A new conformational epitope generated by the binding of recombinant 70-kd protein and U1 RNA to anti-U1 RNP autoantibodies in sera from patients with mixed connective tissue disease

Positive and negative regulation of the Fcγ receptor-stimulating activity of RNA-containing immune complexes by RNase

The U1RNP complex, Ro/SSA, and La/SSB are major RNA-containing autoantigens. Immune complexes (ICs) composed of RNA-containing autoantigens and autoantibodies are suspected to be involved in the pathogenesis of some systemic autoimmune diseases. Therefore, RNase treatment, which degrades RNA in ICs, has been tested in clinical trials as a potential therapeutic agent. However, no studies to our knowledge have specifically evaluated the effect of RNase treatment on the Fcγ receptor-stimulating (FcγR-stimulating) activity of RNA-containing ICs. In this study, using a reporter system that specifically detects FcγR-stimulating capacity, we investigated the effect of RNase treatment on the FcγR-stimulating activity of RNA-containing ICs composed of autoantigens and autoantibodies from patients with systemic autoimmune diseases such as systemic lupus erythematosus. We found that RNase enhanced the FcγR-stimulating activity of Ro/SSA- and La/SSB-containing ICs, but attenuated that of the U1RNP complex-containing ICs. RNase decreased autoantibody binding to the U1RNP complex, but increased autoantibody binding to Ro/SSA and La/SSB. Our results suggest that RNase enhances FcγR activation by promoting the formation of ICs containing Ro/SSA or La/SSB. Our study provides insights into the pathophysiology of autoimmune diseases involving anti-Ro/SSA and anti-La/SSB autoantibodies, and into the therapeutic application of RNase treatment for systemic autoimmune diseases.

SOD1G93A Astrocyte-Derived Extracellular Vesicles Induce Motor Neuron Death by a miRNA-155-5p-Mediated Mechanism

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by upper and lower motor neuron (MN) degeneration. Astrocytes surrounding MNs are known to modulate ALS progression. When cocultured with astrocytes overexpressing the ALS-linked mutant Cu/Zn superoxide dismutase (SOD1G93A) or when cultured with conditioned medium from SOD1G93A astrocytes, MN survival is reduced. The exact mechanism of this neurotoxic effect is unknown. Astrocytes secrete extracellular vesicles (EVs) that transport protein, mRNA, and microRNA species from one cell to another. The size and protein markers characteristic of exosomes were observed in the EVs obtained from cultured astrocytes, indicating their abundance in exosomes. Here, we analyzed the microRNA content of the exosomes derived from SOD1G93A astrocytes and evaluated their role in MN survival. Purified MNs exposed to SOD1G93A astrocyte-derived exosomes showed reduced survival and neurite length compared to those exposed to exosomes derived from non-transgenic (non-Tg) astrocytes. Analysis of the miRNA content of the exosomes revealed that miR-155-5p and miR-582-3p are differentially expressed in SOD1G93A exosomes compared with exosomes from non-Tg astrocytes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicates that miR-155-5p and miR-582-3p predicted targets are enriched in the neurotrophin signaling pathway. Importantly, when levels of miR-155-5p were reduced by incubation with a specific antagomir, SOD1G93A exosomes did not affect MN survival or neurite length. These results demonstrate that SOD1G93A-derived exosomes are sufficient to induce MN death, and miRNA-155-5p contributes to this effect. miRNA-155-5p may offer a new therapeutic target to modulate disease progression in ALS.

Using C. elegans to discover therapeutic compounds for ageing-associated neurodegenerative diseases

Age-associated neurodegenerative disorders such as Alzheimer's disease are a major public health challenge, due to the demographic increase in the proportion of older individuals in society. However, the relatively few currently approved drugs for these conditions provide only symptomatic relief. A major goal of neurodegeneration research is therefore to identify potential new therapeutic compounds that can slow or even reverse disease progression, either by impacting directly on the neurodegenerative process or by activating endogenous physiological neuroprotective mechanisms that decline with ageing. This requires model systems that can recapitulate key features of human neurodegenerative diseases that are also amenable to compound screening approaches. Mammalian models are very powerful, but are prohibitively expensive for high-throughput drug screens. Given the highly conserved neurological pathways between mammals and invertebrates, Caenorhabditis elegans has emerged as a powerful tool for neuroprotective compound screening. Here we describe how C. elegans has been used to model various human ageing-associated neurodegenerative diseases and provide an extensive list of compounds that have therapeutic activity in these worm models and so may have translational potential.

