John Newsom-Davis

Auto-antibodies to the receptor tyrosine kinase MuSK in patients with myasthenia gravis without acetylcholine receptor antibodies

Myasthenia gravis (MG) is an antibody-mediated autoimmune disease of the neuromuscular junction. In approximately 80% of patients, auto-antibodies to the muscle nicotinic acetylcholine receptor (AChR) are present. These antibodies cause loss of AChR numbers and function, and lead to failure of neuromuscular transmission with muscle weakness. The pathogenic mechanisms acting in the 20% of patients with generalized MG who are seronegative for AChR-antibodies (AChR-Ab) have not been elucidated, but there is evidence that they also have an antibody-mediated disorder, with the antibodies directed towards another, previously unidentified muscle-surface–membrane target. Here we show that 70% of AChR-Ab–seronegative MG patients, but not AChR-Ab–seropositive MG patients, have serum auto-antibodies against the muscle-specific receptor tyrosine kinase, MuSK. MuSK mediates the agrin-induced clustering of AChRs during synapse formation, and is also expressed at the mature neuromuscular junction. The MuSK antibodies were specific for the extracellular domains of MuSK expressed in transfected COS7 cells and strongly inhibited MuSK function in cultured myotubes. Our results indicate the involvement of MuSK antibodies in the pathogenesis of AChR-Ab–seronegative MG, thus defining two immunologically distinct forms of the disease. Measurement of MuSK antibodies will substantially aid diagnosis and clinical management.

CONTROLLED TRIAL OF PREDNISOLONE IN ACUTE POLYNEUROPATHY

In a multicentre, randomised trial of prednisolone in acute polyneuropathy of undetermined aetiology (Guillain-Barré syndrome), 21 patients were treated with prednisolone (60 mg daily for one week, 40 mg daily for four days, and then 30 mg daily for three days) and 19 did not have steroid treatment. Patients were graded on a six-point scale by one of two neurologists who had no knowledge of the treatment schedule. Reassessment at one, three, and twelve months consistently showed greater improvement in the control than the prednisolone group but the only statistically significant result was in the improvement at three months among patients entered to the trial within a week of onset of illness. The 6 control patients had improved by 2.5 +/- 0.43 grades by three months from entry to the trial whereas the 10 prednisolone patients had only improved by 0.9 +/- 0.46 grades (P less than 0.05). There was 1 death related to the polyneuropathy in each group, and 1 suicide in a control patient during convalescence. 6 prednisolone patients were left with considerable disability compared with 1 control patient. There were 3 relapses in the prednisolone group, but none in the control group. The results indicate that steroid treatment is not beneficial and can be detrimental in acute neuropathy of undetermined aetiology.

AUTOIMMUNE AETIOLOGY FOR MYASTHENIC (EATON-LAMBERT) SYNDROME

The myasthenic (Eaton-Lambert) syndrome, associated with carcinoma of the bronchus in one patient and with immunological disorders in two others, improved after plasma exchange--observations supported by electromyographic evidence in two cases. Prednisolone and azathioprine treatment led to almost complete remission in one of the non-neoplastic cases and to improvement in the other. The IgG fraction of plasma from all three patients, injected daily (10 mg) into mice for 37-77 days, significantly reduced the initial compound muscle action potential and the quantal content of the end-plate potential measured in the diaphragm, when compared with control human IgG. These results indicate that an IgG autoantibody, binding to nerve terminal determinants, may be responsible for the disorder of neuromuscular transmission in the myasthenic syndrome, and that immunosuppressive drugs may be useful in treating the nonneoplastic form of the disease.

Function of circulating antibody to acetylcholine receptor in myasthenia gravis Investigation by plasma exchange

Plasma exchange has been used to investigate the function of antiacetylcholine receptor (anti-AChR) antibody in seven patients with acquired myasthenia gravis (MG) who had elevated antibody titers and in one patient with congenital MG who had normal titers. There was an inverse association between clinical indices of muscle strength and anti-AChR titers, with a minimum time lag of 2 days for the clinical response. The inverse association of the clinical state with anti-AChR antibody titers was closer than that with total immunoglobulin G or other immunoglobulins, and is consistent with the view that the anti-AChR antibody is the principal factor interfering with neuromuscular transmission in acquired MG.

Myasthenia gravis without acetylcholine-receptor antibody: a distinct disease entity.

Immunoglobulin preparations from eight patients with clinical features of myasthenia gravis, in whom no serum antibody against acetylcholine receptor (AChR) could be detected, were injected intraperitoneally into mice. Neuromuscular transmission was significantly impaired compared with mice receiving control human immunoglobulin. No antibody bound to the mouse AChR was detected, but there was a small loss (9.4%) of AChR in the mouse diaphragms. Mice injected with myasthenic AChR-antibody-positive immunoglobulin and mice growing hybridoma cells secreting monoclonal AChR antibody showed similar impairment of neuromuscular transmission, but 75% and 94%, respectively, of their muscle AChR had antibody bound and AChR losses were 47% and 60%. The results suggest that a pathogenetic immunoglobulin antibody interferes with neuromuscular transmission in these AChR-antibody-negative patients by binding to non-AChR determinants at the neuromuscular junction. This form of myasthenia is immunologically and physiologically distinct from the AChR-antibody-positive form.

Neurological paraneoplastic syndromes in patients with small cell lung cancer. A prospective survey of 150 patients.

