Interaction between fibronectin and C1q in rheumatoid synovial fluid and normal plasma

The interaction between fibronectin and C1q was studied in the presence of normal human plasma and rheumatoid synovial fluid by solid phase binding assay. Fibronectin-C1q binding occurred in the presence of rheumatoid synovial fluid but not in the presence of normal plasma. Binding was strongest at 4 degrees C and in the presence of EDTA. Fibronectin-C1q binding could be induced in the presence of normal plasma by hypotonicity, augmentation of the concentration of solution-phase fibronectin or by the addition of heat-aggregated IgG. The C1q present in rheumatoid synovial fluid bound to both aminoterminal collagen-binding and carboxyterminal noncollagen binding fibronectin fragments although binding to the aminoterminal fragment was stronger. The interaction between fibronectin and C1q in rheumatoid synovial fluid may modulate immune-complex deposition and complement activation in the inflamed joint.