Characterization of the SS-B (La) antigen in adenovirus-infected and uninfected HeLa cells

The multifunctional RNA-binding protein La is required for mouse development and for the establishment of embryonic stem cells

The La protein is a target of autoantibodies in patients suffering from Sjögren's syndrome, systemic lupus erythematosus, and neonatal lupus. Ubiquitous in eukaryotes, La functions as a RNA-binding protein that promotes the maturation of tRNA precursors and other nascent transcripts synthesized by RNA polymerase III as well as other noncoding RNAs. La also associates with a class of mRNAs that encode ribosome subunits and precursors to snoRNAs involved in ribosome biogenesis. Thus, it was surprising that La is dispensable in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, the organisms from which it has been characterized most extensively. To determine whether La is essential in mammals and if so, at which developmental stage it is required, mice were created with a disrupted La gene, and the offspring from La+/-intercrosses were analyzed. La-/- offspring were detected at the expected frequency among blastocysts prior to implantation, whereas no nullizygotes were detected after implantation, indicating that La is required early in development. Blastocysts derived from La+/- intercrosses yielded 38 La+/+ and La+/- embryonic stem (ES) cell lines but no La-/- ES cell lines, suggesting that La contributes a critical function toward the establishment or survival of ES cells. Consistent with this, La-/- blastocyst outgrowths revealed loss of the inner cell mass (ICM). The results indicate that in contrast to the situation in yeasts, La is essential in mammals and is one of a limited number of genes required as early as the development of the ICM.

Molecular characterization of the human La protein.hepatitis B virus RNA.B interaction in vitro

The La protein was recently identified as a host factor potentially involved in the cytokine-induced post-transcriptional down-regulation of hepatitis B virus (HBV) RNA. The La binding site was mapped to a predicted stem-loop structure within a region shared by all HBV RNAs, and it was concluded that the La protein might be an HBV RNA-stabilizing factor. To characterize the RNA binding mediated by the different RNA recognition motifs (RRMs) of the human La protein, several La deletion mutants were produced and analyzed for HBV RNA binding ability. The data demonstrate that the first RRM is not required for binding, whereas the RNP-1 and RNP-2 consensus sequences of the RRM-2 and RRM-3 are separately required for binding, indicating a cooperative function of these two RRMs. Furthermore, the results suggest that multimeric La disassembles into monomeric La upon binding of HBV RNA.B. By gel retardation assay the affinity of the wild type human La.HBV RNA.B interaction was determined in the nanomolar range, comparable to the affinity determined for the mouse La.HBV RNA.B interaction. This study identified small regions within the human La protein mediating the binding of HBV RNA. Hence, these binding sites might represent targets for novel antiviral strategies based on the disruption of the human La.HBV RNA interaction, thereby leading to HBV RNA degradation.

Intracellular trafficking and surface expression of SS-A (Ro), SS-B (La), poly(ADP-ribose) polymerase and alpha-fodrin autoantigens during apoptosis in human salivary gland cells induced by tumour necrosis factor-alpha

Autoantibodies directed against nucleic acid and protein complexes present in cell nuclei characterize autoimmune diseases and are employed in diagnosis. The mechanisms by which these autoantigens escape immunological tolerance are largely unknown, but a number of recent observations suggest that modified self-protein generated during apoptosis my play an important part in the development of autoimmunity. To investigate the possibility that autoantibodies in patients with Sjögren's syndrome are induced by apoptosis and presented on the surface of the cell, the internal distribution of autoantigens in apoptotic human salivary gland cells was studied in vitro. Salivary gland cells were treated with tumour necrosis factor-alpha, an apoptosis inducer. At increasing times after induction, cells were homogenized and cytoplasmic, cell surface membrane and nuclear compartments were fractionated using a sucrose density-gradient system. Autoantigens alpha-fodrin, SS-A (Ro), SS-B (La), and the enzyme poly(ADP-ribose) polymerase, were detected by conventional immunofluorescence and confirmed by Western immunoblotting. At increasing times after apoptosis, nuclear proteins SS-A (Ro) and SS-B (La), but not poly(ADP-ribose) polymerase were relocated from the cell nucleus to the cell surface membrane. Fodrin, a cytoplasmic protein, was also translocated to the cell membrane after cleavage of alpha-fodrin. These results show that autoantigens fodrin, SS-A (Ro) and SS-B (La) in human salivary gland cells undergo a striking redistribution during apoptosis and relocate to the cell membrane of apoptotic cells. The appearance of autoantigens on the surface of induced cells could form the basis of a mechanism for autoantigen presentation, processing and autoantibody induction.

La protein and its associated small nuclear and nucleolar precursor RNAs

After transcription by RNA polymerase (pol) III, nascent Pol III transcripts pass through RNA processing, modification, and transport machineries as part of their posttranscriptional maturation process. The first factor to interact with Pol III transcripts is La protein, which binds principally via its conserved N-terminal domain (NTD), to the UUU-OH motif that results from transcription termination. This review includes a sequence Logo of the most conserved region of La and its refined modeling as an RNA recognition motif (RRM). La protects RNAs from 3' exonucleolytic digestion and also contributes to their nuclear retention. The variety of modifications found on La-associated RNAs is reviewed in detail and considered in the contexts of how La may bind the termini of structured RNAs without interfering with recognition by modification enzymes, and its ability to chaperone RNAs through multiple parts of their maturation pathways. The CTD of human La recognizes the 5' end region of nascent RNA in a manner that is sensitive to serine 366 phosphorylation. Although the CTD can control pre-tRNA cleavage by RNase P, a rate-limiting step in tRNASerUGA maturation, the extent to which it acts in the maturation pathway(s) of other transcripts is unknown but considered here. Evidence that a fraction of La resides in the nucleolus together with recent findings that several Pol III transcripts pass through the nucleolus is also reviewed. An imminent goal is to understand how the bipartite RNA binding, intracellular trafficking, and signal transduction activities of La are integrated with the maturation pathways of the various RNAs with which it associates.

