Conclusions:
The gastrointestinal tissue of patients with IN-COL is characterised by high levels of activated memory CD8+ T cells and MAIT cells. These changes were distinct from the peripheral blood compartment where CD8+ T cell activation reflected an on-treatment effect, and where circulating MAIT cell count was significantly lower than in health but neither could predict the onset of colitis. Further work is required to determine the pathway by which CD8+ T cells and MAIT cells become activated in the gut of patients with ICI-associated colitis and whether MAIT cells contribute to tissue damage or repair. Advancements in this area have the potential to inform novel treatment strategies for this new and distinct inflammatory bowel disease.