Overexpression of rapsyn in rat muscle increases acetylcholine receptor levels in chronic experimental autoimmune myasthenia gravis

The primary autoantigen in myasthenia gravis, the acetylcholine receptor (AChR), is clustered and anchored in the postsynaptic membrane of the neuromuscular junction by rapsyn. Previously, we found that overexpression of rapsyn by cDNA transfection protects AChRs in rat muscles from antibody-mediated loss in passive transfer experimental autoimmune myasthenia gravis (EAMG). Here, we determined whether rapsyn overexpression can reduce or even reverse AChR loss in muscles that are already damaged by chronic EAMG, which mimics the human disease. Active immunization against purified AChR was performed in female Lewis rats. Rapsyn overexpression resulted in an increase in total muscle membrane AChR levels, with some AChR at neuromuscular junctions but much of it in extrasynaptic membrane regions. At the ultrastructural level, most endplates in rapsyn-treated chronic EAMG muscles showed increased damage to the postsynaptic membrane. Although rapsyn overexpression stabilized AChRs in intact or mildly damaged endplates, the rapsyn-induced increase of membrane AChR enhanced autoantibody binding and membrane damage in severe ongoing disease. Thus, these results show the complexity of synaptic stabilization of AChR during the autoantibody attack. They also indicate that the expression of receptor-associated proteins may determine the severity of autoimmune diseases caused by anti-receptor antibodies.

Complement membrane attack is required for endplate damage and clinical disease in passive experimental myasthenia gravis in Lewis rats

Myasthenia gravis (MG) is a debilitating and potentially fatal neuromuscular disease characterized by the generation of autoantibodies reactive with nicotinic acetylcholine receptors (AChR) that cause loss of AChR from the neuromuscular endplate with resultant failure of neuromuscular transmission. A role for complement (C) in the pathology of human MG has been suggested based upon identification of C activation products in plasma and deposited at the endplate in MG. In the rat model, experimental autoimmune MG (EAMG), C depletion or inhibition restricts clinical disease, further implicating C in pathology. The mechanisms by which C activation drives pathology in MG and EAMG are unclear. Here we provide further evidence implicating C and specifically the membrane attack complex (MAC) in the Lewis rat passive EAMG model of MG. Rats deficient in C6, an essential component of the MAC, were resistant to disease induction and endplate destruction was reduced markedly compared to C6-sufficient controls. After reconstitution with C6, disease severity and endplate destruction in the C6-deficient rats was equivalent to that in controls. The data confirm the essential role of the MAC in the destruction of the endplate in EAMG and raise the prospect of specific MAC inhibition as an alternative therapy in MG patients resistant to conventional treatments.

The membrane attack pathway of complement drives pathology in passively induced experimental autoimmune myasthenia gravis in mice

The human neuromuscular disease myasthenia gravis (MG) is characterized by the generation of autoantibodies reactive with nicotinic acetylcholine receptors (AChR) that cause loss of AChR from the neuromuscular end-plate with resultant failure of neuromuscular transmission. A role for complement (C) in AChR loss has been suggested based upon morphological identification of C at the end-plate in MG and from the effects of C inhibition in murine models. Here we provide further evidence implicating C, and specifically the membrane attack complex (MAC), in a mouse model of MG. Mice deficient in the C regulators Daf1 and/or Cd59a were tested in the model. Wild-type mice were resistant to disease while mice deficient in Daf1 had mild disease symptoms with evidence of C activation and AChR loss at end-plates. Cd59a-deficient mice had very mild disease with some muscle inflammation and essentially undamaged end-plates. In contrast, mice deficient in both C regulators developed a severe paralytic disease with marked muscle inflammation and loss of end-plates. Inhibition of MAC assembly abrogated clinical disease in these double-deficient mice, demonstrating conclusively that MAC formation was driving pathology in the model. These findings provoke us to suggest that current anti-C therapeutics targeting MAC assembly will be beneficial in MG patients resistant to conventional therapies.

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.

"Guessing it right," John A. Simpson, and myasthenia gravis: the role of analogy in science

In 1960, John A. Simpson hypothesized that myasthenia gravis "is an 'auto-immune' response of muscle in which an antibody to end-plate protein may be formed." Simpson's speculation was subsequently shown to be correct. A review of Simpson's hypothesis from a historical perspective illustrates the role of analogy in science.

