At least three distinct B cell epitopes reside in the C-terminal half of La protein, as determined by a recombinant DNA approach

Granzyme B cleavage of autoantigens in autoimmunity

The systemic autoimmune diseases are a complex group of disorders characterized by elaboration of high titer autoantibodies and immune-mediated damage of tissues. Two striking features of autoimmune rheumatic diseases are their self-sustaining nature and capacity for autoamplification, exemplified by disease flares. These features suggest the presence of a feed-forward cycle in disease propagation, in which immune effector pathways drive the generation/release of autoantigens, which in turn fuel the immune response. There is a growing awareness that structural modification during cytotoxic granule-induced cell death is a frequent and striking feature of autoantigens, and may be an important principle driving disease. This review focuses on granzyme B (GrB)-mediated cleavage of autoantigens including (i) features of GrB cleavage sites within autoantigens, (ii) co-location of cleavage sites with autoimmune epitopes, and (iii) GrB sensitivity of autoantigens in disease-relevant target tissue. The mechanisms whereby GrB-induced changes in autoantigen structure may contribute to the initiation and propagation of autoimmunity are reviewed and reveal that GrB has the potential to create or destroy autoimmune epitopes. As there remains no direct evidence showing a causal function for GrB cleavage of antigens in the generation of autoimmunity, this review highlights important outstanding questions about the function of GrB in autoantigen selection.

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-

Analysis of expression of the gene encoding for the nuclear autoantigen La/SS-B using reporter gene constructs

In earlier studies mRNA isoforms encoding for the nuclear autoantigen La were identified. In an alternative La mRNA form the exon 1 was replaced with the exon 1'. Moreover, exon 1' La mRNAs were found to start at different 5'-regions. In dependence on the 5'-start the exon 1' La mRNAs encoded for up to three open reading frames upstream of the La frame, which starts in the exon 2. The exon 1' was located in the intron about 70 nts downstream of the exon 1. The exon 1' La mRNA was proposed to be the result of a promoter switch in combination with an alternative splicing mechanism. The commonly used technique to study the expression of a eucaryotic gene is to fuse a reportergene immediately downstream of the proposed regulatory elements. Due to (i) the short distance between exon 1 and exon 1', (ii) the varying 5'-starts of the exon 1' La mRNAs, and (iii) the upstream open reading frames in the exon 1' La mRNAs this technique appeared to be difficult to apply to the La gene. In order to overcome these problems a luciferase reportergene construct was cloned which started about 2500 nts upstream of the exon 1 and contained the exon 1, the intron including the exon 1', and a portion of the exon 2. Luciferase was fused into the exon 2. This construct was used to prepare 5'-deletion mutants. The constructs were transiently transfected into HeLa cells. RNAs were isolated from the transiently transfected cells and analyzed using the 5'-Rapid Amplification of cDNA End technique. The PCR products were subcloned and sequenced. This analysis showed that exon 1 and exon 1' transcripts were correctly transcribed and spliced from the La luciferase fusion construct. Moreover, the 5'-start of the respective transcript allowed to identify those genomic regions in the La gene that were most likely being involved in determining the respective transcription initiation site. In parallel to the estimation of the 5'-start of the transcripts, the luciferase activity was measured. Thereby we detected a cryptic promoter element in the intron between the exon 1 and exon 2.

The nuclear autoantigen La/SS-B: mapping and sequencing of the gene and the three retropseudogenes

One target of autoantibodies in sera of patients with systemic lupus erythematosus or primary Sjögren's syndrome is the nuclear autoantigen La/SS-B. Lambda clones and cosmids were isolated, which contained the sequences of the La gene and the three La pseudogenes. They were used for preparation of a physical map. Finally, the La gene and pseudogenes were sequenced. The pseudogenes were characterized as retropseudogenes. Their evolutionary ages were estimated to be approx. 4, 4.5 and 5 million years. Inserts of 4, 16 and 24 nucleotides, which were mostly A-residues, were found in exon 7 of the respective pseudogene. The oldest pseudogene contained the longest insert, the youngest pseudogene contained the smallest insert. The oligonucleotides seem to be the result of repeated inserts of A-residues in a hot spot region of the La genes. Two La cDNAs were isolated which contained either a deletion or an insert of an A-residue at the same position.