The La protein

Ubiquitous in eukaryotic cells, the La protein associates with the 3' termini of many newly synthesized small RNAs. RNAs bound by the La protein include all nascent transcripts made by RNA polymerase III as well as certain small RNAs synthesized by other RNA polymerases. Recent genetic and biochemical analyses have revealed that binding by the La protein protects the 3' ends of these RNAs from exonucleases. This La-mediated stabilization is required for the normal pathway of pre-tRNA maturation, facilitates assembly of small RNAs into functional RNA-protein complexes, and contributes to nuclear retention of certain small RNAs. Studies of mutant La proteins have given some insights into how the La protein specifically recognizes its RNA targets. However, many questions remain regarding the molecular mechanisms by which La protein binding influences multiple steps in small RNA biogenesis. This review focuses on the roles of the La protein in small RNA biogenesis and also discusses data that implicate the La protein in the translation of specific mRNAs.

Detailed analysis of the phosphorylation of the human La (SS-B) autoantigen. (De)phosphorylation does not affect its subcellular distribution

The La (SS-B) autoantigen is an evolutionarily conserved phosphoprotein which plays an important role, most likely as an RNA chaperone, in various processes, such as the biosynthesis and maturation of RNA polymerase III transcripts in the cell nucleus and (internal) initiation of translation in the cytoplasm. In this study, the phosphorylation state of this protein from human HeLa and HEp-2 cells was characterized by high-resolution two-dimensional IEF/SDS-PAGE analysis, and phosphorylation sites were mapped by nanoelectrospray mass spectrometry. Furthermore, the effect of phosphorylation at the sites identified on the subcellular distribution of the protein was studied by site-directed mutagenesis. At least 14 isoelectric isoforms were discerned on 2-D gels with La protein from both types of cells. Metabolic labeling in combination with alkaline phosphatase treatment revealed that only a limited number of these isoforms could be attributed to phosphorylation. Four phosphorylation sites, Thr-302, Ser-325, Thr-362, and Ser-366, were mapped by mass spectrometric analysis of the isolated La protein from HeLa cells or the carboxy-terminal half of this protein. The analysis of mutants of La, in which the respective phosphorylated residues were replaced by either a neutral (alanine) or an acidic (aspartate) residue, by microinjection into Xenopus laevis oocytes on the one hand and transfection of HEp-2 cells on the other hand revealed that the subcellular distribution of this protein was not affected by these amino acid substitutions. These results strongly suggest that the signals that determine the subcellular distribution of this protein are not regulated by (de)phosphorylation of the target residues examined.

Neonatal lupus syndrome: the heart as a target of the immune system

Neonatal lupus erythematosus (NLE) is an auto-immune disease related to systemic lupus erythematosus (SLE). Unlike SLE it is not a spontaneous syndrome but rather an acquired one. In NLE the most common disease manifestations are a transient cutaneous lesion and cardiac conduction disturbances. The cutaneous lesions and other non-cardiac manifestations of NLE are transient and disappear about six months after birth, at the time when maternal antibodies disappear from the neonatal circulation. This fact suggests that maternal antibodies may cross the placenta leading to an inflammatory reaction in the fetal tissues. NLE is the principal cause of atria-ventricular block, when it is not associated with congenital birth defects. All the clinical studies to date correlate the heart block in NLE with the presence of certain types of circulating maternal antibodies, against the Ro/SSA nuclear proteins, in the serum of the newborn. In this paper we discuss animal models that have been developed by our and others groups to study the participation of the anti-Ro/SSA antibodies in the pathogenesis of the cardiac conduction blockades that occur in NLE.

La autoantigen specifically recognizes a predicted stem-loop in hepatitis B virus RNA

We recently identified three nuclear proteins (p45, p39, and p26) that bind to a 91-nucleotide (nt) RNA element between nt 1243 and 1333 in hepatitis B virus (HBV) RNA, and we showed that these proteins and HBV RNA are regulated coordinately by gamma interferon and tumor necrosis factor alpha. Purification and sequence analysis of tryptic peptides obtained from p39 revealed sequence homology to the mouse La protein. Immunoprecipitation experiments showed that p45, p39, and p26 were recognized by anti-La-specific antiserum, indicating that p45 is the full-length La protein and that p39 and p26 are likely to be proteolytic La cleavage products. Furthermore, in competition experiments we found that all three La proteins bind, in a phosphorylation-dependent manner, to the same predicted stem-loop structure located between nt 1275 and 1291 of HBV, with Kds of approximately 1.0 nM. Collectively, these results support the notion that the La protein may contribute to HBV RNA stability, constitutively and in response to inflammatory cytokines.

