Abstract
The complement system is tightly regulated to safeguard against tissue damage that results from unwanted activation. The key step of C3 cleavage to C3b is regulated by multiple mechanisms that control the initiation and extent of activation. This study demonstrated that C3b:plasma protein complexes form in the fluid-phase during complement activation. Several different plasma proteins displayed a discrete high molecular SDS-resistant band when any of the three complement activating pathways were triggered in normal human serum or plasma. Serum depleted of individual complement proteins revealed that C3 and factors B and D were essential for complex formation. Inactivation of the thioester bond in C3 also prevented complex formation. In vitro, complexes could be generated using four purified proteins-C3, factor B, factor D, and target protein-and Mg(2+) to allow C3 convertase formation. These studies showed that the complexes consisted of a plasma protein covalently bound to C3b in a 1:1 molar ratio; the C3b portion was rapidly degraded by factors H and I. Analysis of plasma samples from patients with dense deposit disease and C3 glomerulonephritis demonstrated that C3b:protein complexes form spontaneously in the blood of patients with dense deposit disease and, to a lesser extent, in C3 glomerulonephritis patients, but not in healthy controls. This finding supports the underlying hypothesis that these C3 glomerulopathies are diseases of fluid-phase complement dysregulation. These complexes could normally function as a passive mechanism to intercept C3b from depositing on host cells. However, excessive generation and/or defective clearance of fluid-phase C3b:protein complexes may have pathological consequences.