Lupus antiribosomal P antisera contain antibodies to a small fragment of 28S rRNA located in the proposed ribosomal GTPase center

Neuronal BC RNA Transport Impairments Caused by Systemic Lupus Erythematosus Autoantibodies

The etiology of the autoimmune disorder systemic lupus erythematosus (SLE) remains poorly understood. In neuropsychiatric SLE (NPSLE), autoimmune responses against neural self-antigens find expression in neurological and cognitive alterations. SLE autoantibodies often target nucleic acids, including RNAs and specifically RNA domains with higher-order structural content. We report that autoantibodies directed against neuronal regulatory brain cytoplasmic (BC) RNAs were generated in a subset of SLE patients. By contrast, anti-BC RNA autoantibodies (anti-BC abs) were not detected in sera from patients with autoimmune diseases other than SLE (e.g., rheumatoid arthritis or multiple sclerosis) or in sera from healthy subjects with no evidence of disease. SLE anti-BC abs belong to the IgG class of immunoglobulins and target both primate BC200 RNA and rodent BC1 RNA. They are specifically directed at architectural motifs in BC RNA 5' stem-loop domains that serve as dendritic targeting elements (DTEs). SLE anti-BC abs effectively compete with RNA transport factor heterogeneous nuclear ribonucleoprotein A2 (hnRNP A2) for DTE access and significantly diminish BC RNA delivery to synapto-dendritic sites of function. In vivo experiments with male BALB/c mice indicate that, upon lipopolysaccharide-induced opening of the blood-brain barrier, SLE anti-BC abs are taken up by CNS neurons where they significantly impede localization of endogenous BC1 RNA to synapto-dendritic domains. Lack of BC1 RNA causes phenotypic abnormalities including epileptogenic responses and cognitive dysfunction. The combined data indicate a role for anti-BC RNA autoimmunity in SLE and its neuropsychiatric manifestations.SIGNIFICANCE STATEMENT Although clinical manifestations of neuropsychiatric lupus are well recognized, the underlying molecular-cellular alterations have been difficult to determine. We report that sera of a subset of lupus patients contain autoantibodies directed at regulatory brain cytoplasmic (BC) RNAs. These antibodies, which we call anti-BC abs, target the BC RNA 5' domain noncanonical motif structures that specify dendritic delivery. Lupus anti-BC abs effectively compete with RNA transport factor heterogeneous nuclear ribonucleoprotein A2 (hnRNP A2) for access to BC RNAs. As a result, hnRNP A2 is displaced, and BC RNAs are impaired in their ability to reach synapto-dendritic sites of function. The results reveal an unexpected link between BC RNA autoantibody recognition and dendritic RNA targeting. Cellular RNA dysregulation may thus be a contributing factor in the pathogenesis of neuropsychiatric lupus.

Characterization of anti-P monoclonal antibodies directed against the ribosomal protein-RNA complex antigen and produced using Murphy Roths large autoimmune-prone mice

Autoantibodies, including anti-ribosomal P proteins (anti-P), are thought to be produced by an antigen-driven immune response in systemic lupus erythematosus (SLE). To test this hypothesis, we reconstituted the ribosomal antigenic complex in vitro using human P0, phosphorylated P1 and P2 and a 28S rRNA fragment covering the P0 binding site, and immunized Murphy Roths large (MRL)/lrp lupus mice with this complex without any added adjuvant to generate anti-P antibodies. Using hybridoma technology, we subsequently obtained 34 clones, each producing an anti-P monoclonal antibody (mAb) that recognized the conserved C-terminal tail sequence common to all three P proteins. We also obtained two P0-specific monoclonal antibodies, but no antibody specific to P1, P2 or rRNA fragment. Two types of mAbs were found among these anti-P antibodies: one type (e.g. 9D5) reacted more strongly with the phosphorylated P1 and P2 than that with their non-phosphorylated forms, whereas the other type (e.g. 4H11) reacted equally with both phosphorylated and non-phosphorylated forms of P1/P2. Both 9D5 and 4H11 inhibited the ribosome/eukaryotic elongation factor-2 (eEF-2)-coupled guanosine triphosphate (GTP)ase activity. However, preincubation with a synthetic peptide corresponding to the C-terminal sequence common to all three P proteins, but not the peptide that lacked the last three C-terminal amino acids, mostly prevented the mAb-induced inhibition of GTPase activity. Thus, at least two types of anti-P were produced preferentially following the immunization of MRL mice with the reconstituted antigenic complex. Presence of multiple copies of the C-termini, particularly that of the last three C-terminal amino acid residues, in the antigenic complex appears to contribute to the immunogenic stimulus.

