Analysis of peripheral B cells and autoantibodies against the anti-nicotinic acetylcholine receptor derived from patients with myasthenia gravis using single-cell manipulation tools

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.

"Untargeting" autoantibodies using genome editing, a proof-of-concept study

Autoantibodies (AAbs) are useful biomarkers and many have direct pathogenic role. Current standard therapies for elimination of specific B/plasma-cell clones are not fully efficient. We apply CRISPR/Cas9 genome-editing to knockout V(D)J rearrangements that produce pathogenic AAbs in vitro. HEK293T cell-lines were established stably expressing a humanized anti-dsDNA Ab (clone 3H9) and a human-derived anti-nAChR-α1 Ab (clone B12L). For each clone, five CRISPR/Cas9 heavy-chain's CDR2/3-targeting guided-RNAs (T-gRNAs) were designed. Non-Target-gRNA (NT-gRNA) was control. After editing, levels of secreted Abs were evaluated, as well as 3H9 anti-dsDNA and B12L anti-AChR reactivities. T-gRNAs editing decreased expression of heavy-chain genes to ~50-60%, compared to >90% in NT-gRNA, although secreted Abs levels and reactivity to their respective antigens in T-gRNAs decreased ~90% and ~ 95% compared with NT-gRNA for 3H9 and B12L, respectively. Sequencing indicated indels at Cas9 cut-site, which could lead to codon jam, and consequently, knockout. Additionally, remaining secreted 3H9-Abs presented variable dsDNA reactivity among the five T-gRNA, suggesting the exact Cas9 cut-site and indels further interfere with antibody-antigen interaction. CRISPR/Cas9 genome-editing was very effective to knockout the Heavy-Chain-IgG genes, considerably affecting AAbs secretion and binding capacity, fostering application of this concept to in vivo models as a potential novel therapeutic approach for AAb-mediated diseases.

Molecular disease mechanisms of human antineuronal monoclonal autoantibodies

Autoantibodies targeting brain antigens can mediate a wide range of neurological symptoms ranging from epileptic seizures to psychosis to dementia. Although earlier experimental work indicated that autoantibodies can be directly pathogenic, detailed studies on disease mechanisms, biophysical autoantibody properties, and target interactions were hampered by the availability of human material and the paucity of monospecific disease-related autoantibodies. The emerging generation of patient-derived monoclonal autoantibodies (mAbs) provides a novel platform for the detailed characterization of immunobiology and autoantibody pathogenicity in vitro and in animal models. This Feature Review focuses on recent advances in mAb generation and discusses their potential as powerful scientific tools for high-resolution imaging, antigenic target identification, atomic-level structural analyses, and the development of antibody-selective immunotherapies.

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.

Non-canonical endogenous expression of voltage-gated sodium channel NaV1.7 subtype by the TE671 rhabdomyosarcoma cell line

Abstract

The human TE671 cell line was originally used as a model of medulloblastoma but has since been reassigned as rhabdomyosarcoma. Despite the characterised endogenous expression of voltage‐sensitive sodium currents in these cells, the specific voltage‐gated sodium channel (VGSC) subtype underlying these currents remains unknown. To profile the VGSC subtype in undifferentiated TE671 cells, endpoint and quantitative reverse transcription–PCR (qRT‐PCR), western blot and whole‐cell patch clamp electrophysiology were performed. qRT‐PCR profiling revealed that expression of the SCN9A gene was ∼215‐fold greater than the SCN4A gene and over 400‐fold greater than any of the other VGSC genes, while western blot confirmed that the dominant SCN9A RNA was translated to a protein with a molecular mass of ∼250 kDa. Elicited sodium currents had a mean amplitude of 2.6 ± 0.7 nA with activation and fast inactivation V₅₀ values of −31.9 ± 1.1 and −69.6 ± 1.0 mV, respectively. The currents were completely and reversibly blocked by tetrodotoxin at concentrations greater than 100 nm (IC₅₀ = 22.3 nm). They were also very susceptible to the Na_V1.7 specific blockers Huwentoxin‐IV and Protoxin‐II with IC₅₀ values of 14.6 nm and 0.8 nm, respectively, characteristic of those previously determined for Na_V1.7. Combined, the results revealed the non‐canonical and highly dominant expression of Na_V1.7 in the human TE671 rhabdomyosarcoma cell line. We show that the TE671 cell line is an easy to maintain and cost‐effective model for the study of Na_V1.7, a major target for the development of analgesic drugs and more generally for the study of pain.

