PDCD1 and glioblastoma: Despite the disappointing early clinical trial results, the use of ICIs for the treatment of glioblastoma remains under ongoing clinical investigation, aiming to reverse the glioblastoma immunosuppressive TME which is mainly attributed to the upregulation of several immune checkpoint molecules, such as PD-1, PD-L1, CTLA-4, LAG-3, TIM-3 and TIGIT.