Seronegative myasthenia gravis: a plasma factor inhibiting agonist-induced acetylcholine receptor function copurifies with IgM

Critical illness-associated weakness and related motor disorders

Weakness of limb and respiratory muscles that occurs in the course of critical illness has become an increasingly common and serious complication of adult and pediatric intensive care unit patients and a cause of prolonged ventilatory support, morbidity, and prolonged hospitalization. Two motor disorders that occur singly or together, namely critical illness polyneuropathy and critical illness myopathy, cause weakness of limb and of breathing muscles, making it difficult to be weaned from ventilatory support, commencing rehabilitation, and extending the length of stay in the intensive care unit, with higher rates of morbidity and mortality. Recovery can take weeks or months and in severe cases, and may be incomplete or absent. Recent findings suggest an improved prognosis of critical illness myopathy compared to polyneuropathy. Prevention and treatment are therefore very important. Its management requires an integrated team approach commencing with neurologic consultation, creatine kinase (CK) measurement, detailed electrodiagnostic, respiratory and neuroimaging studies, and potentially muscle biopsy to elucidate the etiopathogenesis of the weakness in the peripheral and/or central nervous system, for which there may be a variety of causes. These tenets of care are being applied to new cases and survivors of the coronavirus-2 disease pandemic of 2019. This chapter provides an update to the understanding and approach to critical illness motor disorders.

On the path to evidence-based therapy in neuromuscular disorders

Neuromuscular disorders encompass a diverse group of acquired and genetic diseases characterized by loss of motor functionality. Although cure is the goal, many therapeutic strategies have been envisioned and are being studied in randomized clinical trials and entered clinical practice. As in all scientific endeavors, the successful clinical translation depends on the quality and translatability of preclinical findings and on the predictive value and feasibility of the clinical models. This chapter focuses on five exemplary diseases: childhood spinal muscular atrophy (SMA), Charcot-Marie-Tooth (CMT) disorders, chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), acquired autoimmune myasthenia gravis (MG), and Duchenne muscular dystrophy (DMD), to illustrate the progress made on the path to evidenced-based therapy.

MuSK autoantibodies in myasthenia gravis detected by cell based assay--A multinational study

Seronegative myasthenia gravis (MG) presents a serious gap in MG diagnosis and understanding. We applied a cell based assay (CBA) for the detection of muscle specific kinase (MuSK) antibodies undetectable by radioimmunoassay. We tested 633 triple-seronegative MG patients' sera from 13 countries, detecting 13% as positive. MuSK antibodies were found, at significantly lower frequencies, in 1.9% of healthy controls and 5.1% of other neuroimmune disease patients, including multiple sclerosis and neuromyelitis optica. The clinical data of the newly diagnosed MuSK-MG patients are presented. 27% of ocular seronegative patients were MuSK antibody positive. Moreover, 23% had thymic hyperplasia suggesting that thymic abnormalities are more common than believed.

Serological and clinical features of patients with myasthenia gravis in north Indian population

Myasthenia gravis (MG) is a disorder of neuromuscular junction associated with presence of antibodies against nicotinic acetylcholine receptors (nAChRs). Here, we compared the clinical and serological profile of seropositive myasthenia gravis (SPMG) and seronegative myasthenia gravis (SNMG) patients. Anti-AChR antibody was measured using radio receptor immunoassay and correlated with clinical phenotype in 250 MG patients over 2004 and 2006. Out of 250 MG patients, 161 (64.4%) were males (male:female = 1.8:1). SNMG patients formed 40% (n = 101) of our MG patients which is much higher as compared to Caucasian and Oriental population (15%-20%). The median age of disease onset in SPMG was significantly higher than SNMG patients (43 years; range 8-74 vs. 35 years; range 4-72, p = .022). A bimodal peak of age of disease onset in both genders was observed (first peak in second-third decades and second one in fifth-sixth decades). Among the MG patients with late-onset of disease, male were significantly higher compared to Caucasian and Oriental MG population (p = .047). MG patients with thymoma were significantly older and consisted of higher percent of males. Bulbar symptoms and severe grade (IIB+ III+ IV) at disease onset were more frequent in SPMG than SNMG patients.

IgG1 antibodies to acetylcholine receptors in 'seronegative' myasthenia gravis

Only around 80% of patients with generalized myasthenia gravis (MG) have serum antibodies to acetylcholine receptor [AChR; acetylcholine receptor antibody positive myasthenia gravis (AChR-MG)] by the radioimmunoprecipitation assay used worldwide. Antibodies to muscle specific kinase [MuSK; MuSK antibody positive myasthenia gravis (MuSK-MG)] make up a variable proportion of the remaining 20%. The patients with neither AChR nor MuSK antibodies are often called seronegative (seronegative MG, SNMG). There is accumulating evidence that SNMG patients are similar to AChR-MG in clinical features and thymic pathology. We hypothesized that SNMG patients have low-affinity antibodies to AChR that cannot be detected in solution phase assays, but would be detected by binding to the AChRs on the cell membrane, particularly if they were clustered at the high density that is found at the neuromuscular junction. We expressed recombinant AChR subunits with the clustering protein, rapsyn, in human embryonic kidney cells and tested for binding of antibodies by immunofluorescence. To identify AChRs, we tagged either AChR or rapsyn with enhanced green fluorescence protein, and visualized human antibodies with Alexa Fluor-labelled secondary or tertiary antibodies, or by fluorescence-activated cell sorter (FACS). We correlated the results with the thymic pathology where available. We detected AChR antibodies to rapsyn-clustered AChR in 66% (25/38) of sera previously negative for binding to AChR in solution and confirmed the results with FACS. The antibodies were mainly IgG1 subclass and showed ability to activate complement. In addition, there was a correlation between serum binding to clustered AChR and complement deposition on myoid cells in patients' thymus tissue. A similar approach was used to demonstrate that MuSK antibodies, although mainly IgG4, were partially IgG1 subclass and capable of activating complement when bound to MuSK on the cell surface. These observations throw new light on different forms of MG paving the way for improved diagnosis and management, and the approaches used have applicability to other antibody-mediated conditions.

