our overall hypothesis is that IL-23 produced by donor antigen presenting cells (APCs) induces a proinflammatory environment in the GI tract and that this pathway is a clinically viable target for the prevention of GVHD in humans. Experiments in Specific Aim 1 will define the cellular mechanism(s) by which IL-23 promotes inflammation in the GI tract during GVHD in murine transplant recipients. To address this question, we will identify the relevant donor APC populations that produce IL-23 and are functionally important for the induction of inflammation in the colon, examine whether donor APC-derived IL-23 promotes indirect alloantigen presentation which is a critical pathway for the propagation of GVHD in the GI tract, and define whether IL-23 production adversely impacts the regulatory arm of the immune system by deleteriously affecting CD4+ and CD8+ regulatory T cell reconstitution. Studies in Specific Aim 2 will consist of a phase 2 clinical trial to determine whether administration of the IL-23p19-specific antibody, tildrakizumab, attenuates the severity of GVHD in human allogeneic HSCT recipients with underlying hematological malignancies. In addition, we will perform correlative studies to define the effect of this therapeutic approach on immune reconstitution and determine whether tildrakizumab is able to mitigate systemic inflammatory cytokine production that occurs during GVHD. We will also serially examine the microbiome to delineate whether administration of tildrakizumab preserves microbial diversity that is otherwise adversely affected during this disease. The overall goal of these studies is to define the mechanisms by which IL-23 facilitates GVHD in the GI tract and to determine whether blockade of this pathway constitutes a clinically viable strategy for the prevention of GVHD in humans.