PD-L1-expressing exosomes secreted by tumor cells can enter the peripheral circulation and further suppress the function of circulating CD8+ T cells in both patients with metastatic malignant melanoma and tumor-bearing mice models.447 Thus, anti-CTLA-4 monoclonal antibody (anti-CTLA-4 mAb) and anti-PD-1/PD-L1 mAb (anti-PD-1/PD-L1 mAb) that target immune checkpoint molecules have been considered remarkably promising anticancer drugs. Here, PDCD1 is linked to metastatic melanoma.