Autoantibodies From Single Circulating Plasmablasts React With Citrullinated Antigens and Porphyromonas gingivalis in Rheumatoid Arthritis

Anti-Citrullinated Protein Antibodies With Multiple Specificities Ameliorate Collagen Antibody-Induced Arthritis in a Time-Dependent Manner

OBJECTIVE

Anti-citrullinated protein antibodies (ACPAs) are highly specific for rheumatoid arthritis (RA) and have long been regarded as pathogenic. Despite substantial in vitro evidence supporting this claim, reports investigating the proinflammatory effects of ACPAs in animal models of arthritis are rare and include mixed results. Here, we sequenced the plasmablast antibody repertoire of a patient with RA and functionally characterized the encoded ACPAs.

METHODS

We expressed ACPAs from the antibody repertoire of a patient with RA and characterized their autoantigen specificities on antigen arrays and enzyme-linked immunosorbent assays. Binding affinities were estimated by bio-layer interferometry. Select ACPAs (n = 9) were tested in the collagen antibody-induced arthritis (CAIA) mouse model to evaluate their effects on joint inflammation.

RESULTS

Recombinant ACPAs bound preferentially and with high affinity (nanomolar range) to citrullinated (cit) autoantigens (primarily histones and fibrinogen) and to auto-cit peptidylarginine deiminase 4 (PAD4). ACPAs were grouped for in vivo testing based on their predominant cit-antigen specificities. Unexpectedly, injections of recombinant ACPAs significantly reduced paw thickness and arthritis severity in CAIA mice as compared with isotype-matched control antibodies (P ≤ 0.001). Bone erosion, synovitis, and cartilage damage were also significantly reduced (P ≤ 0.01). This amelioration of CAIA was observed for all the ACPAs tested and was independent of cit-PAD4 and cit-fibrinogen specificities. Furthermore, disease amelioration was more prominent when ACPAs were injected at earlier stages of CAIA than at later phases of the model.

CONCLUSION

Recombinant patient-derived ACPAs ameliorated CAIA. Their antiinflammatory effects were more preventive than therapeutic. This study highlights a potential protective role for ACPAs in arthritis.

Periodontitis and periodontopathic bacteria as risk factors for rheumatoid arthritis: A review of the last 10 years

Rheumatoid arthritis (RA) is characterized by chronic inflammatory destruction of joint tissue and is caused by an abnormal autoimmune response triggered by interactions between genetics, environmental factors, and epigenetic and posttranslational modifications. RA has been suggested to be interrelated with periodontitis, a serious form or stage of chronic inflammatory periodontal disease associated with periodontopathic bacterial infections, genetic predisposition, environmental factors, and epigenetic influences. Over the last decade, a number of animal and clinical studies have been conducted to assess whether or not periodontitis and associated periodontopathic bacteria constitute risk factors for RA. The present review introduces recent accumulating evidence to support the associations of periodontitis and periodontopathic bacteria with the risk of RA or the outcome of RA pharmacological treatment with disease-modifying antirheumatic drugs. In addition, the results from intervention studies have suggested an improvement in RA clinical parameters after nonsurgical periodontal treatment. Furthermore, the potential causal mechanisms underlying the link between periodontitis and periodontopathic bacteria and RA are summarized.

Anti-topoisomerase, but not anti-centromere B cell responses in systemic sclerosis display active, Ig-secreting cells associated with lung fibrosis

OBJECTIVES

Almost all patients with systemic sclerosis (SSc) harbour autoantibodies. Anti-topoisomerase antibodies (ATA) and anti-centromere antibodies (ACA) are most prevalent and associate with distinct clinical phenotypes. B cell responses underlying these phenotypes are ill-defined. To understand how B cell autoreactivity and disease pathology connect, we determined phenotypic and functional characteristics of autoreactive B cells in ATA-positive and ACA-positive patients.

METHODS

Levels and isotypes of autoantibodies secreted by ex vivo cultured peripheral blood mononuclear cells from patients with ATA-positive (n=22) and ACA-positive (n=20) SSc were determined. Antibody secreting cells (ASCs) were isolated by cell sorting and cultured separately. Correlations were studied between the degree of spontaneous autoantibody production and the presence and degree of interstitial lung disease (ILD).

RESULTS

Circulating B cells secreting either ATA-immunoglobulin G (IgG) or ACA-IgG on stimulation was readily detectable in patients. The ATA response, but not the ACA response, showed additional secretion of autoreactive IgA. ATA-IgG and ATA-IgA were also secreted spontaneously. Additional cell sorting confirmed the presence of ATA-secreting plasmablasts. The degree of spontaneous ATA-secretion was higher in patients with ILD than in those without (p<0.001) and correlated with the degree of pulmonary fibrosis (p<0.001).

CONCLUSION

In contrast to ACA-positive patients, ATA-positive patients show signs of recent activation of the B cell response that hallmarks this disease. The degree of activation correlates with the presence and severity of ILD, the most deleterious disease manifestation. This could explain differential responsiveness to B cell depleting therapy. The abundant and spontaneous secretion of ATA-IgG and ATA-IgA may point toward a continuously activating trigger.

Characteristics of the (Auto)Reactive T Cells in Rheumatoid Arthritis According to the Immune Epitope Database

T cells are known to be involved in the pathogenesis of rheumatoid arthritis (RA). Accordingly, and to better understand T cells' contribution to RA, a comprehensive review based on an analysis of the Immune Epitope Database (IEDB) was conducted. An immune CD8+ T cell senescence response is reported in RA and inflammatory diseases, which is driven by active viral antigens from latent viruses and cryptic self-apoptotic peptides. RA-associated pro-inflammatory CD4+ T cells are selected by MHC class II and immunodominant peptides, which are derived from molecular chaperones, host extra-cellular and cellular peptides that could be post-translationally modified (PTM), and bacterial cross-reactive peptides. A large panel of techniques have been used to characterize (auto)reactive T cells and RA-associated peptides with regards to their interaction with the MHC and TCR, capacity to enter the docking site of the shared epitope (DRB1-SE), capacity to induce T cell proliferation, capacity to select T cell subsets (Th1/Th17, Treg), and clinical contribution. Among docking DRB1-SE peptides, those with PTM expand autoreactive and high-affinity CD4+ memory T cells in RA patients with an active disease. Considering original therapeutic options in RA, mutated, or altered peptide ligands (APL) have been developed and are tested in clinical trials.

Positive Effects of Biologics on Osteoporosis in Rheumatoid Arthritis

Osteoporosis is a systemic skeletal disorder that causes vulnerability of bones to fracture owing to reduction in bone density and deterioration of the bone tissue microstructure. The prevalence of osteoporosis is higher in patients with autoimmune inflammatory rheumatic diseases, including rheumatoid arthritis (RA), than in those of the general population. In this autoimmune inflammatory rheumatic disease, in addition to known risk factors for osteoporosis, various factors such as chronic inflammation, autoantibodies, metabolic disorders, drugs, and decreased physical activity contribute to additional risk. In RA, disease-related inflammation plays an important role in local or systemic bone loss, and active treatment for inflammation can help prevent osteoporosis. In addition to conventional synthetic disease-modifying anti-rheumatic drugs that have been traditionally used for treatment of RA, biologic DMARDs and targeted synthetic DMARDs have been widely used. These agents can be employed more selectively and precisely based on disease pathogenesis. It has been reported that these drugs can inhibit bone loss by not only reducing inflammation in RA, but also by inhibiting bone resorption and promoting bone formation. In this review, the pathogenesis and research results of the increase in osteoporosis in RA are reviewed, and the effects of biological agents on osteoporosis are discussed.

