Citrullinated proteins: sparks that may ignite the fire in rheumatoid arthritis

T cells in arteritis and atherosclerosis

PURPOSE OF REVIEW

Inflammatory vasculopathies, spanning from atherosclerosis to vasculitides, are driven by innate and adaptive immune responses. Instructed by antigen-presenting cells, T cells have unsurpassed skills to orchestrate protective and pathogenic immunity. Pro-inflammatory and anti-inflammatory T cells regulate master pathogenic pathways, providing a framework for novel immunotherapeutic strategies.

RECENT FINDINGS

The multilayered wall of macrovessels creates a unique tissue niche; professional antigen-presenting cells, specifically dendritic cells, are superior in triggering and maintaining T-cell responses in this tissue milieu. Plaque-residing dendritic cells sense pathogen-derived motifs and edit inflammatory responses. T cells respond to antigen but antigen-nonspecific factors setting cellular response thresholds may be equally important. Dysregulated signal transduction pathways emerge as highly relevant in biasing T cells toward hyperresponsiveness. In the inflamed atheroma and in arteritic lesions, pathogenic T cells coordinate multiple injury pathways. Besides inducing tissue-damaging macrophage functions, they directly inflict cellular injury within the arterial wall. Distinctively, selected T cells induce smooth muscle cell apoptosis, most prominently by upregulating the death-receptor ligand TRAIL.

SUMMARY

Innate sentinels, specifically dendritic cells, populate normal arteries, intramural vasculitic lesions, and the inflamed atheroma. They sense microbial motifs and instruct T cells toward pro-inflammatory and tissue-destructive effector functions. Microenvironmental factors imposed by the unique structure of the arterial wall appear to be highly conserved across disease entities, modulating inflammation in atherosclerosis and arteritis.

An important step towards completing the rheumatoid arthritis cycle

In the previous issue of Arthritis Research & Therapy data are presented showing that circulating immune complexes containing citrullinated fibrin(ogen) are present in anti-citrullinated protein antibody-positive rheumatoid arthritis patients, and that such immune complexes co-localize with complement factor C3 in the rheumatoid synovium. These results corroborate the idea that citrullination is intimately involved in the pathophysiology of rheumatoid arthritis and complete our model (the rheumatoid arthritis cycle) for the development and chronic nature of this disease.

An improved synthesis of haloaceteamidine-based inactivators of protein arginine deiminase 4 (PAD4)

Protein arginine deiminase 4 (PAD4) is an enzyme that hydrolyzes peptidyl arginine residues to form citrulline and ammonia. This enzyme has been implicated in several disease states, e.g. rheumatoid arthritis, and therefore represents a unique target for the development of a novel therapeutic. A solution-phase synthesis of Cl-amidine, the most potent PAD4 inactivator described to date, has been developed. This synthesis proceeds in 80% yield over 4 steps at a significantly (12-fold) lower cost.

Atherosclerotic disease is increased in recent-onset rheumatoid arthritis: a critical role for inflammation

Rheumatoid arthritis (RA) patients have increased mortality and morbidity as a result of cardiovascular and cerebrovascular disease. What is not clear, however, is either how early accelerated atherosclerosis begins in RA or how soon risk factors must be rigorously controlled. Furthermore, given the strong relationship of vascular disease to RA mortality and of inflammation to the accelerated atherosclerosis associated with RA, it is important to evaluate indices that could serially and noninvasively quantify atherosclerotic disease in RA patients. The carotid intima-media thickness (cIMT) and plaque, measured by ultrasound, correlate closely with direct measurement of the local and systemic atherosclerotic burden. To investigate the presence of subclinical atherosclerosis in the early stages of RA, the cIMT and plaque were measured using carotid duplex scanning in 40 RA patients with disease duration < 12 months and in 40 control subjects matched for age, sex and established cardiovascular risk factors. Patients with RA had significantly higher average cIMT values and more plaque than the control group (cIMT 0.64 +/- 0.13 mm versus 0.58 +/- 0.09 mm, respectively; P = 0.03). In RA patients, the cIMT was predicted by age and C-reactive protein level at first presentation to the clinic (R2 = 0.64). C-reactive protein was associated with age of disease onset and history of smoking. Since inflammation has been shown to predate onset of clinical RA, the accelerated atherogenic process related to inflammation may precede RA symptom onset.

Serum IgG activity against cyclic citrullinated peptide in patients evaluated for rheumatoid factor correlates with the IgM isotype

The purpose of this study was to evaluate the frequency of seric antibodies against cyclic citrullinated peptide (a-CCP) in patients tested for rheumatoid factor (RF) reactivity, and to analyze the correlation between their titers. We obtained serum from 112 consecutive patients (85 female), aged 47.2 +/- 13.4 years and from 46 clinically healthy subjects (CHS). Patients where stratified into four subgroups: rheumatoid arthritis (RA), probable RA (PRA), spondylarthropathies and other diagnosis. The a-CCP antibodies were determined by enzyme linked immunoassay (ELISA), RF by nephelometric test (IgM) and ELISA (IgG and IgM). Analysis of variance (ANOVA) showed statistically that a-CCP antibodies differs among RA versus CHS and other diagnosis; PRA versus CHS and other diagnosis. A significant Rho value of 0.84 (P < 0.05, Spearman's correlation) was identified between a-CCP antibodies and RF in PRA subgroup. When a correlation of a-CCP antibodies with RF (both isotypes) was done, the higher correlation was observed against IgM RF. The data suggests different pathways and times for each antibody generation.

