The protective function of human C-reactive protein in mouse models of Streptococcus pneumoniae infection

C-reactive protein: structure, function, regulation, and role in clinical diseases

C-reactive protein (CRP) is a plasma protein that is evolutionarily conserved, found in both vertebrates and many invertebrates. It is a member of the pentraxin superfamily, characterized by its pentameric structure and calcium-dependent binding to ligands like phosphocholine (PC). In humans and various other species, the plasma concentration of this protein is markedly elevated during inflammatory conditions, establishing it as a prototypical acute phase protein that plays a role in innate immune responses. This feature can also be used clinically to evaluate the severity of inflammation in the organism. Human CRP (huCRP) can exhibit contrasting biological functions due to conformational transitions, while CRP in various species retains conserved protective functions in vivo. The focus of this review will be on the structural traits of CRP, the regulation of its expression, activate complement, and its function in related diseases in vivo.

Analysis of factors influencing onset and survival of patients with severe acute pancreatitis: A clinical study

OBJECTIVES

Acute pancreatitis (AP) is an inflammatory disease of the pancreas, and the prognosis of severe AP (SAP) is poor. The study aimed to identify promising biomarkers for predicting the occurrence and survival outcome of SAP patients.

MATERIALS AND METHODS

Two hundred and forty AP patients were retrospectively recruited, in which 72 cases with SAP. Blood test was done for collection of laboratory indicators. After treatment, the mortality of patients was recorded.

RESULTS

Patients in the SAP group had higher intensive care unit admissions and longer hospital stays (p < .001). Among laboratory parameters, significantly high values of C-reactive protein (CRP), triglycerides and glucose (TyG) index, Von willebrand factor antigen (vWF:Ag) and D-dimer were found in SAP groups relative to non-SAP ones. Receiver operating characteristic curve indicated the good performance of CRP, TyG index, vWF:Ag and D-dimer in SAP diagnosis. Among all SAP cases, 51 survived while 21 died. TyG index (odds ratio [OR] = 6.914, 95% confidence interval [CI] = 1.193-40.068, p = .028), vWF:Ag (OR = 7.441, 95% CI = 1.236-244.815, p = .028), and D-dimer (OR = 7.987, 95% CI = 1.251-50.997, p = .028) were significantly related to survival outcome of SAP patients by multiple logistic regression analysis. Both TyG index and vWF showed favorable efficiency in predicting overall prognosis. The area under the curve for the multivariate model (PRE = -35.908 + 2.764 × TyG + 0.021 × vWF:Ag) was 0.909 which was greater than 0.9, indicating its excellent performance in prognosis prediction.

CONCLUSION

CRP, TyG index, vWF:Ag, and D-dimer values on admission may be potential clinical predictors of the development of SAP. Moreover, TyG index and vWF:Ag may be helpful to predict survival outcome.

Pentraxins in invertebrates and vertebrates: From structure, function and evolution to clinical applications

The immune system is divided into two broad categories, consisting of innate and adaptive immunity. As recognition and effector factors of innate immunity and regulators of adaptive immune responses, lectins are considered to be important defense chemicals against microbial pathogens, cell trafficking, immune regulation, and prevention of autoimmunity. Pentraxins, important members of animal lectins, play a significant role in protecting the body from pathogen infection and regulating inflammatory reactions. They can recognize and bind to a variety of ligands, including carbohydrates, lipids, proteins, nucleic acids and their complexes, and protect the host from pathogen invasion by activating the complement cascade and Fcγ receptor pathways. Based on the primary structure of the subunit, pentraxins are divided into short and long pentraxins. The short pentraxins are comprised of C-reactive protein (CRP) and serum amyloid P (SAP), and the most important member of the long pentraxins is pentraxin 3 (PTX3). The CRP and SAP exist in both vertebrates and invertebrates, while the PTX3 may be present only in vertebrates. The major ligands and functions of CRP, SAP and PTX3 and three activation pathways involved in the complement system are summarized in this review. Their different characteristics in various animals including humans, and their evolutionary trees are analyzed. The clinical applications of CRP, SAP and PTX3 in human are reviewed. Some questions that remain to be understood are also highlighted.

Being Eaten Alive: How Energy-Deprived Cells Are Disposed of, Mediated by C-Reactive Protein-Including a Treatment Option

In medicine, C-reactive protein (CRP) has become established primarily as a biomarker, predicting patient prognosis in many indications. Recently, however, there has been mounting evidence that it causes inflammatory injury. As early as 1999, CRP was shown to induce cell death after acute myocardial infarction (AMI) in rats and this was found to be dependent on complement. The pathological effect of CRP was subsequently confirmed in further animal species such as rabbit, mouse and pig. A conceptual gap was recently closed when it was demonstrated that ischemia in AMI or ischemia/hypoxia in the severe course of COVID-19 causes a drastic lack of energy in involved cells, resulting in an apoptotic presentation because these cells cannot repair/flip-flop altered lipids. The deprivation of energy leads to extensive expression on the cell membranes of the CRP ligand lysophosphatidylcholine. Upon attachment of CRP to this ligand, the classical complement pathway is triggered leading to the swift elimination of viable cells with the appearance of an apoptotic cell by phagocytes. They are being eaten alive. This, consequently, results in substantial fibrotic remodeling within the involved tissue. Inhibiting this pathomechanism via CRP-targeting therapy has been shown to be beneficial in different indications.

