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

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.

Role of B cells in systemic lupus erythematosus and rheumatoid arthritis

B cell tolerance to many self-proteins is actively maintained by either purging self-reactive B receptors through clonal deletion and receptor editing, or by functional silencing known as anergy. However, these processes are clearly incomplete as B cell driven autoimmune diseases still occur. The significance of B cells in two such diseases, rheumatoid arthritis and systemic lupus erythematosus, is highlighted by the ameliorative effects of B cell depletion. It remains to be determined, however, whether the key role of the B cell in autoimmune disease is autoantibody production or another antibody-independent function.

The importance of 'Mechnikov's thorn' for an improved understanding of 21st century medicine and immunology: a view from the eye

In 1908, Ehrlich and Mechnikov shared the Nobel Prize in Medicine for their independent studies that set the scene for the modern understanding of innate and adaptive immunity. However, 20th century immunology thinking was dominated by aberrant adaptive immunity but this never adequately explained the full spectrum of inflammatory disease. This article draws on medical observations, from where immunology originated, and uses the example of the eye to illustrate how the integration of medicine and immunology leads to an improved understanding of inflammation against self. The spectrum of ocular inflammation can be viewed as either predominantly adaptive immune mediated (mostly the realm of immunology), or predominantly due to ocular tissues factors that lead to regional innate immune activation (the realm of medicine), or a variable interaction between the two. Just as the thorns that Mechnikov inserted into molluscs lead to localized innate immune activation; ocular inflammation can likewise be driven by non-immune factors that include tissue degeneration or microdamage. The present article emphasizes the importance of such factors in the initiation or phenotypic expression of ocular immunopathology allowing different immunological dogmas including self-non-self discrimination, immunological tolerance and immunoprivilege to be viewed in a different light. This scheme also leads to an appreciation of how the innate immune system may be the sole perpetuator of some ocular immunopathologies. We propose that this integrated view of medicine and immunology is crucial for understanding immunology from a translational angle and has implications far beyond ocular disease.

Insights into spatial configuration of a galactosylated epitope required to trigger arthritogenic T-cell receptors specific for the sugar moiety

The immunodominant epitope of bovine type II collagen (CII256–270) in A^q mice carries a hydroxylysine-264 linked galactose (Gal-Hyl²⁶⁴), the recognition of which is central to the development of collagen-induced arthritis. This study explores the molecular interactions involved in the engagement of T-cell receptors (TCRs) with such epitopes. Responses of three anti-CII T-cell hybridomas and clone A9.2 (all sharing close TCR sequences) to a panel of CII256–270 analogues incorporating Gal-Hyl²⁶⁴ with a modified side chain were determined. Recognition of naturally occurring CII256–270 peptides by either group of T cells depended strictly upon the presence of the carbohydrate and, more precisely, its intact HO-4 group. Modifications of primary amino group on the hydroxylysine side chain eliminated T-cell reactivity, notwithstanding the presence of the galactosyl moiety. Moderate stereochemical changes, such as altered sugar orientation and methylation at the galactose anchor position, were still permissive. Conversely, robust transformations affecting the relative positions of the key elements were detrimental to TCR recognition. To conclude, these data provide strong new experimental evidence that integrity of both galactose HO-4 and hydroxylysine side chain primary amino groups are mandatory for activation of anti-Gal-Hyl²⁶⁴ TCRs. They also indicate that there is a certain degree of TCR plasticity in peptide-TCR interactions.

Cells of the synovium in rheumatoid arthritis. Dendritic cells

Dendritic cells are the major antigen-presenting and antigen-priming cells of the immune system. We review the antigen-presenting and proinflammatory roles played by dendritic cells in the initiation of rheumatoid arthritis (RA) and atherosclerosis, which complicates RA. Various signals that promote the activation of NF-κB and the secretion of TNF and IL-1 drive the maturation of dendritic cells to prime self-specific responses, and drive the perpetuation of synovial inflammation. These signals may include genetic factors, infection, cigarette smoking, immunostimulatory DNA and oxidized low-density lipoprotein, with major involvement of autoantibodies. We propose that the pathogenesis of RA and atherosclerosis is intimately linked, with the vascular disease of RA driven by similar and simultaneous triggers to NF-κB.

