Molecular mimicry and autoantigens in connective tissue diseases

Autoimmune antibodies to hnRNPG protein in dogs with systemic lupus erythematosus: epitope mapping of the antigen

We previously reported the presence of circulating autoantibodies to hnRNPG protein in dogs with systemic lupus erythematosus (Soulard et al. 1993, 1994). These antibodies appeared to be specifically limited to German shepherd dog species. In the present report, we have analysed the nature of the hnRNPG epitopes responsible for autoantibody specificity. By using a set of 11 dog sera selected for their strong reactivity to hnRNPG protein, we have found that these sera had the ability to recognize two epitopes: (1) within a stretch of 33 amino acids located around the central part of protein, that is readily detected by immunoblotting; (2) a N-terminal conformation of the protein close to the RNA binding domain (RBD), that is revealed by immunoprecipitation. Our data strongly support the notion that the dog autoimmune response against hnRNPG protein is antigen-driven.

Parvovirus B19 infection associated with the production of anti-neutrophil cytoplasmic antibody (ANCA) and anticardiolipin antibody (aCL)

We described four patients who had clinical diagnosis of erythema infectiosum and presented with skin rash, polyarthralgia, polyarthritis, and mild fever. Anti-parvovirus B19 IgM and IgG antibodies were found in all four patients and parvovirus B19 DNA was detected in three of the four patients by polymerase chain reaction (PCR) in sera using standard methods. Anticardiolipin antibody (aCL) was positive in three of the four patients included three with anti-beta2 glycoprotein I (beta2GPI). The immunoglobulin isotype of aCL was found to be IgM. Anti-neutrophil cytoplasmic antibody (ANCA) included three p-ANCA and one c-ANCA was detected in all four patients by indirect immunofluoresence (IIF). Both anti-proteinase 3 (PR3) and anti-myeloperoxidase (MPO) antibodies were found in two patients whom had polyarthritis for more than 6 months. These data indicate parvovirus B19 may be linked to the induction of an autoimmune response.

Retroviral sequences in rheumatoid arthritis synovium

There are several relationships between retroviruses and cellular transformation, as well as retroviruses being involved in the development of autoimmune diseases. Retroviruses have been discussed as etiologic agents modulating or triggering certain pathways in the pathogenesis of rheumatoid arthritis (RA). However, none of the currently known retroviruses has been identified as specific for RA. Due to the unique properties of retroviruses, distinct experimental approaches can be used to detect retroviral activity in cells and tissues. Current research in RA using state-of-the-art molecular biology techniques includes both the search for exogenous and endogenous retroviral gene sequences in synovium of patients with RA.

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.

Antiribonucleoprotein antibodies in children with HIV infection: a comparative study with childhood-onset systemic lupus erythematosus

A number of clinical and laboratory features of HIV infection are found in systemic lupus erythematosus (SLE). The objective of this study was to analyze the presence of circulating antibodies to small nuclear ribonucleoproteins (snRNP) in both diseases. Sera from 44 HIV-infected children, from 22 patients with childhood-onset SLE, and from 50 healthy children were studied. Anti-snRNP antibodies were detected by ELISA using recombinant and affinity-purified nuclear antigens, by counterimmunoelectrophoresis (CIE), and by immunoblotting using extractable nuclear antigens. Results included the detection of anti-snRNP antibodies by ELISA in 30 HIV-infected patients (68.1%) and 19 SLE patients (86.3%). These antibodies were directed against U1-RNP (61.3% and 77.2%, respectively), Sm (29.5% and 54.5%, respectively), 60 kDa Ro/SS-A (47.7% and 50%, respectively), and La/SS-B proteins (18.1% and 9%, respectively). None of the HIV-infected children and 11 SLE patients (50%) showed anti-snRNP antibodies by CIE. None of the HIV-infected patients showed anti-70 kDa U1-RNP or anti-D-Sm antibodies by immunoblotting. No differences between the two groups were noted on the presence of nonprecipitating anti-snRNP antibodies. No such reactivities were observed among the normal sera tested. The authors concluded that nonprecipitating anti-snRNP antibodies in HIV-infected children are as frequent as in childhood-onset SLE. The significance of these antibodies is not clear at present. Although polyreactive and low-affinity antibodies and a mechanism of molecular mimicry may explain these results, a specific stimulation of B cells by nuclear antigens could not be excluded.

