B cell epitopes of the heterogeneous nuclear ribonucleoprotein A2: identification of a new specific antibody marker for active lupus disease.

OBJECTIVES: Autoantibody formation and T cell reactivity against the heterogeneous nuclear ribonucleoprotein A2 (hnRNP-A2) has been observed in patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Since no differences in epitope recognition were reported and the usefulness of anti-hnRNP-A2 antibodies as diagnostic markers of SLE is unknown, it was our objective to characterise linear B cell epitopes of hnRNP-A2 and to relate the anti-hnRNP-A2 antibody responses to disease activity and clinical features of SLE. METHODS: Sequential serum samples from 15 patients with SLE and sera from patients with other rheumatic diseases and healthy subjects were investigated by ELISA for autoantibody reactivities against a set of 13 overlapping peptides spanning the RNA-binding region of hnRNP-A2. Antibody reactivity against the complete protein was determined by western immunoblotting and ELISA. SLE disease activity was assessed by European Consensus Lupus Activity Measure scores, by SLE Index scores and the British Isles Lupus Assessment index. RESULTS: Anti-peptide antibody reactivities were found in 60% of SLE sera but in only 5% of control samples, and were mainly directed to four peptides, one of which (p155-175) appeared to be immunodominant. Antibodies to p155-175 were exclusively seen in patients with SLE and correlated with clinical disease activity as well as kidney and skin involvement. No correlations were found for the other anti-peptide antibody responses. CONCLUSION: Peptide p155-175 encompasses a disease-specific immunodominant epitope of hnRNP-A2. Since antibodies to p155-175 correlate with disease activity and nephritis, they may be useful as markers for active SLE.

Murine models of systemic lupus erythematosus: B and T cell responses to spliceosomal ribonucleoproteins in MRL/Fas(lpr) and (NZB x NZW)F(1) lupus mice.

(NZB x NZW)F(1) and MRL/Fas(lpr) lupus mice present a similar phenotype with a spectrum of autoantibodies associated with very severe nephritis. It is thought, however, that in contrast to other lupus-prone mice such as MRL/Fas(lpr) mice, (NZB x NZW)F(1) mice do not generate autoantibodies to ribonucleoproteins (RNP) Sm/RNP. In this study, we demonstrate that contrary to previous reports, the autoimmune response directed against Sm/RNP antigens also occurs in NZB x NZW mice. CD4(+) T cells from unprimed 10-week-old NZB x NZW mice proliferate and secrete IL-2 in response to peptide 131-151 of the U1-70K protein, which is known to contain a T(h) epitope recognized by CD4(+) T cells from MRL/Fas(lpr) mice. Peptide 131-151, which was found to bind I-A(k) and I-E(k) class II MHC molecules, also bound both I-A(d) and I-E(d) molecules. This result led us to also re-evaluate longitudinally the anti-Sm/RNP antibody response in NZB x NZW mice. We found that 25-week-old mice do produce antibodies reacting with several small nuclear and heterogeneous nuclear (hn) RNP proteins, such as SmD1, U1-70K and hnRNP A2/B1 proteins. The fine specificity of these antibodies was studied with overlapping synthetic peptides. The same antigenically positive and negative peptides were characterized in MRL/Fas(lpr) and NZB x NZW mice in the three proteins. This new finding can help to understand the mechanisms involved in the development of the anti-Sm/RNP antibody response and, particularly, the role played by non-MHC genes in this autoimmune response.

B and T cell responses to the spliceosomal heterogeneous nuclear ribonucleoproteins A2 and B1 in normal and lupus mice.

Autoantibodies directed against spliceosomal heterogeneous nuclear ribonucleoproteins (hnRNPs) are a typical feature of rheumatoid arthritis, systemic lupus erythematosus, and mixed-connective tissue disease. With the aim of investigating a potential pathogenic role of these Abs, we have studied the Ab response to A2/B1 hnRNPs in different murine models of lupus. The specificity of anti-A2/B1 Abs was tested with a series of 14 overlapping synthetic peptides covering the region 1-206 of A2 that contains most of the epitopes recognized by patients' Abs. A major epitope recognized very early during the course of the disease by Abs from most of MRL lpr/lpr mice but not from other lupus mice and from mice of different MHC haplotypes immunized against B1 was identified in residues 50-70. This peptide contains a highly conserved sequence RGFGFVTF also present in other hnRNPs and small nuclear ribonucleoproteins. Abs reacting with a second A2 epitope identified in residues 35-55 were detectable several weeks later, suggesting an intramolecular B cell epitope spreading during the course of the disease. We identified several T cell epitopes within the region 35-175 that generated an effective Th cell response with IL-2 and IFN-gamma secretion in nonautoimmune CBA/J mice sharing the same MHC haplotype H-2k as MRL/lpr mice. None of the peptides stimulated T cells primed in vivo with B1. Because Abs to peptide 50-70 were detected significantly earlier than Abs reacting with other A2 peptides and the protein itself, it is possible that within the protein, this segment contains residues playing an initiator role in the induction of the anti-A2/B1 and antispliceosome Ab response.

Conformational restriction of the Tyr53 side-chain in the decapeptide HE.