Immunoregulation in Experimental Autoimmune Myasthenia Gravis-about T Cells, Antibodies, and Endplates

Experimental autoimmune myasthenia gravis (EAMG) can be induced in a large number of animal species by active immunization (AI) AChR, by passive transfer (PT) of anti-AChR antibodies, by autologous bone marrow transplantation and cyclosporin (BMT-Cy), or spontaneously. Depending on the model used, different immunological mechanisms are operational. In the AI model, the T cell is pivotal in directing the anti-AChR antibody production towards pathogenic, that is, cross-linking and complement-fixing antibodies. Injection of anti-AChR antibodies alone suffices to induce EAMG, excluding the role of specific cell-mediated immune responses in the effector phase of the disease. Aged animals are resistant to the induction of AI and PT EAMG. This resistance is localized at the postsynaptic membrane containing more AChR-anchoring proteins, including S-laminin and rapsyn in aged animals. In BMT-CyA EAMG, a dysregulation of the immune system in the absence of immunization is capable of inducing myasthenia. The role of these animal models in relation to pathogenesis and immunotherapy is discussed.

Effect of myasthenic immunoglobulin G on motor end-plate morphology

This study was undertaken to clarify the role of complement in acetylcholine receptor loss and degeneration of the postsynaptic membrane in myasthenia gravis (MG). We examined the end-plate morphology in rats with passively transferred immunoglobulin G (IgG) from myasthenic patients and the effect of complement by treatment of the rats with cobra venom factor. We injected peroxidase-labeled alpha-BuTx (P-BuTx) into the extensor digitorum longus (EDL) muscle to label the motor end-plates. Three hours later, 100 mg of IgG from MG patients or healthy controls was injected into the tail vein. The EDL was removed 48 hours after the injection of IgG. The presence of macrophages and degeneration of the postsynaptic membrane were seen in 4 of 6 IgG samples from MG patients and a decrease in AChRs in the other 2 samples. These changes were reversed completely by treatment with cobra venom factor in all but one case in which the end-plates were severely degenerated. Injection of MG IgG only never induced end-plate morphology changes. The results suggest that complement has a critical role in degeneration of the postsynaptic membrane and AChR loss at the motor end-plates in the passively transferred model and probably in human MG.

[Paraneoplastic diseases of the motor end-plate and muscle]

INTRODUCTION

Some forms of myasthenia gravis and many cases of Eaton-Lambert syndrome are disorders of neuromuscular transmission possibly of paraneoplastic origin. Paraneoplastic necrotizing myopathy and some cases of inflammatory myopathy (polymyositis-dermatomyositis) are paraneoplastic myopathies.

DEVELOPMENT

These are all autoimmune processes. The diagnosis is clinical, based mainly on the characteristics of the muscle weakness and motor disorders, confirmed by serological methods (demonstration of antibodies) pharmacological and electromyographic tests, by imaging studies and in some cases biopsy. Treatment includes specific symptomatic measures (aimed at correcting the defect in neuromuscular transmission in the myasthenic syndromes, reducing axonal auto-excitability in the neuromuscular hyperexcitability syndromes, etc.) and pathogenic treatment (basically immunosuppression). A search for the hidden tumour should also be part of the diagnostic investigation. The paraneoplastic disease may remit if the associated tumour is treated.

Myasthenia gravis, thymoma, intestinal pseudo-obstruction, and neuronal nicotinic acetylcholine receptor antibody

Intestinal pseudo-obstruction occurs rarely in patients with myasthenia gravis (MG) and thymoma. The etiology of the intestinal pseudo-obstruction remains to be elucidated, although an autoimmune mechanism is postulated. We present the first report of neuronal nicotinic acetylcholine receptor (AChR)-specific antibody in a patient with seropositive MG, malignant thymoma, and intestinal pseudo-obstruction. This finding provides evidence that intestinal pseudo-obstruction associated with thymoma and possibly other neoplasms may be related to antibodies against the neuronal nicotinic receptors at autonomic ganglia.

