The main immunogenic region of the acetylcholine receptor. Structure and role in myasthenia gravis

Myasthenia gravis-Pathophysiology, diagnosis, and treatment

Myasthenia gravis (MG) is an autoimmune disease characterized by dysfunction of the neuromuscular junction resulting in skeletal muscle weakness. It is equally prevalent in males and females, but debuts at a younger age in females and at an older age in males. Ptosis, diplopia, facial bulbar weakness, and limb weakness are the most common symptoms. MG can be classified based on the presence of serum autoantibodies. Acetylcholine receptor (AChR) antibodies are found in 80%-85% of patients, muscle-specific kinase (MuSK) antibodies in 5%-8%, and <1% may have low-density lipoprotein receptor-related protein 4 (Lrp4) antibodies. Approximately 10% of patients are seronegative for antibodies binding the known disease-related antigens. In patients with AChR MG, 10%-20% have a thymoma, which is usually detected at the onset of the disease. Important differences between clinical presentation, treatment responsiveness, and disease mechanisms have been observed between these different serologic MG classes. Besides the typical clinical features and serologic testing, the diagnosis can be established with additional tests, including repetitive nerve stimulation, single fiber EMG, and the ice pack test. Treatment options for MG consist of symptomatic treatment (such as pyridostigmine), immunosuppressive treatment, or thymectomy. Despite the treatment with symptomatic drugs, steroid-sparing immunosuppressants, intravenous immunoglobulins, plasmapheresis, and thymectomy, a large proportion of patients remain chronically dependent on corticosteroids (CS). In the past decade, the number of treatment options for MG has considerably increased. Advances in the understanding of the pathophysiology have led to new treatment options targeting B or T cells, the complement cascade, the neonatal Fc receptor or cytokines. In the future, these new treatments are likely to reduce the chronic use of CS, diminish side effects, and decrease the number of patients with refractory disease.

Current Biomarker Strategies in Autoimmune Neuromuscular Diseases

Inflammatory neuromuscular disorders encompass a diverse group of immune-mediated diseases with varying clinical manifestations and treatment responses. The identification of specific biomarkers has the potential to provide valuable insights into disease pathogenesis, aid in accurate diagnosis, predict disease course, and monitor treatment efficacy. However, the rarity and heterogeneity of these disorders pose significant challenges in the identification and implementation of reliable biomarkers. Here, we aim to provide a comprehensive review of biomarkers currently established in Guillain-Barré syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), myasthenia gravis (MG), and idiopathic inflammatory myopathy (IIM). It highlights the existing biomarkers in these disorders, including diagnostic, prognostic, predictive and monitoring biomarkers, while emphasizing the unmet need for additional specific biomarkers. The limitations and challenges associated with the current biomarkers are discussed, and the potential implications for disease management and personalized treatment strategies are explored. Collectively, biomarkers have the potential to improve the management of inflammatory neuromuscular disorders. However, novel strategies and further research are needed to establish clinically meaningful biomarkers.

Therapeutic potential of chimeric antigen receptor based therapies in autoimmune diseases

Chimeric antigen receptor (CAR) based therapies have been adopted as an option for treating autoimmune diseases from the field of blood malignancies by targeting immune cells or rebalancing the pro-inflammatory milieu. Important questions still remained about the efficacy and safety regarding the dynamic and complex autoimmune pathological networks. We here reviewed the emerged developments in basic, translational, and clinical studies of the CAR based therapies in a wide spectrum of autoimmune diseases. The primary goal of the study is to provide some future perspectives on how to optimize the performance of CAR based therapies. The fundamental strategy is to engineer the recognition domains in CAR products for precisely targeting the components in the pro-inflammatory milieu. The second strategy is to incorporate multiple CARs in one carrier, or use fluorescein isothiocyanate (FITC)-CAR T cells for enhancing the therapeutic efficacy. In addition, we reviewed the preclinical evidence in disease-specific context. Overall, we aim to attract more attention in the field of developing future precision CAR based therapies to tailor medial decisions in autoimmune diseases.