A series of conformationally restricted analogs of the hen egg lysozyme (HEL) decapeptide 52-61 in which the conformationally flexible Tyr53 residue was replaced by several more constrained tyrosine and phenylalanine analogs was prepared. Among these tyrosine and phenylalanine analogs were 1,2,3,4-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (Htc), 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic), 4-amino- 1,2,4,5-tetrahydro-8-hydroxy-2-benzazepine-3-one (Hba), 4-amino-1,2,4,5-tetrahydro-2-benzazepine-3-one (Aba), 2-amino-6-hydroxytetralin-2-carboxylic acid (Hat) and 2-amino-5-hydroxyindan-2-carboxylic acid (Hai) in which the rotations around Calpha-Cbeta and Cbeta-Cgamma were restricted because of cyclization of the side-chain to the backbone. Synthesis of Pht-Hba-Gly-OH using a modification of the Flynn and de Laszlo procedure is described. Analogs of beta-methyltyrosine (beta-MeTyr) in which the side-chains were biased to particular side-chain torsional angles because of substitution at the beta-hydrogens were also prepared. These analogs of HEL[52-61] peptide were tested for their ability to bind to the major histocompatibility complex class II I-Ak molecule and to be recognized in this context by two T-cell hybridomas, specific for the parent peptide HEL[52-61]. The data showed that the conformation and also the configuration of the Tyr53 residue influenced both the binding of the peptide to I-Ak and the recognition of the peptide/I-Ak complex by a T-cell receptor.

MHC class II gene associations with autoantibodies to U1A and SmD1 proteins.

Autoantibodies against U small nuclear ribonucleoproteins (snRNP) are frequently present in the serum of patients with systemic rheumatic diseases, and have been reported to be associated with HLA-DR and -DQ genes. To better define the role of HLA genes in the production of such antibodies, we studied immunogenetic associations with autoantibodies reacting with U1 RNP, U1A and SmD1 proteins, and synthetic peptides containing immunodominant linear epitopes of these proteins. Only two out of the 15 overlapping peptides of U1A (i.e. peptides 35-58 and 257-282) and three of 11 peptides of SmD1 (i.e. peptides 1-20, 44-67 and 97-119) were significantly recognized by patients' sera selected on the basis of their antibody positivity with RNP in immunodiffusion. The distribution of DRB1, DQB1 and DPB1 alleles among the anti-RNP antibody-positive patients (n = 28) and healthy control subjects was similar. Antibodies against U1A (tested in Western immunoblotting with HeLa cell extracts) were positively associated to DRB1*06 allele; antibodies reacting with SmD1 peptide 44-67 were negatively associated to DRB1*02 and DQB1*0602 alleles. No association was found between DPB1 alleles and antibodies reacting with U1A and SmD1 antigens. This first study reporting an association between autoantibodies reacting with U1A and SmD1 proteins (and peptides of these proteins), and immunogenetic markers suggest that the production of antibody subsets directed against different components (or regions of these proteins) bound to the same snRNP particle is associated with distinct MHC class II alleles.

At least three linear regions but not the zinc-finger domain of U1C protein are exposed at the surface of the protein in solution and on the human spliceosomal U1 snRNP particle.

No structural information on U1C protein either in its free state or bound to the spliceosomal U1 small nuclear ribonucleoprotein (snRNP) particle is currently available. Using rabbit antibodies raised against a complete set of 15 U1C overlapping synthetic peptides (16-30 residues long) in different immunochemical tests, linear regions exposed at the surface of free and U1 snRNP-bound U1C were identified. Epitopes within at least three regions spanning residues 31-62, 85-103 and 116-159 were recognized on free and plastic-immobilized recombinant human U1C expressed in Escherichia coli, on in vitro translated U1C protein and on U1C bound to the U1 snRNP particle present in HeLa S100 extract. Using a zinc affinity labeling method, we further showed that the N-terminal U1C peptide containing a zinc-finger motif (peptide 5-34) effectively binds65Zn2+. The N-terminal region of U1C, which is functional in U1 snRNP assembly, is apparently not located at the surface of the U1 snRNP particle.

Human monoclonal autoantibody fragments from combinatorial antibody libraries directed to the U1snRNP associated U1C protein; epitope mapping, immunolocalization and V-gene usage.