Autoimmune Channelopathies and Related Neurological Disorders

Ion channels are crucial elements in neuronal signaling and synaptic transmission, and defects in their function are known to underlie rare genetic disorders, including some forms of epilepsy. A second class of channelopathies, characterized by autoantibodies against ligand- and voltage-gated ion channels, cause a variety of defects in peripheral neuromuscular and ganglionic transmission. There is also emerging evidence for autoantibody-mediated mechanisms in subgroups of patients with central nervous system disorders, particularly those involving defects in cognition or sleep and often associated with epilepsy. In all autoimmune channelopathies, the relationship between autoantibody specificity and clinical phenotype is complex. But with this new information, autoimmune channelopathies are detected and treated with increasing success, and future research promises new insights into the mechanisms of dysfunction at neuronal synapses and the determinants of clinical phenotype.

Inhibition of acetylcholine receptor function by seronegative myasthenia gravis non-IgG factor correlates with desensitisation

15% of myasthenia gravis (MG) patients do not have antibodies against the acetylcholine receptor (AChR). Some of these "seronegative" MG patients have antibodies against muscle specific kinase (MuSK), and many have a non-IgG factor that acutely inhibits AChR function in a muscle-like cell line, CN21. Here we show, using mainly one plasma negative for both AChR and MuSK antibodies, that the inhibitory effect of the non-IgG fraction correlates well with the desensitisation caused by 100 microM nicotine, and is found also when AChRs are expressed in a non-muscle cell line (HEK). Moreover, a similar effect was seen with M3C7-a monoclonal antibody against human AChR. The results suggest that, rather than acting indirectly as previously proposed, the SNMG factor may bind directly to an allosteric site that induces or enhances AChR desensitisation.

Beneficial effects of plasmapheresis before thymectomy on the outcome in myasthenia gravis

OBJECTIVE

Since 1980, we have performed plasmapheresis before thymectomy for patients with generalized symptoms in order to protect against myasthenic crisis and to improve patient outcomes after thymectomy. The aim of this study was to evaluate an immediate and a long-term results of plasmapheresis before thymectomy for myasthenia gravis, retrospectively.

METHODS

Between January 1980 and December 1997, 51 patients with Osserman class IIA or IIB symptoms were treated with transsternal thymectomy. Nineteen patients (group 1) were treated with plasmapheresis before thymectomy and 32 patients (group 2) were treated with thymectomy alone.

RESULTS

In group 1, the time of plasmapheresis prior to thymectomy was 3.2 +/- 1.5. Nine (28.1%) patients in group 2 had crisis within 1 year after thymectomy as compared with only one (5.3%) patient in group 1 had crisis (p = 0.049). There was no evidence of crisis within 30 days after thymectomy in group 1 and 5 (15.6%) patients in group 2 (p = 0.0724). There was no postoperative death among patients in group 1. Responses to thymectomy in group 1 improved significantly, the improvement and pharmacologic remission rate had increased up to 100% and 79% at 5-7 years after operation, while the improvement and pharmacologic remission rate of group 2 had increased to 81.3% (p = 0.0466 vs. group 1) and 50.0% at that time (p = 0.0427 vs. group 1).

CONCLUSIONS

The present study demonstrated that preoperative plasmapheresis may facilitate improved outcomes of patients with myasthenia gravis after thymectomy.

Clinical comparison of muscle-specific tyrosine kinase (MuSK) antibody-positive and -negative myasthenic patients

We assayed cryopreserved sera from 38 acetylcholine receptor (AChR) antibody-negative patients with myasthenia gravis (MG) who were followed clinically for muscle-specific tyrosine kinase (MuSK) antibodies and analyzed and compared their clinical characteristics. None of 13 sera from patients with purely ocular MG were positive. Sera from 10 of 25 patients (40%) with generalized MG were positive for MuSK antibodies. The age at onset of myasthenic symptoms was significantly earlier in MuSK antibody-positive patients (P = 0.02). MuSK antibodies were present in AChR antibody-negative patients of either gender, with virtually identical prevalence in women (41.2%) and men (37.5%). The distribution of weakness more commonly involved neck muscles in MuSK antibody-positive patients, and limb muscles in MuSK antibody-negative patients. Patients responded to immunosuppressive treatment regardless of whether MuSK antibody was present. We conclude that MuSK antibodies are present and diagnostically useful in a subset of myasthenic patients without AChR antibodies. Although the distribution of weakness differs somewhat depending on whether MuSK antibodies are present, responses to anticholinesterase and immunosuppressive treatments are similar.

