NOS2 and hepatocellular carcinoma: In HCC studies, MDSCs suppressed cytotoxic activity of cell-based immunotherapy cytokine-induced killer (CIK), and concomitant treatment with a PDE5 inhibitor reduced tumour MDSCs numbers and suppressor function by downregulating Arg1 and iNOS potentiating tumour eradication both in vitro and in vivo [40].