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[FOXP3.sup.+] Tregs are expanded within the inflamed intestine of human Crohn's disease, yet FOXP3-mediated gene repression within these cells is lost. The polycomb repressive complexes play a role in FOXP3 target gene regulation, but deeper mechanistic insight is incomplete. We have now specifically identified the polycomb-repressive complex 1 (PRC1) family member, BMI1 in the regulation of a proinflammatory enhancer network in both human and murine Tregs. Using human Tregs and lamina propria T cells, we inferred PRC1 to regulate Crohn's associated gene networks through assays of chromatin accessibility. Conditional deletion of BMI1 in murine [FOXP3.sup.+] cells led to systemic inflammation. BMI1- deficient Tregs beared a TH1/TH17-like phenotype as assessed by assays of genome wide transcription, chromatin accessibility and proteomic techniques. Finally, BMI1 mutant [FOXP3.sup.+] cells did not suppress colitis in the adoptive transfer model of human inflammatory bowel disease. We propose that BMI1 plays an important role in enforcing Treg identity in vitro and in vivo. Loss of Treg identity via genetic or transient BMI1 depletion perturbs the epigenome and converts Tregs into Th1/Th17- like proinflammatory cells, a transition relevant to human Crohn's disease associated [CD4.sup.+] T cells., Introduction Persistent activation of pathogenic adaptive immune cells is evident in many autoimmune diseases, including inflammatory bowel disease (IBD, including Crohn's disease, CD, and ulcerative colitis [UC], refs. 1-3). In [...]