Antibodies in Myasthenia Gravis and Related Disorders

Acetylcholine receptor (AChR) antibodies are present in around 85% of patients with myasthenia gravis (MG) as measured by the conventional radioimmunoprecipitation assay. Antibodies that block the fetal form of the AChR are occasionally present in mothers who develop MG after pregnancy, especially in those whose babies are born with arthrogryposis multiplex congenita. The antibodies cross the placenta and block neuromuscular transmission, leading to joint deformities and often stillbirth. In these mothers, antibodies made in the thymus are mainly specific for fetal AChR and show restricted germline origins, suggesting a highly mutated clonal response; subsequent spread to involve adult AChR could explain development of maternal MG in those cases who first present after pregnancy. In the 15% of "seronegative" MG patients without AChR antibodies (SNMG), there are serum factors that increase AChR phosphorylation and reduce AChR function, probably acting via a different membrane receptor. These factors are not IgG and could be IgM or even non-Ig serum proteins. In a proportion of SNMG patients, however, IgG antibodies to the muscle-specific kinase, MuSK, are present. These antibodies are not found in AChR antibody-positive MG and are predominantly IgG4. MuSK antibody positivity appears to be associated with more severe bulbar disease that can be difficult to treat effectively.

Animal experimentation in snake venom research and in vitro alternatives

Current experimental techniques used in snake venom research (with and without the use of animals) are reviewed. The emphasis is on the reduction of the use of animals in the development of antivenoms for the clinical treatment of snakebite. Diagnostic and research techniques for the major pathologies of envenoming are described and those using animals are contrasted with non-sentient methods where possible. In particular, LD50 and ED50 assays using animals (in vivo) and fertilised eggs (in vivo, non-sentient) are compared as well as in vitro procedures (ELISA and haemolytic test) for ED50 estimations. The social context of antivenom production, supply and demand is outlined together with the consequent tension between the benefits derived and the increase in opposition to experiments on animals. Stringent regulations governing the use of animals, limited research funds and public pressure all focus the need for progress towards non-animal, or non-sentient, research methods. Some achievements are noted but success is hampered by lack of detailed knowledge of the many constituents of venom which have to be assessed as a whole rather than individually. The only way to evaluate the net pathological effect of venom is to use a living system, usually a rodent, and similarly, the efficacy of antivenoms is also measured in vivo. The pre-clinical testing of antivenoms in animals is therefore a legal requirement in many countries and is strictly monitored by government authorities. New technologies applied to the characterisation of individual venom proteins should enable novel in vitro assays to be designed thus reducing the number of animals required. In the meantime, the principles of Reduce, Refine and Replace relating to animals in research are increasingly endorsed by those working in the field and the many agencies regulating ethical and research policy.

Seronegative generalised myasthenia gravis: clinical features, antibodies, and their targets

Myasthenia gravis (MG) is a well-recognised disorder of neuromuscular transmission that can be diagnosed by the presence of antibodies to the acetylcholine receptor (AChR). However, some patients (about 15%) with generalised MG do not have detectable AChR antibodies. There is some evidence, however, that this "seronegative" MG is an antibody-mediated disorder. Plasma from patients with the disorder seems to contain various distinct humoral factors: IgG antibodies that reversibly inhibit AChR function; a non-IgG (possibly IgM) factor that indirectly inhibits AChR function; and an IgG antibody against the muscle-specific kinase (MuSK). The presence of antibodies against MuSK appears to define a subgroup of patients with seronegative MG who have predominantly localised, in many cases bulbar, muscle weaknesses (face, tongue, pharynx, etc) and reduced response to conventional immunosuppressive treatments. Moreover, muscle wasting may be present, which prevents complete response to these therapies.

Myasthenia gravis: diagnostic and management dilemmas

This review focuses on the diagnostic tests that may help to confirm myasthenia in patients without acetylcholine receptor antibodies, including the newly discovered anti-muscle-specific receptor tyrosine kinase antibody and other investigations. We discuss management dilemmas, particularly those that revolve around treatments whose efficacy is questionable or unproved, such as thymectomy. We review the important issue of how treatments are assessed, and examine future treatment trial designs.

Myasthenia gravis in children: analysis of 18 patients

Myasthenia gravis (MG) in childhood is rare comprising 10 to 20 % of all myasthenic patients. We studied 18 patients with MG whose first symptoms started from 1 to 12 years of age, followed at the Department of Neurology of the UNIFESP-EPM, from January 1983 to August 1997. There were 10 girls and 8 boys (1.2:1). Eleven patients (61%) presented moderate or severe generalized disease and 4 (22%) had at least one myasthenic crisis. EMG with supramaximal repetitive nerve stimulation was diagnostic in 8 (47%) out of 17 patients, and chest CT was normal in 14 patients. Seropositivity to acetylcholine receptor antibodies was found in 81.6% (9 out of 11 tested) and the levels had no relation to clinical severity. Nine out of 16 patients (56%) worsened with pyridostigmine alone and were treated with prednisone. Four out of those nine continued worsening despite steroids and were subjected to thymectomy (all showed thymic lymphoid follicular hyperplasia). Three patients (75%) improved markedly after thymectomy and one (25%) worsened, eventually getting better with intravenous immunoglobulin and oral azathioprine. MG treatment, using all resources available, has to be individualized for each child.

