Effect of complement and its regulation on myasthenia gravis pathogenesis

Delving into Molecular Pathways: Analyzing the Mechanisms of Action of Monoclonal Antibodies Integrated in IMGT/mAb-DB for Myasthenia Gravis

BACKGROUND

Myasthenia Gravis (MG) is a rare autoimmune disease presenting with auto-antibodies that affect the neuromuscular junction. In addition to symptomatic treatment options, novel therapeutics include monoclonal antibodies (mAbs). IMGT®, the international ImMunoGeneTics information system®, extends the characterization of therapeutic antibodies with a systematic description of their mechanisms of action (MOA) and makes them available through its database for mAbs and fusion proteins, IMGT/mAb-DB.

METHODS

Using available literature data combined with amino acid sequence analyses from mAbs managed in IMGT/2Dstructure-DB, the IMGT® protein database, biocuration allowed us to define in a standardized way descriptions of MOAs of mAbs that target molecules towards MG treatment.

RESULTS

New therapeutic targets include FcRn and molecules such as CD38, CD40, CD19, MS4A1, and interleukin-6 receptor. A standardized graphical representation of the MOAs of selected mAbs was created and integrated within IMGT/mAb-DB. The main mechanisms involved in these mAbs are either blocking or neutralizing. Therapies directed to B cell depletion and plasma cells have a blocking MOA with an immunosuppressant effect along with Fc-effector function (MS4A1, CD38) or FcγRIIb engager effect (CD19). Monoclonal antibodies targeting the complement also have a blocking MOA with a complement inhibitor effect, and treatments targeting T cells have a blocking MOA with an immunosuppressant effect (CD40) and Fc-effector function (IL6R). On the other hand, FcRn antagonists present a neutralizing MOA with an FcRn inhibitor effect.

CONCLUSION

The MOA of each new mAb needs to be considered in association with the immunopathogenesis of each of the subtypes of MG in order to integrate the new mAbs as a viable and safe option in the therapy decision process. In IMGT/mAb-DB, mAbs for MG are characterized by their sequence, domains, and chains, and their MOA is described.

Zilucoplan, a macrocyclic peptide inhibitor of human complement component 5, uses a dual mode of action to prevent terminal complement pathway activation

Introduction

The complement system is a key component of the innate immune system, and its aberrant activation underlies the pathophysiology of various diseases. Zilucoplan is a macrocyclic peptide that binds and inhibits the cleavage/activation of human complement component 5 (C5). We present in vitro and ex vivo data on the mechanism of action of zilucoplan for the inhibition of C5 activation, including two clinically relevant C5 polymorphisms at R885.

Methods

The interaction of zilucoplan with C5, including for clinical C5 R885 variants, was investigated using surface plasmon resonance (SPR), hemolysis assays, and ELISA. The interference of C5b6 formation by zilucoplan was investigated by native gel analysis and hemolysis assay. The permeability of zilucoplan in a reconstituted basement membrane was assessed by the partition of zilucoplan on Matrigel-coated transwell chambers.

Results

Zilucoplan specifically bound human complement C5 with high affinity, competitively inhibited the binding of C5 to C3b, and blocked C5 cleavage by C5 convertases and the assembly of the cytolytic membrane attack complex (MAC, or C5b9). Zilucoplan fully prevented the in vitro activation of C5 clinical variants at R885 that have been previously reported to respond poorly to eculizumab treatment. Zilucoplan was further demonstrated to interfere with the formation of C5b6 and inhibit red blood cell (RBC) hemolysis induced by plasmin-mediated non-canonical C5 activation. Zilucoplan demonstrated greater permeability than a monoclonal C5 antibody in a reconstituted basement membrane model, providing a rationale for the rapid onset of action of zilucoplan observed in clinical studies.

Conclusion

Our findings demonstrate that zilucoplan uses a dual mode of action to potently inhibit the activation of C5 and terminal complement pathway including wild-type and clinical R885 variants that do not respond to eculizumab treatment. These data may be relevant to the clinically demonstrated benefits of zilucoplan.

Safety and efficacy of zilucoplan in patients with generalised myasthenia gravis (RAISE): a randomised, double-blind, placebo-controlled, phase 3 study

BACKGROUND

Generalised myasthenia gravis is a chronic, unpredictable, and debilitating rare disease, often accompanied by high treatment burden and with an unmet need for more efficacious and well tolerated treatments. Zilucoplan is a subcutaneous, self-administered macrocyclic peptide complement C5 inhibitor. We aimed to assess safety, efficacy, and tolerability of zilucoplan in patients with acetylcholine receptor autoantibody (AChR)-positive generalised myasthenia gravis.

