Murine T and B cell epitopes within the human recombinant acetylcholine receptor alpha-subunit

Myasthenia gravis is amongst the best characterised of autoimmune diseases and is an antibody mediated T cell dependent disorder. The target antigen is the nicotinic acetylcholine receptor (an integral, five subunit, membrane protein) of the neuromuscular junction. The α-subunit is the major site of antibody attack both in myasthenia gravis and its experimental animal model. To investigate the innate immunogenicity of the human acetylcholine receptor α-subunit, recombinant polypeptide fragments were prepared and purified. Inbred mice of four defined MHC class II haplotypes were immunised with the recombinant α-subunit fragment r37-429. T cell lines were propagated from draining popliteal lymph nodes at ten days and serum antibodies were measured after three inoculations. T and B cell epitopes within r37-429 were identified using recombinant and synthetic peptides. Antigen specific T cells and antibodies were obtained in all four strains. The recombinant α-subunit did not induce high levels of antibody and the T and B cell epitopes were limited. A small number of epitopes were identified in the extracellular sequence; α40-65 and α84-98 for T cells and α138-167 for B cells. An immunodominant region towards the carboxy-terminus was recognised by T cells (α347-429), and in the intracellular sequence (α325-368) by B cells. No mice became clinically weak, probably because only antibodies that bound to intracellular sequences crossreacted with the whole acetylcholine receptor protein. The co-mapping of B and T cell epitopes on a well characterised autoantigen has not previously been reported. This approach provides a suitable system for investigating epitope specific manipulation of the immune response.