Mapping epitopes of U1-70K autoantibodies at single-amino acid resolution

The mechanisms underlying development of ribonucleoprotein (RNP) autoantibodies are unclear. The U1-70K protein is the predominant target of RNP autoantibodies, and the RNA binding domain has been shown to be the immunodominant autoantigenic region of U1-70K, although the specific epitopes are not known. To precisely map U1-70K epitopes, we developed silicon-based peptide microarrays with >5700 features, corresponding to 843 unique peptides derived from the U1-70K protein. The microarrays feature overlapping peptides, with single-amino acid resolution in length and location, spanning amino acids 110-170 within the U1-70K RNA binding domain. We evaluated the serum IgG of a cohort of patients with systemic lupus erythematosus (SLE; n = 26) using the microarrays, and identified multiple reactive epitopes, including peptides 116-121 and 143-148. Indirect peptide ELISA analysis of the sera of patients with SLE (n = 88) revealed that ∼14% of patients had serum IgG reactivity to 116-121, while reactivity to 143-148 appeared to be limited to a single patient. SLE patients with serum reactivity to 116-121 had significantly lower SLE Disease Activity Index (SLEDAI) scores at the time of sampling, compared to non-reactive patients. Minimal reactivity to the peptides was observed in the sera of healthy controls (n = 92). Competitive ELISA showed antibodies to 116-121 bind a common epitope in U1-70K (68-72) and the matrix protein M1 of human influenza B viruses. Institutional Review Boards approved this study. Knowledge of the precise epitopes of U1-70K autoantibodies may provide insight into the mechanisms of development of anti-RNP, identify potential clinical biomarkers and inform ongoing clinical trails of peptide-based therapeutics.

U1-RNP and Toll-like receptors in the pathogenesis of mixed connective tissue diseasePart II. Endosomal TLRs and their biological significance in the pathogenesis of mixed connective tissue disease

Mixed connective tissue disease (MCTD) is a chronic autoimmune immunopathological disease of unknown etiology, which is characterized by the presence of various clinical symptoms and the presence of autoantibodies against U1-RNP particles. The U1-RNP component engages immune cells and their receptors in a complex network of interactions that ultimately lead to autoimmunity, inflammation, and tissue injury. The anti-U1-RNP autoantibodies form an immune complex with self-RNA, present in MCTD serum, which can act as endosomal Toll-like receptor (TLR) ligands. Inhibition of TLRs by nucleic acids is a promising area of research for the development of novel therapeutic strategies against pathogenic infection, tumorigenesis and autoimmunity. In this review we summarize current knowledge of endogenous TLRs and discuss their biological significance in the pathogenesis of MCTD. In part I we described the structure, biological function and significance of the U1-RNP complex in MCTD.

U1-RNP and TLR receptors in the pathogenesis of mixed connective tissue diseasePart I. The U1-RNP complex and its biological significance in the pathogenesis of mixed connective tissue disease

Mixed connective tissue disease (MCTD) is a rare autoimmune syndrome, signified by complex interactions between disease-related phenomena, including inflammation, proliferative vascular arteriopathy, thrombotic events and humoral autoimmune processes. It is still controversial whether MCTD is a distinct clinical entity among systemic connective tissue diseases, although several authors consider that it is distinct and underline characteristic, distinct clinical, serological and immunogenetic features. The putative target of autoimmunity in MCTD is U1-RNP, which is a complex of U1-RNA and small nuclear RNP. Both the U1-RNA component and the specific proteins, particularly U1-70K, engage immune cells and their receptors in a complex network of interactions that ultimately lead to autoimmunity, inflammation, and tissue injury. U1-RNA is capable of inducing manifestations consistent with TLR activation. Stimulation of innate immunity by native RNA molecules with a double-stranded secondary structure may help explain the high prevalence of autoimmunity to RNA binding proteins.

The predictive value of anti-SS-A antibodies titration in pregnant women with fetal congenital heart block

OBJECTIVE

Fetal congenital complete heart block (CHB) is irreversible and is associated with significant mortality and morbidity. Anti-SS-A antibodies in the maternal sera are involved in its pathogenesis; however, the predictive value of the antibody titer and its role in prediction of this complication are controversial. The aim of this study was to determine the predictive value of maternal anti-SS-A antibodies on the development of fetal CHB.

METHODS

A retrospective chart review was performed for 189 cases of positive anti-SS-A antibodies determined by the double immunodiffusion (DID) method, and included 17 patients that developed fetal CHB. The relationship between the appearance of CHB and the anti-SS-A antibodies titer was examined.

