Anatomy of the antigenic structure of a large membrane autoantigen, the muscle-type nicotinic acetylcholine receptor

Detection of Autoantibodies Against the Acetylcholine Receptor, Evaluation of Commercially Available Methodologies: Fixed Cell-Based Assay, Radioimmunoprecipitation Assay and Enzyme-Linked Immunosorbent Assay1

Background/Objective

Myasthenia Gravis (MG) is an autoimmune disorder characterized by pathogenic autoantibodies (AAbs) targeting nicotinic acetylcholine receptors (AChR), disrupting neuromuscular communication. RadioImmunoPrecipitation Assay (RIPA) is recommended to detect AChR AAbs, but its complexity and radioactive requirements limit widespread use. We compare non-RIPA anti-AChR immunoassays, including Cell-Based Assay (CBA) and two ELISA kits, against the gold standard RIPA.

Methods/Results

145 samples were included with medical indication for anti-AChR testing. By the RIPA method, 63 were negative (RIPA-Neg < 0.02 nmol/L), 18 were classified as Borderline (≥0.02 -1 nmol/L), and 64 were positive (RIPA-Pos > 1 nmol/L). The competitive ELISA showed poor agreement with RIPA (Kappa = 0.216). The indirect ELISA demonstrated substantial agreement with RIPA (Kappa = 0.652), with ∼76% sensitivity and ∼94% specificity for MG diagnostic. The CBA, where fixed cells expressing clustered AChR were used as substrate, exhibited almost perfect agreement with RIPA (Kappa = 0.984), yielding ∼98% sensitivity and 96% specificity for MG. In addition, a semiquantitative analysis showed a strong correlation between CBA titration, indirect ELISA, and RIPA levels (r = 0.793 and r = 0.789, respectively).

Conclusions

The CBA displayed excellent analytical performance for MG diagnostic when compared to RIPA, making it a potential replacement for RIPA in clinical laboratories. Some solid-phase assays (such as the indirect ELISA applied here), as well as CBA titration, offer reliable options to estimate anti-AChR AAb levels after confirming positivity by the CBA.∥.

Role of complement in myasthenia gravis

Myasthenia gravis is a prototypic neuroimmune disorder with autoantibodies targeting the acetylcholine receptor complex at the neuromuscular junction. Patients present with mainly ocular muscle weakness and tend to have a generalized muscle weakness later in the clinical course. The weakness can be severe and fatal when bulbar muscles are heavily involved. Acetylcholine receptor antibodies are present in the majority of patients and are of IgG1 and IgG3 subtypes which can activate the complement system. The complement involvement plays a major role in the neuromuscular junction damage and the supporting evidence in the literature is described in this article. Complement therapies were initially studied and approved for paroxysmal nocturnal hemoglobinuria and in the past decade, those have also been studied in myasthenia gravis. The currently available randomized control trial and real-world data on the efficacy and safety of the approved and investigational complement therapies are summarized in this review.

Individual myasthenia gravis autoantibody clones can efficiently mediate multiple mechanisms of pathology

Serum autoantibodies targeting the nicotinic acetylcholine receptor (AChR) in patients with autoimmune myasthenia gravis (MG) can mediate pathology via three distinct molecular mechanisms: complement activation, receptor blockade, and antigenic modulation. However, it is unclear whether multi-pathogenicity is mediated by individual or multiple autoantibody clones. Using an unbiased B cell culture screening approach, we generated a library of 11 human-derived AChR-specific recombinant monoclonal autoantibodies (mAb) and assessed their binding properties and pathogenic profiles using specialized cell-based assays. Five mAbs activated complement, three blocked α-bungarotoxin binding to the receptor, and seven induced antigenic modulation. Furthermore, two clonally related mAbs derived from one patient were each highly efficient at more than one of these mechanisms, demonstrating that pathogenic mechanisms are not mutually exclusive at the monoclonal level. Using novel Jurkat cell lines that individually express each monomeric AChR subunit (α2βδε), these two mAbs with multi-pathogenic capacity were determined to exclusively bind the α-subunit of AChR, demonstrating an association between mAb specificity and pathogenic capacity. These findings provide new insight into the immunopathology of MG, demonstrating that single autoreactive clones can efficiently mediate multiple modes of pathology. Current therapeutic approaches targeting only one autoantibody-mediated pathogenic mechanism may be evaded by autoantibodies with multifaceted capacity.

Immune checkpoint inhibitor-induced myocarditis and myositis in liver cancer patients: A case report and literature review

With the development of immunotherapy, immune checkpoint inhibitors (ICIs) are widely used in clinical oncology and have achieved good results. ICIs could induce immune-related adverse events (irAEs) in cancer treatment, which warrant sufficient attention. Among them, immune myositis can manifest severe symptoms affecting the whole body, and immune myocarditis occurs with a low incidence but high fatality rate. Here we report a case of grade 3/4 adverse reactions in a patient with partial hepatectomy for malignancy after using ICIs and describe the clinical presentation, laboratory results, treatment, and prognosis. It emphasizes that clinicians should focus on being alert to irAEs in liver cancer patients who have received ICI therapy. The case we present is a 56-year-old male diagnosed with hepatocellular carcinoma. Right hepatic lobectomy was performed in April 2019. Postoperative follow-up showed that transcatheter arterial chemoembolization (TACE) combined with sorafenib (400 mg twice daily) failed to stop the recurrence of the tumor. In December 2020, the patient started to use Camrelizumab injections (200mg/injection every 21 days as a cycle). After 3 cycles, the patient had decreased muscle strength in both lower extremities with chest tightness, dyspnea, and expectoration (whitish sputum). The diagnosis was ICIs injection-induced immune myocarditis and myositis accompanied. The patient's condition improved considerably by steroid pulse therapy timely. The case emphasizes that clinicians should focus on being alert to irAEs in liver cancer patients who have received ICI therapy.

Complement and myasthenia gravis

Myasthenia gravis is a neuromuscular disease associated with antibodies against components of the neuromuscular junction, most often against the acetylcholine receptor (AChR). Although several mechanisms have been postulated to explain how these autoantibodies can lead to the pathology of the disease, convincing evidence suggests that destruction of the receptor-bearing postsynaptic membrane by complement membrane attack complex is of central importance. In this review, evidence for the importance of complement, and possible relationships between autoantigen, autoantibodies, complement activation, and the destruction of the membrane are discussed. More recent insights from the results of the complement-inhibiting therapeutic antibody eculizumab are also described, and the mechanisms connecting antibody binding to complement activation are considered from a structural viewpoint.

Heterogeneity of Acetylcholine Receptor Autoantibody-Mediated Complement Activity in Patients With Myasthenia Gravis

BACKGROUND AND OBJECTIVES

Autoantibodies targeting the acetylcholine receptor (AChR), found in patients with myasthenia gravis (MG), mediate pathology through 3 mechanisms: complement-directed tissue damage, blocking of the acetylcholine binding site, and internalization of the AChR. Clinical assays, used to diagnose and monitor patients, measure only autoantibody binding. Consequently, they are limited in providing association with disease burden, understanding of mechanistic heterogeneity, and monitoring therapeutic response. The objective of this study was to develop a cell-based assay that measures AChR autoantibody-mediated complement membrane attack complex (MAC) formation.

METHODS

An HEK293T cell line-modified using CRISPR/Cas9 genome editing to disrupt expression of the complement regulator genes (CD46, CD55, and CD59)-was used to measure AChR autoantibody-mediated MAC formation through flow cytometry.

