The Ro/SSA autoantigen as an immunogen. Some anti-Ro/SSA antibody binds IgG

Anti-TROVE2 Antibody Determined by Immune-Related Array May Serve as a Predictive Marker for Adalimumab Immunogenicity and Effectiveness in RA

Anti-drug antibody (ADAb) development is associated with secondary therapeutic failure in biologic-treated rheumatoid arthritis (RA) patients. With a treat-to-target goal, we aimed to identify biomarkers for predicting ADAb development and therapeutic response in adalimumab-treated patients. Three independent cohorts were enrolled. In Cohort-1, 24 plasma samples (6 ADAb-positive and 6 ADAb-negative patients at baseline and week 24 of adalimumab therapy, respectively) were assayed with immune-related microarray containing 1,636 correctly folded functional proteins. Next, we executed statistically powered autoantibody profiling analysis of 50 samples in Cohort-2 (24 ADAb-positive and 26 ADAb-negative patients). Subsequently, immunofluorescence assay was performed on 48 samples in Cohort-3 to correlate with ADAb titers and drug levels. The biomarkers were identified for predicting ADAb development and therapeutic response using the immune-related microarray and machine learning approach. ADAb-positive patients had lower drug levels at week 24 (median = 0.024 μg/ml) compared with ADAb-negative patients (median = 6.38 μg/ml, p < 0.001). ROC analysis based on the ADAb status revealed the top 20 autoantibodies with AUC ≥ 0.7 in differentiating both groups in Cohort-1. Analysis of Cohort-2 dataset identified a panel of 8 biomarkers (TROVE2, SSB, NDE1, ZHX2, SH3GL1, CARD9, PTPN20, and KLHL12) with 80.6% specificity, 77.4% sensitivity, and 79.0% accuracy in discriminating poor from EULAR responders. Immunofluorescence assay validated that anti-TROVE2 antibody could highly predict ADAb development and poor EULAR response (AUC 0.79 and 0.89, respectively). Multivariate regression analysis proved anti-TROVE2 antibody to be an independent predictor for developing ADAb. Immune-related protein microarray and replication analysis identified anti-TROVE2 antibody as a useful biomarker for predicting ADAb development and therapeutic response in adalimumab-treated patients.

Association of neutropenia in systemic lupus erythematosus (SLE) with anti-Ro and binding of an immunologically cross-reactive neutrophil membrane antigen

SLE is associated with the production of autoantibodies to self-constituents. In particular, certain ribonucleoprotein particles are targeted. Despite the multitude of autoantibodies produced and the remarkable concentrations of these antibodies in the sera of SLE patients, there have been little data that the autoantibodies found in SLE are involved in the pathogenesis of disease or its manifestations. The present work demonstrates that anti-Ro (or SSA) is associated with granulocytopenia, binds the surface of granulocytes and fixes complement to this membrane surface. Binding is a property of anti-Ro Fab fragments and can be inhibited by 60-kD Ro. However, the antigen bound on the surface of granulocytes is a 64 000 mol. wt protein that is a novel autoantigen in SLE. As suggested by inhibition studies, sequence identity between 60-kD Ro and eight tandem repeats in the 64-kD antigen may be responsible for the observed serologic cross-reactivity. These data imply that anti-Ro antibodies that also bind the 64-kD protein mediate neutropenia in patients with SLE.

Lupus erythematosus associated with erythema multiforme: does Rowell's syndrome exist?

We describe a patient with lupus erythematosus who experienced an unusual erythema multiforme-like eruption suggestive of Rowell's syndrome. We compare our case and 9 other reports of lupus erythematosus associated with erythema multiforme to the 4 cases reported by Rowell. Our findings indicate that Rowell's original criteria are not well preserved. The coexistence of lupus erythematosus with erythema multiforme does not impart any unusual characteristics to either disease, and the immunologic disturbances in such patients are probably coincidental.

Electrocardiographic abnormalities in a murine model injected with IgG from mothers of children with congenital heart block

BACKGROUND

It is a widely held view that congenital heart block (CHB) is caused by the transplacental transfer of maternal autoantibodies (anti-SSA/Ro and/or anti-SSB/La) into the fetal circulation. To test this hypothesis and to reproduce human CHB, an experimental mouse model (BALB/c) was developed by passive transfer of human autoantibodies into pregnant mice.

