The prototypic acute-phase reactant C-reactive protein (CRP), discovered as a protein that precipitates C-polysaccharide of Streptococcus pneumoniae, has long been recognized as a sensitive biomarker of inflammation (1). Elevations in baseline serum CRP level have been detected in numerous pathologies, and have been suggested to being useful to monitor disease progression. CRP has received considerable attention as a diagnostic and prognostic marker in autoimmune diseases (2, 3), cardiovascular diseases (4–6), chronic kidney disease (7), cancer (8) and COVID-19 (9, 10) as well as for guiding therapy (8, 11). Although there is a continuing debate over whether CRP is primarily a passive indicator of inflammation or is a “culprit” mediating disease (12–18), CRP plays important roles in host defense against invading pathogens, autoimmunity and inflammation. CRP exhibits many, often conflicting pro- and anti-inflammatory activities (14, 19–21), which makes delineating its pathogenetic roles even more challenging. Results from structure-function studies challenge the long-held and rather simplistic view of CRP as a stable pentameric protein and identified conformationally distinct forms, including native pentameric CRP (pCRP) and modified/monomeric CRP (mCRP), which exhibit distinct functional properties and may explain many of the opposing biological activities attributed to CRP (21, 22). CRP synthesis, structure and biological activities have been reviewed in detail elsewhere (19, 21, 23–25). In this review, we aimed to critically assess the competing views on the role of CRP isomers in disease pathogenesis and therapy, focusing on recent advances that may provide a rationale basis for guiding therapy and/or therapeutic targeting of CRP isomers to limit inflammation underlying various diseases.

Native CRP is member of the pentraxin family, an evolutionarily highly conserved class of pattern recognition molecules. The human CRP gene, located on chromosome 1, q23-q24 encodes for a CRP subunit of a single polypeptide chain of 206 amino acids (23). CRP is composed of five identical non-covalently bound subunits forming a planar ring with a central pore (23). The two opposite faces of the pentamer, and thus each protomer, have distinct binding properties. The A-face binds and activates complement C1q, whereas the B-face contains the Ca²⁺-dependent binding pocket for phosphocholine, expressed by bacterial, fungal and eukaryotic cells (24, 26). CRP also binds to nuclear antigens, the oxidized LDL receptor, apoptotic cell membrane, glycan components of microorganisms, and many other ligands (24, 27, 28), though some of these interactions have been disputed.

Native CRP is predominantly synthesized in hepatocytes under transcriptional control by cytokines (IL-6 and to a lesser extent IL-1β and TNF-α), the transcription factors hepatic nuclear factor (HNF) 1α and HNF3 as part of the “reorchestration” of hepatic gene expression in response to infection or tissue injury (19), promoter methylation and a distal enhancer (29). Hepatic secretion of pCRP accounts for rapid increases in serum pCRP levels during the acute-phase reaction. The serum half-life of pCRP is about 19 h under both physiological and pathological conditions (30), thus directly reflecting the rate of its hepatic synthesis. Ethnicity, sex and polymorphism in the apoliprotein E and CRP genes are known to influence baseline serum pCRP levels in humans (31, 32). CRP gene polymorphism influences gene expression and may predispose to systemic lupus erythematosus (31), but do not appear to be associated with increased risk for cardiovascular diseases (33, 34). Additionally, the kidney has been reported as a second site of pCRP formation in humans (35). Expression of CRP mRNA and pCRP synthesis has also been detected in the diseased vessel wall, coronary artery bypass grafts and neurons (35–37). The contribution of these sites to circulating pCRP remains to be investigated.

Under physiological conditions, pCRP appears to exist in a NaCl concentration-dependent pentamer-decamer equilibrium (38). Another form of CRP, characterized by multiple-size lettuce-like structures of about 300-500kDa, were detected in the serum of obese patients (39). The origin and pathological significance of these CRP forms are not known.

Although not specific for a single disease process, CRP is commonly used as a static measurement and CRP levels have been correlated with disease activity and to some degree, severity and prognosis in several diseases. CRP has been promoted as an independent predictor of cardiovascular events and metabolic syndrome (40–42), though the association is considerably weaker than previously thought (43, 44). The data from Mendelian randomization studies (33, 34) coupled with animal studies with injection of human pCRP and transgenic mice over-expressing human CRP may support an association between CRP and cardiovascular disease, but provide no direct evidence for a causative role for pCRP. In the Dallas Heart study hs-CRP was not independently associated with atherosclerotic burden in the coronary artery and abdominal aorta (45), whereas the REVERSAL trial reported that lower hs-CRP levels were independently and significantly correlated with atherosclerosis progression (46). Other studies have concluded that hs-CRP likely serves as a biomarker of vascular inflammation underlying atherosclerosis (14, 44). Thus, whereas the potential of therapeutic targeting of pCRP in cardiovascular disease remains unresolved, hs-CRP has clinical usefulness in guiding therapy as discussed below.