Autoantibody explosion in systemic lupus erythematosus: more than 100 different antibodies found in SLE patients

OBJECTIVE

Description of the various autoantibodies that can be detected in patients with systemic lupus erythematosus (SLE).

METHODS

A literature review, using the terms "autoantibody" and "systemic lupus erythematosus", was conducted to search for articles on autoantibodies in SLE, their target antigens, association with disease activity, or other clinical associations.

RESULTS

One hundred sixteen autoantibodies were described in SLE patients. These include autoantibodies that target nuclear antigens, cytoplasmic antigens, cell membrane antigens, phospholipid-associated antigens, blood cells, endothelial cells, and nervous system antigens, plasma proteins, matrix proteins, and miscellaneous antigens. The target of autoantibody, the autoantigen properties, autoantibody frequencies in SLE, as well as clinical associations, and correlation with disease activity are described for all 116 autoantibodies.

CONCLUSIONS

SLE is the autoimmune disease with the largest number of detectable autoantibodies. Their production could be antigen-driven, the result of polyclonal B cell activation, impaired apoptotic pathways, or the outcome of idiotypic network dysregulation.

Ribosomal proteins at the stalk region modulate functional rRNA structures in the GTPase center

Replacement of the L10.L7/L12 protein complex and L11 in Escherichia coli ribosomes with the respective rat counterparts P0.P1/P2 and eukaryotic L12 causes conversion of ribosomal specificity for elongation factors from prokaryotic elongation factor (EF)-Tu/EF-G to eukaryotic EF (eEF)-1alpha/eEF-2. Here we have investigated the effects of protein replacement on the structure and function of two rRNA domains around positions 1070 and 2660 (sarcin/ricin loop) of 23 S rRNA. Protein replacement at the 1070 region in E. coli 50 S subunits was demonstrated by chemical probing analysis. Binding of rat proteins to the 1070 region caused increased accessibility of the 2660 and 1070 regions to ligands for eukaryotic ribosomes: the ribotoxin pepocin for the 2660 region (E. coli numbering), anti-28 S autoantibody for the 1070 region, and eEF-2 for both regions. Moreover, binding of the E. coli L10.L7/L12 complex and L11 to the 1070 region was shown to be responsible for E. coli ribosomal accessibility to another ribotoxin, gypsophilin. Ribosomal proteins at the 1070 region appear to modulate the structures and functions of the 2660 and 1070 RNA regions in slightly different modes in prokaryotes and eukaryotes.

Immunoscreening of phage-displayed cDNA-encoded polypeptides identifies B cell targets in autoimmune disease

Characterisation of self-antigens can contribute to an understanding of the aetiology of autoimmune disorders as well as to the development of new therapies and diagnostic methods. The present study was undertaken to investigate the applicability of complementary DNA (cDNA) phage-display technology to the identification of autoantigens recognised by the humoral response in autoimmune disease. Using systemic lupus erythematosus (SLE) as a model system, a pool of patient immunoglobulin G (IgG) was biopanned on a fibroblast cDNA phage-display library constructed in the vector pJuFo. Following three rounds of biopanning, recovered cDNAs were sequenced and then identified using BLAST comparisons with international databases. Both previously reported SLE autoantigens, for example, alpha-enolase and U1 small nuclear ribonucleoprotein-C (U1snRNP-C), and novel autoantibody targets, including ribosomal protein S20 (RPS20), ribosomal protein S13 (RPS13), ubiquitin-like protein PIC1 (PIC1), and transcription factor-like protein MRG15 (MRG15), were recovered from the biopanning procedure. Radiobinding assays were used subsequently to confirm the reactivity of some putative autoantigens to panels of sera from SLE patients, control patient groups, and healthy individuals. SLE patient sera were positive for reactivity to: U1snRNP-C, 4/15 (27%); alpha-enolase, 1/15 (7%); RPS20, 3/15 (20%); RPS13, 1/15 (7%); PIC1, 1/15 (7%); and MRG15, 2/15 (13%). Overall, cDNA phage-display technology appears to be applicable to the identification of autoantigens in autoimmune disease.

Mouse antibody response to a microsporidian parasite following inoculation with a gene coding for parasite ribosomal RNA

This study found that a plasmid construct encoding the small-subunit ribosomal RNA (SSUrRNA) of the microsporidian Microgemma caulleryi generates a humoral response upon intramuscular inoculation in mice. The plasmid used was pCMV, following preliminary trials indicating efficient beta-galactosidase gene expression in mouse muscle cells transfected with pCMV/beta-Gal. The antibodies produced after inoculation with pCMV/SSUDNA recognized parasite spore antigens and reached maximum levels at 30 days postinoculation, subsequently remaining stable for at least 120 days. Due to the highly conserved sequence of the SSUrDNA in different microsporidian species, these results open up interesting prospects for broad-spectrum vaccination.