Expression of a highly antigenic and native-like folded extracellular domain of the human α1 subunit of muscle nicotinic acetylcholine receptor, suitable for use in antigen specific therapies for Myasthenia Gravis

We describe the expression of the extracellular domain of the human α1 nicotinic acetylcholine receptor (nAChR) in lepidopteran insect cells (i-α1-ECD) and its suitability for use in antigen-specific therapies for Myasthenia Gravis (MG). Compared to the previously expressed protein in P. pastoris (y-α1-ECD), i-α1-ECD had a 2-fold increased expression yield, bound anti-nAChR monoclonal antibodies and autoantibodies from MG patients two to several-fold more efficiently and resulted in a secondary structure closer to that of the crystal structure of mouse α1-ECD. Our results indicate that i-α1-ECD is an improved protein for use in antigen-specific MG therapeutic strategies.

Acetylcholine-binding protein in the hemolymph of the planorbid snail Biomphalaria glabrata is a pentagonal dodecahedron (60 subunits)

Nicotinic acetylcholine receptors (nAChR) play important neurophysiological roles and are of considerable medical relevance. They have been studied extensively, greatly facilitated by the gastropod acetylcholine-binding proteins (AChBP) which represent soluble structural and functional homologues of the ligand-binding domain of nAChR. All these proteins are ring-like pentamers. Here we report that AChBP exists in the hemolymph of the planorbid snail Biomphalaria glabrata (vector of the schistosomiasis parasite) as a regular pentagonal dodecahedron, 22 nm in diameter (12 pentamers, 60 active sites). We sequenced and recombinantly expressed two ∼25 kDa polypeptides (BgAChBP1 and BgAChBP2) with a specific active site, N-glycan site and disulfide bridge variation. We also provide the exon/intron structures. Recombinant BgAChBP1 formed pentamers and dodecahedra, recombinant BgAChBP2 formed pentamers and probably disulfide-bridged di-pentamers, but not dodecahedra. Three-dimensional electron cryo-microscopy (3D-EM) yielded a 3D reconstruction of the dodecahedron with a resolution of 6 Å. Homology models of the pentamers docked to the 6 Å structure revealed opportunities for chemical bonding at the inter-pentamer interfaces. Definition of the ligand-binding pocket and the gating C-loop in the 6 Å structure suggests that 3D-EM might lead to the identification of functional states in the BgAChBP dodecahedron.

In pursuit of the high-resolution structure of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAChR) has been studied extensively for well over four decades because of its important physiological roles and medical relevance. A large body of data from biochemical and biophysical studies are now available. The structural information, which is needed to integrate existing data to address the mechanism and function of nAChRs, started to emerge in recent years. Structural studies of acetylcholine binding proteins (AChBPs) have greatly facilitated the study of nAChRs. The recently determined crystal structures of the prokaryotic homologues of nAChRs will probably have similar impact over time. However, a direct structural model of nAChRs at high resolution will be important for mechanistic studies and drug development. Here we will review some of the recent efforts in this area and use the high-resolution structure of the extracellular domains of nAChR alpha1 to illustrate the potential insights one may gain at higher resolution.

Isolation and characterization of human anti-acetylcholine receptor monoclonal antibodies from transgenic mice expressing human immunoglobulin loci

The isolation of human antibodies against muscle acetylcholine receptor (AChR), the autoantigen involved in myasthenia gravis (MG), is important for the development of therapeutically useful reagents. Monovalent antibody fragments from monoclonal antibodies against the main immunogenic region (MIR) of AChR protect the receptor from the destructive activity of MG autoantibodies. Human anti-AChR alpha-subunit antibody fragments with therapeutic potential have been isolated using phage display antibody libraries. An alternative approach for obtaining human mAb has been provided by the development of humanized mice. In this report, we show that immunization of transgenic mouse strains with the extracellular domain of the human AChR alpha-subunit results in antibody responses and isolation of hybridomas producing human mAb. Four specific IgM mAb were isolated and analyzed. mAb170 recognized the native receptor the best and was capable of inducing AChR antigenic modulation, suggesting its specificity for a pathogenic epitope. Moreover, the recombinant antigen-binding (Fab) fragment of this mAb competed with an anti-MIR mAb, revealing that its antigenic determinant lies in or near the MIR. Finally, Fab170 was able to compete with MG autoantibodies and protect the AChR against antigenic modulation induced by MG sera. This approach will be useful for isolating additional mAb with therapeutic potential against the other AChR subunits.