Anti-Citrullinated Peptide Antibodies Control Oral Porphyromonas and Aggregatibacter species in Patients with Rheumatoid Arthritis

Oral microbiome changes take place at the initiation of rheumatoid arthritis (RA); however, questions remain regarding the oral microbiome at pre-RA stages in individuals with clinically suspect arthralgia (CSA). Two cross-sectional cohorts were selected including 84 Tatarstan women (15 early-RA as compared to individuals with CSA ranging from CSA = 0 [n = 22], CSA = 1 [n = 19], CSA = 2 [n = 11], and CSA ≥ 3 [n = 17]) and 42 women with established RA (median: 5 years from diagnosis [IQ: 2–11]). Amplicon sequence variants (ASVs) obtained from oral samples (16S rRNA) were analyzed for alpha and beta diversity along with the abundance at the genus level. A decrease in oral Porphyromonas sp. is observed in ACPA-positive individuals, and this predominates in early-RA patients as compared to non-RA individuals irrespective of their CSA score. In the RA-established cohort, Porphyromonas sp. and Aggregatibacter sp. reductions were associated with elevated ACPA levels. In contrast, no associations were reported when considering individual, genetic and clinical RA-associated factors. Oral microbiome changes related to the genera implicated in post-translational citrullination (Porphyromonas sp. and Aggregatibacter sp.) characterized RA patients with elevated ACPA levels, which supports that the role of ACPA in controlling the oral microbiome needs further evaluation.

Assessment of plasmablast cells in immune thrombocytopenic purpura in children

Primary immune thrombocytopenic purpura (ITP) is an autoimmune disorder with platelet destruction due to B- and T-cell dysregulation and antiplatelet autoantibodies production. Flow cytometry can be used to further characterize the B- and T-cell compartments involved in platelet destruction. This case-control study was to enumerate plasmablast cells in pediatric ITP patients and to correlate their levels with disease course. This study included 30 ITP patients and 10 controls. Identification and enumeration of Plasmablast were done by multicolor flow cytometry using specific antibody panels (CD19, CD27 & CD38) and sequential gating using FACSCanto flow cytometer and FlowJo software. We found that lymphocytes subpopulation in ITP patients and controls revealed increase in frequency of CD19 (B lymphocytes) in acute, persistent, and chronic ITP patients in comparison with controls (p < 0.001, 0.023, 0.001) respectively. Plasmablast cells could play a role in the pathogenesis of ITP and might guide therapy in ITP patients in the future.

Disease mechanisms in preclinical rheumatoid arthritis: A narrative review

In the last decades, the concept of preclinical rheumatoid arthritis (RA) has become established. In fact, the discovery that disease mechanisms start years before the onset of clinical RA has been one of the major recent insights in the understanding of RA pathogenesis. In accordance with the complex nature of the disease, preclinical events extend over several sequential phases. In a genetically predisposed host, environmental factors will further increase susceptibility for incident RA. In the initial steps of preclinical disease, immune disturbance mechanisms take place outside the joint compartment, namely in mucosal surfaces, such as the lung, gums or gut. Herein, the persistent immunologic response to altered antigens will lead to breach of tolerance and trigger autoimmunity. In a second phase, the immune response matures and is amplified at a systemic level, with epitope spreading and widening of the autoantibody repertoire. Finally, the synovial and bone compartment are targeted by specific autoantibodies against modified antigens, initiating a local inflammatory response that will eventually culminate in clinically evident synovitis. In this review, we discuss the elaborate disease mechanisms in place during preclinical RA, providing a broad perspective in the light of current evidence.

Outer membrane vesicles of the oral pathogen Porphyromonas gingivalis promote aggregation and phagocytosis of Staphylococcus aureus

Staphylococcus aureus is an opportunistic Gram-positive bacterial pathogen that causes a wide variety of infectious diseases, including S. aureus bacteremia (SAB). Recent studies showed that rheumatoid arthritis (RA) is a risk factor for SAB, as RA patients appear to be more susceptible to SAB and display higher degrees of disease severity or complications, such as osteoarticular infections. On the other hand, Porphyromonas gingivalis is a Gram-negative bacterial oral pathogen, which is notable for its implication in the etiopathogenesis of RA due to its unique citrullinating enzyme PPAD and its highly effective proteases, known as gingipains. Both PPAD and gingipains are abundant in P. gingivalis outer membrane vesicles (OMVs), which are secreted nanostructures that originate from the outer membrane. Here we show that P. gingivalis OMVs cause the aggregation of S. aureus bacteria in a gingipain- and PPAD-dependent fashion, and that this aggregation phenotype is reversible. Importantly, we also show that the exposure of S. aureus to OMVs of P. gingivalis promotes the staphylococcal internalization by human neutrophils with no detectable neutrophil killing. Altogether, our observations suggest that P. gingivalis can eliminate its potential competitor S. aureus by promoting staphylococcal aggregation and the subsequent internalization by neutrophils. We hypothesize that this phenomenon may have repercussions for the host, since immune cells with internalized bacteria may facilitate bacterial translocation to the blood stream, which could potentially contribute to the association between RA and SAB.

Integrated single cell and spatial transcriptomics reveal autoreactive differentiated B cells in joints of early rheumatoid arthritis

B cells play a significant role in established Rheumatoid Arthritis (RA). However, it is unclear to what extent differentiated B cells are present in joint tissue already at the onset of disease. Here, we studied synovial biopsies (n = 8) captured from untreated patients at time of diagnosis. 3414 index-sorted B cells underwent RNA sequencing and paired tissue pieces were subjected to spatial transcriptomics (n = 4). We performed extensive bioinformatics analyses to dissect the local B cell composition. Select plasma cell immunoglobulin sequences were expressed as monoclonal antibodies and tested by ELISA. Memory and plasma cells were found irrespective of autoantibody status of the patients. Double negative memory B cells were prominent, but did not display a distinct transcriptional profile. The tissue architecture implicate both local B cell maturation via T cell help and plasma cell survival niches with a strong CXCL12–CXCR4 axis. The immunoglobulin sequence analyses revealed clonality between the memory B and plasma cell pools further supporting local maturation. One of the plasma cell-derived antibodies displayed citrulline autoreactivity, demonstrating local autoreactive plasma cell differentiation in joint biopsies captured from untreated early RA. Hence, plasma cell niches are not a consequence of chronic inflammation, but are already present at the time of diagnosis.

