Immunological specificity and cross-reactivity of anti-acetylcholine receptor and anti-presynaptic membrane receptor antibodies in myasthenia gravis

Presynaptic membrane receptor in human brain

Myasthenia gravis (MG) is an autoimmune disease that results from antibody mediated damage of Acetylcholine receptor (AChR) at the neuromuscular junction. The autoimmune character of MG and pathogenic role of AChR antibodies have been established by several workers i.e., the demonstration of anti-AChR antibodies in about 90 % of MG patients. It has been demonstrated that patients with MG also have antibodies against a second protein named presynaptic membrane receptor (PsmR), which is identified by utilizing β-Bgtx, a ligand which binds to PsmR. Using β-Bgtx Sepharose 4B affinity matrix, the PsmR was purified from different regions of human cadaver brain by affinity chromatography. Purified receptor was characterized both by biochemical and immunological procedures. PsmR purified from different regions of the brain shows a specific activity of 0.37 ± 0.01, 0.39 ± 0.02 and 0.43 ± 0.005 nM/ μg of protein in Parietal lobe, Occipital lobe and Frontal lobe respectively. The affinity purified PsmR from the brain of 87 and 68 kd (parietal lobe, occipital lobe and frontal lobe) shows immunoreactivity with myasthenic sera. These findings suggest that PsmR from brain is another antigen against which autoantibodies are developed in Myasthenia gravis patients. Upon treatment with various enzymes we concluded that PsmR from brain is a glycoprotein in which the immunoreactivity resides in the carbohydrate as well as the peptide epitopes. In conclusion the PsmR is another antigen against which autoantibodies are formed in different regions of brain. These can be used as a diagnostic tool for detecting antibodies in the sera or cerebrospinal fluid of MG patients.

The auto-antigen repertoire in myasthenia gravis

Myasthenia Gravis (MG) is an antibody-mediated autoimmune disorder affecting the postsynaptic membrane of the neuromuscular junction (NMJ). MG is characterized by an impaired signal transmission between the motor neuron and the skeletal muscle cell, caused by auto-antibodies directed against NMJ proteins. The auto-antibodies target the nicotinic acetylcholine receptor (nAChR) in about 90% of MG patients. In approximately 5% of MG patients, the muscle specific kinase (MuSK) is the auto-antigen. In the remaining 5% of MG patients, however, antibodies against the nAChR or MuSK are not detectable (idiopathic MG, iMG). Although only the anti-nAChR and anti-MuSK auto-antibodies have been demonstrated to be pathogenic, several other antibodies recognizing self-antigens can also be found in MG patients. Various auto-antibodies associated with thymic abnormalities have been reported, as well as many non-MG-specific auto-antibodies. However, their contribution to the cause, pathology and severity of the disease is still poorly understood. Here, we comprehensively review the reported auto-antibodies in MG patients and discuss their role in the pathology of this autoimmune disease.

Myasthenia and neuroborreliosis with excessively high acetylcholine-receptor antibodies

In a 29-y-old male with neuroborreliosis, partially responsive to ceftriaxone, myasthenia gravis with acetylcholine-receptor antibodies elevated almost 1000 times the upper reference limit was diagnosed. Pyridostigmine resolved all remaining neurological deficits. During a 1-y follow-up the patient remained symptom free, despite persistently high acetylcholine-receptor antibodies. They were attributed to epitope homology of the acetylcholine receptors and Borrelia surface antigens.

beta-Bungarotoxin binding protein is immunogenic but lacks myasthenogenicity in rats

Myasthenia gravis is an autoimmune disease resulting from autoaggressive immunity to the nicotinic acetylcholine receptor (AChR) of the neuromuscular junction. Immunization with AChR leads to experimental autoimmune myasthenia gravis (EAMG). AChR binds to alpha-bungarotoxin, and this property is utilized to purify AChR. The utilization of beta-bungarotoxin (beta-BGT) allows the purification of a different protein of striated muscle. To examine the myasthenogenic activity of the beta-BGT binding protein, we immunized Lewis rats with the beta-BGT binding protein or AChR from Torpedo. While AChR-immunized rats developed typical signs of EAMG, beta-BGT binding protein immunized rats showed no definite clinical signs of muscular weakness during observation periods up to 14 weeks after immunization. High levels of anti-AChR IgG antibodies and low levels of antibodies against beta-BGT binding protein were present in the AChR-immunized rats, while high levels of IgG antibodies to the beta-BGT binding protein and low anti-AChR IgG antibody levels were found in the rats immunized with the beta-BGT binding protein. No relationship was observed between antibody levels and severity of EAMG. The anti-AChR and anti-beta-BGT binding protein IgG antibodies showed only low degrees of cross-reactivity. We conclude that the beta-BGT binding protein is immunogenic also in EAMG, while it is still an open question whether it also has myasthenogenic properties.

