B cell apotopes of the 60-kDa Ro/SSA and La/SSB autoantigens

Isolated anti-Ro52 identifies a severe subset of Sjögren’s syndrome patients

Introduction

Serum autoantibodies targeting the SSA/Ro proteins are a key component of the classification criteria for the diagnosis of Sjögren’s syndrome (SS). Most patients' serum reacts with both Ro60 and Ro52 proteins. Here we compare the molecular and clinical characteristics of patients diagnosed with SS with anti-Ro52 in the presence or absence of anti-Ro60/La autoantibodies.

Methods

A cross-sectional study was performed. Patients in the SS biobank at Westmead Hospital (Sydney, Australia) that were positive for anti-Ro52 were included and stratified based on the absence (isolated) or presence (combined) of anti-Ro60/La, measured by line immunoassay. We examined clinical associations and the serological and molecular characteristics of anti-Ro52 using ELISA and mass spectrometry in serological groups.

Results

A total of 123 SS patients were included for study. SS patients with isolated anti-Ro52 (12%) identified a severe serological subset characterised by higher disease activity, vasculitis, pulmonary involvement, rheumatoid factor (RhF) and cryoglobulinaemia. Serum antibodies reacting with Ro52 in the isolated anti-Ro52 subset displayed less isotype switching, less immunoglobulin variable region subfamily usage and a lower degree of somatic hypermutation than the combined anti-Ro52 subset.

Conclusions

In our cohort of SS patients, isolated anti-Ro52 represents a severe subset of SS, and is associated with the presence of cryoglobulinaemia. We therefore provide clinical relevance to the stratification of SS patients by their sero-reactivities. It is possible that the autoantibody patterns may be immunological epiphenomena of the underlying disease process, and further work is required to unearth the mechanisms of the differential clinical phenotypes.

Transforming mutations in the development of pathogenic B cell clones and autoantibodies

Autoimmune diseases are characterized by serum autoantibodies, some of which are pathogenic, causing severe manifestations and organ injury. However, autoantibodies of the same antigenic reactivity are also present in the serum of asymptomatic people years before they develop any clinical signs of autoimmunity. Autoantibodies can arise during multiple stages of B cell development, and various genetic and environmental factors drive their production. However, what drives the development of pathogenic autoantibodies is poorly understood. Advances in single-cell technology have enabled the deep analysis of rare B cell clones producing pathogenic autoantibodies responsible for vasculitis in patients with primary Sjögren's syndrome complicated by mixed cryoglobulinaemia. These findings demonstrated a cascade of genetic events involving stereotypic immunoglobulin V(D)J recombination and transforming somatic mutations in lymphoma genes and V(D)J regions that disrupted antibody quality control mechanisms and decreased autoantibody solubility. Most studies consider V(D)J mutations that enhance autoantibody affinity to drive pathology; however, V(D)J mutations that increase autoantibody propensity to form insoluble complexes could be a major contributor to autoantibody pathogenicity. Defining the molecular characteristics of pathogenic autoantibodies and failed tolerance checkpoints driving their formation will improve prognostication, enabling early treatment to prevent escalating organ damage and B cell malignancy.

LAMP3 induces apoptosis and autoantigen release in Sjögren's syndrome patients

Primary Sjögren's syndrome (pSS) is a complex autoimmune disease characterized by dysfunction of secretory epithelia with only palliative therapy. Patients present with a constellation of symptoms, and the diversity of symptomatic presentation has made it difficult to understand the underlying disease mechanisms. In this study, aggregation of unbiased transcriptome profiling data sets of minor salivary gland biopsies from controls and Sjögren's syndrome patients identified increased expression of lysosome-associated membrane protein 3 (LAMP3/CD208/DC-LAMP) in a subset of Sjögren's syndrome cases. Stratification of patients based on their clinical characteristics suggested an association between increased LAMP3 expression and the presence of serum autoantibodies including anti-Ro/SSA, anti-La/SSB, anti-nuclear antibodies. In vitro studies demonstrated that LAMP3 expression induces epithelial cell dysfunction leading to apoptosis. Interestingly, LAMP3 expression resulted in the accumulation and release of intracellular TRIM21 (one component of SSA), La (SSB), and α-fodrin protein, common autoantigens in Sjögren's syndrome, via extracellular vesicles in an apoptosis-independent mechanism. This study defines a clear role for LAMP3 in the initiation of apoptosis and an independent pathway for the extracellular release of known autoantigens leading to the formation of autoantibodies associated with this disease.ClinicalTrials.gov Identifier: NCT00001196, NCT00001390, NCT02327884.