Involvement of the Xenopus laevis Ro60 autoantigen in the alternative interaction of La and CNBP proteins with the 5'UTR of L4 ribosomal protein mRNA

In vertebrates the synthesis of ribosomal proteins is co-ordinately regulated at the translational level. The 5'-untranslated region (5'UTR) of this class of mRNAs contains conserved regions that are necessary and sufficient for translational regulation. Recently, we found that two proteins, the Xenopus laevis La autoantigen and the cellular nucleic acid binding protein (CNBP), are able to bind in vitro a pyrimidine tract at the 5' end and a downstream region, respectively. These regions are considered the common cis-acting elements of translational regulation. It was previously observed that the binding of both these putative trans-acting factors to their RNA sequences is assisted by a protease-sensitive factor(s) that dissociates from the complex after its formation. Here we provide evidence that the requirement for an ancillary factor assisting La binding to the pyrimidine tract of ribosomal protein mRNAs is typical of this RNA, and secondly that it may involve an RNA recognition motif of the La protein not clearly characterized previously. We also show that the Ro60 autoantigen is involved in the common factor activity necessary for the binding of La and CNBP proteins to their respective sequences. In addition, our findings suggest that an RNA also participates in this process. We show that CNBP can multimerise and that it binds to the 5'UTR as a dimer. Both La and CNBP compete for the interaction with the factor, and their binding to the 5'UTR is mutually exclusive. Our results from the binding analysis of mutations in the 5'UTR, which are known to disrupt the translational control in vivo, suggest a model in which the protein interactions and the 5'UTR RNA structure may co-operate in regulating the translational fate of ribosomal protein mRNAs.

La autoantigen binding to a 5' cis-element of rubella virus RNA correlates with element function in vivo

Rubella virus genomic RNA contains a 5' stem-loop (5'(+) SL) which is required for efficient translation and replication. The La autoantigen previously was shown to bind this RNA sequence in vitro. Results reported here demonstrate that this cellular RNA-binding protein binds to the RV 5' SL RNA with sufficient specificity for the binding to occur in the presence of excess total cellular RNA. Further, the affinity of purified La for the RV sequence is similar to its affinity for known cellular substrates. To address the functional significance of La binding, mutant forms of the RV 5'(+) SL were analysed which bind La weaker or stronger than the native form. These three forms of the RV 5' SL were incorporated into RV-luciferase constructs which expressed luciferase activity in transient transfection. The level of expression from each construct correlated with the ability of its RV sequence to bind La. The detection of La/RV RNA complexes in infected cells further supported the physiological relevance of this interaction. Possible implications of autoantigen La interaction with RV RNA for rubella virus pathology and vaccine associated adverse reactions are discussed.

Immunological aspects of congenital atrioventricular block

Isolated congenital heart block (CHB) in children is strongly associated with the occurrence of autoantibodies to Ro/SS-A and La/SS-B in the circulation of the mother. Although these antibodies are generally found in patients with generalized autoimmune diseases, they may also occur in symptomless mothers of CHB patients. It is thought that anti-Ro/SS-A and anti-La/SS-B may reach the fetal circulation via transplacental transport, starting around the 20th week of gestation. Although several hypotheses have been put forward, the exact mechanism through which these autoantibodies bind the fetal conduction system and elicit a local inflammatory response is still unclear. Also, the induction of the autoantibody response in the mother remains largely obscure, although a putative role of viral infection has been suggested. In this overview, the structure of the involved antigens will be discussed, followed by the nature of the antibodies. Recent data regarding the etiology of the anti-Ro/SS-A and anti-La/SS-B response and the role of these autoantibodies in the pathogenesis of CHB will be presented.

Specific interaction in vitro and in vivo of glyceraldehyde-3-phosphate dehydrogenase and LA protein with cis-acting RNAs of human parainfluenza virus type 3

Human parainfluenza virus type 3 (HPIV3) genome RNA is transcribed and replicated by the virus-encoded RNA-dependent RNA polymerase, and specific cellular proteins play a regulatory role in these processes. To search for cellular proteins potentially interacting with HPIV3 cis-acting regulatory RNAs, a gel mobility shift assay was used. Two cellular proteins specifically interacted with the viral cis-acting RNAs containing the genomic 3'-noncoding region and the plus-sense leader sequence region. Surprisingly, by biochemical and immunological analyses, one of the cellular proteins was identified as the key glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The other protein was characterized as the autoantigen, LA protein. Both GAPDH and LA protein also interacted with the same cis-acting RNA sequences in vivo and were found to be associated with the HPIV3 ribonucleoprotein complex in the infected cells. By double immunofluorescent labeling, GAPDH was found to be co-localized with viral ribonucleoprotein in the perinuclear region. These observations strongly suggest that cellular GAPDH and LA Protein participate in the regulation of HPIV3 gene expression.

Anti-La monoclonal antibodies recognizing epitopes within the RNA-binding domain of the La protein show differential capacities to immunoprecipitate RNA-associated La protein

The La (SS-B) autoimmune antigen is an RNA-binding protein that is present in both the nucleus and cytoplasm of eukaryotic cells, where it is found associated with RNA polymerase III transcripts. We have investigated the capacity of anti-La monoclonal antibodies SW1, SW3, and SW5 to immunoprecipitate human La ribonucleoprotein particles. Distinct differences were observed for SW3 in comparison with SW1 and SW5. While SW1 and SW5 precipitated ribonucleoproteins containing pre-tRNA, pre-5S rRNA, hY RNAs, pre-U6 snRNA or the viral EBER1 and VA RNAs, SW3 precipitated only ribonucleoproteins containing VA RNAs or (the precursor of) 7-2 RNA. Mapping of the epitopes recognized by SW1, SW3, and SW5 revealed that all three monoclonal antibodies recognize an epitope within the domain of the protein formed by the ribonucleoprotein motif. Cross-competition studies suggested that the epitope recognized by SW1 and SW5 are identical but distinct from the epitope recognized by SW3. Further analyses of the recognition of La from other species by these monoclonal antibodies revealed that they all reacted with bovine La and were not reactive with La from rodents and Xenopus laevis. Replacement of a single amino acid in the human protein by its murine counterpart abolished recognition by SW1 and SW5, but had no effect on recognition by SW3. Taken together, our results indicate that SW1 and SW5 recognize the same epitope and that SW3 recognizes a distinct epitope, both of which are located in the RNA-binding domain of La, and that the accessibility of these epitopes is differentially influenced by the association of La with various RNA polymerase III transcripts.

