Myasthenia gravis AChR antibodies inhibit function of rapsyn-clustered AChRs

Myasthenia gravis: Molecular mechanisms and promising therapeutic strategies

Myasthenia gravis (MG) is a type of autoimmune disease caused by the blockage of neuromuscular junction transmission owing to the attack of autoantibodies on transmission-related proteins. Related antibodies, such as anti-AChR, anti-MuSK and anti-LRP4 antibodies, can be detected in most patients with MG. Although traditional therapies can control most symptoms, several challenges remain to be addressed, necessitating the development of more effective and safe treatment strategies for MG. With the in-depth exploration on the mechanism and immune targets of MG, effective therapies, especially therapies using biologicals, have been reported recently. Given the important roles of immune cells, cytokines and intercellular interactions in the pathological process of MG, B-cell targeted therapy, T-cell targeted therapy, proteasome inhibitors targeting plasma cell, complement inhibitors, FcRn inhibitors have been developed for the treatment of MG. Although these novel therapies exert good therapeutic effects, they may weaken the immunity and increase the risk of infection in MG patients. This review elaborates on the pathogenesis of MG and discusses the advantages and disadvantages of the strategies of traditional treatment and biologicals. In addition, this review emphasises that combined therapy may have better therapeutic effects and reducing the risk of side effects of treatments, which has great prospects for the treatment of MG. With the deepening of research on immunotherapy targets in MG, novel opportunities and challenges in the treatment of MG will be introduced.

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.

Novel pathophysiological insights in autoimmune myasthenia gravis

PURPOSE OF REVIEW

This review summarizes recent insights into the immunopathogenesis of autoimmune myasthenia gravis (MG). Mechanistic understanding is presented according to MG disease subtypes and by leveraging the knowledge gained through the use of immunomodulating biological therapeutics.

RECENT FINDINGS

The past two years of research on MG have led to a more accurate definition of the mechanisms through which muscle-specific tyrosine kinase (MuSK) autoantibodies induce pathology. Novel insights have also emerged from the collection of stronger evidence on the pathogenic capacity of low-density lipoprotein receptor-related protein 4 autoantibodies. Clinical observations have revealed a new MG phenotype triggered by cancer immunotherapy, but the underlying immunobiology remains undetermined. From a therapeutic perspective, MG patients can now benefit from a wider spectrum of treatment options. Such therapies have uncovered profound differences in clinical responses between and within the acetylcholine receptor and MuSK MG subtypes. Diverse mechanisms of immunopathology between the two subtypes, as well as qualitative nuances in the autoantibody repertoire of each patient, likely underpin the variability in therapeutic outcomes. Although predictive biomarkers of clinical response are lacking, these observations have ignited the development of assays that might assist clinicians in the choice of specific therapeutic strategies.

SUMMARY

Recent advances in the understanding of autoantibody functionalities are bringing neuroimmunologists closer to a more detailed appreciation of the mechanisms that govern MG pathology. Future investigations on the immunological heterogeneity among MG patients will be key to developing effective, individually tailored therapies.

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.

Effects of Plasma Exchange Combined with Immunoglobulin Therapy on Consciousness, Immune Function, and Prognosis in Patients with Myasthenia Gravis Crisis: A Prospective Randomized Test

Background

Myasthenia gravis (MG) is an acquired autoimmune disease. The main clinical features of MG are skeletal muscle fatigue and pathological fatigue, which worsen at night or after fatigue, such as dyspnea, dysphagia, and systemic weakness. Plasma exchange (PE) is often used in patients with acute exacerbation of MG. Intravenous immunoglobulin (IVIG) is a collection of immunoglobulins from thousands of donors. IVIG can replace a variety of immunosuppressants or PE. However, the effect of PE or IVIG on patients' consciousness, immune function, and prognosis is not clear.

Objective

A prospective randomized test of the effects of PE combined with immunoglobulin on consciousness, immune function, and prognosis in patients with myasthenia gravis crisis (MGC).

