Specificities of antibodies to acetylcholine receptors

Towards antigen-specific apheresis of pathogenic autoantibodies as a further step in the treatment of myasthenia gravis by plasmapheresis

Myasthenia gravis (MG), a prototypic antibody-mediated autoimmune disease, presents an excellent target for scientific research aimed at a better understanding of the disease itself and the source that triggers an autoimmune reaction in an organism. MG is a neuromuscular disease caused mainly by an autoimmune response against the nicotinic acetylcholine receptor (AChR) which interferes with neuromuscular transmission. This review focuses on our studies on the extracellular domains of human muscle AChR subunits in an effort to develop an approach for the specific therapeutic apheresis of autoantibodies from patients' sera using the immobilized subunits as immunoadsorbents. The ability of the anti-AChR antibodies isolated by this technique, but not of the depleted sera, to induce disease is also described. This review is dedicated to the late Prof. John Newsom-Davis, who was the first to introduce the use of plasmapheresis for MG.

Structure of the acetylcholine receptor and specificities of antibodies to it in myasthenia gravis

Acetylcholine receptors in skeletal muscle and fish electric organs are intrinsic membrane proteins whose function is to bind acetylcholine released from the nerve ending and trigger the opening of a cation-specific channel in the postsynaptic membrane, thereby facilitating transmission of the nerve signal to the muscle. Investigations from several laboratories indicate that acetylcholine receptors from fish electric organs are composed of four homologous glycoprotein subunits of apparent relative molecular masses (Mr) approximating 40, 50, 57 and 64 x 10(3) designated, respectively, alpha, beta, gamma and delta. These subunits are present in receptor monomers in the mole ratio alpha 2 beta gamma delta. Receptor purified from skeletal muscle appears to have a similar structure. The alpha subunits are unknown. It is known that the cation channel regulated by acetylcholine binding is located within the receptor monomer. Experimental autoimmune myasthenia gravis (EAMG) is induced by immunizing animals with purified receptor. The mechanisms by which neuromuscular transmission is impaired in this model are very similar to those in myasthenia gravis (MG). Although there are many immunogenic determinants on receptors, and EAMG can be induced in rats by any of the denatured subunits, there is a main immunogenic region at which most of the antibodies to native receptors are directed. The main immunogenic region is a conformationally dependent part of the external surface of alpha subunits other than the acetylcholine-binding site or the attached carbohydrate. Antisera from MG patients are also directed primarily at this region. No correlation was detected between the specificities of antibodies to receptor in patients' sera and the severity of their weakness.

Myasthenia gravis and its animal model: T cell receptor expression in an antibody mediated autoimmune disease

Myasthenia gravis (MG) is a prototypic antibody-mediated autoimmune disease. Since the primary target antigen of the autoimmune response is known and a well-characterized animal model is available, MG is often considered an excellent situation for the application of novel specific immunotherapies, many of which are directed at T lymphocytes. CD4+ helper T cells are required for the development of the animal model, experimental autoimmune MG (EAMG). Even though the target antigen, acetylcholine receptor (AChR) is immunologically complex, the T cell response to AChR in mice is dominated by recognition of a single peptide by about 50% of the T cells. These T cells, in turn, utilize a restricted set of TCR gene elements and conserved CDR3 regions. While specific therapy directed at the immunodominant T cells is capable of reducing the magnitude of the anti-AChR response, considerable flexibility is apparent and reveals the ability of additional T cells to provide the requisite B cell help. In human MG patients, AChR-specific T cells have been identified but in many studies the frequencies were surprisingly low. In a very few cases, AChR-specific T cells have been cloned from MG patients. Analysis reveals heterogeneity in epitope recognition and MHC restriction. Little information on TCR structure is available. Our own studies using antigen-specific as well as non-specific methods for examining clonal T cell expansions in MG have led to an alternative hypothesis concerning T-B collaboration in MG.

Acquired myasthenia gravis: what we have learned from experimental and spontaneous animal models

Acquired myasthenia gravis (MG) is a disorder of neuromuscular transmission in which muscle weakness results from an autoantibody mediated depletion of acetylcholine receptors (AChR) at the neuromuscular junction. Experimental autoimmune myasthenia gravis, described in rodents and rabbits, has provided a good model of the effects of the autoimmune response against AChR and has shown that the specificities of the immune response in MG are those that would be obtained by immunization with native AChR. It has provided little information, however, about what initiates and sustains the immune response in MG. Acquired MG occurs spontaneously in dogs and may be the most common neuromuscular disorder that can be diagnosed in this species. As in human MG, an autoimmune response against AChR has been demonstrated and AChR autoantibodies have been implicated in the pathogenesis. The variability in clinical presentation, methods of diagnosis, and occurrence with other autoimmune diseases and neoplasia are identical to that of humans. Future studies of spontaneous canine autoimmune MG may provide clues to the determination of what factors initiate and sustain the autoimmune response to AChR, and in the study of specific suppression of the autoimmune response against AChR.

Altered tumor growth factor beta mRNA expression is associated with thymectomy-related clinical remission in myasthenia gravis

Clinical remission in myasthenia gravis after thymectomy and immune suppressive treatment may be reflected by changes of immunoregulatory cytokines. Mononuclear cells from paired blood samples from fifteen patients with myasthenia gravis before and after thymectomy and immunosuppressive therapy were examined by in situ hybridization for acetylcholine receptor-induced mRNA expression of the T helper type 1 proinflammatory cytokines interferon gamma (IFN-gamma), tumor necrosis factor alpha (TNF-alpha) and TNF-beta, the T helper type 2 cytokines interleukin 4 (IL-4) and IL-10 and the immune response downregulating cytokine tumor growth factor beta (TGF-beta). A significant change was observed for TGF-beta, with increased expression after treatment and induction of clinical remission. Augmentation of TGF-beta may play a role in the induction of remission in myasthenia gravis.