Antibodies to a Citrullinated Porphyromonas gingivalis Epitope Are Increased in Early Rheumatoid Arthritis, and Can Be Produced by Gingival Tissue B Cells: Implications for a Bacterial Origin in RA Etiology

Based on the epidemiological link between periodontitis and rheumatoid arthritis (RA), and the unique feature of the periodontal bacterium Porphyromonas gingivalis to citrullinate proteins, it has been suggested that production of anti-citrullinated protein antibodies (ACPA), which are present in a majority of RA patients, may be triggered in the gum mucosa. To address this hypothesis, we investigated the antibody response to a citrullinated P. gingivalis peptide in relation to the autoimmune ACPA response in early RA, and examined citrulline-reactivity in monoclonal antibodies derived from human gingival B cells. Antibodies to a citrullinated peptide derived from P. gingivalis (denoted CPP3) and human citrullinated peptides were analyzed by multiplex array in 2,807 RA patients and 372 controls; associations with RA risk factors and clinical features were examined. B cells from inflamed gingival tissue were single-cell sorted, and immunoglobulin (Ig) genes were amplified, sequenced, cloned and expressed (n=63) as recombinant monoclonal antibodies, and assayed for citrulline-reactivities by enzyme-linked immunosorbent assay. Additionally, affinity-purified polyclonal anti-cyclic-citrullinated peptide (CCP2) IgG, and monoclonal antibodies derived from RA blood and synovial fluid B cells (n=175), were screened for CPP3-reactivity. Elevated anti-CPP3 antibody levels were detected in RA (11%), mainly CCP2+ RA, compared to controls (2%), p<0.0001, with a significant association to HLA-DRB1 shared epitope alleles, smoking and baseline pain, but with low correlation to autoimmune ACPA fine-specificities. Monoclonal antibodies derived from gingival B cells showed cross-reactivity between P. gingivalis CPP3 and human citrullinated peptides, and a CPP3+/CCP2+ clone, derived from an RA blood memory B cell, was identified. Our data support the possibility that immunity to P. gingivalis derived citrullinated antigens, triggered in the inflamed gum mucosa, may contribute to the presence of ACPA in RA patients, through mechanisms of molecular mimicry.

Effects of Biological/Targeted Therapies on Bone Mineral Density in Inflammatory Arthritis

Inflammatory arthritis has been reported to be associated with the development of osteoporosis. Recent research has investigated the mechanisms of bone metabolism in chronic inflammatory arthritis such as rheumatoid arthritis (RA) and spondyloarthritis (SpA). Progress in both animal and clinical studies has provided a better understanding of the osteoclastogenesis-related pathways regarding the receptor activator of nuclear factor-κB ligand (RANKL), anti-citrullinated protein antibodies (ACPAs), and Wnt signaling and Dickkopf-related protein 1 (Dkk-1). The complex interplay between inflammatory cytokines and bone destruction has been elucidated, especially that in the interleukin-17/23 (IL-17/23) axis and Janus kinase and signal transducer and activator of transcription (JAK-STAT) signaling. Moreover, advances in biological and targeted therapies have achieved essential modifications to the bone metabolism of these inflammatory arthritis types. In this narrative review, we discuss recent findings on the pathogenic effects on bone in RA and SpA. Proinflammatory cytokines, autoantibodies, and multiple signaling pathways play an essential role in bone destruction in RA and SpA patients. We also reviewed the underlying pathomechanisms of bone structure in biological and targeted therapies of RA and SpA. The clinical implications of tumor necrosis factor inhibitors, abatacept, rituximab, tocilizumab, Janus kinase inhibitors, and inhibitors of the IL-17/23 axis are discussed. Since these novel therapeutics provide new options for disease improvement and symptom control in patients with RA and SpA, further rigorous evidence is warranted to provide a clinical reference for physicians and patients.

Impact of Posttranslational Modification in Pathogenesis of Rheumatoid Arthritis: Focusing on Citrullination, Carbamylation, and Acetylation

Rheumatoid arthritis (RA) is caused by prolonged periodic interactions between genetic, environmental, and immunologic factors. Posttranslational modifications (PTMs) such as citrullination, carbamylation, and acetylation are correlated with the pathogenesis of RA. PTM and cell death mechanisms such as apoptosis, autophagy, NETosis, leukotoxic hypercitrullination (LTH), and necrosis are related to each other and induce autoantigenicity. Certain microbial infections, such as those caused by Porphyromonasgingivalis, Aggregatibacter actinomycetemcomitans, and Prevotella copri, can induce autoantigens in RA. Anti-modified protein antibodies (AMPA) containing anti-citrullinated protein/peptide antibodies (ACPAs), anti-carbamylated protein (anti-CarP) antibodies, and anti-acetylated protein antibodies (AAPAs) play a role in pathogenesis as well as in prediction, diagnosis, and prognosis. Interestingly, smoking is correlated with both PTMs and AMPAs in the development of RA. However, there is lack of evidence that smoking induces the generation of AMPAs.

The Germinal Center Milieu in Rheumatoid Arthritis: The Immunological Drummer or Dancer?

Rheumatoid Arthritis (RA) is a chronic autoimmune disease characterized by joint inflammation, affecting approximately 1% of the general population. To alleviate symptoms and ameliorate joint damage, chronic use of immunosuppressives is needed. However, these treatments are only partially effective and may lead to unwanted side effects. Therefore, a more profound understanding of the pathophysiology might lead to more effective therapies, or better still, a cure. The presence of autoantibodies in RA indicates that B cells might have a pivotal role in the disease. This concept is further supported by the fact that a diverse antibody response to various arthritis-related epitopes is associated with arthritis development. In this context, attention has focused in recent years on the role of Germinal Centers (GCs) in RA. Since GCs act as the main anatomic location of somatic hypermutations, and, thus, contributing to the diversity and specificity of (auto) antibodies, it has been speculated that defects in germinal center reactions might be crucial in the initiation and maintenance of auto-immune events. In this paper, we discuss current evidence that various processes within GCs can result in the aberrant production of B cells that possess autoreactive properties and might result in the production of RA related autoantibodies. Secondly, we discuss various (pre-)clinical studies that have targeted various GC processes as novel therapies for RA treatment.

Periodontitis and rheumatoid arthritis: What have we learned about their connection and their treatment?

Rheumatoid arthritis and periodontitis are chronic inflammatory diseases defined respectively by the destruction of the articular cartilage and tooth-supporting periodontal tissues. Although the epidemiologic evidence for an association between these two diseases is still scarce, there is emerging scientific information linking specific bacterial periodontal pathogens, such as Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans, in the citrullination process, leading to autoantibody formation and compromised immunotolerance of the susceptible patient to rheumatoid arthritis. In this review, we update the existing information on the evidence, not only regarding the epidemiologic association, but also the biologic mechanisms linking these two diseases. Finally, we review information emerging from intervention studies evaluating whether periodontal treatment could influence the initiation and progression of rheumatoid arthritis.

Physiologic Target, Molecular Evolution, and Pathogenic Functions of a Monoclonal Anti-Citrullinated Protein Antibody Obtained From a Patient With Rheumatoid Arthritis

OBJECTIVE

In plasma from a patient with rheumatoid arthritis (RA), we previously isolated a human monoclonal anti-citrullinated protein antibody (ACPA), CCP-Ab1, that recognizes various citrullinated antigens. In this study, we aimed to explore the physiologic target of CCP-Ab1 and the role of molecular evolution, through affinity maturation, of this ACPA in the onset and the exacerbation of RA.

