Human anti-p68 autoantibodies recognize a common epitope of U1 RNA containing small nuclear ribonucleoprotein and influenza B virus

ANA testing in the presence of acute and chronic infections

Autoantibody testing is performed to help diagnose patients who have clinical symptoms suggestive of possible autoimmune diseases. Antinuclear antibodies (ANA) are present in many systemic autoimmune conditions such as systemic lupus erythematosus (SLE). However, a positive ANA test may also be seen with non-autoimmune inflammatory diseases, including both acute and chronic infections. When the ANA test is used as an initial screen in patients with non-specific clinical symptoms, such as fever, joint pain, myalgias, fatigue, rash, or anemia, the likelihood of a positive result due to infection will increase, especially in children. This article identifies acute and chronic infectious diseases that are likely to produce a positive ANA result and summarizes recent literature addressing both the causes and consequences of these findings.

Mapping epitopes of U1-70K autoantibodies at single-amino acid resolution

The mechanisms underlying development of ribonucleoprotein (RNP) autoantibodies are unclear. The U1-70K protein is the predominant target of RNP autoantibodies, and the RNA binding domain has been shown to be the immunodominant autoantigenic region of U1-70K, although the specific epitopes are not known. To precisely map U1-70K epitopes, we developed silicon-based peptide microarrays with >5700 features, corresponding to 843 unique peptides derived from the U1-70K protein. The microarrays feature overlapping peptides, with single-amino acid resolution in length and location, spanning amino acids 110-170 within the U1-70K RNA binding domain. We evaluated the serum IgG of a cohort of patients with systemic lupus erythematosus (SLE; n = 26) using the microarrays, and identified multiple reactive epitopes, including peptides 116-121 and 143-148. Indirect peptide ELISA analysis of the sera of patients with SLE (n = 88) revealed that ∼14% of patients had serum IgG reactivity to 116-121, while reactivity to 143-148 appeared to be limited to a single patient. SLE patients with serum reactivity to 116-121 had significantly lower SLE Disease Activity Index (SLEDAI) scores at the time of sampling, compared to non-reactive patients. Minimal reactivity to the peptides was observed in the sera of healthy controls (n = 92). Competitive ELISA showed antibodies to 116-121 bind a common epitope in U1-70K (68-72) and the matrix protein M1 of human influenza B viruses. Institutional Review Boards approved this study. Knowledge of the precise epitopes of U1-70K autoantibodies may provide insight into the mechanisms of development of anti-RNP, identify potential clinical biomarkers and inform ongoing clinical trails of peptide-based therapeutics.

Escape of pathogens from the host immune response by mutations and mimicry. Possible means to improve vaccine performance

The ability of certain pathogens, such as human immunodeficiency, hepatitis C, herpes simplex, influenza viruses, Plasmodium falciparum, etc., to escape from host immune response is generally ascribed to high mutation rate of their genome. We challenge this assumption and propose that molecular mimicry of host antigens by these pathogens could also participate to this resistance. Several studies show that there is no correlation between the mutation rate value of a pathogen and the possibility to develop an effective vaccine. On the other hand, pathogens which do not respond to vaccine are usually reported to display host protein mimicry. We propose to suppress in the thymus the epitopes of the self which are in common with the pathogen. This could be achieved by intrathymic injection of antibodies against this microorganism. These antibodies would be obtained by vaccination of a foreign animal species. It is expected that the negative selection of the CD4(+) and CD8(+) T lymphocytes specific for these epitopes would be prevented, that the number of epitopes recognized as foreign to the host would be increased and that the immune response diversity would be enhanced.

[New aspects on the pathogenesis of myositis]

Idiopathic inflammatory myopathies (IIM) are chronic inflammatory diseases of muscle characterized by proximal muscle weakness. There are three main groups of diseases, dermatomyositis, polymyositis and inclusion body myositis. The muscle tissue is invaded by the humoral autoantibody producing immune system (B-cells) and by the cellular immune system with autoaggressive and inflammation modulating cells (e.g. dendritic cells, monocytes/macrophages, CD4 + and CD8 + T-cells and natural killer cells). The presence of specific or associated autoantibodies and inflammatory cellular infiltrates with cytotoxic and immune autoreactive properties are characteristic for IIM diseases. The pathogenesis is still unknown; nevertheless, there are several hints that exogenic factors might be involved in initiation and disease progression and bacterial, fungal and viral infections are thought to be possible initiators. Up to now information on prognostic markers to help with decision-making for individual treatment are limited. In addition, there has been only limited therapeutic success including conventional or novel drugs and biologicals and comparative validation studies are needed using similar outcome measurements. Moreover, to facilitate the use and development of novel therapies, elaboration of intracellular and cell-specific regulation could be useful to understand the etiopathogenesis and allow a better diagnosis, prognosis and possibly also a prediction for individualized subgroup treatment.

Potential role of molecular mimicry between human U1-70 kDa and fungal proteins in the development of T-cell mediated anti-U1-70 kDa autoimmunity

CONTEXT

Molecular mimicry between human and microbial antigens is a possible trigger of autoimmunity. The possible role of this mechanism in the onset of autoimmunity against the human autoantigen U1-70 kDa, typical of mixed connective tissue disease, is not fully elucidated.

