Direct proof of the in vivo pathogenic role of the AChR autoantibodies from myasthenia gravis patients.

Several studies have suggested that the autoantibodies (autoAbs) against muscle acetylcholine receptor (AChR) of myasthenia gravis (MG) patients are the main pathogenic factor in MG; however, this belief has not yet been confirmed with direct observations. Although animals immunized with AChR or injected with anti-AChR monoclonal Abs, or with crude human MG Ig fractions exhibit MG symptoms, the pathogenic role of isolated anti-AChR autoAbs, and, more importantly, the absence of pathogenic factor(s) in the autoAb-depleted MG sera has not yet been shown by in vivo studies. Using recombinant extracellular domains of the human AChR α and ß subunits, we have isolated autoAbs from the sera of four MG patients. The ability of these isolated anti-subunit Abs and of the Ab-depleted sera to passively transfer experimental autoimmune MG in Lewis rats was investigated. We found that the isolated anti-subunit Abs were at least as efficient as the corresponding whole sera or whole Ig in causing experimental MG. Abs to both α- and ß-subunit were pathogenic although the anti-α-subunit were much more efficient than the anti-ß-subunit ones. Interestingly, the autoAb-depleted sera were free of pathogenic activity. The later suggests that the myasthenogenic potency of the studied anti-AChR MG sera is totally due to their anti-AChR autoAbs, and therefore selective elimination of the anti-AChR autoAbs from MG patients may be an efficient therapy for MG.

Anti-MuSK- and anti-AChR-positive myasthenia gravis induced by d-penicillamine.

BACKGROUND: Myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction usually caused by antibodies to the nicotinic acetylcholine receptor (AChR) and occasionally to muscle-specific kinase (MuSK). D-penicillamine is a therapeutic agent for several diseases, but can also induce a number of immune-mediated disorders, including MG, as a side-effect. In most patients with D-penicillamine-induced MG, anti-AChR antibodies are detected, but the presence of anti-MuSK antibodies has not been reported previously. CASE: The case reported was a female patient who presented with myasthenic symptoms after D-penicillamine administration for scleroderma. RESULTS: Both anti-AChR and anti-MuSK antibodies were identified in the patient's serum. The anti-MuSK antibodies were of the IgG4 subclass, as in idiopathic MG. Both types of antibody gradually disappeared after discontinuation of D-penicillamine. A significant improvement in symptoms was observed and the patient gradually became free of MG symptoms, without requiring any treatment for MG. Another four double-positive (anti-AChR and anti-MuSK antibodies) patients were identified during a retrospective study, but none had been treated with D-penicillamine. CONCLUSION: D-penicillamine can cause anti-AChR and anti-MuSK antibody-positive MG, a rare phenomenon which is reversed after discontinuation of D-penicillamine treatment.

Recent approaches to the development of antigen-specific immunotherapies for myasthenia gravis.

Acquired autoimmune myasthenia gravis (MG) is the most common disease that affects the neuromuscular junction (NMJ). MG is associated with autoantibodies (auto-Abs) to components of the NMJ. About 85-90% of MG patients have auto-Abs against the muscle nicotinic acetylcholine receptor (AChR), while about half of the remaining patients have auto-Abs against muscle-specific kinase. Auto-Abs, in combination with local deposition of complement, reduce the number of available post-synaptic nicotinic AChRs and thereby impair neuromuscular transmission. Current medications for MG are non-specific and include acetylcholinesterase inhibitors, immunosuppressants, plasma exchange, intravenous Ig administration and thymectomy. Treatments that selectively target the anti-AChR auto-Abs may prove to be more effective and free of side-effects. We here review two approaches aimed at the development of antigen-specific therapies for MG. The first is specific apheresis of Abs from patients' sera using immobilised recombinant AChR domains as immunoadsorbents. Indeed, we have recently shown that the combined recombinant extracellular domains of all human AChR subunits are capable of specifically immunoadsorbing the majority of pathogenic auto-Abs from several MG sera. The second therapeutic approach is the development of non-pathogenic anti-AChR monoclonal Abs that could potentially be used as protective agents by blocking the binding of patients' auto-Abs to the AChR.

Towards antigen-specific apheresis of pathogenic autoantibodies as a further step in the treatment of myasthenia gravis by plasmapheresis.

Myasthenia gravis (MG), a prototypic antibody-mediated autoimmune disease, presents an excellent target for scientific research aimed at a better understanding of the disease itself and the source that triggers an autoimmune reaction in an organism. MG is a neuromuscular disease caused mainly by an autoimmune response against the nicotinic acetylcholine receptor (AChR) which interferes with neuromuscular transmission. This review focuses on our studies on the extracellular domains of human muscle AChR subunits in an effort to develop an approach for the specific therapeutic apheresis of autoantibodies from patients' sera using the immobilized subunits as immunoadsorbents. The ability of the anti-AChR antibodies isolated by this technique, but not of the depleted sera, to induce disease is also described. This review is dedicated to the late Prof. John Newsom-Davis, who was the first to introduce the use of plasmapheresis for MG.

Isolation and functional characterization of anti-acetylcholine receptor subunit-specific autoantibodies from myasthenic patients: receptor loss in cell culture.

The muscle nicotinic acetylcholine receptor (nAChR) is the major autoantigen in the autoimmune disease myasthenia gravis (MG), in which autoantibodies bind to, and cause loss of, nAChRs. Antibody-mediated nAChR loss is caused by the action of complement and by the acceleration of nAChR internalization caused by antibody-induced cross-linking of nAChR molecules (antigenic modulation). To obtain an insight into the role of the various anti-nAChR antibody specificities in MG, we have studied nAChR antigenic modulation caused by isolated anti-subunit autoantibodies. Autoantibodies against the nAChR alpha or beta subunits were isolated from four MG sera by affinity chromatography on columns carrying immobilized recombinant extracellular domains of human nAChR expressed in the yeast Pichia pastoris. The isolated anti-alpha and anti-beta autoantibodies, as well as untreated MG sera, induced nAChR antigenic modulation in TE671 cells. Partially antibody-depleted sera exhibited reduced modulating activity, whereas a serum completely depleted of anti-nAChR antibodies exhibited no nAChR modulation. Interestingly, the anti-alpha autoantibodies were, on average, approximately 4.3 times more effective than the anti-beta autoantibodies. The present work supports the notion that anti-nAChR autoantibodies may be the sole nAChR-reducing factor in anti-nAChR antibody-seropositive MG, and that anti-alpha-subunit autoantibodies are the dominant pathogenic autoantibody specificity.