An investigation of the added value of an ACPA multiplex assay in an early rheumatoid arthritis setting

Anti-citrullinated protein antibodies as biomarkers in rheumatoid arthritis

INTRODUCTION

The serological biomarker anti-citrullinated protein antibodies (ACPAs) may have several functions but is especially important for the diagnosis of rheumatoid arthritis (RA) along with clinical symptoms.

AREAS COVERED

This review provides an overview of ACPAs, which are useful in RA diagnostics and may improve our understanding of disease etiology. PubMed was searched with combinations of words related to antibodies recognizing epitopes containing the post-translationally modified amino acid citrulline in combination with rheumatoid arthritis; cyclic citrullinated peptide, CCP, anti-CCP, anti-citrullinated protein antibodies, ACPA, citrullination, peptide/protein arginine deiminase, PAD, filaggrin, vimentin, keratin, collagen, perinuclear factor, EBNA1, EBNA2, and others. From this search, we made a qualitative extract of publications relevant to the discovery, characterization, and clinical use of these antibodies in relation to RA. We highlight significant findings and identify areas for improvement.

EXPERT OPINION

ACPAs have high diagnostic sensitivity and specificity for RA and recognize citrullinated epitopes from several proteins. The best-performing single epitope originates from Epstein-Barr Virus nuclear antigen 2 and contains a central Cit-Gly motif, which is recognized by ACPAS when located in a flexible peptide structure. In addition, ACPAs may also have prognostic value, especially in relation to early treatment, although ACPAs' main function is to aid in the diagnosis of RA.

Haemophilus parasuis (Glaesserella parasuis) as a Potential Driver of Molecular Mimicry and Inflammation in Rheumatoid Arthritis

Background:Haemophilus parasuis (Hps; now Glaesserella parasuis) is an infectious agent that causes severe arthritis in swines and shares sequence similarity with residues 261–273 of collagen type 2 (Coll_261−273), a possible autoantigen in rheumatoid arthritis (RA).

Objectives/methods: We tested the presence of Hps sequencing 16S ribosomal RNA in crevicular fluid, synovial fluids, and tissues in patients with arthritis (RA and other peripheral arthritides) and in healthy controls. Moreover, we examined the cross-recognition of Hps by Coll_261−273-specific T cells in HLA-DRB1^*04^pos RA patients, by T-cell receptor (TCR) beta chain spectratyping and T-cell phenotyping.

Results:Hps DNA was present in 57.4% of the tooth crevicular fluids of RA patients and in 31.6% of controls. Anti-Hps IgM and IgG titers were detectable and correlated with disease duration and the age of the patients. Peripheral blood mononuclear cells (PBMCs) were stimulated with Hps virulence-associated trimeric autotransporter peptide (VtaA10_755−766), homologous to human Coll_261−273 or co-cultured with live Hps. In both conditions, the expanded TCR repertoire overlapped with Coll_261−273 and led to the production of IL-17.

Discussion: We show that the DNA of an infectious agent (Hps), not previously described as pathogen in humans, is present in most patients with RA and that an Hps peptide is able to activate T cells specific for Coll_261−273, likely inducing or maintaining a molecular mimicry mechanism.

Conclusion: The cross-reactivity between VtaA10_755−766 of a non-human infectious agent and human Coll_261−273 suggests an involvement in the pathogenesis of RA. This mechanism appears emphasized in predisposed individuals, such as patients with shared epitope.

Clinical Value of Detecting Anti-Mutated Citrullinated Vimentin, Anti-Cyclic Citrullinated Peptide, Red Cell Distribution Width and 25-Hydroxyvitamin D in the Diagnosis of Rheumatoid Arthritis

OBJECTIVES

To investigate the clinical value of detecting anti-mutated citrullinated vimentin (anti-MCV), anti-citrullinated peptide (anti-CCP), red-blood-cell distribution width (RDW), and 25-hydroxyvitamin D (25-[OH]D) in the diagnosis of rheumatoid arthritis (RA).