Therapeutic potential of curcumin in ARDS and COVID-19

Curcumin is a safe, non-toxic, readily available and naturally occurring compound, an active constituent of Curcuma longa (turmeric). Curcumin could potentially treat diseases, but faces poor physicochemical and pharmacological characteristics. To overcome these limitations, we developed a stable, water-soluble formulation of curcumin called cyclodextrin-complexed curcumin (CDC). We have previously shown that direct delivery of CDC to the lung following lipopolysaccharides exposure reduces acute lung injury (ALI) and effectively reduces lung injury, inflammation and mortality in mice following Klebsiella pneumoniae. Recently, we found that administration of CDC led to a significant reduction in angiotensin-converting enzyme 2 and signal transducer and activator of transcription 3 expression in gene and protein levels following pneumonia, indicating its potential in treating coronavirus disease 2019 (COVID-19). In this review, we consider the clinical features of ALI and acute respiratory distress syndrome (ARDS) and the role of curcumin in modulating the pathogenesis of bacterial/viral-induced ARDS and COVID-19.

Identification of Lipid Biomarkers for Chronic Joint Pain Associated with Different Joint Diseases

Lipids, especially lysophosphatidylcholine LPC16:0, have been shown to be involved in chronic joint pain through the activation of acid-sensing ion channels (ASIC3). The aim of the present study was to investigate the lipid contents of the synovial fluids from controls and patients suffering from chronic joint pain in order to identify characteristic lipid signatures associated with specific joint diseases. For this purpose, lipids were extracted from the synovial fluids and analyzed by mass spectrometry. Lipidomic analyses identified certain choline-containing lipid classes and molecular species as biomarkers of chronic joint pain, regardless of the pathology, with significantly higher levels detected in the patient samples. Moreover, correlations were observed between certain lipid levels and the type of joint pathologies. Interestingly, LPC16:0 levels appeared to correlate with the metabolic status of patients while other choline-containing lipids were more specifically associated with the inflammatory state. Overall, these data point at selective lipid species in synovial fluid as being strong predictors of specific joint pathologies which could help in the selection of the most adapted treatment.

A novel phosphocholine-mimetic inhibits a pro-inflammatory conformational change in C-reactive protein

C-reactive protein (CRP) is an early-stage acute phase protein and highly upregulated in response to inflammatory reactions. We recently identified a novel mechanism that leads to a conformational change from the native, functionally relatively inert, pentameric CRP (pCRP) structure to a pentameric CRP intermediate (pCRP*) and ultimately to the monomeric CRP (mCRP) form, both exhibiting highly pro-inflammatory effects. This transition in the inflammatory profile of CRP is mediated by binding of pCRP to activated/damaged cell membranes via exposed phosphocholine lipid head groups. We designed a tool compound as a low molecular weight CRP inhibitor using the structure of phosphocholine as a template. X-ray crystallography revealed specific binding to the phosphocholine binding pockets of pCRP. We provide in vitro and in vivo proof-of-concept data demonstrating that the low molecular weight tool compound inhibits CRP-driven exacerbation of local inflammatory responses, while potentially preserving pathogen-defense functions of CRP. The inhibition of the conformational change generating pro-inflammatory CRP isoforms via phosphocholine-mimicking compounds represents a promising, potentially broadly applicable anti-inflammatory therapy.

Streptococcus pneumoniae interactions with the complement system

Host innate and adaptive immunity to infection with Streptococcus pneumoniae is critically dependent on the complement system, demonstrated by the high incidence of invasive S. pneumoniae infection in people with inherited deficiency of complement components. The complement system is activated by S. pneumoniae through multiple mechanisms. The classical complement pathway is activated by recognition of S. pneumoniae by C-reactive protein, serum amyloid P, C1q, SIGN-R1, or natural or acquired antibody. Some S. pneumoniae strains are also recognised by ficolins to activate the mannose binding lectin (MBL) activation pathway. Complement activation is then amplified by the alternative complement pathway, which can also be activated by S. pneumoniae directly. Complement activation results in covalent linkage of the opsonic complement factors C3b and iC3b to the S. pneumoniae surface which promote phagocytic clearance, along with complement-mediated immune adherence to erythrocytes, thereby protecting against septicaemia. The role of complement for mucosal immunity to S. pneumoniae is less clear. Given the major role of complement in controlling infection with S. pneumoniae, it is perhaps unsurprising that S. pneumoniae has evolved multiple mechanisms of complement evasion, including the capsule, multiple surface proteins, and the toxin pneumolysin. There is considerable variation between S. pneumoniae capsular serotypes and genotypes with regards to sensitivity to complement which correlates with ability to cause invasive infections. However, at present we only have a limited understanding of the main mechanisms causing variations in complement sensitivity between S. pneumoniae strains and to non-pathogenic streptococci.

C-Reactive Protein: Friend or Foe? Phylogeny From Heavy Metals to Modified Lipoproteins and SARS-CoV-2

Animal C-reactive protein (CRP) has a widespread existence throughout phylogeny implying that these proteins have essential functions mandatory to be preserved. About 500 million years of evolution teach us that there is a continuous interplay between emerging antigens and components of innate immunity. The most archaic physiological roles of CRP seem to be detoxication of heavy metals and other chemicals followed or accompanied by an acute phase response and host defense against bacterial, viral as well as parasitic infection. On the other hand, unusual antigens have emerged questioning the black-and-white perception of CRP as being invariably beneficial. Such antigens came along either as autoantigens like excessive tissue-stranded modified lipoprotein due to misdirected food intake linking CRP with atherosclerosis with an as yet open net effect, or as foreign antigens like SARS-CoV-2 inducing an uncontrolled CRP-mediated autoimmune response. The latter two examples impressingly demonstrate that a component of ancient immunity like CRP should not be considered under identical “beneficial” auspices throughout phylogeny but might effect quite the reverse as well.