Immunogenic properties of synthetic fragments of Sm-D protein in normal and lupus mice

Antibodies against the Sm antigen are characteristic of systemic lupus erythematosus (SLE). They are found in 20-30% of SLE patients and it has been shown previously that up to 70% of SLE sera react with synthetic fragments 1-20 and 44-67 of the Sm-D polypeptide. To determine whether injections of these peptides might be pathogenic both were administered intraperitoneally into normal mouse strains BALB/c (H-2d), B10/brown (H-2k) and C57BL/6 (H-2b) and an autoimmune strain MRL/lpr (H-2k). IgG antibodies against peptide 1-20 were detected by ELISA in the sera of BALB/c and MRL/lpr mice but not in the sera of B10/brown and C57BL/6 mice. IgG antibodies against peptide 44-67 were found in the sera of BALB/c, B10/brown and MRL/lpr mice but not in the sera of C57BL/6 mice. Neither fragment induced a response against the whole Sm-D antigen as detected by Western blotting. Reactivity to synthetic fragments from other nuclear antigens was however detected in the sera of MRL/lpr mice, especially in those mice injected with Sm-D peptide 44-67 emulsified in Freund's adjuvant. Following immunization with Sm-D peptides, antibodies to ssDNA or dsDNA were not detected in the sera of BALB/c, B10/brown and C57BL/6 mice and in the MRL/lpr mice the naturally occurring production of these antibodies was not enhanced. No difference in IgG deposition in the renal glomeruli of the mice injected with the peptides compared with the control groups was observed. These results suggest that the humoral response to the Sm-D fragment is, at least partially, controlled by the MHC haplotype of the recipient mice, is related to dose and type of immunogen, and is also influenced by the presence of Freund's adjuvant. It is evident that although the sera of many SLE patients recognize either or both the 1-20 and 44-67 peptides, these peptides when injected into MRL/lpr mice are not directly pathogenic.

Characteristic epitope recognition pattern of autoantibodies against eukaryotic ribosomal protein L7 in systemic autoimmune diseases

OBJECTIVE

To define the epitope-recognition pattern and the fine specificity of the autoantibody response to protein L7 in patients with rheumatic diseases.

METHODS

The epitope-recognition pattern was studied by enzyme-linked immunosorbent assay utilizing overlapping fragments of L7. The fine specificity was examined by binding inhibition and isoelectric focusing.

RESULTS

We observed a disease-specific epitope-recognition pattern of anti-L7 autoantibodies. There was one immunodominant epitope that was recognized by all anti-L7-positive sera from patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and systemic sclerosis (SSc). Additional recognition of minor epitopes was observed; it arises by intramolecular epitope spreading and was correlated with disease activity in SLE patients. SSc patients differed from SLE and RA patients in that their sera did not recognize certain minor epitopes. The major epitope was recognized by high-affinity autoantibodies of limited heterogeneity. Minor epitopes were recognized by heterogeneous low-affinity autoantibodies.

CONCLUSION

The anti-L7 autoantibody response is oligoclonal. Additional B cell clones are activated by antigen during active phases of disease.

Random phage mimotopes recognized by monoclonal antibodies against the pyruvate dehydrogenase complex-E2 (PDC-E2)

Dihydrolipoamide acetyltransferase, the E2 component of the pyruvate dehydrogenase complex (PDC-E2), is the autoantigen most commonly recognized by autoantibodies in primary biliary cirrhosis (PBC). We identified a peptide mimotope(s) of PDC-E2 by screening a phage-epitope library expressing random dodecapeptides in the pIII coat protein of fd phage using C355.1, a murine monoclonal antibody (mAb) that recognizes a conformation-dependent epitope in the inner lipoyl domain of PDC-E2 and uniquely stains the apical region of bile duct epithelium (BDE) only in patients with PBC. Eight different sequences were identified in 36 phage clones. WMSYPDRTLRTS was present in 29 clones; WESYPFRVGTSL, APKTYVSVSGMV, LTYVSLQGRQGH, LDYVPLKHRHRH, AALWGVKVRHVS, KVLNRIMAGVRH and GNVALVSSRVNA were singly represented. Three common amino acid motifs (W-SYP, TYVS, and VRH) were shared among all peptide sequences. Competitive inhibition of the immunohistochemical staining of PBC BDE was performed by incubating the peptides WMSYPDRTLRTS, WESYPDRTLRTS, APKTYVSVSGMV, and AALWGVKVRHVS with either C355.1 or a second PDC-E2-specific mAb, C150.1. Both mAbs were originally generated to PDC-E2 but map to distinct regions of PDC-E2. Two of the peptides, although selected by reaction with C355.1, strongly inhibited the staining of BDE by C150.1, whereas the peptide APKTYVSVSGMV consistently inhibited the staining of C355.1 on biliary duct epithelium more strongly than the typical mitochondrial staining of hepatocytes. Rabbit sera raised against the peptide WMSYPDRTLRTS stained BDE of livers and isolated bile duct epithelial cells of PBC patients more intensively than controls. The rabbit sera stained all size ducts in normals, but only small/medium-sized ductules in PBC livers. These studies provide evidence that the antigen present in BDE is a molecular mimic of PDC-E2, and not PDC-E2 itself.