Key points

Undifferentiated TE671 cells produce a voltage‐sensitive sodium current when depolarised.

The voltage‐gated sodium channel isoform expressed in undifferentiated TE671 cells was previously unknown.

Through qRT‐PCR, western blot and toxin pharmacology, it is shown that undifferentiated TE671 cells dominantly (>99.5%) express the Na_V1.7 isoform that is strongly associated with pain.

The TE671 cell line is, therefore, a very easy to maintain and cost‐effective model to study Na_V1.7‐targeting drugs.

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.

A Stable Cell Line Expressing Clustered AChR: A Novel Cell-Based Assay for Anti-AChR Antibody Detection in Myasthenia Gravis

Cell-based assays (CBAs) and radioimmunoprecipitation assay (RIPA) are the most sensitive methods for identifying anti-acetylcholine receptor (AChR) antibody in myasthenia gravis (MG). But CBAs are limited in clinical practice by transient transfection. We established a stable cell line (KL525) expressing clustered AChR by infecting HEK 293T cells with dual lentiviral vectors expressing the genes encoding the human AChR α1, β1, δ, ϵ and the clustering protein rapsyn. We verified the stable expression of human clustered AChR by immunofluorescence, immunoblotting, and real-time PCR. Fluorescence-activated cell sorting (FACS) was used to detect anti-AChR antibodies in 103 MG patients and 58 healthy individuals. The positive results of MG patients reported by the KL525 was 80.6% (83/103), 29.1% higher than the 51.4% (53/103) of RIPA. 58 healthy individuals tested by both the KL525 CBA and RIPA were all negative. In summary, the stable expression of clustered AChR in our cell line makes it highly sensitive and advantageous for broad clinical application in CBAs.

Immunopathology of Autoimmune Myasthenia Gravis: Implications for Improved Testing Algorithms and Treatment Strategies

Myasthenia gravis (MG) is a heterogeneous condition, characterized by autoantibodies (Abs) that target functionally important structures within neuromuscular junctions (NMJ), thus affecting nerve-to-muscle transmission. MG patients are more often now subgrouped based on the profile of serum autoantibodies, which segregate with clinical presentation, immunopathology, and their response to therapies. The serological testing plays an essential role in confirming MG diagnosis and guiding disease management, although a small percentage of MG patients remain negative for antibodies. With the advancements in new highly effective pathophysiologically-specific immunotherapeutic options, it has become increasingly important to identify the specific Abs responsible for the pathogenicity in individual MG patients. There are several new assays and protocols being developed for the improved detection of Abs in MG patients. This review focuses on the divergent immunopathological mechanisms in MG, and discusses their relevance to improved diagnostic and treatment. We propose a comprehensive "reflex testing," algorithm for the presence of MG autoantibodies, and foresee that in the near future, the convenience and specificity of novel assays will permit the clinicians to consider them into routine systematic testing, thus stimulating laboratories to make these tests available. Moreover, adopting treatment driven testing algorithms will be crucial to identify subgroups of patients potentially benefiting from novel immunotherapies for MG.

Emerging Strategies for Therapeutic Antibody Discovery from Human B Cells

Monoclonal antibodies from human sources are being increasingly recognized as valuable options in many therapeutic areas. These antibodies can show exquisite specificity and high potency while maintaining a desirable safety profile, having been matured and tolerized within human patients. However, the discovery of these antibodies presents important challenges, since the B cells encoding therapeutic antibodies can be rare in a typical blood draw and are short-lived ex vivo. Furthermore, the unique pairing of V_H and V_L domains in each B cell contributes to specificity and function; therefore, maintaining antibody chain pairing presents a throughput limitation. This work will review the various approaches aimed at addressing these challenges with an eye to next-generation methods for high-throughput discovery from the human B-cell repertoire.

