PDCD1 and melanoma: In solid tumors, such as gastric cancer and melanoma, high levels of M2 macrophage infiltration are associated with higher expression levels of immune checkpoints, such as programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1), suggesting that macrophages can be used as a potential therapeutic target for tumors (Ju et al., 2020; Kim et al., 2020).