Antibodies against carbamylated proteins are present in primary Sjögren's syndrome and are associated with disease severity

Anti-Carbamylated Protein Antibodies in ACPA-Negative and ACPA-Positive Patients with Rheumatoid Arthritis

The objective of this study was to assess the level of antibodies to carbamylated proteins and analyze the clinical and immunological associations in patients with ACPA-negative and ACPA-positive variants of rheumatoid arthritis.

MATERIALS AND METHODS

. The study involved 150 patients with a reliable diagnosis of rheumatoid arthritis and 25 patients as healthy controls. Depending on ACPA values, two groups of patients were recruited: ACPA-positive (n = 75) and ACPA-negative (n = 75). RA activity was assessed by the DAS28 index. Determination of antibodies to carbamylated proteins was performed by enzyme-linked immunosorbent assay (BlueGene Biotech, China). Quantitative determination of ACPA in serum was performed by enzyme immunoassay using a commercial reagent kit (AxisShield, UK; upper limit of normal 5.0 U/mL; Orgentec, Germany; upper limit of normal 20.0 U/mL).

RESULTS AND DISCUSSION

. Median anti-CarP in patients with RA was 126.2 [100.83; 157.41] ng/mL and was statistically significantly higher (p < 0.001) than in healthy controls (88.89 [70.53; 107.75] ng/mL). Among all patients with RA, 50 (33.3%) were anti-Carp-positive (22 (29.3%) in the ACPA(+) group and 28 (37.3%) in the ACPA(-) group), and one (2%) volunteer from healthy controls was anti-CarP(+) (p = 0.002). In ROC analysis performed to assess the diagnostic significance of anti-CarP for RA for all patients with RA, the area under the curve was 0.783 ± 0.047 with 95% CI: 0.691-0.874 (p < 0.001), with a cut-off point of 143 ng/mL, specificity 96%, sensitivity 36.7%. In the ACPA(+) RA group, the erosion count was statistically significantly higher (p = 0.044) in anti-CarP(+) patients than in anti-CarP(-) patients. A weak direct correlation between anti-CarP and DAS28 was found in the ACPA(-) RA group.

CONCLUSIONS

. We studied the predictive value of anti-CarP as an auxiliary biomarker in ACPA(+) and ACPA(-) subtypes of RA. ACPA(+) anti-CarP(+) patients have a more "erosive" subtype of the disease than ACPA(+) anti-CarP(-) patients. In ACPA(-) patients, anti-CarP helps to identify a more erosive subtype of the disease, and among ACPA(-) patients it helps to reduce the proportion of seronegative patients. Further studies are required to determine the optimal standards for the laboratory diagnosis of anti-CarP and to clarify the diagnostic potential of these ABs as part of the differential diagnosis of arthritis in other rheumatic diseases.

G protein-coupled receptor kinase 2 as a novel therapeutic target for gland fibrosis of Sjögren's syndrome

Sjogren's syndrome (SS) is a chronic, progressive autoimmune disorder characterized by gland fibrosis. We previously found a close correlation between gland fibrosis and the expression of G protein-coupled receptor kinase 2 (GRK2). In this study we explored the pathological and therapeutic significance of GRK2 in SS. Submandibular gland (SMG) antigen-induced SS mouse model was established in WT and GRK2+/- mice. We showed that the expression levels of GRK2 were significantly up-regulated in glandular tissue and positively correlated with fibrotic morphology in SS patients and mice. Hemizygous knockout of GRK2 significantly inhibited the gland fibrosis. In mouse salivary gland epithelial cells (SGECs), we demonstrated that GRK2 interacted with Smad2/3 to positively regulate the activation of TGF-β-Smad signaling with a TGF-β-GRK2 positive feedback loop contributing to gland fibrosis. Hemizygous knockout of GRK2 attenuated TGF-β-induced collagen I production in SGECs in vitro and hindered gland fibrosis in murine SS though preventing Smad2/3 nuclear translocation. Around 28 days post immunization with SMG antigen, WT SS mice were treated with a specific GRK2 inhibitor paroxetine (Par, 5 mg·kg-1·d-1, i.g. for 19 days). We found that Par administration significantly attenuated gland fibrosis and alleviated the progression of SS in mice. We conclude that genetic knockdown or pharmacological inhibition of GRK2 significantly attenuates gland fibrosis and alleviates the progression of SS. GRK2 binds to Smad2/3 and positively regulates the activation of TGF-β-Smad signaling. A TGF-β-GRK2 positive feedback loop contributes to gland fibrosis. Our research points out that GRK2 could be a promising therapeutic target for treating SS.

Distribution and clinical significance of anti-carbamylation protein antibodies in rheumatological diseases among the Chinese Han population

Objective

Several studies have demonstrated that anti-carbamylation protein antibodies (Anti-CarPA) are persistent in patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), systemic sclerosis (SSC), primary Sjögren's syndrome (pSS), and interstitial lung disease associated with RA (RA-ILD). However, the relationship between anti-CarPA and other rheumatic diseases (RDs) and non-RA-ILD is not known till now. This study sought to examine the presence of anti-CarPA in Chinese Han patients with RDs and its clinical significance.

Methods

The study included 90 healthy controls (HCs) and 300 patients with RDs, including RA, SLE, polymyositis/dermatomyositis (PM/DM), pSS, SSC, spondyloarthritis (SpA), anti-neutrophil cytoplasmic autoantibodies associated with vasculitis (AAV), undifferentiated connective tissue disease (UCTD), and Behcet's disease (BD). Antibodies against carbamylated human serum albumin were detected using commercial enzyme-linked immunosorbent assay kits. Correlations between clinical and laboratory parameters were analyzed.

Result

Serum levels of anti-CarPA in RA (34.43 ± 33.34 ng/ml), SLE (21.12 ± 22.23 ng/ml), pSS (16.32 ± 13.54 ng/ml), PM/DM (30.85 ± 17.34 ng/ml), SSC (23.53 ± 10.70 ng/ml), and UCTD (28.35 ± 21.91 ng/ml) were higher than those of anti-CarPA in the HCs (7.30 ± 5.05 ng/ml). The concentration of serum anti-CarPA was higher in patients with rheumatic disease-related interstitial lung disease (RD-ILD), especially RA-ILD, PM/DM-ILD, and pSS-ILD. Patients with RD-ILD who tested positive for anti-CarPA were more likely to have a more severe radiographic classification (grades II, p = 0.045; grades III, p = 0.003). Binary logistic regression analysis suggested that anti-CarPA had an association with ILD in RA (p = 0.033), PM/DM (p = 0.039), and pSS (p = 0.048). Based on receiver operating characteristics (ROC) analysis, anti-CarPA cutoffs best discriminated ILD in RA (>32.59 ng/ml, p = 0.050), PM/DM (>23.46 ng/ml, p = 0.038), and pSS (>37.08 ng/ml, p = 0.040). Moreover, serum levels of anti-CarPA were correlated with antibodies against transcription intermediary factor 1 complex (anti-TIF1) (R = -0.28, p = 0.044), antibodies against glycyl-transfer ribonucleic acid synthetase (anti-EJ) (R = 0.30, p = 0.031), and antibodies against melanoma differentiation-associated gene 5 (anti-MDA5) (R = 0.35, p = 0.011).