The nuclear autoantigen La/SS-associated antigen B: one gene, three functional mRNAs

Transcription of the gene encoding for the nuclear autoantigen La resulted in three mRNA forms. A promoter switching combined with an alternative splicing pathway replaced exon 1 with either exon 1' or exon 1'. The exon 1' donor splice site was located 4 nts downstream of the exon 1' donor splice site. All three La mRNA forms were expressed in all the tissues analysed including peripheral blood lymphocytes, liver, fetal spleen, cultured primary endothelial cells, and mouse LTA cell lines permanently transfected with the human La gene. Both the exons 1' and 1' had unusual structures. They contained GC-rich regions and an oligo(U)-tail of 23 uridine residues. Moreover, they encoded for three open reading frames upstream of the La protein reading frame. In spite of this unusual structure, when exon 1' or exon 1' La mRNAs were expressed in transfected mouse LTA cells, both La mRNAs were translated to nuclear La protein, indicating that all La mRNA forms are functional mRNAs.

Analysis of expression of an alternative La (SS-B) cDNA and localization of the encoded N- and C-terminal peptides

A deletion of an (A)-residue was detected in a cDNA encoding for the nuclear autoantigen La/SS-B. The cDNA was recently isolated from a cDNA library made from peripheral blood lymphocytes of a patient with primary Sjögren's Syndrome. The region, where the deletion occurred, represents a hot spot region in the La gene(s). It leads to a frame shift mutation and a premature stop codon eleven amino acids downstream of the deletion site within one of the protease sensitive regions of the La protein. In spite of the frame shift mutation expression of full length La protein occurred efficiently in E. coli. Full length La protein was also made in SF9 cells infected with recombinant baculoviruses, although the efficiency of full length protein production was less. Two major peptides with molecular weights of 29 kDa and 25 kDa were made. The size of these peptides was similar to the known proteolytic degradation products of La protein. The N-terminal 29 kDa fragment containing the RNP consensus sequence located in the cytoplasm. The 25 kDa C-terminal fragment containing the nuclear location signal entered in the nucleus and associated with nuclear speckles. In conclusion, the ability to (i) enter, (ii) remain in the nucleus and (iii) assemble with nuclear speckles resides in the C-terminal domain of La protein and does not depend on the N-terminal RNP-consensus motif.

Molecular biology of autoantigens in rheumatic diseases

The advent of molecular biologic techniques has provided new approaches that are of great utility to the study of autoimmune-mediated responses. In the past few years, there has been a remarkable accumulation of knowledge concerning the molecular identity and function of autoantigens, and further consolidation for the use of autoantibodies as diagnostic markers in clinical rheumatology. The understanding of basis methodologies in molecular biology applied to the study of autoantigens, in particular, techniques for cloning and analyzing genes that are important in rheumatic diseases, is valuable for both basic scientists and clinicians interested in diagnostic and prognostic markers of various connective tissue diseases.

One gene, two transcripts: isolation of an alternative transcript encoding for the autoantigen La/SS-B from a cDNA library of a patient with primary Sjögrens' syndrome

A cDNA library was prepared from peripheral blood lymphocytes of an autoimmune patient with primary Sjögrens' syndrome. The cDNA library was screened with the patients own autoimmune serum being monospecific for the nuclear autoantigen La/SS-B. Thereby an alternative type of La mRNA was identified that differed from the known La mRNA due to an exchange of the exon 1. Sequencing of the genomic region between the exons 1 and 2 showed that the alternative 5'-end is a part of the intron. In addition, the presence of an alternative promoter site, which exists within the intron downstream of the exon 1, became evident. In consequence, the alternative La mRNA is the result of a promoter switching combined with an alternative splicing mechanism. In the intron, further transcription factor binding sites, including a NF-kappa B element, were identified leading to the suggestion that the expression of the gene encoding for the nuclear autoantigen La/SS-B alters in dependence on disease conditions.