IgV peptide mapping of native Ro60 autoantibody proteomes in primary Sjögren's syndrome reveals molecular markers of Ro/La diversification

We have used high-resolution mass spectrometry to sequence precipitating anti-Ro60 proteomes from sera of patients with primary Sjögren's syndrome and compare immunoglobulin variable-region (IgV) peptide signatures in Ro/La autoantibody subsets. Anti-Ro60 were purified by elution from native Ro60-coated ELISA plates and subjected to combined de novo amino acid sequencing and database matching. Monospecific anti-Ro60 Igs comprised dominant public and minor private sets of IgG1 kappa and lambda restricted heavy and light chains. Specific IgV amino acid substitutions stratified anti-Ro60 from anti-Ro60/La responses, providing a molecular fingerprint of Ro60/La determinant spreading and suggesting that different forms of Ro60 antigen drive these responses. Sequencing of linked anti-Ro52 proteomes from individual patients and comparison with their anti-Ro60 partners revealed sharing of a dominant IGHV3-23/IGKV3-20 paired clonotype but with divergent IgV mutational signatures. In summary, anti-Ro60 IgV peptide mapping provides insights into Ro/La autoantibody diversification and reveals serum-based molecular markers of humoral Ro60 autoimmunity.

Secreted autoantibody repertoires in Sjögren's syndrome and systemic lupus erythematosus: A proteomic approach

The structures of epitopes bound by autoantibodies against RNA-protein complexes have been well-defined over several decades, but little is known of the clonality, immunoglobulin (Ig) variable (V) gene usage and mutational status of the autoantibodies themselves at the level of the secreted (serum) proteome. A novel proteomic workflow is presented based on affinity purification of specific Igs from serum, high-resolution two-dimensional gel electrophoresis, and de novo and database-driven sequencing of V-region proteins by mass spectrometry. Analysis of anti-Ro52/Ro60/La proteomes in primary Sjögren's syndrome (SS) and anti-Sm and anti-ribosomal P proteomes in systemic lupus erythematosus (SLE) has revealed that these antibody responses are dominated by restricted sets of public (shared) clonotypes, consistent with common pathways of production across unrelated individuals. The discovery of shared sets of specific V-region peptides can be exploited for diagnostic biomarkers in targeted mass spectrometry platforms and for tracking and removal of pathogenic clones.

General Approach for Tetramer-Based Identification of Autoantigen-Reactive B Cells: Characterization of La- and snRNP-Reactive B Cells in Autoimmune BXD2 Mice

Autoreactive B cells are associated with the development of several autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis. The low frequency of these cells represents a major barrier to their analysis. Ag tetramers prepared from linear epitopes represent a promising strategy for the identification of small subsets of Ag-reactive immune cells. This is challenging given the requirement for identification and validation of linear epitopes and the complexity of autoantibody responses, including the broad spectrum of autoantibody specificities and the contribution of isotype to pathogenicity. Therefore, we tested a two-tiered peptide microarray approach, coupled with epitope mapping of known autoantigens, to identify and characterize autoepitopes using the BXD2 autoimmune mouse model. Microarray results were verified through comparison with established age-associated profiles of autoantigen specificities and autoantibody class switching in BXD2 and control (C57BL/6) mice and high-throughput ELISA and ELISPOT analyses of synthetic peptides. Tetramers were prepared from two linear peptides derived from two RNA-binding proteins (RBPs): lupus La and 70-kDa U1 small nuclear ribonucleoprotein. Flow cytometric analysis of tetramer-reactive B cell subsets revealed a significantly higher frequency and greater numbers of RBP-reactive marginal zone precursor, transitional T3, and PDL-2(+)CD80(+) memory B cells, with significantly elevated CD69 and CD86 observed in RBP(+) marginal zone precursor B cells in the spleens of BXD2 mice compared with C57BL/6 mice, suggesting a regulatory defect. This study establishes a feasible strategy for the characterization of autoantigen-specific B cell subsets in different models of autoimmunity and, potentially, in humans.

