Mark B. Pepys

C-reactive protein: a critical update

In the mid 1990s, immunoassays for C-reactive protein (CRP), with greater sensitivity than those previously in routine use, revealed that increased CRP values, even within the range previously considered normal, strongly predict future coronary events. These findings triggered widespread interest, especially, remarkably, in the US, where the clinical use of CRP measurement had been largely ignored for about 30 years. CRP production is part of the nonspecific acute-phase response to most forms of inflammation, infection, and tissue damage and was therefore considered not to provide clinically useful information. Indeed, CRP values can never be diagnostic on their own and can only be interpreted at the bedside, in full knowledge of all other clinical and pathological results. However, they can then contribute powerfully to management, just as universal recording of the patient’s temperature, an equally nonspecific parameter, is of great clinical utility. The present torrent of studies of CRP in cardiovascular disease and associated conditions is facilitated by the ready commercial availability of automated CRP assays and of CRP itself as a research reagent. However, unlike the earlier rejection in the US of CRP as an empirical test because of its perceived lack of specificity, the current enthusiasm over CRP in cardiovascular disease is widely characterized by failure to recognize appropriately the nonspecific nature of the acute-phase response, and by lack of critical biological judgment. Quality control of the source, purity, and structural and functional integrity of the CRP, and the relevance of experimental design before ascribing pathophysiological functions, are also often ignored. This article provides information about CRP as a protein and an acute-phase reactant, and a knowledge-based framework for interpretation and analysis of clinical observations of CRP in relation to cardiovascular and other diseases. We also review the properties of CRP, its possible role in pathogenesis of disease, and our own observations that identify it as a possible therapeutic target.

The Prognostic Value of C-Reactive Protein and Serum Amyloid A Protein in Severe Unstable Angina

BACKGROUND The pathogenesis of unstable angina is poorly understood, and predicting the prognosis at the time of hospital admission is problematic. Recent evidence suggests that there may be active inflammation, possibly in the coronary arteries, in this syndrome. We therefore studied the prognostic value of measurements of the circulating acute-phase reactants C-reactive protein and serum amyloid A protein, which are sensitive indicators of inflammation. METHODS We measured C-reactive protein, serum amyloid A protein, creatine kinase, and cardiac troponin T in 32 patients with chronic stable angina, 31 patients with severe unstable angina, and 29 patients with acute myocardial infarction. RESULTS At the time of hospital admission, creatine kinase and cardiac troponin T levels were normal in all the patients, but the levels of C-reactive protein and serum amyloid A protein were > or = 0.3 mg per deciliter (exceeding the 90th percentile of the normal distribution) in 4 of the patients with stable angina (13 percent), 20 of the patients with unstable angina (65 percent), and 22 of the patients with acute myocardial infarction (76 percent). The 20 patients with unstable angina who had levels of C-reactive protein and serum amyloid A protein > or = 0.3 mg per deciliter had more ischemic episodes in the hospital than those with levels < 0.3 mg per deciliter (mean [+/- SD] number of episodes per patient, 4.8 +/- 2.5 vs. 1.8 +/- 2.4; P = 0.004); 5 patients subsequently had a myocardial infarction, 2 died, and 12 required immediate coronary revascularization. In contrast, no deaths or myocardial infarction occurred among the 11 patients with levels of C-reactive protein and serum amyloid A protein < 0.3 mg per deciliter, and only 2 of them required coronary revascularization. Among the patients admitted with a diagnosis of acute myocardial infarction, unstable angina preceded infarction in 14 of the 22 patients (64 percent) with levels of C-reactive protein and serum amyloid A protein > or = 0.3 mg per deciliter but in none of the 7 patients with levels < 0.3 mg per deciliter. CONCLUSIONS Elevation of the sensitive acute-phase proteins C-reactive protein and serum amyloid A protein at the time of hospital admission predicts a poor outcome in patients with unstable angina and may reflect an important inflammatory component in the pathogenesis of this condition.