Identification of a human-specific epitope in a conserved region of the La/SS-B autoantigen

Human anti-La/SS-B autoantibodies are known to react with highly conserved epitopes suggested to be functional or active sites on the La/SS-B polypeptide. This study was designed to determine whether the autoantibodies also react with poorly conserved regions of La/SS-B as predicted by an antigen-driven autoimmune response. Binding of human autoantibodies to purified human, mouse, and bovine recombinant fragments representing immunodominant regions of the La/SS-B polypeptide was compared using Western blotting and ELISA. A cross-reactive epitope was located in the highly conserved NH2-terminal region of La/SS-B. Significantly, human-specific epitopes were identified in both the conserved RNA-recognition motif and a poorly conserved COOH-terminal fragment, providing direct evidence for an autoantigen-driven response. The lack of autoantibody cross-reactivity with a conserved domain of mouse and bovine La/SS-B implies that a small number of residues in human autoepitopes may be critical for autoimmunogenicity.

Isolation of rat cDNA clones coding for the autoantigen SS-B/La: detection of species-specific variations

Clones of cDNA coding for the autoantigen La (or SS-B) were isolated from a library made from rat liver. A comparison of the rat La cDNA (encoding from nt 38 to 1281 for rat La protein) with the sequences known for human and bovine La protein resulted in the identification of species-specific inserts. The inserts seem to be the result of multiplication of flanking sequences during evolution. In addition to these variations, we observed that rat La cDNAs exhibit non-canonical polyadenylation sites. Finally, a databank search resulted in the identification of a DNA sequence originally termed as TAG or TSG20X (GenBank accession No. X61893) which represents the C terminus of mouse La/SS-B protein.

Recurrent annular erythema associated with anti-SS-B/La antibodies: analysis of the disease-specific epitope

We have found that anti-SS-B/La antibodies are present in a group of patients with a characteristic recurrent annular erythema, and immunological abnormalities. The presence of a disease-specific epitope for this entity has been examined by comparing immunological reactivity of anti-SS-B/La antibodies between these patients and patients with classical Sjögren's syndrome who have anti-SS-B/La antibodies but do not have this characteristic erythema. In immunoblotting using three different antigen sources, all the sera from both groups exhibited the same reaction patterns. V8 protease mapping showed that the sera could be divided into three groups by the difference in reactivity to two main digested peptide fragments. However, no significant relationship of the reactivity in peptide mapping between the two patient groups was observed. Further studies are necessary to elucidate the role of anti-SS-B/La antibodies in disease pathogenesis in these patients, in particular by more precise peptide mapping using recombinant SS-B/La antigen proteins.

The autoantigen La/SSB: detection on and uptake by mitotic cells

The nuclear autoantigen La, a transcription/termination factor of RNA polymerase III, was recently shown to translocalize to the cell surface of growth-stimulated cells during transition from G0- to G1-phase. Here we describe the staining of living mitotic cells with the anti-La mab La11G7. Moreover, La protein added to cell culture medium was able to enter into synchronized mitotic cells. Uptake was inhibited by the anti-La mab. La protein taken up into prophase cells assembled into a fibrillar network. Taken up byu ana/telophase cells, La protein was preferentially transported into the newly forming or formed nuclei. This import allowed us to study directly the intranuclear localization of La protein in living cells by the use confocal laser scanning microscopy (cLSM). Adsorbed La protein was found in the nucleoplasm but also assembled into nuclear speckles. Some of these speckles surrounded the nucleolus like a ring.

