Dietary L-tryptophan converted by host IDO1/2 enzymes, but not by gut microbiota, induces G-protein coupled receptor (Gpr) 15 transcription preferentially in Tregs via the AHR, increases the colonic number of Tregs and attenuates the ongoing inflammation in a mouse colitis model generated by Citrobacter rodentium infection or DSS treatment [32]. This evidence concerns the gene AHR and colitis.