Autoantibodies to citrullinated proteins in rheumatoid arthritis: clinical performance and biochemical aspects of an RA-specific marker

Role of Liquid Biopsies in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease caused by genetic and environmental factors. Early diagnosis is crucial for effective therapy and prognosis of RA, while biomarkers play important roles in early diagnosis. Traditional laboratory tests include rheumatoid factor, anti-cyclic citrullinated peptide antibody, which are inadequate in the ability of early diagnosis. Liquid biopsy technology is a technique using biomarkers found in the blood, urine, and other biological samples from patients, including DNA, RNA, exosome, etc. Evidence indicates that these biomarkers are involved in pathological and physiological conditions of RA. We reviewed the effects of liquid biopsy technology in the early diagnosis of RA and may provide new ideas for effective and precise treatment.

Improved classification of rheumatoid arthritis with a score including anti-acetylated ornithine antibodies

The presence of rheumatoid factor (RF) or anti-cyclic citrullinated peptide (anti-CCP) autoantibodies contributes to the current rheumatoid arthritis (RA) classification criteria. These criteria involve stratification on antibody levels, which limits reproducibility, and underperform in the RA patients without RF and anti-CCP. Here, we have explored if two anti-acetylated peptide antibodies (AAPA), anti-acetylated lysine (AcLys) and anti-acetylated ornithine (AcOrn), could improve the performance of the current criteria. The analysis was done in 1062 prospectively-followed early arthritis (EA) patients. The anti-AcOrn were more informative than the anti-AcLys, the conventional RA antibodies and the anti-carbamylated protein antibodies. The anti-AcOrn produced a classification that did not require antibody levels and showed improved specificity (77.6% vs. 72.6%, p = 0.003) and accuracy (79.0% vs. 75.8%, p = 0.002) over the current criteria. These improvements were obtained with a scoring system that values concordance between anti-AcOrn, RF and anti-CCP. No significant gain was obtained in sensitivity (80.2% vs. 78.8%, p = 0.25) or in improving the classification of the RA patients lacking RF and anti-CCP, although the anti-AcOrn ranked first among the analysed new antibodies. Therefore, the anti-AcOrn antibodies could contribute to the improvement of RA classification criteria by exploiting antibody concordance.

Post-translational modifications such as citrullination are excellent targets for cancer therapy

Under conditions of cellular stress, proteins can be post-translationally modified causing them to be recognized by the immune system. One such stress-induced post-translational modification (siPTM) is citrullination, the conversion of arginine residues to citrulline by peptidylarginine deiminase (PAD) enzymes. PAD enzymes are activated by millimolar concentrations of calcium which can occur during apoptosis, leading to precipitation of proteins, their subsequent uptake by B cells and stimulation of antibody responses. Detection of anti-citrullinated protein antibodies (ACPAs) is a diagnostic of rheumatoid arthritis (RA), where immune complexes stimulate inflammation around the joints. More recently, autophagy has been shown to play a role in the presentation of citrullinated peptides on MHC class II molecules to CD4⁺ helper T cells, suggesting that citrullination may be a way of alerting immune cells to cellular stress. Additionally, inflammation-induced IFNγ and concomitant MHC class II expression on target cells contributes to immune activation. Stressful conditions in the tumor microenvironment induce autophagy in cancer cells as a pro-survival mechanism. Cancer cells also over express PAD enzymes and in light of this the hypothesis that citrullinated peptides stimulate CD4⁺ T cell responses that would recognize these siPTM's produced during autophagy has been investigated. The induction of potent citrullinated peptide-specific CD4 responses has been shown in both humans and HLA transgenic mouse models. Responses in mouse models resulted in potent anti-tumour responses against tumours expressing either constitutive or IFNγ-inducible MHC class II. The anti-tumour effect relied upon direct recognition of tumours by specific CD4 T cells suggesting that citrullinated peptides are attractive targets for cancer vaccines.

An Overview of the Intrinsic Role of Citrullination in Autoimmune Disorders

A protein undergoes many types of posttranslation modification. Citrullination is one of these modifications, where an arginine amino acid is converted to a citrulline amino acid. This process depends on catalytic enzymes such as peptidylarginine deiminase enzymes (PADs). This modification leads to a charge shift, which affects the protein structure, protein-protein interactions, and hydrogen bond formation, and it may cause protein denaturation. The irreversible citrullination reaction is not limited to a specific protein, cell, or tissue. It can target a wide range of proteins in the cell membrane, cytoplasm, nucleus, and mitochondria. Citrullination is a normal reaction during cell death. Apoptosis is normally accompanied with a clearance process via scavenger cells. A defect in the clearance system either in terms of efficiency or capacity may occur due to massive cell death, which may result in the accumulation and leakage of PAD enzymes and the citrullinated peptide from the necrotized cell which could be recognized by the immune system, where the immunological tolerance will be avoided and the autoimmune disorders will be subsequently triggered. The induction of autoimmune responses, autoantibody production, and cytokines involved in the major autoimmune diseases will be discussed.

Probing the Roles of Calcium-Binding Sites during the Folding of Human Peptidylarginine Deiminase 4

Our recent studies of peptidylarginine deiminase 4 (PAD4) demonstrate that its non-catalytic Ca²⁺-binding sites play a crucial role in the assembly of the correct geometry of the enzyme. Here, we examined the folding mechanism of PAD4 and the role of Ca²⁺ ions in the folding pathway. Multiple mutations were introduced into the calcium-binding sites, and these mutants were termed the Ca1_site, Ca2_site, Ca3_site, Ca4_site and Ca5_site mutants. Our data indicate that during the unfolding process, the PAD4 dimer first dissociates into monomers, and the monomers then undergo a three-state denaturation process via an intermediate state formation. In addition, Ca²⁺ ions assist in stabilizing the folding intermediate, particularly through binding to the Ca3_site and Ca4_site to ensure the correct and active conformation of PAD4. The binding of calcium ions to the Ca1_site and Ca2_site is directly involved in the catalytic action of the enzyme. Finally, this study proposes a model for the folding of PAD4. The nascent polypeptide chains of PAD4 are first folded into monomeric intermediate states, then continue to fold into monomers, and ultimately assemble into a functional and dimeric PAD4 enzyme, and cellular Ca²⁺ ions may be the critical factor governing the interchange.

