CTLA4 and inflammatory bowel disease: However, we cannot exclude the possibility that both CTLA-4 and PD-1/PD-L1 play similar key roles in IBD regulation, although mucosal homeostasis is largely controlled by CTLA-4 in the healthy gut.5 Johnson et al16 reported that ipilimumab was associated with a moderate risk for GI adverse events, with 2 of 6 patients having an IBD flare.