Conclusion

Serum anti-CarPA could be detected in patients with RA, PM/DM, pSS, SSC, and UCTD among the Chinese Han population. And it may also assist in identifying ILD in patients with RA, PM/DM, and pSS, which emphasized attention to the lung involvement in anti-CarPA-positive patients.

Not all autoantibodies are clinically relevant. Classic and novel autoantibodies in Sjögren’s syndrome: A critical review

Sjögren’s syndrome (SjS) is a heterogeneous systemic disease. The abnormal responses to La/SSB and Ro/SSA of both B-cells and T-cells are implicated as well as others, in the destruction of the epithelium of the exocrine glands, whose tissue characteristically shows a peri-epithelial lymphocytic infiltration that can vary from sicca syndrome to systemic disease and lymphoma. Despite the appearance of new autoantibodies, anti-Ro/SSA is still the only autoantibody included in the American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) classification criteria and is used extensively as a traditional biomarker in clinical practice. The study and findings of new autoantibodies in SjS has risen in the previous decade, with a central role given to diagnosis and elucidating new aspects of SjS physiopathology, while raising the opportunity to establish clinical phenotypes with the goal of predicting long-term complications. In this paper, we critically review the classic and the novel autoantibodies in SjS, analyzing the methods employed for detection, the pathogenic role and the wide spectrum of clinical phenotypes.

AGE/Non-AGE Glycation: An Important Event in Rheumatoid Arthritis Pathophysiology

Rheumatoid arthritis (RA) is a chronic inflammatory, autoimmune disease that gradually affects the synovial membrane and joints. Many intrinsic and/or extrinsic factors are crucial in making RA pathology challenging throughout the disease. Substantial enzymatic or non-enzymatic modification of proteins driving inflammation has gained a lot of interest in recent years. Endogenously modified glycated protein influences disease development linked with AGEs/non-AGEs and is reported as a disease marker. In this review, we summarized current knowledge of the differential abundance of glycated proteins by compiling and analyzing a variety of AGE and non-AGE ligands that bind with RAGE to activate multi-faceted inflammatory and oxidative stress pathways that are pathobiologically associated with RA-fibroblast-like synoviocytes (RA-FLS). It is critical to comprehend the connection between oxidative stress and inflammation generation, mediated by glycated protein, which may bind to the receptor RAGE, activate downstream pathways, and impart immunogenicity in RA. It is worth noting that AGEs and non-AGEs ligands play a variety of functions, and their functionality is likely to be more reliant on pathogenic states and severity that may serve as biomarkers for RA. Screening and monitoring of these differentially glycated proteins, as well as their stability in circulation, in combination with established pre-clinical characteristics, may aid or predict the onset of RA.

Early diagnosis and treatment for Sjögren's syndrome: current challenges, redefined disease stages and future prospects

There are some challenges and unmet needs in the early diagnosis and management of Sjögren's syndrome (SjS) such as prominent glandular dysfunction at diagnosis and long diagnostic delay. Those challenges are partly attributed to the lack of a good knowledge of the early stages of SjS, which is a major obstacle to delivering appropriate care to SjS patients. Findings from both clinical and experimental studies suggest the plausibility of a redefined SjS course consisting of 4 stages, which includes initiation stage, preclinical stage, asymptomatic SjS stage and overt SjS stage. More studies focusing on the pathological processes and changes during the early stages of SjS are needed. To enable early diagnosis and treatment for SjS, more useful biomarkers of the early stages of SjS need to be identified, and individuals at high risk of SjS development need to be identified. Appropriate screening can be performed to facilitate the early diagnosis of SjS among those high-risk individuals.

Antibodies against carbamylated proteins: prevalence and associated disease characteristics in Belgian patients with rheumatoid arthritis or other rheumatic diseases

Objectives: Anti-carbamylated protein antibodies (anti-CarP) are reported to be associated with increased disease activity and with more severe joint damage in rheumatoid arthritis (RA) patients. The present study investigated the presence of anti-CarP in various rheumatic diseases, and their specific clinical significance in RA, in Belgian rheumatology patients.Method: We tested sera from 254 RA patients, 56 healthy controls, and 153 patients with different rheumatic conditions: juvenile idiopathic arthritis (JIA), axial spondyloarthritis, systemic sclerosis, and Sjögren's syndrome (SS). An in-house enzyme-linked immunosorbent assay was used to detect immunoglobulin G antibodies against carbamylated foetal calf serum.Results: Anti-CarP were detected in 88 RA patients (34.6%), of whom 82% were also positive for anti-citrullinated protein antibodies (ACPAs) and 81% were also rheumatoid factor (RF) positive. Of note, 11 anti-CarP single-positive patients were detected (4.3%). The previously reported association with joint erosions was not detected. However, in ACPA- and RF-negative RA patients, the presence of anti-CarP was associated with higher disease activity and disability. Fifteen per cent of JIA patients and 30% of SS patients also tested positive for anti-CarP and their antibody levels did not differ significantly from those of anti-CarP-positive RA patients. Anti-CarP levels were, however, significantly higher in ACPA- or RF-positive patients.Conclusion: Anti-CarP antibodies were detected in the sera of a cohort of Belgian RA patients. Moreover, they were also detected in primary SS patients and in JIA patients. In the seronegative subset of RA patients, anti-CarP antibodies showed prognostic value.