METHODS

The target protein of CCP-Ab1 was identified in the plasma of a patient with RA and purified under native conditions. Germline-reverted (GL-rev) CCP-Ab1 was generated, and its reactivity was compared to that of mature CCP-Ab1. The functions of CCP-Ab1 and GL-rev CCP-Ab1 in the onset or exacerbation of autoimmune arthritis were analyzed using autoimmune arthritis-prone SKG mice.

RESULTS

CCP-Ab1 bound citrullinated fibrinogen under native conditions. In cultures with GL-rev CCP-Ab1, the binding affinity to citrullinated fibrinogen was drastically reduced (P < 0.05). The elements implicated in GL-rev CCP-Ab1 binding to a citrullinated peptide, cfc1-cyc, were almost identical to those implicated in CCP-Ab1 binding. In arthritis-prone SKG mice, CCP-Ab1, but not GL-rev CCP-Ab1, induced significant exacerbation of experimental arthritis (P < 0.05). Increased production of interleukin-6, both in the joint tissue and in the serum, was observed in SKG mice treated with CCP-Ab1 compared to those treated with GL-rev CCP-Ab1 (P < 0.05). Furthermore, the immune complex formed by CCP-Ab1 and fibrinogen was detected at higher concentrations in the synovial tissue of SKG mice administered CCP-Ab1 (P < 0.05 versus control treatment groups).

CONCLUSION

These data show that germline-encoded CCP-Ab1, which binds weakly to citrullinated fibrinogen, undergoes hypermutation through the activation of naive B cells by citrullinated peptides/proteins, thereby stimulating high reactivity to citrullinated fibrinogen. These findings deepen our understanding of the role of molecular evolution of ACPAs in the onset and exacerbation of RA.

Production of Autoreactive Heavy Chain-Only Antibodies in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is characterized by the overproduction of high-affinity autoreactive antibodies. Here, we show that more than 65.8% of 222 recombinant antibodies derived from 8 SLE patients can be secreted as heavy chain-only antibodies (HCAbs) when expressed in HEK-293T cells. The secretion of HCAbs follows the conventional endoplasmic reticulum-Golgi apparatus pathway, despite triggering a weaker unfolded protein response (UPR). Many of the purified SLE HCAbs remain autoreactive and have an even higher affinity for dsDNA, Sm, nucleosome, and cardiolipin than HCAbs from healthy individuals. Extended analyses of the CDR3 region and the heavy chain variable (VH) region of HCAb F3 show that the VH region is responsible for IgH secretion, while the CDR3 region determines its reactivity. Such a high frequency of HCAb secretion cannot fully concur with our current understanding of antibody assembly and secretion. The presence of a large proportion of autoreactive HCAbs in SLE reveals a novel mechanism for the generation of autoreactive antibodies in lupus.

B cells in rheumatoid arthritis synovial tissues encode focused antibody repertoires that include antibodies that stimulate macrophage TNF-α production

Rheumatoid arthritis (RA) is characterized by the production of anti-citrullinated protein antibodies (ACPAs). To gain insights into the relationship between ACPA-expressing B cells in peripheral blood (PB) and synovial tissue (ST), we sequenced the B cell repertoire in paired PB and ST samples from five individuals with established, ACPA+ RA. Bioinformatics analysis of paired heavy- and light-chain sequences revealed clonally-related family members shared between PB and ST. ST-derived antibody repertoires exhibited reduced diversity and increased normalized clonal family size compared to PB-derived repertoires. Functional characterization showed that seven recombinant antibodies (rAbs) expressed from subject-derived sequences from both compartments bound citrullinated antigens and immune complexes (ICs) formed using one ST-derived rAb stimulated macrophage TNF-α production. Our findings demonstrate B cell trafficking between PB and ST in subjects with RA and ST repertoires include B cells that encode ACPA capable of forming ICs that stimulate cellular responses implicated in RA pathogenesis.

The etiology of rheumatoid arthritis

Rheumatoid arthritis is a heterogeneous disease, which can be, based on data combining genetic risk factors and autoantibodies, sub-classified into ACPA-positive and -negative RA. Presence of ACPA and RF as well as rising CRP-levels in some patients years before onset of clinical symptoms indicate that relevant immune responses for RA development are initiated very early. ACPA are highly specific for RA, whereas RF can also be found among healthy (elderly) individuals and patients with other autoimmune diseases or infection. The most important genetic risk factor for RA development, the shared epitope alleles, resides in the MHC class II region. Shared epitope alleles, however, only predispose to the development of ACPA-positive RA. Smoking is thus far the most important environmental risk factor associated with the development of RA. Studies on synovitis have shown the importance not only of adaptive but also of innate immune responses. In summary of the various results from immunological changes in blood and synovial tissue, the extension of the immune response from a diffuse myeloid to a lympho-myeloid inflammation appears to be associated with a more successful therapeutic response to biologics. With respect to advances in synovitis research, new targets for treatment against pathological subsets of immune cells or fibroblasts are already on the horizon. However, alternative strategies involving the microbiome may play an important role as well and research in this field is growing rapidly.

Iguratimod dose dependently inhibits the expression of citrullinated proteins and peptidylarginine deiminases 2 and 4 in neutrophils from rheumatoid arthritis patients

INTRODUCTION

Anti-citrullinated protein antibodies (ACPAs) play an important role in rheumatoid arthritis (RA). Citrullinated proteins (CPs), which are produced by post-translational modification via peptidylarginine deiminase (PAD), are the target antigen of ACPAs and promote the generation thereof. Herein, we investigated whether iguratimod (IGU) affects the generation of CPs via PAD.

METHODS

Neutrophils and peripheral blood mononuclear cells (PBMCs) were isolated from three patients diagnosed with RA and treated with various concentrations of IGU, methotrexate (MTX), or dexamethasone (DXM) or without any drugs as a control for 8 h. The levels of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and IL-8 in culture supernatants were tested by ELISA. CPs were measured by western blot, and the expression of PAD2 and PAD4 in cells was detected by qRT-PCR and western blot.

RESULTS

PAD2 and PAD4 expressions in neutrophils but not in PBMCs were decreased by IGU at both the protein and mRNA levels (P < 0.05). CP expression in neutrophils but not in PBMCs was also inhibited by IGU. The inhibitory effect of IGU was dose-dependent. IGU, MTX, and DXM dose dependently decreased the secretion of TNF-α, IL-1β, IL-6, and IL-8 in neutrophils and PBMCs (P < 0.05); the inhibitory effect of IGU was not significantly different from that of MTX and DXM.

CONCLUSIONS

IGU inhibited the expression of CPs by downregulating PADs in neutrophils from RA patients, and the effect was comparable to that of MTX and DXM at appropriate concentrations. These findings may provide guidance for more appropriate treatment of RA.Key Points• Iguratimod inhibited citrullinated protein expression in neutrophils from rheumatoid arthritis patients similarly to methotrexate and dexamethasone at appropriate concentrations.• The inhibitory effect was mediated by downregulation of peptidylarginine deiminases, providing insight into the mechanism of iguratimod as a treatment for rheumatoid arthritis.• This study may guide rheumatoid arthritis treatment and facilitate identification of other therapeutic targets.