Autoimmune Pathology in Myasthenia Gravis Disease Subtypes Is Governed by Divergent Mechanisms of Immunopathology

Myasthenia gravis (MG) is a prototypical autoantibody mediated disease. The autoantibodies in MG target structures within the neuromuscular junction (NMJ), thus affecting neuromuscular transmission. The major disease subtypes of autoimmune MG are defined by their antigenic target. The most common target of pathogenic autoantibodies in MG is the nicotinic acetylcholine receptor (AChR), followed by muscle-specific kinase (MuSK) and lipoprotein receptor-related protein 4 (LRP4). MG patients present with similar symptoms independent of the underlying subtype of disease, while the immunopathology is remarkably distinct. Here we highlight these distinct immune mechanisms that describe both the B cell- and autoantibody-mediated pathogenesis by comparing AChR and MuSK MG subtypes. In our discussion of the AChR subtype, we focus on the role of long-lived plasma cells in the production of pathogenic autoantibodies, the IgG1 subclass mediated pathology, and contributions of complement. The similarities underlying the immunopathology of AChR MG and neuromyelitis optica (NMO) are highlighted. In contrast, MuSK MG is caused by autoantibody production by short-lived plasmablasts. MuSK MG autoantibodies are mainly of the IgG4 subclass which can undergo Fab-arm exchange (FAE), a process unique to this subclass. In FAE IgG4, molecules can dissociate into two halves and recombine with other half IgG4 molecules resulting in bispecific antibodies. Similarities between MuSK MG and other IgG4-mediated autoimmune diseases, including pemphigus vulgaris (PV) and chronic inflammatory demyelinating polyneuropathy (CIDP), are highlighted. Finally, the immunological distinctions are emphasized through presentation of biological therapeutics that provide clinical benefit depending on the MG disease subtype.

Characterization of pathogenic monoclonal autoantibodies derived from muscle-specific kinase myasthenia gravis patients

Myasthenia gravis (MG) is a chronic autoimmune disorder characterized by muscle weakness and caused by pathogenic autoantibodies that bind to membrane proteins at the neuromuscular junction. Most patients have autoantibodies against the acetylcholine receptor (AChR), but a subset of patients have autoantibodies against muscle-specific tyrosine kinase (MuSK) instead. MuSK is an essential component of the pathway responsible for synaptic differentiation, which is activated by nerve-released agrin. Through binding MuSK, serum-derived autoantibodies inhibit agrin-induced MuSK autophosphorylation, impair clustering of AChRs, and block neuromuscular transmission. We sought to establish individual MuSK autoantibody clones so that the autoimmune mechanisms could be better understood. We isolated MuSK autoantibody-expressing B cells from 6 MuSK MG patients using a fluorescently tagged MuSK antigen multimer, then generated a panel of human monoclonal autoantibodies (mAbs) from these cells. Here we focused on 3 highly specific mAbs that bound quantitatively to MuSK in solution, to MuSK-expressing HEK cells, and at mouse neuromuscular junctions, where they colocalized with AChRs. These 3 IgG isotype mAbs (2 IgG4 and 1 IgG3 subclass) recognized the Ig-like domain 2 of MuSK. The mAbs inhibited AChR clustering, but intriguingly, they enhanced rather than inhibited MuSK phosphorylation, which suggests an alternative mechanism for inhibiting AChR clustering.

A fluorescent tetrameric antigen allows isolation of human myasthenia gravis monoclonal antibodies that interrupt acetylcholine receptor signaling.

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.

Antigen Extraction and B Cell Activation Enable Identification of Rare Membrane Antigen Specific Human B Cells

Determining antigen specificity is vital for understanding B cell biology and for producing human monoclonal antibodies. We describe here a powerful method for identifying B cells that recognize membrane antigens expressed on cells. The technique depends on two characteristics of the interaction between a B cell and an antigen-expressing cell: antigen-receptor-mediated extraction of antigen from the membrane of the target cell, and B cell activation. We developed the method using influenza hemagglutinin as a model viral membrane antigen, and tested it using acetylcholine receptor (AChR) as a model membrane autoantigen. The technique involves co-culturing B cells with adherent, bioorthogonally labeled cells expressing GFP-tagged antigen, and sorting GFP-capturing, newly activated B cells. Hemagglutinin-specific B cells isolated this way from vaccinated human donors expressed elevated CD20, CD27, CD71, and CD11c, and reduced CD21, and their secreted antibodies blocked hemagglutination and neutralized viral infection. Antibodies cloned from AChR-capturing B cells derived from patients with myasthenia gravis bound specifically to the receptor on cell membrane. The approach is sensitive enough to detect antigen-specific B cells at steady state, and can be adapted for any membrane antigen.

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.