RESULTS

An anti-SS-A antibodies titer of 1:32 or higher was identified by analyzing the receiver-operating characteristics (area under curve 0.72) curve. An anti-SS-A antibodies titer of 32 or more times greater than the upper limit by DID was a risk factor for fetal CHB (odds ratio 27.77, 95% confidence interval (CI) 1.91-21.02, P < 0.05) in the multivariate analysis. Among 107 cases of anti-SS-A antibodies titers of 1:32 or higher, 65 patients (60.7%) were treated with oral steroids. Of these, four patients had CHB (6.2%). This rate of CHB was significantly lower (P < 0.01) than the rate in patients not treated with steroids.

CONCLUSION

An anti-SS-A antibodies titer of 1:32 or higher in the maternal sera by DID was an independent risk factor for fetal CHB. In these patients, either antenatally administered prednisolone or betamethasone, was associated with a lower risk of fetal CHB.

Serum antibodies against the 70k polypeptides of the U1 ribonucleoprotein complex are associated with psychiatric syndromes in systemic lupus erythematosus: a retrospective study

OBJECTIVES

We assessed the association between serum autoantibodies against the 70-kDa polypeptide of the U1-ribonucleoprotein (RNP) complex (U1-70k) and the central nervous system (CNS) syndromes in systemic lupus erythematosus (SLE) patients.

METHODS

We studied 106 hospitalized patients with active SLE, comparing those with (n = 32) and without (n = 74) CNS syndromes. CNS syndromes were further classified into neurologic (n = 21) and psychiatric (n = 15) disorders. Immunoglobulin G (IgG) anti-U1-70k antibodies were measured by enzyme-linked immunosorbent assay (ELISA) using recombinant antigens. IgG antibodies against whole U1-RNP were measured using commercial ELISA kits.

RESULTS

Although there was no significant difference in the levels of serum anti-U1-70k antibodies in SLE patients with or without CNS syndromes (p = 0.83), the levels were significantly elevated in SLE patients compared with patients without psychiatric syndromes (p = 0.030). In contrast, no significant difference was observed in the levels of serum anti-U1-RNP antibodies in SLE patients with or without psychiatric syndromes (p = 0.555).

CONCLUSIONS

These results indicate that serum anti-U1-70k antibodies are associated with psychiatric syndromes in SLE but that they are not associated with CNS syndromes as a whole or with neurologic syndromes. The anti-U1-70k antibodies might be involved in the pathological mechanisms of psychiatric syndromes in SLE.

Sm peptides in differentiation of autoimmune diseases

Systemic lupus erythematosus (SLE) and related diseases are characterized by circulating autoantibodies to defined intracellular targets. Among the earliest identified autoantibodies were those directed to components of U2-U6 small nuclear ribonucleoproteins (snRNPs) known as Smith (Sm) antigen, which are highly specific for SLE. The Sm-antigen is composed of at least nine different polypeptides with molecular weights ranging from 9 to 29.5 kDa (B (B1, 28 kDa), B' (B2, 29 kDa), N (B3, 29.5 kDa), D1 (16 kDa), D2 (16.5 kDa), D3 (18 kDa), E (12 kDa), F (11 kDa), and G (9 kDa)). All of the nine core proteins, but most frequently the B and D polypeptides, are targets of the anti-Sm autoimmune response. However, since SmBB' and U1 specific RNPs share the cross-reactive epitope motif PPPGMRPP, SmD is regarded as the most SLE specific Sm-antigen. It has been shown that the polypeptides D1, D3, and BB' contain symmetrical dimethylarginine, constituting a major autoepitope within the C-terminus of SmD1 and SmD3. Several synthetic peptides have been used for the detection of anti-Sm antibodies and thus for the diagnosis of SLE. Anti-Sm antibodies have been reported to occur later than other SLE associated autoantibodies and, on average, around 1 year before the clinical onset of SLE. The present review provides a comprehensive summary on the history of anti-Sm antibodies and their use as biochemical tools to study cellular processes and as biomarker in the diagnosis of SLE. Additionally, a meta-analysis focused on recent data analyzes the prevalence of anti-Sm antibodies in SLE.

