IDO1 and neoplasm: In TME, MDSCs can contribute to tumor progression by promoting its immune escape, cell growth, and angiogenesis, as well as invasion and metastasis (92), which as a class of regulatory cells can inhibit T-cell function by producing immunosuppressive metabolites (reactive nitroxide intermediates), immunosuppressive cytokines (e.g., TGF-β, IL-10), immunoactive enzymes (ARG, IDO, aminopeptidase), and immunosuppressive PGE2 (93).