Mapping of citrullinated fibrinogen B-cell epitopes in rheumatoid arthritis by imaging surface plasmon resonance

Introduction  

Rheumatoid arthritis (RA) frequently involves the loss of tolerance to citrullinated antigens, which may play a role in pathogenicity. Citrullinated fibrinogen is commonly found in inflamed synovial tissue and is a frequent target of autoantibodies in RA patients. To obtain insight into the B-cell response to citrullinated fibrinogen in RA, its autoepitopes were systematically mapped using a new methodology.     

Targeting of anti-citrullinated protein/peptide antibodies in rheumatoid arthritis using peptides mimicking endogenously citrullinated fibrinogen antigens

IntroductionWe have previously identified endogenously citrullinated peptides derived from fibrinogen in rheumatoid arthritis (RA) synovial tissues. In this study, we have investigated the auto-antigenicity of four of those citrullinated peptides, and explored their feasibility to target anti-citrullinated protein/peptide antibodies (ACPA).MethodsThe autoantigenic potential of the fibrinogen peptides was investigated by screening 927 serum samples from the Epidemiological Investigation of RA (EIRA) cohort on a peptide microarray based on the ImmunoCAP ISAC® system. In order to assay for ACPA blocking, two independent pools of purified ACPA were incubated with the respective targeting peptide prior to binding to cyclic citrullinated peptide (CCP)2 using the CCPlus® ELISA kit.ResultsTwo peptides derived from the fibrinogen α chain, Arg573Cit (563-583) and Arg591Cit (580-600), referred to as Cit573 and Cit591, and two peptides from the fibrinogen β chain, Arg72Cit (62-81) and Arg74Cit (62-81) (Cit72 and Cit74), displayed 65 %, 15 %, 35 %, and 53 % of immune reactivity among CCP2-positive RA sera, respectively. In CCP2-negative RA sera, a positive reactivity was detected in 5 % (Cit573), 6 % (Cit591), 8 % (Cit72), and 4 % (Cit74). In the competition assay, Cit573 and Cit591 peptides reduced ACPA binding to CCP2 by a maximum of 84 % and 63 % respectively. An additive effect was observed when these peptides were combined. In contrast, Cit74 and Cit72 were less effective. Cyclization of the peptide structure containing Cit573 significantly increased the blocking efficiency.ConclusionsHere we demonstrate extensive autoantibody reactivity against in vivo citrullinated fibrinogen epitopes, and further show the potential use of these peptides for antagonizing ACPA.     

Citrullinated vimentin as an important antigen in immune complexes from synovial fluid of rheumatoid arthritis patients with antibodies against citrullinated proteins

Introduction  

Rheumatoid arthritis (RA) is an inflammatory disease, which results in destruction of the joint. The presence of immune complexes (IC) in serum and synovial fluid of RA patients might contribute to this articular damage through different mechanisms, such as complement activation. Therefore, identification of the antigens from these IC is important to gain more insight into the pathogenesis of RA. Since RA patients have antibodies against citrullinated proteins (ACPA) in their serum and synovial fluid (SF) and since elevated levels of citrullinated proteins are detected in the joints of RA patients, citrullinated antigens are possibly present in IC from RA patients.     

Diagnostic utility of anti-cyclic citrullinated peptide and anti-modified citrullinated vimentin antibodies in rheumatoid arthritis

