Redefining CRP in tissue injury and repair: more than an acute pro-inflammatory mediator

Most early studies investigating the role of C-reactive protein (CRP) in tissue damage determined it supported pro-hemostatic and pro-inflammatory activities. However, these findings were not universal, as other data suggested CRP inhibited these same processes. A potential explanation for these disparate observations finally emerged with the recognition that CRP undergoes context-dependent conformational changes in vivo, and each of its three isoforms - pentameric CRP (pCRP), modified pentameric CRP (pCRP*), and monomeric CRP (mCRP) - have different effects. In this review, we consider this new paradigm and re-evaluate the role of CRP and its isoforms in the tissue repair process. Indeed, a growing body of evidence points toward the involvement of CRP not just in hemostasis and inflammation, but also in the resolution of inflammation and in tissue regeneration. Additionally, we briefly discuss the shortcomings of the currently available diagnostic tests for CRP and highlight the need for change in how CRP is currently utilized in clinical practice.

Association of high-sensitivity C-reactive protein with hepatic fibrosis in patients with metabolic dysfunction-associated steatotic liver disease

Objective

This study aimed to investigate the association between high-sensitivity C-reactive protein (hsCRP) levels and hepatic fibrosis in patients with metabolic dysfunction-associated steatotic liver disease (MASLD) and assess its predictive efficacy.

Methods

The study included 1,477 participants from the United States and 1,531 from China diagnosed with MASLD. Liver stiffness measurement (LSM) and controlled attenuation parameter (CAP) were assessed by vibration-controlled transient elastography (VCTE) to evaluate the presence and degree of hepatic fibrosis and steatosis. The relationship between hsCRP levels and hepatic fibrosis in MASLD patients was examined using multivariable-adjusted and restricted cubic spline (RCS) models. Additionally, subgroup analyses were conducted to investigate the potential heterogeneity among different characteristic subgroups.

Results

The results demonstrated a significant correlation between elevated hsCRP levels and an increased risk of significant fibrosis, advanced fibrosis, and cirrhosis in the US cohort of MASLD patients (OR 2.22, 1.69, and 2.85, respectively; all P <0.05). The results of the Chinese cohort were consistent with those of the US cohort, and there was a significant and positive correlation between hsCRP levels and the risk of hepatic fibrosis in patients with MASLD (OR 2.53, 3.85, and 3.78, respectively, all P <0.001). The RCS analysis revealed a significant non-linear relationship between hsCRP levels and the degree of hepatic fibrosis, with disparate inflection point values observed across different cohorts (approximately 9 mg/L in the US cohort and 4 mg/L in the Chinese cohort). The impact of hsCRP levels on the risk of hepatic fibrosis varied across different subgroups with distinct characteristics.

Conclusion

The present study demonstrated a significant correlation between hsCRP levels and the degree of hepatic fibrosis in patients with MASLD, with notable dose-response relationships and subgroup differences.

A conformational change of C-reactive protein drives neutrophil extracellular trap formation in inflammation

BACKGROUND

C-reactive protein (CRP) represents a routine diagnostic marker of inflammation. Dissociation of native pentameric CRP (pCRP) into the monomeric structure (mCRP) liberates proinflammatory features, presumably contributing to excessive immune cell activation via unknown molecular mechanisms.

RESULTS

In a multi-translational study of systemic inflammation, we found a time- and inflammation-dependent pCRP dissociation into mCRP. We were able to confirm that mCRP co-localizes with leukocytes at the site of injury after polytrauma and therefore assessed whether the CRP conformation potentiates neutrophil activation. We found mCRP-induced neutrophil-extracellular trap formation in vitro and ex vivo involving nicotinamide adenine dinucleotide phosphate oxidase activation, p38/mitogen-activated protein kinase signaling, and histone H3 citrullination. Mimicking the trauma milieu in a human ex vivo whole blood model, we found significant mCRP generation as well as NET formation, prevented by blocking pCRP conformational changes.

CONCLUSIONS

Our data provide novel molecular insights how CRP dissociation contributes to neutrophil activation as driver of various inflammatory disorders.

C-Reactive Protein: Pathophysiology, Diagnosis, False Test Results and a Novel Diagnostic Algorithm for Clinicians

The current literature provides a body of evidence on C-Reactive Protein (CRP) and its potential role in inflammation. However, most pieces of evidence are sparse and controversial. This critical state-of-the-art monography provides all the crucial data on the potential biochemical properties of the protein, along with further evidence on its potential pathobiology, both for its pentameric and monomeric forms, including information for its ligands as well as the possible function of autoantibodies against the protein. Furthermore, the current evidence on its potential utility as a biomarker of various diseases is presented, of all cardiovascular, respiratory, hepatobiliary, gastrointestinal, pancreatic, renal, gynecological, andrological, dental, oral, otorhinolaryngological, ophthalmological, dermatological, musculoskeletal, neurological, mental, splenic, thyroid conditions, as well as infections, autoimmune-supposed conditions and neoplasms, including other possible factors that have been linked with elevated concentrations of that protein. Moreover, data on molecular diagnostics on CRP are discussed, and possible etiologies of false test results are highlighted. Additionally, this review evaluates all current pieces of evidence on CRP and systemic inflammation, and highlights future goals. Finally, a novel diagnostic algorithm to carefully assess the CRP level for a precise diagnosis of a medical condition is illustrated.

A biofunctional review of C-reactive protein (CRP) as a mediator of inflammatory and immune responses: differentiating pentameric and modified CRP isoform effects

C-reactive protein (CRP) is an acute phase, predominantly hepatically synthesized protein, secreted in response to cytokine signaling at sites of tissue injury or infection with the physiological function of acute pro-inflammatory response. Historically, CRP has been classified as a mediator of the innate immune system, acting as a pattern recognition receptor for phosphocholine-containing ligands. For decades, CRP was envisioned as a single, non-glycosylated, multi-subunit protein arranged non-covalently in cyclic symmetry around a central void. Over the past few years, however, CRP has been shown to exist in at least three distinct isoforms: 1.) a pentamer of five identical globular subunits (pCRP), 2.) a modified monomer (mCRP) resulting from a conformational change when subunits are dissociated from the pentamer, and 3.) a transitional isoform where the pentamer remains intact but is partially changed to express mCRP structural characteristics (referred to as pCRP* or mCRPm). The conversion of pCRP into mCRP can occur spontaneously and is observed under commonly used experimental conditions. In careful consideration of experimental design used in published reports of in vitro pro- and anti-inflammatory CRP bioactivities, we herein provide an interpretation of how distinctive CRP isoforms may have affected reported results. We argue that pro-inflammatory amplification mechanisms are consistent with the biofunction of mCRP, while weak anti-inflammatory mechanisms are consistent with pCRP. The interplay of each CRP isoform with specific immune cells (platelets, neutrophils, monocytes, endothelial cells, natural killer cells) and mechanisms of the innate immune system (complement), as well as differences in mCRP and pCRP ligand recognition and effector functions are discussed. This review will serve as a revised understanding of the structure-function relationship between CRP isoforms as related to inflammation and innate immunity mechanisms.

Extracellular vesicles opsonized by monomeric C-reactive protein (CRP) are accessible as autoantigens in patients with systemic lupus erythematosus and associate with autoantibodies against CRP

The pentraxin C-reactive protein (CRP) is a pentameric protein now known to be able to undergo dissociation into a monomeric, modified isoform, referred to as mCRP. In carefully assessing the bioactivities of each isoform, mCRP has strong pro-inflammatory activities while pCRP has mild anti-inflammatory activities. Systemic lupus erythematosus (SLE) is a disease characterized by a vast number of autoantibodies, including anti-CRP autoantibodies which have been associated with SLE disease activity and lupus nephritis. The origin of these autoantibodies is currently unknown. Extracellular vesicles (EVs) have been implicated in SLE pathogenesis as they can expose nuclear antigens on their outside surface, thereby being a potential adjuvant for the generation of autoantibodies. Herein, we studied exposure of both pCRP and mCRP on EVs in SLE plasma and the implications of each in disease activity, organ damage and clinical manifestations. We used flow cytometry to detect CRP isoforms on EV surfaces in 67 well-characterized SLE patients and 60 sex- and age-matched healthy controls. Autoantibodies against mCRP were measured using ELISA. We found an abundance of both pCRP and mCRP on SLE EVs compared to controls. Furthermore, mCRP+ but not pCRP+ EVs were elevated in patients with active disease and in anti-CRP positive patients. The proportions of mCRP+ EVs were lower in patients with acquired organ damage, especially in patients with lupus nephritis (LN), and displayed an inverse relationship with disease duration in LN and patients with active disease. Speculatively, these data suggest EV-bound mCRP as a relevant factor in SLE pathogenesis, which could contribute to development of anti-CRP autoantibodies by stimulating an immune response.

