In vitro detection of cytotoxic cellular immunity against tumor-specific antigens by a radioisotopic technique.

[(3)H]Thymidine-labeled tumor cells are used to evaluate the cytotoxic cellular immune response against tumor-specific antigens; the loss of label due to destruction and detachment of target cells from the surface of the culture vessel is measured. Spleen cells from mice immunized against Moloney virus-induced rhabdomyosarcoma specifically destroyed the sarcoma cells, while cells from normal syngeneic mice did not. Peripheral blood lymphocytes from patients with malignant tumors were specifically cytotoxic to autologous tumor cells and to allogeneic tumor cells histopathologically identical to the autologous tumor, but not to autologous nonmalignant fibroblasts, or to allogeneic tumor cells from a histologically dissimilar tumor. Serum from the same patients specifically protected autologous tumor cells from lymphocyte cytotoxicity. This serum-mediated protection of tumor cells against autologous cellular immunocytotoxicity also extended to histologically identical allogeneic tumor cells. Cross-reactivity of anti-tumor cellular immunocytotoxicity in vitro, and its "blocking" by autologous serum, strongly suggest the presence of common tumor antigens. The antagonism demonstrated in vitro between serum and cellular immunity may explain the continued growth of malignant tumors in the face of demonstrable cellular immunity.