Roberto Carrasco and Anne Barton
Arthritis Research UK, The University of Manchester, Manchester, UK
Correspondence: Dr Anne Barton, Arthritis Research UK Epidemiology Unit, Manchester Academic Health Sciences Centre, The University of Manchester, Stopford Building, Oxford Road, Manchester, M13, 9PT, UK. E-mail: anne.barton@manchester.ac.uk
Key words: biomarkers, rheumatic diseases, rheumatoid arthritis.
Conflict of interest: the authors report no conflicts of interest.
Received for publication: 11 March 2010.
Revision received: 28 April 2010.
Accepted for publication: 19 May 2010.
©Copyright R. Carrasco and A. Barton, 2010
Licensee PAGEPress, Italy
Rheumatology Reports 2010; 2:e3
doi:10.4081/rr.2010.e3
Rheumatoid arthritis (RA) is the most common autoimmune inflammatory arthritis, affecting approximately 1% of the population. This systemic disease is marked by chronic inflammation that predominantly affects the synovial membrane of diarthrodal joints. Its etiology is unknown, but it is presumed to be an immunological disease with contributing genetic1 and environmental factors.2 Evidence suggests that RA develops in 3 phases: an asymptomatic period of genetic risk, a pre-clinical period in which RA-related antibodies can be detected, and a clinical phase with acute signs and symptoms of inflammatory arthritis.3
Evidence suggests that the sooner RA patients are treated, the better is their prognosis.4 The availability of markers that could help to identify patients with more aggressive, rapidly progressive RA with poorer prognosis would offer a rational basis for early and aggressive treatment. In that way it may be possible to avoid many irreversible clinical complications.5 The number of disease modifying anti-rheumatic drugs (DMARD) available has increased in recent years. The effectiveness, cost and toxicity of the new agents vary widely and the natural history of RA itself includes spontaneous remission, remission with medical treatment or continuously progressive disease despite medication. Hence, there is a clear need to identify efficient diagnostic and prognostic indicators of disease to assist clinicians and help them in making initial treatment choices.
This article is a literature review of current biomarkers to aid in the diagnosis of RA and emerging predictive markers of prognosis, but is non-systematic and therefore may be subject to publication and selection biases. No attempt has been made to assess the validity of studies or to compare risks between studies. Our aim was to review the literature in an inclusive manner.
The intensive search for markers of prediction and prognosis in RA has been the subject of a large number of studies and a huge variety of possible markers have been reported, although not always validated. They can be classified in different groups or categories depending on their function and their location (Figure 1). A discussion of clinical and imaging biomarkers is beyond the scope of the current review, which will focus on biochemical markers.
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Figure 1. Classification schema for RA markers. |
For a long time, the diagnosis of RA was mainly based on clinical manifestations. However, it is often difficult to diagnose RA in the very early phases of the disease and in many cases irreversible damage had occurred by the time the diagnosis was confirmed. Therefore, laboratory tests which are sensitive and specific early in the disease course are desirable to allow earlier diagnosis and intervention. A multitude of such biomarkers have been investigated focusing on analytes found in the different cellular compartments including biomarkers involved in the synthesis and degradation of bone/cartilage, inflammation or autoimmune processes in order to identify those that could be clinically useful.
Genetic markers in RA have been widely reviewed elsewhere.1 At least 12 genetic loci have been confidently identified as associated with susceptibility to RA but each locus confers a relatively modest effect and, even in combination, cannot be used to diagnose the condition as variants are common in the healthy population (Table 1). Only 2 show evidence of association with severity or outcome of disease. Firstly, variants within the human leukocyte antigen (HLA)-DRB1 gene show the strongest association with RA.6 In particular alleles encoding a shared amino acid motif are collectively called the shared epitope (SE)7 and not only increase the risk of RA but are also associated with more severe disease. These alleles have been also reported to be associated with anti-citrullinated protein antibodies (ACPA) as measured by anti-cyclic citrullinated peptide (anti-CCP) antibodies. More severe disease with an increased rate of joint destruction has been found in patients with both anti-CCP antibodies and SE alleles.8,9,10,11,12,13,14,15,16,17,As anti-CCP antibodies are cheaper and easier to measure, testing for SE alleles carriage is not used clinically. Secondly, variants within the TRAF1/C5 susceptibility locus have been reported in two studies to be associated with erosive change independently of the SE.19,20 However, the risk conferred by carriage at the susceptibility variants is only in the order of 1.5 and so not clinically useful at the current time.
