Indoleamine 2,3-dioxygenase (IDO): Only an enzyme or a checkpoint controller?

Tryptophan (Trp) is one of the important essential amino acids and Kynurenine pathway (KP) is the first and rate limiting step of degrading Trp to active metabolites. Recent studies showed that KP metabolites have antimicrobial and immunosuppressive roles. The main controller enzyme of this pathway is indoleamine-2,3-dioxygenase (IDO). IDO is an intracellular monomeric, heme-containing enzyme that controls the Trp breakdown in the KP. In this review, we discuss IDO functions on cancer progression and prognosis, and possible therapeutic aspects of inhibition of IDO on cancer immunotherapy.