OBJECTIVE

We aimed to identify microbial proteins highly similar to U1-70 kDa and potentially triggering anti-U1-70 kDa autoimmunity.

MATERIALS AND METHODS

We compared in silico the amino acid sequence of human U1-70 kDa and those of all the sequenced fungal, viral and bacterial proteins.

RESULTS

Human U1-70 kDa shares highly significant (E<10(-20)) amino acid sequence homology, spanning a segment containing T-cell epitopes, with 13 fungal (but no viral or bacterial) proteins, belonging to human pathogens. Nine of these proteins include the amino acid sequence VLVDVERGRTV, identical to the most frequent U1-70 kDa T-cell epitope in anti-U1-70 kDa positive patients, and sequences highly similar to the epitope DAFKTLFVARVN (identical residues or conservative residue substitutions in positions crucial for epitope binding).

DISCUSSION AND CONCLUSION

Cross-reactivity between human U1-70 kDa and microbial proteins was demonstrated for B-cell epitopes, but never investigated before for T-cell epitopes. Our data identify some fungal proteins as possible triggers of anti-U1-70 kDa autoimmunity via molecular mimicry. Research in this field could improve the understanding of the mechanisms leading to anti-U1-70 kDa autoimmunity, with potential consequences on prevention.

Infection—genetics relationship in systemic lupus erythematosus

Genetic, environmental, and hormonal factors contribute to disease susceptibility in systemic lupus erythematosus. Among environmental factors, infectious agents play a major role. When considering the complex relationship between genetic predisposition and infections in the pathogenesis of systemic lupus erythematosus, we have to consider that infectious agents can interact with the immune system in several ways. For example, molecular mimicry, altered apoptosis of the host cells, exposure of as yet masked antigens to the immune system by a given microorganism, and direct viral invasion of immunocompetent cells are all mechanisms that may give rise to dysfunction of the immune system; in addition, some genetically determined deficit of the immune system, such as complement deficiency or deficit of mannose binding lectine, may cause insufficient clearance of infectious agents, whose persistence in the host may determine autoimmunity. Finally, evidence has been emerging suggesting that the production of autoantibodies, by infected B-lymphocytes, may be drawn by altered expression of particular microRNA in these cells. In this paper, we review some of the distinct scenarios that can account for the role of infectious agents, acting on a genetically prone host, in determining systemic lupus erythematosus.

Autoantibodies to intracellular autoantigens and their B-cell epitopes: molecular probes to study the autoimmune response

A common laboratory finding in systemic autoimmune diseases is the presence of autoantibodies against intracellular autoantigens. Although their pathogenesis is not fully understood, autoantibodies are important tools for establishing diagnosis, classification, and prognosis of autoimmune diseases. Autoantibodies mainly target multicomponent complexes containing both protein antigens and (ribo)-nucleic acid(s), such as the spliceosome or Ro/La RNPs. In this review, we address the main characteristics and the clinical value of the main autoantibody types with respect to their disease association, and we describe the corresponding autoantigens, their biologic function, and their B-cell antigenic determinants (epitopes). The structural characteristics and clinical associations of these epitopes, and their utility as tools to investigate the autoimmune response, are discussed in detail. New insights into the pathogenetic role of epitopes in systemic autoimmunity are also examined. In this regard, using the defined structures of the B-cell antigenic epitopes, complementary epitopes can be designed according to the "molecular recognition" theory. These complementary epitopes can be used as probes to study pathogenetic and immunoregulatory aspects of the anti-idiotypic response. The origin of humoral autoimmunity and the spreading of the epitopes in systemic lupus erythematosus are also discussed. Finally, the ability of post-translational modifications to induce autoreactive immune attack via the generation of neo-epitopes is summarized.

[The clinical spectrum of dermatomyositis]

Dermatomyositis is characterized by marked clinical heterogeneity. Many classification schemes are available, based on internal organ involvement, presence of circulating autoantibodies and additional symptoms characteristic for other connective tissue diseases. Viral infections and underlying malignancies are possible mechanisms to trigger this autoimmune disease. Intensive clinical investigations to detect such malignancies are therefore required. The prognosis of the individual patient depends on the involvement of internal organs and the characterization of circulating autoantibodies. There are patients with rapidly developing subsets of dermatomyositis and those with slowly progressive disease. Therefore treatment has to be adjusted for the disease severity. Sometimes high dosages of corticosteroids in combination with immunosuppressive agents or immunoglobulins are required.

