IDO1 and hepatocellular carcinoma: At one time, HCC cells could evade host immune surveillance via several mechanisms including silencing the expression of tumor-related antigens, increasing the infiltration of suppressive immune cells, such as MDSC and tumor-associated macrophages (TAM), and expressing immunoinhibitory factors like PD-1, PD-L1, CTLA-4, and indoleamine 2, 3-dioxygenase (IDO1) [43–45].