C-reactive protein: a target for therapy to reduce inflammation

C-reactive protein (CRP) is well-recognized as a sensitive biomarker of inflammation. Association of elevations in plasma/serum CRP level with disease state has received considerable attention, even though CRP is not a specific indicator of a single disease state. Circulating CRP levels have been monitored with a varying degree of success to gauge disease severity or to predict disease progression and outcome. Elevations in CRP level have been implicated as a useful marker to identify patients at risk for cardiovascular disease and certain cancers, and to guide therapy in a context-dependent manner. Since even strong associations do not establish causality, the pathogenic role of CRP has often been over-interpreted. CRP functions as an important modulator of host defense against bacterial infection, tissue injury and autoimmunity. CRP exists in conformationally distinct forms, which exhibit distinct functional properties and help explaining the diverse, often contradictory effects attributed to CRP. In particular, dissociation of native pentameric CRP into its subunits, monomeric CRP, unmasks "hidden" pro-inflammatory activities in pentameric CRP. Here, we review recent advances in CRP targeting strategies, therapeutic lowering of circulating CRP level and development of CRP antagonists, and a conformation change inhibitor in particular. We will also discuss their therapeutic potential in mitigating the deleterious actions attributed to CRP under various pathologies, including cardiovascular, pulmonary and autoimmune diseases and cancer.

Monomeric C-reactive protein evokes TCR Signaling-dependent bystander activation of CD4+ T cells

Bystander activation of T cells is defined as induction of effector responses by innate cytokines in the absence of cognate antigens and independent of T cell receptor (TCR) signaling. Here we show that C-reactive protein (CRP), a soluble pattern-recognition receptor assembled noncovalently by five identical subunits, can instead trigger bystander activation of CD4 + T cells by evoking allosteric activation and spontaneous signaling of TCR in the absence of cognate antigens. The actions of CRP depend on pattern ligand-binding induced conformational changes that result in the generation of monomeric CRP (mCRP). mCRP binds cholesterol in plasma membranes of CD4 + T cells, thereby shifting the conformational equilibrium of TCR to the cholesterol-unbound, primed state. The spontaneous signaling of primed TCR leads to productive effector responses manifested by upregulation of surface activation markers and release of IFN-γ. Our results thus identify a novel mode of bystander T cell activation triggered by allosteric TCR signaling, and reveal an interesting paradigm wherein innate immune recognition of CRP transforms it to a direct activator that evokes immediate adaptive immune responses.

Fueling the flames of colon cancer – does CRP play a direct pro-inflammatory role?

Background

Systemic inflammation, diagnostically ascribed by measuring serum levels of the acute phase reactant C-reactive protein (CRP), has consistently been correlated with poor outcomes across cancer types. CRP exists in two structurally and functionally distinct isoforms, circulating pentameric CRP (pCRP) and the highly pro-inflammatory monomeric isoform (mCRP). The aim of this pilot study was to map the pattern of mCRP distribution in a previously immunologically well-defined colon cancer (CC) cohort and explore possible functional roles of mCRP within the tumor microenvironment (TME).

Methods

Formalin-fixed, paraffin-embedded (FFPE) tissue samples from 43 stage II and III CC patients, including 20 patients with serum CRP 0-1 mg/L and 23 patients with serum CRP &gt;30 mg/L were immunohistochemically (IHC) stained with a conformation-specific mCRP antibody and selected immune and stromal markers. A digital analysis algorithm was developed for evaluating mCRP distribution within the primary tumors and adjacent normal colon mucosa.

Results

mCRP was abundantly present within tumors from patients with high serum CRP (&gt;30 mg/L) diagnostically interpreted as being systemically inflamed, whereas patients with CRP 0-1 mg/L exhibited only modest mCRP positivity (median mCRP per area 5.07‰ (95%CI:1.32-6.85) vs. 0.02‰ (95%CI:0.01-0.04), p&lt;0.001). Similarly, tissue-expressed mCRP correlated strongly with circulating pCRP (Spearman correlation 0.81, p&lt;0.001). Importantly, mCRP was detected exclusively within tumors, whereas adjacent normal colon mucosa showed no mCRP expression. Double IHC staining revealed colocalization of mCRP with endothelial cells and neutrophils. Intriguingly, some tumor cells also colocalized with mCRP, suggesting a direct interaction or mCRP expression by the tumor itself.

Conclusion

Our data show that the pro-inflammatory mCRP isoform is expressed in the TME of CC, primarily in patients with high systemic pCRP values. This strengthens the hypothesis that CRP might not only be an inflammatory marker but also an active mediator within tumors.

Monomeric C-Reactive Protein in Atherosclerotic Cardiovascular Disease: Advances and Perspectives

This review aimed to trace the inflammatory pathway from the NLRP3 inflammasome to monomeric C-reactive protein (mCRP) in atherosclerotic cardiovascular disease. CRP is the final product of the interleukin (IL)-1β/IL-6/CRP axis. Its monomeric form can be produced at sites of local inflammation through the dissociation of pentameric CRP and, to some extent, local synthesis. mCRP has a distinct proinflammatory profile. In vitro and animal-model studies have suggested a role for mCRP in: platelet activation, adhesion, and aggregation; endothelial activation; leukocyte recruitment and polarization; foam-cell formation; and neovascularization. mCRP has been shown to deposit in atherosclerotic plaques and damaged tissues. In recent years, the first published papers have reported the development and application of mCRP assays. Principally, these studies demonstrated the feasibility of measuring mCRP levels. With recent advances in detection techniques and the introduction of first assays, mCRP-level measurement should become more accessible and widely used. To date, anti-inflammatory therapy in atherosclerosis has targeted the NLRP3 inflammasome and upstream links of the IL-1β/IL-6/CRP axis. Large clinical trials have provided sufficient evidence to support this strategy. However, few compounds target CRP. Studies on these agents are limited to animal models or small clinical trials.

C-reactive protein, immunothrombosis and venous thromboembolism

C-reactive protein (CRP) is a member of the highly conserved pentraxin superfamily of proteins and is often used in clinical practice as a marker of infection and inflammation. There is now increasing evidence that CRP is not only a marker of inflammation, but also that destabilized isoforms of CRP possess pro-inflammatory and pro-thrombotic properties. CRP circulates as a functionally inert pentameric form (pCRP), which relaxes its conformation to pCRP* after binding to phosphocholine-enriched membranes and then dissociates to monomeric CRP (mCRP). with the latter two being destabilized isoforms possessing highly pro-inflammatory features. pCRP* and mCRP have significant biological effects in regulating many of the aspects central to pathogenesis of atherothrombosis and venous thromboembolism (VTE), by directly activating platelets and triggering the classical complement pathway. Importantly, it is now well appreciated that VTE is a consequence of thromboinflammation. Accordingly, acute VTE is known to be associated with classical inflammatory responses and elevations of CRP, and indeed VTE risk is elevated in conditions associated with inflammation, such as inflammatory bowel disease, COVID-19 and sepsis. Although the clinical data regarding the utility of CRP as a biomarker in predicting VTE remains modest, and in some cases conflicting, the clinical utility of CRP appears to be improved in subsets of the population such as in predicting VTE recurrence, in cancer-associated thrombosis and in those with COVID-19. Therefore, given the known biological function of CRP in amplifying inflammation and tissue damage, this raises the prospect that CRP may play a role in promoting VTE formation in the context of concurrent inflammation. However, further investigation is required to unravel whether CRP plays a direct role in the pathogenesis of VTE, the utility of which will be in developing novel prophylactic or therapeutic strategies to target thromboinflammation.

