Immunological heterogeneity and cellular mechanisms in myasthenia gravis

Sex bias in lymphocytes: Implications for autoimmune diseases

Autoimmune diseases are characterized by a significant sex dimorphism, with women showing increased susceptibility to disease. This is, at least in part, due to sex-dependent differences in the immune system that are influenced by the complex interplay between sex hormones and sex chromosomes, with contribution from sociological factors, diet and gut microbiota. Sex differences are evident in the number and function of lymphocyte populations. Women mount a stronger pro-inflammatory response than males, with increased lymphocyte proliferation, activation and pro-inflammatory cytokine production, whereas men display expanded regulatory cell subsets. Ageing alters the immune landscape of men and women in differing ways, resulting in changes in autoimmune disease susceptibility. Here we review the current literature on sex differences in lymphocyte function, the factors that influence this, and the implications for autoimmune disease. We propose that improved understanding of sex bias in lymphocyte function can provide sex-specific tailoring of treatment strategies for better management of autoimmune diseases.

Genomic Insights into Myasthenia Gravis Identify Distinct Immunological Mechanisms in Early and Late Onset Disease

OBJECTIVE

The purpose of this study was to identify disease relevant genes and explore underlying immunological mechanisms that contribute to early and late onset forms of myasthenia gravis.

METHODS

We used a novel genomic methodology to integrate genomewide association study (GWAS) findings in myasthenia gravis with cell-type specific information, such as gene expression patterns and promotor-enhancer interactions, in order to identify disease-relevant genes. Subsequently, we conducted additional genomic investigations, including an expression quantitative analysis of 313 healthy people to provide mechanistic insights.

RESULTS

We identified several genes that were specifically linked to early onset myasthenia gravis including TNIP1, ORMDL3, GSDMB, and TRAF3. We showed that regulators of toll-like receptor 4 signaling were enriched among these early onset disease genes (fold enrichment = 3.85, p = 6.4 × 10^-3 ). In contrast, T-cell regulators CD28 and CTLA4 were exclusively linked to late onset disease. We identified 2 causal genetic variants (rs231770 and rs231735; posterior probability = 0.98 and 0.91) near the CTLA4 gene. Subsequently, we demonstrated that these causal variants result in low expression of CTLA4 (rho = -0.66, p = 1.28 × 10^-38 and rho = -0.52, p = 7.01 × 10^-22 , for rs231735 and rs231770, respectively).

INTERPRETATION

The disease-relevant genes identified in this study are a unique resource for many disciplines, including clinicians, scientists, and the pharmaceutical industry. The distinct immunological pathways linked to early and late onset myasthenia gravis carry important implications for drug repurposing opportunities and for future studies of drug development. ANN NEUROL 2021;90:455-463.

Late onset myasthenia gravis is associated with HLA DRB1*15:01 in the Norwegian population

Background

Acquired myasthenia gravis (MG) is a rare antibody-mediated autoimmune disease caused by impaired neuromuscular transmission, leading to abnormal muscle fatigability. The aetiology is complex, including genetic risk factors of the human leukocyte antigen (HLA) complex and unknown environmental factors. Although associations between the HLA complex and MG are well established, not all involved components of the HLA predisposition to this heterogeneous disease have been revealed. Well-powered and comprehensive HLA analyses of subgroups in MG are warranted, especially in late onset MG.

Methodology/Principal Findings

This case-control association study is of a large population-based Norwegian cohort of 369 MG patients and 651 healthy controls. We performed comprehensive genotyping of four classical HLA loci (HLA-A, -B, -C and -DRB1) and showed that the DRB1*15:01 allele conferred the strongest risk in late onset MG (LOMG; onset ≥60years) (OR 2.38, p_c7.4×10⁻⁵). DRB1*13:01 was found to be a protective allele for both early onset MG (EOMG) and LOMG (OR 0.31, p_c 4.71×10⁻⁴), a finding not previously described. No significant association was found to the DRB1*07:01 allele (p_nc = 0.18) in a subset of nonthymomatous anti-titin antibody positive LOMG as reported by others. HLA-B*08 was mapped to give the strongest contribution to EOMG, supporting previous studies.

Conclusion

The results from this study provide important new information concerning the susceptibility of HLA alleles in Caucasian MG, with highlights on DRB1*15:01 as being a major risk allele in LOMG.