Extensive Citrullination Promotes Immunogenicity of HSP90 through Protein Unfolding and Exposure of Cryptic Epitopes

Post-translational protein modifications such as citrullination have been linked to the breach of immune tolerance and clinical autoimmunity. Previous studies from our laboratory support this concept, demonstrating that autoantibodies targeting citrullinated isoforms of heat shock protein 90 (HSP90) are associated with rheumatoid arthritis complicated by interstitial lung disease. To further explore the relationship between citrullination and structural determinants of HSP90 immunogenicity, we employed a combination of ELISA-based epitope profiling, computational modeling, and mass-spectrometric sequencing of peptidylarginine deiminase (PAD)-modified protein. Remarkably, ELISAs involving selected citrullinated HSP90β/α peptides identified a key epitope corresponding to an internal Arg residue (R502 [HSP90β]/R510 [HSP90α]) that is normally buried within the crystal structure of native/unmodified HSP90. In vitro time/dose-response experiments reveal an ordered pattern of PAD-mediated deimination events culminating in citrullination of R502/R510. Conventional as well as scaled molecular dynamics simulations further demonstrate that citrullination of selected Arg residues leads to progressive disruption of HSP90 tertiary structure, promoting exposure of R502/R510 to PAD modification and subsequent autoantibody binding. Consistent with this process, ELISAs incorporating variably deiminated HSP90 as substrate Ag indicate a direct relationship between the degree of citrullination and the level of ex vivo Ab recognition. Overall, these data support a novel structural paradigm whereby citrullination-induced shifts in protein structure generate cryptic epitopes capable of bypassing B cell tolerance in the appropriate genetic context.

Microvesicles in Autoimmune Diseases

During apoptosis or activation, cells can release a subcellular structure, called a membrane microvesicle (also known as microparticle) into the extracellular environment. Microvesicles bud-off as a portion of cell membrane with its associated proteins and lipids surrounding a cytosolic core that contains intracellular proteins, lipids, and nucleic acids (DNA, RNA, siRNA, microRNA, lncRNA). Biologically active molecules on the microvesicle surface and encapsulated within can act on recipient cells as a novel mode of intercellular communication. Apoptosis has long been known to be involved in the development of diseases of autoimmunity. Abnormally persistent microvesicles, particularly apoptotic microvesicles, can accelerate autoimmune responses locally in specific organs and tissues as well as systemically. In this review, we focus on studies implicating microvesicles in the pathogenesis of autoimmune diseases and their complications.

Progress in understanding and utilizing TNF-α inhibition for the treatment of psoriatic arthritis

The improved recognition of pathogenetic molecular mechanisms has led to the use of drugs targeting cytokines in different inflammatory arthropathies as well psoriatic arthritis (PsA). In particular, the progress in knowledge on tumor necrosis factor (TNF)-α in the pathogenesis of PsA has changed the therapeutic approach by use of direct and receptor cytokine antagonists. Currently, infliximab (IFX), adalimumab, etanercept, golimumab and certolizumab pegol represent the five anti-TNF-α available for the treatment of PsA. This review describes evidence on treatment aimed at neutralizing TNF-α in PsA patients, from the first study in 2000 until today, mainly derived from randomized clinical trials. In comparison with traditional therapies, anti-TNF-α agents have shown to have more efficacy both in treating clinical aspects, including enthesitis, dactylitis, joint pain and swelling, axial involvement, nail and skin lesions, and in reducing radiographic progression. Moreover, anti-TNF-α agents have been demonstrated to be reasonably safe in PsA, as confirmed by data derived by different registries.

Structure and mechanism of a bacterial host-protein citrullinating virulence factor, Porphyromonas gingivalis peptidylarginine deiminase

Citrullination is a post-translational modification of higher organisms that deiminates arginines in proteins and peptides. It occurs in physiological processes but also pathologies such as multiple sclerosis, fibrosis, Alzheimer’s disease and rheumatoid arthritis (RA). The reaction is catalyzed by peptidylarginine deiminases (PADs), which are found in vertebrates but not in lower organisms. RA has been epidemiologically associated with periodontal disease, whose main infective agent is Porphyromonas gingivalis. Uniquely among microbes, P. gingivalis secretes a PAD, termed PPAD (Porphyromonas peptidylarginine deiminase), which is genetically unrelated to eukaryotic PADs. Here, we studied function of PPAD and its substrate-free, substrate-complex, and substrate-mimic-complex structures. It comprises a flat cylindrical catalytic domain with five-fold α/β-propeller architecture and a C-terminal immunoglobulin-like domain. The PPAD active site is a funnel located on one of the cylinder bases. It accommodates arginines from peptide substrates after major rearrangement of a “Michaelis loop” that closes the cleft. The guanidinium and carboxylate groups of substrates are tightly bound, which explains activity of PPAD against arginines at C-termini but not within peptides. Catalysis is based on a cysteine-histidine-asparagine triad, which is shared with human PAD1-PAD4 and other guanidino-group modifying enzymes. We provide a working mechanism hypothesis based on 18 structure-derived point mutants.

Decreased serum cell-free DNA levels in rheumatoid arthritis

Purpose

Recent studies have demonstrated that serum/plasma DNA and RNA molecules in addition to proteins can serve as biomarkers. Elevated levels of these nucleic acids have been found not only in acute, but also in chronic conditions, including autoimmune diseases. The aim of this study was to assess cell-free DNA (cfDNA) levels in sera of rheumatoid arthritis (RA) patients compared to controls.

Methods

cfDNA was extracted from sera of patients with early and established RA, relapsing-remitting multiple sclerosis patients (RRMS) and healthy subjects, and its concentration was determined by quantitative PCR using two amplicons, Alu115 and β-actin205, corresponding to Alu repetitive elements and the β-actin single-copy gene, respectively. Serum DNase activity was measured by a single radial enzyme diffusion method.

Results

Reduced levels of cfDNA were observed in patients with established RA in comparison with healthy controls, early RA patients and RRMS patients. There were no significant differences in cfDNA concentration between healthy controls, early RA and RRMS patients. Total DNase activity appeared to be similar in the sera of all tested groups.

Conclusions

Our results demonstrate that cfDNA levels are strongly reduced in the sera of established RA patients, which is not caused by changes in DNase activity. Measurement of cfDNA can distinguish established RA patients from early RA patients. Thus, cfDNA may serve as a biomarker in RA.

Electronic supplementary material

The online version of this article (doi:10.1007/s13317-015-0066-6) contains supplementary material, which is available to authorized users.

