Intra- and intermolecular spreading of autoimmunity involving the nuclear self-antigens La (SS-B) and Ro (SS-A)

Determinant spreading: lessons from animal models and human disease

Spreading of the immune response is a common theme in organ-specific and systemic autoimmune diseases. We evaluated whether some of the mixed antinuclear antibody patterns characteristic of systemic autoimmunity might be the result of determinant spreading from a single initiating event. Immunisation of healthy mice with individual protein components of the La/Ro ribonucleoprotein (RNP) targeted in systemic lupus erythematosus and primary Sjögren's syndrome induced autoantibodies recognising Ro60 (SS-A), Ro52 (SS-A) and La (SS-B) and in some cases the molecular chaperones calreticulin and Grp78. The endogenous antigen(s) driving determinant spreading might be derived from physiological apoptosis which could explain the involvement of some chaperone proteins in the autoimmune response. Diversified anti-La/Ro antibody responses were initiated by challenge with a single subdominant T epitope of La even though some self epitopes of La were efficiently tolerised. The pattern of autoantibody responses in primary Sjögren's syndrome was strongly influenced by HLA class II phenotype which we speculate controls activation of T cells recognising defined peptides from the La/Ro RNP. In this way, HLA class II alleles may be critical in influencing initiation and spreading of systemic autoimmune reactions. Molecular mimicry of such determinants by exogenous agents might readily initiate spreading of an autoimmune response in genetically susceptible hosts.

Immunization with a peptide surrogate for double-stranded DNA (dsDNA) induces autoantibody production and renal immunoglobulin deposition

Anti-double-stranded DNA (dsDNA) antibodies are the serologic abnormality characteristically associated with systemic lupus erythematosus (SLE) and may play an important role in disease pathogenesis. Although the anti-dsDNA antibodies present in SLE are indicative of an antigen-driven response, the antigen has not been conclusively identified. By screening a phage peptide display library, we demonstrated previously that the decapeptide DWEYSVWLSN is specifically bound by the pathogenic murine IgG2b anti-dsDNA antibody R4A. To investigate the possibility that a protein antigen might trigger lupus-like autoimmunity, we immunized BALB/c mice with DWEYSVWLSN in adjuvant. Mice developed significant titers of IgG anti-dsDNA antibodies 2-3 wk after the initial immunization. Immunized mice also developed antibodies against some other lupus autoantigens, and immunoglobulin deposition was present in renal glomeruli at 49 d. Although an immune response to peptide and dsDNA was evident in BALB/c mice, there was little response in other inbred strains. This study demonstrates that lupus-like anti-dsDNA reactivity can be generated in nonautoimmune mice by immunization with a peptide antigen. Peptide-induced autoimmunity may prove useful in understanding the spreading of antigenic specificities targeted in SLE. However, most importantly, the demonstration that a peptide antigen can initiate a SLE-like immune response opens a new chapter on the potential antigenic stimuli that might trigger SLE.

Influence of antigen organization on the development of lupus autoantibodies

OBJECTIVE

To investigate the reason for grouping of antibodies against small nuclear RNP (snRNP) particles, which are major autoantigens in systemic lupus erythematosus (SLE).

METHODS

Mice were immunized with biochemically purified native snRNP particles or recombinant proteins, followed by assessment of antibody and T cell responses. Since mouse (self) snRNPs are not immunogenic in mice, a eukaryotic expression vector was constructed to induce high-level expression of the human U1 snRNP-associated A protein in murine cells. Native chimeric (mouse/human) snRNP particles were used to immunize normal mice of both H-2k and H-2b backgrounds. We also disrupted the native snRNPs by digestion with ribonuclease and used this mixture of proteins to immunize mice.

RESULTS

Immunization with native chimeric snRNPs resulted in the development of antibodies against a set of snRNP-associated proteins, a response which was accompanied by breakdown in T cell tolerance to mouse snRNPs in mice immunized with chimeric snRNPs. We also demonstrated that the ordered production of these antibodies was due to the fact that snRNP-associated proteins are grouped together in snRNP particles, since disruption of the particles resulted in development of antibodies in a random order, distinct from antibodies seen with intact particles.

CONCLUSION

Our findings directly demonstrate that the pattern of development of antibodies to native snRNPs is similar to that which is commonly observed in SLE, and that disruption of the particles results in disappearance of this ordered pattern. These results suggest that the autoimmune response to snRNPs, and possibly to other autoantigens, in lupus is a specific reaction similar to that seen in a typical immune response to foreign immunogens.