METHODS

We enrolled 119 patients with RA, 114 control individuals without RA (disease controls), and 40 healthy controls in our study (Han Chinese). Anti-CCP and anti-MCV were detected by enzyme-linked immunosorbent assay (ELISA), 25-(OH)D was detected by electrochemical luminescence, and RDW was calculated by erythrocyte parameters detected via the electric resistance method.

RESULTS

The serum levels of anti-CCP and anti-MCV in RA were higher than those in disease controls and healthy controls (P <.01). The areas under the curve (AUCs) of anti-MCV, anti-CCP, RDW, and 25-(OH)D were 0.857, 0.890, 0.611, and 0.569 respectively (P <.05). In various combinations of indicators, when RDW, 25-(OH)D, and anti-CCP; or RDW, 25-(OH)D, anti-CCP, and anti-MCV were connected in parallel, the sensitivity was the highest (all 94.1%). Also, when RDW, 25-(OH)D, anti-CCP, and anti-MCV were connected in series, the sensitivity was the lowest (13.4%).

CONCLUSIONS

Anti-CCP and anti-MCV are ideal indices for RA diagnosis. Also, in combination with RDW and 25-(OH)D, the diagnostic level will be improved, as well as the sensitivity and specificity, which is significant for the differential diagnosis of RA.

Advances in the diagnosis of autoimmune diseases based on citrullinated peptides/proteins

Introduction: Autoimmune diseases are still one of the hard obstacles associated with humanity. There are many exogenous and endogenous etiological factors behind autoimmune diseases, which may be combined or dispersed to stimulate the autoimmune responses. Protein citrullination represents one of these factors. Harnessing specific citrullinated proteins/peptides could early predict and/or diagnose some of the autoimmune diseases. Many generations of diagnostic tools based on citrullinated peptides with comparable specificity/sensitivity are available worldwide.Areas covered: In this review, we discuss the deimination reaction behind the citrullination of most known autoantigens targeted, different generations of diagnostic tools based on citrullinated probes with specificity/sensitivity of each as well as newly developed assays. Furthermore, the most advanced molecular analytical tools to detect the citrullinated residues in the biological fluid and their performance are also evaluated, providing new avenues to early detect autoimmune diseases with high accuracy.Expert opinion: With the current specificity/sensitivity tools available for autoimmune disease detection, emphasis must be placed on developing more advance and effective, early, rapid, and simple diagnostic devices for autoimmune disease monitoring (similar to a portable device for sugar test at home). The molecular analytical devices with dual and/or multiplexe functions should be more simplified and invested in clinical laboratories.

Bacterial citrullinated epitopes generated by Porphyromonas gingivalis infection-a missing link for ACPA production

OBJECTIVES

Porphyromonas gingivalis (P.g.) is discussed to be involved in triggering self-reactive immune responses. The aim of this study was to investigate the autocitrullinated prokaryotic peptidylarginine deiminase (PPAD) from P.g. CH2007 (RACH2007-PPAD) from a rheumatoid arthritis (RA) patient and a synthetic citrullinated PPAD peptide (CPP) containing the main autocitrullination site as potential targets for antibody reactivity in RA and to analyse the possibility of citrullinating native human proteins by PPAD in the context of RA.

METHODS

Recombinant RACH2007-PPAD was cloned and expressed in Escherichia coli. Purified RACH2007-PPAD and its enzymatic activity was analysed using two-dimensional electrophoresis, mass spectrometry, immunoblot and ELISA. Autoantibody response to different modified proteins and peptides was recorded and bioinformatically evaluated.

