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

Titin antibody positive myasthenia gravis patients have a cellular immune response against the main immunogenic region of titin

Some myasthenia gravis (MG) patients have antibodies against non-acetylcholine receptor (AChR) epitopes of skeletal muscle including titin. Peripheral blood lymphocytes from 11 MG patients and 13 blood-donors were tested for lymphocyte proliferation after stimulation with the titin peptide MGT-30, which represents the main immunogenic region. Four out of seven titin antibody positive patients had significant stimulation defined as a stimulation index (SI) above 2. Neither of the four titin antibody negative patients nor the 13 blood-donors had SI above 2 (p = 0.001). Mean SI was significantly higher for T-cells from titin antibody positive MG patients, SI = 2.2 ± 0.8, compared to titin antibody negative patients, SI = 0.9 ± 0.2 (p = 0.01), and blood-donors, SI = 0.8 ±0.3 (p > 0.0005). After MGT-30 stimulation, IL-4 was detected in the blood lymphocyte culture supernatant from four of the five MG patients examined, but from none of the eight blood-donors. Thus, MG patients with anti-titin antibodies have a T-cell mediated immune reaction against titin.

Modeling the intrathymic pathogenesis of myasthenia gravis

Myasthenia gravis is (MG) a prototypic autoimmune disease; the immune effector mechanisms and autoantigenic target have been delineated. However, the events that lead to the abrogation of self-tolerance to neuromuscular acetylcholine receptors (nAChRs) remain a mystery. The thymus gland has long been considered to hold the key to solving this mystery, although the nature of its involvement remains to be elucidated. The nAChR was one of the first self-proteins associated with a defined autoimmune disease that was found to be expressed on thymic stromal populations. The studies described herein represent our efforts to determine how this "promiscuous" autoantigen expression may be involved in the immunopathogenesis of MG. We review our work, characterizing the expression of the nAChR alpha subunit in the thymus, and advance a hypothesis and experimental model, which explore how intrathymic expression of this autoantigen may contribute to the immunopathogenesis of this autoimmune disease.

Subtle differences in HLA DQ haplotype-associated presentation of AChR α-chain peptides may suffice to mediate myasthenia gravis

The HLA DQA1 and DQB1 alleles were determined on a set of 24 myasthenia gravis patients that had previously been examined for their T-cell proliferative responses to the 18 overlapping peptides representing the extracellular domain of hAChR alpha-chain. Patient responses according to assumed cis or trans haplotypes were significantly higher in most cases relative to normal controls. Comparisons of in vitro peptide-stimulated T-cell responses of patient pairs which had DQA1:DQB1 in common displayed responses in tighter distribution relative to comparisons in which patient pairs did not share the same DQA1:DQB1 haplotype. Similar haplotypes, such as DQA1*0102:DQB1*0602 and DQA1*0102:DQB1*0604, tended to exhibit similar responses and were grouped according to this similarity. Modified F-test and Student's T-test analyses on DQ isoform bearing groups revealed that high responses to peptide alpha34-49 were associated with A1*0102:B1*0602/0604, A1*0301:B1*0302 and A1*0401/0303:B1*0301. Peptide alpha146-162 showed higher responses in A1*0301:B1*0302 group and moderate responses in A1*0401/0303:B1*0301 groups. Differences in the age of disease onset relative to DQ haplotypes were also observed. Groups of A1*0301:B1*0302, A1*0501:B1*0201 and A1*0102:B1*0604 showed earlier ages of disease onset relative to those of A1*0102:B1*0602 or A1*0505:B1*0301.

Reflections on the "intrathymic pathogenesis" of myasthenia gravis

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.