Spreading of the immune response from 52 kDaRo and 60 kDaRo to calreticulin in experimental autoimmunity

Calreticulin (CR) is widely recognized as a new human autoantigen but there are conflicting data concerning its relationship with the Ro(SS-A) ribonucleoprotein (RNP). Recent evidence suggests that CR binds to 52 kDaRo (Ro52) by a protein/protein interaction and binds to hY RNA and rubella virus RNA. Other studies have shown that initiation of immunity to either Ro52 or 60 kDaRo (Ro60) can lead to reciprocal spreading of autoimmunity to Ro60 or Ro52, respectively, and induce anti-La autoantibodies in some strains of mice. These findings support a physical association of these polypeptides in Ro/La complexes. To test the hypothesis that CR is physically associated with Ro52 and/or Ro60 we examined the sera of Ro52-, Ro60- and La-immunized mice for intermolecular spreading to CR. Immune sera from BALB/c and C3H/HeJ mice immunized with recombinant 6xHis-mouse Ro52, 6xHis-human Ro60 or 6xHis-human La were tested for reactivity by ELISA and immunoblotting with a full-length human CR protein expressed as a soluble maltose binding protein fusion protein (CR-MBP). Five of the six Ro52-immunized C3H/HeJ mice sera and all six Ro60-immunized C3H/HeJ mice sera reacted with the CR-MBP (but not a MBP control) on ELISA. In the BALB/c group, the responder rate was lower with one in six of the Ro52-immunized and one in five of the Ro60-immunized mice spreading to CR. In contrast, none of the BALB/c or C3H/HeJ mice which was immunized with La showed evidence of a recruited anti-CR antibody response. Immunoblotting of the different recombinant proteins with immune sera from the C3H/HeJ mice confirmed the specificity of the initial and recruited antibody responses. The spreading of immunity from Ro52 and Ro60 to CR in Ro-immunized mice suggests that a subpopulation of CR or CR-like molecules must associate under certain circumstances with Ro52 and Ro60 polypeptides in vivo, possibly as Ro/CR complexes concentrated in surface membrane blebs of apoptotic cells. The lack of spreading to CR in La-immunized mice suggests that CR may be associated with a subpopulation of Ro particles from which La has already dissociated.

Mechanism of ovarian autoimmunity: induction of T cell and antibody responses by T cell epitope mimicry and epitope spreading

Autoimmune diseases are often manifested as organ inflammation with loss of function, and detectable autoreactive T cell and autoantibody responses. In the proper genetic context, we have shown that these parameters of autoimmunity can result from a single pivotal event: the induction of a strong and persistent T cell response for a foreign or unrelated self peptide that mimics the target self peptide. This may apply to organ-specific and systemic autoimmunity, independent of whether the tissue inflammation results from T cell immune mechanism or antibodies. T cell peptide mimicry, through sharing of critical residues or by a less defined mechanism, can result in autoimmune disease. Once triggered, the helper T cell response leads rapidly to a concomitant autoantibody response spreading to distant B cell determinants of the self protein antigen. Evidently, with T cell help, endogenous antigens can stimulate B cells to provoke a functional autoantibody response against conformational antigenic determinants. These findings are based on recent studies on a novel autoimmune ovarian disease model induced by a self peptide with well-defined T and B cell epitopes. However, studies reported on systemic lupus erythematosus models have shown that similar events may result in autoantibody response in systemic autoimmunity.

Ro/SS-A and La/SS-B antibody level variation in patients with Sjögren's syndrome and systemic lupus erythematosus

To examine both possible correlations between anti-Ro/SS-A and anti-La/SS-B levels and their correlation with clinical disease activity in patients with Sjögren's syndrome (SS) or systemic lupus erythematosus (SLE), an ELISA was developed using purified recombinant Ro 60 kDa, Ro 52 kDa and La antigens. The ELISA was used for testing sequential serum samples from 16 patients with either SS or SLE. The patients were followed for periods between 15 and 128 months, and 3-15 serum samples per patient were analysed and compared with clinically apparent disease activity at the time of sampling in 14 patients. A temporal correlation of antibody levels to Ro and La antigens was found, and antibodies to different epitopes of the Ro 60 kDa protein showed parallel variation in seven of eight patients tested. Co-variation of autoantibody levels and disease activity was found in 11 of 14 patients. In seven of these 11 patients the anti-Ro and anti-La levels were stable and changes in disease activity were minimal during the observation period. In the other four of these 11, changes in disease activity were noted, with an associated change in autoantibody levels. The results suggest that the serological response to Ro and La antigens, as well as to different epitopes of the Ro 60 kDa protein, is antigen driven and regulated by common mechanisms, and indicate a correlation of Ro and La antibodies with pathogenic events.

