Molecular recognition of acetylcholine receptor. Recognition by alpha-neurotoxins and by immune and autoimmune responses and manipulation of the responses

Myasthenia gravis induced in mice by immunization with the recombinant extracellular domain of rat muscle-specific kinase (MuSK)

Myasthenia gravis (MG) is mostly caused by anti-acetylcholine receptor (AChR) auto-antibodies (Abs). Such Abs are undetectable in 10-15% of MG patients, but many have anti-muscle-specific kinase (MuSK) Abs. We injected recombinant rat-MuSK extracellular domain in H-2(a), H-2(b), H-2(bm12) and H-2(d) mice. Certain strains exhibited exercise-induced fatigue, tremors, weight loss, and some died after 2-3 injections. Compound muscle action potentials showed decrement with low-frequency repetitive nerve stimulation. Miniature endplate potentials decreased, suggesting lower numbers of endplates functional AChRs. Myasthenic sera inhibited agrin-induced AChR aggregation in C2C12 myotubes.

CONCLUSION

Anti-MuSK Abs induce MG, which might also result from blocking the agrin-signaling pathway.

Split tolerance in a novel transgenic model of autoimmune myasthenia gravis

Because it is one of the few autoimmune disorders in which the target autoantigen has been definitively identified, myasthenia gravis (MG) provides a unique opportunity for testing basic concepts of immune tolerance. In most MG patients, Abs against the acetylcholine receptors (AChR) at the neuromuscular junction can be readily identified and have been directly shown to cause muscle weakness. T cells have also been implicated and appear to play a role in regulating the pathogenic B cells. A murine MG model, generated by immunizing mice with heterologous AChR from the electric fish Torpedo californica, has been used extensively. In these animals, Abs cross-react with murine AChR; however, the T cells do not. Thus, to study tolerance to AChR, a transgenic mouse model was generated in which the immunodominant Torpedo AChR (T-AChR) alpha subunit is expressed in appropriate tissues. Upon immunization, these mice showed greatly reduced T cell responses to T-AChR and the immunodominant alpha-chain peptide. Limiting dilution assays suggest the likely mechanism of tolerance is deletion or anergy. Despite this tolerance, immunization with intact T-AChR induced anti-AChR Abs, including Abs against the alpha subunit, and the incidence of MG-like symptoms was similar to that of wild-type animals. Furthermore, evidence suggests that this B cell response to the alpha-chain receives help from T cells directed against the other AChR polypeptides (beta, gamma, or delta). This model offers a novel opportunity to elucidate mechanisms of tolerance regulation to muscle AChR and to clarify the role of T cells in MG.