Internal translation initiation on poliovirus RNA: further characterization of La function in poliovirus translation in vitro

Initiation of poliovirus RNA translation by internal entry of ribosomes is believed to require the participation of trans-acting factors. The mechanism of action of these factors is poorly defined. The limiting amount of one of these factors, La protein, in rabbit reticulocyte lysates (RRL) has been postulated to partially explain the inefficient translation of poliovirus RNA in this system. To further characterize La activity in translation and to identify other potential limiting factors, we assayed the ability of La protein as well as purified initiation factors, eIF-2, guanine nucleotide exchange factor (GEF), eIF-4A, eIF-4B, eIF-4F, and eIF-3, to stimulate the synthesis of P1, the capsid precursor protein, in poliovirus type 1 (Mahoney) RNA-programmed RRL. Of the proteins tested, only La, GEF, and to some extent eIF-2 stimulated the synthesis of P1. The enhanced translation of P1 in response to La occurred concomitantly with the inhibition of synthesis of most aberrant polypeptides, resulting from initiation in the middle of the genome. Deletion of the carboxy-terminal half (214 amino acids) of La did not decrease its binding to the poliovirus 5' untranslated region but abrogated the stimulatory and correcting activity in translation. In contrast to La, GEF and eIF-2 stimulated the overall translation and increased the synthesis of aberrant products as well as P1. Neither La, GEF, nor any other factor stimulated translation of encephalomyocarditis virus RNA in RRL. The implications of these findings for the mechanism of internal translation initiation on picornavirus RNAs are discussed.

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.

Refined definition of the 56K and other autoantigens in the 50-60 kDa region

Alteration of the acrylamide:bisacrylamide ratio in the SDS-polyacrylamide gel used for Western blotting strongly improved the unambiguous detection of antibodies against 50-60 kDa autoantigens present in autoimmune patient sera. The relative migration of Ro52, the 56K autoantigen and calreticulin increased with reduced acrylamide:bisacrylamide ratios in contrast to that of Ro60, La and Jo-1. These analyses indicated that these six autoantigens correspond to six distinct polypeptides. Further analyses using recombinant calreticulin showed that (i) the 56K autoantigen is neither identical nor related to calreticulin and (ii) calreticulin is not a Ro autoantigen. A series of experiments designed to better characterize the 56K autoantigen showed that (i) the antigen is not detectable in fixed cells, presumably due to masking of the epitopes; (ii) about equal amounts of the antigen were recovered in nuclear and cytoplasmic cell fractions after enucleation of the cells; (iii) the 56K autoantigen is not stably associated with either RNA or other proteins.

Detection and occurrence of the 60- and 52-kD Ro (SS-A) antigens and of autoantibodies against these proteins

The simultaneous detection of anti-La, anti-60-kD Ro and anti-52-kD Ro antibodies by immunoblotting is greatly improved by changing the crosslinking level in the gel to an acrylamide/bisacrylamide ratio of 19:1. Using this method for the analysis of a number of systemic lupus erythematosus (SLE) and Sjögren's syndrome patient sera it was observed that antibody to the 52-kD Ro protein without anti-60-kD Ro antibody was restricted to Sjögren's syndrome patients (9/26), whereas antibody to the 60-kD Ro protein without contaminating anti-52-kD Ro antibody was only found in SLE patients (8/38). Moreover, in Sjögren's syndrome patient sera anti-Ro antibody was found only in combination with anti-La antibody (20/26), whereas in SLE patient sera anti-Ro antibody could be found without detectable anti-La specificity (4/38). Double immunofluorescence microscopy revealed that the 52-kD Ro and the 60-kD Ro proteins co-localize in the cytoplasm as well as in the nucleus, whereas immunoprecipitation of [32P]-labelled HeLa cell extract with monospecific anti-52-kD Ro and anti-60-kD Ro sera showed that both proteins are associated with the Ro RNAs. These data suggest the presence of both the 52-kD and the 60-kD Ro proteins in the same ribonucleoprotein complexes. To study the evolutionary conservation of the 52-kD Ro, the 60-kD Ro and the La proteins, extracts of cell lines derived from various mammalian species were analysed on Western blots using monospecific human antibodies. In contrast to the 60-kD Ro and the La antigens which are well conserved in evolution, the 52-kD Ro antigen could be detected in primate cells only by this immunological approach.

The consensus workshops for the detection of autoantibodies to intracellular antigens in rheumatic diseases

In 1988 and in 1989 consensus workshops were organized in order to define the interlaboratory concordance in detecting autoantibody specificities in selected sera from patients with rheumatoid disorders and to determine the possible causes of discrepancies. In total 20 sera were tested for the presence of antibodies against nRNP, Sm, Ro (SS-A), La (SS-B), Scl-70, centromeric antigens, ribosomal RNP and Jo-1. The methods used for detection by the 28 European laboratories who participated included immunofluorescence, counter-immunoelectrophoresis, immunodiffusion, immunoblotting and ELISA. The results showed that only a combination of two or more techniques was able to detect all specificities with an adequate efficiency. Recommendations to improve the efficiency of autoantibody detection and to standardize laboratory protocols are given.

