IDO1 and glioblastoma: A lot of immune-suppressing cells, like Tregs, MDSCs, and TAM, invade the GBM microenvironment and upregulate multiple immune checkpoints, like PD-1, Tim-3, CTLA-4, and IDO-1, and immune-suppressing ligands, like PD-L1 on GBM cells and tumor-infiltrating myeloid cells, which conceal GBM tumor antigens.