In general, glioma cells are surrounded by a predominantly immunosuppressed environment, characterized by a low number of infiltrating lymphocytes, high concentrations of immunosuppressive cytokines such as TGFβ and IL-10 (released by brain stromal cells), and of indoleamine 2,3-dioxygenase 1 and tryptophan 2,3-dioxygenase 2 (IDO/TDO), both of which could stimulate the accumulation of regulatory T cells (Tregs). Here, IDO1 is linked to central nervous system cancer.