Sera from patients with autoimmune disease recognize conformational determinants on the 60-kd Ro/SS-A protein

Anti-Ro antibodies are found in a large proportion of patients with systemic lupus erythematosus and primary Sjögren's syndrome. These antibodies also characterize neonatal lupus, subacute cutaneous lupus erythematosus, and vasculitis associated with Sjögren's syndrome and rheumatoid arthritis. Anti-Ro-positive sera may contain either or both of 2 sets of antibodies, recognizing either a 60-kd or a 52-kd polypeptide component of the Ro particle. We found in this study that the immune response to the 60-kd Ro antigen is heterogeneous. Some sera specifically recognize the native Ro antigen but fail to bind the corresponding denatured polypeptides. In addition, after immunodepletion using the denatured 60-kd Ro polypeptide, all anti-Ro-positive sera tested still contained high titers of antibodies recognizing conformational determinants on the Ro antigen. The frequent immunodominance of anti-Ro antibodies targeted to conformational determinants suggests that native autoantigens may directly drive the autoimmune response.

The use of adenovirus-infected HeLa cells for the detection of low titer autoantibodies

Following infection of HeLa cells with adenovirus type 5 the cellular La protein becomes predominantly associated with the virally encoded RNA polymerase III products VAI, and VAII, while most of the host RNA polymerase II (e.g. U1, U2, U4, U5 and mRNA) and RNA polymerase III transcription (e.g. U6 and pre-tRNAs) ceases. Other RNA polymerase III products such as the cellular Ro RNAs continue to be transcribed and assembled into ribonucleoprotein complexes containing the Ro (SS-A) antigens. Using a 32P-pulse chase-labeled, adenovirus-infected HeLa cellular extract as a source of antigen, anti-La (SS-B) and anti-Ro (SS-A) antibodies can be detected simultaneously using an immunoprecipitation assay. In the present study this method was found to be more sensitive in detecting anti-La antibodies then counter immunoelectrophoresis and immunoblotting. In studies of sera from patients suffering from rheumatic diseases the percentage positive for anti-La antibody was significantly elevated using this method, especially in patients with systemic lupus erythematosus.

La/SSB ribonucleoprotein levels increased in transformed cells

Autoantibodies to the La/SSB ribonucleoprotein are commonly found in patients with Sjögren's syndrome. Previous studies have shown that La/SSB accumulates in cells shortly after viral infection. We have extended these studies by investigating levels of the La/SSB antigen in virally and spontaneously transformed cell lines (contact-insensitive and tumourigenic) relative to their non-transformed counterpart cell lines (contact-sensitive and non-tumourigenic). Transformed BALB/3T12-3 and KNRK fibroblasts were visibly brighter by immunofluorescence assay than non-transformed BALB/3T3 and NRK fibroblasts respectively, when reacted with anti-La/SSB specific sera. This was confirmed by flow cytometry, as La/SSB levels were elevated in the transformed counterparts of the same cell lines. An anti-Sm monoclonal antibody and normal human serum reacted with these cell lines failed to show a significant increase by flow cytometry. Finally, a two-fold increase in the La/SSB antigen was demonstrated in cell lysates of these cell lines by a capture ELISA. These data show that La/SSB is elevated in transformed cell lines compared with non-transformed counterpart cell lines and suggest that this increase is not restricted to viral transformation.

Virus infection induces redistribution and membrane localization of the nuclear antigen La (SS-B): a possible mechanism for autoimmunity

To investigate the possibility that anti-La (SS-B) antibodies in Sjögren's syndrome were induced by virus infection we studied the distribution of La in virus-infected human cell lines. Three monoclonal antibodies to La were used with monoclonal anti-Sm (derived from MRL/lpr lupus mice) and anti-rat immunoglobulin antibodies as controls. In uninfected cells La was predominantly in the nucleus. Twenty-four hours after infection of HEp-2 cells with adenovirus 2, the La and Sm antigens appeared to aggregate and accumulate in the periphery of the nucleus and, after 48 h, La was seen in the cytoplasm and cell membrane. No cytoplasmic or membrane expression of Sm was seen. Infection with adenovirus or cytomegalovirus caused a 2-13-fold increase in the concentration of La in three cell lines. Treatment of HE--2 cells with interferon-gamma (IFN-gamma) and infection with Epstein-Barr virus and cytomegalovirus caused cytoplasmic, but no definite membrane expression of La. The appearance of La on the surface of virally infected epithelial cells together with IFN-gamma induced class II expression could form the basis of a T cell dependent mechanism for anti-La autoantibody induction.

The diagnostic value of several immunological tests for anti-nuclear antibody in predicting the development of connective tissue disease in patients presenting with Raynaud's phenomenon

One-hundred-and-one patients referred because of Raynaud's phenomenon (RP) were prospectively followed for a mean period of 42 months. At presentation they were screened for signs and symptoms of connective tissue disease (CTD) according to a detailed protocol. At presentation 37 patients had primary RP (PRP), nine had RP in combination with vascular occlusive disease (RP-VOD), 25 had one symptom of a CTD (questionable PRP), 13 had two or more symptoms (undifferentiated CTD, UCTD) and 17 had definite CTD. Progression from one of these groups to another was seen in 24 patients and from PRP, RP-VOD or questionable PRP towards a (U)CTD was seen in 19 patients. Patients with one sign of CTD showed a high tendency (56%) to develop CTD. The presence of ANA as detected by immunofluorescence and by immunoblotting at the start of the study was associated with the future development of symptoms of CTD; positive predictive value 65% and 71% and negative predictive value 93% and 83%, respectively. ANA-testing by immunoblotting was of special help in predicting the development of scleroderma, the CREST syndrome and mixed connective tissue disease. In conclusion, testing for ANA by indirect immunofluorescence helps to discriminate between patients with persisting PRP and those who will develop a CTD, while testing for ANA by the immunoblotting technique helps to predict the development of a specific CTD.