Immune response to chemically modified proteome

Both enzymatic and nonenzymatic PTMs of proteins involve chemical modifications. Some of these modifications are prerequisite for the normal functioning of cell, while other chemical modifications render the proteins as "neo-self" antigens, which are recognized as "non-self" leading to aberrant cellular and humoral immune responses. However, these modifications could be a secondary effect of autoimmune diseases, as in the case of type I diabetes, hyperglycemia leads to protein glycation. The enigma of chemical modifications and immune response is akin to the "chick-and-egg" paradox. Nevertheless, chemical modifications regulate immune response. In some of the well-known autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and multiple sclerosis, chemically modified proteins act as autoantigens forming immune complexes. In some instances, chemical modifications are also involved in regulating immune response during pathogen infection. Further, the usefulness of proteomic analysis of immune complexes is briefly discussed.

SERS-based immunoassay of anti-cyclic citrullinated peptide for early diagnosis of rheumatoid arthritis

We report a highly sensitive detection method for anti-CCP autoantibodies using a SERS-based magnetic immunosensor. The proposed immunoassay technique is expected to be a new clinical tool for the early diagnosis of rheumatoid arthritis (RA).

PADI2 is significantly associated with rheumatoid arthritis

Citrullination, a posttranslational modification of peptidyl arginine to citrulline, plays an essential role in rheumatoid arthritis (RA). Citrullination is catalyzed by a group of peptidylarginine deiminases (PADs) including PADI 1, 2, 3, 4 and 6. Many studies have indicated that the gene encoding PADI4 is a factor in susceptibility to RA. Some studies have detected PADI2 expression in RA synovial tissues, suggesting that PADI2 also plays an important role in the disease. This study evaluated the possible association between the PADI2-encoding gene and RA. Seventeen tag SNPs across the PAD locus were genotyped using a custom-designed Illumina 96-SNP VeraCode microarray. Peripheral blood samples were collected from patients with RA (n = 267), ankylosing spondylitis (AS, n = 51) and healthy controls (n = 160). The results of genotyping were verified using Sequenom MassARRAY in an independent cohort of 307 patients with RA, 324 patients with AS and 509 healthy controls. A western blot analysis was performed using synovial tissue from patients with RA (n = 7), osteoarthritis (OA, n = 7) and AS (n = 5) to determine the levels of expression of PADI2. A microarray analysis revealed a significant association between three selected PADI2 SNPs (rs2235926, rs2057094, rs2076616) and the presence of RA. The increased susceptibility to RA associated with rs2235926 (OR = 1.706733, 95% CI = [1.576366–1.866587], p = 0.000839) and rs2057094 (OR = 1.360432, 95% CI = [1.065483–1.869482], p = 0.003291) was further confirmed by the Sequenom MassARRAY. No tag SNPs in the PADI2 locus showed a significant association with AS. Increased expression of PADI2 was detected in RA synovial tissues compared with samples from patients with OA and AS. PADI2 is significantly associated with RA and may be involved in the pathogenesis of the disease.

Identification of citrullinated hsp90 isoforms as novel autoantigens in rheumatoid arthritis-associated interstitial lung disease

OBJECTIVE

Subsets of patients with rheumatoid arthritis (RA) develop extraarticular complications that include interstitial lung disease (ILD). Because standardized algorithms for identification of RA patients at risk of developing clinically significant ILD are lacking, the purpose of this study was to elucidate unique serologic markers of RA-associated ILD (RA-ILD).

METHODS

Sera from RA patients with ILD and from RA patients without ILD were used to immunoprecipitate citrullinated and uncitrullinated proteins derived from K562 cell extracts. Mass spectrometry was performed to facilitate identification of citrullinated proteins differentially immunoprecipitated by RA-ILD patient sera. These candidate proteins were then used as substrate antigens in custom enzyme-linked immunosorbent assays (ELISAs) for high-throughput screening of sera obtained from cohorts of patients with RA, RA-ILD mixed connective tissue disease (MCTD), or idiopathic pulmonary fibrosis (IPF).

RESULTS

Differential immunoprecipitation and subsequent mass spectrometric sequencing identified citrullinated Hsp90α and citrullinated Hsp90β as candidate autoantigens in patients with RA-ILD. ELISAs incorporating uncitrullinated and citrullinated isoforms of Hsp90 as substrate antigens demonstrated that sera from patients with RA-ILD preferentially recognized citrullinated versions of Hsp90 with moderate sensitivity (range 20-30%) and great specificity (>95%) relative to sera derived from patients with RA alone (without ILD), patients with MCTD, or patients with IPF.

CONCLUSION

These studies demonstrate the utility of a novel autoantigen discovery method based on differential immunoprecipitation of citrullinated protein extracts. Application of these techniques identified citrullinated versions of Hsp90α and Hsp90β as autoantibody targets distinguishing RA-ILD from RA without ILD, MCTD, and IPF, suggesting that anti-citrullinated Hsp90α/β autoantibodies may serve as effective biomarkers for the potentially devastating pulmonary manifestations of RA-ILD.

Role of N- or C-terminal biotinylation in autoantibody recognition of citrullin containing filaggrin epitope peptides in rheumatoid arthritis

Here, we report on the synthesis, conformational analysis, and autoantibody binding properties of new sets of rheumatoid arthritis (RA) specific biotin-peptide conjugates derived from filaggrin epitope peptides. The biotin with or without a linker was attached to the Cit or Arg containing epitope core ((311)TXGRS(315)) or epitope region ((306)SHQESTXGXSXGRSGRSGS(324)) peptide (where X = Cit), through an amide bond at the N- or C-terminal of the epitopes. Antibody binding was detected by indirect enzyme-linked immunosorbent assay (ELISA) using sera from RA, Systemic lupus erythematosus (SLE) patients, as well as healthy individuals, and the secondary structure of conjugates was investigated by electronic circular dichroism (ECD). We found that autoantibodies from RA patients recognize specifically both filaggrin epitope region ((306)SHQESTXGXSXGRSGRSGS(324)) and short epitope core ((311)TXGRS(315)) peptides. Our data also indicate that the positioning of the biotin label within a peptide sequence can markedly influence the antibody binding, but the length of the linker incorporated has essentially no effect on the recognition. ECD experiments demonstrate that the Arg/Cit change does not influence the solution conformation of the peptide conjugates. However, the presence and position of the biotin moiety has a pronounced effect on the conformation of the 5-mer epitope core peptides, while it does not alter the secondary structure of the 19-mer epitope region peptides.

