PDCD1 and central nervous system cancer: In glioma, which is classified as a “cold” tumor with fewer CD3 T cells, a greater frequency of myeloid cells (monocytes, macrophages, and microglia), and lower expression of PD-L1 and PD-1-expressing TILs (5–7), a phase III clinical trial of PD-1 for glioma revealed an overall treatment response rate of only 8% and did not significantly prolong the overall survival of glioma patients (8).