Epitope mapping with synthetic peptides of 52-kD SSA/Ro protein reveals heterogeneous antibody profiles in human autoimmune sera

Roles of TRIM21/Ro52 in connective tissue disease-associated interstitial lung diseases

Multiple factors contribute to the development of connective tissue diseases (CTD), often alongside a range of interstitial lung diseases (ILD), including Sjögren's syndrome-associated ILD, systemic sclerosis-associated ILD, systemic lupus erythematosus-associated ILD, idiopathic inflammatory myositis-associated ILD. TRIM21(or Ro52), an E3 ubiquitin ligase, plays a vital role in managing innate and adaptive immunity, and maintaining cellular homeostasis, and is a focal target for autoantibodies in various rheumatic autoimmune diseases. However, the effectiveness of anti-TRIM21 antibodies in diagnosing CTD remains a matter of debate because of their non-specific nature. Recent studies indicate that TRIM21 and its autoantibody are involved in the pathogenesis of CTD-ILD and play an important role in diagnosis and prognosis. In this review, we focus on the contribution of TRIM21 in the pathogenesis of CTD-ILD, as well as the potential diagnostic value of its autoantibodies in different types of CTD-ILD for disease progression and potential as a novel therapeutic target.

A Interacting Model: How TRIM21 Orchestrates with Proteins in Intracellular Immunity

Tripartite motif-containing protein 21 (TRIM21), identified as both a cytosolic E3 ubiquitin ligase and FcR (Fragment crystallizable receptor), primarily interacts with proteins via its PRY/SPRY domains and promotes their proteasomal degradation to regulate intracellular immunity. But how TRIM21 involves in intracellular immunity still lacks systematical understanding. Herein, it is probed into the TRIM21-related literature and raises an interacting model about how TRIM21 orchestrates proteins in cytosol. In this novel model, TRIM21 generally interacts with miscellaneous protein in intracellular immunity in two ways: For one, TRIM21 solely plays as an E3, ubiquitylating a glut of proteins that contain specific interferon-regulatory factor, nuclear transcription factor kappaB, virus sensors and others, and involving inflammatory responses. For another, TRIM21 serves as both E3 and specific FcR that detects antibody-complexes and facilitates antibody destroying target proteins. Correspondingly delineated as Fc-independent signaling and Fc-dependent signaling in this review, how TRIM21's interactions contribute to intracellular immunity, expecting to provide a systematical understanding of this important protein and invest enlightenment for further research on the pathogenesis of related diseases and its prospective application is elaborated.

Predictive factors for progressive fibrosing interstitial lung disease in anti-synthetase syndrome

OBJECTIVES

Interstitial lung disease (ILD) is common in anti-synthetase syndrome (ASS). Progressive fibrosing ILD (PF-ILD) may develop in ILD with autoimmune features. Data on PF-ILDs in ASS as a group are scarce. This study aimed to explore the characteristics and predictors of PF-ILD in ASS patients.

METHODS

This retrospective study enrolled 96 ASS-ILD patients. Baseline clinical data were collected. PF-ILD assessments were conducted at every hospital visit during windows of 24 months after initial diagnosis. Phenotypic, survival features and predictors of PF-ILD were estimated through SPSS 22.0.

RESULTS

The results revealed that 35.42% (34/96) were evaluated to be PF-ILD with a median interval time of 14.73 months. Nonspecific interstitial pneumonia was the most common radiological pattern of PF-ILD. Ground glass opacity (GGO), traction bronchiectasis and reticulation were representative high-resolution computed tomography findings of this group. Compared with the non-progressive group, PF-ILD patients had higher frequencies of anti-Ro-52 antibodies (91.18% vs 66.13%, P = 0.007) and GGO in the lower + middle and lower + middle + upper zones of the left lung, as well as lower + middle zones in the right lung (85.30% vs 54.84%, P = 0.003; 64.71% vs 38.71%, P = 0.015; 82.35% vs 58.06%, P = 0.016). Multivariate Cox analysis identified that anti-Ro-52 antibody (hazards ratio [HR] 3.55, 95% CI 1.06-11.90, P = 0.040) and GGO in left lower + middle lung zones (HR 22.11, 95% CI 1.95-250.90, P = 0.012) were independent risk factors for PF-ILD.

CONCLUSIONS

PF-ILD was associated with poor prognosis. Over one-third of ASS-ILD patients may develop to PF-ILD. Anti-Ro-52 antibody positivity and GGO in left lower + middle lung zones were independent risk factors for PF-ILD in ASS patients.

Anti-Ro52 Autoantibody Is Common in Systemic Autoimmune Rheumatic Diseases and Correlating with Worse Outcome when Associated with interstitial lung disease in Systemic Sclerosis and Autoimmune Myositis

This review highlights the 30 plus years research progress since the discovery of autoantibody to Ro52/TRIM21 in patients with systemic lupus erythematosus (SLE) and Sjögren's syndrome (SjS). After the initial expression cloning of the Ro52 cDNA, it has taken many years to the current understanding in the interesting biological function of Ro52 as an E3 ubiquitin ligase and its role in innate immune clearance of intracellular IgG-bound complex. Early observations show that anti-Ro52, mostly associated with anti-SS-A/Ro60 and/or anti-SS-B/La, is commonly found in SLE (40-70%), SjS (70-90%), neonatal lupus erythematosus (NLE, 75-90%), and subacute cutaneous lupus erythematosus (50-60%). Anti-Ro52 has long been postulated to play a direct pathogenic role in congenital heart block in NLE as well as in the QT interval prolongation in some adults. The widespread availability of the anti-Ro52 assay has led to the detection of anti-Ro52 in other diseases including autoimmune hepatitis (20-40%), systemic sclerosis (10-30%), and autoimmune myositis (20-40%). More than ten studies have pointed to an association of anti-Ro52 with interstitial lung disease and, more importantly, correlating with poor outcome and worse survival. Other studies are implicating an interesting role for anti-Ro52 in the diagnosis of certain cancers. Future studies are needed to examine the mechanism in the pathogenesis of anti-Ro52 and carefully documenting its causal relationships in different disease conditions.