Recognition of Amino Acid Motifs, Rather Than Specific Proteins, by Human Plasma Cell-Derived Monoclonal Antibodies to Posttranslationally Modified Proteins in Rheumatoid Arthritis

OBJECTIVE

Antibodies against posttranslationally modified proteins are a hallmark of rheumatoid arthritis (RA), but the emergence and pathogenicity of these autoantibodies are still incompletely understood. The aim of this study was to analyze the antigen specificities and mutation patterns of monoclonal antibodies (mAb) derived from RA synovial plasma cells and address the question of antigen cross-reactivity.

METHODS

IgG-secreting cells were isolated from RA synovial fluid, and the variable regions of the immunoglobulins were sequenced (n = 182) and expressed in full-length mAb (n = 93) and also as germline-reverted versions. The patterns of reactivity with 53,019 citrullinated peptides and 49,211 carbamylated peptides and the potential of the mAb to promote osteoclastogenesis were investigated.

RESULTS

Four unrelated anti-citrullinated protein autoantibodies (ACPAs), of which one was clonally expanded, were identified and found to be highly somatically mutated in the synovial fluid of a patient with RA. The ACPAs recognized >3,000 unique peptides modified by either citrullination or carbamylation. This highly multireactive autoantibody feature was replicated for Ig sequences derived from B cells from the peripheral blood of other RA patients. The plasma cell-derived mAb were found to target distinct amino acid motifs and partially overlapping protein targets. They also conveyed different effector functions as revealed in an osteoclast activation assay.

CONCLUSION

These findings suggest that the high level of cross-reactivity among RA autoreactive B cells is the result of different antigen encounters, possibly at different sites and at different time points. This is consistent with the notion that RA is initiated in one context, such as in the mucosal organs, and thereafter targets other sites, such as the joints.

The B side of rheumatoid arthritis pathogenesis

In the last years, a dramatic amount of research has been performedincreasing the knowledge about the biological mechanism underpinning Rheumatoid Arthritis (RA) inflammation, putting B lymphocytes in the center of RA pathogenesis. Nowadays, B cell phenotypes and autoantibodies positivity arose as important biomarkers in early and long-standing disease. Moreover, comparative analysis of peripheral blood and synovial tissue compartments enables the identification of novel physiopathological mechanisms promoting inflammation. In this narrative review we will discuss the biological relevance of B cell derived autoimmunity and in RA course, from disease onset to remission achievement.

Rheumatoid Arthritis-Associated Mechanisms of Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans

Rheumatoid arthritis (RA) is an autoimmune disease of unknown etiology characterized by immune-mediated damage of synovial joints and antibodies to citrullinated antigens. Periodontal disease, a bacterial-induced inflammatory disease of the periodontium, is commonly observed in RA and has implicated periodontal pathogens as potential triggers of the disease. In particular, Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans have gained interest as microbial candidates involved in RA pathogenesis by inducing the production of citrullinated antigens. Here, we will discuss the clinical and mechanistic evidence surrounding the role of these periodontal bacteria in RA pathogenesis, which highlights a key area for the treatment and preventive interventions in RA.

Mucosa Biology and the Development of Rheumatoid Arthritis: Potential for Prevention by Targeting Mucosal Processes

As the goal in rheumatoid arthritis (RA) management shifts toward the prevention of joint disease, it is important to consider the role of mucosal sites in the pathogenesis of RA because they may be potential targets for preventive interventions. Multiple mucosal sites demonstrate immune dysregulation and inflammation in individuals with classifiable RA as well as, importantly, in individuals with systemic autoimmunity related to RA. The lung, gingival, and gastrointestinal mucosae are most strongly implicated in RA pathogenesis and may be sites where autoimmunity in RA initially develops. Targeting the exact site where the initial immune dysregulation in RA occurs is an appealing approach to prevention because it could avoid unwanted side effects of systemic therapies. However, several challenges must be addressed before mucosa-targeted interventions are a readily available option for RA prevention. Studies are needed to determine whether all RA-related immune dysregulation at mucosal sites will progress to joint disease and whether one or multiple mucosal sites demonstrate dysregulation prior to the development of classifiable RA. These areas of future research are likely to provide crucial pieces in the understanding of RA pathogenesis and ultimately RA prevention.

The Influence of Porphyromonas Gingivalis Bacterium Causing Periodontal Disease on the Pathogenesis of Rheumatoid Arthritis: Systematic Review of Literature

Periodontal pathologies are highly widespread throughout the world. Epidemiological studies have shown that as much as 1% of the population is suffering from periodontal disease. In recent years, there has been a growing number of studies linking these diseases with autoimmune diseases, especially with rheumatoid arthritis. This literature review evaluates changes in the relationship between periodontal pathologies caused by the bacterium Porphyromonas gingivalis and rheumatoid arthritis. The systematic review of the literature was performed according to the PRISMA analysis protocol. The review was performed with articles from the PubMed database. Searched articles were not older than 5 years. Only full texts and research performed with people were selected. A total of 56 results were received. A review and analysis of their full texts have been carried out and 10 articles were selected according to the established criteria. They were analyzed and results were presented. The results obtained from the literature were based on the influence of Porphyromonas gingivalis on the pathogenesis of rheumatoid arthritis. In the literature, the activity of this bacterium is explained by the analysis of its enzyme peptidylarginine deiminase and its principle of action. Studies have also been found to prove the presence of Porphyromonas gingivalis not only in the oral cavity but its DNA is also found in synovial fluid and plasma. In the researched articles, direct links between Porphyromonas gingivalis and rheumatoid arthritis have led doctors to draw attention to patients' oral hygiene and the condition of parodentium, as this may be the cause of autoimmune lesions. Treatment of periodontal disease will not only help maintain a healthy oral cavity but prevent the spread of bacteria to the surrounding tissues.

Affinity Maturation of the Anti-Citrullinated Protein Antibody Paratope Drives Epitope Spreading and Polyreactivity in Rheumatoid Arthritis

OBJECTIVE

Anti-citrullinated protein antibodies (ACPAs) are a hallmark of rheumatoid arthritis (RA). While epitope spreading of the serum ACPA response is believed to contribute to RA pathogenesis, little is understood regarding how this phenomenon occurs. This study was undertaken to analyze the antibody repertoires of individuals with RA to gain insight into the mechanisms leading to epitope spreading of the serum ACPA response in RA.

METHODS

Plasmablasts from the blood of 6 RA patients were stained with citrullinated peptide tetramers to identify ACPA-producing B cells by flow cytometry. Plasmablasts were single-cell sorted and sequenced to obtain antibody repertoires. Sixty-nine antibodies were recombinantly expressed, and their anticitrulline reactivities were characterized using a cyclic citrullinated peptide enzyme-linked immuosorbent assay and synovial antigen arrays. Thirty-six mutated antibodies designed either to represent ancestral antibodies or to test paratope residues critical for binding, as determined from molecular modeling studies, were also tested for anticitrulline reactivities.

