FAS and glioblastoma: There are, however, significant problems in the application of immunotherapy for brain tumor treatment, such as the blood-brain barrier and multiple well-pronounced immunosuppressive mechanisms, including FAS-Ligand expression by GBM that could induce FAS-L/FAS-mediated apoptosis of immune lymphocytes in the brain [35], as well as triggering T-lymphocyte checkpoints by PD-L1 surface expression in GBM cells and an additional permanent expression of immunosuppressors TGFβ1 and TGFβ2 by GBM cells [59, 60].