The peptidylarginine deiminase 4 (PADI4) gene is of particular interest as it has been tested in Asian, European and North American populations and its relative effect in relation to RA susceptibility across these groups remains controversial.21,22,23,24,25,26,27 An interesting study suggests that the PADI4 genotype critically modulates the effect of anti-CCPs on clinical characteristics of RA and that the PADI4 genotype itself appears to influence joint destruction.24 However, in the largest study performed to date, the PADI4 genotype was not a significant risk factor for RA in people of European ancestry, in contrast to Asian populations.21 New studies are needed to clarify the role of PADI4 as RA marker.
In summary, HLA-DRB1 together with anti-CCP antibodies remains the most reliable genetic marker for a more severe progress of RA. However, a number of new genetic loci are emerging and it is expected that some of them will also contribute to predict outcome in RA.
Although genetic markers have the advantage that they are stable over time, a number of serological markers have been investigated as diagnostic or prognostic aids in the management of RA. These are outlined below.
The receptor activator of nuclear factor κB ligand (RANKL) is a member of the TNF superfamily; it is expressed on various cell populations and also exists as a soluble molecule. It is a key factor in the stimulation of osteoclast formation and activation. In turn, osteoclasts play a central role in the mechanism of joint destruction in RA.28 It has been demonstrated that T cells express RANKL and that there is overexpression of RANKL mRNA in the synovium of RA patients at the site of the bone resorption.29 The soluble receptor-like molecule osteoprotegerin (OPG), a decoy receptor, is a natural inhibitor of RANKL. Both RANKL and OPG can be measured in serum and the ratio RANKL/OPG is thought to regulate bone resorption. The COmbinatie therapie Bij Rheumatoide Artritis (COBRA) trial showed that a baseline value of a low ratio RANKL/OPG predicted a higher 5-year radiographic progression of joint damage.30 An independent study found that baseline levels of the RANKL:OPG-ratio in patients with early, active, untreated RA are strong independent predictors of rapid and persistent damage progression over 11 years follow up.31 Interestingly, another research group found that increased levels of OPG are effective in compensating for the action of soluble RANKL, but do not directly prevent bone degradation.32 A fourth study found that baseline levels of OPG and RANKL were not associated with radiographic progression at five or ten years.33 This discrepancy with the COBRA study might be related to differences in patient characteristics. Dysregulation of the RANK/RANKL/OPG system has been implicated in the pathophysiology of multiple bone remodeling disorders including osteoporosis, glucocorticoid-induced bone loss, multiple myeloma and rheumatoid arthritis. Therefore RANK/RANKL/OPG may eventually be used as markers of bone metabolism, though the broad role of RANKL signaling in the immune system may limit its specificity.
Collagen cross-linked C-telopeptide (CTX-I) is a degradation product of C-terminal cross-linking telopeptide of type I collagen which has been suggested to be a candidate biomarker for joint destruction. A prospective study of patients with early RA showed that high baseline levels of urinary CTX-I predict increased risk of radiological progression and correlation with more rapid progression of joint destruction34 whilst another long-term follow-up study noted correlation of serum CTX-I levels and subsequent joint destruction.33 These results suggest that CTX-I might be useful for identifying patients with a high risk of joint damage progression before damage can be detected radiographically.
Bone contains non-collagenous proteins such as bone sialoprotein (BSP) which is released during bone damage and can reflect the rate of bone turnover.35 Serum levels of bone sialoprotein were measured in the chronic and destructive arthritis induced in dark agouti (DA) rats after immunization with autologous rat native collagen type II in Freund's incomplete adjuvant. Increased serum levels BSP were seen on day 21 after immunization and even higher levels were observed on day 28 at termination of the experiment, paralleling increases in the clinical joint score and histopathological signs of cartilage and bone erosions suggesting that it may be a marker of joint damage.36 In a study by Mansson et al. of 42 patients, BSP was quantified by enzyme linked immunosorbent (ELISA) assays in longitudinally collected knee joint synovial fluid (SF) samples of RA patients rapidly developing destruction in knees or hips, the "destructive" group (n=18), and in patients slowly developing destruction, the "non-destructive" group (n=25). The destructive group was characterized by rising BSP concentrations in synovial fluid with time comparing with the initial levels (P<0.001).37 The authors conclude that this molecule contributes to the assessment of extent of tissue destruction and may help in the early identification of patients at risk of rapidly progressing destruction.
Cathepsin K is expressed by osteoclasts and synovial fibroblasts to degrade type I collagen during bone resorption and is hypothesized to play a role in the pathology of RA.38 Some studies suggest that it may be interesting as a marker of bone resorption.39 For example, a study of 100 patients with active, longstanding RA found that the radiological destruction correlated with the levels of cathepsin K.32 However, this has not yet been validated so further studies will be required to investigate the role of cathepsin K as a marker of bone resorption in RA.