Targeted therapies in rheumatoid arthritis: Focus on rituximab

B-cell depletion is a new strategy for treating patients with rheumatoid arthritis (RA). In the past years, several studies have proven the efficacy of anti-CD20 mediated B-cell depletion with rituximab (Mabthera((R))) in RA patients who failed TNF-blocking therapy. The important role of B-cells in the pathogenesis of RA is deducted from the specific detection of autoantibodies in RA and infiltration of B-cells and plasma cells in inflamed synovium. Pharmacological studies in RA patients treated with rituximab showed that half-life was approximately 3 weeks leading to a 6- to 8-month period of B-cell depletion in peripheral blood. Rituximab treatment led to significant improvements in disease activity of RA patients and the current review summarizes the results from phase III, randomized clinical trials that have been performed. Lastly, data on safety and quality of life are summarized. Although relatively low numbers of RA patients have been treated and long-term data are lacking, current data thus far suggest a relatively good safety profile for rituximab. Future studies will need to focus on predicting responsiveness to rituximab, investigating efficacy of re-treatment with rituximab and extending data on safety and patient-focused outcomes.

Activity-based protein profiling reagents for protein arginine deiminase 4 (PAD4): synthesis and in vitro evaluation of a fluorescently labeled probe

Protein arginine deiminase 4 (PAD4), which catalyzes the post-translational conversion of peptidyl arginine to peptidyl citrulline, is widely regarded as one of the best new targets for the development of a novel rheumatoid arthritis therapeutic. In addition to its presumed role in this disease, PAD4 is also a calcium-dependent histone deiminase that acts as a transcriptional co-repressor. Herein we describe the design, synthesis, and in vitro evaluation of two fluorescently labeled activity-based protein profiling (ABPP) reagents that specifically and irreversibly modify the active, that is, calcium-bound, form PAD4 with equal affinity to previously described small molecule chemical probes of PAD4 function. These fluorescently tagged ABPPs will be useful for identifying the conditions under which this enzyme is activated in vivo and may prove to be useful RA diagnostics.

Anti-citrulline antibodies in the diagnosis and prognosis of rheumatoid arthritis: evolving concepts

Citrulline is a non-standard amino acid that can be incorporated into proteins only by post-translational modification of arginine by peptidylarginine deiminase (PAD) enzymes during a variety of biologic processes, including inflammation. Rheumatoid arthritis (RA) is an inflammatory autoimmune disease, with a prevalence of 0.3 to 1% worldwide, which leads to progressive joint erosion and substantial disability if not treated early. A reliable and specific test for a marker present early in the disease would be useful to identify RA patients prior to the occurrence of joint damage. A new group of autoantibodies, the anti-cyclic citrullinated peptide antibodies (anti-CCP), can be detected in up to 80% of patients with RA, are highly specific for the disease, and may be of value for both the diagnosis and the prognosis of RA. The fact that these antibodies may appear before the onset of the disease suggests a potential role in primary prevention. Interestingly, they may also play a role in the pathophysiology of this disabling disease. The process of citrullination, its physiologic role, and citrullination-related pathologies, as well as the use of anti-citrullinated protein antibody tests (ACPA) for the early diagnosis and prognosis of RA and their potential role in the pathophysiology of the disease, are discussed.

Protein arginine deiminase 4: evidence for a reverse protonation mechanism

The presumed role of an overactive protein arginine deiminase 4 (PAD4) in the pathophysiology of rheumatoid arthritis (RA) suggests that PAD4 inhibitors could be used to treat an underlying cause of RA, potentially offering a mechanism to stop further disease progression. Thus, the development of such inhibitors is of paramount importance. Toward the goal of developing such inhibitors, we initiated efforts to characterize the catalytic mechanism of PAD4 and thereby identify important mechanistic features that can be exploited for inhibitor development. Herein we report the results of mutagenesis studies as well as our efforts to characterize the initial steps of the PAD4 reaction, in particular, the protonation status of Cys645 and His471 prior to substrate binding. The results indicate that Cys645, the active site nucleophile, exists as the thiolate in the active form of the free enzyme. pH studies on PAD4 further suggest that this enzyme utilizes a reverse protonation mechanism.

Predicting factors for severity of rheumatoid arthritis: a prospective multicenter cohort study of 172 patients over 3 years

Rheumatoid arthritis may take an unfavourable course leading to rapid functional decline in a certain percentage of patients. Early identification of these patients is desirable. The aim of this study was to evaluate clinical and laboratory parameters for their value in the prediction of bad outcome. A total of 172 patients with early arthritis were followed for 3 years. Higher initial values for erythrocyte sedimentation rate, IgG and IgM rheumatoid factor, serum concentration of cartilage oligomeric matrix protein, Health Assessment Questionnaire score, Larsen score of feet, disease activity score, and swollen and tender joint count predicted worse outcome. An association with the presence of IgA rheumatoid factor or anti-cyclic-citrullinated peptide could not be established. We conclude that prognosis in an individual with rheumatoid arthritis depends on many factors. The determination of independent prognostic factors for progression of rheumatoid arthritis is a valuable tool in early arthritis to select patients for more aggressive therapy.

