Pathogenic role of antibodies to citrullinated proteins in rheumatoid arthritis

Amelioration of autoimmunity and inflammation by zinc oxide nanoparticles in experimental rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease that affects the lining of the synovial joints and approximately affects 0.5 - 1% of the total population imposing a socioeconomic burden. The current study aimed at investigating the novel possible beneficial effects of using zinc oxide nanoparticles (ZnO NPs) on such devastating disease. The complete Freund's adjuvant (CFA) model was used to mimic RA in rats where ZnO NPs were given orally (2 mg/kg/day) daily for 14 days; and diclofenac Na, the standard drug, was given intraperitoneally (1 mg/kg/day) the day after CFA, daily for 14 days. Our results displayed that ZnO NPs attenuated adjuvant-induced increased production of inflammatory mediators interleukin-1β (IL-1β), tumor necrosis factor alpha (TNF-α), interleukin-10 (IL-10), and total leukocyte count. Besides, they ameliorated autoimmunity through suppression of anti-citrullinated protein auto antibodies (anti-CCP) levels in rats. In conclusion our results highlight the benefits which could be obtained of nanoparticles either alone or in combination with the known anti-arthritic and/or anti-inflammatory agents, giving rise to new protocols to maximize the control of RA.

Moving target: shifting the focus to pulmonary sarcoidosis as an autoimmune spectrum disorder

Despite more than a century of research, the causative agent(s) in sarcoidosis, a heterogeneous granulomatous disorder mainly affecting the lungs, remain(s) elusive. Following identification of genetic factors underlying different clinical phenotypes, increased understanding of CD4⁺ T-cell immunology, which is believed to be central to sarcoid pathogenesis, as well as the role of B-cells and other cells bridging innate and adaptive immunity, contributes to novel insights into the mechanistic pathways influencing disease resolution or chronicity. Hopefully, new perspectives and state-of-the-art technology will help to shed light on the still-elusive enigma of sarcoid aetiology. This perspective article highlights a number of recent advances in the search for antigenic targets in sarcoidosis, as well as the main arguments for sarcoidosis as a spectrum of autoimmune conditions, either as a result of an external (microbial) trigger and/or due to defective control mechanisms regulating the balance between T-cell activation and inhibition.

Antibodies to Cyclic Citrullinated Peptides in Patients With Juvenile Idiopathic Arthritis and Patients With Rheumatoid Arthritis: Shared Expression of the Inherently Autoreactive 9G4 Idiotype

OBJECTIVE

Antibodies to cyclic citrullinated peptides (anti-CCP) in rheumatoid arthritis (RA) can express the inherently autoreactive gene V_H 4-34, detected using the rat monoclonal antibody 9G4. Patients with the polyarticular subtype of juvenile idiopathic arthritis (JIA) share some but not all of the features of adult patients with RA. This study was undertaken to compare serologic findings for rheumatoid factor (RF), anti-CCP, and 9G4-expressing anti-CCP in a large JIA cohort with a cohort of adult RA patients.

METHODS

Serum from 88 patients with polyarticular JIA, 29 patients with enthesitis-related arthritis, 38 patients with extended oligoarthritis, 31 adolescent controls, 35 patients with RA, and 30 adult controls were tested for RF, for IgG, IgA, and IgM anti-CCP, and for 9G4-expressing anti-CCP by enzyme-linked immunosorbent assay. Total serum 9G4-positive IgM was also measured.

RESULTS

Of 65 patients with RF-negative polyarticular JIA, 4 (6.2%) were IgG anti-CCP positive. Sera from 20 of 23 patients with RF-positive polyarticular JIA (87.0%), 24 of 35 patients with RA (68.6%), and 1 patient with extended oligoarthritis contained IgG anti-CCP. IgA and IgM anti-CCP levels were lower in the adolescent group (P < 0.01). Levels of 9G4-expressing anti-CCP were higher in patients with RF-positive polyarticular JIA than in those with RF-negative polyarticular JIA (P < 0.0001). Median levels of 9G4-expressing anti-CCP in patients with RF-positive polyarticular JIA and those with RA did not differ. Expression of 9G4 on serum total IgM was greater in patients with RF-positive polyarticular JIA than other adolescent groups (P < 0.01), but similar to adult RF-positive RA.