[Neonatal lupus syndrome: Literature review]

Neonatal lupus syndrome is associated with transplacental passage of maternal anti-SSA/Ro and anti-SSB/La antibodies. Children display cutaneous, hematological, liver or cardiac features. Cardiac manifestations include congenital heart block (CHB); endocardial fibroelastosis and dilated cardiomyopathy. The prevalence of CHB in newborns of anti-Ro/SSA positive women with known connective tissue disease is between 1 and 2% and the risk of recurrence is around 19%. Skin and systemic lesions are transient, whereas CHB is definitive and associated with significant morbidity and a mortality of 18%. A pacemaker must be implanted in 2/3 of cases. Myocarditis may be associated or appear secondly. Mothers of children with CHB are usually asymptomatic or display Sjogren's syndrome or undifferentiated connective tissue disease. In anti-Ro/SSA positive pregnant women, fetal echocardiography should be performed at least every 2 weeks from the 16th to 24th week gestation. An electrocardiogram should be performed for all newborn babies. The benefit of fluorinated corticosteroid therapy for CHB detected in utero remains unclear. Maternal use of hydroxychloroquine may be associated with a decreased recurrent CHB risk in a subsequent offspring. A prospective study is actually ongoing to confirm these findings.

Circulating levels of soluble MER in lupus reflect M2c activation of monocytes/macrophages, autoantibody specificities and disease activity

INTRODUCTION

Systemic lupus erythematosus (SLE) is characterized by impaired efferocytosis and aberrant activation of innate immunity. We asked if shedding of MER receptor tyrosine kinase (MerTK) and AXL into soluble (s) ectodomains was related to immunological and clinical aspects of SLE.

METHODS

Levels of sMER and sAXL in the plasma of 107 SLE patients and 45 matched controls were measured by ELISA. In 40 consecutive SLE patients, we examined potential correlations between either sMER or sAXL and plasma levels of sCD163, a marker of M2 activation. All three soluble receptors were measured in supernatants of monocytes/macrophages cultured in various immunological conditions. Membrane expression of MerTK, AXL and CD163 was assessed by flow cytometry.

RESULTS

Both sMER and sAXL were associated with anti-chromatin and anti-phospholipid autoantibodies, and with hematological and renal involvement. However, sMER and sAXL did not significantly correlate with each other; sAXL correlated with growth arrest-specific 6 (Gas6), whereas sMER correlated with reduced free protein S (PROS) levels. Only sMER showed significant associations with lupus-specific anti-dsDNA, anti-Sm, anti-ribonucleoprotein (anti-RNP) and anti-Ro60 autoantibodies. Strong correlations with disease activity indices (Systemic Lupus Erythematosus Disease Activity Index (SLEDAI), complement reduction, titer of circulating anti-dsDNA) were found for sMER, not for sAXL. Patients with active SLEDAI, nephritis, anti-dsDNA and anti-Ro60 positivity showed higher levels of sMER compared to controls. Levels of sMER, not sAXL, correlated with sCD163 levels, and these correlated with SLEDAI. Production of sMER and sCD163 occurred under “M2c” polarizing conditions, whereas sAXL was released upon type-I IFN exposure.

CONCLUSIONS

Alterations in homeostasis of anti-inflammatory and efferocytic “M2c” monocytes/macrophages may have a role in immunopathogenesis of SLE.