Use of a molecularly cloned human SS.B antigen to detect anti-SS.B antibodies

The aim of this study was to examine the utility of diagnostic assays based on recombinant SS.B/La (rSS.B). Using this antigen, we have developed an ELISA and an immunoblot and compared these recombinant antigen-based assays with traditional thymus extract-based counterimmunoelectrophoresis (CIEP). Using the recombinant ELISA, the incidence of anti-SS.B in 184 normal blood donors was 2.2% (four sera). These four sera were all low titre, i.e., 3-5 SD above the mean. Of 38 sera positive for anti-SS.B by CIEP, 37 were positive in both recombinant assays (97.4% concordance). Anti-SS.B titre in CIEP correlated strongly with results of the rSS.B-based ELISA, but the ELISA was 3,000-fold more sensitive. In an analysis of 152 autoimmune sera containing anti-DNA, anti-RNP, anti-centromere, anti-SS.A/Ro or anti-cardiolipin, all of which were negative for anti-SS.B/La by CIEP, the recombinant assays detected 17 new anti-SS.B positives. These positive results were found only in sera which had previously been characterised by CIEP as anti-SS.A/Ro positive. Anti-SS.B/La antibodies detected by recombinant SS.B assays were found to be highly predictive of primary Sjögren's syndrome. Our results show that rSS.B can have an important role in the design of sensitive and specific assays for anti-SS.B. The diagnostic significance of anti-SS.B/La as a guide to primary Sjögren's syndrome is not diminished by the increased sensitivity of recombinant SS.B assays.

Recently characterised autoantibodies and their clinical significance

Multisystem autoimmune diseases such as systemic lupus erythematosus (SLE), primary Sjögren's syndrome (SS), scleroderma and polymyositis are characterised by the presence of antinuclear antibodies (ANAs). Immunoblotting and cDNA cloning studies reveal that the autoantigens of the multisystem autoimmune diseases are important proteins involved in nucleic acid metabolism, including tRNA charging, intron splicing, DNA uncoiling, and RNA polymerase co-factors. Each specific syndrome associates with a restricted variety of ANAs, e.g. anti-La with primary SS, anti-Sm with SLE, anti-synthetase enzymes with myositis, anti-topoisomerase 1 (Scl 70) with scleroderma, and anti-centromere with CREST. Precise characterisation of an ANA provides valuable diagnostic and prognostic information, and should be performed when an ANA is detected.

Restricted epitope recognition by precipitin-negative anti-La/SS-B-positive sera

OBJECTIVE

To determine whether anti-La/SS-B-positive sera that are precipitin negative show a distinct B cell epitope pattern.

METHODS

Serum reactivity was tested with recombinant La/SS-B fusion proteins.

RESULTS

Among the 18 precipitin-negative anti-La/SS-B-positive sera, reactivity was confined to the full-length recombinant protein (La33.3) in 8 (44%); 5 of 18 (28%) reacted only with La33.3 and with the first 107 N-terminal amino acids (LaA), and 4 (22%) reacted with La33.3, LaA, and the middle region of the La molecule (LaC; amino acids 111-242). One serum reacted with La33.3 and LaC. None of the 18 precipitin-negative sera was positive on a carboxy-terminal fragment (LaL2/3; amino acids 346-408). In contrast, all 26 precipitin-positive anti-La/SS-B-positive sera reacted with La33.3, LaA, and LaC, and 92% reacted with LaL2/3. Rheumatoid factor and serum IgG levels were significantly lower in the precipitin-negative group, providing further evidence of a distinct serologic subset.

CONCLUSION

The restricted epitope recognition by these sera may explain the lack of precipitin formation and may represent an early autoantibody response to La/SS-B.

Definition of a discontinuous immunodominant epitope at the NH2 terminus of the La/SS-B ribonucleoprotein autoantigen