Epigenetic Regulations of AhR in the Aspect of Immunomodulation

Environmental factors contribute to autoimmune disease manifestation, and as regarded today, AhR has become an important factor in studies of immunomodulation. Besides immunological aspects, AhR also plays a role in pharmacological, toxicological and many other physiological processes such as adaptive metabolism. In recent years, epigenetic mechanisms have provided new insight into gene regulation and reveal a new contribution to autoimmune disease pathogenesis. DNA methylation, histone modifications, chromatin alterations, microRNA and consequently non-genetic changes in phenotypes connect with environmental factors. Increasing data reveals AhR cross-roads with the most significant in immunology pathways. Although study on epigenetic modulations in autoimmune diseases is still not well understood, therefore future research will help us understand their pathophysiology and help to find new therapeutic strategies. Present literature review sheds the light on the common ground between remodeling chromatin compounds and autoimmune antibodies used in diagnostics. In the proposed review we summarize recent findings that describe epigenetic factors which regulate AhR activity and impact diverse immunological responses and pathological changes.

Current State of Knowledge on Primary Sjögren's Syndrome, an Autoimmune Exocrinopathy

Primary Sjögren's syndrome (pSS) is a chronic systemic autoimmune rheumatic disease characterized by lymphoplasmacytic infiltration of the salivary and lacrimal glands, whereby sicca syndrome and/or systemic manifestations are the clinical hallmarks, associated with a particular autoantibody profile. pSS is the most frequent connective tissue disease after rheumatoid arthritis, affecting 0.3-3% of the population. Women are more prone to develop pSS than men, with a sex ratio of 9:1. Considered in the past as innocent collateral passive victims of autoimmunity, the epithelial cells of the salivary glands are now known to play an active role in the pathogenesis of the disease. The aetiology of the "autoimmune epithelitis" still remains unknown, but certainly involves genetic, environmental and hormonal factors. Later during the disease evolution, the subsequent chronic activation of B cells can lead to the development of systemic manifestations or non-Hodgkin's lymphoma. The aim of the present comprehensive review is to provide the current state of knowledge on pSS. The review addresses the clinical manifestations and complications of the disease, the diagnostic workup, the pathogenic mechanisms and the therapeutic approaches.

Antibodies against carbamylated vimentin exist in systemic lupus erythematosus and correlate with disease activity

OBJECTIVES

Antibodies against carbamylated protein (anti-CarP) were found to be a promising marker to evaluate joint damage and disease activity in patients with rheumatoid arthritis (RA). However, whether anti-CarP antibodies were present in systemic lupus erythematosus (SLE) remained ambiguous. We have therefore undertaken this study to assess the levels of serum anti-CarP antibodies and to evaluate their clinical value in SLE.

METHODS

Serum levels of antibodies against carbamylated-vimentin (anti-Carp) were measured by enzyme immunosorbent assay in 100 patients with SLE, 76 with RA, 17 with primary Sjögren syndrome (pSS), and 68 healthy controls. Data analyses between anti-Carp antibodies and other laboratory measures were performed using SPSS 24 software for Windows.

RESULTS

The levels of serum anti-CarP antibodies in patients with SLE were significantly higher than those in healthy controls. In addition, anti-CarP antibodies were present in SLE patients lacking the disease-specific antibodies, including anti-Smith-negative patients (24.4%, 21/86), anti-dsDNA-negative patients (29.3%, 12/41), anti-nucleosome-negative patients (21.4%, 9/42), and antiribosomal P protein antibody-negative patients (23.7%, 18/76). There were significant differences between the anti-CarP-positive and anti-CarP-negative SLE patients in clinical and laboratory features, such as age, erythrocyte sedimentation rate (ESR), C-reactive protein, rheumatoid factor, third-generation cyclic citrullinated peptide (CCP3), anticardiolipin, D-dipolymer, complement 3, immunoglobulin G (IgG), red blood cell count (RBC) and hemoglobin. After adjusting for age and disease duration, the high levels of anti-CarP antibodies were still correlated with low RBC, hemoglobin and high ESR, IgG and CCP3. Active SLE patients demonstrated higher anti-CarP IgG than inactive patients. Moreover, the levels of anti-CarP were significantly higher in SLE patients with arthralgia and/or arthritis than in those without joint involvement.

CONCLUSIONS

Anti-CarP antibodies were present in SLE patients and associated with the disease severity. These might provide a potential supplement to other specific autoantibodies for diagnosis of SLE and serve as a promising marker for measuring joint damage in the disease.

[Significance of anti-carbamylated fibrinogen antibodies in systemic lupus erythematosus]

OBJECTIVE

Antibodies against carbamylated protein (anti-CarP) were found to be a promising marker to evaluate joint damage and disease activity in patients with rheumatoid arthritis (RA). However, whether anti-CarP antibodies were present in systemic lupus erythematosus (SLE) remained ambiguity. We have therefore undertaken this study to assess the levels of serum anti-CarP antibodies and to evaluate their clinical value in SLE.

METHODS

Serum levels of antibodies against carbamylatedfibrinogen (anti-CarP) were measured by enzyme-linked immunosorbent assay (ELISA) in 105 SLE patients and 73 healthy controls. Other clinical and laboratory measurements of the SLE patients were collected from medical records. Data analyses between anti-CarP antibodies and other laboratory measurements were performed using SPSS software for Windows 24.0.

RESULTS

The levels of serum anti-CarP antibodies in the patients with SLE were significantly higher than those in the healthy controls (P<0.05). There were significant differences between the anti-CarP-positive group and anti-CarP-negative group in many clinical features. The disease duration, values of ESR, CRP, RF, anti-cardiolipin, anti-dsDNA, D-dipolymer, IgA and IgG were significantly higher in the anti-CarP-positive group compared with the negative group (P<0.05). Conversely, the values of complement 3, complement 4, peripheral blood RBC, and hemoglobin were significantly lower in anti-CarP-positive group than in the negative group(P<0.05). Moreover, the incidence of increase of erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), rheumatoid factor (RF), D-dipolymer, decrease of peripheral blood RBC, hemoglobin, complement 3, complement 4, and positive rate of anti-dsDNA were significant different between the two groups(P<0.05). The positive rate of anti-CarP (21.9%) was higher than that of anti-Sm (15.24%), and close to anti-ribosomal P protein (22.86%) in our SLE patients. In addition, anti-CarP antibody was present in the SLE patients lacking the disease specific antibodies, including anti-Sm (anti-CarP positive rate 20.2%, 18/89), anti-dsDNA (anti-CarP positive rate 9.3%, 4/43), anti-nucleosome (anti-CarP positive rate 12.5%, 6/48), and anti-ribosomal P protein antibody (anti-CarP positive rate 20.9%, 17/81). Moreover, the high levels of anti-CarP antibodies were correlated with short disease duration, low C3, C4, RBC, and hemoglobin (P<0.05), high ESR, CRP, IgA, IgG, RF, anti-cardiolipin, anti-dsDNA, and D-dipolymer (P<0.05).