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.

Molecular targets for autoimmune and genetic disorders of neuromuscular transmission

The neuromuscular junction is the target of a variety of autoimmune, neurotoxic and genetic disorders, most of which result in muscle weakness. Most of the diseases, and many neurotoxins, target the ion channels that are essential for neuromuscular transmission. Myasthenia gravis is an acquired autoimmune disease caused in the majority of patients by antibodies to the acetylcholine receptor, a ligand-gated ion channel. The antibodies lead to loss of acetylcholine receptor, reduced efficiency of neuromuscular transmission and muscle weakness and fatigue. Placental transfer of these antibodies in women with myasthenia can cause fetal or neonatal weakness and occasionally severe deformities. Lambert Eaton myasthenic syndrome and acquired neuromyotonia are caused by antibodies to voltage-gated calcium or potassium channels, respectively. In the rare acquired neuromyotonia, reduced repolarization of the nerve terminal leads to spontaneous and repetitive muscle activity. In each of these disorders, the antibodies are detected by immunoprecipitation of the relevant ion channel labelled with radioactive neurotoxins. Genetic disorders of neuromuscular transmission are due mainly to mutations in the genes for the acetylcholine receptor. These conditions show recessive or dominant inheritance and result in either loss of receptors or altered kinetics of acetylcholine receptor channel properties. Study of these conditions has greatly increased our understanding of synaptic function and of disease aetiology.

Characterization of human anti-acetylcholine receptor monoclonal autoantibodies from the peripheral blood of a myasthenia gravis patient using combinatorial libraries

Myasthenia gravis (MG) is an autoimmune disease caused by autoantibodies against the nicotinic acetylcholine receptor (AChR). Using phage-display technology we have characterized the largest panel of anti-AChR monoclonal antibodies thus far isolated from a single patient. Despite having been isolated with either Torpedo AChR or a human peptide, the recombinant antibodies shared with the donor's serum the ability to recognize human AChR expressed in its native configuration on the surface of TE671 cells. Their specificity for the main immunogenic region (MIR) of the AChR was demonstrated using a synthetic peptide corresponding to the region 67-76 of the human AChR alpha subunit and by inhibition of a highly pathogenic rat anti-MIR monoclonal antibody (mAb35). This work demonstrates the value of combinatorial libraries in isolating pathogenic autoantibodies from peripheral blood lymphocytes. Future genetic, structural, and functional analyses of the monoclonal antibodies reported herein should enhance our understanding of the pathogenesis of MG.

Myasthenic crisis with delayed recovery after plasmapheresis

We report on 2 elderly patients with myasthenia gravis in whom recovery from crisis was prolonged despite intensive plasmapheresis (PP). In both patients, the anti-acetylcholine (anti-AChR) titer failed to fall sufficiently after completing PP. These patients might have had antibodies that produced a more pronounced effect on the degradation of AChR, or the synthesis of AChR might have been reduced by aging. The anti-AChR titer did not correlate with a reduction of IgG after PP in 1 patient. Successful treatment was achieved by keeping the anti-AChR titer at a low level via the concomitant use of prednisolone with PP.

Plasma autoantibodies against platelet glycoprotein IIb/IIIa from patients with autoimmune thrombocytopenic purpura may recognize different antigenic determinants

Autoantibodies against platelet glycoprotein (GP) GPIIb/IIIa have been demonstrated in patients with autoimmune thrombocytopenic purpura. Recently, it has been shown that plasma autoantibodies from some patients bind to the cytoplasmic domain of GPIIIa. Our aim was to evaluate further the binding specificity of these plasma autoantibodies. From 7 patients with detectable plasma antibodies against intact GPIIb/IIIa, 1 showed strong antibody binding to a synthetic C-terminal peptide of GPIIIa. Ig class analysis of affinity purified anti-GPIIb/IIIa autoantibodies from this patient revealed an IgM antibody that reacted with intact GPIIb/IIIa as well as with recombinant GPIIb/IIIa lacking the C-terminal domains, and an IgG antibody that bound to intact GPIIb/IIIa but not to GPIIb/IIIa lacking the C-terminal region. These data indicate that this patient has at least 2 autoantibodies, an IgG directed against the cytoplasmic domain of GPIIIa and an IgM reacting with the extracellular part of GPIIIa. This may support the hypothesis that plasma IgG antibodies directed against the C-terminal domain of GPIIIa may be due to the exposition of cytoplasmic epitopes of GPIIIa as a result of increased cell lysis by IgM autoantibodies.

