TAMs also influence T-cell cytotoxic activity by stimulating immune checkpoint molecules such as programmed death-ligand 1 (PD-L1) in cancer cells and T cell immunoglobulin and mucin domain-containing protein 3 (TIM-3), programmed cell death protein-1 (PD-1) and cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) on the T cell surface, leading to an impaired immune response (Cassetta and Kitamura, 2018; Liu et al., 2018; Xiao et al., 2018). Here, PDCD1 is linked to cancer.