Acetylcholine receptor-reactive T cells in myasthenia gravis: evidence for the involvement of different subpopulations of T helper cells

Patients with myasthenia gravis have a high prevalence of acetylcholine receptor-specific T lymphocytes in the peripheral blood. Our earlier study shows that these T lymphocytes are stimulated to secrete interferon (IFN)-gamma and interleukin (IL)-2 in response to the autoantigen. Such stimulated T cells may be subdivided into different subsets according to the pattern of cytokine production. In the present study we have investigated the subpopulations of cells by analyzing their IL-4, IFN-gamma and IL-2 secretion pattern. Autoantigen-stimulated IL-4 secretion was found in 55% of the patients, IFN-gamma secretion in 86% and IL-2 secretion in 72%. T lymphocytes from all patients who responded with increased IL-2 secretion also showed increased IFN-gamma secretion. Stimulated IL-4 secretion was detected both in the presence and absence of stimulated IFN-gamma secretion. Depletion of monocytes/macrophages from peripheral blood mononuclear cell preparation and treatment of the cells with a mouse anti-human HLA-DR antibody abolished the secretion of IFN-gamma and IL-4. There were positive correlations between the numbers of IFN-gamma- and IL-2-secreting T cells and the numbers of B cells secreting antibodies against the acetylcholine receptor. Our results show that acetylcholine receptor-stimulated T lymphocytes secrete IL-4, IFN-gamma and/or IL-2. This T cell response is major histocompatibility complex (MHC) class II-restricted and monocyte/macrophage-dependent. Our study indicates that both Th1/Th2 or Th0 subpopulations of the T lymphocytes are involved in the autoimmune response in myasthenia gravis.

Detection of anti-acetylcholine receptor antibody by an ELISA using human receptor from a rhabdomyosarcoma cell line

A simple and reliable enzyme-linked immunosorbent assay (ELISA) has been developed for the measurement of anti-acetylcholine receptor (AChR) antibodies. The test utilizes a membrane-bound AChR obtained from a human rhabdomyosarcoma cell line (TE671) as antigen and employs an affinity-purified rabbit anti-human immunoglobulin G alkaline phosphatase-conjugated antibody as labelled antibody. To assess the sensitivity and the specificity of our assay we tested serum samples from 13 anti-AChR antibody-positive myasthenia gravis (MG) patients known to contain between 2 and 120 nmol/l of anti-AChR antibody, three anti-AChR antibody-negative MG patients, and 70 control subjects including patients with other neurological and autoimmune diseases. A panel of six different anti-AChR monoclonal antibodies and membranes from a AChR-negative rat adrenal pheochromocytoma cell line (PC 12) were also used in competitive studies. The test showed to be specific and able to detect as low as 2.0 nmol/l of anti-AChR antibodies. Moreover, we found a good correspondence between anti-AChR antibody levels measured in the serum samples tested by our assay and levels measured by the routinely adopted radioimmuno assay (RIA) using human-AChR (r = 0.96). Cross-reaction phenomena were observed only using serum samples containing high-titer anti-DNA antibodies. The proposed ELISA, circumventing the limitation of the commonly used RIA (radioactivity and amputated legs as source of human antigen), can be considered as an useful screening test for the diagnosis of myasthenia gravis.

Ocular myasthenia gravis. A critical review of clinical and pathophysiological aspects

Myasthenia gravis (MG) is probably the best studied autoimmune disease caused by autoantibodies against the acetylcholine receptor (AChR) at the neuromuscular junction, subsequently leading to abnormal fatigability and weakness of skeletal muscle. Extraocular muscle weakness with droopy eyelids and double vision is present in about 90% of MG patients, being the initial complaint in about 50%. In approximately 20% of the patients the disease will always be confined to the extraocular muscles. The single most important diagnostic test is the detection of serum antibodies against AChR which is positive in 90% of patients with generalized MG, but only in 65% with purely ocular MG. Electromyographic studies and the Tensilon test are of diagnostic value in clear-cut cases, but may be equivocal in purely ocular myasthenia, especially the latter not rarely producing false-positive results. Treatment response to corticosteroids and anti-cholinesterase agents is satisfactory in many patients with ocular MG, however other immunosuppressive drugs may also be needed. Pathogenetically relevant steps of the underlying autoimmune process have been elucidated during the last few years; nevertheless a number of questions remain open, especially what starts off the autoimmune process, and why are eye muscles so frequently involved in MG?

Primary structure of the human muscle acetylcholine receptor. cDNA cloning of the gamma and epsilon subunits

cDNA sequences encompassing the full coding region for the human muscle acetylcholine receptor (AChR) epsilon and gamma subunits have been isolated. The deduced amino-acid sequences indicate that the mature epsilon subunit contains 473 amino acids and is preceded by a 20-amino-acid signal peptide. As predicted from genomic clones, the gamma subunit contains 495 amino acids preceded by a 22-amino-acid signal peptide. In common with the human alpha, beta, gamma and delta subunits the epsilon subunit is highly conserved between mammalian species. The epsilon subunit gene is not closely linked to the gamma and delta subunits on chromosome 2 but rather is located with the beta subunit on chromosome 17. Expression of the alpha-, beta-, gamma-, delta- and epsilon-subunit cRNAs in rabbit-reticulocyte lysates followed by analysis on SDS/PAGE show glycosylated proteins with apparent molecular masses of 44-60 kDa.

