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Background Immune checkpoint inhibitors (CPI) have revolutionised cancer treatment, with previously untreatable disease now amenable to potential cure. Combination regimens of anti-CTLA4 and anti-PD-1 show enhanced efficacy but are prone to off target immune-mediated tissue injury, particularly at the barrier surfaces. CPI-induced colitis is a common a serious complication. Methods To probe the impact of immune checkpoints on intestinal homeostasis mice were challenged with combination anti-CTLA4 and anti-PD-1 immunotherapy, manipulation of the intestinal microbiota and antibody blockade/depletion studies. Colonic immune responses were profiled using RNA-sequencing, including high-resolution single cell analyses, and flow cytometry. Results CPI colitis was dependent on the composition of the intestinal microbiota and was characterized by remodelling of mucosal lymphocytes with induction of polyfunctional lymphocyte responses characterized by expression of interferon-g (IFNg), other pro-inflammatory cytokines/chemokines (Il22, Il17a Ccl3, Ccl4 and Ccl9) and cytotoxicity molecules (Gzmb, Gzma, Prf1, Nkg7). In comparison with mucosal lymphocytes in the steady state, polyfunctional lymphocytes from both CD4+ and CD8+ lineages upregulated costimulatory molecules and checkpoint molecules in CPI-colitis, indicating that these cells are usually very tightly regulated. CPI-colitis was attenuated following depletion of effector lymphocytes or following blockade of the IL23/IFNg axis. Conclusion This study provides new mechanistic insights into CPI-colitis, identifying polyfunctional, cytotoxic lymphocytes as key mediators of disease. Therapeutic targeting of their effector response or regulatory networks, including the IL23/IFNg axis holds the key to preventing and reversing CPI-colitis.

P001 Immune checkpoint inhibitor-induced colitis is mediated by polyfunctional lymphocytes and is dependent on the IL23/IFNg axis