Prolactin may be a promising therapeutic target for myasthenia gravis: hypothesis and importance

Myasthenia gravis (MG) is an autoimmune disease that affects the transmission signals from nerves to muscles. The basic pathology is the production of anti-acetylcholine receptor (AChR) antibodies (AChRAb) which is the consequence for the generation of autoreactive T lymphocytes responsing to AChR. However, the molecular mechanism of MG and the production of autoreactive T lymphocytes remain elusive. Recently beside its pivotal role in reproduction, the pituitary hormone prolactin (PRL) has been attributed to an immunomodulatory function. Furthermore it has been shown to be expressed in T cells and conversely it also affects the function of T cells, such as directly stimulating the proliferation and survival of T lymphocytes. In addition, elevated PRL levels frequently are described in autoimmune diseases, such as systemic lupus erythematosus (SLE) and multiple sclerosis (MS). So we hypothesized that the stimulating effect of PRL on T-cells function may be implied in the pathogenesis of MG and, perhaps, prolactin may be a promising therapeutic target for MG.

Vaccines against myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease mediated by antibodies to nicotinic acetylcholine receptor (AChR) interfering with the neuromuscular transmission. Experimental autoimmune MG serves as an excellent animal model to study possible therapeutic modalities for MG. This review will focus on the different ways to turn off the autoimmune response to AChR, which results in suppression of myasthenia. This paper will describe the use of fragments or peptides derived from the AChR, antigen-presenting cells and anti-T cell receptor antibodies, and will discuss the underlying mechanisms of action. Finally, the authors propose new promising therapeutic prospects, including treatment based on the modulation of regulatory T cells, which have recently been found to be functionally defective in MG patients.

HLA-DQ Polymorphism in Turkish Patients With Myasthenia Gravis

Genetic susceptibility to myasthenia gravis (MG) is reported frequently and varies depending on the clinical presentation of the patients. HLA-DQ genotyping was performed in 132 patients using polymerase chain reaction and sequence-specific oligonucleotide hybridizations in the Turkish population for the first time in this study. Antibody positivities against acetylcholine receptor and titin were 81 and 27%, respectively. Sixty-five percent of the patients had disease onset before 40 years of age (EOMG). Overall distribution of DQA1*0103 (odds ratio (OR): 0.5) and DQB1*0502 (OR: 1.9) alleles was different in patients and an ethnically matched healthy control group. Among the subgroups, DQB1*02 was significantly more frequent in EOMG (OR: 1.8), in women with MG (OR: 2.4), and in women with EOMG (OR: 2.8), whereas DQA1*0102 and DQB1*502 (OR: 2.3 for both) were increased and DQA1*0103 (OR: 0.04) was decreased in men with MG. Seropositivity was associated with both DQA1*03 (OR: 12.1) and DQB1*0302 (OR: 14.2) in the patient group. DQA1*02 (OR: 4.9) was associated with the presence of anti-titin antibodies, whereas DQA1*0101 (OR: 3.7) and *0102 (OR: 2.9) were more frequent in patients without this antibody. The presence of thymoma in MG was positively associated with DQB1*0301 (OR: 2.8), and DQB1*02 (OR: 0.3) was significantly less frequent in this group. The HLA-DQ associations in subgroups of MG suggest that the heterogeneity of the disease may be influenced by different genes or even by different alleles. DQ alleles have proved to be relatively informative polymorphisms in studying MG.

Vaccination with a MHC class II peptide in Alum and inactive pertussis strongly ameliorates clinical MG in C57BL/6 mice

We have investigated the efficacy of immunization against peptides from predisposing MHC class II molecules in human-compatible adjuvants for ameliorating experimental autoimmune myasthenia gravis (EAMG). C57BL/6 mice were immunized three times with the peptide I-Abetab62-76 in Alum+killed pertussis organisms (PT) prior to two injections with tAChR. The treatment greatly reduced the occurrence and severity of clinical MG relative to controls that received saline/Alum+PT or none. It also reduced antibody and T-cell responses against tAChR. The results have important implications for the possible immunotherapy of MG by targeting disease-associated MHC.