METHODS AND RESULTS

Timed pregnant mice (n=54) were injected with a single intravenous bolus of purified IgG containing human anti-SSA/Ro and anti-SSB/La antibodies from mothers of children with CHB. To parallel the "window period" of susceptibility to CHB in humans, 3 groups of mice were used: 8, 11, and 16 days of gestation. Within each group, we tested 10, 25, 50, and 100 microg of IgG. At delivery, ECGs were recorded and analyzed for conduction abnormalities. Bradycardia and PR interval were significantly increased in 8-, 11-, and 16-day gestational groups when compared with controls (P<0.05). QRS duration was not significantly different between all groups. Antibody levels measured by ELISA in both mothers and their offspring confirmed the transplacental transfer of the human antibodies to the pups.

CONCLUSIONS

The passive transfer model demonstrated bradycardia, first-degree but not complete atrioventricular block in pups. The greater percentage and degree of bradycardia and PR prolongation in the 11-day mouse group correlates with the "window period" of susceptibility observed in humans. The high incidence of bradycardia suggests possible sinoatrial node involvement. All together, these data provide relevant insights into the pathogenesis of CHB.

Induction of Autoimmunity by Multivalent Immunodominant and Subdominant T Cell Determinants of La (SS-B)

We investigated the consequences of altering the form and valence of defined autodeterminants on the initiation and spreading of experimentally induced La/Ro autoimmunity. Anti-La and Ro (SS-A) Ab responses were monitored following immunization of healthy mice with defined immunodominant and subdominant T cell determinants of the La (SS-B) autoantigen synthesized as either monomeric or multiple antigenic (MAP) peptides. Abs to mouse La (mLa) developed faster and were of higher titer in mice immunized with the subdominant mLa25–44 MAP compared with mice immunized with the 25–44 monomer. Rapid intermolecular spreading of the autoimmune response to 60-kDa Ro was observed in AKR/J mice immunized with mLa25–44 MAP, but not in mice immunized repeatedly with monomeric peptide. A/J mice immunized and boosted with the known tolerogenic mLa287–301 determinant delivered as monomeric peptide failed to develop Abs to either intact mLa or mLa287–301 peptide. However, immunization with the multivalent mLa287–301 peptide led to the rapid production of high titer mLa autoantibodies associated with a proliferative T cell response to the mLa287–301 peptide. The data suggested that the enhanced immunogenicity of MAPs was not due to augmented Ag presentation or T cell stimulation. However, MAP-, but not monomer peptide-, containing immune complexes were potent substrates for Ab-dependent fixation of complement. These results demonstrate that the form of Ag responsible for inducing autoimmunity can profoundly influence the nature and magnitude of the immune response. Thus, molecular mimicry of tolerogenic and nontolerogenic self determinants might trigger autoimmunity under conditions of altered valence.

Characterization of human telomerase complex

Telomerase, a ribonucleoprotein complex, adds hexameric repeats called "telomeres" to the growing ends of chromosomal DNA. Characterization of mammalian telomerase has been elusive because of its low level of expression. We describe a bioinformatics approach to enrich and characterize the human telomerase complex. Using local sequence homology search methods, we detected similarity of the Tetrahymena p80 subunit of telomerase with the autoantigen Ro60. Antibodies to Ro60 immunoprecipitated the telomerase activity. Ro60 and p80 proteins were cross-recognizable by antibodies to either protein. Telomerase activity and the RNA component of telomerase complex were localized to a doublet in a native gel from the Ro60 antibody-precipitated material. The enriched material showed specific binding to a TTA GGG probe in vitro in an RNA template-dependent manner. Polyclonal antibodies to the doublet also immunoprecipitated the telomerase activity. These results suggest an evolutionary conservation of the telomerase proteins.