Pathogenic implications for autoantibodies against C-reactive protein and other acute phase proteins

Systemic lupus erythematosus (SLE) is a systemic rheumatic disease characterized clinically by multiorgan involvement and serologically by the occurrence of antinuclear antibodies. SLE patients may present with multiple autoantibodies to cytoplasmic and cell surface antigens as well as to circulating plasma proteins. Another feature of SLE is that serum levels of C-reactive protein (CRP) often remain low despite high disease activity and despite high levels of other acute phase proteins and interleukin-6, i.e. the main CRP inducing cytokine. Apart from its important role as a laboratory marker of inflammation, CRP attracts increasing interest due to its many intriguing biological functions, one of which is a role as an opsonin contributing to the elimination of apoptotic cell debris, e.g. nucleosomes, thereby preventing immunization against autoantigens. Recently, autoantibodies against CRP and other acute phase proteins have been reported in certain rheumatic conditions, including SLE. Although the presence of anti-CRP autoantibodies does not explain the failed CRP response in SLE, antibodies directed against acute phase proteins have several implications of pathogenetic interest. This paper thus highlights the biological and clinical aspects of native and monomeric CRP and anti-CRP, as well as autoantibodies against mannose-binding lectin, serum amyloid A and serum amyloid P component.

The HLA-DRB1 shared epitope alleles differ in the interaction with smoking and predisposition to antibodies to cyclic citrullinated peptide

OBJECTIVE

The HLA shared epitope (SE) alleles are primarily a risk factor for the presence of antibodies to cyclic citrullinated peptide (anti-CCP antibodies) rather than for the development of rheumatoid arthritis (RA). The SE alleles interact with the environmental risk factor tobacco exposure (TE) for predisposition to anti-CCP+ RA. The objectives of this study were to determine 1) whether different SE subtypes contribute differently to the presence of anti-CCP antibodies, 2) whether different SE subtypes all interact with TE for the development of anti-CCP antibodies, and 3) the effect of TE in relation to the SE alleles and anti-CCP antibodies on the risk of progression from undifferentiated arthritis (UA) to RA.

METHODS

We assessed the effect of SE subtypes and TE on the presence and level of anti-CCP antibodies and on the risk of progression from UA to RA in 977 patients with early arthritis who were included in the Leiden Early Arthritis Clinic.

RESULTS

The HLA-DRB1*0401, *0404, *0405, or *0408 SE alleles conferred the highest risk of developing anti-CCP antibodies (odds ratio [OR] 5.0, compared with an OR of 2.0 for the HLA-DRB1*0101 or *0102 SE alleles and an OR of 1.7 for the HLA-DRB1*1001 SE allele). Conversely, the TE-SE allele interaction was the strongest for the HLA-DRB1*0101 or *0102 SE alleles and the HLA-DRB1*1001 SE allele. TE in SE+, anti-CCP+ patients correlated with higher levels of anti-CCP antibodies and with progression from UA to RA. In logistic regression analysis, only the presence and level of anti-CCP antibodies were associated independently with RA development.

CONCLUSION

The HLA-DRB1 SE subtypes differ in their interaction with smoking and in their predisposition to anti-CCP antibodies. TE contributes to the development of RA in SE+, anti-CCP+ patients, which is explained by its effect on the level of anti-CCP antibodies.

Auto-antibodies and autoreactive T-cells in rheumatoid arthritis

Rheumatoid arthritis (RA) is not only the most severe of all joint diseases but also the most common systemic autoimmune disease affecting approximately 1% of the world-wide adult population. RA is characterized by the presence of autoantibodies in serum and synovial fluid distinguishing the disease from other arthritides such reactive arthritis or osteoarthritis. Since the historical description of rheumatoid factor (RF), which is an autoantibody directed to immunoglobulin G, numerous additional autoantibodies have been discovered in sera of RA patients. These antibodies may be directed to cartilage components, stress proteins, enzymes, nuclear proteins and, most importantly, citrullinated proteins such as fibrin or vimentin. In contrast to other antibodies including RF, anti-citrullinated protein antibodies are targeted almost exclusively by RA patients thus being the most specific serological markers of RA. Even though most other antibodies are not used for diagnostics, they may contribute to the patholophysiology of RA by forming immune complexes in the joint. Furthermore, autoreactive T-cells in serum and synovial fluid may initiate or enhance the disease process via production of proinflammatory cytokines leading to autoantibody secretion, stimulation of macrophages and activation of bone resorbing osteoclasts. Identification of novel autoantigens, particularly citrullinated proteins, and the characterization of the cellular and molecular processes underlying the autoimmune reactions against them has provided new insights into the complex pathogenesis of RA. This has made possible the development of novel therapeutic concepts that may allow to treat the disease more effectively in its early stages where the chances are highest to interrupt the deleterious processes.