Myositis specific antibodies are associated with isolated anti-Ro-52 associated interstitial lung disease

OBJECTIVES

Anti-Ro-52 antibody positivity might be associated with the presence of interstitial lung disease (ILD) among patients with autoimmune features. However, the clinical significance of isolated anti-Ro-52 positivity (i.e., the presence of anti-Ro52 antibodies but the absence of anti-Ro60 antibodies; anti-Ro52+-Ro60-) in patients with ILD is not clear.

METHODS

This is a prospective and observational study of Chinese ILD patients with isolated anti-Ro-52 positivity. According to their myositis-specific antibody (MSA) status, patients were split into groups, and their clinical and radiological features were compared.

RESULTS

Of the 158 enrolled patients with ILD and isolated anti-Ro-52 positivity (isolated anti-Ro-52-ILD), there were 130 patients with a positive MSA status and 28 patients with a negative MSA status. Anti-synthetase antibodies (ASAs) were found in 61.5% of patients with MSA+ ILD, and anti-melanoma differentiated-associated protein 5 (MDA-5) antibodies were found in the remaining 38.5% of patients. The anti-nuclear antibody (ANA) pattern was associated with ASA and anti-MDA-5 positivity (χ2=70.7, P < 0.001; Cramer's value 0.47, P < 0.001): ANA negativity was associated with anti-MDA-5 positivity, and cytoplasmic ANA positivity was associated with ASA positivity. There were statistically significant differences in the high-resolution CT patterns between patients with isolated anti-Ro-52 positivity with different MSA statuses (χ2=29.8, P < 0.001; Cramer's value 0.31, P < 0.001): OP pattern was more common in patients with anti-MDA-5 antibodies than in those without anti-MDA-5 antibodies.

CONCLUSIONS

Patients with isolated anti-Ro-52-ILD showed high positivity of MSA. Isolated anti-Ro-52 positivity with cytoplasmic ANA positivity was strongly associated with ASA+-ILD, while ANA negativity was associated with anti-MDA-5+-ILD.

Anti-Ro60 Seropositivity Determines Anti-Ro52 Epitope Mapping in Patients With Systemic Sclerosis

Epitope mapping of anti-Ro52 antibodies (Abs) has been extensively studied in patients with Sjögren's syndrome (SjS) and systemic lupus erythematosus (SLE). Comprehensive epitope mapping in systemic sclerosis (SSc), where anti-Ro52 antibodies are also frequently detected, has not been performed. The aim of the present study was to fully characterize Ro52 epitopes in anti-Ro52-positive SSc using Ro52 fragments spanning the full antigen. Further analysis was made according to anti-Ro60 status. Epitope mapping was performed in 43 anti-Ro52-positive SSc patients. Seventy eight anti-Ro52-positive pathological controls, including 20 patients with SjS, 28 patients with SLE, 15 patients with dermatomyositis (DM), and 15 patients with primary biliary cholangitis (PBC), and 20 anti-Ro52-negative healthy individuals as normal controls were also tested. Five recombinant Ro52 fragments [Ro52-1 (aa 1-127), Ro52-2 (aa 125-268), Ro52-3 (aa 268-475), Ro52-4 (aa 57-180), and Ro52-5 (aa 181-320) were used to test reactivity by line-immunoassay and in house ELISA. Anti-Ro60 reactivity was tested by ELISA. All anti-Ro52 positive sera reacted with Ro52-2; none recognized Ro52-3. Antibodies against Ro52-1 were less frequently found in SSc than in SjS/SLE (11.6 vs. 41.7%, p = 0.001); and antibodies against Ro52-4 were less frequently found in SSc than in SjS/SLE (27.9 vs. 50%, p = 0.03). In SSc patients, reactivity against Ro52-1 was more frequent in anti-Ro52+/anti-Ro60+ than in anti-Ro52+/anti-Ro60-patients (33.3 vs. 0%, p = 0.003). In this comprehensive analysis of Ro52 epitope mapping in SSc, the coiled coil domain remains the predominant epitope on Ro52. Contrary to SjS and SLE, patients with SSc fail to identify epitopic regions within the N-terminus of the protein, especially if they lack con-current anti-Ro60 reactivity.

Umbilical cord blood levels of maternal antibodies reactive with p200 and full-length Ro 52 in the assessment of risk for cardiac manifestations of neonatal lupus

OBJECTIVE

Maternal anti-Ro autoantibodies are associated with cardiac manifestations of neonatal lupus (cardiac NL), yet only 2% of women with this reactivity have an affected child. Identification of a more specific marker would channel intense monitoring to fetuses at greater risk. This study aimed to determine whether autoantibodies against Ro 52 amino acids 200-239 (p200) confer added risk over autoantibodies to full-length Ro 52, Ro 60, or La.

METHODS

Anti-Ro-exposed pregnancies resulting in cardiac NL or no cardiac manifestations were identified from the Research Registry for Neonatal Lupus and the PR Interval and Dexamethasone Evaluation study. Umbilical cord (n = 123) and maternal (n = 115) samples were evaluated by enzyme-linked immunosorbent assay.

RESULTS

The frequencies of p200, Ro 52, Ro 60, and La autoantibodies were not significantly different between affected and unaffected children. However, neonatal anti-Ro 52 and Ro 60 titers were highest in cardiac NL and their unaffected siblings compared to unaffected neonates without a cardiac NL sibling. Although both maternal anti-Ro 52 and p200 autoantibodies were less than 50% specific for cardiac NL, anti-p200 was the least likely of the Ro autoantibodies to be false-positive in mothers who have never had an affected child. Titers of anti-Ro 52 and p200 did not differ during a cardiac NL or unaffected pregnancy from the same mother.

CONCLUSION

Maternal reactivity to p200 does not confer an added risk to fetal conduction defects over full-length Ro 52 or Ro 60 autoantibodies. Mothers who may never be at risk for having an affected child have lower anti-Ro 60 titers and may require less stringent echocardiographic monitoring compared to women with high-titer autoantibodies.

Anti-Ro/SSA-associated corrected QT interval prolongation in adults: the role of antibody level and specificity

OBJECTIVE

Recent evidence suggests that anti-Ro/SSA antibodies, strongly associated with the development of congenital heart block, may also be arrhythmogenic for the adult heart. In fact, anti-Ro/SSA-positive patients with connective tissue disease (CTD) frequently display corrected QT (QTc) prolongation associated with an increase in ventricular arrhythmias. However, QTc prolongation prevalence markedly differs throughout the studies (10-60%), but the reason why is not yet clear. The aim of this study was to evaluate whether anti-Ro/SSA-associated QTc prolongation in adult patients with CTD is related to antibody level and specificity.