RESULTS

Serum samples (n = 155) from 96 clinically confirmed AChR MG patients, representing a wide range of disease burden and autoantibody titer, were tested along with 32 healthy donor (HD) samples. AChR autoantibodies were detected in 139 of the 155 (89.7%) MG samples through a cell-based assay. Of the 139 AChR-positive samples, autoantibody-mediated MAC formation was detected in 83 (59.7%), whereas MAC formation was undetectable in the HD group or AChR-positive samples with low autoantibody levels. MAC formation was positively associated with autoantibody binding in most patient samples; ratios (mean fluorescence intensity) of MAC formation to AChR autoantibody binding ranged between 0.27 and 48, with a median of 0.79 and an interquartile range of 0.43 (0.58-1.1). However, the distribution of ratios was asymmetric and included extreme values; 16 samples were beyond the 10-90 percentile, with high MAC to low AChR autoantibody binding ratio or the reverse. Correlation between MAC formation and clinical disease scores suggested a modest positive association (rho = 0.34, p = 0.0023), which included a subset of outliers that did not follow this pattern. MAC formation did not associate with exposure to immunotherapy, thymectomy, or MG subtypes defined by age-of-onset.

DISCUSSION

A novel assay for evaluating AChR autoantibody-mediated complement activity was developed. A subset of patients that lacks association between MAC formation and autoantibody binding or disease burden was identified. The assay may provide a better understanding of the heterogeneous autoantibody molecular pathology and identify patients expected to benefit from complement inhibitor therapy.

Novel treatment strategies for acetylcholine receptor antibody-positive myasthenia gravis and related disorders

The presence of autoantibodies directed against the muscle nicotinic acetylcholine receptor (AChR) is the most common cause of myasthenia gravis (MG). These antibodies damage the postsynaptic membrane of the neuromuscular junction and cause muscle weakness by depleting AChRs and thus impairing synaptic transmission. As one of the best-characterized antibody-mediated autoimmune diseases, AChR-MG has often served as a reference model for other autoimmune disorders. Classical pharmacological treatments, including broad-spectrum immunosuppressive drugs, are effective in many patients. However, complete remission cannot be achieved in all patients, and 10% of patients do not respond to currently used therapies. This may be attributed to production of autoantibodies by long-lived plasma cells which are resistant to conventional immunosuppressive drugs. Hence, novel therapies specifically targeting plasma cells might be a suitable therapeutic approach for selected patients. Additionally, in order to reduce side effects of broad-spectrum immunosuppression, targeted immunotherapies and symptomatic treatments will be required. This review presents established therapies as well as novel therapeutic approaches for MG and related conditions, with a focus on AChR-MG.

Receptor clustering and pathogenic complement activation in myasthenia gravis depend on synergy between antibodies with multiple subunit specificities

Myasthenia gravis is an autoimmune disorder defined by muscle weakness and fatigability associated with antibodies against proteins of the neuromuscular junction (NMJ). The most common autoantibody target is the acetylcholine receptor (AChR). Three mechanisms have been postulated by which autoantibodies might interfere with neurotransmission: direct antagonism of the receptor, complement-mediated destruction of the postsynaptic membrane, and enhanced internalization of the receptor. It is very likely that more than one of these mechanisms act in parallel. Dissecting the mechanisms of autoantibody-mediated pathology requires patient-derived, monoclonal antibodies. Using membrane antigen capture activated cell sorting (MACACS), we isolated AChR-specific B cells from patients with myasthenia gravis, and produced six recombinant antibodies. All AChR-specific antibodies were hypermutated, including isotypes IgG₁, IgG₃, and IgG₄, and recognized different subunits of the AChR. Despite clear binding, none of the individual antibodies showed significant antagonism of the AChR measured in an in vitro neuromuscular synapse model, or AChR-dependent complement activation, and they did not induce myasthenic signs in vivo. However, combinations of antibodies induced strong complement activation in vitro, and severe weakness in a passive transfer myasthenia gravis rat model, associated with NMJ destruction and complement activation in muscle. The strongest complement activation was mediated by combinations of antibodies targeting disparate subunits of the AChR, and such combinations also induced the formation of large clusters of AChR on the surface of live cells in vitro. We propose that synergy between antibodies of different epitope specificities is a fundamental feature of this disease, and possibly a general feature of complement-mediated autoimmune diseases. The importance of synergistic interaction between antibodies targeting different subunits of the receptor can explain the well-known discrepancy between serum anti-AChR titers and clinical severity, and has implications for therapeutic strategies currently under investigation.

Disordered development of gut microbiome interferes with the establishment of the gut ecosystem during early childhood with atopic dermatitis

The gut microbiome influences the development of allergic diseases during early childhood. However, there is a lack of comprehensive understanding of microbiome-host crosstalk. Here, we analyzed the influence of gut microbiome dynamics in early childhood on atopic dermatitis (AD) and the potential interactions between host and microbiome that control this homeostasis. We analyzed the gut microbiome in 346 fecal samples (6-36 months; 112 non-AD, 110 mild AD, and 124 moderate to severe AD) from the Longitudinal Cohort for Childhood Origin of Asthma and Allergic Disease birth cohort. The microbiome-host interactions were analyzed in animal and in vitro cell assays. Although the gut microbiome maturated with age in both AD and non-AD groups, its development was disordered in the AD group. Disordered colonization of short-chain fatty acids (SCFA) producers along with age led to abnormal SCFA production and increased IgE levels. A butyrate deficiency and downregulation of GPR109A and PPAR-γ genes were detected in AD-induced mice. Insufficient butyrate decreases the oxygen consumption rate of host cells, which can release oxygen to the gut and perturb the gut microbiome. The disordered gut microbiome development could aggravate balanced microbiome-host interactions, including immune responses during early childhood with AD.

A Functional Human-on-a-Chip Autoimmune Disease Model of Myasthenia Gravis for Development of Therapeutics

Myasthenia gravis (MG) is a chronic and progressive neuromuscular disease where autoantibodies target essential proteins such as the nicotinic acetylcholine receptor (nAChR) at the neuromuscular junction (NMJ) causing muscle fatigue and weakness. Autoantibodies directed against nAChRs are proposed to work by three main pathological mechanisms of receptor disruption: blocking, receptor internalization, and downregulation. Current in vivo models using experimental autoimmune animal models fail to recapitulate the disease pathology and are limited in clinical translatability due to disproportionate disease severity and high animal death rates. The development of a highly sensitive antibody assay that mimics human disease pathology is desirable for clinical advancement and therapeutic development. To address this lack of relevant models, an NMJ platform derived from human iPSC differentiated motoneurons and primary skeletal muscle was used to investigate the ability of an anti-nAChR antibody to induce clinically relevant MG pathology in the serum-free, spatially organized, functionally mature NMJ platform. Treatment of the NMJ model with the anti-nAChR antibody revealed decreasing NMJ stability as measured by the number of NMJs before and after the synchrony stimulation protocol. This decrease in NMJ stability was dose-dependent over a concentration range of 0.01-20 μg/mL. Immunocytochemical (ICC) analysis was used to distinguish between pathological mechanisms of antibody-mediated receptor disruption including blocking, receptor internalization and downregulation. Antibody treatment also activated the complement cascade as indicated by complement protein 3 deposition near the nAChRs. Additionally, complement cascade activation significantly altered other readouts of NMJ function including the NMJ fidelity parameter as measured by the number of muscle contractions missed in response to increasing motoneuron stimulation frequencies. This synchrony readout mimics the clinical phenotype of neurological blocking that results in failure of muscle contractions despite motoneuron stimulations. Taken together, these data indicate the establishment of a relevant disease model of MG that mimics reduction of functional nAChRs at the NMJ, decreased NMJ stability, complement activation and blocking of neuromuscular transmission. This system is the first functional human in vitro model of MG to be used to simulate three potential disease mechanisms as well as to establish a preclinical platform for evaluation of disease modifying treatments (etiology).