Autoimmune epitopes in messenger RNA

Patients with systemic autoimmune disorders produce autoantibodies against sequence-specific conformational RNA epitopes on U1 snRNA, 28S rRNA, and transfer RNAs. The molecular basis for immunological reactivity with these highly abundant and stable RNAs is not understood. Here, we report the existence of discrete RNA epitopes in messenger RNAs that are generally less abundant and less stable than snRNAs and tRNAs. An iterative selection and amplification procedure using pooled autoimmune patient sera identified immunoreactive mRNA species. Following deconvolution of the pools to identify the reactive sera, several mRNAs recognized by these autoantibodies were cloned and sequenced. Detailed analysis using one particular serum indicated reactivity against the messages encoding alternative splicing factor (ASF/SF2) and calmodulin. Deletion and site-directed mutagenesis determined that an epitope recognized by this serum is located in a 17-base stem-loop structure common to both messages. This serum was then used to immunoprecipitate native mRNAs encoding ASF/SF2 and calmodulin from total HeLa cell RNA. Our results demonstrate that despite its low abundance and instability, messenger RNA is capable of reacting with autoantibodies generated during an autoimmune response. These data are consistent with direct presentation as a model to explain the generation of RNA conformation-specific autoantibodies.

Cryptic peripheral ribosomal domains distributed intermittently along mammalian myelinated axons

A growing body of metabolic and molecular evidence of an endogenous protein-synthesizing machinery in the mature axon is a challenge to the prevailing dogma that the latter is dependent exclusively on slow axoplasmic transport to maintain protein mass in a steady state. However, evidence for a systematic occurrence of ribosomes in mature vertebrate axons has been lacking until recently, when restricted ribosomal domains, called "periaxoplasmic plaques," were described in goldfish CNS myelinated axons. Comparable restricted RNA/ribosomal "plaque" domains now have been identified in myelinated axons of lumbar spinal nerve roots in rabbit and rat on the basis of RNase sensitivity of YOYO-1-binding fluorescence, immunofluorescence of ribosome-specific antibodies, and ribosome phosphorus mapping by electron spectroscopic imaging (ESI). The findings were derived from examination of the axoplasm isolated from myelinated fibers as axoplasmic whole mounts and delipidated spinal nerve roots. Ribosomal periaxoplasmic plaque domains in rabbit axons were typically narrow ( approximately 2 microm), elongated ( approximately 10 microm) sites that frequently were marked by a protruding structure. The domain complexity included an apparent ribosome-binding matrix. The small size, random distribution, and variable intermittent axial spacing of plaques around the periphery of axoplasm near the axon-myelin border are likely reasons why their systematic occurrence has remained undetected in ensheathed axons. The periodic but regular incidence of ribosomal domains provides a structural basis for previous metabolic evidence of protein synthesis in myelinated axons.

Autoantibodies to the 20-kDa ribosomal proteins: identification, characterization, and new aspects on prevalence in systemic Lupus erythematosus

Autoantibodies to the 20-kDa ribosomal proteins (L12/S10) are not well studied, especially in juveniles with systemic lupus erythematosus (SLE). Randomly selected sera from American juveniles and adults with SLE were screened for antibodies to either 20-kDa protein and P proteins and then assayed for anti-L12 and anti-S10 by immunoblot assays. In a pilot study of patients with anti-P (Cohort 1), IgG antibodies to either 20-kDa protein and, specifically, to L12 were observed in 72 and 42% of juveniles and adults, respectively. IgG antibodies to S10 were detected less frequently. In Cohort 2 patients who were chosen irrespective of autoantibody status, twice as many juveniles as adults had IgG antibodies to either 20-kDa protein. Prevalences of IgG anti-L12 and IgG anti-S10 antibodies in the juveniles were 28 and 16% and in the adults were 13 and 12%, respectively. Anti-L12 were strongly but not invariably associated with anti-P, and usually arose temporally to these antibodies. Anti-S10 activity was due to anti-Sm antibodies. We conclude that IgG anti-L12 are more prevalent in SLE than previously reported, and are responsible for the majority of activity toward the 20-kDa ribosomal proteins, especially in juveniles.