METHODS

Forty-nine adult patients with CTD underwent a resting 12-lead electrocardiogram recording to measure QTc interval, and a venous withdrawal to determine anti-Ro/SSA antibody level and specificity (anti-Ro/SSA 52 kd and anti-Ro/SSA 60 kd) by immunoenzymatic methods and Western blotting.

RESULTS

In our population, a direct correlation was demonstrated between anti-Ro/SSA 52-kd level and QTc duration (r = 0.38, P = 0.007), patients with a prolonged QTc had higher levels of anti-Ro/SSA 52 kd with respect to those with a normal QTc (P = 0.003), and patients with a moderate to high level (≥50 units/ml) of anti-Ro/SSA 52 kd showed a longer QTc interval (P = 0.008) and a higher QTc prolongation prevalence (P = 0.008) than those with a low positive/negative level (<50 units/ml). On the contrary, no association was found between QTc and anti-Ro/SSA 60-kd level.

CONCLUSION

In anti-Ro/SSA-positive adult patients with CTD, the occurrence of QTc prolongation seems strictly dependent on the anti-Ro/SSA 52-kd level. This finding, possibly explaining the different QTc prolongation prevalence reported, strengthens the hypothesis that an extremely specific autoimmune cross-reaction is responsible for the anti-Ro/SSA-dependent interference on ventricular repolarization.

Prevalence of antibodies to Ro-52 in a serologically defined population of patients with systemic sclerosis

Background

Antibodies against Ro-52 have been described in patients with a broad spectrum of autoimmune disease, most commonly in association with anti-Ro-60 in systemic lupus erythematosus and Sjogrens syndrome. However, in inflammatory myositis anti-Ro-52 is frequently present without anti-Ro-60 and is closely linked to the presence of aminoacyl-tRNA synthetase (aats) antibodies. To date there have been no comprehensive reports on the frequency of anti-Ro-52 in systemic sclerosis (SSc), a disease characterised by hallmark autoantibodies that occur in non-overlapping subsets. Clinically, each antibody-defined group has a distinct pattern of organ involvement, some featuring myositis.

Objectives

To determine the frequency of anti-Ro-52 in serologically defined groups of SSc patients and to investigate a possible link with myositis-associated autoantibodies.

Methods

Serum samples from 1010 patients with SSc and 55 and 32 patients with anti-aats and anti-Ku respectively were tested for the presence of anti-Ro-52 using a commercial ELISA.

Results

The prevalence of anti-Ro-52 was 15–38% in nine of the eleven sub-groups. There were no significant differences in mean anti-Ro-52 levels in these groups with the exception of that defined by the presence of anti-U1-RNP. In the remaining groups defined by anti-Ro-60 and anti-aats, anti-Ro-52 was present in 92% and 100% respectively. In sera from non-SSc patients with anti-aats, anti-Ro-52 was detected in 64%.

Conclusion

Anti-Ro-52 is present throughout the SSc population. It is neither more prevalent in the myositis-associated antibody groups nor does it segregate with any other major SSc-specific autoantibodies. The co-existence of anti-Ro-52 with both anti-Ro-60 and anti-aats is confirmed.

52-kDa Ro/SSA epitopes preferentially recognized by antibodies from mothers of children with neonatal lupus and congenital heart block

Neonatal lupus erythematosus is a rare disorder caused by the transplacental passage of maternal autoantibodies. The 52-kDa Ro/SSA antigen (Ro52) ribonucleoprotein represents an antigenic target strongly associated with the autoimmune response in mothers whose children have neonatal lupus and cardiac conduction disturbances, mainly congenital heart block. The objective of this study was to identify putative Ro52/60-kDa Ro/SSA antigen (Ro60) epitopes associated with neonatal lupus and congenital heart block. The reactivity of IgG antibodies present in the sera from mothers with systemic lupus erythematosus and Sjögren's syndrome and in the sera from asymptomatic mothers (a longitudinal study of 192 samples from 66 subjects) was investigated by ELISA using Ro52, Ro60 and 48-kDa La/SSB antigen proteins, as well as 45 synthetic peptides, 13–24 residues long, of Ro52/Ro60 proteins. One to 19 samples collected before, during and after pregnancy were available for each mother. Forty-three disease controls selected randomly and normal sera were tested in parallel. Although no differences were found between Sjögren's syndrome and asymptomatic mothers of group I, who had at least one infant with neonatal lupus, and of group II, who had healthy babies only, significant differences were observed between lupus mothers from both groups. In the former group of lupus mothers, a significantly higher frequency of antibodies to Ro52 peptides 107–122 and 277–292 was observed. Between 18 and 30 weeks of gestation, the period of risk, there was clearly an elevated level of antibodies reacting with Ro52 peptides 1–13, 277–292 and 365–382. Antibodies to Ro52 peptide 365–382 have been shown previously to cross-react with residues 165–185 of the heart 5-HT₄ serotoninergic receptor, and might be pathologically important. The level of these Ro52 antibody subsets decreased at the end of pregnancy and after delivery. IgG antibodies to Ro52 peptides 1–13, 107–122, 277–292 and 365–382 may therefore represent important biomarkers to predict a complication in pregnant lupus women with Ro52 antibodies.

Structural Organization and Zn2+-dependent Subdomain Interactions Involving Autoantigenic Epitopes in the Ring-B-box-Coiled-coil (RBCC) Region of Ro52

Ro52 is one of the major autoantigens targeted in the autoimmune disease Sjögren syndrome. By sequence similarity, Ro52 belongs to the RING-B-box-coiled-coil (RBCC) protein family. Disease-related antibodies bind Ro52 in a conformation-dependent way both in the coiled-coil region and in the Zn2+-binding Ring-B-box region. Primarily associated with Sjögren syndrome, Ro52 autoantibodies directed to a specific, partially structured epitope in the coiled-coil region may also induce a congenital heart block in the fetus of pregnant Ro52-positive mothers. To improve our understanding of the pathogenic effects of autoantibody binding to the Zn2+-binding region, a multianalytical mapping of its structural, biophysical, and antigenic properties is presented. Structure content and ligand binding of subregions, dissected by peptide synthesis and subcloning, were analyzed by fluorescence and circular dichroism spectroscopy. A novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry strategy for time-resolved proteolysis experiments of large protein domains was developed to facilitate analysis and to help resolve the tertiary arrangement of the entire RBCC subregion. The linker region between the RING and B-box motifs is crucial for full folding, and Zn2+ affinity of the RING-B-box region is further protected in the entire RBCC region and appears to interact with the coiled-coil region. Murine monoclonal antibodies raised toward the RING-B-box region were primarily directed toward the linker, further supporting a highly functional role for the linker in a cellular environment. Taken together with our previous analysis of autoantigenic epitopes in the coiled-coil region, localization of autoantigenic epitopes in Ro52 appears closely related to molecular functionalities.