Autoimmune autonomic ganglionopathy: Ganglionic acetylcholine receptor autoantibodies

Autoimmune Autonomic Ganglionopathy (AAG) is a rare immune-mediated disease of the autonomic nervous system. The incidence of AAG is unknown and diagnosis is often difficult due to the multicompartmental nature of the autonomic nervous system - sympathetic, parasympathetic and enteric components - with variable severity and number of components affected. Diagnostic confidence is increased when ganglionic acetylcholine receptor (gnACHR) autoantibodies are detected. Three gnACHR autoantibody diagnostic assays have been described (two binding assays, one receptor immunomodulation assay), but cross-validation between assays is limited. The prevalence of gnACHR autoantibodies in AAG is not known, with application of different clinical and laboratory criteria in the few studies of AAG cohorts and large retrospective laboratory studies of positive gnACHR autoantibodies lacking adequate clinical characterisation. Furthermore, the rate of unexpected gnACHR autoantibody positivity in conditions without overt autonomic dysfunction (false positive results) adds to the complexity of their interpretation. We review the pathophysiology of gnACHR autoantibodies and assays for their detection, with immunomodulation and high titer radioimmunoprecipitation results likely offering better AAG disease identification.

Characterization of LRP4/Agrin Antibodies From a Patient With Myasthenia Gravis

BACKGROUND AND OBJECTIVE

To determine whether human anti-LRP4/agrin antibodies are pathogenic in mice and to investigate underpinning pathogenic mechanisms.

METHODS

Immunoglobulin (Ig) was purified from a patient with myasthenia gravis (MG) with anti-LRP4/agrin antibodies and transferred to mice. Mice were characterized for body weight, muscle strength, twitch and tetanic force, neuromuscular junction (NMJ) functions including compound muscle action potential (CMAP) and endplate potentials, and NMJ structure. Effects of the antibodies on agrin-elicited muscle-specific tyrosine kinase (MuSK) activation and AChR clustering were studied and the epitopes of these antibodies were identified.

RESULTS

Patient Ig-injected mice had MG symptoms, including weight loss and muscle weakness. Decreased CMAPs, reduced twitch and tetanus force, compromised neuromuscular transmission, and NMJ fragmentation and distortion were detected in patient Ig-injected mice. Patient Ig inhibited agrin-elicited MuSK activation and AChR clustering. The patient Ig recognized the β3 domain of LRP4 and the C-terminus of agrin and reduced agrin-enhanced LRP4-MuSK interaction.

DISCUSSION

Anti-LRP4/agrin antibodies in the patient with MG is pathogenic. It impairs the NMJ by interrupting agrin-dependent LRP4-MuSK interaction.

Fc-Receptor Targeted Therapies for the Treatment of Myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease in which immunoglobulin G (IgG) antibodies (Abs) bind to acetylcholine receptors (AChR) or to functionally related molecules in the postsynaptic membrane at the neuromuscular junction. IgG crystallizable fragment (Fc)-mediated effector functions, such as antibody-dependent complement deposition, contribute to disease development and progression. Despite progress in understanding Ab-mediated disease mechanisms, immunotherapy of MG remained rather unspecific with corticosteroids and maintenance with immunosuppressants as first choice drugs for most patients. More specific therapeutic IgG Fc-based platforms that reduce serum half-life or effector functions of pathogenic MG-related Abs are currently being developed, tested in clinical trials or have recently been successfully translated into the clinic. In this review, we illustrate mechanisms of action and clinical efficacies of emerging Fc-mediated therapeutics such as neonatal Fc receptor (FcRn)-targeting agents. Furthermore, we evaluate prospects of therapies targeting classical Fc receptors that have shown promising therapeutic efficacy in other antibody-mediated conditions. Increased availability of Fc- and Fc receptor-targeting biologics might foster the development of personalized immunotherapies with the potential to induce sustained disease remission in patients with MG.

Myasthenia Gravis: Epidemiology, Pathophysiology and Clinical Manifestations

Myasthenia gravis (MG) is an autoimmune neurological disorder characterized by defective transmission at the neuromuscular junction. The incidence of the disease is 4.1 to 30 cases per million person-years, and the prevalence rate ranges from 150 to 200 cases per million. MG is considered a classic example of antibody-mediated autoimmune disease. Most patients with MG have autoantibodies against the acetylcholine receptors (AChRs). Less commonly identified autoantibodies include those targeted to muscle-specific kinase (MuSK), low-density lipoprotein receptor-related protein 4 (Lrp4), and agrin. These autoantibodies disrupt cholinergic transmission between nerve terminals and muscle fibers by causing downregulation, destruction, functional blocking of AChRs, or disrupting the clustering of AChRs in the postsynaptic membrane. The core clinical manifestation of MG is fatigable muscle weakness, which may affect ocular, bulbar, respiratory and limb muscles. Clinical manifestations vary according to the type of autoantibody, and whether a thymoma is present.

Oxidative stress and the cholinergic system in non-segmental vitiligo: Effect of narrow band ultraviolet b

BACKGROUND

Despite exhaustive research, melanocyte disappearance and the evolution of vitiligo remain enigmatic, and although multi-factorial, oxidative stress appears as a major player. The role of cutaneous cholinergic system in vitiligo pathogenesis has also been reported in some studies.

OBJECTIVE

To evaluate and correlate the influence of phototherapy on cutaneous cholinergic system and oxidative stress in vitiligo.

METHODS

Acetyl choline (ACh), its receptors; nicotinic (nAChR) and muscarinic (mAChR); acetylcholine esterase (AChE) and H₂ O₂ levels were estimated in de-pigmented and re-pigmented lesions of 30 vitiligo patients before and after NB-UVB phototherapy and in 30 controls. ACh and H₂ O₂ levels were measured by colorimetry. AChE and acetylcholine receptors expression were measured by quantitative real-time PCR.

RESULTS

Mean ACh and H₂ O₂ levels were significantly higher in vitiligo lesions before NB-UVB (P < .001) whereas AChE enzyme level was significantly lower (P < .001) compared to both re-pigmented and control skin. Additionally, mean mAChR was significantly higher and mean nAChR was significantly lower in vitiligo lesions before NB-UVB versus controls and re-pigmented skin (P < .001). Also, H₂ O₂ and AChE showed negative correlation whereas ACh and mAChR showed significant positive correlation. Although all the studied parameters showed significant changes after treatment and subsequent re-pigmentation, a significant difference continued to exist between all vitiligo skin and controls.

CONCLUSION

Cholinergic system is strongly involved in vitiligo pathogenesis through H₂ O₂ inhibition of AChE which could be reversed by NB-UVB. Moreover, the strong activation of mAChRs may reflect genetic and/or acquired errors, direct up-regulation by ACh and H₂ O₂ or both.

