PDCD1 and glioblastoma: While immune checkpoint blockade by anti-PD1 or anti-PD-L1 antibodies have improved outcomes with objective responses in a variety of other cancers, including those in the brain such as metastatic melanoma3, they have had limited efficacy as monotherapy for recurrent glioblastoma where the microenvironment is innately immunosuppressive (that is, immunologically ‘cold’)4,5.