RESULTS

RACH2007-PPAD was capable to citrullinate major RA autoantigens, such as fibrinogen, vimentin, hnRNP-A2/B1, histone H1 and multiple peptides, which identify a common RG/RGG consensus motif. 33% of RA patients (n=30) revealed increased reactivity for α-cit-RACH2007-PPAD before RA onset. 77% of RA patients (n=99) presented α-cit-specific signals to CPP amino acids 57-71 which were positively correlated to α-CCP2 antibody levels. Interestingly, 48% of the α-CPP-positives were rheumatoidfactor IgM/anti-citrullinated peptide/protein antibodies (ACPA)-negative. Anti-CPP and α-RACH2007-PPAD antibody levels increase with age. Protein macroarrays that were citrullinated by RACH2007-PPAD and screened with RA patient sera (n=6) and controls (n=4) uncovered 16 RACH2007-PPAD citrullinated RA autoantigens and 9 autoantigens associated with lung diseases. We showed that the α-CPP response could be an important determinant in parenchymal changes in the lung at the time of RA diagnosis (n=106; p=0.018).

CONCLUSIONS

RACH2007-PPAD induced internal citrullination of major RA autoantigens. Anti-RACH2007-PPAD correlates with ACPA levels and interstitial lung disease autoantigen reactivity, supporting an infection-based concept for induction of ACPAs via enzymatic mimicry.

Multiple biomarker approach for the diagnosis and therapy of rheumatoid arthritis

The lack of specific clinical symptoms for patients in the early stage of rheumatoid arthritis (RA) has created strong interest in the laboratory diagnosis of RA. The main laboratory markers of RA, rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPAs), can be found in patients with other pathologies and in healthy donors. Even today, there is no single laboratory test that can diagnosis RA with high sensitivity and specificity. To improve the diagnosis and treatment of RA, alternative biomarkers, including 14-3-3η protein, connective tissue growth factor (CTGF), antibodies against PAD4, antibodies against BRAF, and anti-acetylated and anti-carbamylated protein antibodies have been studied extensively. The use of a multiple biomarker approach, the simultaneous measurement of a set of biomarkers, is an alternative strategy for the diagnosis of RA and for predicting the therapeutic effect of biological disease-modifying antirheumatic drugs (DMARDs). However, despite the large number of studies, only a few biomarker combinations have been validated and can be applied in clinical practice. In this article, results of studies focused on the multiple biomarker approach (both multiplex and combined single-analyte assays) to diagnose RA and to predict response to biological drug therapy are reviewed. Additionally, general factors limiting the use of multiplex analysis in RA diagnostics and therapy are discussed.

Response to comment on "Aggregatibacter actinomycetemcomitans-induced hypercitrullination links periodontal infection to autoimmunity in rheumatoid arthritis"

Antibodies to leukotoxin A are markers that link Aggregatibacter actinomycetemcomitans-associated periodontitis and rheumatoid arthritis.

Post-Translational Modifications of Proteins: Novel Insights in the Autoimmune Response in Rheumatoid Arthritis

Post-translational modifications (PTM) are chemical changes mostly catalyzed by enzymes that recognize specific target sequences in specific proteins. These modifications play a key role in regulating the folding of proteins, their targeting to specific subcellular compartments, their interaction with ligands or other proteins, and eventually their immunogenic properties. Citrullination is the best characterized PTM in the field of rheumatology, with antibodies anticyclic citrullinated peptides being the gold standard for the diagnosis of rheumatoid arthritis (RA). In recent years, growing evidence supports not only that a wide range of proteins are subject to citrullination and can trigger an autoimmune response in RA, but also that several other PTMs such as carbamylation and acetylation occur in patients with this disease. This induces a wide spectrum of autoantibodies, as biomarkers, with different sensitivity and specificity for diagnosis, which may be linked to peculiar clinical manifestations and/or response to treatment. The purpose of this review article is to critically summarize the available literature on antibodies against post-translationally modified proteins, in particular antibodies against citrullinated proteins (ACPA) and antibodies against modified proteins (AMPA), and outline their diagnostic and prognostic role to be implemented in clinical practice for RA patients.