Several autoantibodies found in RA are directed to epitopes in citrullinated proteins. One of them is anti modified citrullinated vimentin (Anti-MCV). We tested the value a newly developed ELISA for the detection of antibodies against a genetically modified citrullinated vimentin (anti-MCV) in comparison with an anti-CCP based ELISA system for the diagnosis of RA. Thirty-five patients with RA (mean age; 42.6 +/- 10.87 years, mean disease duration; 9.37 +/- 3.98 years) were enrolled in this study. Twenty -five ankylosing spondylitis (mean age; 35.88 +/- 6.64 years, mean disease duration; 10.25 +/- 4.61 years), and 19 healthy subjects (mean age; 40.26 +/- 5.11 years) served as controls. Anti-CCP antibodies and Anti-MCV antibodies were measured using ELISA. In all RA patients, mean anti- CCP level was 69.07 +/- 90.43 U/ml and anti-MCV level was 665.77 +/- 1040.19 U/ml. In patients with AS, the mean anti-CCP level was 10.7 +/- 5.22 U/ml and anti-MCV level was 40.54 +/- 20.15 U/ml. In healthy controls, the mean anti-CCP level was 11.11 +/- 7.65 U/ml, anti-MCV level was 23.12 +/- 12.04 U/ml. In patients with active RA, the mean serum anti-CCP level was 100.54 +/- 98.07 U/ml and anti-MCV level was 998.74 +/- 1154.93 U/ml. In patients with inactive RA, the mean serum anti-CCP level was 8.77 +/- 1.55 U/ml and anti-MCV level was 27.59 +/- 23.10 U/ml. According to these results; In patients with RA, the mean serum anti-MCV and anti-CCP levels were significantly high compared to patients with AS and healthy controls (p=0.002, p=0.001, p=0.002, p=0.001 respectively). The mean serum anti-MCV and anti- CCP levels were significantly higher in active patients with RA than in inactive patients with RA patients (p=0.001 and p=0.001 respectively). In inactive patients with RA, the mean serum anti-MCV and anti-CCP levels were similar in patients with AS and patients (p=0.484, p=0.308, p=0.09 and p=0.222 respectively). The mean serum anti-MCV levels were correlated with DAS 28 (r=0.531, p=0.001), VAS score (r=0.332, p=0.01), ESR (r=0.458, p=0.001), serum CRP levels (r=0.568, p=0.01), serum RF levels (r=0.529, p=0.001), swollen joints number (r=0.525, p=0.001) and tender joints number (r=0.638, p=0.001). As a result; measurement of serum anti-MCV levels is useful for diagnosis of RA and combined use of anti-MCV and RF may be more useful prognostic factor than either method alone, RF and anti-CCP.     

Detection of autoantibodies to citrullinated BiP in rheumatoid arthritis patients and pro-inflammatory role of citrullinated BiP in collagen-induced arthritis

Introduction

Anti-citrullinated protein/peptide antibodies (ACPAs) are highly specific to rheumatoid arthritis (RA) patients and are thought to have a close relationship with the pathogenesis of arthritis. Several proteins, including fibrinogen, vimentin, and alpha-enolase, were reported as ACPA-target antigens, and their importance in RA pathogenesis was widely proposed. We identified citrullinated immunoglobulin binding protein (citBiP) as another ACPA target in RA patients and examined its pro-inflammatory role in arthritis.

Methods

We measured the levels of anti-citBiP, anti-BiP, and anti-cyclic citrullinated peptide (CCP) antibodies in the serum of RA patients (n = 100), systemic lupus erythematosus (SLE) patients (n = 60), and healthy controls (n = 30) using ELISA and immunoblotting. Epitope mapping was performed using 27 citBiP-derived peptides. In the mouse study, after DBA/1J mice were immunized with BiP or citBiP, serum titers of ACPAs were measured by ELISA and immunohistochemistry. The development of collagen-induced arthritis (CIA) was observed in BiP- or citBiP-pre-immunized mice.

Results

The serum levels of anti-BiP and anti-citBiP antibodies were significantly increased in RA patients, although only anti-BiP antibodies were slightly increased in SLE patients. Interestingly, anti-citBiP antibody levels were higher than anti-BiP antibody levels in 72% of RA patients, whereas no significant increase in anti-citBiP antibody levels was detected in SLE patients and healthy controls. The serum levels of anti-CCP antibodies were correlated with those of anti-citBiP antibodies in RA patients (R² = 0.41). Several citrulline residues of citBiP were determined to be major epitopes of anti-citBiP antibodies, one of which showed cross-reactivity with CCP. Immunization of DBA/1J mice with citBiP induced several kinds of ACPAs, including anti-CCP and anti-citrullinated fibrinogen antibodies. Pre-immunization with citBiP exacerbated CIA, and anti-CCP antibody levels were increased in citBiP-pre-immunized CIA mice.

Conclusions

CitBiP is a newly described ACPA target that may play a pro-inflammatory role in arthritis.     

Circulating immune complexes in pre-eclampsia.

Sixteen patients with severe pre-eclampsia had more IgG-containing and C1q-binding circulating soluble immune complexes than did 16 matched women with normal pregnancies. The clinical features of preeclampsia may be explained by damage due to such complexes, although raised complex levels were also present in a few women with normal pregnancies. As immune complexes are so heterogenous in terms of the type of antigen, class and subclass of immunoglobulin, size, and complement-binding capacity, further investigations are needed to determine their role in normal and pre-eclamptic pregnancies.     