Relevance of lipoproteins, membranes, and extracellular vesicles in understanding C-reactive protein biochemical structure and biological activities

Early purification protocols for C-reactive protein (CRP) often involved co-isolation of lipoproteins, primarily very low-density lipoproteins (VLDLs). The interaction with lipid particles was initially attributed to CRP’s calcium-dependent binding affinity for its primary ligand—phosphocholine—the predominant hydrophilic head group expressed on phospholipids of most lipoprotein particles. Later, CRP was shown to additionally express binding affinity for apolipoprotein B (apo B), a predominant apolipoprotein of both VLDL and LDL particles. Apo B interaction with CRP was shown to be mediated by a cationic peptide sequence in apo B. Optimal apo B binding required CRP to be surface immobilized or aggregated, treatments now known to structurally change CRP from its serum soluble pentamer isoform (i.e., pCRP) into its poorly soluble, modified, monomeric isoform (i.e., mCRP). Other cationic ligands have been described for CRP which affect complement activation, histone bioactivities, and interactions with membranes. mCRP, but not pCRP, binds cholesterol and activates signaling pathways that activate pro-inflammatory bioactivities long associated with CRP as a biomarker. Hence, a key step to express CRP’s biofunctions is its conversion into its mCRP isoform. Conversion occurs when (1) pCRP binds to a membrane surface expressed ligand (often phosphocholine); (2) biochemical forces associated with binding cause relaxation/partial dissociation of secondary and tertiary structures into a swollen membrane bound intermediate (described as mCRP_m or pCRP*); (3) further structural relaxation which leads to total, irreversible dissociation of the pentamer into mCRP and expression of a cholesterol/multi-ligand binding sequence that extends into the subunit core; (4) reduction of the CRP subunit intrachain disulfide bond which enhances CRP’s binding accessibility for various ligands and activates acute phase proinflammatory responses. Taken together, the biofunctions of CRP involve both lipid and protein interactions and a conformational rearrangement of higher order structure that affects its role as a mediator of inflammatory responses.

Regulation of Conformational Changes in C-reactive Protein Alters its Bioactivity

The acute phase C-reactive protein (CRP) is mainly synthesized and secreted by the liver in a cytokine-mediated response to infection or inflammation and circulates as a pentamer (pCRP) in plasma. Recent studies indicate that CRP is not only a marker but is directly involved in inflammation. CRP has a vital role in host defense and inflammation, metabolic function and scavenging through its ability for calcium depended binding to exogenous and endogenous molecules having phosphocholine followed by activation of the classical complement pathway. Accumulating evidence indicates that pCRP dissociates into monomeric CRP (mCRP) and most proinflammatory actions of CRP are only expressed following dissociation of its native pentameric assembly into mCRP. The dissociation of CRP into mCRP altogether promotes the ligand-binding capability. mCRP emerges to be the main conformation of CRP that participates in the regulation of local inflammation, however, little is identified concerning what triggers the significantly enhanced actions of mCRP and their binding to diverse ligands. The separation of mCRP from pCRP may be a direct relationship between CRP and inflammation. Here we review the current literature on CRP dissociation and its interaction with different ligands. The possibility to avoid the generation of the proinflammatory potential of mCRP has driven therapeutic approaches by targeting the dissociation mechanism of pCRP or inhibition of mCRP itself during inflammation.

Transitional changes in the structure of C-reactive protein create highly pro-inflammatory molecules: Therapeutic implications for cardiovascular diseases

C-reactive protein (CRP) is the prototypic acute-phase reactant that has long been recognized almost exclusively as a marker of inflammation and predictor of cardiovascular risk. However, accumulating evidence indicates that CRP is also a direct pathogenic pro-inflammatory mediator in atherosclerosis and cardiovascular diseases. The 'CRP system' consists of at least two protein conformations with distinct pathophysiological functions. The binding of the native, pentameric CRP (pCRP) to activated cell membranes leads to a conformational change resulting in two highly pro-inflammatory isoforms, pCRP* and monomeric CRP (mCRP). The deposition of these pro-inflammatory isoforms has been shown to aggravate the localized tissue injury in a broad range of pathological conditions including atherosclerosis and thrombosis, myocardial infarction, and stroke. Here, we review recent findings on how these structural changes contribute to the inflammatory response and discuss the transitional changes in the structure of CRP as a novel therapeutic target in cardiovascular diseases and overshooting inflammation.

mCRP as a Biomarker of Adult-Onset Still’s Disease: Quantification of mCRP by ELISA

Background

C-reactive protein (CRP) is a dynamic protein that undergoes conformational changes between circulating native pentameric CRP (pCRP), pentameric symmetrical forms (pCRP*) and monomeric (or modified) CRP (mCRP) forms. mCRP exhibits strong pro-inflammatory activity and activates platelets, leukocytes, and endothelial cells. Abundant deposition of mCRP in inflamed tissues plays a role in several disease conditions, such as ischemia/reperfusion injury, Alzheimer’s disease, and cardiovascular disease. Although pCRP is typically quantified rather than mCRP for clinical purposes, mCRP may be a more appropriate disease marker of inflammatory diseases. Therefore, simple methods for quantifying mCRP are needed.

Methods

We developed a specific enzyme-linked immunosorbent assay (ELISA) to measure plasma levels of mCRP. Plasma mCRP concentration was measured in patients with adult-onset Still’s disease (AOSD) (n=20), polymyalgia rheumatica (PMR) (n=20), rheumatoid arthritis (RA) (n=30), infection (n=50), and in control subjects (n=30) using the developed ELISA.

Results

We demonstrated that mCRP is elevated in some inflammatory autoimmune diseases, particularly AOSD. The mCRP concentration was also significantly higher among AOSD patients than RA, PMR patients and controls (477 ng/ml, 77 ng/ml, 186 ng/ml, and 1.2 ng/ml, respectively). Also, the mCRP (×1,000)/pCRP ratio was significantly higher among AOSD patients than RA, PMR, and infection patients (3.5, 0.6, 1,6, and 2.0, respectively).

Conclusion

The plasma mCRP levels are elevated in some autoimmune diseases, particularly AOSD. The plasma mCRP levels may therefore be a potentially useful biomarker for AOSD.

Secretory quality control constrains functional selection-associated protein structure innovation

Biophysical models suggest a dominant role of structural over functional constraints in shaping protein evolution. Selection on structural constraints is linked closely to expression levels of proteins, which together with structure-associated activities determine in vivo functions of proteins. Here we show that despite the up to two orders of magnitude differences in levels of C-reactive protein (CRP) in distinct species, the in vivo functions of CRP are paradoxically conserved. Such a pronounced level-function mismatch cannot be explained by activities associated with the conserved native structure, but is coupled to hidden activities associated with the unfolded, activated conformation. This is not the result of selection on structural constraints like foldability and stability, but is achieved by folding determinants-mediated functional selection that keeps a confined carrier structure to pass the stringent eukaryotic quality control on secretion. Further analysis suggests a folding threshold model which may partly explain the mismatch between the vast sequence space and the limited structure space of proteins.

The mismatch in the conserved structure but different expression levels of C-reactive protein (CRP) in distinct species is reconciled by functional selection on hidden activities of unfolded CRPs.

Biochemically prepared C-reactive protein conformational states differentially affect C1q binding

C-reactive protein (CRP) is commonly measured as an inflammatory marker in patient studies for coronary heart disease, autoimmune disease and recent acute infections. Due to a correlation of CRP to a vast number of disease states, CRP is a well-studied protein in medical literature with over 16000 references in PubMed [1]. However, the biochemical and structural variations of CRP are not well understood in regards to their binding of complement immune response proteins. Conformations of CRP are thought to affect disease states differently, with a modified form showing neoepitopes and activating the complement immune response through C1q binding. In this work, we compare the unfolding of CRP using chemical denaturants and identify which states of CRP bind a downstream complement immune response binding partner (C1q). We used guanidine HCl (GndHCl), urea/EDTA, and 0.01% SDS with heat to perturb the pentameric state. All treatments give rise to a monomeric state in non-denaturing polyacrylamide gel electrophoresis experiments, but only treatment with certain concentrations of denaturant or dilute SDS with heat maintains CRP function with a key downstream binding partner, C1q, as measured by enzyme-linked immunosorbent assays. The results suggest that the final form of modified CRP and its ability to mimic biological binding is dependent on the preparation method.