Prevention of autoimmune attack by targeting specific T-cell receptors in a severe combined immunodeficiency mouse model of myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease targeting the skeletal muscle acetylcholine receptor. We have previously demonstrated a selection bias of CD4+ T cells expressing the Vbeta5.1 T-cell receptor gene in the thymus of HLA-DR3 patients with MG. To evaluate the pathogenicity of these cells, severe combined immunodeficiency mice engrafted with MG thymic lymphocytes were treated with anti-Vbeta5.1 antibody. Signs of pathogenicity (eg, acetylcholine receptor loss and complement deposits at the muscle end plates of chimeric mice) were prevented in anti-Vbeta5.1-treated severe combined immunodeficiency chimeras. Pathogenicity was mediated by autoantibodies against acetylcholine receptor. Thymic cells depleted of Vbeta5.1-positive cells in vitro before cell transfer were nonpathogenic, indicating that Vbeta5.1-positive cells are involved in the production of pathogenic autoantibodies. Acetylcholine receptor loss was prevented by Vbeta5.1 targeting in HLA-DR3 patients only, demonstrating specificity for HLA-DR3-peptide complexes. The action of the anti-Vbeta5.1 antibody involved both the in vivo depletion of Vbeta5.1-expressing cells and an increase in the interferon-gamma/interleukin-4 ratio, pointing to an immune deviation-based mechanism. This demonstration that a selective and specific T-helper cell population is involved in controlling pathogenic autoantibodies in MG holds promise for the treatment of MG.

Normal human immunoglobulin suppresses experimental myasthenia gravis in SCID mice

Serum IgM has been shown to participate in the control of IgG autoreactivity in healthy subjects. We have recently shown that an immunoglobulin preparation of pooled normal human IgM (IVIgM) contains anti-idiotypic antibodies against disease-associated IgG autoantibodies in autoimmune patients and protects rats from experimental autoimmunity. The aim of the present study was to asses the in vitro and in vivo immunomodulatory effects of IVIgM in comparison with IgG, in SCID mice reconstituted with thymic cells from a myasthenia gravis patient. Non-leaky SCID mice were injected i.p. with 60 x 10(6) thymic cells from a patient with myasthenia gravis and 1 day later boosted with 10(6) irradiated acetylcholine receptor (AchR)-expressing TE671 cells. On days 14, 21 and 28, mice were treated with IVIgM or with equimolar amounts of human serum albumin. The level of anti-AchR antibodies in the sera of three out of four IgM-treated animals was less than 1 nM. Further, there was a significant decrease in the loss of endplate AchR on the diaphragms of IgM-treated SCID mice. These findings indicate that pooled normal IgM exerts an immunoregulatory role in experimental myasthenia gravis, and suggests that IgM may be considered as an alternative approach in the therapy of autommune diseases.

Miller Fisher anti-GQ1b antibodies: alpha-latrotoxin-like effects on motor end plates

In the Miller Fisher syndrome (MFS) variant of the Guillain-Barré syndrome, weakness is restricted to extraocular muscles and occasionally other craniobulbar muscles. Most MFS patients have serum antibodies against ganglioside type GQ1b of which the pathophysiological relevance is unclear. We examined the in vitro effects of MFS sera, MFS IgG, and a human monoclonal anti-GQ1b IgM antibody on mouse neuromuscular junctions (NMJs). It was found that anti-GQ1b antibodies bind at NMJs where they induce massive quantal release of acetylcholine from nerve terminals and eventually block neuromuscular transmission. This effect closely resembled the effect of the paralytic neurotoxin alpha-latrotoxin at the mouse NMJs, implying possible involvement of alpha-latrotoxin receptors or associated downstream pathways. By using complement-deficient sera, the effect of anti-GQ1b antibodies on NMJs was shown to be entirely dependent on activation of complement components. However, neither classical pathway activation nor the formation of membrane attack complex was required, indicating the effects could be due to involvement of the alternative pathway and intermediate complement cascade products. Our findings strongly suggest that anti-GQ1b antibodies in conjunction with activated complement components are the principal pathophysiological mediators of motor symptoms in MFS and that the NMJ is an important site of their action.

Acquired myasthenia gravis

Myasthenia gravis, an antibody-mediated disorder of neuromuscular transmission that produces clinical weakness, may be ocular or generalized. Clinical diagnostic evaluation may be supplemented by electrophysiologic studies and antibody testing. Therapeutic options, including anticholinesterase inhibitors, immunosuppressive agents, plasmapheresis and thymectomy, are tailored for the individual patient. This article emphasizes the key aspects of the clinical evaluation, diagnosis, and therapy.

Galanin-immunoreactive nerve terminals innervating the striated muscle fibers of the rat esophagus

Galanin (GAL) immunohistochemistry combined with acetylcholinesterase (AChE) histochemistry was applied to demonstrate the innervation of the rat esophageal muscle coats. GAL immunoreactivity was found in a number of nerve cell bodies in the myenteric ganglia and in numerous varicose and non-varicose nerve fibers in the myenteric plexus and around blood vessels. Many GAL-positive varicose fibers ran in the internodal strands and along the striated muscle fibers. They often ramified and terminated on the muscle fibers to form arborizing structures, which were most abundant in the thoracic portion of the esophagus. Such GAL-positive terminals were localized in most (87.7%) of AChE-reactive motor endplates on the esophageal striated muscles. Left supranodose vagotomy caused a significant decrease of the GAL-arborizing terminals on the striated muscles of the esophagus. This suggests that they are terminals of efferent fibers in the vagus nerve.

