PDCD1 and neoplasm: Blockade with different immune checkpoint inhibitors may activate the body’s immune system and weaken immunosuppression in TME against tumor cells by promoting the activation and proliferation of T cells, including PD-1, programmed cell death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) inhibitors.525 As mentioned before, the tumor microenvironment is composed of tumor cells, cancer stem cells, immune cells, fibroblasts and other cells and their secretions, as well as non-cellular components such as extracellular matrix.