Mucosal‐associated invariant T (MAIT) cells are activated in the gastrointestinal tissue of patients with combination ipilimumab and nivolumab therapy‐related colitis in a pathology distinct from ulcerative colitis

The aim of this study was to investigate the pathogenesis of combination ipilimumab and nivolumab‐associated colitis (IN‐COL) by measuring gut‐derived and peripheral blood mononuclear cell (GMNC; PBMC) profiles. We studied GMNC and PBMC from patients with IN‐COL, IN‐treated with no adverse‐events (IN‐NAE), ulcerative colitis (UC) and healthy volunteers using flow cytometry. In the gastrointestinal‐derived cells we found high levels of activated CD8(+) T cells and mucosal‐associated invariant T (MAIT) cells in IN‐COL, changes that were not evident in IN‐NAE or UC. UC, but not IN‐C, was associated with a high proportion of regulatory T cells (T(reg)). We sought to determine if local tissue responses could be measured in peripheral blood. Peripherally, checkpoint inhibition instigated a rise in activated memory CD4(+) and CD8(+) T cells, regardless of colitis. Low circulating MAIT cells at baseline was associated with IN‐COL patients compared with IN‐NAE in one of two cohorts. UC, but not IN‐COL, was associated with high levels of circulating plasmablasts. In summary, the alterations in T cell subsets measured in IN‐COL‐affected tissue, characterized by high levels of activated CD8(+) T cells and MAIT cells and a low proportion of T(reg), reflected a pathology distinct from UC. These tissue changes differed from the periphery, where T cell activation was a widespread on‐treatment effect, and circulating MAIT cell count was low but not reliably predictive of colitis.