Responses in vitro of peripheral blood lymphocytes from patients with myasthenia gravis to stimulation with human acetylcholine receptor α-chain peptides: Analysis in relation to age, thymic abnormality, and ethnicity

Peripheral blood lymphocytes (PBLs) were isolated from 24 patients with myasthenia gravis of three ethnic groups (Caucasian, African American, and Hispanic) and ten healthy individuals. We determined the in vitro proliferative responses of the PBL samples to each of 18 overlapping synthetic peptides corresponding to the entire main extracellular domain (residues 1-210) of the alpha-subunit of human acetylcholine receptor. The profiles of the T-cell responses (expressed in stimulation index [SI]) to the peptides varied among the 24 patient samples. There was a significant difference in the overall patient responses relative to controls toward 17 of 18 peptides. T cells from the patients gave responses greater than control mean SI + 4 standard deviation (Z(SI) > 4) to 2 approximately 9 peptides/sample. Six peptides, alpha 23-38, alpha 34-49, alpha 78-93, alpha 122-138, alpha 146-162, and alpha 182-198, were recognized with Z > 4 level by 42% to 58% of the patients' PBLs. The grouped patient responses, divided according to age, thymic diagnosis, or ethnicity, were compared with controls and with each other. Significant differences were observed between early- and late-onset cases in recognition of residues alpha 34-49 (p = 0.015) and alpha 78-93 (p = 0.053), and in recognition of residues alpha 12-27, alpha 56-71, alpha 134-150, and alpha 146-162 (0.0072 < p < 0.064) when two ethnic groups were compared with each other.

Intrathymic expression of neuromuscular acetylcholine receptors and the immunpathogenesis of myasthenia gravis

The thymus has been considered to play an important role in the pathogenesis of myasthenia gravis (MG), an autoimmune disease characterized by skeletal muscle weakness. However, the pathogenic role of the thymus still remains a mystery. The neuromuscular type of acetylcholine receptor (AChR) was the first self-protein associated with a defined autoimmune disease that was found to be expressed by thymic stromal populations. The studies described herein represent our efforts to determine how this "promiscuous" autoantigen expression may be involved in the immunopathogenesis of MG. We review our work, characterizating the expression of the alpha subunit of AChR (AChRalpha) in the thymus, and advance a new hypothesis that examines the intrathymic expression of this autoantigen in disease pathogenesis.

A New Model Linking Intrathymic Acetylcholine Receptor Expression and the Pathogenesis of Myasthenia Gravis

The thymus is thought to play an important role in the pathogenesis of myasthenia gravis (MG), an autoimmune disease characterized by skeletal muscle weakness. However, its role remains a mystery. The studies described represent our efforts to determine how intrathymic expression of the neuromuscular type of acetylcholine receptors (nAChRs) is involved in the immunopathogenesis of MG. We review our work characterizing the expression of the alpha subunit of nAChR (nAChRalpha) in the thymus and advance a new hypothesis that examines the intrathymic expression of this autoantigen in disease pathogenesis.

Evidence of a diverse T cell receptor repertoire for acetylcholine receptor, the autoantigen of myasthenia gravis

We utilized two methods to look for T cell clonal expansions in myasthenia gravis (MG). We analyzed TCRBV CDR3 length polymorphism (spectratyping) to look for evidence of clonal expansion of CD4 or CD8 T cells directly from peripheral blood of MG patients. No statistically significant differences were found between the diversity of TCR repertoires in MG patients compared to normal control individuals when analyzed as groups. Rare oligoclonal expansions were detected in some individual MG patients but the significance of these findings is unclear. Next, we analyzed a panel of T cell hybridomas from acetylcholine receptor (AChR) immunized, MG-susceptible HLA-DR3 transgenic mice. The epitope specificity, TCRBV gene usage and CDR3 sequences of these hybridomas were highly diverse. We conclude there is only limited evidence for restricted TCR repertoire usage in human MG and suggest this may be due to the inability of HLA-DR molecules to select for restricted TCR recognition of AChR epitopes.

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.

Myasthenia gravis: selective enrichment of antiacetylcholine receptor antibody production in untransformed human B cell cultures

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.

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.