Antibodies as Predictors of Complex Autoimmune Diseases

Emerging evidence has suggested environmental factors such as infections and xenobiotics and some dietary proteins and peptides in the pathogenesis of many autoimmune diseases. Considering the fact that autoantibodies can often be detected prior to the onset of a disease, in this study an enzyme immunoassay was used for measurement of antibodies against different highly purified antigens or synthetic peptides originating not only from human tissue, but also from cross-reactive epitopes of infectious agents, dietary proteins and xenobiotics. The measurement of antibodies against a panel of antigens allows for identification of patterns or antibody signatures, rather than just one or two markers of autoimmunity, thus establishing the premise for increased sensitivity and specificity of prediction, as well as positive predictive values. This panel of different autoantibodies was applied to 420 patients with different autoimmune diseases, including pernicious anemia, celiac disease, thyroiditis, lupus, rheumatoid arthritis, osteoarthritis, Addison's disease, type 1 diabetes, cardiovascular disease and autoimmunity, which are presented in this article. In all cases, the levels of these antibodies were significantly elevated in patients versus controls. Antibody patterns related to neuroautoimmune disorders, cancer, and patients with somatic hypermutation will be shown in a subsequent article. We believe that this novel 96 antigen-specific autoantibody or predictive antibody screen should be studied for its incorporation into routine medical examinations. Clinicians should be aware that the detection of antibodies should not automatically mean that a patient will definitely become ill, but would rather give a percentage of risk for autoimmune disease over subsequent months or years.

Pathogenic complement activation in collagen antibody-induced arthritis in mice requires amplification by the alternative pathway

Immune complex-induced inflammation can be mediated by the classical pathway of complement. However, using mice genetically deficient in factor B or C4, we have shown that the collagen Ab-induced model of arthritis requires the alternative pathway of complement and is not dependent on the classical pathway. We now demonstrate that collagen Ab-induced arthritis is not altered in mice genetically deficient in either C1q or mannose-binding lectins A and C, or in both C1q and mannose-binding lectins. These in vivo results prove the ability of the alternative pathway to carry out pathologic complement activation in the combined absence of intact classical and lectin pathways. C3 activation was also examined in vitro by adherent collagen-anti-collagen immune complexes using sera from normal or complement-deficient mice. These results confirm the ability of the alternative pathway to mediate immune complex-induced C3 activation when C4 or C1q, or both C1q and mannose-binding lectins, are absent. However, when all three activation pathways of complement are intact, initiation by immune complexes occurs primarily by the classical pathway. These results indicate that the alternative pathway amplification loop, with its ability to greatly enhance C3 activation, is necessary to mediate inflammatory arthritis induced by adherent immune complexes.

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.

Mutation and citrullination modifies vimentin to a novel autoantigen for rheumatoid arthritis

OBJECTIVE

Modification of antigens represents a trigger for the generation of autoantibodies. In the pathogenesis of rheumatoid arthritis (RA), citrullination of proteins has been shown to be a critical process, and the determination of antibodies against citrullinated antigens has been a diagnostic milestone. We undertook this study to determine whether antibodies to mutated and citrullinated vimentin (MCV) could serve as a diagnostic and prognostic marker for RA.

METHODS

We identified novel isoforms of human MCV in the synovial fluid of RA patients. The significance of these disease-related modifications was investigated by the analysis of autoantibody reactivities. In a group of 1,151 RA patients, the diagnostic significance and the prognostic value of an anti-MCV enzyme-linked immunosorbent assay (ELISA) were compared with that of an anti-cyclic citrullinated peptide (anti-CCP) ELISA.

RESULTS

In RA, sensitivities of 82% and 72% were calculated for the anti-MCV and anti-CCP assays, respectively. The specificity of both assays was comparable (98% and 96%, respectively). In followup analyses of 16 RA patients with moderate disease activity (mean Disease Activity Score in 28 joints [DAS28] of 2.72) and 26 RA patients with active disease (mean DAS28 of 5.07), disease stratification of RA was possible using the anti-MCV assay (P = 0.0084). A significant correlation of anti-MCV antibodies with the DAS28 was documented (r = 0.5334, P = 0.0003), in 42 RA patients (n = 427 antibody determinations at different time points).

CONCLUSION

Antigenic properties of vimentin were determined by mutation and citrullination. Anti-MCV antibodies are a novel diagnostic marker for RA. Furthermore, they may allow monitoring and-if confirmed in even larger series of patients-stratification of disease.