METHODS

RAISE was a randomised, double-blind, placebo-controlled, phase 3 trial that was done at 75 sites in Europe, Japan, and North America. We enrolled patients (aged 18-74 years) with AChR-positive generalised myasthenia gravis (Myasthenia Gravis Foundation of America disease class II-IV), a myasthenia gravis activities of daily living (MG-ADL) score of least 6, and a quantitative myasthenia gravis score of at least 12. Participants were randomly assigned (1:1) to receive subcutaneous zilucoplan 0·3 mg/kg once daily by self-injection, or matched placebo, for 12 weeks. The primary efficacy endpoint was change from baseline to week 12 in MG-ADL score in the modified intention-to-treat population (all randomly assigned patients who received at least one dose of study drug and had at least one post-dosing MG-ADL score). Safety was mainly assessed by the incidence of treatment-emergent adverse events (TEAEs) in all patients who had received at least one dose of zilucoplan or placebo. This trial is registered at ClinicalTrials.gov, NCT04115293. An open-label extension study is ongoing (NCT04225871).

FINDINGS

Between Sept 17, 2019, and Sept 10, 2021, 239 patients were screened for the study, of whom 174 (73%) were eligible. 86 (49%) patients were randomly assigned to zilucoplan 0·3 mg/kg and 88 (51%) were assigned to placebo. Patients assigned to zilucoplan showed a greater reduction in MG-ADL score from baseline to week 12, compared with those assigned to placebo (least squares mean change -4·39 [95% CI -5·28 to -3·50] vs -2·30 [-3·17 to -1·43]; least squares mean difference -2·09 [-3·24 to -0·95]; p=0·0004). TEAEs occurred in 66 (77%) patients in the zilucoplan group and in 62 (70%) patients in the placebo group. The most common TEAE was injection-site bruising (n=14 [16%] in the zilucoplan group and n=8 [9%] in the placebo group). Incidences of serious TEAEs and serious infections were similar in both groups. One patient died in each group; neither death (COVID-19 [zilucoplan] and cerebral haemorrhage [placebo]) was considered related to the study drug.

INTERPRETATION

Zilucoplan treatment showed rapid and clinically meaningful improvements in myasthenia gravis-specific efficacy outcomes, had a favourable safety profile, and was well tolerated, with no major safety findings. Zilucoplan is a new potential treatment option for a broad population of patients with AChR-positive generalised myasthenia gravis. The long-term safety and efficacy of zilucoplan is being assessed in an ongoing open-label extension study.

FUNDING

UCB Pharma.

CD59 Expression in Skeletal Muscles and Its Role in Myasthenia Gravis

Background and Objectives

Complement regulatory proteins at the neuromuscular junction (NMJ) could offer protection against complement-mediated damage in myasthenia gravis (MG). However, there is limited information on their expression at the human NMJ. Thus, this study aimed at investigating the expression of the cluster of differentiation 59 (CD59) at the NMJ of human muscle specimens and demonstrating the overexpression ofCD59mRNA and protein in the muscles of patients with MG.

Methods

In this observational study, muscle specimens from 16 patients with MG (9 and 7 patients with and without thymoma, respectively) and 6 nonmyopathy control patients were examined. Immunohistochemical stains, Western blot analysis, and quantitative real-time reverse transcription PCR were used to evaluate the CD59 expression.

Results

A strong localized expression of CD59 was observed at the NMJ in both patients with and without MG. Moreover, the CD59/glyceraldehyde-3-phosphate dehydrogenase protein ratio in patients with MG was significantly higher than that in the nonmyopathy controls (MG; n = 16, median 0.16, interquartile range (IQR) 0.08–0.26 and nonmyopathy controls; n = 6, median 0.03, IQR 0.02–0.11,p= 0.01). The proportion ofCD59mRNA expression relative toAChRmRNA expression (ΔCtCD59/AChR) was associated with the quantitative MG score, MG activities of daily living score, and MG of Foundation of America Clinical Classification (r= 0.663,p= 0.01;r= 0.638,p= 0.014; andr= 0.715,p= 0.003, respectively).

Discussion

CD59, which acts as a complement regulator, may protect the NMJ from complement attack. Our findings could provide a basis for further research that investigates the underlying pathogenesis in MG and the immunomodulating interactions of the muscle cells.