T lymphocyte activation in systemic lupus erythematosus analysed by proliferative response to nucleoplasmic proteins on nitrocellulose immunoblots

Polyclonal B cell activity in systemic lupus erythematosus (SLE) may be under T cell control. The use of nitrocellulose immunoblots for the analysis of recognition by peripheral blood lymphocytes of nucleoplasmic proteins in SLE patients led to the characterization of significant proliferative responses to 68K (U1 RNP); SS-B; B-B' and D (Sm) antigen in 15 of 20 patients. Variations of proliferative response were parallel to disease activity over a follow-up period of greater than or equal to 6 months, conferring some prognostic value to the assay of lymphocyte response to nucleoplasmic antigens. The pattern of reactivity differs from the corresponding serum antibody profile, and purified T cell suspensions (greater than 95% pure) were shown to proliferate in response to soluble nucleoplasmic antigens, indicating that T and B cell repertoires against nucleoplasmic proteins may differ. This suggests that activated helper T cells contribute to the fine modulation of B cell reactivity to subcellular particles to determine the particular antibody profile of the patients.

Epitope specificity of anti-La antibodies from patients with Sjögren's syndrome

To investigate patterns of autoreactivity in Sjögren's syndrome, the epitope specificity of anti-La antibodies was determined using recombinant antigens bearing sequences of the amino, middle, and carboxyl portions of the La molecule. Sera from patients with primary as well as secondary Sjögren's syndrome reacted with all three fragments, although the magnitude of the responses varied markedly among individuals. Furthermore, the proportion of antibody binding directed to the different La epitopes showed considerable individual variations, but these patterns were not correlated with specific clinical manifestations. These results suggest that quantitative and qualitative aspects of anti-La responses in Sjögren's syndrome are determined by factors distinct from those determining the clinical expression of disease.

The RNA binding protein La influences both the accuracy and the efficiency of RNA polymerase III transcription in vitro

The autoantigen La binds the U-rich 3' ends of all nascent RNA polymerase III transcripts. Here, we demonstrate that this abundant nuclear phosphoprotein not only binds these RNAs but appears to be required for their synthesis. HeLa cell extracts immunochemically depleted of La by either patient or mouse monoclonal antibodies lose greater than 99% of their transcription activity on class III genes. The few transcripts synthesized in the absence of La have fewer uridylate residues at their 3' ends than those made in its presence. Reconstitution of La-depleted extracts with biochemically purified HeLa La protein stimulates transcription levels and completely restores transcript length. A model coupling transcription levels to the action of La at the RNA polymerase III termination signal is presented.

Function of the mammalian La protein: evidence for its action in transcription termination by RNA polymerase III

We have tested the hypothesis that the mammalian La protein, which appears to be required for accurate and efficient RNA polymerase III transcription, is a transcription termination factor. Our data suggest that 3' foreshortened transcripts generated in La's absence are components of a novel transcription intermediate containing a paused polymerase. These transcripts are produced by fractionated transcription complexes, are synthesized with kinetics different from full-length transcripts, and are chasable to completion from the stalled transcription complexes. Together, these findings argue that termination by RNA polymerase III requires auxilliary factor(s) and implicate La as such a factor. Since La appears to facilitate transcript completion and release and also binds the resulting RNA product, it may be a regulator of RNA polymerase III transcription.

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

The La antigen is a nuclear protein that is one of the major target antigens of autoantibodies found in the sera of patients with primary Sjögren's syndrome. The formation of such autoantibodies is therefore likely to reflect the basic immunopathogenesis of this disorder. A recombinant DNA strategy has been used to examine the La protein for sequences that encode autoimmunizing B cell epitopes. We have isolated and characterized a 1.2-kb-long cDNA from a human liver cDNA library encoding a region of the La protein; this region contains 296 amino acids, including the C terminus. A sub-library of recombinant DNA in the expression vector pEX was made from portions of the La cDNA. Individual fusion proteins were tested by immunoblotting and enzyme-linked immunosorbent assay for their ability to react with anti-La autoantibodies contained in sera from patients with Sjögren's syndrome. In this way, we have identified at least three distinct epitopes in the C-terminal half of the La protein. Every anti-La serum tested contained antibodies against all three of the antigenic regions identified. Furthermore, most of the sera display similar ratios between the titers of antibodies with the three kinds of specificity. Our data suggest that the production of anti-La autoantibodies may be antigen driven.

Quantitative immunoassay of anti-La antibodies using purified recombinant La antigen

A purified recombinant La fusion protein was tested in an enzyme-linked immunosorbent assay to quantitate anti-La responses. This protein contained the immunodominant region of the La molecule fused to beta-galactosidase. In solid-phase assays, recombinant La protein was solubilized in urea and bound to polystyrene wells without loss of immunoreactivity. The recombinant-based enzyme-linked immunosorbent assay proved to be a sensitive method for the detection of anti-La binding, and it accurately distinguished anti-La precipitin positive sera from normal sera.

