Indoleamine 2,3-dioxygenase 1 (IDO1) was specifically induced in corneal endothelial cells and is among the 10 most highly expressed enzymes in the whole genome. Indoleamine 2,3-dioxygenase/IDO is interferon (IFN) inducible as is the intracellular haem enzyme which is the rate-limiting enzyme for the degradation of tryptophan.
Indoleamine 2,3-dioxygenase/IDO Immune Checkpoint Pathway functions as an antimicrobial agent that acts against a wide range of intracellular pathogens including viruses and protozoans. However, the antiviral role of Indoleamine 2,3-dioxygenase/IDO has been challenged by the results of a study on an infectious disease model. On the other hand, Indoleamine 2,3-dioxygenase/IDO immune Checkpoint Pathway can induce regulatory T cells or T-cell anergy as has been observed in HIV. Because of its strong regulatory property, an Indoleamine 2,3-dioxygenase/IDO inhibitor has been developed as a promising anticancer drug. Clinical trials are being conducted to determine whether it can reduce tumour tolerance in conjunction with the anti-programmed death-ligand 1 (PD-L1) antibody for melanomas. Thus, Indoleamine 2,3-dioxygenase/IDO might have distinctive activities of limiting inflammatory responses, which are used by tumour cells to promote their own survival or to prevent the activation of antitumour responses.
David H. Munn et al. Indoleamine 2,3-dioxygenase and tumor-induced tolerance.J Clin Invest. 2007 May 1; 117(5): 1147–1154.