Myasthenia Gravis: Autoantibody Specificities and Their Role in MG Management

Myasthenia gravis (MG) is the most common autoimmune disorder affecting the neuromuscular junction, characterized by skeletal muscle weakness and fatigability. It is caused by autoantibodies targeting proteins of the neuromuscular junction; ~85% of MG patients have autoantibodies against the muscle acetylcholine receptor (AChR-MG), whereas about 5% of MG patients have autoantibodies against the muscle specific kinase (MuSK-MG). In the remaining about 10% of patients no autoantibodies can be found with the classical diagnostics for AChR and MuSK antibodies (seronegative MG, SN-MG). Since serological tests are relatively easy and non-invasive for disease diagnosis, the improvement of methods for the detection of known autoantibodies or the discovery of novel autoantibody specificities to diminish SN-MG and to facilitate differential diagnosis of similar diseases, is crucial. Radioimmunoprecipitation assays (RIPA) are the staple for MG antibody detection, but over the past years, using cell-based assays (CBAs) or improved highly sensitive RIPAs, it has been possible to detect autoantibodies in previously SN-MG patients. This led to the identification of more patients with antibodies to the classical antigens AChR and MuSK and to the third MG autoantigen, the low-density lipoprotein receptor-related protein 4 (LRP4), while antibodies against other extracellular or intracellular targets, such as agrin, K_v1.4 potassium channels, collagen Q, titin, the ryanodine receptor and cortactin have been found in some MG patients. Since the autoantigen targeted determines in part the clinical manifestations, prognosis and response to treatment, serological tests are not only indispensable for initial diagnosis, but also for monitoring treatment efficacy. Importantly, knowing the autoantibody profile of MG patients could allow for more efficient personalized therapeutic approaches. Significant progress has been made over the past years toward the development of antigen-specific therapies, targeting only the specific immune cells or autoantibodies involved in the autoimmune response. In this review, we will present the progress made toward the development of novel sensitive autoantibody detection assays, the identification of new MG autoantigens, and the implications for improved antigen-specific therapeutics. These advancements increase our understanding of MG pathology and improve patient quality of life by providing faster, more accurate diagnosis and better disease management.

A challenging case of hypercapnic respiratory failure during pregnancy

We describe a 40-year-old female who presented with progressive breathlessness and hypercapnic respiratory failure during pregnancy secondary to undiagnosed muscle-specific kinase myasthenia gravis. Her presentation was progressive and protracted, having over five contacts with healthcare professionals over nine months, many of these predating her pregnancy. Her atypical presentation for myasthenia with minimal limb weakness led to consideration of other causes of hypercapnic respiratory failure. Once diagnosed, she was treated with intravenous immunoglobulin and non-invasive ventilation. She gave birth to a pre-term infant by planned caesarean section. Her insidious presentation and the progressive nature of her breathlessness were unusual and our report highlights the predominant involvement of respiratory muscles in muscle-specific kinase myasthenia. Her pregnancy may have further delayed her diagnosis due the attribution of some symptoms to normal pregnancy. Early recognition and treatment of myasthenia gravis are important to prevent life-threatening complications.

The Structure, Function, and Physiology of the Fetal and Adult Acetylcholine Receptor in Muscle

The neuromuscular junction (NMJ) is a highly developed synapse linking motor neuron activity with muscle contraction. A complex of molecular cascades together with the specialized NMJ architecture ensures that each action potential arriving at the motor nerve terminal is translated into an action potential in the muscle fiber. The muscle-type nicotinic acetylcholine receptor (AChR) is a key molecular component located at the postsynaptic muscle membrane responsible for the generation of the endplate potential (EPP), which usually exceeds the threshold potential necessary to activate voltage-gated sodium channels and triggers a muscle action potential. Two AChR isoforms are found in mammalian muscle. The fetal isoform is present in prenatal stages and is involved in the development of the neuromuscular system whereas the adult isoform prevails thereafter, except after denervation when the fetal form is re-expressed throughout the muscle. This review will summarize the structural and functional differences between the two isoforms and outline congenital and autoimmune myasthenic syndromes that involve the isoform specific AChR subunits.

Autoantibody Specificities in Myasthenia Gravis; Implications for Improved Diagnostics and Therapeutics

Myasthenia gravis (MG) is an autoimmune disease characterized by muscle weakness and fatiguability of skeletal muscles. It is an antibody-mediated disease, caused by autoantibodies targeting neuromuscular junction proteins. In the majority of patients (~85%) antibodies against the muscle acetylcholine receptor (AChR) are detected, while in 6% antibodies against the muscle-specific kinase (MuSK) are detected. In ~10% of MG patients no autoantibodies can be found with the classical diagnostics for AChR and MuSK antibodies (seronegative MG, SN-MG), making the improvement of methods for the detection of known autoantibodies or the discovery of novel antigenic targets imperative. Over the past years, using cell-based assays or improved highly sensitive immunoprecipitation assays, it has been possible to detect autoantibodies in previously SN-MG patients, including the identification of the low-density lipoprotein receptor-related protein 4 (LRP4) as a third MG autoantigen, as well as AChR and MuSK antibodies undetectable by conventional methods. Furthermore, antibodies against other extracellular or intracellular targets, such as titin, the ryanodine receptor, agrin, collagen Q, K_v1.4 potassium channels and cortactin have been found in some MG patients, which can be useful biomarkers. In addition to the improvement of diagnosis, the identification of the patients' autoantibody specificity is important for their stratification into respective subgroups, which can differ in terms of clinical manifestations, prognosis and most importantly their response to therapies. The knowledge of the autoantibody profile of MG patients would allow for a therapeutic strategy tailored to their MG subgroup. This is becoming especially relevant as there is increasing progress toward the development of antigen-specific therapies, targeting only the specific autoantibodies or immune cells involved in the autoimmune response, such as antigen-specific immunoadsorption, which have shown promising results. We will herein review the advances made by us and others toward development of more sensitive detection methods and the identification of new antibody targets in MG, and discuss their significance in MG diagnosis and therapy. Overall, the development of novel autoantibody assays is aiding in the more accurate diagnosis and classification of MG patients, supporting the development of advanced therapeutics and ultimately the improvement of disease management and patient quality of life.

Myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease caused by antibodies against the acetylcholine receptor (AChR), muscle-specific kinase (MuSK) or other AChR-related proteins in the postsynaptic muscle membrane. Localized or general muscle weakness is the predominant symptom and is induced by the antibodies. Patients are grouped according to the presence of antibodies, symptoms, age at onset and thymus pathology. Diagnosis is straightforward in most patients with typical symptoms and a positive antibody test, although a detailed clinical and neurophysiological examination is important in antibody-negative patients. MG therapy should be ambitious and aim for clinical remission or only mild symptoms with near-normal function and quality of life. Treatment should be based on MG subgroup and includes symptomatic treatment using acetylcholinesterase inhibitors, thymectomy and immunotherapy. Intravenous immunoglobulin and plasma exchange are fast-acting treatments used for disease exacerbations, and intensive care is necessary during exacerbations with respiratory failure. Comorbidity is frequent, particularly in elderly patients. Active physical training should be encouraged.

