IDO1 and neoplasm: In addition to these immunomodulatory functions, type I IFNs can promote different inhibitory mechanisms to regulate the amplitude and duration of the response, including the production of the indoleamine 2,3-dioxygenase (IDO) enzyme and, of special interest to our group, the expression of PD-L1 in both tumors and tumor-infiltrating immune cells 28,126.