PDCD1 and glioblastoma: Interaction between the programmed cell death protein 1 (PD-1) and its ligand (PD-L1) could create an immunoregulatory axis that promotes the invasiveness of GBM tumor cells [17] because PD-L1 expression on the tumor surface activates PD-1 receptor in the microglia, leading to a negative regulation of T cell responses [18].