Thymoma and paraneoplastic myasthenia gravis

Paraneoplastic autoimmune diseases associate occasionally with small cell lung cancers and gynecologic tumors. However, myasthenia gravis (MG) occurs in at least 30% of all patients with thymomas (usually present at MG diagnosis). These epithelial neoplasms almost always have numerous admixed maturing polyclonal T cells (thymocytes). This thymopoiesis-and export of mature CD4(+)T cells-particularly associates with MG, though there are rare/puzzling exceptions in apparently pure epithelial WHO type A thymomas. Other features potentially leading to inefficient self-tolerance induction include defective epithelial expression of the autoimmune regulator (AIRE) gene and/or of major histocompatibility complex class II molecules in thymomas, absence of myoid cells, failure to generate FOXP3(+) regulatory T cells, and genetic polymorphisms affecting T-cell signaling. However, the strong focus on MG/neuromuscular targets remains unexplained and suggests some biased autoantigen expression, T-cell selection, or autoimmunization within thymomas. There must be further clues in the intriguing serological and cellular parallels in some patients with late-onset MG but without thymomas-and in others with AIRE mutations-and in the contrasts with early-onset MG, as discussed here.

Reflections on the "intrathymic pathogenesis" of myasthenia gravis

The beneficial effects of thymectomy argue for a causal role of the thymus in myasthenia gravis (MG). The MG thymus contains acetylcholine receptor (AChR), which is expressed by myoid cells (whole AChR), and by medullary thymic epithelial cells (AChR subunits). The myoid cells are closely associated with antigen-presenting dendritic cells, helper T cells, and antibody-producing B cells in lymphoid follicles ("lymphofollicular hyperplasia"). Thus, all the cellular components required to initiate and maintain an autoimmune response to AChR are present in the MG thymus. It is unlikely that the cellular alterations in the thymus are secondary to an ongoing peripheral immune response, because they are absent in experimental autoimmune myasthenia gravis.

Myasthenia gravis: a long term follow-up study of Swedish patients with specific reference to thymic histology

BACKGROUND

Myasthenia gravis (MG) is an autoimmune disorder of neuromuscular transmission. The majority of patients show abnormal thymic histology.

SETTING

The study was performed at the Myasthenia Gravis Centre, Karolinska University Hospital, and at the Immunological Research Laboratory, Centre for Molecular Medicine, Karolinska Institutet, Stockholm, Sweden.

PATIENTS AND METHODS

Information was collected retrospectively from 1956 and prospectively from 1975 on clinical data, concomitant diseases, concentration of serum acetylcholine receptor antibodies (AChR-abs), immunosuppressive treatment (IS) and response to it, in 537 patients of whom 326 were thymectomised. Follow-up time was 1.5-50 years.

RESULTS

Age at onset of MG increased from a median age of 24 years before 1975 to a median age of 61 years after 2000. Thymoma was found in 65, hyperplasia (HPL) in 185 and a normal thymus in 76 patients. The trans-sternal surgical approach for thymectomy was used in 255 patients (78%). In five patients with thymoma, MG appeared after thymectomy. Of 537 patients, 466 (87%) had circulating AChR-abs. IS was given to 300 (56%) patients, mostly those with thymoma (85%). In total, 441 patients (82%) showed an improvement. One-third of patients with HPL, a quarter of those with thymoma, one-fifth of those with a normal thymus and one-seventh of those not operated on went into remission.

CONCLUSION

The prognosis for the majority of patients with MG is favourable, irrespective of thymic histology. The cause may be the use of immunomodulating therapy.

Deficiency of the effector mechanisms of the immune response and autoimmunity

It has long been the conventional wisdom that most autoimmune responses represent a pathological aberration of the immune system and a great deal of effort has been devoted to investigating how such abnormal responses may be induced. It seems likely, however, particularly in the form of autoimmunity seen in diseases like systemic lupus erythematosus (LE) and associated with immune complex disease, that it is not the induction of the autoimmune response that is primarily abnormal, but its persistence, and that this abnormal persistence is a consequence of a failure of the proper functioning of the effector mechanisms of the immune response. The strongest reason for so believing is the striking incidence of these diseases in subjects with deficiencies of the early components of the classical complement pathway. Total, homozygous deficiencies are rare and account for only a small proportion of patients with systemic LE. However, partial, heterozygous deficiency of these components is much commoner and also carries an increased susceptibility to these diseases. An explanation for this association is given. In the presence of an adequately functioning complement system immune complexes remain soluble and of relatively small size. It is proposed that this is a result of the incorporation of C4 and C3 into the antigen-antibody lattice leading to a reduction in the effective valency of antigen and antibody. The Goldberg theory of immune precipitation predicts that a reduction in valency would inhibit precipitation and the formation of large complexes. In the absence of adequate complement function this mechanism will fail and large, potentially insoluble complexes with little C4 and C3 on them will be formed. These large immune complexes without sufficient C4 and C3 bound on them will also not be bound normally to erythrocyte CR1 and will therefore be transported in the (peripheral) plasma stream rather than in the (central) erythrocyte stream. It is proposed that this will result in the deposition of immune complexes in peripheral small blood vessels rather than in the sinusoids of the liver and spleen; and that this peripheral deposition gives rise to inflammation, with the release of autoantigens and the formation of further autoantibodies. The importance of CR1 in relation to these diseases is emphasized by the reduction in CR1 numbers that accompanies their active phase. This appears to be due to proteolysis of the receptor while the immune complex-bearing erythrocyte is sequestered in the reticuloendothelial system.