Mechanisms of disease: Genetic susceptibility and environmental triggers in the development of rheumatoid arthritis

Rheumatoid arthritis (RA) is a complex disease in which environmental agents are thought to interact with genetic factors that influence susceptibility. This interaction triggers immunologic events that eventually result in the clinical signs of arthritis. Knowledge of the chain of etiological events that lead to the development of RA is incomplete. In this review, we describe the experimental approaches that are used to address the issue of gene-environment interactions in the etiology of RA, and discuss relevant examples of such interactions. We focus on how smoking, the best-known environmental risk factor for RA, interacts with HLA-DR shared epitope genes, the main genetic risk factors for RA, and result in a high risk of RA in individuals exposed to both of these risk factors. From these and other related findings, we can begin to define the distinct environmental risk factors (such as smoking) that in certain genetic contexts (for example, the presence of HLA-DR shared epitope alleles) can trigger immune reactions (such as autoantibodies to citrullinated peptides) many years before onset of RA, and consider how these immune reactions might contribute to clinical symptoms in a subset of affected patients. Increased knowledge about these and other events involved in the development of RA should enable the design of new tools for suppressing RA pathogenesis before the onset of disease.

Changes in avidity and specificity of IgG during electro-oxidation. Relevance of binding of antibodies to β2-GPI

The immune response may be changed due to altered proteins or modifications of immunoglobulins, including oxidative processes. The susceptibility to oxidative modifications depends greatly on amino-acid moiety composition due to chemical characteristics (instability) of their side-chains. Initial steps of oxidation may change the specificity and avidity of immunoglobulins due to chemical alteration of the hypervariable region. The oxidation of antibodies increases the hydrophilic nature of the paratopes and makes them more susceptible for the binding to cationic surfaces even without the strong surface-to-surface fitting. The electro-oxidation of IgG significantly changes the immunoreactivity and specificity of IgG fractions, regardless of the initial immunoreactivity to a specific autoantigen also in healthy persons. Data are presented on changes in the immunoreactivity as well as the avidity of antibodies against beta2-glycoprotein I after being exposed to direct current. ELISA measurements showed increased reactivity of anti-beta2-glycoprotein I antibodies at the beginning and various, fluctuating results after prolonged exposure to electro-oxidation. Inter-individual differences in chemical stability of immunoglobulins and patient's antioxidative status may influence the range of their alterations and their impact on health/disease balance.

Impact of Treatment with Infliximab on Anticyclic Citrullinated Peptide Antibody and Rheumatoid Factor in Patients with Rheumatoid Arthritis

OBJECTIVE

To investigate the impact of infliximab treatment on anticyclic citrullinated peptide (anti-CCP) antibody and rheumatoid factor (RF) levels in patients with rheumatoid arthritis (RA).

METHODS

Sera from 33 RA patients receiving infliximab and disease modifying antirheumatic drugs were tested for anti-CCP antibody, IgA-, IgG- and IgM-RF using a commercially available semiquantitative ELISA at baseline, 30 and 54 weeks after treatment.

RESULTS

The serum levels of anti-CCP antibody and IgA-RF decreased significantly after 30 weeks (P = 0.002 and 0.024); however, the decrease was not significant at week 54 (P = 0.147 and 0.207). The decrease in IgG-RF level was not significant at 30 and 54 weeks (P = 0.059 and 0.097). IgM-RF levels, however decreased significantly at 30 and 54 weeks (P = 0.002 and 0.004). A strong correlation between anti-CCP and IgA-, IgG- and IgM-RF was observed at baseline (r(s) = 0.48, 0.43, 0.65, P = < 0.05) and after infliximab treatment at 30 (r(s) = 0.45, 0.46, 0.62, P = < 0.05) and 54 (r(s) = 0.49, 0.45, 0.60, P = < 0.05) weeks.

CONCLUSION

Treatment with infliximab results in decreased anti-CCP antibody and IgA-RF early in the course of therapy that is not sustained. IgM-RF declines and remains decreased for at least 54 weeks. Investigations in larger cohorts of RA patients (especially early RA) with longer follow-up are needed to assess the impact of specific therapeutic interventions on anti-CCP antibody and RF levels and the relationship of their levels to disease activity.

Proteomic biomarkers for autoimmune disease

Autoimmune diseases affect 3% of the world population, yet the diagnosis and classification of autoimmune diseases remain based on clinical examination combined with traditional laboratory tests and imaging studies. The development of genomic and proteomic technologies provides an unprecedented ability to identify novel biosignatures to diagnose, classify, and guide therapeutic decision making in patients with autoimmune disease. In this article, we review recent advances in proteomics technologies and their application to autoimmune disease.

Synthesis of Overlapping Fibrin Citrullinated Peptides and their use for Diagnosing Rheumatoid Arthritis

With the aim of developing a new enzyme-linked immunosorbent assay test to detect autoantibodies in the sera of rheumatoid arthritis patients with a high sensitivity and specificity using synthetic citrullinated peptides of fibrin (which is abundant in rheumatoid synovium) as antigenic substract, peptides belonging to alpha- and beta-fibrin chains were selected by computer-aided prediction of antigenicity and epitope mapping and synthesized in solid phase. We analysed by enzyme-linked immunosorbent assay 133 sera from patients with well-characterized rheumatic diseases, including 67 patients with rheumatoid arthritis. The results of the immunoassays reported highlight the usefulness of fibrin-related peptides in rheumatoid arthritis diagnosis and, especially, the ability and specificity of the [Cit(621,627,630)]alpha-fibrin(617-631) (alpha fib617) peptide sequence to recognize the autoantibodies that are present in rheumatoid arthritis patients.