Monoclonal antibodies derived from BALB/c mice immunized with apoptotic Jurkat T cells recognize known autoantigens

It has been postulated that post-translational modifications and relocalization of proteins during apoptosis may lead to presentation of these molecules to the immune system in such a way that normal mechanisms of tolerance are bypassed. In the present study, Jurkat cells were induced to undergo apoptosis by treatment with the chemotherapeutic agent Ara-C. BALB/c mice were then immunized with the apoptotic cells and hybridomas were generated. Using an indirect immunofluorescence assay, the monoclonal antibodies produced were screened by flow cytometry for those monoclonal antibodies demonstrating reactivity with permeabilized apoptotic Jurkat cells but not with non-permeabilized normal or apoptotic Jurkat cells. Of 281 monoclonal antibodies, 20 monoclonal antibodies with these properties were selected for further analysis. Using 32P- or 35S-metabolically labelled Jurkat cells, these selected monoclonal antibodies were screened for their ability to recognize autoantigens by immunoprecipitation and Western blotting. Well characterized autoimmune sera were then used to confirm the identity of autoantigens by immunoblotting. We demonstrate that immunization of normal mice with apoptotic Jurkat cells results in the formation of antibodies targeting multiple autoantigens or autoantigen complexes, including Ku, rRNPs, snRNPs and vimentin. These findings are consistent with the hypothesis that apoptosis can contribute to the development of autoimmunity.

Cryptic peripheral ribosomal domains distributed intermittently along mammalian myelinated axons

A growing body of metabolic and molecular evidence of an endogenous protein-synthesizing machinery in the mature axon is a challenge to the prevailing dogma that the latter is dependent exclusively on slow axoplasmic transport to maintain protein mass in a steady state. However, evidence for a systematic occurrence of ribosomes in mature vertebrate axons has been lacking until recently, when restricted ribosomal domains, called "periaxoplasmic plaques," were described in goldfish CNS myelinated axons. Comparable restricted RNA/ribosomal "plaque" domains now have been identified in myelinated axons of lumbar spinal nerve roots in rabbit and rat on the basis of RNase sensitivity of YOYO-1-binding fluorescence, immunofluorescence of ribosome-specific antibodies, and ribosome phosphorus mapping by electron spectroscopic imaging (ESI). The findings were derived from examination of the axoplasm isolated from myelinated fibers as axoplasmic whole mounts and delipidated spinal nerve roots. Ribosomal periaxoplasmic plaque domains in rabbit axons were typically narrow ( approximately 2 microm), elongated ( approximately 10 microm) sites that frequently were marked by a protruding structure. The domain complexity included an apparent ribosome-binding matrix. The small size, random distribution, and variable intermittent axial spacing of plaques around the periphery of axoplasm near the axon-myelin border are likely reasons why their systematic occurrence has remained undetected in ensheathed axons. The periodic but regular incidence of ribosomal domains provides a structural basis for previous metabolic evidence of protein synthesis in myelinated axons.

Caspase-dependent cleavage of nucleic acids

Autoimmune diseases are frequently characterized by the presence of autoantibodies directed against nucleic acid-protein complexes present in the nucleus of the cell. The mechanisms by which these autoantigenic molecules escape immunological tolerance are largely unknown, although a number of recent observations suggest that modified self-proteins generated during apoptosis may play an important role in the development of autoimmunity. It has been hypothesized that the recognition of these modified self-proteins by the immune system may promote autoantibody production. While apoptosis is specifically characterized by posttranslational modification of proteins, recent findings also show that nucleic acids are modified. This review summarizes the specific cleavages of some of these key nucleic acids, i.e. chromosomal DNA, ribosomal RNA and small structural RNAs (U1 snRNA, Y RNA), in apoptotic cells.

[Clinical significance of antiribosomal antibodies. Study Group on Autoimmunity (GEAI)]

PURPOSE

Autoantibodies directed against the ribosomal P proteins, P0, P1 and P2 (anti-P), have been related to lupus-related psychosis and/or depression. The diagnostic value of antibodies directed against other ribosomal proteins or 28S RNA (anti-no-P) remains unknown. A multicenter study including ten centers belonging to the study group for autoimmune diseases (GEAI) was conducted in order to determine the diagnostic value of anti-P and anti-no-P antibodies in a large population of patients.

METHODS

The patients were selected on the basis of the presence of serum anti-ribosomal antibodies detected by indirect immunofluorescence (IF) on rat liver/kidney/stomach/pancreas sections and human HEp2 cells. The clinical course of all patients was studied using a predetermined survey. The specificity of anti-P antibodies were determined by Western blot.