Acquired slow-channel syndrome: a form of myasthenia gravis with prolonged open time of the acetylcholine receptor channel

A 32-year-old female presented with a 2-year history of fluctuating generalized weakness including extraocular, bulbar, and limb muscles, suggesting myasthenia gravis, but with poor response to pyridostigmine and unusual electromyographic findings. After rest, power increased on repeated maximal contractions, followed by progressive weakness. There were decremental responses at low-frequency stimulation, but incremental responses at high frequencies, and single stimuli evoked repetitive compound muscle action potentials. Plasmapheresis was ineffective. In a conventional assay, antibodies against acetylcholine receptors (AChRs) were borderline. However, in an assay using cells expressing mainly adult-type human AChRs, the patient's serum was positive. Thymectomy revealed a hyperplastic thymus. An intercostal muscle specimen revealed small miniature end-plate potentials, 0.22+/-0.02 mV instead of 0.56+/-0.05 mV in controls. The number of 125I-alpha-bungarotoxin binding sites was normal. The decay time constant of end-plate potentials was increased from 5.3+/-0.6 msec in controls to 23+/-3.6 msec in the patient. Ultrastructurally, there was no destruction of the end plate. Transfer of the patient's plasma to mice in vivo produced similar physiological changes in their diaphragms. We conclude that the patient has an immune-mediated disorder, in which an antibody specific to the adult form of the AChRs alters the channel properties, reducing total current and slowing the closure. We propose the name "acquired slow-channel syndrome" for this variant of myasthenia gravis.

Anticholinesterase induces nicotinic receptor modulation

The effects of carbamate anticholinesterases, pyridostigmine and physostigmine, on the function of the nicotinic receptor (nAChR) in TE671 cells was studied, precluding their inhibition of acetylcholine hydrolysis by carbachol usage. In radioassay, the simultaneous application of carbachol and carbamates dose-dependently decreased carbachol-induced 22Na+ influx, compared with carbachol activation alone. Increasing cell preincubation in the presence of carbamates, however, potentiated influx at low concentrations in a time-dependent manner. This facilitating effect of carbamates, even at high concentrations, was significantly increased by washing out these drugs and was blocked by pretreatment with diisopropylfluorophosphate. Similar results were also obtained in whole-cell patch-clamp study. There were insignificant changes in desensitization properties during facilitation. It is thus supposed that facilitation cannot be explained by the inhibition of acetylcholine hydrolysis. These results support a previous hypothesis that acetylcholinesterase might modulate nAChR by an unknown mechanism. In addition, the clinical effects of carbamates may be partly attributed to this facilitation.

Selective impairment of serum antibody repertoires toward muscle and thymus antigens in patients with seronegative and seropositive myasthenia gravis

We analyzed the antibody (Ab) repertoires of IgM and IgG of patients with seropositive and patients with seronegative myasthenia gravis (MG) toward self antigens by means of a quantitative immunoblotting technique using normal human tissue extracts as sources of self antigens. Repertoires of reactivities of IgG and IgM with liver, kidney and stomach antigens were conserved between myasthenic patients and controls. IgG and IgM Ab repertoires toward muscle antigens differed significantly between patients with seropositive MG and healthy donors, as assessed by multiparametric statistical analysis. Patterns of Ab reactivities to muscle antigens were similar in patients with seronegative MG and healthy controls. Antibody repertoires of IgG and IgM toward thymus antigens of both seropositive and seronegative MG patients, differed significantly from those of healthy individuals. Our results indicate that MG is characterized by a selective impairment of self-reactive Ab repertoires toward muscle and thymus antigens. The observation that self-reactive Ab repertoires toward thymus antigens are similar in patients with seropositive and seronegative MG suggests that both forms of MG share common immunopathological features.

Antibodies affecting ion channel function in acquired neuromyotonia, in seropositive and seronegative myasthenia gravis, and in antibody-mediated arthrogryposis multiplex congenita

A new autoimmune disease affecting the neuromuscular junction has been defined. Acquired neuromyotonia is associated with antibodies to voltage-gated potassium channels that act, at least in part, by reducing potassium channel function with resulting neuronal hyperactivity. This condition is quite frequently associated with thymoma and, in many cases, antibodies to acetylcholine receptors are present as well as antibodies to VGKC. Improvements in techniques and the availability of cloned DNA and recombinant forms of the AChR subunits have led to new observations concerning the specificity and roles of antibodies in myasthenia gravis. The transfection of a cell line with the epsilon subunit means that we can now accurately compare antibodies reactive with adult and fetal human AChR. This may help to determine the relationship between AChR subunit expression in different tissues and the induction of antibodies that bind specifically to the two forms, as well as to clarify the role of antibodies to fetal or adult AChR in causing ocular muscle symptoms. Serum antibodies from a few mothers with obstetric histories of recurrent arthrogryposis multiplex congenita in their babies specifically inhibit the function of fetal AChR. These observations not only explain the cause of some cases of arthrogryposis multiplex congenita, but also suggest that other fetal-specific antibodies might be responsible for other fetal or neonatal conditions. An animal model has been established to enable us to investigate the role of maternal serum factors in causing such disorders. Seronegative MG has been the subject of many studies from our laboratory over the last ten years. The transience of the effects of SNMG plasmas on AChR function strongly suggests that the plasma antibodies do not bind directly to the AChR, but inhibit function by some indirect mechanism. They do not appear to act via the cAMP-dependent protein kinase pathway, and studies are in progress to investigate the involvement of other second messenger systems.

