PDCD1 and neoplasm: However, the ligands of these immunomodulatory receptors are often expressed by the tumor cells–for example, the expression of programmed death ligand-1 (PD-L1), the ligand of PD-1, is increased in tumor cells upon stimulation with interferon-γ (IFN-γ), a cytokine secreted by pro-inflammatory cells in the tumor microenvironment [9].