CTLA4 and colitis: The anti-CTLA-4 antibody, ipilimumab, inhibits Treg function by abolishing CTLA-4-mediated suppressive signaling and depletes Treg cells in the tumor microenvironment3,4, but the nonspecific abrogation of Treg function by ipilimumab often leads to substantial immune-related adverse effects such as colitis, which can occur in over 50% of patients.