PDCD1 and central nervous system cancer: Notably, in murine models of high-grade gliomas, systemic administration of anti-PD-1 therapy triggered phenotypic changes of the myeloid cells and microglia within the TME indicating that: (1) biomarkers of response need to include the innate immune compartment; and (2) that there may be important immune-modulatory roles within the TME mediated by these drugs [40].