Alpha-Bungarotoxin binding to human muscle acetylcholine receptor: measurement of affinity, delineation of AChR subunit residues crucial to binding, and protection of AChR function by synthetic peptides

Alpha-Bungarotoxin (alpha-BuTx) binds with high affinity to the nicotinic acetylcholine receptor (AChR) of most species, mainly to sequences around the two cysteines at positions 192 and 193 of the alpha-subunit, but other sequences of the alpha-subunit and of the adjacent gamma- or epsilon- and delta-subunits are also important in the native molecule. Alpha-BuTx binds strongly to human AChR but the short alpha neurotoxins, for instance Erabutoxin B, are relatively ineffective at the human neuromuscular junction. In this article we compare the affinity of 125I-alpha-BuTx for Torpedo and human muscle AChR and the ability of neurotoxins to inhibit this binding. We examine the contribution to alpha-BuTx binding of the three amino acids that differ between human and Torpedo AChR alpha-185-196. In addition, we show that an alpha-185-199, peptide that binds strongly to 125I-alpha-BuTx and can inhibit its binding in solution, is also capable of protecting the AChR on a cell line or at the neuromuscular junction. Such peptides might be useful in the treatment of acute envenoming or the autoantibody-mediated block of AChR function that can occur in human disorders.

Purified IgG from seropositive and seronegative patients with mysasthenia gravis reversibly blocks currents through nicotinic acetylcholine receptor channels

The mechanism of block of nicotinic acetylcholine receptor (nAChR) channels by purified antibodies from patients with myasthenia gravis (MG) was investigated by using an ultrafast system for solution exchange at outside-out patches. IgG of MG patients and controls was purified by using protein A-Sepharose columns. Probes from 9 seropositive MG patients and 3 seronegative MG patients were tested. As a preparation, cultured mouse myotubes expressing the embryonic-type nAChR channels were used. Twenty-millisecond pulses of 1.0 mM ACh were applied repetitively to outside-out patches. Outside-out patches were preexposed with IgG in concentrations between 0.1 and 200 mg/L during application of ACh pulses. The peak current amplitude was reduced to values between 6% and 71% of control for the 9 seropositive and 3 seronegative MG patients. The block was concentration dependent and fully reversible after washout of antibodies. Incubation with IgG from different control patients did not reduce the peak current amplitude. In addition, our findings with purified IgG from seronegative MG patients support the idea of the immunopathogenesis of this disorder and may allow the development of a diagnostic test for seronegative MG patients.

Autoantibody-Mediated Channelopathies at the Neuromuscular Junction

The neuromuscular junction is vulnerable to antibody-mediated autoimmune attack, probably because it lacks the protection of the blood-brain barrier. This review focuses on three disorders: myasthenia gravis (MG) and the Lambert-Eaton myasthenic syndrome (LEMS), in both of which there is fatiguable muscle weakness, and acquired neuromyotonia (ANMT), in which hyperexcitable peripheral nerves lead to continuous muscle fiber activity and sometimes parasthesias. Each can occur as a paraneoplastic disorder (thymoma in MG and ANMT, and small cell lung cancer in LEMS). The clinical abnormalities are improved following plasmapheresis (which removes circulating antibodies), and injection of experimental animals with immunoglobulins of patients transfers the pathophysiological changes. The ion channel targets in these three disorders are the muscle acetylcholine receptor (a ligand-gated cation channel) in MG, nerve terminal and autonomic voltage-gated calcium channels in LEMS, and peripheral nerve voltage-gated potassium channels in ANMT. The autoantibody attack results in a reduced number of functional channels. Each of the autoantibodies can be detected in serum by immunoassay. These discoveries have allowed new approaches to treatment and suggest that there may be other undiscovered antibody-mediated ion channelopathies. NEUROSCIENTIST 3:337–346, 1997

Detection of antibody classes and subpopulations in myasthenia gravis patients using a new nonradioactive enzyme immunoassay

To investigate the presence of antibodies in myasthenia gravis (MG) patients, we have developed a new reproducible and sensitive enzyme immunoassay (EIA-AChR), in which a beta subunit-specific monoclonal antibody (mAb 73) immobilizes fetal calf acetylcholine receptors (AChRs). We tested 92 MG patients (42 with positive and 50 with negative antibody titers), 60 healthy controls, and 40 controls with other autoimmune diseases. EIA-AChR detected immunoglobulin G (IgG) titers in all of the seropositive samples, with a significant correlation between these and those obtained using the traditional immunoprecipitation method. Moreover, 5 seronegative patients at immunoprecipitation assay were positive at EIA-AChR. EIA-AChR was also useful in revealing: (1) a seropositive patient subpopulation with generalized MG who had Abs directed against alpha-Bungarotoxin binding sites; and (2) patients with IgM directed against fetal calf AChR (detected in 13 seronegative and 16 seropositive MG patients, and in 6 of the patients with other autoimmune diseases).

