Acetylcholine receptor antibody in myasthenia gravis

Radioimmunoassay techniques were used to detect antibodies to the acetylcholine receptor (AAChR) in 164 patients with adult-onset myasthenia gravis. AAChR levels above 0.6 nM/l were considered pathological and were found in 67% of the patients with an average value of 58.99 +/- 125.02 nM/l (0.6-900.0). Correlation, with clinical functional status, the histopathological thymus alterations and the different therapeutics used did not disclose any statistically significant differences.     

Acetylcholine receptor antibodies in myasthenia gravis

A multivariate statistical analysis of levels of serum acetylcholine receptor antibody (AChR Ab) obtained from 197 patients with various clinical forms of myasthenia gravis (MG) was performed. Elevated AChR Ab levels are specific for MG, but normal AChR Ab levels do not rule out MG. Patients in remission or with purely ocular MG had the lowest incidence of elevation of serum AChR Ab levels, while patients with generalized, severe MG, particularly in the presence of thymoma, tended to have the greatest antibody elevations. Corticosteroids depressed AChR Ab levels, but thymectomy did not exert a consistent effect on antibody levels within a 24- to 30-month postoperative period. The relatively low 55% positivity of antibody elevations in all 197 patients probably reflects the use of heterologous (rat) AChR.     

Acetylcholine release in diaphragm of rats with chronic experimental autoimmune myasthenia gravis.

Recent evidence indicates that in chronic experimental autoimmune myasthenia gravis (EAMG) and in human myasthenia gravis, the defect of neuromuscular transmission results from immune-mediated destruction of post-synaptic membrane at the neuromuscular junction, with a reduction in the density of acetylcholine (ACh) receptors and decreased sensitivity to ACh released by nerve impulses. In the present study, the amount of ACh released by nerve impulse in rats with chronic EAMG and control rats of the same age, weight, and sex was compared. Phrenic nerve-hemidiaphragm preparations were stimulated in vitro, and the amount of ACh released was measured by bioassay. Despite a marked reduction in the amplitude of miniature end-plate potentials in chronic EAMG, ACh output at rest and during stimulation was not different from that of control rats. These data support the concept that the defect of neuromuscular transmission is due to a reduction of postsynaptic sensitivity to ACh.     

Immunohistochemical study of utrophin and dystrophin at the motor end-plate in myasthenia gravis

We studied the densities of utrophin and dystrophin at the motor end-plates of patients with myasthenia gravis (MG) using immunohistochemical analysis. The densities were compared with those found in patients with amyotrophic lateral sclerosis, Lambert-Eaton myasthenic syndrome and normal controls. Utrophin was reduced at the motor end-plates of MG patients, in association with a reduction of α-bungarotoxin binding sites. In contrast, the density of dystrophin at the motor end-plate of MG patients was not significantly different from that found in the controls. We conclude that, at the motor end-plate, utrophin may be more closely associated than dystrophin with the acetylcholine receptor, and that it plays a different role. Received: 8 June 1995 / Revised: 13 October 1995 / Accepted: 16 January 1996     

Study of antibodies to motor end-plate in ocular myasthenia gravis

This study was undertaken to investigate whether serum obtained from ocular MG with undetectable antibodies contains antibodies which bind to normal motor end-plates immunohistochemically. Nineteen patients with ocular MG were studied. Anti-AChR antibodies in serum were assayed by an immunoprecipitation method using human junctional AChR as the antigen. Anti-AChR antibodies in serum which bind to the junctional AChR at the motor end-plates were detected immunohistochemically by incubating the muscle with each serum. Bound IgG was detected by peroxidase labeled protein A (P-PA). IgG deposit at the own limb muscle motor end-plates (biceps brachii) was also detected by P-PA. Anti-AChR antibodies in serum were positive in 9 out of 19 patients and IgG antibodies bound to the junctional AChR were demonstrated in 16 of 19 patients. IgG bound to own end-plates was observed in 13 of 14 patients studied. In 2 patients IgG was detected at the own end-plates, but not at the not-self end-plates. These findings indicate that detection of IgG at the limb muscle end-plates serves for the diagnosis of ocular MG with undetectable antibodies in serum and anti-AChR antibodies in some patients may react exclusively with the autologous AChR.     

Congenital canine myasthenia gravis: II. Acetylcholine receptor metabolism

Acetylcholine receptor (AChR) metabolism was studied in muscle from juvenile and adult dogs with congenital myasthenia gravis (CMG) and their unaffected littermates. Although the amount of AChR in the junctional region of innervated CMG muscle fibers was 25% of normal, or less, denervation of CMG fibers resulted in the appearance of AChR in extrajunctional membranes at as high a concentration as in denervated normal fibers. The rate of degradation of junctional AChR in CMG fibers explanted to organ culture did not differ significantly from normal. In monolayer cultures derived from enzyme-dissociated CMG muscle, myotubes of normal morphology developed, and the synthesis and degradation of AChR did not differ from normal. Addition of sera from dogs with the acquired autoimmune form of MG accelerated the degradation of AChR on cultured myotubes, but CMG dog sera were without effect. These data suggest that the low junctional membrane density of AChR in CMG does not reflect a primary inability of muscle to synthesize AChR, nor an accelerated degradation of AChR in the postsynaptic membrane, but rather a low insertion rate of AChR in the postsynaptic membrane.     