Histone citrullination by protein arginine deiminase: is arginine methylation a green light or a roadblock?

Protein citrullination, a once-obscure post-translational modification (PTM) of peptidylarginine, has recently become an area of significant interest because of its suspected role in human disease states, including rheumatoid arthritis and multiple sclerosis, and also because of its newfound role in gene regulation. One protein isozyme responsible for this modification, protein arginine deiminase 4 (PAD4), has also been proposed to "reverse" epigenetic histone modifications made by the protein arginine methyltransferases. Here, we review the in vivo and in vitro studies of transcriptional regulation by PAD4, evaluate conflicting evidence for its ability to use methylated peptidylarginine as a substrate, and highlight promising areas of future work. Understanding the interplay of multiple arginine PTMs is an emerging area of importance in health and disease and is a topic best addressed by novel tools in proteomics and chemical biology.

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Antibodies against cyclic citrullinated peptide are associated with the DRB1 shared epitope and predict joint erosion in rheumatoid arthritis

OBJECTIVE

To evaluate antibodies against cyclic citrullinated peptide (anti-CCP antibodies) for their predictive value for severe joint destruction in rheumatoid arthritis (RA) and to examine their relationship to shared epitope (SE)-positive DRB1 alleles.

METHODS

Concentrations of anti-CCP antibodies were determined in sera from 126 patients with recent onset RA who had been followed prospectively for 6 yr. Progression of joint destruction was evaluated according to Larsen by scoring radiographs from the hand and feet taken at baseline and after 1, 2, 4 and 6 yr of observation. In addition to clinical parameters, the presence of SE-positive DRB1 alleles and of rheumatoid factor IgM and IgA was determined.

RESULTS

Anti-CCP antibodies were found more frequently and in higher concentrations in both DRB1*01-positive and in DRB1*04-positive SE-positive patients compared with SE-negative patients. Severe joint destruction as defined by a Larsen score in the upper third of the study population was predicted by positivity for anti-CCP antibodies, by the presence of SE-positive DRB1*04 alleles and by the presence of erosive disease at initial presentation. Multiple logistic regression analysis revealed that SE-positive DRB1*04 alleles and anti-CCP antibodies exerted a significant influence on the progression of joint destruction.

CONCLUSION

The association of anti-CCP antibodies with DRB1*01 and with SE-positive DRB1*04 alleles implies a functional role for the SE sequence motif. The determination of SE-positive DRB1*04 alleles and of anti-CCP antibody positivity facilitates the prediction of disease course and prognosis at the time of initial presentation.

Biological therapies directed against cells in autoimmune disease

Among the cells of the immune system involved in the pathogenesis of autoimmune disease, T cells have received the most attention. The central role of these cells in several animal models of autoimmune diseases and in human disease counterparts has provided the rationale for specific therapeutic targeting of T cell subsets, especially CD4 T cells. So far, the applicability of this approach has not been clearly evident in clinical trials, which was also the case when nondepleting "coating" anti-CD4 monoclonal antibodies was used. In the past several years, experimental evidence supporting a major role of B cells in systemic autoimmune disease has grown. This includes the pathogenicity of certain autoantibodies, the potential of B cells to present antigen in the context of MHC Class II and to signal via costimulatory molecules, and to secrete proinflammatory cytokines. In some instances, engagement of the B cell receptor and other surface receptors is sufficient to stimulate B cells to produce antibodies. The depletion of B cells by targeting the surface marker CD20 has been shown to be effective in treating rheumatoid arthritis with a good side effect profile. Series of cases with other systemic autoimmune diseases indicate that this strategy may be effective in these conditions too. The clinical data add weight to the importance of B cells in the pathogenesis of autoimmune diseases.

A new model for an etiology of rheumatoid arthritis: smoking may trigger HLA-DR (shared epitope)-restricted immune reactions to autoantigens modified by citrullination

OBJECTIVE

To investigate whether smoking and HLA-DR shared epitope (SE) genes may interact in triggering immune reactions to citrulline-modified proteins.

METHODS

In a case-control study involving patients with recent-onset rheumatoid arthritis (RA), we studied interactions between a major environmental risk factor (smoking), major susceptibility genes included in the SE of HLA-DR, and the presence of the most specific autoimmunity known for RA (i.e., antibodies to proteins modified by citrullination). Immunostaining for citrullinated proteins in cells from bronchoalveolar lavage fluid was used to investigate whether smoking is associated with citrullination in the lungs.

RESULTS

Previous smoking was dose-dependently associated with occurrence of anticitrulline antibodies in RA patients. The presence of SE genes was a risk factor only for anticitrulline-positive RA, and not for anticitrulline-negative RA. A major gene-environment interaction between smoking and HLA-DR SE genes was evident for anticitrulline-positive RA, but not for anticitrulline-negative RA, and the combination of smoking history and the presence of double copies of HLA-DR SE genes increased the risk for RA 21-fold compared with the risk among nonsmokers carrying no SE genes. Positive immunostaining for citrullinated proteins was recorded in bronchoalveolar lavage cells from smokers but not in those from nonsmokers.

