Expression of citrulline and homocitrulline residues in the lungs of non-smokers and smokers: implications for autoimmunity in rheumatoid arthritis

Contribution of neutrophils in the pathogenesis of rheumatoid arthritis

Neutrophils are major innate immune effector cells for host defense and have been a topic of active research for their participation in the pathogenesis of autoimmune inflammatory diseases including rheumatoid arthritis (RA) due to recently discovered neutrophil extracellular trap (NET) formation. NET formation and other mechanisms leading to the release of neutrophil nuclear and cytoplasmic contents are implicated as a source of citrullinated antigens in RA. Further investigations are required to delineate what factors diverge neutrophils from host defense to autoimmune response in RA.

Insights into the study and origin of the citrullinome in rheumatoid arthritis

The presence of autoantibodies and autoreactive T cells to citrullinated proteins and citrullinating enzymes in patients with rheumatoid arthritis (RA), together with the accumulation of citrullinated proteins in rheumatoid joints, provides substantial evidence that dysregulated citrullination is a hallmark feature of RA. However, understanding mechanisms that dysregulate citrullination in RA has important challenges. Citrullination is a normal process in immune and non-immune cells, which is likely activated by different conditions (eg, inflammation) with no pathogenic consequences. In a complex inflammatory environment such as the RA joint, unique strategies are therefore required to dissect specific mechanisms involved in the abnormal production of citrullinated proteins. Here, we will review current models of citrullination in RA and discuss critical components that, in our view, are relevant to understanding the accumulation of citrullinated proteins in the RA joint, collectively referred to as the RA citrullinome. In particular, we will focus on potential caveats in the study of citrullination in RA and will highlight methods to precisely detect citrullinated proteins in complex biological samples, which is a confirmatory approach to mechanistically link the RA citrullinome with unique pathogenic pathways in RA.

Post-translational modifications of vimentin reflect different pathological processes associated with non-small cell lung cancer and chronic obstructive pulmonary disease

INTRODUCTION

Vimentin has shown to be highly implicated in cancer initiation and progression. Vimentin is often a target of post-translational modifications (PTMs) which can be disease specific, thus targeting these specific modifications can be of high biomarker potential. In this study we set out to evaluate the biological relevance and serum biomarker potential of citrullinated vimentin (VICM) and non-citrullinated vimentin (VIM) in non-small cell lung cancer (NSCLC) and chronic obstructive pulmonary disease (COPD).

METHODS

A competitive ELISA targeting VIM was developed and quantified in serum from patients with NSCLC and COPD. VIM was compared with levels of VICM in the same indications.

RESULTS

VIM was significantly increased in NSCLC (n = 100) compared to healthy controls (n = 67) in two independent cohorts (p = 0.0003 and p < 0.0001). Furthermore, VIM was highly increased in late stages of NSCLC (p = 0.001), however VIM was not increased in COPD patients (n = 10). Contrarily, serum levels of VICM was not increased in late stages of NSCLC, but highly elevated in patients with COPD (p < 0.0001).

CONCLUSIONS

These findings suggest a biomarker potential of VIM in NSCLC. Our findings also indicate that PTMs of vimentin are highly relevant and that targeting these modifications can have differential biomarker potential.

Asthma and elevation of anti-citrullinated protein antibodies prior to the onset of rheumatoid arthritis

BACKGROUND

Anti-citrullinated protein antibodies (ACPA) are central to rheumatoid arthritis (RA) pathogenesis and may develop at inflamed mucosa. We investigated whether asthma, a disease of airway mucosal inflammation, was associated with elevated ACPA before RA diagnosis.

METHODS

We performed a nested case-control study among women in two prospective cohorts, the Nurses' Health Study (NHS; 1976-2014) and NHSII (1989-2015). Blood was obtained on a subset (NHS: 1989-1990; NHSII: 1996-1999). Cases met 1987 ACR or 2010 ACR/EULAR RA criteria by medical record review and were classified as seropositive (ACPA+ or rheumatoid factor positivity) or seronegative by clinical laboratory testing at diagnosis. We identified RA cases with blood drawn before the date of RA diagnosis (index date), matching each to three controls by age, cohort, year, time from blood draw to index date, and menopause. Pre-RA ACPA elevation for cases was defined as >99th percentile of the control distribution on a research assay composed of autoantibodies targeting citrullinated protein epitopes or positivity on the second-generation commercial assay for cyclic citrullinated peptide. Asthma status and covariates were obtained through biennial questionnaires before blood draw. Conditional logistic regression estimated ORs and 95%CIs for RA by pre-RA ACPA and clinical serostatus, adjusted for matching factors, smoking pack-years, passive smoking, and body mass index (BMI).

RESULTS

We identified 284 incident RA cases and 849 matched controls; mean age at the index date was 61.2 years (SD 10.1). Blood was drawn 9.7 years (mean; SD 5.8) before the index date. We identified 96 (33.8%) RA cases with elevated pre-RA ACPA. At blood draw, 17.7% of pre-RA ACPA+ cases and 6.3% of matched controls (p = 0.0008) reported clinician-diagnosed asthma. After adjusting for matching factors, smoking pack-years, passive smoking, and BMI, asthma was significantly associated with pre-RA ACPA+ RA (OR 3.57, 95%CI 1.58,8.04). Asthma was not associated with overall RA (OR 1.45, 95%CI 0.91,2.31), but was significantly associated with seropositive RA (OR 1.79, 95%CI 1.01,3.18).

CONCLUSIONS

Asthma was strongly associated with ACPA elevation in blood drawn prior to RA diagnosis, independent of smoking. Chronic mucosal airway inflammation may contribute to ACPA development and RA pathogenesis.

[Pulmonary involvement in rheumatoid arthritis]

Pulmonary involvement in patients with rheumatoid arthritis, in particular interstitial lung diseases (RA-ILD) is of great clinical importance. Patients should be asked about symptoms of pulmonary involvement and the lungs should be clinically examined even during the diagnostic procedure and regularly during the course of the disease. Before initiation of a basic pharmacological treatment an X‑ray examination of thoracic organs is obligatory. In cases of conspicuous clinical or radiological findings, extended diagnostic procedures with lung function testing (body plethysmography with diffusion measurement) and high resolution computed tomography (CT) should be performed, depending on the findings. The differential diagnosis of interstitial lung alterations in patients with RA is broad and should consider side effects of the basis medication in addition to infectious causes. The optimal pharmacological treatment of RA-ILD is not sufficiently clarified. The value of methotrexate (MTX) has changed because, in contrast to previous assumptions, a better course could be observed under MTX treatment, at least in mild to moderate courses of RA-ILD. In the case of a clinically relevant RA-ILD, tumor necrosis factor (TNF) blockers should be avoided because a dramatic deterioration of pulmonary function has sometimes been observed. Among biological disease-modifying antirheumatic drugs (DMARD), rituximab and abatacept are currently preferred. The role of Janus kinase (JAK) inhibitors in RA-ILD is currently being discussed but limited data are available. Patients with RA-ILD benefit from a close collaboration between pulmonologists and rheumatologists.

