Evidence that fibronectin-immunoglobulin complexes occur normally in plasma

This study was done to ascertain whether complexes of plasma fibronectin (PFn) and immunoglobulins (Igs) are normally present in plasma. PFn preparations from plasmas of six random human donors were analyzed for IgG, IgM, and IgA by competitive ELISA procedures. To look for direct evidence of circulating complexes, two of the plasma samples were chromatographed on Sephacryl S-300. PFn prepared from a pool of three plasmas by cryoprecipitation was studied by crossed immunoelectrophoresis to detect PFn-Ig complexes. The Ig content of PFn ranged from 0.78% to 9.0% IgG; 0.25% to 4.9% IgM, and 0.15% to 0.71% IgA. The relative amounts were IgG > IgM > IgA in every sample, but the total Ig content of PFn varied with the plasma donor. The Sephacryl S-300 chromatogram of the plasma from which Ig-rich PFn was made showed distortion of the PFn peak by Ig peaks. PFn prepared by cryoprecipitation contained PFn-immunoglobulin complexes. Plasma from adult beagle dogs was used to prepare canine PFn. IgM was detected in canine PFn by SDS-Page and by double diffusion experiments against antiserum to canine IgM. By crossed immunoelectrophoresis, a PFn-IgM complex was detected in canine plasma. The cryoprecipitate recovered from a Sephacryl S-300 chromatogram of canine plasma contained equivalent amounts of IgM and PFn. These data suggest that complexes of Igs with PFn are normally present in plasma.