Review
Expert Review of Gastroenterology & Hepatology Downloaded from informahealthcare.com by Emory University on 08/08/15 For personal use only.
C-reactive protein in Crohn’s disease: how informative is it? Expert Rev. Gastroenterol. Hepatol. 8(4), 393–408 (2014)
Fernando Magro*1–4‡, Paula Sousa5‡ and Paula Ministro5 1 Department of Pharmacology and Therapeutics, Faculty of Medicine, University of Porto, Alameda Prof. Hernani Monteiro, 420-319 Porto, Portugal 2 Gastroenterology Department, Faculty of Medicine, Hospital de Sa˜o Joa˜o, Porto, Portugal 3 Institute for Molecular and Cell Biology, Porto, Portugal 4 MedInUP – Center for Drug Discovery and Innovative Medicines, University of Porto, Porto, Portugal 5 Gastroenterology Department, Centro Hospitalar Tondela-Viseu, Viseu, Portugal *Author for correspondence: Tel.: +351 225 513 600 Fax: +351 225 513 601 [email protected] ‡
Authors contributed equally
C-reactive protein (CRP) is an important acute-phase marker, produced mainly in the liver. Its production by mesenteric adipocytes has been recently stressed in Crohn’s disease (CD). There are many factors affecting CRP levels, both environmental and genetics. The short-life of this biomarker makes it of pertinent use in the assessment of inflammation. There are inconsistent results concerning the association of clinical activity indices, mucosal healing, histological activity and CRP. This review summarizes the role of CRP in CD, namely its importance in the differential diagnosis of CD; its relationship with clinical activity indices, other markers of inflammation and endoscopic and radiological cross sectional imaging; prediction of response to anti-TNF treatment and prediction of outcome. KEYWORDS: anti-TNF • biomarkers • clinical activity indices • C-reactive protein • Crohn’s disease • differential diagnosis • endoscopic indices of activity • outcome
C-reactive protein (CRP) was first detected in 1930 by William S Tillet and Thomas Francis, who identified a substance that precipitated the ‘C’ polysaccharide of the cell wall of Streptococcus pneumoniae in patients with pneumonia [1]. Subsequently, it was found that this reaction was not unique to pneumococcal pneumonia but could be found with a variety of other acute infections. This led to the characterization of other so-called ‘acute-phase proteins’ [2,3]. CRP structure & functions Structure & synthesis of CRP
CRP is a member of the phylogenetically ancient ‘pentraxin’ family of proteins and consists of a cyclical arrangement of five identical noncovalently-bound subunits (protomers) arranged symmetrically around a central pore. The pentameric structure of CRP (FIGURE 1) imparts a high degree of stability to the molecule and resistance to enzymatic attack [4,5]. Circulating CRP is synthesized primarily by the liver at very low levels constitutively [6]. Under physiological circumstances, the median concentration of CRP is 0.8 mg/l, but it can exceed 300 mg/l within 48 h after an acute stimulus. CRP synthesis is mainly regulated at the transcriptional level. The principal inducer
informahealthcare.com
10.1586/17474124.2014.893821
of the CRP gene is IL-6, and this response is enhanced by IL-1b. CRP synthesis is also regulated by post-transcriptional mechanisms: after a stimulus, there is a pronounced acceleration of the secretion of CRP from the endoplasmic reticulum, with half time for exit reduced from 18 h to 75 min. This explains the rapid rise in concentration, with its peak in 48 h [7–10]. Although the major site of CRP synthesis is the hepatocyte, extrahepatic expression has also been documented [7,9] Recently, PeyrinBiroulet et al. described mesenteric fat as an important source of CRP in Crohn’s disease (CD). CRP production by mesenteric adipocytes was triggered by inflammatory and bacterial stimuli [11]. Most of CRP is degraded in the hepatocyte, but a small part (bound to its ligands) is taken up and processed by neutrophils and macrophages. Its biological half-life is approximately 19 h, and it is independent of the CRP levels or of any physiological or pathophysiological circumstances. This means that the only significant determinant of plasma CRP levels is the rate of synthesis. As a consequence, only liver failure or therapies affecting the acute-phase stimulus are able to decrease CRP. The short half-life of CRP also ensures that the concentrations quickly decrease once the acute-phase stimulus
2014 Informa UK Ltd
ISSN 1747-4124
393
Expert Review of Gastroenterology & Hepatology Downloaded from informahealthcare.com by Emory University on 08/08/15 For personal use only.
