Cross-reactivity of IgM anti-modified protein antibodies in rheumatoid arthritis despite limited mutational load

Background Anti-modified protein antibodies (AMPA) targeting citrullinated, acetylated and/or carbamylated self-antigens are hallmarks of rheumatoid arthritis (RA). Although AMPA-IgG cross-reactivity to multiple post-translational modifications (PTMs) is evident, it is unknown whether the first responding B cells, expressing IgM, display similar characteristics or if cross-reactivity is crucially dependent on somatic hypermutation (SHM). We now studied the reactivity of (germline) AMPA-IgM to further understand the breach of B cell tolerance and to identify if cross-reactivity depends on extensive SHM. Moreover, we investigated whether AMPA-IgM can efficiently recruit immune effector mechanisms. Methods Polyclonal AMPA-IgM were isolated from RA patients and assessed for cross-reactivity towards PTM antigens. AMPA-IgM B cell receptor sequences were obtained by single cell isolation using antigen-specific tetramers. Subsequently, pentameric monoclonal AMPA-IgM, their germline counterparts and monomeric IgG variants were generated. The antibodies were analysed on a panel of PTM antigens and tested for complement activation. Results Pentameric monoclonal and polyclonal AMPA-IgM displayed cross-reactivity to multiple antigens and different PTMs. PTM antigen recognition was still present, although reduced, after reverting the IgM into germline. Valency of AMPA-IgM was crucial for antigen recognition as PTM-reactivity significantly decreased when AMPA-IgM were expressed as IgG. Furthermore, AMPA-IgM was 15- to 30-fold more potent in complement-activation compared to AMPA-IgG. Conclusions We provide first evidence that AMPA-IgM are cross-reactive towards different PTMs, indicating that PTM (cross-)reactivity is not confined to IgG and does not necessarily depend on extensive somatic hypermutation. Moreover, our data indicate that a diverse set of PTM antigens could be involved in the initial tolerance breach in RA and suggest that AMPA-IgM can induce complement-activation and thereby inflammation.Pathophysiology and treatment of rheumatic disease

Surface Ig variable domain glycosylation affects autoantigen binding and acts as threshold for human autoreactive B cell activation

The hallmark autoantibodies in rheumatoid arthritis are characterized by variable domain glycans (VDGs). Their abundant occurrence results from the selective introduction of N-linked glycosylation sites during somatic hypermutation, and their presence is predictive for disease development. However, the functional consequences of VDGs on autoreactive B cells remain elusive. Combining crystallography, glycobiology, and functional B cell assays allowed us to dissect key characteristics of VDGs on human B cell biology. Crystal structures showed that VDGs are positioned in the vicinity of the antigen-binding pocket, and dynamic modeling combined with binding assays elucidated their impact on binding. We found that VDG-expressing B cell receptors stay longer on the B cell surface and that VDGs enhance B cell activation. These results provide a rationale on how the acquisition of VDGs might contribute to the breach of tolerance of autoreactive B cells in a major human autoimmune disease.Medicinal Chemistr

A genome-wide association study of rheumatoid arthritis without antibodies against citrullinated peptides

Introduction. Rheumatoid arthritis (RA) patients can be classified based on presence or absence of anticitrullinated peptide antibodies (ACPA) in their serum. This heterogeneity among patients may reflect important biological differences underlying the disease process. To date, the majority of genetic studies have focused on the ACPA-positive group. Therefore, our goal was to analyse the genetic risk factors that contribute to ACPA-negative RA. Methods. We performed a large-scale genome-wide association study (GWAS) in three Caucasian European cohorts comprising 1148 ACPA-negative RA patients and 6008 controls. All patients were screened using the Illumina Human Cyto-12 chip, and controls were genotyped using different genome-wide platforms. Population-independent analyses were carried out by means of logistic regression. Meta-analysis with previously published data was performed as follow-up for selected signals (reaching a total of 1922 ACPA-negative RA patients and 7087 controls). Imputation of classical HLA alleles, aminoacid residues and single nucleotide polymorphisms was undertaken. Results. The combined analysis of the studied cohorts resulted in identification of a peak of association in the HLA-region and several suggestive non-HLA associations. Meta-analysis with previous reports confirmed the association of the HLA region with this subset and an observed association in the CLYBL locus remained suggestive. The imputation and deep interrogation of the HLA region led to identification of a two aminoacid model (HLA-B at position 9 and HLA-DRB1 at position 11) that accounted for the observed genome-wide associations in this region. Conclusions. Our study shed light on the influence of the HLA region in ACPA-negative RA and identified a suggestive risk locus for this condition

The Absolute Concentration of Anti Citrullinated Protein Antibodies in Serum and Synovial Fluid in Relation to Total Immunoglobulin-Concentrations

Pathophysiology and treatment of rheumatic disease

The concentration of anticitrullinated protein antibodies in serum and synovial fluid in relation to total immunoglobulin concentrations

Transplantation and autoimmunit

Genetic variants of C1q are a risk for rheumatoid arthritis

Pathophysiology and treatment of rheumatic disease

THE ACPA IGM FINE SPECIFICITY DIFFERS FROM THE ACPA IGG ANTIGEN-RECOGNITION PROFILE

Pathophysiology and treatment of rheumatic disease

GENETIC VARIANTS OF C1Q ARE A RISK FOR RHEUMATOID ARTHRITIS

Development and application of statistical models for medical scientific researc

GENETIC VARIANTS OF C1Q ARE A RISK FOR RHEUMATOID ARTHRITIS

Pathophysiology and treatment of rheumatic disease

Diagnostic value of anti-MCV antibodies in differentiating early inflammatory arthritis

Objectives To evaluates the diagnostic performance of the anti-CCP2, anti-CCP3 and anti-mutated citrullinated vimentin (anti-MCV) tests in differentiating rheumatoid arthritis (RA) from other forms of arthritis in a clinical setting of early arthritis. Methods In 917 patients with recent-onset arthritis (566 RA, 351 other diseases) and in 99 healthy controls the anti-MCV, anti-CCP2 and anti-CCP3.1 tests were performed and the test characteristics compared. Results Comparison of the tests for differentiating RA from other causes of arthritis showed a lower specificity for anti-MCV (82.9%) than for anti-CCP2 (93.4%) and anti-CCP3.1 (90.0%). Similarly, the positive likelihood ratio for anti-MCV was also lower (3.6, compared with 8.7, 5.8 for anti-CCP2 and anti-CCP3.1). The anti-MCV test had a higher sensitivity (62% vs 56.9% and 58.1%, respectively). In psoriatic arthritis, spondyloarthropathy and other arthritis anti-MCV antibodies had a prevalence of 15.2%, 13.9% and 19.4%. Conclusion The diagnostic performance of the antitest in the differential diagnosis of early arthritis is lower than that of the anti-CCP tests.Pathophysiology and treatment of rheumatic disease