Acetylcholine receptor alpha subunit mRNA expression in human thymus: augmented expression in myasthenia gravis and upregulation by interferon-gamma

Previous studies by us and others have demonstrated the expression of acetylcholine receptors on epithelial cells in the thymus of myasthenia gravis (MG) and control subjects. In the present experiments, we used a reverse transcription-polymerase chain reaction (RT-PCR) to analyze the profile of the two major isoforms of the alpha chain of these receptors (AChRalpha), P3A- and P3A+, in thymus tissue obtained from MG and control subjects and a human thymic epithelial cell line (TEC9). In addition, using a semiquantitative RT-PCR, we compared the amounts of P3A- and P3A+ mRNA expressed in thymic tissue obtained from these two sources and determined if their expression in TEC9 is modulated by cytokines. We found that mRNAs encoding P3A- and P3A+ are expressed at approximately a 5:1 ratio in both MG and control thymus tissue. This contrasts with skeletal muscle where mRNAs encoding these isoforms are expressed equally. A pattern of preferential P3A- vs P3A+ mRNA expression was also observed in TEC9. We observed 2.8-fold greater expression of both isoforms in MG than in control thymus. Expression of both isoforms in TEC9 was enhanced significantly by treatment with interferon-gamma whereas IL-1alpha, IL-4, and IL-6 had no effect. Thus, there is differential regulation of AChRalpha variants in thymus and TEC relative to muscle and interferon-gamma represents a novel regulator of AChRalpha mRNA expression. MG thymus is distinguished by increased expression of both isoforms of this autoantigen, a finding that may reflect enhancement of transcription by local microenvironmental factors.

Mice with IFN-γ Receptor Deficiency Are Less Susceptible to Experimental Autoimmune Myasthenia Gravis

IFN-γ can either adversely or beneficially affect certain experimental autoimmune diseases. To study the role of IFN-γ in the autoantibody-mediated experimental autoimmune myasthenia gravis (EAMG), an animal model of myasthenia gravis in humans, IFN-γR-deficient (IFN-γR−/−) mutant C57BL/6 mice and congenic wild-type mice were immunized with Torpedo acetylcholine receptor (AChR) plus CFA. IFN-γR−/− mice exhibited significantly lower incidence and severity of muscle weakness, lower anti-AChR IgG Ab levels, and lower Ab affinity to AChR compared with wild-type mice. Passive transfer of serum from IFN-γR−/− mice induced less muscular weakness compared with serum from wild-type mice. In contrast, numbers of lymph node cells secreting IFN-γ and of those expressing IFN-γ mRNA were strongly augmented in the IFN-γR−/− mice, reflecting a failure of negative feedback circuits. Cytokine studies by in situ hybridization revealed lower levels of lymphoid cells expressing AChR-reactive IL-1β and TNF-α mRNA in AChR + CFA-immunized IFN-γR−/− mice compared with wild-type mice. No differences were found for AChR-reactive cells expressing IL-4, IL-10, or TGF-β mRNA. These results indicate that IFN-γ promotes systemic humoral responses in EAMG by up-regulating the production and the affinity of anti-AChR autoantibodies, thereby contributing to susceptibility to EAMG in C57BL/6-type mice.

Decrease of LFA-1 is associated with upregulation of TGF-beta in CD4(+) T cell clones derived from rats nasally tolerized against experimental autoimmune myasthenia gravis

Tolerance to experimental autoimmune myasthenia gravis by nasal administration of microgram amounts of acetylcholine receptor (AChR) has been reported. To elucidate the mechanisms behind tolerance induction via the respiratory tract and the involvement of CD4(+) T cells, we established AChR-specific CD4(+)CD8(-) T cell clones from nasally tolerized rats. Nasal tolerance decreased leukocyte function-associated antigen-1 (LFA-1) expression in CD4(+) T cells from tolerized rats. There was no difference between nasally tolerized and control rats in expression of intercellular adhesion molecule-1. The levels of transforming growth factor-beta (TGF-beta) mRNA-expressing cells were upregulated in CD4(+) T cell clones after tolerance induction. These findings suggest that decreased LFA-1 expression in CD4(+) T cells contributes to reduction of the infiltration of inflammatory CD4(+) T cells, while upregulated TGF-beta may inhibit lymphocyte functions.

