PDCD1 and glioblastoma: Apart from the PD-1/PD-L1 pathway, GBM is characterized by the up-regulation of multiple alternative immune checkpoints such as T-cell immunoglobulin and mucin domain 3 (TIM-3), B7 homolog 3 (B7-H3), indoleamine 2,3-dioxygenase-1 (IDO1), and lymphocyte activation gene 3 (LAG3), which usually mediate T transition into a dysfunctional, exhausted status (177–179).