HLA class II phenotype controls diversification of the autoantibody response in primary Sjögren's syndrome (pSS)

The coexistence of anti-La (SS-B) and anti-Ro (SS-A) autoantibodies in pSS is probably explained by intermolecular spreading of autoimmunity toward different components of the La/Ro ribonucleoprotein (RNP). In order to evaluate the role of the HLA class II phenotype in controlling diversification of this autoantibody response, 80 patients with pSS were typed by polymerase chain reaction sequence-specific oligonucleotide (PCR-SSO) at the HLA class II loci DRB1, DQA1 and DQB1. Serum samples were examined for anti-La and anti-Ro by counterimmunoelectrophoresis and by ELISA using purified recombinant La and 60-kD Ro proteins. Patient sera were classified according to the extent of diversification of the anti-La, anti-Ro response including the presence or absence of precipitating anti-La antibodies. Immunogenic characteristics of these stratified groups were then studied. All patients with pSS, with or without autoantibodies to Ro and La, were found to have at least one of the HLA-DRB1 types DR2, DR3 or DR5. The HLA DR3-DQA1*0501-DQB1*02 (DR3-DQ2) haplotype was primarily associated with a diversified La/Ro RNP response containing precipitating autoantibodies to La (P<0.001); whereas the haplotype HLA DR2-DQA1*0102-DQB1*0602 (DR2-DQ1) was associated with a less diversified La/Ro RNP response containing non-precipitating (restricted epitope) anti-La autoantibodies (P<0.001). Anti-La-positive patients lacking both HLA-DR2 and HLA-DR3 all expressed the HLA-DQA1*0501 allele, which was present at increasing frequency with greater diversification of the anti-La/Ro autoantibody response. The association of distinct HLA haplotypes with different degrees of autoantibody diversification in patients with pSS suggests a model of HLA-restricted presentation of La/Ro peptide determinants to autoreactive helper T cells. We propose that non-precipitating anti-La responses are driven by limited intermolecular help from DR2-DQ1-restricted T helper cells recognizing Ro determinants. On the other hand, we speculate that the more diversified, precipitating anti-La responses obtain more efficient cognate T help from DR3-DQ2-restricted T helper cells recognizing La determinants, where HLA-DQA1*0501 may be a critical determinant for antigen presentation.

Epitope spreading: lessons from autoimmune skin diseases

Autoimmune diseases are initiated when patients develop aberrant T and/or B cell responses against self proteins. These responses presumably are directed to single immunogenic epitopes on these proteins. Recent data in animal models of autoimmune diseases suggest that the targets of immune responses in autoimmunity do not remain fixed, but can be extended to include other epitopes on the same protein or other proteins in the same tissue, a phenomenon termed "epitope spreading." The "epitope spreading" phenomenon also applies to situations in which tissue damage from a primary inflammatory process causes the release and exposure of a previously "sequestered" antigen, leading to a secondary autoimmune response against the newly released antigen. In experimental autoimmune animal diseases, "epitope spreading" seems to have significant physiologic importance in determining the course and duration of disease. In this paper, we review the current concepts in animal models of autoimmune diseases in order to define the "epitope spreading" phenomenon, and we then propose how this phenomenon might play a significant role in the development and the course of autoimmune skin diseases. Hopefully, an understanding of "epitope spreading" will help the dermatology community to better understand the pathogenesis of autoimmune skin diseases and to rationally fashion disease-specific immune therapy in the future.

Antibody and T cell responses of patients with adenocarcinoma immunized with mannan-MUC1 fusion protein

Mucin 1 (MUC1) is a large complex glycoprotein that is highly expressed in breast cancer, and as such could be a target for immunotherapy. In mice, human MUC1 is highly immunogenic, particularly when conjugated to mannan, where a high frequency of CD8(+) MHC-restricted cytotoxic T lymphocytes is induced, accompanied by tumor protection. On this basis, a clinical trial was performed in which 25 patients with advanced metastatic carcinoma of breast, colon, stomach, or rectum received mannan-MUC1 in increasing doses. After 4 to 8 injections, large amounts of IgG1 anti-MUC1 antibodies were produced in 13 out of 25 patients (with antibody titers by ELISA of 1/320-1/20,480). Most of the antibodies reacted to the epitopes STAPPAHG and PAPGSTAP. In addition, T cell proliferation was found in 4 out of 15 patients, and CTL responses were seen in 2 out of 10 patients. Mannan-MUC1 can immunize patients, particularly for antibody formation, and to a lesser extent, cellular responses. It remains to be seen whether such responses have antitumor activity.

