Rous sarcoma virus-induced tumors in mice--I. Macrophage-mediated natural cytotoxicity.

Lymphoid cells from normal untreated A.SW mice showed a cell-mediated cytotoxicity (CMC) towards Rous sarcoma virus-induced tumor lines in a 48-hr microcytotoxicity assay in vitro, using 3[H]-thymidine prelabeled tumor cells. The tissue distribution of this natural CMC shows high levels of activity in the peritoneal cavity, intermediate levels in bone marrow and spleen, and low levels in lymph node, peripheral blood and thymus. Natural CMC of unstimulated resident peritoneal cells against tumor cells was also found in several strains of mice. This was not dependent on the donor's age, since similar levels appeared in mice aged 1-40 weeks. Passage through nylon wool columns or adherence to the plastic surface of resident peritoneal cells removed cytotoxic activity. By contrast, adherent cell displayed significant CMC levels. Treatment with carbonyl iron powder and magnet decreased peritoneal cell cytotoxicity when performed once and abolished it when repeated. All these results suggest that in mice, natural cytotoxicity against Rous sarcoma virus-induced tumors is primarily mediated by macrophages or macrophage-like cells.