Acetylcholine receptor antibody and clinical response to thymectomy in myasthenia gravis

We studied serum anti-acetylcholine receptor (AChR) antibody and clinical response to thymectomy in myasthenia gravis for 1 to 3 1/2 years postoperatively in 25 patients who did not receive immunosuppressive drugs. Clinical grade was assessed "blind." Mean final anti-AChR values were significantly reduced compared with thymectomy values (69.6 +/- 7.5% SEM; p less than 0.05). Anti-AChR fell steadily to 42-15% in the six patients who developed remission. Overall, there was a significant correlation between changes in anti-AChR and in clinical grade at 1 year (p less than 0.01) and at final assessment (p less than 0.001). An association between fall in anti-AChR and clinical improvement was absent in five individuals and not accounted for by change in antibody characteristics.     

Acetylcholine receptors loss and postsynaptic damage in MuSK antibody-positive myasthenia gravis

Muscle-specific tyrosine kinase (MuSK) antibodies are found in some patients with "seronegative" myasthenia gravis (MG), but how they cause myasthenic symptoms is not clear. We visualized acetylcholine receptors (AChRs) and complement component 3 (C3) in muscle biopsies from 10 Japanese MG patients with MuSK antibodies, compared with 42 with AChR antibodies. The AChR density was not significantly decreased in MuSK antibody (Ab)-positive end-plates compared with AChR antibody-positive end-plates, and C3 was detected in only two of eight MuSK Ab-positive patients. MuSK antibodies do not appear to cause substantial AChR loss, complement deposition, or morphological damage. Effects on MuSK function need to be explored.     

Acetylcholine receptor antibodies in the diagnosis of human and experimental myasthenia gravis

Aspects of acetylcholine receptor immunology and circulating receptor antibodies are reviewed with regard to both human and experimental myasthenia gravis. Receptor antibodies that have negligible cross-reactivity with skeletal muscle receptor can nonetheless cause destruction of the postsynaptic motor endplate area. Antibodies to mammalian skeletal muscle receptor can bind in-situ receptors. Transfer of myasthenic IgG increases neurophysiologic symptoms in rabbits showing slight sings of experimental myasthenia gravis, suggesting a block of in-situ receptors. Determination of receptor antibodies using RIA tests and partially purified human skeletal muscle receptor has been evaluated in the diagnosis of myasthenia gravis. Ninety percent of Swedish myasthenic patients in one study were found to have receptor antibodies. A rough correlation has been found between antibody titer and the severity of disease. Immunosuppressive treatment and thymectomy decrease the titer. In patients with thymoma, high antibody titers remain. When taken together, antibody-titer determination and electrophysiologic tests–particularly single-fiber electromyography–are very valuable diagnostic tools.     

Acetylcholine receptor antibodies in myasthenia gravis: use of a qualitative assay for diagnostic purposes

We have modified the techniques of Lindstrom and of Tindall to measure serum acetylcholine receptor antibody using human antigen bound to 125I-alpha Bungarotoxin. By using 10 microliters of serum and precipitating antigen-antibody complexes with an excess of staph A, we found that only one out of 43 patients with clinically diagnosed active generalized Myasthenia Gravis had no antibodies. In pooling these results with the results of tests done for diagnostic purposes we found positive results in 54/55 generalized active MG, 8/21 MG in remission, 16/37 ocular MG and 0/55 healthy controls. Two out of 38 non MG were also positive and their clinical diagnosis of botulism and penicillamine treated rheumatoid arthritis have been confirmed by a one year follow-up. Most of these sera were also tested for reactivity with fetal calf AchR. Six out of 49 samples positive with the human receptor were negative with calf receptor. We conclude that our technique is extremely useful for the diagnosis of Myasthenia Gravis and that fetal calf antigen cannot replace human antigen in the assay.     

Acetylcholine receptor in myasthenia gravis: Increased affinity for α-bungarotoxin

Studies of the binding of ¹²⁵I-labeled α-bungarotoxin to myasthenic motor end-plates have been interpreted as showing a decrease in the number of acetylcholine (ACh) receptors at these end-plates. Equilibrium binding studies of ¹²⁵I-tagged α-bungarotoxin to detergent-extracted ACh receptors from normal and myasthenic intercostal muscle were carried out to determine whether the reduced toxin binding previously reported could be due to a reduced affinity of myasthenic receptors for α-bungarotoxin rather than to a decreased number of receptors. Our results show increased rather than decreased affinity of myasthenic receptors for α-bungarotoxin and also suggest that the number of ACh receptors is indeed reduced. The presence of a change in binding affinity, in addition to the reduced number of ACh receptors, suggests the presence of membrane changes that may contribute to the pathogenesis of myasthenia gravis.     