Nonstereotyped lymphoma B cell receptors recognize vimentin as a shared autoantigen

Ag activation of the BCR may play a role in the pathogenesis of human follicular lymphoma (FL) and other B cell malignancies. However, the nature of the Ag(s) recognized by tumor BCRs has not been well studied. In this study, we used unbiased approaches to demonstrate that 42 (19.35%) of 217 tested FL Igs recognized vimentin as a shared autoantigen. The epitope was localized to the N-terminal region of vimentin for all vimentin-reactive tumor Igs. We confirmed specific binding to vimentin by using recombinant vimentin and by performing competitive inhibition studies. Furthermore, using indirect immunofluorescence staining, we showed that the vimentin-reactive tumor Igs colocalized with an anti-vimentin mAb in HEp-2 cells. The reactivity to N-terminal vimentin of IgG FL Igs was significantly higher than that of IgM FL Igs (30.4 versus 10%; p = 0.0022). However, vimentin-reactive FL Igs did not share CDR3 motifs and were not homologous. Vimentin was expressed in the T cell-rich regions of FL, suggesting that vimentin is available for binding with tumor BCRs within the tumor microenvironment. Vimentin was also frequently recognized by mantle cell lymphoma and multiple myeloma Igs. Our results demonstrate that vimentin is a shared autoantigen recognized by nonstereotyped FL BCRs and by the Igs of mantle cell lymphoma and multiple myeloma and suggest that vimentin may play a role in the pathogenesis of multiple B cell malignancies. These findings may lead to a better understanding of the biology and natural history of FL and other B cell malignancies.

Functional roles of the non-catalytic calcium-binding sites in the N-terminal domain of human peptidylarginine deiminase 4

This study investigated the functional roles of the N-terminal Ca²⁺ ion-binding sites, in terms of enzyme catalysis and stability, of peptidylarginine deiminase 4 (PAD4). Amino acid residues located in the N-terminal Ca²⁺-binding site of PAD4 were mutated to disrupt the binding of Ca²⁺ ions. Kinetic data suggest that Asp155, Asp157 and Asp179, which directly coordinate Ca3 and Ca4, are essential for catalysis in PAD4. For D155A, D157A and D179A, the k_cat/K_m,BAEE values were 0.02, 0.63 and 0.01 s⁻¹mM⁻¹ (20.8 s⁻¹mM⁻¹ for WT), respectively. Asn153 and Asp176 are directly coordinated with Ca3 and indirectly coordinated with Ca5 via a water molecule. However, N153A displayed low enzymatic activity with a k_cat value of 0.3 s⁻¹ (13.3 s⁻¹ for wild-type), whereas D176A retained some catalytic power with a k_cat of 9.7 s⁻¹. Asp168 is the direct ligand for Ca5, and Ca5 coordination by Glu252 is mediated by two water molecules. However, mutation of these two residues to Ala did not cause a reduction in the k_cat/K_m,BAEE values, which indicates that the binding of Ca5 may not be required for PAD4 enzymatic activity. The possible conformational changes of these PAD4 mutants were examined. Thermal stability analysis of the PAD4 mutants in the absence or presence of Ca²⁺ indicated that the conformational stability of the enzyme is highly dependent on Ca²⁺ ions. In addition, the results of urea-induced denaturation for the N153, D155, D157 and D179 series mutants further suggest that the binding of Ca²⁺ ions in the N-terminal Ca²⁺-binding site stabilizes the overall conformational stability of PAD4. Therefore, our data strongly suggest that the N-terminal Ca²⁺ ions play critical roles in the full activation of the PAD4 enzyme.

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.

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.

Comparative assessment of essential and toxic metals in the blood of rheumatoid arthritis patients and healthy subjects

The present study deals with the comparative evaluation of essential and toxic metals in rheumatoid arthritis and healthy donors. Blood samples collected from rheumatoid arthritis patients and healthy subjects were analysed for selected essential and toxic metals (Ca, Mg, Fe, Zn, Cu, Co, Mn, Cr, Cd and Pb). The samples were digested in nitric acid and perchloric acid mixture, followed by quantification of the metals using atomic absorption spectrometry. Mean levels of Ca, Mg, Fe and Zn were significantly higher in the blood of healthy donors; however, elevated levels of Cd, Co, Cr, Cu and Pb were observed in blood of the patients. The correlation coefficients among the selected metals in the blood of arthritis patients were significantly different compared with the healthy counterparts. Multivariate cluster analysis revealed mutual apportionment of the essential and toxic metals in blood of the patients, whereas, in controls, the essential and toxic metals revealed diverse apportionment. Variations in the metal levels with gender, residence and smoking habits were also evaluated in both donor groups. Relative distribution, correlation and apportionment of the essential and toxic metals in the blood of the patients were significantly different than of controls.

Mutated citrullinated vimentin antibodies in rheumatoid arthritis

Rheumatoid arthritis (RA) is the most common inflammatory systemic autoimmune disease, primarily affecting the peripheral joints. The past decade has been marked with revolutionary changes both in the therapeutic and diagnostic perspectives of RA. The discovery of an RA-specific citrullination-driven immune reaction gave a substantial contribution in the diagnostic approach to RA. Efforts directed towards the identification of the antigenic target specifically recognized by these autoantibodies resulted in the identification of vimentin in citrullinated form as the potential native antigen, among other proteins. Furthermore, it was found that the mutation of vimentin represents an independent trigger of antigenic properties, in addition to citrullination. As a result of this discovery, a commercial ELISA using mutated citrullinated vimentin (MCV) was developed. Increasingly, data now support the use of anti-MCV in RA diagnosis and prognosis for errosion. This review summarizes the research to date on the use of anti-MCV in RA diagnosis and prognosis and its potential use as a therapeutic marker. The pathologic role of these antibodies in RA disease is also discussed.

A systematic review of serum biomarkers anti-cyclic citrullinated Peptide and rheumatoid factor as tests for rheumatoid arthritis

This systematic review assesses the current status of anti-cyclic citrullinated peptide (anti-CCP) and rheumatoid factor (RF) tests in the diagnosis and prognosis of rheumatoid arthritis (RA). We reviewed publications on tests and biomarkers for early diagnosis of RA from English-language MEDLINE-indexed journals and non-MEDLINE-indexed sources. 85 publications were identified and reviewed, including 68 studies from MEDLINE and 17 non-MEDLINE sources. Anti-CCP2 assays provide improved sensitivity over anti-CCP assays and RF, but anti-CCP2 and RF assays in combination demonstrate a positive predictive value (PPV) nearing 100%, greater than the PPV of either of the tests alone. The combination also appears to be able to distinguish between patients whose disease course is expected to be more severe and both tests are incorporated in the 2010 ACR Rheumatoid Arthritis Classification Criteria. While the clinical value of anti-CCP tests has been established, differences in cut-off values, sensitivities and specificities exist between first-, second- and third-generation tests and harmonization efforts are under way. Anti-CCP and RF are clinically valuable biomarkers for the diagnosis and prognosis of RA patients. The combination of the two biomarkers in conjunction with other clinical measures is an important tool for the diagnosis and management of RA patients.