Intensive Care Management of the Neuromuscular Patient

Neuromuscular emergencies are a distinct group of acute neurological diseases with distinct characteristic presentations. Patients who suffer from this group of diseases are at immediate risk of losing protection of their native airway as well as aspirating orogastric contents. This is secondary to weakness of the muscles of the oropharynx and respiratory muscles. Although some neuromuscular emergencies such as myasthenia gravis or Guillain-Barré syndrome are well understood, others such as critical illness myopathy and neuropathy are less well characterized. In this chapter, we have discussed the pathophysiology, diagnostic evaluation, and management options in patients who are admitted to the intensive care unit. We have also emphasized the importance of a thorough understanding of the use of pharmacological anesthetic agents in this patient population.

Heterogeneity of auto-antibodies against nAChR in myasthenic serum and their pathogenic roles in experimental autoimmune myasthenia gravis

Many myasthenia gravis (MG) patients have auto-antibodies against the nicotinic acetylcholine receptor (nAChR), and monoclonal antibodies against the main immunogenic region (MIR) of nAChR can induce experimental autoimmune MG (EAMG). We investigated whether Fab fragment of MIR antibody (Fab35) could block the pathogenicity of polyclonal antibodies. Fab35 partially inhibited nAChR downmodulation, blocked EAMG serum-induced binding of polyclonal antibodies and complement deposition in vitro. Moreover, Fab35 did not ameliorate the EAMG serum-induced EAMG phenotype in rats. These results suggested that the EAMG serum possessed several different pathogenic antibodies that might be sufficient to induce the EAMG phenotype.

Muscle satellite cells are functionally impaired in myasthenia gravis: consequences on muscle regeneration

Myasthenia gravis (MG) is a neuromuscular disease caused in most cases by anti-acetyl-choline receptor (AChR) autoantibodies that impair neuromuscular signal transmission and affect skeletal muscle homeostasis. Myogenesis is carried out by muscle stem cells called satellite cells (SCs). However, myogenesis in MG had never been explored. The aim of this study was to characterise the functional properties of myasthenic SCs as well as their abilities in muscle regeneration. SCs were isolated from muscle biopsies of MG patients and age-matched controls. We first showed that the number of Pax7+ SCs was increased in muscle sections from MG and its experimental autoimmune myasthenia gravis (EAMG) mouse model. Myoblasts isolated from MG muscles proliferate and differentiate more actively than myoblasts from control muscles. MyoD and MyoG were expressed at a higher level in MG myoblasts as well as in MG muscle biopsies compared to controls. We found that treatment of control myoblasts with MG sera or monoclonal anti-AChR antibodies increased the differentiation and MyoG mRNA expression compared to control sera. To investigate the functional ability of SCs from MG muscle to regenerate, we induced muscle regeneration using acute cardiotoxin injury in the EAMG mouse model. We observed a delay in maturation evidenced by a decrease in fibre size and MyoG mRNA expression as well as an increase in fibre number and embryonic myosin heavy-chain mRNA expression. These findings demonstrate for the first time the altered function of SCs from MG compared to control muscles. These alterations could be due to the anti-AChR antibodies via the modulation of myogenic markers resulting in muscle regeneration impairment. In conclusion, the autoimmune attack in MG appears to have unsuspected pathogenic effects on SCs and muscle regeneration, with potential consequences on myogenic signalling pathways, and subsequently on clinical outcome, especially in the case of muscle stress.

Structural insights into the molecular mechanisms of myasthenia gravis and their therapeutic implications

The nicotinic acetylcholine receptor (nAChR) is a major target of autoantibodies in myasthenia gravis (MG), an autoimmune disease that causes neuromuscular transmission dysfunction. Despite decades of research, the molecular mechanisms underlying MG have not been fully elucidated. Here, we present the crystal structure of the nAChR α1 subunit bound by the Fab fragment of mAb35, a reference monoclonal antibody that causes experimental MG and competes with ~65% of antibodies from MG patients. Our structures reveal for the first time the detailed molecular interactions between MG antibodies and a core region on nAChR α1. These structures suggest a major nAChR-binding mechanism shared by a large number of MG antibodies and the possibility to treat MG by blocking this binding mechanism. Structure-based modeling also provides insights into antibody-mediated nAChR cross-linking known to cause receptor degradation. Our studies establish a structural basis for further mechanistic studies and therapeutic development of MG.

DOI:http://dx.doi.org/10.7554/eLife.23043.001

Myasthenia gravis is a disease that causes chronic weakness in muscles. It affects more than 20 in every 100,000 people and diagnosis is becoming more common due to increased awareness of the disease. However, most current treatments only temporarily relieve symptoms so there is a need to develop more effective therapies.

The disease occurs when the immune system produces molecules called antibodies that bind to and destroy a receptor protein called nAChR. This receptor is normally found at the junctions between nerve cells and muscle cells, and its destruction disrupts communication between the nervous system and the muscle. However, it is not known exactly how these antibodies bind to nAChR, partly due to the lack of a detailed three-dimensional structure of the antibodies and nAChR together.

The human nAChR protein is made up of several subunits, including one called alpha1 that is the primary target of Myasthenia gravis antibodies. Noridomi et al. used a technique known as X-ray crystallography to generate a highly detailed three-dimensional model of the structure of the alpha1 subunit with an antibody from rats that acts as in a similar way to human Myasthenia gravis antibodies. The structure reveals the points of contact between the antibodies and a core region of the nAChR alpha1 subunit and suggests that many different Myasthenia gravis antibodies may bind to nAChR in the same way.

These findings may aid the development of drugs that bind to and disable Myasthenia gravis antibodies to relieve the symptoms of the disease.

DOI:http://dx.doi.org/10.7554/eLife.23043.002

Mapping autoantigen epitopes: molecular insights into autoantibody-associated disorders of the nervous system

Background

Our knowledge of autoantibody-associated diseases of the central (CNS) and peripheral (PNS) nervous systems has expanded greatly over the recent years. A number of extracellular and intracellular autoantigens have been identified, and there is no doubt that this field will continue to expand as more autoantigens are discovered as a result of improved clinical awareness and methodological practice. In recent years, interest has shifted to uncover the target epitopes of these autoantibodies.

Main body

The purpose of this review is to discuss the mapping of the epitope targets of autoantibodies in CNS and PNS antibody-mediated disorders, such as N-methyl-D-aspartate receptor (NMDAR), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), leucine-rich glioma-inactivated protein 1 (Lgi1), contactin-associated protein-like 2 (Caspr2), myelin oligodendrocyte glycoprotein (MOG), aquaporin-4 (AQP4), 65 kDa glutamic acid decarboxylase (GAD65), acetylcholine receptor (AChR), muscle-specific kinase (MuSK), voltage-gated calcium channel (VGCC), neurofascin (NF), and contactin. We also address the methods used to analyze these epitopes, the relevance of their determination, and how this knowledge can inform studies on autoantibody pathogenicity. Furthermore, we discuss triggers of autoimmunity, such as molecular mimicry, ectopic antigen expression, epitope spreading, and potential mechanisms for the rising number of double autoantibody-positive patients.

Conclusions

Molecular insights into specificity and role of autoantibodies will likely improve diagnosis and treatment of CNS and PNS neuroimmune diseases.