Clinical associations with autoantibody reactivities to individual components of U1 small nuclear ribonucleoprotein

The reactivities to individual U1 small nuclear ribonucleoprotein (snRNP) components and their relationship to clinical features in patients with anti-U1 snRNP antibodies were examined. We evaluated 114 patients with connective tissue disease whose sera were positive for anti-U1 snRNP antibodies, but negative for anti-Sm antibodies. Antibodies to the U1 snRNP polypeptides 70K, A, and C were detected using subunit-specific enzyme-linked immunosorbent assays and antibodies to U1 small nuclear RNA (snRNA) were identified by an immunoprecipitation assay using deproteinized HeLa cell extracts. The clinical features were retrospectively obtained by chart review and prospectively collected after study entry. The pattern of antibody reactivities to U1 snRNP components varied among patients. The frequency of anti-70K, anti-A, anti-C, and anti-U1 snRNA antibodies was 60%, 86%, 74%, and 46%, respectively. There was no relationship between each reactivity and the clinical findings, but the presence of reactivities to increasing numbers of U1 snRNP components was correlated with sclerodactyly, shortness of the sublingual frenulum, esophageal dysfunction, and a lack of persistent proteinurea (p < 0.05 for all comparisons). The detection of autoantibody reactivities to individual components of the U1 snRNP particle is potentially useful for predicting the clinical course in patients with connective tissue disease and anti-U1 snRNP antibodies. Lupus (2010) 19, 307—312.

Immune pathogenesis of Mixed Connective Tissue Disease: a short analytical review

Mixed Connective Tissue Disease (MCTD) was first described 35 years ago by Gordon C. Sharp and his colleagues. In the ensuing decades, a clearer understanding of the clinical and serologic features of MCTD has emerged. Classification criteria now exist to define MCTD for study purposes, the long-term outcome of the disease has been established, and novel genetic associations within the major histocompatibility complex on chromosome 6 and select regions on chromosome 3 have been identified. Studies on immune pathogenesis have made substantial progress in advancing our understanding of MCTD. In MCTD, there is a complex interaction of the innate and adaptive immune system that culminates in autoimmune disease. Antigenic structural modification occurring during apoptosis or other modifications of self antigens leads to an autoantigen driven immune process with innate immune activation, immunoglobulin G autoantibody production directed against select components of the spliceosome, B lymphocyte activation, and CD4 and CD8 T lymphocyte participation.

Prevalence of disease-specific antinuclear antibodies in general population: estimates from annual physical examinations of residents of a small town over a 5-year period

The aim of this study was to investigate the types and prevalence of disease-specific antinuclear antibodies (ANAs) and their relationship to rheumatic diseases in the general Japanese population. An immunofluorescence (IF) method was used for the first screening of ANA levels in serum samples obtained from 2181 residents of a small Japanese town. Individuals positive for IF-ANA were then further tested for disease-specific ANAs using eight enzyme immunoassays. Physical status and the presence of illness were determined by means of questionnaires and medical examinations. Based on the result of the IF-ANA assay, the rates of positive samples at 1:40 and 1:160 dilutions were 26.0 and 9.5%, respectively, with females have significantly higher positivity rates than males (P < 0.0001). Among 566 IF-ANA-positive individuals, 100 individuals were found to have 114 disease-specific ANAs. Anti-SSA/Ro, anti-centromere, and anti-U1RNP antibodies were detected in 58, 30, and 11 individuals, respectively, but anti-Sm, anti-Scl-70, and anti-Jo-1 antibodies were undetectable. Questionnaires and medical examinations revealed that among 60 disease-specific ANA-positive individuals that were available for testing, six had Sjögren's syndrome (SS), five were suspected of having SS, and five had rheumatoid arthritis. Surprisingly, 34 (57%) of the disease-specific ANA-positive individuals were clinically healthy. Anti-SSA/Ro, anti-centromere, and anti-U1RNP antibodies were quite frequent among clinically healthy Japanese subjects, although anti-Sm, anti-Scl-70, and anti-Jo-1 antibodies were not. Of the 60 individuals who tested positive for disease-specific ANAs, 30% (18/60) actually manifested systemic rheumatic diseases, while 50% showed no detectable signs or symptoms of rheumatic diseases.

A murine model of mixed connective tissue disease induced with U1 small nuclear RNP autoantigen

OBJECTIVE

To test whether immunizing mice with autoantigens closely linked to mixed connective tissue disease (MCTD) could induce an MCTD-like clinical syndrome distinguishable from systemic lupus erythematosus (SLE).

METHODS

Transgenic and knockout C57BL/6-derived mice were immunized subcutaneously at age 8-12 weeks with U1-70-kd small nuclear RNP (70K) fusion protein along with either Freund's complete adjuvant (CFA) or U1 RNA. After 2 months, mice were killed and analyzed histologically and serologically.