Immunopathology of primary biliary cirrhosis

Our understanding of the immunopathology of PBC has dramatically changed with the application of molecular biology techniques in clinical medicine. This has allowed, not only the possibility of characterizing mitochondrial autoantigens fully at the molecular level, but also the identification of specific sites on these molecules that are targetted by autoreactive B and T cells. In addition, the expression of cloned antigens has facilitated the development of the most reliable assays currently available for the detection of mitochondrial autoantibodies. The assessment of the pathogenic capacity of autoreactive T cells, as well as the characterization the PDC-E2 'look alike' molecule expressed on the cell membrane of PBC biliary epithelial cells, remain the major unsolved issues in this disease. Ideally, the continuous effort from both basic and clinical scientist in understanding the pathogenic mechanisms of PBC will lead to more specific, effective, and safer modalities of treatment.

Characterization of human variable domain antibody fragments against the U1 RNA-associated A protein, selected from a synthetic and patient-derived combinatorial V gene library

This is the first study describing recombinant human antibody fragments directed to the U1 RNA-associated A protein (U1A). Three anti-U1A antibody fragments (Fab) were isolated from a semi-synthetic human Fab library and one anti-U1A single-chain variable fragment (scFv) was isolated from a library which was derived from the IgG-positive splenic lymphocytes of an autoimmune patient. Competition studies with autoantibodies against the U1 small nuclear ribonucleoprotein (snRNP) particle from patients with systemic lupus erythematosus (SLE) and SLE-overlap syndromes revealed that U1A binding of these antibody fragments can be inhibited by about 40% of the patient sera. All antibody fragments recognized the native U1 snRNP in immunoprecipitation assays. Two of three Fab clones as well as the scFv clone derived from the repertoire of an autoimmune patient use the same heavy chain germ-line gene DP-65. Epitope mapping revealed that these three clones appear to recognize an identical epitope domain present on the C-terminal RNP motif of the U1A protein. The DP-65 heavy chain gene is used in less than 1% of the B cells in healthy individuals, while three out of four anti-U1A antibody fragments use this gene. This points to a restricted VH gene usage in the case of U1A, suggesting that the DP-65 heavy chain has a natural shape complementarity to the U1A protein.

Molecular biology of autoantigens in rheumatic diseases

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

Human endogenous retroviruses: nature, occurrence, and clinical implications in human disease

Retroviral diagnostics have become standard in human laboratory medicine. While current emphasis is placed on the human exogenous viruses (human immunodeficiency virus and human T-cell leukemia virus), evidence implicating human endogenous retroviruses (HERVs) in various human disease entities continues to mount. Literature on the occurrence of HERVs in human tissues and cells was analyzed. Substantial evidence documents that retrovirus particles were clearly demonstrable in various tissues and cells in both health and disease and were abundant in the placenta and that their occurrence could be implicated in some of the reproductive diseases. The characteristics of HERVs are summarized, mechanisms of replication and regulation are outlined, and the consistent hormonal responsiveness of HERVs is noted. Clear evidence implicating HERV gene products as participants in glomerulonephritis in some cases of systemic lupus erythematosus is adduced. Data implicating HERVs as etiologic factors in reproductive diseases, in some of the autoimmune diseases, in some forms of rheumatoid arthritis and connective tissue disease, in psoriasis, and in some of the inflammatory neurologic diseases are reviewed. The current major needs are to improve methods for HERV detection, to identify the most appropriate HERV prototypes, and to develop diagnostic reagents so that the putative biologic and pathologic roles of HERVs can be better evaluated.