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

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

A pathogenetic role for the thymoma in myasthenia gravis. Autosensitization of IL-4- producing T cell clones recognizing extracellular acetylcholine receptor epitopes presented by minority class II isotypes

Myasthenia gravis (MG) is caused by helper T cell-dependent autoantibodies against the muscle acetylcholine receptor (AChR). Thymic epithelial tumors (thymomas) occur in 10% of MG patients, but their autoimmunizing potential is unclear. They express mRNAs encoding AChR alpha and epsilon subunits, and might aberrantly select or sensitize developing thymocytes or recirculating peripheral T cells against AChR epitopes. Alternatively, there could be defective self-tolerance induction in the abundant maturing thymocytes that they usually generate. For the first time, we have isolated and characterized AChR-specific T cell clones from two MG thymomas. They recognize extracellular epitopes (alpha75-90 and alpha149-158) which are processed very efficiently from muscle AChR. Both clones express CD4 and CD8alpha, and have a Th-0 cytokine profile, producing IL-4 as well as IFN-gamma. They are restricted to HLA-DP14 and DR52a; expression of these minority isotypes was strong on professional antigen-presenting cells in the donors' tumors, although it is generally weak in the periphery. The two clones' T cell receptor beta chains are different, but their alpha chain sequences are very similar. These resemblances, and the striking contrasts with T cells previously cloned from non-thymoma patients, show that thymomas generate and actively induce specific T cells rather than merely failing to tolerize them against self antigens.

Cytokines and the pathogenesis of myasthenia gravis

Myasthenia gravis (MG) and its animal model experimental autoimmune myasthenia gravis (EAMG) are caused by autoantibodies against nicotinic acetylcholine receptor (AChR) in skeletal muscle. The production of anti-AChR antibodies is mediated by cytokines produced by CD4+ and CD8+ T helper (Th) cells. Emerging investigations of the roles of cytokines in MG and EAMG have revealed that the Th2 cell related cytokine interleukin 4 (IL-4), an efficient growth promoter for B-cell proliferation and differentiation, is important for anti-AChR antibody production. IL-6 and IL-10 have similar effects. The Th1 cytokine IFN-gamma is important in inducing B-cell maturation and in helping anti-AChR antibody production and, thereby, for induction of clinical signs and symptoms. Results from studies of time kinetics of cytokines imply that IFN-gamma is more agile at the onset of EAMG, probably being one of the initiating factors in the induction of the disease, and IL-4 may be mainly responsible for disease progression and persistance. Even though other Th1 cytokines like IL-2, tumor necrosis factor alpha (TNF-alpha), and TNF-beta as well as the cytolytic compound perforin do not directly play a role in T-cell-mediated help for anti-AChR antibody production, they are actually involved in the development of both EAMG and MG, probably by acting in concert with other cytokines within the cytokine network. In contrast, transforming growth factor beta (TGF-beta) exerts immunosuppressive effects which include the down-regulation of both Th1 and Th2 cytokines in MG as well as EAMG. Suppressive effects are also exerted by interferon alpha (IFN-alpha). Based on elucidation of the role of cytokines in EAMG and MG, treatments that up-modulate TGF-beta or IFN-alpha and/or suppress cytokines that help B-cell proliferation could be useful to improve the clinical outcome.

Linomide suppresses both Th1 and Th2 cytokines in experimental autoimmune myasthenia gravis

Suppressive effects of the synthetic immunomodulatory drug Linomide have been shown in several autoimmune models, including antibody-mediated experimental autoimmune myasthenia gravis (EAMG), a model for human myasthenia gravis (MG). To define the mechanisms underlying EAMG suppression, we injected Linomide subcutaneously at different doses into Lewis rats immunized with Torpedo acetylcholine receptor (AChR) in complete Freund's adjuvant (CFA), and investigated AChR-specific T and B cell responses, and the levels of lymph node cells expressing mRNA of different cytokines after AChR stimulation in vitro. Both 160 and 16, but not 1.6, mg/kg/day of Linomide effectively suppressed clinical muscle weakness, accompanied by decreased AChR-induced T and B cell responses. Linomide also suppressed the mRNA expression of the Th1 cytokines IFN-gamma, IL-12 and TNF-alpha as well as the Th2 cytokines IL-4 and IL-10, which are important in the immunopathogenesis of EAMG by promoting antibody production. There were no differences for IL-1beta, IL-6, lymphotoxin or TGF-beta expression in Linomide-treated vs nontreated control EAMG rats. We conclude that Linomide suppresses clinical EAMG as well as B and T cell responses to AChR by counteracting the production of AChR-induced Th1 and Th2 cytokines.

