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Myasthenia gravis (MG) is an autoimmune antibody-mediated disorder of neuromuscular synaptic transmission. The clinical hallmark of MG consists of fluctuating fatigability and weakness affecting ocular, bulbar and (proximal) limb skeletal muscle groups. MG may either occur as an autoimmune disease with distinct immunogenetic characteristics or as a paraneoplastic syndrome associated with tumors of the thymus. Impairment of central thymic and peripheral self-tolerance mechanisms in both cases is thought to favor an autoimmune CD4(+) T cell-mediated B cell activation and synthesis of pathogenic high-affinity autoantibodies of either the IgG1 and 3 or IgG4 subclass. These autoantibodies bind to the nicotinic acetylcholine receptor (AchR) itself, or muscle-specific tyrosine-kinase (MuSK), lipoprotein receptor-related protein 4 (LRP4) and agrin involved in clustering of AchRs within the postsynaptic membrane and structural maintenance of the neuromuscular synapse. This results in disturbance of neuromuscular transmission and thus clinical manifestation of the disease. Emphasizing evidence from clinical trials, we provide an updated overview on immunopathogenesis, and derived current and future treatment strategies for MG divided into: (a) symptomatic treatments facilitating neuromuscular transmission, (b) antibody-depleting treatments, and, ((c) immunotherapeutic treatment strategies.)

The beneficial effects of thymectomy argue for a causal role of the thymus in myasthenia gravis (MG). The MG thymus contains acetylcholine receptor (AChR), which is expressed by myoid cells (whole AChR), and by medullary thymic epithelial cells (AChR subunits). The myoid cells are closely associated with antigen-presenting dendritic cells, helper T cells, and antibody-producing B cells in lymphoid follicles ("lymphofollicular hyperplasia"). Thus, all the cellular components required to initiate and maintain an autoimmune response to AChR are present in the MG thymus. It is unlikely that the cellular alterations in the thymus are secondary to an ongoing peripheral immune response, because they are absent in experimental autoimmune myasthenia gravis.

Here we show that in myasthenic thymus several cell types, including thymic epithelial cells (TEC) and immune cells, were the source and the target of the neurotrophic factor brain-derived growth factor (BDNF). Interestingly, many actively proliferating medullary thymocytes expressed the receptor TrkB in vivo in involuted thymus, while this population was lost in hyperplastic or neoplastic thymuses. Furthermore, in hyperplastic thymuses the robust coordinated expression of BDNF in the germinal centers together with the receptor p75NTR on all proliferating B cells strongly suggests that this factor regulates germinal center reaction. Finally, all TEC dying of apoptosis expressed BDNF receptors, indicating that this neurotrophin is involved in TEC turnover. In thymomas both BDNF production and receptor expression in TEC were strongly hindered. This may represent an attempt of tumour escape from cell death.