Osteocalcin is a large peptide that is synthesized by osteoclasts, odontoblasts and some chondrocytes. Circulating osteocalcin and its fragments reflect both bone formation and resorption.40 In an experiment to study the changes in markers of bone metabolism, including osteocalcin, during anti-TNF treatment, it was found that the levels of osteocalcin were appreciably increased at 14 weeks compared with baseline, but not at 30 and 46 weeks, and no significant association was found between changes in disease activity and levels of osteocalcin at any of the time intervals.41 However, another study reported that osteocalcin fragments are released during osteoclastic bone resorption and that the quantification of specific age-modified osteocalcin fragments can provide an index of bone resorption suggesting that it may be a marker of joint erosion rather than disease activity.42
In summary, although many bone markers have been tested as potential biomarkers, the evidence, to date, suggests that the RANKL/OPG ratio is most reliable in predicting bone erosions. However, further larger studies are needed to confirm this and to assess the clinical utility.
Cartilage oligomeric matrix protein (COMP) is an extracellular matrix glycoprotein member of the thrombospondin family of calcium binding proteins. Several studies indicate that changes in COMP serum concentration are related to processes in cartilage.43 In a prospective study of 183 patients with early RA, it was found that early determination of serum COMP predicted the development of joint damage in the hands and feet by five years.44 Another study showed that changes in COMP levels appear to reflect the cartilage destruction process.45 In a longitudinal study of early RA in which serum concentration of COMP was measured in two well-defined patient groups with distinctly different disease outcomes it was reported that compared with a matched normal population, increased concentrations of COMP were found in all patients who developed rapid hip joint destruction.46 However, conflicting results were found in a number of other studies examining COMP as a predictor of joint damage.33,47,48,49 The cause for the disparity between these results might be due to the fact that different COMP assays were used in these trials. In conclusion, serum COMP remains a strong candidate biomarker of RA and may be a valuable tool for identifying patients at high risk for rapid joint destruction but further studies are required to confirm this.
Aggrecan is a major structural component of cartilage. It is cleaved by metalloproteinases to produce fragments that express neoepitopes due to the exposure of new terminal amino acid sequences. These fragments are found in normal, OA and RA cartilage and in synovial fluids.50 Because aggrecan is one of the most abundant proteins of the cartilage matrix, current cartilage turnover markers are based mainly on immunological reagents detecting their synthesis and degradation.51 Aggrecan fragments are increased in the synovial fluid of RA patients38 and the levels have been related to disease severity.52 In one study, aggrecan was quantified by ELISA in longitudinally collected knee joint synovial fluid samples of both patients rapidly developing destruction in knee or hip joints and in patients slowly developing destruction and it was found that the aggrecan concentrations were initially highest in the group developing destruction (P<0.001).37 In a more recent study, it was found that levels of aggrecan fragments in human synovial fluid are increased in RA, OA and after knee injury.53 Conversely, the data from a third study strongly suggests that total aggrecan levels are lower in RA patients than in healthy controls, and that RA patients have at least one specific subpopulation of aggrecan fragments, namely aggrecanse generated 374ARGSVI fragments54 which, if confirmed, could help in differentiating RA from other types of arthritis.
The chondroitin sulphate epitope of aggrecan recognized by the monoclonal antibody 846 (CS846-epitope) is present in hardly detectable amounts in the normal adult cartilage. Preliminary in vitro studies indicate that the level of the epitope in cartilage is directly correlated with aggrecan biosynthesis. The available evidence suggests that fragments of aggrecan containing the 846 epitope reflect the degradation of newly synthesized aggrecan molecules. Serum concentration of CS846-epitope was measured in two patient groups with early RA but with distinctly different disease outcomes.46 The results showed that concentrations of CS846-epitope in patients with slow joint destruction were increased, whilst low levels were observed in patients with rapid joint destruction, concomitant with elevated levels of C-propeptide of type II procoliagen (CPII), suggesting a selective increase in collagen synthesis. Further clinical studies are needed to investigate the potential of aggrecan as a biochemical marker in destructive joint-diseases.