RESULTS

Anti-ribosomal antibodies were found in 82 patients. Fifty-five of them had systemic lupus erythematosus and 27 had another disease. Only 54% of the anti-ribosomal antibodies detected by IF were anti-P and were found in 69% of the patients with systemic lupus erythematosus. Anti-no-P antibodies (46%) were preferably detected in patients who suffered from another disease (78%). In patients with systemic lupus erythematosus, neurological and psychiatric disorders were more frequent in the no-P group (47% vs. 16%, P < 0.01) than arthritis, which was found more frequently in the P group (78% vs. 53%, P < 0.05).

CONCLUSION

Anti P antibodies do not constitute a specific diagnostic marker of systemic lupus erythematosus, and lupus-related neuropsychiatric disorders would be preferably associated with the presence of anti no-P antibodies.

Inosine and N1-methylinosine within a synthetic oligomer mimicking the anticodon loop of human tRNA(Ala) are major epitopes for anti-PL-12 myositis autoantibodies

Sera of some patients afflicted with the inflammatory disease myositis contain antibodies of the anti-PL-12 type. A fraction of these polyclonal autoantibodies specifically precipitates the fully matured human tRNA(Ala) bearing the anticodon IGC (PL-12 antigen). Earlier work (Bunn & Mathews, 1987, Science 238:116-119) had shown that the epitopes are located entirely within the anticodon stem-loop of the tRNA(Ala). Here we demonstrate that human anti-tRNA(Ala) autoantibodies immunoprecipitate a synthetic polyribonucleotide containing inosine (I) and N1-methylinosine (m1I) separated by 2 nt as in the anticodon stem-loop of human tRNA(Ala). The shortest polyribonucleotide that can be immunoprecipitated corresponds to the pentanucleotide IpGpCpm1IpUp, which corresponds to part of the anticodon loop of human tRNA(Ala) and lacks the stem-loop structure. The efficiency of immunoprecipitation was about four times greater with longer polyribonucleotides capable of forming a stem-loop structure, and was abolished by altering the relative positions of I and m1I within the synthetic polynucleotide. Synthetic oligodeoxyribonucleotide analogs of the tRNA(Ala) stem-loop, containing the sequence dIpdGdCdm1Ip, are not antigenic. Our results show that human anti-tRNA(Ala) autoantibodies selectively recognize chemical details of modified nucleotides (the 6-keto group of inosine-34 and the 6-keto group and the N1-methyl groups of N1-methylinosine-37) within an anticodon loop structure of a tRNA molecule. We also describe the chemical synthesis of the phosphoramidite derivatives corresponding to N1-methylinosine and N1-methyl-2'-deoxyinosine for use in the automatic chemical synthesis of oligonucleotides containing N1-methylinosine and N1-methyl-2'-deoxyinosine.

Neuronal polarity: vectorial cytoplasmic flow precedes axon formation

Axon formation in multipolar neurons is believed to depend on the existence of precise sorting mechanisms for axonal membrane and membrane-associated proteins. Conclusive evidence in living neurons, however, is lacking. In the present study, we use light and video microscopy to address this issue directly. We show that axon formation is preceded by the appearance in one of the multiple neurites of (1) a larger growth cone, (2) a higher amount and greater transport of membrane organelles, (3) polarized delivery of TGN-derived vesicles, (4) a higher concentration of mitochondria and peroxisomes, (5) a higher concentration of a cytosolic protein, and (6) a higher concentration of ribosomes. These results provide evidence for the involvement of bulk cytoplasmic flow as an early determinant of neuronal morphological polarization. Molecular sorting events would later trigger the establishment of functional polarity.

Binding of mammalian ribosomal protein complex P0.P1.P2 and protein L12 to the GTPase-associated domain of 28 S ribosomal RNA and effect on the accessibility to anti-28 S RNA autoantibody

We have investigated binding of rat ribosomal proteins to the "GTPase domain" of 28 S rRNA and its effect on accessibility to the anti-28 S autoantibody, which recognizes a unique tertiary structure of this RNA domain. Ribosomal protein L12 and P protein complex (P complex) consisting of P0, P1, and P2 both bound to the GTPase domain of rat 28 S rRNA in a buffer containing Mg2. Chemical footprinting analysis of their binding sites revealed that the P complex mainly protected a conserved internal loop region comprising residues 1855-1861 and 1920-1922, whereas L12 protected an adjacent helix region encompassing residues 1867-1878 and 1887-1899. These sites are close to but distinct from the binding site for anti-28 S antibody determined previously. The bindings of P complex and L12 increased the anti-28 S accessibility, as revealed by gel retardation and quantitative immunoprecipitation analyses. In a Mg2+-eliminated condition, the RNA failed to bind to either anti-28 S or L12 but assembled into a complex under their coexistence. However, the RNA retained a property of binding to the P complex even in the absence of Mg2+, and this binding conferred high anti-28 S accessibility. These results indicated that the bindings of the P complex and L12 to their respective sites influenced the GTPase domain to increase the accessibility to anti-28 S. A possible RNA conformation adjusted by the protein bindings is discussed.