The ultrastructural localization of 60-kDa Ro protein and human cytoplasmic RNAs: association with novel electron-dense bodies

The 60-kDa Ro ribonucleoprotein is an important target of humoral autoimmune responses. However, the ultrastructural locations of the 60-kDa Ro protein and its associated small cytoplasmic RNAs (Y RNAs) have not been previously determined, and the functions of the Ro protein and RNAs are not known. In this study, the cellular locations of the 60-kDa Ro protein and the Ro Y1 and Y4 RNAs are determined by immunoelectron microscopy and in situ hybridization electron microscopy, respectively. Both Ro protein and Y RNAs are concentrated in discrete areas of the nucleoplasm, nucleolus, and cytoplasm of cultured cells and human skin sections. The 60-kDa Ro protein and Y RNAs are also present diffusely in the cytoplasm, where they occur in ribosome-rich regions, and in the nucleus. The presence of Ro ribonucleoprotein components in nucleoli and in ribosome-rich cytoplasmic areas suggests a potential for the involvement of Y RNAs and/or 60-kDa Ro protein in ribosome synthesis, assembly, or transport. Double labeling experiments show that Ro protein and Y RNAs colocalize in the nucleoplasm, nucleolus, and cytoplasm. In addition, aggregates of Y RNA occur unassociated with 60-kDa Ro protein, and aggregates of 60-kDa Ro protein occur unassociated with Y RNA. Aggregates of both Ro protein and Y RNAs label previously unreported nuclear and cytoplasmic electron-dense bodies. We propose that these distinctive Ro-associated electron-dense bodies may represent structure(s) important for cellular transport and/or Ro function.

Significance of the anti-Ro (SS-A) antibody in evaluation of patients with cutaneous manifestations of a connective tissue disease

The anti-Ro(SS-A) antibody is arguably the most important antibody determination except for antinuclear antibodies in evaluation of patients suspected of having lupus erythematosus. During the past 25 years, studies have established the importance of this antibody in the evaluation of patients with atypical lupus erythematosus, who have a photosensitive dermatitis as the presenting sign. The purpose of this review is to demonstrate the utility and the necessity of this antibody determination in the evaluation of all patients with cutaneous manifestations who are suspected of having a connective tissue disease.

One hundred anti-Ro (SS-A) antibody positive patients: a 10-year follow-up

To explore further the varied clinical expression of anti-Ro(SS-A) antibody positive patients and to determine the outcomes of these patients, we followed 100 anti-Ro(SS-A) antibody positive patients, originally seen at the Johns Hopkins Medical Institutions in 1982 and 1983, over a 10-year period. The results of this study indicate that anti-Ro(SS-A) antibody positive patients have a diverse clinical presentation and that the anti-Ro(SS-A) antibody response generally persists for years. Some of these patients appear to have a static disease process for years. However, 65% (51, including 13 deaths, of 78 patients) of the patients for whom we had follow-up data had a chronic (10 years or greater) progressive disease process. Black patients, in general, have an earlier onset of disease and may have a more severe disease than white patients. At least 25% of our anti-Ro(SS-A) antibody positive patients demonstrated a dynamic change in clinical presentation with the development of Sjögren syndrome and/or a progressive "rheumatoid-like" arthritis. Interstitial pulmonary disease, central nervous system disease, and vasculitic insults occur frequently in these patients. Renal disease occurred in 19 anti-Ro(SS-A) positive patients, and in 47% of these renal disease patients, no anti-DNA antibodies (dsDNA or ssDNA) were detected. Cutaneous manifestations are prominent in anti-Ro(SS-A) antibody positive patients with lupus. Photosensitivity and a malar dermatitis were the most common features. Twenty percent of lupus patients had discoid lesions, and 20% had SCLE lesions. Based on this study, we believe that anti-Ro(SS-A) antibody positive patients should be routinely evaluated for the emergence of systemic features. Since these systemic features are at least in part, if not solely, the result of inflammation, early treatment with steroids and/or immunosuppressive agents may minimize the damage and influence in a positive manner the significant morbidity and mortality observed in some anti-Ro(SS-A) antibody positive patients.

