IDO1 and hepatocellular carcinoma: In the TMEs of HCC, the high expression of several regulatory molecules including VEGF, C-X-C motif chemokine ligand 17, indoleamine 2,3-dioxygenase (IDO), and interleukin-10 (IL-10), the lack of CD4+ cells, metabolic competition with HCC cells competition, and hypoxia limit the specific responses to tumor-associated antigens and cause poor release of interferon (IFN)-γ by CTLs, and the presence of CTLs in HCC has positive impact on the prognosis for patients [32,33,34,35].