Breakage of tolerance to hidden cytoplasmic epitopes of the acetylcholine receptor in experimental autoimmune myasthenia gravis

The acetylcholine receptor (AChR) is the major autoantigen in the antibody-mediated disease myasthenia gravis (MG) and its animal model experimental autoimmune myasthenia gravis (EAMG). This study demonstrates that rats immunized with a recombinant fragment corresponding to the normally exposed extracellular region of the rat AChR alpha-subunit first develop antibodies to the injected extracellular portion only, but later develop antibodies to intracellular cytoplasmic epitopes of AChR. The presence of autoantibodies to intracellular epitopes seems to be correlated with development of clinical signs of disease. We propose that a similar process of epitope spreading may take place in the natural course of myasthenia.

Prevention of passively transferred experimental autoimmune myasthenia gravis by a phage library-derived cyclic peptide

Many pathogenic antibodies in myasthenia gravis (MG) and its animal model, experimental autoimmune MG (EAMG), are directed against the main immunogenic region (MIR) of the acetylcholine receptor (AcChoR). These antibodies are highly conformation dependent; hence, linear peptides derived from native receptor sequences are poor candidates for their immunoneutralization. We employed a phage-epitope library to identify peptide-mimotopes capable of preventing the pathogenicity of the anti-MIR mAb 198. We identified a 15-mer peptide (PMTLPENYFSERPYH) that binds specifically to mAb 198 and inhibits its binding to AcChoR. A 10-fold increase in the affinity of this peptide was achieved by incorporating flanking amino acid residues from the coat protein as present in the original phage library. This extended peptide (AEPMTLPENYFSERPYHPPPP) was constrained by the addition of cysteine residues on both ends of the peptide, thus generating a cyclic peptide that inhibited the binding of mAb 198 to AcChoR with a potency that is three orders of magnitude higher when compared with the parent library peptide. This cyclic peptide inhibited the in vitro binding of mAb 198 to AcChoR and prevented the antigenic modulation of AcChoR caused by mAb 198 in human muscle cell cultures. The cyclic peptide also reacted with several other anti-MIR mAbs and the sera of EAMG rats. In addition, this peptide blocked the ability of mAb 198 to passively transfer EAMG in rats. Further derivatization of the cyclic peptide may aid in the design of suitable synthetic mimotopes for modulation of MG.

Pathogenic autoantibodies to neuronal proteins in neurological disorders

Autoantibodies to acetylcholine receptors and to voltage-gated calcium and potassium channels are thought to be pathogenic in three peripheral neurological disorders: myasthenia gravis, the Lambert Eaton syndrome and acquired neuromyotonia. However, evidence for the role of antibodies in conditions involving the central nervous system, is scanty or unclear. This review describes the ways in which the roles of autoantibodies have been defined in the peripheral diseases, and discusses the more controversial evidence for involvement of autoantibodies in some central disorders such as multiple sclerosis.

High affinity single-chain Fv antibody fragments protecting the human nicotinic acetylcholine receptor

Univalent antibody fragments directed against the main immunogenic region (MIR) of the human acetylcholine receptor (AChR) are capable of protecting the AChR against loss induced by antibodies from myasthenia gravis (MG) patients. Our aim was to construct single-chain Fv (scFv) antibody fragments as a first step towards the production of therapeutic protecting molecules, from two high-affinity anti-MIR monoclonal antibodies (mAb 192 and mAb 195). During the construction of scFv192 fragment, two light chains co-secreted from the hybridoma mAb192 were identified. N-terminal amino acid and cDNA sequence analysis showed that one of the two light chains corresponded to the antigen binding molecule while the other originated from the non-secreting myeloma S194/5.XXO.BU.1 which was used in the production of the hybridoma. Functional scFv 192 and 195 fragments were constructed, expressed in Escherichia coli and affinity purified. The binding affinities of scFv192 and scFv195 (K(D) = 0.6 and 0.8 nM for human AChR) were two orders of magnitude higher than that of the earlier constructed scFv198. The scFv192 almost completely protected human AChR against binding of intact anti-MIR mAbs. Human AChR was also very efficiently protected (74-85%) by the scFv192 against binding of autoantibodies from MG sera with high anti-alpha subunit antibody fractions. These scFvs are good candidates for protection of MG patients after appropriate genetic modifications.