The Complex Role of C-Reactive Protein in Systemic Lupus Erythematosus

C-reactive protein (CRP) is well-known as a sensitive albeit unspecific biomarker of inflammation. In most rheumatic conditions, the level of this evolutionarily highly conserved pattern recognition molecule conveys reliable information regarding the degree of ongoing inflammation, driven mainly by interleukin-6. However, the underlying causes of increased CRP levels are numerous, including both infections and malignancies. In addition, low to moderate increases in CRP predict subsequent cardiovascular events, often occurring years later, in patients with angina and in healthy individuals. However, autoimmune diseases characterized by the Type I interferon gene signature (e.g., systemic lupus erythematosus, primary Sjögren’s syndrome and inflammatory myopathies) represent exceptions to the general rule that the concentrations of CRP correlate with the extent and severity of inflammation. In fact, adequate levels of CRP can be beneficial in autoimmune conditions, in that they contribute to efficient clearance of cell remnants and immune complexes through complement activation/modulation, opsonization and phagocytosis. Furthermore, emerging data indicate that CRP constitutes an autoantigen in systemic lupus erythematosus. At the same time, the increased risks of cardiovascular and cerebrovascular diseases in patients diagnosed with systemic lupus erythematosus and rheumatoid arthritis are well-established, with significant impacts on quality of life, accrual of organ damage, and premature mortality. This review describes CRP-mediated biological effects and the regulation of CRP release in relation to aspects of cardiovascular disease and mechanisms of autoimmunity, with particular focus on systemic lupus erythematosus.

Monomeric C reactive protein (mCRP) regulates inflammatory responses in human and mouse chondrocytes

C-reactive protein (CRP) is an acute-phase protein that is used as an established biomarker to follow disease severity and progression in a plethora of inflammatory diseases. However, its pathophysiologic mechanisms of action are still poorly defined and remain elusive. CRP, in its pentameric form, exhibits weak anti-inflammatory activity. On the contrary, the monomeric isoform (mCRP) exhibits potent pro-inflammatory properties in endothelial cells, leukocytes, and platelets. So far, no data exists regarding mCRP effects in human or mouse chondrocytes. This work aimed to verify the pathophysiological relevance of mCRP in the etiology and/or progression of osteoarthritis (OA). We investigated the effects of mCRP in cultured human primary chondrocytes and in the chondrogenic ATDC5 mouse cell line. We determined mRNA and protein levels of relevant factors involved in inflammatory responses and the modulation of nitric oxide synthase type II (NOS2), an early inflammatory molecular target. We demonstrate, for the first time, that monomeric C reactive protein increases NOS2, COX2, MMP13, VCAM1, IL-6, IL-8, and LCN2 expression in human and murine chondrocytes. We also demonstrated that NF-kB is a key factor in the intracellular signaling of mCRP-driven induction of pro-inflammatory and catabolic mediators in chondrocytes. We concluded that mCRP exerts a sustained catabolic effect on human and murine chondrocytes, increasing the expression of inflammatory mediators and proteolytic enzymes, which can promote extracellular matrix (ECM) breakdown in healthy and OA cartilage. In addition, our results implicate the NF-kB signaling pathway in catabolic effects mediated by mCRP.

Purification of Recombinant Mouse C-Reactive Protein from Pichia Pastoris GS115 by Nickel Chelating Sepharose Fast-Flow Affinity Chromatography and P-Aminophenyl Phosphoryl Choline Agarose Resin Affinity Chromatography in Tandem

C-reactive protein (CRP) is a circulating marker of inflammation yet with ill-defined biological functions. This is partly due to the uncharacterized activities of endogenous CRP in mice, the major animal model used to define protein function. The hurdles for purification and characterization of mouse CRP are its low circulating levels and the lack of specific antibodies. To clear these hurdles, here we developed an efficient expression system by constructing recombinant Pichia pastoris cells for secretion of native conformation mouse CRP. The recombinant expression of mouse CRP in Escherichia coli failed to yield sufficient amount of native protein, reflecting the importance of post-translational modification of glycosylation in aiding proper folding. By contrast, sufficient amount of native mouse CRP was successfully purified from P. pastoris. Preliminary purification was performed by Nickel Chelating Sepharose Fast-Flow affinity chromatography with 6 × His tags attached to the protein. Subsequently, p-Aminophenyl Phosphoryl Choline Agarose resin affinity chromatography was used for tandem purification. The purified mouse CRP showed native pentamer and capabilities of PC binding. Moreover, the 6 × His tag provides a convenient tool for detecting the interactions of mouse CRP with ligands.

Anti-Monomeric C-Reactive Protein Antibody Ameliorates Arthritis and Nephritis in Mice

Conformation-specific Ags are ideal targets for mAb-based immunotherapy. Here, we demonstrate that the monomeric form of C-reactive protein (mCRP) is a specific therapeutic target for arthritis and nephritis in a murine model. Screening of >1800 anti-mCRP mAb clones identified 3C as a clone recognizing the monomeric, but not polymeric, form of CRP. The anti-mCRP mAb suppressed leukocyte infiltration in thioglycollate-induced peritonitis, attenuated rheumatoid arthritis symptoms in collagen Ab-induced arthritis model mice, and attenuated lupus nephritis symptoms in MRL/Mp-lpr/lpr lupus-prone model mice. These data suggest that the anti-mCRP mAb 3C has therapeutic potential against rheumatoid arthritis and lupus nephritis.

C-Reactive Protein and Cancer: Interpreting the Differential Bioactivities of Its Pentameric and Monomeric, Modified Isoforms

C-reactive protein (CRP) was first recognized in the 1940s as a protein that appeared in blood during acute episodes of infectious disease. Its presence and pharmacodynamics were found in essentially all diseases that involved tissue damage and inflammation. Identified as a major component of the innate, unlearned immunity, it became a useful diagnostic marker for the extent of inflammation during disease exacerbation or remission. Efforts to define its true biological role has eluded clear definition for over a half-century. Herein, a unifying concept is presented that explains both pro-inflammatory and anti-inflammatory activities of CRP. This concept involves the recognition and understanding that CRP can be induced to undergo a pronounced, non-proteolytic reorganization of its higher-level protein structures into conformationally distinct isomers with distinctive functional activities. This process occurs when the non-covalently associated globular subunits of the pentameric isoform ("pCRP") are induced to dissociate into a monomeric isoform ("mCRP"). mCRP consistently and potently provides pro-inflammatory activation and amplification activities. pCRP provides weak anti-inflammatory activities consistent with low-level chronic inflammation. mCRP can spontaneously form in purified pCRP reagents in ways that are not immediately recognized during purification and certification analyses. By now understanding the factors that influence pCRP dissociate into mCRP, many published reports investigating CRP as a biological response modifier of host defense can be reevaluated to include a discussion of how each CRP isoform may have affected the generated results. Specific attention is given to in vitro and in vivo studies of CRP as an anti-cancer agent.

Better prognosis in females with severe COVID-19 pneumonia: possible role of inflammation as potential mediator

OBJECTIVES

Sex differences in COVID-19 severity and mortality have been described. Key aims of this analysis were to compare the risk of invasive mechanical ventilation (IMV) and mortality by sex and to explore whether variation in specific biomarkers could mediate this difference.

METHODS

This was a retrospective, observational cohort study among patients with severe COVID-19 pneumonia. A survival analysis was conducted to compare time to the composite endpoint of IMV or death according to sex. Interaction was formally tested to compare the risk difference by sex in sub-populations. Mediation analysis with a binary endpoint IMV or death (yes/no) by day 28 of follow-up for a number of inflammation/coagulation biomarkers in the context of counterfactual prediction was also conducted.

RESULTS

Among 415 patients, 134 were females (32%) and 281 males (67%), median age 66 years (IQR 54-77). At admission, females showed a significantly less severe clinical and respiratory profiles with a higher PaO2/FiO2 (254 mmHg vs. 191 mmHg; p 0.023). By 28 days from admission, 49.2% (95% CI 39.6-58.9%) of males vs. 31.7% (17.9-45.4%) of females underwent IMV or death (log-rank p < 0.0001) and this amounted to a difference in terms of HR of 0.40 (0.26-0.63, p 0.0001). The area under the curve in C-reactive protein (CRP) over the study period appeared to explain 85% of this difference in risk by sex.

DISCUSSION

Our analysis confirms a difference in the risk of COVID-19 clinical progression by sex and provides a hypothesis for potential mechanisms leading to this. Specifically, CRP showed a predominant role to mediate the difference in risk by sex.

