C1q, a subunit of the first component of complement, enhances binding of plasma fibronectin to bacteria.

The interaction of plasma fibronectin with C1q of the complement system has been demonstrated in the past several years. In addition, the antibody-independent binding of C1q to bacteria, as well as the binding of plasma fibronectin to bacteria, is well documented. This study examines whether the binding of C1q to bacteria enhances the interaction of C1q and bacteria with plasma fibronectin. Highly purified 125I-C1q bound to several species of bacteria in the absence of antibody. The binding of 125I-C1q to bacteria was saturable and specific since the addition of unlabeled C1q inhibited binding while the presence of bovine serum albumin did not. Bacteria which had been pretreated with either buffer or unlabeled C1q were tested for their ability to bind 125I-fibronectin. When bacteria were preincubated with buffer, Staphylococcus aureus bound fivefold more 125I-fibronectin than did Escherichia coli. However, preincubation of E. coli with C1q increased the binding of 125I-fibronectin by up to 20-fold, whereas pretreatment of S. aureus with C1q increased fibronectin binding by only twofold. These results were confirmed by immunoblotting studies which demonstrated the presence of C1q, as well as an increase in fibronectin antigens on the C1q-treated bacteria as compared with the level of fibronectin on buffer-treated bacteria. In addition, preincubation of 3H-labeled bacteria with C1q enhanced their attachment to fibronectin-coated surfaces but not to albumin-coated surfaces. The biological consequences of these observations are discussed.