C-Reactive Protein, a Sensitive Marker of Inflammation, Predicts Future Risk of Coronary Heart Disease in Initially Healthy Middle-Aged Men Results From the MONICA (Monitoring Trends and Determinants in Cardiovascular Disease) Augsburg Cohort Study, 1984 to 1992

BACKGROUND Inflammatory reactions in coronary plaques play an important role in the pathogenesis of acute atherothrombotic events; inflammation elsewhere is also associated with both atherogenesis generally and its thrombotic complications. Recent studies indicate that systemic markers of inflammation can identify subjects at high risk of coronary events. METHODS AND RESULTS We used a sensitive immunoradiometric assay to examine the association of serum C-reactive protein (CRP) with the incidence of first major coronary heart disease (CHD) event in 936 men 45 to 64 years of age. The subjects, who were sampled at random from the general population, participated in the first MONICA Augsburg survey (1984 to 1985) and were followed for 8 years. There was a positive and statistically significant unadjusted relationship, which was linear on the log-hazards scale, between CRP values and the incidence of CHD events (n=53). The hazard rate ratio (HRR) of CHD events associated with a 1-SD increase in log-CRP level was 1.67 (95% CI, 1.29 to 2. 17). After adjustment for age, the HRR was 1.60 (95% CI, 1.23 to 2. 08). Adjusting further for smoking behavior, the only variable selected from a variety of potential confounders by a forward stepping process with a 5% change in the relative risk of CRP as the selection criterion, yielded an HRR of 1.50 (95% CI, 1.14 to 1.97). CONCLUSIONS These results confirm the prognostic relevance of CRP, a sensitive systemic marker of inflammation, to the risk of CHD in a large, randomly selected cohort of initially healthy middle-aged men. They suggest that low-grade inflammation is involved in pathogenesis of atherosclerosis, especially its thrombo-occlusive complications.

Low grade inflammation and coronary heart disease : prospective study and updated meta-analyses

Abstract Objective: To assess associations between baseline values of four different circulating markers of inflammation and future risk of coronary heart disease, potential triggers of systemic inflammation (such as persistent infection), and other markers of inflammation. Design: Nested case-control comparisons in a prospective, population based cohort. Setting: General practices in 18 towns in Britain. Participants: 506 men who died from coronary heart disease or had a non-fatal myocardial infarction and 1025 men who remained free of such disease until 1996 selected from 5661 men aged 40–59 years who provided blood samples in 1978-1980. Main outcome measures: Plasma concentrations of C reactive protein, serum amyloid A protein, and serum albumin and leucocyte count. Information on fatal and non-fatal coronary heart disease was obtained from medical records and death certificates. Results: Compared with men in the bottom third of baseline measurements of C reactive protein, men in the top third had an odds ratio for coronary heart disease of 2.13 (95% confidence interval 1.38 to 3.28) after age, town, smoking, vascular risk factors, and indicators of socioeconomic status were adjusted for. Similar adjusted odds ratios were 1.65 (1.07 to 2.55) for serum amyloid A protein; 1.12 (0.71 to 1.77) for leucocyte count; and 0.67 (0.43 to 1.04) for albumin. No strong associations were observed of these factors with Helicobacter pylori seropositivity, Chlamydia pneumoniae IgG titres, or plasma total homocysteine concentrations. Baseline values of the acute phase reactants were significantly associated with one another (P<0.0001), although the association between low serum albumin concentration and leucocyte count was weaker (P=0.08). Conclusion: In the context of results from other relevant studies these findings suggest that some inflammatory processes, unrelated to the chronic infections studied here, are likely to be involved in coronary heart disease.

Production of C-reactive protein and risk of coronary events in stable and unstable angina

BACKGROUND Inflammation is an important feature of atherosclerotic lesions, and increased production of the acute-phase reactant. C-reactive protein (CRF), is associated with a poor prognosis in severe unstable angina. We have investigated the existence and possible significance of the acute-phase responses of CRP and another sensitive reactant, serum amyloid A protein (SAA), in patients with unstable or stable angina. METHODS We used new ultrasensitive immunoassays to measure CRP and SAA concentrations in plasma from 2121 outpatients with angina (1030 unstable, 743 stable, the rest atypical) enrolled in the European Concerted Action on Thrombosis and Disabilities (ECAT) Angina Pectoris Study. All patients underwent coronary angiography and extensive clinical and laboratory assessment at study entry, and were then followed up for 2 years. All suspected coronary events during follow-up were reviewed by an independent endpoint committee. FINDINGS 75 individuals (41 with unstable, 29 with stable, and 5 with atypical angina) had a coronary event during follow-up. Concentrations of CRP at study entry were associated with coronary events in patients with stable or unstable angina: there was about a two-fold increase in the risk of a coronary event in patients whose CRP concentration was in the fifth quintile (> 3.6 mg/L), compared with the first four quintiles. A third of the events occurred among patients who had a CRP concentration of more than 3.6 mg/L. CRP concentrations were positively correlated with age, smoking, body-mass index, triglycerides, extent of coronary stenosis, history of myocardial infarction, and lower ejection fraction. By contrast, concentrations of SAA were not associated with risk of a coronary event. INTERPRETATION We found that raised circulating concentrations of CRP are predictors of coronary events in patients with stable or unstable angina. The modest acute-phase responses of CRP were probably not the result of myocardial necrosis. Whatever the underlying mechanisms, the sensitive measurement of CRP as a prognostic marker may be useful in the management of coronary heart disease.

