PDCD1 and ulcerative colitis: CTLA-4 haploinsufficiency is associated with severe inflammation in the GI tract, among other organs,10-12 and polymorphisms in the CTLA-4 gene have been linked to ulcerative colitis risk in Asian populations.13 PD-1 and PD-L1 are expressed by the colonic epithelium, and surface expression of PD-1/PD-L1 is higher in patients with IBD, suggesting a potential regulatory function.14,15