Background: Although widely accepted as an appropriate immunosuppressive therapy, the efficacy of glucocorticosteroid treatment has only rarely been tested in controlled studies., Objectives: To assess the efficacy of glucocorticosteroids or adrenocorticotrophic hormone (ACTH) medication in autoimmune myasthenia gravis., Search Strategy: We searched the Cochrane Neuromuscular Disease Group Trials Register in July 2004, MEDLINE (from January 1966 to June 2004) and EMBASE (from January 1980 to June 2004). We also checked the bibliographies in reviews and the randomised trials and contacted their authors to identify additional published and unpublished data., Selection Criteria: From the articles identified we selected those open or controlled studies which allowed us to assess the outcome of treated and untreated patients at definite endpoints. Types of studies: quasi-randomised or randomised controlled trials., Types of Participants: patients with myasthenia gravis of all ages and all degrees of severity. Types of interventions: any form of glucocorticosteroids or adrenocorticotrophic hormone treatment. Types of outcome measures:Primary outcome(1) improvement after at least three months in either the weakest muscles or all muscles. Secondary outcomes(1) proportion of patients improved after at least six months(2) proportion of patients in remission(3) number of episodes of worsening during the first six months(4) acetylcholine receptor antibody titres after at least three months of therapy., Data Collection and Analysis: Three authors extracted the data from the selected articles and one other checked them., Main Results: A trial of adrenocorticotrophic hormone (43 patients) did not show any advantage compared with placebo for the treatment of ocular myasthenia gravis. Two double-blind trials compared prednisone with placebo for generalised myasthenia gravis. In the first (13 patients), the improvement was slightly greater in the prednisone group at six months. In the second (20 patients) which was a short-term trial, the improvement was significantly greater at two weeks. Two trials compared glucocorticosteroids with azathioprine (41 and 10 patients respectively). In one of these the rate of treatment failure was greater in the prednisone group. In a trial of glucocorticosteroids versus intravenous immunoglobulin (33 patients) no differences in treatment responses were encountered during a treatment period of 14 days. An open trial (39 patients) evaluating different corticosteroid doses revealed a shorter time to improvement in the high-dose group. However only limited evidence can be drawn from the available randomised controlled trials due to numerous and important methodological flaws., Authors' Conclusions: Limited evidence from randomised controlled trials suggests that corticosteroid treatment offers significant short-term benefit in myasthenia gravis compared with placebo. This supports the conclusions of observational studies and expert opinion. Limited evidence from randomised controlled trials does not show any difference in efficacy between corticosteroids and either azathioprine or intravenous immunoglobulin.

B cells producing antibodies against the acetylcholine receptor (AchR) play a central role in the pathogenesis of myasthenia gravis (MG). Although anti-AchR autoantibodies have been studied extensively, not much is known about autoimmune B cells and their antigen-driven activation. This has mainly been due to difficulties in establishing and maintaining untransformed antigen-specific B cells in vitro. In this study, we show that highly enriched B cells from peripheral blood and thymus of MG patients can be maintained in culture over a period of 4 weeks when grown on the AchR-expressing rhabdomyosarcoma cell line TE671 together with an anti-CD40 stimulus and lymphokines. Anti-AchR antibody secretion could be detected in the majority of B cell cultures on TE671 cells up to 4 weeks. In contrast, B cells cultured on CDw32-transfected L cells binding anti-CD40 antibodies (the CD40 system) produced only small amounts of anti-AchR antibodies at single time points, whereas the overall IgG production was higher than on TE671 cells. The expression of the relevant autoantigen on the adherent cell line in addition to other growth stimuli could account for this difference and may provide a useful tool for investigating antigen-dependent B cell activation in MG and other B cell-mediated autoimmune conditions.

It has been suggested that antibodies against non-acetylcholine receptor proteins of striated muscle are markers of the presence of a thymic epithelial tumor in patients with myasthenia gravis (MG). These antibodies may be measured using an immunofluorescence assay against striated muscle (anti-STR) or an ELISA with a recombinant 30-kd titin fragment (anti-MGT30). To directly compare anti-STR with anti-MGT30, we examined the sera of 276 consecutive patients with known or suspected MG. Definite diagnoses and thymic histology, if available, were correlated with the antibody assays. Of the 276 patients, 164 had MG. Thymic histology was obtained in 44 patients: 18 had lymphofollicular hyperplasia, 13 thymic epithelial tumors, 8 atrophy, and 5 were normal. When compared with anti-STR, anti-MGT30 showed a sensitivity of 69% (STR 77%), specificity of 100% (STR 56%, p = 0.026), negative predictive value of 82% (STR 77%), and positive predictive value of 100% (STR 56%, p = 0.003) for the identification of a thymic epithelial tumor versus thymic hyperplasia. We conclude that the anti-MGT30 ELISA is better than the anti-STR immunofluorescence assay for the diagnosis of paraneoplastic MG.

