IDO1 and neoplasm: Esophageal tumor cells can also induce production of the tryptophan-catabolizing enzyme, indoleamine 2,3-dioxygenase 1 (IDO1) and/or expression of PD-L1 by immunosuppressive DCs, which can promote immune tolerance by inhibiting CD8+ T cell infiltration and inducing immunosuppressive Tregs, and are associated with poor prognosis in EC patients (3, 66–68).