Treatment of Pneumococcal Infection by Using Engineered Human C-Reactive Protein in a Mouse Model

C-reactive protein (CRP) binds to several species of bacterial pathogens including Streptococcus pneumoniae. Experiments in mice have revealed that one of the functions of CRP is to protect against pneumococcal infection by binding to pneumococci and activating the complement system. For protection, however, CRP must be injected into mice within a few hours of administering pneumococci, that is, CRP is protective against early-stage infection but not against late-stage infection. It is assumed that CRP cannot protect if pneumococci got time to recruit complement inhibitor factor H on their surface to become complement attack-resistant. Since the conformation of CRP is altered under inflammatory conditions and altered CRP binds to immobilized factor H also, we hypothesized that in order to protect against late-stage infection, CRP needed to change its structure and that was not happening in mice. Accordingly, we engineered CRP molecules (E-CRP) which bind to factor H on pneumococci but do not bind to factor H on any host cell in the blood. We found that E-CRP, in cooperation with wild-type CRP, was protective regardless of the timing of administering E-CRP into mice. We conclude that CRP acts via two different conformations to execute its anti-pneumococcal function and a model for the mechanism of action of CRP is proposed. These results suggest that pre-modified CRP, such as E-CRP, is therapeutically beneficial to decrease bacteremia in pneumococcal infection. Our findings may also have implications for infections with antibiotic-resistant pneumococcal strains and for infections with other bacterial species that use host proteins to evade complement-mediated killing.

Complement Activation by C-Reactive Protein Is Critical for Protection of Mice Against Pneumococcal Infection

C-reactive protein (CRP), a component of the innate immune system, is an antipneumococcal plasma protein. Human CRP has been shown to protect mice against infection with lethal doses of Streptococcus pneumoniae by decreasing bacteremia. in vitro, CRP binds to phosphocholine-containing substances, such as pneumococcal C-polysaccharide, in a Ca²⁺-dependent manner. Phosphocholine-complexed human CRP activates the complement system in both human and murine sera. The mechanism of antipneumococcal action of CRP in vivo, however, has not been defined yet. In this study, we tested a decades-old hypothesis that the complement-activating property of phosphocholine-complexed CRP contributes to protection of mice against pneumococcal infection. Our approach was to investigate a CRP mutant, incapable of activating murine complement, in mouse protection experiments. We employed site-directed mutagenesis of CRP, guided by its three-dimensional structure, and identified a mutant H38R which, unlike wild-type CRP, did not activate complement in murine serum. Substitution of His³⁸ with Arg in CRP did not affect the pentameric structure of CRP, did not affect the binding of CRP to pneumococci, and did not decrease the stability of CRP in mouse circulation. Employing a murine model of pneumococcal infection, we found that passively administered H38R CRP failed to protect mice against infection. Infected mice injected with H38R CRP showed no reduction in bacteremia and did not survive longer, as opposed to infected mice treated with wild-type CRP. Thus, the hypothesis that complement activation by phosphocholine-complexed CRP is an antipneumococcal effector function was supported. We can conclude now that complement activation by phosphocholine-complexed CRP is indeed essential for CRP-mediated protection of mice against pneumococcal infection.

Evolution of C-Reactive Protein

C-reactive protein (CRP) is an evolutionarily conserved protein. From arthropods to humans, CRP has been found in every organism where the presence of CRP has been sought. Human CRP is a pentamer made up of five identical subunits which binds to phosphocholine (PCh) in a Ca²⁺-dependent manner. In various species, we define a protein as CRP if it has any two of the following three characteristics: First, it is a cyclic oligomer of almost identical subunits of molecular weight 20–30 kDa. Second, it binds to PCh in a Ca²⁺-dependent manner. Third, it exhibits immunological cross-reactivity with human CRP. In the arthropod horseshoe crab, CRP is a constitutively expressed protein, while in humans, CRP is an acute phase plasma protein and a component of the acute phase response. As the nature of CRP gene expression evolved from a constitutively expressed protein in arthropods to an acute phase protein in humans, the definition of CRP became distinctive. In humans, CRP can be distinguished from other homologous proteins such as serum amyloid P, but this is not the case for most other vertebrates and invertebrates. Literature indicates that the binding ability of CRP to PCh is less relevant than its binding to other ligands. Human CRP displays structure-based ligand-binding specificities, but it is not known if that is true for invertebrate CRP. During evolution, changes in the intrachain disulfide and interchain disulfide bonds and changes in the glycosylation status of CRP may be responsible for different structure-function relationships of CRP in various species. More studies of invertebrate CRP are needed to understand the reasons behind such evolution of CRP. Also, CRP evolved as a component of and along with the development of the immune system. It is important to understand the biology of ancient CRP molecules because the knowledge could be useful for immunodeficient individuals.

Structure-Function Relationships of C-Reactive Protein in Bacterial Infection

One host defense function of C-reactive protein (CRP) is to protect against Streptococcus pneumoniae infection as shown by experiments employing murine models of pneumococcal infection. The protective effect of CRP is due to reduction in bacteremia. There is a distinct relationship between the structure of CRP and its anti-pneumococcal function. CRP is functional in both native and non-native pentameric structural conformations. In the native conformation, CRP binds to pneumococci through the phosphocholine molecules present on the C-polysaccharide of the pneumococcus and the anti-pneumococcal function probably involves the known ability of ligand-complexed CRP to activate the complement system. In the native structure-function relationship, CRP is protective only when given to mice within a few hours of the administration of pneumococci. The non-native pentameric conformation of CRP is created when CRP is exposed to conditions mimicking inflammatory microenvironments, such as acidic pH and redox conditions. In the non-native conformation, CRP binds to immobilized complement inhibitor factor H in addition to being able to bind to phosphocholine. Recent data using CRP mutants suggest that the factor H-binding function of non-native CRP is beneficial: in the non-native structure-function relationship, CRP can be given to mice any time after the administration of pneumococci irrespective of whether the pneumococci became complement-resistant or not. In conclusion, while native CRP is protective only against early stage infection, non-native CRP is protective against both early stage and late stage infections. Because non-native CRP displays phosphocholine-independent anti-pneumococcal activity, it is quite possible that CRP functions as a general anti-bacterial molecule.