To study the possible role of thymomas and of extrathymomal thymic tissue in the development and maintenance of myasthenia gravis, we transplanted fragments of either tissue into SCID mice and monitored the production of anti-acetylcholine receptor antibodies in the recipients. Furthermore, the transplants were characterized by immunohistochemistry. Unlike after transplantation of thymus with lymphofollicular hyperplasia that induced high titers of anti-acetylcholine receptor antibodies, thymoma transplants never produced autoantibodies. Mice transplanted with extrathymomal thymic tissue also failed to produce anti-acetylcholine receptor antibodies except one group that received transplants containing hyperplastic extrathymomal tissue. These findings may explain the refractoriness of thymomatous myasthenia to thymectomy.

Thymic myoid cells (TMCs) bearing acetylcholine receptors (AchR) on their surface have a central role in the concept of intrathymic autosensitization in the pathogenesis of myasthenia gravis. In a SCID mouse model of myasthenia gravis, solid pieces of thymuses with lymphofollicular hyperplasia were transplanted into SCID mice. The chimeric mice displayed long-term secretion of anti-AchR antibodies. Here, we traced the fate of TMCs contained in transplanted myasthenia gravis thymuses. Unexpectedly, the number of thymic TMCs in transplanted tissue was slightly higher than in untransplanted thymus. More strikingly, the transplanted TMCs were more highly differentiated than their nontransplanted counterparts. This was demonstrated by a more than 10-fold increase of AchR-main immunogenic region epitopes recognized by monoclonal antibody 198. Some TMCs had even differentiated into striated muscle cells. The abundance of AchRs in human thymic transplants in the SCID mouse model of myasthenia gravis may help to study the mechanisms of autosensitization against the AchR in vivo.

The experimental work discussed here endorses the hypothesis that in the pathogenesis of MG the initial and essential steps take place within the thymus. Most, if not all, thymuses of MG patients contain B cells capable of producing AChR-specific autoantibody along with appropriate stroma elements. This is especially pertinent in hyperplastic thymuses with germinal centers, which characteristically contain cellular complexes formed by AChR-producing MCs and surrounding interdigitating dendritic cells. The source of the myasthenogenic autoantigen is more complex in thymomas. There are data suggesting that thymoma epithelium express a protein that shares certain peptide epitopes with the AChR alpha chain, although there is no further molecular similarity. A unique type of "molecular self mimicry" could be the starter of thymoma-associated MG.

The experimental work discussed here supports the hypothesis that in the pathogenesis of MG the initial and essential steps take place within the thymus. Most if not all thymuses of MG patients contain B cells capable of producing AChR specific autoantibody along with appropriate stroma elements. Hyperplastic thymuses characteristically contain germinal centers with cellular complexes of AChR-producing MC and surrounding interdigitating dendritic cells. In thymomas, the source of the myasthenogenic autoantigen is less obvious. There are data suggesting that thymoma epithelium expresses a protein sharing certain peptide epitopes with the AChR alpha chain, although there is no further molecular similarity. A unique type of 'molecular self-mimicry' cold be involved in the initiation of thymoma-associated MG.

To study the role of the thymus in the cellular pathogenesis of myasthenia gravis (MG) we transplanted thymus tissue fragments from MG thymuses beneath the kidney capsule of severe combined immunodeficiency (SCID) mice. Immunocytochemical studies documented that the human thymus tissues are accepted as long-term grafts in the host SCID mice, with human lymphocytes, thymic stroma, and thymic myoid cells demonstrable in transplanted thymus for at least 15 weeks after transplantation. Human anti-acetylcholine receptor antibodies became detectable 1 to 2 weeks after transplantation, and in most chimeras the titers increased over at least 11 weeks to reach levels typically found in severe human MG. Human Ig deposits were detected at skeletal muscle end-plates, demonstrating that the human (auto)antibodies bound to murine acetylcholine receptor. In contrast, transfers of dissociated thymus cells only lead to a transient increase of anti-acetylcholine receptor antibodies. Our data prove that myasthenia gravis thymus is able to induce and maintain autoantibody production in immunodeprived host animals, and that this tissue contains all cellular components required for autoantibody production. Transplantation of solid thymus tissue seems to transfer an autoimmune microenvironment, which will allow direct studies of the mechanism of autosensitization inside the thymus.