Review
Magro, Sousa & Ministro
carbohydrates. CRP also binds endogenous ligands that are exposed after cell damage or death, including histones, chromatin, ribonucleoproteins and lysophosphatidylcholine [15]. This ability to participate in the clearance and processing of nuclear antigens led to the ‘waste disposal’ theory that CRP could prevent autoimmune diseases [12]. After binding, the CRP–ligand complex will activate the complement cascade via C1q, resulting in opsonization and phagocytosis. However, in contrast to immunoglobulin-mediated activation of the complement cascade, activation by CRP is limited to the early stages of the classical complement pathway, with a failure to generate the membrane-attack complex. As such, complement activation initiated by CRP efficiently results in the recruitment of the opsonic function of the system, but not its proinflammatory and membrane-damaging effects [16]. Binding to Fcg receptors
Figure 1. C-reactive protein pentamer – surface view of the ligand-binding face. Each protomer contains a binding site for two calcium and one phosphocholine molecules. The structure is taken from structure file PDB ID: 1B09 from the NCBI [95].
disappears, making CRP a very valuable marker to detect and follow-up inflammation [7,9]. Factors that influence CRP levels
There are many socioeconomic and lifestyle factors that can influence baseline concentrations of CRP, such as smoking, BMI, coffee consumption and oral contraceptives [12]. In the National Heart, Lung and Blood Institute Family Heart Study, a large cross-sectional cohort evaluated the family aggregation of three systemic markers of inflammation including CRP. The combination of sociodemographic factors (age, center, education), behavioral and lifestyle (cigarette smoking status, alcohol intake, hormone replacement therapy), obesity and diabetes explained 30 and 22% of the interindividual variability of CRP levels in women and men, respectively [13]. There is also now evidence suggesting that CRP levels are strongly influenced by inherited genetic variation [8,12]. The findings in the National Heart, Lung and Blood Institute Family Heart Study suggested substantial familial and genetic influences on the levels of basal CRP, with an estimated heritability of about 0.4 [13]. CRP levels are affected by single nucleotide polymorphisms and haplotypes associated with high and low acute-phase CRP expression [14]. CRP functions Complement activation
The main ligand for CRP is phosphocholine, a component of the teichoic acids of Gram-positive bacteria, the lipopolysaccharides of Gram-negative bacteria and microbial capsular 394
Phagocytosis may also occur via complement-independent pathways through binding of CRP to the Fcg receptors I and IIa localized on macrophages and neutrophils [9]. Whereas attachment of CRP to these receptors promotes the release of proinflammatory cytokines, its binding to FcgIIb leads to the inhibition of cell activation [8,10]. Monomers of CRP
Even though only intact pentameric CRP can be detected in the plasma, some of the CRP molecules undergo processes of proteolysis or denaturation, resulting in dissociation of CRP into monomeric subunits (mCRP). mCRP was shown to have both proinflammatory and anti-inflammatory effects [7,8]. In sum, CRP bridges innate and adaptive immune pathways by activation of complement and interaction with Fcg receptors. It can have both pro- and anti-inflammatory effects, although it is still not known how each is determined in any given physiological or pathological situation. It functions as a first line of innate host defense by binding to a number of pathogens and promoting their elimination by phagocytic cells. In addition to bacteria, CRP has been shown to play an important role in the clearance of apoptotic and necrotic cells, which would contribute to the restitution of normal function and structuring of injured tissues, and may be important in preventing autoimmune response by the nuclear constituents of apoptotic blebs. On the other hand, CRP can cause tissue damage, as it was hypothesized for acute myocardial infarction [12]. CRP & Crohn’s disease
Inflammatory bowel disease (IBD) is a multifactorial condition of uncertain etiology, with significant heterogeneity in its onset, outcome and response to treatment. There is an inherent complexity in the assessment of IBD. Physicians still face many challenges in the diagnosis, assessment of disease activity and prognosis of both CD and ulcerative colitis (UC). A single gold standard test for diagnosis and follow-up of these diseases is still inexistent. A combination of different methods, such as Expert Rev. Gastroenterol. Hepatol. 8(4), (2014)
Expert Review of Gastroenterology & Hepatology Downloaded from informahealthcare.com by Emory University on 08/08/15 For personal use only.
