MDSCs have been demonstrated to conduct their suppressive function by stimulating the formation of indoleamine 2, 3-dioxygenase (IDO), which diminishes local tryptophan (TRP) and creates cytotoxic metabolites such as kynurenine in the TME and lymphatic drainage areas, which causes a rise in Tregs, inhibition of antigen-specific immune responses, and suppression of tumor-specific CTLs (52). Here, IDO1 is linked to neoplasm.