[Auto-antibodies to anti-ENA SSA/RO (52 and 60 kDa): an auto-immunity laboratory's experience]

PURPOSE

Anti-SSA/Ro and anti-SSB/La autoantibodies are frequently encountered in SLE or SGS where anti-SSA subtypes 52 and 60 kDa seems to be differently found in connection with the disease type: anti-SSA/Ro 60 kDa more frequently found in SLE and anti-SSA/Ro 52 kDa in SGS. We try to find if it was interesting in identifying these specificities for all anti-ENA screening.

METHOD

The study included 162 patients' sera found anti-SSA 52 and/or anti-SSA 60 and/or anti-SSB positive among 1600 screening tests from the different hospital's services. We used two assays: first, dotblot (Innolia-Ana Update INGEN) as a screening test and second, an Elisa (ENA-LISA BMD) as confirmation. Thirty-eight control sera were found negative with dotblot.

RESULTS

Only one subtype of anti-SSA (52 or 60 kD) or anti-SSB was found for 55 sera (44 anti-SSA 52, 10 anti-SSA 60, 1 anti-SSB) and 107 sera were found positive for two or more (73 anti-SSA 52 + 60 and 34 anti-SSA 52 or 60 with another anti-ENA). While anti-SSA 60 kDa alone or not was always positive with the Elisa test, neither anti-SSA 52 alone was anti-SSA Elisa's positive. Diseases associations results show a greater linking of anti-SSA 60 kDa with SLE, a frequent linking of combined reactivity anti-SSA 52/60 in SLE and SGS and a greater spreading of anti-SSA 52 kDa alone among pathological groups, showing an autoimmune disease's linking in 68%. Among SGS, 29% had only anti-SSA 52 kDa.

CONCLUSION

We suggest screening specific tests for identifying anti-SSA/Ro 52 kDa reactivity which are missed by routine testing (tests using animal's antigens) and could represent an additional serum marker in Connective Tissue Diseases.

Advances in B-cell epitope analysis of autoantigens in connective tissue diseases

The characterization of autoantibody specificities in rheumatic diseases is important in both diagnostic and basic research areas. Identification of the epitopes recognized by autoantibodies and their clinical and biological significance is not a trivial task. Epitopes may range in complexity from simple linear sequences of amino acids to complex quaternary structures. In addition to this structural complexity the frequency with which an autoantigen and its epitopes are recognized in a patient population may be useful in diagnosis, defining disease subgroups, and may offer information on disease prognosis. In this review recent advances in the epitope mapping of autoantigens in connective tissue diseases are discussed, with particular emphasis placed on the methodologies used to identify epitopes and the classification of the structural features of epitopes. To illustrate the identification of epitope structure, clinically relevant autoantigens, including CENP-A, PM/Scl-100, fibrillarin, filaggrin, Ro-52, and dsDNA, are discussed as examples of each type of epitope.

Cardiac 5-HT(4) serotoninergic receptors, 52kD SSA/Ro and autoimmune-associated congenital heart block

It was recently reported that sera from patients with systemic lupus erythematosus contain antibodies reactive with the second extracellular loop of the serotoninergic 5-HT(4) receptor expressed in the human heart. This antibody response was associated with antibodies to 52kD SSA/Ro, a reactivity prevalent in mothers of children with congenital heart block (CHB). The current study was undertaken to determine whether the 5-HT(4) receptor is a target of the immune response in these mothers. Initial experiments demonstrated mRNA expression of the 5-HT(4) receptor in the human foetal atrium. Electrophysiologic studies established that human foetal atrial cells express functional 5-HT(4) receptors. Sera from 116 mothers enrolled in the Research Registry for Neonatal Lupus, whose children have CHB, were evaluated. Ninety-nine (85%) of these maternal sera contained antibodies to SSA/Ro, 84% of which were reactive with the 52kD SSA/Ro component by immunoblot. None of the 116 sera were reactive with the peptide spanning aa165-185 of the serotoninergic receptor. Rabbit antisera which recognized this peptide did not react with 52kD SSA/Ro or peptide aa365-382 in the C terminus. Although 5-HT(4) receptors are present and functional in the human foetal heart, maternal antibodies to the 5-HT(4) receptor are not associated with the development of CHB.

Autoantibodies and overlap syndromes in autoimmune rheumatic disease

Many patients diagnosed with autoimmune rheumatic disease cannot be categorised easily into one of the established clinical entities such as systemic lupus erythematosus, dermatomyositis, or systemic sclerosis. The term "overlap syndrome" has been increasingly used to identify such patients and is useful in terms of clarifying prognosis and facilitating disease management. This article reviews overlap syndrome in autoimmune rheumatic disease, with particular emphasis on the associated serological markers.

Autoantibody to the leucine zipper region of 52 kDa Ro/SSA binds native 60 kDa Ro/SSA: identification of a tertiary epitope with components from 60 kDa Ro/SSA and 52 kDa Ro/SSA

Anti-Ro (or SSA) is found in the sera of patients with autoimmune rheumatic illnesses. All patients with anti-Ro defined by precipitation bind a 60 000 Da antigen (60 kDa Ro), whereas some patients also bind a 52 000 Da molecule (52 kDa Ro). In general, antibody binding is directed against native 60 kDa Ro and denatured 52 kDa Ro. The mechanism by which anti-52 kDa Ro arises in the setting of anti-60 kDa Ro is unknown. Conflicting data exist as to the existence of a physical interaction between the two proteins in cells and as to cross-reacting antibodies. Antibodies were affinity purified from a peptide within the leucine zipper region of 52 kDa Ro. These purified antibodies binding the 197-207 peptide from 52 kDa Ro (anti-52LZ) bound native 60 kDa Ro as well as denatured 52 kDa Ro. In addition, anti-52LZ also bound up to four regions from the sequence of 60 kDa Ro and a single conformational epitope of 60 kDa Ro. Thus, these primary sites represent components of the tertiary epitope. We hypothesized that if this was the case, these peptides making up a tertiary epitope would show molecular interaction. In fact, peptides from 60 kDa Ro have a molecular interaction with the 52 kDa Ro peptide as well as full-length 52 kDa Ro when assessed by surface plasmon resonance. The leucine-zipper region peptide from 52 kDa Ro bound three of the four peptides from 60 kDa Ro. These data suggest that these two molecular species, 60 and 52 kDa Ro, form a conformational epitope. This relationship may explain why anti-52 kDa Ro is found in association with anti-60 kDa Ro.