The B-cell repertoire in myasthenia gravis includes all four acetylcholine receptor subunits

By enumerating cells secreting IgG antibodies of particular specificities using an enzyme-linked immunospot (ELISPOT) assay, the B-cell responses to Torpedo acetylcholine receptor (AChR) and its alpha-, beta-, gamma- and delta-subunits in peripheral blood from patients with myasthenia gravis (MG), and controls with other neurological diseases (OND) as well as healthy subjects were determined. Compared to controls, the patients with MG had elevated numbers of B cells secreting antibodies against AChR and its alpha-, beta-, gamma- and delta-subunits in peripheral blood in parallel. The mean numbers of anti-AChR antibody secreting cells were about 17 per 10(5) blood MNC, and for the subunits 10 to 15 in MG patients, compared to between 0.8 and 1.9 per 10(5) blood MNC in OND patients, and 0.1 to 0.3 in healthy controls. Such B cells detected in controls probably represent naturally occurring B cells responded to AChR and its subunits. The finding that most (60%) MG patients had B cells predominantly recognizing the alpha-subunit may be an indirect argument for the existence of a main immunogenic region (MIR). In the remaining 40% of patients with MG the predominant B-cell responses were directed to beta-, gamma- or delta-subunit. The data suggest that all four AChR subunits may function as strong immunogens in MG, though the alpha-subunit may be the major immune target in a substantial proportion of MG patients.

Role of the target organ in determining susceptibility to experimental autoimmune myasthenia gravis

Injection of anti-AChR antibodies in passive transfer experimental autoimmune myasthenia gravis (EAMG) results in increased degradation of acetylcholine receptor (AChR) and increased synthesis of AChR alpha-subunit mRNA. Passive transfer of anti-Main Immunogenic Region (MIR) mAb 35 in aged rats does not induce clinical signs of disease nor AChR loss. The expression of the AChR subunit genes was analyzed in susceptible and resistant rats. In aged EAMG resistant rats, no increase in the amount of AChR alpha-subunit mRNA was measured. In vivo AChR degradation experiments did not show any increase in AChR degradation rates in aged resistant rats, in contrast to young susceptible rats. Taken together, these data demonstrate that resistance of the AChR protein to antibody-mediated degradation is the primary mechanism that accounts for the resistance to passive transfer EAMG in aged rats.

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

The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

Antibodies affecting ion channel function in acquired neuromyotonia, in seropositive and seronegative myasthenia gravis, and in antibody-mediated arthrogryposis multiplex congenita

A new autoimmune disease affecting the neuromuscular junction has been defined. Acquired neuromyotonia is associated with antibodies to voltage-gated potassium channels that act, at least in part, by reducing potassium channel function with resulting neuronal hyperactivity. This condition is quite frequently associated with thymoma and, in many cases, antibodies to acetylcholine receptors are present as well as antibodies to VGKC. Improvements in techniques and the availability of cloned DNA and recombinant forms of the AChR subunits have led to new observations concerning the specificity and roles of antibodies in myasthenia gravis. The transfection of a cell line with the epsilon subunit means that we can now accurately compare antibodies reactive with adult and fetal human AChR. This may help to determine the relationship between AChR subunit expression in different tissues and the induction of antibodies that bind specifically to the two forms, as well as to clarify the role of antibodies to fetal or adult AChR in causing ocular muscle symptoms. Serum antibodies from a few mothers with obstetric histories of recurrent arthrogryposis multiplex congenita in their babies specifically inhibit the function of fetal AChR. These observations not only explain the cause of some cases of arthrogryposis multiplex congenita, but also suggest that other fetal-specific antibodies might be responsible for other fetal or neonatal conditions. An animal model has been established to enable us to investigate the role of maternal serum factors in causing such disorders. Seronegative MG has been the subject of many studies from our laboratory over the last ten years. The transience of the effects of SNMG plasmas on AChR function strongly suggests that the plasma antibodies do not bind directly to the AChR, but inhibit function by some indirect mechanism. They do not appear to act via the cAMP-dependent protein kinase pathway, and studies are in progress to investigate the involvement of other second messenger systems.

Aza-peptides. III. Experimental structural analysis of aza-alanine and aza-asparagine-containing peptides

To determine the structural perturbations induced by the C alpha H-->N alpha exchange in aza-peptides, we have examined by 1H NMR and IR spectroscopy various derivatives of the aza-analogues of alanine, aspartic acid and asparagine in different organic solvents with increasing polarity. Their general formulas are: R1-AzXaa-NR2R3, R1-Pro-AzXaa-NR2R3 and R1-AzXaa-Pro-NR2R3 (where AzXaa denotes the aza-analogue of the amino acid residue Xaa = Ala, Asp, Asn; R1 = Boc, Z; R2, R3 = H, Me, iPr). The aza-analogue of an amino acid residue appears to be a strong beta-turn-inducing motif, and the AzAsn carboxamide side-chain is capable of interacting, as a proton donor, with the preceding peptide carbonyl group.

