STAT3 and hepatocellular carcinoma: Another study found that melatonin inhibits the synthesis of pro-inflammatory cytokines and reduces inflammation which is important to promote tumor invasion in the TME and that exosomes derived from melatonin-treated HCC cells downregulate PD-L1 and cytokine expression on macrophages through the STAT3 pathway, thus attenuating the immunosuppressive state of macrophages (55).