PDCD1 and glioblastoma: Apart from cell-based vaccines, immune checkpoint inhibitors that target PD-1/PD-L1 (nivolumab, pembrolizumab) and CTLA-4 (ipilimumab) are also increasingly being used, but clinical efficacy is limited due to the immunosuppressive microenvironment and the low mutational burden in GBM [28,29].