Anti-acetylcholine receptor (AChR) antibodies in myasthenia gravis (MG) can be quantitated using AChR extracted from the human rhabdomyosarcoma cell line TE671 (AChRTE671) as a practical alternative to AChR from human amputated limbs (AChRAMP). We compared the two antigen preparations using serum samples from different clinical groups of MG patients (n = 112) and various controls (n = 189). With two exceptions, both tests were positive or negative in the same patients. However, in the generalized MG group, the TE671 assay yielded significantly lower titers than the AChRAMP assay.

In 82 non-related patients with myasthenia gravis the following immunogenetic marker systems were investigated: HLA-A, -B, -C, -DR antigens, complement polymorphisms for Bf, C3 and C4 as well as immunoglobulin allotypes G1m, G3m and Km. In 78 patients the level of circulating acetylcholin receptor autoantibody was measured. The HLA antigens showed no significant differences in frequency when compared with healthy controls. This was also true for the HLA-bound Bf and C4 polymorphism, the non-HLA-bound C3 polymorphism and the immunoglobulin allotypes. A significant (P less than 0.005) increase in heterozygote frequency of 39.7% in the Gm allotype combination G1m (1), (17); (3) when compared with an expected frequency as calculated from the gene frequency of 19.8% was found. HLA-B8 and/or HLA-DR3 positive myasthenia gravis patients showed a significantly (P less than 0.025 and less than 0.005) earlier average onset of the disease whereas in HLA-DR2 positive patients the average onset was later (P less than 0.05). The same HLA antigens correlated with quantitative differences in the acetylcholine receptor autoantibodies: HLA-B8 and/or -DR3 positive patients had on average higher antibody levels than HLA-B8 or -DR3-negative patients. The difference for HLA-DR3 was statistically significant (P less than 0.05). On the other hand patients with HLA-DR2 antigens had significantly lower antibody levels (P less than 0.025). Increased mean antibody levels were also seen in G1m(1), (17) allotype (P less than 0.005) and G3m(21) (P less than 0.05). As the HLA antigens and Gm allotype genes are situated on different chromosomes (C6 and C14) two distinct gene regions are identified in myasthenia gravis. These are associated with quantitative differences in acetylcholine receptor antibody titres.

Myasthenia gravis is a classic autoaggression disease in which autoantibodies against the acetylcholine receptors (AChR) of the neuromuscular end-plate have a decisive patho-genetic significance. With sensitive radioimmunoassays anti-AChR antibodies are demonstrable in over 90% of myasthenics and consequently are of great diagnostic significance. The treatment possibilities to date have distinctly improved the prognosis but are either symptomatic or unspecific. More recent knowledge on pathophysiology and pathogenesis and the strategies for development of a specific immunotherapy deducible from this are discussed. X

Autoimmune helper T lymphocytes were selected from the blood of two myasthenic patients of different HLA-DR type, using acetylcholine receptor (AChR) from Torpedo californica. These polyclonal T cell lines were tested for reactivity with three synthetic peptides corresponding to the NH2-terminal region of the human AChR alpha subunit. This segment is a good candidate for T cell epitopes since it has a propensity to form an amphipathic alpha helix. The peptides elicited 10-30% of the response induced by native Torpedo AChR. Different peptides were recognized by the autoreactive T cells of the two patients. These results suggest that the NH2-terminal region of the AChR alpha chain contains T cell-stimulating epitopes, and that the T cell autoimmune response in myasthenia gravis, like the B cell response, is heterogeneous.