RNA recognition by autoantigens and autoantibodies

The La, Ro, Sm and RNP autoantigens have been intensely studied over the past decade since cDNAs encoding autoantigens have been available. Most of these autoantigens are closely associated with RNA in RNP particles and molecular studies have provided insights into their modes of recognition and binding to RNA. For example, a common RNA Recognition Motif (RRM) was found to be a critical component of the RNA-binding domain of these autoantigens and the three dimensional structure of the RRM has been solved. As described in other articles in this series, the presence of La, Ro, Sm and RNP autoantibodies correlates with disease subsets, such as Sjogren's syndrome, systemic lupus erythematous and other connective tissue diseases. Immunological analysis of sera from autoimmune patients using recombinant autoantigens has revealed that multiple epitopes reside along the proteins and these represent both continuous and discontinuous (conformational) autotopes. Findings to date support a model of autoantibody induction which involves the direct presentation of proteinaceous autoantigens to the immune system. Circumstantial evidence has suggested that immunological crossreactivity between systemic autoantigens and structural components of infectious agents may play an initial role in the autoimmune response to certain antigens. However, the etiology of autoimmune diseases is probably multifactoral with genetic and other immune features acting on the organismal level. In addition, RNA molecules themselves can be autoantigens with higher order structural conformations which are recognized by RNP-type autoantibodies. Immune crossreactivity and/or direct presentation may generate autoantibodies reactive with conformational RNA epitopes. If crossreactivity with components of cellular or infectious agents give rise to RNA epitopes, they may represent structural or functional mimetics of the primary epitopes that actually drive the response. These ideas are discussed with respect to the role of mimetic processes in molecular recognition during autoimmunity.

Tryptophanyl-tRNA synthetase as a human autoantigen

Autoantibodies to highly purified tryptophanyl-tRNA synthetase, consisting of two approximately 60-kDa subunits (6.1.1.2, TrpRS), were detected in some sera of donors and patients with various diagnosis using the newly developed 125I-TrpRS-radiodot, 125I-TrpRS-radioblot, ELISA and Western immunoblotting. The percentage of positive sera appears to be dependent upon the method of sera testing. The autoimmune sera recognized both the native and denatured TrpRS forms. The binding of the human serum to the 60-kDa band of tissue extract was demonstrable by the 125I-TrpRS-blot as well as Western blot techniques. The possible role of infections in the induction of anti-TrpRS antibodies and maintenance of the autoimmune response is discussed.

The etiopathogenesis of Sjögren's syndrome

With increasing awareness and improved diagnostic tests, Sjögren's syndrome (SS) is becoming recognized as a common autoimmune disease, affecting as many as 3% of women over age 55 years. Apart from keratoconjunctivitis sicca, systemic features are common, leading to considerable morbidity and occasionally mortality. Predisposing factors for SS include HLA determinants that have been linked to DR3 and heterozygosity for DQ-1 and DQ-2. There is accumulating evidence that activated epithelial cells and their interaction with T cells play a central role in pathogenesis. Some restriction of T-cell receptor gene usage to V beta 6.7b and V beta 13.2 and a profile of cytokine production consistent with Th-1-type cells has been observed in affected tissues. Antibodies to Ro (SS-A) and La (SS-B) are found in about 50% of patients and are associated with more severe glandular and extraglandular manifestations. There is evidence that the antibodies are pathogenic, not only in patients, but in their infants born with congenital heart block. Studies of herpesviruses have led to conflicting results, and interest has recently focussed on retroviruses, based on the findings of the expression of retroviral elements in salivary glands of SS patients and antiretrovial antibodies in serum. Mice infected with or transgenic for retroviruses develop SS-like pathology and are currently being studied as animal models of the disease. In the last few years, considerable progress has been made in the understanding of the pathogenesis of SS, and the disease has become the prototype for the investigation of a viral etiology for autoimmune rheumatic disease. Study of its etiopathogenesis may be the key to understanding autoimmune disease in general.