Urinary C-terminal crosslinking telopeptide of type II collagen (CTX-II) is a marker of cartilage destruction.33 A prospective study of a cohort comprising 116 patients with early RA investigated the relationship between levels of urinary Glucosyl-Galactosyl-Pyridinoline (Glc-Gal-Pyd), urinary CTX-II and serum matrix metalloproteinase (MMP)-3 and the progression of joint destruction. The levels of Glc-Gal-Pyd, CTX-II and MMP-3 were elevated compared with the levels in 76 healthy controls and high baseline levels of Glc-Gal-Pyd, CTX-II, and MMP-3 were associated with increased risk of progression of joint destruction over one year in early RA.55 Later, the same group investigated the relationship between baseline levels of urinary CTX-I and CTX-II and the mean annual progression of joint destruction over a median of four years and concluded that high baseline levels of urinary CTX-I and CTX-II independently predict an increased risk of radiological progression over four years in patients with early RA, especially those without radiological joint damage at onset.34 Another longitudinal analysis found that cartilage degradation as measured by urinary CTX-II and, to a lesser extent, bone degradation as measured by CTX-I closely follow indices of RA activity. Clinically perceptible arthritis is responsible for immediate damage which will become visible on plain X-rays only much later.56 In experiments using experimental arthritis models (collagen-induced arthritis (CIA) model and mono-iodoacetate-induced arthritis (MIA) model) in rats, markedly increased levels of CTX-II were detected in the synovial fluid and the serum and both showed strong correlations with the microscopic severity scores of joint lesions at day 22 post-induction in the CIA model (synovial fluid CTX-II, r=0.76; P<0.0001; serum CTX-II r=0.85; P<0.0001). In the MIA model, CTX-II concentration in the synovial fluid (r=0.53; P<0.0001), but not in the serum, correlated with the microscopic severity score.57 In a recent prospective study of 66 RA patients and 76 healthy controls, in which measurements of urinary Glc-Gal-Pyd and CTX-II were performed at baseline and at one year, it was found that baseline levels of Glc-Gal-Pyd and CTX-II were increased in patients with RA and correlated with progression of joint damage.58
Further support for CTX-II as a biomarker of joint progression was provided by a study of 148 RA patients receiving conventional DMARDs who participated in the SAMURAI trial.59 The 28 joint count disease activity score (DAS28), clinical improvement in signs and symptoms of RA, tender joint count, swollen joint count, and modified health assessment questionnaire (MHAQ) were assessed at baseline and 26 biological markers were measured. Among of a panel of 40 different variables, the investigators identified baseline joint damage, urinary CTX-II, the PYD/DPD (total pyridinoline /urinary total deoxypyridinoline) ratio and Body Mass Index (BMI) as strong and independent predictors of radiological progression in patients with RA receiving conventional DMARDs. Therefore, there are several lines of evidence to suggest that urinary CTX-II levels could be useful for detecting patients who are at high risk of joint damage progression very early in the disease before abnormalities can be detected radiographically.
In RA, degradation of articular cartilage is caused by proteinases derived from both the inflamed synovium and stimulated chondrocytes. In RA matrix metalloproteinase (MMP)-1,60 MMP-3,61 MMP-962 and membrane type 1 (MT1)-MMP63 are over-produced. These MMPs are secreted into the synovium and attack the cartilage immersed with synovial fluid. Interestingly, MMP-3 levels in rheumatoid synovial fluid are higher than other MMPs and in some studies have been shown to be predictive of joint destruction.64,65 For example, in a longitudinal study of 132 patients with early RA, levels of MMP-1, MMP-13, MMP-3, TIMP-1, and COMP were assessed in serially obtained serum samples. Levels of MMP-3, CTX-II, COMP and TIMP-1 correlated with radiographic progression at entry and longitudinally as assessed by area under the curve (AUC). By multivariate analysis, a model including MMP-3 and CTX-II was identified as providing the best prediction of radiographic progression at baseline and a combination of MMP-3, CTX-II, and swollen joint count formed the best longitudinal AUC model.66 In another experiment involving patients with RA and systemic lupus erythematosus (SLE), it was found that levels of MMP-2 and MMP-9 were higher than controls in both groups of patients, supporting the involvement of MMP proteins in these autoimmune disorders.67 Baseline serum levels of 2 elements of the metalloproteinase network, MMP-1 and TIMP-1, reflected subsequent joint destruction.68 All these data support the important potential role of MMP proteins as biomarkers in RA.
Type II collagen (CII) is the main collagen of articular cartilage and is highly degraded in RA due to its cleavage by collagenases, exposing a neoepitope Col2-3/4Clong mono (C2C), which can be detected by immunoassay. Two studies have found that C2C as a measure of CII synthesis may be useful as a clinical prognostic biomarker for disease onset and activity in RA and as a predictor for joint space narrowing and radiographic damage progression.69,70 By contrast, a third study showed no association between baseline serum levels of C2C and radiographic progression.33 A related neoepitope Col2-3/4Cshort (C1,2C) is a marker for degradation for type I collagen and type II collagen in cartilage,69 and is related to specific joint space narrowing and erosion in RA.70 Thus, these biomarkers may be of value when studying progression of joint damage in RA patients but large, definitive studies are required.
Glucosyl-Galactosyl-Pyridinoline (Glc-Gal-PYD) is a recently described marker of synovial tissue (ST) destruction, which correlates with joint damage in RA.55,71 These findings were confirmed in a prospective study of 66 RA patients treated with infliximab and metotrexate. It was found that baseline urinary levels of Glc-Gal-PYD were elevated compared with the healthy controls and were associated with erosive changes.58 Measurement of this specific molecular marker may thus be useful in identifying patients with early RA at high risk of rapid progression of joint damage.
