PDCD1 and glioblastoma: “Removing the brakes” from a dysfunctional, inadequate immune response leaves an uninhibited, though still dysfunctional and inadequate, immune response; thus, given the highly complex network of immunosuppression commonly present in GBM, single-agent therapy with PD-1 inhibition is unlikely to address and alleviate all factors contributing to T-cell dysfunction, and therefore would not be expected to result in durable, sustained tumor regression.