The human-severe combined immunodeficiency myasthenic mouse model: a new approach for the study of myasthenia gravis

We have established a new chimeric human-mouse model of myasthenia gravis in severe combined immunodeficiency mice, using human peripheral blood lymphocytes that survive in the mouse and produce specific antibodies that mediate pathological changes typical of human myasthenia gravis. Mice given peripheral blood lymphocytes from both anti-acetylcholine receptor (AChR) antibody-positive and -negative patients with myasthenia gravis showed circulating anti-acetylcholine receptor antibodies, deposition of human IgG at muscle end-plates, and simplification of the postsynaptic membrane, findings characteristic of human myasthenia gravis. Mice given human peripheral blood lymphocytes from healthy donors and simultaneously immunized with Torpedo acetylcholine receptor showed the same changes. This chimeric model, utilizing human cells to reproduce the immunopathological findings of human myasthenia gravis in a nonhuman environment, offers new opportunities to study immune regulation in autoimmunity.

Differences in acetylcholine receptor-antibody interactions between extraocular and extremity muscle fibers

There are two types of motor nerve innervation patterns and AChR distributions in human EOM: single and multiple. The latter is further divided into two subgroups that are restricted to EOM and are not found in limb muscles. Epitopes that are unique to EOM end-plates exist. Some OMG patients have antibodies that are specifically targeted to those epitopes. These antibodies are functionally active and can cause AChR loss in EOM end-plates. In addition to AChRs, specific components constructing the microenvironment surrounding them may also be involved in the susceptibility of EOM in myasthenia gravis.

The beta 2-adrenergic agonist terbutaline suppresses acute passive transfer experimental autoimmune myasthenia gravis (EAMG)

Treatment of Lewis rats with the beta 2-adrenergic agonist terbutaline suppressed clinical symptoms of acute passive transfer EAMG induced with monoclonal anti-acetylcholine receptor antibody and accelerated clinical recovery in affected animals. Electrophysiological studies showed that the amplitude of the first compound muscle action potential was significantly larger in terbutaline-treated rats as compared to controls. In both groups, a comparable number of inflammatory cells at the muscle endplates was seen.

Myasthenogenicity of a human acetylcholine receptor ?-subunit peptide: Morphology and immunology

Each of 10 rats inoculated with a synthetic peptide comprising residues 125-147 (without a disulfide bond) of human acetylcholine receptor (AChR) alpha-subunit (H alpha) had deposits of IgG and C3 (immune complexes) and showed morphological changes in the fine structure at the motor end-plates 5 weeks after a single immunization. Antibody to the H alpha peptides was elevated 1 week after immunization, but, antibody levels to solubilized human or rat AChR were very low in 8 of the 10 rats. These results suggest that the immune response to peptide H alpha is the myasthenogenic site, which induces morphological change at the end-plates.

[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.

A myasthenia gravis plasma immunoglobulin reduces miniature endplate potentials at human endplates in vitro

A particular myasthenia gravis (MG) plasma Ig has previously been shown to block a single alpha-bungarotoxin (alpha-BuTx) binding site on embryonic rat muscle acetylcholine receptor (AChR). We have investigated its effect on embryonic/denervated and adult human AChR both in extracts and in situ. Plasma Ig blocked 125I-alpha-BuTx binding by greater than 85% to the AChR extracted from denervated muscle, but only by 55% to AChR extracted from normal human muscle. Incubation of intact human muscle fibers with the plasma Ig reduced 125I-alpha-BuTx binding to the endplate AChRs by 63%, and substantially decreased the amplitude of miniature endplate potentials. We conclude that anti-alpha-BuTx site antibodies, when present, can be important in the pathophysiology of the disease.