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

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

Epitopes of the Onchocerca volvulus RAL1 antigen, a member of the calreticulin family of proteins, recognized by sera from patients with onchocerciasis

RAL1 is an antigen (Ag) encoded by the filarial nematode Onchocerca volvulus, the parasite causing onchocerciasis (river blindness). RAL1 shares 64.4% identity with the autoantigen calreticulin. The striking similarity of the parasite Ag and the human autoantigen has led to the hypothesis that RAL1 may induce a cross-reactive immune response to calreticulin, which in turn may be involved in the pathogenesis of onchocerciasis. To test this hypothesis, we explored the immune response to RAL1 recombinant Ag (RAL1 rAg) and human calreticulin in patients with O. volvulus infection. A total of 86% of the O. volvulus-infected individuals produced antibodies recognizing RAL1 rAg. Antibody reactivity to RAL1 rAg in patient sera was confined primarily to the central and carboxyl-terminal parts of the molecule. No significant correlations were found to associate recognition of RAL1 rAg, or any particular portion thereof, with a particular disease state. Antibodies against RAL1 thus appear to be produced as a general immune reaction to O. volvulus infection and do not necessarily lead to a cross-reacting response with the host protein. In contrast, 33% of the patient sera tested bound recombinant human calreticulin. All of these sera also recognized a polypeptide encompassing the carboxyl-terminal portion of the RAL1 rAg. These results suggest that recognition of an epitope encoded in the carboxyl-terminal portion of RAL1 is at least in part responsible for inducing a cross-reacting immune response to the host protein.

Antigenic cross-reactivity between human T lymphotropic virus type I (HTLV-I) and retinal antigens recognized by T cells

To investigate the relationship between uveitis and HTLV-I infection, we examined the cross-reactivity between HTLV-I antigens and retinal antigens recognized by T cells in B10.BR mice immunized with human HTLV-I-infected MT-2 cells. We found that T cells obtained from MT-2 immune mouse spleen responded not only to HTLV-I antigens but also to retinal antigens of various species. However, they did not respond to HTLV-I-negative lymphoid cell lines. Furthermore, established T cell lines from MT-2 immune spleen cells also responded to both HTLV-I and retinal antigens. The phenotype of the immune cells that responded to both HTLV-I and retinal antigens was CD4+, CD8-, and CD3+. The proliferative response of T cell lines to HTLV-I as well as various retinal antigens, was clearly blocked by addition of anti-CD3, anti-CD4, or anti-I-Ak MoAbs, but not by anti-CD8 antibody. The established T cell lines from HTLV-I immune spleen cells were all found to be CD3+, TCR beta +, CD4+, CD8- cells by flow cytometric analysis. These results indicate that an epitope of HTLV-I antigens is cross-reactive to an epitope of retinal antigens extracted from either human retinoblastoma or normal murine, rat, and bovine retinae at a T cell recognition level.

Sera from JRA patients contain antibodies against a defined epitope in chromosomal protein HMG-17

Autoantibodies against the nonhistone nucleosomal protein HMG-17 have been detected in a high percentage of ANA-positive patients with pauciarticular-onset JRA4. Here we report on the epitope mapping of the HMG-17 autoantigen with a set of overlapping and nested synthetic peptides spanning the entire amino acid sequence of the human HMG-17 protein. Competition ELISA experiments defined a proline and lysine rich octapeptide PKPEPKPK as the major epitope recognized by more than 70% of the HMG-17 positive JRA sera. Point mutations introduced in the autoimmune peptide determined the amino acid residues important for autoantibody recognition. Computer based sequence comparison shows close homology between the HMG-17 autoimmune epitope and certain infectious organisms, supporting the possibility that molecular mimicry is an important factor in the etiology of JRA.

Rare scleroderma autoantibodies to the U11 small nuclear ribonucleoprotein and to the trimethylguanosine cap of U small nuclear RNAs

We have identified a scleroderma serum (Ru) with a previously undescribed specificity to protein components of the U11 small nuclear ribonucleoprotein particle (snRNP), a low-abundance member of the Sm class of U RNPs. The U11 RNP can be specifically immunoprecipitated from sonicated HeLa cells with Ru serum. In nuclear extracts, a fraction of the U11 particle is found complexed to the U12 RNP, an even lower abundance Sm snRNP. In glycerol gradient fractions, Ru serum identifies a 65-kDa protein that cosediments with the U11-U12 complex and is shifted upon targeted degradation of the U12 RNA. The 65-kDa protein therefore appears to be a component of the U11-U12 snRNP complex, whereas another Ru-reactive (140 kDa) protein may be associated with the free U11 RNP. The Ru serum also contains autoantibodies directed against the trimethylguanosine cap of U RNAs. This rare specificity has been described previously in only three other scleroderma patients.