Anti-SSA/Ro52 autoantibodies blocking the cardiac 5-HT4 serotoninergic receptor could explain neonatal lupus congenital heart block

The 52-kDa SSA/Ro (Ro52) ribonucleoprotein is an antigenic target strongly associated with the autoimmune response in mothers whose children develop neonatal lupus and congenital heart block. When sera from patients with systemic lupus erythematosus were used as autoimmune controls in an enzyme immunoassay to screen for antibodies against the human serotoninergic 5-HT4-receptor, a high correlation was found between the presence of anti-Ro52 protein antibodies in such sera and antibodies reacting with a synthetic peptide, corresponding to the second extracellular loop of the human 5-HT4 receptor (amino acid residues 165-185). Homology scanning between the 5-HT4 peptide and the sequence of the Ro52 protein indicated two potential common epitopes located between residues 365 and 396 of the Ro52 protein. Cross-reactivity was found between the peptide derived from the 5-HT4 receptor, and a peptide corresponding to residues 365-382 of the Ro52 protein. Autoantibodies, affinity-purified on the 5-HT4 receptor peptide, specifically recognized both the Ro52 protein and the 5-HT4 receptor protein in immunoblots. The affinity-purified antibodies antagonized the serotonin-induced L-type Ca channel activation on human atrial cells. This effect could explain the electrophysiological abnormalities in neonatal lupus.

Comparison of the anti-Ro/SSA autoantibody profile between patients with primary and secondary Sjögren's syndrome

OBJECTIVE

To measure serum levels of anti-Ro52-kD/SSA, anti-Ro60-kD/SSA and anti-La/SSB autoantibodies in patients with primary and secondary Sjögren's syndrome. To examine if there is any connection between the disease and the subtype-spectrum of these antibodies.

METHODS

We measured serum levels of anti-Ro52-kD/SSA, anti-Ro60-kD/SSA and anti-La/SSB autoantibodies by ELISA, in the sera of patients with primary Sjögren's syndrome with or without extraglandular manifestations and with or without anti-La/SSB positivity and of patients with systemic lupus erythematosus/Sjögren's syndrome overlapping disease with or without anti-La/SSB positivity.

RESULTS

Differences of the distribution of the anti-Ro52-kD/SSA and the anti-Ro60-kD/SSA were found between the primary and secondary Sjögren's syndrome patients' groups; when Sjögren's syndrome is accompanied by systemic lupus erythematosus, the occurrence of anti-Ro60-kD/SSA autoantibodies is significantly higher than in primary Sjögren'syndrome.

CONCLUSION

Our results suggest that there is a possible connection between the distribution of the subtypes of the anti-Ro/SSA autoantibodies and the disease type in primary/secondary Sjögren's syndrome.

Analysis of B-cell epitopes of the Ro/SS-A autoantigen

Autoantibodies to the Ro and La ribonucleoprotein antigens are found in several rheumatoid diseases. The important epitopes have been studied using synthetic peptides and recombinant antigens to understand how they arise and their implication in disease. Here, we analyse the results of epitope mapping studies of Ro60 and Ro52 autoantigens and focus on the major findings to date.

MHC class II gene associations with autoantibodies to U1A and SmD1 proteins

Autoantibodies against U small nuclear ribonucleoproteins (snRNP) are frequently present in the serum of patients with systemic rheumatic diseases, and have been reported to be associated with HLA-DR and -DQ genes. To better define the role of HLA genes in the production of such antibodies, we studied immunogenetic associations with autoantibodies reacting with U1 RNP, U1A and SmD1 proteins, and synthetic peptides containing immunodominant linear epitopes of these proteins. Only two out of the 15 overlapping peptides of U1A (i.e. peptides 35-58 and 257-282) and three of 11 peptides of SmD1 (i.e. peptides 1-20, 44-67 and 97-119) were significantly recognized by patients' sera selected on the basis of their antibody positivity with RNP in immunodiffusion. The distribution of DRB1, DQB1 and DPB1 alleles among the anti-RNP antibody-positive patients (n = 28) and healthy control subjects was similar. Antibodies against U1A (tested in Western immunoblotting with HeLa cell extracts) were positively associated to DRB1*06 allele; antibodies reacting with SmD1 peptide 44-67 were negatively associated to DRB1*02 and DQB1*0602 alleles. No association was found between DPB1 alleles and antibodies reacting with U1A and SmD1 antigens. This first study reporting an association between autoantibodies reacting with U1A and SmD1 proteins (and peptides of these proteins), and immunogenetic markers suggest that the production of antibody subsets directed against different components (or regions of these proteins) bound to the same snRNP particle is associated with distinct MHC class II alleles.