Eculizumab versus rituximab in generalised myasthenia gravis

OBJECTIVE

Myasthenia gravis (MG) is the most common autoimmune disorder affecting the neuromuscular junction. However, evidence shaping treatment decisions, particularly for treatment-refractory cases, is sparse. Both rituximab and eculizumab may be considered as therapeutic options for refractory MG after insufficient symptom control by standard immunosuppressive therapies.

METHODS

In this retrospective observational study, we included 57 rituximab-treated and 20 eculizumab-treated patients with MG to compare the efficacy of treatment agents in generalised, therapy-refractory anti-acetylcholine receptor antibody (anti-AChR-ab)-mediated MG with an observation period of 24 months. Change in the quantitative myasthenia gravis (QMG) score was defined as the primary outcome parameter. Differences between groups were determined in an optimal full propensity score matching model.

RESULTS

Both groups were comparable in terms of clinical and demographic characteristics. Eculizumab was associated with a better outcome compared with rituximab, as measured by the change of the QMG score at 12 and 24 months of treatment. Minimal manifestation of disease was more frequently achieved in eculizumab-treated patients than rituximab-treated patients at 12 and 24 months after baseline. However, the risk of myasthenic crisis (MC) was not ameliorated in either group.

INTERPRETATION

This retrospective, observational study provides the first real-world evidence supporting the use of eculizumab for the treatment of refractory, anti-AChR-ab positive MG. Nonetheless, the risk of MC remained high and prompts the need for intensified monitoring and further research effort aimed at this vulnerable patient cohort.

A bioassay for neuromuscular junction-restricted complement activation by myasthenia gravis acetylcholine receptor antibodies

BACKGROUND

Myasthenia gravis (MG) is an autoimmune neuromuscular disorder hallmarked by fluctuating fatigable muscle weakness. Most patients have autoantibodies against acetylcholine receptors (AChRs) at the neuromuscular junction (NMJ). These are thought to have three possible pathogenic mode-of-actions: 1) cross-linking and endocytosis of AChRs, 2) direct block of AChRs and 3) complement activation. The relative contributions of these mechanisms to synaptic block and muscle weakness of individual patients cannot be determined. It likely varies between patients and perhaps also with disease course, depending on the nature of the circulating AChR antibodies.

NEW METHOD

We developed a new bioassay which specifically enables functional characterization and quantification of complement-mediated synaptic damage at NMJs, without interference of the other pathogenic mechanisms. To this end, we pre-incubated mouse hemi-diaphragm muscle-nerve preparations with mAb35-hG1, a humanized rat AChR monoclonal and subsequently exposed the preparation to normal human serum as a complement source. NMJ-restricted effects were studied.

RESULTS

Clearly NMJ-restricted damage occurred. With immunohistology we showed complement deposition at NMJs, and synaptic electrophysiological measurements demonstrated transmission block. In whole-muscle contraction experiments we quantified the effect and characterized its onset and progression during the incubation with normal human serum.

COMPARISON WITH EXISTING METHODS

With this new assay the complement-mediated component of myasthenic NMJ pathology can be studied separately.

CONCLUSIONS

Our assay will be of importance in detailed mechanistic studies of local complement activation at NMJs, investigations of new complement inhibitors, and laboratory pre-screening of therapeutic efficacy for individual MG patients to optimize care with clinically approved complement inhibitors.

Molecular Therapy in Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune disorder caused by antibodies that act against the myoneural junction. Conventional immunosuppressants such as corticosteroids, azathioprine and mycophenolate are associated with long-term side effects and many patients do not achieve remission and may become refractory. Thus, there is an unmet need for target-specific therapies that act faster, have fewer side effects and lead to stable disease remission. However, many of the novel therapeutic agents being described are not meeting their primary endpoints. We reviewed the current status of novel immunotherapies for MG, their mechanisms of action, along with the side effect profiles. Fast onset of action, sustained disease remission and relatively low frequency of side effects of the new agents are attractive. However, the unknown long-term safety and high cost are precluding factors. Better preclinical studies and more randomized trials are needed before novel agents are routinely employed.

Zilucoplan: An Investigational Complement C5 Inhibitor for the Treatment of Acetylcholine Receptor Autoantibody-Positive Generalized Myasthenia Gravis

INTRODUCTION

Generalized myasthenia gravis (gMG) is an autoimmune disorder in which pathogenic autoantibodies damage the neuromuscular junction, causing disabling or life-threatening muscle weakness. Most treatments nonspecifically inhibit aspects of the immune system, do not directly address the causal mechanisms of tissue damage, and often have side-effect profiles that negatively impact patients. Understanding of the central pathogenic role of the complement cascade in gMG is advancing, and a new complement-targeting treatment is under investigation.

