Project Summary Monoclonal antibodies against co-inhibitory immune receptors PD-1 and CTLA-4 [immune checkpoint inhibitors (ICIs)] have revolutionized immuno-oncology by reversing T cell exhaustion and harnessing the power of the immune system to fight solid tumors. Although ICI therapy has improved the survival of patients with metastatic cancer, treatments are unfortunately limited by the development of immune-related adverse events. Colitis is the most common, severe side effect of ICIs – seen in 5-10% of patients on PD-1 and up to 50% of patients on dual PD-1/CTLA-4 blockade – and is marked by epithelial injury and apoptosis. To date, little is understood about the immunologic underpinnings of PD-1 colitis, in part because mice treated with PD-1-blocking antibodies do not readily develop gastrointestinal toxicities. This proposal presents a five-year research career development program focused on understanding the immune drivers of human colitis that develops after PD-1 blockade [i.e., immunotherapy-related colitis (irColitis)]. Dr. Molly Thomas is an Instructor of Medicine at Harvard Medical School in the Division of Gastroenterology at Massachusetts General Hospital. This award will provide Dr. Thomas with the support necessary to test the hypothesis that irColitis is caused by the expansion of specific colon CD8+ T-resident memory (Trm) cells into cytotoxic effectors that produce IL-26 and IL-17A and home directly to damaged epithelium where they have deleterious effects on epithelial turnover and absorptive function. This hypothesis will be tested through the following aims: (Aim 1) Define the epigenetic landscapes and effector functions of distinct colonic CD8+ Trm and Trm-derived effectors; (Aim 2) Determine if CD8+ T effectors detected in the colon mucosa of irColitis patients circulate in blood and which blood immune cell states track with disease; (Aim 3) Map interactions between CD8+ Trms and damaged epithelial cells to understand how PD-1 inhibition leads to interferon-dependent defects in CD8+ Trm homing and epithelial absorptive function. Through these proposed aims, Dr. Thomas will map the complex interactions between CD8+ Trms and damaged colonic epithelium following PD-1 blockade. She will work under the guidance of her primary mentor Dr. Nir Hacohen, an expert in immuno-oncology and autoimmunity, and her co-mentor Dr. Alexandra-Chloé Villani, an expert in leveraging single cell genomics to understand immune heterogeneity in health and disease. The proposed experiments, analyses, and didactic work will provide Dr. Thomas with a unique set of skills that will enable her to transition to independence as a physician-scientist studying immune- mediated gastrointestinal diseases. These studies will define mechanisms by which IL-26+ and IL-17A+ CD8+ T cell effectors orchestrate pathologic inflammation and will allow for the development of diagnostic tools and treatment strategies that will be broadly applicable to irColitis patients...