Fine specificity of the autoimmune response to the Ro/SSA and La/SSB ribonucleoproteins

The fine specificity of the Ro and La proteins has been studied by several techniques. In general, there is agreement in a qualitative sense that autoantibodies bind multiple epitopes. For some specific antibody binding, different studies agree quantitatively, for instance, the binding of the carboxyl terminus of 60-kd Ro as described by 2 studies using different techniques and the presence of an epitope within the leucine zipper of 52-kd Ro. In addition, there is general agreement about the location of a prominent epitope at the RRM motif region of the La molecule. On the other hand, the many specific epitope regions of the molecules differ among these studies. These discrepancies are likely the result of using different techniques, sera, and peptide constructs as well as a result of inherent advantages and disadvantages in the individual approaches. Several theories concerning the origin of not only the antibodies, but also the diseases themselves, have been generated from studies of the fine specificity of antibody binding. These include a theory of a primordial foreign antigen for anti-Ro autoimmunity, molecular mimicry with regard to La and CCHB, as well as the association of anti-Ro with HLA. These remain unproven, but are of continuing interest. An explanation for the association of anti-60-kd Ro and anti-52-kd Ro in the sera of patients has sprung from evaluating antibody binding. Data demonstrating multiple epitopes are part of a large body of evidence that strongly suggests an antigen-driven immune response. This means that the autoantigens are directly implicated in initiating and sustaining autoimmunity in their associated diseases. A number of studies have investigated the possibility of differences in the immune response to these antigens in SS and SLE sera. While several differences have been reported, none have been reproduced in a second cohort of patients. Furthermore, none of the reported differences may be sufficiently robust for clinical purposes, such as distinguishing between SS with systemic features and mild SLE, although some might be promising. For instance, in at least 3 groups of SLE patients, no binding of residues spanning amino acids 21-41 of 60-kd Ro has been found. Meanwhile, 1 of those studies found that 41% of sera from patients with primary SS bound the 60-kd Ro peptide 21-41. Perhaps future studies will elaborate a clinical role of such a difference among SS and SLE patients. Study of the epitopes of these autoantigens has, in part, led to a new animal model of anti-Ro and anti-La. Non-autoimmune-prone animals are immunized with proteins or peptides that make up the Ro/La RNP. Such animals develop an autoimmune response to the entire particle, not just the immunogen. This response has been hypothesized to arise from autoreactive B cells. In another, older animal model of disease, the MRL-lpr/lpr mouse, B cells have recently been shown to be required for the generation of abnormal, autoreactive T cells. Thus, there are now powerful data indicating that B cells that produce autoantibodies are directly involved in the pathogenesis of disease above and beyond the formation of immune complexes. Given that the autoreactive B cell is potentially critical to the underlying pathogenesis of disease, then studying these cells will be crucial to further understanding the origin of diseases associated with Ro and La autoimmunity. Hopefully, an increased understanding will eventually lead to improved treatment of patients. Progress in the area of treatment will almost surely be incremental, and studies of the fine specificity of autoantibody binding will be a part of the body of basic knowledge contributing to ultimate advancement. In the future, the animal models will need to be examined with regard to immunology and immunochemistry as well as genetics. The development of these autoantibodies has not been studied extensively because upon presentation to medical care, virtually all patients have a full-

At least three linear regions but not the zinc-finger domain of U1C protein are exposed at the surface of the protein in solution and on the human spliceosomal U1 snRNP particle

No structural information on U1C protein either in its free state or bound to the spliceosomal U1 small nuclear ribonucleoprotein (snRNP) particle is currently available. Using rabbit antibodies raised against a complete set of 15 U1C overlapping synthetic peptides (16-30 residues long) in different immunochemical tests, linear regions exposed at the surface of free and U1 snRNP-bound U1C were identified. Epitopes within at least three regions spanning residues 31-62, 85-103 and 116-159 were recognized on free and plastic-immobilized recombinant human U1C expressed in Escherichia coli, on in vitro translated U1C protein and on U1C bound to the U1 snRNP particle present in HeLa S100 extract. Using a zinc affinity labeling method, we further showed that the N-terminal U1C peptide containing a zinc-finger motif (peptide 5-34) effectively binds65Zn2+. The N-terminal region of U1C, which is functional in U1 snRNP assembly, is apparently not located at the surface of the U1 snRNP particle.

New methods for detection of anti-nuclear antibodies

Many different autoantibodies which react with a variety of different nuclear and cytoplasmic antigens have been described. Detection of some these antibodies has been shown to be clinically useful in a number of different autoimmune diseases. For many years, the detection of most of the clinically relevant antibodies was done with by immunofluorescence on tissue substrates and human cultured cell lines. Within the past few years, a number of technical advances has now made it possible to convert to enzyme immunoassay. The paper reviews the clinically relevant antibodies and discusses the variety of new methods which are now available for ANA detection in diagnostic laboratories.

Zinc is an essential cofactor for recognition of the DNA binding domain of poly(ADP-ribose) polymerase by antibodies in autoimmune rheumatic and bowel diseases

OBJECTIVE

To characterize autoantibody response to poly(ADP-ribose) polymerase (PARP) and to assess the significance of autoantibodies to the 2 zinc fingers of this enzyme in patients with autoimmune rheumatic and bowel diseases.

METHODS

The specificity of antienzyme autoantibodies was established by dot immunoassay with recombinant human PARP and by enzyme-linked immunosorbent assay using the recombinant N-terminal fragment containing the DNA binding domain of PARP, the recombinant C-terminal catalytic domain (40-kd fragment), a peptide containing the nuclear localization signal (NLS) of PARP, 2 synthetic peptides (and mutated peptides) corresponding to zinc-finger motifs F1 and F2 that are present in the DNA binding domain, zinc fingers from other self antigens (e.g., peptides from Ro60, Ro52, and U1C proteins), and poly(ADP-ribose). Sera from patients with autoimmune rheumatic and bowel diseases were tested, as were affinity-purified antibodies. Histocompatibility typing of systemic lupus erythematosus (SLE) patients was performed by serology.

RESULTS

Antibodies from the patient sera reacted only weakly with the recombinant N- and C-terminal domains and with the NLS peptide. In contrast, the 2 synthetic peptides corresponding to zinc-finger motifs F1 and F2 represented immunodominant targets for IgG antibodies from patients with SLE, mixed connective tissue disease (MCTD), Crohn's disease, and ulcerative colitis. The sera from patients with SLE and MCTD showed much weaker reactivity with mutant peptides F1 and F2, which contain mutations at the cysteine residues involved in zinc coordination. F1/F2 antibodies did not cross-react with zinc fingers from other self proteins. No correlation was found between the presence of F1/F2 autoantibodies in SLE sera and the presence of other autoantibodies typical of this disease (e.g., anti-double-stranded DNA and poly[ADP-ribose] antibodies). The presence of F2 antibodies in the serum of SLE patients was negatively associated with HLA-DR6.

CONCLUSION

An autoimmune response to PARP is potentially important because this enzyme is involved in DNA repair and is rapidly cleaved during the "execution phase" of apoptosis. The high prevalence in certain autoimmune rheumatic and bowel diseases of antibodies to F1 and F2, which are directly involved in this process, is further evidence implicating involvement of the DNA repair system in chronic inflammatory diseases.