AREAS COVERED

We provide an overview of gMG etiology, the complement cascade, current treatments, and the investigational gMG therapy zilucoplan. Zilucoplan is a small, subcutaneously administered, macrocyclic peptide that inhibits cleavage of complement component C5 and the subsequent formation of the membrane attack complex.

EXPERT OPINION

In a randomized, double-blind, placebo-controlled, phase 2 clinical trial, zilucoplan demonstrated clinically meaningful complement inhibition in patients with acetylcholine receptor-positive gMG. Zilucoplan, a first-of-its-kind cyclic peptide targeting C5, appears to be a therapeutic option for the treatment of gMG based on available pharmacokinetic/pharmacodynamic data and phase 1 and 2 efficacy, safety, and tolerability data with limited long-term follow-up. Zilucoplan use earlier in the treatment paradigm would be suitable in this population should phase 3 efficacy and safety data be equally favorable.

Cholesterol in myasthenia gravis

The cholinergic neuromuscular junction is the paradigm peripheral synapse between a motor neuron nerve ending and a skeletal muscle fiber. In vertebrates, acetylcholine is released from the presynaptic site and binds to the nicotinic acetylcholine receptor at the postsynaptic membrane. A variety of pathologies among which myasthenia gravis stands out can impact on this rapid and efficient signaling mechanism, including autoimmune diseases affecting the nicotinic receptor or other synaptic proteins. Cholesterol is an essential component of biomembranes and is particularly rich at the postsynaptic membrane, where it interacts with and modulates many properties of the nicotinic receptor. The profound changes inflicted by myasthenia gravis on the postsynaptic membrane necessarily involve cholesterol. This review analyzes some aspects of myasthenia gravis pathophysiology and associated postsynaptic membrane dysfunction, including dysregulation of cholesterol metabolism in the myocyte brought about by antibody-receptor interactions. In addition, given the extensive therapeutic use of statins as the typical cholesterol-lowering drugs, we discuss their effects on skeletal muscle and the possible implications for MG patients under chronic treatment with this type of compound.

Changes in serum complements and their regulators in generalized myasthenia gravis

OBJECTIVE

To investigate changes in serum complements and their regulators in the pathogenesis of myasthenia gravis (MG).

METHODS

Forty-four patients with acetylcholine receptor antibody-positive MG, as well as 20 patients with non-inflammatory neurological disorders were enrolled. Serum complements (C3, C4 and soluble C5b-9) and complement regulators (vitronectin, clusterin and properdin) were extensively analysed by enzyme-linked immunosorbent assay and their associations with clinical profiles of MG were examined.

RESULTS

Serum C3, C4 and clusterin levels were not significantly different between patients with MG and controls. The patients with MG had higher soluble C5b-9 (P = 0.09) and vitronectin (P = 0.001) levels than the controls; moreover, vitronectin levels decreased after treatment (P = 0.09). Serum properdin (P = 0.03) levels were lower in the patients with MG than in the controls, and negatively correlated with the MG Activities of Daily Living score (rs = -0.26, P = 0.09) and with the presence of bulbar palsy (P = 0.04).

CONCLUSION

Our results show that activation of complements and an altered complement network could contribute to the inflammatory pathogenesis of MG.

C3, C5a and anti-acetylcholine receptor antibody as severity biomarkers in myasthenia gravis

Background

Although the pathogenesis of myasthenia gravis (MG) is well known, prognostic markers are not yet available. We assessed the utility of anti-acetylcholine receptor (AChR) antibody (AChR-ab) titer and concentration of C3, C4, and C5a as potential severity biomarkers in MG.

Methods

Levels of C3, C4, C5a, and AChR-ab were measured in 60 AChR-ab-positive patients with MG. Their relationship with clinical severity was analyzed using the activities of daily living (ADL) and MG composite (MGC) scales.

Results

AChR-ab titer correlated with severity of MG according to ADL (p = 0.002) and MGC scales (p = 0.001). When patients were classified according to disease duration, a statistically significant correlation between AChR-ab titer and clinical severity was only found in the subgroup of patients with fewer than 5 years from symptoms onset. C5a levels showed a positive correlation with MG severity according to the ADL scale (p = 0.041; τb = 0.18), although C5a levels were not different from the control group.

Discussion

AChR-ab titers and C5a levels could potentially be considered markers of severity in patients with MG.