Myasthenia gravis: an autoimmune response against the acetylcholine receptor

Myasthenia gravis (MG) is an organ-specific autoimmune disease caused by an antibody-mediated assault on the muscle nicotinic acetylcholine receptor (AChR) at the neuromuscular junction. Binding of antibodies to the AChR leads to loss of functional AChRs and impairs the neuromuscular signal transmission, resulting in muscular weakness. Although a great deal of information on the immunopathological mechanisms involved in AChR destruction exists due to well-characterized animal models, it is not known which etiological factors determine the susceptibility for the disease. This review gives an overview of the literature on the AChR, MG and experimental models for this autoimmune disease.

Human muscle acetylcholine receptor reactive T and B lymphocytes in the peripheral blood of patients with myasthenia gravis

The prevalence of T and B cells reactive with the acetylcholine receptor (AChR) of human skeletal muscle was studied in 33 patients with myasthenia gravis (MG), 18 patients with other neurological diseases (OND) or autoimmune disorders (AD) and 27 age- and sex-matched healthy controls. T cell stimulation was estimated by enumerating cells secreting interferon (IFN)-gamma and interleukin (IL)-2 in response to the AChR, whereas B cell reactivity was estimated by enumerating cells secreting IgG antibodies binding to the AChR. AChR-reactive T cells were increased in the peripheral blood of patients with MG as compared to patients with OND, AD and healthy individuals. Of the patients with MG, 29/33 (87.7%) had numbers of IFN-gamma secreting cells higher than the mean +/- 2 SD of the mean of controls as compared to 4/18 (22.2%) of patients with OND or AD and 2/27 (7.4%) of the controls. The mean value of the numbers of AChR-reactive T cells in the patients with MG was 19.6/10(5) PBMC, corresponding to 1/5100 PBMC. Comparable results were obtained also for IL-2-secreting cells. Anti-AChR IgG antibody-secreting cells were detected in the blood of 30/33 (91%) of the patients with MG, 3/18 (16.7%) of the patients with OND or AD and 2/25 (8%) of the controls. The mean value of the antibody-secreting cells in MG was 11.7 cells/10(6) PBMC corresponding to 1/70,400 PBMC in the patients with MG, compared to a mean value of antibody-secreting cells in the patients with OND or AD of 0.33 and controls of 0.16 cells/10(6) PBMC.

Stimulation of human T cells by sparse antigens captured on immunomagnetic particles

The acetylcholine receptor (AChR) of muscle is the target of the pathogenic antibodies in the human autoimmune disease myasthenia gravis (MG). For studies on the autoreactive T cells presumed to be responsible, use of intact human autoantigen would be optimal, but it was thought to be prohibitively scarce. However, adsorption to the surface of immunomagnetic particles (Dynabeads) of intact AChR from whole muscle extracts or from affinity-purified preparations, using mouse anti-human AChR Mabs, largely overcomes this problem. Together with antigen presenting cells (APC), this bead-bound AChR has consistently and maximally stimulated an established MG T cell line (previously selected with recombinant human AChR alpha subunit) that recognises the 144-156 region of the human alpha sequence (Ong et al., 1991). For equivalent T cell stimulation, bead-bound AChR was at least 10(3) times more potent than soluble AChR or recombinant alpha subunit, and 10(6) times more potent than peptide 144-156, implying that antigen in this form is targetted very efficiently to APC and thus to T cells. Finally, we have obtained similar results with T cells specific for other antigens suggesting that this method may have wider applications.

T cells from normal and myasthenic individuals recognize the human acetylcholine receptor: heterogeneity of antigenic sites on the alpha-subunit

The alpha-subunit of the nicotinic acetylcholine receptor is the major target of the autoimmune response in myasthenia gravis. We investigated the proliferative response of T cells from patients with myasthenia gravis and healthy volunteers to recombinant polypeptides of the human acetylcholine receptor including the full-length alpha-subunit (alpha 1-437). T cells from 20 (71%) of 28 patients and 7 (37%) of 19 healthy volunteers responded in primary cultures. Subsequently, specific T-cell lines were established: CD4+, CD8-, UCHL1+, and major histocompatibility complex (MHC) class II-restricted. Using a set of fragments of the alpha-subunit, major antigenic sites could be localized on the extracellular, N-terminal part of the molecule as well as close to the C-terminus. The T-cell response was heterogeneous, both among different individuals and among T-cell lines from a single donor. These T cells did not cross-react with Torpedo acetylcholine receptor, which was previously used as a substitute for human muscle acetylcholine receptor, suggesting that the T cells had a bias for unique human sequences. A single antigenic fragment could be presented in the context of different MHC class II molecules, and different fragments could be presented in the context of the same MHC molecule. This supports earlier observations of considerable heterogeneity in dealing with acetylcholine receptor as an autoantigen on the level of both T cells and antigen-presenting cells. The data also demonstrate that acetylcholine receptor-specific T cells are present in the normal immune repertoire, and emphasize the role of immune regulation for maintaining a state of tolerance.

Steroids induce acetylcholine receptors on cultured human muscle: implications for myasthenia gravis

Antibodies to the acetylcholine receptor (AChR), which are diagnostic of the human autoimmune disease myasthenia gravis, block AChR function and increase the rate of AChR degradation leading to impaired neuromuscular transmission. Steroids are frequently used to alleviate symptoms of muscle fatigue and weakness in patients with myasthenia gravis because of their well-documented immunosuppressive effects. We show here that the steroid dexamethasone significantly increases total surface AChRs on cultured human muscle exposed to myasthenia gravis sera. Our results suggest that the clinical improvement observed in myasthenic patients treated with steroids is due not only to an effect on the immune system but also to a direct effect on muscle. We propose that the identification and development of pharmacologic agents that augment receptors and other proteins that are reduced by human genetic or autoimmune disease will have broad therapeutic applications.

