PDCD1 and neoplasm: Checkpoint pathways are intimately linked with the metabolic status of both tumor cells and T-cells; non-responsiveness to anti-PD-1 mAb has been linked to the upregulation of a metabolic gene-signature in tumors (35), whereas successful PD-L1 blockade has been demonstrated to block tumor cell glycolysis, thereby enhancing T-cell fitness (34).