CONCLUSION

We identified an environmental factor, smoking, that in the context of HLA-DR SE genes may trigger RA-specific immune reactions to citrullinated proteins. These data thus suggest an etiology involving a specific genotype, an environmental provocation, and the induction of specific autoimmunity, all restricted to a distinct subset of RA.

Citrullination is an inflammation-dependent process

OBJECTIVES

To study the presence of citrullinated proteins in inflammatory conditions and in clinically non-affected tissues of controls.

METHODS

Synovial biopsy specimens from 19 patients with rheumatoid arthritis and 10 healthy controls were investigated by immunohistochemistry. Additionally, muscle tissue from 5 patients with polymyositis and from 7 healthy controls, intestinal tissue from macroscopically affected and non-affected areas from 10 patients with inflammatory bowel disease (IBD) and tonsil tissues from 4 chronically inflamed tonsils were studied.

RESULTS

Citrullinated proteins were present in all synovial biopsy specimens from patients with rheumatoid arthritis, whereas only three of 10 healthy synovial biopsy specimens showed scarce amounts of citrullination. Citrullination was also present in all myositis-affected muscles, whereas it was absent in the muscle tissues of controls. All tonsil biopsy specimens studied were positive for citrulline. Even though more frequently detected in the macroscopically affected colonic areas, no marked difference was observed in the pattern or extent of citrullination between the macroscopically affected and non-affected intestinal IBD tissues.

CONCLUSION

Citrullination is present in a wide range of inflammatory tissues, suggesting that this process is inflammation dependent rather than disease dependent.

IL-32, a proinflammatory cytokine in rheumatoid arthritis

IL-32 is a recently discovered cytokine that induces TNFalpha, IL-1beta, IL-6, and chemokines. We investigated whether IL-32 is expressed in the synovia of patients with rheumatoid arthritis (RA) and studied associations with disease severity and the presence of other cytokines. Immunohistochemistry revealed that IL-32 is highly expressed in RA synovial tissue biopsies, whereas IL-32 was not observed in synovial tissues from patients with osteoarthritis. Moreover, in synovial biopsies from 29 RA patients with active disease, the level of IL-32 staining correlated with erythrocyte sedimentation rate, a marker of systemic inflammation (R = 0.63 and P < 0.0003). Synovial staining of IL-32 also correlated with indices of synovial inflammation (R = 0.80 and P < 0.0001) as well as synovial presence of TNFalpha (R = 0.68 and P < 0.004), IL-1beta (R = 0.79 and P < 0.0001), and IL-18 (R = 0.82 and P < 0.001). IL-32 was a potent inducer of prostaglandin E(2) release in mouse macrophages and human blood monocytes, an important property for inflammation. After the injection of human IL-32gamma into the knee joints of naïve mice, joint swelling, with pronounced influx of inflammatory cells and cartilage damage, was observed. In TNFalpha-deficient mice, IL-32-driven joint swelling was absent and cell influx was markedly reduced, but loss of proteoglycan was unaffected, suggesting that IL-32 activity is, in part, TNFalpha-dependent. IL-32, strongly associated with TNFalpha, IL-1beta, and IL-18, appears to play a role in human RA and may be a novel target in autoimmune diseases.

The role of B cells and autoantibodies in rheumatoid arthritis

In this article, we will review B lymphocyte development and function, then discuss the role of B cells in RA, including immune complex formation; the K/BxN mouse model of RA; toll-like receptors; B cells as antigen presenting cells; germinal center-like structures in RA synovium; and influence on T cell activation, leukocyte infiltration, and angiogenesis. With regard to autoantibody production, we will focus on rheumatoid factor (RF) and anti-CCP antibodies, particularly mechanisms of their production; sensitivity and specificity in RA; and their roles as prognostic factors. Other autoantibodies will be discussed, as will treatment implications and future areas of investigation related to B cells and autoantibodies in RA.

The genetics of psoriasis and autoimmunity

Psoriasis is an inflammatory/autoimmune disease and, as with many autoimmune diseases, is associated with alleles from the major histocompatibility complex (MHC). With psoriasis and autoimmune disease, the penetrance of the MHC-associated alleles is never 100%, even for monozygotic twins. This may be because development requires additional environmental and/or genetic modifiers or requires specific T-cell receptor arrangements. Families segregating single or multilocus susceptibility alleles other than the MHC have also been reported. Overlapping genetic locations of loci for different autoimmune diseases have been known for several years and are starting to reveal common genes or genetic variants. These include genes normally involved in preventing spontaneous T-cell activation or proliferation, immune synapse formation, or cytokine production via pathways such as those mediated by NFkappaB and those involved in thymic selection. Autoimmunity may also involve dysregulation of genes or pathways regulated by the RUNX family of transcription factors. RUNX is involved in hematopoietic cell development, development of T cells in the thymus, chromatin remodeling, and gene silencing. Hence, its effect on cells of the immune system may be due to variable changes in gene expression and could account for variable body surface involvement and waxing and waning of disease.

