Regulation of Complement Activation by C-Reactive Protein: Targeting of the Inhibitory Activity of C4b-Binding Protein

Abstract

C-reactive protein (CRP) is the major acute phase protein in humans. It has been shown that CRP interacts with factor H, an inhibitor of the alternative pathway of complement, and now we demonstrate binding of CRP to the fluid-phase inhibitor of the classical pathway, C4b-binding protein (C4BP). C4BP bound to directly immobilized recombinant CRP as well as CRP attached to phosphorylcholine. The binding was sensitive to ionic strength and was enhanced in the presence of calcium. C4BP lacking β-chain and protein S, which is a form of C4BP increasing upon inflammation, bound CRP with higher affinity than the C4BP-protein S complex. The binding could not be blocked with mAbs directed against peripheral parts of the α-chains of C4BP while the isolated central core of C4BP obtained by partial proteolytic digestion bound CRP, indicating that the binding site for CRP is localized in the central core of the C4BP molecule. Furthermore, we found complexes in serum from a patient with an elevated CRP level and trace amounts of CRP were also identified in a plasma-derived C4BP preparation. We were also able to detect C4BP-CRP complexes in solution and established that C4BP retains full complement regulatory activity in the presence of CRP. In addition, we found that C4BP can compete with C1q for binding to immobilized CRP and that it inhibits complement activation locally. We hypothesize that CRP limits excessive complement activation on targets via its interactions with both factor H and C4BP.

C4b-binding Protein and Factor H Compensate for the Loss of Membrane-bound Complement Inhibitors to Protect Apoptotic Cells against Excessive Complement Attack

“…Although fH (31,46) and C4BP (32) have been reported to interact with CRP, we did not observe enhanced binding of fH or C4BP in the presence of CRP. This does not mean that these molecules do not interact with CRP but shows that they do not need CRP for binding to dead cells.…”

“…It was reported previously that both fH and C4BP bind to C-reactive protein (CRP) (31,32) and that CRP binds to dead cells (33,34). Therefore, we investigated to what extent CRP would influence the binding of C4BP and fH to dead cells and what the effect would be on overall complement activation.…”

Monomeric CRP contributes to complement control in fluid phase and on cellular surfaces and increases phagocytosis by recruiting factor H

“…This blockade is likely because of the attachment of the soluble regulators, Factor H and C4BP, which inhibit complement at the level of C3 convertase and block further progression. 18,19 In this study, we characterize the Factor H-CRP interaction and compare the binding of the two CRP isoforms with immobilized Factor H. mCRP, but not pCRP, binds to Factor H in a calcium-independent manner. A novel third mCRPbinding site was localized to the C-terminus of Factor H. We show for the first time that mCRP enhances the regulatory activity of Factor H both in the fluid phase and on surfaces.…”

Functional Transformation of C-reactive Protein by Hydrogen Peroxide

“…When native CRP, or any protein for that matter, is immobilized directly to a surface, the immobilized protein is no longer native and is aggregated on the surface. Aggregated CRP has also been shown to exhibit ligand binding properties overlapping with that of H 2 O 2 -CRP (38,(41)(42)(43)(44)(45)(46). Aggregation of CRP on a PCh-coated surface also changes the ligand-binding function of CRP; it binds C1q (41,47).…”