Citrullination of fibronectin alters integrin clustering and focal adhesion stability promoting stromal cell invasion

The extracellular matrix (ECM) microenvironment is increasingly implicated in the instruction of pathologically relevant cell behaviors, from aberrant transdifferentation to invasion and beyond. Indeed, pathologic ECMs possess a panoply of alterations that provide deleterious instructions to resident cells. Here we demonstrate the precise manner in which the ECM protein fibronectin (FN) undergoes the posttranslational modification citrullination in response to peptidyl-arginine deiminase (PAD), an enzyme associated with innate immune cell activity and implicated in systemic ECM-centric diseases, like cancer, fibrosis and rheumatoid arthritis. FN can be citrullinated in at least 24 locations, 5 of which reside in FN's primary cell-binding domain. Citrullination of FN alters integrin clustering and focal adhesion stability with a concomitant enhancement in force-triggered integrin signaling along the FAK-Src and ILK-Parvin pathways within fibroblasts. In vitro migration and in vivo wound healing studies demonstrate the ability of citrullinated FN to support a more migratory/invasive phenotype that enables more rapid wound closure. These findings highlight the potential of ECM, particularly FN, to "record" inflammatory insults via post-translational modification by inflammation-associated enzymes that are subsequently "read" by resident tissue fibroblasts, establishing a direct link between inflammation and tissue homeostasis and pathogenesis through the matrix.

Galectin-9 Is a Possible Promoter of Immunopathology in Rheumatoid Arthritis by Activation of Peptidyl Arginine Deiminase 4 (PAD-4) in Granulocytes

The aetiology of rheumatoid arthritis (RA) is unknown, but citrullination of proteins is thought to be an initiating event. In addition, it is increasingly evident that the lung can be a potential site for the generation of autoimmune triggers before the development of joint disease. Here, we identified that serum levels of galectin-9 (Gal-9), a pleiotropic immunomodulatory protein, are elevated in RA patients, and are even further increased in patients with comorbid bronchiectasis, a lung disease caused by chronic inflammation. The serum concentrations of Gal-9 correlate with C-reactive protein levels and DAS-28 score. Gal-9 activated polymorphonuclear leukocytes (granulocytes) in vitro, which was characterized by increased cytokine secretion, migration, and survival. Further, granulocytes treated with Gal-9 upregulated expression of peptidyl arginine deiminase 4 (PAD-4), a key enzyme required for RA-associated citrullination of proteins. Correspondingly, treatment with Gal-9 triggered citrullination of intracellular granulocyte proteins that are known contributors to RA pathogenesis (i.e., myeloperoxidase, alpha-enolase, MMP-9, lactoferrin). In conclusion, this study identifies for the first time an immunomodulatory protein, Gal-9, that triggers activation of granulocytes leading to increased PAD-4 expression and generation of citrullinated autoantigens. This pathway may represent a potentially important mechanism for development of RA.

Mucosa-Environment Interactions in the Pathogenesis of Rheumatoid Arthritis

Mucosal surfaces play a central role in the pathogenesis of rheumatoid arthritis (RA). Several risk factors, such as cigarette smoking, environmental pollution, and periodontitis interact with the host at the mucosal level, triggering immune system activation. Moreover, the alteration of microbiota homeostasis is gaining increased attention for its involvement in the disease pathogenesis, modulating the immune cell response at a local and subsequently at a systemic level. Currently, the onset of the clinical manifest arthritis is thought to be the last step of a series of pathogenic events lasting years. The positivity for anti-citrullinated protein antibodies (ACPAs) and rheumatoid factor (RF), in absence of symptoms, characterizes a preclinical phase of RA-namely systemic autoimmune phase- which is at high risk for disease progression. Several immune abnormalities, such as local ACPA production, increased T cell polarization towards a pro-inflammatory phenotype, and innate immune cell activation can be documented in at-risk subjects. Many of these abnormalities are direct consequences of the interaction between the environment and the host, which takes place at the mucosal level. The purpose of this review is to describe the humoral and cellular immune abnormalities detected in subjects at risk of RA, highlighting their origin from the mucosa-environment interaction.

Release of active peptidylarginine deiminase into the circulation during acute inflammation induced by coronary artery bypass surgery

Purpose: Peptidylarginine deiminase (PAD) catalyzes citrullination, a post-translational modification that can alter structure, function and antigenicity of proteins. Citrullination in the lungs due to smoking is believed to initiate an anti-citrulline immune response in rheumatoid arthritis. Citrullination in other inflamed organs has also been demonstrated, but it is not known whether smoking or inflammatory processes in general result in release of relevant amounts of PAD into the circulation with potential to cause citrullination of proteins at various anatomical sites. Coronary artery bypass grafting (CABG) using cardiopulmonary bypass (CPB) induces an acute systemic inflammation response. In the present study, we investigate whether smoking or acute systemic inflammation causes release of PAD into the circulation.

Patients and methods: This study included 36 patients with coronary heart disease (16 smokers and 20 non-smokers) undergoing CABG surgery with CPB. Circulating levels of PAD2 and PAD4, PAD activity, the neutrophil activation markers MPO, MMP-9 and lipocalin-2, the cytokines IL-6 and IL-10, and the chemokine CXCL8 were measured 2 hrs preoperatively and 2 hrs postoperatively.

Results: At baseline, serum PAD2 and PAD4 concentration did not differ between smokers and non-smokers. However, serum from non-smokers contained higher PAD activity than serum from smokers. Circulating PAD2 levels and PAD activity increased markedly in both groups after surgery, as did all neutrophil activation markers, cytokines and chemokine. PAD2 levels correlated with neutrophil activation markers, but not with cytokine and chemokine levels.

Conclusion: Blood levels of PAD2 did not differ significantly between smokers and non-smokers, but smokers had decreased PAD activity in the circulation. PAD2 levels and PAD activity increased in blood during inflammation induced by CABG with CPB. This suggests that acute inflammation, ischemia or reperfusion, or a combination of these, leads to systemic spreading of enzymatically active PAD, which may affect protein function and induce generation of citrullinated self-antigens.

"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.