Other clinical studies reported positive correlation between elevated plasma CRP levels and myocardial infarct size (47), reduced lung function in chronic obstructive pulmonary disease (48) or the severity of COVID-19-evoked respiratory distress (49, 50). Higher plasma CRP levels were found to predict flares in systemic lupus erythematosus (2) and to portend poor prognosis in melanoma (51). Patients with non-small cell lung cancer who received the immune checkpoint PD-1 inhibitor nivolumab, early increases in hs-CRP and IL-6 were predictive for the efficacy of treatment (52). Nivolumab evoked a CRP flare-response (defined as a rapid, more than twofold increase in CRP levels followed by decrease below baseline values within 3 months) in about 25% of patients with metastatic renal cell carcinoma and this was associated with significant tumor shrinkage and improved 1-year survival rate (53).

The association of CRP with prognosis should, however, be interpreted with caution as an indication of direct causal contribution of CRP to disease pathogenesis. A definitive way to test this is the use of compounds that specifically block binding of CRP to its ligands and/or receptors to assess its pro-inflammatory effects in vivo.

While pCRP has been postulated to be stable under physiological conditions (24), compelling evidence indicates that CRP exists in conformationally distinct forms and conformational changes in pCRP results in expression of potent pro-inflammatory activities ( Figure 1 ) (21). The mild acidic environment within inflamed tissues confers pCRP binding specificities for factor H (55) and conformationally altered proteins, such as oxidized LDL and complement C3b (56) that do not bind to pCRP at physiological pH. Binding of circulating pCRP to phosphocholine or phosphoethanolamine head groups of membrane lipids expressed on the surface of activated platelets or apoptotic cells induces the formation of a partially dissociated pentamer (pCRP*), which then dissociates into the monomeric subunits, mCRP (57–59). pCRP* and mCRP exhibit potent pro-inflammatory activities, including stimulation of IL-8 secretion from neutrophils and human coronary artery endothelial cells (60, 61), promote neutrophil adhesion to platelets and endothelial cells (62, 63), delay neutrophil apoptosis (64) and trigger extrusion of neutrophil extracellular traps (65), characteristic features of the inflammatory response. pCRP* binds and activate complement C1q (66), which, in turn, can amplify pre-existing inflammation and tissue damage (21, 57, 67). Accumulation of pro-inflammatory CRP isoforms with in inflamed/injured but not in healthy tissues, and local expression of mCRP, for example within arteriosclerotic plaques (68) and in circulating microparticles (69, 70) would ensure localization of inflammation (57) and precipitate tissue injury (21).

While purified human pCRP itself does not evoke inflammation when injected into healthy individuals (71), it can amplify tissue injury in animal models, induce the expression of adhesion molecules and production of IL-6 and IL-8 (24, 72–75). Caution should be exercised in interpreting these observations because many of these effects can be attributed to contaminants (endotoxin or the preservative sodium azide) in commercial CRP preparations not to pCRP itself (13, 21). By contrast, other studies documented pCRP protection against the assembly of the terminal complement attack complex (27) and bacterial sepsis (76), reversal of proteinuria in autoimmune mice (77), and prevention of autoimmunity (78). Recently, pCRP was found to reduce immune complex-triggered type I interferon response, consistent with a protective action in systemic lupus erythematosus (79). This action was lost in mCRP, further highlighting the complexity of regulation of immune response by CRP isomers in autoimmune diseases.

CRP isomers bind to distinct receptors. Thus, pCRP binds primarily to the low affinity Ig receptor FcγRIIa (CD32) and to some extent to the high affinity IgG receptor FCγRI (CD64) on phagocytes and endothelial cells (21, 80), and αI_Ibβ₃integrin on platelets (81), whereas mCRP binds to FcγRIIIb (CD64) on phagocytes and lipid rafts on human endothelial cells (82).

The concept of activation-induced conformational changes could explain why pCRP itself is not pro-inflammatory in the absence of infection or tissue injury. Conformational changes in pCRP, and generation of pCRP* and mCRP, would unmask “hidden” pro-inflammatory activities that may collectively amplify the initial inflammatory signal evoked by infection or tissue injury, leading to exacerbation of tissue damage and more severe disease (21, 73). However, further studies are needed to explore the clinical importance of mCRP or other CRP conformers.

CRP is a well-established biomarker of inflammation and much written about its association with disease state. Circulating hs-CRP may identify patients at risk, predict disease progression and outcome, and guide therapy in a context-dependent manner. Nevertheless, since even strong associations do not establish causality, further studies are clearly warranted to elucidate its potential pathogenic roles. Activation-induced conformational changes in pCRP would unmask “hidden” pro-inflammatory properties as opposed to the largely protective role of pCRP. CRP lowering strategies yielded promising, but often inconclusive data on altering disease progression. Development of CRP antagonists, and in particular recent development of a phosphocholine mimetic that binds to pCRP and inhibits conformation change-mediated expression of pro-inflammatory activities without impairing pCRP’s defense function, should facilitate future investigations into the full spectrum of the roles of CRP isomers in inflammatory pathologies. This approach has the potential of opening a novel therapeutic avenue for preventing or limiting the deleterious actions attributed to CRP.

All authors contributed to the article and approved the submitted work.