High-titer IgG autoantibodies to the La/SS-B ribonucleoprotein (RNP) are a hallmark of patients with primary Sjogren's syndrome. Anti-La/SS-B-positive human sera bind to multiple epitopes on recombinant La/SS-B, although the initial response is against an immunodominant epitope within the first 107 NH2-terminal amino acids (aa). Sequence analysis has identified a striking homology between aa 88-101 in this NH2-terminal region of La/SS-B and a feline retroviral gag polypeptide suggesting the anti-La/SS-B response may be initiated by cross-reactivity with an exogenous agent. In the present study, detailed mapping of this NH2-terminal epitope, using recombinant La/SS-B purified from the expression of overlapping DNA fragments spanning aa 1-107, has shown that this immunodominant epitope is a complex conformational or discontinuous epitope dependent upon both aa 12-28 and 82-99 for expression, even though these regions share no homology with each other. This requirement questions the significance of the homology between La/SS-B and a retroviral gag polypeptide in the generation of the B cell response to La/SS-B and is in accord with the general concept that B cells recognize conformational epitopes on antigens rather than small linear peptide sequences. The finding also reinforces the notion that native autoantigen could be the initiator of the autoimmune response.

Estimation of amounts of anti-La(SS-B) antibody directed against immunodominant epitopes of the La(SS-B) autoantigen

The contribution of circulating anti-La(SS-B) antibody to the hypergammaglobulinaemia seen in primary Sjögren's syndrome is unknown. In this study levels of anti-La(SS-B) antibody directed against three immunodominant epitopes of the anti-La(SS-B) autoantigen were measured by ELISA in 84 anti-La(SS-B)+ sera using purified recombinant protein and antibody affinity-purified against the three anti-La(SS-B) fusion proteins. There was marked variation in the amounts of IgG anti-La(SS-B) antibody detected, with levels ranging from 0.02 mg/ml to 11 mg/ml. The anti-La(SS-B) levels were greater than 1 mg/ml in 61% of patients; in 18% of sera the anti-La(SS-B) level constituted 10% or more of the total serum IgG. However, other patients were seen with marked hypergammaglobulinaemia and low anti-La(SS-B) concentrations. These results support an antigen-driven mechanism for the anti-La(SS-B) response and suggest that anti-La(SS-B) antibody production is regulated independently of other immunoglobulins.

Immunoaffinity purification of a [U4/U6.U5] tri-snRNP from human cells

We describe the isolation and biochemical characterization of [U4/U6.U5] tri-snRNP complexes from HeLa cells under nondenaturing conditions using a monoclonal antibody reacting with the U5-specific 100-kD protein. We show that the [U4/U6.U5] complex contains five previously unobserved proteins with molecular masses of 90, 60, 27, 20, and 15.5 kD, in addition to the core proteins, common to the U4/U6, U5, U1, and U2 snRNPs, and the U5-specific proteins, as found in 20S U5 snRNPs. With approximately 20 distinct snRNP proteins the complexity of the [U4/U6.U5] tri-snRNP is surprising. One or more of the five proteins found exclusively in the 25S [U4/U6.U5] tri-snRNP appears to be involved in the assembly of the tri-snRNP complex, as, in an in vitro reconstitution assay, purified 20S U5 and 10S U4/U6 snRNPs formed stable 25S [U4/U6.U5] complexes only in the presence of the free tri-snRNP-specific proteins. The formation of the [U4/U6.U5] complex in vitro does not require ATP, and the stability of the purified tri-snRNP complex is not affected by ATP to a measurable extent. However, the native [U4/U6.U5] displays a kinase activity that is absent in isolated U5: A 52-kD protein present in both U5 and [U4/U6.U5] is phosphorylated only in the latter. The function of this phosphorylation is unclear thus far; it may be involved in the activation of [U4/U6.U5] in the spliceosome.

Shuttling of the autoantigen La between nucleus and cell surface after uv irradiation of human keratinocytes

During the past years we have established that the nuclear autoantigen La shuttles between the nucleus and the cytoplasm in tumor cells after inhibition of transcription or virus infection. We reinvestigated this shuttling using primary human keratinocytes from both healthy donors and patients with xeroderma pigmentosum. Ultraviolet irradiation resulted in both an inhibition of transcription and a translocation of La protein from the nucleus to the cytoplasm. After a prolonged inhibition of transcription La protein relocated into the nucleus and assembled with nuclear storage regions. The uv-induced shuttling included a translocation to the cell surface, where La protein colocalized with epidermal growth factor receptors.