Acetylcholine receptor expression in human extraocular muscles and their susceptibility to myasthenia gravis

In myasthenia gravis (MG), extraocular muscle (EOM) weakness is often an initial and persisting symptom. It has been proposed that acetylcholine receptor (AChR) from EOM is antigenically different from AChR of other innervated muscles and that the presence of antibodies to fetal AChR expressed in EOM causes their weakness. We have (1) studied mRNA expression for each of the AChR subunits (alpha, beta, gamma, delta, and epsilon) in human muscle, including EOM, and (2) compared the binding of sera from ocular myasthenia gravis (OMG) patients with fetal (alpha2 beta gamma delta) and adult (alpha2 beta epsilon delta) human AChRs. RNase protection assays showed that expression of the AChR gamma-subunit (fetal-type) mRNA in EOM was comparable with that in other innervated muscle types. By contrast, epsilon-subunit (adult-type) mRNA was expressed at much higher levels in EOM than in other muscles studied. Moreover, some OMG sera bound specifically to adult AChR. These results do not support the contention that susceptibility of EOM in MG results from expression of fetal AChR and indicate that the inclusion of antigen from a source rich in adult AChR in the MG diagnostic assay will increase the yield of positive results in OMG patients.

Functional and non-functional isoforms of the human muscle acetylcholine receptor

1. The properties of a recently identified isoform of the human muscle nicotinic acetylcholine receptor (AChR) alpha subunit (alpha +), which in muscle is expressed at similar levels to the alpha subunit, were investigated by both electrophysiological and biochemical approaches following expression in Xenopus laevis oocytes. The single-channel properties of adult (alpha 2 beta delta epsilon) and fetal (alpha 2 beta delta gamma) forms of the human AChR were also investigated. 2. The mean burst duration of adult channels (4.1 +/- 0.3 ms, mean +/- S.E.M., n = 5) is half that of fetal channels (7.9 +/- 0.6 ms, n = 4), while the single-channel conductance is larger (62.2 +/- 0.8 and 37.9 +/- 1.6 pS for adult and fetal channels, respectively), comparable to the developmental changes in single-channel properties observed for other mammalian species. 3. In contrast to the alpha isoform, the alpha + subunit does not bind 125I-labelled alpha-bungarotoxin or monoclonal antibodies directed against the AChR 'main immunogenic region' (MIR), illustrating why the alpha + subunit was first detected through screening of cDNA libraries. 4. By using site-directed mutagenesis to produce subunits that conferred different single-channel conductances on the AChR, we demonstrate that the alpha + isoform is not integrated into functional AChRs. 5. The mutagenesis experiments also revealed that the two alpha subunits within an AChR pentamer are not equivalent within the pore lining region.

Response to human acetylcholine receptor alpha 138-199: determinant spreading initiates autoimmunity to self-antigen in rabbits

NZW rabbits immunised with a mixture of synthetic peptides representing alpha 138-199 of the human acetylcholine receptor (AChR) alpha-subunit exhibited clinical, biochemical and electrophysiological signs of experimental autoimmune myasthenia gravis (EAMG), with raised levels of anti-rabbit AChR antibodies. Surprisingly, these were partly directed at the main immunogenic region (MIR, thought to be alpha 67-76) and alpha-Bungarotoxin binding sites on rabbit AChR, and reacted less well with human AChR. Moreover, they could be separated from the anti-peptide antibodies by fractionation on immobilised peptide. We conclude that immunisation with these peptides led, by 'determinant spreading', to a response directed at self-AChR. Similar phenomena may have been overlooked in previous studies of responses to synthetic or recombinant AChR sequences. These findings suggest that autoimmunity could be induced by low-affinity, cross-reacting epitopes even when the observed serum response is highly specific for the autoantigen.