CONCLUSION

In healthy individuals, 9G4-positive B cells comprise 5-10% of the peripheral blood pool but serum immunoglobulins utilizing V_H 4-34 are disproportionately low. The idiotope recognized by 9G4 was detected on anti-CCP antibodies in >80% of patients with RF-positive polyarticular JIA. V_H 4-34 usage by anti-CCP in both JIA and RA patients suggest elicitation of these autoantibodies through shared pathogenic B cell selection processes.

Rheumatoid arthritis onset in postmenopausal women: Does the ACPA seropositive subset result from genetic effects, estrogen deficiency, skewed profile of CD4(+) T-cells, and their interactions?

Rheumatoid arthritis (RA) incidence displays a differentiated age-dependent female-to-male ratio in which women outnumber men. Evidence that the peak incidence of RA in women coincides with menopause age, suggests a potential estrogenic role to disease etiology. Estrogens exert physiologically both stimulatory and inhibitory effects on the immune system. Epidemiologic and animal model studies with estrogen deprivation or supplementation suggested estrogens as to play, mainly, a protective role in RA immunopathology. In this review, we propose that some yet unidentified disturbances associated with estrogen circulating levels, differentiated by the menopausal status, play a major role in women's RA susceptibility. We focus on the interaction between estrogen deprivation and genetic risk alleles for anti-citrullinated protein antibodies (ACPA) seropositive RA, as a major driving force for increased immune reactivity and RA susceptibility, in postmenopausal women. This opens up new fields for research concerning the association among different irregular estrogenic conditions, the cytokine milieu, and age/menopausal status bias in RA.

Rheumatoid Arthritis is Associated with IgG Antibodies to Human Endogenous Retrovirus Gag Matrix: A Potential Pathogenic Mechanism of Disease?

Objective.

Human endogenous retrovirus (HERV)-K10 has been implicated in the etiology and pathogenesis of rheumatoid arthritis (RA). A secondary immune response to this virus might suggest an antigen-driven response in patients. The Gag region of HERV-K10 could provide a key epitope important for immunological reactivity. We investigated the presence of IgG antibodies to this region and assessed its significance in RA.

Methods.

We determined an antigenic peptide on the matrix segment of HERV-K10 and developed an ELISA system to detect IgG antibodies in patients with RA and controls. The presence of antibodies to the matrix peptide (denoted as MAG1: RIGKELKQAGRKGNI) was correlated with patient details.

Results.

On screening patients’ serum, we found a significantly higher mean IgG antibody response to MAG1 in 30 patients with RA as compared to 23 patients with inflammatory bowel disease (p = 0.003), 29 patients with osteoarthritis (p = 0.001), and 43 healthy individuals (p = 0.002). Reactivity was not observed to a control peptide possessing a nonhomologous amino acid sequence. On evaluating clinical details with serological activity, no correlation with disease duration (p = 0.128), sex (p = 0.768), or rheumatoid factor status (p = 0.576) was found.

Conclusion.

A significantly elevated IgG antibody response to an HERV-K10 Gag matrix peptide was observed in patients with RA, suggesting that the exposure of HERV-K10 may cause a secondary, antigenic driven immune response in RA.

Screening citrullinated proteins in synovial tissues of rheumatoid arthritis using 2-dimensional western blotting

OBJECTIVE

Citrullination, a reaction converting arginine residue into citrulline residue, is essential for autoimmunity of rheumatoid arthritis (RA). We conducted 2-dimensional Western blot analyses (2-D WB) to screen for novel citrullinated proteins in synovial tissues from patients with RA.

METHODS

Total proteins were extracted from the synovial membranes of patients with RA (n = 10) and pooled. Four identical 2-D electrophoresis (2-DE) gels were prepared, and 2 gels were transblotted to polyvinylidene fluoride membranes that were separately probed with sera from patients with RA (n = 10) or an anticitrulline antibody. The protein profiles of the 2-DE gels were compared with the hybridization results on a global level. The immunoreactive protein spots were collected from the 2-DE gels and identified using mass spectrometry. Proteins that were detected by both RA sera and anticitrulline antibody were considered citrullinated proteins. The result was confirmed through routine WB, immunoprecipitation, and ELISA. Autoantibodies against these potential antigens were also examined in the blood of patients with RA by ELISA.

RESULTS

RA sera and the anticitrulline antibody on 2-D WB detected α-1-antitrypsin (A1AT), dynein heavy-chain 3, fibrinogen β chain, keratin type II cuticular Hb4 (KRT84), lumican, tubulin β-chain (TUBB), and vimentin. A1AT, KRT84, and TUBB had high expression in the synovial membranes (n = 5) of patients with RA and A1AT and KRT84 had high expression in RA synovial fluids (n = 40). A1AT, KRT84, and TUBB immunoprecipitated from synovial tissues showed citrullination. A high level of autoantibodies against KRT84 was detected in the blood of patients with RA (n = 92) compared to that of healthy controls (n = 92).