Stereotypical chronic lymphocytic leukemia B-cell receptors recognize survival promoting antigens on stromal cells

Chronic lymphocytic leukemia (CLL) is the most common leukemia in the Western world. Survival of CLL cells depends on their close contact with stromal cells in lymphatic tissues, bone marrow and blood. This microenvironmental regulation of CLL cell survival involves the stromal secretion of chemo- and cytokines as well as the expression of adhesion molecules. Since CLL survival may also be driven by antigenic stimulation through the B-cell antigen receptor (BCR), we explored the hypothesis that these processes may be linked to each other. We tested if stromal cells could serve as an antigen reservoir for CLL cells, thus promoting CLL cell survival by stimulation through the BCR. As a proof of principle, we found that two CLL BCRs with a common stereotyped heavy chain complementarity-determining region 3 (previously characterized as “subset 1”) recognize antigens highly expressed in stromal cells – vimentin and calreticulin. Both antigens are well-documented targets of autoantibodies in autoimmune disorders. We demonstrated that vimentin is displayed on the surface of viable stromal cells and that it is present and bound by the stereotyped CLL BCR in CLL-stroma co-culture supernatant. Blocking the vimentin antigen by recombinant soluble CLL BCR under CLL-stromal cell co-culture conditions reduces stroma-mediated anti-apoptotic effects by 20–45%. We therefore conclude that CLL BCR stimulation by stroma-derived antigens can contribute to the protective effect that the stroma exerts on CLL cells. This finding sheds a new light on the understanding of the pathobiology of this so far mostly incurable disease.

Interstitial lung disease in rheumatoid arthritis

Rheumatoid arthritis (RA) is the most common systemic autoimmune disease in the United States, affecting 1% to 2% of the adult population. Although joints and synovium are the primary targets in this disorder, extra-articular manifestations involving the lungs can lead to significant morbidity and excess mortality. Among the various pulmonary complications that occur in RA, interstitial lung disease (ILD) is the most damaging, with effects ranging from subclinical inflammation/scarring to end-stage pulmonary fibrosis. New insights during the past several years have underscored the epidemiologic impact of clinically/functionally significant RA-associated ILD (RA-ILD) and have begun to identify factors contributing to the pathogenesis of this potentially devastating complication of RA. Despite these advancements, the complexity of RA-ILD and the lack of reliable predictors for disease progression highlight the need for improved biomarker development. Establishing such detailed molecular signatures will ultimately guide the application and timing of therapeutic agents that include immunomodulators as well as newly studied antifibrotic agents.

Anti-cyclic citrullinated peptide antibody predicts functional disability in patients with rheumatoid arthritis in a large prospective observational cohort in Japan

To clarify the clinical significance of anti-cyclic citrullinated peptide antibody (anti-CCP) in the long-term outcome of RA, we established a large observational cohort of RA patients (IORRA) in our institute beginning in 2000. Essentially all RA patients who consulted our institute were registered, and clinical parameters, including disease activity and drug use, were assessed biannually based on patient reports, physician examinations, and laboratory data. In the third phase (October 2001) of the IORRA survey, anti-CCP levels were measured in 1,226 RA patients. In a cross-sectional analysis, clinical variables were compared in anti-CCP-positive versus -negative patients and in RF-positive versus -negative patients. In a longitudinal analysis, subsequent progression of disability was analyzed in anti-CCP-positive versus -negative and in RF-positive versus -negative patients. A verified Japanese version of the Health Assessment Questionnaire (J-HAQ) was used to measure functional disability. In the cross-sectional analysis, anti-CCP-positive patients (84.2%) had a significantly longer disease duration and higher disease activity score and more frequently used corticosteroids and methotrexate compared to anti-CCP-negative patients statistically. Similar phenomena were noted between RF-positive and -negative patients. In contrast, the longitudinal analysis revealed that J-HAQ slopes-a measure of progression of functional disability-were strongly associated with anti-CCP positivity but not with RF positivity. In a linear regression model, J-HAQ scores significantly worsened in anti-CCP-positive patients compared to anti-CCP-negative patients at the third year (annual progression 0.0317, P = 0.001) and the fifth year (annual progression 0.0199, P = 0.0012); however, J-HAQ progression was not influenced by RF status. Anti-CCP is a better predictive and discriminative marker for progression of disability in the long-term outcome of RA patients compared to RF.

Peptidyl arginine deiminase: A novel immunohistochemical marker for liver fibrosis in patients with chronic hepatitis

Peptidylarginine deiminase (PAD) is an enzyme known to be involved in the pathogenesis of rheumatoid arthritis (RA). Since many of the molecular events present in the joints in RA also take place in the injured liver, we postulated in this study that PAD may be involved in liver fibrosis. The objectives of this study therefore were to find out if PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and if it is associated with METAVIR activity and fibrosis scores. Liver biopsies were obtained from 100 patients with chronic liver diseases between September 2006 and 2007. The biopsies were scored by two histopathologists according to the METAVIR activity and fibrosis scores after histological preparation. Immunohistochemistry for PAD was performed on the biopsies using a monoclonal antibody against PAD. PAD could not be demonstrated in normal liver biopsies but was found in the hepatocytes of patients with chronic hepatitis. PAD labeling could distinguish patients with no fibrosis from either F1 or F2 or F3 or F4 fibrosis. Similarly, PAD labeling could separate patients with no inflammatory activity from those with mild or moderate or severe activity. We concluded that PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and that its immunodetection was significantly associated with Metavir activity and fibrosis scores.