Soluble fragments of Hyaluronic Acid (HA) in sera of RA patients can be elevated from 10 to 20-fold compared with healthy controls and correlate with disease activity.72 However, HA levels vary significantly during the daytime and peak levels correspond to times of physical activity so the diagnostic utility of HA could be questioned. In a prospective study, 62 patients were followed for five years with radiographic examinations at baseline and at one, two and five years. It was found that HA at baseline correlated with the one, two and five year radiographic score. Interestingly, it was found that HA also correlated with other markers including ESR and CRP.47 An experiment including RA and OA patients showed that the highest concentration of HA was found in RA SF compared with asymptomatic donor and OA SF samples and that HA correlated with osteogenic protein 1 (OP-1) which, in turn, is correlated with cartilage degeneration.73
Of the synovial and cartilage markers, therefore, COMP, Aggrecan, CTX-II and MMPs show promise as possible biomarkers of rapidly progressive joint damage in RA patients. However, further studies are needed to confirm the associations and to determine whether a single or a combination of markers provides most accuracy in prediction.
Rheumatoid factor (RF) consists of antibodies directed against the Fc region of human IgG immunoglobulin.74 They are the hallmark of RA and can be detected in 60-80% of RA patients in hospital series.74 RF was included in the 1987 laboratory criteria by the American Rheumatism Association (ARA).75 However, RF is not specific for RA and is found in other rheumatic diseases, other chronic inflammatory disorders, infections and even in healthy people.76,77 The sensitivity for rheumatoid factor in RA varies from 19-53% and the variation in specificity is from 91.7-98.6%.78 RF is widely used for diagnosis and prognosis in RA and several studies have shown that it correlates with radiographic outcome.38,79,80 Its predictive value is dependent on the stage of the disease.76,81,82 Thus, in the early stages of RA, RF positivity is associated with more active disease and the development of erosions. However, in later phases of the disease, it is less predictive of subsequent joint progression.38,79 The RF assay is used as a marker for RA diagnosis, since its presence is associated with an increased risk of developing RA.83
Anti-critrullinated protein/peptide antibodies (ACPA) are antibodies to peptides or proteins containing citrulline, a modified form of the amino acid arginine. Several citrullinated antigens have been found.
Perhaps even more predictive of disease progress in RA is the presence of antibodies against cyclic citrullinated peptide (anti-CCP). In recent years, several studies have demonstrated that the anti-CCP antibodies are predictive for RA and that they are associated with joint destruction.83,84,85,86,87,88,89 Anti-CCP can be detected years before RA onset and its specificity is higher than RF.8,84,85,87,89,90 Reported diagnostic sensitivities and specificities ranged from 39-94% and 81-100%, respectively.91 In 2007, anti-CCP antibodies were included in the European League Against Rheumatism (EULAR) guidelines for the diagnosis of early RA92 and in the American College Rheumatology (ACR) Criteria for RA classification.93 Anti-CCP positive RA shows a higher rate of local joint destruction and is associated with HLA-DRB1 alleles containing the shared epitope, while anti-CCP negative RA is associated with HLA-DR3.11,18,94,95,96,97 The anti-CCP test also enables clinicians to distinguish RA patients from other arthritic diseases in cases where the RF is not useful since the anti-CCP test is highly specific for RA.
Antiperinuclear factor (APF) and antikeratin antibodies (AKA) have been found to be as specific for RA as RF.98 The APF are present in 49-91% of RA patients with specificity from 73-99%. Antikeratin antibodies have been identified in the serum of RA patients with an estimated incidence of positive AKA reactions in RA sera from 36-59%.38 The presence of APF and AKA is independent of the disease duration. Both of them appear early and are sometimes present before the onset of RA.99,100 Thus, APF and AKA may help to identify RA to enable early intervention with treatment. These antibodies are correlated with each other, with the presence of RF and with the activity and severity of RA.101 Other studies have confirmed that measurement of anti-CCP, AKA, APF, and individual isotypes of RFs are useful for prediction of structural damage early in the disease course102 but it is not clear which is most useful. For example, it was reported that AKA and anti-CCP were similarly useful for diagnosing RA and were better than APF and the authors suggested that rheumatologists can choose either AKA or anti-CCP to test for and to help with the diagnosis of RA.103 The major issue is that the test for APF is difficult to perform and not routinely available.