Monomeric C-reactive protein promotes platelets to release mitochondrial DNA in anti-neutrophil cytoplasmic antibody-associated vasculitis

Although high level of circulating C-reactive protein (pCRP) is considered as a biomarker for disease activity, the significance of CRP in the pathogenesis of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) has not been clarified. We once reported in AAV, pentameric CRP (pCRP) could dissociate into monomeric CRP (mCRP) and activate platelets. Recent studies have demonstrated that the activated platelets can release mitochondrial DNA (mtDNA). The purpose of this study was to further study the relationship between mCRP and platelets in AAV. We found the plasma level of mCRP in AAV patients was significantly higher than that of normal control and positively correlated with the proportion of mCRP-positive platelets. Platelets isolated from one normal donor could be activated by plasma from 5 AAV patients and this effect could be attenuated when mCRP had been removed. Only 0.1 μg/mL of recombinant mCRP was needed for inducing platelets to release mtDNA via interaction with lipid raft and through p38 MAPK/NF-κB pathway. The mCRP binding on platelets depended on the C-terminal octapeptide (aa 199-206). The released mtDNA did not induce respiratory burst alone, but enhanced the ANCA-induced neutrophils respiratory burst after binding Toll-like receptor 9 (TLR9). The mtDNA released by mCRP-activated platelets also enhanced thrombin generation of plasma. In conclusion, our data demonstrate that mCRP can bind platelets via interaction with lipid raft and induce the release of mtDNA. The released mtDNA can enhance the pathogenicity of ANCA and promote activation of coagulation system in AAV.

Antibody Protection against Long-Term Memory Loss Induced by Monomeric C-Reactive Protein in a Mouse Model of Dementia

Monomeric C-reactive protein (mCRP), the activated isoform of CRP, induces tissue damage in a range of inflammatory pathologies. Its detection in infarcted human brain tissue and its experimentally proven ability to promote dementia with Alzheimer's disease (AD) traits at 4 weeks after intrahippocampal injection in mice have suggested that it may contribute to the development of AD after cerebrovascular injury. Here, we showed that a single hippocampal administration of mCRP in mice induced memory loss, lasting at least 6 months, along with neurodegenerative changes detected by increased levels of hyperphosphorylated tau protein and a decrease of the neuroplasticity marker Egr1. Furthermore, co-treatment with the monoclonal antibody 8C10 specific for mCRP showed that long-term memory loss and tau pathology were entirely avoided by early blockade of mCRP. Notably, 8C10 mitigated Egr1 decrease in the mouse hippocampus. 8C10 also protected against mCRP-induced inflammatory pathways in a microglial cell line, as shown by the prevention of increased generation of nitric oxide. Additional in vivo and in vitro neuroprotective testing with the anti-inflammatory agent TPPU, an inhibitor of the soluble epoxide hydrolase enzyme, confirmed the predominant involvement of neuroinflammatory processes in the dementia induced by mCRP. Therefore, locally deposited mCRP in the infarcted brain may be a novel biomarker for AD prognosis, and its antibody blockade opens up therapeutic opportunities for reducing post-stroke AD risk.

Current Position on the Role of Monomeric C-reactive Protein in Vascular Pathology and Atherothrombosis

C-reactive Protein (CRP) is an acute phase reactant, belonging to the pentraxin family of proteins. Its level rises up to 1000-fold in response to acute inflammation. High sensitivity CRP level is utilized as an independent biomarker of inflammation and cardiovascular disease. The accumulating data suggests that CRP has two distinct forms. It is predominantly produced in the liver in a native pentameric form (nCRP). At sites of local inflammation and tissue injury it may bind to phosphocholine-rich membranes of activated and apoptotic cells and their microparticles, undergoing irreversible dissociation to five monomeric subunits, termed monomeric CRP (mCRP). Through dissociation, CRP deposits into tissues and acquires distinct proinflammatory properties. It activates both classic and alternative complement pathways, binding complement component C1q and factor H. mCRP actively participates in the development of endothelial dysfunction. It activates leukocytes, inducing cytokine release and monocyte recruitment. It may also play a role in the polarization of monocytes and T cells into proinflammatory phenotypes. It may be involved in low-density lipoproteins (LDL) opsonization and uptake by macrophages. mCRP deposits were detected in samples of atherosclerotic lesions from human aorta, carotid, coronary and femoral arteries. mCRP may also induce platelet aggregation and thrombus formation, thus contributing in multiple ways in the development of atherosclerosis and atherothrombosis. In this mini-review, we will provide an insight into the process of conformational rearrangement of nCRP, leading to dissociation, and describe known effects of mCRP. We will provide a rationalization for mCRP involvement in the development of atherosclerosis and atherothrombosis.

CRP Is Transported by Monocytes and Monocyte-Derived Exosomes in the Blood of Patients with Coronary Artery Disease

The objective of this work was to study the ability of blood cells and their microparticles to transport monomeric and pentameric forms of C-reactive protein (mCRP and pCRP) in the blood of patients with coronary artery disease (CAD). Blood was obtained from 14 patients with CAD 46 ± 13 years old and 8 healthy volunteers 49 ± 13.6 years old. Blood cells and microparticles with mCRP and pCRP on their surface were detected by flow cytometry. Messenger RNA (mRNA) of CRP was extracted from peripheral blood monocytes stimulated with lipopolysaccharide (LPS) and granulocyte-macrophage colony-stimulating factor (GM-CSF). mRNA of CRP in monocytes was detected with PCR. Monocytes were predominantly pCRP-positive (92.9 ± 6.8%). mCRP was present on 22.0 ± 9.6% of monocyte-derived exosomes. mCRP-positive leukocyte-derived microparticle counts were significantly higher (8764 ± 2876/µL) in the blood of patients with CAD than in healthy volunteers (1472 ± 307/µL). LPS and GM-CSF stimulated monocytes expressed CRP mRNA transcripts levels (0.79 ± 0.73-fold), slightly lower relative to unstimulated hepatocytes of the HepG2 cell line (1.0 ± 0.6-fold), but still detectable. The ability of monocytes to transport pCRP in blood flow, and monocyte-derived exosomes to transmit mCRP, may contribute to the maintenance of chronic inflammation in CAD.

Endocytosis Is a Key Mode of Interaction between Extracellular β-Amyloid and the Cell Membrane

Interactions between amyloid-β peptide (Aβ) and the cell membrane include interaction with membrane lipids and binding to membrane receptors, both of which are considered to be the toxicity mechanisms of Aβ. However, it is unclear whether both mechanisms lead to cytotoxicity. Thus, we aimed to analyze these two mechanisms of Aβ42 interaction with cell membranes under different Aβ aggregation states. To this end, model membrane experiments were conducted. Quantitative analysis of Aβ42 monomers or oligomers bound to the membrane of neuro-2a cells was also performed, and laser confocal microscopy was employed to assess endocytosis of FITC-Aβ42 monomers or oligomers by neuro-2a cells. We found that the binding capacity of Aβ42 to membrane lipids was weak and that the amount of Aβ42 bound to membrane lipids was low. Moreover, clathrin-mediated endocytosis of Aβ42 oligomers by neuro-2a cells was observed. Endocytosis serves as a key mode of interaction between extracellular Aβ42 and neurons. These findings provide insights into the mechanisms underlying Aβ oligomer metabolism.

How C-Reactive Protein Structural Isoforms With Distinctive Bioactivities Affect Disease Progression

C-reactive protein (CRP) is a widely known, hepatically synthesized protein whose blood levels change rapidly and pronouncedly in response to any tissue damaging event associated with an inflammatory response. The synthesis and secretion of CRP is stimulated by interleukin-6, an early pleiotropic cytokine released by macrophages, endothelial, and other cells that are activated when localized normal tissue structures are compromised by trauma or disease. Serum CRP levels can change rapidly and robustly from 10-100-fold within 6–72 h of any tissue damaging event. Elevated blood levels correlate with the onset and extent of both activated inflammation and the acute phase biochemical response to the tissue insult. Because its functional bioactivity as the prototypic acute phase reactant has eluded clear definition for decades, diagnosticians of various conditions and diseases use CRP blood levels as a simple index for ongoing inflammation. In many pathologies, which involves many different tissues, stages of disease, treatments, and responses to treatments, its interpretive diagnostic value requires a deeper understanding of the localized tissue processes and events that contribute signals which regulate protective or pathological host defense bioactivities. This report presents concepts that describe how local tissue activation events can lead to a non-proteolytic, conformational rearrangement of CRP into a unique isoform with distinctive solubility, antigenicity, binding reactivities and bioactivities from that protein widely known and measured in serum. By describing factors that control the expression, tissue localization, half-life and pro-inflammatory amplification activity of this CRP isoform, a unifying explanation for the diagnostic significance of CRP measurement in disease is advanced.

