Macrophage accumulation in murine ascites tumors. I. Cytoxan-induced dominance of macrophages over tumor cells and the anti-tumor effect of endotoxin

The host cellular response to i.p. growth of the SA-1 spindle cell sarcoma was studied in semisyngeneic AB6F1 hybrid mice. At progressive stages of tumor growth mice were sacrificed, their ascites were collected, and changes in the numbers of tumor cells, macrophages, lymphocytes, and granulocytes were determined. It was found that macrophages were the predominant host cell that accumulated locally in response to tumor growth. Cytoxan treatment caused a 90% reduction in tumor cells without affecting the accumulation of macrophages. Six days after treatment with cytoxan, macrophages exceeded all other cell types present in the peritoneal cavity and outnumbered tumor cells by 20:1. However, in spite of this macrophage dominance, the tumor cells ultimately regrew to kill the host. Ascites tumors could be completely regressed, however, by following cytoxan treatment with an appropriately timed intratumor injection of endotoxin. The relatively long delay before endotoxin caused the tumor to regress suggests that endotoxin acted by stimulating the generation of anti-tumor immunity, rather than by directly augmenting the tumoricidal activity of macrophages.