Myasthenia gravis as a prototype autoimmune receptor disease

Myasthenia gravis (MG) is an organ-specific autoimmune disease in which autoantibodies against nicotinic acetylcholine receptors (AChR) at the postsynaptic membrane cause loss of functional AChR and disturbed neuromuscular transmission. The immunopathogenic mechanisms responsible for loss of functional AChR include antigenic modulation by anti-AChR antibodies, complement-mediated focal lysis of the postsynaptic membrane, and direct interference with binding of acetylcholine to the AChR or with ion channel function. The loss of AChR and subsequent defective neuromuscular transmission is accompanied by increased expression of the different AChR subunit genes, suggesting a role for the target organ itself in determining susceptibility and severity of disease. Experimental autoimmune myasthenia gravis (EAMG) is an animal model for the disease MG, and is very suitable to study the immunopathogenic mechanisms leading to AChR loss and the response of the AChR to this attack. In this article the current concepts of the structure and function of the AChR and the immunopathological mechanisms in MG and EAMG are reviewed.

Both CD4+ and CD8+ T cells are essential to induce experimental autoimmune myasthenia gravis

CD4+ T cells have been shown to be crucial in the development of experimental autoimmune myasthenia gravis (EAMG). The role of CD8+ T cells in EAMG is less well established. We previously showed that antibody depletion of CD8+ T cells in rats effectively suppresses EAMG. To further study the role and relationship of CD4+ versus CD8+ T cells in induction of EAMG, CD4-/-, CD8-/-, and CD4-8- mutant C57BL/6 mice and the parent CD4+8- wild-type mice were immunized with Torpedo acetylcholine receptor (AChR) plus complete Freund's adjuvant. Clinical EAMG was nearly completely prevented in CD4-8-, CD4-/-, and CD8-/- mice. This was associated with strongly reduced AChR-specific T and B cell responses, and with reduced levels of AChR-reactive interferon gamma (IFN-gamma) and interleukin 4 (IL-4) mRNA-expressing cells in lymphoid organs when compared with CD4+8+ wild-type mice. We conclude that (a) both CD4+ and CD8+ T cells are essential for development of EAMG, and a collaboration between these cell types may be necessary; (b) CD4+ as well as CD8+ T cells secrete IFN-gamma and IL-4, and both cytokines are involved in the development of EAMG; and (c), besides T cells, other immune cells might also be responsible for help of anti-AChR antibody production.

Overexpression of select T cell receptor V beta gene families within CD4+ and CD8+ T cell subsets of myasthenia gravis patients: a role for superantigen(s)?

BACKGROUND

The principal symptoms of myasthenia gravis (MG), muscle weakness and fatigue due to impaired neuromuscular transmission, are caused by autoantibodies to the muscle nicotinic acetylcholine receptor (AChR). The mechanisms underlying the autoimmune response, however, appear to be initiated by activation of specific HLA class II-restricted CD4+ T lymphocytes. Thus, central to elucidating the causation of MG is determining how T cells are recruited to contribute to misguided immunological assaults on the major autoantigenic target, AChR.

MATERIALS AND METHODS

By combining a polymerase chain reaction (PCR)-based strategy and Southern blot technique, we have analyzed the frequency of expression of 22 individual T cell receptor (TCR) V beta gene subfamilies in CD4+ and CD8+ peripheral blood T cell subsets derived from eight MG patients and seven healthy controls. The quantification of relative usage of individual TCR J beta gene segments was performed by hybridization of PCR-amplified products (specifically V beta 1-C beta) with a complete panel of 32P-5'-end-labeled J beta-specific oligonucleotide probes, followed by scanning analysis of autoradiographs.

RESULTS

Comparisons of data obtained from V beta analyses of T cells from MG patients with those from healthy individuals established that MG patients significantly overexpressed V beta 1, V beta 13.2, V beta 17, and V beta 20 gene family members within both CD4+ and CD8+ T cell subpopulations. Moreover, analysis of the relative utilization of individual TCR J beta gene segments in V beta 1+/CD4+ and V beta 1+/CD8+ T lymphocytes revealed distribution patterns in patients indistinguishable from those recorded in the corresponding cell subsets derived from controls.