To study the localization and function of the U1snRNP associated U1C protein, so far only human sera from systemic lupus erythematosus (SLE) overlap syndrome patients have been used. Here we report for the first time the isolation of human monoclonal anti-UIC autoantibody fragments from IgG derived combinatorial and semi-synthetic human antibody libraries. Two classes of human monoclonal anti-UIC (auto)antibodies were found: specific anti-U1C autoantibodies, recognizing U1C only, and cross-reactive antibodies which also react with U1A and Sm-B/B'proteins. The heavy chains (V(H)genes) of all five antibodies from the semi-synthetic libraries and two of the three U1C-specific patient derived autoantibody fragments are encoded by V(H)3 genes, in which V(H) 3-30 (DP-49) was overrepresented. The heavy chain of the two cross-reactive autoantibodies are derived from the 3-07 (DP-54) gene. Three epitope regions on the U1C protein are targeted by these antibodies. (1) Four U1C specific antibodies recognize an N-terminal region of U1C in which amino acids 30-63 are essential for recognition, (2) two antibodies recognize only the complete U1C protein, and (3) two cross-reactive and one U1C specific antibody recognize the C-terminal domain in which amino acids 98-126 are critical for recognition. The two cross-reactive antibodies (K 11 and K 15) recognize the proline-rich region of the U1C protein (amino acids 98 126) and cross-react with proline-rich regions in Sm-B/B' (amino acids 163-184) and U1A (amino acids 187-204). All 10 antibody fragments are able to immunoprecipitate the native U1snRNP particle. The two cross-reactive antibodies immunoprecipitate the other Sm containing snRNPs as well. Using confocal immunofluorescence microscopy we could show that the major part of the U1C protein is localized within the coiled body structure.

In vivo T helper cell response to retro-inverso peptidomimetics.

Peptide analogues containing reversed peptide bonds between each residue along the peptide sequence (retro-inverso modification) have been analyzed for their antigenic and in vivo immunogenic properties in the MHC II and Th cell response context. Two antigenic peptides were selected for this study, namely peptide 103-115 of poliovirus VP1, which is involved in the production of Abs that neutralize the infectivity of the virus, and peptide 435-446 from the third constant region of mouse heavy chain IgG2a allopeptide gamma 2ab, which mimics a corneal Ag implicated in autoimmune keratitis. In a competition assay performed in vitro using reference hybridomas of known MHC class II restriction, both retro-inverso analogues bound (although more weakly in our test) to I-Ad and/or I-Ed class II molecules. However, in both cases, this lower affinity was apparently largely compensated in vivo, as a T cell response (with IL-2 secretion), equivalent to that obtained with the wild-type peptides, was observed following immunization of BALB/c mice with the retro-inverso analogues. Moreover, these T cells proliferated and produced IL-2 in response to the cognate peptides. It is concluded that the T cell receptors of T cells primed in vivo with the retro-inverso analogues readily cross-react with parent and retro-inverso analogue-MHC complexes. The approach of using pseudopeptides containing changes involving the backbone, and not the orientation of side chains, may thus be promising to design potent immunogens for class II-restricted T cells.

Comparison of two different methods using overlapping synthetic peptides for localizing linear B cell epitopes in the U1 snRNP-C autoantigen.

We have compared the performances of two different approaches using overlapping synthetic peptides to identify the location of linear epitopes of the U1 snRNP-C autoantigen. The first method was based on the use of 15 overlapping peptides (16-30 residue-long) synthesized using conventional Fmoc chemistry, removed from the resin by a standard cleavage procedure, and tested by ELISA after direct coating to polyvinyl microtiter plates. The second approach used a commercial kit (SPOT) to synthesize 75 overlapping decapeptides on cellulose membrane which were assayed by a direct immunoenzymatic test. Both standard and SPOTscan methods were evaluated with antibodies raised in rabbits against synthetic peptides of U1C and sera from patients with autoimmune diseases. In addition to inherent problems linked to the SPOT synthesis (in particular the impossibility of checking the quality of peptides), a number of limitations in the SPOTscan method were identified (e.g. a certain lack of sensitivity and, in one case, the complete lack of peptide reactivity due to the removal of charged end groups at both extremities). However, we found no background with sera from autoimmune patients in the SPOTscan and the antigenic maps obtained using the two approaches generally agreed. This study shows that the SPOTscan approach represents a simple, relatively non expensive and rapid method for initial screening to identify candidate sequences that may be dominant linear epitopes in a protein. Subsequent analysis and controls should include the preparation of conventionally synthesized peptides for formal immunochemical investigations.

Retro-inverso peptidomimetics as new immunological probes. Validation and application to the detection of antibodies in rheumatic diseases.

Retro-inverso peptides which contain NH-CO bonds instead of CO-NH peptide bonds are much more resistant to proteolysis than L-peptides. Moreover, they have been shown recently to be able to mimic natural L-peptides with respect to poly- and monoclonal antibodies (Guichard, G., Benkirane, N., Zeder-Lutz, G., Van Regenmortel, M. H. V., Briand, J. P., and Muller, S. (1994b) Proc. Natl. Acad. Sci. U.S.A. 91, 9765-9769). We have further tested the capacity of retro-inverso peptidomimetics to serve as possible targets for antibodies produced by lupus mice and by patients with rheumatic autoimmune diseases. Several retro-inverso peptides corresponding to sequences known to be recognized by autoantibodies were synthesized, namely peptides 28-45 and 130-135 of H3, 277-291 of the Ro/SSA 52-kDa protein, and 304-324 of the Ro/SSA 60-kDa protein, and tested with autoimmune sera by enzyme-linked immunosorbent assay. We have found that retro-inverso peptides are recognized as well as or even better than natural peptides by antibodies from autoimmune patients and lupus mice. This new approach may lead to important progress in the future development of immunodiagnostic assays, particularly in the case of diseases characterized by inflammatory reactions in the course of which the level of degradative enzymes is increased.