Pathogenicity and proteomic signatures of autoantibodies to Ro and La

Ro/SSA and La/SSB comprise a linked set of autoantigens that are clinically important members of the extractable nuclear antigen family and key translational biomarkers for lupus and primary Sjögren's syndrome. Autoantibodies directed against the Ro60 and La polypeptide components of the Ro/La ribonucleoprotein complex, and the structurally unrelated Ro52 protein, mediate tissue damage in the neonatal lupus syndrome, a model of passively acquired autoimmunity in humans in which the most serious manifestation is congenital heart block (CHB). Recent studies have concentrated on two distinct pathogenic mechanisms by which maternal anti-Ro/La autoantibodies can cause CHB: by forming immune complexes with apoptotic cells in developing fetal heart; and/or by acting as functional autoantibodies that cross-react with and inhibit calcium channels. Although the precise role of the individual autoantibodies is yet to be settled, maternal anti-Ro60 and anti-Ro52 remain the most likely culprits. This article will discuss the molecular pathways that culminate in the development of CHB, including the recent discovery of β2 glycoprotein I as a protective factor, and present a proteomic approach based on direct mass spectrometric sequencing, which may give a more representative snapshot of the idiotype repertoire of these autoantibodies than genomic-based technologies.

Primary biliary cirrhosis and autoimmune hepatitis: apotopes and epitopes

Autoimmune liver diseases (ALDs) represent a wide spectrum of chronic inflammatory diseases that are characterized by an immune-mediated attack against either hepatocytes (in the case of autoimmune hepatitis types 1 and 2, AIH-1, 2) or cholangiocytes (in primary biliary cirrhosis, PBC). PBC is considered a model autoimmune disease due to the homogeneity of patients, the high specificity of antimitochondrial antibodies (AMAs), and the specificity of biliary epithelial cell (BEC) destruction. It ensues from a multi-lineage loss of tolerance to the E2 component of the pyruvate dehydrogenase complex (PDC-E2). One of the major unanswered questions in the pathogenesis of PBC is the specificity of small intrahepatic bile duct attack while PDC-E2 is present in mitochondria of nucleated cells. Recent findings suggest that the apoptosis of BECs may be of considerable importance for understanding PBC, and that they are more than simply an innocent victim of an immune attack. Rather, they attract immune attack by virtue of the unique biochemical mechanisms by which they handle PDC-E2. The role of apoptotic cells in AIH is not well defined, but advances in the study of autoreactive T cells stem mostly from AIH type 2, where the main autoantigen (CYP2D6) is known, enabling the characterization of antigen-specific immune responses. This review article is intended to provide a critical overview of current evidence on tissue specificity in ALDs, as well as the characteristics of the relevant epitopes and apotopes and their biological and clinical significance.

Biliary apotopes and anti-mitochondrial antibodies activate innate immune responses in primary biliary cirrhosis

Our understanding of primary biliary cirrhosis (PBC) has been significantly enhanced by the rigorous dissection of the multilineage T and B cell response against the immunodominant mitochondrial autoantigen, the E2 component of the pyruvate dehydrogenase complex (PDC-E2). PDC-E2 is a ubiquitous protein present in mitochondria of nucleated cells. However, the damage of PBC is confined to small biliary epithelial cells (BECs). We have previously demonstrated that BECs translocate immunologically intact PDC-E2 to apoptotic bodies and create an apotope. To define the significance of this observation, we have studied the ability of biliary or control epithelial apotopes to induce cytokine secretion from mature monocyte-derived macrophages (MDMphis) from either patients with PBC or controls in the presence or absence of anti-mitochondrial antibodies (AMAs). We demonstrate that there is intense inflammatory cytokine production in the presence of the unique triad of BEC apotopes, macrophages from patients with PBC, and AMAs. The cytokine secretion is inhibited by anti-CD16 and is not due to differences in apotope uptake. Moreover, MDMphis from PBC patients cultured with BEC apoptotic bodies in the presence of AMAs markedly increase tumor necrosis factor-related apoptosis-inducing ligand expression.

CONCLUSION

These results provide a mechanism for the biliary specificity of PBC, the recurrence of disease after liver transplantation, and the success of ursodiol in treatment. They further emphasize the critical role of the innate immune system in the perpetuation of this autoimmune disease.