Experimental autoimmune myasthenia gravis induced in mice without adjuvant: genetic susceptibility and adoptive transfer of weakness

The induction of experimental autoimmune myasthenia gravis (EAMG) in mice in the absence of adjuvant was investigated. The essential biochemical features of the disease were induced using a protocol including a primary intrasplenic immunization (1 microgram Torpedo acetylcholine receptor, AChR) followed by booster immunizations with the same dose at 2 and 6 weeks (intraperitoneal) and at 14 weeks (intrasplenic). These features included serum antibodies reactive with mouse AChR, antibody complexed with AChR in vivo, and AChR loss from motor endplates. None of the mice immunized with AChR in adjuvant developed weakness. By contrast, a few mice (less than 5%) immunized without adjuvant became overtly weak and these clinical signs could be adoptively transferred to irradiated recipients, suggesting a possible model for testing immunotherapeutic strategies. Of the nine mouse strains immunized (on six different H-2 haplotypes: a, b, d, k, q, and s) only those with the H-2k haplotype did not develop the biochemical features associated with EAMG. In an F1 cross the genotype (H-2b) was dominant in conferring susceptibility to CBA mice (H-2k).

Antigenic difference of acetylcholine receptor between single and multiple form endplates of human extraocular muscle

The acetylcholine receptor (AChR) of human extraocular muscle (EOM) has been studied by the immunological method using anti-AChR antibodies obtained from the sera of patients with myasthenia gravis (MG) of ocular type, whose symptoms have been restricted to EOM. Those antibodies could distinguish the AChR of multiple (en grappe) form endplates from that of single (en plaque) form endplates. This result indicates the antigenic difference of AChR between those two forms of endplates of human EOM.

Passive transfer of myasthenia gravis by immunoglobulins: lack of correlation between AChR with antibody bound, acetylcholine receptor loss and transmission defect

The effects of serum Ig from 7 myasthenia gravis patients on neuromuscular transmission was investigated by passive transfer to mice. A protocol of 60 mg/day for 3 days produced mouse serum levels of anti-mouse AChR that were similar to those in the MG patients, and resulted in corresponding levels (2-76%) of mouse muscle AChR with antibody bound in situ. However, AChR loss was only greater than 20% with one MG preparation. Nevertheless, there was a marked neuromuscular defect in mice injected with 3 preparations which did not necessarily correlate with the degree of AChR loss or the amount of AChRs with antibody bound in situ. We conclude that in some MG patients part of the defect in neuromuscular transmission may result from antibodies binding to other components of the neuromuscular junction.

Passively transferred Lambert-Eaton syndrome in mice receiving purified IgG

Purified IgG antibodies were prepared by ion-exchange chromatography from the plasma of a patient with nonneoplastic form of the Lambert-Eaton myasthenic syndrome (LES). The antibodies were injected into mice with daily doses of 0.15-10 mg for 20-22 days, following which the integrity of neuromuscular transmission was assessed in vitro in phrenic nerve-diaphragm muscle preparations. The injected animals manifested electrophysiologic features of human LES, which were characterized by: dose-dependent reduction in the quantal content of nerve-evoked endplate potentials, an abnormally small increase in the frequency of spontaneous miniature endplate potentials (MEPPs) with elevated [K+]o, and normal MEPP amplitude with no evidence of postjunctional deficiency. Crude immunoglobulins (Igs) from the same patient and two LES patients with associated malignancy similarly transferred the defects in quantal transmitter release. In contrast, animals receiving Igs from control subjects or from a patient with small-cell carcinoma of the lung manifested no functional impairment of neuromuscular transmission. Instead, the evoked release in these animals was significantly enhanced relative to that found in normal untreated mice. These results suggest that an IgG antibody produces the presynaptic impairment that is characteristic of LES and support the concept that LES with and without cancer has an autoimmune pathogenesis.

Suppression of terminal axonal sprouting at the neuromuscular junction by monoclonal antibodies against a muscle-derived antigen of 56,000 daltons

After the partial denervation or paralysis of a muscle, the remaining motor axon terminals may sprout fine, neuritic processes (terminal sprouts) which escape the endplate region of the neuromuscular junction. We previously identified a muscle-derived, protein antigen of 56,000 daltons (56 kD) which plays a necessary role in terminal sprouting. A panel of monoclonal antibodies have been produced against the 56-kD antigen, some of which also partially suppress motor axon terminal sprouting. These monoclonal antibodies define at least two different epitopes upon the surface of the antigen, one of which is necessary for it to effect its biological role in vivo.

Myasthenia gravis: demonstration of membrane attack complex in muscle end-plates

The membrane attack complex (MAC) assembles from C5b-9 complement components and has neoantigenic properties. Antihuman-MAC rabbit immunserum was applied in order to localize the MAC in myasthenic muscles. Using the indirect immunoperoxidase method MAC was demonstrated at the motor end-plates in eleven myasthenic patients who underwent thymectomy. This result provides direct evidence of antibody-dependent complement-mediated injury of acetylcholine receptors in myasthenia gravis.

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.

