PDCD1 and neoplasm: Moreover, a close association between PD-1 expression and the density of CD3+, Foxp3+, and CD8+ lymphocytes, as well as the ratio of Foxp3+/CD4+ lymphocytes [73], in line with the findings of Pia Zapka et al., which demonstrated the dominance of CD3+ T cells in the tumor microenvironment, with an elevated regulatory T cell population and a significant presence of PD-1-positive immune cells [69], suggests an immune-suppressive state characterized by T cell dysfunction in CNS GCTs.