CONCLUSION

The level of anti-CarP antibody was increased in the serum of patients with SLE. There were correlations between anti-CarP antibodies and clinical and laboratory indicators of SLE patients.

"NETtling" the host: Breaking of tolerance in chronic inflammation and chronic infection

How and why we break tolerance to self-proteins still remains a largely unanswered question. Neutrophils have been identified as a rich source of autoantigens in a wide array of autoimmune diseases that arise as a consequence of different environmental and genetic factors, e.g. rheumatoid arthritis (RA), lupus, vasculitis, cystic fibrosis (CF) etc. Specifically, neutrophil extracellular trap (NET) formation has been identified as a link between innate and adaptive immune responses in autoimmunity. Autoantigens including neutrophil granular proteins (targeted by anti-neutrophil cytoplasmic antibodies, ANCA) as well as post-translationally modified proteins, i.e. citrullinated and carbamylated proteins targeted by anti-citrullinated protein antibodies (ACPA) and anti-carbamylated protein antibodies (ACarPA), respectively, localize to the NETs. Moreover, NETs provide stimuli to dendritic cells that potentiate adaptive autoimmune responses. However, while NETs promote inflammation and appear to induce humoral autoreactivity across autoimmune diseases, the antigen specificity of autoantibodies found in these disorders is striking. These unique autoantigen signatures suggest that not all NETs are created equal and that the environment in which NETs arise shapes their disease-specific character. In this review article, we discuss the effects of different stimuli on the mechanism of NET formation as well as how they contribute to antigen specificity in the breaking of immune tolerance. Specifically, we compare and contrast the autoreactive nature of NETs in two settings of chronic airway inflammation: one triggered by smoking, a recognized environmental NET stimulus in RA patients, and one mediated by Pseudomonas aeruginosa, the most prevalent lung pathogen in CF patients. Finally, we draw attention to novel findings that, together with the specific environmental/chemical stimuli, should be taken into account when investigating how and why antigen specificity arises in the context of NET formation.

Clinical and etiological meaning of anti-carbamylated protein antibodies in rheumatoid arthritis

Several autoantibodies against proteins with post-translational modifications have been detected in patients with rheumatoid arthritis (RA) and are called anti-modified protein antibodies (AMPAs). Anti-carbamylated protein antibodies (Anti-CarP Ab) are the second most vigorously researched AMPAs following anti-citrullinated protein/peptide antibodies (ACPA). Anti-CarP Ab and ACPA show cross-reactivity to some extent and frequently co-exist with each other in RA, but are two distinct antibodies. Although the diagnostic efficacy of anti-CarP Ab is inferior to that of ACPA, the diagnostic specificity of RA may improve when used in combination with ACPA and rheumatoid factor. Anti-CarP Ab and ACPA are also useful for identifying patients at high risk of more severe joint destruction and cardiovascular diseases. The high prevalence of the co-existence of both antibodies suggests a common factor in their production, and this is important for the development of RA because both antibodies emerge before the onset of clinical symptoms. Neutrophils may also be crucially involved. It is important to distinguish citrullinated antigens from carbamylated antigens because the methods commonly used to detect the former are now known to be cross-reactive with the latter. Research on anti-CarP Ab will provide novel insights into the pathology and etiology of RA.

Anti-carbamylated protein antibodies and skin involvement in patients with systemic sclerosis: An intriguing association

Carbamylation is a post-translational modification that mostly affects proteins with low turnover, such as dermal proteins. Carbamylated proteins accumulate in skin in an age-dependent manner, contributing to tissue alterations. As dermis is affected by systemic sclerosis (SSc) and anti-carbamylated protein antibodies (anti-CarP Ab) are found in SSc patients, we sought to evaluate the specificity of anti-CarP Ab and their relationship with clinical parameters reflecting skin involvement in SSc. This study investigated serum samples and clinical data from 124 patients with SSc. Anti-CarP Ab were affinity purified from pooled SSc sera, and their specificity was assessed by western blotting and ELISA with carbamylated proteins from two species (human and bovine albumin; human fibrinogen). Anti-CarP Ab were measured in SSc serum samples and in 41 healthy aged-matched individuals. Affinity-purified anti-CarP Ab recognized carbamylated epitopes irrespective of the protein type or species origin. Anti-CarP Ab levels inversely correlated with the modified Rodnan skin score (mRss) (Spearman's R = -0.32, p<0.001), independently of patients' age. Receiver operating characteristics (ROC) analysis identified anti-CarP Ab cut-offs that best discriminated dichotomized clinical variables related to skin involvement: the only clinical variables that were significantly different between groups were mRss (p = 0.001) and scleredema (p<0.001). Low anti-CarP Ab levels were associated with worse skin involvement. Future prospective studies are needed to assess their usefulness in the clinical setting.

Novel autoantibodies in Sjögren's syndrome: A comprehensive review

Sjögren's syndrome is a systemic autoimmune disease characterized by immune- mediated injury of exocrine glands, as well as a diverse array of extraglandular manifestations. B cell over-activation is a key feature of the disease, attested by the wide spectrum of autoantibodies detected in these patients. Up to date, anti- Ro/SSA and anti-La/SSB antibodies are traditional biomarkers for disease classification and diagnosis. On the other hand, the detection of novel autoantibodies in SS has increased in the last years, opening a window of opportunity to denote particular stages of the disease, to establish clinical phenotypes, and to predict long-term complications such as lymphoma. For instance, anti-SP-1, anti-CA6 and anti-PSP antibodies occur in an earlier stage than anti-Ro/La antibodies, and may identify a subset of primary Sjögren's syndrome patients with mild or incomplete disease, whereas anti-cofilin-1, anti- alpha-enolase and anti-RGI2 antibodies are potential biomarkers of MALT lymphoma. Antibody detection is also important to elucidate new aspects of SS pathophysiology, and in the future to permit a phenotype-specific patient approach. Herein we review the literature regarding new autoantibodies in SS and attempt to dissect their usefulness as diagnostic tools, pathogenic role, identification of clinical phenotypes and as predictors of an overlap syndrome.