C-reactive protein concentration and risk of coronary heart disease, stroke, and mortality: an individual participant meta-analysis.

Summary Background Associations of C-reactive protein (CRP) concentration with risk of major diseases can best be assessed by long-term prospective follow-up of large numbers of people. We assessed the associations of CRP concentration with risk of vascular and non-vascular outcomes under different circumstances. Methods We meta-analysed individual records of 160 309 people without a history of vascular disease (ie, 1·31 million person-years at risk, 27 769 fatal or non-fatal disease outcomes) from 54 long-term prospective studies. Within-study regression analyses were adjusted for within-person variation in risk factor levels. Results Loge CRP concentration was linearly associated with several conventional risk factors and inflammatory markers, and nearly log-linearly with the risk of ischaemic vascular disease and non-vascular mortality. Risk ratios (RRs) for coronary heart disease per 1-SD higher loge CRP concentration (three-fold higher) were 1·63 (95% CI 1·51–1·76) when initially adjusted for age and sex only, and 1·37 (1·27–1·48) when adjusted further for conventional risk factors; 1·44 (1·32–1·57) and 1·27 (1·15–1·40) for ischaemic stroke; 1·71 (1·53–1·91) and 1·55 (1·37–1·76) for vascular mortality; and 1·55 (1·41–1·69) and 1·54 (1·40–1·68) for non-vascular mortality. RRs were largely unchanged after exclusion of smokers or initial follow-up. After further adjustment for fibrinogen, the corresponding RRs were 1·23 (1·07–1·42) for coronary heart disease; 1·32 (1·18–1·49) for ischaemic stroke; 1·34 (1·18–1·52) for vascular mortality; and 1·34 (1·20–1·50) for non-vascular mortality. Interpretation CRP concentration has continuous associations with the risk of coronary heart disease, ischaemic stroke, vascular mortality, and death from several cancers and lung disease that are each of broadly similar size. The relevance of CRP to such a range of disorders is unclear. Associations with ischaemic vascular disease depend considerably on conventional risk factors and other markers of inflammation. Funding British Heart Foundation, UK Medical Research Council, BUPA Foundation, and GlaxoSmithKline.

Acute phase proteins with special reference to C-reactive protein and related proteins (pentaxins) and serum amyloid A protein.

The acute phase response among plasma proteins is a normal response to tissue injury and is therefore a fundamental aspect of many diverse disease processes. It probably usually has a beneficial net function in limiting damage and promoting repair but in some circumstances it may have pathological consequences. Sustained high levels of acute phase proteins and especially SAA are associated with the development of amyloidosis in some individuals. Increased concentrations of CRP may, by activating the complement system, contribute to inflammation and enhance tissue damage. Failure of the normal or appropriate CRP response may also possibly have deleterious effects. SAA is a polymorphic protein which is normally present only in trace amounts but which, during the acute phase response, becomes one of the major apolipoproteins associated with high-density lipoprotein particles. The function of apoSAA is not known but it must have considerable physiological significance apart from its role as the putative precursor of amyloid A protein fibrils. CRP and SAP have been very stably conserved throughout vertebrate evolution and homologous proteins are apparently present even in vertebrates. This strongly suggests that they have important functions although these have not yet been precisely delineated. The main role of CRP may be to provide for enhanced clearance of inappropriate materials from the plasma whether these are of extrinsic origin, such as microorganisms and their products, or the autologous products of cell damage and death. The interaction between aggregated CRP and plasma low-density lipoprotein may play a significant part in the normal function of CRP and may also have a role in lipoprotein metabolism, clearance, and deposition. SAP is a normal tissue protein as well as being a plasma protein. Aggregated SAP selectively binds fibronectin and this may represent an aspect of the normal function of SAP. The deposition of SAP in amyloid is evidently not a normal function but it is not known whether this deposition is involved in the pathogenesis of amyloid or whether it is merely an epiphenomenon. In any case immunohistochemical staining for SAP is useful in the diagnosis of amyloid, in investigation of glomerulonephritis, and in studying disorders of elastic tissue. Regardless of its physiological or pathophysiological functions, the assay of serum CRP is a valuable aid to clinical management in a number of different situations and in different diseases provided results are interpreted in the light of full clinical information.(ABSTRACT TRUNCATED AT 400 WORDS)

C-reactive protein fifty years on.