Autoantibodies as prognostic markers in autoimmune liver disease

Certain autoantibodies in autoimmune liver disease have prognostic implications that are under-utilized and under-developed. The goals of this review are to indicate progress in characterizing the autoantibodies with prognostic connotations and to indicate the feasibility and importance of discovering other markers. Prime source and review articles in English were selected by a Medline search through 2010. Antibodies to soluble liver antigen, actin, liver cytosol type 1, asialoglycoprotein receptor, chromatin, cyclic citrullinated peptide, and uridine glucuronosyltransferases have been associated with the occurrence, severity, and progression of autoimmune hepatitis, and antibodies to Sp100, gp210, and centromere have had similar implications in primary biliary cirrhosis. Antibodies to soluble liver antigen have shown the most promise in autoimmune hepatitis as they have been associated with severe histological changes, long durations of treatment, relapse after drug withdrawal, and high frequency of liver failure. Antibodies to the nuclear rim pore protein, gp210, have shown the most promise in primary biliary cirrhosis as they have been associated with severe interface hepatitis, lobular inflammation, and progression to liver failure. The major limitations of the autoantibodies have been their lack of standardized assays, low negative predictabilities, and fluctuating levels. Performance parameters will improve as critical pathogenic pathways, comprehensive testing batteries, and standardized assays through international exchange workshops are developed. Progress has been made in identifying the serological markers of prognosis in autoimmune liver disease, and they promise to reflect critical disease mechanisms and enhance patient management.

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.

Pathogenic role of antibodies to citrullinated proteins in rheumatoid arthritis

In the last 10 years, the discovery that antibodies to citrullinated proteins are highly specific for rheumatoid arthritis has led to a model of pathogenesis that ties together the genetic and environmental risk factors for susceptibility and severity of disease. The authors propose that the chronic inflammation is driven by two phases of an immune response. The first phase is the priming of autoimmunity, which may occur many years before the onset of disease and is caused by environmental factors, such as smoking and infectious agents, in the context of disease susceptibility alleles. This may occur in sites outside the joint, such as the respiratory tract. The second phase is the induction of arthritis, which is associated with the generation of citrullinated proteins within the joint, which is then perpetuated as the erosive disease by a local chronic immune response. The identity of candidate synovial citrullinated antigen(s), whether fibrin, vimentin, alpha-enolase, collagen type II or others yet to be described, may be the key to the pathogenesis of the destructive disease characteristic of rheumatoid arthritis. There is emerging evidence that citrullination may already be modified by established therapy in rheumatoid arthritis, but more specific inhibitors of deimination may provide new agents for future treatments.

Autoimmunity to specific citrullinated proteins gives the first clues to the etiology of rheumatoid arthritis

Rheumatoid arthritis (RA) is now clearly a true autoimmune disease with accumulating evidence of pathogenic disease-specific autoimmunity to citrullinated proteins. Citrullination, also termed deimination, is a modification of arginine side chains catalyzed by peptidylarginine deiminase (PAD) enzymes. This post-translational modification has the potential to alter the structure, antigenicity, and function of proteins. In RA, antibodies to cyclic citrullinated peptides are now well established for clinical diagnosis, though we argue that the identification of specific citrullinated antigens, as whole proteins, is necessary for exploring pathogenic mechanisms. Four citrullinated antigens, fibrinogen, vimentin, collagen type II, and alpha-enolase, are now well established, with others awaiting further characterization. All four proteins are expressed in the joint, and there is evidence that antibodies to citrullinated fibrinogen and collagen type II mediate inflammation by the formation of immune complexes, both in humans and animal models. Antibodies to citrullinated proteins are associated with HLA 'shared epitope' alleles, and autoimmunity to at least one antigenic sequence, the CEP-1 peptide from citrullinated alpha-enolase (KIHAcitEIFDScitGNPTVE), shows a specific association with HLA-DRB1*0401, *0404, 620W PTPN22, and smoking. Periodontitis, in which Porphyromonas gingivalis is a major pathogenic bacterium, has been linked to RA in epidemiological studies and also shares similar gene/environment associations. This is also the only bacterium identified that expresses endogenous citrullinated proteins and its own bacterial PAD enzyme, though the precise molecular mechanisms of bacterial citrullination have yet to be explored. Thus, both smoking and Porphyromonas gingivalis are attractive etiological agents for further investigation into the gene/environment/autoimmunity triad of RA.

Antibodies to mutated citrullinated vimentin and antibodies to cyclic citrullinated peptides in juvenile idiopathic arthritis

BACKGROUND

The goal of the study was to assess the presence of antibodies to mutated citrullinated vimentin (anti-MCV) and cyclic citrullinated peptides (anti-CCP) in patients with juvenile idiopathic arthritis (JIA) compared with patients with other juvenile onset rheumatic diseases.

METHODS

The study included 56 patients who fulfilled the International League of Associations for Rheumatology (ILAR) classification criteria for JIA, and 17 control patients with other juvenile onset rheumatic diseases. Data on six core outcome variables and the Sharp score were collected for patients with JIA. Sera and synovial fluid, if available, were tested for anti-CCP and anti-MCV antibodies using a commercial enzyme-linked immunosorbent assay (ELISA).

RESULTS

Anti-MCV antibodies were positive in 3/56 (5.4%) and anti-CCP in 1/56 (1.8%) of patients with JIA. Two out of three anti-MCV positive patients (one of them also anti-CCP positive) were found to be rheumatoid factor (RF)-positive with polyarticular disease. Within the control group, anti-MCV was positive in 4/17 (23.5%) patients, while anti-CCP positivity was not observed. No correlation between anti-MCV with anti-CCP antibody levels was found for any of the six core outcome variables or for the adapted Sharp score.

CONCLUSIONS

Our results show that antibodies targeting citrullinated proteins are not a useful diagnostic marker for JIA, but can indicate severe patterns of disease in JIA.

Terminal monosaccharide screening of synovial immunoglobulins G and A for the early detection of rheumatoid arthritis

The expressions of some terminal glycotopes of synovial immunoglobulins G, A, and M were analysed in relation to rheumatoid arthritis (RA) progression defined according to early and advanced radiological changes in patients' hands. The relative amounts of terminal monosaccharides were determined by lectin-immunoblotting of immunoglobulin preparations using appropriate lectins able to recognize alpha2,6-linked (Sambucus nigra agglutinin) and alpha2,3-linked (Maackia amurensis agglutinin) sialic acid, galactose (Ricinus communis agglutinin I), N-acetylglucosamine (Griffonia simplicifolia agglutinin II) as well as alpha1,6-linked (Aleuria aurantia lectin), alpha1,3-linked (Lotus tetragonolobus agglutinin), and alpha1,2-linked (Ulex europaeus agglutinin) fucose. The results indicate differences between early and advanced RA stages in the terminal sugar exposition of synovial IgG and IgA, but not IgM. The galactose-deficient glycotope with exposed N-acetylglucosamine of the synovial 33.1-kDa IgG fragment appeared exclusively in the early stage of RA. In contrast, this glycotope of intact synovial IgG and IgA was present in both groups, although with higher proportions in advanced RA. The proportions of the sialyl and fucosyl determinants of intact synovial A and G immunoglobulins were clearly lower in the early RA group than in the advanced. The analysis of terminal oligosaccharide exposition in IgG, IgG fragments, and IgA present in the synovial fluid of RA patients might be applicable as a stage-specific marker in the diagnosis and therapy of RA patients.