Understanding autoimmunity: The ion channel perspective

Ion channels are integral membrane proteins that orchestrate the passage of ions across the cell membrane and thus regulate various key physiological processes of the living system. The stringently regulated expression and function of these channels hold a pivotal role in the development and execution of various cellular functions. Malfunction of these channels results in debilitating diseases collectively termed channelopathies. In this review, we highlight the role of these proteins in the immune system with special emphasis on the development of autoimmunity. The role of ion channels in various autoimmune diseases is also listed out. This comprehensive review summarizes the ion channels that could be used as molecular targets in the development of new therapeutics against autoimmune disorders.

Cellular plasticity induced by anti-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor encephalitis antibodies

Objective

Autoimmune-mediated anti–α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) encephalitis is a severe but treatment-responsive disorder with prominent short-term memory loss and seizures. The mechanisms by which patient antibodies affect synapses and neurons leading to symptoms are poorly understood.

Methods

The effects of patient antibodies on cultures of live rat hippocampal neurons were determined with immunostaining, Western blot, and electrophysiological analyses.

Results

We show that patient antibodies cause a selective decrease in the total surface amount and synaptic localization of GluA1- and GluA2-containing AMPARs, regardless of receptor subunit binding specificity, through increased internalization and degradation of surface AMPAR clusters. In contrast, patient antibodies do not alter the density of excitatory synapses, N-methyl-D-aspartate receptor (NMDAR) clusters, or cell viability. Commercially available AMPAR antibodies directed against extracellular epitopes do not result in a loss of surface and synaptic receptor clusters, suggesting specific effects of patient antibodies. Whole-cell patch clamp recordings of spontaneous miniature postsynaptic currents show that patient antibodies decrease AMPAR-mediated currents, but not NMDAR-mediated currents. Interestingly, several functional properties of neurons are also altered: inhibitory synaptic currents and vesicular γ-aminobutyric acid transporter (vGAT) staining intensity decrease, whereas the intrinsic excitability of neurons and short-interval firing increase.

Interpretation

These results establish that antibodies from patients with anti-AMPAR encephalitis selectively eliminate surface and synaptic AMPARs, resulting in a homeostatic decrease in inhibitory synaptic transmission and increased intrinsic excitability, which may contribute to the memory deficits and epilepsy that are prominent in patients with this disorder.

IL-6 and Akt are involved in muscular pathogenesis in myasthenia gravis

Introduction

Anti-acetylcholine receptor (AChR) autoantibodies target muscles in spontaneous human myasthenia gravis (MG) and its induced experimental autoimmune model MG (EAMG). The aim of this study was to identify novel functional mechanisms occurring in the muscle pathology of myasthenia.

Results

A transcriptome analysis performed on muscle tissue from MG patients (compared with healthy controls) and from EAMG rats (compared with control rats) revealed a deregulation of genes associated with the Interleukin-6 (IL-6) and Insulin-Like Growth Factor 1 (IGF-1) pathways in both humans and rats. The expression of IL-6 and its receptor IL-6R transcripts was found to be altered in muscles of EAMG rats and mice compared with control animals. In muscle biopsies from MG patients, IL-6 protein level was higher than in control muscles. Using cultures of human muscle cells, we evaluated the effects of anti-AChR antibodies on IL-6 production and on the phosphorylation of Protein Kinase B (PKB/Akt). Most MG sera and some monoclonal anti-AChR antibodies induced a significant increase in IL-6 production by human muscle cells. Furthermore, Akt phosphorylation in response to insulin was decreased in the presence of monoclonal anti-AChR antibodies.

Conclusions

Anti-AChR antibodies alter IL-6 production by muscle cells, suggesting a putative novel functional mechanism of action for the anti-AChR antibodies. IL-6 is a myokine with known effects on signaling pathways such as Akt/mTOR (mammalian Target of Rapamycin). Since Akt plays a key role in multiple cellular processes, the reduced phosphorylation of Akt by the anti-AChR antibodies may have a significant impact on the muscle fatigability observed in MG patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-014-0179-6) contains supplementary material, which is available to authorized users.

Direct proof of the in vivo pathogenic role of the AChR autoantibodies from myasthenia gravis patients

Several studies have suggested that the autoantibodies (autoAbs) against muscle acetylcholine receptor (AChR) of myasthenia gravis (MG) patients are the main pathogenic factor in MG; however, this belief has not yet been confirmed with direct observations. Although animals immunized with AChR or injected with anti-AChR monoclonal Abs, or with crude human MG Ig fractions exhibit MG symptoms, the pathogenic role of isolated anti-AChR autoAbs, and, more importantly, the absence of pathogenic factor(s) in the autoAb-depleted MG sera has not yet been shown by in vivo studies. Using recombinant extracellular domains of the human AChR α and β subunits, we have isolated autoAbs from the sera of four MG patients. The ability of these isolated anti-subunit Abs and of the Ab-depleted sera to passively transfer experimental autoimmune MG in Lewis rats was investigated. We found that the isolated anti-subunit Abs were at least as efficient as the corresponding whole sera or whole Ig in causing experimental MG. Abs to both α- and β-subunit were pathogenic although the anti-α-subunit were much more efficient than the anti-β-subunit ones. Interestingly, the autoAb-depleted sera were free of pathogenic activity. The later suggests that the myasthenogenic potency of the studied anti-AChR MG sera is totally due to their anti-AChR autoAbs, and therefore selective elimination of the anti-AChR autoAbs from MG patients may be an efficient therapy for MG.

Therapeutic Plasma Exchange in Neurologic Diseases: An Experience with 91 Patients in Seven Years

INTRODUCTION

In this study, we report the results of our experience of therapeutic plasma exchange (TPE) for neuroimmunologic disorders performed at our hospital over a seven-year period.

METHODS

We retrospectively reviewed the medical records of 91 patients (53 male, 38 female) who had been treated at our center with TPE.

RESULTS

60 patients with Guillain-Barrè syndrome (GBS), 23 with myasthenia gravis (MG), 4 with chronic inflammatory demyelinating polyneuropathy (CIDP) and 1 patient each with polymyositis, septic encephalopathy, acute disseminated encephalomyelitis (ADEM) and Opsoclonus-Myoclonus syndrome (OMS) received TPE. 26.7% of GBS patient's made complete recovery, 61.7% had partial recovery and 11.7% patients died due to respiratory failure. Despite our best efforts and effective TPE treatments, 13.4% of MG patients deceased, however, 78% had full recovery. Three patients with CIDP were discharged with full and 1 patient with partial recovery. The patient with ADEM had partial recovery with TPE at first, but deceased 2 months later due to pneumonia-related respiratory insufficiency. While, patient with polymyositis had slight-partial recovery, we obtained full recovery with TPE in septic encephalopathy and OMS patients. The side effects and complications of treatments with TPE, which included hypotension, hypocalcaemia and anemia, were mild and manageable.

CONCLUSION

The improvement rates were encouraging and we concluded that significant benefit can be achieved with TPE for the treatment of neuroimmunological disorders.

Prospects for a T-cell receptor vaccination against myasthenia gravis

T-cell receptor (TCR) vaccination has been proposed as a specific therapy against autoimmune diseases. It is already used in clinical trials, which are supported by pharmaceutical companies for the treatment of multiple sclerosis, rheumatoid arthritis and psoriasis. Current vaccine developments are focusing on enhancement of immunogenicity as well as selecting the best route of immunization and adjuvant to favor the therapeutic effect. In the meantime, academic laboratories are tackling the regulatory mechanisms involved in the beneficial effect of the vaccines to further understand how to control the therapeutic tool. Indeed, several examples in experimental models of autoimmune diseases indicate that any specific therapy may rely on a delicate balance between the pathogenic and regulatory mechanisms. This review presents a critical analysis of the potential of such therapy in myasthenia gravis, a prototype antibody-mediated disease. Indeed, a specific pathogenic T-cell target population and a TCR-specific regulatory mechanism mediated by anti-TCR antibodies and involved in protection from the disease have recently been identified in a patient subgroup. The presence of spontaneous anti-TCR antibodies directed against the pathogenic T-cells that may be boosted by a TCR vaccine provides a rationale for such therapy in myasthenia gravis. The development of this vaccine may well benefit from experience gained in the other autoimmune diseases in which clinical trials are ongoing.

