Immunobiology of metastases.

The role of immunity in the control of tumor metastasis is unclear, although various evidence suggests its existence. Immunosuppressive treatment is associated with increase in metastasis in both experimental animals and humans. Infiltration by T-lymphocytes is substantial in primary tumors while minimal or absent in their metastases. The capacity for metastasis is related to histologic type and grade of differentiation; small cell carcinomas of the lung are more metastatic than large cell carcinomas; small cell lymphomas are more metastatic than large cell lymphomas. Organ selectivity is evident in the patterns of metastasis; the spleen is common site of metastasis for lymphomas but not for carcinomas. In an experimental system, a virus-induced lymphoma invariably metastasized to the thymus while chemical-induced lymphomas metastasized to the liver; immunosuppression did not alter the patterns. Malignant tumors may exhibit years-long intervals of dormancy before metastasis and established metastases may regress spontaneously, both phenomena being altered by changes in immune status. Malignant tumors in persons with immune deficiency, particularly AIDS, like the opportunistic infections, have a tendency for early dissemination, including organs not usually affected.