Current concepts on Sjögren's syndrome - classification criteria and biomarkers

Sjögren's syndrome is a lymphoproliferative disease with autoimmune features characterized by mononuclear cell infiltration of exocrine glands, notably the lacrimal and salivary glands. These lymphoid infiltrations lead to dryness of the eyes (keratoconjunctivitis sicca), dryness of the mouth (xerostomia), and, frequently, dryness of other surfaces connected to exocrine glands. Sjögren's syndrome is associated with the production of autoantibodies because B-cell activation is a consistent immunoregulatory abnormality. The spectrum of the disease extends from an organ-specific autoimmune disorder to a systemic process and is also associated with an increased risk of B-cell lymphoma. Current treatments are mainly symptomatic. As a result of the diverse presentation of the syndrome, a major challenge remains to improve diagnosis and therapy. For this purpose an international set of classification criteria for primary Sjögren's syndrome has recently been developed and validated and seems well suited for enrolment in clinical trials. Salivary gland biopsies have been examined and histopathology standards have been developed, to be used in clinical trials and patient stratification. Finally, ultrasonography and saliva meet the need of non-invasive imaging and sampling methods for discovery and validation of disease biomarkers in Sjögren's syndrome.

Triple Positivity for Anti-Citrullinated Protein Autoantibodies, Rheumatoid Factor, and Anti-Carbamylated Protein Antibodies Conferring High Specificity for Rheumatoid Arthritis: Implications for Very Early Identification of At-Risk Individuals

OBJECTIVE

In rheumatoid arthritis (RA), anti-citrullinated protein antibodies (ACPAs) and rheumatoid factor (RF) are commonly used to aid in the diagnosis. Although these autoantibodies are mainly found in RA, their specificity is not optimal. It is therefore difficult to identify RA patients, especially in very early disease, based on the presence of ACPAs and RF alone. In addition, anti-carbamylated protein (anti-CarP) antibodies have diagnostic and prognostic value, since their presence is associated with joint damage in RA patients and also associated with the future development of RA in patients with arthralgia. Therefore, the aim of the present study was to investigate the value of combined antibody testing in relation to prediction and diagnosis of (early) RA.

METHODS

A literature search resulted in identification of 12 relevant studies, consisting of RA patients, pre-RA individuals, disease controls, healthy first-degree relatives of RA patients, and healthy control subjects, in which data on RF, ACPAs, and anti-CarP antibody status were available. Using these data, random effects meta-analyses were carried out for several antibody combinations.

RESULTS

The individual antibodies were highly prevalent in patients with RA (34-80%) compared to the control groups, but were also present in non-RA controls (0-23%). For the classification of most subjects correctly as having RA or as a non-RA control, the combination of ACPAs and/or RF often performed well (specificity 65-100%, sensitivity 59-88%). However, triple positivity for ACPAs, RF, and anti-CarP antibodies resulted in a higher specificity for RA (98-100%), accompanied by a lower sensitivity (11-39%).

CONCLUSION

As the rheumatology field is moving toward very early identification of RA and possible screening for individuals at maximum risk of RA in populations with a low pretest probability, an autoantibody profile of triple positivity for ACPAs, RF, and anti-CarP provides interesting information that might help identify individuals at risk of developing RA.

Rheumatoid factor isotype and Ro epitope distribution in primary Sjögren syndrome and rheumatoid arthritis with keratoconjunctivitis sicca

Primary Sjögren syndrome (pS) is associated with autoantibodies such as rheumatoid factor (RF) and anti-nuclear antibodies such as anti-Ro (SS-A) and/or La (SS-B). Recent developments within autoimmune diagnostics allow quantitation of RF subclasses and anti-Ro epitopes. Will this refinement by autoimmune diagnostics help predicting development of extraglandular manifestations (EGM) in pS patients? A cohort of pS and rheumatoid arthritis (RA) patients with keratoconjunctivitis sicca (n = 35 and 16, resp) was included. Of the pS patients, 54% developed one or more EGM. Antibodies quantitated were IgM-RF, IgA-RF, IgG-RF, anti-Ro52, and anti-Ro60. Upon analysis of RF isotypes, pS patients generally displayed higher IgA-RF concentrations than RA patients (126 versus 49 U/ml, p = 0.015), while the dominant RF isotype in RA patients was IgM-RF (82.5 versus 38 U/ml, p = 0.012). No differences were observed regarding IgG-RF concentrations. In pS without/with EGM, the median RF IgM concentrations were similar, while RF IgA and IgG concentrations tended to be lower in pS patients with EGM > 1. Both Ro epitopes were strongly recognized by almost all pS patients, independent from EGM, while these antibodies were absent in RA patients. Primary Sjögren syndrome and RA patients have distinct serological profiles when analysing RF and Ro-specific antibodies. A longitudinal study of switched RF isotypes in pS patients is worthwhile from an immunological point of view, but its value is limited regarding identification of pS patients prone to developing EGM or RA patients prone to developing secondary sicca symptoms.

Relatedness of Antibodies to Peptides Containing Homocitrulline or Citrulline in Patients with Rheumatoid Arthritis

Objective.

Antibodies that target citrullinated protein/peptide (ACPA) and homocitrullinated/carbamylated protein/peptide (AHCPA) are associated with rheumatoid arthritis (RA). The relationship between ACPA and AHCPA remains unclear. We examined the expression and cross-reactivity of these antibodies using citrulline- and homocitrulline-containing synthetic peptides, CitJED and HomoCitJED, respectively, which have equal numbers of citrulline or homocitrulline residues on the same peptide backbone.

Methods.

Serum from healthy subjects (n = 51) and patients with RA (n = 137), systemic lupus erythematosus (SLE; n = 37), and psoriatic arthritis (PsA; n = 37) were screened for IgG anti-CitJED and anti-HomoCitJED antibodies by ELISA. Cross-reactivity of these antibodies was examined by inhibition with various concentrations of CitJED and HomoCitJED.

Results.

Out of 137 patients with RA, antibodies to CitJED and HomoCitJED were detected in 69 (50%) and 78 (57%), respectively. Anti-CitJED and HomoCitJED antibodies were 77% concordant and their levels were strongly correlated [Spearman correlation coefficient (rs) = 0.6676]. Sera from 25/27 patients (93%) with RA were inhibited by both CitJED and HomoCitJED with equal or higher affinity for the cognate (homologous) peptide.

Conclusion.

Antibodies to CitJED and HomoCitJED frequently occurred in RA, but were not found in SLE or PsA, suggesting that these antibodies are specific to RA. Cross-reactivity between anti-HomoCitJED and anti-CitJED antibodies suggests that ACPA and AHCPA are derived from the same B cell population and both may contribute to the pathogenesis of RA.