Purification of recombinant C-reactive protein mutants

C-reactive protein (CRP) is an evolutionarily conserved protein, a component of the innate immune system, and an acute phase protein in humans. In addition to its raised level in blood in inflammatory states, CRP is also localized at sites of inflammation including atherosclerotic lesions, arthritic joints and amyloid plaque deposits. Results of in vivo experiments in animal models of inflammatory diseases indicate that CRP is an anti-pneumococcal, anti-atherosclerotic, anti-arthritic and an anti-amyloidogenic molecule. The mechanisms through which CRP functions in inflammatory diseases are not fully defined; however, the ligand recognition function of CRP in its native and non-native pentameric structural conformations and the complement-activating ability of ligand-complexed CRP have been suggested to play a role. One tool to understand the structure-function relationships of CRP and determine the contributions of the recognition and effector functions of CRP in host defense is to employ site-directed mutagenesis to create mutants for experimentation. For example, CRP mutants incapable of binding to phosphocholine are generated to investigate the importance of the phosphocholine-binding property of CRP in mediating host defense. Recombinant CRP mutants can be expressed in mammalian cells and, if expressed, can be purified from the cell culture media. While the methods to purify wild-type CRP are well established, different purification strategies are needed to purify various mutant forms of CRP if the mutant does not bind to either calcium or phosphocholine. In this article, we report the methods used to purify pentameric recombinant wild-type and mutant CRP expressed in and secreted by mammalian cells.

Energetics and the immune system: Trade-offs associated with non-acute levels of CRP in adolescent Gambian girls

Background and objectives: The human immune system is an ever-changing composition of innumerable cells and proteins, continually ready to respond to pathogens or insults. The cost of maintaining this state of immunological readiness is rarely considered. In this paper we aim to discern a cost to non-acute immune function by investigating how low levels of C-reactive protein (CRP) relate to other energetic demands and resources in adolescent Gambian girls.

Methodology: Data from a longitudinal study of 66 adolescent girls was used to test hypotheses around investment in immune function. Non-acute (under 2 mg/L) CRP was used as an index of immune function. Predictor variables include linear height velocity, adiposity, leptin, and measures of energy balance.

Results: Non-acute log CRP was positively associated with adiposity (β = 0.16, P < 0.001, R² = 0.17) and levels of the adipokine leptin (β = 1.17, P = 0.006, R² = 0.09). CRP was also negatively associated with increased investment in growth, as measured by height velocity (β = −0.58, P < 0.001, R² = 0.13) and lean mass deposition β = −0.42, P = 0.005, R² = 0.08). Relationships between adiposity and growth explained some, but not all, of this association. We do not find that CRP was related to energy balance.

Conclusions and implications: These data support a hypothesis that investment in non-acute immune function is facultative, and sensitive to energetic resources and demands. We also find support for an adaptive association between the immune system and adipose tissue.

CD14 is associated with biliary stricture formation

The pathogenesis of intrahepatic biliary stricture formation in patients with primary sclerosing cholangitis (PSC) or after liver transplantation (LTx) remains elusive. CD14 receptor signaling is a key mediator of the innate immune system; its common genetic variant is associated with alcoholic liver disease. PSC and LTx cohort patients and primary biliary cirrhosis (PBC) control patients were genotyped for the CD14 -260C>T (rs2569190) polymorphism, and genotypes were correlated with long-term clinical outcome. Biliary tissue, bile, and whole blood of PSC patients and healthy controls were screened for markers of the innate immune system and bacterial infection. In 121 PSC patients, the CD14 -260C>T genotype was associated with development of dominant bile duct strictures (P = 0.02). In 365 LTx patients, TT carriers (4.1%) were protected against the formation of nonanastomotic biliary strictures versus CC/CT patients (12.6%; P = 0.01). Chemokine ligand 8 (P = 0.04) and chemokine receptor 6 (P = 0.004) were up-regulated in biliary tissue of PSC patients with the TT versus the CC/CT genotype. Lipopolysaccharide whole-blood stimulation resulted in a significant change in interleukin (IL)-8 (P = 0.05) and IL-12p40 levels (P = 0.04) in healthy control subjects carrying the TT genotype. TT PSC patients were protected against Gram-negative bacterial biliary infection (TT: 0% vs.

CC/CT

22.5%; P = 0.02). Serum-soluble CD14 levels correlated with the CD14 -260C>T genotype (P = 0.02), representing an independent risk indicator of survival in PSC patients (hazard ratio, 0.40; 95% confidence interval, 0.19-0.86; P =0.01).

CONCLUSIONS

The function of the innate immune response by CD14 is crucial during biliary infection and stricture formation. The benefits of CD14 signaling modification should be addressed in future studies. (Hepatology 2016;64:843-852).

Extremely elevated C-reactive protein levels are associated with unfavourable outcomes, including death, in paediatric patients

AIM

The aim of this study was to investigate the clinical significance of extremely elevated C-reactive protein (CRP) levels in hospitalised children.