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.

Antibodies against a peptide sequence located in the linker region of the HMG-1/2 box domains in sera from patients with juvenile rheumatoid arthritis

OBJECTIVE

To extend our work on the mapping of B cell epitopes on nucleosomal high mobility group (HMG) proteins in the sera of patients with juvenile rheumatoid arthritis (JRA).

METHODS

Seventy-seven pauciarticular-onset JRA serum samples from antinuclear antibody (ANA)-positive patients and 42 polyarticular-onset JRA patient sera found to react with HMG-2 by immunoblotting were used in this study. To identify B cell epitopes on HMG-2, recombinant HMG-2 protein fragments were used in enzyme-linked immunosorbent assay (ELISA) and in competition ELISA experiments with a set of overlapping synthetic peptides. Fine epitope mapping was achieved by oligopeptide synthesis, followed by immunoblotting.

RESULTS

Pauciarticular, but not polyarticular, JRA patient sera were found to recognize a lysine-rich major epitope (KKGKKKDP), which is located in the linker region of the HMG box domains of the HMG-2 nonhistone chromosomal protein. No significant immunoreactions were observed in sera from ANA-negative JRA patients and in sera from children with nonrheumatic diseases, indicating that this epitope seems to be specific for pauciarticular-onset JRA.

CONCLUSION

In addition to our previous finding that JRA sera will react with a defined epitope on HMG-17, pauciarticular JRA patient sera were also found to recognize a defined epitope on the HMG-2 protein, thus suggesting the importance of this epitope in the etiology of JRA.

Analysis of the molecular basis of synovial rheumatoid factors in rheumatoid arthritis

The objective of this study was to better understand the molecular basis of IgM rheumatoid factor in rheumatoid arthritis (RA). We recently generated 10 different monoclonal IgM RF (mRF) molecules isolated from the synovium of a single patient with RA. The heavy (H) and light chain (L) variable region (V) genes of these 10 mRFs were cloned and sequenced. Six mRFs used kappa light chains and 4 mRFs used lambda light chains. Of particular interest, 8 of 10 heavy chains used the JH4 joining region gene, and all five VH4 heavy chains used the DK4 diversity region gene with the JH4. Four of the VH4 clones used the same germline gene, likely representing a novel but closely related germline gene to VH4.18, and may be clonally related because of the extensive homology in their heavy chain sequence. Two VH4 clones shared the same light chain gene, VkappaIIIb kv325 (99% homology) and the same JK4 joining region gene, while three VH4 clones used two different light chain genes, an uncommon Vkappa4 and a Vlambda4 gene, respectively. In this RA patient, there was recurrent utilization of VH4-DK4-21/10-JH4 genes and a recurring association with gene elements Vkappa3 and Vlambda4. Recurring usage of Vkappa3 (kv325) and Vlambda4 (lv418) gene elements may result from a light chain editing process whereby immature autoreactive B cells encountering self-antigen attempt, and often succeed, in altering their specificities through secondary Ig light chain gene rearrangement. Moreover, the oligoclonality of these RFs suggest clonal relatedness secondary to an antigen-driven response.

The 52-kd protein as a target of intermolecular spreading of the immune response to components of the SS-A/Ro-SS-B/La complex

OBJECTIVE

To determine whether immunization of healthy non-autoimmune mice with 52-kd SS-A/Ro induces a secondary antibody response to other components of the 48-kd SS-B/La-60-kd SS-A/Ro RNP complex and vice versa, since anti-52-kd antibodies have been invariably linked to these antigens in patients with Sjogren's syndrome and in mothers whose children have neonatal lupus.

METHODS

Female BALB/c mice were immunized with 100 microg of 6xHis recombinant human 48-kd SS-B/La, 52-kd SS-A/Ro, or 60-kd SS-A/Ro proteins, or the 6xHis polypeptide control, each purified by Ni2+ affinity chromatography. Mice subsequently received booster injections with 50 microg of the same antigen every 10-21 days. Immune responses were measured by enzyme-linked immunosorbent assay (ELISA), immunoblotting of recombinant antigens, and immunoprecipitation of 35S-methionine-labeled in vitro translation products.