Cytokines and the pathogenesis of myasthenia gravis

Myasthenia gravis (MG) and its animal model experimental autoimmune myasthenia gravis (EAMG) are caused by autoantibodies against nicotinic acetylcholine receptor (AChR) in skeletal muscle. The production of anti-AChR antibodies is mediated by cytokines produced by CD4+ and CD8+ T helper (Th) cells. Emerging investigations of the roles of cytokines in MG and EAMG have revealed that the Th2 cell related cytokine interleukin 4 (IL-4), an efficient growth promoter for B-cell proliferation and differentiation, is important for anti-AChR antibody production. IL-6 and IL-10 have similar effects. The Th1 cytokine IFN-gamma is important in inducing B-cell maturation and in helping anti-AChR antibody production and, thereby, for induction of clinical signs and symptoms. Results from studies of time kinetics of cytokines imply that IFN-gamma is more agile at the onset of EAMG, probably being one of the initiating factors in the induction of the disease, and IL-4 may be mainly responsible for disease progression and persistance. Even though other Th1 cytokines like IL-2, tumor necrosis factor alpha (TNF-alpha), and TNF-beta as well as the cytolytic compound perforin do not directly play a role in T-cell-mediated help for anti-AChR antibody production, they are actually involved in the development of both EAMG and MG, probably by acting in concert with other cytokines within the cytokine network. In contrast, transforming growth factor beta (TGF-beta) exerts immunosuppressive effects which include the down-regulation of both Th1 and Th2 cytokines in MG as well as EAMG. Suppressive effects are also exerted by interferon alpha (IFN-alpha). Based on elucidation of the role of cytokines in EAMG and MG, treatments that up-modulate TGF-beta or IFN-alpha and/or suppress cytokines that help B-cell proliferation could be useful to improve the clinical outcome.

Association of arthrogryposis multiplex congenita with maternal antibodies inhibiting fetal acetylcholine receptor function

Arthrogryposis multiplex congenita (AMC), characterized by multiple joint contractures developing in utero, results from lack of fetal movement. Some cases are genetically determined, but AMC occasionally complicates pregnancy in patients with myasthenia gravis (MG) suggesting involvement of circulating maternal antibodies. We previously demonstrated antibodies that inhibited the function of fetal acetylcholine receptor (AChR) in one healthy woman with an obstetric history of recurrent AMC. Here we study sera from this woman, from one other with a similar history, and from three (one asymptomatic) whose babies had neonatal MG and AMC. All five maternal sera had high titers of antibodies that inhibited alpha-Bungarotoxin (alpha-BuTx) binding to fetal AChR, and their sera markedly inhibited fetal AChR function with little effect on adult AChR function. Moreover, in a further survey, 3 of 20 sera from anti-AChR negative AMC mothers inhibited fetal AChR function significantly at 1:100 dilution. These results demonstrate the role of antibodies to fetal AChR and perhaps other muscle antigens in some cases of AMC. More generally, they suggest that placental transfer of antibodies directed at fetal antigens should be considered as a cause of other recurrent fetal or perinatal disorders.

Clinical heterogeneity of seronegative myasthenia gravis

We studied 38 patients affected by seronegative myasthenia gravis (SNMG) with age at the onset of the disease ranging from 6 to 66 yr. Clinical follow-up lasted at least 2 yr. Patients' lymphocyte cultures showed in no case anti-acetylcholine receptor antibody production; HLA associations did not differ significantly from those in seropositive MG. In most cases (33 out of 38) the disease differed from seropositive MG only in the low incidence of thymic pathology. In five cases the clinical picture was characterized by prevalent involvement of ocular and bulbar muscles and by a more stable course. These last group of patients did not respond satisfactorily to immunosuppressants, but they clearly improved with plasma-exchange. Mice injected with plasma IgG from four patients (two with typical clinical picture and two with prevalent oculobulbar involvement) showed a defect of neuromuscular transmission. In our experience, SNMG is a heterogeneous disease. A humorally mediated pathogenesis appears to be operating in both groups of patients we described.

Modulation of acetylcholine receptor function in TE671 (rhabdomyosarcoma) cells by non-AChR ligands: possible relevance to seronegative myasthenia gravis

The acetylcholine receptor (AChR) is the main target antigen in myasthenia gravis (MG), but about 15% of patients with typical immunologically mediated MG do not have detectable anti-AChR antibodies. Previous studies showed that plasma from these 'seronegative' patients (SNMG) reduced AChR function in the human AChR-expressing TE671 cell line, and it was proposed that SNMG plasmas may act indirectly via phosphorylation of AChR. We show here that substances such as the beta 2-adrenergic agonist, salbutamol, calcitonin-gene-related-peptide (CGRP), and cholera toxin, that increase intracellular cAMP via binding to specific cell-surface receptors, reduced AChR function in TE671 cells. Moreover, non-specific activation of cell surface proteins by lectins achieved similar results. These observations lead us to hypothesise that SNMG immunoglobulins act in TE671 cells by cross-linking of specific cell surface antigen(s) resulting in generation of intracellular cAMP and/or other second messengers. The role of such antibodies at the neuromuscular junction in vivo could be reduction in AChR function by desensitization and/or damage to the postsynaptic membrane following complement activation.

Arthrogryposis multiplex congenita with maternal autoantibodies specific for a fetal antigen

Fetal arthrogryposis multiplex congenita (AMC) is characterised by non-progressive multiple joint contractures, which may result in fetal death, and is heterogeneous in origin. It can associate with maternal myasthenia gravis and autoantibodies to muscle acetylcholine receptor (AChR). We found maternal antibodies that selectively inhibit the fetal form of the AChR in a mother who herself had no features of myasthenia gravis. Maternal autoantibodies specific for fetal antigens could be an unrecognised cause of other congenital disorders.

