Project Summary: While immune checkpoint inhibitors (ICIs) have revolutionized cancer care across multiple tumor types, their widespread use has also brought new clinical challenges in the form of immune-related adverse events (irAEs). Up to 40% of cancer patients receive ICIs, and of those receiving combination CTLA-4 and PD-1 inhibition, ~60% will require immune- suppressive drugs along with temporary or permanent cessation of ICIs. IrAEs can be irreversible or fatal, and their management may permit tumor progression due to both withdrawal of the tumor-controlling agents and direct dampening of anti-tumor immunity. One of the most common irAEs is ICI-induced diarrhea and colitis, occurring in 20- 40% of patients receiving combination ICI therapy. Prediction, prevention, and management of ICI-colitis are limited by an incomplete understanding of underlying mechanisms. Current therapeutic regimens are based on clinical experience with inflammatory bowel disease (IBD), with few prospective trials assessing interventions in the context of active malignancy. The impact of immune suppressive therapies on tumor progression is unknown. Further, there is increasing evidence that the molecular and cellular underpinnings of ICI-colitis differ from IBD. Thus, a more detailed portrait of ICI-colitis pathogenesis and therapeutic response is urgently needed. Our preliminary data from longitudinal colon biopsies and matched blood samples show a colon-infiltrating cytotoxic T cells that selectively diminish in response to drugs that target T cell trafficking and homing mechanisms. Thus, our central hypothesis is that targetable T cell subsets recruited from circulation promote inflammation in ICI-colitis and mediate therapeutic response. This proposal has two aims that extend our initial observations of T cell dynamics in the colon to define mechanisms of colon inflammation and investigate targeted therapies for mitigation of colitis. We will expand our study of a single patient to include a larger cohort treated with infliximab for ICI-colitis to characterize immune phenotypes and TCR repertoire shifts in colon tissue before and after treatment. We will also address the lack of animal models that faithfully recapitulate both tissue-specific inflammation and malignancy by optimizing a novel, tightly controlled animal model of antigen specific inflammation paired with ICIs and tumor cells expressing the same antigen. We will use this model to establish a platform for preclinical drug evaluation to more efficiently advance promising agents to human trials. By defining mechanisms governing ICI-colitis and utilizing these insights to inform drug target selection, this proposal has the immediate potential to help improve upon current immunotherapy and ICI-colitis treatment protocols, yielding better quality of life and survival outcomes.