IDO1/IDO2-mediated tryptophan catabolism. The essential amino acid, tryptophan, is converted to the immunosuppressive catabolite, kynurenine, via IDO1 and IDO2. Kynurenine is further catabolized to other downstream substrates. KAT Kynurenine amino-transferase (I, II, III), KMO Kynurenine 3-monooxygenase, KYNU Kynureninase, 3-HAO 3-Hydroxyanthranilate 3,4-dioxygenase, ACMSD 2-Amino-3-carboxymuconate smialdehyde carboxylase. Red enzymes. Blue crosses the blood brain barrier (BBB). Underlined immunosuppressive