RESULTS

Clonally related monoclonal ACPAs and their shared ancestral antibodies each exhibited differential reactivity against citrullinated antigens. Molecular modeling identified residues within the complementarity-determining region loops and framework regions predicted to be important for citrullinated antigen binding. Affinity maturation resulted in mutations of these key residues, which conferred binding to different citrullinated epitopes and/or increased polyreactivity to citrullinated epitopes.

CONCLUSION

These results demonstrate that the different somatic hypermutations accumulated by clonally related B cells during affinity maturation alter the antibody paratope to mediate epitope spreading and polyreactivity of the ACPA response in RA, suggesting that these may be key properties that likely contribute to the pathogenicity of ACPAs.

Talk to your gut: the oral-gut microbiome axis and its immunomodulatory role in the etiology of rheumatoid arthritis

Microbial communities inhabiting the human body, collectively called the microbiome, are critical modulators of immunity. This notion is underpinned by associations between changes in the microbiome and particular autoimmune disorders. Specifically, in rheumatoid arthritis, one of the most frequently occurring autoimmune disorders worldwide, changes in the oral and gut microbiomes have been implicated in the loss of tolerance against self-antigens and in increased inflammatory events promoting the damage of joints. In the present review, we highlight recently gained insights in the roles of microbes in the etiology of rheumatoid arthritis. In addition, we address important immunomodulatory processes, including biofilm formation and neutrophil function, which have been implicated in host-microbe interactions relevant for rheumatoid arthritis. Lastly, we present recent advances in the development and evaluation of emerging microbiome-based therapeutic approaches. Altogether, we conclude that the key to uncovering the etiopathogenesis of rheumatoid arthritis will lie in the immunomodulatory functions of the oral and gut microbiomes.

Pathogenic Citrulline-Multispecific B Cell Receptor Clades in Rheumatoid Arthritis

OBJECTIVE

Anti-citrullinated protein antibodies (ACPAs) have proven highly useful as biomarkers for rheumatoid arthritis (RA). However, composition and functionality of the associated autoreactive B cell repertoire have not been directly assessed. We aimed to selectively investigate citrullinated autoantigen-specific B cell receptors (BCRs) involved in RA and initiate studies on their pathogenicity.

METHODS

Blood samples were obtained from patients in a University of Minnesota cohort with ACPA-positive RA (n = 89). Tetramer sets bearing citrullinated filaggrin peptide cfc1 or citrullinated α-enolase peptide were constructed to specifically capture autoreactive B cells from the unaltered, polyclonal repertoire in RA patients. Citrullinated peptide tetramer-bound B cells were subjected to flow cytometric cell sorting and single-cell IGH, IGK, and IGL gene sequencing for B cell lineage determinations. BCR gene sequences were also expressed as recombinant monoclonal antibodies (mAb) for direct evaluation of citrullinated autoantigen binding and effector functionality.

RESULTS

Using citrullinated peptide tetramer enrichment to investigate single autoreactive blood B cells, we identified biased V-region gene usage and conserved junction arrangements in BCRs from RA patients. Parsimonious clustering of related immunoglobulin gene nucleotide sequences revealed clonal expansions of rare individual B cell clades, in parallel with divergent sequence mutations. Correspondingly, recombinant mAb generated from such BCR lineages demonstrated citrulline-dependent cross-reactivity extending beyond the citrullinated peptides used for B cell capture. A pair of citrullinated autoantigen-specific mAb with cross-reactive binding profiles also promoted arthritis in mice.

CONCLUSION

Our findings suggest that broad ACPA specificities in RA arise from a restricted repertoire of evolving citrulline-multispecific B cell clades with pathogenic potential.

Rheumatoid arthritis and the mucosal origins hypothesis: protection turns to destruction

Individuals at high risk of developing seropositive rheumatoid arthritis (RA) can be identified for translational research and disease prevention studies through the presence of highly informative and predictive patterns of RA-related autoantibodies, especially anti-citrullinated protein antibodies (ACPAs), in the serum. In serologically positive individuals without arthritis, designated ACPA positive at risk, the presence of mucosal inflammatory processes associated with the presence of local ACPA production has been demonstrated. In other at-risk populations, local RA-related autoantibody production is present even in the absence of serum autoantibodies. Additionally, a proportion of at-risk individuals exhibit local mucosal ACPA production in the lung, as well as radiographic small-airway disease, sputum hypercellularity and increased neutrophil extracellular trap formation. Other mucosal sites in at-risk individuals also exhibit autoantibody production, inflammation and/or evidence of dysbiosis. As the proportion of individuals who exhibit such localized inflammation-associated ACPA production is substantially higher than the likelihood of an individual developing future RA, this finding raises the hypothesis that mucosal ACPAs have biologically relevant protective roles. Identifying the mechanisms that drive both the generation and loss of externally focused mucosal ACPA production and promote systemic autoantibody expression and ultimately arthritis development should provide insights into new therapeutic approaches to prevent RA.

Antibody Responses to Citrullinated and Noncitrullinated Antigens in the Sputum of Subjects With Rheumatoid Arthritis and Subjects at Risk for Development of Rheumatoid Arthritis

OBJECTIVE

The location and mechanisms involved in the initial generation of autoantibodies to citrullinated and noncitrullinated proteins/peptides during the natural history of rheumatoid arthritis (RA) development is incompletely understood. This study sought to explore individual antibody responses to citrullinated and noncitrullinated proteins/peptides in the sputum and associations with neutrophil extracellular traps (NETs) in subjects at risk for the future development of RA.

METHODS

Serum and sputum samples were obtained from 41 RA-free subjects who were considered at risk for the development of RA based on familial or serologic risk factors, from 20 subjects classified as having RA, and from 22 healthy control subjects. Samples were evaluated using a bead-based array for IgG reactivity to 29 citrullinated proteins/peptides and 21 noncitrullinated proteins/peptides. Cutoff levels for antibody positivity were established in a separate control group. NET levels in the sputum were measured using sandwich enzyme-linked immunosorbent assays that quantitate DNA-myeloperoxidase and DNA-neutrophil elastase complexes.

RESULTS

In at-risk subjects, antibody responses to the citrullinated forms of fibrinogen, apolipoprotein E, and fibronectin were highly prevalent. The most citrulline-specific antibodies in the sputum of at-risk subjects were those to fibrinogen, vimentin, and peptides of fibrinogen A and apolipoprotein A1. Patterns of sputum autoantibody positivity differed between at-risk subjects and subjects with RA. In at-risk subjects, increasing sputum NET levels significantly correlated with several citrullinated and some noncitrullinated antibody reactivities.

CONCLUSION

These findings suggest that sputum antibody reactivity to particular citrullinated and noncitrullinated proteins/peptides is specific for RA and for subjects at risk of RA, and the association of these proteins/peptides with NETs may be a key feature of early RA-related autoimmunity in the lung. These results further support the hypothesis that the lung plays a role in early RA-related autoimmunity.