The clinical value of intracellular autoantigens B-cell epitopes in systemic rheumatic diseases

A hallmark of autoimmune diseases is the production of autoantibodies against intracellular autoantigens. Although their pathogenetic and their etiologic relationship are not fully understood, these autoantibodies are important tools for establishing the diagnosis, classification and prognosis of autoimmune diseases. Systemic rheumatic diseases are among the most complex disorders because their clinical presentation and constellation of findings are in part reflected by the wide spectrum of autoantibodies found in the sera of patients suffering from these disorders. These autoantibodies usually target large complexes consisting of protein antigens noncovalently associated with (ribo)-nucleic acid(s), like the spliceosome or Ro/La-RNPs. In this review, we first address the main characteristics and the clinical value of several autoantibodies, with respect to their diagnostic sensitivity and specificity. Subsequently, we provide a brief overview of the antigenic determinant types that have been identified on the corresponding autoantigens. The antibody targets of autontigens include primary, secondary, tertiary and quarternary structure epitopes, as well as cryptotopes, neoepitopes and mimotopes. We next focus on antigenic structures corresponding to B-cell epitopes with high disease specificity and sensitivity for all the major autoantigens in systemic autoimmunity including the Ro/La and U1 ribonucleoprotein complexes and the Ku70/80, ribosomal P, DNA topoisomerase I, filaggrin, Jo-1 and PM/SCl-100 autoantigens. These epitopes, defined at the peptide level, can be chemically synthesized and engineered for the development of new inexpensive and easier to perform assays and the improvement of the methods for autoantibody detection. Specific examples of newly developed assays that incorporate (i) epitopes with high disease specificity and sensitivity, (ii) modified epitopes, (iii) conformational epitopes and (iv) complementary epitopes are discussed in detail. Finally, we examine the potential of combining these synthetic epitopes for future development of multiplex diagnostic tests based on miniaturized autoantigen arrays.

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

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

Therapy of systemic lupus erythematosus: a look into the future

The prognosis for patients with systemic lupus erythematosus has greatly improved over the past two decades. However, therapies that are more effective and that have fewer sequelae are needed to rescue patients from organ failure and further reduce mortality. Research under way, including that into induction of tolerance to self-antigens, prevention of the consequences of pathogenic autoantibody production, interference with the cytokine network and signal transduction, the identification and treatment of any infectious triggers, and stem cell therapy, offers hope of improved remedies or even of cure. Given the fact that a number of biological therapies for rheumatologic disease are already in use or are in the development stage, such progress may come soon.

Key sequences involved in the spreading of the systemic autoimmune response to spliceosomal proteins

Immune spreading to multiple intracellular antigens is likely to be of primary importance in organ-specific and systemic autoimmune diseases. A number of mechanisms by which immune spreading may occur from only a single autoreactive epitope have been proposed. Search for an initiator or early epitope thus represents an important area of investigation. For example, many studies have focused on the identification of epitopes recognized by the antibodies from both patients with systemic lupus erythematosus (SLE) and lupus-prone mice. Recently, an autoepitope present in the 70K U1 ribonucleo protein (RNP) and recognized by CD4+ T cells from lupus mice has also been identified. Here, we analyze the results of B- and T-cell-epitope mapping studies of several RNPs present in the spliceosome and propose a model of epitope spreading. In this model, a consensus sequence (the RNP motif) conserved in many nuclear, nucleolar and cytoplasmic antigens, might play a role as 'driver' epitope. This hypothesis is based on the observation that this sequence is recognized by CD4+ T cells from lupus mice and is often targeted by autoantibodies, very early during the course of the disease. Targeting this region that is repeated in different self-antigens, might represent an interesting strategy to interfere with the continuous T-cell stimulation and exposure to specific antigens.

The appearance of U1 RNP antibody specificities in sequential autoimmune human antisera follows a characteristic order that implicates the U1-70 kd and B'/B proteins as predominant U1 RNP immunogens

OBJECTIVE

To observe the order of development of anti-U1 RNP peptide antibodies in humans.

METHODS

Immunoblots against Jurkat cell lysates were performed on 5,882 serum samples from 3,668 patients referred on clinical grounds for RNP antibody testing to a reference laboratory between 1989 and 1999. In patients from whom multiple samples were drawn, we determined the order in which IgG antibodies to the U1 RNP peptides A, B'/B, C, D, and 70 kd appeared.

RESULTS

One hundred sixty-three patients with serial samples were identified in whom antibodies to at least one U1 RNP peptide initially were not present but later appeared. The first RNP antibodies to appear were most often directed against the 70 kd and B'/B peptides (P < 0.01). Antibodies to the A and C peptides usually developed after other RNP peptide antibodies, and antibodies to D often emerged only after immunity to multiple other U1 RNP proteins had appeared. B'/B, but not 70 kd, was a frequent early target of spreading after initial immunity to other RNP peptides.

CONCLUSION

Orderly patterns of emergence of U1 RNP peptide antibodies appear to exist in humans. Two peptides, 70 kd and B'/B, show characteristics of early immunogens in the development of human RNP immunity.

Splice variants of the nuclear dot-associated Sp100 protein contain homologies to HMG-1 and a human nuclear phosphoprotein-box motif

Sp100 and PML are interferon-inducible proteins associated with a new class of nuclear domains (known as nuclear dots or PML bodies) which play a role in tumorigenesis, virus infections, and autoimmunity. While PML is extensively alternatively spliced, only two splice variants are known for Sp100. Here we describe the identification and characterization of several Sp100 splice variant proteins and support their existence by elucidation of the 3'-end of the Sp100 gene. Some of the splice variants contain a domain of significant sequence similarity with two previously described highly related interferon-inducible nuclear phosphoproteins as well as to suppressin and DEAF-1, which altogether define a novel protein motif, termed HNPP-box. One class of splice variants contains an almost complete and highly conserved copy of the DNA-binding high mobility group 1 protein sequence and thus represent novel HMG-box proteins. When expressed transiently, both major classes of Sp100 splice variant proteins localize in part to nuclear dots/PML bodies and in addition to different nuclear domains. Furthermore, PML was occasionally redistributed. These data indicate that alternatively spliced Sp100 proteins are expressed, differ in part in localization from Sp100, and might bind to chromatin via the HMG domain.