Mutated citrullinated vimentin (MCV) has been described as an important autoantigen expressed in synovial tissue and was formerly known as Sa. Anti-Sa antibodies have a high specificity for the diagnosis of RA (98%) but a moderate sensitivity from 20-40%. Anti-Sa also have a high predictive value for severe joint involvement and extraarticular disease in RA.104 Recently, a new ELISA test has been developed for anti-MCV testing, and can be used for the diagnosis of RA with similar specificity and sensitivity to anti-CCP antibodies.105 In a two year follow-up study of 210 patients with early RA, antibodies against MCV, CCP type 2 and 3 (both of IgG isotype) and 3.1 (of both IgG and IgA isotype) were analyzed at baseline and disease activity was evaluated at baseline and regularly for 24 months. Radiographs of the hands and feet were graded using the Larsen score. This study showed that anti-MCV antibodies were associated with more severe disease, as measured by the DAS28, ESR and swollen joint count over time, compared with anti-CCP2, CCP3 and CCP3.1 tests. Interestingly, it was found that radiological progression was predicted equally by all 4 tests.106 In another study, where anti-MCV antibody levels were measured at baseline and at one year and two year follow up in 162 patients with early arthritis, it was found that anti-MCV had a specificity of 92.3% and a sensitivity of 59.3% when using the recommended cut-off of 20 U/mL and that anti-MCV-positive patients had a higher Sharp-van der Heijde score, ESR and CRP levels than did anti-MCV-negative patients at all time points.107 Recently, a number of publications have concluded that anti-MCV antibodies are a specific and sensitive marker for the diagnosis of RA and could provide an alternative to anti-CCP testing.108,109,110
Antibodies to stress protein immunoglobulin heavy-chain binding protein or chaperone (anti-BIP or antip68) are found in the sera of more than 60% of RA patients111 and are also found in experimental animal models of arthritis.111,112,113 Moreover, BIP is over-expressed in the rheumatoid joint and is present in early and pre-disease sera.114 These findings suggest that BIP may be an important autoantigen in RA but its role as a potential biomarker has yet to be fully evaluated.
RF and anti-CCP antibodies are established biomarkers used in both diagnosis and prognosis of RA. Anti-MCV antibodies have been suggested as an alternative to anti-CCP antibody testing due to their high specificity and selectivity, similar or in some cases even higher than anti-CCP tests but it is not yet clear whether they are sufficiently superior to recommend routine testing in clinical practice.
A number of studies have suggested that high erythrocyte sedimentation rate (ESR) levels at onset of early RA predicts long-term radiological progression.92,115 It is, together with CRP, the most frequently used laboratory measure reflecting disease activity. ESR measurements tend to reflect disease activity of the previous weeks116 and has been incorporated in several disease activity scoring systems, but its measurements are influenced by confounding factors including age, sex, fibrinogen levels, RF, hypergammaglobulinemia, and anemia.117 Nonetheless, the ESR level has been incorporated into a risk prediction model for rapid radiographic progression (RRP) that in the future could be used to predict the risk of joint damage progression in RA patients.118
Human C-reactive protein (CRP) is one of the most responsive acute-phase serum reactants and its levels in patients with RA has been correlated with clinical disease activity, radiological progression and response to therapy.38,116,119 As with ESR, CRP has been incorporated into several disease activity scoring systems, but CRP is considered to be more specific biomarker for RA disease activity than ESR, since the hepatic production of CRP reflects the effects of inflammatory cytokines in the liver.120 Interestingly, CRP reflects more short-term changes in disease activity compared with ESR.116 The serum levels of CRP not only reflect the extent of disease activity but are also associated with joint destruction.66,121 In an inception cohort of patients with early inflammatory polyarthritis followed prospectively for outcome, CRP was found to be the best predictor of radiographic damage by five years.122 In addition, CRP determination is widely available, easy to perform and of low cost, making it the preferred biomarker of disease activity.
Calprotectin is a major leukocyte protein that has been shown to correlate with laboratory and clinical assessments in inflammatory diseases.123 High levels of calprotectin have been reported in SF of patients with RA124 and one study found it to be highly significantly correlated with joint damage measures in addition to laboratory and clinical markers of inflammation.125 This has been supported by the findings of a recent study in which calprotectin was an independent predictor of clinical and radiographic joint damage after ten years, suggesting that calprotectin may be a prognostic biomarker for erosive disease in patients with RA.126
Serum amyloid-associated protein (SAA) is a precursor for Amyolid A and, similar to CRP, correlates with several clinical parameters in inflammatory disease127 but SAA levels are generally higher and vary more among individuals.128 One study reported that SAA is an inducer of MMPs in the synovial tissue fibroblasts of RA patients.129 Therefore, SAA may play a significant role in the degradation of extracellular matrix in joint tissues of RA patients. However, SAA measurements correlate significantly with CRP levels in sera of patients with RA, but not in healthy subjects suggesting that they do not provide additional information.129 More recently, another study showed that the incorporation of SAA together with anti-CCP in a RA diagnosis model, increased the sensitivity compared with the anti-CCP alone, but decreased the selectivity of the model.86
Others markers of inflammation such as E-Selectin,130,131 Thioredoxin,132,133 intracellular adhesion molecule (I-CAM)1 130,134,135,136 and vascular cell adhesion molecule (V-CAM)1 130,135,137 have been found to correlate with disease activity in RA but there is little evidence to suggest that additional information is gained over and above measurement of ESR and CRP. Thus, ESR and CRP remain the two biomarkers used routinely in clinical practice.