Circulating immune complexes in cynomolgus macaques

Nonhuman primates can be used as models for the study of immune-complex-associated diseases. Recognizing that very little is known about the levels of circulating immune complexes (CICs) in normal monkeys, we have used three assays to measure the levels in serum collected from 313 adult and 106 juvenile cynomolgus macaques (Macaca fascicularis). The prevalence was higher than expected. There was a strong statistical association between CIC levels and country of origin. Monkeys from Indonesia were more likely to have elevated CICs than those from Malaya or the Philippines. This relationship was observed with all three assays. Furthermore, juvenile macaques tended to have lower levels than did adults. This study indicates that it may be important to consider genetic factors, the country of origin, or both when selecting cynomolgus macaques for research on immune-complex-associated diseases.     

Identification of citrullinated alpha-enolase as a candidate autoantigen in rheumatoid arthritis

Antibodies against citrullinated proteins are highly specific for rheumatoid arthritis (RA), but little is understood about their citrullinated target antigens. We have detected a candidate citrullinated protein by immunoblotting lysates of monocytic and granulocytic HL-60 cells treated with peptidylarginine deiminase. In an initial screen of serum samples from four patients with RA and one control, a protein of molecular mass 47 kDa from monocytic HL-60s reacted with sera from the patients, but not with the serum from the control. Only the citrullinated form of the protein was recognised. The antigen was identified by tandem mass spectrometry as alpha-enolase, and the positions of nine citrulline residues in the sequence were determined. Serum samples from 52 patients with RA and 40 healthy controls were tested for presence of antibodies against citrullinated and non-citrullinated alpha-enolase by immunoblotting of the purified antigens. Twenty-four sera from patients with RA (46%) reacted with citrullinated alpha-enolase, of which seven (13%) also recognised the non-citrullinated protein. Six samples from the controls (15%) reacted with both forms. Alpha-enolase was detected in the RA joint, where it co-localised with citrullinated proteins. The presence of antibody together with expression of antigen within the joint implicates citrullinated alpha-enolase as a candidate autoantigen that could drive the chronic inflammatory response in RA.     

Circulating immune complexes in Graves' disease.

Circulating immune complexes in Graves' disease sera were detected by the 125I Clq deviation test. High titers of immune complexes were detected and correlated significantly with the microsomal antibody but not with the thyroglobulin antibody titer nor with serum thyroxine levels. Serum fractionation studies in a patient with high titer of immune complexes revealed these to be heterogeneous in size, sedimenting in 19S, intermediate and 7S regions. The data suggest a role for immune complexes in the pathogenesis of Graves' disease.     

Antibodies to citrullinated proteins and differences in clinical progression of rheumatoid arthritis

Antibodies to citrullinated proteins (anti-cyclic-citrullinated peptide [anti-CCP] antibodies) are highly specific for rheumatoid arthritis (RA) and precede the onset of disease symptoms, indicating a pathogenetic role for these antibodies in RA. We recently showed that distinct genetic risk factors are associated with either anti-CCP-positive disease or anti-CCP-negative disease. These data are important as they indicate that distinct pathogenic mechanisms are underlying anti-CCP-positive disease or anti-CCP-negative disease. Likewise, these observations raise the question of whether anti-CCP-positive RA and anti-CCP-negative RA are clinically different disease entities. We therefore investigated whether RA patients with anti-CCP antibodies have a different clinical presentation and disease course compared with patients without these autoantibodies. In a cohort of 454 incident patients with RA, 228 patients were anti-CCP-positive and 226 patients were anti-CCP-negative. The early symptoms, tender and swollen joint count, and C-reactive protein level at inclusion, as well as the swollen joint count and radiological destruction during 4 years of follow-up, were compared for the two groups. There were no differences in morning stiffness, type, location and distribution of early symptoms, patients' rated disease activity and C-reactive protein at inclusion between RA patients with and without anti-CCP antibodies. The mean tender and swollen joint count for the different joints at inclusion was similar. At follow-up, patients with anti-CCP antibodies had more swollen joints and more severe radiological destruction. Nevertheless, the distribution of affected joints, for swelling, bone erosions and joint space narrowing, was similar. In conclusion, the phenotype of RA patients with or without anti-CCP antibodies is similar with respect to clinical presentation but differs with respect to disease course.     