Human anti-Ro autoantibodies bind peptides accessible to the surface of the native Ro autoantigen

The relationship between fine specificity of linear epitopes and conformational determinants has been explored in a naturally arising human autoimmune response. In particular, the hypothesis tested is that the linear epitopes of the human Ro autoantigen are components of its conformational epitopes. Twenty groups among the 531 overlapping octapeptides 60 kDa Ro are variably bound by anti-Ro precipitin positive lupus sera whose reactivity was easily distinguished from sera of normal controls and of anti-Ro precipitin negative lupus patients. The specific activities of anti-peptide antibodies and of anti-native Ro autoantibodies are similarly increased after affinity enrichment using native human Ro as ligand. Moreover, affinity-enriched anti-native Ro autoantibodies bind virtually the same 20 groups of epitopes recognized by whole anti-Ro positive sera. Two peptides (residues 274-290 and 480-494) from the defined 60 kDa Ro octapeptide epitopes have been prepared and used as ligands for affinity purification of peptide specific autoantibodies. The binding of both whole IgG and affinity-enriched peptide specific autoantibodies is inhibited by native Ro autoantigen. Thus, none of the available data can be construed to support the existence of cryptic linear epitopes in this system. Indeed, the data are only consistent with the conclusion that all of the anti-Ro octapeptide autoantibodies are part of the population of anti-native Ro autoantibodies in this naturally arising autoimmune response.

Antibodies to EYRKK vesicular stomatitis virus-related peptide account only for a minority of anti-Ro60kD antibodies

Previous studies demonstrated a possible antigenic relation between the carboxyl terminal portion of anti-Ro60kD autoantigen and a nucleocapsid protein (N) of vesicular stomatitis virus (VSV). In order to investigate whether anti-Ro60kD autoantibodies react with the VSV homologous region of the Ro60kD protein we synthesized, according to Merrifield's method, the EYRKKMDI octapeptide (8p) sharing a common sequence with the N protein of VSV. Sera from 61 patients with autoimmune rheumatic diseases (34 systemic lupus erythematosus (SLE), 21 Sjörgren's syndrome (SS) and six rheumatoid arthritis (RA)) as well as 59 from normal blood donors were tested for the presence of anti-Ro60kD autoantibodies by ELISA and immunoblot (IB) and anti-8p antibodies by ELISA. Antibodies to 8p were found in 9/31 of anti-Ro60kD IB-positive sera, 5/30 of anti-Ro60kD-negative sera and 2/59 of normal control sera. The concordance between the anti-8p ELISA and the anti-Ro60kD IB was very poor (chi 2 = 0.71, P = 0.4) in contrast to the anti-Ro60kD ELISA and the anti-Ro IB (chi 2 = 27.6, P = 10(-7)). Subsequent affinity purification of the anti-8p antibodies from a strong positive anti-8p and anti-Ro60kD SLE serum yielded 95% depletion of the anti-8p activity and 37% reduction of the anti-Ro60kD activity. Inhibition assays with the affinity-purified anti-8p antibodies demonstrated that the octapeptide gave 94.5% inhibition of the anti-Ro60kD activity, while Ro60kD protein led to 42.3% inhibition of the anti-8p. Preincubation of the serum with the octapeptide produced 4% inhibition of anti-Ro60kD ELISA. These results indicate that the anti-8p antibodies account only for a minority of the anti-Ro60kD autoantibodies.

Characterization of human-human hybridoma monoclonal anti-Ro(SS-A) autoantibodies derived from normal tonsil lymphoid cells

Human-human hybridomas obtained from the separate fusion of tonsillar lymphoid cells from three different normal individuals to the lymphoblastoid cell line GM 4672 were screened by ELISA for the presence of autoantibody to Ro(SS-A). Those anti-Ro(SS-A) reactive hybridomas were then cloned by limiting dilution. Nineteen monoclonal IgM anti-Ro(SS-A) antibodies were obtained, which showed specificity to Ro(SS-A) by ELISA and Western blotting (60 kDa). Some of these monoclonal anti-Ro(SS-A) antibodies showed reactivity to DNA (2/19), cardiolipin (9/19), Sm/RNP (15/19) by ELISA, and to IgG (12/19) and La(SS-B) (19/19) by ELISA and Western blotting. None showed reactivity to the unrelated proteins casein and BSA, nor to RNA. Inhibition studies revealed that the binding to Ro(SS-A) of both IgM hybridoma monoclonal and SLE serum polyclonal IgM anti-Ro(SS-A) antibodies was inhibited with Ro(SS-A), La(SS-B) and Sm/RNP but not with IgG, DNA, RNA and BSA. These data indicate that (1) normal humans have the genetic potential to express antibodies to Ro(SS-A) and (2) the normally derived monoclonal and SLE serum IgM anti-Ro(SS-A) antibodies share similar antigen binding properties and therefore may possibly originate from a common pool of precursor B cells.