RESULTS

Immunization with 48-kd SS-B/La resulted in anti-48-kd SS-B/La antibodies within 45 days, followed 10 days later by a secondary response to 52-kd SS-A/Ro, as measured by ELISA. Antibody spreading to 60-kd SS-A/Ro was not detected. Immunization with 52-kd SS-A/Ro resulted in rapid high-titer anti-52-kd SS-A/Ro responses within 27 days. Spreading to 48-kd SS-B/La occurred in only 1 mouse and 60-kd SS-A/Ro was detected in a minority of the mice after prolonged antigen exposure. Immunization with 60-kd SS-A/Ro led to anti-60-kd SS-A/Ro responses within 37 days, followed 3 months later by low-titer anti-48-kd SS-B/La and anti-52-kd SS-A/Ro antibodies. All primary immune responses were confirmed by immunoblotting and immunoprecipitation. While immunoblotting of the recombinant proteins revealed reciprocal intermolecular spreading in the majority of mice, immunoprecipitation clearly demonstrated that predominant spreading was generated after immunization with 48-kd SS-B/La, which consistently resulted in antibodies to 52-kd SS-A/Ro.

CONCLUSION

The murine responses observed in the present study, demonstrating reciprocal intermolecular spreading to 48-kd SS-B/La, 52-kd SS-A/Ro, and 60-kd SS-A/Ro, support the linkage of 52-kd SS-A/Ro with the other proteins, despite their as-yet-undetected association in vivo. The marked recruitment of anti-52-kd SS-A/Ro responses elicited by 48-kd SS-B/La may provide a lead to exploring the physical interaction, direct or indirect, of 52-kd SS-A/Ro with the SS-A/Ro-SS-B/La RNP particle and its presentation to the immune system. These data should facilitate the establishment of a murine model of neonatal lupus.

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.

Arrhythmogenicity of IgG and anti-52-kD SSA/Ro affinity-purified antibodies from mothers of children with congenital heart block

An important advance in the description and understanding of congenital heart block (CHB) came in the 1970s with the observation that mothers of affected infants frequently had autoimmune diseases and, in particular, that many maternal sera contained antibodies to SSA/Ro and SSB/La ribonucleoproteins. Although the molecular biology of the candidate antigens has been extensively defined, the arrhythmogenic and electrophysiological effects of their cognate antibodies on the human fetal heart are unknown. In the present study, we provide evidence that IgG-enriched fractions and anti-52-kD SSA/Ro antibodies affinity-purified from sera of mothers whose children have CHB induce complete atrioventricular (AV) block in the human fetal heart perfused by the Langendorff technique and inhibit L-type Ca2+ currents at the whole-cell and single-channel level. Immunization of female BALB/c mice with recombinant 52-kD SSA/Ro protein generated high-titer antibodies that crossed the placenta during pregnancy and were associated with varying degrees of AV conduction abnormalities, including complete AV block, in the pups. These findings strongly suggest that anti-52-kD SSA/Ro antibodies are causally related to the development of CHB.

Production of anti-Ro/SS-A and anti-La/SS-B autoantibodies is closely coordinated in systemic lupus erythematosus and independent of anti-dsDNA production

Autoimmune diseases such as Sjögren's Syndrome and systemic lupus erythematosus (SLE) are characterized by serum autoantibodies against protein components of small cytoplasmic ribonucleoproteins (scRNPs). The origin and regulation of these anti-Ro/SS-A and anti-La/SS-B antibodies is not well understood. Regular co-occurrence of these two autoantibodies in humans together with murine studies on antibody responses against scRNPs after immunization suggest a role for scRNPs as common antigen. We sought additional evidence for this hypothesis in a longitudinal serological study of patients with SLE. Quantitative measurement of the antibody responses against Ro/SS-A and La/SS-B proteins and for comparison to dsDNA in 852 serum samples was performed. These samples were collected from nine patients during an average observation period of more than 10 years. A significant and strong correlation between the two anti-scRNP responses emerged during 90% of follow-up. In contrast, correlation of anti-scRNP with anti-dsDNA responses was remarkably absent in the same patients. Our results confirm the unique relationship between anti-Ro/SS-A and anti-La/ SS-B responses and could thus be interpreted as support for a model wherein induction and perpetuation of autoantibody production is dependent on scRNPs containing both proteins as antigen.