Development of a multiplex assay based on chimeric citrullinated peptides as proof of concept for diagnosis of rheumatoid arthritis

Anti-citrullinated peptide/protein antibodies (ACPAs) are the most specific serological biomarkers for rheumatoid arthritis (RA). They have both diagnostic and prognostic value, and are related to more aggressive joint disease in RA. However, a single biomarker cannot differentiate RA subtypes. So, simultaneous analysis of target citrullinated peptides, using a multiplex array based on chimeric peptides composed of several domains of human proteins, could be useful. In this work, eight chimeric peptides and the corresponding native arginine-containing control peptides were obtained by solid-phase peptide synthesis. The study included RA and psoriatic arthritis (PsA) patients attending the Rheumatology Unit of the Hospital Clinic in Barcelona, as well as healthy blood donors (BD) at the same hospital. Our main aim was to explore the diagnostic value of the novel multiplex array compared to a commercial ELISA-based ACPA assay in a serum-saving way. Using the combination of the eight chimeric peptide antigens in the multiplex array, 61.4% of the RA cohort were positive for 3 or more peptides; while, the healthy BD and PsA cohorts did not show any reactivity with the tested peptides. These results indicate that we have developed a highly specific multiplex assay based of chimeric citrullinated peptides derived from filaggrin, fibrin, vimentin and human enolase proteins for the detection of ACPAs in a serum-saving way.

The role of anti-citrullinated protein antibody reactivities in an inception cohort of patients with rheumatoid arthritis receiving treat-to-target therapy

BACKGROUND

Anti-citrullinated protein antibody (ACPA) reactivities precede clinical onset of rheumatoid arthritis (RA), and it has been suggested that ACPA reactivities towards distinct target proteins may be associated with differences in RA phenotypes. We aimed to assess the prevalence of baseline ACPA reactivities in an inception cohort of patients with early RA, and to investigate their associations with disease activity, treatment response, ultrasound findings and radiographic damage.

METHODS

Disease-modifying antirheumatic drug (DMARD)-naïve patients with early RA, classified according to the 2010 American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) criteria, were included in the ARCTIC trial and assessed in the present analysis. During follow up, patients were monitored frequently and treatment was adjusted according to a predetermined protocol, starting with methotrexate monotherapy with prednisolone bridging. Analysis of 16 different ACPA reactivities targeting citrullinated peptides from fibrinogen, alpha-1 enolase, vimentin, filaggrin and histone was performed using a multiplex chip-based assay. Samples from 0, 3, 12 and 24 months were analysed. Controls were blood donors with similar characteristics to the patients (age, gender, smoking status).

RESULTS

A total of 217 patients and 94 controls were included. Median [25, 75 percentile] number of ACPA reactivities in all patients was 9 [4, 12], and were most prevalent in anti-cyclic citrullinated peptide /rheumatoid factor-positive patients 10 [7, 12]. Disease activity measures and ultrasound scores at baseline were lower in ACPA reactivity-positive compared to ACPA reactivity-negative patients. ACPA reactivity levels decreased after 3 months of DMARD treatment, most pronounced for fibrinogenβ 60-74 to 62% of baseline antibody level, with least change in filaggrin 307-324 to 81% of baseline antibody level, both p < 0.001. However, outcomes in disease activity measures, ultrasound and radiographic scores after 12 and 24 months were not associated with baseline levels or changes in ACPA reactivity levels and/or seroreversion after 3 months.

CONCLUSIONS

The clinical relevance of analysing ACPA reactivities in intensively treated and closely monitored early RA was limited, with no apparent associations with disease activity, prediction of treatment response or radiographic progression. Further studies in larger patient materials are needed to understand the role of ACPA reactivities in patients with RA classified according to the 2010 ACR/EULAR criteria and treated according to modern treatment strategies.

TRIAL REGISTRATION

www.ClinicalTrials.gov, NCT01205854 . Registered on 21 September 2010.