Synthetic peptides for diagnostic use

Synthetic peptides representing relevant B-cell epitopes are, potentially, ideal antigens to be used in diagnostic assays because of their superior properties with respect to quality control as compared to those of biologically derived molecules and the much higher specificity that sometimes can be obtained. It is thus not a surprise than an increasing number of synthetic peptide based diagnostic assays are being developed or already are commercially available. In this report methods to define useful synthetic peptides, and diagnostic systems that are under development or on the market are discussed.

Clinical significances of antibodies to Ro/SS-A autoantigens and its subtypes in primary Sjögren's syndrome

OBJECTIVE

To evaluate the patterns of Ro autoantigen recognition in Korean patients with primary Sjögren's syndrome (SS) and to investigate its clinical significance in SS.

METHODS

Sera from primary SS (n = 51) and systemic lupus erythematosus (SLE) (n = 132) were tested by double immunodiffusion test and immunoblotting for reactivity with 60 kDa and 52 kDa Ro/SS-A proteins. Clinical manifestations were evaluated on the basis of the presence of anti-Ro/SS-A antibodies and anti-60 kDa/52 kDa proteins.

RESULTS

The prevalence of anti-Ro/SS-A antibodies in Korean patients with primary SS was 64.7%. In immunoblotting analysis, the incidence of anti-60 kDa without anti-52 kDa was lower in patients with SS(3.0% vs. 11.6%, p > 0.05), whereas anti-52 kDa without anti-60 kDa was more common in SS patients than in SLE patients(42.5% vs. 4.3%, p < 0.001). Patients with anti-Ro/SS-A antibody were significantly associated with the presence of vasculitis, hyperglobulinemia and rheumatoid factor in primary SS (p < 0.05).

CONCLUSION

The patterns of 52 kDa and 60 kDa Ro autoantigen recognition were quite different in the SLE and primary SS. Anti-52 kDa without anti-60 kDa antibody may be used as a diagnostic marker for primary SS. Although the presence of anti-Ro/SS-A antibody was closely associated with certain clinical features in SS, these clinical manifestations were not correlated with the presence of antibodies against each 52 kDa and 60 kDa proteins. Extended studies with a large population are required to determine the clinical correlation of autoantibodies against each peptides or epitopes of Ro/SS-A proteins.

Comparison of two different methods using overlapping synthetic peptides for localizing linear B cell epitopes in the U1 snRNP-C autoantigen

We have compared the performances of two different approaches using overlapping synthetic peptides to identify the location of linear epitopes of the U1 snRNP-C autoantigen. The first method was based on the use of 15 overlapping peptides (16-30 residue-long) synthesized using conventional Fmoc chemistry, removed from the resin by a standard cleavage procedure, and tested by ELISA after direct coating to polyvinyl microtiter plates. The second approach used a commercial kit (SPOT) to synthesize 75 overlapping decapeptides on cellulose membrane which were assayed by a direct immunoenzymatic test. Both standard and SPOTscan methods were evaluated with antibodies raised in rabbits against synthetic peptides of U1C and sera from patients with autoimmune diseases. In addition to inherent problems linked to the SPOT synthesis (in particular the impossibility of checking the quality of peptides), a number of limitations in the SPOTscan method were identified (e.g. a certain lack of sensitivity and, in one case, the complete lack of peptide reactivity due to the removal of charged end groups at both extremities). However, we found no background with sera from autoimmune patients in the SPOTscan and the antigenic maps obtained using the two approaches generally agreed. This study shows that the SPOTscan approach represents a simple, relatively non expensive and rapid method for initial screening to identify candidate sequences that may be dominant linear epitopes in a protein. Subsequent analysis and controls should include the preparation of conventionally synthesized peptides for formal immunochemical investigations.

Random phage mimotopes recognized by monoclonal antibodies against the pyruvate dehydrogenase complex-E2 (PDC-E2)

Dihydrolipoamide acetyltransferase, the E2 component of the pyruvate dehydrogenase complex (PDC-E2), is the autoantigen most commonly recognized by autoantibodies in primary biliary cirrhosis (PBC). We identified a peptide mimotope(s) of PDC-E2 by screening a phage-epitope library expressing random dodecapeptides in the pIII coat protein of fd phage using C355.1, a murine monoclonal antibody (mAb) that recognizes a conformation-dependent epitope in the inner lipoyl domain of PDC-E2 and uniquely stains the apical region of bile duct epithelium (BDE) only in patients with PBC. Eight different sequences were identified in 36 phage clones. WMSYPDRTLRTS was present in 29 clones; WESYPFRVGTSL, APKTYVSVSGMV, LTYVSLQGRQGH, LDYVPLKHRHRH, AALWGVKVRHVS, KVLNRIMAGVRH and GNVALVSSRVNA were singly represented. Three common amino acid motifs (W-SYP, TYVS, and VRH) were shared among all peptide sequences. Competitive inhibition of the immunohistochemical staining of PBC BDE was performed by incubating the peptides WMSYPDRTLRTS, WESYPDRTLRTS, APKTYVSVSGMV, and AALWGVKVRHVS with either C355.1 or a second PDC-E2-specific mAb, C150.1. Both mAbs were originally generated to PDC-E2 but map to distinct regions of PDC-E2. Two of the peptides, although selected by reaction with C355.1, strongly inhibited the staining of BDE by C150.1, whereas the peptide APKTYVSVSGMV consistently inhibited the staining of C355.1 on biliary duct epithelium more strongly than the typical mitochondrial staining of hepatocytes. Rabbit sera raised against the peptide WMSYPDRTLRTS stained BDE of livers and isolated bile duct epithelial cells of PBC patients more intensively than controls. The rabbit sera stained all size ducts in normals, but only small/medium-sized ductules in PBC livers. These studies provide evidence that the antigen present in BDE is a molecular mimic of PDC-E2, and not PDC-E2 itself.