Autoreactive human T cell lines recognizing ribosomal protein L7

Sera of patients suffering from systemic lupus erythematosus (SLE) frequently contain oligoclonal IgG autoantibodies with high affinity for the ribosomal protein L7 (rpL7). The humoral autoimmune response to rpL7 apparently is driven by antigen and T cell dependent. In order to analyze the T cell response to rpL7 we cultured peripheral blood lymphocytes of healthy individuals and SLE patients in the presence of recombinant rpL7. After 10 days, the cytokine response to re-stimulation with rpL7 was examined using a spot-ELISA. Measuring IFN-gamma secretion, the T cells of two patients and four healthy donors showed a significant increase in the number of spots as compared to control cells. Secretion of IL-4 or IL-10 was not detected. From the antigen-stimulated primary cultures we established by limiting dilution cloning six rpL7-reactive, IFN-gamma-secreting T cell lines which show a CD3+CD4+CD8- phenotype. One line additionally was shown to be positive for HLA-DR and CD45R0, but negative for CD27 and CD31. The cell lines carry alphabeta TCR chains which differ from each other in sequence and specificity. rpL7 fragments rich in basic amino acids could be identified as epitopes recognized by the TCR of three cell lines. Recognition of rpL7 is HLA-DR6 restricted or respectively HLA-DP restricted in the two cell lines analyzed.

Correlation between chlamydial infection and autoimmune response: molecular mimicry between RNA polymerase major sigma subunit from Chlamydia trachomatis and human L7

L7 is one of the ribosomal proteins frequently targeted by autoantibodies in rheumatic autoimmune diseases. A computer search revealed a region within the immunodominant epitope of L7 (peptide II) that is highly homologous to amino acid sequence 264-286 of the RNA polymerase major sigma factor of the eubacterium Chlamydia trachomatis. Anti-L7 autoantibodies affinity purified from the immunodominant epitope were able to recognize this sequence as they reacted with purified recombinant sigma factor. Immunofluorescence labeling experiments on C. trachomatis lysates revealed a punctate staining pattern of numerous spots when incubated with the affinity-purified anti-peptide II autoantibodies. Binding of autoantibodies to peptide II was inhibited by the homologous sigma peptide. This is the first demonstration of epitope mimicry between a human and a chlamydial protein on the level of B cells. Antibody screening revealed a significant correlation between the presence of anti-L7 autoantibodies and C. trachomatis infection in patients with systemic lupus erythematosus and mixed connective tissue disease. Our results suggest that molecular mimicry is involved in the initiation of anti-L7 autoantibody response and may represent a first glance into the immunopathology of Chlamydia with respect to systemic rheumatic diseases.

An epitope on Ki antigen recognized by autoantibodies from lupus patients shows homology with the SV40 large T antigen nuclear localization signal

OBJECTIVE

Epitopes on Ki antigen were analyzed using synthetic peptides, including KILT, a 16-mer peptide with an amino acid sequence homologous to the SV40 large T antigen nuclear localization signal (SV40 T NLS).

METHODS

In addition to KILT, 4 synthetic peptides, all potential epitopes on Ki antigen according to computer analysis, were prepared and tested for reactivity with 49 anti-Ki-positive lupus sera by enzyme-linked immunosorbent assay.

RESULTS

Eighteen sera reacted with KILT, but not with other peptides. The reaction of anit-Ki sera with KILT was specifically inhibited by recombinant Ki antigen. Eight of 49 anti-Ki sera reacted with a 7-mer synthetic peptide of SV40 T NLS, and the reaction was specifically inhibited by KILT.

CONCLUSION

The 16-mer Ki peptide containing the sequence homologous to the SV40 T NLS is one of the antigenic epitopes recognized by anti-Ki antibodies in lupus sera.

T cell lines recognizing the 70-kD protein of U1 small nuclear ribonucleoprotein (U1snRNP)

In sera of patients with mixed connective tissue disease (MCTD) high titres of IgG autoantibodies to U1snRNP-specific proteins (70 kD, A, C) are found, suggesting an antigen-driven and T-cell-dependent process. In order to establish U1snRNP-specific T cell lines we cultured under various culture conditions mononuclear cells from MCTD patients and healthy donors with a highly purified UsnRNP preparation from HeLa cells. Nine T cell lines were established by limiting dilution cloning from two MCTD patients and five T cell lines from a healthy individual. All T cell lines expressed the TCR alpha beta/CD3 complex. Surprisingly, most of the T cells lines exhibited the CD8 phenotype. Irrespective of this phenotype, all T cell lines showed a proliferative response to an N-terminal part (aa 51-195) of recombinant U1-specific 70-kD protein. One CD8+ T cell clone exhibited cytotoxic activity against an autologous B cell line pulsed with snRNP or recombinant fragments (aa 51-95 and aa 51-88). Interestingly, two T cell lines proliferated in response to four recombinant polypeptides representing different parts of the U1snRNP 70-kD protein. Since regions of sequence homology are distributed over the 70-kD molecule, it is suggested that conserved motifs may be recognized by the T cell lines.