Interferon gamma- and interleukin-4-secreting cells in multiple sclerosis

Interleukin 4 (IL-4)- and interferon gamma (IFN gamma)-secreting peripheral blood cells were enumerated by immunospot assay in 13 multiple sclerosis (MS) patients during exacerbations, in 24 patients with progressive multiple sclerosis (CPMS), and in 20 controls. Cells that spontaneously secreted IFN gamma were significantly higher in MS patients experiencing an attack (P < 0.001) than in controls or in CPMS (P < 0.04). IL-4-secreting cell numbers were elevated significantly and to a comparable extent in both MS groups compared to controls. Our finding of increased numbers of IFN gamma-secreting cells is in keeping with prior work showing increased IFN gamma levels in the circulation prior to and during MS attacks and increased release of IFN gamma to the supernatant in bulk cultured blood cells from MS patients. What role an increase in IL-4-secreting cells might play in MS is unclear, but it could relate to immune system regulation. Following in vitro exposure to MBP, IFN gamma-secreting cell number rose above levels observed in the absence of stimulation in controls and in both MS groups with the rise in acutely exacerbating MS patients being significantly greater than in controls. Our results provide further evidence for reactivity to MBP in MS, the significance of which in terms of pathogenesis remains clouded.

Nicotinic neuronal acetylcholine receptor alpha-3 subunit transcription in normal and myasthenic thymus

Thymic transcription of the alpha-3 subunit of the AChR was studied through sequencing and PCR analysis of thymic cDNA clones, Northern blotting, and ribonuclease protection assays. This analysis revealed at least three, 3' end sequence variants for the alpha-3 subunit as well as a variant that results from the alternative splicing of an antisense 122 bp Alu sequence between exons 5 and 6 of the normal transcript. The spliced Alu sequence not only shifts the exon 6 reading frame but also carries an in-frame stop codon. If translated, this variant transcript would produce a truncated peptide lacking the fourth transmembrane domain of the subunit and carrying a carboxy terminus dodecapeptide not found in any other known AChR subunit sequence. The putative variant subunit may lack biological activity and should differ antigenically from its normal counterpart. In comparing the normal, the MG hypertrophic, and the MG thymoma for transcription of the alpha-3 subunit and its 122 bp variant, it was found that there were no qualitative or quantitative changes in alpha-3 transcript expression in the MG hypertrophic thymi. Thymomas, however, showed an overall decrease in alpha-3 transcription and a comparative increase in beta-amyloid precursor transcription. The decrease in the levels of alpha-3 transcription in thymomas may be related to the proliferation of thymic epithelial cells.

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.

Antibody-secreting cells to acetylcholine receptor and to presynaptic membrane receptor in seronegative myasthenia gravis

Peripheral blood and bone marrow from seronegative and seropositive myasthenics were evaluated for antibody-secreting cells (ASC). Cells secreting antibody to acetylcholine receptor (AchR) and to presynaptic membrane receptor (prsmR) were counted using an immunospot assay. Immunoglobulin G (IgG) anti-AchR ASC were present in peripheral blood lymphocytes (PBL) from nine of 13 seronegative and nine of 12 seropositive myasthenics and in bone marrow lymphocytes (BML) from nine of 13 seronegative and eight of 12 seropositive myasthenics. The mean number of IgG anti-AchR ASC was lower for seronegative than for seropositive patients (P < 0.01 for PBL and P < 0.0001 for BML). In seropositive patients the mean number of IgG anti-AchR ASC was higher for BML than for PBL (P < 0.01); in seronegative patients it was not. IgG anti-prsmR ASC were detected in PBL from four of eight seronegative and six of eight seropositive myasthenics and in BML from three of eight seronegative and five of eight seropositive patients. The mean number of IgG-anti-prsmR ASC did not differ between seronegative and seropositive patients for PBL but for BML the value was higher for seropositive than for seronegative patients (P < 0.01). We conclude that seronegative myasthenia gravis is an autoimmune disease and that ASC to AchR and to prsmR are present both in the blood and the bone marrow in seronegative patients as in seropositive ones. A major difference between the groups lies in the significantly greater number of ASC found in the bone marrow in the seropositive cohort.

Myasthenia gravis: T and B cell reactivities to the beta-bungarotoxin binding protein presynaptic membrane receptor

Antibodies against acetylcholine receptor (AChR) can be detected in most patients with myasthenia gravis (MG) and are considered to be involved in the immunopathogenesis of this disease. AChR are isolated from crude receptor preparations by binding to alpha-bungarotoxin (alpha-BuTx). Patients with MG have also antibodies against a second protein tentatively named presynaptic membrane receptor (PsmR), which has been isolated from crude receptor utilizing beta-bungarotoxin (beta-BuTx). PsmR could represent another antigen besides AChR relevant for the development of MG. We have now evaluated the T cell reactivity to PsmR in MG and controls by analysing the frequencies of cells which in response to PsmR in short-term cultures secreted interferon-gamma (IFN-gamma). The B cell response to PsmR was analysed in parallel by counting cells secreting anti-PsmR antibodies. Most patients with MG had PsmR reactive T cell in blood with a median number of about 1 per 44,000 mononuclear cells. Cells secreting anti-PsmR antibodies belonging to the IgG and IgA isotypes, less frequently of the IgM isotype were detected in most MG patients. A positive correlation was found between T cells reactive with PsmR and anti-PsmR IgG antibody secreting cells. PsmR reactive T and B cells were also detected in control patients, but at much lower numbers. Our results indicate that MG is accompanied by T as well as B cell responses to PsmR, in addition to the previously recognized responses to AChR. It remains to be shown whether these PsmR reactivities are of pathogenetic importance in MG.