Lung alveolar tissue destruction and protein citrullination in diesel exhaust-exposed mouse lungs

Humanity faces an increasing impact of air pollution worldwide, including threats to human health. Air pollutants prompt and promote chronic inflammation, tumourigenesis, autoimmune and other destructive processes in the human body. Post-translational modification of proteins, for example citrullination, results from damaging attacks of pollutants, including smoking, air pollution and others, rendering host tissues immunogenic. Citrullinated proteins and citrullinating enzymes, deiminases, are more prevalent in patients with COPD and correlate with ongoing inflammation and oxidative stress. In this study, we installed an in-house-designed diesel exhaust delivery and cannabidiol vaporization system where mice were exposed to relevant, urban traffic-related levels of diesel exhaust for 14 days and assessed integrity of alveolar tissue, gene expression shifts and changes in protein content in the lungs and other tissues of exposed mice. Systemic presence of modified proteins was also tested. The protective effect of phytocannabinoids was investigated as well. Data obtained in our study show subacute effects of diesel exhaust on mouse lung integrity and protein content. Emphysematous changes are documented in exposed mouse lungs. In parallel, increased levels of citrulline were detected in the alveolar lung tissue and peripheral blood of exposed mice. Pre-treatment with vaporized cannabidiol ameliorated some damaging effects. Results reported hereby provide new insights into subacute lung tissue changes that follow diesel exhaust exposure and suggest possible dietary and/or other therapeutic interventions for maintaining lung health and healthy ageing.

NETopathic Inflammation in Chronic Obstructive Pulmonary Disease and Severe Asthma

Neutrophils play a central role in innate immunity, inflammation, and resolution. Unresolving neutrophilia features as a disrupted inflammatory process in the airways of patients with chronic obstructive pulmonary disease (COPD) and severe asthma. The extent to which this may be linked to disease pathobiology remains obscure and could be further confounded by indication of glucocorticoids or concomitant respiratory infections. The formation of neutrophil extracellular traps (NETs) represents a specialized host defense mechanism that entrap and eliminate invading microbes. NETs are web-like scaffolds of extracellular DNA in complex with histones and neutrophil granular proteins, such as myeloperoxidase and neutrophil elastase. Distinct from apoptosis, NET formation is an active form of cell death that could be triggered by various microbial, inflammatory, and endogenous or exogenous stimuli. NETs are reportedly enriched in neutrophil-dominant refractory lung diseases, such as COPD and severe asthma. Evidence for a pathogenic role for respiratory viruses (e.g., Rhinovirus), bacteria (e.g., Staphylococcus aureus) and fungi (e.g., Aspergillus fumigatus) in NET induction is emerging. Dysregulation of this process may exert localized NET burden and contribute to NETopathic lung inflammation. Disentangling the role of NETs in human health and disease offer unique opportunities for therapeutic modulation. The chemokine CXCR2 receptor regulates neutrophil activation and migration, and small molecule CXCR2 antagonists (e.g., AZD5069, danirixin) have been developed to selectively block neutrophilic inflammatory pathways. NET-stabilizing agents using CXCR2 antagonists are being investigated in proof-of-concept studies in patients with COPD to provide mechanistic insights. Clinical validation of this type could lead to novel therapeutics for multiple CXCR2-related NETopathologies. In this Review, we discuss the emerging role of NETs in the clinicopathobiology of COPD and severe asthma and provide an outlook on how novel NET-stabilizing therapies via CXCR2 blockade could be leveraged to disrupt NETopathic inflammation in disease-specific phenotypes.

Colorectal cancer liver metastatic growth depends on PAD4-driven citrullination of the extracellular matrix

Citrullination of proteins, a post-translational conversion of arginine residues to citrulline, is recognized in rheumatoid arthritis, but largely undocumented in cancer. Here we show that citrullination of the extracellular matrix by cancer cell derived peptidylarginine deiminase 4 (PAD4) is essential for the growth of liver metastases from colorectal cancer (CRC). Using proteomics, we demonstrate that liver metastases exhibit higher levels of citrullination and PAD4 than unaffected liver, primary CRC or adjacent colonic mucosa. Functional significance for citrullination in metastatic growth is evident in murine models where inhibition of citrullination substantially reduces liver metastatic burden. Additionally, citrullination of a key matrix component collagen type I promotes greater adhesion and decreased migration of CRC cells along with increased expression of characteristic epithelial markers, suggesting a role for citrullination in promoting mesenchymal-to-epithelial transition and liver metastasis. Overall, our study reveals the potential for PAD4-dependant citrullination to drive the progression of CRC liver metastasis.

Autoimmune, rheumatic, chronic inflammatory diseases: Neutrophil extracellular traps on parade

Rheumatic diseases are a group of inflammatory conditions that affect joints and connective tissues and are often accompanied by pain and restriction of motility. In many of these diseases, autoantibodies develop that react with molecules/structures commonly found hidden in neutrophils. Neutrophil extracellular trap (NET) formation and release is considered a defense mechanism against pathogens or endogenous danger signals and it has been associated with initial inflammatory responses. NETs are also endowed with an important resolution potential based on its intrinsic enzymatic activity, but in the case they are not timely removed from the crime scene they might modulate subsequent immune responses and contribute to the pathogenesis of chronic inflammatory diseases. In this review, we will summarize the actual knowledge about the multifaceted roles of NETs in the etiology and pathogenesis of rheumatic autoimmune diseases.

Cadmium nanoparticles citrullinate cytokeratins within lung epithelial cells: cadmium as a potential cause of citrullination in chronic obstructive pulmonary disease

Objective

The objective of the study was to determine whether the cadmium-derived materials induce intracellular protein citrullination.

Methods

Human A549 lung epithelial cells were exposed to cadmium in soluble and nanoparticulate forms represented by cadmium chloride (CdCl₂) and cadmium oxide (CdO), respectively, and their combinations with ultrafine carbon black (ufCB) produced by high temperature combustion, imitating cigarette burning. Protein citrullination in cell lysates was analyzed by Western immunoblotting and verified by immunofluorescent confocal microscopy. Target citrullinated proteins were identified by proteomic analysis.

Results

CdO, ufCB and its combination with CdCl₂ and CdO after high temperature combustion induced protein citrullination in cultured human lung epithelial cells, as detected by immunoblotting with anti-citrullinated protein antibody. Cytokeratins of type II (1, 2, 5, 6A, 6B and 77) and type I (9, 10) were identified as major intracellular citrullination targets. Immunofluorescent staining confirmed the localization of citrullinated proteins both in the cytoplasm and cell nuclei.

Conclusion

Cadmium oxide nanoparticle exposure facilitated post-translational citrullination of proteins.

Upregulation of citrullination pathway: From Autoimmune to Idiopathic Lung Fibrosis

Background

Increased protein citrullination and peptidylarginine deiminases (PADIs), which catalyze the citrullination process, are central in Rheumatoid arthritis pathogenesis and probably involved in the initial steps towards autoimmunity. Approximately, 10% of RA patients develop clinically significantly ILD. A possible shared role of protein citrullination in rheumatoid arthritis associated interstitial lung disease (RA-ILD), and idiopathic pulmonary fibrosis (IPF) pathogenesis remains unclear.

Methods

We evaluated PADI2 and PADI4 mRNA expression in bronchoalveolar lavage fluid (BALF) cells of 59 patients with IPF, 27 patients RA-ILD and 10 healthy controls. PADI 2 and 4 expression was analyzed by western blot and immunohistochemistry. Citrullinated protein levels were also quantified.