Epitope spreading

Epitope (determinant) spreading is the development of immune responses to endogenous epitopes secondary to the release of self antigens during a chronic autoimmune or inflammatory response. The past year has seen considerable advances in our understanding of the contribution of epitope spreading to the chronic pathogenesis of experimental T-cell-mediated and antibody-mediated autoimmune diseases. Most significantly, conclusive functional evidence for a major role for epitope spreading in the chronic pathogenesis of murine relapsing-remitting experimental autoimmune encephalomyelitis, a CD4+ T-cell-mediated model of multiple sclerosis, was forthcoming.

Hierarchical self-tolerance to T cell determinants within the ubiquitous nuclear self-antigen La (SS-B) permits induction of systemic autoimmunity in normal mice

Systemic autoimmune diseases are frequently associated with clustering of high titer autoantibody responses towards nuclear self-antigens. Little is known, however, about the extent of immune tolerance to the target nuclear antigens or the events leading to the complex autoantibody responses that are characteristic of systemic autoimmunity. To address these issues, we have examined the mouse immune response to La autoantigen (mLa) and the homologous human La antigen (hLa), which are components of the La(SS-B)/Ro(SS-A) ribonucleoprotein (RNP) complex targeted in systemic lupus erythematosus and primary Sjögren's syndrome. The findings reveal the presence of hierarchical T cell tolerance involving multiple autodeterminants within the La autoantigen expressed by normal H-2k and H-2a mice. At one end of this spectrum, there was no detectable T or B cell autoimmunity observed in mice that were immunized with the immunodominant mLa287-301 determinant, which differed by a single residue in its core sequence from the homologous but highly immunogenic human La288-302 determinant. Interestingly, the mLa287-301 peptide acted as an altered peptide ligand that specifically antagonized the activation of an hLa288-302-specific T cell hybridoma. In contrast to the tolerogenic mLa287-301 determinant, a range of autoimmune potential was identified among poorly tolerizing, subdominant self-peptides present within mouse La autoantigen. Notably, immunization of normal mice with the autologous subdominant La25-44 and La106-129 determinants resulted in limited or no detectable autoantibody response. In contrast, immunization with the subdominant mouse La13-30 determinant induced a proliferative T cell response associated with the appearance of specific autoantibodies recognizing multiple intrastructural (La) and intermolecular components (Ro) of the murine La/Ro RNP. The findings suggest how diversified autoimmunity might follow initiation of immunity to simple peptide mimics of poorly tolerogenic determinants that are present within ubiquitous self-antigens.

Increased expression of CD40 ligand on systemic lupus erythematosus lymphocytes

The specificity of T cell help for B cell activation and differentiation is maintained by the brief expression on the T cell surface, following T cell receptor-mediated triggering, of CD40 ligand (CD40L). Interaction of T helper (Th) cell CD40L with B cell CD40 induces B cell activation, cell surface expression of activation antigens, proliferation, and initiation of immunoglobulin isotype switch. We predicted that in patients with systemic lupus erythematosus (SLE), in whom Th cell-dependent production of autoantibodies results in immune complex-mediated tissue damage, CD40L expression might be augmented, prolonged, or abnormally regulated. Baseline expression of CD40L was increased in some SLE patients studied, when compared with control subjects. While Th cells from normal subjects (n = 14) and rheumatic disease control patients (n = 9) showed maximal expression of CD40L, after in vitro activation with phorbol myristate acetate (PMA) and ionomycin, at 6 h of culture with diminished levels observed at 24 and 48 h, Th cells from SLE patients (n = 19) maintained high level cell surface expression of CD40L through 24 and 48 h of culture. The prolonged expression of CD40L was functionally significant, as 24 h-activated SLE T cells, when cocultured with target B cells, induced greater B cell surface CD80 (B7-1) expression than did 24 h-activated normal T cells. These results document impaired regulation of CD40L expression in SLE T cells and identify an important potential target for therapy in this systemic autoimmune disease.

Late development of anti-La/SS-B antibodies in a patient with Sjogren's syndrome and high titer anti-Ro/SS-A antibodies

The frequent coexistence of anti-Ro and anti-La autoantibodies is well described, however, there is little evidence of sequential development of these two autoantibodies. We report a case of typical Sjogren's syndrome with high titer anti-Ro antibodies, who subsequently developed anti-La antibodies later in the course. This case suggests that the anti-La antibodies may actually follow the anti-Ro antibodies in some cases as hypothesized in the concept of linked set of autoantibodies, analogous to development of anti-Sm in certain anti-nRNP antibody positive SLE patients and animal models.