RESULTS

Immunization of C57BL/6-derived mice transgenic for human HLA-DR4 with 70K and either CFA or U1 RNA led to anti-70K antibodies in 62% of mice (21 of 34), and diversified anti-RNP immune responses. MCTD-like lung disease also developed in 50% of immunized mice (17 of 34), and anti-70K antibodies were strongly correlated with lung disease. CFA and U1 RNA were comparably able to induce this syndrome. Mice deficient in Toll-like receptor 3 (TLR-3) also developed this same syndrome when immunized with 70K and CFA. However, TLR-3(-/-) mice failed to develop MCTD-like lung disease when treated with 70K and U1 RNA. Rather, TLR-3(-/-) mice immunized with 70K and U1 RNA developed an autoimmune syndrome characterized by glomerulonephritis typical of SLE.

CONCLUSION

Exposure to 70K in an appropriate context is sufficient to induce autoimmunity and target organ injury consistent with MCTD. This system represents a new model of autoimmune interstitial lung disease, and establishes a closer link between anti-70K immunity and MCTD-like lung disease. Of note, changes in innate immune signaling can cause the same trigger to lead to the development of SLE-like nephritis rather than MCTD-like lung disease.

Autoantibodies in the Pathogenesis of Mixed Connective Tissue Disease

Antibodies to U1-RNP are part of the clinical definition of mixed connective tissue disease (MCTD). These antibodies and other well-defined antibodies tend to arise together in affected patients. Although still speculative, hypotheses that link U1-RNP antibodies to the development of autoimmunity in MCTD and that associate U1-RNP antibodies with mechanisms of tissue injury in MCTD have emerged and are being tested. Salient features of these hypotheses include: (1) an antigen-driven response that is due to impaired clearance of potentially immunogenic self-antigens, (2) inadequate B- and T-cell tolerance to RNP autoantigens, and (3) immunogenic properties of the RNA component of targeted ribonucleoproteins. Further studies are needed to establish whether anti-RNP antibodies have prognostic importance that is relevant to practicing clinicians.

Enfermedad mixta del tejido conjuntivo, conectivopatía indiferenciada y síndromes de superposición

Mixed connective tissue disease (MCTD), undifferentiated connective tissue disease and overlap syndromes are autoimmune systemic diseases that must be differentiated. Antibodies against the U1-ribonucleoprotein complex --spliceosome-- allows the diagnosis of mixed connective tissue disease. Links between the immunologic and clinical phenomena are emerging. Longitudinal studies of patients with MCTD highlight the impact of pulmonary hypertension and contribute to define the disease. Immunogenetic studies hold MCTD as an independent disease.

The clinical value of intracellular autoantigens B-cell epitopes in systemic rheumatic diseases

A hallmark of autoimmune diseases is the production of autoantibodies against intracellular autoantigens. Although their pathogenetic and their etiologic relationship are not fully understood, these autoantibodies are important tools for establishing the diagnosis, classification and prognosis of autoimmune diseases. Systemic rheumatic diseases are among the most complex disorders because their clinical presentation and constellation of findings are in part reflected by the wide spectrum of autoantibodies found in the sera of patients suffering from these disorders. These autoantibodies usually target large complexes consisting of protein antigens noncovalently associated with (ribo)-nucleic acid(s), like the spliceosome or Ro/La-RNPs. In this review, we first address the main characteristics and the clinical value of several autoantibodies, with respect to their diagnostic sensitivity and specificity. Subsequently, we provide a brief overview of the antigenic determinant types that have been identified on the corresponding autoantigens. The antibody targets of autontigens include primary, secondary, tertiary and quarternary structure epitopes, as well as cryptotopes, neoepitopes and mimotopes. We next focus on antigenic structures corresponding to B-cell epitopes with high disease specificity and sensitivity for all the major autoantigens in systemic autoimmunity including the Ro/La and U1 ribonucleoprotein complexes and the Ku70/80, ribosomal P, DNA topoisomerase I, filaggrin, Jo-1 and PM/SCl-100 autoantigens. These epitopes, defined at the peptide level, can be chemically synthesized and engineered for the development of new inexpensive and easier to perform assays and the improvement of the methods for autoantibody detection. Specific examples of newly developed assays that incorporate (i) epitopes with high disease specificity and sensitivity, (ii) modified epitopes, (iii) conformational epitopes and (iv) complementary epitopes are discussed in detail. Finally, we examine the potential of combining these synthetic epitopes for future development of multiplex diagnostic tests based on miniaturized autoantigen arrays.