Region of peptide 125-147 of acetylcholine receptor alpha subunit is exposed at neuromuscular junction and induces experimental autoimmune myasthenia gravis, T-cell immunity, and modulating autoantibodies

A major antigenic region of native nicotinic acetylcholine receptors (AcChoR) has been identified by using a synthetic disulfide-looped peptide corresponding to alpha-subunit residues 125-147 of Torpedo electric organ AcChoR: Lys-Ser-Tyr-Cys-Glu-Ile-Ile-Val-Thr-His-Phe- Pro-Phe-Asp-Gln-Gln-Asn-Cys-Thr-Met-Lys-Leu-Gly. The peptide bound 26-56% of polyclonal antibodies induced in rat, rabbit, and dog by immunization with native AcChoR. Rats inoculated with 50 micrograms of unconjugated peptide developed helper T-cell responses, delayed hypersensitivity, and antibodies to native AcChoR. Anti-peptide antibodies were more reactive with native than denatured AcChoR and bound to the alpha subunit. Some reacted exclusively with mammalian muscle AcChoR, some induced modulation of AcChoR on cultured myotubes, but none inhibited binding of alpha-bungarotoxin to solubilized or membrane-associated AcChoR. Repeated immunization induced experimental autoimmune myasthenia gravis: clinical signs in one rat and electrophysiologic and/or biochemical signs in 10 of 11 rats. Thus, at least part of the corresponding region of the mammalian AcChoR alpha subunit is extracellular at the neuromuscular junction and a potential target for pathogenic autoantibodies in patients with acquired myasthenia gravis.

Myasthenic serum selectively blocks acetylcholine receptors with long channel open times at developing rat endplates

We have examined the physiological effects of antibodies from a highly specific myasthenic serum on acetylcholine receptors at developing rat endplates. The antibodies reduced the amplitude of miniature endplate potentials by 60% in 3- to 6-day-old animals but had no effect after day 14. Between days 7 and 12 the antibodies had an intermediate effect. This is the same period during which acetylcholine receptors with long channel open times (slow channels) disappear and receptors with short open times (fast channels) increase in number. Therefore, we examined the effect of the antibodies at endplates with a mixture of channel types more carefully. At all times tested, both noise analysis and analysis of miniature endplate currents showed that the antibodies reduced slow channel activity selectively. Single-channel recordings indicated that acetylcholine receptors that remained active after antibody treatment had normal gating properties. Thus, the antibodies appeared to silence slow channels selectively.

A postnatal change in the immunological properties of the acetylcholine receptor at rat muscle endplates

We have used a myasthenic serum that in adult rat muscle is specific for acetylcholine receptors (AChRs) in the extra-junctional membrane to characterize the AChRs at developing endplates. Immunocytochemical experiments show that AChRs at endplates in the rat diaphragm bind the myasthenic antibodies during the first week after birth but lose their reactivity during the second and third postnatal weeks. AChRs at endplates in adult rat diaphragm do not bind the antibodies even after denervation; in contrast, AChRs at endplates in an adult chicken muscle (anterior latissimus dorsi) are recognized by the antibodies. The loss of immunological reactivity thus may be correlated with a change in the channel properties of the AChR and with the appearance of synaptic folds, two postnatal developmental changes that occur at the endplates of rats, but not of chickens.

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.

T-lymphocytes in experimental autoimmune myasthenia gravis. Isolation of T-helper cell lines

The role of T-lymphocytes in Experimental Autoimmune Myasthenia Gravis (EAMG) was investigated. We generated highly purified, acetylcholine receptor (AChR)-specific T-cell populations and subsequently characterized these cell lines with respect to their membrane phenotype and their function. Using a series of mouse monoclonal antibodies directed against rat lymphocyte surface differentiation antigens, the vast majority of line cells was shown to express a leucocyte common antigen, a T-common antigen and a T-helper antigen. Small subpopulations were Ia or T suppressor antigen-positive. Adaptive transfer to sublethally irradiated, thymectomized recipients revealed that 1 X 10(6) AChR-specific line cells could cooperate effectively with 10 X 10(6) AChR-primed, complement (C3) receptor-bearing (B-cell enriched) spleen cells in the production of anti-AChR autoantibodies. Recipients of B-cells along with relevant line cells developed an acute myasthenic syndrome 6-7 days after cell transfer. Electron-microscopical examination revealed the typical features of "acute phase" EAMG with heavy mononuclear infiltration. There was, however, no evidence antibody-independent cytotoxic activity exerted by AChR-specific line cells.

