RESIDUES WITHIN THE ALPHA-SUBUNIT SEQUENCE-304-322 OF MUSCLE ACETYLCHOLINE-RECEPTOR FORMING AUTOIMMUNE CD4(+) EPITOPES IN BALB C MICE/

BALB/c mice develop myasthenic symptoms after immunization with rodent acetylcholine receptor (AChR). After immunization with Torpedo AChR ( TAChR), their CD4(+) cells become strongly sensitized against a conser ved region of the TAChR alpha subunit sequence (residues alpha 304-322 ), and cross-react vigorously with the homologous sequences of mouse a nd human AChR, which are almost identical. Therefore AChR-specific pot entially autoreactive CD4(+) cells exist in this strain. We immunized BALB/c mice with the synthetic TAChR sequence alpha 304-322. The CD4() cells thus sensitized responded to TAChR, indicating that they recog nize an epitope(s) produced upon TAChR processing. They recognized pep tide alpha 304-322 in association with the I-A(d) molecule. Anti-alpha 304-322 CD4(+) cells cross-reacted well with the corresponding murine and human synthetic sequences. To identify residues involved in forma tion of an autoimmune epitope(s), CD4(+) cells from mice immunized wit h peptide alpha 304-322 were challenged in vitro with single residue g lycine-substituted analogues of this sequence. Substitution of residue W-311, and of any residue within the sequence alpha 313-319 (RKVFIDT) , consistently and, in some cases, strongly affected the CD4(+) cells response. Substitution of residues in the region alpha 311-319 had var iable effects in different experiments, and in general affected modera tely the CD4(+) response. These results suggest that anti-alpha 304-32 2 CD4(+) cells comprise several clones, recognizing overlapping epitop es which share residues alpha 311-319. The importance of the sequence region alpha 311-319 for formation of CD4(+) cell epitope(s) was verif ied by testing CD4(+) cells sensitized to T alpha 304-322 with analogu es of this sequence, carrying nonconservative substitutions at positio ns Q(310), K-314 and D-318 Substitution of Q(310) had minimal or no ef fects, while those of K-314 or D-318 strongly affected the CD4(+) cell response.