PDCD1 and central nervous system neoplasm: Since functions of immune checkpoint signals, such as the PD-1/PD-L1 axis, were discovered in cancer, most studies have focused on correlations between PD-1, PD-L1 and TILs using semiquantitatively analyses, but limited studies have reported the details of infiltrating T-lymphocyte subtypes, particularly in central nervous system tumors.