The use of immunoblotting to detect antibodies to nuclear and cytoplasmic antigens. Clinical and serological associations in rheumatic diseases

Using the immunoblotting technique, sera from 433 patients with rheumatic diseases were screened for the presence of antibodies against several nuclear and cytoplasmic antigens, such as RNP, Sm, Ro(SSA), La(SSB), CR-19 (centromeric antigen), Topo-1 (Scl-70), Jo-1, histone and 56 kD. At the same time clinical data from these patients were collected without prior knowledge of the immunoblotting results. Syndrome-specific autoantibodies were found for mixed connective tissue disease (antibodies against the RNP related 70 kD antigen), for CREST (anti-CR-19 antibodies), for diffuse scleroderma (anti-Topo-1 antibodies) and for polymyositis (anti-Jo-1 antibodies). Almost all specific autoantibodies were present exclusively in patients with a connective tissue disease. Controls were only in a few cases positive for antihistone and anti-56 kD antibodies. Associations of specific autoantibodies with clinical and laboratory features of the patients were mostly as expected. However, some unexpected associations were found, for example polymyositis and calcinosis with anti-Sm antibodies, sicca symptoms with anti-centromere antibodies and leucopenia with Ro(SSA) and La(SSB).

Purification of human autoantibodies from cross-linked antigen immunosorbents

A method is described whereby autoantibodies to the Sjögren's syndrome antigen La (SS-B) can be purified from re-usable immunosorbent columns constructed from covalently linked human autoantibodies to which the antigen is cross-linked. Previous attempts to link the antigen directly to CNBr-Sepharose beads resulted in loss of biological activity and thus each purification of antibody required fresh batches of antigen. The present technique is a significant improvement since the cross-linked immunosorbents prepared from a single batch of antigen can be re-used several times over a 6 month period. Furthermore F(ab')2 fragments of anti-La antibodies can be purified from pepsin-digested serum samples. These antibodies react in ELISA, Western blot and immunofluorescence in an identical way to serum and murine monoclonal anti-La antibodies and show no reaction with the Ro antigen. However, being of human origin the affinity-purified anti-La antibodies have the advantages of bearing the same idiotypes and reacting with the same antigenic epitopes as naturally occurring serum autoantibodies.

Human SS-B/LA autoantigen contains a covalent protein-RNA linkage

The chemical nature of association of RNA in immunoprecipitates of human SS-B/La ribonucleoprotein, an autoantigen expressed in various autoimmune disorders, was investigated. A fraction of RNA associated with SS-B/La immunoprecipitates was readily dissociated by SDS-polyacrylamide gel electrophoresis, yielding four main subfractions, R1-4, with chain lengths in the range of 90-130 nucleotides (R4), 140-175 nucleotides (R2 and R3) and above 200 nucleotides (R1). Moreover, the immunoreactive protein component, migrating with a molecular mass of 49 kDa, contained a very tightly bound RNA co-migrating with the protein unless the protein was proteolytically degraded. Most of the RNA molecules in this fraction, represented by about 20 components, had a free 3'-terminus but a blocked 5'-terminus and showed chain lengths between 10 and 125 nucleotides. After pretreatment with alkaline phosphatase and a mixture of ribonucleases T1 + T2 + A, adenosine 3',5'-biphosphate (pAp) was liberated by phosphodiesterase (Crotalus durissus) as the blocked 5'-end of the RNA. The chemical nature of the blockage was revealed after alternative treatment of the protein-pAp component with phosphodiesterase or nuclease S7 followed by acid hydrolysis and phosphoamino acid analysis which showed that a threonine residue must be directly involved in the RNA-protein linkage of 49 kDa SS/La antigen, indicating the presence of a covalent threonine-pAp bond.

The effect of La(SSB) on PWM-induced immunoglobulin synthesis by anti-La(SSB) positive SLE patients and healthy controls

The effect of affinity purified La(SSB) on immunoglobulin synthesis in vitro by mononuclear cells (MNC) from anti-La(SSB)-positive systemic lupus erythematosus (SLE) patients and healthy controls was studied. La(SSB) was prepared from calf thymus extract and characterized by SDS-polyacrylamide gel electrophoresis and immunoblotting. Silver staining of gels reveals nine major bands at 68 kD, 43-48 kD and 30-33 kD, of which six were recognized on immunoblots by sera from anti-La(SSB) positive SLE patients. Studies in vitro showed that La(SSB) alone did not stimulate total IgG or IgM synthesis in controls or SLE patients. Low concentrations of La(SSB) (optimal dose less than 0.02 ng/ml) suppressed pokeweed mitogen (PWM)-driven IgG synthesis by controls but not by anti-La(SSB) positive SLE patients. IgM responses were unaffected. Anti-La(SSB) and anti-DNA were detected in PWM-stimulated cultures from both study groups. In the presence of La(SSB) IgM anti-La(SSB) synthesis was enhanced in anti-La(SSB)-positive patients in a dose-dependent manner. In contrast, La(SSB) inhibited anti-La(SSB) production by controls (maximal at 2 ng/ml). La(SSB) had no effect on anti-DNA production in either group. Pre-incubation of control or anti-La(SSB)-positive SLE MNC with La(SSB) before addition to autologous PWM-driven cultures did not induce suppressor cells, although pre-incubation with Concanavalin A (ConA) did. Thus we suggest that La(SSB)-induced suppression of IgG synthesis in PWM-driven control cultures may not be due to induction of regulator cells, possibly missing from SLE cultures, but perhaps is a direct effect on B cells.

SS-B (La) nuclear antigen. Organization in structural domains of the protein moiety

Stable degradation products, obtained by digestion with endogenous and V8 proteases of calf thymus SS-B (La) antigenic protein, have been studied. The most characteristic fragments have molecular masses of 47, 30, 23 and 17 kDa. The 47-kDa and 30-kDa fragments are complex and are constituted of a number of species of different isoelectric points, as has been described for the SS-B protein molecule from other sources. Degradation products from the entire SS-B nuclear antigen still contain the 30-kDa SS-B fragment, suggesting that the 30-kDa region of the SS-B protein molecule is firmly attached to the RNA moiety. A model is presented that implies the presence of two hinge regions sensitive to proteases and three structural domains that correspond to segments of 30 kDa, 17 kDa and 5-6 kDa.