One hundred and fifty patients presenting with small cell lung cancer (SCLC) to chest physicians, were assessed neurologically. Neuromuscular or autonomic deficits were common and occurred in up to 44% of cases. Weakness, dry mouth, and weight loss were not mutually independent and may represent the syndrome formerly described as carcinomatous neuromyopathy. By contrast, undoubted paraneoplastic syndromes were much less commonly detected. Two patients had the Lambert-Eaton myasthenic syndrome (LEMS) and one had subacute sensory neuropathy (SSN). In these patients, neurological symptoms antedated other manifestations of cancer, by between six and 17 months. The 95% confidence interval for the prevalence of LEMS or SSN among SCLC patients was 0-4%, consistent with the results of previous retrospective or smaller studies: summing these, the overall prevalence of LEMS among SCLC patients is close to 3%, which implies about 250 new cases per annum in England and Wales. If LEMS and SSN are the least uncommon neurological paraneoplastic syndromes in SCLC patients, this may reflect the accessibility of motor nerve terminals and dorsal root ganglia to cross-reactive anti-tumour cell antibodies.

Antibodies to motor nerve terminals: an electrophysiological study of a human myasthenic syndrome transferred to mouse.

Immunoglobulin G(IgG) prepared from the plasma of patients with a presynaptic disorder of neuromuscular transmission (Lambert‐Eaton myasthenic syndrome, l.e.m.s.), or from normal pooled control human plasma, was injected into mice (10 mg daily) for up to 99 days. Micro‐electrodes were used to record end‐plate potentials from the diaphragm muscle bathed in normal Krebs solution containing tubocurarine (1.0‐4.6 microM). At 0.5 Hz nerve stimulation frequency, the quantal content was significantly reduced (P less than 0.01‐P less than 0.001) in mice treated with six l.e.m.s. patients’ IgG each compared with paired controls. The pooled quantal content was 55 +/‐ 3 (n = 110 end‐plates) for all test animals and 131 +/‐ 9 (n = 47) for all controls (P less than 0.001). During short trains at 20 or 40 Hz nerve stimulation, control muscles showed marked depression, while test muscles showed either facilitation or less marked depression. Quantal content throughout these trains remained lower than in controls. The results indicate that IgG antibody from l.e.m.s. patients can induce a similar physiologic disorder in injected mice, and they support the view that this antibody interferes with evoked release of transmitter in l.e.m.s. by binding to nerve terminal determinants.

Serum factor in Miller-Fisher variant of Guillain-Barré syndrome and neurotransmitter release.

Serum IgG autoantibodies to GQ1b ganglioside are associated with the acute phase of the Miller-Fisher syndrome (MFS). We investigated the effects of three anti-GQ1b-positive MFS sera in the mouse phrenic-nerve/diaphragm preparation. Miniature endplate potential frequencies increased eight-fold within 25 min, declined rapidly, and ceased altogether after 3 h, when nerve stimulation no longer evoked a response. One MFS convalescent serum (anti-GQ1b negative) and sera from healthy controls and from patients with other neurological diseases were without effect. Thus muscle weakness in MFS may be caused by a serum factor, likely to be GQ1b antibody, that leads to failure of acetylcholine release from motor nerve terminals.

LONG-TERM EFFECTS OF REPEATED PLASMA EXCHANGE IN MYASTHENIA GRAVIS

Plasma exchange produces a short-term clinical improvement in myasthenia gravis (M.G.) which may be attributable to removal of acetylcholine receptor (AChR) antibody. The possibility that repeated plasma exchanges might confer cumulative long-term benefits was investigated. Serum-AChR-antibody and clinical response were followed for 4--12 months (mean 8 months) in six M.G. patients receiving 4--25 plasma exchanges of 2--4 1 together with immunosuppressive drugs (azathioprine [2.5 mg/kg] with or without alternate-day prednisone therapy), and in seven M.G. patients on immunosuppressive drugs alone. Percentage decrease in AChR antibody was not significantly different in the two treatment groups. Decline in antibody titre was associated with clinical improvement. Eight patients with previous thymoma showed significantly greater decline in antibody than the remaining five patients, irrespective of plasma exchange. Since repeated plasma exchange had no cumulative long-term benefit, the value of this treatment as used here lies only in short-term control of severe M.G. symptoms.

Mature, long-lived CD4+ and CD8+ T cells are generated by the thymoma in myasthenia gravis

Antibodies to muscle acetylcholine receptors, to other muscle antigens, and to some cytokines are found in the majority of patients with thymic tumors (thymomas) and myasthenia gravis (MG). The role of the tumor in initiating autoimmunity, however, is unclear; in particular, it is not known whether the thymoma exports mature and long‐lived T cells, which could provide help for antibody production in the periphery. Here, we quantified recently exported thymic T cells using the approach of measuring episomal DNA fragments [T‐cell receptor excision circles (TRECs)], generated by T‐cell receptor gene rearrangement. Compared to values in healthy individuals (n = 10) or in patients with late‐onset MG (n = 8), TREC levels were significantly raised in both the CD4+ and CD8+ peripheral blood compartments of patients with thymoma and MG (n = 14, p = 0.002 and p = 0.0004 compared to healthy controls) but only in the CD8+ compartment of the 3 patients with thymoma without MG (p = 0.4 and p = 0.01 for CD4+ and CD8+). TREC levels decreased following thymectomy to values similar to controls but were substantially raised in patients who had developed tumor recurrence (n = 6, p = 0.04 and p = 0.02 for CD4+ and CD8+); this was associated with increased antibodies to interferon‐α and interleukin‐12 in the one case studied serially. Collectively, these results support the hypothesis that the neoplastic thymoma tissue itself can generate and export mature, long‐lived T cells and that these T cells reflect the thymic pathology and are likely to be related to the associated autoimmune diseases. The results also provide a new approach for early diagnosis of thymoma recurrence.