PDCD1 and neoplasm: GB cells are also able to avoid anti-tumor T cell responses by several mechanisms, such as prevention of antigen processing and presentation on MHC-class I, up-regulation of PD-L1 expression, the ligand of the negative regulator receptor of T cell activation PD-1 (programmed cell death protein 1), or by increasing infiltration of Tregs and T cells with high expression of PD-1 and CTLA-4 (cytotoxic T-lymphocyte-associated protein 4) [22,31,32].