GPR15 and colitis: Surprisingly, Tgfb1, Il17ra, Il23a, Il6ra, Ror1, Tnf, Tgfbr2, Ccr2, and Il17c were downregulated by cigarette smoke exposure in TNBS-induced colitis Gpr15−/− mice compared with similarly treated Gpr15+/+ mice (Fig. 4a).