Anti-carbamylated protein antibodies in systemic lupus erythematosus patients with articular involvement

Objective Several studies have evaluated the prevalence of rheumatoid factor (RF) and anti-citrullinated proteins antibodies (ACPA) in systemic lupus erythematosus (SLE) patients but no data are available on the anti-carbamylated proteins (anti-CarP), a new biomarker for rheumatoid arthritis (RA). We evaluated the anti-CarP prevalence in SLE patients with joint involvement and the associations with different phenotypes. Methods Seventy-eight SLE patients with joint involvement were enrolled (F/M 73/5; mean ± SD age 47.6 ± 11.2 years; mean ± SD disease duration 214.3 ± 115.6 months). As control groups, we evaluated SLE patients without joint manifestations ( N = 15), RA ( N = 78) and healthy individuals (HS, N = 98). Anti-CarP were assessed by home-made ELISA in all patients and controls, RF and ACPA in SLE patients with joint involvement (commercial ELISA kit). Results The prevalence of anti-CarP in SLE patients with joint involvement was similar to RA ( p = NS) and significantly higher compared with SLE without joint involvement and HS ( p < 0.0001, p < 0.0001, respectively). Four patients were positive for all three antibodies: seventy-five percent of these showed Jaccoud arthropathy. Fourty-five percent of ACPA-ve/RF-ve patients were anti-CarP + ve. Conclusions The evaluation of anti-CarP in SLE joint involvement demonstrated a prevalence of almost 50%, similar to RA and significantly higher than SLE without joint involvement and HS.

Anti-carbamylated Protein Antibodies Are Detectable in Various Connective Tissue Diseases

Objective.

Anti-carbamylated protein (anti-CarP) antibodies are possible diagnostic biomarkers of anticitrullinated protein antibody (ACPA)-negative rheumatoid arthritis (RA). We aimed to elucidate the prevalence of anti-CarP antibodies in non-RA connective tissue diseases (CTD) because CTD are important in the differential diagnosis of ACPA-negative RA.

Methods.

The sera from 266 patients with RA and 616 patients with CTD and 80 healthy controls were examined using an in-house anti-CarP ELISA.

Results.

The prevalence and the level of anti-CarP antibodies in several CTD were comparable to those in ACPA-negative RA.

Conclusion.

Anti-CarP antibodies are not useful for differentiating ACPA-negative RA from CTD.

Impact of fibrinogen carbamylation on fibrin clot formation and stability

Carbamylation is a non-enzymatic post-translational modification induced upon exposure of free amino groups to urea-derived cyanate leading to irreversible changes of protein charge, structure and function. Levels of carbamylated proteins increase significantly in chronic kidney disease and carbamylated albumin is considered as an important biomarker indicating mortality risk. High plasma concentrations and long half-life make fibrinogen a prime target for carbamylation. As aggregation and cross-linking of fibrin monomers rely on lysine residues, it is likely that carbamylation impacts fibrinogen processing. In this study we investigated carbamylation levels of fibrinogen from kidney disease patients as well as the impact of carbamylation on fibrinogen cleavage by thrombin, fibrin polymerisation and cross-linking in vitro. In conjunction, all these factors determine clot structure and stability and thus control biochemical and mechanical properties. LC-MS/MS analyses revealed significantly higher homocitrulline levels in patient fibrinogen than in fibrinogen isolated from control plasma. In our in vitro studies we found that although carbamylation does not affect thrombin cleavage per se, it alters fibrin polymerisation kinetics and impairs cross-linking and clot degradation. In addition, carbamylated fibrin clots had reduced fiber size and porosity associated with decreased mechanical stability. Using mass spectroscopy, we discovered that N-terminally carbamylated fibrinopeptide A was generated in this process and acted as a strong neutrophil chemoattractant potentially mediating recruitment of inflammatory cells to sites of fibrin(ogen) turnover. Taken together, carbamylation of fibrinogen seems to play a role in aberrant fibrin clot formation and might be involved in haemostatic disorders associated with chronic inflammatory diseases.

The Potential Role for Early Biomarker Testing as Part of a Modern, Multidisciplinary Approach to Sjögren's Syndrome Diagnosis

Sjögren's syndrome (SS) is a chronic and progressive multisystem autoimmune disease typically managed by rheumatologists. Diagnostic delays are common, due in large part to the non-specific and variable nature of SS symptoms and the slow progression of disease. The hallmark characteristics of SS are dry eye and dry mouth, but there are a broad range of other possible symptoms such as joint and muscle pain, skin rashes, chronic dry cough, vaginal dryness, extremity numbness or tingling, and disabling fatigue. Given that dry eye and dry mouth are typically the earliest presenting complaints, eye care clinicians and dental professionals are often the first point of medical contact and can provide critical collaboration with rheumatologists to facilitate both timely diagnosis and ongoing care of patients with SS. Current diagnostic criteria advocated by the American College of Rheumatology are predicated on the presence of signs/symptoms suggestive of SS along with at least two objective factors such as traditional biomarker positivity, salivary gland biopsy findings, and/or presence of keratoconjunctivitis sicca. Traditional biomarkers for SS include the autoantibodies anti-Sjögren's syndrome-related antigen A (SS-A/Ro), anti-Sjögren's syndrome-related antigen B (SS-B/La), antinuclear antibody (ANA) titers, and rheumatoid factor (RF). While diagnostically useful, these biomarkers have low specificity for SS and are not always positive, especially in early cases of SS. Several newly-identified biomarkers for SS include autoantibodies to proteins specific to the salivary and lacrimal glands [SP-1 (salivary gland protein-1), PSP (parotid secretory protein), CA-6 (carbonic anhydrase VI)]. Data suggest that these novel biomarkers may appear earlier in the course of disease and are often identified in cases that test negative to traditional biomarkers. The Sjö^® test is a commercially available diagnostic panel that incorporates testing for traditional SS biomarkers (anti-SS-A/Ro, anti-SS-B/La, ANA, and RF), as well as three novel, proprietary early biomarkers (antibodies to SP-1, PSP, and CA-6) which provide greater sensitivity and specificity than traditional biomarker testing alone. Timely diagnosis of SS requires appropriate clinical vigilance for potential SS symptoms, referral and collaborative communication among rheumatology, ophthalmology, and oral care professions, and proactive differential work-up that includes both physical and laboratory evaluations.

