AHR and infection: Infection-induced inflammation triggers catabolism of Trp via IDO/AhR.16 In human, Trp catabolism through the Kyn pathway is regulated by three distinct enzymes: IDO-1, IDO-2, which are inducible in many tissues, and TDO, which is expressed in liver, brain and in cancer cells.41, 42 In physiological conditions, TDO is the main enzyme degrading Trp (reviewed in Platten et al.15 and Mehraj and Routy16), while in the context of infections: IDO-1 is induced and becomes the major intracellular enzyme mediating Trp degradation.