METHODS

We searched the electronic database of a tertiary paediatric medical centre for all patients admitted in 2010-2013 with a CRP of ≥ 30 mg/dL, and these comprised the study group. The controls were the other admissions. Data were collected on demographics, admission details, pre-existing conditions, discharge diagnosis, laboratory results and clinical outcomes.

RESULTS

Our study group comprised 435 (0.72%) of the 59,997 patients hospitalised during the study period. The mortality rate and mean hospital stays were significantly higher in the study group, and infectious diseases were the most common diagnoses, affecting 389 patients (89.4%), particularly pneumonia (47.1%). Higher CRP was correlated with low albumin levels (p < 0.01). Bacteraemia was the most prevalent diagnosis (38%) in the 84 oncology patients, with a crude mortality rate of 17.6%.

CONCLUSION

Infectious diseases, mainly bacterial, were the most common diagnoses in previously healthy children with CRP ≥ 30 mg/dL. Extremely elevated CRP levels were associated with an unfavourable clinical outcome, including high mortality, particularly in oncology patients. Paediatricians should be aware of the significance of extremely elevated CRP levels.

Increased Baseline C-Reactive Protein Concentrations Are Associated with Increased Risk of Infections: Results from 2 Large Danish Population Cohorts

BACKGROUND

The acute-phase reactant C-reactive protein (CRP) increases rapidly during an infection. We tested the hypothesis that chronic low-level increases in CRP are associated with an increased risk of infectious disease.

METHODS

We studied 9660 individuals from a prospective general population cohort, including 3592 in whom infectious disease developed, and another 60 896 individuals from a cross-sectional general population study, of whom 13 332 developed infectious disease; 55% were women, and the mean age was 57 years. Hospital diagnoses of infections in 1977-2010 were based on International Classification of Diseases-coded discharge records from the national Danish Patient Registry. We measured CRP concentrations and conducted genotyping for 4 CRP polymorphisms that increase CRP. Individuals with CRP >10 mg/L were excluded because of possible ongoing infection at the time of testing.

RESULTS

Individuals with CRP >3 mg/L had 1.2 and 1.7 times increased risk of infectious disease, in the prospective general population cohort and the cross-sectional general population study, respectively, compared with individuals with CRP <1 mg/L. In the combined populations, individuals in the highest CRP tertile (compared with the lowest) had an increased risk of bacterial diseases (hazard ratio 1.7, 95% CI 1.6-1.8), but not viral, mycosis, and parasitic diseases. The increased risk was mainly carried by pneumonia, sepsis, and particularly gram-negative infections. None of the genotype combinations examined conferred an increased risk of infectious disease.

CONCLUSIONS

Chronic low-level CRP increases were associated with increased risk of bacterial infections, gram-negative infections in particular. Genotypes associated with increases in CRP were not associated with increased risk of infection.

Low C-reactive protein values at admission predict mortality in patients with severe community-acquired pneumonia caused by Streptococcus pneumoniae that require intensive care management

Purpose

To identify risk factors associated with mortality in patients with severe community-acquired pneumonia (CAP) caused by S. pneumoniae who require intensive care unit (ICU) management, and to assess the prognostic values of these risk factors at the time of admission.

Methods

Retrospective analysis of all consecutive patients with CAP caused by S. pneumoniae who were admitted to the 32-bed medico-surgical ICU of a community and referral university hospital between 2002 and 2011. Univariate and multivariate analyses were performed on variables available at admission.

Results

Among the 77 adult patients with severe CAP caused by S. pneumoniae who required ICU management, 12 patients died (observed mortality rate 15.6 %). Univariate analysis indicated that septic shock and low C-reactive protein (CRP) values at admission were associated with an increased risk of death. In a multivariate model, after adjustment for age and gender, septic shock [odds ratio (OR), confidence interval 95 %; 4.96, 1.11–22.25; p = 0.036], and CRP (OR 0.99, 0.98–0.99 p = 0.034) remained significantly associated with death. Finally, we assessed the discriminative ability of CRP to predict mortality by computing its receiver operating characteristic curve. The CRP value cut-off for the best sensitivity and specificity was 169.5 mg/L to predict hospital mortality with an area under the curve of 0.72 (0.55–0.89).

Conclusions

The mortality of patients with S. pneumoniae CAP requiring ICU management was much lower than predicted by severity scores. The presence of septic shock and a CRP value at admission <169.5 mg/L predicted a fatal outcome.

C-reactive protein protects mice against pneumococcal infection via both phosphocholine-dependent and phosphocholine-independent mechanisms

The mechanism of action of C-reactive protein (CRP) in protecting mice against lethal Streptococcus pneumoniae infection is unknown. The involvement of the phosphocholine (PCh)-binding property of CRP in its antipneumococcal function previously has been explored twice, with conflicting results. In this study, using three different intravenous sepsis mouse models, we investigated the role of the PCh-binding property of CRP by employing a CRP mutant incapable of binding to PCh. The ability of wild-type CRP to protect mice against infection was found to differ in the three models; the protective ability of wild-type CRP decreased when the severity of infection was increased, as determined by measuring mortality and bacteremia. In the first animal model, in which we used 25 μg of CRP and 10(7) CFU of pneumococci, both wild-type and mutant CRP protected mice against infection, suggesting that the protection was independent of the PCh-binding activity of CRP. In the second model, in which we used 25 μg of CRP and 5 × 10(7) CFU of pneumococci, mutant CRP was not protective while wild-type CRP was, suggesting that the protection was dependent on the PCh-binding activity of CRP. In the third model, in which we used 150 μg of CRP and 10(7) CFU of pneumococci, mutant CRP was as protective as wild-type CRP, again indicating that the protection was independent of the PCh-binding activity of CRP. We conclude that both PCh-dependent and PCh-independent mechanisms are involved in the CRP-mediated decrease in bacteremia and the resulting protection of mice against pneumococcal infection.