Activation of C-reactive protein proinflammatory phenotype in the blood retinal barrier in vitro: implications for age-related macular degeneration

The retinal pigment epithelium (RPE) is considered one of the main targets of age-related macular degeneration (AMD), the leading cause of irreversible vision loss among the ageing population worldwide. Persistent low grade inflammation and oxidative stress eventually lead to RPE dysfunction and disruption of the outer blood-retinal barrier (oBRB). Increased levels of circulating pentameric C-reactive protein (pCRP) are associated with higher risk of AMD. The monomeric form (mCRP) has been detected in drusen, the hallmark deposits associated with AMD, and we have found that mCRP induces oBRB disruption. However, it is unknown how mCRP is generated in the subretinal space. Using a Transwell model we found that both pCRP and mCRP can cross choroidal endothelial cells and reach the RPE in vitro and that mCRP, but not pCRP, is able to cross the RPE monolayer in ARPE-19 cells. Alternatively, mCRP can originate from the dissociation of pCRP in the surface of lipopolysaccharide-damaged RPE in both ARPE-19 and primary porcine RPE lines. In addition, we found that the proinflammatory phenotype of mCRP in the RPE depends on its topological localization. Together, our findings further support mCRP contribution to AMD progression enhancing oBRB disruption.

Predicting the Risk of Recurrent Venous Thromboembolism: Current Challenges and Future Opportunities

Acute venous thromboembolism (VTE) is a commonly diagnosed condition and requires treatment with anticoagulation to reduce the risk of embolisation as well as recurrent venous thrombotic events. In many cases, cessation of anticoagulation is associated with an unacceptably high risk of recurrent VTE, precipitating the use of indefinite anticoagulation. In contrast, however, continuing anticoagulation is associated with increased major bleeding events. As a consequence, it is essential to accurately predict the subgroup of patients who have the highest probability of experiencing recurrent VTE, so that treatment can be appropriately tailored to each individual. To this end, the development of clinical prediction models has aided in calculating the risk of recurrent thrombotic events; however, there are several limitations with regards to routine use for all patients with acute VTE. More recently, focus has shifted towards the utility of novel biomarkers in the understanding of disease pathogenesis as well as their application in predicting recurrent VTE. Below, we review the current strategies used to predict the development of recurrent VTE, with emphasis on the application of several promising novel biomarkers in this field.

Matrix sieving-enforced retrograde transcytosis regulates tissue accumulation of C-reactive protein

Aims

Circulating proteins larger than 3 nm can be transported across continuous endothelial barrier of blood vessels via transcytosis. However, excessive accumulation of serum proteins within the vessel walls is uncommon even for those abundant in the circulation. The aim of this study was to investigate how transcytosis regulates tissue accumulation of the prototypical acute-phase reactant C-reactive protein (CRP) and other serum proteins.

Methods and results

Transcytosis of CRP as well as of transferrin and low-density lipoprotein across aortic endothelial cells is bidirectional with directional preference from the apical (blood) to basolateral (tissue) direction both in vitro and in vivo. This directional preference is, however, reversed by the basement membrane (BM) matrix underlying the basolateral surface of endothelial cells. This is due to the sieving effect of the BM that physically hinders the diffusion of transcytosed proteins from the apical compartment towards underlying tissues, resulting in immediate retrograde transcytosis that limits basolateral protein accumulation. Conversely, CRP produced within vessel wall lesions can also be transported into the circulation.

Conclusion

Our findings identify matrix sieving-enforced retrograde transcytosis as a general mechanism that prevents excessive tissue accumulation of blood-borne proteins and suggest that lesion-derived CRP might also contribute to elevated serum CRP levels associated with increased risk for cardiovascular diseases.

Monomeric C-Reactive Protein in Serum With Markedly Elevated CRP Levels Shares Common Calcium-Dependent Ligand Binding Properties With an in vitro Dissociated Form of C-Reactive Protein

A monomeric form of C-reactive protein (CRP) which precipitates with cell wall pneumococcal C polysaccharide (CWPS) and retains the ability to reversibly bind to its ligand phosphocholine has been produced through urea-induced dissociation at an optimized concentration of 3 M urea over a 10 weeks period. Dissociated samples were purified via size exclusion chromatography and characterized by western blot, phosphocholine affinity chromatography and CWPS precipitation. Human serum samples from patients with raised CRP levels (>100 mg/L as determined by the clinical laboratory assay) were purified by affinity and size exclusion chromatography and analyzed (n = 40) to determine whether circulating monomeric CRP could be detected ex vivo. All 40 samples tested positive for pentameric CRP via western blot and enzyme linked immunosorbent assay (ELISA) analysis. Monomeric C-reactive protein was also identified in all 40 patient samples tested, with an average level recorded of 1.03 mg/L (SE = ±0.11). Both the in vitro monomeric C-reactive protein and the human serum monomeric protein displayed a molecular weight of approximately 23 kDa, both were recognized by the same anti-CRP monoclonal antibody and both reversibly bound to phosphocholine in a calcium-dependent manner. In common with native pentameric CRP, the in vitro mCRP precipitated with CWPS. These overlapping characteristics suggest that a physiologically relevant, near-native monomeric CRP, which retains the structure and binding properties of native CRP subunits, has been produced through in vitro dissociation of pentameric CRP and also isolated from serum with markedly elevated CRP levels. This provides a clear route toward the in-depth study of the structure and function of physiological monomeric CRP.

C-Reactive Protein and Its Structural Isoforms: An Evolutionary Conserved Marker and Central Player in Inflammatory Diseases and Beyond

C-reactive protein (CRP) is an evolutionary highly conserved member of the pentraxin superfamily of proteins. CRP is widely used as a marker of inflammation, infection and for risk stratification of cardiovascular events. However, there is now a large body of evidence, that continues to evolve, detailing that CRP directly mediates inflammatory reactions and the innate immune response in the context of localised tissue injury. These data support the concept that the pentameric conformation of CRP dissociates into pro-inflammatory CRP isoforms termed pCRP* and monomeric CRP. These pro-inflammatory CRP isoforms undergo conformational changes that facilitate complement binding and immune cell activation and therefore demonstrate the ability to trigger complement activation, activate platelets, monocytes and endothelial cells. The dissociation of pCRP occurs on the surface of necrotic, apoptotic, and ischaemic cells, regular β-sheet structures such as β-amyloid, the membranes of activated cells (e.g., platelets, monocytes, and endothelial cells), and/or the surface of microparticles, the latter by binding to phosphocholine. Therefore, the deposition and localisation of these pro-inflammatory isoforms of CRP have been demonstrated to amplify inflammation and tissue damage in a broad range of clinical conditions including ischaemia/reperfusion injury, Alzheimer's disease, age-related macular degeneration and immune thrombocytopaenia. Given the potentially broad relevance of CRP to disease pathology, the development of inhibitors of CRP remains an area of active investigation, which may pave the way for novel therapeutics for a diverse range of inflammatory diseases.

Intra-subunit Disulfide Determines the Conversion and Structural Stability of CRP Isoforms

C-reactive protein (CRP) is a major human acute-phase reactant that is composed of five identical subunits. CRP dissociates into subunits at inflammatory loci forming monomeric CRP (mCRP) with substantially enhanced activities, which can be further activated by reducing the intra-subunit disulfide bond. However, conformational changes underlying the activation process of CRP are less well understood. Conformational changes accompanying the conversion of CRP to mCRP with or without reduction were examined with circular dichroism spectroscopy, fluorescence spectroscopy, electron microscopy, size-exclusion chromatography, and neoepitope expression. The conversion of CRP to mCRP follows a two-stage process. In the first stage, CRP dissociates into molten globular subunits characterized by intact secondary structure elements with greatly impaired tertiary packing. In the second stage, these intermediates completely lose their native subunit conformation and assemble into high-order aggregates. The inclusion of reductant accelerates the formation of molten globular subunits in the first step and promotes the formation of more compact aggregates in the second stage. We further show a significant contribution of electrostatic interactions to the stabilization of native CRP. The conformational features of dissociated subunits and the aggregation of mCRP may have a key impact on their activities.

