CD8A and neoplasm: The permissive immune environment in pancreatic cancer is complex, primarily resulting from the failure in innate immunity exerted by natural killer (NK) T-cells and the suppression of adaptive immunity by the immune checkpoint machinery between CD8+ T-cells and tumor cells, mediated by the programmed death (PD)-1/PD ligand (PD-L)1 axis and cytotoxic T-lymphocyte-associated protein (CTLA)-4.