The Role of Thymomas in the Development of Myasthenia Gravis

Thymic pathology occurs in 80-90% of myasthenia gravis patients. Significant associations between different thymic alterations and clinical findings are discussed. To highlight peculiarities in thymoma-associated myasthenia gravis, we briefly review myasthenia gravis associated with thymic lymphofollicular hyperplasia (TFH) and thymic atrophy.

Circulating regulatory anti-T cell receptor antibodies in patients with myasthenia gravis

Serum anti-T cell receptor (TCR) Ab's are involved in immune regulation directed against pathogenic T cells in experimental models of autoimmune diseases. Our identification of a dominant T cell population expressing the Vbeta5.1 TCR gene (TCRBV5-1), which is responsible for the production of pathogenic anti-acetylcholine receptor (AChR) autoantibodies in HLA-DR3 patients with early-onset myasthenia gravis (EOMG), prompted us to explore the occurrence, reactivity, and regulatory role of anti-TCR Ab's in EOMG patients and disease controls with clearly defined other autoantibodies. In the absence of prior vaccination against the TCR, EOMG patients had elevated anti-Vbeta5.1 Ab's of the IgG class. This increase was restricted largely to EOMG cases with HLA-DR3 and with less severe disease, and it predicted clinical improvement in follow-up studies. EOMG patient sera containing anti-TCR Ab's bound specifically the native TCR on intact Vbeta5.1-expressing cells and specifically inhibited the proliferation and IFN-gamma production of purified Vbeta5.1-expressing cells to alloantigens in mixed lymphocyte reaction and the proliferation of a Vbeta5.1-expressing T cell clone to an AChR peptide, indicating a regulatory function for these Ab's. This evidence of spontaneously active anti-Vbeta5.1 Ab's in EOMG patients suggests dynamic protective immune regulation directed against the excess of pathogenic Vbeta5.1-expressing T cells. Though not sufficient to prevent a chronic, exacerbated autoimmune process, it might be boosted using a TCR peptide as vaccine.

Polymorphic markers in MHC class II/III region: a study on Italian patients with myasthenia gravis

With an Italian case series of 81 Italian patients and 130 controls, we analysed associations between myasthenia gravis (MG) and genetic polymorphisms in the MHC class II/III region. Increases in the frequency of the TNF-B*1, C4A*Q0, C4B*1, DRB1*03 supratype, which is likely part of the 8.1 ancestral haplotype, were maximal in females with early onset (EO) MG vs. controls [p<0.05, relative risk (RR)=9.9]. These patients showed neither a significantly high frequency of thymic hyperplasia, nor high levels of serum anti-acethylcholine receptor antibodies. The DRB1*03 allele was absent in patients with thymoma; however, in comparison with controls, occurrence of this marker was frequent in MG patients (p<0.005; RR=6.2), more frequent in females (p<0.005; RR=7.8) and most frequent in EOMG female patients (p<0.005; RR=15.1). Analysis of the TNF-B*1, C4A*Q0, C4B*1, DRB1*03 supratype and its recombinants showed that the MHC region between C4 and TNF might contain genes that influence susceptibility to MG in females. Polymorphic markers within the supratype, e.g. TNF-B*1 and C4A*Q0, might contribute to pathogenetically significant abnormalities in immune responses in a subset of female MG patients. The combined effect of other intervening genes cannot be excluded.

HLA class I and II in Lambert-Eaton myasthenic syndrome without associated tumor

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder, in which antibodies against voltage-gated calcium channels located at nerve terminals cause muscle weakness and autonomic dysfunction. In approximately half of the patients the autoimmune process is initiated by a tumor. In the other half of patients no tumor is found and the etiology is unknown. The aims of this study were to investigate the strength of HLA-associations with nontumor LEMS (NT-LEMS) and to study the relation of HLA-haplotypes with age at onset of LEMS and other clinical features. Therefore, typing of HLA class I and II was performed in 19 patients with NT-LEMS, who were clinically evaluated. NT-LEMS was significantly associated with alleles of both HLA-class I (i.e. HLA-B8) as well as -class II (i.e. HLA-DR3 and -DQ2). HLA-B8+ patients had significantly younger age at onset of LEMS and tended to be female. This study shows that HLA-class I haplotype is associated with a distinct phenotype in NT-LEMS.