Retrospective Analysis of Eculizumab in Patients with Acetylcholine Receptor Antibody-Negative Myasthenia Gravis: A Case Series

Background:

The role of the complement cascade in acetylcholine receptor antibody-negative (AChR–) myasthenia gravis (MG) is unclear.

Objective:

To assess the efficacy and tolerability of eculizumab (terminal complement inhibitor) in patients with AChR–MG.

Methods:

Retrospective chart review of data from six patients treated for 12 months with eculizumab for treatment-refractory, AChR–(by radioimmunoassay) generalized MG (gMG). The eculizumab dose was 900 mg/week for 4 weeks then 1200 mg every 2 weeks. Outcome measures were Myasthenia Gravis–Activities of Daily Living (MG-ADL) scores, number of exacerbations, and qualitative physical assessments based on selected items of the Quantitative Myasthenia Gravis evaluation (ptosis, double vision, eye closure, duration of ability to stretch out limbs).

Results:

All patients were female (mean age, 50.8 years). In the 12 months before eculizumab initiation, all measures were relatively stable. After its initiation, clinically meaningful reductions (≥2 points) in total MG-ADL scores were observed before or at 5 months and were maintained to Month 12 in all patients; mean (standard deviation [SD]) scores were 11.3 (0.9) and 5.0 (0.9), respectively. There was also a reduction in the mean (SD) number of exacerbations per patient, from 2.8 (1.2) to 0.3 (0.5) in the 12 months before and after eculizumab initiation, respectively. Physical assessment ratings were improved in all patients. Adverse events were reported in four patients, but all were mild and none were treatment-related.

Conclusions:

This small retrospective analysis provides preliminary evidence for the efficacy of eculizumab in treatment-refractory gMG that was AChR–according to radioimmunoassay. Larger, more robust studies are warranted to evaluate this further.

In vitro models of neuromuscular junctions and their potential for novel drug discovery and development

Introduction: Neuromuscular Junctions (NMJs) are the synapses between motor neurons and skeletal muscle fibers, and they are responsible for voluntary motor function. NMJs are affected at early stages of numerous neurodegenerative and neuroimmunological diseases. Due to the difficulty of systematically studying and manipulating NMJs in live subjects, in vitro systems with human tissue models would provide a powerful complement to simple cell cultures and animal models for mechanistic and drug development studies.Areas covered: The authors review the latest advances in in vitro models of NMJs, from traditional cell co-culture systems to novel tissue culture approaches, with focus on disease modeling and drug testing.Expert opinion: In recent years, more sophisticated in vitro models of human NMJs have been established. The combination of human stem cell technology with advanced tissue culture systems has resulted in systems that better recapitulate the human NMJ structure and function, and thereby allow for high-throughput quantitative functional measurements under both healthy and diseased conditions. Although they still have limitations, these advanced systems are increasingly demonstrating their utility for evaluating new therapies for motoneuron and autoimmune neuromuscular diseases, and we expect them to become an integral part of the drug discovery process in the near future.

Temporal correlation between serum CH50 level and symptom severity of myasthenia gravis during eculizumab therapy

The correlation between serum 50 % hemolytic complement (CH50) level and myasthenic symptom severity has not been known in patients with anti-acetylcholine receptor (anti-AChR)-positive myasthenia gravis (MG) during eculizumab treatment. A patient with anti-AChR-positive MG showed severe bulbar symptoms. Eculizumab administration decreased CH50 level and improved the symptoms. However, shortly after the second administration of eculizumab was postponed due to the development of pneumonia, his serum CH50 level returned almost to the level it was at before the initiation of eculizumab therapy and myasthenic symptoms worsened. Even after his pneumonia was completely cleared in response to an antibiotic, the severe myasthenic symptoms persisted. After eculizumab was resumed, serum CH50 level was reduced to below the limit of detection within 24 h, and the symptom steadily improved. His symptom severity was correlated temporally with serum CH50 level during eculizumab therapy. Our case suggests that serum CH50 level may be a marker of eculizumab-induced complement blockade and an indicator of a potential worsening of myasthenic symptoms during eculizumab treatment.

