Lymphocytes are key mediators of inflammation and IBD, including ulcerative colitis (UC). GPR15 is a chemoattractant receptor for lymphocyte homing to the colon and is implicated in mouse models of colitis. It is expressed by subsets of CD4 and CD8 effector and of CD4 regulatory T cells in both human and mouse. In humans the receptor is highly expressed by presumed pathogenic Th2 cells in the colonic lamina propria of UC patients. In mice it is expressed by colon-homing pathogenic Th17 cells, and deficiency of GPR15 suppresses colitis in the naïve T cell transfer model.