A sensitive rosetting assay for detection of acetylcholine receptor antibodies using BC3H-1 cells: positive results in 'antibody-negative' myasthenia gravis

Antibodies to acetylcholine receptor (AChR) were measured in a group of patients with myasthenia gravis (MG), some of whom had previously been classified as 'antibody negative' using the standard anti-AChR radioimmunoassay (RIA). AChR antibodies were measured using the rosetting assay, a new detection method which utilizes protein A-coated red blood cells and live BC3H-1 cells, a murine cell line which expresses muscle nicotinic AChR. The results of the rosetting assay were compared with those obtained in the anti-AChR RIA. 76% of all myasthenic sera tested showed rosetting at titers higher than any of the control sera (from patients with non-myasthenic neurologic disease and normal individuals). Of the myasthenic patients previously classified as 'antibody negative' in the RIA using human AChR, 71% demonstrated positive rosetting. There was no correlation between the anti-AChR antibody titer obtained in the rosetting assay and that obtained in the RIA using either human or denervated rat AChR. The results suggest that the rosetting assay may measure a subpopulation of antibodies that differs from those detected in the RIA.

Fab fragments of monoclonal antibodies protect the human acetylcholine receptor against antigenic modulation caused by myasthenic sera

The human cell line TE671 produces large amounts of muscle nicotinic acetylcholine receptor (AChR). TE671 cells were used to determine the specificity of antibodies which can increase the internalization rate of AChR (antigenic modulation) and to test procedures for protecting AChR against this mechanism. The half-life of AChR both in the absence and the presence of anti-AChR antibodies was very similar to that of AChR on human muscle cell cultures. The relative contribution of different anti-AChR antibody fractions to the total antigenic modulation capacity of human myasthenic sera was investigated by competition experiments between Fab fragments of anti-AChR monoclonal antibodies (MoAbs) and intact antibodies (MoAb or myasthenic sera). Fab fragments, which do not induce antigenic modulation, were allowed to shield the corresponding regions of the AChR. Intact antibodies were subsequently added. It was found that protection of the main immunogenic region (MIR), but not of a region on the beta-subunit, essentially blocked the modulatory effect of the intact anti-MIR MoAbs, and approximately 80% of that of myasthenic sera. These data suggest that anti-MIR antibodies are mainly responsible for the loss of human AChR via antigenic modulation. Furthermore the observation that Fab fragments of anti-MIR MoAbs can efficiently protect AChR against antigenic modulation may have therapeutic implications.

Autoimmune T lymphocytes in myasthenia gravis. Determination of target epitopes using T lines and recombinant products of the mouse nicotinic acetylcholine receptor gene

Oligoclonal and cloned T lines from peripheral blood or thymuses of patients with myasthenia gravis (MG) were selected for reactivity against nicotinic acetylcholine receptors (AChR) from Torpedo california, or against a recombinant fusion peptide, X4, representing the extracellular portion of the mouse AChR alpha-chain. All cell lines expressed the CD4 membrane phenotype, and their antigen reactivity was blocked by antibodies against monomorphic HLA DR/DP determinants. Using a panel of fusion proteins of different, overlapping mouse AChR alpha-chain sequences, a major T cell epitope was localized between amino acid positions 85 and 142. This determinant was distinct from the humoral main immunogenic region, which has been identified on the sequence 61-76. The response pattern of uncloned T lines from three patients with different HLA haplotypes suggests, however, that in any one MG patient T lymphocytes may recognize more than one autoantigenic epitope on the AChR alpha-chain, and that the T lymphocyte response profiles vary among individual patients.

Role of the macrophage in the pathogenesis of experimental autoimmune myasthenia gravis

To clarify the role of Ia antigen positive macrophages which invade motor end-plates in the induction of the chronic phase, experimental autoimmune myasthenia gravis (EAMG) rats were injected intraperitoneally with silica dust on day 6 after immunization. Silica injection partially inhibited the invasion of motor end-plates with macrophages as compared with a saline injection. The titer of antibodies to the Narke acetylcholine receptor (AChR) in the chronic phase did not differ in either the saline or the silica groups, while the titer of antibodies to rat-AChR (non-crossreactive antibodies) was lower in the silica group than in the saline group. The silica group survived longer than the saline group. These results suggest that Ia antigen positive macrophages in the acute phase act as antigen-presenting cells and play an important role in the production of antibodies to self-AChR in the chronic phase.

Comparative biochemical and genetic characterization of clonally related human B-cell lines secreting pathogenic anti-Pr2 cold agglutinins

To study the biology of cold autoimmune hemolytic anemia, Epstein-Barr virus (EBV)-transformed B-cell clones were established from a patient with splenic lymphoma associated with immune hemolysis due to an anti-Pr2 cold autoantibody. Studies were performed comparing the cold autoantibody present in culture supernatants of these cell lines to the pathogenic cold autoantibodies present in the patient's plasma. Cytogenetic studies of splenic lymphocytes demonstrated an abnormal karyotype (51XX, +3, +9, +12, +13, +18). After EBV transformation, eight clones secreting IgM, kappa anti-Pr were isolated; each clone had the same abnormal karyotype as above. DNA isolated from the clones and spleen was analyzed by Southern blot hybridization with JH, C mu, and C kappa probes; identical gene rearrangements were seen in each case. Anti-Pr antibodies, isolated from culture supernatant and serum were compared by isoelectric focusing (IEF) and demonstrated similar banding patterns. Distinctive binding patterns, however, were observed in 2/8 clones, suggesting structural differences. Adsorption studies with red blood cells further showed that the observed IEF banding patterns were solely due to anti-Pr cold autoantibody. With a thin-layer chromatography method, the biochemical determinants recognized by the cold autoantibodies were defined as glycolipids containing Neu Ac alpha 2-3Gal beta 1-4Glc sequences. The data demonstrate that the autoantibodies of the EBV-transformed B-cell lines were similar to the pathogenic monoclonal serum autoantibody in both structure and specificity. These clonal cell lines may thus serve to further study the biology of human B-cell lymphomas with defined autoantibody specificity.