Autoimmune Attack of the Neuromuscular Junction in Myasthenia Gravis: Nicotinic Acetylcholine Receptors and Other Targets

The nicotinic acetylcholine receptor (nAChR) family, the archetype member of the pentameric ligand-gated ion channels, is ubiquitously distributed in the central and peripheral nervous systems, and its members are the targets for both genetic and acquired forms of neurological disorders. In the central nervous system, nAChRs contribute to the pathological mechanisms of neurodegenerative disorders, such as Alzheimer and Parkinson diseases. In the peripheral nerve-muscle synapse, the vertebrate neuromuscular junction, "classical" myasthenia gravis (MG) and other forms of neuromuscular transmission disorders are antibody-mediated autoimmune diseases. In MG, antibodies to the nAChR bind to the postsynaptic receptors and activate the classical complement pathway culminating in the formation of the membrane attack complex, with the subsequent destruction of the postsynaptic apparatus. Divalent nAChR-antibodies also cause internalization and loss of the nAChRs. Loss of receptors by either mechanism results in the muscle weakness and fatigability that typify the clinical manifestations of the disease. Other targets for antibodies, in a minority of patients, include muscle specific kinase (MuSK) and low-density lipoprotein related protein 4 (LRP4). This brief Review analyzes the current status of muscle-type nAChR in relation to the pathogenesis of autoimmune diseases affecting the peripheral cholinergic synapse.

A Neurologist's Perspective on Understanding Myasthenia Gravis: Clinical Perspectives of Etiologic Factors, Diagnosis, and Preoperative Treatment

Myasthenia gravis (MG) is a disease of neuromuscular transmission caused by antibodies directed toward proteins concentrated at the neuromuscular junction. Mild to life-threatening weakness varies in severity over time and with level of activity. Therefore, clinical diagnosis is often challenging. MG may be categorized by autoantibody type, thymic pathologic condition, and age of onset. Treatments are tailored for each group. A key management concern is severe exacerbation of weakness resulting from infections or exposure to certain medications, including antibiotics, which may be severe enough to produce respiratory decompensation. The article reviews key diagnostic issues and treatment options.

Myasthenia gravis: the role of complement at the neuromuscular junction

Generalized myasthenia gravis (gMG) is a rare autoimmune disorder characterized by skeletal muscle weakness caused by disrupted neurotransmission at the neuromuscular junction (NMJ). Approximately 74-88% of patients with gMG have acetylcholine receptor (AChR) autoantibodies. Complement plays an important role in innate and antibody-mediated immunity, and activation and amplification of complement results in the formation of membrane attack complexes (MACs), lipophilic proteins that damage cell membranes. The role of complement in gMG has been demonstrated in animal models and patients. Studies in animals lacking specific complement proteins have confirmed that MAC formation is required to induce experimental autoimmune MG (EAMG) and NMJ damage. Complement inhibition in EAMG models can prevent disease induction and reverse its progression. Patients with anti-AChR⁺ MG have autoantibodies and MACs present at NMJs. Damaged NMJs are associated with more severe disease, fewer AChRs, and MACs in synaptic debris. Current MG therapies do not target complement directly. Eculizumab is a humanized monoclonal antibody that inhibits cleavage of complement protein C5, preventing MAC formation. Eculizumab treatment improved symptoms compared with placebo in a phase II study in patients with refractory gMG. Direct complement inhibition could preserve NMJ physiology and muscle function in patients with anti-AChR⁺ gMG.

Viral-derived complement inhibitors: current status and potential role in immunomodulation

The complement system is one of the body's major innate immune defense mechanisms in vertebrates. Its function is to detect foreign bodies and promote their elimination through opsonisation or lysis. Complement proteins play an important role in the immunopathogenesis of several disorders. However, excessive complement activation does not confer more protection but instead leads to several autoimmune and inflammatory diseases. With inappropriate activation of the complement system, activated complement proteins and glycoproteins may damage both healthy and diseased tissues. Development of complement inhibitors represents an effective approach in controlling dysregulated complement activity and reducing disease severity, yet few studies have investigated the nature and role of novel complement inhibitory proteins of viral origin. Viral complement inhibitors have important implications in understanding the importance of complement inhibition and their role as a promising novel therapeutic approach in diseases caused by dysregulated complement function. In this review, we discuss the role and importance of complement inhibitors derived from several viruses in the scope of human inflammatory and autoimmune diseases.

Applying complement therapeutics to rare diseases

Around 350 million people worldwide suffer from rare diseases. These may have a genetic, infectious, or autoimmune basis, and several include an inflammatory component. Launching of effective treatments can be very challenging when there is a low disease prevalence and limited scientific insights into the disease mechanisms. As a key trigger of inflammatory processes, complement has been associated with a variety of diseases and has become an attractive therapeutic target for conditions involving inflammation. In view of the clinical experience acquired with drugs licensed for the treatment of rare diseases such as hereditary angioedema and paroxysmal nocturnal hemoglobinuria, growing evidence supports the safety and efficacy of complement therapeutics in restoring immune balance and preventing aggravation of clinical outcomes. This review provides an overview of the candidates currently in the pharmaceutical pipeline with potential to treat orphan diseases and discusses the molecular mechanisms triggered by complement involved with the disease pathogenesis.

