IDO1 and central nervous system cancer: In gliomas, Tregs can inhibit dendritic cells, antigen presenting cells and other lymphocytes inhibiting the secretion of IL-2 and IFN-γ and promoting the secretion of TGF-β and indoleamine-2,3-dioxygenase (IDO), thus maintaining the immunosuppressive microenvironment [19] (Figure 7).