Papel de la Porphyromonas gingivalis en la patogenicidad de la Artritis Reumatoide: revisión de la literatura

Introducción: la evidencia cientí ca sobre la Artritis Reumatoide (AR) y la Enfermedad Periodontal (EP) se ha orientado hacia la presencia del periodontopatógeno Porphyromonas gingivalis (P.g). Se ha establecido, a partir de diferentes estudios, su relación con el proceso de citrulinación y producción de anticuerpos antipéptidos citrulinados. Actualmente, hay una heterogénea evidencia cientí ca con nuevos aportes y variabilidad de hallazgos entre las dos enfermedades, pero los estudios en humanos son los que más generan interés. Objetivo: revisar evidencia cientí ca sobre estudios clínicos relacionados con la patogenicidad de la Enfermedad Periodontal y Porphyromonas gingivalis en la Artritis Reumatoide. Metodología: por medio de una búsqueda se identi caron las publicaciones realizadas con los tópicos de nidos y te- niendo en cuenta el análisis de contenido de estudios clínicos. Se revisó en el período de 2012-2016, los términos de búsqueda empleados fueron: rheumatoid arthritis y Porphyromonas gingivalis, con una lectura inicial basada en títulos y resúmenes, se excluyeron revisiones, reporte de casos, estudios in vitro y en animales. Resultados: después de realizar la búsqueda en tres bases de datos (PubMed, Lilacs y Embase), se encontraron 166 artículos, de los cuales 140 fueron rechazados y 25 fueron incluidos, pues describían estudios clínicos entre AR y P.g. La mayoría mostró una investigación de tipo cuantitativo, determinando la presencia de P.g en los pacientes con AR. Es clara la presencia de anticuerpos de P.g en suero y se ha reportado poca presencia en líquido sinovial, se ha identi cado ADN bacteriano en pacientes con AR y enfermedad periodontal. Nueva evidencia sugiere asociaciones con otros patógenos y detección en artritis de aparición temprana. 

The role of anti-citrullinated protein antibodies (ACPA) in the pathogenesis of rheumatoid arthritis

The most specific autoimmunity known for rheumatoid arthritis (RA) is reflected by generation of anti-citrullinated protein antibodies (ACPA). Presence of ACPA in established RA is associated with disease severity, while generation of ACPA at early developmental phases of RA can have a strong predictive value for progressing to the full-blown disease. Hence, development of ACPA may be of crucial importance to the pathogenesis of RA. Therefore, a lot of effort has been put recently to investigate the feature of ACPA at early developmental stages of RA (before disease onset) and functional activities of these autoantibodies. Results of these studies enlarged the knowledge about the nature of ACPA, which is essential for planning the therapeutic or preventive strategies interfering with their development and pathogenic functions. In this review we describe recent evidence for a role of ACPA in the etiopathogenesis of RA and indicate key unresolved issues regarding ACPA biology that need to be clarified in the future.

Affinity maturation shapes the function of agonistic antibodies to peptidylarginine deiminase type 4 in rheumatoid arthritis

OBJECTIVES

The citrullinating enzyme peptidylarginine deiminase type 4 (PAD4) is the target of a polyclonal group of autoantibodies in patients with rheumatoid arthritis (RA). A subgroup of such antibodies, initially identified by cross-reactivity with peptidylarginine deiminase type 3 (PAD3), is strongly associated with progression of radiographic joint damage and interstitial lung disease and has the unique ability to activate PAD4. The features of these antibodies in terms of their T cell-dependent origin, genetic characteristics and effect of individual antibody specificities on PAD4 function remain to be defined.

METHODS

We used PAD4 tagged with the monomeric fluorescent protein mWasabi to isolate PAD4-specific memory B cells from anti-PAD4 positive patients with RA and applied single cell cloning technologies to obtain monoclonal antibodies.

RESULTS

Among 44 single B cells, we cloned five antibodies with PAD4-activating properties. Sequence analysis, germline reversion experiments and antigen specificity assays suggested that autoantibodies to PAD4 are not polyreactive and arise from PAD4-reactive precursors. Somatic mutations increase the agonistic activity of these antibodies at low calcium concentrations by facilitating their interaction with structural epitopes that modulate calcium-binding site 5 in PAD4.

CONCLUSIONS

PAD4-activating antibodies directly amplify a key process in disease pathogenesis, making them unique among other autoantibodies in RA. Understanding the molecular basis for their functionality may inform the design of future PAD4 inhibitors.

The Infectious Basis of ACPA-Positive Rheumatoid Arthritis

Rheumatoid arthritis (RA) is associated with HLA-DRB1 shared epitope (HLA-DRB1SE) and anti-citrullinated protein autoantibodies (ACPAs). ACPAs precedes the onset of clinical and subclinical RA. There are strong data for three infectious agents as autoimmunity triggers in RA, namely Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans causes of periodontal disease (PD), and Epstein-Barr virus (EBV). P. gingivalis expresses arginine gingipains, that cleave proteins at the arginine residues, and peptidyl arginine deiminase (PPAD), which citrullinates arginine residues of proteins, thus forming neoantigens that lead to ACPA production. Peripheral blood plasmablasts from ACPA+RA patients produce ACPAs the majority of which react against P. gingivalis. A. actinocycetemcomitans produces leukotoxin A, a toxin that forms pores in the neutrophil membranes and leads to citrullination and release of citrullinated autoantigens in the gums. EBV can infect B cells and epithelial cells and resides as latent infection in resting B cells. Abs against citrullinated peptides derived from EBV nuclear antigen appear years before RA and cross-react with human citrullinated fibrin. Citrullinated proteins are potential arthritogenic autoantigens in RA. The conversion of arginine to citrulline increases the peptide binding affinity to HLA-DRB1SE. Also, citrullinated fibrinogen induces arthritis in HLA-DRB1^*0401 transgenic mice, and transfer of their splenic T cells causes arthritis to recipient mice.

Is ACPA positivity the main driver for rheumatoid arthritis treatment? Pros and cons

Rheumatoid Arthritis (RA) is an autoimmune chronic disease that is characterized by the positivity of various antibodies, the most specific being autoantibodies against citrullinated antigens (ACPA). Despite ACPA are not arthritogenic by themselves, ACPA positive individuals have high risk of RA development and ACPA positivity is associated with severe erosive phenotype and higher mortality rate compared to seronegative RA. Moreover, ACPA status is associated with favorable response to biologics targeting pathways involving autoantibody producing cells as B lymphocytes. In the current review we have discussed the pros and cons on the available scientific evidences, regarding the diagnostic, prognostic and management implications of ACPAs in RA.

Autoantibody-producing plasmablasts after B cell depletion identified in muscle-specific kinase myasthenia gravis

Myasthenia gravis (MG) is a B cell-mediated autoimmune disorder of neuromuscular transmission. Pathogenic autoantibodies to muscle-specific tyrosine kinase (MuSK) can be found in patients with MG who do not have detectable antibodies to the acetylcholine receptor (AChR). MuSK MG includes immunological and clinical features that are generally distinct from AChR MG, particularly regarding responsiveness to therapy. B cell depletion has been shown to affect a decline in serum autoantibodies and to induce sustained clinical improvement in the majority of MuSK MG patients. However, the duration of this benefit may be limited, as we observed disease relapse in MuSK MG patients who had achieved rituximab-induced remission. We investigated the mechanisms of such relapses by exploring autoantibody production in the reemerging B cell compartment. Autoantibody-expressing CD27+ B cells were observed within the reconstituted repertoire during relapse but not during remission or in controls. Using two complementary approaches, which included production of 108 unique human monoclonal recombinant immunoglobulins, we demonstrated that antibody-secreting CD27hiCD38hi B cells (plasmablasts) contribute to the production of MuSK autoantibodies during relapse. The autoantibodies displayed hallmarks of antigen-driven affinity maturation. These collective findings introduce potential mechanisms for understanding both MuSK autoantibody production and disease relapse following B cell depletion.