Epitope mapping of B-cell determinants on the 15-kilodalton lipoprotein of Treponema pallidum (Tpp15) with synthetic peptides

The antigenicity of the 15-kDa lipoprotein of Treponema pallidum (Tpp15 or TpN15) was comprehensively evaluated in epitope-scanning studies with overlapping deca- and octapeptides and polygonal rabbit and human infant immunoglobulins (Igs) and antisera. This approach enabled us to identify potentially important regions and to determine the optimal dilutions of Igs or antisera for use in further studies. IgM and IgG from both species were capable of recognizing multiple, continuous epitopes. A total of 13 peptides, principally clustered in the central regions of the protein, were recognized by all syphilitic sera and Ig fractions. On the basis of window analyses, frequency profiles, and alanine substitution studies, five heptapeptides were selected for mimetic studies. Two of these five immunodominant, continuous epitopes initially appeared to be species specific; however, antisera elicited against mimetics of all five epitopes were polyspecific, recognizing similar motifs on several other treponemal proteins, including those of avirulent organisms. The only mimetic which yielded positive reactions with infant IgM and syphilitic sera in the absence of cross-reactions with rabbit antisera to avirulent treponemes was the variant of the VMYASSG motif. These findings are relevant to the development of simple, inexpensive assays for the serodiagnosis of active syphilis.

Distinctive immune response patterns of human and murine autoimmune sera to U1 small nuclear ribonucleoprotein C protein

The Ul small nuclear ribonucleoprotein (snRNP), a complex of nine proteins with Ul RNA, is a frequent target of autoantibodies in human and murine systemic lupus erythematosus (SLE). Anti-Sm antibodies recognizing the B'/B, D, E, F, and G proteins of Ul snRNPs are highly specific for SLE, and are nearly always accompanied by anti-nRNP antibodies recognizing the Ul snRNP-specific 70K, A, and/or C proteins. Previous studies suggest that human anti-nRNP antibodies recognize primarily the U1-70K and Ul-A proteins, whereas recognition of Ul-C is less frequent. We report here that autoantibodies to U1-C are more common in human autoimmune sera than believed previously. Using a novel immunoprecipitation technique to detect autoantibodies to native Ul-C, 75/78 human sera with anti-nRNP/ Sm antibodies were anti-Ul-C (+). In striking contrast, only 1/65 anti-nRNP/Sm (+) MRL mouse sera of various Igh allotypes was positive. Two of ten anti-nRNP/Sm (+) sera from BALB/c mice with a lupus-like syndrome induced by pristane recognized Ul-C. Thus, lupus in MRL mice was characterized by a markedly lower frequency of anti-U1-C antibodies than seen in human SLE or pristane-induced lupus. The results may indicate different pathways of intermolecular-intrastructural diversification of autoantibody responses to the components of Ul snRNPs in human and murine lupus, possibly mediated by alterations in antigen processing induced by the autoantibodies themselves.

Stability of immunoblot profile of anti-SSA/Ro-SSB/La antibodies over time in mothers whose children have neonatal lupus

Neonatal lupus is strongly associated with antibodies reactive with SSA/Ro-SSB/La proteins, independent of maternal disease activity or classification. We sought to determine whether the fine specificity of antibody profiles remains stable or evolves over time and whether these findings relate to clinical status. Sera from 23 mothers whose children had neonatal lupus (22 heart block, one skin) were evaluated by SDS-immunoblot. For each mother two samples were available at least 13 months apart; the mean duration of time between testing was 45 months +/- 27 S.D. (range 13-108 months). Twenty-two of the 23 initial profiles were identical to the results obtained in a later sample. The health status of seven (30%) of 23 mothers changed after the birth of the affected infant but the immunoblot specificity of the antibodies remained unchanged. SLE was the initial and final diagnosis in the only mother whose profiles differed, with development of weak reactivity to 48 kD SSB/La in addition to the 52kD SSA/Ro after 14 months. In conclusion, the fine specificity of anti-SSA/Ro-SSB/La antibodies as assessed by immunoblot is highly stable for years. Progression of clinical status was not associated with a concomitant change in antibody profile.

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.