[Antibodies to citrullinated peptides in rheumathoid arthritis]

Rheumatoid arthritis is a systemic autoimmune disease characterized by chronic inflammation of the synovial joints leading to progressive joint destruction. The serum of these patients contains a large repertoire of autoantibodies, mainly rheumatoid factor, which is part of the ACR classification criteria in spite of having only moderate specificity. Antibodies directed to citrullinated proteins provide clinicians with a valuable tool for early diagnosis. It has been shown that these antibodies can be detected years before presentation of the first symptom and are very useful for diagnosis and prognosis, due to good sensitivity and specificity and prediction of development of erosive disease. The immune response against citrullinated antigens is characteristic of an immuno-genetic subtype of disease, in which the combined role of genes, environmental factors and autoimmunity has become the prime suspected for disease pathogenesis. A model is proposed of how these antibodies are produced and lead to chronic synovial inflammation.

A candidate gene approach identifies the TRAF1/C5 region as a risk factor for rheumatoid arthritis

BACKGROUND

Rheumatoid arthritis (RA) is a chronic autoimmune disorder affecting approximately 1% of the population. The disease results from the interplay between an individual's genetic background and unknown environmental triggers. Although human leukocyte antigens (HLAs) account for approximately 30% of the heritable risk, the identities of non-HLA genes explaining the remainder of the genetic component are largely unknown. Based on functional data in mice, we hypothesized that the immune-related genes complement component 5 (C5) and/or TNF receptor-associated factor 1 (TRAF1), located on Chromosome 9q33-34, would represent relevant candidate genes for RA. We therefore aimed to investigate whether this locus would play a role in RA.

METHODS AND FINDINGS

We performed a multitiered case-control study using 40 single-nucleotide polymorphisms (SNPs) from the TRAF1 and C5 (TRAF1/C5) region in a set of 290 RA patients and 254 unaffected participants (controls) of Dutch origin. Stepwise replication of significant SNPs was performed in three independent sample sets from the Netherlands (ncases/controls = 454/270), Sweden (ncases/controls = 1,500/1,000) and US (ncases/controls = 475/475). We observed a significant association (p < 0.05) of SNPs located in a haplotype block that encompasses a 65 kb region including the 3' end of C5 as well as TRAF1. A sliding window analysis revealed an association peak at an intergenic region located approximately 10 kb from both C5 and TRAF1. This peak, defined by SNP14/rs10818488, was confirmed in a total of 2,719 RA patients and 1,999 controls (odds ratiocommon = 1.28, 95% confidence interval 1.17-1.39, pcombined = 1.40 x 10(-8)) with a population-attributable risk of 6.1%. The A (minor susceptibility) allele of this SNP also significantly correlates with increased disease progression as determined by radiographic damage over time in RA patients (p = 0.008).

CONCLUSIONS

Using a candidate-gene approach we have identified a novel genetic risk factor for RA. Our findings indicate that a polymorphism in the TRAF1/C5 region increases the susceptibility to and severity of RA, possibly by influencing the structure, function, and/or expression levels of TRAF1 and/or C5.

B cells in rheumatoid arthritis

Though its etiology remains unknown thus far, the role that autoimmune processes play in rheumatoid arthritis (RA) pathogenesis has been widely proven. Given the easier accessibility of humoral components, the first feature of this contribution to be recognized has been the occurrence of the so-called rheumatoid factor in a large proportion of RA patients. This antibody recognizes the Fc portion of human IgG. By investigating RA pathologic processes and also through experimental models where immune complexes play a fundamental role, many other autoantibodies have then come to our knowledge to be associated with the disease. Their presence and persistence implies that clones of autoreactive B cells survive and proliferate in RA patients under a continuous stimulation. Whether this is a mechanism of disease initiation or just an epiphenomenon is still unclear but no doubt exists that autoantibodies represent a very useful tool in both diagnostic and prognostic terms. Being much more than simple autoantibody producers, B cells are able to secrete many important cytokines and to efficiently present antigens to T lymphocytes in the synovial environment. All of these functions are essential in the development of RA, and lately have claimed attention as B cell depletion has become a common and effective strategy of treatment in RA.

Infection and musculoskeletal conditions: Viral causes of arthritis

Several viruses cause postinfectious arthritis. The disease is a typical manifestation of arthritogenic alphaviruses, rubella virus and human parvovirus B19. In addition, arthritis is not uncommon after infection by HIV, cytomegalovirus, hepatitis B virus, hepatitis C virus, or Epstein-Barr virus (EBV). Also prolonged arthritis may result from viral infections, particularly with alphaviruses and human parvovirus B19. Viruses such as EBV and B19 may have significant roles in initiating chronic arthropathies, which in some cases may be indistinguishable from rheumatoid arthritis.

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.