Insights into native epitopes of proliferating cell nuclear antigen using recombinant DNA protein products

A cDNA clone encoding full-length human proliferating cell nuclear antigen (PCNA) was used to generate a panel of in vitro translated labeled protein products with COOH-terminal deletions and to construct a set of fusion proteins with COOH- and NH2-terminal deletions. A rabbit antiserum raised against an NH2-terminal peptide, a well-characterized murine monoclonal antibody (mAb), and 14 human lupus sera with autoantibody to PCNA were analyzed for their reactivity with the constructs using both immunoprecipitation and immunoblotting techniques. The rabbit antiserum reacted in immunoprecipitation and immunoblotting with constructs containing the appropriate NH2-terminal sequence and mAb reacted with a sequence from the midregion of PCNA. These experimentally induced antibodies also reacted with 15-mer synthetic peptides in enzyme-linked immunosorbent assay (ELISA). In contrast, none of the lupus sera reacted with synthetic peptides in ELISA. 9 of the 14 lupus sera also failed to react in Western immunoblotting with any recombinant fusion protein, although they all immunoprecipitated in vitro translated full-length protein. Four of the nine had variable patterns of immunoprecipitation with shorter constructs. The remaining five lupus sera were able to immunoprecipitate translation products as well as Western blot recombinant fusion proteins. From analysis of the patterns of reactivity of human lupus sera, it was deduced that the apparent heterogeneity of human autoantibodies to PCNA could be explained by immune response to highly conformational epitopes. These observations demonstrate that there might be special features in "native" epitopes of intranuclear antigens that are recognized by autoantibodies, and that these special features of native epitopes might not be present in prepared antigen used for experimental immunization. These features may be related to protein folding or to association of the antigen with other intranuclear proteins or nucleic acids, as might occur with antigens that are components of subcellular particles.

Fine epitope mapping of the human SS-B/La protein. Identification of a distinct autoepitope homologous to a viral gag polyprotein

To analyze the autoepitopes on the SS-B/La protein, a cDNA covering the entire region coding the protein was isolated from a human cDNA library. The cDNA was subcloned into an expression plasmid vector, pEX, to express its protein product as a fusion protein with cro-beta-galactosidase in Escherichia coli. A recombinant pEX plasmid expressing three-fourths of the protein (amino acid 112-408) was also constructed. The antigenicities of these recombinant proteins were confirmed with a patient's serum. Their various deletion mutants were produced with exonuclease III treatment from the 3' ends of the cDNAs without changing the proper translational frame. Immunoblot analysis and enzyme-linked immunosorbent assay were used to evaluate the reactivities of the recombinant proteins with patients' sera to determine the autoepitopes. A narrow segment (amino acid 88-101) and the region where several epitopes were located (amino acid 283-338) on the SS-B/La protein were universally recognized by all the sera with anti-SS-B/La antibodies examined. An additional epitope region (amino acid 179-220) was recognized by some patients' sera. Computer analysis revealed that the most distinct autoepitope, amino acid 88-101, had a striking homology to a retroviral gag polyprotein. These findings indicate that exogenous or endogenous retroviruses may play a role in initiation of the anti-SS-B/La autoimmunity.

Antinuclear antibody determination methods

Screening rheumatology patients for anti-nuclear and anti-cytoplasmic antibodies is easily done in a qualitative manner using the IF, CIE and ID assays. The immunoblot is of use for anti-La and anti-RNP assays but gives anomalous results for Sm binding by anti-RNP sera and is not easily quantitated. These deficiencies of the immunoblot do not apply to the ELISA. Advances in cloning of autoantigens will enable standardisation of antigen preparations used for these ELISAs. The quantitation of autoantibody appears significant since disease flares occur together with elevations in specific autoantibody. IgM anti-Sm autoantibody was detected with a different disease distribution to IgG anti-Sm but the prognostic implications for this remain to be determined.