Features of autoantigens

The major cellular antigens recognized by autoantibodies in SLE and other systemic autoimmune diseases have been identified and characterized over the past 25 years. The pioneering studies of Eng Tan demonstrate the importance of autoantibodies as diagnostic markers. However, why certain autoantibodies, such as anti-Sm, are pathognomonic of SLE, while others are markers of other autoimmune disease subsets, remains unanswered. This central question continues to drive much current research into the pathogenesis of SLE. Features of the autoantigens recognized by autoantibodies may provide important clues to the causes of lupus. Most autoantigens in systemic autoimmunity are multicomponent nucleoprotein complexes. These particles are encountered by the immune system as units, resulting in the tandem production of autoantibodies recognizing several components of the same complex. However, the intermolecular-intrastructural spreading of autoimmunity is regulated by mechanisms that at present are defined poorly. Also unexplained is the observation that the antigenic determinants recognized by autoantibodies are restricted and frequently correspond to active sites or functional domains. Analysis of experimental models of autoimmunity suggests that altering the structure of autoantigens, due to abnormal protein-protein interactions, hapten binding, altered degradation, or other mechanisms, could help to explain both the restricted patterns of autoantibody spreading and the selective targeting of antigenic sites. This may be a worthwhile area for further investigation of the pathogenesis of systemic autoimmune diseases.

Ribonucleoprotein complexes as autoantigens

Many intracellular proteins and nucleic acids, that are involved in important biosynthetic pathways, are targeted by autoantibodies occurring spontaneously in the sera of patients with systemic autoimmune diseases. Frequently, the autoantigens are assembled into multicomponent complexes containing both nucleic acid(s) and proteins. Recently, progress has been made in the study of autoantigenic ribonucleoprotein complexes, the most important of which are spliceosomal ribonucleoproteins, nucleolar ribonucleoproteins, Ro/La ribonucleoproteins and complexes of aminoacyl-tRNA synthetase and tRNA. In addition to new structural and functional information, important results have been obtained on epitope spreading, as well as on a potential role for apoptosis during the development of an autoimmune response against these complexes.

Antibodies to synovial antigens in recent-onset rheumatoid arthritis

OBJECTIVE

To identify synovial antigens that bind to serum antibodies from subjects with recent-onset rheumatoid arthritis (RA) (< 6 months of synovitis).

METHODS

Soluble and insoluble fractions of Triton X-100 extracts of RA and normal synovial tissue, and normal spleen and placenta, were immunoblotted with sera from 27 patients with recent-onset RA, 13 autoimmune disease control subjects, and 13 blood bank control donors. Bound immunoglobulin was probed with 125I-labeled protein A.

RESULTS

Antibodies in the sera of 20 (74%) patients with recent-onset RA recognized at least 1 of 5 antigens in both a disease- and tissue-specific and nonspecific manner. Anti-La antibodies, usually associated with primary Sjögren's syndrome, were detected in 2 sera. Eight sera had increased reactivity to an IgG heavy and light chain dimer. There was strong binding of 8 sera with a 35-kd doublet and of 3 sera with a 55-kd species in RA and normal synovial lysates (insoluble fractions). Two sera uniquely recognized a 45-kd protein only in the RA synovial lysate (soluble fraction).

CONCLUSION

IgG antibodies in the sera of patients with recent-onset RA show positive immunoblots for 3 novel synovial antigens of 35-kd, 55-kd, and 45-kd, as well as for 2 previously characterized antigens (La and IgG). Thus, a variety of synovial antigens appear to be recognized by B cells even early in the clinical course of RA.

Human autoantibodies directed against the RNA recognition motif of La (SS-B) bind to a conformational epitope present on the intact La (SS-B)/Ro (SS-A) ribonucleoprotein particle

In systemic autoimmunity, the human B cell response to the La (SS-B) autoantigen is polyclonal and directed to both conserved and human-specific epitopes. This study has further characterized the B cell epitope(s) present within the conserved central region of the La protein, LaC (amino acids 111-242) containing the RNA recognition motif (RRM, aa 111-187). Ten overlapping and non-overlapping protein fragments spanning LaC were expressed in bacteria as NH2-terminal fusions with glutathione-S-transferase. The fusion proteins were tested by ELISA for reactivity with a panel of human anti-La sera in order to define the nature of the epitopes. Ninety-two percent of patient sera containing anti-La antibodies reacted with the region of La containing the RRM. Fine mapping of this reactivity using deletion mutants indicated that the deletion of 19 amino acids from either the NH2-terminal or COOH-terminal region of the RRM was associated with loss of antibody reactivity, suggesting that the immunodominant epitope expressed in this region is discontinuous. Autoantibodies affinity-purified from the La RRM fragment to remove other specificities immunoprecipitated newly synthesized native La (SS-B)/Ro (SS-A) complexes, providing additional evidence that autoantibodies were recognizing a conformational epitope. The findings indicate that the human autoantibody response to La involves recognition of a conformational determinant involving the conserved RRM region without necessarily interfering with the RNA-dependent association of the La/Ro ribonucleoprotein.