PDCD1 and glioblastoma: GBM cells can produce anti-inflammatory cytokines such as transforming growth factor beta (TGB-β), the vascular endothelial growth factor (VEGF), interleukin (IL) 6, and IL-10, as well as to express cell surface inhibitory molecules like the cytotoxic T lymphocyte antigen-4 (CTLA-4) and programmed cell death-1 (PD-1) [19].