Are spontaneous anti-idiotypic antibodies against anti-acetylcholine receptor antibodies present in myasthenia gravis?

The presence of anti-acetylcholine receptor anti-idiotypic antibodies in sera from 102 myasthenia gravis patients and from 33 first-degree relatives was investigated by: (a) Enzyme linked immunosorbent assay (ELISA) using monoclonal antibodies raised against human acetylcholine receptor, (b) immunoprecipitation of 125I-monoclonal anti-acetylcholine receptor antibodies; (c) inhibition of anti-acetylcholine receptor monoclonal antibody binding to the receptor and/or (d) inhibition of autologous and heterologous anti-acetylcholine receptor antibody binding to the receptor. No clear evidence for the presence of abnormal levels of spontaneous anti-idiotypic antibodies to anti-acetylcholine receptor antibodies was found.

The human medulloblastoma cell line TE671 expresses a muscle-like acetylcholine receptor. Cloning of the alpha-subunit cDNA

Nicotinic acetylcholine receptors (AChRs) from muscle bind alpha-bungarotoxin (alpha Bgt) and are composed of four kinds of subunits, whereas AChRs from mammalian brains do not bind alpha Bgt and are composed of two kinds of subunits. alpha Bgt-binding proteins whose function is unknown are also found in brain. All these proteins belong to the same gene family. The human medulloblastoma cell line TE671 expresses a functional AChR which binds alpha Bgt. Surprisingly, the AChR of this neuron-derived cell line has electrophysiological, immunological and biochemical properties different from neuronal AChRs and very similar to muscle AChRs. The TE671 AChR binds alpha Bgt, but is different from alpha Bgt-binding proteins in brain. Here we show that TE671 expresses the alpha-subunit mRNA coding for the muscle AChR, thereby proving that TE671 expresses a muscle-type AChR that is not expressed in adult brain. The isolated cDNA clones should prove useful for expression of large amounts of human muscle-type AChR alpha-subunit protein for studies of the autoimmune response to muscle AChRs in human myasthenia gravis.

Monoclonal anti-acetylcholine receptor antibodies as probes for human acetylcholine-receptor in myasthenia gravis

Monoclonal antibodies have been shown to bind to five regions on human acetylcholine receptor, each probably consisting of a discrete epitope on the extracellular surface. Two of these regions are equivalent to the 'main immunogenic region', and the other three appear to be close to the a-Bungarotoxin binding sites. These antibodies have been used to probe differences in myasthenia gravis anti-acetylcholine receptor antibodies, to locate acetylcholine receptor in thymic tissue, and to look for naturally-occurring anti-idiotype antibodies. Anti-acetylcholine receptor antibody specificities differ between groups of patients defined by their age of onset, thymic pathology and HLA associations. Anti-AChR synthesised by the thymus in young onset patients has similar specificity to that found in the individual's serum, and may be stimulated by the presence of AChR on thymic myoid cells. However, myoid cells (defined by staining with anti-troponin and anti-myosin antibodies) do not appear to differ between control and myasthenia gravis patients and show no obvious involvement in an immunological reaction. There was no convincing evidence for the presence of anti-idiotype antibodies in myasthenia gravis patients.

Epitopes on human acetylcholine receptor defined by monoclonal antibodies and myasthenia gravis sera

The mouse monoclonal antibody (m.ab) binding sites on human acetylcholine receptor have been mapped by inhibition by F(ab')2 m.ab fragments, by competition with a rat m.ab against the main immunogenic region (anti-m.i.r), and by their ability to protect the a-Bungarotoxin (a-BuTx) binding sites from inhibition by a myasthenia gravis (MG) plasma. Two m.abs (C3 and D6) that bind to two distinct but overlapping regions on the AChR, were inhibited by the anti-m.i.r. m.ab M35. M.abs binding to three other regions protected the a-BuTx sites by up to 50%. In further inhibition assays these m.abs were used to define the binding sites for MG anti-AChR antibodies. There was considerable heterogeneity in the antigenic specificity of the MG sera. The inhibition of MG anti-AChR by anti-m.i.r. M35 correlated highly with inhibition by mouse m.ab D6, but not with inhibition by m.ab C3. There was a correlation between inhibition by m.abs F8 and B3, although these m.abs bind to two non-overlapping regions. Some MG anti-AChR antibodies bound to epitopes that only partially overlapped those defined by m.abs. Inhibition of human antibody binding by m.abs raised against specific antigens is a useful approach that should help define the epitopes to which autoantibodies bind.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

Human T-helper lymphocytes in myasthenia gravis recognize the nicotinic receptor alpha subunit

Myasthenia gravis is a human disease caused by an autoimmune response against the nicotinic acetylcholine receptor (AcChoR). Since the molecular structure of AcChoR is well known, myasthenia gravis is an excellent system for studying the recognition of a complex membrane antigen in the human immune system. Human T-helper (TH) cell lines reactive to the AcChoR were isolated from four myasthenic patients by selection with native AcChoR from Torpedo californica. The selected TH cells could efficiently recognize native and fully denatured AcChoR. The vast majority of the TH-stimulating AcChoR epitopes were located on the denatured alpha subunit of AcChoR. Antibody competition experiments using a panel of rat anti-AcChoR monoclonal antibodies showed that 39-45% of the autoantibodies present in the sera of these same patients bound to the conformation-sensitive "main immunogenic region" (MIR), also located on the alpha subunit. However, AcChoR-induced stimulation of the T cells could not be inhibited with up to 20-fold molar excess of different rat anti-MIR monoclonal antibodies. These results suggest that the Torpedo AcChoR alpha subunit contains conformation-insensitive epitopes that play a role in the autosensitization of TH cells and that seem to be physically separated from the MIR. The specificity of the TH cell response may contribute to directing the B-cell response to other alpha-subunit determinants, such as the MIR itself.

