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Serum and Plasma Fibronectin Binds to Complement Reacted Immune Complexes Primarily via C1q

The binding of fibronectin to human Clq, C3b, and complement‐reacted immune complexes (1C) was investigated by enzyme‐linked immunosorbent assays. Microplates were coated with BSA followed by incubation with rabbit‐anti‐BSA IgG or F(ab')2 fragments of rabbit anti‐BSA. Incubation of the solid phase with serum a 37°C caused attachment of Clq and C3b. Addition of EDTA to the serum inhibited the binding of C3b, but not Clq, whereas substitution of the anti‐BSA IgG on the solid phase with the F(ab')2 fragments abrogated the Clq, but not the C3b binding Fibronectin binding was observed after incubation of the solid‐phase IC with serum or plasma at conditions where Clq was also bound, whereas only minor amounts of fibronectin were bound to the solid phase IC via C3b. Purified fibronectin showed a dose‐dependent binding to solid‐phase IC pretreated with Clq or fibronectin‐depleted serum, confirming that the binding of fibronectin to IC largely occurred via Clq. Significantly smaller amounts of fibronectin were bound to solid‐phase IC incubated with plasma instead of serum, despite the higher fibronectin concentration in plasma. This difference was found not lo be due to a fibrinogen‐fibronectin interaction in plasma. Binding of fibronectin to preformed fluid phase IC incubated with serum was demonstrated by SDS‐PAOE analysis of PEG‐precipitated IC.