The discovery of C-reactive protein (CRP) half a century ago led to the description of the acute-phase reaction which is a fundamental response of the body to injury. Recent work on the structure and function of CRP has revealed the existence of a unique plasma protein family, including CRP and serum amyloid P component (SAP). These proteins have been conserved throughout vertebrate evolution. CRP binds specifically to a wide range of substances derived both from damaged autologous cells and from microorganisms. Complexed CRP can activate the complement system and, by virtue of its dramatically increased production in response to tissue injury, it probably acts primarily as a protective mechanism. However, in some circumstances CRP may also initiate or exacerbate inflammatory lesions. Clinical measurement of serum CRP is valuable as a screening test for organic disease and as a sensitive object index of disease activity and response to therapy in some inflammatory, infective, and ischaemic conditions. SAP closely resembles CRP in structure but not an acute-phase reactant in man. An apparently identical protein, amyloid P component (AP), is always found in amyloid deposits. AP is also found in normal tissues, as an integral constituent of vascular basement membranes and is located on the peripheral microfibrillar mantle of elastic fibres throughout the body.

Targeting C-reactive protein for the treatment of cardiovascular disease

Complement-mediated inflammation exacerbates the tissue injury of ischaemic necrosis in heart attacks and strokes, the most common causes of death in developed countries. Large infarct size increases immediate morbidity and mortality and, in survivors of the acute event, larger non-functional scars adversely affect long-term prognosis. There is thus an important unmet medical need for new cardioprotective and neuroprotective treatments. We have previously shown that human C-reactive protein (CRP), the classical acute-phase protein that binds to ligands exposed in damaged tissue and then activates complement, increases myocardial and cerebral infarct size in rats subjected to coronary or cerebral artery ligation, respectively. Rat CRP does not activate rat complement, whereas human CRP activates both rat and human complement. Administration of human CRP to rats is thus an excellent model for the actions of endogenous human CRP. Here we report the design, synthesis and efficacy of 1,6-bis(phosphocholine)-hexane as a specific small-molecule inhibitor of CRP. Five molecules of this palindromic compound are bound by two pentameric CRP molecules, crosslinking and occluding the ligand-binding B-face of CRP and blocking its functions. Administration of 1,6-bis(phosphocholine)-hexane to rats undergoing acute myocardial infarction abrogated the increase in infarct size and cardiac dysfunction produced by injection of human CRP. Therapeutic inhibition of CRP is thus a promising new approach to cardioprotection in acute myocardial infarction, and may also provide neuroprotection in stroke. Potential wider applications include other inflammatory, infective and tissue-damaging conditions characterized by increased CRP production, in which binding of CRP to exposed ligands in damaged cells may lead to complement-mediated exacerbation of tissue injury.

The physiological structure of human C-reactive protein and its complex with phosphocholine.

BACKGROUND Human C-reactive protein (CRP) is the classical acute phase reactant, the circulating concentration of which rises rapidly and extensively in a cytokine-mediated response to tissue injury, infection and inflammation. Serum CRP values are routinely measured, empirically, to detect and monitor many human diseases. However, CRP is likely to have important host defence, scavenging and metabolic functions through its capacity for calcium-dependent binding to exogenous and autologous molecules containing phosphocholine (PC) and then activating the classical complement pathway. CRP may also have pathogenic effects and the recent discovery of a prognostic association between increased CRP production and coronary atherothrombotic events is of particular interest. RESULTS The X-ray structures of fully calcified C-reactive protein, in the presence and absence of bound PC, reveal that although the subunit beta-sheet jellyroll fold is very similar to that of the homologous pentameric protein serum amyloid P component, each subunit is tipped towards the fivefold axis. PC is bound in a shallow surface pocket on each subunit, interacting with the two protein-bound calcium ions via the phosphate group and with Glu81 via the choline moiety. There is also an unexpected hydrophobic pocket adjacent to the ligand. CONCLUSIONS The structure shows how large ligands containing PC may be bound by CRP via a phosphate oxygen that projects away from the surface of the protein. Multipoint attachment of one planar face of the CRP molecule to a PC-bearing surface would leave available, on the opposite exposed face, the recognition sites for C1q, which have been identified by mutagenesis. This would enable CRP to target physiologically and/or pathologically significant complement activation. The hydrophobic pocket adjacent to bound PC invites the design of inhibitors of CRP binding that may have therapeutic relevance to the possible role of CRP in atherothrombotic events.