Diagnostic value of antibodies against a modified citrullinated vimentin in rheumatoid arthritis

Antibodies directed against citrullinated vimentin are members of the family of autoantibodies reactive with citrullinated proteins and are among the most specific serological markers for the diagnosis of rheumatoid arthritis (RA). This study was performed to test the diagnostic value of a newly developed enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies against a genetically modified citrullinated vimentin (anti-MCV) in comparison with a second-generation anti-cyclic citrullinated peptides (anti-CCP2) ELISA test system. Blinded sera from 631 patients (409 consecutive out-patients and 222 randomly selected stored sera) with RA (n = 164) and non-RA (osteoarthritis [n = 120], polymyalgia rheumatica/giant cell arteritis [n = 80], spondyloarthritis [n = 36], and other inflammatory rheumatic or non-inflammatory disease [n = 67]) were tested for the presence of anti-MCV and anti-CCP2 antibodies according to the manufacturers' instructions. The diagnostic performance of the anti-MCV was comparable with the anti-CCP2 assay for the diagnosis of RA according to the calculated area under the curve (0.824; 95% confidence interval (CI) 0.778-0.870 versus 0.818; 95% CI 0.767-0.869) as analysed by receiving operating characteristic curve. When categorised with a cutoff value of 20.0 U/ml (as recommended by the manufacturer), sensitivity and specificity of the anti-MCV ELISA were 69.5% (95% CI 61.9%-76.5%) and 90.8% (86.9%-93.8%), respectively, compared with 70.1% (62.5%-77.0%) and 98.7% (96.7%-99.6%) of the anti-CCP2 assay. Using the cutoff values of 19.0 U/ml and 81.5 U/ml for the anti-MCV test to obtain a sensitivity and specificity identical to the anti-CCP2 assay, showed a reduced specificity (89.8%; 85.8%-92.9%) and sensitivity (53.7%; 45.7%-61.5%), respectively, of the anti-MCV ELISA compared with the anti-CCP2 test. In conclusion, the serum ELISA testing for anti-MCV antibodies as well as the anti-CCP-2 assay perform comparably well in the diagnosis of RA. In the high-specificity range, however, the anti-CCP2 assay appears to be superior to the anti-MCV test.     

Circulating immune complexes in rodent and simian malaria

The circulating immune complexes have been detected in the sera of albino rats infected withPlasmodium berghei and rhesus monkeys infected with P.knowlesi by (i) quantitative cryoprecipitation assay and (ii) polyethylene glycol assay. In the rodent model, the levels of circulating immune complexes increased during infection and decreased considerably in the post-infection period. In the simian system, high levels were detected during peak parasitaemia. Polyethylene glycol precipitate obtained from the sera during acuteP. knowlesi infection when analysed by Immunoelectrophoresis was found to contain (i) monkey IgG, (ii) four other components of monkey plasma, (iii) two components of normal monkey erythrocytes and (iv) antigen(s) ofP. knowlesi.     

Circulating immune complexes and autoantibodies in canine leishmaniasis

In sera of 35 dogs with generalised leishmaniasis elevated levels of circulating immune complexes were detected by the 125I-C1q binding test. Furthermore, anti-heart muscle and anti-smooth muscle antibodies were detected by an indirect immunofluorescence technique in sera of infected dogs.     

Microbial pathways to subvert host immunity generate citrullinated neoantigens targeted in rheumatoid arthritis

The specific association between antibodies to citrullinated proteins and rheumatoid arthritis (RA) has centered interest on understanding why citrullinated proteins become immunogenic in this disease, which is believed to inform the origins of autoimmunity in RA. Since citrullination is a physiologic post-translational modification (PTM), one theory is that conditions promoting abnormal citrullination are initiators of self-reactive immune responses to citrullinated proteins in RA. Foremost candidates that dysregulate the normal balance of citrullination are microbial agents, which can exploit citrullination as an effector mechanism to subvert host antimicrobial activities and maximize their progeny. Here, we will use the host-pathogen interface as a unifying model to link microbe-induced citrullination and the loss of immunological tolerance to citrullinated antigens in RA.     

Circulating immune complexes in patients treated with Pollinex vaccine

Twenty six patients with rhinitis were desensitized with Pollinex. Circulating immune complexes were determined in these patients before and after immunotherapy. Two assay techniques - precipitation and PEG-CCT were applied to assay circulating immune complexes. It was found that specific immunotherapy with Pollinex does not produce circulating immune complexes in patients with hay fever.