CTLA4 and ulcerative colitis: Inflammation of the small intestinal and colonic mucosa (immune-mediated enterocolitis) is one of the most common adverse events associated with CTLA-4 or PD-1/PD-L1 inhibition.5,8,9 Immune-mediated enterocolitis is a distinct clinical and pathologic entity but has many features resembling inflammatory bowel diseases (IBDs), such as ulcerative colitis and Crohn’s disease.5,8,9 The roles of CTLA-4 and PD-1/PD-L1 in IBD are unclear.