Teratogen update: maternal myasthenia gravis as a cause of congenital arthrogryposis

BACKGROUND

Arthrogryposis multiplex congenita (AMC) is defined as nonprogressive congenital contractures that generally result from lack of fetal movement in utero. AMC is a feature of many congenital disorders caused by genetic, environmental, or other factors. One rare cause of AMC is maternal myasthenia gravis (MG). This is an autoimmune disorder, caused by antibodies to the nicotinic acetylcholine receptor (AChR), and resulting in weakness of voluntary muscles. In 10-15% of babies born to MG mothers, transient signs of MG are noted after placental transfer of anti-AChR antibodies. In a few cases, AMC predominates.

METHODS

We review the role of antibodies to AChR in MG and in AMC associated with maternal antibodies to AChR.

RESULTS

In anti-AChR antibody-associated AMC, fetal or neonatal death is common; other deformities or CNS abnormalities are common as well. The condition usually recurs in each pregnancy unless the mother is treated for MG, but some mothers are asymptomatic. The maternal antibodies cross the placenta and block the function of the fetal isoform of the AChR leading to fetal paralysis. Injection of maternal plasma into pregnant mice results in AMC in mouse fetuses. Some women with recurrent AMC in their babies have no detectable anti-AChR suggesting the presence of antibodies to other fetal muscle or neuronal proteins.

CONCLUSIONS

Although rare, anti-AChR-associated AMC is potentially treatable and can be diagnosed by a routine antibody test. The mouse model can be used to investigate the role of these and other maternal antibodies in causing congenital conditions.

Acetylcholine receptors and myasthenia

Much progress has been made in the 26 years since initial studies of the first purified acetylcholine receptors (AChRs) led to the discovery that an antibody-mediated autoimmune response to AChRs causes the muscular weakness and fatigability characteristic of myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG). Now, the structure of muscle AChRs is much better known. Monoclonal antibodies to muscle AChRs, developed as model autoantibodies for studies of EAMG, were used for initial purifications of neuronal AChRs, and now many homologous subunits of neuronal nicotinic AChRs have been cloned. There is a basic understanding of the pathological mechanisms by which autoantibodies to AChRs impair neuromuscular transmission. Immunodiagnostic assays for MG are used routinely. Nonspecific approaches to immunosuppressive therapy have been refined. However, fundamental mysteries remain regarding what initiates and sustains the autoimmune response to muscle AChRs and how to specifically suppress this autoimmune response using a practical therapy. Many rare congenital myasthenic syndromes have been elegantly shown to result from mutations in muscle AChRs. These studies have provided insights into AChR structure and function as well as into the pathological mechanisms of these diseases. Evidence has been found for autoimmune responses even to some central nervous system neurotransmitter receptors, but only one neuronal AChR has so far been implicated in an autoimmune disease. Thus far, only two neuronal AChR mutations have been found to be associated with a rare form of epilepsy, but many more neuronal AChR mutations will probably be found to be associated with disease in the years ahead.

A susceptibility region for myasthenia gravis extending into the HLA-class I sector telomeric to HLA-C

We have analyzed a series of HLA region markers in 207 UK Caucasoids with early-onset myasthenia gravis (EOMG, onset before age 40), where there is a strong female bias. The well known associations with HLA-DR3 and -B8 have now proved to be significantly stronger in the 165 females than in the 42 males. In patients (of either sex) lacking -DR3, there was also a significant increase in HLA-DR2. Although the muscle weakness in EOMG is clearly mediated by autoantibodies, the associations are consistently stronger with HLA-B8 (in class I) than with HLADR3 (in class II), as confirmed here. We therefore typed 87-137 cases for polymorphisms at four loci in the intervening class III region, and also at three in the adjacent stretch of class I. At each locus, one allele tended to co-occur with HLA-B8 and showed strong and highly significant associations in the patients. There appeared to be a region of maximal susceptibility extending from HSP70 (in class III) past HLA-B and HLA-C at least 600 kb telomerically into the class I region, which is now being mapped in detail. Any candidate genes here that act shortly after puberty may allow more precise localization of susceptibility.

Acquired binocular horizontal diplopia

Binocular horizontal diplopia is an important symptom that may foretell or help localize and characterize various neurologic and neuromuscular disorders. An appropriate evaluation requires a careful and complete neuro-ophthalmic history and examination. This review focuses on the differential diagnosis of binocular horizontal diplopia.

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.

Absence of Oligoclonal IgG Bands and Anti-Achr Antibodies in the Cerebrospinal Fluid of Patients with Myasthenia Gravis

Myasthenia gravis (MG) is a neuromuscular disorder in which antibodies are directed against the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Some investigators describe the existence of oligoclonal IgG bands and anti-AChR antibodies in the cerebrospinal fluid (CSF) of MG patients while other refuse it. This study was performed in 15 patients with clinical and electrophysiological diagnosis of MG. Oligoclonal IgG bands (OCB) and antibodies to the AChR from human skeletal muscle were determined in the serum and the CSF of the above MG patients. The last one was done in order to investigate any possible central nervous system (CNS) involvement.