Acetylcholine receptor expression in human extraocular muscles and their susceptibility to myasthenia gravis

In myasthenia gravis (MG), extraocular muscle (EOM) weakness is often an initial and persisting symptom. It has been proposed that acetylcholine receptor (AChR) from EOM is antigenically different from AChR of other innervated muscles and that the presence of antibodies to fetal AChR expressed in EOM causes their weakness. We have (1) studied mRNA expression for each of the AChR subunits (α, β, γ, δ, and ?) in human muscle, including EOM, and (2) compared the binding of sera from ocular myasthenia gravis (OMG) patients with fetal (α₂βγδ) and adult (α₂β?δ) human AChRs. RNase protection assays showed that expression of the AChR γ-subunit (fetal-type) mRNA in EOM was comparable with that in other innervated muscle types. By contrast, ?-subunit (adult-type) mRNA was expressed at much higher levels in EOM than in other muscles studied. Moreover, some OMG sera bound specifically to adult AChR. These results do not support the contention that susceptibility of EOM in MG results from expression of fetal AChR and indicate that the inclusion of antigen from a source rich in adult AChR in the MG diagnostic assay will increase the yield of positive results in OMG patients.     

Acetylcholine receptor-reactive T lymphocytes from healthy subjects and myasthenia gravis patients.

Peripheral blood lymphocytes from 23 of 114 (20%) myasthenia gravis (MG) patients showed positive T-cell proliferative responses to native acetylcholine receptor (AChR) purified from the electric fish Torpedo, compared with two of 25 (8%) healthy or other neurologic disease controls. Responsiveness appeared to fluctuate seasonally. Long-term T-cell lines and clones could be selected as readily from the two healthy responders as from the MG cases and showed similar culture behavior, CD4+ phenotype, and HLA class II restrictions. One clone from a control cross-reacted with recombinant human AChR alpha chain (r37-429A) and with the synthetic peptide 125-143(S-S) from its sequence. Both these human antigens stimulated primary proliferative responses at substantially higher frequencies (26 to 59%) than native xeno-AChR--in both patients and controls--demonstrating that truly autoreactive T cells are not inevitably deleted during normal T-cell development.     

Acetylcholine receptor antibody production by bone marrow cells in a patient with myasthenia gravis

We briefly report antiacetylcholine receptor antibody production by cultured bone marrow cells (4.9 +/- 0.9 fmol/10(6) cells/wk, mean +/- SD) in a 65-year-old man with myasthenia gravis without thymoma. Lymphocytes from peripheral blood, thymus, and lymph nodes produced less antibody (1.8 +/- 0.1, 0.9 +/- 0.3, and 1.0 +/- 0.4, respectively). The thymus was involuted, and the effect of thymectomy was doubtful 6 months postoperatively.     

Neuromuscular transmission in myasthenia gravis studied with single fibre electromyography

Patients with myasthenia gravis have been investigated with single fibre electromyography. In all myasthenic muscles recordings were obtained where there was an increase of the jitter and occasional blocking of different degree of single fibre action potentials. The jitter is the variability at consecutive discharges in the time interval between action potentials from two muscle fibres from the same motor unit. During prolonged activity the jitter increased and blockings occurred more and more frequently. The size of the jitter and the degree of blocking also depended on the discharge rate. At a low rate the jitter was lower and the degree of blocking less than at the high rate. Injection of edrophonium could decrease or increase the jitter or leave it unaffected. Single fibre electromyography is a sensitive way of studying the transmission in individual motor end-plates in myasthenia gravis and is also a valuable diagnostic aid.     

Diagnostic significance of IgG, C3, and C9 at the limb muscle motor end-plate in minimal myasthenia gravis

We detected deposits of IgG, C3, and C9 (immune complexes) at the limb muscle motor end-plates (biceps brachii muscle) in 16 of 19 patients who exhibited only ocular signs and symptoms of myasthenia gravis that were improved by intravenous injections of edrophonium chloride. Circulating anti-acetylcholine receptor (anti-AChR) antibodies were negative in 6 of the 16 patients, but the motor end-plate fine structure in the postsynaptic regions was abnormal in all 16. Single-fiber EMG revealed no abnormalities in 8 of 13 patients studied. Our results indicate that the detection of immune complexes at the limb muscle end-plate provides a highly sensitive and confirmative method for diagnosing patients with minimal or atypical myasthenia gravis who have no detectable anti-AChR antibodies in their serum.     

Acetylcholine receptor antibody titer, sensitivity to curare electromyogram and disease severity in myasthenia gravis

All of 39 rabbits immunized with acetylcholine receptors invariably formed anti-AChR Abs and some of them developed muscular weakness or flaccid paralysis. Pharmacological, physiological and ultrastructural studies indicated that the pathology of experimental autoimmune myasthenia gravis in rabbits resembled that of human myasthenia gravis. The titer of anti-AChR Abs correlated poorly with disease severity and did not simply predicted the muscular weakness. The results ruled out the possibility that antigenic modulation of AChR was sufficient to account for the induction of myasthenia gravis. There was a close relationship between the sensitivity to curare and disease severity, but only part of immunized animals appeared electromyogram change. This led us to conclude that serum anti-AChR concentration would not be the single pathological factor in myasthenia gravis and disease severity may correlate with amount of normal AChR.