Results

PADI4 mRNA and protein levels were higher in RA-ILD and IPF than controls. Furthermore, PADI4 mRNA levels showed an increase among smokers in RA-ILD. PADI4 expression was detected in granulocytes and macrophages in all groups, with the strongest cytoplasmic expression observed in granulocytes in RA-ILD and IPF. PADI2 mRNA and immunostaining of BAL cells, were similar in all groups among smokers. Overall, stronger staining was observed in current smokers. Citrullinated peptides were significantly increased in IPF compared to RA-ILD and controls. In RA-ILD, protein citrullination strongly correlated with PADI4 expression and anti-citrullinated protein antibodies (ACPAs).

Conclusions

These results suggest that the citrullination pathway is upregulated in IPF and in RA-ILD.

Electronic supplementary material

The online version of this article (10.1186/s12931-017-0692-9) contains supplementary material, which is available to authorized users.

Association of Distinct Fine Specificities of Anti-Citrullinated Peptide Antibodies With Elevated Immune Responses to Prevotella intermedia in a Subgroup of Patients With Rheumatoid Arthritis and Periodontitis

OBJECTIVE

In addition to the long-established link with smoking, periodontitis (PD) is a risk factor for rheumatoid arthritis (RA). This study was undertaken to elucidate the mechanism by which PD could induce antibodies to citrullinated peptides (ACPAs), by examining the antibody response to a novel citrullinated peptide of cytokeratin 13 (CK-13) identified in gingival crevicular fluid (GCF), and comparing the response to 4 other citrullinated peptides in patients with RA who were well-characterized for PD and smoking.

METHODS

The citrullinomes of GCF and periodontal tissue from patients with PD were mapped by mass spectrometry. ACPAs of CK13 (cCK13), tenascin-C (cTNC5), vimentin (cVIM), α-enolase (CEP-1), and fibrinogen β (cFIBβ) were examined by enzyme-linked immunosorbent assay in patients with RA (n = 287) and patients with osteoarthritis (n = 330), and cross-reactivity was assessed by inhibition assays.

RESULTS

A novel citrullinated peptide cCK13-1 (444 TSNASGR-Cit-TSDV-Cit-RP458 ) identified in GCF exhibited elevated antibody responses in RA patients (24%). Anti-cCK13-1 antibody levels correlated with anti-cTNC5 antibody levels, and absorption experiments confirmed this was not due to cross-reactivity. Only anti-cCK13-1 and anti-cTNC5 were associated with antibodies to the periodontal pathogen Prevotella intermedia (P = 0.05 and P = 0.001, respectively), but not with antibodies to Porphyromonas gingivalis arginine gingipains. Levels of antibodies to CEP-1, cFIBβ, and cVIM correlated with each other, and with smoking and shared epitope risk factors in RA.

CONCLUSION

This study identifies 2 groups of ACPA fine specificities associated with different RA risk factors. One is predominantly linked to smoking and shared epitope, and the other links anti-cTNC5 and cCK13-1 to infection with the periodontal pathogen P intermedia.

Carbamylation of vimentin is inducible by smoking and represents an independent autoantigen in rheumatoid arthritis

OBJECTIVES

Smoking has been connected to citrullination of antigens and formation of anti-citrullinated peptide antibodies (ACPAs) in rheumatoid arthritis (RA). Since smoking can modify proteins by carbamylation (formation of homocitrulline), this study was conducted to investigate these effects on vimentin in animal models and RA.

METHODS

The efficiency of enzymatic carbamylation of vimentin was characterised. B-cell response was investigated after immunisation of rabbits with different vimentin isoforms. Effects of tobacco smoke exposure on carbamylation of vimentin and formation of autoantibodies were analysed in mice. The antibody responses against isoforms of vimentin were characterised with respect to disease duration and smoking status of patients with RA.

RESULTS

Enzymatic carbamylation of vimentin was efficiently achieved. Subsequent citrullination of vimentin was not disturbed by homocitrullination. Sera from rabbits immunised with carbamylated vimentin (carbVim), in addition to carbVim also recognised human IgG-Fc showing rheumatoid factor-like reactivity. Smoke-exposed mice contained detectable amounts of carbVim and developed a broad immune response against carbamylated antigens. Although the prevalence of anti-carbamylated antibodies in smokers and non-smokers was similar, the titres of carbamylated antibodies were significantly increased in sera of smoking compared with non-smoking RA. CarbVim antibodies were observed independently of ACPAs in early phases of disease and double-positive patients for anti-mutated citrullinated vimentin (MCV) and anti-carbVim antibodies showed an extended epitope recognition pattern towards MCV.

CONCLUSIONS

Carbamylation of vimentin is inducible by cigarette smoke exposure. The polyclonal immune response against modified antigens in patients with RA is not exclusively citrulline-specific and carbamylation of antigens could be involved in the pathogenesis of disease.

TRIAL REGISTRATION NUMBER

ISRCTN36745608; EudraCT Number: 2006-003146-41.

Influence of Cigarette Smoking on Rheumatoid Arthritis Risk in the Han Chinese Population

OBJECTIVES

Cigarette smoking has been shown in European populations to be associated with rheumatoid arthritis (RA) susceptibility. This study aims to examine the association of smoking with RA in the Han Chinese population.

METHODS

718 Han Chinese RA patients and 404 healthy controls were studied. The associations of cigarette smoking (current, former or ever vs. never smokers, and pack-years of exposure) with RA, anti-cyclic citrullinated peptide antibody (ACPA) positive RA, IgM rheumatoid factor (RF) positive RA, and baseline radiographic erosions (modified van der Heijde-Sharp scores) were assessed. The interaction between smoking and the HLA-DRB1 shared epitope (SE) in RA was also examined.

RESULTS

In this study, 11 (1.53%) cases and 6 (1.49%) controls were former smokers (p = 0.95), while 95 (13.23%) cases and 48 (11.88%) controls were current smokers (p = 0.52). Trends toward associations between smoking status (ever vs. never) with RA-overall (p = 0.15, OR = 1.44), ACPA-positive RA (p = 0.24, OR = 1.37), RF-positive RA (p = 0.14, OR = 1.46), or the presence of radiographic erosions (p = 0.66, OR = 1.28) were observed although individually here were not statistically significant. There was no evidence of statistical interaction between smoking status (ever vs. never) and SE for all RA, ACPA-positive RA, ACPA-negative RA, RF-positive RA, RF-negative RA (p = 0.37, 0.50, 0.24, 0.26, and 0.81 respectively), and the 95% CI for the attributable proportion for all interactions included 0.

CONCLUSION

This is the first study to examine the association of cigarette smoking with RA in the Han Chinese population. This study shows a trend toward an interaction between smoking and SE carriage influencing the risk of RA, though findings were not statistically significant. It is possible that in the presence of universal exposure to heavy air pollution the effect of smoking on RA risk may be obscured.