The immunopathology of acquired myasthenia gravis

Specific probes (alpha-bungarotoxin for acetylcholine receptor (AChR), staphylococcal protein A for IgG, monospecific antibodies against C3 and C9) labelled with peroxidase were applied to study of the ultrastructure of the MG end plate. In each case of MG there was postsynaptic AChR deficiency, usually greatest at end plates with marked degeneration of junctional folds. Morphometric estimates of postsynaptic AChR correlated linearly with the MEPP amplitude. In each case of MG, IgG was localized on the postsynaptic membrane where AChR is known to be located and on debris in the synaptic space. The abundance of antibody was proportionate to the amount of AChR remaining at the end plate. The localization of C3 was essentially identical with that of IgG. For most cases of MG it can be inferred that binding of IgG and C3 to AChR does not interfere with receptor function. C9, the terminal lytic complement component, was localized on debris in the synaptic space and on remnants of junctional folds. This proves that complement mediated destruction of junctional folds occurs in human MG. Studies in experimental auto-immune MG indicate that antibody-dependent internalization of AChR occurs in subclinical, mild and more severe diseases but increased AChR synthesis can compensate for this in subclinical and mild myasthenia. Complement-mediated injury of the postsynaptic membrane appears to be a requirement for induction of more severe MG.

[Myasthenia gravis. Prototype of an anti-receptor autoaggression disease (author's transl)]

Myasthenia gravis is a classic autoaggression disease in which autoantibodies against the acetylcholine receptors (AChR) of the neuromuscular end-plate have a decisive patho-genetic significance. With sensitive radioimmunoassays anti-AChR antibodies are demonstrable in over 90% of myasthenics and consequently are of great diagnostic significance. The treatment possibilities to date have distinctly improved the prognosis but are either symptomatic or unspecific. More recent knowledge on pathophysiology and pathogenesis and the strategies for development of a specific immunotherapy deducible from this are discussed. X

Immunochemical properties of junctional and extrajunctional acetylcholine receptor

Immunological assays were performed to compare two distinct forms of the nicotinic acetylcholine receptor (AChR): junctional (JR) and extrajunctional receptor (EJR). Antibodies from myasthenia gravis patients' sera inhibited the binding of [125I]alpha-bungarotoxin (BGT), to EJR more effectively than binding to JR. Immunological differences between JR and EJR were confirmed by other assay methods. In all cases, EJR appeared to have antigenic determinants not found on JR. It was established that enzymatic removal of carbohydrates from EJR caused it to more closely resemble JR. Thus differences between JR and EJR may be due, in part, to carbohydrate residues found on EJR that are absent on JR. The extent of antibody binding to EJR was examined by gel filtration methods. Immunochemical studies of bands from SDS gels showed that antibodies are present in myasthenic serum which react with the 3 subunits (42, 53, 64 kdaltons) of AChR to varying degrees.

Myasthenia gravis: passive transfer to mice of antibody to human and mouse acetylcholine receptor

Mice were given intraperitoneal injections of serum from three patients with myasthenia gravis who had different titers of antibody against mouse acetylcholine receptor (AChR). Almost all mice treated with a high titer of serum antigens showed generalized paralysis, reduced amplitudes of miniature endplate potential (MEPP), or reduced numbers of AChRs. The effects were less marked in serum with lower titers of antibody to mouse AChR, when there was no relation to the severity of effects to the titer of antibody to human AChR.

Properties of end-plate channels in rats immunized against acetylcholine receptors