It was found that all the MG patients who had a high titre of anti- AChR antibodies in the serum (mean titre 29.2±24.3 nM, range 1.8 to 62 nM) did not present OCB and anti-AChR antibodies in their CSF. On the same time, in a group of 10 patients with a definite multiple sclerosis it was found that eight of them presented OCB in their CSF while the results were negative in another group of 10 patients without evidence of CNS disease. The last two groups served as control groups.

Our findings are in accordance with the concept that MG is a pure neuromuscular disorder.

Expression of CD23 in the germinal center of thymus from myasthenia gravis patients

In order to investigate a pathogenic role of germinal centers which appear in the hyperplastic thymus of myasthenia gravis (MG) patients, we performed an immunohistochemical study using various monoclonal antibodies including CD23. In contrast with tonsilar germinal centers from non-MG individuals, CD23 was strongly and diffusely expressed in the whole area of germinal centers of MG thymi, including the outer zone. In addition, we measured the serum level of soluble CD23 (sCD23) in MG patients at various clinical stages. The high serum sCD23 levels, which were noted in the unthymectomized patients, fell to within normal range over 5 years after thymectomy, and the decline of serum sCD23 correlated well with clinical improvement. CD23 is thought to be responsible for preventing unselected germinal center B cells from entering apoptosis and, in turn, leads to the survival of auto-reactive B cell clones.

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.

A practical guide to the recognition and management of myasthenia gravis

Myasthenia gravis is an autoimmune disorder of neuromuscular transmission. It is characterised by muscular weakness and fatiguability. Extrinsic ocular muscles are frequently involved at the onset of the disease; in about 15% of cases clinical signs remain confined to these areas, while in the other patients weakness becomes generalised. Diagnosis relies on clinical history and signs, improvement on anticholinesterase injection, electromyographic signs of impaired neuromuscular transmission and serum antiacetylcholine receptor (anti-AChR) antibody titration. Apart from the anti-AChR antibody assay, no single test is specific for myasthenia gravis; it is rather their combined results that can confirm diagnosis. The therapeutic approach varies from patient to patient and, as far as possible, should be adapted to each individual. Symptomatic drugs such as anticholinesterases improve muscle strength but seldom restore it to normal. Immunosuppressive therapy is very effective but entails the risk of severe adverse effects. Thymectomy improves the course of the disease and can increase remission rate; short term treatments are of benefit in the management of myasthenic emergencies.

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.

Familial autoimmune myasthenia gravis: report of four families

Four families each with two patients with autoimmune myasthenia gravis or related conditions are reported. All clinical forms of myasthenia gravis were represented and different disease types were found within the same family. Either one or two generations could be affected and no association with a single HLA haplotype was found. The frequency of familial autoimmune myasthenia gravis is very low and the genetic factors involved seem to be different from MHC genes.

Myasthenia gravis-like syndrome induced by expression of interferon gamma in the neuromuscular junction

Abnormal humoral responses toward motor end plate constituents in muscle induce myasthenia gravis (MG). To study the etiology of this disease, and whether it could be induced by host defense molecules, we examined the consequences of interferon (IFN) gamma production within the neuromuscular junction of transgenic mice. The transgenic mice exhibited gradually increasing muscular weakness, flaccid paralysis, and functional disruption of the neuromuscular junction that was reversed after administration of an inhibitor of acetylcholinesterase, features which are strikingly similar to human MG. Furthermore, histological examination revealed infiltration of mononuclear cells and autoantibody deposition at motor end plates. Immunoprecipitation analysis indicated that a previously unidentified 87-kD target antigen was recognized by sera from transgenic mice and also by sera from the majority of human MG patients studied. These results suggest that expression of IFN-gamma at motor end plates provokes an autoimmune humoral response, similar to human MG, thus linking the expression of this factor with development of this disease.

Ocular myasthenia: a protean disorder

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

Augmented interferon-gamma, interleukin-4 and transforming growth factor-beta mRNA expression in blood mononuclear cells in myasthenia gravis

The abnormal T lymphocyte-dependent production of antibodies to the nicotinic acetylcholine receptor (AChR) in myasthenia gravis (MG) suggests a role for immunoregulatory cytokines. We examined the T helper type 1 (Th1) cell-associated interferon-gamma (IFN-gamma) that promotes cell-mediated immunity, the Th2 cell-related interleukin-4 (IL-4) that augments B cell immunity, and transforming growth factor-beta (TGF-beta) that downregulates immune responses but enhances isotype switching. Blood mononuclear cells (MNC) expressing cytokine mRNA were enumerated after in situ hybridization with labelled complementary DNA oligonucleotide probes for IFN-gamma, IL-4 and TGF-beta. MG patients had elevated numbers of cells expressing IFN-gamma and IL-4 compared to patients with non-inflammatory neurological diseases and healthy controls, implying that both Th1- and Th2-like cells are involved in MG. TGF-beta-positive cells were also elevated in MG. The levels of cytokine-positive MNC were similar in MG and in control patients with other inflammatory neurological diseases. There were no associations between numbers of cytokine-positive blood MNC and clinical variables of MG, but individual patients need to be studied over the course of MG to clarify a relation between the cytokines under study and clinical or laboratory variables of MG.