Pathogenic immune mechanisms at the neuromuscular synapse: the role of specific antibody-binding epitopes in myasthenia gravis

Autoantibodies against three different postsynaptic antigens and one presynaptic antigen at the neuromuscular junction are known to cause myasthenic syndromes. The mechanisms by which these antibodies cause muscle weakness vary from antigenic modulation and complement-mediated membrane damage to inhibition of endogenous ligand binding and blocking of essential protein-protein interactions. These mechanisms are related to the autoantibody titre, specific epitopes on the target proteins and IgG autoantibody subclass. We here review the role of specific autoantibody-binding epitopes in myasthenia gravis, their possible relevance to the pathophysiology of the disease and potential implications of epitope mapping knowledge for new therapeutic strategies.

Structural characterization of the main immunogenic region of the Torpedo acetylcholine receptor

To develop antigen-specific immunotherapies for autoimmune diseases, knowledge of the molecular structure of targeted immunological hotspots will guide the production of reagents to inhibit and halt production of antigen specific attack agents. To this end we have identified three noncontiguous segments of the Torpedo nicotinic acetylcholine receptor (AChR) α-subunit that contribute to the conformationally sensitive immunological hotspot on the AChR termed the main immunogenic region (MIR): α(1-12), α(65-79), and α(110-115). This region is the target of greater than 50% of the anti-AChR Abs in serum from patients with myasthenia gravis (MG) and animals with experimental autoimmune myasthenia gravis (EAMG). Many monoclonal antibodies (mAbs) raised in one species against an electric organ AChR cross react with the neuromuscular AChR MIR in several species. Probing the Torpedo AChR α-subunit with mAb 132A, a disease inducing anti-MIR mAb raised against the Torpedo AChR, we have determined that two of the three MIR segments, α(1-12) and α(65-79), form a complex providing the signature components recognized by mAb 132A. These two segments straddle a third, α(110-115), that seems not to contribute specific side chains for 132A recognition, but is necessary for optimum antibody binding. This third segment appears to form a foundation upon which the three-dimensional 132A epitope is anchored.

Scale up and safety parameters of antigen specific immunoadsorption of human anti-acetylcholine receptor antibodies

Myasthenia gravis is an autoimmune disease usually caused by autoantibodies against the muscle nicotinic acetylcholine receptor (nAChR). Current treatments are not specific, and thus often cause side effects. Here, we elaborate on our previous findings on antigen specific immunoadsorption towards scaling up the method as well as testing whole blood apheresis. The average percent of plasma or whole blood immunoadsorption was up to 79.5%±2.9. Moreover, neither pyrogens were co-administered nor did complement activation occur after immunoadsorption. Thus, antigen-specific apheresis of anti-AChR autoantibodies seems a safe and effective treatment for myasthenia gravis that can be scaled up for clinical testing.

Recent advances in genetic predisposition of myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease mediated by the presence of autoantibodies that bind to components of the neuromuscular junction, causing the symptoms of muscular weakness and fatigability. Like most autoimmune disorders, MG is a multifactorial, noninherited disease, though with an established genetic constituent. The heterogeneity observed in MG perplexes genetic analysis even more, as it occurs in various levels, including diverse autoantigens, thymus histopathology, and age at onset. In this context of distinct subgroups, a plethora of association studies, discussed in this review, have assessed the involvement of various HLA and non-HLA related loci in MG susceptibility, over the past five years. As expected, certain HLA alleles were strongly associated with MG. Many of the non-HLA genes, such as PTPN22 and CTLA-4, have been previously studied in MG and other autoimmune diseases and their association with MG has been reevaluated in more cohesive groups of patients. Moreover, novel risk or protective loci have been revealed, as in the case of TNIP1 and FOXP3. Although the majority of these results have been derived from candidate gene studies, the focal point of all recent genetic studies is the first genome-wide association study (GWAS) conducted on early-onset MG patients.

Serological diagnostics in myasthenia gravis based on novel assays and recently identified antigens

Myasthenia gravis (MG) is the most common immune-mediated disorder of the neuromuscular junction with a prevalence of 200-300/million population and its study has established paradigms for exploring other antibody-mediated diseases. Most MG patients (~85%) have autoantibodies against the muscle acetylcholine receptor (AChR-MG), whereas about 6% of MG patients have autoantibodies against the muscle specific kinase (MuSK-MG). Until recently no autoantibodies could be detected in the remaining patients (seronegative MG). Probably, the most sensitive assays for the detection of the autoantibodies in MG sera have been the radioimmunoprecipitation assays (RIPA) for both types of MG. However, with recent novel methods, not yet used routinely, it has been shown that the "seronegative" MG group includes patients with low levels of autoantibodies or of low affinity, against the known autoantigens, or even with antibodies to recently identified autoantigens. Since MG is heterogeneous in terms of pathophysiology, depending on the autoantigen targeted and on other factors (e.g. presence of thymoma), the serological tests are crucial in verifying the initial clinical diagnosis, whereas frequent measurement of autoantibody levels is important in monitoring the course of the disease and the efficacy of treatment. In addition, in AChR-MG, autoantibodies against the muscle proteins titin and ryanodin receptor have been identified; these antibodies are useful for the classification of MG, indicating the concomitant presence of thymoma, and as prognostic markers.

Beneficial effect of crotamine in the treatment of myasthenic rats

INTRODUCTION

Crotamine is a basic, low-molecular-weight peptide that, at low concentrations, improves neurotransmission in isolated neuromuscular preparations by modulating sodium channels. In this study, we compared the effects of crotamine and neostigmine on neuromuscular transmission in myasthenic rats.

METHODS

We used a conventional electromyographic technique in in-situ neuromuscular preparations and a 4-week treadmill program.

RESULTS

During the in-situ electromyographic recording, neostigmine (17 μg/kg) caused short-term facilitation, whereas crotamine induced progressive and sustained twitch-tension enhancement during 140 min of recording (50 ± 5%, P < 0.05). On the treadmill evaluation, rats showed significant improvement in exercise tolerance, characterized by a decrease in the number of fatigue episodes after 2 weeks of a single-dose treatment with crotamine.

CONCLUSIONS

These results indicate that crotamine is more efficient than neostigmine for enhancing muscular performance in myasthenic rats, possibly by improving the safety factor of neuromuscular transmission.

Myasthenia gravis: analysis of serum autoantibody reactivities to 1827 potential human autoantigens by protein macroarrays

BACKGROUND

Myasthenia gravis is a disorder of neuromuscular transmission associated with autoantibodies against the nicotinic acetylcholine receptor. We have previously developed a customized protein macroarray comprising 1827 potential human autoantigens, which permitted to discriminate sera of patients with different cancers from sera of healthy controls, but has not yet been evaluated in antibody-mediated autoimmune diseases.