Methods

Sixty patients with MGC treated from February 2019 to April 2021 were enrolled in our hospital. The cases who received PE were set as the PE group, and those who received PE combined with immunoglobulin were set as the PE+immunoglobulin group. The efficacy, clinical score, state of consciousness, immune function, acetylcholine receptor antibody (AChR-Ab), lymphocyte (LYM), albumin (ALB) levels, and the incidence of adverse reactions were compared.

Results

The improvement rate was 100.005% in the treatment group and 83.33% in the PE group. After treatment, the clinical score of the PE+immunoglobulin group was lower than that of the PE group, and the clinical relative score of the PE+immunoglobulin group was higher than that of the PE group (P < 0.05). The number of conscious people in the PE+immunoglobulin group was more than that in the PE group (P < 0.05). Immunoglobulin A, immunoglobulin M, immunoglobulin G, and immunoglobulin G in the PE+immunoglobulin group were higher than those in the PE group (P < 0.05). The levels of AChR-Ab and ALB in the PE+immunoglobulin group were higher than those in the PE group, while the level of LYM in the PE+immunoglobulin group was lower than that in the PE group. The incidence of skin system, gastrointestinal system, nervous system, and systemic damage in the PE+immunoglobulin group was lower than that in the PE group (P < 0.05).

Conclusion

The treatment of MGC with PE combined with immunoglobulin can not only effectively enhance the consciousness and immune function of patients but also effectively promote the prognosis, and the safety of treatment can be guaranteed.

Rate of change in acetylcholine receptor antibody levels predicts myasthenia gravis outcome

OBJECTIVE

To investigate the association between changes in anti-acetylcholine receptor antibody (AChR Ab) levels induced by immunosuppressive treatment and myasthenia gravis (MG) prognosis at 1-year post-treatment in patients with MG.

METHODS

We included 53 consecutive AChR Ab-positive patients with MG whose AChR Ab levels were remeasured within 100 days of initiating immunosuppressive treatment (median remeasuring time post-treatment: 71 (55-84) days). The AChR Ab level reduction rate (RR-AChRAb, %/day) adjusted for the time between treatment initiation, and AChR Ab level remeasurement was calculated as follows: (pretreatment-post-treatment AChR Ab level)/pretreatment AChR Ab level/days between therapy initiation and AChR Ab level remeasurement ×100. Participants were divided into two groups based on the cut-off value of RR-AChR Ab, determined using receiver operating characteristic analyses for achieving minimal manifestation (MM) or better status at 1-year postimmunosuppressive treatment. The Myasthenia Gravis Foundation of America postintervention status and MG activity of daily living (MG-ADL) score at 1-year post-treatment were compared between the two groups.

RESULTS

The RR-AChRAb cut-off value was 0.64%/day. The high RR-AChRAb group had a higher ratio of MM or better status (90% vs 65%, p=0.03) and lower MG-ADL score (median; 1 vs 2, p=0.04) than the low RR-AChRAb group. Kaplan-Meier analyses showed the early MM achievement in the high RR-AChRAb group (p=0.002, log-rank test).

CONCLUSIONS

High RR-AChRAb is associated with a favourable outcome at 1-year post-treatment. AChR Ab remeasurement within 100 days of therapy may be useful for predicting AChR Ab-positive MG outcomes at 1-year post-treatment.

Improving laboratory diagnostics in myasthenia gravis

Introduction: Myasthenia gravis (MG) is a prototypical autoimmune disease, characterized by pathogenic autoantibodies targeting structures of the neuromuscular junction. Radioimmunoprecipitation assays (RIPAs) represent the gold standard for their detection. However, new methods are emerging to complement, or overcome RIPAs, also with the perspective of eliminating the use of radioactive reagents.Areas covered: We discuss advances in laboratory methods, prompted especially by cell-based assays (CBAs), for the detection of the autoantibodies of MG diagnostics, above all those to the nicotinic acetylcholine receptor (AChR), muscle-specific kinase (MuSK), and low molecular-weight receptor-related low-density lipoprotein-4 (LRP4).Expert opinion: CBA technology makes AChRs aggregate on cell membranes, thus allowing to detect autoantibodies to clustered AChRs, with reduction of seronegative MG cases. The diagnostic relevance of RIPA/CBA-measurable LRP4 antibodies is still unclear, in Caucasian patients at least. Live CBAs for the detection of AChR, MuSK, and LRP4 antibodies might represent an alternative to RIPAs, but first require full validation. CBAs could be used as screening tests, limiting RIPAs for antibody quantification. To this end, ELISAs might be an alternative.Fixation procedures preserving enough degree of antigen conformationality could yield AChR and MuSK CBAs suitable for a wide use in clinical-chemistry laboratories.