Human L-ficolin recognizes phosphocholine moieties of pneumococcal teichoic acid

Human L-ficolin is a soluble protein of the innate immune system able to sense pathogens through its fibrinogen (FBG) recognition domains and to trigger activation of the lectin complement pathway through associated serine proteases. L-Ficolin has been previously shown to recognize pneumococcal clinical isolates, but its ligands and especially its molecular specificity remain to be identified. Using solid-phase binding assays, serum and recombinant L-ficolins were shown to interact with serotype 2 pneumococcal strain D39 and its unencapsulated R6 derivative. Incubation of both strains with serum triggered complement activation, as measured by C4b and C3b deposition, which was decreased by using ficolin-depleted serum. Recombinant L-ficolin and its FBG-like recognition domain bound to isolated pneumococcal cell wall extracts, whereas binding to cell walls depleted of teichoic acid (TA) was decreased. Both proteins were also shown to interact with two synthetic TA compounds, each comprising part structures of the complete lipoteichoic acid molecule with two PCho residues. Competition studies and direct interaction measurements by surface plasmon resonance identified PCho as a novel L-ficolin ligand. Structural analysis of complexes of the FBG domain of L-ficolin and PCho revealed that the phosphate moiety interacts with amino acids previously shown to define an acetyl binding site. Consequently, binding of L-ficolin to immobilized acetylated BSA was inhibited by PCho and synthetic TA. Binding of serum L-ficolin to immobilized synthetic TA and PCho-conjugated BSA triggered activation of the lectin complement pathway, thus further supporting the hypothesis of L-ficolin involvement in host antipneumococcal defense.

C-reactive protein is essential for innate resistance to pneumococcal infection

No deficiency of human C-reactive protein (CRP), or even structural polymorphism of the protein, has yet been reported so its physiological role is not known. Here we show for the first time that CRP-deficient mice are remarkably susceptible to Streptococcus pneumoniae infection and are protected by reconstitution with isolated pure human CRP, or by anti-pneumococcal antibodies. Autologous mouse CRP is evidently essential for innate resistance to pneumococcal infection before antibodies are produced. Our findings are consistent with the significant association between clinical pneumococcal infection and non-coding human CRP gene polymorphisms which affect CRP expression. Deficiency or loss of function variation in CRP may therefore be lethal at the first early-life encounter with this ubiquitous virulent pathogen, explaining the invariant presence and structure of CRP in human adults.

Recognition functions of pentameric C-reactive protein in cardiovascular disease

C-reactive protein (CRP) performs two recognition functions that are relevant to cardiovascular disease. First, in its native pentameric conformation, CRP recognizes molecules and cells with exposed phosphocholine (PCh) groups, such as microbial pathogens and damaged cells. PCh-containing ligand-bound CRP activates the complement system to destroy the ligand. Thus, the PCh-binding function of CRP is defensive if it occurs on foreign pathogens because it results in the killing of the pathogen via complement activation. On the other hand, the PCh-binding function of CRP is detrimental if it occurs on injured host cells because it causes more damage to the tissue via complement activation; this is how CRP worsens acute myocardial infarction and ischemia/reperfusion injury. Second, in its nonnative pentameric conformation, CRP also recognizes atherogenic low-density lipoprotein (LDL). Recent data suggest that the LDL-binding function of CRP is beneficial because it prevents formation of macrophage foam cells, attenuates inflammatory effects of LDL, inhibits LDL oxidation, and reduces proatherogenic effects of macrophages, raising the possibility that nonnative CRP may show atheroprotective effects in experimental animals. In conclusion, temporarily inhibiting the PCh-binding function of CRP along with facilitating localized presence of nonnative pentameric CRP could be a promising approach to treat atherosclerosis and myocardial infarction. There is no need to stop the biosynthesis of CRP.

Correlations between peripheral blood Coxiella burnetii DNA load, interleukin-6 levels, and C-reactive protein levels in patients with acute Q fever

From 2007 to 2010, the Netherlands experienced the largest reported Q fever outbreak, with >4,000 notified cases. We showed previously that C-reactive protein is the only traditional infection marker reflecting disease activity in acute Q fever. Interleukin-6 is the principal inducer of C-reactive protein. We questioned whether increased C-reactive protein levels in acute Q fever patients coincide with increased interleukin-6 levels and if these levels correlate with the Coxiella burnetii DNA load in serum. In addition, we studied their correlation with disease severity, expressed by hospital admission and the development of fatigue. Interleukin-6 and C-reactive protein levels were analyzed in sera from 102 patients diagnosed with seronegative PCR-positive acute Q fever. Significant but weak negative correlations were observed between bacterial DNA loads expressed as cycle threshold values and interleukin-6 and C-reactive protein levels, while a significant moderate-strong positive correlation was present between interleukin-6 and C-reactive protein levels. Furthermore, significantly higher interleukin-6 and C-reactive protein levels were observed in hospitalized acute Q fever patients in comparison to those in nonhospitalized patients, while bacterial DNA loads were the same in the two groups. No marker was prognostic for the development of fatigue. In conclusion, the correlation between interleukin-6 and C-reactive protein levels in acute Q fever patients points to an immune activation pathway in which interleukin-6 induces the production of C-reactive protein. Significant differences in interleukin-6 and C-reactive protein levels between hospitalized and nonhospitalized patients despite identical bacterial DNA loads suggest an important role for host factors in disease presentation. Higher interleukin-6 and C-reactive protein levels seem predictive of more severe disease.