C-reactive protein in Crohn’s disease
clinical, laboratory, endoscopic, histopathological or radiological techniques, is required. Many biomarkers have been studied in IBD in order to obtain an objective measurement of disease activity and also to avoid invasive procedures. Ideally, a marker should be easy to perform, inexpensive, reproducible, non- or minimally invasive and disease-specific. It should also be able to evaluate different points in disease management, such as diagnosis, prognosis, assessment of severity, monitoring of response to treatment and relapse. Unfortunately, no such biomarker exists in IBD [17]. Nevertheless, CRP is used with diagnosis purposes and to measure disease activity, response to therapy and follow-up. Diagnosis of CD
Even with the rapidly evolving knowledge of matters related to CD and with the current arsenal of laboratory tools, the diagnosis of CD can still be a challenge. The clinical onset is often nonspecific and overlaps with other diseases, with indolent symptoms that are characteristic of both organic and nonorganic disorders [18]. One must be able to discriminate CD from infectious colitis, irritable bowel syndrome (IBS) and UC, among other diseases. CRP could be used to distinguish CD from functional disorders as shown in TABLE 1.
Review
control group [25]. However, in this latter study, clinical remission was subjectively defined by one of two observers. Normal CRP levels have been described in subsets with active CD patients. In Florin et al. study [30], 10% of patients with Crohn’s disease activity index (CDAI) >200 points had a CRP value 10 mg/l) at diagnosis. More recently, Peyrin-Biroulet et al. [32] investigated the relationship between clinical disease activity, CRP normalization and mucosal healing in CD using a large population of patients who were included in SONIC trial. Among 188 patients with moderate-to-severe disease (CDAI >220 and
Expert Review of Gastroenterology & Hepatology Downloaded from informahealthcare.com by Emory University on 08/08/15 For personal use only.
C-reactive protein in Crohn’s disease: how informative is it? Expert Rev. Gastroenterol. Hepatol. 8(4), 393–408 (2014)
Fernando Magro*1–4‡, Paula Sousa5‡ and Paula Ministro5 1 Department of Pharmacology and Therapeutics, Faculty of Medicine, University of Porto, Alameda Prof. Hernani Monteiro, 420-319 Porto, Portugal 2 Gastroenterology Department, Faculty of Medicine, Hospital de Sa˜o Joa˜o, Porto, Portugal 3 Institute for Molecular and Cell Biology, Porto, Portugal 4 MedInUP – Center for Drug Discovery and Innovative Medicines, University of Porto, Porto, Portugal 5 Gastroenterology Department, Centro Hospitalar Tondela-Viseu, Viseu, Portugal *Author for correspondence: Tel.: +351 225 513 600 Fax: +351 225 513 601 [email protected] ‡
Authors contributed equally
C-reactive protein (CRP) is an important acute-phase marker, produced mainly in the liver. Its production by mesenteric adipocytes has been recently stressed in Crohn’s disease (CD). There are many factors affecting CRP levels, both environmental and genetics. The short-life of this biomarker makes it of pertinent use in the assessment of inflammation. There are inconsistent results concerning the association of clinical activity indices, mucosal healing, histological activity and CRP. This review summarizes the role of CRP in CD, namely its importance in the differential diagnosis of CD; its relationship with clinical activity indices, other markers of inflammation and endoscopic and radiological cross sectional imaging; prediction of response to anti-TNF treatment and prediction of outcome. KEYWORDS: anti-TNF • biomarkers • clinical activity indices • C-reactive protein • Crohn’s disease • differential diagnosis • endoscopic indices of activity • outcome