The risk of individual autoantibodies, autoantibody combinations and levels for arthritis development in clinically suspect arthralgia

Objectives

Autoantibody testing is helpful for predicting the risk of progression to clinical arthritis in subjects at risk. Previous longitudinal studies have mainly selected autoantibody-positive arthralgia patients, and consequently the predictive values of autoantibodies were evaluated relative to one another. This study assessed the risks for arthritis development of ACPA, RF and/or anti-carbamylated protein antibodies (anti-CarP) in arthralgia patients considered at risk for RA by rheumatologists, based on clinical characteristics (clinically suspect arthralgia, CSA).

Methods

The baseline ACPA, RF and anti-CarP autoantibody status of 241 patients, consecutively included in the CSA cohort, was studied for risk of developing clinical arthritis during a median follow-up of 103 (interquartile range: 81-114) weeks.

Results

Univariable associations for arthritis development were observed for ACPA, RF and anti-CarP antibodies; hazard ratios (HRs) (95% CI) were 8.5 (4.7-15.5), 5.1 (2.8-9.3) and 3.9 (1.9-7.7), respectively. In multivariable analysis, only ACPA was independently associated (HR = 5.1; 2.0-13.2). Relative to autoantibody-negative CSA patients, ACPA-negative/RF-positive patients had HRs of 2.6 (1.04-6.6), ACPA-positive/RF-negative patients 8.0 (2.4-27.4) and ACPA-positive/RF-positive patients 10.5 (5.4-20.6). Positive predictive values for development of clinical arthritis within 2 years were: 38% for ACPA-negative/RF-positive, 50% for ACPA-positive/RF-negative and 67% for ACPA-positive/RF-positive patients. Higher ACPA levels were not significantly associated with increased progression to clinical arthritis, in contrast to higher RF levels. Autoantibody levels were stable during follow-up.

Conclusion

ACPA conferred the highest risk for arthritis development and had an additive value to RF. However, >30% of ACPA-positive/RF-positive CSA patients did not develop arthritis during the 2-year follow-up. Thus, CSA and information on autoantibodies is insufficient for accurately identifying imminent autoantibody-positive RA.

Anticitrullinated protein/peptide antibody multiplexing defines an extended group of ACPA-positive rheumatoid arthritis patients with distinct genetic and environmental determinants

INTRODUCTION

The second generation anticycliccitrullinated peptide (anti-CCP2) assay detects the majority but not all anticitrullinated protein/peptide antibodies (ACPA). Anti-CCP2-positive rheumatoid arthritis (RA) is associated with HLA-DRB1* shared epitope (SE) alleles and smoking. Using a multiplex assay to detect multiple specific ACPA, we have investigated the fine specificity of individual ACPA responses and the biological impact of additional ACPA reactivity among anti-CCP2-negative patients.

METHODS

We investigated 2825 patients with RA and 551 healthy controls with full data on anti-CCP2, HLA-DRB1* alleles and smoking history concerning reactivity against 16 citrullinated peptides and arginine control peptides with a multiplex array.

RESULTS

The prevalence of the 16 ACPA specificities ranged from 9% to 58%. When reactivity to arginine peptides was subtracted, the mean diagnostic sensitivity increased by 3.2% with maintained 98% specificity. Of the anti-CCP2-negative patients, 16% were found to be ACPA positive. All ACPA specificities associated with SE, and all but one with smoking. Correction for arginine reactivity also conveyed a stronger association with SE for 13/16 peptides. Importantly, when all ACPA specificities were analysed together, SE and smoking associated with RA in synergy among ACPA positive, but not among ACPA-negative subjects also in the anti-CCP2-negative subset.

CONCLUSIONS

Multiplexing detects an enlarged group of ACPA-positive but anti-CCP2-negative patients with genetic and environmental attributes previously assigned to anti-CCP2-positive patients. The individual correction for arginine peptide reactivity confers both higher diagnostic sensitivity and stronger association to SE than gross ACPA measurement.