Anti-Citrullinated Protein Antibodies Are Associated With Neutrophil Extracellular Traps in the Sputum in Relatives of Rheumatoid Arthritis Patients

OBJECTIVE

Studies suggest that rheumatoid arthritis (RA)-related autoimmunity is initiated at a mucosal site. However, the factors associated with the mucosal generation of this autoimmunity are unknown, especially in individuals who are at risk of future RA. Therefore, we tested anti-cyclic citrullinated peptide (anti-CCP) antibodies in the sputum of RA-free first-degree relatives (FDRs) of RA patients and patients with classifiable RA.

METHODS

We evaluated induced sputum and serum samples from 67 FDRs and 20 RA patients for IgA anti-CCP and IgG anti-CCP, with cutoff levels for positivity determined in a control population. Sputum was also evaluated for cell counts, neutrophil extracellular traps (NETs) using sandwich enzyme-linked immunosorbent assays for protein/nucleic acid complexes, and total citrulline.

RESULTS

Sputum was positive for IgA and/or IgG anti-CCP in 14 of 20 RA patients (70%) and 17 of 67 FDRs (25%), including a portion of FDRs who were serum anti-CCP negative. In the FDRs, elevations of sputum IgA and IgG anti-CCP were associated with elevated sputum cell counts and NET levels. IgA anti-CCP was associated with ever smoking and with elevated sputum citrulline levels.

CONCLUSION

Anti-CCP is elevated in the sputum of FDRs, including seronegative FDRs, suggesting that the lung may be a site of anti-CCP generation in this population. The association of anti-CCP with elevated cell counts and NET levels in FDRs supports a hypothesis that local airway inflammation and NET formation may drive anti-CCP production in the lung and may promote the early stages of RA development. Longitudinal studies are needed to follow the evolution of these processes relative to the development of systemic autoimmunity and articular RA.

Pre-symptomatic autoimmunity in rheumatoid arthritis: when does the disease start?

It is well recognised that a state of autoimmunity, in which immunological tolerance is broken, precedes the development of symptoms in the majority of patients with rheumatoid arthritis (RA). For individuals who will later develop seropositive disease, this manifests as autoantibodies directed against proteins that have undergone specific post-translational modifications. There is evidence that the induction of this autoantibody response occurs at peripheral extra-articular mucosal sites, such as the periodontium and lung. In addition to their utility as diagnostic markers, these autoantibodies may have a pathogenic role that helps localise disease to the synovium. Alongside the development of autoantibodies, other factors contributing to pre-symptomatic autoimmunity may include dysbiosis of the gastrointestinal tract, abnormal development of lymphoid tissue, and dysregulated autonomic and lipid-mediated anti-inflammatory signalling. These factors combine to skew the balance between pro-inflammatory and anti-inflammatory signalling in a manner that is permissive for the development of clinical arthritis. We present data to support the concept that the transitions from at-risk states to systemic autoimmunity and then to classifiable RA depend on multiple “switches”. However, further prospective studies are necessary to define the molecular basis of these switches and the specific features of pre-symptomatic autoimmunity, so that preventative treatments can be targeted to individuals at high risk for RA. In this review, we analyse mechanisms that may contribute to the development of autoimmunity in at-risk individuals and discuss the relationship between this pre-symptomatic state and subsequent development of RA.

Experimental silicosis does not aggravate collagen-induced arthritis in mice

Objectives

To investigate the effect of chronic lung inflammation on the incidence and severity of collagen-induced arthritis in mice.

Methods

Chronic lung inflammation in the form of silicosis was induced via intranasal application of silica particles. Immunization with collagen Type II commenced one week later and mice were sacrificed six weeks after booster immunization. Thereafter, silicosis was confirmed via flow cytometry and arthritis was evaluated performing knee and paw histology.

Results

Pronounced lung inflammation in the silica-treated compared to PBS-treated control mice was demonstrated by significantly elevated broncho-alveolar lavage (BAL) cell count, attributable to increased numbers of macrophages and granulocytes. Inflammation in the lungs was not associated with elevated PAD2 and PAD4 expression, yet silica treated animals had significantly higher aCCP serum titers. However, lung inflammation did not lead to an increase in the incidence of arthritis, nor did it exacerbate the macroscopic or histologic joint scores.

Conclusions

Chronic lung inflammation resulting from silicosis does not aggravate collagen-induced arthritis in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s12952-017-0071-6) contains supplementary material, which is available to authorized users.

Mechanisms leading from systemic autoimmunity to joint-specific disease in rheumatoid arthritis

A key unanswered question in the pathophysiology of rheumatoid arthritis (RA) is how systemic autoimmunity progresses to joint-specific inflammation. In patients with seropositive RA (that is, characterized by the presence of autoantibodies) evidence is accumulating that immunity against post-translationally modified (such as citrullinated) autoantigens might be triggered in mucosal organs, such as the lung, long before the first signs of inflammation are seen in the joints. However, the mechanism by which systemic autoimmunity specifically homes to the joint and bone compartment, thereby triggering inflammation, remains elusive. This Review summarizes potential pathways involved in this joint-homing mechanism, focusing particularly on osteoclasts as the primary targets of anti-citrullinated protein antibodies (ACPAs) in the bone and joint compartment. Osteoclasts are dependent on citrullinating enzymes for their normal differentiation and are unique in displaying citrullinated antigens on their cell surface in a non-inflamed state. The binding of ACPAs to osteoclasts releases the chemokine IL-8, leading to bone erosion and pain. This process initiates a chain of events that could lead to attraction and activation of neutrophils, resulting in a complex series of proinflammatory processes in the synovium, eventually leading to RA.

Causes and consequences of endoplasmic reticulum stress in rheumatic disease

Rheumatic diseases represent a heterogeneous group of inflammatory conditions, many of which involve chronic activation of both innate and adaptive immune responses by multiple genetic and environmental factors. These immune responses involve the secretion of excessive amounts of cytokines and other signalling mediators by activated immune cells. The endoplasmic reticulum (ER) is the cellular organelle that directs the folding, processing and trafficking of membrane-bound and secreted proteins, including many key components of the immune response. Maintaining homeostasis in the ER is critical to cell function and survival. Consequently, elaborate mechanisms have evolved to sense and respond to ER stress through three main signalling pathways that together comprise the unfolded protein response (UPR). Activation of the UPR can rapidly resolve the accumulation of misfolded proteins, direct permanent changes in the size and function of cells during differentiation, and critically influence the immune response and inflammation. Recognition of the importance of ER stress and UPR signalling pathways in normal and dysregulated immune responses has greatly increased in the past few years. This Review discusses several settings in which ER stress contributes to the pathogenesis of rheumatic diseases and considers some of the therapeutic opportunities that these discoveries provide.