Two short autoepitopes on the nuclear dot antigen are similar to epitopes encoded by the Epstein-Barr virus

To understand the relationship between antibodies present in patients with anti-nuclear dot (ND) autoimmune disease and the proteins they recognize, epitopes that react with the autoantibodies were mapped. A panel of fusion proteins containing different portions of the ND protein were overproduced in Escherichia coli. Immunoblot analysis with anti-ND antibodies revealed that most (10 of 12) sera recognize two major autoepitopes that are each a maximum of 8 amino acids long. The other two sera recognize one of the two epitopes. In addition to the short linear autoepitopes, a conformational epitope appears to be present on the ND antigen. Each of the two linear epitope sequences shares sequence similarities with those of several viral proteins found in the databases. Furthermore, two fusion proteins containing short Epstein-Barr virus (EBV) protein sequences that are similar to the ND epitopes were recognized by the human autoimmune sera, indicating that the autoepitopes are present in EBV protein sequences. Our results are consistent with the hypothesis that ND autoimmune disease might be associated with EBV infections.

Sequential autoantigenic determinants of the small nuclear ribonucleoprotein Sm D shared by human lupus autoantibodies and MRL lpr/lpr antibodies

Autoantibodies directed against the Sm proteins of the spliceosome complex are found in approximately 25% of systemic lupus erythematosus (SLE) patients sera. To determine which regions of the Sm D polypeptide are involved in the lupus autoimmune response, binding to overlapping octapeptides of Sm D has been evaluated with sera from nine Sm D-positive patients, six patients with other autoimmune serology, and five normal human sera. Lupus patient sera which are Sm precipitin-positive bind various combinations of five regions of the peptide. The major antigenic region, Epitope 5 (REAVA(GR)10GGPRR), is bound by eight of nine Sm precipitin-positive sera tested. This region of Sm D shows significant sequence homology with Epstein-Barr nuclear antigen-1. To determine the fine specificity of the murine Sm response, four unique Sm D MoAbs derived from MRL lpr/lpr mice and three adult anti-Sm-positive MRL lpr/lpr mouse sera have been analysed. Two of these monoclonals, KSm 4 and Y12, as well as the MRL lpr/lpr sera tested, show binding with Epitope 5. Another of these monoclonals, KSm 2, binds octapeptides 84-91, DVEPKVKSKKREAVAG, which corresponds to Epitope 4 of this study. Antibodies from SLE patients with autoimmune serology other than anti-Sm bind the carboxyl glycine-arginine repeat (GR)10 peptides of Sm D. However, none of the antibodies tested from patients who do not have lupus and who have different autoimmune serology binds any of the Sm D octapeptides. Normal controls did not significantly bind any of the Sm D octapeptides. These results describe two major regions of shared antigenicity of Sm D between sera from SLE patients and MRL lpr/lpr mice, thereby establishing a basis for the cross-species similarity of autoimmunity to the Sm autoantigen in SLE.

Basic amino acids predominate in the sequential autoantigenic determinants of the small nuclear 70K ribonucleoprotein

Autoantibodies binding the 70K nRNP polypeptide are commonly found in the serum of patients with systemic lupus erythematosus. IgG antibodies binding overlapping octapeptides of 70K nRNP have been evaluated in 10 patients with anti-nRNP precipitins, seven patients with other autoimmune serology, and four normal human sera. Neither normal controls nor patients without an anti-nRNP precipitin significantly bind any of the 70K nRNP octapeptides. Sera containing an anti-nRNP precipitin strongly bind various combinations of eleven different regions of the 70K nRNP protein. One antigenic region is consistently the most reactive in nine of ten nRNP precipitin positive sera tested. This sequence, KDKDRDRKRRSSRSR, is highly charged and has a similar pattern of alternating basic amino acids also present in seven of the other purported humoral autoimmune epitopes of the 70K nRNP polypeptide. The closely related DRKR and ERKR are important components of two of these epitopes. All regions of the 70K peptide bound by human anti-nRNP precipitin positive sera are very rich in the basic amino acids, especially lysine (chi-square = 23.03, odds ratio = 13.3, P < 0.000001).

Characterization of an HLA-DR4-restricted T cell clone recognizing a protein moiety of small nuclear ribonucleoproteins (UsnRNP)

In sera of patients with mixed connective tissue disease (MCTD, Sharp Syndrome) high titres of IgG autoantibodies to U1snRNP-specific proteins are found. The isolated occurrence of these autoantibodies is highly associated with the HLA-DR4 haplotype. snRNP-specific T cells are supposed to be involved in this autoantibody production. To address this question we cultured mononuclear cells from MCTD patients and healthy donors with a highly purified UsnRNP preparation from HeLa cells using bulk or limiting dilution cultures. Secondary responses to snRNP were detected only rarely with T cell lines from two patients and two controls, and turned out to be unstable during further expansion. One T cell line derived from a healthy individual retained its snRNP reactivity upon limiting dilution cloning and could be characterized in detail. The CD4+ T cell clone recognized native snRNP particles presented by monocytes in an HLA-DR4 (B1*0401)-restricted manner. Separation of the protein and RNA moieties of snRNP particles revealed that the T cell clone responded specifically to the protein fraction, but not to RNA and diverse control antigens. Sequencing of the T cell receptor alpha and beta chain cDNAs revealed that the clone used the V alpha 14.2 and V beta 14 elements. Upon antigen-specific and mitogenic stimulation the T cell clone showed a Th1-specific cytokine pattern, and did not provide helper activity for in vitro immunoglobulin production. This study demonstrate the presence of self-reactive snRNP-specific T cells in a healthy donor. The T cell clone may not represent a helper T cell for the formation of U1snRNP-specific autoantibodies.

