Proteasome inhibition with bortezomib depletes plasma cells and autoantibodies in experimental autoimmune myasthenia gravis.

Bortezomib, an inhibitor of proteasomes, has been reported to reduce autoantibody titers and to improve clinical condition in mice suffering from lupus-like disease. Bortezomib depletes both short- and long-lived plasma cells; the latter normally survive the standard immunosuppressant treatments targeting T and B cells. These findings encouraged us to test whether bortezomib is effective for alleviating the symptoms in the experimental autoimmune myasthenia gravis (EAMG) model for myasthenia gravis, a disease that is characterized by autoantibodies against the acetylcholine receptor (AChR) of skeletal muscle. Lewis rats were immunized with saline (control, n = 36) or Torpedo AChR (EAMG, n = 54) in CFA in the first week of an experimental period of 8 wk. After immunization, rats received twice a week s.c. injections of bortezomib (0.2 mg/kg in saline) or saline injections. Bortezomib induced apoptosis in bone marrow cells and reduced the amount of plasma cells in the bone marrow by up to 81%. In the EAMG animals, bortezomib efficiently reduced the rise of anti-AChR autoantibody titers, prevented ultrastructural damage of the postsynaptic membrane, improved neuromuscular transmission, and decreased myasthenic symptoms. This study thus underscores the potential of the therapeutic use of proteasome inhibitors to target plasma cells in Ab-mediated autoimmune diseases.

Silencing rapsyn in vivo decreases acetylcholine receptors and augments sodium channels and secondary postsynaptic membrane folding.

The receptor-associated protein of the synapse (rapsyn) is required for anchoring and stabilizing the nicotinic acetylcholine receptor (AChR) in the postsynaptic membrane of the neuromuscular junction (NMJ) during development. Here we studied the role of rapsyn in the maintenance of the adult NMJ by reducing rapsyn expression levels with short hairpin RNA (shRNA). Silencing rapsyn led to the average reduction of the protein levels of rapsyn (31% loss) and AChR (36% loss) at the NMJ within 2 weeks, corresponding to previously reported half life of these proteins. On the other hand, the sodium channel protein expression was augmented (66%) in rapsyn-silenced muscles. Unexpectedly, at the ultrastructural level a significant increase in the amount of secondary folds of the postsynaptic membrane in silenced muscles was observed. The neuromuscular transmission in rapsyn-silenced muscles was mildly impaired. The results suggest that the adult NMJ can rapidly produce postsynaptic folds to compensate for AChR and rapsyn loss.

Immunosuppression of experimental autoimmune myasthenia gravis by mycophenolate mofetil.

Currently used non-specific immunosuppressive drugs often require intervention in myasthenia gravis (MG) and clinical improvement varies widely. To analyze the therapeutic effect of mycophenolate mofetil (MMF) in experimental autoimmune MG (EAMG), rats were immunized with acetylcholine receptors (AChRs) and subsequently treated with MMF or vehicle. MMF treatment resulted in a significant suppression of anti-rat AChR antibody titers. Interestingly, no abnormalities of neuromuscular transmission and adverse side effects were detected in MMF-treated EAMG animals. Moreover, anti-rat AChR antibody titers correlated to an improvement of clinical outcome. In conclusion, our data suggest that MMF acts as a potent immunosuppressant drug in EAMG.

Treatment of myasthenia gravis by preventing acetylcholine receptor modulation.

Myasthenia gravis (MG) is an autoimmune disease caused by antibodies mainly directed to the acetylcholine receptor (AChR) of the neuromuscular junction. Induction of antigenic modulation and complement activation by such autoantibodies leads to ultrastructural damage of the postsynaptic membrane and loss of AChR and associated proteins. Reduction of antigenic modulation by increasing the expression of the receptor-associated anchor protein, rapsyn, or by functionally monovalent competing IgG4 anti-AChR antibodies was shown to prevent MG disease activity. We propose that preventing antigenic modulation can be used as a treatment strategy for MG.