In RA, the balance between pro- and anti-inflammatory cytokines is tilted toward continued inflammation. The normally low levels of pro-inflammatory cytokines become chronically increased causing prolonged inflammation. Therefore, inappropriate activity of the main inflammatory cytokines and their receptors could potentially have an important role in the RA pathogenesis. A broad array of macrophage and fibroblast cytokines, including interleukin (IL) -1, IL-6, IL-15, IL-18, tumor necrosis factor (TNF)-α, granulocyte macrophage-colony stimulatory factor (GM-CSF), various chemokines and many others are produced by the rheumatoid synovium.138 Many of them, such as IL-1α and β, IL-6, M-CSF, IL-15, IL-17 and TNF-α, enhance osteoclast formation, activity and survival.139,140,141 Pro-inflammatory cytokines such as IL-6, TNF-α and IL-1 are present at higher levels in the SF of RA patients compared with other arthritic disorders or normal patients. Levels of both TNF-α and IL-6 are elevated in serum and joints during active RA and are thought to contribute to the joint destruction in RA.140,142,143 IL-6 is the most abundant cytokine in the serum and SF of patients with RA and its levels have been shown to correlate with disease activity in both fluids. However, reports are conflicting about whether IL-6 levels correlate with joint destruction with some suggesting they do144 but others showing no association with progression.145,146 Moreover, baseline IL-6 concentrations may vary almost 100-fold between different individuals, can increase with exercise147 and are elevated in several other diseases characterized by inflammation. In a multivariate analysis of biomarkers in RA, it was found that the combination of anti-CCP with IL-6 had the greatest discrimination for the diagnosis of established RA86 but whether it is a useful biomarker for outcome remains questionable.
Soluble TNF receptor (sTNFR)-II parallels TNF-α levels and is a surrogate marker for inflammation. One study suggested that levels of sTNFRII can be elevated up to 12 years prior to the development of RA and so it may be a potential biomarker of future disease development.148 Similarly, in an extensive analysis of 16 cytokines or cytokine-related markers, 7 of the analytes (IL-1α, IL-1β, IL1-receptor antagonist (Ra), IL-4, IL-10, TNF-α and sTNFRI) were found to be significantly elevated at five years before the diagnosis of RA.149 In another study, a panel of 23 cytokines and chemokines was measured in SF from patients with early synovitis, who where subsequently followed up. It was reported that patients who develop RA had a distinct but transient SF cytokine profile. The levels of cytokines such as IL-2, IL-4, IL-13, IL-17, IL-15, basic fibroblast growth factor (bFGF) and endothelial growth factor (EGF), were significantly elevated in these patients three months after symptom onset, when compared with early arthritis patients who did not develop RA.150 In addition, this profile was no longer present in established RA. Other cytokines such as IL-4, interferon (IFN)-γ, IL-13 and IL-18 inhibit osteoclastogenesis, whereas suppressive cytokines like transforming growth factor (TGF)-β and IL-1Ra, as well as the suppressor of cytokines signaling-3 (SOCS3), are expressed in RA synovium but at levels that are inadequate to block synovitis.138
Production of IL-17 is now believed to play a crucial role in the development of joint lesions in RA as IL-17 can induce the production of pro-inflammatory cytokines (IL-1 and TNF-α), the upregulation of RANKL and stimulate the activity of MMPs, matrix catabolism and bone resorption.151,152 In RA patients, high local levels of IL-17A are present in both synovium and synovial fluid.141 Furthermore, a recent clinical study demonstrated that synovial IL-17A expression is associated with joint destruction.153 Adipocytokines are produced in adipose tissue and modulate inflammatory responses and radiographic joint damage. In a recent study, concentrations of adipocytokines such as, leptin, visfatin, resistin and adiponectin were measured in RA patients and healthy controls.154 It was found that concentrations of adipocytokines are elevated in patients with RA and may modulate radiographic joint damage. For example, visfatin was associated with increased, and leptin with reduced, levels of radiographic joint damage.