Monomeric C-reactive protein regulates fibronectin mediated monocyte adhesion

The acute phase reactant C-reactive protein (CRP) binds with high affinity to fibronectin (FN), but this binding occurs only at pH 6.5 or lower, and the binding is inhibited by calcium ions at physiological pH. Since CRP in the circulating blood exists in a calcium-binding form, the interaction between CRP and FN in vivo has been uncertain. CRP can undergo a conformational rearrangement in the absence of calcium or in the local microenvironment (e.g., acidic pH) of inflamed tissue to dissociate into monomeric CRP (mCRP). Therefore, we tested whether these discrepancies can be explained by the different isoforms and locations of CRP. Surface plasmon resonance and ELISA assays showed that mCRP binds with high affinity to FN, and the binding of mCRP to FN was unaffected by calcium or pH. Peptide competition assay, deletion mutant binding assay and protein docking analyse verified that the binding site of mCRP to FN is residues a.a.35-47. Furthermore, mCRP can significantly enhance the adhesion of monocytes to FN as well as upregulate the adhesion molecules expression on endothelial cell. Colocalization of mCRP with FN was observed in mice with DSS-induced colitis, whereas there was very little signal orcolocalization of CRP. These results provide in vitro and in vivo evidence that mCRP formed by local dissociation from circulating CRP is the major isoform that interacts with FN and regulates FN-mediated monocyte adhesion, which is involved in the pro-inflammatory process.

Regulation of C-reactive protein conformation in inflammation

C-reactive protein (CRP) is a non-specific diagnostic marker of inflammation and an evolutionarily conserved protein with roles in innate immune signaling. Natural CRP is composed of five identical globular subunits that form a pentamer, but the role of pentameric CRP (pCRP) during inflammatory pathogenesis remains controversial. Emerging evidence suggests that pCRP can be dissociated into monomeric CRP (mCRP) that has major roles in host defenses and inflammation. Here, we discuss our current knowledge of the dissociation mechanisms of pCRP and summarize the stepwise conformational transition model to mCRP to elucidate how CRP dissociation contributes to proinflammatory activity. These discussions will evoke new understanding of this ancient protein.

Acute phase protein response to viral infection and vaccination

Organisms respond in multiple ways to microbial infections. Pathogen invasion tipically triggers an inflammatory response where acute phase proteins (APP) have a key role. Pentraxins (PTX) are a family of highly conserved APP that play a part in the host defense against infection. The larger proteins of the family are simply named pentraxins, while c-reactive proteins (CRP) and serum amyloid proteins (SAA, SAP) are known as short pentraxins. Although high APP levels have been broadly associated with bacterial infections, there is a growing body of evidence revealing increased PTX, CRP and SAP expression upon viral infection. Furthermore, CRP, PTX and SAP have shown their potential as diagnostic markers and predictors of disease outcome. Likewise, the measurement of APP levels can be valuable to determine the efficacy of antiviral therapies and vaccines. From the practical point of view, the ability of APP to reduce viral infectivity has been observed in several virus-host models. This has prompted investigation efforts to assess the role of acute phase response proteins as immunoregulatory molecules and their potential as therapeutic reagents. This work aims to present an overview of the APP response to viral infections reviewing the current knowledge in the field.

The Multiple Faces of C-Reactive Protein-Physiological and Pathophysiological Implications in Cardiovascular Disease

C-reactive protein (CRP) is an intriguing protein which plays a variety of roles in either physiological or pathophysiological states. For years it has been regarded merely as a useful biomarker of infection, tissue injury and inflammation, and it was only in the early 80s that the modified isoforms (mCRP) of native CRP (nCRP) appeared. It soon became clear that the roles of native CRP should be clearly discriminated from those of the modified form and so the impacts of both isoforms were divided to a certain degree between physiological and pathophysiological states. For decades, CRP has been regarded only as a hallmark of inflammation; however, it has since been recognised as a significant predictor of future episodes of cardiovascular disease, independent of other risk factors. The existence of modified CRP isoforms and their possible relevance to various pathophysiological conditions, suggested over thirty years ago, has prompted the search for structural and functional dissimilarities between the pentameric nCRP and monomeric mCRP isoforms. New attempts to identify the possible relevance between the diversity of structures and their opposing functions have initiated a new era of research on C-reactive protein. This review discusses the biochemical aspects of CRP physiology, emphasizing the supposed relevance between the structural biology of CRP isoforms and their differentiated physiological and pathophysiological roles.

A redox sensitivity-based method to quantify both pentameric and monomeric C-reactive protein in a single assay

C-reactive protein (CRP) can exist in both pentameric (pCRP) and monomeric conformation (mCRP). Though serum pCRP is an established marker of inflammation, the diagnostic significance of mCRP remains unknown largely due to the lack of a reliable assay. The power and specificity of antibody-based assays are limited by the antibody reagents used and by the degree of cross-reactivity that may exist in detecting each antigen, as mCRP is known to be formed from the pentameric and both conformations usually coexist in clinical samples. Here, we describe an assay that measures both CRP conformations in simple samples in a single assay. This assay depends on the rationale that the intra-molecular disulfide bonds in pCRP resist reduction, while those in mCRP can be readily reduced. The distinct sensitivity of pCRP and mCRP to reduction can be easily detected and separated by electrophoresis. This assay may provide a means to study clinical correlation between pCRP and mCRP in clinical samples in the future and to evaluate their respective significance as disease markers.

Hyper-acidic fusion minipeptides escort the intrinsic antioxidative ability of the pattern recognition receptor CRP in non-animal organisms

C-reactive protein (CRP) is widely used as a biomarker of inflammation. It plays important roles in innate immunity response as a member of pattern recognition receptors, by binding oxidation-specific epitopes including some intermediates of lipid oxidative chain reaction. The inferred antioxidative ability of CRP was ever demonstrated by only few in vitro evidences, and needs to be clarified especially in vivo. Herein, we expressed human CRP in three representative non-animal organisms (Escherichia coli, Saccharomyces cerevisiae, and tobacco) inherently lacking the milieu for CRP signalling, and found CRP did possess an intrinsic antioxidative ability. Heterologous CRP could confer increased oxidative resistance in its recombinant E. coli and yeast cells and transgenic tobaccos. We also revealed a positive correlation between the antioxidative effect of CRP and its solubility. Only soluble CRP could exhibit distinct antioxidative activity, while the CRP aggregates might be instead toxic (probably pro-oxidative) to cells. Moreover, fusion with hyper-acidic minipeptides could remarkably improve CRP solubility, and meanwhile guarantee or enhance CRP antioxidative ability. These results not only provide a new insight for understanding the etiology of CRP-involved inflammations and diseases, and also endorse a potential of CRP biotechnological applications in developing new pharmaceutical therapies and improving plant oxidative resistance.

Acidic pH promotes oxidation-induced dissociation of C-reactive protein

BACKGROUND

Circulating levels of the systemic inflammation marker C-reactive protein (CRP) have been associated with increased risk and poor outcomes of many diseases, such as cardiovascular events and cancer. Accumulating evidence has indicated that the conformational rearrangement of human pentameric CRP (pCRP) to monomeric CRP (mCRP) is a prerequisite for participation in the pathogenesis. Therefore, determining the mechanism of the dissociation of pCRP into pro-inflammatory mCRP under physiological/pathological circumstances has been intriguing.

METHODS

The effects of oxidative and acidic stress occurring in inflammation on pCRP were examined by electrophoresis, electron microscopy, protein fluorescence, neoepitope expression and endothelial cell responses.

RESULTS

Reactive oxygen species (ROS) generated by the copper-hydrogen peroxide system could rapidly induce the dissociation of CRP at mild acidic pH within four hours, but not at physiological pH of 7.4. Meanwhile, mannitol, a ROS scavenger, could not protect against dissociation, which implied that local ROS from accessible histidine residues may be crucially beneficial to the formation of mCRP in a redox-balanced microenvironment. Furthermore, mCRP generated by ROS could be reduced by DTT, which indicated the exposure of functional motif aa35-47, and showed potent proinflammatory actions on endothelial cells, comparable to mCRP generated by urea.

CONCLUSION

dissociation of pCRP to mCRP could be rapidly induced by ROS from copper- hydrogen peroxide system in dependence on mildly acidic stress regardless of a redox-balanced microenvironment.

