PDCD1 and neoplasm: Studies of the immunomodulatory function of TAMs have shown that they can inhibit CTLs and reduce their tumor-killing capacity in a number of ways.165 For example, the expression of PD-L1 and B7-1 (or CD80), which are ligands for the inhibitory checkpoint receptors PD-1 and CTLA4 expressed by activated T cells, leads to T-cell functionality by binding to PD-1 on CD8+ T cells “exhaustion”.