mCRP-Induced Focal Adhesion Kinase-Dependent Monocyte Aggregation and M1 Polarization, Which Was Partially Blocked by the C10M Inhibitor.

Monomeric C-reactive protein (mCRP) has recently been implicated in the abnormal vascular activation associated with development of atherosclerosis, but it may act more specifically through mechanisms perpetuating damaged vessel inflammation and subsequent aggregation and internalization of resident macrophages. Whilst the direct effects of mCRP on endothelial cells have been characterized, the interaction with blood monocytes has, to our knowledge, not been fully defined. Here we showed that mCRP caused a strong aggregation of both U937 cell line and primary peripheral blood monocytes (PBMs) obtained from healthy donors. Moreover, this increase in clustering was dependent on focal adhesion kinase (FAK) activation (blocked by a specific inhibitor), as was the concomitant adhesive attachment to the plate, which was suggestive of macrophage differentiation. Confocal microscopy confirmed the increased expression and nuclear localization of p-FAK, and cell surface marker expression associated with M1 macrophage polarization (CD11b, CD14, and CD80, as well as iNOS) in the presence of mCRP. Inclusion of a specific CRP dissociation/mCRP inhibitor (C10M) effectively inhibited PBMs clustering, as well as abrogating p-FAK expression, and partially reduced the expression of markers associated with M1 macrophage differentiation. mCRP also increased the secretion of pro-inflammatory cytokines Interleukin-8 (IL-8) and Interleukin-1ß (IL-1ß), without notably affecting MAP kinase signaling pathways; inclusion of C10M did not perturb or modify these effects. In conclusion, mCRP modulates PBMs through a mechanism that involves FAK and results in cell clustering and adhesion concomitant with changes consistent with M1 phenotypical polarization. C10M has potential therapeutic utility in blocking the primary interaction of mCRP with the cells-for example, by protecting against monocyte accumulation and residence at damaged vessels that may be predisposed to plaque development and atherosclerosis.

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.

Monomeric C-reactive protein: A novel biomarker predicting neurodegenerative disease and vascular dysfunction.

Circulating C-reactive protein (pCRP) concentrations rise dramatically during both acute (e.g., following stroke) or chronic infection and disease (e.g., autoimmune conditions such as lupus), providing complement fixation through C1q protein binding. It is now known, that on exposure to the membranes of activated immune cells (and microvesicles and platelets), or damaged/dysfunctional tissue, it undergoes lysophosphocholine (LPC)-phospholipase-C-dependent dissociation to the monomeric form (mCRP), concomitantly becoming biologically active. We review histological, immunohistochemical, and morphological/topological studies of post-mortem brain tissue from individuals with neuroinflammatory disease, showing that mCRP becomes stably distributed within the parenchyma, and resident in the arterial intima and lumen, being "released" from damaged, hemorrhagic vessels into the extracellular matrix. The possible de novo synthesis via neurons, endothelial cells, and glia is also considered. In vitro, in vivo, and human tissue co-localization analyses have linked mCRP to neurovascular dysfunction, vascular activation resulting in increased permeability, and leakage, compromise of blood brain barrier function, buildup of toxic proteins including tau and beta amyloid (Aß), association with and capacity to "manufacture" Aß-mCRP-hybrid plaques, and, greater susceptibility to neurodegeneration and dementia. Recently, several studies linked chronic CRP/mCRP systemic expression in autoimmune disease with increased risk of dementia and the mechanisms through which this occurs are investigated here. The neurovascular unit mediates correct intramural periarterial drainage, evidence is provided here that suggests a critical impact of mCRP on neurovascular elements that could suggest its participation in the earliest stages of dysfunction and conclude that further investigation is warranted. We discuss future therapeutic options aimed at inhibiting the pCRP-LPC mediated dissociation associated with brain pathology, for example, compound 1,6-bis-PC, injected intravenously, prevented mCRP deposition and associated damage, after temporary left anterior descending artery ligation and myocardial infarction in a rat model.

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 >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 (>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<0.001). Similarly, tissue-expressed mCRP correlated strongly with circulating pCRP (Spearman correlation 0.81, p<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.

Peripheral apolipoprotein E proteins and their binding to LRP1 antagonize Alzheimer's disease pathogenesis in the brain during peripheral chronic inflammation.

C-reactive protein (CRP) impacts apolipoprotein E4 (ApoE4) allele to increase Alzheimer's disease (AD) risk. However, it is unclear how the ApoE protein and its binding to LRP1 are involved. We found that ApoE2 carriers had the highest but ApoE4 carriers had the lowest concentrations of blood ApoE in both humans and mice; blood ApoE concentration was negatively associated with AD risk. Elevation of peripheral monomeric CRP (mCRP) reduced the expression of ApoE in ApoE2 mice, while it decreased ApoE-LRP1 binding in the brains of ApoE4 mice that was characterized by Proximity Ligation Assay. Both serum ApoE and brain ApoE-LRP1 binding were positively associated with the expression of pericytes that disappeared after mCRP treatment, and negatively associated with brain tauopathy and neuroinflammation in the presence of mCRP. In ApoE-/- mice, mCRP reduced the brain expression levels of synaptophysin and PSD95 and the positive relationship between ApoE-LRP1 binding and synaptophysin or PSD95 expression disappeared. Our study suggests that blood ApoE protects against AD pathogenesis by binding to LRP1 during peripheral chronic inflammation.