Acetylcholine Inhibits Monomeric C-Reactive Protein Induced Inflammation, Endothelial Cell Adhesion, and Platelet Aggregation; A Potential Therapeutic?

Objectives: In this study, we examined the possibility of using targeted antibodies and the potential of small molecular therapeutics (acetylcholine, nicotine and tacrine) to block the pro-inflammatory and adhesion-related properties of monomeric C-reactive protein (mCRP). Methods: We used three established models (platelet aggregation assay, endothelial leucocyte binding assay and monocyte inflammation via ELISA and Western blotting) to assess the potential of these therapeutics. Results: The results of this study showed that monocyte induced inflammation (raised tumor necrosis factor-alpha-TNF-α) induced by mCRP was significantly blocked in the presence of acetylcholine and nicotine, whilst tacrine and targeted antibodies (clones 8C10 and 3H12) had less of or no significant effects. Western blotting confirmed the ability of acetylcholine to inhibit mCRP-induced cell signaling phosphorylation of extracellular signal regulated kinase 1/2 (ERK1/2), p38 and nuclear factor-kappa B (NF-κB). There was no evidence of direct binding between small molecules and mCRP. mCRP also induced endothelial cell-monocyte adhesion in a dose dependent fashion, however, both acetylcholine and nicotine as well as targeting antibodies notably inhibited adhesion. Finally, we investigated their effects on mCRP-induced platelet aggregation. All three small molecules significantly attenuated platelet aggregation as did the antibody 8C10, although 3H12 had a weaker effect. Discussion: Acetylcholine and to a lesser extent nicotine show potential for therapeutic inhibition of mCRP-induced inflammation and cell and platelet adhesion. These results highlight the potential of targeted antibodies and small molecule therapeutics to inhibit the binding of mCRP by prevention of membrane interaction and subsequent activation of cellular cascade systems, which produce the pro-inflammatory effects associated with mCRP.

Monomeric C-Reactive Protein and Cerebral Hemorrhage: From Bench to Bedside

C-reactive protein (CRP) is an important mediator and a hallmark of the acute-phase response to inflammation. High-sensitivity assays that accurately measure levels of CRP have been recommended for use in risk assessment in ischemic stroke patients. Elevation of CRP during the acute-phase response in intracerebral hemorrhage (ICH) is also associated with the outcomes such as death and vascular complications. However, no association has been found with the increased risk of ICH. The aim of this review is to synthesize the published literature on the associations of CRP with acute ICH both as a risk biomarker and predictor of short- and long-term outcomes as well as its role as a pathogenic determinant. We believe before any clinical utility, a critical appraisal of the strengths and deficiencies of the accumulated evidence is required both to evaluate the current state of knowledge and to improve the design of future clinical studies.

The Effect of C-Reactive Protein Isoforms on Nitric Oxide Production by U937 Monocytes/Macrophages

Inflammation is regulated by many endogenous factors including estrogen, a steroid hormone that declines with increasing age, leading to excessive inflammation in the elderly. C-reactive protein (CRP) is an acute phase inflammatory protein that exists in two forms, native CRP (nCRP) and monomeric CRP (mCRP), which mediate distinct biological activities. It is unclear how each CRP isoform mediates nitric oxide (NO), a signaling molecule generated by NO synthase (NOS). This study investigated whether CRP isoforms have distinct effects on NO production by unstimulated and lipopolysaccharide (LPS)-activated monocytes/macrophages and whether estrogen mediates CRP-induced NO production in an in vitro model of aging. NO and inducible NOS (iNOS) were measured (n = 12) by the Griess assay and an enzyme-linked immunosorbent assay, respectively following incubation (24 h) of human-derived U937 monocytes/macrophages with CRP isoforms [(nCRP) = 500 and 1,000 µg/ml; (mCRP) = 100 and 250 µg/ml] in the absence or presence of 17 beta-estradiol (1 × 10-7, 1 × 10-8, and 1 × 10-9 M). The response to each CRP isoform and estrogen was dependent on the differentiation and activation status of cells. Monocytes with or without prior LPS-activation significantly increased (P < 0.01) NO/iNOS production when treated with mCRP. The mCRP isoform had no effect (P > 0.05) on NO/iNOS production by unactivated or LPS-activated macrophages, whereas nCRP significantly (P < 0.05) reduced NO/iNOS production by macrophages, with or without prior LPS-activation. The nCRP isoform had opposing actions on monocytes, significantly (P < 0.01) increasing and reducing NO/iNOS by unactivated and LPS-activated monocytes, respectively. Estrogen significantly (P < 0.01) reversed nCRP-mediated NO inhibition by unactivated macrophages but decreased CRP-induced NO by unactivated monocytes treated with nCRP or mCRP and LPS-activated monocytes treated with mCRP. NO was differentially mediated by CRP isoforms in a cell-type/state-specific manner, with production corresponding to concomitant changes in iNOS levels. Collectively, the findings indicate nCRP and estrogen predominantly reduce NO production, whereas mCRP increases NO production. This supports growing evidence that mCRP exacerbates inflammation while nCRP and estrogen dampen the overall inflammatory response. Therapeutic strategies that restore estrogen levels to those found in youth and promote the stability of nCRP or/and prevent the formation of mCRP may reduce NO production in age-related inflammatory conditions.

Dissociation of C-Reactive Protein Localizes and Amplifies Inflammation: Evidence for a Direct Biological Role of C-Reactive Protein and Its Conformational Changes

C-reactive protein (CRP) is a member of the pentraxin superfamily that is widely recognized as a marker of inflammatory reactions and cardiovascular risk in humans. Recently, a growing body of data is emerging, which demonstrates that CRP is not only a marker of inflammation but also acts as a direct mediator of inflammatory reactions and the innate immune response. Here, we critically review the various lines of evidence supporting the concept of a pro-inflammatory “CRP system.” The CRP system consists of a functionally inert circulating pentameric form (pCRP), which is transformed to its highly pro-inflammatory structural isoforms, pCRP* and ultimately to monomeric CRP (mCRP). While retaining an overall pentameric structure, pCRP* is structurally more relaxed than pCRP, thus exposing neoepitopes important for immune activation and complement fixation. Thereby, pCRP* shares its pro-inflammatory properties with the fully dissociated structural isoform mCRP. The dissociation of pCRP into its pro-inflammatory structural isoforms and thus activation of the CRP system occur on necrotic, apoptotic, and ischemic cells, regular β-sheet structures such as β-amyloid, the membranes of activated cells (e.g., platelets, monocytes, and endothelial cells), and/or the surface of microparticles, the latter by binding to phosphocholine. Both pCRP* and mCRP can cause activation of platelets, leukocytes, endothelial cells, and complement. The localization and deposition of these pro-inflammatory structural isoforms of CRP in inflamed tissue appear to be important mediators for a range of clinical conditions, including ischemia/reperfusion (I/R) injury of various organs, cardiovascular disease, transplant rejection, Alzheimer’s disease, and age-related macular degeneration. These findings provide the impetus to tackle the vexing problem of innate immunity response by targeting CRP. Understanding the “activation process” of CRP will also likely allow the development of novel anti-inflammatory drugs, thereby providing potential new immunomodulatory therapeutics in a broad range of inflammatory diseases.

pCRP-mCRP Dissociation Mechanisms as Potential Targets for the Development of Small-Molecule Anti-Inflammatory Chemotherapeutics

Circulating C-reactive protein (CRP) is a key acute-phase protein and one of the main clinical biomarkers for inflammation and infection. CRP is an important upstream mediator of inflammation and is associated with the onset of a number of important disease states including cardiovascular disease and neurodegenerative disorders such as Alzheimer’s disease. This pentraxin exerts pro-inflammatory properties via dissociation of the pentamer (pCRP) to a monomeric form (mCRP). This dissociation is induced by binding of pCRP to cell surface phosphocholine residues exposed by the action of phospholipase A₂ (PLA₂). Given the association of CRP with the onset of a range of serious disease states this CRP dissociation process is a tempting drug target for the development of novel small-molecule therapeutics. This review will discuss potential targets for chemotherapeutic intervention elucidated during studies of CRP-mediated inflammation and provide an up-to-date summary of the development of small molecules, not only targeted directly at inhibiting conversion of pCRP to mCRP, but also those developed for activity against PLA₂, given the key role of this enzyme in the activation of CRP.