Sjögren syndrome or sjögren disease? The histological and immunological bias caused by the 2002 criteria

The current 2002 classification criteria do not cover the broad clinical and immunological heterogeneity of primary Sjögren syndrome (SS), since five of the six criteria focus exclusively on glandular involvement and the remaining criterion is the mandatory presence of anti-Ro/La antibodies. The aim of this study was to analyze the clinical features of patients with a well-established diagnosis of primary SS who do not fulfill the 2002 classification criteria. Five hundred seven patients diagnosed with primary SS (1993 criteria) were consecutively included and followed up. Two hundred twenty-one (44%) patients did not fulfill the 2002 criteria. These patients were older at diagnosis (p < 0.001) and had a lower frequency of parotid enlargement (p = 0.002), fever (p = 0.041), arthritis (p = 0.041), vasculitis (p = 0.050), peripheral neuropathy (p = 0.002), cranial nerve involvement (p = 0.015), raised erythrocyte sedimentation rate ( ESR) levels (p < 0.001), anemia (p < 0.001), leukopenia (p = 0.037), hypergammaglobulinemia (p < 0.001), positive rheumatoid factor ( RF; p = 0.002), and cryoglobulinemia (p = 0.049) in comparison with those fulfilling 2002 criteria. However, there were no significant differences in the prevalence of sicca features, diagnostic tests, overall systemic involvement, antinuclear antibodies , complement levels, development of B-cell lymphoma, or survival. Patients with anti-Ro antibodies had the highest frequencies of systemic features, hematological abnormalities, and altered immunological markers. In conclusion, patients fulfilling the 2002 criteria, who have either a specific histological diagnosis (lymphocytic infiltration) or highly specific autoantibodies (Ro/La), might well be considered to have Sjögren "disease." In contrast, etiopathogenic mechanisms other than lymphocytic-mediated epithelial damage could be involved in patients with negative Ro and negative biopsy, in whom the term Sjögren "syndrome" seems more adequate.

Reactivity with dichotomous determinants of Ro 60 stratifies autoantibody responses in lupus and primary Sjögren's syndrome

OBJECTIVE

Analysis of B cell determinants of Ro 60 exposed on the surface of apoptotic cells (apotopes) or intracellular epitopes provides insight into the structural forms of the autoantigen that break immune tolerance. This study was initiated to compare anti-Ro 60 responses in systemic lupus erythematosus (SLE) and primary Sjögren's syndrome (SS) against membrane-bound and intracellular forms of Ro 60.

METHODS

The reactivity of autoantibodies from patients with SLE and primary SS to Ro 60 apotopes and epitopes was assessed by multiparameter flow cytometry and solid-phase immunoassay. Anti-Ro 60 IgG was eluted from early apoptotic cells or recombinant Ro 60 immobilized on nitrocellulose, and binding to membrane-bound and intracellular forms of Ro 60 was quantitated by flow cytometry.

RESULTS

An immunodominant apotope, which was recognized by IgG from a subset of SLE patients with anti-Ro, but not anti-La, autoantibodies, was mapped to a region forming a helix-loop-helix at the apical tip of the Ro 60 molecule. Immobilization of this region to the solid phase exposed an epitope that was recognized by IgG from primary SS and SLE patients whose sera had both anti-Ro and anti-La autoantibodies. Autoantibodies eluted from either the surface of apoptotic cells or the Ro 60 epitope on the solid phase were non-cross-reactive and specifically recognized membrane-bound or cytoplasmic forms of Ro 60.

CONCLUSION

This is the first example of a dichotomy of human autoantibody responses against mutually exclusive determinants linked to a single domain of a systemic autoantigen and supports a model in which tolerance is broken by different immunogenic forms of Ro 60.

Prevention of complete heart block in children of mothers with anti-SSA/Ro and anti-SSB/La autoantibodies: detection and treatment of first-degree atrioventricular block

PURPOSE OF REVIEW

To describe the results of two recent prospective studies that may indicate how to monitor, diagnose, and treat fetuses with neonatal lupus manifesting with heart involvement and to summarize additional research reports regarding the pathophysiology and outcomes of this rare condition.

