The Role of Indoleamine 2, 3-Dioxygenase 1 in Regulating Tumor Microenvironment

SIMPLE SUMMARY: Tumor microenvironment is a complex and dynamically changing entity, which is crucial for tumor development. Indoleamine 2, 3-dioxygenase 1 is elevated in the tumor microenvironment and is strongly associated with tumor histological malignancy. Therefore, the Indoleamine 2, 3-dioxygenase 1 metabolic pathway as a potential tumor immune escape could be used as a novel strategy for cancer therapy. However, the current phase III clinical trials did not achieve a desired result. Thus, it is imperative to further explore the immunosuppressive mechanism mediated by indoleamine 2,3-dioxygenase 1 in the tumor microenvironment to optimize clinical trial treatment strategies. ABSTRACT: Indoleamine 2, 3-dioxygenase 1 (IDO1) is a rate-limiting enzyme that metabolizes an essential amino acid tryptophan (Trp) into kynurenine (Kyn), and it promotes the occurrence of immunosuppressive effects by regulating the consumption of Trp and the accumulation of Kyn in the tumor microenvironment (TME). Recent studies have shown that the main cellular components of TME interact with each other through this pathway to promote the formation of tumor immunosuppressive microenvironment. Here, we review the role of the immunosuppression mechanisms mediated by the IDO1 pathway in tumor growth. We discuss obstacles encountered in using IDO1 as a new tumor immunotherapy target, as well as the current clinical research progress.