IDO1 and central nervous system cancer: This may be due, in part, to increase expression of immunosuppressive factors, like programmed cell death 1 ligand (PD-L1), interleukin 10 (IL-10), indolamine 2,3-dioxygenase (IDO), and transforming-growth factor β (TGF-β), in the glioma microenvironment [19–22].