CD274 and glioblastoma: GBM cells employ multiple immunosuppressive strategies, including the overexpression of immune checkpoint molecules such as programmed death-ligand 1 (PD-L1), the secretion of immunosuppressive cytokines such as transforming growth factor-beta (TGF-β) and interleukin-10 (IL-10), and the recruitment of regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) [9].