DETECTION OF ALPHA-SUBUNIT ISOFORMS IN HUMAN MUSCLE ACETYLCHOLINE-RECEPTOR BY SPECIFIC T-CELLS FROM A MYASTHENIA-GRAVIS PATIENT

The nicotinic acetylcholine receptor (AChR) is both the best-character ized transmitter receptor-ion channel and the target for the pathogeni c antibodies in the human autoimmune disease myasthenia gravis (MG). I n cloning and sequencing its components in man, we found that the a-su bunit was transcribed in two isoforms, with (P3A(+)) or without (P3A(- )) a 75 base pair exon that had not been described in other species. W hile studying the human T lymphocyte response to recombinant AChR, we found that part of this P3A insert was recognized by one T cell line ( from an MG patient), whereas another line only recognized the uninterr upted insertion site. To establish whether this exon is also translate d in normal human muscle, we initially raised anti-peptide antibodies to the relevant amino acid sequences, but these failed to bind native AChR (affinity-purified from muscle on alpha-neurotoxin columns). We t herefore exploited the great sensitivity and specificity of these T ce lls to detect the two isoforms after unfolding by antigen-presenting c ells, and have been able to show that both are expressed in affinity-p urified human muscle AChR.