Conformational state of C-reactive protein is critical for reducing immune complex-triggered type I interferon response: Implications for pathogenic mechanisms in autoimmune diseases imprinted by type I interferon gene dysregulation.

Presence of autoantibodies targeting nuclear constituents, i.e., double-stranded DNA and small nuclear ribonucleoproteins (snRNPs), remain a cornerstone in systemic lupus erythematosus (SLE). Fcγ receptor IIa (FcγRIIa) dependent uptake of nucleic acid containing immune complexes (ICs) by plasmacytoid dendritic cells (PDCs) can activate toll-like receptors (TLRs) such as TLR7 and TLR9 resulting in type I interferon (IFN) production. Previously, the classical liver-derived acute-phase reactant C-reactive protein (CRP) has been suggested to reduce IC-induced type I IFN production, whereas monomeric (mCRP) vs. pentameric (pCRP) mediated effects have not yet been unraveled. Herein, peripheral blood mononuclear cells (PBMCs) or enriched blood DCs from healthy volunteers were stimulated with SLE sera, snRNP-IgG (ICs), or TLR ligands with or without pCRP, mCRP, or anti-FcγRIIa antibody. Type I IFNs and cytokine responses were investigated using quantitative PCR, ELISA, and flow cytometry. pCRP inhibited IFN gene expression in PBMCs and enriched DCs after incubation with ICs, compared to ICs alone, whereas mCRP had significantly less inhibitory effect. The effect was independent on the order in which IC or CRP was added to the cells. In addition, pCRP inhibited IFN induced by other TLR stimulators, implicating broader inhibitory effects induced by pCRP. We demonstrate pronounced immunoregulatory functions of CRP whereas the inhibitory properties were evidently dependent on CRP's intact conformational state. The inhibition of type I IFNs was not due to competition of FcγRs, or binding of CRP to the ICs. Our findings have implications for autoimmune IC-mediated conditions imprinted by type I IFN gene dysregulation.

Monomeric C-reactive protein as a biomarker for major depressive disorder.

Neuroinflammation has been postulated to be a key factor in the pathogenesis of major depressive disorder (MDD). With this is mind, there has been a wave of research looking into pro-inflammatory mediators as potential biomarkers for MDD. One such mediator is the acute phase protein, C-reactive protein (CRP). While several studies have investigated the potential of CRP as a biomarker for MDD, the results have been inconsistent. One explanation for the lack of consistent findings may be that the high-sensitivity CRP tests utilized in these studies only measure the pentameric isoform of CRP (pCRP). Recent research, however, has indicated that the monomeric isoform of CRP (mCRP) is responsible for the pro-inflammatory function of CRP, while pCRP is weakly anti-inflammatory. The objective of this minireview is to re-examine the evidence of CRP involvement in MDD with a view of mCRP as a potential biomarker.

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.

Monomeric C-reactive protein induces the cellular pathology of Alzheimer's disease.

Introduction: Human study shows that elevated C-reactive protein (CRP) in blood impacts apolipoprotein E (APOE) ε4, but not APOE ε3 or APOE ε2, genotype to increase the risk of Alzheimer's disease (AD). However, whether CRP is directly involved in cellular AD pathogenesis and in which type of neuronal cells of APOE ε4 carriers are unknown. Methods: We aimed to use different primary neuronal cells and investigate if CRP induces cellular AD pathology depending on APOE genotypes. Here the different primary neuronal cells from the different APOE genotype knock-in mice cortex were isolated and used. Results: Monomeric CRP (mCRP) increased amyloid beta production and, in parallel, induced tau phosphorylation in addition to their related proteins in the primary neurons in a pattern of APOE ε4 > APOE ε3 > APOE ε2 in a dose- and time-dependent manner. Consistently, mCRP induced the staining of other neurodegenerative biomarkers, including Fluoro-Jade B stain (FjB), TUNEL and Cleaved Caspase-3, in primary neurons in a similar pattern of APOE ε4 > APOE ε3 > APOE ε2. In contrast, pentameric CRP (pCRP) had a tendency to induce cellular AD pathology but did not reach statistical significance. On the other hand, it is intriguing that regardless of APOE genotype, mCRP did not influence the expressions of Iba-1 and CD68 in primary microglia or the expression of glial fibrillary acidic protein in primary astrocytes, and additionally mCRP did not affect the secretions of interleukin (IL)-1α, IL-1ß, and tumor necrosis factor α from these cells. Discussion: This is the first report to demonstrate that mCRP directly induces cellular AD pathogenesis in neurons in an APOE genotype-dependent pattern, suggesting that mCRP plays a role as a mediator involved in the APOE ε4-related pathway for AD during chronic inflammation. Highlights: Pentameric C-reactive protein (pCRP) can be dissociated irreversibly to form free subunits or monomeric CRP (mCRP) during and after the acute phase.mCRP increased amyloid beta production in the primary neurons in a pattern of apolipoprotein E (APOE) ε4 > APOE ε3 > APOE ε2 in a dose-dependent manner.mCRP induced the expression of phosphorylated tau in the primary neurons in a pattern of APOE ε4 > APOE ε3 > APOE ε2 in a dose- and time-dependent manner.mCRP plays an important mediator role in the APOE ε4-related pathway of Alzheimer's disease risk.