RECENT FINDINGS

The PR Interval and Dexamethasone Evaluation study found 10 cases of neonatal lupus (10%) with three first-degree atrioventricular blocks (AVBs) and three complete heart blocks. The study included 98 pregnancies in 95 women with anti-SSA/Ro antibodies who completed weekly fetal Doppler echocardiogram-based evaluation. The authors concluded that they were unable to detect first-degree AVB before progression to complete heart block. A similar observational prospective study was performed in 70 fetuses of 56 mothers using tissue velocity fetal kinetocardiogram for measurement of PR prolongation. In this study, six fetuses (8.5%) showed first-degree AVB, and fast normalization of heart function was achieved through maternal treatment with fluorinated steroids. The authors concluded that fetal kinetocardiogram can detect first-degree AVB in the fetus exposed to maternal anti-SSA/Ro or anti-SSB/La antibodies or both and that fluorinated steroids given on detection were associated with normalized atrioventricular conduction in fetuses with first-degree AVB.

SUMMARY

Echo Doppler seems a less reliable method for early detection of fetus first-degree AVB, and it is suggested that fetal kinetocardiogram or fetal electrocardiography are preferred. Although atrioventricular block reverses spontaneously in some fetuses, parents and treating physicians should consider immediate treatment with fluorinated steroids once a first-degree AVB is detected due to the high risk of rapid progression to complete blockage.

Characteristics of the minor salivary gland infiltrates in Sjögren's syndrome

Sjögren's syndrome (SS) is a chronic autoimmune exocrinopathy associated with variable degree of lymphocytic infiltration of the affected organs (primarily salivary and lacrimal glands) and broad clinical manifestations. Minor salivary gland (MSG) lesions mainly consist of T and B cells, while antigen-presenting cells have been reported in heavy infiltrates. Evidence suggests that the infiltrate composition differs according to lesion severity; however, these differences are not well-defined. To investigate the differential distribution of the major infiltrating mononuclear cell (MNC) types in SS-lesions of variable severity, total-T, CD4(+)-T, CD8(+)-T, Treg, and B cell, macrophage (MPhi), interdigitating (iDC) and follicular dendritic cell (fDC), and natural-killer (NK)-cell incidence (%-total infiltrating MNC) was analyzed in MSG biopsies with mild (n = 11), intermediate (n = 13) or severe (n = 15) lesions. T cells, CD4(+)-T cells and Tregs, B lymphocytes, MPhis and iDCs were significantly different among MSG tissues with mild, intermediate or severe inflammatory lesions, while CD8(+)-T cell, fDC and NK cell incidence was not correlated with lesion severity. T cell, CD4(+)-T cell, T/B cell ratio and iDC incidence was negatively, whereas B cell and MPhi incidence was positively correlated with infiltration grade and biopsy focus score. Tregs predominated in intermediate lesions. Multivariate analysis revealed several associations between the incidence of each infiltrating MNC-type and disease manifestations, implying an involvement of local immune responses in systemic disease features. Our findings support that the distribution of infiltrating MNCs at the SS-lesions varies according to lesion severity and correlates with disease manifestations. The significance of this differential distribution and the underlying aetiopathogenic factors need to be elucidated.

The weight of interleukin-6 in B cell-related autoimmune disorders

Interleukin (IL)-6 is a prevailing factor of polyclonal B-cell activation of B cells, and thereby of their tolerance breach. Its receptor (R) complex consists of a transducing unit, and a membrane-bound or soluble protein. Many activities ascribed to this cytokine are generated by the soluble IL-6R. Evidence has however been gleaned in autoimmune diseases that the system is instrumental in rheumatoid arthritis (RA), Sjögren's syndrome and systemic lupus erythematosus (SLE). To gain insight into the understanding of the mechanisms behind these observations, a prime example is the recombination-activating gene (Rag) machinery in B lymphocytes. It is interesting that the expression of Rags is favored by IL-6, and repressed by anti-IL-6R antibody (Ab) in RA and SLE. Not surprisingly, clinical benefits are reported in the treatment of autoimmune disorders with anti-IL-6R Ab, and other perspectives about to be open in biotherapy.