Regulation of complement activation by C-reactive protein: targeting of the inhibitory activity of C4b-binding
Andreas P Sjöberg1, Leendert A Trouw, Fabian D G McGrath
1Department of Laboratory Medicine, Section of Clinical Chemistry, Wallenberg Laboratory, University Hospital Malmö, Lund University, Sweden.
Insights
C-reactive protein (CRP) binds to C4b-binding protein (C4BP), a complement inhibitor. This interaction, crucial for regulating inflammation, involves CRP limiting excessive complement activation.
Area of Science:
- Immunology
- Biochemistry
- Complement System
Background:
- C-reactive protein (CRP) is a key acute phase protein in human immune responses.
- CRP is known to interact with Factor H, regulating the alternative complement pathway.
- The interaction of CRP with classical pathway regulators remained largely unexplored.
Purpose of the Study:
- To investigate the binding interaction between C-reactive protein (CRP) and C4b-binding protein (C4BP).
- To identify the binding site of CRP on C4BP and characterize the functional consequences of this interaction.
- To explore the role of CRP-C4BP complexes in modulating complement activation.
Main Methods:
- Surface plasmon resonance to assess CRP-C4BP binding kinetics.
- Analysis of CRP binding to immobilized CRP and CRP-ligand complexes.
- Characterization of C4BP variants and fragments for binding site mapping.
- Detection of CRP-C4BP complexes in human serum and plasma.
Main Results:
- CRP directly binds to C4b-binding protein (C4BP), with binding enhanced by calcium ions.
- The binding site for CRP is located in the central core of the C4BP molecule.
- C4BP retains its complement regulatory activity in the presence of CRP.
- C4BP competes with C1q for CRP binding, suggesting a role in local complement inhibition.
Conclusions:
- CRP interacts with C4BP, a fluid-phase inhibitor of the classical complement pathway.
- This interaction localizes complement regulation, potentially preventing excessive inflammatory responses.
- CRP's dual interaction with Factor H and C4BP suggests a broader role in controlling complement activation.
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 beta-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 alpha-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.
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