Immunopathology of acetylcholine receptors in myasthenia gravis

Ocular myasthenia: a protean disorder

Ocular myasthenia is a localized form of myasthenia clinically involving only the extraocular, levator palpebrae superioris, and/or orbicularis oculi muscles. Ocular manifestations can masquerade as a variety of ocular motility disorders, including cranial nerve and gaze palsies. A history of variable and fatiguable muscle weakness suggests this diagnosis, which may be confirmed by the edrophonium (Tensilon) test and acetylcholine receptor antibody titer. Anticholinesterases, corticosteroids and other immunosuppressive agents, and other therapeutic modalities, including thymectomy and plasmapheresis, are used in treatment. As the pathophysiology of myasthenia has been elucidated in recent years, newer treatment strategies have evolved, resulting in a much more favorable prognosis than several decades ago. This review provides historical background, pathophysiology, immuno-genetics, diagnostic testing, and treatment options for ocular myasthenia, as well as a discussion of drug-induced myasthenic syndromes.

Selective removal of anti-acetylcholine receptor antibody in the low temperature operation of membrane plasma fractionation

The removal of anti-acetylcholine receptor antibody (anti-AChR Ab) by the plasma fractionator, Kuraray EVAL 2A, was measured as a function of the filtration temperature (4 and 37 degrees C). Plasmas procured from eight myasthenia gravis (MG) patients undergoing routine plasma exchanges (PE: membrane plasma filtration: n = 4, and centrifugation: n = 4) were used in the studies. Plasma flow rate was 20 ml/min, and plasma perfusion in single pass was terminated when the transmembrane pressure of the filter reached 300 mm Hg. Solute concentration data from before and after perfusion were used to assess the sieving coefficient (SC) of each solute. Results show that the SCs for anti-AChR Ab and albumin were significantly (P < .0002) lower at 4 degrees C (0.15 and 0.52, respectively) as compared to 37 degrees C (0.44 and 0.72, respectively). The SC of anti-AChR Ab (0.15) was significantly lower than the SC of IgG (0.42; P < .006) at 4 degrees C, even though it also belongs to the IgG class. The ratio of the SC of anti-AChR Ab to albumin at 4 degrees C (0.29) was significantly lower than that at 37 degrees C (0.57; P < .003). These data indicate that the selectivity of removal of anti-AChR Ab from albumin is higher at 4 degrees C than that at 37 degrees C. The volume treated at 4 degrees C was significantly lower than that treated at 37 degrees C, and it was less than that required for a clinical treatment; however, it was shown that filter backwashing is possible without loss of solute removal selectivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Spleen cell dialyzable extracts (SCDE) mediated immunotherapy for patients with myasthenia gravis

We studied 14 cases of myasthenia gravis (MG) with regard to their anti-acetylcholine receptor antibody (AChR-Ab) production, peripheral blood CD4+/CD8+ ratio, and responses of peripheral lymphocytes to PHA and PWM stimulations. We found that the clinical severity was not proportional to AChR-Ab level in serum in terms of whole myasthenic population, whereas in one individual patient there was a close correlation between clinical severity and the AChR-Ab level in serum. The CD4+/CD8+ ratio in myasthenic patients was significantly higher than that of normal controls. The responses of T- and B-cells to mitogenic stimulations were impaired in myasthenic patients. The immune abnormalities found in the majority of the patients were restored to about normal after spleen cell dialyzable extracts (SCDE) therapy was applied to all patients. The results implicated that SCDE, working as an immunomodulator, could serve as a promising agent to normalize the abnormal immune functions of MG patients or other autoimmune diseases.

