AHR and colitis: Manipulation of the feedback control of AHR signaling cytochrome P4501 (CYP1) enzymes results in a loss of AHR-dependent ILC3 and Th17 cells, which can be reversed by supplementation of AHR ligands in the diet.32 Recently, impaired tryptophan metabolism by the microbiome due to a deficiency in the CARD-9 gene has also been associated with susceptibility to colonic inflammation.33 Other metabolites, such as inosine, may be remodeled by Lactobacillus reuteri, which then suppress the autoimmunity caused by Treg deficiency.34