NETosis as Source of Autoantigens in Rheumatoid Arthritis

In neutrophils (but also in eosinophils and in mast cells), different inflammatory stimuli induce histone deimination, chromatin decondensation, and NET formation. These web-like structures that trap and kill microbes contain DNA, cationic granule proteins, and antimicrobial peptides, but the most abundant proteins are core histones. Histones contained in NETs have been deiminated, and arginines are converted in citrullines. While deimination is a physiological process amplified in inflammatory conditions, only individuals carrying genetic predisposition to develop rheumatoid arthritis (RA) make antibodies to deiminated proteins. These antibodies, collectively identified as anti-citrullinated proteins/peptides antibodies (ACPA), react with different deiminated proteins and display partially overlapping specificities. In this paper, we will summarize current evidence supporting the role of NETosis as critical mechanism in the breach of tolerance to self-antigens and in supporting expansion and differentiation of autoreactive cells. In fact, several lines of evidence connect NETosis with RA: RA unstimulated synovial fluid neutrophils display enhanced NETosis; sera from RA patients with Felty's syndrome bind deiminated H3 and NETs; a high number of RA sera bind deiminated H4 contained in NETs; human monoclonal antibodies generated from RA synovial B cells decorate NETs and bind deiminated histones. In RA, NETs represent on one side an important source of autoantigens bearing posttranslational modifications and fueling the production of ACPA. On the other side, NETs deliver signals that maintain an inflammatory milieu and contribute to the expansion and differentiation of ACPA-producing B cells.

Association of anti-peptidyl arginine deiminase antibodies with radiographic severity of rheumatoid arthritis in African Americans

Background

Evidence suggests that the presence of peptidyl arginine deiminase type 4 (PAD4) antibodies is associated with radiographic-severity rheumatoid arthritis (RA) among Caucasian patients. The presence of anti-PAD4 antibodies that were cross-reactivity against PAD3 was associated with more aggressive erosive disease (compared with the presence of anti-PAD4 antibodies without anti-PAD3 crossreactivity) in Caucasian RA patients. The objectives of this study were to determine the prevalence of serum anti-PAD4 and anti-PAD4/PAD3 cross-reactive autoantibodies in African Americans with RA and whether these antibodies associate with radiographic severity and radiographic progression.

Methods

Serum anti-PAD4 and anti-PAD4/PAD3 antibodies were measured by immunoprecipitation, and the temporal trends in titers were analyzed. We compared total radiographic scores among anti-PAD4-positive, anti-PAD4/PAD3-positive, and anti-PAD4-negative patients and used a zero-inflated negative binomial model to determine associations between radiographic severity and antibody status. Logistic regression was used to analyze radiographic progression.

Results

Of 192 African-American patients with RA, 73 % were anti-citrullinated peptide/protein antibody (ACPA)-positive, 46 out of 192 (24 %) of whom had serum anti-PAD4 antibodies. Median (interquartile range) total Sharp van der Heijde radiographic scores were 2 (1–97.5) in ACPA-positive patients and 0 (0–3) in ACPA-negative patients (P < 0.001). Of the 46 anti-PAD4-positive patients, 20 had anti-PAD4 antibodies that cross-reacted with PAD3. In patients with early RA, anti-PAD4 and anti-PAD4/PAD3 antibody titers increased over time (P = 0.006, P = 0.001, respectively). Median (interquartile range) total radiographic scores were higher for anti-PAD4-positive than for anti-PAD4-negative patients (3 (1–115) versus 2 (0–11), respectively; P = 0.005). Median (interquartile range) total radiographic score for anti-PAD4/PAD3-positive patients was 76 (3–117) (P < 0.001) versus anti-PAD4-negative patients. Only anti-PAD4/PAD3 antibodies associated with radiographic severity (incidence rate ratio = 2.81; 95 % confidence interval 1.23, 6.43).

Conclusion

This analysis suggests that autoantibodies against PAD4 and PAD3 proteins may serve as biomarkers for identifying African-American patients with RA and higher radiographic severity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-1126-7) contains supplementary material, which is available to authorized users.

Mechanisms involved in triggering rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory syndrome with a strong autoimmune component. The autoantigens in RA are neither tissue nor organ-specific, but comprise a broad collection of post-translational modified proteins, such as citrullinated proteins. These modifications are likely to be triggered by innate stimuli. In genetically susceptible hosts, they can lead to a more substantiated secondary autoimmune reaction targeting the joints and precipitating the clinical onset of RA. Both innate and adaptive mechanisms will then closely interplay to promote chronic joint inflammation in the several absence of appropriate treatment. This scenario, is shared with other autoimmune diseases where potentially pathogenic immune responses are present already before disease onset. Better understanding of these processes will allow both earlier diagnosis of RA and identification of those healthy individuals that are at risk of developing disease, opening possibilities for disease prevention. In this review, we discuss the iterative processes of innate and adaptive immunity responsible for the (longitudinal) development of immune reactions that may contribute to the development of RA.

Tumor necrosis factor alpha, citrullination, and peptidylarginine deiminase 4 in lung and joint inflammation

Background

The relationship between lung and joint inflammation in rheumatoid arthritis is poorly understood. Lung inflammation with resultant protein citrullination may trigger anti-citrullinated protein antibodies, inflammation, and arthritis. Alternatively, lung and joint inflammation may be two manifestations of a single underlying pathology. The lung has increased citrullination and TNF-α levels are high in rheumatoid arthritis; however, it is unknown if TNF-α can induce lung protein citrullination. The citrullinating enzyme peptidylarginine deiminase 4 (PAD4) exacerbates TNF-α-induced arthritis, but a role for PAD4 in lung citrullination and TNF-α-induced lung inflammation has not been explored. Our aim was to use TNF-α-overexpressing mice to clarify the intersection of TNF-α, citrullination, PAD4, arthritis, and lung inflammation.

Methods

Lung protein citrullination in wild-type mice, mice that overexpress TNF-α systemically (TNF⁺), TNF⁺PAD4^+/+, and TNF⁺PAD4^-/- mice was quantified by both gel electrophoresis using a citrulline probe and western blot. Hematoxylin and eosin (H&E)-stained lung sections from TNF⁺PAD4^+/+ and TNF⁺PAD4^-/- mice were scored for lung inflammation. H&E-stained ankle joint sections from mice that overexpress TNF-α only in the lungs were assessed for arthritis.

Results

TNF⁺ mice have increased lung protein citrullination. TNF⁺PAD4^-/- mice do not have significantly reduced lung protein citrullination, but do have decreased lung inflammation compared to TNF⁺PAD4^+/+ mice. Mice that overexpress TNF-α only in the lungs do not develop arthritis.

Conclusions

PAD4 exacerbates lung inflammation downstream of TNF-α without having a major role in generalized protein citrullination in inflamed lungs. Also, TNF-α-induced lung inflammation is not sufficient to drive murine arthritis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-1068-0) contains supplementary material, which is available to authorized users.

The lung in rheumatoid arthritis, cause or consequence?

PURPOSE OF REVIEW

Recent discoveries implicate the lungs as a possible extra-articular mucosal site for initiating rheumatoid arthritis-associated immunity.