CONCLUSIONS

T lymphocytes from MG patients displayed a biased overexpression of four TCR V beta gene segments: V beta 1, V beta 13.2, V beta 17, and V beta 20. The relative frequencies of association of individual V beta 1 (D beta) J beta combinations revealed that J beta gene usage in the V beta 1-over-represented T cell subsets had normal distribution patterns. It can thus be deduced that J beta gene segment products appear not to have a selective effect on the process leading to overexpression of V beta 1 exons in MG patients. Hence, our observations suggest a possible role for superantigen(s) in the T cell activation in MG patients.

Presentation of endogenous acetylcholine receptor antigen to a specific CD4+ T-cell line by a transfected B-cell line

Currently, the limited supply and stability of some human autoantigens pose formidable difficulties in characterizing patients' T cells specific for them; recombinant preparations may contain bacterial contaminants, and synthetic peptides have arbitrarily chosen start and stop points. In order to provide a stable antigen source with naturally processed epitopes, a full-length acetylcholine receptor (AChR) alpha subunit construct was transfected into B-lymphoblastoid cell lines (B-LCL). Expression was much easier to detect at the mRNA level than the protein level. Nevertheless, this transfectant also stimulated a T-cell line that recognized the alpha 149-156 region in the context of HLA-DR4 at high sensitivity. The responses were specific both for the antigen transfected and for the presenting HLA-DR allele. This study thus confirms the potential of autologous B-LCL expressing natural epitopes in the context of HLA class II molecules for characterizing established T-cell lines, and perhaps also for initiating new ones.

Binding of monoclonal antibodies against the carboxyl terminal segment of the nicotinic receptor delta subunit suggests an unusual transmembrane disposition of this sequence region

Monoclonal antibodies (mAbs) specific for the carboxyl terminal region of the delta subunit of Torpedo nicotinic acetylcholine receptor (AChR), derived from mice immunized with AChR or a synthetic carboxyl terminal sequence of the delta subunit (C delta-mAbs), were used to determine the transmembrane disposition of their epitope(s) by immunoelectron microscopy, using AChR-rich postsynaptic membrane fragments from Torpedo electroplax. Some C delta-mAbs recognized only the cytoplasmic side of the membranes, some both sides to a similar extent, and others bound mostly, but not exclusively, to the cytoplasmic side. Binding of C delta-mAbs to the membranes was specifically blocked by synthetic peptides containing the carboxyl terminal region of the delta subunit. Control anti-AChR mAbs specific for the alpha or the delta subunits, whose epitopes have known transmembrane topology, uniquely recognized the expected side of the postsynaptic membrane. Residues involved in C delta-mAb binding were identified using single residue substituted peptide analogues of the sequence delta 481-501. All C delta-mAbs recognized epitopes within the same sequence segment, delta 485-493, at the carboxyl terminal of the AChR delta subunit. These results suggest that the delta subunit of the AChR might have alternative conformations, leading to exposure of the same sequence region on the extracellular or the cytoplasmic surface. Several Pro residues are present in this region. The alternative cis or trans conformation of one or more of them might result in different folding patterns of the carboxyl terminal sequence of the delta subunit, as described for a viral protein [Liddington, R. C., Yan, Y., Moulai, J., Sahli, R., Benjamin, T. L., & Harrison, S. C. (1991) Nature 354, 278-284.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Approaches for studying the pathogenic T cells in autoimmune patients