Immunogenicity of new heterobifunctional cross-linking reagents used in the conjugation of synthetic peptides to liposomes

We have investigated the immunogenicity of six thiol-reactive heterobifunctional cross-linking reagents that permit the conjugation of cysteine carrying peptides to the surface of liposome containing monophosphoryl lipid A. Such constructs elicit an immune response against short synthetic peptides and our aim was to find the least immunogenic linkers to limit potential carrier-induced epitopic suppression. For that purpose the properties of three new polyoxyethylene linkers of different lengths and thiol-reactive moieties (maleimide, bromoacetyl, dithiopyridine) were compared to known derivatives obtained by reacting the classical reagents SMPB and SPDP or N-succinimidyl bromoacetate with phosphatidylethanolamine. The least immunogenic linkers were the bromoacetate derivatives whereas those containing a maleimide group evoked a significant anti-linker immune response. In addition, using IRGERA as a model peptide, we found that all six liposomal constructs strongly elicited the production of anti-peptide IgG antibodies. This immune response was therefore independent of the length of the linkers (ranging between 0.3 and 1.6 nm) and of the nature of the linkage. between the peptide and the thiol-reactive moieties of the cross-linkers, i.e. stable thioether or bio-reducible disulfide bonds.

Retro-inverso peptidomimetics as new immunological probes. Validation and application to the detection of antibodies in rheumatic diseases

Retro-inverso peptides which contain NH-CO bonds instead of CO-NH peptide bonds are much more resistant to proteolysis than L-peptides. Moreover, they have been shown recently to be able to mimic natural L-peptides with respect to poly- and monoclonal antibodies (Guichard, G., Benkirane, N., Zeder-Lutz, G., Van Regenmortel, M. H. V., Briand, J. P., and Muller, S. (1994b) Proc. Natl. Acad. Sci. U.S.A. 91, 9765-9769). We have further tested the capacity of retro-inverso peptidomimetics to serve as possible targets for antibodies produced by lupus mice and by patients with rheumatic autoimmune diseases. Several retro-inverso peptides corresponding to sequences known to be recognized by autoantibodies were synthesized, namely peptides 28-45 and 130-135 of H3, 277-291 of the Ro/SSA 52-kDa protein, and 304-324 of the Ro/SSA 60-kDa protein, and tested with autoimmune sera by enzyme-linked immunosorbent assay. We have found that retro-inverso peptides are recognized as well as or even better than natural peptides by antibodies from autoimmune patients and lupus mice. This new approach may lead to important progress in the future development of immunodiagnostic assays, particularly in the case of diseases characterized by inflammatory reactions in the course of which the level of degradative enzymes is increased.

Autoepitopes of the 52-kd SS-A/Ro molecule

OBJECTIVE

This study was undertaken to clarify the mechanisms responsible for the generation of anti-52-kd SS-A/Ro autoantibodies and to elucidate why, as has recently been reported, anti-52-kd autoantibodies preferentially recognize the denatured form rather than the native 52-kd molecule.

METHODS

Using a series of truncated 52-kd autoantigens, produced as beta-galactosidase fusion proteins in Escherichia coli, the B cell epitope distribution was probed with 18 anti-Ro-positive sera by immunoblotting and by enzyme-linked immunosorbent assay.

RESULTS

Nearly all the antigenicity of the molecule was found to be linked to its leucine zipper region. In a further study using 9 of the 18 sera, the antigenicity of the molecule was found to be mainly formed by multiple conformational epitopes, and one of these epitopes appeared to be universally recognized by all the sera tested.

CONCLUSION

The recognition of multiple epitopes indicates that the Ro 52-kd antigen itself drives the autoimmunity to this molecule. Further, the concentration of the antigenicity at the leucine zipper region may explain why anti-52-kd antibodies preferentially recognize the denatured protein rather than its native form.

Stress-induced cell surface expression and antigenic alteration of the Ro/SSA autoantigen

Recent studies have shown that Ro/SSA autoantigen is heterogeneous. There are two isoform families; the 60 kD forms (Ro60) and the 52 kD forms (Ro52). Recently we have found that autoantibodies to the Ro/SSA proteins are conformation dependent. Anti-Ro60 antibodies are mainly directed to the native protein and conversely anti-Ro52 antibodies are directed only to the denatured protein. It has been known that UV irradiation to cultured keratinocytes induces cell surface expression of Ro/SSA and this phenomenon has been thought to be related with photosensitivity in patients with anti-Ro/SSA antibodies. We studied the quantitative and qualitative changes of the Ro/SSA protein induced by stress, such as with heat shock and UV irradiation, and found that only Ro52 could be expressed on the cell surface of human peripheral lymphocytes by either heat shock or UV irradiation. Moreover, flow cytometric analysis revealed that HS-treated and UV-treated lymphocytes could be stained with patient sera, and by using a technique which combined immunoprecipitation and Western immunoblotting, it has been confirmed that Ro52 expressed on the cell surface can be recognized by anti-Ro/SSA antibodies in native form while cytoplasmic Ro52 cannot be recognized. These data suggest that Ro52 can be antigenic in vivo when expressed on the cell surface and may explain the mechanism of direct tissue damage by anti-Ro/SSA antibodies.

Effect of histone and histone-RNA complexes on the disease process of murine systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterised by the production of a variety of autoantibodies against cell surface, nuclear and cytoplasmic antigens. The antigen or antigens responsible for the induction of this disease is/are unknown. We have analysed the antigenicity and pathogenicity of free histones and histones complexed with RNA in Balb/c, B10 Br, C57BL/6 and MRL-lpr/lpr mice by giving 1 microgram and 25 micrograms of each antigen intraperitoneally in complete and incomplete Freund's adjuvant. The same number of control animals were injected with either adjuvant or PBS. In the initial experiment we gave three doses of antigen at three weekly intervals. B10 Brown and C57BL/6 mice had no response to the antigens. Balb/c mice developed a mild transient antibody response against H1 histone, branched peptide of ubiquitinated H2A (peptide T4) and also against ssDNA. However in repeated experiments when the histone-RNA complex was injected into young MRL-lpr/lpr animals at two weekly intervals, a significantly increased antibody response was detected against H1, peptide T4 and some histone peptide residues (204-218 of H1, 1-20 and 65-85 of H2A, 1-25 of H2B, 1-21 of H3 and 1-29 of H4) compared to the control groups. Moreover, this group also showed elevated serum anti-DNA antibody levels and early impairment of renal function assessed by the urine protein levels. These experiments have demonstrated that there is a genetic variation in antibody responses against histones and histone-RNA complexes and that histone-RNA complexes exaggerate the disease in young MRL-lpr/lpr mice by inducing antibodies to basic regions of histones and other autoantigens.