Dietary Trp can be metabolized to kynurenine (Kyn) by indoleamine-2,3-dioxygenase 1 (IDO1) activity in intestinal epithelial cells (IECs) and dendritic cells (DCs)10,11 and contributes to the differentiation of naïve CD4+ T cells into Foxp3+ Tregs12 through activation of the aryl hydrocarbon receptor (AhR)13, a ligand-activated transcription factor and immune sensor14 that has been implicated in IBD pathogenesis15. Here, AHR is linked to inflammatory bowel disease.