Acetylcholine receptor antibodies in myasthenia gravis: use of a qualitative assay for diagnostic purposes

We have modified the techniques of Lindstrom and of Tindall to measure serum acetylcholine receptor antibody using human antigen bound to 125I-alpha Bungarotoxin. By using 10 microliters of serum and precipitating antigen-antibody complexes with an excess of staph A, we found that only one out of 43 patients with clinically diagnosed active generalized Myasthenia Gravis had no antibodies. In pooling these results with the results of tests done for diagnostic purposes we found positive results in 54/55 generalized active MG, 8/21 MG in remission, 16/37 ocular MG and 0/55 healthy controls. Two out of 38 non MG were also positive and their clinical diagnosis of botulism and penicillamine treated rheumatoid arthritis have been confirmed by a one year follow-up. Most of these sera were also tested for reactivity with fetal calf AchR. Six out of 49 samples positive with the human receptor were negative with calf receptor. We conclude that our technique is extremely useful for the diagnosis of Myasthenia Gravis and that fetal calf antigen cannot replace human antigen in the assay.

Antibody heterogeneity and specificity in myasthenia gravis

Anti-AChR is heterogeneous within individuals and between individuals. Anti-AChR idiotypes are not shared to any large extent. Ten monoclonal antibodies raised against human AChR: (a) bind to five partially overlapping regions; (b) are not idiotypically identical even within a region; (c) do not all bind to the main immunogenic region; (d) four distinguish between normal and denervated human AChR; (e) can be used to define the antigenic determinants in MG. Antigenic specificities vary in different clinical groups. Antigenic specificities can change during the course of the disease, but some remain relatively constant. Thymus cultures make antibodies with the same specificity as those present in the serum of the individual. All monoclonal antibodies bind to myoid cells of normal and MG thymus. We find no convincing evidence of naturally occurring antiidiotype antibodies in MG sera.

Behavioral analysis of opiate-mediated inhibition in the early chick embryo

Morphine (opiate agonist) produced a dose-dependent decrease in the spontaneous motility of 5- and 9-day chick embryos. Naloxone (opiate antagonist) appeared to reverse competitively the inhibition of motility caused by morphine. The effects of morphine on spontaneous motility in 5-day embryos were also reversed stereospecifically by the opiate antagonist pairs WIN 44441-3/WIN 44441-2 and levallorphan/dextrallorphan. Levorphanol (opiate agonist) also produced a dose-dependent decrease in the motility of 5-day embryos while its inactive (+)-isomer, dextrophan, was not effective. Etorphine (opiate agonist) was more than 1000-fold more effective than morphine in inhibiting the motility of 5-day embryos. The effectiveness of several opiate agonists and antagonists on the spontaneous motility of 5-day embryos was similar to their effectiveness in radioligand-binding studies on isolated membrane receptors from either adult mammalian brain or ileum. Levorphanol was more effective than dextrophan and etorphine was substantially more effective than morphine in decreasing the spontaneous motility of 4-day embryos. WIN 44441-3 was more effective than WIN 44441-2 in reversing the inhibition of motility in 4-day embryos caused by morphine. Morphine inhibited spontaneous hind-limb motility in both thoracic spinal and sham-operated 7-day embryos; the inhibition of motility caused by morphine was reversed by WIN 44441-3 in both thoracic spinal and sham-operated 7-day embryos. [Leu5]enkephalin-like immunoreactivity in the lumbar spinal cord was concentrated in the superficial laminae of the dorsal horn and along the midline rostral to the central canal. A lesser concentration of immunoreactive processes occurred in the medial and lateral motor columns where labelled varicosities appeared to contact motoneurons. Opiate receptors appear to be present at least as early as day 5 (and perhaps as early as day 4) in the chick embryo. Opiate receptors appear to be present in the lumbar spinal cord of the chick embryo at least as early as day 7. The structural requirements for ligand binding to opiate receptors in the 5-day chick embryo are similar to the requirements for ligand binding to opiate receptors in the adult.

Complement activation in myasthenia gravis plasma by Torpedo acetylcholine receptor liposomes

The purpose of this study was to determine if antibodies from patients with myasthenia gravis (MG) react in a biologically relevant manner with membrane bound Torpedo californica acetylcholine receptor (AChR). Antibody-mediated complement activation was measured in plasma from 15 patients with MG and 9 controls following incubation with Torpedo AChR-rich membrane vesicles (AChR liposomes). Significantly more (p less than 0.001) complement activation occurred in the patient samples compared with the controls. Torpedo AChR liposomes are well characterized and have been widely used to study the structure and function of AChR. These results suggest that Torpedo AChR liposomes may also be used in immunologic studies with autoantibodies from patients with MG.