Increased antiretroviral antibody reactivity in sera from a defined population of patients with systemic lupus erythematosus. Correlation with autoantibodies and clinical manifestations

OBJECTIVE

The implied role of retroviruses in the pathogenesis of murine systemic lupus erythematosus (SLE) led us to study antiretroviral antibodies in a population-based SLE cohort.

METHODS

Immunoassays using whole virus and synthetic peptides were performed on sera from 72 patients with SLE and 88 control subjects.

RESULTS

Reactions with whole baboon endogenous virus occurred more frequently in patients with SLE, and correlated with the presence of anti-RNP and anti-Sm. Some retroviral env and gag peptides, several of which were similar to U1 small nuclear RNP, reacted more strongly in patients with SLE, and their presence was correlated with discoid rash, hematologic disorder, and other symptoms.

CONCLUSION

These results provide circumstantial evidence for involvement of retroviruses in the pathogenesis of human SLE; further studies should be carried out using other techniques for measurement of retroviral expression.

Divergent evolution may link human immunodeficiency virus GP41 to human CD4

A local sequence similarity of HIV envelope proteins (gp120 and gp41) to immunoglobulins suggests that a mimicry phenomenon may form the basis of the HIV-cell membrane interaction and of HIV-induced autoimmune reaction. We explored the hypothesis of any deeper relationship between HIV env proteins and immunoglobulin family members. An overall DNA sequence similarity between gp41 coding region of env gene and the HIV-receptor CD4 gene was observed and a 14-base-long oligonucleotide, almost unique in the GenBank, was found in gp41 and CD4 genes. The alignment of env gene to CD4 gene and to 84 different sequences showed a significantly higher homology score and a nonrandom similarity in the CD4-env alignment. A significant similarity was also found between the env protein and the sequence encoded by an alternate reading frame of CD4 gene. Our observations suggest that gp41 coding region might have a different origin than the gp120 coding region of the env gene, and that a divergent evolution might link gp41 to CD4 or immunoglobulin family members. In this study the analysis of alternate-reading-frame products is also proposed as a novel approach to investigate evolutionary links and structure-function relationships.

Autoantibodies from patients with systemic lupus erythematosus bind a shared sequence of SmD and Epstein-Barr virus-encoded nuclear antigen EBNA I

SmD is one of the small nuclear ribonucleoproteins frequently targeted by autoantibodies in systemic lupus erythematosus. We isolated and characterized the antibodies present in lupus sera that are specific for the C-terminal region of SmD (sequence 95-119). This region is highly homologous to sequence 35-58 of the EBNA I antigen, one of the nuclear antigens induced by infection with Epstein-Barr virus. Antibodies affinity purified over a peptide 95-119 column were able to recognize this sequence in the context of the whole SmD molecule, as they reacted with blotted recombinant SmD. Anti-SmD 95-119 antibodies bound also the EBNA I 35-58 peptide and detected the EBNA I molecule in a total cell extract from Epstein-Barr virus-infected lines. A population of anti-SmD antibodies is, therefore, able to bind an epitope shared by the autoantigen and the viral antigen EBNA I. To investigate the involvement of this shared epitope in the generation of anti-SmD antibodies, we immunized mice with the EBNA I 35-58 peptide. Sera from immunized animals displayed the same pattern of reactivity of spontaneously produced anti-SmD antibodies. They reacted in fact with the EBNA peptide as well as with SmD 95-119 and recombinant SmD. These data suggest that molecular mimicry may play a role in the induction of anti-SmD autoantibodies.

B cell epitope on the U1 snRNP-C autoantigen contains a sequence similar to that of the herpes simplex virus protein

The mechanism of autoantibody production in autoimmune diseases is not well understood. In the present study we performed the B cell epitope mapping of the U1 small nuclear ribonucleoprotein (snRNP)-C, one of the target molecules of anti-nRNP autoantibody to investigate how B cells respond to the autoantigen. After cloning and expression of a full length complementary DNA (cDNA) encoding the U1-C protein, several truncated mutants of the cDNA were constructed and expressed in E. coli. Although a few epitopes were distributed on the whole molecule, all anti-C protein antibody-positive patients' sera tested recognized the region between amino acid residues 102 and 125 of the coding sequence. This universal epitope region contains an amino acid sequence similar to that of the herpes simplex virus type 1 ICP4 protein. The peptides representing each molecule were cross-reactive to each other. In addition this region cross-reacted to the B/B' protein. These observations suggest that molecular mimicry might be involved in the initiation of autoantibody production, followed by cross-reactive events between the autoantigens and by antigen-driven mechanisms to generate more complicated autoantibody patterns against the U1 snRNP complexes.