Immunogenicity of human recombinant acetylcholine receptor alpha subunit: cytoplasmic epitopes dominate the antibody response in four mouse strains

In mysathenia gravis (MG) autoantibodies directed against acetylcholine receptors (AChR), at the neuromuscular junction lead to muscle weakness. These antibodies are directed against extracellular determinants, predominantly on the AChR alpha subunits. Similar antibodies can be induced in animals by immunisation with purified AChR, but immunisation of mice with recombinant human alpha subunit or its extracellular domain has produced conflicting results. To study further the immunogenicity of the human alpha subunit we immunised four inbred stains (C57B1/6, SJL, BALB/c, SWR) with almost full-length recombinant alpha subunit, r37-429, and looked at B cell epitopes by mapping with smaller recombinant fragments and synthetic peptides. The majority of anti-r37-429 antibodies bound to sequences within a region thought to be cytoplasmic, alpha 325-368, and reacted with human AChR. In two C57B1/6 sera, only, most antibodies were directed against an extracellular region, alpha 138-167, but the r37-429 used for immunisation of these two mice appeared to have lost the integrity of its cytoplasmic domain during preparation. Our results suggest that the antigenicity of the cytoplasmic region of the recombinant alpha subunit dominates the immune response in each of the four strains, and may even suppress the formation of antibodies to the extracellular domain. Moreover, although C57B1/6 and SJL mice were able to produce antibodies to alpha 138-167, these antibodies did not react with intact AChR, and none of the mice became weak.

B cell autoimmunity to acetylcholine receptor and its subunits in Lewis rats over the course of experimental autoimmune myasthenia gravis

Experimental autoimmune myasthenia gravis (EAMG) is induced by a single injection of acetylcholine receptor (AChR) with complete Freund's adjuvant and represents a useful animal model for studying the mechanisms by which autoimmune responses to AChR and its subunits are coupled to the development of human myasthenia gravis. Using an immunospot assay, we enumerated cells secreting IgG antibodies against Torpedo AChR and the alpha-, beta-, gamma- and delta-subunits of Torpedo AChR in lymph nodes, spleen and thymus from Lewis rats over the course of EAMG. Cells secreting IgG antibodies to AChR and to all four subunits were detected at higher numbers in the three immune organs in EAMG compared to controls. Numbers were highest in lymph nodes followed by spleen and thymus. Cells secreting IgG antibodies against native AChR were always higher than those against individual subunits. The immunogenicity between the four subunits did not differ, with the exception that the alpha-subunit induced a slightly higher B cell response in thymus and lymph nodes. The patterns of B cell responses were similar when analyzed over the course of EAMG from week 2 to week 5, and there was no restriction of the B cell repertoire early in EAMG. Anti-AChR and anti-subunit antibody-secreting cells were also detected in control animals immunized with adjuvant only, but at numbers which were much lower, and which were within the same level as numbers of cells secreting IgG antibodies to the control antigen myelin basic protein, probably reflecting naturally occurring autoimmune B cells.

T-cell immunity to acetylcholine receptor and its subunits in Lewis rats over the course of experimental autoimmune myasthenia gravis

Lymph nodes, spleen and thymus obtained from Lewis rats were examined over the course of experimental autoimmune myasthenia gravis (EAMG) for the distribution and the number of antigen-reactive CD4+ T helper cells which, upon recognition of Torpedo acetylcholine receptor (AChR) or the alpha, beta, gamma or delta subunits of Torpedo AChR, responded by secretion of interferon-gamma (IFN-gamma). T cells with these specificities were detected in these three immune organs. Numbers were highest in lymph nodes. In spleen and thymus, numbers of antigen-reactive T cells did not differ. T cells reacting against the intact AChR were more frequent than T cells recognizing any of the subunits. The immunogenicity between the four subunits did not differ, with the exception that the alpha subunit induced a slightly higher T-cell response. No restriction of the T-cell repertoire to the four subunits was detected during early compared to late phases of EAMG. The AChR and subunit-reactive T cells could--via secretion of effector molecules including IFN-gamma--play an important role in the initiation and perpetuation of EAMG, and consequently also of human myasthenia gravis. T cells with the same specificities were also detected in control animals injected with adjuvant only, but at much lower numbers which were within the range of T cells recognizing the control antigen myelin basic protein. They could represent naturally occurring autoimmune T cells.

Antigen-specific therapy of experimental myasthenia gravis with acetylcholine receptor-gelonin conjugates in vivo

Rats suffering from experimental autoimmune myasthenia gravis (EAMG) induced by previous immunization with foreign acetylcholine receptor (AChR) were treated with AChR-toxin conjugates using the plant toxin gelonin. This led to a marked improvement of clinical symptoms as well as a significant increase in functional AChR compared to untreated rats with EAMG as determined 6 to 10 weeks later. No therapeutic effect was observed after treatment with gelonin or AChR alone. The immune response to irrelevant control antigens was not altered by this treatment.