C-reactive protein (CRP) was first detected in 1930 by William S Tillet and Thomas Francis, who identified a substance that precipitated the ‘C’ polysaccharide of the cell wall of Streptococcus pneumoniae in patients with pneumonia [1]. Subsequently, it was found that this reaction was not unique to pneumococcal pneumonia but could be found with a variety of other acute infections. This led to the characterization of other so-called ‘acute-phase proteins’ [2,3]. CRP structure & functions Structure & synthesis of CRP
CRP is a member of the phylogenetically ancient ‘pentraxin’ family of proteins and consists of a cyclical arrangement of five identical noncovalently-bound subunits (protomers) arranged symmetrically around a central pore. The pentameric structure of CRP (FIGURE 1) imparts a high degree of stability to the molecule and resistance to enzymatic attack [4,5]. Circulating CRP is synthesized primarily by the liver at very low levels constitutively [6]. Under physiological circumstances, the median concentration of CRP is 0.8 mg/l, but it can exceed 300 mg/l within 48 h after an acute stimulus. CRP synthesis is mainly regulated at the transcriptional level. The principal inducer
informahealthcare.com
10.1586/17474124.2014.893821
of the CRP gene is IL-6, and this response is enhanced by IL-1b. CRP synthesis is also regulated by post-transcriptional mechanisms: after a stimulus, there is a pronounced acceleration of the secretion of CRP from the endoplasmic reticulum, with half time for exit reduced from 18 h to 75 min. This explains the rapid rise in concentration, with its peak in 48 h [7–10]. Although the major site of CRP synthesis is the hepatocyte, extrahepatic expression has also been documented [7,9] Recently, PeyrinBiroulet et al. described mesenteric fat as an important source of CRP in Crohn’s disease (CD). CRP production by mesenteric adipocytes was triggered by inflammatory and bacterial stimuli [11]. Most of CRP is degraded in the hepatocyte, but a small part (bound to its ligands) is taken up and processed by neutrophils and macrophages. Its biological half-life is approximately 19 h, and it is independent of the CRP levels or of any physiological or pathophysiological circumstances. This means that the only significant determinant of plasma CRP levels is the rate of synthesis. As a consequence, only liver failure or therapies affecting the acute-phase stimulus are able to decrease CRP. The short half-life of CRP also ensures that the concentrations quickly decrease once the acute-phase stimulus
2014 Informa UK Ltd
ISSN 1747-4124
393
Expert Review of Gastroenterology & Hepatology Downloaded from informahealthcare.com by Emory University on 08/08/15 For personal use only.
Review
Magro, Sousa & Ministro
carbohydrates. CRP also binds endogenous ligands that are exposed after cell damage or death, including histones, chromatin, ribonucleoproteins and lysophosphatidylcholine [15]. This ability to participate in the clearance and processing of nuclear antigens led to the ‘waste disposal’ theory that CRP could prevent autoimmune diseases [12]. After binding, the CRP–ligand complex will activate the complement cascade via C1q, resulting in opsonization and phagocytosis. However, in contrast to immunoglobulin-mediated activation of the complement cascade, activation by CRP is limited to the early stages of the classical complement pathway, with a failure to generate the membrane-attack complex. As such, complement activation initiated by CRP efficiently results in the recruitment of the opsonic function of the system, but not its proinflammatory and membrane-damaging effects [16]. Binding to Fcg receptors
Figure 1. C-reactive protein pentamer – surface view of the ligand-binding face. Each protomer contains a binding site for two calcium and one phosphocholine molecules. The structure is taken from structure file PDB ID: 1B09 from the NCBI [95].
