IDO1 and glioma: The majority of endogenous AHR ligands are tryptophan metabolites, and the most abundant tryptophan metabolite in glioma is kynurenine, which is produced by enzymes 2,3-dioxygenase (IDO/TDO), binds to AHR and inhibits cytotoxic immune cell activity against glioma cells [6], forming a pathway of Tryptophan-Kynurenine-AHR-Impairment of Immunity.