Monomeric C‐reactive protein modulates classic complement activation on necrotic cells

Abstract

The acute-phase protein C-reactive protein (CRP) recruits C1q to the surface of damaged cells and thereby initiates complement activation. However, CRP also recruits complement inhibitors, such as C4b-binding protein (C4bp) and factor H, which both block complement progression at the level of C3 and inhibits inflammation. To define how CRP modulates the classic complement pathway, we studied the interaction of CRP with the classic pathway inhibitor C4bp. Monomeric CRP (mCRP), but not pentameric CRP (pCRP), binds C4bp and enhances degradation of C4b and C3b. Both C1q, the initiator, and C4bp, the inhibitor of the classic pathway, compete for mCRP binding, and this competition adjusts the local balance of activation and inhibition. After attachment of pCRP to the surface of necrotic rat myocytes, generation of mCRP was demonstrated over a period of 18 h. Similarly, a biological role for mCRP, C1q, and C4bp in the disease setting of acute myocardial infarction was revealed. In this inflamed tissue, mCRP, pCRP, C4bp, C1q, and C4d were detected in acetone-fixed and in unfixed tissue. Protein levels were enhanced 6 h to 5 d after infarction. Thus, mCRP bound to damaged cardiomyocytes recruits C1q to activate and also C4bp to control the classic complement pathway.

C-reactive protein, immunoglobulin G and complement co-localize in renal immune deposits of proliferative lupus nephritis

“…However, tissue-bound mCRP may also modulate classical complement activation by recruiting C4-binding protein [35]. Taken …”

“…Such mCRP has been detected in vessels, skeletal muscle, liver and in tissue of chronic renal disease, possibly with extrahepatically produced CRP [29][30][31][32]. The well-known fact that pentameric CRP activates the classical complement pathway, has also been reported to account for mCRP [33][34][35]. Two studies have indeed identified CRP deposited in renal specimens from LN patients by means of conventional immunofluorescence microscopy [36,37].…”

Regulated conformation changes in C-reactive protein orchestrate its role in atherogenesis

“…The proposal of disturbed membranes being the major sites for mCRP generation has been further corroborated by elegant studies that demonstrate the binding and dissociation of CRP on the surface of necrotic cells [65,66] and activated platelets [76,79] in a lysoPC-dependent manner [76]. Moreover, Dr. Peter and colleagues [80] recently identified novel pathways for mCRP generation upon binding of CRP to amyloid aggregates, the typical pathological feature found in brains of persons with Alzheimer's disease, and lysoPC-enriched microparticles released by activated cells or isolated from blood of patients with myocardial infarction [81].…”

“…Since mCRP (but not CRP) also binds to Factor H, the activation of CCP by mCRP is largely restricted to the opsonic C3 level without proceeding to the more inflammatory C5 stage [60]. Subsequent studies confirmed the enhanced interactions of mCRP with modified lipoproteins [61] and complement [62][63][64][65][66][67] and further revealed the important contributions of mCRP in promoting non-inflammatory clearance of apoptotic or necrotic cells by controlling complement activation on the cell surface through balanced recruitment of C1q, Factor H and C4bp [64][65][66]. Because inappropriate handling of CRP, including lyophilization [68], storage in the absence of calcium [69] and immobilization onto microtiter wells [60,70], leads to the disruption of the pentameric structure, the reported properties of CRP suffering these technical pitfalls should be re-evaluated with Factor H interaction as a prominent example [60,64,71].…”

“…All of the dissociation pathways of CRP identified thus far are primarily associated with local inflammatory conditions, such as damaged membranes [65,66,73,76,79], pathological protein aggregates [80] and microparticles released by activated cells [81]. Interestingly, the acidic pH frequently found in inflamed tissues can also boost the binding capacity of CRP even in the absence of pentamer dissociation [83]; however, after binding to immobilized ligands at a low pH, a prominent exposure of the mCRP epitope has been observed [83].…”