Anti-Ro autoantibody with cross-reactive binding to the heavy chain of immunoglobulin G

Autoantibodies directed at the intracellular Ro ribonucleoprotein complex are found in the serum of patients with systemic lupus erythematosus (SLE) and related autoimmune diseases. The antigenic stimulus for the induction of these autoantibodies is unknown, although we have previously demonstrated that the Ro protein and immunoglobulin G (IgG) share immunologic determinants bound by anti-Ro antibodies. The present study further defines the fine specificity of this cross-reactive binding. Using both patient autoanti-Ro antibodies and antigen-induced rabbit anti-Ro serum, the binding specificity for IgG was located to the heavy chains of IgG outside the Fc domain. F(ab')2 fragments of IgG were observed to inhibit specific Ro binding by either human or antigen-induced rabbit sera, while Fc fragments of IgG failed to inhibit Ro binding. Anti-Ro sera were found to bind the heavy chains of IgG in immunoblots, and the antibodies eluted from these heavy chains were capable of immunoprecipitating the Ro particle from human cell extracts. Not all patient sera with anti-Ro antibodies possessed IgG binding antibodies. Studies of cyanogen bromide digestion fragments of IgG implicate the hinge region of IgG as the region cross-reactive with the Ro protein. The nature of this cross-reactivity may be important in understanding the induction and/or perpetuation of the anti-Ro response in patients with autoimmune disease.

A common autoepitope near the carboxyl terminus of the 60-kD Ro ribonucleoprotein: sequence similarity with a viral protein

The Ro ribonucleoprotein is composed of hY RNA and a 60.7-kD peptide that is antigenic for autoantibodies produced by many patients with systemic lupus erythematosus or Sjögren's syndrome and mothers of newborns with complete congenital heart block. A major immunoreactive fragment (13 kD) of the 60-kD Ro is bound by 28 of 45 (62%) of the anti-Ro sera tested. Amino acid sequence analysis localizes this fragment to the carboxyl end of the 60-kD Ro peptide. All possible overlapping octapeptides of this 13-kD peptide of 60-kD Ro have been assessed for antigenicity. Sera that bind the 13-kD peptide fragment in immunoblot generally also bind the octapeptides of Ro spanning the sequence AIALREYRKKMDIPA (P less than 0.01). Inhibition studies with synthetic peptides and purified Ro have established specificity for reference serum antibody binding to an antigenic octapeptide, EYRKKMDI, from this region. The closely related sequence EYRKKLMD is found in the nucleocapsid protein of vesicular stomatitis virus and may portend an immunologic link to this or a related viral antigen. These results also demonstrate that despite fine specificity variation between human sera, there are recurring patterns of anti-Ro binding shared by some patients who have precipitating anti-Ro autoantibodies.

Polyclonal human rheumatoid factors cross-reacting with histone H3: characterization of an idiotope on the H3 binding site

The development of highly sensitive immunoassays has made the detection of the multireactivity of antibodies a relatively common phenomenon. Polyreactivity is frequent in human auto antibodies, especially in rheumatoid factors (RFs), but the structural basis and the significance of this phenomenon remain substantially unknown. Recently, we showed that the double reactivity of a human monoclonal RF with histones was probably due to two distinct binding sites. However, cross-reactivity seems more frequent among polyclonal RFs occurring during autoimmune diseases than with monoclonal RFs. We studied double-reactive (IgG and histone H3) polyclonal RFs in a patient suffering from primary Sjögren's syndrome. We showed by means of affinity chromatographies that H3 cross-reactive RFs were only a small subset of the total patient's RFs and that this subset was enriched in IgA class. Competitive inhibition experiments suggested the existence of two distinct binding sites for IgG and H3. These results were confirmed by showing the selective sensitivity to acid treatment of the histone binding site and by producing a murine antiidiotope monoclonal antibody BII 2.1 defining an idiotope on bireactive RF apparently linked to the H3 binding site. This idiotope was absent in a panel of monoclonal RF, one of them cross-reacting with histone H3. This report extends previous results concerning a monoclonal RF to the polyclonal RFs which occur during autoimmune diseases.