A functional haplotype of PADI4 gene in rheumatoid arthritis: positive correlation in a French population

OBJECTIVE

A functional haplotype of peptidyl arginine deiminase 4 (PADI4) was associated with susceptibility to rheumatoid arthritis (RA) in Asian populations, but the results are contradictory in Europeans. We investigated (1) the association of 2 single-nucleotide polymorphisms (SNP) located in exon 2 of PADI4 with RA in another Caucasian population; and (2) the association between PADI4 and anti-citrullinated protein (anti-CCP) antibodies.

METHODS

DNA samples were obtained from 405 French RA patients and 275 controls. All RA patients met the revised criteria of the American College of Rheumatology. PADI4_89 163(G-->A) and PADI4_90 245(T-->C) SNP were genotyped using a PCR-RFLP method confirmed by direct sequencing. All patients and controls were genotyped for HLA-DRB1. The presence of anti-CCP antibodies was tested in 243 RA patients using an ELISA technique.

RESULTS

We focused on PADI4_89 163(G-->A) and PADI4_90 245(T-->C) SNP that distinguished 2 main haplotypes: AC haplotype (PADI4_89*A PADI4_90*C) and GT haplotype (PADI4_89*G PADI4_90*T), described, respectively, as "nonsusceptible" and "susceptible." A positive association between RA and presence of the GT haplotype was found in the heterozygous state (p = 0.002) and the homozygous state (RA patients 22%, controls 13%; p = 0.005). A correlation was observed between the presence but not the level of anti-CCP antibodies and the GT heterozygous (p = 0.03) and homozygous (p = 0.05) haplotypes. No correlation was found between the HLA-DRB1 shared epitope and any of the PADI4 haplotypes.

CONCLUSION

Our findings confirm those of Japanese, Korean, and Canadian studies and suggest that PADI4 may be a new susceptibility gene independent of HLA-DRB1 for RA in Caucasian populations.

Epitope spreading to citrullinated antigens in mouse models of autoimmune arthritis and demyelination

Introduction

Anti-citrullinated protein antibodies have a diagnostic role in rheumatoid arthritis (RA); however, little is known about their origins and contribution to pathogenesis. Citrullination is the post-translational conversion of arginine to citrulline by peptidyl arginine deiminase, and increased citrullination of proteins is observed in the joint tissue in RA and in brain tissue in multiple sclerosis (MS).

Methods

We applied synovial and myelin protein arrays to examine epitope spreading of B cell responses to citrullinated epitopes in both the collagen-induced arthritis (CIA) model for RA and the experimental autoimmune encephalomyelitis (EAE) model for MS. Synovial and myelin protein arrays contain a spectrum of proteins and peptides, including native and citrullinated forms, representing candidate autoantigens in RA and MS, respectively. We applied these arrays to characterise the specificity of autoantibodies in serial serum samples derived from mice with acute and chronic stages of CIA and EAE.

Results

In samples from pre-disease CIA and acute-disease EAE, we observed autoantibody targeting of the immunising antigen and responses to a limited set of citrullinated epitopes. Over the course of diseases, the autoantibody responses expanded to target multiple citrullinated epitopes in both CIA and EAE. Using immunoblotting and mass spectrometry analysis, we identified citrullination of multiple polypeptides in CIA joint and EAE brain tissue that have not previously been described as citrullinated.

Conclusions

Our results suggest that anti-citrulline antibody responses develop in the early stages of CIA and EAE, and that autoimmune inflammation results in citrullination of joint proteins in CIA and brain proteins in EAE, thereby creating neoantigens that become additional targets in epitope spreading of autoimmune responses.

Accumulation of citrullinated proteins by up-regulated peptidylarginine deiminase 2 in brains of scrapie-infected mice: a possible role in pathogenesis

Peptidylarginine deiminases (PADs), which are a group of posttranslational modification enzymes, are involved in protein citrullination (deimination) by the conversion of peptidylarginine to peptidylcitrulline in a calcium concentration-dependent manner. Among the PADs, PAD2 is widely distributed in various tissues and is the only type that is expressed in brain. To elucidate the involvement of protein citrullination by PAD2 in the pathogenesis of brain-specific prion diseases, we examined the profiles of citrullinated proteins using the brains of scrapie-infected mice as a prion disease model. We found that, compared with controls, increased levels of citrullinated proteins of various molecular weights were detected in different brain sections of scrapie-infected mice. In support of this data, expression levels of PAD2 protein as well as its enzyme activity were significantly increased in brain sections of scrapie-infected mice, including hippocampus, brain stem, and striatum. Additionally, the expression levels of PAD2 mRNA were increased during scrapie infection. Moreover, PAD2 immunoreactivity was increased in scrapie-infected brains, with staining detected primarily in reactive astrocytes. Using two-dimensional electrophoresis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry, various citrullinated proteins were identified in the brains of scrapie-infected mice, including glial fibrillary acidic protein, myelin basic protein, enolases, and aldolases. This study suggests that accumulated citrullinated proteins and abnormal activation of PAD2 may function in the pathogenesis of prion diseases and serve as potential therapeutic targets.

Association of autoimmunity to peptidyl arginine deiminase type 4 with genotype and disease severity in rheumatoid arthritis

OBJECTIVE

Protein citrullination is an important posttranslational modification recognized by rheumatoid arthritis (RA)-specific autoantibodies. One of the citrullinating enzymes, peptidyl arginine deiminase type 4 (PAD-4), is genetically associated with development of RA in some populations, although the mechanism(s) mediating this effect are not yet clear. There have been descriptions of anti-PAD-4 autoantibodies in different rheumatic diseases. This study was undertaken to investigate whether anti-PAD-4 antibodies are specific to RA, are associated with disease phenotype or severity, and whether PAD-4 polymorphisms influence the anti-PAD-4 autoantibody response.

