C-reactive protein (CRP) is a predictor of cardiovascular risk. It circulates as a pentamer (pentameric CRP) in plasma. The in vivo existence of monomeric (m)CRP has been postulated, but its function and source are not clear. We show that mCRP is deposited in human aortic and carotid atherosclerotic plaques but not in healthy vessels. pCRP is found neither in healthy nor in diseased vessels. As source of mCRP, we identify a mechanism of dissociation of pCRP to mCRP. We report that activated platelets, which play a central role in cardiovascular events, mediate this dissociation via lysophosphatidylcholine, which is present on activated but not resting platelets. Furthermore, the dissociation of pCRP to mCRP can also be mediated by apoptotic monocytic THP-1 and Jurkat T cells. The functional consequence is the unmasking of proinflammatory effects of CRP as demonstrated in experimental settings that are pathophysiologically relevant for atherogenesis: compared to pCRP, mCRP induces enhanced monocyte chemotaxis; monocyte activation, as determined by conformational change of integrin Mac-1; generation of reactive oxygen species; and monocyte adhesion under static and physiological flow conditions. In conclusion, we demonstrate mCRP generation via pCRP dissociation on activated platelets and H ₂ O ₂ -treated apoptotic THP-1 and Jurkat T cells, thereby identifying a mechanism of localized unmasking of the proinflammatory properties of CRP. This novel mechanism provides a potential link between the established cardiovascular risk marker, circulating pCRP, and localized platelet-mediated inflammatory and proatherogenic effects.

Background—: The relevance of the dissociation of circulating pentameric C-reactive protein (pCRP) to its monomeric subunits (mCRP) is poorly understood. We investigated the role of conformational C-reactive protein changes in vivo. Methods and Results—: We identified mCRP in inflamed human striated muscle, human atherosclerotic plaque, and infarcted myocardium (rat and human) and its colocalization with inflammatory cells, which suggests a general causal role of mCRP in inflammation. This was confirmed in rat intravital microscopy of lipopolysaccharide-induced cremasteric muscle inflammation. Intravenous pCRP administration significantly enhanced leukocyte rolling, adhesion, and transmigration via localized dissociation to mCRP in inflamed but not noninflamed cremaster muscle. This was confirmed in a rat model of myocardial infarction. Mechanistically, this process was dependent on exposure of lysophosphatidylcholine on activated cell membranes, which is generated after phospholipase A2 activation. These membrane changes could be visualized intravitally on endothelial cells, as could the colocalized mCRP generation. Blocking of phospholipase A2 abrogated C-reactive protein dissociation and thereby blunted the proinflammatory effects of C-reactive protein. Identifying the dissociation process as a therapeutic target, we stabilized pCRP using 1,6-bis(phosphocholine)-hexane, which prevented dissociation in vitro and in vivo and consequently inhibited the generation and proinflammatory activity of mCRP; notably, it also inhibited mCRP deposition and inflammation in rat myocardial infarction. Conclusions—: These results provide in vivo evidence for a novel mechanism that localizes and aggravates inflammation via phospholipase A2–dependent dissociation of circulating pCRP to mCRP. mCRP is proposed as a pathogenic factor in atherosclerosis and myocardial infarction. Most importantly, the inhibition of pCRP dissociation represents a promising, novel anti-inflammatory therapeutic strategy.

C-reactive protein (CRP) concentrations rise in response to tissue injury or infection. Circulating pentameric CRP (pCRP) localizes to damaged tissue where it leads to complement activation and further tissue damage. In-depth knowledge of the pCRP activation mechanism is essential to develop therapeutic strategies to minimize tissue injury. Here we demonstrate that pCRP by binding to cell-derived microvesicles undergoes a structural change without disrupting the pentameric symmetry (pCRP*). pCRP* constitutes the major CRP species in human-inflamed tissue and allows binding of complement factor 1q (C1q) and activation of the classical complement pathway. pCRP*–microvesicle complexes lead to enhanced recruitment of leukocytes to inflamed tissue. A small-molecule inhibitor of pCRP (1,6-bis(phosphocholine)-hexane), which blocks the pCRP–microvesicle interactions, abrogates these proinflammatory effects. Reducing inflammation-mediated tissue injury by therapeutic inhibition might improve the outcome of myocardial infarction, stroke and other inflammatory conditions.

Recent evidence suggests that the prototypic acute phase reactant C-reactive protein (CRP) is not only a marker but also a potential contributor to inflammatory diseases. CRP belongs to the family of pentraxins and as such consists of five identical non-covalently linked subunits. Contradictory data on the characteristics of CRP as either being pro- or anti-inflammatory may be explained by the existence of two conformations of the protein: the circulating native, pentameric CRP (pCRP) and the monomeric isoform (mCRP), formed as a result of a dissociation process of pCRP. In vitro both isoforms exhibit a very distinct inflammatory profile. We recently identified a localized, physiologically relevant pCRP dissociation mechanism by activated platelets and apoptotic cells and showed the deposition of mCRP in inflamed tissue. Here we review the literature on the causal role of p- and mCRP in the light of our findings and critically analyze the current controversies around CRP. The novel understanding of the lo...

C-reactive protein (CRP) is a predictor of cardiovascular risk. It circulates as a pentameric protein in plasma. Recently, a potential dissociation mechanism from the disc-shaped pentameric CRP (pCRP) into single monomers (monomeric or mCRP) has been described. It has been shown that mCRP has strong pro-inflammatory effects on monocytes. To further define the role of mCRP in determining monocyte phenotype, the effects of CRP isoforms on THP-1 protein expression profiles were determined. The hypothesis to be tested was that mCRP induces specific changes in the protein expression profile of THP-1 cells that differ from that of pCRP.