Induction of macrophage growth by heat-stable factor in tumorous ascitic fluid

Peritoneal macrophages from mice synthesized DNA and proliferated in vitro in the presence of cell-free tumorous ascites. The growth stimulating activity of ascitic fluid was stable on heat treatment (100 degrees C, 30 min). In contrast, the macrophage growth stimulating activity of L cell-conditioned medium was inactivated by heat treatment. The growth factor(s) in heat-treated ascitic fluid was stable at pH 2 and 10, and on treatment with 2-mercaptoethanol or urea. Treatments, with enzymes such as trypsin, pronase, phospholipase and neuraminidase, and with sodium periodate oxidation partially inactivated the macrophage growth stimulating activity of the heat-stable factor(s). The adherent cultured cells that proliferated in heat-treated ascitic fluid were macrophages, as indicated by their nonspecific esterase staining, pinocytosis and phagocytosis. These macrophages showed cytolytic activity against a murine tumor in the presence of wheat germ agglutinin, but not antitumor antibody. These results indicate that tumorous ascitis fluid contains a heat-stable macrophage growth factor(s) distinct from the colony stimulating factor and that the proliferated macrophages killed tumor cells in cooperation with a lectin.