Myasthenia Gravis: paradox versus paradigm in autoimmunity

Myasthenia Gravis (MG) is a paradigm of organ-specific autoimmune disease (AID). It is mediated by antibodies that target the neuromuscular junction. The purpose of this review is to place MG in the general context of autoimmunity, to summarize the common mechanisms between MG and other AIDs, and to describe the specific mechanisms of MG. We have chosen the most common organ-specific AIDs to compare with MG: type 1 diabetes mellitus (T1DM), autoimmune thyroid diseases (AITD), multiple sclerosis (MS), some systemic AIDs (systemic lupus erythematous (SLE), rheumatoid arthritis (RA), Sjogren's syndrome (SS)), as well as inflammatory diseases of the gut and liver (celiac disease (CeD), Crohn's disease (CD), and primary biliary cirrhosis (PBC)). Several features are similar between all AIDs, suggesting that common pathogenic mechanisms lead to their development. In this review, we address the predisposing factors (genetic, epigenetic, hormones, vitamin D, microbiota), the triggering components (infections, drugs) and their interactions with the immune system [1,2]. The dysregulation of the immune system is detailed and includes the role of B cells, Treg cells, Th17 and cytokines. We particularly focused on the role of TNF-α and interferon type I whose role in MG is very analogous to that in several other AIDS. The implication of AIRE, a key factor in central tolerance is also discussed. Finally, if MG is a prototype of AIDS, it has a clear specificity compared to the other AIDS, by the fact that the target organ, the muscle, is not the site of immune infiltration and B cell expansion, but exclusively that of antibody-mediated pathogenic mechanisms. By contrast, the thymus in the early onset subtype frequently undergoes tissue remodeling, resulting in the development of ectopic germinal centers surrounded by high endothelial venules (HEV), as observed in the target organs of many other AIDs.

Targeting therapy to the neuromuscular junction: proof of concept

INTRODUCTION

The site of pathology in myasthenia gravis (MG) is the neuromuscular junction (NMJ). Our goal was to determine the ability to direct complement inhibition to the NMJ.

METHODS

A single-chain antibody directed against the alpha subunit of the acetylcholine receptor was synthesized (scFv-35) and coupled to decay-accelerating factor (DAF, scFv-35-DAF). scFv-35-DAF was tested in a passive model of experimentally acquired MG.

RESULTS

Administration of scFv-35-DAF to mice deficient in intrinsic complement inhibitors produced no weakness despite confirmation of its localization to the NMJ and no evidence of tissue destruction related to complement activation. Rats with experimentally acquired MG treated with scFV-35-DAF showed less weakness and a reduction of complement deposition.

CONCLUSIONS

We demonstrate a method to effectively target a therapeutic agent to the NMJ.

Biomarker development for myasthenia gravis

Biomarkers are defined as characteristics (e.g., proteins, RNA, single nucleotide polymorphisms, imaging) that are objectively measured and evaluated as indicators of pathogenic processes or pharmacologic responses to therapeutic intervention. Biomarkers are important in clinical trials where the robust biomarker reflects the underlying disease process in a sensitive and reliable manner. For myasthenia gravis (MG), acetylcholine receptor and muscle-specific kinase antibodies, as well as single-fiber electromyography, serve as excellent biomarkers for diagnosis but do not adequately substitute for clinical evaluations to predict treatment response. New technologies are emerging that enable broad biomarker discovery in biological fluids. Biomarker evaluation is ideally done in the context of longitudinal clinical trials. The MGTX trial has collected plasma and serum for RNA and protein analysis and thymus, which will allow robust biomarker discovery. The ultimate goal will be to identify candidates for a reliable substitute for a clinically meaningful end point that is a direct measure of the effectiveness of a therapy in the context of a continuum of disease natural history and a patient's overall well-being.

Cell surface complement regulators moderate experimental myasthenia gravis pathology

INTRODUCTION

Intrinsic mouse complement regulators influence the severity of passively induced experimental acquired myasthenia gravis (EAMG). To assess the potential influence of CD59b in the absence of CD59a background, we used the mCD59ab(-/-) mouse model to re-evaluate mCD59 in protecting the neuromuscular junction (NMJ).

METHODS

EAMG was induced with monoclonal antibody to the acetylcholine receptor (AChR) in Daf1(-/-) , CD59ab(-/-) , Daf1(-/-) CD59ab(-/-) , and wild-type C57Bl/6 mice. Animals were monitored throughout the experiment. Diaphragms were analyzed for NMJ injury.