Aberrant Expression of the Autoantigen Heterogeneous Nuclear Ribonucleoprotein-A2 (RA33) and Spontaneous Formation of Rheumatoid Arthritis-Associated Anti-RA33 Autoantibodies in TNF-α Transgenic Mice

Human TNF-alpha transgenic (hTNFtg) mice develop erosive arthritis closely resembling rheumatoid arthritis (RA). To investigate mechanisms leading to pathological autoimmune reactions in RA, we examined hTNFtg animals for the presence of RA-associated autoantibodies including Abs to citrullinated epitopes (anti-cyclic citrullinated peptide), heterogeneous nuclear ribonucleoprotein (hnRNP)-A2 (anti-RA33), and heat shock proteins (hsp) (anti-hsp). Although IgM anti-hsp Abs were detected in 40% of hTNFtg and control mice, IgG anti-hsp Abs were rarely seen, and anti-cyclic citrullinated peptide Abs were not seen at all. In contrast, >50% of hTNFtg mice showed IgG anti-RA33 autoantibodies, which became detectable shortly after the onset of arthritis. These Abs were predominantly directed to a short epitope, which was identical with an epitope previously described in MRL/lpr mice. Incidence of anti-RA33 was significantly decreased in mice treated with the osteoclast inhibitor osteoprotegerin and also in c-fos-deficient mice lacking osteoclasts. Pronounced expression of hnRNP-A2 and a smaller splice variant was seen in joints of hTNFtg mice, whereas expression was low in control animals. Although the closely related hnRNP-A1 was also overexpressed, autoantibodies to this protein were infrequently detected. Because expression of hnRNP-A2 in thymus, spleen, brain, and lung was similar in hTNFtg and control mice, aberrant expression appeared to be restricted to the inflamed joint. Finally, immunization of hTNFtg mice with recombinant hnRNP-A2 or a peptide harboring the major B cell epitope aggravated arthritis. These findings suggest that overproduction of TNF-alpha leads to aberrant expression of hnRNP-A2 in the rheumatoid joint and subsequently to autoimmune reactions, which may enhance the inflammatory and destructive process.

Antigen microarray profiling of autoantibodies in rheumatoid arthritis

OBJECTIVE

Because rheumatoid arthritis (RA) is a heterogeneous autoimmune disease in terms of disease manifestations, clinical outcomes, and therapeutic responses, we developed and applied a novel antigen microarray technology to identify distinct serum antibody profiles in patients with RA.

METHODS

Synovial proteome microarrays, containing 225 peptides and proteins that represent candidate and control antigens, were developed. These arrays were used to profile autoantibodies in randomly selected sera from 2 different cohorts of patients: the Stanford Arthritis Center inception cohort, comprising 18 patients with established RA and 38 controls, and the Arthritis, Rheumatism, and Aging Medical Information System cohort, comprising 58 patients with a clinical diagnosis of RA of <6 months duration. Data were analyzed using the significance analysis of microarrays algorithm, the prediction analysis of microarrays algorithm, and Cluster software.

RESULTS

Antigen microarrays demonstrated that autoreactive B cell responses targeting citrullinated epitopes were present in a subset of patients with early RA with features predictive of the development of severe RA. In contrast, autoimmune targeting of the native epitopes contained on synovial arrays, including several human cartilage gp39 peptides and type II collagen, were associated with features predictive of less severe RA.

CONCLUSION

Proteomic analysis of autoantibody reactivities provides diagnostic information and allows stratification of patients with early RA into clinically relevant disease subsets.

Cytoskeletal rearrangements in synovial fibroblasts as a novel pathophysiological determinant of modeled rheumatoid arthritis

Rheumatoid arthritis is a chronic inflammatory disease with a high prevalence and substantial socioeconomic burden. Despite intense research efforts, its aetiology and pathogenesis remain poorly understood. To identify novel genes and/or cellular pathways involved in the pathogenesis of the disease, we utilized a well-recognized tumour necrosis factor-driven animal model of this disease and performed high-throughput expression profiling with subtractive cDNA libraries and oligonucleotide microarray hybridizations, coupled with independent statistical analysis. This twin approach was validated by a number of different methods in other animal models of arthritis as well as in human patient samples, thus creating a unique list of disease modifiers of potential therapeutic value. Importantly, and through the integration of genetic linkage analysis and Gene Ontology–assisted functional discovery, we identified the gelsolin-driven synovial fibroblast cytoskeletal rearrangements as a novel pathophysiological determinant of the disease.

Rheumatoid arthritis (RA) is a chronic destructive disease that affects 1–3% of the general population, exacting substantial personal, social, and economic costs. Current treatments alleviate the symptoms and offer immediate relief for many patients but do not cure the disease. While the cause of the disease remains poorly understood, the completion of the Human Genome Project and the emergence of functional genomics and high-throughput technologies offer intriguing new possibilities. For example, expression profiling creates a molecular fingerprint of the disease status by quantifying the expression levels of thousand of genes simultaneously. Similarly, reverse genetics (the genetic modification of a particular gene in search of its function) allow for the creation of animal models of disease. To discover novel genes and/or cellular pathways involved in the development of the disease, the authors used two methods in an animal model of RA for large-scale expression profiling. They identified a large number of genes and molecular processes that are deregulated in the disease. Using this information, the authors described pathophysiologic determinants of RA and created a unique list of disease modifiers of potential therapeutic value.