RECENT FINDINGS

Individuals at risk for developing arthritis and patients with early untreated rheumatoid arthritis show signs of lung involvement on high-resolution computer tomography. Rheumatoid arthritis-associated antibodies are present in patients with respiratory complains such as bronchiectasis and unexplained dyspnea even in the absence of joint disease and might predict future development of rheumatoid arthritis in these patients. Rheumatoid arthritis-associated antibodies are detected in the sputum of individuals at risk for developing rheumatoid arthritis but not yet having disease. Signs of bronchial mucosal inflammation with germinal center formation and local production of antibodies have been described in patients with early untreated rheumatoid arthritis. Shared antigenic targets have been identified and characterized in the lungs and joints of rheumatoid arthritis patients.

SUMMARY

Recent findings support an important role for the lung as a mucosal extra-articular place were inflammation induced by external triggers (such as smoking) leads to tolerance break and generation of rheumatoid arthritis-specific immunity already before disease onset with only secondary targeting of the joints.

How the microbiota shapes rheumatic diseases

The human gut harbours a tremendously diverse and abundant microbial community that correlates with, and even modulates, many health-related processes. The mucosal interfaces are particularly active sites of microorganism-host interplay. Growing insight into the characteristic composition and functionality of the mucosal microbiota has revealed that the microbiota is involved in mucosal barrier integrity and immune function. This involvement affects proinflammatory and anti-inflammatory processes not only at the epithelial level, but also at remote sites such as the joints. Here, we review the role of the gut microbiota in shaping local and systemic immune responses and how disturbances in the host-microorganism interplay can potentially affect the development and progression of rheumatic diseases. Increasing our understanding of how to promote host-microorganism homeostasis could therefore reveal novel strategies for the prevention or alleviation of rheumatic disease.

Abrogation of collagen-induced arthritis by a peptidyl arginine deiminase inhibitor is associated with modulation of T cell-mediated immune responses

Proteins containing citrulline, a post-translational modification of arginine, are generated by peptidyl arginine deiminases (PAD). Citrullinated proteins have pro-inflammatory effects in both innate and adaptive immune responses. Here, we examine the therapeutic effects in collagen-induced arthritis of the second generation PAD inhibitor, BB-Cl-amidine. Treatment after disease onset resulted in the reversal of clinical and histological changes of arthritis, associated with a marked reduction in citrullinated proteins in lymph nodes. There was little overall change in antibodies to collagen or antibodies to citrullinated peptides, but a shift from pro-inflammatory Th1 and Th17-type responses to pro-resolution Th2-type responses was demonstrated by serum cytokines and antibody subtypes. In lymph node cells from the arthritic mice treated with BB-Cl-amidine, there was a decrease in total cell numbers but an increase in the proportion of Th2 cells. BB-Cl-amidine had a pro-apoptotic effect on all Th subsets in vitro with Th17 cells appearing to be the most sensitive. We suggest that these immunoregulatory effects of PAD inhibition in CIA are complex, but primarily mediated by transcriptional regulation. We suggest that targeting PADs is a promising strategy for the treatment of chronic inflammatory disease.

Smoking and Air Pollution as Pro-Inflammatory Triggers for the Development of Rheumatoid Arthritis

INTRODUCTION

Smoking is now well recognized not only as a risk factor for rheumatoid arthritis (RA), but also as a determinant of disease activity, severity, response to therapy, and possibly mortality.

METHODS

Studies, mostly recent, which have provided significant insights into the molecular and cellular mechanisms which underpin the pathogenesis of smoking-related RA, as well as the possible involvement of other types of outdoor and indoor pollution form the basis of this review.

RESULTS

Smoking initiates chronic inflammatory events in the lungs. These, in turn, promote the release of the enzymes, peptidylarginine deiminases 2 and 4 from smoke-activated, resident and infiltrating pulmonary phagocytes. Peptidylarginine deiminases mediate conversion of various endogenous proteins to putative citrullinated autoantigens. In genetically susceptible individuals, these autoantigens trigger the production of anti-citrullinated peptide, pathogenic autoantibodies, an event which precedes the development of RA.

CONCLUSIONS

An increasing body of evidence has linked chronic inflammatory events in the lungs of smokers, to the production of anti-citrullinated peptide autoantibodies and development of RA. Creation of awareness of the associated risks, assessment of smoking status and implementation of compelling antismoking strategies must be included in the routine clinical management of patients presenting with suspected RA.

IMPLICATIONS

Chronic inflammatory mechanisms operative in the lungs of smokers lead to the production of anti-citrullinated protein antibodies which, in turn, drive the development of RA. These mechanistic insights not only reinforce the association between smoking and risk for RA, but also the necessity to increase the level of awareness in those at highest risk.

Unmet challenges in immune-mediated hepatobiliary diseases

It is ironic that the liver, which serves a critical function in immune tolerance, itself becomes the victim of an autoimmune attack. Indeed, liver autoimmunity and the autoimmune diseases associated with both innate and adaptive responses to hepatocytes and/or cholangiocytes are models of human autoimmunity. For example, in primary biliary cirrhosis, there exists a well-defined and characteristic autoantibody and considerable homogeneity between patients. In autoimmune hepatitis, there are clinical characteristics that allow a rigorous subset definition and well-defined inflammatory infiltrates. In both cases, there are defects in a variety of immune pathways and including regulatory cells. In primary sclerosing cholangitis, with its characteristic overlap with inflammatory bowel disease, there are unique defects in innate immunity and particular important contribution of lymphoid homing to disease pathogenesis. In these diseases, as with other human autoimmune processes, there is the critical understanding that pathogenesis requires a genetic background, but is determined by environmental features, and indeed the concordance of these diseases in identical twins highlights the stochastic nature of immunopathology. Unfortunately, despite major advances in basic immunology and in immunopathology in these diseases, there remains a major void in therapy. The newer biologics that are so widely used in rheumatology, neurology, and gastroenterology have not yet seen success in autoimmune liver disease. Future efforts will depend on more rigorous molecular biology and systems analysis in order for successful application to be made to patients.

Identification of an immunodominant peptide from citrullinated tenascin-C as a major target for autoantibodies in rheumatoid arthritis

Objectives

We investigated whether citrullinated tenascin-C (cTNC), an extracellular matrix protein expressed at high levels in the joints of patients with rheumatoid arthritis (RA), is a target for the autoantibodies in RA.

Methods

Citrullinated sites were mapped by mass spectrometry in the fibrinogen-like globe (FBG) domain of tenascin-C treated with peptidylarginine deiminases (PAD) 2 and 4. Antibodies to cyclic peptides containing citrullinated sites were screened in sera from patients with RA by ELISA. Potential cross-reactivity with well-established anticitrullinated protein antibody (ACPA) epitopes was tested by inhibition assays. The autoantibody response to one immunodominant cTNC peptide was then analysed in 101 pre-RA sera (median 7 years before onset) and two large independent RA cohorts.