CONCLUSION

Our study identified some new citrullinated proteins in RA synovial tissues using 2-D WB.

B cells as effectors and regulators of sex-biased arthritis

B cells have been implicated both with pathogenic as well as protective capabilities in induction and regulation of autoimmune diseases. Rheumatoid arthritis (RA) is an autoimmune disease that occurs more often in women than men. A significant role of B cells as antibody producing and antigen-presenting cells has been demonstrated in RA. Predisposition to RA is associated with the presence of certain HLA class II alleles that share sequences with DRB1*0401. To determine the role of HLA genes and B cells in vivo, we have generated transgenic mice carrying HLA genes, DRB1*0401 and DQ8, known to be associated with susceptibility to RA. Humanized mice can be induced to develop arthritis that mimics human disease in clinical, histopathological and sex bias. Effect of hormones on immune cells and their function has been described in humans and mice and has been suggested to be the major reason for female bias of autoimmune diseases. An immune response to an antigen requires presentation by HLA molecules thus suggesting a critical role of MHC in combination with sex hormones in susceptibility to develop rheumatoid arthritis. Based on our observations, we hypothesize that modulation of B cells by estrogen, presentation of modified antigens by DR4 and production of antigen-specific B cell modulating cytokines leads to autoreactivity in females. These data suggest that considering patient's sex may be crucial in selecting the optimal treatment strategy. Humanized mice expressing RA susceptible and resistant haplotype provide a means to investigate mechanism sex-bias of arthritis and future strategies for therapy.

Neuronal PAD4 expression and protein citrullination: possible role in production of autoantibodies associated with neurodegenerative disease

Peptidyl arginine deiminases (PADs) catalyze a post-translational protein modification reaction called citrullination, where arginine is converted to citrulline. This modification has been linked to the pathogenesis of autoimmune diseases including rheumatoid arthritis (RA). More recently, several studies have suggested that Alzheimer's disease (AD), a devastating neurodegenerative disorder, may have an autoimmune component. In the present study, we have investigated the possibility that expression of PADs and protein citrullination plays a role in the production of brain-reactive autoantibodies that may contribute to Alzheimer's-related brain pathology. Here, we report the selective expression of the PAD isoforms, PAD2 and PAD4, in astrocytes and neurons, respectively, and the concomitant accumulation of citrullinated proteins within PAD4-expressing cells, including neurons of the hippocampus and cerebral cortex. Expression of PADs and citrullinated proteins is prominent in brain regions engaged in neurodegenerative changes typical for AD pathology. Furthermore, we also demonstrate that the pentatricopeptide repeat domain2 (PTCD2) protein, an antigen target of a prominent AD diagnostic autoantibody, is present in a citrullinated form in AD brains. Our results suggest that disease-associated neuronal loss results in the release of cellular contents, including citrullinated proteins, into the brain interstitium. We propose that these citrullinated proteins and their degradation fragments enter into the blood and lymphatic circulation, and some are capable of eliciting an immune response that results in the production of autoantibodies. The long-term and progressive nature of AD and other neurodegenerative diseases results in chronic exposure of the immune system to these citrullinated products and may drive the continual production of autoantibodies.

Immunization with Porphyromonas gingivalis enolase induces autoimmunity to mammalian α-enolase and arthritis in DR4-IE-transgenic mice

OBJECTIVE

To examine the hypothesis that the subset of rheumatoid arthritis (RA) characterized by antibodies to citrullinated α-enolase is mediated by Porphyromonas gingivalis enolase in the context of DR4 alleles.

METHODS

Recombinant human α-enolase and P gingivalis enolase, either citrullinated or uncitrullinated, were used to immunize DR4-IE-transgenic mice and control mice (class II major histocompatibility complex-deficient [class II MHC(-/-)] and C57BL/6 wild-type mice). Arthritis was quantified by measurement of ankle swelling in the hind paws and histologic examination. Serum IgG reactivity with α-enolase and citrullinated α-enolase was assayed by Western blotting and enzyme-linked immunosorbent assay (ELISA). Antibodies to peptide 1 of citrullinated α-enolase (CEP-1) and its arginine-bearing control peptide, REP-1, were also assessed by ELISA.