The phosphocholine-binding pocket on C-reactive protein is necessary for initial protection of mice against pneumococcal infection

Human C-reactive protein (CRP) protects mice from lethal Streptococcus pneumoniae infection when injected into mice within the range of 6 h before to 2 h after the administration of pneumococci. Because CRP binds to phosphocholine-containing substances and subsequently activates the complement system, it has been proposed that the antipneumococcal function of CRP requires the binding of CRP to phosphocholine moieties present in pneumococcal cell wall C-polysaccharide. To test this proposal experimentally, in this study, we utilized a new CRP mutant incapable of binding to phosphocholine. Based on the structure of CRP-phosphocholine complexes, which showed that Phe(66), Thr(76), and Glu(81) formed the phosphocholine-binding pocket, we constructed a CRP mutant F66A/T76Y/E81A in which the pocket was blocked by substituting Tyr for Thr(76). When compared with wild-type CRP, mutant CRP bound more avidly to phosphoethanolamine and could be purified by affinity chromatography using phosphoethanolamine-conjugated Sepharose. Mutant CRP did not bind to phosphocholine, C-polysaccharide, or pneumococci. Mutant CRP was free in the mouse serum, and its rate of clearance in vivo was not faster than that of wild-type CRP. When either 25 μg or 150 μg of CRP was administered into mice, unlike wild-type CRP, mutant CRP did not protect mice from lethal pneumococcal infection. Mice injected with mutant CRP had higher mortality rates than mice that received wild-type CRP. Decreased survival was due to the increased bacteremia in mice treated with mutant CRP. We conclude that the phosphocholine-binding pocket on CRP is necessary for CRP-mediated initial protection of mice against lethal pneumococcal infection.

Exposing a hidden functional site of C-reactive protein by site-directed mutagenesis

C-reactive protein (CRP) is a cyclic pentameric protein whose major binding specificity, at physiological pH, is for substances bearing exposed phosphocholine moieties. Another pentameric form of CRP, which exists at acidic pH, displays binding activity for oxidized LDL (ox-LDL). The ox-LDL-binding site in CRP, which is hidden at physiological pH, is exposed by acidic pH-induced structural changes in pentameric CRP. The aim of this study was to expose the hidden ox-LDL-binding site of CRP by site-directed mutagenesis and to generate a CRP mutant that can bind to ox-LDL without the requirement of acidic pH. Mutation of Glu(42), an amino acid that participates in intersubunit interactions in the CRP pentamer and is buried, to Gln resulted in a CRP mutant (E42Q) that showed significant binding activity for ox-LDL at physiological pH. For maximal binding to ox-LDL, E42Q CRP required a pH much less acidic than that required by wild-type CRP. At any given pH, E42Q CRP was more efficient than wild-type CRP in binding to ox-LDL. Like wild-type CRP, E42Q CRP remained pentameric at acidic pH. Also, E42Q CRP was more efficient than wild-type CRP in binding to several other deposited, conformationally altered proteins. The E42Q CRP mutant provides a tool to investigate the functions of CRP in defined animal models of inflammatory diseases including atherosclerosis because wild-type CRP requires acidic pH to bind to deposited, conformationally altered proteins, including ox-LDL, and available animal models may not have sufficient acidosis or other possible modifiers of the pentameric structure of CRP at the sites of inflammation.

C-reactive protein as a predictor of mortality in critically ill patients: a meta-analysis and systematic review

C-reactive protein is a marker of inflammatory response and has been widely investigated in cardiovascular and infectious diseases, especially to monitor therapeutic success. However, its role as a predictor of clinical outcome in critically ill patients remains uncertain and controversial. The objective of this study was to investigate the predictive value of C-reactive protein in critically ill patients. The databases of PubMed, the Cochrane clinical trial database and EMBASE (from inception to August 2010) were searched. Prospective non-randomised clinical studies comparing C-reactive protein concentrations between survivors and non-survivors were included. Pooled mean difference in C-reactive protein concentrations between survivors and non-survivors was calculated. Heterogeneity was analysed by I2. Sensitivity and subgroup analyses were conducted to explore the heterogeneity. Fourteen studies containing a total of 1969 patients were finally included in our analysis. The weighted mean difference in the C-reactive protein levels between survivors and non-survivors was 9.15 mg/l (95% confidence interval -6.50 to 24.81). The heterogeneity was large with I2 = 92%. Subsequent investigation of the heterogeneity with sensitivity analyses yielded no significant differences. The subgroup analysis showed that the weighted mean difference in early (within 48 hours) C-reactive protein levels between survivors and non-survivors was not significantly different, in contrast to the late (beyond 48 hours) C-reactive protein level. This was significantly greater in non-survivors with a weighted mean difference of 63.80 mg/l (95% confidence interval 35.67 to 91.93). Our systematic review shows that while the early C-reactive protein concentration is not a good predictor of survival in critically ill patients, the late C-reactive protein concentration may help to identify patients who are at risk of death.