Identification of carbamylated alpha 1 anti-trypsin (A1AT) as an antigenic target of anti-CarP antibodies in patients with rheumatoid arthritis

In 2011 a novel autoantibody system, anti-carbamylated protein (anti-CarP) antibodies, was described in rheumatoid arthritis (RA) patients. Anti-CarP antibody positivity associates with a more severe disease course, is observed years before disease onset, and may predict the development of RA in arthralgia patients. Although many clinical observations have been carried out, information on the antigenic targets of anti-CarP antibodies is limited. Most studies on anti-CarP antibodies utilize an ELISA-based assay with carbamylated fetal calf serum (Ca-FCS) as antigen, a complex mixture of proteins. Therefore, we analysed the molecular identity of proteins within Ca-FCS that are recognized by anti-CarP antibodies. Ca-FCS was fractionated using ion exchange chromatography, selecting one of the fractions for further investigation. Using mass-spectrometry, carbamylated alpha-1-antitrypsin (Ca-A1AT) was identified as a potential antigenic target of anti-CarP antibodies in RA patients. A1AT contains several lysines on the protein surface that can readily be carbamylated. A large proportion of the RA patients harbour antibodies that bind human Ca-A1AT in ELISA, indicating that Ca-A1AT is indeed an autoantigen for anti-CarP antibodies. Next to the Ca-A1AT protein, several homocitrulline-containing peptides of A1AT were recognized by RA sera. Moreover, we identified a carbamylated peptide of A1AT in the synovial fluid of an RA patient using mass spectrometry. We conclude that Ca-A1AT is not only a target of anti-CarP antibodies but is also present in the synovial compartment, suggesting that Ca-A1AT recognized by anti-CarP antibodies in the joint may contribute to synovial inflammation in anti-CarP-positive RA.

Antibodies against carbamylated proteins and cyclic citrullinated peptides in systemic lupus erythematosus: results from two well-defined European cohorts

BACKGROUND

Articular manifestations are common in systemic lupus erythematosus (SLE) whereas erosive disease is not. Antibodies to cyclic citrullinated peptide (anti-CCP) are citrulline-dependent in rheumatoid arthritis (RA), whereas the opposite is suggested in SLE, as reactivity with cyclic arginine peptide (CAP) is typically present. Antibodies targeting carbamylated proteins (anti-CarP) may occur in anti-CCP/rheumatoid factor (RF)-negative cases long before clinical onset of RA. We analysed these antibody specificities in sera from European patients with SLE in relation to phenotypes, smoking habits and imaging data.

METHODS

Cases of SLE (n = 441) from Linköping, Sweden, and Leiden, the Netherlands, were classified according to American College of Rheumatology (ACR) and/or Systemic Lupus Erythematosus International Collaborating Clinics (SLICC) criteria. IgG anti-CCP, anti-CAP and anti-CarP were analysed by immunoassays. Radiographic data from 102 Swedish patients were available.

RESULTS

There were 16 Linköping (6.8%) and 11 Leiden patients (5.4%) who were anti-CCP-positive, of whom approximately one third were citrulline-dependent: 40/441 (9.1%) were anti-CarP-positive, and 33% of the anti-CarP-positive patients were identified as anti-CCP-positive. No associations were found comparing anti-CCP or anti-CarP with ACR-defined phenotypes, immunologic abnormalities or smoking habits. Radiographically confirmed erosions were found in 10 patients, and were significantly associated with anti-CCP, anti-CarP and RF. Musculoskeletal ultrasonography scores were higher in anti-CCP-positive compared to anti-CCP-negative patients.

CONCLUSIONS

In the hitherto largest anti-CarP study in SLE, we demonstrate that anti-CarP is more prevalent than anti-CCP and that the overlap is limited. We obtained some evidence that both autoantibodies seem to be associated with erosivity. Similar pathogenetic mechanisms to those seen in RA may be relevant in a subgroup of SLE cases with a phenotype dominated by arthritis.

Prevalence, sensitivity and specificity of antibodies against carbamylated proteins in a monocentric cohort of patients with rheumatoid arthritis and other autoimmune rheumatic diseases

Background

Antibodies against carbamylated proteins (anti-CarP) have been recently identified in the sera of patients with rheumatoid arthritis (RA). The objective of the study was to evaluate the prevalence, sensitivity and specificity of anti-CarP compared to anti-citrullinated peptide antibodies (ACPA) and rheumatoid factor (RF), replicating the existing data in a large cohort of Italian patients with RA and extending the evaluation to other autoimmune rheumatic diseases (AIRDs).

Methods

Serum samples (n = 607) from 309 patients with RA, 200 disease controls and 98 normal healthy subjects (NHS) were evaluated. Anti-CarP were detected using carbamylated fetal calf serum as the antigen. ACPAs were detected using second-generation ELISA and IgM RF was assessed as part of routine analysis.

Results

Anti-CarP antibodies were detected in 117 patients with RA (34.4%), ACPA in 190 patients (61.4%) and RF in 202 patients (65.3%). Two (2.04%) of the NHS were positive for anti-CarP, one NHS (1.02%) was positive for ACPA and three NHS were positive for RF (3.06%). Among disease controls, anti-CarP antibodies were detected in 33 patients (16.5%), ACPA in 29 patients (14.5%) and RF in 64 patients (32%). In particular, 16.8% of patients with systemic lupus erythematosus and 31.1% of patients with Sjögren syndrome were positive for anti-CarP. The sensitivity of anti-CarP, ACPA and RF was 46.8%, 61.8% and 64.4%, respectively and specificity was 91.95%, 89.93% and 76.51%, respectively.

Conclusions

The present study extends the knowledge of anti-CarP antibodies, confirming previous data on the diagnostic accuracy of anti-CarP in RA in a large cohort of Italian patients. Anti-CarP antibodies demonstrated relatively low sensitivity and slightly higher specificity compared to ACPA and RF. Even if predominantly present in RA, anti-CarP was detected in a variable percentage of patients with other autoimmune rheumatic diseases and their generation could be attributed to the inflammatory status; the clinical relevance of anti-CarP antibodies in these latter patients should be further determined.

The role for neutrophil extracellular traps in cystic fibrosis autoimmunity

While respiratory failure in cystic fibrosis (CF) frequently associates with chronic infection by Pseudomonas aeruginosa, no single factor predicts the extent of lung damage in CF. To elucidate other causes, we studied the autoantibody profile in CF and rheumatoid arthritis (RA) patients, given the similar association of airway inflammation and autoimmunity in RA. Even though we observed that bactericidal permeability-increasing protein (BPI), carbamylated proteins, and citrullinated proteins all localized to the neutrophil extracellular traps (NETs), which are implicated in the development of autoimmunity, our study demonstrates striking autoantibody specificity in CF. Particularly, CF patients developed anti-BPI autoantibodies but hardly any anti-citrullinated protein autoantibodies (ACPA). In contrast, ACPA-positive RA patients exhibited no reactivity with BPI. Interestingly, anti-carbamylated protein autoantibodies (ACarPA) were found in both cohorts but did not cross-react with BPI. Contrary to ACPA and ACarPA, anti-BPI autoantibodies recognized the BPI C-terminus in the absence of posttranslational modifications. In fact, we discovered that P. aeruginosa-mediated NET formation results in BPI cleavage by P. aeruginosa elastase, which suggests a novel mechanism in the development of autoimmunity to BPI. In accordance with this model, autoantibodies associated with presence of P. aeruginosa on sputum culture. Finally, our results provide a role for autoimmunity in CF disease severity, as autoantibody levels associate with diminished lung function.