RNA surfaces as functional mimetics of proteins

Accumulating evidence suggests that RNA molecules can form surfaces that mimic those of proteins. Reactivity of autoantibodies with RNA surfaces may be due to cross-reactivity between a protein epitope and the RNA. The structural mimicry detected by an autoantibody may reflect functional mimicry.

Analysis of anti-U1 RNA antibodies in patients with connective tissue disease. Association with HLA and clinical manifestations of disease

OBJECTIVE

To determine the prevalence of anti-U1 RNA antibodies in connective tissue disease (CTD) patients and evaluate immunogenetic and clinical features of patients possessing these antibodies.

METHODS

RNA immunoprecipitation was used to analyze patient and healthy control sera for the presence of anti-R1 RNA antibodies. Enzyme-linked immunosorbent assay and immunoblotting were used to determine small nuclear RNP (snRNP) polypeptide antibodies. HLA polymorphisms were determined by microcytotoxicity and DNA typing.

RESULTS

Anti-U1 RNA IgM and IgG antibodies were found in 60% of anti-RNP positive patients. All of the anti-U1 RNA positive patients had anti-70K, and most had anti-A, (U1)snRNP polypeptide antibodies. HLA-DR2/DR4, as well as Raynaud's phenomenon and synovitis, were significantly increased in the anti-U1 RNA positive group.

CONCLUSION

The presence of anti-U1 RNA antibodies correlates with anti-70K and anti-A polypeptide antibodies. In addition, the anti-U1 RNA positive CTD patient group is immunogenetically and clinically distinctive from the anti-U1 RNA negative patient group.

Non-exclusive Fas control and age dependence of viral superantigen-induced clonal deletion in lupus-prone mice

To investigate the role of Fas in the induction of tolerance by viral superantigen (SAG), we infected MRL-+/+ and MRL-lpr (Fas mutant) mice with mouse mammary tumor virus (MMTV) (SW), a virus encoding an SAG with the same specificity as endogenous Mtv-7-SAG. In normal mice, this infection has two distinct consequences on specific V beta 6+CD4+ T cells, consisting of activation followed by clonal deletion. MMTV (SW)-SAG-induced activation in vivo was identical in MRL-+/+ and MRL-lpr mice. In contrast, clonal deletion showed age-dependent impairment. Early infection (5 weeks) led to identical clonal deletion of specific T cells in blood lymphocytes from MRL-+/+ and MRL-lpr mice, although clonal deletion was slightly impaired in the MRL-lpr lymph nodes. Late infection (10 weeks) of MRL-lpr mice led to markedly delayed and reduced clonal deletion. V beta 6+CD4+ T cells which escaped clonal deletion in aging MRL-lpr mice were not anergized by interaction with SAG. These results show that peripheral clonal deletion induced by viral SAG in adult mice is controlled by Fas, but not exclusively so.

The basis of autoimmunity: Part II. Genetic predisposition

In Part II of his review of the basis of autoimmunity, Argyrios N. Theofilopoulos summarizes current knowledge on the genetic factors that contribute to autoimmune disease predisposition. The findings indicate that multiple genes contribute to the induction of pathogenic autoimmunity, and that no single genetic abnormality is sufficient in itself to induce disease. The definition of these genetically complex diseases is about to be revolutionized by the development of genome scanning approaches, such as dense chromosomal maps based on polymorphic microsatellite DNA and other informative markers. These will allow the loci and genes that predispose to these diseases to be identified broadly.

Autoantibodies directed against ribosomal P proteins: use of a multiple antigen peptide as the coating agent in ELISA

Autoantibodies directed against the ribosomal proteins P0, P1 and P2 (P proteins) are specific for systemic lupus erythematosus (SLE) and there are some evidences that they could be related to the neuropsychiatric manifestations of the disease. In this study, a multiple antigen peptide (MAP) carrying four copies of the C-terminal peptide (13 residues) of the P2 protein, which is a common epitope of the three P proteins, was prepared for use in an ELISA assay. It was employed to detect antibodies directed against the ribosomal P proteins in 102 SLE patients and the results were compared with those obtained using immunoblotting (IB). With this new ELISA, antiribosomal P protein antibodies were found in 15/102 SLE sera. These results correlated well with the results of IB. Furthermore, we confirmed that naturally occurring antiribosomal P protein antibodies are directed mainly against the epitope containing the C-terminal sequence and shared by the three P proteins. MAP appears to be an excellent coating agent for ELISA assays designed to detect anti-P antibodies. Further experiments showed the superiority of MAP, compared to the free peptide, in the detection of weakly positive sera. This ELISA can also be used for the serological follow-up of SLE patients.

