FIBRONECTIN FRAGMENTS STIMULATE TUMOR-NECROSIS-FACTOR SECRETION BY HUMAN MONOCYTES

Fibronectin (Fn) is a high molecular-weight glycoprotein that can influence many aspects of monocyte function. The purpose of this study was to determine whether Fn could stimulate monocyte tumor necrosis factor (TNF) secretion. Monocytes were isolated from the peripheral blood of healthy volunteers by density gradient centrifugation and adherence to plastic (2 h). Plasma Fn was purified from the blood by gelatin-Sepharose affinity chromatography. Monocytes were stimulated with Fn for 18 h and the supernatants were assayed for TNF activity using the L929 bioassay. Intact Fn stimulated the secretion of TNF in a dose-dependent manner. Intact Fn-induced TNF secretion by monocytes was inhibited (50%) but not eliminated by the addition of the R-G-D-containing peptide GRGDSP. Limited proteolysis of the Fn molecule using insoluble chymotrypsin resulted in a fragment preparation that was dramatically more stimulatory than the intact Fn preparation. High-performance liquid chromatographic (HPLC) purification of the fragments demonstrated that at least two fragments were capable of stimulating TNF secretion. Further purification by affinity chromatography and HPLC localized the stimulatory activity to the 120-kd cell-binding fragment. The possibility that the stimulatory activity was the result of endotoxin co contamination was ruled out using macrophages from C3H/Hej mice. These results suggest that Fn fragments are potentially important molecules for activation of monocytes and may stimulate monocyte cytotoxic activity.