PDCD1 and glioblastoma: Cytotoxic T lymphocytes (CTLs) in GBM express multiple terminal exhaustion markers (e.g., TIM3, LAG3, PD1, CTLA-4 and TIGIT), many of which contribute to T cell dysfunction upon binding to their respective ligands (e.g., phosphatidyl serine, PD-L1, CD155), which are also present within the TME [19,25].