PDCD1 and glioblastoma: The PD-1/PD-L1 axis plays a key role in the escape of cancer from immunosurveillance; in fact, PD-1 is highly expressed in effector T cells present in the tumor microenvironment and PD-L1 is expressed in cell surfaces in different types of cancer including bladder, lung, colon, breast, kidney, ovary, cervix, melanoma, glioblastoma, multiple myeloma and lymphoma T-cell [53].