Antigenicity of a recombinant Ro (SS-A) fusion protein

The antigenicity of the 60-kd human Ro (SS-A) synthesized in vitro from its complementary DNA as a beta-galactosidase fusion protein (beta-gal-Ro) was evaluated by Western blotting. In this analysis, almost all the anti-Ro (SS-A)-positive sera that bound beta-gal-Ro also bound affinity-purified 60-kd human Ro (SS-A) (P less than 0.005). Three of the 27 anti-Ro (SS-A) precipitin-positive sera, however, did not show reactivity on Western blot analysis, which suggests that in some sera, antigenicity to Ro (SS-A) is destroyed by denaturation. Of the 22 sera that were reactive with beta-gal-Ro, 2 were not reactive with affinity-purified human Ro (SS-A). Two serum samples that did not react with beta-gal-Ro were also reactive with affinity-purified human Ro (SS-A). Nevertheless, except for a small percentage of Ro (SS-A) precipitin-positive sera, the frequency of antibody binding to the fusion protein was similar to the frequency of binding to the purified antigen in Western blots. Recombinant Ro (SS-A) antigen may therefore be valuable in the serologic evaluation of anti-Ro (SS-A) autoantibodies.

Differences in the fine specificity of anti-Ro (SS-A) in relation to the presence of other precipitating autoantibodies

In sera from patients with systemic lupus erythematosus or Sjögren's syndrome, we determined the fraction of antibody that remained reactive with human Ro (SS-A) after absorption with bovine spleen extract, and the reactivity with the 60-kd and 54-kd red blood cell Ro (SS-A) bands by Western blot. Of the 3 groups of sera studied, those containing anti-Ro (SS-A) alone had the highest degree of reactivity with human Ro (SS-A) after absorption with bovine spleen extract, followed, in descending order, by sera containing anti-Ro (SS-A) and anti-La (SS-B), and sera containing anti-Ro (SS-A) and anti-nuclear RNP. The groups of sera could be distinguished on this basis. Sera with anti-Ro (SS-A) and anti-nuclear RNP could also be distinguished from the other 2 types of sera by their uniform and preferential reactivity with the 60-kd red blood cell Ro (SS-A), by Western blot analysis. These findings indicate that there are both qualitative and quantitative differences, associated with the presence of other autoantibodies, in the fine specificity of anti-Ro (SS-A) sera.

The Ro ribonucleoprotein particle: induction of autoantibodies and the detection of Ro RNAs among species

High titers of autoantibody specific for the Ro(SSA) ribonucleoprotein are frequently found in patients with systemic lupus erythematosus and Sjogren's syndrome. In this study we have analyzed the immune responses to the Ro particle when utilized as an immunogen in animal hosts. Anti-Ro autoantibodies which bound autologous Ro ribonucleoprotein particles were induced in rabbits. In immunodiffusion studies using crude rabbit tissue extracts, the rabbit antibody made a precipitin line of identity with a prototype human anti-Ro serum. In solid-phase assays, the human autoimmune serum and the antigen-induced rabbit serum competed for similar or overlapping epitopes on the Ro particle. The rabbit and human sera precipitated the four Ro RNAs from human cells as well as four previously uncharacterized Ro RNAs from a bovine cell line, three Ro RNAs from a rabbit cell line, and two Ro RNAs from duck cells. While total numbers of cellular Ro RNAs differ among species, all possess an RNA of common size which comigrated with the hY1 of human cells.

