We have identified in an extracellular segment of the alpha1 subunit of
nicotinic acetylcholine receptor (AChR) of homologous muscle a 17-residue
autoantigen that, without conjugation to a carrier, activates Th lymphocytes and induces production of
autoantibodies that cause electrophysiologic signs of
experimental autoimmune myasthenia gravis. A panel of overlapping synthetic
peptides revealed two
T cell epitopes, one encompassed by residues 121-136 and the other by 129-145. Residues 129 (
glutamic acid) and 130 (
isoleucine) were implicated in a pathogenic
B cell epitope.
Peptide 129-145 (Glu-Ile-
Ile-Val-Thr-
His-Phe-Pro-Phe-
Asp-Glu-Gln-Asn-Cys-
Ser-Met-Lys, a conserved sequence in rat and human AChR) induced
autoantibody production in 76% of rats. All seropositive rats had evidence of
experimental autoimmune myasthenia gravis; five of five tested had electrophysiologic signs, and all had loss of immunochemically measured autologous muscle AChR. Analogues of 129-145, with single residues substituted by
alanine, revealed
phenylalanine 135,
phenylalanine 137, and
glutamic acid 139 as most important determinants of Ag/MHC-II/TCR interactions;
phenylalanine 137 is critical for T cell activation. B cells were the major MHC-II-positive cell type to which the self-
peptide 129-145 bound in a population of nonimmune splenic cells. More efficient processing and presentation of the Th cell
epitope by an expanded population of immune B cells selected by specifically binding another
epitope of the same
peptide would greatly amplify the production of
autoantibodies.
Peptide autoantigens of this type could plausibly perpetuate the
autoantibody response in
myasthenia gravis, and are a rational target for strategies aimed at Ag-specific therapeutic intervention.