Tumoristatic Effects of Nonimmune BALB/c Peritoneal Macrophages on Syngeneic Lymphoma Cells in Vitro

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Abstract
Peritoneal macrophages from unstimulated nonimmune BALB/c mice exerted nonspecific cytostatic effects on tumor cells in vitro. Incubation of syngeneic B40 lymphoma or allogeneic B16 melanoma cells with peritoneal macrophages or with macrophage culture supernatant fluids (SF) resulted in decreased target cell DNA synthesis and growth; in contrast, nontumorigenic syngeneic 3T3 fibroblasts were stimulated by macrophages but SF exerted no significant effects. Lipopolysaccharide activation of macrophages did not induce cytotoxicity for any target cell tested, and did not enhance the observed tumoristatic effects. The level of tumor inhibition associated with macrophage culture SF was dependent upon the concentration of fetal calf serum present in the medium as well as upon the numbers of macrophages cultured. Inhibitory SF could be harvested continuously from macrophage cultures for at least 7 days with no appreciable loss of activity.