The thymus is an important site of sensitization of autoreactive B and T lymphocytes in myasthenia gravis (MG). We have investigated clonal diversity of B or T cells in the thymus of patients with MG by Southern blot experiments using probes specific for immunoglobulin heavy (IgH) and light chain (IgL) genes and for T cell receptor (TCR) beta- and gamma-chain genes. This method allows to detect individual clones of B or T cells if they represent at least 1% of the total cell population. We investigated thymus glands from 14 patients who underwent thymectomy. Single rearranged fragments could be demonstrated with a TCR gamma-specific probe in DNAs from both normal donors and MG patients. TCR beta gene rearrangements occurred mainly in the C beta 1 region. However, single rearranged bands could not be detected with either TCR beta, JH or with J kappa specific probes. Thus any single autoimmune B and T cell clone present in the myasthenic thymus represents presumably less than 1% of all thymocytes.

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.

Human autoreactive helper T lymphocytes with specificity for acetylcholine receptor (AChR) were isolated from three HLA-DR3-positive patients who had myasthenia gravis (MG), an autoimmune disease known to be associated with HLA-DR3 in the North European population. The antigen-specific T cells were evaluated for genetic restriction. Antigen presentation studies were performed with mitomycin C-treated accessory cells from a panel of HLA-typed unrelated donors. AChR-induced proliferation of the autoreactive T cells was maximal in the presence of autologous or HLA-DR-compatible antigen-presenting cells. In two DR-heterozygous patients both parental DR specificities served as restriction elements of the polyclonal AChR-reactive T cell populations. Preferential restriction to HLA-DR3 was observed in one patient, but this was also seen with PPD-specific T cells from the same donor. A series of monoclonal antibodies against HLA class II molecules was used for inhibition experiments. The inhibitory effects of the antibodies were not due to unspecific toxicity and could be observed after separate treatment of the antigen-presenting cells but not of the responding T cells. Several monoclonal antibodies against monomorphic HLA-DR determinants (DA 231, MAS 53, MAS 54, L243, OKIa1) had pronounced inhibitory effects. Anti-HLA-DQ(DC) monoclonal antibodies (Leu-10; TU 22) had only mild or no inhibitory effects in two patients but significantly inhibited AChR-specific T cells in one patient. A monoclonal antibody against HLA-DR3 (antibody 16.23) was not or was only weakly inhibitory in the DR3-positive patients, although it bound to autologous T line cells and B cells by indirect immunofluorescence. In one patient it was possible to compare the inhibition patterns of AChR-specific and PPD-specific T cells. Most of the monoclonal antibodies affected AChR- and PPD-specific T cells to a similar extent, but three antibodies (TU 22, 36, 39) inhibited PPD-specific T cells more than AChR-specific T cells, indicating the possibility of differential restriction of antigen- and autoantigen-specific T cells. It is suggested that the in vitro system described here may be helpful for the evaluation of anti-HLA class II antibodies as potential immunotherapeutic reagents.

Myasthenia gravis is one of the best characterized human autoimmune disorders. Circulating autoantibodies to the nicotinic acetylcholine receptor (AChR) at the neuromuscular junction play a prominent part in the effector phase, that is, the immunoregulation. Indirect evidence, such as thymic abnormalities and the association with certain histocompatibility antigens (for example HLA-B8,-DR3) suggests a defect of immunoregulation at the level of thymus-dependent (T) lymphocytes. We report here on the isolation of autoreactive T cells from six patients with myasthenia gravis. From one of these patients, who is homozygous for HLA-DR3, we established a long-term T-cell line. The line cells are specific for purified fish and human AChR, display the surface phenotype of inducer/helper T cells and are genetically restricted to HLA-DR3. AChR-induced proliferation could be inhibited with two monoclonal antibodies against monomorphic DR determinants and also with DR3-specific alloantiserum.

