Binding of human fibronectin to antigen-antibody complexes

We examined the capacity of human fibronectin (FN) to bind to antigen-antibody (IgG) complexes (IC) in the absence of complement components. Binding of FN to IC was not significantly different at 37 degrees C and 4 degrees C and reached equilibrium after 60 minutes of incubation. The binding was markedly enhanced by decreasing the ionic strength and lowering the pH of the incubation media. The effect of ionic strength on binding was caused at least in part by an increase in the percent of the total FN able to bind to IC. Binding of radiolabeled FN to IC was saturable and was inhibited by the addition of an excess of unlabeled FN. In addition, soluble IC prepared in antigen excess but not antigen or antibody alone inhibited the binding of FN to precipitable IC. FN binding to precipitable IC increased with increasing antibody concentrations in the IC. Heparin and gelatin, which bind to specific locations in the FN molecule, did not inhibit the binding of FN to IC. In conclusion, human FN binds to IC in the absence of C1q, and the binding is enhanced under conditions of antibody excess.