HLA-DPB1 polymorphisms in patients with hyperthyroid Graves' disease and early onset myasthenia gravis

Using the technique of in vitro enzymatic DNA amplification and dot blot hybridization with sequence-specific oligonucleotide (SSO) probes, a study of genetic polymorphism of HLA-DPB1 was performed in 83 unrelated patients with Graves' disease (GD), 48 patients with early onset myasthenia gravis (EOMG) and 100 normal British caucasoid subjects who were also tissue typed for HLA-A, B and DR antigens. HLA-DPB1*0401 was the commonest allele in both patient and control groups with gene frequencies of 0.380, 0.333 and 0.445 for GD, EOMG and controls, respectively. No significant independent association was found with any HLA-DPB1 allele. As expected, HLA-DR17 is significantly associated with Graves' disease (pc < 8 x 10(-3), RR = 2.9), while both HLA-B8 and DR17 are significantly associated with EOMG (pc < 2 x 10(-7), RR = 10.3 and pc < 0.02, RR = 3.4, respectively)] HLA-DR2 is also significantly increased in EOMG patients who were negative for HLA-DR17 (pc < 0.02, RR = 6.4). In addition, the co-occurrence of HLA-B8 with DPB1*0402 was significantly commoner in patients with GD (p < 0.021, RR = 6.2) and EOMG (p < 0.0007, RR = 10.8) than in controls, although the HLA-DPB1*0402 by itself showed no significant increase.

Combined effects of a thymic peptide, thymopoietin and myasthenic patient sera in rat myotube culture

We investigated in a rat myotube assay the combined effect of 26 myasthenic (MG) patient sera and a thymic peptide, thymopoietin (Tpo) which had previously been shown to bind Torpedo and human AChR and to compete with alpha-bungarotoxin (alpha-Bgt) binding. Cultures were first exposed to Tpo alone for 3 h (0.3, 7.5, 15 nM), then MG sera (5% final dilution) were added for an additional 18 h. Reduction in the amount of 125I-alpha-Bgt binding sites in the presence of various concentrations of Tpo were similar with control sera and in all the patients with low or undetectable anti-AChR Ab (11 cases). In cultures exposed to Tpo and sera with high anti-AChR Ab titre (15 cases), Tpo and anti-AChR Ab have an additive capacity to reduce the number of alpha-Bgt binding sites. The results are compatible with the hypothesis that anti-AChR Ab and Tpo could impair neuromuscular transmission by complementary mechanisms.

Myasthenia gravis: T and B cell reactivities to the beta-bungarotoxin binding protein presynaptic membrane receptor

Antibodies against acetylcholine receptor (AChR) can be detected in most patients with myasthenia gravis (MG) and are considered to be involved in the immunopathogenesis of this disease. AChR are isolated from crude receptor preparations by binding to alpha-bungarotoxin (alpha-BuTx). Patients with MG have also antibodies against a second protein tentatively named presynaptic membrane receptor (PsmR), which has been isolated from crude receptor utilizing beta-bungarotoxin (beta-BuTx). PsmR could represent another antigen besides AChR relevant for the development of MG. We have now evaluated the T cell reactivity to PsmR in MG and controls by analysing the frequencies of cells which in response to PsmR in short-term cultures secreted interferon-gamma (IFN-gamma). The B cell response to PsmR was analysed in parallel by counting cells secreting anti-PsmR antibodies. Most patients with MG had PsmR reactive T cell in blood with a median number of about 1 per 44,000 mononuclear cells. Cells secreting anti-PsmR antibodies belonging to the IgG and IgA isotypes, less frequently of the IgM isotype were detected in most MG patients. A positive correlation was found between T cells reactive with PsmR and anti-PsmR IgG antibody secreting cells. PsmR reactive T and B cells were also detected in control patients, but at much lower numbers. Our results indicate that MG is accompanied by T as well as B cell responses to PsmR, in addition to the previously recognized responses to AChR. It remains to be shown whether these PsmR reactivities are of pathogenetic importance in MG.