In this uncontrolled study, 104 patients with generalized myasthenia gravis treated with azathioprine for a median period of 29 months (range, 1 month to 12 years) were surveyed for possible adverse reactions. These occurred in 36 patients (35%) in the following order of frequency: hematologic (18%), gastrointestinal (13%), infectious diseases (13%), and elevation of liver enzymes (6%). No allergic skin reactions were observed. Azathioprine had to be discontinued temporarily in a total of 11 patients (11%) because of possible side effects. The cause of death in the nine patients who died during the period of observation (up to 12 years) was related to myasthenic crisis in two patients. In five patients, a malignant tumor was diagnosed (two carcinoma of the prostate, one ovarian carcinoma, one bronchial carcinoma, and one renal lymphoma) after 2.5 years, 6 months, 3 months, 5 years, and 6 years of treatment, respectively. A causal relationship seems unlikely in the first four cases, but cannot be excluded in the one case of late lymphoma.

In 15 patients with myasthenia gravis who were in stable clinical remission while receiving azathioprine, we monitored disease severity and serial autoantibody titers before and after discontinuation of azathioprine. Cellular immunoreactivity against tuberculin (PPD) and against Torpedo acetylcholine receptor (AChR) was measured serially in 11 patients. Eight of 15 patients (53%) had a clinical relapse after 3 to 11 months, necessitating the reinstitution of immunosuppressive treatment in 6 patients. Seven patients have remained clinically stable during an observation period of 20 to 40 months. Anti-AChR autoantibody titers correlated closely with the clinical course in the majority of patients, and rose markedly in 7 of the 8 patients who relapsed. Cellular stimulation indices correlated less closely with the clinical severity. Only in 3 patients did the clinical score, antibody titer, and cellular stimulation index rise concurrently. In 4 patients who had high cellular stimulation indices after the discontinuation of azathioprine, it was possible to isolate AChR-reactive inducer/helper T lymphocytes.

Chronic experimental autoimmune myasthenia gravis (EAMG) was induced in rats by immunization with acetylcholine receptor (AChR) purified from the electroplax of Torpedo californica. 35--40 days after immunization, serum anti-AChR antibody titers were about 40 nM. At this stage, electrophysiology was performed on isolated M. omohyoideus muscle-preparations from myasthenic and from normal (control) rats. For the study of the equilibrium interaction between acetylcholine (ACh) and AChR, dose-response curves were obtained by quantitative ionophoretic application of ACh to voltage-clamped end-plates. Analysis of dose-response curves yielded the following parameters: maximum end-plate conductance per unit surface gmax (EAMG) = 10.3 +/- 1.1 nS/micrometer 2, gmax (normal) = 20.2 +/- 1.8 nS/micrometer 2; apparent dissociation constant K (EAMG) = 96 +/- 5 microM, K (normal) = 58 +/- 6 microM; Hill-coefficient nH (EAMG) = 2.3 +/- 0.1, nH (normal) = 2.3 +/- 0.1. Single channel properties were derived from an analysis of ACh-induced end-plate current noise: the mean single channel conductance was gamma (EAMG) = 20.1 +/- 2.2 pS, gamma (normal) = 27.6 +/- 1.8 pS and the mean channel life-time tau (EAMG) = 1.39 +/- 0.09 ms, tau (normal) = 1.32 +/- 0.08 ms (T = 22 degrees C). The electrophysiological data are interpreted as follows: (1) At myasthenic end-plates there is a 50--60% reduction of functioning AChR (decrease of gmax). A total number of about 2 x 10(6) (1 x 10(6)) channels per end-plate was calculated for control (myasthenic) rats. (2) The affinity of AChR for ACh is reduced and/or there is an impediment of the conformational change from the closed- to the open-channel configuration (increase of K). (3) Single channel properties are essentially unaffected.

