Th1 epitope repertoire on the alpha subunit of human muscle acetylcholine receptor in myasthenia gravis.

In myasthenia gravis (MG), CD4+ T helper cells recognize the muscle acetylcholine receptor (AChR) alpha subunit. We investigated the epitope repertoire of anti-AChR blood CD4+ Th1 cells from 13 myasthenic patients and three healthy controls, using overlapping synthetic peptides screening the alpha subunit sequence and an enzyme-linked immunospot (ELISPOT) assay that detects antigen-induced interferon-gamma secretion of individual Th1 cells. All patients recognized a pool of the alpha subunit peptides. All but one patient recognized numerous peptides. Each patient had an individual pattern of peptide recognition, but most or all patients recognized four sequences (residues 48-67, 101-137, 304-322, and 403-437) that stimulated relatively large numbers of Th1 cells. They include previously identified "immunodominant" sequences recognized by AChR-specific CD4+ T cell lines from myasthenic patients. Peptide 1-14 was also recognized frequently. The controls recognized, with a low precursor frequency, the peptide pool and a few peptides that frequently included the immunodominant sequences described above. The present results demonstrate that Th1 cells are involved in the anti-AChR response in MG and that their epitope repertoire is very complex. This indicates that when MG is clinically evident, the AChR itself is the sensitizing antigen and the target of the autoimmune Th1 cells, although it does not exclude that molecular mimicry between one AChR epitope and a microbial structure may have triggered this autoimmune response. Although the complexity of the Th1 repertoire suggests that development of specific immunosuppressive treatments targeted on epitopes recognized by autoimmune T cells will be difficult, the existence of immunodominant T epitope sequences might facilitate that task.