The potential for induction of autoimmune disease by a randomly-mutated self-antigen

The pathology of most autoimmune diseases is well described. However, the exact event that triggers the onset of the inflammatory cascade leading to disease is less certain and most autoimmune diseases are complex idiopathic diseases with no single gene known to be causative. In many cases, a relation to an infectious disease is described, and it is thought that microbes can play a direct role in induction of autoimmunity, for instance by molecular mimicry or bystander activation of autoreactive T cells. In contrast, less attention has been given to the possibility that modified self-antigens can be immunogenic and lead to autoimmunity against wildtype self-antigens. In theory, modified self-antigens can arise by random errors and mutations during protein synthesis and would be recognized as foreign antigens by naïve B and T lymphocytes. Here, it is postulated that the initial auto-antigen is not a germline self-antigen, but rather a mutated self-antigen. This mutated self-antigen might interfere with peripheral tolerance if presented to the immune system during an infection. The infection lead to bystander activation of naïve T and B cells with specificity for mutated self-antigen and this can lead to epitopespreading in which T and B cells with specificity for wildtype self-antigens are activated as a result of general inflammation.

Alternative Complement Pathway Activation Is Essential for Inflammation and Joint Destruction in the Passive Transfer Model of Collagen-Induced Arthritis

Activation of each complement initiation pathway (classical, alternative, and lectin) can lead to the generation of bioactive fragments with resulting inflammation in target organs. The objective of the current study was to determine the role of specific complement activation pathways in the pathogenesis of experimental anti-type II collagen mAb-passive transfer arthritis. C57BL/6 mice were used that were genetically deficient in either the alternative pathway protein factor B (Bf(-/-)) or in the classical pathway component C4 (C4(-/-)). Clinical disease activity was markedly decreased in Bf(-/-) compared with wild-type (WT) mice (0.5 +/- 0.22 (n = 6) in Bf(-/-) vs 8.83 +/- 0.41 (n = 6) in WT mice (p < 0.0001)). Disease activity scores were not different between C4(-/-) and WT mice. Analyses of joints showed that C3 deposition, inflammation, pannus, cartilage, and bone damage scores were all significantly less in Bf(-/-) as compared with WT mice. There were significant decreases in mRNA levels of C3, C4, CR2, CR3, C3aR, and C5aR in the knees of Bf(-/-) as compared with C4(-/-) and WT mice with arthritis; mRNA levels for complement regulatory proteins did not differ between the three strains. These results indicate that the alternative pathway is absolutely required for the induction of arthritis following injection of anti-collagen Abs. The mechanisms by which these target organ-specific mAbs bypass the requirements for engagement of the classical pathway remain to be defined but do not appear to involve a lack of alternative pathway regulatory proteins.

HLA and RA revisited: citrullinated food for the SE hypothesis, the DR6 effect, and NIMA

An obvious way to unravel the apparently complex association between human leukocyte antigen (HLA) and rheumatoid arthritis (RA) is to reduce the heterogeneity of this multifactorial disease. Recently we have discovered that shared epitope (SE)-positive HLA-DRB1 alleles are exclusively associated with a subgroup of RA patients that test positive for auto-antibodies against cyclic citrullinated peptides. Further studies suggested that SE-positive alleles are classical immune response genes for the development of these antibodies. On the basis of these and other data we formulated a two-hit model for the pathogenesis of RA which incorporates a novel "citrullinated" SE hypothesis. About 5 years ago Zanelli et al. reported that HLA-DR6 (*1301 and *1302) and some other DR alleles that share the DERAA-sequence on amino acids 70-74 of their third hypervariable region are associated with protection from (severe) RA. Recently we corroborated these data in a large prospective cohort study and demonstrated that protection was observed both in the presence and in the absence of a SE susceptibility allele on the other haplotype. Finally we review the state of the art of the association of noninherited maternal HLA antigens with both susceptibility to and protection from RA.

A gut feeling for joint inflammation - using coeliac disease to understand rheumatoid arthritis

Major advances have been made in the molecular understanding of coeliac disease, initiated by the identification of intestinal gluten-reactive T cells. It is now clear that this common intestinal disorder, which is precipitated by the ingestion of wheat gluten, is mediated by DQ2-restricted T cells specific for gluten peptides modified by transglutaminase 2, the same enzyme that is targeted by disease-specific autoantibodies. Interestingly, many of the important features identified in coeliac disease, including HLA association, target organ T-cell infiltration, disease-specific autoantibodies and the distinct targeting of in vivo modified antigens, are also present in rheumatoid arthritis. The experiences from coeliac disease should therefore help identify disease-relevant T-cell epitopes in rheumatoid arthritis.