Protein-mediated nuclear export of RNA: 5S rRNA containing small RNPs in xenopus oocytes

We have analyzed RNP formation and nucleocytoplasmic migration of 5S RNA and 5S RNA variants transcribed from microinjected genes in Xenopus oocytes. Using antisera against three different proteins we find that newly transcribed nuclear 5S rRNA transiently interacts with La antigen. The La protein is then replaced by either ribosomal protein L5 or the 5S gene-specific transcription factor IIIA (TFIIIA), and each of these two RNPs migrates out of the nucleus and accumulates in the cytoplasm. RNA molecules that are impaired in their ability to interact with L5 and TFIIIA are retained in the nucleus. Thus, L5 and TFIIIA define a new functional class of proteins involved in the nuclear export of RNA. In addition, we show that RNP migration depletes the nucleus of TFIIIA, resulting in a loss of transcription competence for newly injected 5S rRNA genes.

Temporal correlation of antibody responses to different epitopes of the human La autoantigen

To investigate the temporal relationship of antibody responses to different La epitopes, sequential sera from nine patients with systemic lupus erythematosus and Sjogren's syndrome were tested by enzyme-linked immunosorbent assay for antibody binding to a series of recombinant fusion proteins containing different regions of the La molecule. The results of this analysis indicate that antibody responses to four different La fragments vary in parallel over time. This finding is supported by a statistical analysis indicating that the changes in antibody levels between the six pairs of responses were highly correlated (P less than 0.001). Furthermore, we show by immunoaffinity purification that antibodies to the three nonoverlapping La protein fragments do not cross-react with other fragments and, hence, represent independent populations. These results suggest that anti-La antibodies are coordinately produced to different epitopes on the La molecule, possibly reflecting an antigen-driven mechanism.

Analysis of autoantibodies to recombinant La (SS-B) peptides in systemic lupus erythematosus and primary Sjogren's syndrome

Autoantibodies to a polymerase III transcription factor, La (SS-B), are frequently detected in the serum of patients with Sjogren's syndrome and systemic lupus erythematosus. To define the humoral immune response to this protein, we analyzed the patterns of antibody recognition toward 13 recombinant La peptides by immunoblotting and determined the heterogeneity of antibodies reactive with the immunodominant epitopes. The smallest epitopes that were strongly antigenic and recognized by greater than 70% of sera tested (immunodominant) were encoded by the subclones BgX and XA located in the 5' and 3' halves of the La cDNA, respectively. Conformation of the immunodominant La peptides played a major role in antibody recognition. Although greater diversity in antibody binding to carboxyl-terminal La peptides was observed, the overall pattern of peptide recognition by anti-La antibodies was similar in different diseases. The antibody responses to the immunodominant peptides were strongly correlated (r = 0.68, P less than 0.001). One- and two-dimensional isoelectric focusing of affinity purified IgG anti-La peptide antibodies revealed restricted heterogeneity and oligoclonal bands (kappa light chains). These observations suggest that anti-La antibodies are induced and/or maintained by the self antigen and that their diversity is constrained either by mechanisms related to tolerance or by affinity maturation of the humoral immune response.

Characterization of the autoantigen La as a nucleic acid-dependent ATPase/dATPase with melting properties

The autoantigen La, a known transcription termination factor of RNA polymerase III, was purified to homogeneity from mouse 3T3 cells and calf thymus by different isolation procedures. The La protein from calf thymus was separated into RNA binding and nonbinding subclasses. The murine La protein and the RNA binding subclass of calf thymus La protein showed ATPase/dATPase activity in the presence of DNA-RNA or RNA-RNA hybrids. A novel monoclonal anti-La antibody (La11G7) and patients' anti-La antibodies immunoadsorbed to homogeneously purified La protein were able to inhibit the enzyme activity of La protein. La protein was able to melt a synthetic DNA-RNA hybrid in a reaction that required ATP hydrolysis. The RNA binding ability of the nonbinding subclass was restored by treatment with sialidase. This treatment also restored the protein's ATP-dependent melting activity.