disappears, making CRP a very valuable marker to detect and follow-up inflammation [7,9]. Factors that influence CRP levels
There are many socioeconomic and lifestyle factors that can influence baseline concentrations of CRP, such as smoking, BMI, coffee consumption and oral contraceptives [12]. In the National Heart, Lung and Blood Institute Family Heart Study, a large cross-sectional cohort evaluated the family aggregation of three systemic markers of inflammation including CRP. The combination of sociodemographic factors (age, center, education), behavioral and lifestyle (cigarette smoking status, alcohol intake, hormone replacement therapy), obesity and diabetes explained 30 and 22% of the interindividual variability of CRP levels in women and men, respectively [13]. There is also now evidence suggesting that CRP levels are strongly influenced by inherited genetic variation [8,12]. The findings in the National Heart, Lung and Blood Institute Family Heart Study suggested substantial familial and genetic influences on the levels of basal CRP, with an estimated heritability of about 0.4 [13]. CRP levels are affected by single nucleotide polymorphisms and haplotypes associated with high and low acute-phase CRP expression [14]. CRP functions Complement activation
The main ligand for CRP is phosphocholine, a component of the teichoic acids of Gram-positive bacteria, the lipopolysaccharides of Gram-negative bacteria and microbial capsular 394
Phagocytosis may also occur via complement-independent pathways through binding of CRP to the Fcg receptors I and IIa localized on macrophages and neutrophils [9]. Whereas attachment of CRP to these receptors promotes the release of proinflammatory cytokines, its binding to FcgIIb leads to the inhibition of cell activation [8,10]. Monomers of CRP
Even though only intact pentameric CRP can be detected in the plasma, some of the CRP molecules undergo processes of proteolysis or denaturation, resulting in dissociation of CRP into monomeric subunits (mCRP). mCRP was shown to have both proinflammatory and anti-inflammatory effects [7,8]. In sum, CRP bridges innate and adaptive immune pathways by activation of complement and interaction with Fcg receptors. It can have both pro- and anti-inflammatory effects, although it is still not known how each is determined in any given physiological or pathological situation. It functions as a first line of innate host defense by binding to a number of pathogens and promoting their elimination by phagocytic cells. In addition to bacteria, CRP has been shown to play an important role in the clearance of apoptotic and necrotic cells, which would contribute to the restitution of normal function and structuring of injured tissues, and may be important in preventing autoimmune response by the nuclear constituents of apoptotic blebs. On the other hand, CRP can cause tissue damage, as it was hypothesized for acute myocardial infarction [12]. CRP & Crohn’s disease
Inflammatory bowel disease (IBD) is a multifactorial condition of uncertain etiology, with significant heterogeneity in its onset, outcome and response to treatment. There is an inherent complexity in the assessment of IBD. Physicians still face many challenges in the diagnosis, assessment of disease activity and prognosis of both CD and ulcerative colitis (UC). A single gold standard test for diagnosis and follow-up of these diseases is still inexistent. A combination of different methods, such as Expert Rev. Gastroenterol. Hepatol. 8(4), (2014)
Expert Review of Gastroenterology & Hepatology Downloaded from informahealthcare.com by Emory University on 08/08/15 For personal use only.
C-reactive protein in Crohn’s disease
clinical, laboratory, endoscopic, histopathological or radiological techniques, is required. Many biomarkers have been studied in IBD in order to obtain an objective measurement of disease activity and also to avoid invasive procedures. Ideally, a marker should be easy to perform, inexpensive, reproducible, non- or minimally invasive and disease-specific. It should also be able to evaluate different points in disease management, such as diagnosis, prognosis, assessment of severity, monitoring of response to treatment and relapse. Unfortunately, no such biomarker exists in IBD [17]. Nevertheless, CRP is used with diagnosis purposes and to measure disease activity, response to therapy and follow-up. Diagnosis of CD
Even with the rapidly evolving knowledge of matters related to CD and with the current arsenal of laboratory tools, the diagnosis of CD can still be a challenge. The clinical onset is often nonspecific and overlaps with other diseases, with indolent symptoms that are characteristic of both organic and nonorganic disorders [18]. One must be able to discriminate CD from infectious colitis, irritable bowel syndrome (IBS) and UC, among other diseases. CRP could be used to distinguish CD from functional disorders as shown in TABLE 1.
Review
control group [25]. However, in this latter study, clinical remission was subjectively defined by one of two observers. Normal CRP levels have been described in subsets with active CD patients. In Florin et al. study [30], 10% of patients with Crohn’s disease activity index (CDAI) >200 points had a CRP value 10 mg/l) at diagnosis. More recently, Peyrin-Biroulet et al. [32] investigated the relationship between clinical disease activity, CRP normalization and mucosal healing in CD using a large population of patients who were included in SONIC trial. Among 188 patients with moderate-to-severe disease (CDAI >220 and