Myasthenia gravis: immunological mechanisms and immunotherapy

This review of the immunological aspects of myasthenia gravis and the immunotherapy of the disease emphasizes the current state of knowledge of the immunological events at the neuromuscular junction, and the immunoregulatory abnormalities noted in myasthenic patients. The treatment modalities available to the clinician are discussed in an attempt to provide information that will allow for a rational approach to therapy.

Autoantibodies, neurotoxins and the nervous system

Myasthenia gravis, the Lambert-Eaton myasthenic syndrome, and acquired neuromyotonia are three disorders of the neuromuscular junction or motor nerve that are caused by autoantibodies to ion channel proteins: acetylcholine receptors, voltage-gated calcium channels and voltage-gated potassium channels, respectively. The antibody titres can be measured using the relevant 125I-neurotoxins to label the extracted channels. Other disorders of the peripheral motor nerve are associated with antibodies to gangliosides. Sera with raised levels of anti-ganglioside antibodies have direct effects on the function of the distal motor nerve and motor nerve terminal. These conditions can be improved by therapies designed to reduce circulating antibodies. Antibodies that bind to neuronal surface antigens are proving to be of great clinical importance and interest in neurological disorders.

Passive transfer of seronegative myasthenia gravis to mice

Muscle weakness in myasthenia gravis is due to autoantibody-induced loss of functional acetylcholine receptors (AChR). About 15% of myasthenia gravis patients, however, do not have detectable anti-AChR antibodies. To investigate the effect of their plasma immunoglobulins on neuromuscular transmission, mice were injected with plasma (and in some cases purified immunoglobulin G (IgG)) from 7 "seronegative" myasthenia gravis (SMG) patients, and neuromuscular transmission parameters were examined. When injected for 15 days, all patients' plasma caused reductions in miniature endplate potential amplitudes, while endplate potential quantal content was significantly reduced by plasma from 4 of the 7 patients. There were no changes in ACh-induced depolarization or single channel properties, and 125I-alpha-bungarotoxin binding studies showed no effect on AChR number, except in 1 case. Purified IgG injected for 3 days had similar effects to plasma injected for 15 days. Our findings confirm that SMG is autoantibody mediated and that there are pathogenic IgG antibodies. SMG appears to be a heterogeneous disorder and the target(s) for the antibodies may be diverse.

Plasma from patients with seronegative myasthenia gravis inhibit nAChR responses in the TE671/RD cell line

Myasthenia gravis (MG) is an autoimmune disorder in which anti-acetylcholine receptor (AChR) antibodies cause muscle weakness. In 10-15% of MG patients anti-AChR antibodies are undetectable (seronegative MG, SMG), though clinical and experimental evidence points to causative circulating factors. Using whole-cell patch-clamp techniques, we investigated the effects of heat-inactivated plasma from SMG patients (n = 7) on voltage-gated sodium [INa(V)] and ACh-induced nicotinic AChR (nAChR) currents in the human rhabdomyosarcoma cell line TE671/RD, comparing the results to those obtained with plasma from healthy individuals (HC, n = 6), patients with Guillain-Barré syndrome (GBS, n = 3) or those with other neurological diseases (OND, n = 3). None of the plasma samples inhibited INa(V). nAChR currents were rapidly (< 1 min) and significantly (P < 0.01) reduced by a 1:10 dilution of plasma from SMG patients compared with plasma from healthy controls and were not restored by washing. The inhibition appeared in some cases to be calcium dependent since for one of three plasmas it was prevented by 10 mM EGTA in the patch pipette. Currents were also reduced by two of three plasmas obtained from GBS patients at 1:3 dilution, but not by the three plasmas from patients with ONDs. The rapid action of plasma from SMG patients argues against an antibody-induced reduction in nAChR numbers; its calcium dependence in one case suggests action by a second messenger that might involve nAChR phosphorylation.

Anti-acetylcholine receptor (AChR) antibodies measurement in myasthenia gravis: the use of cell line TE671 as a source of AChR antigen

Acetylcholine receptor (AChR) from the human rhabdomyosarcoma cell line TE671 was compared with that of human ischaemic muscle AChR as a source of the antigen for the diagnosis of myasthenia gravis (MG). The sera, which were anti-TE671 cell AChR antibody-negative, all came from patients with low anti-human muscle AChR antibody titers. None of the sera that were seronegative as a result of the human muscle AChR RIA became positive with TE671 cell AChR. The overall sensitivity was 7% less using TE671 cell AChR. The lower sensitivity was observed irrespective of the clinical form of MG. It also appeared from this study that epitopes specific to the junctional isoform of human AChR are essential for the detection of low antibody titers, which accounts for this feature, since TE671 cells only express the extrajunctional isoform of AChR in the surface membrane. Accordingly, AChR from cell line TE671 cannot replace human muscle AChR in the conventional diagnostic immunoprecipitation RIA. There are, however, many other useful implications of AChR from cell line TE671.