RESULTS

Significant hind-ankle swelling (≥0.3 mm) occurred in DR4-IE-transgenic mice immunized with citrullinated human α-enolase (9 of 12 mice), uncitrullinated human α-enolase (9 of 12 mice), citrullinated P gingivalis enolase (6 of 6 mice), and uncitrullinated P gingivalis enolase (6 of 6 mice). Swelling peaked on day 24. None of the control groups developed arthritis. The arthritic joints showed synovial hyperplasia and erosions, but there was a paucity of leukocyte infiltration. Antibodies to human α-enolase, both citrullinated and unmodified, and to CEP-1 and REP-1 were detectable in all immunized mice except the class II MHC(-/-) control mice.

CONCLUSION

This is the first animal model that links an immune response to P gingivalis enolase to an important subset of RA, defined by antibodies to citrullinated α-enolase in the context of DR4. The fact that arthritis and anti-CEP-1 antibodies were induced independent of citrullination of the immunizing antigen suggests that the unmodified form of α-enolase may be important in initiating the corresponding subset of human RA.

Citrullinated collagens in the pathogenesis of rheumatoid arthritis

Citrulline formation through the deimination of arginine residues has recently been identified as an enzymatic post-translational modification of proteins that could be related to breaking the tolerance to self-antigens in the development of rheumatoid arthritis. Autoantibodies recognizing citrullinated peptides are highly specific indicators of early rheumatoid arthritis. However, current knowledge of the natural proteins that are citrullinated does not explain the main manifestations of the disease, that is, progressive damage to cartilage and bone - tissues rich in extracellular matrix. Several collagen types are present in the joint, where they provide the structural scaffold of the tissues. Recent findings indicate that both the ubiquitous Type I collagen and the cartilage-specific Type II collagen can be enzymatically citrullinated in vitro. Synthetic citrullinated peptides related to collagen sequences can be used as antigens. Autoantibodies to these citrullinated forms are found in patients with early rheumatoid arthritis. This review summarizes what is known regarding the occurrence of citrulline in a major class of extracellular matrix proteins, the fibrillar collagens and the significance of autoantibodies to citrulline in these proteins.

Mechanism by which HLA-DR4 regulates sex-bias of arthritis in humanized mice

HLA class II allele DRB1*0401 is associated with predisposition to Rheumatoid Arthritis in humans as well as collagen-induced arthritis in mice. Predominantly females develop arthritis in humans and DR4 transgenic mice; however the mechanism of sex-bias is still unknown. We have investigated the molecular basis by which DR4 is associated with sex-bias of arthritis. Here we show that differential antigen-specific immune mechanisms in DR4 male and female mice lead to increased susceptibility in female mice. B cells are hyperactive and present DR-restricted peptides robustly in females compared to males. Antigen-specific response showed that females produced B cell modulating cytokines like IL-13 while males produced IFNgamma. Male transgenic mice have higher number of T and B regulatory cells. An exogenous supply of 17beta estradiol in male mice led to enhanced expression of DR4 and antigen-specific response to DR4-restricted peptides. On the other hand, castration increased the incidence of arthritis. We propose that sex-bias in arthritis involves B cells and presentation of antigen by HLA-DR4 leading to activation of autoreactive cells and autoantibodies production in females, while regulatory B cells in males protect them from pathogenesis. The transgenic mice expressing RA susceptible haplotype simulate human RA and may be valuable to study gender differences observed in patients.

Epitope spreading in animal models: array of hope in rheumatoid arthritis and multiple sclerosis

The paradigm for pathogenic autoimmunity is the generation of high-titre, affinity-matured autoantibodies to a restricted family of autoantigens, in the appropriate genetic context. Genetic determinants of autoimmunity are largely found within the major histocompatibility complex (MHC) and the 'genotype to serotype to phenotype' concept is supported in a number of autoimmune diseases, where both genotype and serotype are well established. The serotype is autoantigen-driven, with evidence of epitope spreading as the disease evolves from asymptomatic to pathogenic autoimmunity. In rheumatoid arthritis and multiple sclerosis, where the autoantigens are poorly characterised, the use of an array in animal models may produce a hint of what happens in human disease. A more complete picture will be obtained from animals transgenic for human MHC, immunised with known human autoantigens.