METHODS

Sera from patients with established RA, patients with other rheumatic diseases, and healthy adults were assayed for anti-PAD-4 autoantibodies by immunoprecipitation of in vitro-translated PAD-4. The epitope(s) recognized by PAD-4 autoantibodies were mapped using various PAD-4 truncations. PAD-4 genotyping was performed on RA patients with the TaqMan assay. Joint erosions were scored from hand and foot radiographs using the Sharp/van der Heijde method.

RESULTS

PAD-4 autoantibodies were found in 36-42% of RA patients, and were very infrequent in controls. Recognition by anti-PAD-4 autoantibodies required the 119 N-terminal amino acids, which encompass the 3 nonsynonymous polymorphisms associated with disease susceptibility. Strikingly, the anti-PAD-4 immune response was associated with the RA susceptibility haplotype of PADI4. Anti-PAD-4 antibodies were associated with more severe joint destruction in RA.

CONCLUSION

Our findings indicate that anti-PAD-4 antibodies are specific markers of RA, independently associated with more severe disease, suggesting that an anti-PAD-4 immune response may be involved in pathways of joint damage in this disease. Polymorphisms in the PADI4 gene influence the immune response to the PAD-4 protein, potentially contributing to disease propagation.

Immunity to citrullinated proteins in rheumatoid arthritis

Antibodies to citrullinated proteins (ACPA), i.e., to peptides posttranslationally modified by the conversion of arginine to citrulline, are specific serological markers for rheumatoid arthritis (RA). Studies on anticitrulline immunity, summarized in this review, demonstrate that the criterion-based syndrome RA should be subdivided into at least two distinct subsets (ACPA-positive and ACPA-negative disease). A new etiological model is proposed for ACPA-positive RA, built on MHC class II-dependent activation of adaptive immunity. Fundamentals of this model include the following: (a) ACPA antedate onset of arthritis; (b) ACPA may aggravate arthritis in rodents; (c) ACPA are triggered in the context of genes that confer susceptibility to RA (HLA-DRB1 SE) and by environmental agents triggering RA (smoking or bacterial stimuli); (d) ACPA may complex with citrullinated proteins present in target tissue as part of a multistep process for arthritis development. The model provides a new basis for molecular studies on the pathogenesis of ACPA-positive arthritis.

Crucial Roles of MZF1 and Sp1 in the Transcriptional Regulation of the Peptidylarginine Deiminase Type I Gene (PADI1) in Human Keratinocytes

Peptidylarginine deiminases (PADs) catalyze the conversion of protein-bound arginine residues into citrulline residues in a calcium-dependent manner. The PAD1 gene (PADI1) is expressed in a few tissues, including the epidermis, where the protein is detected with a higher level in the more differentiated keratinocytes. Using quantitative reverse transcription-PCR experiments, we show that PADI1 mRNAs are more abundant in keratinocytes cultured with 1.2 than 0.15 mM calcium. We cloned and characterized the promoter region using human keratinocytes transfected with variously deleted fragments of the 5'-upstream region of PADI1 coupled to the luciferase gene. We found that as few as 195 bp upstream from the transcription initiation site were sufficient to direct transcription of the reporter gene. Mutations of MZF1- or Sp1-binding sites markedly reduced PADI1 promoter activity. Chromatin immunoprecipitation assays revealed that MZF1 and Sp1/Sp3 bind to this region in vivo. Furthermore, MZF1 or Sp1 small interfering RNAs (siRNAs) effectively diminished PADI1 expression in keratinocytes cultured in both low- and high-calcium-containing medium. In addition, the expression of MZF1 and PAD1 increased in parallel when normal human epidermal keratinocytes underwent differentiation. These data indicate that MZF1 and Sp1/Sp3 binding to the promoter region drive the PADI1 expression.

A new perspective: molecular motifs on oxidized LDL, apoptotic cells, and bacteria are targets for chronic lymphocytic leukemia antibodies

The restricted immunoglobulin (Ig) repertoire found in B-cell chronic lymphocytic leukemia (CLL) implies a role for antigen(s) in the leukemogenesis. The nature of the antigens has, however, not been characterized, although examples of autoantigens have been demonstrated. We have analyzed a panel of 28 CLL cell lines and primary cultures, producing monoclonal Ig with different Ig heavy-chain variable region gene usage and mutational status, including several complementarity determining region 3 homology subset members. Using mass-spectrometry, immunoassays, or protein macroarrays, we have discovered novel antigens binding to CLL Igs. These antigens included cytoskeletal proteins vimentin, filamin B, and cofilin-1, but also phosphorylcholine-containing antigens (eg, Streptococcus pneumoniae polysaccharides and oxidized low-density lipoprotein [oxLDL]). Additional new antigens identified were cardiolipin and proline-rich acidic protein-1. Remarkably, these antigens represent molecular motifs exposed on apoptotic cells/blebs and bacteria, and several CLL Igs bound to apoptotic Jurkat cells. In conclusion, these intriguing data, showing a limited target structure recognition, indicate that CD5+ CLL B cells are derived from a cell compartment that produces "natural antibodies," which may be instrumental in elimination and scavenging of apoptotic cells and pathogenic bacteria.

The Diagnostic Value of Anti-Cyclic Citrullinated Peptide Antibodies (anti-CCP) in Patients with Rheumatoid Arthritis

A cross-sectional study was carried out to analyze the prevalence of anti-CCP antibodies and IgM RF in the sera of 160 randomly selected patients from the Rheumatology Department: 60 with RA, 50 with other rheumatic diseases (non-RA), 50 healthy controls (HC). The mean age of the group was 50.06+/-11.9 years. There were 141 females (88.1%) and 19 males (11.9%). RA patients fulfilled the revised ACR criteria. The mean duration of the disease was 82.4 months. Anti-CCP ELISA kit and IgM RF Latex test were used. The mean anti-CCP values were as follows: RA 60.4+/-57.6, non-RA 2.1+/-3.6, HC 1.3 +/-0.4 U/mL. Respectively, the mean values of IgM RF were: RA 515.8+/-525, non-RA 102+/-294, HC 15+/-57.5. Forty out of 60 (66.6%) RA patients were anti-CCP positive. Forty one out of 60 (68.3%) RA patients were positive for IgM RF. As expected, anti-CCP showed comparable sensitivity (66.8% vs. 68.3%) and higher specificity (98% vs. 87%) than IgM RF, at optimal cut-off values. The presence of either anti-CCP or IgM RF increased the testing sensitivity for the diagnosis of RA to 76.6%. AUC was greater for anti-CCP than for IgM RF (0.92 vs. 0.82).

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.