| Abstract: | Macrophage-mediated cytolysis of thymidine-prelabelled murine A9 fibrosarcoma cells was compared to the level of cytolytic factor (CF) present in the cultures by assaying supernatant aliquots on actinomycin (AcD)-treated A9 fibrosarcoma cells. A good correlation between the level of A9 killing and CF titer was observed when different concentrations of lipopolysaccharide (LPS) were added to various macrophage populations: murine peritoneal cells, short-term bone-marrow (BM)-derived macrophages and JBM phi macrophage lines. Optimal A9 killing and CF secretion, equivalent to the killing of about 1000 AcD-pretreated A9 cells by a single macrophage, were obtained following activation of JBM phi by LPS. CF production by BM-derived macrophages was enhanced in serum-free medium when compared to its release in the presence of fetal calf serum. The LPS-activated macrophages could be restimulated by the activating agent to produce CF following one week of propagation in the absence of LPS. On the other hand, CF activity was absent from the supernatants of activated macrophages co-cultured with normal embryonic fibroblasts, which are resistant to macrophage-mediated killing. This effect could be attributed to a factor, secreted by normal fibroblasts but not by A9 cells, which suppressed CF release from the activated macrophages. Our data strongly support earlier observations, suggesting that CF [which appears to resemble the tumor necrosis factor (TNF)] is responsible for LPS-induced macrophage-mediated tumor cell lysis. It is suggested that suppression of the latter process by the fibroblast-derived factor proceeds via inhibition of CF/TNF production from the macrophage. |