Humoral immunity in polymyositis/dermatomyositis

Autoantibodies are found in most patients with polymyositis (PM) or dermatomyositis (DM) and 35-40% of these patients have myositis-specific antibodies. Twenty-five to thirty percent have anti-aminoacyl-tRNA synthetases, of which anti-Jo-1, directed at histidyl-tRNA synthetase, is by far the most common. Patients with anti-synthetases have a high frequency of myositis, interstitial lung disease, Raynaud's phenomenon, and other features constituting an "anti-synthetase syndrome." Anti-synthetases tend to react with conformational epitopes and to inhibit enzymatic activity, suggesting reaction with conserved regions. Sera with antibodies to alanyl-tRNA synthetase (anti-PL-12) also have antibodies to tRNA(ala), whereas most sera with other anti-synthetases do not react directly with tRNA. Production of the antibodies appears to be antigen-driven, and is influenced by HLA genes, although an initiating factor, possibly a viral infection, may be important. Antibodies to other cytoplasmic antigens, most notably the signal recognition particle (anti-SRP), are seen in a small percentage of patients. Patients with anti-SRP do not tend to develop the anti-synthetase syndrome, but may have very severe disease. Antibodies to the nuclear antigen Mi-2 are also specific for myositis, and are strongly associated with DM. Several autoantibodies, including anti-PM-Scl, anti-Ku, and anti-U1 and U2 RNP, have been associated with scleroderma-PM overlap. The role of humoral immunity in the myositis of PM and DM has not yet been clarified. Capillary loss and ischemic damage are important in DM, and seem to be mediated by humoral mechanisms, whereas cell-mediated attack on muscle fibers is important in PM. The mechanism of skin injury in cutaneous lesions is not known, but antibody deposition is inconsistent and uncommon. Whether the myositis-specific antibodies are involved in disease pathogenesis is not yet known, although there is no direct evidence for this. An understanding of the reasons for production of these antibodies, however, should provide insight into the etiology and pathogenesis of PM and DM.

Autoantibodies to the nuclear Sp100 protein in primary biliary cirrhosis and associated diseases: epitope specificity and immunoglobulin class distribution

Sp100, a protein with a dot-like intranuclear localization in immunofluorescence microscopy, is a major target for patient autoantibodies in primary biliary cirrhosis (PBC) and occasionally in rheumatic disorders. The human Sp100 cDNA has recently been cloned, and the deduced amino acid sequence was found to contain sequence similarities with an MHC class I domain and several transacting regulatory proteins, including HIV-1 nef proteins. In this study, recombinant Sp100 fusion proteins were used to differentiate the immunoglobulin isotypes and to map the epitopes involved in the anti-Sp100 autoimmune response. PBC patients developed IgG as well as IgM and/or IgA class anti-Sp100 autoantibodies whereas most patients with rheumatic diseases developed IgG class autoantibodies only. For epitope mapping, truncated versions of the Sp100 protein were probed for immunoreactivity in ELISA and immunoblotting. With 55 sera, 17 different reaction patterns were obtained, and at least three non-overlapping major autoantigenic domains were recognized by the majority of sera. One domain, which contains the sequence similarity with HIV nef proteins, was recognized by all anti-Sp100 sera and harbours multiple, in part discontinuous, epitopes. These data demonstrate a heterogeneous and patient-specific anti-Sp100 autoimmune response which is antigen-driven and, at least in terms of isotype composition, different in PBC and non-PBC patients.

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

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

Systemic lupus erythematosus

Systemic lupus erythematosus is a prototypic autoimmune disease characterized by antinuclear antibody production. In recent investigations, the contributions of various polymorphic immune response gene systems to disease pathogenesis have been analyzed. Unique cellular and molecular studies have also established the role of 'autoantigen drive' in autoantibody induction and its relationship to polyclonal B-cell activation.

Characterization of antibodies reacting with HIV gag proteins occasionally found in the serum of non-infected subjects

The use of serological tests for the diagnosis of HIV infection has revealed that some non-infected persons have antibodies that react with HIV-1 gag proteins. Here, the sera of three non-infected subjects reacting with p17 and 11 non-infected subjects reacting with p24 were investigated, using an enzyme immunoassay (EIA) with six recombinant gag antigens and Western blot analysis of proteolytic peptides of two of these gag antigens. The results indicate that whereas all p17-reactive sera could react with an unique epitope, individual p24-reactive sera recognize different epitopes. Investigations by EIA also demonstrated the role of sequences located far from the epitopes in making these epitopes accessible to the antibodies or in providing them with an antigenic conformation. In addition to the 14 subjects mentioned above, another subject was shown to have antibodies reacting with the p9 (NC) gag protein. Several proteins are known as having homology with HIV-1 gag proteins. Their possible role in eliciting cross-reactive antibodies is discussed.