Multiple phosphorylation of human SS-B/LA autoantigen and its effect on poly(U) and autoantibody binding

The metabolic turnover rates and the effect of in vitro phosphorylation on poly(U) and autoantibody binding of human SS-B/La ribonucleoprotein, an autoantigen expressed in various autoimmune disorders, were studied. The determination of the metabolic turnover rates of SS-B/La protein, SS-B/La protein phosphorylation and RNA binding yielded values of 12.1 h, 3.6 h and 3.7 h, respectively, indicating a possible functional correlation of RNA-binding and phosphorylation. This assumption was confirmed by studies of in vitro phosphorylation using purified SS-B/La protein and purified casein kinase type II as a model system. A high degree of phosphorylation of the SS-B/La protein (molecular mass 49 kDa) substantially diminished its binding capacity for poly[3H]U. However, binding of human autoantibodies against SS-B/La antigen increases 2-fold with increased SS-B/La phosphorylation. Complete phosphorylation in vitro led to partial molecular transformation, yielding an antigenically cross-reacting component with an apparent molecular mass of 51 kDa which could not be detected during in vivo phosphorylation.

Nature of immune complexes in autoimmune chronic active hepatitis

Immune complexes containing antinuclear antibodies have been observed in autoimmune chronic active hepatitis. Using a library of nuclear constituents (deoxyribonucleic acid, small nuclear ribonucleoproteins, and histones), we analyzed the antigens involved in formation of immune complexes. Immune complexes were defined in dissociation experiments, and after separation under dissociating conditions. Immune complexes composed of histones and immunoglobulin G were observed in four of nine immune complex-positive autoimmune chronic active hepatitis sera. Immune complexes containing the small nuclear ribonucleoprotein U1-RNP and immunoglobulin G were observed in two additional samples. Kidney eluates obtained from 2 patients with autoimmune chronic active hepatitis and membranous glomerulonephritis revealed enrichment of anti-U1-RNP, suggesting specific deposition of this antibody in complexed form. Circulating immune complexes containing histones were observed only in patients with autoimmune chronic active hepatitis-associated sicca syndrome; those containing U1-RNP were restricted to patients with autoimmune chronic active hepatitis associated with kidney disease.

Antibodies to La, Jo-1, nRNP and Sm detected by multi-track immunoblotting using a novel filter holder: a comparative study with counterimmunoelectrophoresis and immunodiffusion using sera from patients with systemic lupus erythematosus and Sjögren's syndrome

The use of Western blotting or immunoblotting to detect autoantibodies in the serum of patients with autoimmune connective tissue diseases was investigated. An apparatus suitable for simultaneously screening 16 sera on immunoblots was used to show that a complex pattern of antibody binding polypeptides was present in whole HeLa cells. A simpler and readily interpreted pattern of binding was achieved using affinity-purified rabbit thymus antigens. Seventy-seven patients with systemic lupus erythematosus, 44 with primary Sjögren's syndrome and 50 normals were screened for anti-Sm, anti-La, anti-nRNP and anti-Jo-1 by immunoblotting and the results compared with those obtained by counterimmunoelectrophoresis and immunodiffusion. It was shown that both IgG and IgM antibodies must be analysed on immunoblots to detect the maximum number of positive sera, and that the immunoblot detects many anti-La sera which do not form precipitins.

Nuclear antigens recognized by antibodies present in liver disease sera

Nuclear and nuclear matrix proteins of HeLa cells were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, and subsequently transferred onto nitrocellulose. Antibodies present in sera of patients with primary biliary cirrhosis and autoimmune chronic active hepatitis reacted with some of the blotted proteins. The antibodies were mainly directed against chromatin-associated proteins and protein constituents of discrete RNP particles. In addition, antibodies found in autoimmune liver disease sera detected a hitherto undescribed nuclear protein of 54 kD, and a nuclear matrix protein of approximately 150 kD. Antibodies recognizing a nuclear 25 kD doublet apparently constituted a marker antibody for autoimmune liver disease. Those directed at the 17 kD centromere protein were associated with the primary biliary cirrhosis-related CREST syndrome, while those recognizing La antigen were related to cases of sicca syndrome associated with autoimmune liver diseases.

Antibody class differences are detected in anti-nRNP and anti-Sm antibodies directed against distinct antigen subunits

Antibodies to nRNP, Sm and La were detected and characterised by immunoblot analysis. A comparison was made between IgG and IgM autoantibodies in 77 patients with systemic lupus erythematosus (SLE) and 50 normal subjects. No antibodies were detected in the normal subjects. In all 3 antigen specificity groups, a heterogeneity of antibody class was observed between patients. Antibodies to the 2 nRNP-specific polypeptides (33 and 67 kD) were approximately equally frequent. Although IgG antibodies to the 67 kD polypeptide were detected in 88% of patients with antibodies to this polypeptide, IgG antibodies to the 33 kD polypeptide were only detected in 43% of patients with antibodies to this polypeptide. This suggested either that anti-33 kD antibody is produced by a B cell which cannot mature to an IgG-secretor, or that anti-33 kD antibody production succeeds an initial immune response producing anti-67 kD. Reactivity with the 29 kD, Sm-specific polypeptide appeared to be the most frequent in anti-Sm sera compared with the 16 kD polypeptide suggesting that this polypeptide may be the primary immunogenic component of Sm.