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.

Myasthenia Gravis: From the Viewpoint of Pathogenicity Focusing on Acetylcholine Receptor Clustering, Trans-Synaptic Homeostasis and Synaptic Stability

Myasthenia gravis (MG) is a disease of the postsynaptic neuromuscular junction (NMJ) where nicotinic acetylcholine (ACh) receptors (AChRs) are targeted by autoantibodies. Search for other pathogenic antigens has detected the antibodies against muscle-specific tyrosine kinase (MuSK) and low-density lipoprotein-related protein 4 (Lrp4), both causing pre- and post-synaptic impairments. Agrin is also suspected as a fourth pathogen. In a complex NMJ organization centering on MuSK: (1) the Wnt non-canonical pathway through the Wnt-Lrp4-MuSK cysteine-rich domain (CRD)-Dishevelled (Dvl, scaffold protein) signaling acts to form AChR prepatterning with axonal guidance; (2) the neural agrin-Lrp4-MuSK (Ig1/2 domains) signaling acts to form rapsyn-anchored AChR clusters at the innervated stage of muscle; (3) adaptor protein Dok-7 acts on MuSK activation for AChR clustering from “inside” and also on cytoskeleton to stabilize AChR clusters by the downstream effector Sorbs1/2; (4) the trans-synaptic retrograde signaling contributes to the presynaptic organization via: (i) Wnt-MuSK CRD-Dvl-β catenin-Slit 2 pathway; (ii) Lrp4; and (iii) laminins. The presynaptic Ca²⁺ homeostasis conditioning ACh release is modified by autoreceptors such as M1-type muscarinic AChR and A2A adenosine receptors. The post-synaptic structure is stabilized by: (i) laminin-network including the muscle-derived agrin; (ii) the extracellular matrix proteins (including collagen Q/perlecan and biglycan which link to MuSK Ig1 domain and CRD); and (iii) the dystrophin-associated glycoprotein complex. The study on MuSK ectodomains (Ig1/2 domains and CRD) recognized by antibodies suggested that the MuSK antibodies were pathologically heterogeneous due to their binding to multiple functional domains. Focussing one of the matrix proteins, biglycan which functions in the manner similar to collagen Q, our antibody assay showed the negative result in MG patients. However, the synaptic stability may be impaired by antibodies against MuSK ectodomains because of the linkage of biglycan with MuSK Ig1 domain and CRD. The pathogenic diversity of MG is discussed based on NMJ signaling molecules.

ANTIBODIES AND RECEPTORS: From Neuromuscular Junction to Central Nervous System

Myasthenia gravis (MG) is a relatively rare neurological disease that is usually associated with antibodies to the acetylcholine receptor (AChR). These antibodies (Abs) cause loss of the AChRs from the neuromuscular junction (NMJ), resulting in muscle weakness that can be life-threatening. Another form of the disease is caused by antibodies to muscle specific kinase (MuSK) that result in impaired AChR clustering and numbers at the NMJ, and may also interfere with presynaptic adaptive mechanisms. Other autoimmune disorders, Lambert Eaton myasthenic syndrome and acquired neuromyotonia, are associated with antibodies to presynaptic voltage-gated calcium and potassium channels respectively. All four conditions can be diagnosed by specific clinical features, electromyography and serum antibody tests, and can be treated effectively by a combination of pharmacological approaches and procedures that reduce the levels of the IgG antibodies. They form the first of a spectrum of diseases in which serum autoantibodies bind to extracellular domains of neuronal proteins throughout the nervous system and lead to constellations of clinical features including paralysis, sensory disturbance and pain, memory loss, seizures, psychiatric disturbance and movement disorders. This review will briefly summarize the ways in which this field has developed, since the 1970s when considerable contributions were made in Ricardo Miledi's laboratory at UCL.