In vitro cultured human monocytes release fibroblast proliferation factor(s) different from interleukin 1

Human monocytes cultured in vitro released factors with growth stimulatory effect upon human dermal fibroblasts. Monocytes in RPMI 1640 released a growth enhancing activity in nearly constant amounts during in vitro differentiation to macrophage-like cells. The growth stimulatory effect mediated by supernatants was reduced when lipopolysaccharide (LPS) or opsonized zymosan particles (OpZ) were added to monocytes during the first 5 days of in vitro culture, thereafter reaching similar levels. The release of interleukin 1 (IL-1) from monocytes was restricted to the first 2 days of culture. IL-1 production by unstimulated monocytes was negligible, while LPS induced a high release of IL-1 from monocytes. A rabbit antibody against human IL-1 abolished the IL-1 activity of the monocyte supernatants as assessed in a mouse thymocyte proliferation assay, but caused only a small reduction of the fibroblast proliferation activity. Thus, the fibroblast growth stimulatory activity mediated by monocytes was caused by factor(s) in addition to IL-1.