The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.
Background: Autoantigen La/SSB is molecular target of humoral autoimmunity in patients with primary Sjogren's Syndrome (pSS) and systemic lupus erythematosus (SLE). In this study, we investigated the existence and possible influence of anti-idiotypic response to anti-La/SSB antibodies. Materials and Methods: Synthetic peptide analogs (pep) of the major antigenic determinants of La/SSB (289-308aa and 349-364aa) were prepared. Based on "molecular recognition" theory, complementary peptides (cpep), derived by anti-parallel readings of the noncoding strand of La/SSB DNA encoding for its antigenic determinants, were constructed. Sera from 150 patients with anti-La/SSB antibodies, 30 patients without anti-La/SSB antibodies, and 42 normal individuals were tested against all four peptides. F(abЈ) 2 fragments from anti-peptide IgG were prepared and F(abЈ) 2 -IgG interactions were evaluated using a specific anti-idiotypic ELISA. Results: All four peptides were recognized by anti-La positive sera (83% and 51% for pep and cpep 349-364 and 51% and 28% for pep and cpep289-308, respectively). Anti-cpep F(abЈ) 2 bound to a common idiotype (Id) located within or spatially close to the antigen combining site of anti La/SSB (anti-pep) antibodies. Homologous and crossinhibition experiments further confirmed this relation. The anti-idiotypic antibodies inhibited the anti-La/SSB antibody binding to recombinant La/SSB by 91%. To overcome the anti-idiotypic interference in anti-La/SSB detection, a specific assay was developed. Sera were heated for dissociation of Id-anti-Id complexes, anti-Id antibodies blocked with cpep, and anti-La/SSB reactivity was recovered. Application of this method to anti-Ro positive-anti-La/SSB "negative" sera showed that all anti-Ro/SSA positive autoimmune sera also possess anti-La/SSB antibodies. This reaction was not observed in 14 anti-Ro negative-anti-Sm/RNP positive sera from patients with SLE. Conclusions: Autoimmune sera from patients with pSS and SLE contain anti-idiotypic antibodies targeting a common anti-La/SSB idiotype. These antibodies can be detected using complementary peptides of La/SSB epitopes. The antiidiotypic antibodies mask the anti-La/SSB response. Hidden anti-La/SSB antibodies can be released and detected using complementary epitope analogs.
Myasthenia gravis (MG) is caused by autoantibodies against the nicotinic acetylcholine receptor (AChR) of the neuromuscular junction. The anti-AChR antibodies are heterogeneous. However, a small region on the extracellular part of the AChR alpha subunit, called the main immunogenic region (MIR), seems to be the major target of the anti-AChR antibodies, but not of the specific T-cells, in experimental animals and possibly in MG patients. The major loop of the overlapping epitopes for all testable anti-MIR monoclonal antibodies (MAbs) was localized within residues 67-76 (WNPADYGGIK for Torpedo and WNPDDYGGVK for human AChR) of the alpha subunit. The N-terminal half of alpha 67-76 is the most critical, Asn68 and Asp71 being indispensable for binding. Yet anti-MIR antibodies are functionally and structurally quite heterogeneous. Anti-MIR MAbs do not affect channel gating, but they are very potent in mediating acceleration of AChR degradation (antigenic modulation) in cell cultures and in transferring experimental MG in animals. Fab fragments of anti-MIR MAbs bound to the AChR prevent the majority of the MG patients' antibodies from binding to and causing loss of the AChR. Whether this inhibition means that most MG antibodies bind on the same small region or is a result of broad steric/allosteric effects is under current investigation.
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