GPR15 and colitis: In DSS-induced colitis following cigarette smoke exposure, the six genes with the most significant differences (Cxcl1, Ccl2, Il10, Tgfb1, Foxp3, and Cxcl10), all of which are Treg signature genes, were consistently downregulated in Gpr15−/− mice compared with those in Gpr15+/+ mice under identical experimental conditions (Fig. 4b).