Pattern of cutaneous immunoglobulin G deposition in subacute cutaneous lupus erythematosus is reproduced by infusing purified anti-Ro (SSA) autoantibodies into human skin-grafted mice

Subacute cutaneous lupus and neonatal lupus are closely associated with the presence of anti-Ro (SSA) autoantibodies, but there is no direct evidence establishing a role for anti-Ro (SSA) in these diseases. After parental injection into mice, IgG from sera containing anti-Ro (SSA) will bind human skin grafted onto the mice. To determine whether the antibody binding is due to anti-Ro (SSA), affinity-purified anti-Ro (SSA) and serum depleted of anti-Ro (SSA) were prepared. After injection into human skin-grafted mice, purified anti-Ro (SSA) antibodies bound an antigen in the human skin graft, while preabsorbing anti-Ro (SSA) serum with Ro (SSA) virtually abolished binding to the human skin graft. Moreover, the pattern of IgG deposition was primarily epidermal and was identical in the human skin-grafted mice injected with purified anti-Ro (SSA) when compared with that found in five patients with subacute lupus (four adults, one neonate). These data directly show that anti-Ro (SSA) antibodies bind to the skin, and support the hypothesis that anti-Ro (SSA) autoantibodies are involved in the disease process that produces subacute cutaneous lupus and neonatal lupus.

Heterogeneity of the Ro/SSA antigen. Different molecular forms in lymphocytes and red blood cells

Ro(SSA) is an intracellular ribonucleoprotein against which autoantibodies are found in a portion of patients with Sjögren's syndrome and systemic lupus erythematosus. A form of Ro(SSA) is described in red blood cells that shares a line of identity with purified Ro(SSA) from bovine spleen and human lymphocytes in counterimmunoelectrophoresis, but has different molecular properties. Ro(SSA) from red blood cells exists in association with only two small RNAs as opposed to four in other cell types, as determined by RNA extraction of protein A-assisted immunoprecipitates. In addition to the common 60-kD Ro(SSA) protein, Western blot analysis revealed an additional 52-kD protein in lymphocytes and a 54-kD protein in red blood cells. The 60-kD form of Ro(SSA) in red cells was found to be antigenically distinct from that in the lymphocyte, because sera were identified that bound each exclusively. Finally, a rabbit antibovine Ro(SSA) serum distinguished red cell from lymphocyte Ro(SSA). These results suggest two distinctive populations of Ro(SSA) proteins and distributions of Ro(SSA) RNAs in the lymphocyte and red blood cell.

Immunochemical studies of polyspecific natural autoantibodies: charge, lipid reactivity, Fab'2 fragments activity and complement fixation

Polyspecific natural autoantibodies (NAAb) are antibodies present in normal unimmunized animals and are able to react with very dissimilar antigens (Ag). To better delineate the characteristics of polyspecificity, we subjected monoclonal NAAb to four different immunochemical studies: (1) Two-dimensional gel electrophoresis performed on eight NAAb did not reveal any obvious relationship between charge and antigen specificities; (2) NAAb widely polyspecific on proteins and nucleic acid were reactive with lipids bearing either phosphate, sulfate or carboxyl polar groups; (3) pepsin digestion of polyspecific IgM NAAb yielded Fab'2 fragments which maintained their multireactivities, but exhibited a decrease in reactivity as compared to that seen with monospecific mAb (induced); (4) two different assays were used to analyse the complement fixation ability of IgM NAAb. While very weak or no complement fixation was observed with a classical complement fixation test (fluid phase), when a complement enzyme immunoassay was used where Ag is immobilized on a solid phase, polyspecific NAAb fixed reproducible and easily detectable amounts of complement.

The double reactivity of a human monoclonal rheumatoid factor to IgG and histones is related to distinct binding sites

The structural basis of the double reactivity of a human monoclonal rheumatoid factor (RF) with both human IgG and histones H1 and H3 was investigated by means of competitive inhibition experiments. The monoclonal RF binding to solid-phase histones was inhibited by increasing concentrations of heat-aggregated IgG. However, increasing concentrations of purified histones were almost unable to reduce the RF binding to solid-phase IgG. Inhibition of antigen binding with two murine monoclonal anti-idiotopes reacting with distinct idiotopes on the monoclonal RF indicated that the fixation to the different antigens was mediated by distinct binding sites. This result was confirmed by showing the selective sensitivity to mild acid treatment of the histone binding site but not of the IgG binding site. This report provides a structural basis for the existence of polyfunctional combining regions on a human autoantibody.

