TGFB1 and glioblastoma: Beyond these indirect effects, glioblastoma cells are also able to inhibit cytotoxic T cells directly by at least three additional mechanisms: (1) T-cell inhibition by glioblastoma cell secretion of TGFβ [82]; (2) upregulation of checkpoint signals such as PD-L1 on glioblastoma cells [83, 84]; (3) aggressive glioblastoma cell competition with T cells for glucose in the tumor microenvironment [85].