PEPTIDE-SELECTED T-CELL LINES FROM MYASTHENIA-GRAVIS PATIENTS AND CONTROLS RECOGNIZE EPITOPES THAT ARE NOT PROCESSED FROM WHOLE ACETYLCHOLINE-RECEPTOR

To study pathogenic T helper cells in myasthenia gravis (MG) reacting against the acetylcholine receptor (AChR), we have previously selected five CD4(+) T cell lines/clones from MG patients (or healthy controls ) against full-length recombinant human AChR alpha subunit (alpha 1-43 7); these can all recognize AChR solubilized from human muscle. Recent ly, T cells selected with pooled AChR subunit synthetic peptides have shown greater heterogeneity than above. Hoping to validate that, we ha ve characterized three MG and six control T cell lines selected with p ooled peptides (averaging 33 residues long) covering the alpha subunit sequence; recurring responses to three particular peptides each showe d preferred HLA class II restrictions-p75-115/DR4, p138-167/DR4, and p 309-344/DR3 (or DR52a). However, none of three lines from MG patients recognized p138-167-even one from a previous responder to this epitope in full-length alpha 1-437; otherwise they resembled those from contr ols. Moreover, no peptide-selected line responded significantly to who le AChR, alpha 1-437, or even to shorter polypeptides sharing one term inus with the peptide, suggesting specificity for epitopes not natural ly processed by APCs from blood. Of 20 sublines maintained with indivi dual peptides, at least 10 responded to independently synthesized over lapping sequences, but four others depended on contaminants in the ori ginal peptides. A single line did recognize one longer polypeptide, bu t only after tryptic digestion; the processing of this cryptic epitope was evidently the limiting factor here rather than its concentration or the T cell sensitivity. Therefore, while synthetic peptides are ess ential for mapping epitopes, assessment of the pathogenic MG T cell re pertoire requires full-length Ag processed naturally.