B cells expressing the IgA receptor FcRL4 participate in the autoimmune response in patients with rheumatoid arthritis

The clinical efficacy of B cell targeting therapies highlights the pathogenic potential of B cells in inflammatory diseases. Expression of Fc Receptor like 4 (FcRL4) identifies a memory B cell subset, which is enriched in the joints of patients with rheumatoid arthritis (RA) and in mucosa-associated lymphoid tissue. The high level of RANKL production by this B cell subset indicates a unique pathogenic role. In addition, recent work has identified a role for FcRL4 as an IgA receptor, suggesting a potential function in mucosal immunity. Here, the contribution of FcRL4+ B cells to the specific autoimmune response in the joints of patients with RA was investigated.

Single FcRL4+ and FcRL4- B cells were sorted from synovial fluid and tissue from RA patients and their immunoglobulin genes characterized. Levels of hypermutation in the variable regions in both populations were largely consistent with memory B cells selected by an antigen- and T cell-dependent process. Recombinant antibodies were generated based on the IgH and IgL variable region sequences and investigated for antigen specificity. A significantly larger proportion of the recombinant antibodies generated from individual synovial FcRL4+ B cells showed reactivity towards citrullinated autoantigens. Furthermore, both in analyses based on heavy chain sequences and flow cytometric detection, FcRL4+ B cells have significantly increased usage of the IgA isotype. Their low level of expression of immunoglobulin and plasma cell differentiation genes does not suggest current antibody secretion. We conclude that these activated B cells are a component of the local autoimmune response, and through their RANKL expression, can contribute to joint destruction. Furthermore, their expression of FcRL4 and their enrichment in the IgA isotype points towards a potential role for these cells in the link between mucosal and joint inflammation.

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Memory B cells expressing the IgA receptor FcRL4 are found in the joints of patients with RA.

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B cell receptors expressed on synovial FcRL4+ B cells more frequently belong to the IgA class.

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Among recombinant antibodies cloned from FcRL4+ B cells there is more reactivity with citrullinated proteins.

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The gene transcription profile of FcRL4+ B cells shows a low level of differentiation to plasma cells.

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These cells may be involved in the link between mucosal and joint autoimmunity.

Beyond citrullination: other post-translational protein modifications in rheumatoid arthritis

The presence of autoantibodies is one of the hallmarks of rheumatoid arthritis (RA). In the past few decades, rheumatoid factors (autoantibodies that recognize the Fc-tail of immunoglobulins) as well as anti-citrullinated protein antibodies (ACPAs) have been studied intensively. ACPAs recognize post-translationally modified proteins in which the amino acid arginine has been converted into a citrulline. More recently, other autoantibody systems recognizing post-translationally modified proteins have also gained attention, including autoantibodies recognizing fragmented immunoglobulin (anti-hinge antibodies), autoantibodies recognizing acetylated proteins and autoantibodies recognizing proteins that are modified by adducts formed under oxidative stress. In particular, detailed insights have been obtained on the presence and properties of autoantibodies recognizing carbamylated proteins, commonly called anti-carbamylated protein (anti-CarP) antibodies. In this Review, we summarize the current knowledge relating to these emerging autoantibodies that recognize post-translationally modified proteins identified in RA, with an emphasis on anti-CarP antibodies.

The immunopathogenesis of seropositive rheumatoid arthritis: from triggering to targeting

Patients with rheumatoid arthritis can be divided into two major subsets characterized by the presence versus absence of antibodies to citrullinated protein antigens (ACPAs) and of rheumatoid factor (RF). The antibody-positive subset of disease, also known as seropositive rheumatoid arthritis, constitutes approximately two-thirds of all cases of rheumatoid arthritis and generally has a more severe disease course. ACPAs and RF are often present in the blood long before any signs of joint inflammation, which suggests that the triggering of autoimmunity may occur at sites other than the joints (for example, in the lung). This Review summarizes recent progress in our understanding of this gradual disease development in seropositive patients. We also emphasize the implications of this new understanding for the development of preventive and therapeutic strategies. Similar temporal and spatial separation of immune triggering and clinical manifestations, with novel opportunities for early intervention, may also occur in other immune-mediated diseases.

Major involvement of bacterial components in rheumatoid arthritis and its accompanying oxidative stress, systemic inflammation and hypercoagulability

We review the evidence that infectious agents, including those that become dormant within the host, have a major role to play in much of the etiology of rheumatoid arthritis and the inflammation that is its hallmark. This occurs in particular because they can produce cross-reactive (auto-)antigens, as well as potent inflammagens such as lipopolysaccharide that can themselves catalyze further inflammagenesis, including via β-amyloid formation. A series of observables coexist in many chronic, inflammatory diseases as well as rheumatoid arthritis. They include iron dysregulation, hypercoagulability, anomalous morphologies of host erythrocytes, and microparticle formation. Iron dysregulation may be responsible for the periodic regrowth and resuscitation of the dormant bacteria, with concomitant inflammagen production. The present systems biology analysis benefits from the philosophical idea of "coherence," that reflects the principle that if a series of ostensibly unrelated findings are brought together into a self-consistent narrative, that narrative is thereby strengthened. As such, we provide a coherent and testable narrative for the major involvement of (often dormant) bacteria in rheumatoid arthritis.

NETosis as Source of Autoantigens in Rheumatoid Arthritis

In neutrophils (but also in eosinophils and in mast cells), different inflammatory stimuli induce histone deimination, chromatin decondensation, and NET formation. These web-like structures that trap and kill microbes contain DNA, cationic granule proteins, and antimicrobial peptides, but the most abundant proteins are core histones. Histones contained in NETs have been deiminated, and arginines are converted in citrullines. While deimination is a physiological process amplified in inflammatory conditions, only individuals carrying genetic predisposition to develop rheumatoid arthritis (RA) make antibodies to deiminated proteins. These antibodies, collectively identified as anti-citrullinated proteins/peptides antibodies (ACPA), react with different deiminated proteins and display partially overlapping specificities. In this paper, we will summarize current evidence supporting the role of NETosis as critical mechanism in the breach of tolerance to self-antigens and in supporting expansion and differentiation of autoreactive cells. In fact, several lines of evidence connect NETosis with RA: RA unstimulated synovial fluid neutrophils display enhanced NETosis; sera from RA patients with Felty's syndrome bind deiminated H3 and NETs; a high number of RA sera bind deiminated H4 contained in NETs; human monoclonal antibodies generated from RA synovial B cells decorate NETs and bind deiminated histones. In RA, NETs represent on one side an important source of autoantigens bearing posttranslational modifications and fueling the production of ACPA. On the other side, NETs deliver signals that maintain an inflammatory milieu and contribute to the expansion and differentiation of ACPA-producing B cells.