Characterization of effects of anti-beta and anti-beta' monoclonal antibodies on the activity of the RNA polymerase from Escherichia coli

Monoclonal antibodies directed against antigenic determinants on the beta and beta' subunits of the Escherichia coli RNA polymerase were characterized by using d(A-T)n-directed transcription assays. Antibodies were prepared by using purified subunits as immunogens, and seven anti-beta and five anti-beta' monoclonal antibodies were generated. Inhibitory anti-beta monoclonal antibodies were found to affect RNA polymerase during synthesis of r(A-U)n, abortive initiation of pApU and UpApU, and elongation by preformed ternary complexes. A comparative enzyme study of r(A-U)n synthesis showed the core polymerase to be more sensitive to inhibition by the anti-beta monoclonal antibody than was the holoenzyme. In contrast, the inhibition effected by the anti-beta' monoclonal antibody was found to be 90% or greater for each of the d(A-T)n-directed assays used. The different inhibitory patterns exhibited by the anti-beta and anti-beta' monoclonal antibodies suggest that the beta and beta' subunits engage in different roles during transcription. Kinetic analysis of the abortive initiation reaction in the presence and absence of the inhibitory antibodies resulted in distinctive but complex modes of inhibition. Inhibition by the anti-beta monoclonal antibody 210E8 was noncompetitive with regard to UTP and competitive for UpA incorporation; at increased UpA concentration, the inhibition was completely reversed. Inhibition of the abortive synthesis of UpApU by the anti-beta' monoclonal antibody 311G2 was noncompetitive with regard to both UpA and UTP incorporation. When the preformed ternary elongation complex was used, inhibition by the anti-beta monoclonal antibody was mixed with regard to the ribonucleoside triphosphate substrates.(ABSTRACT TRUNCATED AT 250 WORDS)

A practical enzyme immunoassay for anti-acetylcholine receptor antibodies in myasthenia gravis

A practical enzyme immunoassay (EIA) has been developed for the measurement of anti-acetylcholine receptor (AChR) antibodies in sera from patients with myasthenia gravis (MG). This system is based on the double antibody technique, using denervated rat muscle AChR labeled with horseradish peroxidase-linked alpha-bungarotoxin (HRP-alpha BGT). This method has the following advantages compared to conventional radioimmunoassay (RIA): (1) HRP-alpha BGT is more stable than [125I]alpha BGT and can be used for at least one year without any loss of the binding activity to AChR and enzymatic activity, (2) the procedure avoids the use of radioactive isotopes, and (3) the equipment for our EIA is more economical than that for RIA.

Physiochemical and immunological properties of acetylcholine receptors from human muscle

The acetylcholine receptor protein from human muscle was extracted with the non-ionic detergent Triton X-100 and purified by affinity chromatography on alpha-Naja toxin sepharose 4B. Further purification on Dicap-MP sepharose 4B, a choline analog compound, led to ACHR preparations with specific activities of 2-7 nmol/mg protein. The isolated receptor, labeled with 125I-alpha-bungarotoxin was characterized by different methods and compared to ACHRs from Torpedo californica electroplax and rat-denervated skeletal muscle. Gel filtration on Ultrogel AcA 34 resulted in a stokes radius of 70 A for the receptor monomer and 99 A for the dimeric form. Sucrose density gradient centrifugation showed sedimentation coefficients of 9.1 S and 13.5 S. From these data the molecular weight of the ACHR monomer was estimated as 254 000 D and 540 000 D for the receptor dimer. The isoelectric point of the 125I-alpha-bgt-ACHR complex was determined by thin-layer isoelectric focussing to be pH 5. Purified ACHRs were used for immunization of rats and mice which developed an EAMG as verified by clinical observation and electrophysical measurements. Sera from the immunized animals as well as from myasthenia gravis patients were subsequently used to compare the cross-reactivity of ACHR preparations from different sources. While antibodies of rats immunized with Torpedo ACHRs cross-reacted with ACHR preparations from rat and human skeletal muscle, antibodies from mice immunized with rat ACHR only reacted with preparations from rats and mice. Antibodies from mice immunized with ACHR of human origin exhibited a broad cross-reactivity, as did antibodies from MG patients.

[Heterogeneity of acetylcholine receptor antibodies in patients with myasthenia gravis]

Antibodies against the nicotinic acetylcholine receptor in sera of 21 myasthenia gravis patients were checked for their ability to block or split binding of alpha-bungarotoxin to the human acetylcholine receptor. Affinity-purified acetylcholine receptors from human skeletal muscle were used in parallel in the common precipitation assay and an inhibition assay. Cross-reactivity of acetylcholine receptor antibodies was analyzed with receptor preparation from different species (calf, rat, Torpedo c. and Electrophorus e.), purified identically to high specific activity. An antibody pattern was set up for each patient and related to the clinical state of the disease. alpha-Bungarotoxin-inhibiting antibodies were demonstrable in 74% of myasthenia gravis patients, alpha-bgt displacing antibodies were found in 39% of the investigated sera. Broad cross-reactivity with acetylcholine receptors from other mammalian muscle was evident (calf 75%, rat 90%) only very few sera reacted with acetylcholine receptors from electric fish (Torpedo c. 14%, Electrophorus e. 38%). Antibody concentrations determined by using xenoantigens were much lower than those obtained by human acetylcholine receptor. The lack of a clear-cut correlation between the amount of serum antibodies and the clinical state of myasthenia gravis can be explained by the established antibody-heterogeneity, shown by a constant antibody pattern characteristic for each patient. However, between this specific antibody pattern and the state of the disease no correlation could be established either.

Immunisation with Torpedo acetylcholine receptor

Acetylcholine mediates the transfer of information between neurons in the electric organ of, for example, Torpedo as well as in vertebrate skeletal muscle. The nicotinic acetylcholine receptor complex translates the binding of acetylcholine into ion permeability changes. This leads to an action potential in the muscle fibre. The nicotinic acetylcholine receptor protein has been purified from Torpedo by use of affinity chromatography. The receptor is an intrinsic membrane glycoprotein composed of five polypeptide chains. When various animals are immunised with the receptor they demonstrate clinical signs of severe muscle weakness coincident with high antibody titres in their sera. The symptoms resemble those found in the autoimmune neuromuscular disease myasthenia gravis in humans. This animal model has constituted a unique model for studying autoimmune diseases. This paper reviews some of the work using Torpedo acetylcholine receptor in order to increase the understanding of the motor nervous system function and myasthenia gravis. It is now known that the nicotinic acetylcholine receptor protein is the antigen involved in myasthenia gravis. The mechanism of immune damage involves a direct block of the receptor function. This depends on the presence of antibodies which crosslink the postsynaptic receptors leading to their degradation. The questions to be answered in the future are; (a) what initiates or triggers the autoimmune response, (b) how do the antibodies cause the symptoms--is there a steric hindrance of the interaction of acetylcholine and the receptor, (c) why is there not a strict relationship between antibody titre and severity of symptoms, and (d) why are some muscles affected and other spared? With help of the experimental model, answers to these questions may result in improved strategies for the treatment of the autoimmune disease myasthenia gravis.