ST and SF from RA patients contain elevated concentrations of several chemokines such as monocyte chemo-attractant protein (MCP)-1, macrophage inflammatory protein (MIP)1α, RANTES, MCP-4, pulmonary and activation-related chemokine (PARC), MIP-3α, growth-related oncogene (GRO), IL-8, monocyte induced by IFN-γ (Mig), interferon-γ-inducible protein (IP)-10 and stromal cell-derived factor (SDF)-1.155 A recent study found that some of these chemokines (MIP-3, transforming growth factor (TGF), CXCL13, monocyte colony stimulating factor (M-CSF)) were predictive of RA disease activity.156 These chemokines are involved in RA pathogenesis via the recruitment and retention of leukocytes in the joints. However, a number of chemokines have been shown to have other biological functions, such as release of mediators of inflammation, cell proliferation and angiogenesis. Therefore, cytokines and cytokine-related molecules are key molecules in the pathogenesis of RA. Further prospective studies are needed to understand their role in the disease and their possible use as biomarkers in RA.
The traditional markers, ESR and CRP, remain the most reliable inflammatory markers in the clinical setting. Due to the complexity of the cytokine network and the instability of these cytokine markers it is not clear whether they can be used in a clinical setting as biomarkers. However, further studies including families of cytokines, chemokines and adipocitokines are necessary to assess the value of more complex models as predictors of outcome in RA.
In RA, the prominent T-cell infiltrate suggests that the rheumatoid synovium contains large number of CD4+ T cells. An accepted model is that the chronic inflammation in established RA is mainly driven by interactions between T cells, macrophages and fibroblasts in an abnormal environment.157 Several types of Treg cells have been described. Thymically-derived CD4+CD25+ Treg cells play a role in the maintenance of self tolerance and prevention of autoimmune disease.158 Treg cells can be divided into subpopulations such as T helper 3 (Th3) cells, which play an important role in mediating tolerance.159 These cells might be insufficiently effective or even defective in RA.160,161 A number of studies have reported that CD4+CD25+ Tregs are increased in the inflamed joint and present a more powerful suppressor activity than peripheral CD4+CD25+ Tregs.162,163,164 However, RA is not suppressed despite this increased suppressor activity. The opinion from the authors is that the effector cells might be less sensitive for the suppression of CD4+CD25+ Tregs. Several studies have compared CD4+CD25+ Tregs in patients with RA and other types of inflammatory arthritis162,163,165,166,167 and a number have reported an enrichment of CD4+CD25+ Tregs in SF of patients with RA and other types of inflammatory arthritis. This enrichment is independent of disease duration and severity. In addition SF CD4+CD25+ Tregs showed suppressive activity in terms of cytokine production and proliferation. The role of Tregs as potential biomarkers of disease state or outcome remains controversial, however, as different studies have reported similar,166,167 increased163 or decreased 162,165 frequencies of CD4+CD25+ Tregs in SF when compared to normal controls.
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Table 1. Previously known rheumatoid arthritis risk associated SNPs in Europeans. Listed are the chromosome, SNP ID, and candidate gene(s) in the region. The table includes susceptibility loci with a PGWAS<1¥10–5. |
During recent years, a huge number of potential biomarkers of prognosis (Table 2a, b) and diagnosis (Table 3) in RA have been investigated but only the autoantibody tests (RF and anti-CCP) and inflammatory markers (ESR and CRP) are currently tested routinely in clinical practice. The best marker for the development of erosions remains the anti-CCP2 antibody test but this lacks sensitivity in early disease. Correlation of outcome with other biomarkers, including CTX-I, CTX-II, COMP, Glc-Gal-Pyd, MMP3 and C2C in urine or serum samples have been reported in more than one study or patient group (Table 2a, b). Whether these markers correlate with each other or add any information over and above that provided by autoantibodies and inflammatory markers is not known but it would seem reasonable to investigate these as a panel of biomarkers may provide more information than the single testing largely attempted to date.
The prospective cohort studies provide an opportunity to find new and better biomarkers, improving prognosis and diagnosis of the disease, making it possible to distinguish patients who will develop more aggressive or rapidly progressive disease from the patients who will develop a milder disease who could be spared potentially toxic medications. Prospective cohort studies also permit the study of the effects of different therapies throughout time and to assess the evolution of the disease in each case. New complex models involving several biomarkers and clinical markers may result in a biomarker signature capable of predicting and monitoring a variety of outcomes important to patients. In turn, this has the potential to allow tailoring of treatment regimens to groups of patients according to their biomarker signature and would truly represent personalized medicine.
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Table 2. Biomarkers of outcome in rheumatoid arthritis. |
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Table 2. Biomarkers of outcome in rheumatoid arthritis. |
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Table 3. Biomarkers in rheumatoid arthritis vs. other diseases. |
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