The role of T-lymphocytes in Experimental Autoimmune Myasthenia Gravis (EAMG) was investigated. We generated highly purified, acetylcholine receptor (AChR)-specific T-cell populations and subsequently characterized these cell lines with respect to their membrane phenotype and their function. Using a series of mouse monoclonal antibodies directed against rat lymphocyte surface differentiation antigens, the vast majority of line cells was shown to express a leucocyte common antigen, a T-common antigen and a T-helper antigen. Small subpopulations were Ia or T suppressor antigen-positive. Adaptive transfer to sublethally irradiated, thymectomized recipients revealed that 1 X 10(6) AChR-specific line cells could cooperate effectively with 10 X 10(6) AChR-primed, complement (C3) receptor-bearing (B-cell enriched) spleen cells in the production of anti-AChR autoantibodies. Recipients of B-cells along with relevant line cells developed an acute myasthenic syndrome 6-7 days after cell transfer. Electron-microscopical examination revealed the typical features of "acute phase" EAMG with heavy mononuclear infiltration. There was, however, no evidence antibody-independent cytotoxic activity exerted by AChR-specific line cells.

We studied autoreactive acetylcholine receptor (AChR)-specific T cell lines from two patients with myasthenia gravis. Anti-AChR autoantibody production by peripheral blood mononuclear cells (PBM) from the donors of the T cell lines was measured with an enzyme-linked immunoadsorbent assay, using purified human AChR as antigen. Freshly isolated PBM produced barely detectable amounts of anti-AChR autoantibodies. If, however, autologous AChR-specific T cells were added to the cultures, the production of anti-AChR autoantibodies, and of total IgM and IgG, was markedly stimulated, depending on the number of T line cells and on the amount of AChR present in the cultures. AChR-specific functional helper T-lymphocytes may have a role in the immunoregulation of myasthenia gravis.

Using the quantitative ionophoresis technique and the mouse passive transfer model of myasthenia gravis, end-plate function was measured in mice transferred with myasthenic IgG, Fab, or F(ab')2 hybrid molecules prepared by recombination of one acetylcholine receptor (AChR) specific Fab and one Fab directed against irrelevant antigen. The Hill coefficient (a measure for the cooperativity between AChR subunits) and the apparent dissociation constant K for the ACh-AChR interaction were essentially unaltered after passive transfer of IgG or IgG fragments. In contrast, myasthenic IgG and Fab, but not control IgG or Fab, markedly reduced the mean number of end-plate channels. A similar effect was observed after passive transfer of F(ab')2 hybrid molecules. These results show that passive transfer of myasthenic IgG or IgG fragments leads to a quantitative reduction of ACh-controlled end-plate channels, whereas the function of the remaining receptors/channels remains unchanged. The results suggest further that cross-linking of AChR by divalent antibody may not be an absolute requirement for the reduction of AChR at the functional neuromuscular synapse.

The HLA-DR3 haplotype is associated with increased risk of myasthenia gravis (MG) and a number of other autoimmune diseases, including insulin-dependent diabetes mellitus (IDDM), coeliac disease, and premature ovarian failure (POF). With a cDNA probe for a DQ beta gene, a 15 kb Hinc II restriction fragment has been demonstrated in genomic DNA from 7 of 16 HLA-DR3 patients with MG, 1 of 19 healthy DR3 controls, and none of 24 DR3 patients with IDDM, coeliac disease, or POF. The HLA-DQ polymorphism may be closely linked to a genetic locus regulating immune responsiveness to acetylcholine receptor and susceptibility to MG.

Sixty patients with myasthenia gravis were examined prospectively by measuring serial titers of antibodies against human acetylcholine receptor, and these were correlated with a quantitative clinical score. Serial titers of antibodies detected by the standard immunoprecipitation assay (binding antibodies) correlated with the clinical score in most patients. Antibodies blocking the binding of alpha-bungarotoxin to receptors (blocking antibodies) were detected in 29 patients. Serial blocking antibody titers correlated with changes in muscle weakness less often than binding antibody titers. Titers of both classes of antibodies often followed a divergent course, suggesting that the autoimmune B-cell clones that formed these classes of antibodies may have been activated asynchronously.