1. Rats injected with purified acetylcholine receptors (AChR) extracted from electric organs of Torpedo marmorata showed clinical symptoms consistent with the development of experimental myasthenia gravis.2. Sera of rats with this disease contain high levels of anti-AChR antibodies. However, no simple correlation was found between antibody titre and miniature end-plate current (m.e.p.c.) amplitude.3. M.e.p.c.s. at the end-plates of rats injected with AChR (Anti-R), emulsified in complete Freund Adjuvant (CFA), were reduced to about one third the size of controls taken from rats injected only with CFA (Anti-CFA). Mean m.e.p.c. (Anti-R) = 0.73 +/- 0.06 nA; mean m.e.p.c. (Anti-CFA) = 2.43 +/- 0.12 nA (V(m) = -80 mV, T = 20 degrees C).4. The m.e.p.c. decay time constant, tau(m.e.p.c.), is similar at immunized and control rat end-plates. tau(m.e.p.c.) (Anti-R) = 1.32 +/- 0.06 msec; tau(m.e.p.c.) (Anti-CFA) = 1.31 +/- 0.06 msec (V(m) = -80 mV, T = 20 degrees C).5. The end-plate current decay time constant, tau(e.p.c.), is similar at immunized and control end-plates and in both cases depends exponentially on membrane potential. The change in membrane potential required to produce an e-fold change in tau(e.p.c.) is 102.0 +/- 5.72 mV at immunized (Anti-R) end-plates and 92.3 +/- 6.14 mV at control (Anti-CFA) end-plates at T = 10 degrees C.6. Acetylcholine noise was examined at immunized and control rat end-plates at 10 degrees C. Analysis of noise indicates that the single channel conductance, gamma, and mean channel life-time, tau(noise), are essentially unchanged by immunization against AChR. gamma (Anti-R) = 13.15 +/- 0.53 pS; gamma (Anti-CFA) = 12.50 +/- 0.50 pS; tau(noise) (Anti-R) = 2.9 +/- 0.18 msec; tau(noise) (Anti-CFA) = 2.68 +/- 0.14 msec (V(m) = -80 mV, T = 10 degrees C).7. Mean quantal content and Ca(2+) dependence of the end-plate potential are unchanged at immunized end-plates.8. It is concluded that at immunized end-plates the number of activated receptor-channel complexes is reduced without modification of single channel properties. In this respect the immunized rat end-plate is a good model for myasthenia gravis affected human end-plates.

Passively transferred myasthenia gravis: protection of mouse endplates by Fab fragments from human myasthenic IgG

Using the mouse passive transfer model, the effects of purified human myasthenic immunoglobulin G and of the monovalent Fab fragment on neuromuscular junctions were investigated. Treatment with IgG markedly reduced amplitudes of miniature endplate potentials. When Fab fragments were transferred alone or with subsequent addition of IgG no neuromuscular transmission block was induced. Myasthentic IgG and Fab were specifically demonstrated at the neuromuscular junctions by immunohistochemistry. On electronmicroscopy endplate structure was normal in transfer experiments using IgG for up to 30 days. It is suggested that Fab fragments bind to acetylcholine receptors without affecting transmission and protect them from the attack of complete IgG antibodies.

Anti-acetylcholine receptor antibodies

Early suggestions that a humoral factor might be implicated in the disorder of neuromuscular transmission in myasthenia gravis have been confirmed by the detection of anti-AChR antibody in 85-90% of the patients with generalised disease and in 75% of cases with restricted ocular myasthenia. Plasma exchange reveals that serum anti-AChR usually has an inverse relationship to muscle strength and present evidence indicates that patients responding to thymectomy and immunosuppressive drug treatment usually show a consistent decline in serum anti-AChR titres. The antibody is heterogeneous and can lead to a loss of muscle AChR by several mechanisms. Anti-AChR is produced in the thymus in relatively small amounts. Anti-AChR antibody synthesis by thymic lymphocytes and pokeweed stimulated peripheral lymphocytes in culture provides a means of studying the effect of different lymphocyte populations in vitro. Analysis of clinical, immunological and HLA antigen characteristics in MG suggest that more than one mechanism may underlie the breakdown in tolerance to AChR, leading to the production of anti-AChR antibodies.

Ultrastructural localization of the terminal and lytic ninth complement component (C9) at the motor end-plate in myasthenia gravis

The terminal and lytic complement component (C9) was localized at the motor end-plate in acquired autoimmune myasthenia gravis (MG) by the immunoperoxidase method, with adequate preservation of fine structure and negligible background staining. C9 was localized on short segments of the postsynaptic membrane on degenerated fragments of the junctional folds shed into the synaptic space, and on disintegrating junctional folds. An inverse relationship was noted between the structural integrity of the junctional folds and the abundance of C9 at a given end-plate region. Destruction of junctional folds by complement may induce relocation of the nerve terminal and increased spatial separation of end-plate regions on the muscle fiber. Destruction of junctional folds by the complement membrane attack complex is a cause of the acetylcholine receptor deficiency at the MG end-plate, but antibody-dependent modulation of the receptor may also contribute to deficiency of the receptor. In certain disorders other than autoimmune MG, pathological mechanisms other than complement-mediated lysis may affect the structural integrity of the postsynaptic region.

Antibodies to acetylcholine receptor in patients with thymoma but without myasthenia gravis

Antibodies to acetylcholine receptor were found in 3 of 11 patients with a thymoma removed by operation but without myasthenia gravis. Because masthenia gravis may appear after removal of the thymoma, detection of antiacetylcholine receptor antibodies may have predictive value.