Antibodies to acetylcholine receptors in myasthenia gravis. In vitro synthesis by peripheral blood lymphocytes before and after thymectomy

Pokeweed mitogen (PWM)-driven in vitro synthesis of antibodies to the acetylcholine receptor (PSA) was studied in non-thymoma patients with myasthenia gravis. In a group of 46 patients, the occurrence of PSA was related to the presence of the thymus or, in operated patients, the absence of a clinical effect of thymectomy. Sixteen patients were followed before and soon after thymectomy. PSA disappeared in all patients, at least temporarily, between 6 weeks and 1 year afterwards, independent of the clinical course and eventual clinical effect of the operation. A recurrence was found only in one of the five patients who derived no benefit from the operation. These findings support the hypothesis that the therapeutic effect of thymectomy can be explained by removal of a source of autoreactive lymphocytes. There was no correlation between the changes in serum levels of a-AChR and clinical improvement, suggesting a minor role of circulating peripheral blood lymphocytes (PBL) and the thymus in the total production of a-AChR.

Immunoglobulin heavy chain gene associations in myasthenia gravis: new evidence for disease heterogeneity

Susceptibility to myasthenia gravis (MG) is known to involve genes residing in the major histocompatibility complex class I and II regions (HLA-B8 and DR3). Immunoglobulin heavy chain constant region (IgCH) allotypes have also shown some associations with MG. We have used restriction fragment length polymorphism analysis with probes to the IgCH switch (S) regions mu and alpha 1 and the downstream marker D14S1 to investigate 189 Caucasoid patients with well-defined MG. A highly significant increase in the frequency of the 2.6 kilobase (kb) S mu homozygous genotype and the 2.6 kb S mu allele was found in patients with disease onset after the age of 40 years (late onset) compared with normal controls (P less than 0.00075 and P less than 0.025 respectively). No association was found at the S alpha 1 or D14S1 loci. In patients with an associated thymoma there was a moderate increase in the frequency of the 2.6 kb S mu and 7.4 kb S alpha 1 genotypes. These results independently support the previous separation of the late-onset subgroup. Finally, the stronger association at S mu rather than at the downstream S alpha 1, Gm and D14S1 loci suggest that the genes predisposing to MG are located within the variable region of the Ig heavy chain loci.

The thymus in seronegative myasthenia gravis patients

In 5-10% of all patients with typical generalised myasthenia gravis (MG), serum antibody to the acetylcholine receptor (AChR) is undetectable. To determine whether these represent a distinct subgroup, we have compared the thymuses of 14 seronegatives, 70 seropositives and 12 non-myasthenic controls. By quantitative immunohistology on coded sections, the 7 seronegative samples were clearly distinguishable from the controls by the pronounced lymph node-type T-cell areas in the medulla. While these closely resembled those in the seropositive cases, germinal centres were significantly sparser, and total in vitro IgG production was disproportionately low (per B cell) in the 12 cases tested. Furthermore, specific anti-AChR production was never detected in any of these cultures. The data support the view that the medullary T-cell areas are the most consistent abnormality in the MG thymus (though it may not be primary), and they strongly imply that seronegative and seropositive MG are distinct entities.

Seronegative myasthenia gravis: clinical features, response to therapy and synthesis of acetylcholine receptor antibodies in vitro

Circulating autoantibodies against the acetylcholine receptor (AChR-Ab) are an important diagnostic tool in myasthenia gravis (MG). Lack of antibodies may cast doubt upon the diagnosis, the immune-mediated mechanism and the nature of the antigen. We examined clinical and laboratory features, response to immunotherapy and production of AChR-Ab in vitro, in 12 seronegative MG patients who were followed up for 2-30 years. It was possible to divide those patients into 2 groups: 7 patients with systemic muscle weakness, with a severe disease and with response to immunosuppressive therapies. The other group of 5 patients was characterized by oculobulbar symptomatology, a relatively benign course and immunotherapy was ineffective in 3 treated patients. Five patients underwent thymectomy and gland histology was normal in all of them. In none of 9 patients examined, were AChR-Ab synthesized in vitro (compared to 65% of seropositive myasthenic patients). Thus seronegative generalized MG is probably an autoimmune disease though the autoantigen is presently unknown and is responsive to immunosuppressive treatment. Seronegative oculobulbar MG might represent a separate disease entity in which immunological mechanisms play no significant role.

A sensitive rosetting assay for detection of acetylcholine receptor antibodies using BC3H-1 cells: positive results in 'antibody-negative' myasthenia gravis

Antibodies to acetylcholine receptor (AChR) were measured in a group of patients with myasthenia gravis (MG), some of whom had previously been classified as 'antibody negative' using the standard anti-AChR radioimmunoassay (RIA). AChR antibodies were measured using the rosetting assay, a new detection method which utilizes protein A-coated red blood cells and live BC3H-1 cells, a murine cell line which expresses muscle nicotinic AChR. The results of the rosetting assay were compared with those obtained in the anti-AChR RIA. 76% of all myasthenic sera tested showed rosetting at titers higher than any of the control sera (from patients with non-myasthenic neurologic disease and normal individuals). Of the myasthenic patients previously classified as 'antibody negative' in the RIA using human AChR, 71% demonstrated positive rosetting. There was no correlation between the anti-AChR antibody titer obtained in the rosetting assay and that obtained in the RIA using either human or denervated rat AChR. The results suggest that the rosetting assay may measure a subpopulation of antibodies that differs from those detected in the RIA.