OBJECTIVE

To determine whether autoantibody signatures obtained by protein macroarray separate sera of patients with myasthenia gravis from healthy controls.

METHODS

Sera of patients with acetylcholine receptor antibody-positive myasthenia gravis (n = 25) and healthy controls (n = 32) were analyzed by protein macroarrays comprising 1827 peptide clones.

RESULTS

Autoantibody signatures did not separate patients with myasthenia gravis from controls with sufficient sensitivity, specificity, and accuracy. Intensity values of one antigen (poly A binding protein cytoplasmic 1, p = 0.0045) were higher in patients with myasthenia gravis, but the relevance of this and two further antigens, 40S ribosomal protein S13 (20.8% vs. 0%, p = 0.011) and proteasome subunit alpha type 1 (25% vs. 3.1%, p = 0.035), which were detected more frequently by myasthenia gravis than by control sera, currently remains uncertain.

CONCLUSION

Seroreactivity profiles of patients with myasthenia gravis detected by a customized protein macroarray did not allow discrimination from healthy controls, compatible with the notion that the autoantibody response in myasthenia gravis is highly focussed against the acetylcholine receptor.

Antigen-specific apheresis of autoantibodies in myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder affecting the neuromuscular junction, usually caused by autoantibodies against the acetylcholine receptor (AChR) or the muscle-specific kinase (MuSK). Our aim is the development of a therapy based on the selective extracorporeal elimination of anti-AChR or anti-MuSK antibodies. To this end, the extracellular domains of the AChR subunits and MuSK have been expressed in yeast to be used as adsorbents, after optimization, and to obtain large quantities of proteins with near-native structure. We have characterized these proteins with respect to their use as specific immunoadsorbents for MG autoantibodies, and have begun large-scale experiments in order to verify the feasibility of application of the method for therapy. Furthermore, we have initiated animal studies to test possible toxicity and safety issues of the adsorbents or the procedure itself. The successful completion of the scale-up and safety tests will allow the initiation of clinical trials.

A comparison of epitope repertoires associated with myasthenia gravis in humans and nonhuman hosts

Here we analyzed the molecular targets associated with myasthenia gravis (MG) immune responses, enabled by an immune epitope database (IEDB) inventory of approximately 600 MG-related epitopes derived from 175 references. The vast majority of epitopes were derived from the α-subunit of human AChR suggesting that other MG-associated autoantigens should be investigated further. Human α-AChR was mostly characterized in humans, whereas reactivity primarily to T. californica AChR was examined in animal models. While the fine specificity of T-cell response was similar in the two systems, substantial antibody reactivity to the C-terminus was detected in the nonhuman system, but not in humans. Further analysis showed that the reactivity of nonhuman hosts to the C-terminus was eliminated when data were restricted to hosts tested in the context of autoimmune disease (spontaneous or induced), demonstrating that the epitopes recognized in humans and animals were shared when disease was present. Finally, we provided data subsets relevant to particular applications, including those associated with HLA typing or restriction, sets of epitopes recognized by monoclonal antibodies, and epitopes associated with modulation of immunity or disease. In conclusion, this analysis highlights gaps, differences, and similarities in the epitope repertoires of humans and animal models.

Anti-NMDA receptor encephalitis antibody binding is dependent on amino acid identity of a small region within the GluN1 amino terminal domain

Anti-NMDA receptor (NMDAR) encephalitis is a newly identified autoimmune disorder that targets NMDARs, causing severe neurological symptoms including hallucinations, psychosis, and seizures, and may result in death (Dalmau et al., 2008). However, the exact epitope to which these antibodies bind is unknown. A clearly defined antigenic region could provide more precise testing, allow for comparison of immunogenicity between patients to explore potential clinically relevant variations, elucidate the functional effects of antibodies, and make patients' antibodies a more effective tool with which to study NMDAR function. Here, we use human CSF to explore the antigenic region of the NMDAR. We created a series of mutants within the amino terminal domain of GluN1 that change patient antibody binding in transfected cells in stereotyped ways. These mutants demonstrate that the N368/G369 region of GluN1 is crucial for the creation of immunoreactivity. Mass spectrometry experiments show that N368 is glycosylated in transfected cells and rat brain regions; however, this glycosylation is not directly required for epitope formation. Mutations of residues N368/G369 change the closed time of the receptor in single channel recordings; more frequent channel openings correlates with the degree of antibody staining, and acute antibody exposure prolongs open time of the receptor. The staining pattern of mutant receptors is similar across subgroups of patients, indicating consistent immunogenicity, although we have identified one region that has a variable role in epitope formation. These findings provide tools for detailed comparison of antibodies across patients and suggest an interaction between antibody binding and channel function.

Clinical significance of detection of antibodies to fetal and adult acetylcholine receptors in myasthenia gravis

OBJECTIVE

To evaluate the frequency, distribution and clinical significance of the antibodies to the fetal and/or adult acetylcholine receptor (AChR) in patients with myasthenia gravis (MG).

METHODS

AChR antibodies were detected by cell-based assay in the serum of ocular MG (OMG) (n = 90) and generalized MG (GMG) patients (n = 110). The fetal-type (2α: β: γ: δ) and adult-type (2α: β: ε: δ) AChR were used as antigens, and their relevance to disease presentation was assessed.

RESULTS

The overall frequencies of anti-adult and anti-fetal AChR antibodies were similar in all 200 patients examined, with 14 having serum specific to the AChR-Γ subunit, and 22 to the AChR-ε subunit. The overall sensitivity when using the fetal and adult AChR antibodies was higher than that when using the fetal AChR antibody only (P = 0.015). Compared with OMG patients, the mean age at disease onset and the positive ratio of antibodies to both isoforms of the AChR were significantly higher in patients who subsequently progressed to GMG. Older patients and patients with both anti-fetal and anti-adult AChR antibodies had a greater risk for developing generalized disease [odds ratio (OR), 1.03; 95% confidence interval (CI), 1.01-1.06 and OR, 5.09; 95% CI, 2.23-11.62].

CONCLUSION

Using both fetal- and adult-type AChRs as the antigens may be more sensitive than using either subtype. Patients with serum specific to both isoforms are at a greater risk of progressing to GMG. Patients with disease onset at an advanced age appear to have a higher frequency of GMG conversion.

Protective effect of scFv-DAF fusion protein on the complement attack to acetylcholine receptor: a possible option for treatment of myasthenia gravis

INTRODUCTION

Autoantibody-induced complement activation, which causes disruption of the postsynaptic membrane, is recognized as a key pathogenic factor in myasthenia gravis (MG). Therefore, specific targeting of complement inhibitors to the site of complement activation is a potential therapeutic strategy for treatment of MG.

METHODS

We assessed expression of single-chain antibody fragment-decay accelerating factor (scFv-DAF), comprising a single-chain fragment scFv1956 based on the rat complement inhibitor DAF in prokaryotic systems, and studied its inhibitory effect on complement deposition in vitro.

RESULTS

The recombinant conjugate scFv-DAF completely retained the wild-type binding activity of scFv1956 to AChR and inhibited complement activation of DAF in vitro.

CONCLUSIONS

We found that scFv-DAF could bind specifically to TE671 cells, and it is significantly more potent at inhibiting complement deposition than the untargeted parent molecule DAF. scFv-DAF may be a candidate for in vivo protection of the AChR in MG.