Autoantibodies in human anti-Ro sera specifically recognize deproteinized hY5 Ro RNA

We report the existence of a novel autoantibody specificity linked to anti-Ro antibodies. Sera from two patients with anti-Ro ribonucleoprotein (RNP) antibodies also contained antibodies that immunoprecipitated specifically either the deproteinized RNA component of the RohY5 RNP particle, or intact in vitro transcribed hY5 RNA. No serum recognized specifically the other hY RNAs. A mutant hY5 RNA with additional nucleotides (nt) at both extremities was not immunoprecipitated, possibly because of altered secondary structure. Following digestion of hY5 RNA with ribonuclease T1, the smallest immunoprecipitable RNA fragments were 27 and 31 nt long, and respectively mapped to the 5' and 3' ends of hY5 RNA, excluding the La-binding region. Base pairing between the 27 and 31 nt long fragments was required for recognition by antibodies. Our data indicate that the epitope bound by anti-hY5 RNA antibodies is conformational. We have previously reported that most anti-Ro sera contain a population of antibodies specific for the RohY5 RNP. Since antibodies to the deproteinized hY RNAs within anti-Ro sera are also restricted to anti-hY5 RNA, a direct role for the human-specific RohY5 particles in the immunization process leading to the production of anti-Ro antibodies is suggested.

Serological association of lupus autoantibodies to a limited functional domain of 28S ribosomal RNA and to the ribosomal proteins bound to the domain

Site-specific anti-RNA antibodies were sought in 120 sera of patients with autoimmune diseases by ribonuclease-protection assay using six fragments covering 28S ribosomal RNA (rRNA) as antigens. Fifteen of 90 sera from patients with systemic lupus erythematosus (SLE), but none of 30 sera of the other autoimmune diseases, provided a 60 nucleotide fragment within a region termed the 'GTPase domain' of 28S rRNA. These sera had potency to precipitate 0.42-69.3 nmol of the RNA domain per ml serum, which was higher than 15 control sera of healthy donors. No other specific antigenic site was detected in 28S rRNA under conditions used. All of the 15 sera having this anti-RNA antibody showed reactivity to ribosomal P proteins (anti-P), and two of them contained an additional antibody to ribosomal protein L12. These results suggested a strong association of the production of these three antibodies. Since P and L12 proteins form a stable complex with the GTPase domain, this serological association may result from an immune response to epitopes clustered on a single RNA-protein complex domain in ribosomes.

Autoantigen-specific T cell proliferation induced by the ribosomal P2 protein in patients with systemic lupus erythematosus

A role for helper T cells in the induction of pathogenic lupus autoantibodies is increasingly supported by data from studies of murine lupus and patients with systemic lupus erythematosus (SLE). However, the poor in vitro function of SLE T cells has hampered the identification and characterization of autoantigen-specific T cells. We used recombinant fusion proteins to study the T cell proliferative response of 31 lupus patients and 27 healthy subjects to a well-characterized SLE autoantigen, the ribosomal P2 protein. Although PBMC from SLE patients showed marked impairment in the proliferative response to the common recall antigen tetanus toxoid when compared with normal subjects, a significantly greater proportion of SLE patients (32%) than normal individuals (0%) showed a T cell response to a recombinant P2 fusion protein. When the SLE patients were subgrouped according to the presence of serum anti-P autoantibody, 7 of 10 anti-P antibody-positive patients, but 0 of 20 anti-P antibody-negative SLE patients, demonstrated > 2,000 cpm [3H]thymidine incorporation and a P2 stimulation index > 5. The specificity of the T cell proliferative response for the P2 protein was confirmed by studies using a second recombinant human P2 fusion protein and by the specific activation of P2-primed T cells by recombinant P2 in secondary cultures. Moreover, the T cell proliferative response to the P2 autoantigen was mediated by CD4-positive T cells and was inhibited by anti-MHC class II antibodies. These data demonstrate the presence of autoantigen-specific T helper cells in patients with SLE and suggest that these T cells drive the production of autoantibodies by B lymphocytes.