Evolutionarily conserved antigens in autoimmune disease: implications for an infective aetiology

The immune system has evolved to eliminate or inactivate infectious organisms. An inappropriate response against self-components (autoantigens) can result in autoimmune disease. Here we examine the hypothesis that some evolutionarily conserved proteins, present in pathogenic and commensal organisms and their hosts, provide the stimulus that initiates autoimmune disease in susceptible individuals. We focus on seven autoantigens, of which at least four, glutamate decarboxylase, pyruvate dehydrogenase, histidyl-tRNA synthetase and alpha enolase, have orthologs in bacteria. Citrullinated alpha-enolase, a target for autoantibodies in 40% of patients with rheumatoid arthritis, is our main example. The major epitope is highly conserved, with over 90% identity to human in some bacteria. We propose that this reactivity of autoantibodies to shared sequences provides a model of autoimmunity in rheumatoid arthritis, which may well extend to other autoimmune disease in humans.

Synovial fluid is a site of citrullination of autoantigens in inflammatory arthritis

OBJECTIVE

To examine synovial fluid as a site for generating citrullinated antigens, including the candidate autoantigen citrullinated alpha-enolase, in rheumatoid arthritis (RA).

METHODS

Synovial fluid was obtained from 20 patients with RA, 20 patients with spondylarthritides (SpA), and 20 patients with osteoarthritis (OA). Samples were resolved using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by staining with Coomassie blue and immunoblotting for citrullinated proteins, alpha-enolase, and the deiminating enzymes peptidylarginine deiminase type 2 (PAD-2) and PAD-4. Proteins from an RA synovial fluid sample were separated by 2-dimensional electrophoresis, and each protein was identified by immunoblotting and mass spectrometry. Antibodies to citrullinated alpha-enolase peptide 1 (CEP-1) and cyclic citrullinated peptide 2 were measured by enzyme-linked immunosorbent assay.

RESULTS

Citrullinated polypeptides were detected in the synovial fluid from patients with RA and patients with SpA, but not in OA samples. Alpha-enolase was detected in all of the samples, with mean levels of 6.4 ng/microl in RA samples, 4.3 ng/microl in SpA samples, and <0.9 ng/microl in OA samples. Two-dimensional electrophoresis provided evidence that the alpha-enolase was citrullinated in RA synovial fluid. The citrullinating enzyme PAD-4 was detected in samples from all 3 disease groups. PAD-2 was detected in 18 of the RA samples, in 16 of the SpA samples, and in none of the OA samples. Antibodies to CEP-1 were found in 12 of the RA samples (60%), in none of the SpA samples, and in 1 OA sample.

CONCLUSION

These results highlight the importance of synovial fluid for the expression of citrullinated autoantigens in inflammatory arthritis. Whereas the expression of citrullinated proteins is a product of inflammation, the antibody response remains specific for RA.

Autoantibodies binding to citrullinated telopeptide of type II collagen and to cyclic citrullinated peptides predict synergistically the development of seropositive rheumatoid arthritis

OBJECTIVES

To find out whether autoantibodies to citrullinated telopeptides of type I and II collagens and to cyclic citrullinated peptides (CCPs) predict the development of rheumatoid arthritis (RA).

METHODS

A case-control study (matched for sex, age and municipality) was nested within a Finnish cohort of 19072 adults who had neither arthritis nor a history of it at the baseline examination during 1973-7. 124 subjects developed RA by late 1989, and of these, 89 were positive for rheumatoid factor (RF). Preillness serum specimens were analysed for autoantibodies against arginine (A)- or citrulline (C)-containing synthetic telopeptides using a chemiluminescence method and for anti-CCPs Mark2 with an enzyme-linked immunosorbent assay method.

RESULTS

The mean levels of autoantibodies to citrulline-containing telopeptides and the C/A ratios of type I and II collagens and to CCP were higher in subjects who later developed RF-positive RA. In the highest tertiles of C/A (I), C/A (II) ratios and anti-CCPs levels, the relative risk of RF-positive RA was significantly increased. In the multifactorial model, only anti-CCPs retained its statistical significance. However, the interaction term of C/A (II) ratio and anti-CCPs proved to be statistically significant (p = 0.02). The subjects ranked into the highest tertiles of both C/A (II) ratio and anti-CCPs had an odds ratio of 20.06 (95% confidence interval, 4.37 to 92.06) of developing RF-positive RA compared with those in the lowest tertiles of these antibodies. None of the autoantibodies predicted RF-negative RA.

CONCLUSION

Autoantibodies to citrullinated telopeptides of type I and II collagen and to CCPs exert a synergistic effect on the risk of seropositive RA.