Differing specificities and isotypes of anti-citrullinated peptide/protein antibodies in palindromic rheumatism and rheumatoid arthritis

Background

To analyze differences in the recognition of anti-citrullinated peptide/protein antibody (ACPA) citrullinated epitopes and isotypes in patients with palindromic rheumatism (PR) and rheumatoid arthritis (RA).

Methods

ACPA fine specificities (citrullinated peptides of enolase, fibrin, and vimentin) and isotypes (IgG, IgM, and IgA) were analyzed in 54 patients with longstanding PR and 54 patients with established RA.

Results

CCP2 tested positive in 66.7% of patients with PR and RA. The ACPA distribution of fine specificities and isotypes differed between PR and RA patients. PR patients had a lower frequency of fine ACPA specificities than RA patients, which was significant in the case of a peptide derived from vimentin (PR 24.1% vs. 59.3% RA; p < 0.001). The mean number of ACPA specificities was lower in PR than in RA patients, and only 25.9% of PR patients recognized ≥2 additional specificities compared with 46.3% of RA patients. Significantly less isotype usage, especially IgA, was observed in PR patients.

Conclusion

The ACPA immune response differed in patients with PR and RA, with fewer fine specificities and isotype usage in patients with PR. Some patients with PR may have impaired maturation of the B-cell response against citrullinated peptides with no progression to RA.

Breach of autoreactive B cell tolerance by post-translationally modified proteins

Objectives

Over 50% of patients with rheumatoid arthritis (RA) harbour a variety of anti-modified protein antibodies (AMPA) against different post-translationally modified (PTM) proteins, including anti-carbamylated protein (anti-CarP) antibodies. At present, it is unknown how AMPA are generated and how autoreactive B cell responses against PTM proteins are induced. Here we studied whether PTM foreign antigens can breach B cell tolerance towards PTM self-proteins.

Methods

Serum reactivity towards five carbamylated proteins was determined for 160 patients with RA and 40 healthy individuals. Antibody cross-reactivity was studied by inhibition experiments. Mass spectrometry was performed to identify carbamylated self-proteins in human rheumatic joint tissue. Mice were immunised with carbamylated or non-modified (auto)antigens and analysed for autoantibody responses.

Results

We show that anti-CarP antibodies in RA are highly cross-reactive towards multiple carbamylated proteins, including modified self-proteins and modified non-self-proteins. Studies in mice show that anti-CarP antibody responses recognising carbamylated self-proteins are induced by immunisation with carbamylated self-proteins and by immunisation with carbamylated proteins of non-self-origin. Similar to the data observed with sera from patients with RA, the murine anti-CarP antibody response was, both at the monoclonal level and the polyclonal level, highly cross-reactive towards multiple carbamylated proteins, including carbamylated self-proteins.

Conclusions

Self-reactive AMPA responses can be induced by exposure to foreign proteins containing PTM. These data show how autoreactive B cell responses against PTM self-proteins can be induced by exposure to PTM foreign proteins and provide new insights on the breach of autoreactive B cell tolerance.

[Knee joint pain with signs of arthrosis]

Chronic pain in the knee joint is most commonly caused by osteoarthritis, especially in elderly patients but can be due to other causes, such as rheumatoid arthritis. The diagnostics include an exact patient medical history and a clinical examination, which often already provide clear indications of the cause of the knee pain. Subsequently, further diagnostics can then be considered, such as radiological procedures and laboratory diagnostics. The treatment is determined by the cause and the individual patient and aims to reduce pain and to preserve the mobility of the joint. Generally, therapy consists of pain management and physiotherapy as well as alternative therapeutic procedures, mostly in combination. Proximal tibial opening wedge osteotomy can be useful; however, partial or total knee arthroplasty should only be considered when conservative treatment options have been exhausted.