Fibronectin dependent macrophage fibrin binding.

Plasma fibronectin has been shown to increase the binding of fibrin monomer to macrophages in vitro. In the present study we began characterization of the mechanism underlying this fibronectin activity. Fragments of fibronectin containing the amino terminus enhanced macrophage fibrin binding to the same extent as intact fibronectin on an equimolar basis. However, fibronectin fragments containing the gelatin-binding domain or the cell-binding domain, but lacking the amino terminus, had no effect on fibrin binding. Fibronectin enhanced fibrin binding was not affected by the addition of synthetic peptides containing the RGD adhesion sequence. The ability of fibronectin to augment fibrin binding remained after paraformaldehyde fixation of macrophage monolayers. Fixation did not alter the basal levels of fibrin binding by macrophages. Preincubation of macrophages with exogenous fibronectin did not increase the binding of fibrin. Fibronectin enhanced fibrin binding remained unaltered after the removal of endogenous cell surface fibronectin by capping with F(ab')2 fragments of antibodies to fibronectin. These results suggest that the amino terminus of fibronectin supports the attachment of fibrin to macrophages by an initial fluid-phase interaction that precedes cellular binding and does not require a cellular response.