Present and future of biologic drugs in primary Sjögren's syndrome

INTRODUCTION

Primary Sjögren's (pSS) syndrome is a chronic, autoimmune, and systemic disease characterized by xerostomia, xerophthalmia, muscle pain and fatigue. The disease may be complicated by a systemic involvement, such as a pulmonary fibrosis or the development of lymphoma which severely worsens the prognosis. Actually, there are no recommendations for the management of pSS. However, recent advances in the understanding of its pathogenesis have uncovered some pathways that have potential as therapeutic targets. Areas covered: In this review, the authors present the biologic drugs potentially valuable to the treatment of pSS in light of its physiopathology with a 'bird's eye' view of future prospects. The authors took into account relevant studies published from 2004 to 2016. Expert opinion: Biological treatment in pSS is a promising opportunity to potentially control disease activity and prevent its complication. Currently, inhibition of B-cell and IL-17 pathways seem to be the most promising avenues. New achievements in the knowledge of pSS pathophysiology are necessary in order to try to simultaneously predict the predominant pathogenic pathway, the kind of patients at major risk to develop a more severe disease, and the appropriate biological therapy to use.

Increased Serum Levels of Anti-Carbamylated 78-kDa Glucose-Regulated Protein Antibody in Patients with Rheumatoid Arthritis

The objective of this study was to investigate the presence and titer of anti-carbamylated 78-kDa glucose-regulated protein (anti-CarGRP78) antibody in serum from controls, and patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and primary Sjögren syndrome (pSS). Thirty-three RA patients, 20 SLE patients, 20 pSS patients, and 20 controls were enrolled from our outpatient clinic. GRP78 was cloned and carbamylated. Serum titers of anti- cyclic citrullinated peptides (anti-CCP), anti-GRP78, and anti-CarGRP78 were measured with an enzyme-linked immunosorbent assay. No differences in serum titers of anti-GRP78 antibody in patients with RA, SLE, or pSS compared with the controls were observed. Serum levels of anti-carGRP78 antibody in patients with RA, but not SLE or pSS, were significantly higher compared with the controls (OD₄₀₅ 0.15 ± 0.08 versus 0.11 ± 0.03, p = 0.033). There was a positive correlation between the serum levels of anti-GRP78 antibody, but not anti-CarGRP78 antibody, with the levels of anti-CCP antibody in patients with RA. Both anti-GRP78 and anti-carGRP78 antibodies failed to correlate with C-reactive protein levels in patients with RA. In conclusion, we demonstrated the presence of anti-CarGRP78 antibody in patients with RA. In addition, the serum titer of anti-CarGRP78 antibody was significantly elevated in patients with RA compared with the controls. Anti-CarGRP78 antibody could also be detected in patients with SLE or pSS.

Sjögren syndrome

Sjögren syndrome (SjS) is a systemic autoimmune disease that primarily affects the exocrine glands (mainly the salivary and lacrimal glands) and results in the severe dryness of mucosal surfaces, principally in the mouth and eyes. This disease predominantly affects middle-aged women, but can also be observed in children, men and the elderly. The clinical presentation of SjS is heterogeneous and can vary from sicca symptoms to systemic disease (characterized by peri-epithelial lymphocytic infiltration of the affected tissue or the deposition of the immune complex) and lymphoma. The mechanism underlying the development of SjS is the destruction of the epithelium of the exocrine glands, as a consequence of abnormal B cell and T cell responses to the autoantigens Ro/SSA and La/SSB, among others. Diagnostic criteria for SjS include the detection of autoantibodies in patient serum and histological analysis of biopsied salivary gland tissue. Therapeutic approaches for SjS include both topical and systemic treatments to manage the sicca and systemic symptoms of disease. SjS is a serious disease with excess mortality, mainly related to the systemic involvement of disease and the development of lymphomas in some patients. Knowledge of SjS has progressed substantially, but this disease is still characterized by sicca symptoms, the systemic involvement of disease, lymphocytic infiltration to exocrine glands, the presence of anti-Ro/SSA and anti-La/SSB autoantibodies and the increased risk of lymphoma in patients with SjS.

Antibodies to carbamylated α-enolase epitopes in rheumatoid arthritis also bind citrullinated epitopes and are largely indistinct from anti-citrullinated protein antibodies

Background

In addition to anti-citrullinated protein antibodies (ACPAs), antibodies targeting carbamylated (i.e., homocitrullinated) proteins (anti-CarP antibodies) have been described in rheumatoid arthritis (RA). However, the extent to which anti-CarP antibodies are truly distinct from ACPA remains unclear, and few studies have focused on specific autoantigens. Here, we examine cross-reactivity between ACPA and anti-CarP antibodies, in the context of the candidate autoantigen α-enolase.

Methods

Cross-reactivity was examined by immunoblotting of citrullinated and carbamylated proteins using purified ACPA; and by peptide absorption experiments, using the citrullinated α-enolase peptide CEP-1 and a homocitrulline-containing version (carb-CEP-1) in ELISA. The population-based case-control cohort EIRA (n = 2836 RA; 373 controls) was screened for reactivity with CEP-1 and carb-CEP-1, using the ISAC multiplex array. Associations between anti-CarP antibodies, smoking and genetic risk factors were analysed using unconditional logistic regression models. Differences in antibody levels were investigated using the Mann-Whitney U test.

Results

Affinity-purified ACPA was found to bind carbamylated proteins and homocitrulline-containing peptides, demonstrating definitive cross-reactivity between ACPA and anti-CarP antibodies. Anti-carb-CEP-1 reactivity in EIRA was almost exclusively confined to the CEP-1-positive subset, and this group of RA patients (21 %) displayed a particularly strong ACPA response with marked epitope spreading. The small RA subset (3 %) with homocitrulline reactivity in the absence of citrulline reactivity did not associate with smoking or risk genes, and importantly had significantly lower anti-carb-CEP-1 antibody levels.

Conclusion

Our data presented herein cast doubt on the specificity of anti-CarP antibodies in RA, which we posit may be a subset of cross-reactive ACPA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-1001-6) contains supplementary material, which is available to authorized users.