VH gene family utilization of anti-acetylcholine receptor antibodies in experimental autoimmune myasthenia gravis

The immunoglobulin heavy chain (VH) gene family usage in experimental autoimmune myasthenia gravis (EAMG) model was investigated by RNA slot blot hybridization using VH gene family specific probes. Anti-acetylcholine receptor (AChR) monoclonal antibodies (mAbs) isolated from susceptible C57BL/6 and resistant BALB/c mice were found to be encoded by VH genes from at least six different families. The Vgam3.8 family was overrepresented in alpha-bungarotoxin blocking mAbs. Expression of cross-reactive idiotypes by anti-AChR mAbs was irrespective of the VH gene family usage. Strain dependent differences in susceptibility for EAMG were not reflected in an aberrant VH gene family usage of anti-AChR mAbs.

Use of Torpedo-mouse hybrid acetylcholine receptors reveals immunodominance of the alpha subunit in myasthenia gravis antisera

The nicotinic acetylcholine receptor (AChR), a pentameric complex of alpha 2 beta gamma delta subunits, is the autoantigen in the human autoimmune disease myasthenia gravis (MG). Anti-AChR antibodies are found in approximately 90% of MG patients and using indirect methods (competitive binding to solubilized AChR), peptides, or synthetic peptides, the majority of these antibodies have been shown to bind to the AChR alpha subunit. In order to determine directly the AChR subunit specificities of MG antibodies, we employed as antigens a novel set of hybrid AChR composed of species cross-reacting and non-cross-reacting subunits stably expressed in fibroblasts. Sequence similarities of homologous subunits among species can vary widely, with mammalian subunits having 87%-96% identity and Torpedo-mammalian subunits having 54%-80% identity. These findings are reflected in antigenic specificities, with human anti-AChR antisera frequently recognizing mouse AChR but rarely recognizing Torpedo. By establishing separate cell lines stably expressing all-Torpedo, all-mouse, and different combinations of Torpedo and mouse subunits, we were able to provide the first direct evidence of a predominant anti-alpha subunit specificity in MG antisera. Functional hybrid AChR stably expressed in an intact cell membrane provide us with a system that best mimics the in vivo environment of the MG antibody in a binding assay. Such a system allows us to investigate a perplexing observation in the field: a poor correlation between the patient's clinical status and antibody titer. Those antibodies which can interfere with AChR function, such as ones with the ability to cross-link AChR and induce their accelerated internalization and degradation (antigenic modulation) might represent a subpopulation of MG antibodies important in disease induction or maintenance. In this report, we demonstrate that wild-type and hybrid AChR expressed in fibroblasts can be antigenically modulated by intermolecular cross-linking antibodies as AChR are in native muscle cells. Because we can monitor dynamic interactions between AChR and MG antibodies, this system may allow us to define crucial pathogenic epitopes in MG by expressing hybrid, chimeric, and mutant AChR.

Passive transfer of experimental myasthenia gravis via antigenic modulation of acetylcholine receptor

Antigenic modulation of acetylcholine receptor (AChR) is considered to contribute to the reduction of endplate AChR in myasthenia gravis (MG). Yet, the pathogenic significance of this mechanism is unclear. To investigate the in vivo role of AChR antigenic modulation we examined the ability of bivalent F(ab')2 and monovalent Fab fragments of monoclonal antibody (mAb) 35 to passively transfer experimental autoimmune MG (EAMG) in rats. mAb 35 which binds at the main immunogenic region (MIR) of the AChR causes severe EAMG without being involved in channel function. Compared to the intact mAb, F(ab')2 35 proved to be less potent but still capable of inducing moderate EAMG, whereas Fab 35 were totally ineffective. Furthermore, both intact and F(ab')2 35 induced mild EAMG in complement-depleted rats. These results (a) provide evidence that antigenic modulation of endplate AChR is sufficient to generate passive transfer of EAMG and (b) further support the pathogenic potential of the anti-MIR antibodies in MG.

Antibody-mediated neurological disease

The number of neurological disorders in which autoantibodies are thought to play a pathogenic role continues to increase although the strength of the evidence varies. Many of the disorders are tumour associated.