PDCD1 and neoplasm: They can suppress the activity of T lymphocytes to facilitate tumor immune escape by overexpressing PD-1 and cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) receptors.122 Furthermore, M2-like TAMs can also increase vascular extravasation, promote the survival and growth of metastases, suppress cytotoxic T lymphocytes, and maintain immunosuppressive Tregs to enhance tumor invasion and metastasis.123 As a result, the premetastatic niche is formed at distant lesions in specific metastatic sites, including bone, lung, and liver, with the assistance of TAMs.124