PDCD1 and neoplasm: However, some studies found that the successful clearance of cancer cells by tumor-infiltrating lymphocytes (TILs) is impeded by a series of immune inhibitory mechanisms that are active in the tumor microenvironment (2–4), including the upregulation of immune checkpoint proteins such as PD-1, CTLA-4, Tim-3 and Lag-3 on TILs (5).