Lack of cross-reactivity of a monoclonal antibody against the neurotransmitter site of Torpedo nicotinic acetylcholine receptor with muscle receptors from several sources

A monoclonal antibody raised against Torpedo nicotinic acetylcholine receptor has been used to study the neurotransmitter binding site of the acetylcholine receptor from several sources. When tested on Torpedo receptor, this monoclonal antibody inhibited binding of alpha-bungarotoxin to 50% of the available sites. The failure to completely inhibit binding of the toxin is attributed to the orientation of the determinant for the monoclonal antibody on the receptor molecule. This monoclonal antibody bound very poorly to the acetylcholine receptor from muscle tissues. This suggests either a modification or a reduced accessibility of the determinant to the monoclonal antibody.

Specific involvement of peripheral T lymphocytes against acetylcholine receptors in myasthenia gravis

A specific sensitisation of blood T lymphocytes against the nicotinic acetylcholine receptor has been demonstrated in myasthenic patients. After thymectomy the T lymphocytes response was quickly decreased and in most patients it reached normal values. The T response remained at low or normal levels for at least two years. In patients who had undergone thymectomy three or more years before, a T lymphocyte response against acetylcholine receptors was frequently detected.

Acetylcholine receptor turnover in mice with passively transferred myasthenia gravis. I. Receptor degradation

The in vivo degradation of endplate acetylcholine receptors was investigated in mice treated daily with IgG from 10 myasthenia gravis patients. Four preparations increased the rate of degradation. The number of endplate acetylcholine receptors after 7 days of IgG treatment was greater than that predicted on the basis of the increased degradation rate, suggesting a compensatory increase in receptor synthesis.

Immunology of myasthenia gravis

Anti-acetylcholine-receptor antibody is demonstrable in more than 90 per cent of patients with myasthenia gravis. Serum antibody titers do not show a direct correlation with disease severity, although in certain patients antibody levels increase in association with disease activity. Impairment of neuromuscular transmission results from the loss of junctional receptors, either as a result of receptor internalization or destruction of junctional folds containing the acetylcholine receptor. Myasthenia gravis manifests immunologic, genetic, and clinical similarities to rheumatic syndromes, suggesting a generic immune dysfunction common to these disorders.

Reduction of miniature end-plate potential amplitude in extraocular and limb muscles in an animal model of myasthenia gravis

Miniature end-plate potential (MEPP) amplitudes of the superior oblique extraocular muscle (EOM) and forelimb flexor digitorum longus muscle (FDL) of rats with experimental autoimmune myasthenia gravis (EAMG) were expressed as reductions from mean MEPP amplitudes of the respective muscles of pair-matched control animals. Postjunctional blockade of neuromuscular transmission at EOM end-plates was no greater than at limb muscle end-plates. In contrast, the amount of reduction measured in EAMG-affected FDLs (52.3%) was marginally, but significantly, greater than that in EOMs (38.3%). A weak correlation was present between the MEPP reduction observed in an animal's FDL and that recorded in its EOM.

Anti-acetylcholine receptor antibodies in myasthenia gravis. Part 1. Relation to clinical parameters in 250 patients

We examined the significance of the presence or absence of anti-acetylcholine receptor (anti-AChR) antibodies in 250 myasthenia gravis (MG) patients and the relation between clinical features and anti-AChR levels. We found high anti-AChR levels in 2 out of 11 thymoma patients without MG, while 37 out of 250 MG patients had no detectable anti-AChR. The absence of these antibodies was related to purely ocular disease and to steroid therapy and/or thymectomy. Differences in anti-AChR levels did not correspond significantly to differences in disease activity when single measurements in patients were analysed. However, the results were influenced by both the presence or absence of a thymoma, the age at onset of disease and by steroid therapy. The thymic pathology and age at onset seemed to act independently. Early onset of disease was associated with high anti-AChR levels and absence of antibodies to striated muscle (anti-SM), whereas late onset was associated with low anti-AChR and the presence of anti-SM. Thymomas both have high anti-AChR and high anti-SM. The effect of steroid therapy on antibody levels was seen in all patient groups but was strongest in thymoma patients with early onset of disease.

Immunopathology of acetylcholine receptors in myasthenia gravis

It is now clear that the muscular weakness and fatigability seen in MG result from an antibody-mediated immune response to AChR. The mechanisms by which antibodies impair transmission are moderately well understood and detection of antibodies in patient's sera is a reliable diagnostic test for the disease. The spectrum of antibody specificities produced in MG is also beginning to be understood, largely through the use of antibodies produced in the experimental model EAMG. Treatment for MG continues to rely heavily on the symptomatic relief afforded by acetylcholinesterase inhibitors. However, the recent recognition of the autoimmune nature of MG has led to increased emphasis on immunosuppression and antibody removal with some beneficial effects. Despite all that has been learned, the level of ignorance has just been pushed back one step--from the neuromuscular junction to the immune system. What initiates the immune response to AChR in MG and how to specifically suppress this aberrant response remain completely unknown.