Antiribosomal antibodies in SLE, infection, and following deliberate immunization

ARA occur in approximately 10% of randomly selected SLE patients but in up to 40% of patients with active disease. Anti-P antibodies appear to be a highly specific diagnostic marker for SLE since they are rarely detected in other multisystem autoimmune disorders. ARA are most frequently directed against the P proteins and the shared conserved C-terminus of the P proteins is immunodominant in almost all sera tested. Anti-P antibodies increase in titer in patients with active disease and have been reported to be detected more frequently in patients with severe behavioral disturbances. This may be particularly true of patients with affective disorders. The clinical utility of serological tests for anti-P in central nervous system lupus must await large, prospective studies. Other ARA antibodies have been detected in patients with SLE. These antibodies include anti-28S rRNA, anti-S10, and anti-L12. In all cases, the frequency with which these antibodies are detected is increased in sera containing anti-P. The P proteins and the 28S rRNA epitope play essential, but as yet undefined, roles in GTPase activity on the ribosome. The L12 protein is the mammalian homologue of the E. coli and yeast proteins known to bind to the 28S rRNA epitope. These findings indicate that some SLE patients produce autoantibodies against multiple components of a functionally related domain of the ribosome. This, in turn, supports the notion that the ribosome initiates and/or maintains autoantibody production. Despite the evidence supporting an antigen driven immune response, attempts to induce anti-P antibodies by immunization with autologous ribosomes in the autoimmune strain of mouse, MRL, have been unsuccessful. It therefore seems likely that the ribosomal components must be altered in some way to break tolerance or that other abnormalities of the immune system are necessary for autoantibody production. Immunization with foreign ribosomes induce anti-P autoantibodies in mice and in apparently normal humans infected with the hemoflaggelate, T. cruzi. The ability of the P proteins to break tolerance in these situations is, most likely, explained by the provision of a T cell epitope (the foreign P protein) together with the multivalency of the P proteins on the ribosome (which activate autoreactive B cells). We therefore propose (Fig. 5) a two-signal model for autoantibody production similar to that suggested for T-B collaboration in the normal immune response and also in the GVHD model of lupus.(ABSTRACT TRUNCATED AT 400 WORDS)

The Fas protein is expressed at high levels on CD4+CD8+ thymocytes and activated mature lymphocytes in normal mice but not in the lupus-prone strain, MRL lpr/lpr

The lymphoproliferation (lpr) mutation in the MRL strain of mice is caused by the insertion of the early transposable element ETn in the Fas gene. The insertion causes a striking decrease in Fas mRNA expression and is associated clinically with marked acceleration of the lupus-like disease. To further explore the role of the Fas protein in T-cell selection in the thymus and tolerance in the peripheral immune system, we produced a monospecific polyclonal anti-murine Fas antibody that binds to a polymorphic region of the protein. Fas protein expression was detected on approximately 90% of BALB/c and MRL +/+ thymocytes, and the expression was highest on CD4+CD8+ thymocytes, the stage at which most thymocytes die by apoptosis. In contrast to the high level of expression of Fas on thymocytes, Fas was detected on < 10% of normal splenic T cells. After activation of splenic T cells with Con A or anti-CD3 and interleukin 2, Fas expression increased approximately 10-fold. Fas expression on splenic B cells was also markedly up-regulated after activation with lipopolysaccharide or anti-mu antibodies. The Fas protein was not detected on resting or activated lymphocytes obtained from MRL lpr/lpr mice. Together, these findings suggest that Fas plays a role in both thymic selection and T-cell survival in the periphery and that the accelerated autoimmunity in MRL lpr/lpr mice results from a defect in both of these pathways.

B-cell responses in autoimmune diseases

Autoantibodies are detected in most autoimmune diseases. Beyond their direct role in mediating tissue damage in some models, the characterization of several intracellular autoantigens and the study of the autoimmune B-cell repertoire favor an antigen-driven B-cell response in most autoimmune models studied.

Epitope regions on U1 small nuclear RNA recognized by anti-U1RNA-specific autoantibodies

Autoantibodies specifically directed to U1RNA were found in patients suffering from systemic lupus erythematosus (SLE) overlap syndromes. To obtain more insight in the mechanism responsible for this U1RNA-specific antibody formation and to use the antibodies eventually as a tool to study U1RNA-protein (U1RNP) interactions, the B cell epitopes on U1RNA were mapped. Using in vitro synthesized domains of U1RNA, the main epitope regions were found in stemloops II and IV. Furthermore, 3'-end or 5'-end truncation of both stemloop II and stemloop IV showed that the conformation of the stemloops is critical for antibody recognition. Mutant studies on both stemloops indicated that in the case of stemloop II the stem is the main antigenic region, whereas in stemloop IV, the loop (E-loop) is a main target. The results of this study support the idea that the anti-U1RNA autoantibody could be the result of a process driven by the human U1RNP complex itself (antigen-driven process).