Repetitive P68-autoantigen specific epitopes recognized by human anti-(U1) small nuclear ribonucleoprotein autoantibodies

The major target of anti-(U1)snRNP autoantibodies, a serological marker of patients with mixed connective tissue disease and related rheumatic disorders, is a 68 kDa protein (p68) associated with (U1)RNA-containing small nuclear ribonucleoprotein particles. With recombinant p68 fusion proteins, multiple autoepitopes have been identified, and one of these has been mapped to the pentamer sequence ERKRR, which is located within antigenic domain A in the amino-terminal half of p68. The lysine residue (K) of this epitope can be replaced by isoleucine without loss of autoantibody binding. Here we have investigated whether other variants of this epitope are present on the p68 autoantigen and if these are recognized by anti-p68 autoantibodies. We identified four related motifs in the carboxy-terminal half of the p68-protein, and three of these (all containing glutamic acid instead of lysine (ERERR] mapped to the previously characterized autoantigenic domains C and D. Immunoreaction of anti-ERKRR autoantibodies, affinity-purified from domain A with recombinant fusion proteins containing either domain C or domain D of p68, revealed that anti-ERKRR autoantibodies cross-react with the ERERR-motifs. This finding, which was confirmed by competitive inhibition-ELISA with solid-phase coupled domain A-, C- and D-fusion proteins and ERKRR-containing synthetic peptides as competitors, suggests that a subset of patient autoantibodies is directed against repetitive structures on a single snRNP component.

Multiple overlapping homologies between two rheumatoid antigens and immunosuppressive viruses

Amino acid (aa) sequence homologies between viruses and autoimmune nuclear antigens are suggestive of viral involvement in disorders such as systemic lupus erythematosus (SLE) and scleroderma. We analyzed the frequency of exact homologies of greater than or equal to 5 aa between 61 viral proteins (19,827 aa), 8 nuclear antigens (3813 aa), and 41 control proteins (11,743 aa). Both pentamer and hexamer homologies between control proteins and viruses are unexpectedly abundant, with hexamer matches occurring in 1 of 3 control proteins (or once every 769 aa). However, 2 nuclear antigens, the SLE-associated 70-kDa antigen and the scleroderma-associated CENP-B protein, are highly unusual in containing multiple homologies to a group of synergizing immunosuppressive viruses. Two viruses, herpes simplex virus 1 (HSV-1) and human immunodeficiency virus 1 (HIV-1), contain sequences exactly duplicated at 15 sites in the 70-kDa antigen and at 10 sites in CENP-B protein. The immediate-early (IE) protein of HSV-1, which activates HIV-1 regulatory functions, contains three homologies to the 70-kDa antigen (two hexamers and a pentamer) and two to CENP-B (a hexamer and pentamer). There are four homologies (including a hexamer) common to the 70-kDa antigen and Epstein-Barr virus, and three homologies (including two hexamers) common to CENP-B and cytomegalovirus. The majority of homologies in both nuclear antigens are clustered in highly charged C-terminal domains containing epitopes for human autoantibodies. Furthermore, most homologies have a contiguous or overlapping distribution, thereby creating a high density of potential epitopes. In addition to the exact homologies tabulated, motifs of matching sequences are repeated frequently in these domains. Our analysis suggests that coexpression of heterologous viruses having common immunosuppressive functions may generate autoantibodies cross-reacting with certain nuclear proteins.

Very long charge runs in systemic lupus erythematosus-associated autoantigens

Systemic lupus erythematosus and other chronic systemic autoimmune diseases are associated with circulating autoantibodies reactive with a limited set of mostly nuclear proteins. Using rigorous statistical methods we have identified segments of highly significant charge concentration in the majority of the characteristic nuclear and cytoplasmic autoantigens. Extremely long runs of charged residues, including some sequences of greater than 20 consecutive charged residues (purely acidic or mixed basic and acidic), occur in about a third of these proteins, whereas equivalent runs are found in less than 3% of other mammalian proteins. The other sequences have less extreme charge clusters, the type and location of which are often conserved between several otherwise nonsimilar antigens. We propose that supercharged surfaces render the targeted host proteins strongly immunogenic and that antinuclear antibody profiles might result from chronic exposure to intracellular contents, possibly in conjunction with crossreactive viral products. The limited number of potential systemic autoantigens may partly be due to the rarity of requisite charge properties.

Major autoantigenic sites of the (U1) small nuclear ribonucleoprotein-specific 68-kDa protein

A 68-kDa protein associated with (U1)snRNP is a major target for human autoantibodies to small ribonucleoprotein particles (snRNP) prevalent in a variety of inflammatory rheumatic diseases. The epitopes recognized by these antibodies were mapped by expression of subfragments of p68 cDNA in Escherichia coli and testing of the corresponding recombinant proteins for immunoreactivity with sera of patients with autoimmune diseases. Three of four antigenic regions were analysed in detail. The immunodominant autoantigenic region was found to coincide with the RNA-binding domain of the p68 protein and was shown to contain a nested set of overlapping discontinuous epitopes. Two additional non-overlapping major antigenic domains were localized in the carboxy-terminal half of the p68 protein. Each of these two carboxy-terminal domains was shown to contain more than one conformation-dependent epitope. Taking into account previous mapping studies, the data demonstrate that p68 contains at least four antigenic regions, each of which harbours multiple epitopes which are recognized in a patient-specific manner.