The Devil in the Details: The Emerging Role of Anticitrulline Autoimmunity in Rheumatoid Arthritis

Rheumatoid arthritis is a chronic inflammatory autoimmune disease of unknown cause. The immune response against citrullinated Ags has recently become the prime suspect for disease pathogenesis. Immunity against citrullinated Ags is thought to play a pivotal role in the disease for several reasons: 1) citrullinated Ags are expressed in the target organ, the inflamed joint; 2) anti-citrullinated protein Abs are present before the disease becomes manifest; and 3) these Abs are highly specific for rheumatoid arthritis. In this review, data from clinical, genetic, biochemical, and animal studies is combined to create a profile of this remarkable autoantibody response. Moreover, a model is proposed of how the anti-citrullinated proteins response is generated and how it could eventually lead to chronic inflammation.

Rheumatoid arthritis

Therapeutic efficacy of depleting B cells or blocking T-cell costimulation in rheumatoid arthritis (RA) has confirmed the critical pathogenic role of adaptive immune responses. Yet, RA preferentially affects elderly individuals, in whom adaptive immunity to exogenous antigens begins to fail. Here, we propose that senescence of the immune system is a risk factor for RA, with chronic inflammation resulting from the accumulation of degenerate T cells that have a low threshold for activation and utilize a spectrum of novel receptors to respond to microenvironmental cues. The process of immunosenescence is accelerated in RA and precedes the onset of disease, the acceleration, in part, being conferred by the HLA-DR4 haplotype. Naive CD4(+) T cells in RA are contracted in diversity and restricted in clonal burst. Senescence of effector CD4(+) T cells is associated with the loss of CD28 and the de novo expression of KIR2DS2, NKG2D, and CX(3)CR1, all of which function as costimulatory molecules and reduce the threshold for T-cell activation. The synovial microenvironment promotes chronic persistent immune responses by facilitating ectopic lymphoid neogenesis, such as the formation of aberrant germinal centers. With the propensity to develop complex lymphoid architectures and to provide optimal activation conditions for senescent CD4(+) T cells, the synovium becomes a natural target for pathogenic immune responses in prematurely aged individuals.

What precedes development of rheumatoid arthritis?

Studies on aetiology of inflammatory diseases such as rheumatoid arthritis (RA) need to investigate the potential environmental triggers that are active before onset of disease, the genetic context in which these triggers act, and whether the presence of such triggers in an arthritis prone genetic context will give rise to the immune reactions associated with/preceding RA. Such knowledge would help not only to address much better the issue of causality of these potential triggers and the immune reactions, but also to carry out various interventions aimed at influencing the disease provoking immune events before development of clinical signs of disease. This short report summarises recent data demonstrating (a) the presence of anticitrullin antibodies or rheumatoid factors in between a third and half of patients with RA before development of clinical signs; (b) long term smoking is associated with a high risk of future development of seropositive but not seronegative RA; and (c) a strong gene-environment interaction between smoking and SE genes in the development of seropositive RA. We conclude that, in a certain genetic context, smoking is a potential trigger of RA, and a combination of the two factors is associated with the occurrence of immune reactions long before the onset of RA.

The presence of citrullinated proteins is not specific for rheumatoid synovial tissue

OBJECTIVE

Antibodies directed toward citrullinated proteins (e.g., anti-cyclic citrullinated peptide antibodies) are highly specific for rheumatoid arthritis (RA) and are produced locally at the site of inflammation. Although the presence of citrullinated proteins in rheumatoid synovium has been described in the literature, it is uncertain whether their presence is specific for RA. The present study was undertaken to investigate this.

METHODS

The local production of the anti-citrullinated protein antibodies was investigated by comparing the concentration of the antibodies (corrected for the total amount of IgG present) in paired samples of serum and synovial fluid from RA patients. The presence of citrullinated proteins in the synovial tissue was investigated by immunohistochemical analysis of synovial tissue from RA patients and from patients with other arthropathies, using a variety of specific antibodies to citrullinated proteins.

RESULTS

In RA patients, anti-citrullinated protein antibodies constituted a 1.4-fold higher proportion of IgG in synovial fluid compared with serum, which is indicative of a local production of the antibodies. Immunohistochemical staining of citrullinated proteins was observed in the lining layer, the sublining layer, and in extravascular fibrin deposits in inflamed synovial tissue from RA as well as non-RA patients.

CONCLUSION

The presence of citrullinated proteins in the inflamed synovium is not specific for RA, but rather, it may be an inflammation-associated phenomenon. The high specificity of the anti-citrullinated protein antibodies is, therefore, most likely the result of an abnormal humoral response to these proteins.

Update on autoantibodies in rheumatoid arthritis

Detection of rheumatoid factor (RF) in the serum of patients with rheumatoid arthritis (RA) was one of the first indications of autoimmunity in RA. The role of RF in the diagnosis of RA has been well-documented, but it has suboptimal sensitivity and specificity. Although patients with RF-positive RA generally have more severe disease than those with RF-negative RA, RF is not a reliable predictor of disease severity in individual patients. Multiple other autoantibodies have been found in RA, with recent interest focused on those directed at cyclic citrullinated peptides. Panels of autoantibodies may ultimately prove useful in preclinical diagnosis and prediction of clinical course in patients with RA and other forms of arthritis, and provide insights into the pathogenesis of the disease.