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.

Monomeric C-reactive protein via endothelial CD31 for neurovascular inflammation in an ApoE genotype-dependent pattern: A risk factor for Alzheimer's disease?

In chronic peripheral inflammation, endothelia in brain capillary beds could play a role for the apolipoprotein E4 (ApoE4)-mediated risk for Alzheimer's disease (AD) risk. Using human brain tissues, here we demonstrate that the interactions of endothelial CD31 with monomeric C-reactive protein (mCRP) versus ApoE were linked with shortened neurovasculature for AD pathology and cognition. Using ApoE knock-in mice, we discovered that intraperitoneal injection of mCRP, via binding to CD31 on endothelial surface and increased CD31 phosphorylation (pCD31), leading to cerebrovascular damage and the extravasation of T lymphocytes into the ApoE4 brain. While mCRP was bound to endothelial CD31 in a dose- and time-dependent manner, knockdown of CD31 significantly decreased mCRP binding and altered the expressions of vascular-inflammatory factors including vWF, NF-κB and p-eNOS. RNAseq revealed endothelial pathways related to oxidative phosphorylation and AD pathogenesis were enhanced, but endothelial pathways involving in epigenetics and vasculogenesis were inhibited in ApoE4. This is the first report providing some evidence on the ApoE4-mCRP-CD31 pathway for the cross talk between peripheral inflammation and cerebrovasculature leading to AD risk.

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.

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.

Corrigendum: Monomeric C-Reactive Protein Binds and Neutralizes Receptor Activator of NF-κB Ligand-Induced Osteoclast Differentiation.

[This corrects the article DOI: 10.3389/fimmu.2018.00234.].

C-Reactive Protein and Cancer-Diagnostic and Therapeutic Insights.

Cancer disease describes any pathology involving uncontrolled cell growth. As cells duplicate, they can remain localized in defined tissues, forming tumor masses and altering their microenvironmental niche, or they can disseminate throughout the body in a metastatic process affecting multiple tissues and organs. As tumors grow and metastasize, they affect normal tissue integrity and homeostasis which signals the body to trigger the acute phase inflammatory response. C-reactive protein (CRP) is a predominant protein of the acute phase response; its blood levels have long been used as a minimally invasive index of any ongoing inflammatory response, including that occurring in cancer. Its diagnostic significance in assessing disease progression or remission, however, remains undefined. By considering the recent understanding that CRP exists in multiple isoforms with distinct biological activities, a unified model is advanced that describes the relevance of CRP as a mediator of host defense responses in cancer. CRP in its monomeric, modified isoform (mCRP) modulates inflammatory responses by inserting into activated cell membranes and stimulating platelet and leukocyte responses associated with acute phase responses to tumor growth. It also binds components of the extracellular matrix in involved tissues. Conversely, CRP in its pentameric isoform (pCRP), which is the form quantified in diagnostic measurements of CRP, is notably less bioactive with weak anti-inflammatory bioactivity. Its accumulation in blood is associated with a continuous, low-level inflammatory response and is indicative of unresolved and advancing disease, as occurs in cancer. Herein, a novel interpretation of the diagnostic utility of CRP is presented accounting for the unique properties of the CRP isoforms in the context of the developing pro-metastatic tumor microenvironment.

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.

Insights into the Use of C-Reactive Protein as a Diagnostic Index of Disease Severity in COVID-19 Infections.

Approximately 20% of patients infected with SARS-CoV-2 (COVID-19) develop potentially life-threatening pathologies involving hyperinflammation, cytokine storm, septic shock complications, coagulation dysfunction, and multiple organ failure. Blood levels of the prototypic acute phase reactant, C-reactive protein (CRP), which is hepatically synthesized and released in response to interleukin-6 stimulation, is markedly elevated in patients with COVID-19. Markedly high CRP levels correlate with poor prognosis for survival. Insights into CRP structure-function relationships have uncovered both pro- and anti-inflammatory isoforms that may be used to monitor the extent of tissue damage associated with COVID-19 pathologies and prognoses. Herein, rationale is given for interpretation of CRP blood levels as a simple, rapid, and cost-effective way to assess disease severity and help guide therapeutic options in COVID-19 patients.

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 ß-sheet structures such as ß-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.

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.