Identification of acidic pH-dependent ligands of pentameric C-reactive protein

C-reactive protein (CRP) is a phylogenetically conserved protein; in humans, it is present in the plasma and at sites of inflammation. At physiological pH, native pentameric CRP exhibits calcium-dependent binding specificity for phosphocholine. In this study, we determined the binding specificities of CRP at acidic pH, a characteristic of inflammatory sites. We investigated the binding of fluid-phase CRP to six immobilized proteins: complement factor H, oxidized low-density lipoprotein, complement C3b, IgG, amyloid β, and BSA immobilized on microtiter plates. At pH 7.0, CRP did not bind to any of these proteins, but, at pH ranging from 5.2 to 4.6, CRP bound to all six proteins. Acidic pH did not monomerize CRP but modified the pentameric structure, as determined by gel filtration, 1-anilinonaphthalene-8-sulfonic acid-binding fluorescence, and phosphocholine-binding assays. Some modifications in CRP were reversible at pH 7.0, for example, the phosphocholine-binding activity of CRP, which was reduced at acidic pH, was restored after pH neutralization. For efficient binding of acidic pH-treated CRP to immobilized proteins, it was necessary that the immobilized proteins, except factor H, were also exposed to acidic pH. Because immobilization of proteins on microtiter plates and exposure of immobilized proteins to acidic pH alter the conformation of immobilized proteins, our findings suggest that conformationally altered proteins form a CRP-ligand in acidic environment, regardless of the identity of the protein. This ligand binding specificity of CRP in its acidic pH-induced pentameric state has implications for toxic conditions involving protein misfolding in acidic environments and favors the conservation of CRP throughout evolution.

Genetic polymorphism of the C-reactive protein (CRP) gene and a deep infection focus determine maximal serum CRP level in Staphylococcus aureus bacteremia

C-reactive protein (CRP) is widely used in early detection of sepsis or organ dysfunction. Several single nucleotide polymorphisms (SNPs) in the CRP gene are shown to be associated with variability of basal CRP. To clarify the effect of these SNPs to CRP response in systemic infections, we compared genetic and clinical data on patients with Staphylococcus aureus bacteremia (SAB). Six SNPs in the CRP gene region (rs2794521, rs30912449, rs1800947, rs1130864, rs1205 and rs3093075) were genotyped in 145 patients and analyzed for associations with CRP and various clinical outcomes. We found that the rare minor A-allele of triallelic SNP rs30912449 (C > T > A) and presence of a deep infection focus were strongly associated to the higher maximal CRP during the first week of SAB. Median of the maximal CRP in patients who had the A-minor allele was 282 mg/L (interquartile range [IQR, defined as the difference between the third quartile and the first quartile], 169 mg/L) but only 179 mg/L (IQR, 148 mg/L) in patients without this allele (P = 0.004), and CRP in patients who had deep infection focus was higher 208 mg/L (IQR, 147 mg/L) than in other patients 114 mg/L (IQR, 121 mg/l) (P < 0.0001). Mortality, degree of leucocytosis, time to defervescence or number of deep infections were not affected by CRP gene SNPs. The maximal CRP during the first week in SAB was partly determined by variation in the CRP gene and partly by presence of deep infection focus. This finding suggests cautiousness in interpreting exceptionally high CRPs from SAB patients and comparison between patients.

Inhibitory effects of soluble MD-2 and soluble CD14 on bacterial growth

The effects of the soluble forms of the endotoxin receptor molecules sMD-2 and sCD14 on bacterial growth were studied. When Escherichia coli and Bacillus subtilis were incubated at 37 degrees C for 18 hr with either sMD-2 or sCD14, growth of these bacteria was significantly inhibited as evaluated by viable cell counts and NADPH/NADH activity. A mutant of sCD14 (sCD14d57-64) lacking a region essential for LPS binding did not inhibit the growth of E. coli, whereas this mutant did inhibit the growth of B. subtilis. Addition of excess PG to the bacterial culture reversed the inhibitory effect of sMD-2 on the growth of B. subtilis, but not on the growth of E. coli. Furthermore, when evaluated by ELISA, both sMD-2 and sCD14 bound specifically to PG. Taken together, these results indicate that sMD-2 and sCD14 inhibit the growth of both Gram-positive and Gram-negative bacteria and further suggest that binding to PG and LPS is involved in the inhibitory effect of sMD-2 on Gram-positive bacteria and of sCD14 on Gram-negative bacteria, respectively.

Probing the phosphocholine-binding site of human C-reactive protein by site-directed mutagenesis

Human C-reactive protein (CRP) can activate the classical pathway of complement and function as an opsonin only when it is complexed to an appropriate ligand. Most known CRP ligands bind to the phosphocholine (PCh)-binding site of the protein. In the present study, we used oligonucleotide-directed site-specific mutagenesis to investigate structural determinants of the PCh-binding site of CRP. Eight mutant recombinant (r) CRP, Y40F; E42Q; Y40F, E42Q; K57Q; R58G; K57Q, R58G; W67K; and K57Q, R58G, W67K were constructed and expressed in COS cells. Wild-type and all mutant rCRP except for the W67K mutants bound to solid-phase PCh-substituted bovine serum albumin (PCh-BSA) with similar apparent avidities. However, W67K rCRP had decreased avidity for PCh-BSA and the triple mutant, K57Q, R58G, W67K, failed to bind PCh-BSA. Inhibition experiments using PCh and dAMP as inhibitors indicated that both Lys-57 and Arg-58 contribute to PCh binding. They also indicated that Trp-67 provides interactions with the choline group. The Y40F and E42Q mutants were found to have increased avidity for fibronectin compared to wild-type rCRP. We conclude that the residues Lys-57, Arg-58, and Trp-67 contribute to the structure of the PCh-binding site of human CRP. Residues Tyr-40 and Glu-42 do not appear to participate in the formation of the PCh-binding site of CRP, however, they may be located in the vicinity of the fibronectin-binding site of CRP.