Treatment of experimental autoimmune myasthenia gravis in rabbits with leupeptin, a protease inhibitor

We injected 12 New Zealand white rabbits intraperitoneally with 15 mg/kg Leupeptin on alternative days for about 4 months. After 1 week of Leupeptin treatment, they were challenged with purified acetylcholine receptor (AChR) from Torpedo californica in Freund's complete adjuvant. All control animals died within 60 days. Six animals treated with Leupeptin did not develop EAMG in spite of repeated AChR injections. Three animals developed clinical signs of EAMG after 65 days. The clinical course was short in the one that survived and prolonged in the 2 that finally died. All animals (Leupeptin-treated and controls) had circulating anti-AChR antibodies. Among the survivors, titers were slightly lower and EMG repetitive stimulation tests were normal. Leupeptin (0.02-200 mM) did not prevent curaremimetic [3H]toxin binding to AChR in membranes or in solution, nor dissociate AChR-toxin-antibody complexes. Immune response to antigens other than receptor remained intact in Leupeptin-treated animals. Leupeptin was not toxic at the doses given. The mechanism of this protection is not well understood. Leupeptin seems to decelerate the turnover rate of AChR induced by anti-AChR antibodies and/or to decrease the complement-mediated immune attack against the muscle end-plate.

Thymectomy in myasthenia with pure ocular symptoms

Eighteen patients with exclusively ocular symptoms of myasthenia were thymectomised. Suspected thymoma, resistance to pyridostigmine therapy or relapse following immunosuppressive therapy were taken as indications for surgery. The mean preoperative observation period before operation was 40 months, and after operation was 26 months. There was no operative or postoperative morbidity or mortality. Histological thymic abnormalities were found in all patients (in one case, thymoma; in four, persistent thymus; in 13, thymic hyperplasia). The histological abnormalities were identical to those found in generalised myasthenia. This included the distribution of T-cell subtypes as identified by use of monoclonal antibodies. The severity of ocular symptoms was rated using a score developed for this purpose. The score progressively declined after surgery to an average of 70% of its initial amount in 80% of patients. Full remission occurred in three cases. No patient developed generalized myasthenia. Antibody titres against acetylcholine receptors if elevated preoperatively also dropped following surgery, with one exception. Clear criteria for the expected therapeutic success of thymectomy could not be identified. Based on our results, and on the assumed significance of the thymus gland for pathogenesis, thymectomy should be considered in patients with pure ocular symptoms.

Immunisation with Torpedo acetylcholine receptor

Acetylcholine mediates the transfer of information between neurons in the electric organ of, for example, Torpedo as well as in vertebrate skeletal muscle. The nicotinic acetylcholine receptor complex translates the binding of acetylcholine into ion permeability changes. This leads to an action potential in the muscle fibre. The nicotinic acetylcholine receptor protein has been purified from Torpedo by use of affinity chromatography. The receptor is an intrinsic membrane glycoprotein composed of five polypeptide chains. When various animals are immunised with the receptor they demonstrate clinical signs of severe muscle weakness coincident with high antibody titres in their sera. The symptoms resemble those found in the autoimmune neuromuscular disease myasthenia gravis in humans. This animal model has constituted a unique model for studying autoimmune diseases. This paper reviews some of the work using Torpedo acetylcholine receptor in order to increase the understanding of the motor nervous system function and myasthenia gravis. It is now known that the nicotinic acetylcholine receptor protein is the antigen involved in myasthenia gravis. The mechanism of immune damage involves a direct block of the receptor function. This depends on the presence of antibodies which crosslink the postsynaptic receptors leading to their degradation. The questions to be answered in the future are; (a) what initiates or triggers the autoimmune response, (b) how do the antibodies cause the symptoms--is there a steric hindrance of the interaction of acetylcholine and the receptor, (c) why is there not a strict relationship between antibody titre and severity of symptoms, and (d) why are some muscles affected and other spared? With help of the experimental model, answers to these questions may result in improved strategies for the treatment of the autoimmune disease myasthenia gravis.

Immunology of myasthenia gravis

Anti-acetylcholine-receptor antibody is demonstrable in more than 90 per cent of patients with myasthenia gravis. Serum antibody titers do not show a direct correlation with disease severity, although in certain patients antibody levels increase in association with disease activity. Impairment of neuromuscular transmission results from the loss of junctional receptors, either as a result of receptor internalization or destruction of junctional folds containing the acetylcholine receptor. Myasthenia gravis manifests immunologic, genetic, and clinical similarities to rheumatic syndromes, suggesting a generic immune dysfunction common to these disorders.