Fig. 1.

A simplified diagram of the KP. Majority of TRP in the body is catabolized through KP to synthesize the vital energy cofactor, nicotinamide adenine dinucleotide. This pathway is frequently elevated in the inflammatory environment such as cancer and has been shown to promote immune evasion. This immune modulation results from the overactivation of IDO1/trytophan 2,3-dioxygenase 2 that leads to the depletion of local TRP, an essential amino acid for T cell proliferation, and production of bioactive metabolites (KYN, 3HK, AA, and 3HAA) that promotes selective apoptosis of tumor-targeting T cells