Indoleamine 2,3-dioxygenase (IDO) is a heme-containing oxidoreductase that catalyzes the initial and rate-limiting step in the breakdown of non-dietary tryptophan. The biology and immunomodulatory role for IDO is discussed in this review with a focus on its interaction with immune cells and its potential therapeutic target in the clinic. IDO has been revealed to be a central regulator of immune responses in a broad variety of physiological and pathological settings, mostly serving as a multifaceted negative feedback mechanism, to self-regulate immune responses. IDO is considered a therapeutic target in cancer and the use of IDO inhibitors as single agent or in combination with other treatment modalities are under active investigation.
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