Results

Nine arginine residues within FBG were citrullinated by PAD2 and PAD4. Two immunodominant peptides cTNC1 (VFLRRKNG-cit-ENFYQNW) and cTNC5 (EHSIQFAEMKL-cit-PSNF-cit-NLEG-cit-cit-KR) were identified. Antibodies to both showed limited cross-reactivity with ACPA epitopes from α-enolase, vimentin and fibrinogen, and no reactivity with citrullinated fibrinogen peptides sharing sequence homology with FBG. cTNC5 antibodies were detected in 18% of pre-RA sera, and in 47% of 1985 Swedish patients with RA and 51% of 287 North American patients with RA. The specificity was 98% compared with 160 healthy controls and 330 patients with osteoarthritis.

Conclusions

There are multiple citrullination sites in the FBG domain of tenascin-C. Among these, one epitope is recognised by autoantibodies that are detected years before disease onset, and which may serve as a useful biomarker to identify ACPA-positive patients with high sensitivity and specificity in established disease.

Bronchiectasis is a Model for Chronic Bacterial Infection Inducing Autoimmunity in Rheumatoid Arthritis

Objective

To examine the potential of chronic severe bacterial infection to generate rheumatoid factor (RF) and anti–citrullinated protein antibodies (ACPAs), by studying patients with bronchiectasis (BR) alone and BR patients with rheumatoid arthritis (BR/RA).

Methods

We studied 122 patients with BR alone, 50 patients with BR/RA, and 50 RA patients without lung disease, as well as 87 patients with asthma and 79 healthy subjects as controls. RF levels were measured using an automated analyzer, and cyclic citrullinated peptide 2 (CCP‐2) was used to detect ACPAs. The fine specificities of citrullinated α‐enolase peptide 1 (CEP‐1), Cit‐vimentin, and Cit‐fibrinogen to their arginine‐containing control peptides (arginine‐containing α‐enolase peptide 1 [REP‐1], vimentin, and fibrinogen) were measured by enzyme‐linked immunosorbent assay.

Results

Among the BR patients and control subjects, 39% and 42%, respectively, were ever‐smokers. The frequency of RF positivity in serum was increased in BR patients compared with controls (25% versus 10%), as were the frequencies of antibodies to CCP‐2 (5% versus 0%), CEP‐1 (7% versus 4%), Cit‐vimentin (7% versus 4%), and Cit‐fibrinogen (12% versus 4%), although only the differences for RF and Cit‐fibrinogen were significant (P < 0.05). We observed a corresponding increase in the frequency of antibodies to the arginine‐containing control peptides in BR patients compared with controls (for REP‐1, 19% versus 4% [P < 0.01]; for vimentin, 16% versus 4% [P < 0.05]), demonstrating that the ACPA response in patients with BR is not citrulline specific. The lack of citrulline specificity was further confirmed by absorption studies. In BR/RA patients, all ACPA responses were highly citrulline specific.

Conclusion

Bronchiectasis is an unusual but potent model for the induction of autoimmunity in RA by bacterial infection in the lung. Our study suggests that the ACPA response is not citrulline specific during the early stages of tolerance breakdown but becomes more specific in patients with BR in whom BR/RA develops.

Signs of immune activation and local inflammation are present in the bronchial tissue of patients with untreated early rheumatoid arthritis

OBJECTIVES

Events in the lungs might contribute to generation of anticitrullinated protein antibodies (ACPA) in rheumatoid arthritis (RA). We investigated if signs of immune activation are present in bronchial biopsies and bronchoalveolar lavage (BAL) of patients with early-untreated RA without clinical signs of lung involvement.

METHODS

Twenty-four patients with RA with symptom duration <1 year and naïve to disease-modifying antirheumatic drugs were subjected to bronchoscopy where BAL and mucosal bronchial biopsies were retrieved. For comparison, 15 bronchial biopsies and 79 BAL samples from healthy volunteers were available. Histological examination was performed to evaluate lymphocyte infiltration, presence of immune cells (T and B cells, plasma cells, dendritic cells and macrophages) and immune activation markers. Cell composition of BAL samples was analysed by differential counting and T cell subsets by flow cytometry.

RESULTS

Lymphocyte infiltration was more frequently found in ACPA-positive patients (50%) as compared with ACPA-negative patients (17%) and controls (13%). Germinal centres, B cells and plasma cells were only found in ACPA-positive patients. The frequency of T cells in bronchial biopsies of patients with ACPA-positive RA was positively associated with expression of immune activation markers. BAL samples of patients with ACPA-positive, but not ACPA-negative, RA had significantly higher relative numbers of lymphocytes and expressed higher levels of activation markers compared with controls.

CONCLUSIONS

Signs of immune cell accumulation and activation are present both in the bronchial tissue and in BAL of untreated patients with early RA without concomitant lung disease, strengthening the role of the lung compartment as an important player in ACPA-positive RA.

Crystal structure of Porphyromonas gingivalis peptidylarginine deiminase: implications for autoimmunity in rheumatoid arthritis

BACKGROUND

Periodontitis (PD) is a known risk factor for rheumatoid arthritis (RA) and there is increasing evidence that the link between the two diseases is due to citrullination by the unique bacterial peptidylarginine deiminase (PAD) enzyme expressed by periodontal pathogen Pophyromonas gingivalis (PPAD). However, the precise mechanism by which PPAD could generate potentially immunogenic peptides has remained controversial due to lack of information about the structural and catalytic mechanisms of the enzyme.

OBJECTIVES

By solving the 3D structure of PPAD we aim to characterise activity and elucidate potential mechanisms involved in breach of tolerance to citrullinated proteins in RA.

METHODS

PPAD and a catalytically inactive mutant PPAD(C351A) were crystallised and their 3D structures solved. Key residues identified from 3D structures were examined by mutations. Fibrinogen and α-enolase were incubated with PPAD and P. gingivalis arginine gingipain (RgpB) and citrullinated peptides formed were sequenced and quantified by mass spectrometry.

RESULTS

Here, we solve the crystal structure of a truncated, highly active form of PPAD. We confirm catalysis is mediated by the following residues: Asp130, His236, Asp238, Asn297 and Cys351 and show Arg152 and Arg154 may determine the substrate specificity of PPAD for C-terminal arginines. We demonstrate the formation of 37 C-terminally citrullinated peptides from fibrinogen and 11 from α-enolase following incubation with tPPAD and RgpB.

CONCLUSIONS

PPAD displays an unequivocal specificity for C-terminal arginine residues and readily citrullinates peptides from key RA autoantigens. The formation of these novel citrullinated peptides may be involved in breach of tolerance to citrullinated proteins in RA.

Autoimmunity: lungs and citrullination

Anti-citrullinated protein antibodies are a distinctive feature of a subset of patients with rheumatoid arthritis (RA). A new report investigates how lung inflammation in patients with chronic obstructive pulmonary disease affects protein citrullination, providing an additional piece of information on the potential link between airway inflammation and RA.