Enhancement of tumor growth by peritoneal macrophages of normal and tumor-bearing mice.

The effect of peritoneal macrophages on tumor growth was investigated in an in vivo transfer assay (Winn test). A significant acceleration of tumor appearance was observed in animals injected with tumor cells together with peritoneal exudate cells (PEC) from normal mice, as compared to animals injected with tumor cells alone. Macrophages were shown to be the cells responsible for tumor enhancement. PEC from tumor-bearing mice also caused a strong tumor-enhancing effect in the Winn test. These results indicate that non-activated macrophages can enhance tumor growth in in vivo transfers and raise the question of whether a similar effect takes place in the primary host of a tumor.