Molecular characterization of human Ro/SS-A antigen. Amino terminal sequence of the protein moiety of human Ro/SS-A antigen and immunological activity of a corresponding synthetic peptide

The Ro/SS-A antigen was purified from an Epstein-Barr virus-transformed human B lymphoblastoid cell line. The amino terminal amino acid sequence of the 60-kD polypeptide bearing this antigenic epitope was determined to be: (formula; see text) A peptide composed of residue 6-19 was synthesized by the solid-phase method. Immunodiffusion-defined monospecific autoimmune sera to Ro/SS-A reacted with this synthetic peptide in ELISA, whereas autoantibodies with other specificities such as anti-La/SS-B and anti-Sm, as well as normal human sera, were not reactive. In addition, rabbit anti-peptide 6-19 antisera reacted specifically with native human Ro/SS-A antigen in ELISA. Furthermore, this synthetic peptide inhibited the binding of rabbit anti-peptide antiserum to native human Ro/SS-A. An additional synthetic peptide corresponding to residues 7-24 partially inhibited the binding of a patient anti-Ro/SS-A serum to native Ro/SS-A. These results suggest that the amino terminal portion of the molecule represents a major epitope of Ro/SS-A. The determination of the amino acid sequence of Ro/SS-A and the availability of synthetic peptide(s) bearing this antigen should provide additional approaches to further characterize the autoimmune response to this antigen.

Anti-Ro (SS-A) antibodies from Ro (SS-A)-immunized mice

Immunization of BALB/c mice with immunoaffinity-purified bovine or human Ro (SS-A) induces the production of antibodies reactive with Ro (SS-A). Fusion of spleen cells from the hyperimmunized mice to SP2/0 cells resulted in hybridoma cell lines that produced anti-Ro (SS-A) antibodies. Anti-Ro (SS-A) binding was established by solid-phase immunosorbent assay, immunoblotting, or RNA immunoprecipitation. Most of the anti-Ro (SS-A) antibodies bound to both human and bovine Ro (SS-A) in the solid phase, but only one of the monoclonal antibodies selectively bound to human Ro (SS-A); this suggests that there are species differences between the bovine and human Ro (SS-A) antigens. Indirect immunofluorescence studies demonstrated that most anti-Ro (SS-A) antibodies bound to cytoplasmic or nuclear HEp-2 cellular antigens, whereas others did not bind to fixed HEp-2 tissue culture cells. Nuclear staining of mouse substrates by one of the sera containing anti-Ro (SS-A) demonstrated that autoantibodies were induced by immunization with human Ro (SS-A).

Human autoantibody-reactive epitopes of SS-B/La are highly conserved in comparison with epitopes recognized by murine monoclonal antibodies

SS-B/La, an ubiquitous nuclear protein of 46-48 kD, is a target antigen of autoantibodies in SLE and Sjogren's syndrome and is involved in the maturation of RNA polymerase III transcripts such as 5S RNA and tRNAs. We have previously shown (14, 15) that SS-B consists of two protease-resistant domains of 23 and 28 kD, with the latter containing the RNA binding site. The epitopes of SS-B/La reactive with human autoantibodies are conserved among several mammalian species examined. BALB/c mice immunized with affinity-purified calf thymus SS-B produce IgG anti-SS-B/La antibodies, which reacted with bovine, human, and rabbit SS-B but not with mouse SS-B/La. The spleen of a mouse with the highest antibody titer was selected for fusion with P3 myeloma. Five IgG1k mAbs (A1-5) were selected by ELISA and immunoblotting. All except A3 reacted with the 28-kD domain. A1, A2, and A3 were capable of immuno-precipitating the 48-kD SS-B protein and its associated RNAs. A1, A2, and A3 also gave fine nuclear speckled staining on human, monkey, bovine, and rabbit cells that was similar in appearance to that with human autoantibodies, but in contrast to staining with human autoantibodies, they did not stain cells from rat, mouse, or rat kangaroo. It appears that human autoantibodies target highly conserved epitopes that can be distinguished from epitopes recognized by immunization-induced murine mAbs. Taken together with other data, it appears that human autoantibodies may be recognizing epitopes that are active or catalytic sites of molecules subserving important cellular functions.