Oligoclonal immunoglobulin G in cerebrospinal fluid of myasthenia gravis patients

Immunoglobulin G (IgG) band patterns were investigated in cerebrospinal fluid (CSF) from 19 patients with myasthenia gravis (MG) in a search for abnormalities indicating central nervous system (CNS) involvement in this disorder. Using the isoelectric focusing (IEF) technique and antiserum immunoblotting against IgG, we found no evidence of the presence of oligoclonal IgG in CSF from most of MG patients. In 2 cases, the positive findings of oligoclonal IgG in CSF may have reflected a manifestation of an associated disease, which has already been associated with immune abnormalities within the CNS. Further investigations with more sophisticated techniques are required to give additional insight into humoral immune events within the CNS in MG patients.

Theoretical mechanisms by which immunoglobulin therapy might benefit myasthenia gravis

The mechanisms underlying any clinical improvement observed in some patients with myasthenia gravis (MG) treated with normal immunoglobulin (Ig) are not defined. The pathologic alterations in the postsynaptic motor end plate in MG are likely due at least in part to one or more actions of antibodies against epitopes on the nicotinic acetylcholine receptor (anti-AChR Ab). Such anti-AChR Ab are secreted by B lymphocytes and are increased in the serum of MG patients but not of controls. The stimulus for anti-AChR Ab production in MG is unknown with evidence for a role of thymic abnormalities, immunoregulatory disturbances, and some possible molecular mimicry of exogenous antigens (microbial?). Postulated mechanisms underlying Ig effects in MG include: (i) competing with anti-AChR for binding to AChR; (ii) preventing attachment of Fc receptor-positive inflammatory cells to the anti-AChR Ab bound to the motor end plate; (iii) decreasing synthesis of anti-AChR Ab; and (iv) exerting an anti-idiotypic effect. Evidence for these mechanisms are discussed.

Analysis of immunoglobulin and T cell receptor gene rearrangements in the thymus of myasthenia gravis patients

The thymus is an important site of sensitization of autoreactive B and T lymphocytes in myasthenia gravis (MG). We have investigated clonal diversity of B or T cells in the thymus of patients with MG by Southern blot experiments using probes specific for immunoglobulin heavy (IgH) and light chain (IgL) genes and for T cell receptor (TCR) beta- and gamma-chain genes. This method allows to detect individual clones of B or T cells if they represent at least 1% of the total cell population. We investigated thymus glands from 14 patients who underwent thymectomy. Single rearranged fragments could be demonstrated with a TCR gamma-specific probe in DNAs from both normal donors and MG patients. TCR beta gene rearrangements occurred mainly in the C beta 1 region. However, single rearranged bands could not be detected with either TCR beta, JH or with J kappa specific probes. Thus any single autoimmune B and T cell clone present in the myasthenic thymus represents presumably less than 1% of all thymocytes.

Myasthenia gravis thymus: clinical, histological and culture correlations

This paper attempts to quantitate immunohistological changes in the myasthenia gravis (MG) thymus and to correlate them with clinical and culture parameters in 40 untreated young onset patients covering a wide range of durations and serum anti-acetylcholine receptor (AChR) antibody titers. Total cellularities of both the thymic cortex and the medulla declined significantly with age. There was some hyperplasia of subcapsular and of medullary epithelial cells, often at the expense of cortex. A combined index of all hyperplastic changes correlated significantly with serum anti-AChR titre. Otherwise histological indices, e.g. of germinal centres (GC) were largely unrelated to any clinical parameters, especially duration of symptoms. Specific anti-AChR synthesis in culture (very closely related to serum titer) correlated better with the medullary lymph node-type T-cell areas; these were more widely prevalent and MG-specific. In contrast, basal and mitogen-stimulated total IgG productivity followed the GC indices more closely. We propose that the variability of GC is due to their dependence on extraneous immune complexes, and we discuss whether they or the T-cell areas are primary or secondary abnormalities. Finally, we conclude that autosensitization in MG with thymic hyperplasia and neoplasia probably arises through separate mechanisms.

Familial autoimmune myasthenia gravis with different pathogenetic antibodies

Two cases of familial myasthenia gravis are reported. One patient is a typical case of autoimmune myasthenia with positive anti acetylcholine receptor antibodies, while in the second patient the impairment of neuromuscular transmission is likely to be due to antibodies directed against determinants other than the acetylcholine receptors.