RESULTS

Daf1(-/-) CD59ab(-/-) mice required euthanasia 24 hours after disease induction because of severe weakness. Histological assessment demonstrated reduced AChR density, simplification of synaptic folds, and disrupted mitochondria. CD59ab-deficient mice demonstrated mild weakness and reduction in weight after 24 hours. In contrast, Daf1(-/-) had more severe weakness at 60 hours. The NMJ of EAMG-induced Daf1(-/-) and CD59ab(-/-) mice demonstrated similar AChR density.

CONCLUSION

NMJs of CD59 and DAF mice are protected from complement-mediated injury of passive EAMG.

DAF/CD55 and Protectin/CD59 modulate adaptive immunity and disease outcome in experimental autoimmune myasthenia gravis

The role of regulators of complement activity (RCA) involving CD55 and CD59 in the pathogenesis of experimental autoimmune myasthenia gravis (EAMG) remains unclear. CD55 and CD59 restrict complement activation by inhibiting C3/C5 convertases' activities and membrane attack complex formation, respectively. Actively immunized EAMG mice deficient in either CD55 or CD59 showed significant differences in adaptive immune responses and worsened disease outcome associated with increased levels of serum cytokines, modified production of acetylcholine receptor antibodies, and more complement deposition at the neuromuscular junction. We conclude that modulation of complement activity by RCA represents an alternative in controlling of autoimmune processes in EAMG.

Novel complement inhibitor limits severity of experimentally myasthenia gravis

OBJECTIVE

Complement mediated injury of the neuromuscular junction is considered a primary disease mechanism in human myasthenia gravis and animal models of experimentally acquired myasthenia gravis (EAMG). We utilized active and passive models of EAMG to investigate the efficacy of a novel C5 complement inhibitor rEV576, recombinantly produced protein derived from tick saliva, in moderating disease severity.

METHODS

Standardized disease severity assessment, serum complement hemolytic activity, serum cytotoxicity, acetylcholine receptor (AChR) antibody concentration, IgG subclassification, and C9 deposition at the neuromuscular junction were used to assess the effect of complement inhibition on EAMG induced by administration of AChR antibody or immunization with purified AChR.

RESULTS

Administration of rEV576 in passive transfer EAMG limited disease severity as evidenced by 100% survival rate and a low disease severity score. In active EAMG, rats with severe and mild EAMG were protected from worsening of disease and had limited weight loss. Serum complement activity (CH(50)) in severe and mild EAMG was reduced to undetectable levels during treatment, and C9 deposition at the neuromuscular junction was reduced. Treatment with rEV576 resulted in reduction of toxicity of serum from severe and mild EAMG rats. Levels of total AChR IgG, and IgG(2a) antibodies were similar, but unexpectedly, the concentration of complement fixing IgG(1) antibodies was lower in a group of rEV576-treated animals, suggesting an effect of rEV576 on cellular immunity.

INTERPRETATION

Inhibition of complement significantly reduced weakness in two models of EAMG. C5 inhibition could prove to be of significant therapeutic value in human myasthenia gravis.

Extraocular muscle susceptibility to myasthenia gravis: unique immunological environment?

Extraocular muscle (EOM) is susceptible to neuromuscular junction disorders, in particular, myasthenia gravis (MG). While EOM physiological characteristics and the ocular motor system requirements contribute to the propensity of ocular motor deficits observed among patients with MG, the authors propose that EOM have immunological features that place the muscles at risk for immune attack. Genomic profiling studies have demonstrated that genes associated with the immune response are differentially expressed in EOM, with particular differences in both classical and alternative complement-mediated immune response pathways. Intrinsic complement regulators are expressed at lower levels at rodent EOM neuromuscular junctions, which would put them at risk for the complement-mediated injury that occurs in MG. In fact, systemic C inhibition in experimental autoimmune MG (EAMG) induced by administration of acetylcholine receptor (AChR) antibodies or immunization with AChR will eliminate complement deposition at junctions of other skeletal muscle, but not EOM. Also, EOM junctions have greater injury in active and passive EAMG by several measures, suggesting that the lack of complement inhibition puts the EOM at risk. Among ocular myasthenia patients, serum AChR antibody levels are low, which would support the concept that EOM junctions are more susceptible to antibody injury than are other junctions. These observations suggest that complement inhibitory therapies may prove to be particularly effective in treatment of ocular myasthenia.