Promotion of fibrosarcoma cell growth by products of syngeneic host macrophages

Cells from a C57BL/cbi chemically induced fibrosarcoma (FS6) require exogenous platelet-derived growth factor (PDGF) for in vitro proliferation (as do normal "untransformed" fibroblasts) whereas cells obtained from the FS6M1 tumour, a spontaneous metastasizing subline, show autonomy from PDGF in vitro. Furthermore, the FS6 cells exhibit very low colony formation in an anchorage-independent growth assay. In vivo, this tumour is immunogenic, rarely metastasizes and is heavily infiltrated by host macrophages. Studies of in vitro cell proliferation and anchorage-independent growth show that syngeneic host macrophages from the peritoneal cavity or from the growing tumour release a diffusible factor(s) which has (1) growth-stimulating activity on FS6 cells in monolayer cultures in PDGF-poor medium and (2) potent colony-stimulating activity on FS6 cell cultured in methyl-cellulose-containing medium. These macrophage supernatants stimulate proliferation of quiescent normal fibroblasts in monolayer culture as well as FS6 sarcoma cells, but do not stimulate anchorage-independent growth of normal cells. Supernatants from BCG-elicited macrophages were shown to contain abundant arginase, and were cytolytic to FS6 cells but not to normal cells. Heat inactivation abrogated the arginase and cytotoxicity, revealing heat-stable mitogenicity for FS6 cells and normal fibroblasts. The stimulatory effect of macrophages on FS6 sarcoma cells can be mimicked by the addition of the tumour promoter 12-tetradecanoyl-phorbol-13-acetate (TPA) and supports the hypothesis that macrophages could play a significant role in multistage carcinogenesis by providing a source of endogenous promoter.