Our provisional conclusions from this work are as follows. (1) For screening responses of established lines, native human AChR is not prohibitively scarce, especially if it is concentrated onto beads, and class II-transfected TE671 cells may be useful too; both may give vital evidence of AChR-specificity, but it is still crucial to confirm that with synthetic peptides. (2) For mapping epitopes, panels of full-length and shorter recombinant human polypeptides, and of synthetic peptides, are invaluable complementary material: longer peptides tend to stimulate particularly strongly. (3) Initial selection with pooled synthetic peptides can easily generate interesting lines from both patients and controls, but they may depend on the artificial processing sites that are an inevitable consequence of arbitrarily chosen start and stop points. Of course, these might conceivably be employed in unusual antigen-presenting cells (such as thymic myoid cells), so we cannot totally dismiss such "cryptic" epitopes. This system can sometimes select T cells responding to "natural" epitopes too, as now reported for tetanus toxin. Nevertheless, for these and other reasons, at present, we strongly favor using the longest human recombinant material possible, because it is apparently processed more naturally. This must be combined with rigorous screening for reactivity to E. coli-derived contaminants plus concomitant mapping of epitopes as above. Use of intact AChR for initiating lines may yet become feasible. (4) The T cells thus isolated and characterized so far are proving to be heterogeneous in the epitopes and presenting class II molecules they recognize, and in their T-cell receptor gene usage. It is premature to claim key myasthenogenic epitopes or clonotypes, but HLA-DR3 and the linked -DQw2 do not appear to monopolize presentation. (5) Assessing the disease-relevance of these T cells is a separate problem, highlighted by their apparent similarity in healthy controls. In the meantime, to test their potential pathogenicity, we are assaying their cytokine profiles and ability to help specific antibody production in vitro. In the hope that they do prove to be relevant, we are also using some of them to test possible therapeutic strategies that might prove applicable in the patients.

T-helper epitopes on human nicotinic acetylcholine receptor in myasthenia gravis

The synthesis of AChR antibodies requires intervention of AChR-specific Th cells. Because of the paucity of anti-AChR Th cells in the blood of myasthenia gravis (MG) patients, direct studies of these autoimmune cells in the blood are seldom possible. Propagation in vitro of anti-AChR T cells from MG patients by cycles of stimulation with AChR antigens selectively enriches and expands the autoimmune T-cell clones, allowing investigation of their function and epitope specificity. Torpedo electroplax AChR was initially used for propagation of anti-AChR T-cell lines. Those studies demonstrated the feasibility of in vitro propagation of AChR-specific T cells. These are bona fide CD4+ Th cells, which stimulate production in vitro of anti-AChR antibodies by B cells of myasthenic patients and recognize equally well denatured and native AChR, suggesting the usefulness of synthetic human AChR sequences as antigens for propagation of the autoimmune Th cells. We used pools of overlapping synthetic peptides, corresponding to the complete sequences of the human AChR alpha-, beta-, gamma-, and delta-subunits, to propagate AChR-specific Th cells from the blood of MG patients. The AChR sequence regions forming epitopes recognized by the autoimmune T cells were determined by challenging the lines with individual synthetic peptides, 20 residues long, screening the AChR subunit sequences. Although each line had an individual pattern of epitope recognition--as expected from their different HLA-DR haplotype--some peptides were recognized by most of all the CD4+ T-cell lines, irrespective of their DR haplotype. The existence of immunodominant regions of the AChR sequence was verified by investigating the response of unselected CD4+ cells from the blood of a relatively large number of MG patients to the individual peptides screening the human alpha-, gamma-, and delta-subunit sequences. Those studies confirmed that each patient has an individual pattern of peptide recognition. The studies also identified a large number of T epitopes of the human AChR and verified the existence of sequence regions immunodominant for T-helper sensitization, because a limited number of sequence regions, including all those immunodominant for the T-helper lines, were recognized by most patients. Anti-AChR CD4+ T lines could be propagated from some healthy controls only for a brief period of time. They recognized AChR sequences poorly, suggesting a low affinity of their T-cell receptors for the corresponding AChR epitopes.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

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.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

Myasthenia gravis. CD4+ T epitopes on the embryonic gamma subunit of human muscle acetylcholine receptor

In myasthenia gravis (MG) an autoimmune response against muscle acetylcholine receptor (AChR) occurs. Embryonic muscle AChR contains a gamma subunit, substituted in adult muscle by a homologous epsilon subunit. Antibodies and CD4+ cells specific for embryonic AChR have been demonstrated in MG patients. We identified sequence segments of the human gamma subunit forming epitopes recognized by four embryonic AChR-specific CD4+ T cell lines, propagated from MG patients' blood by stimulation with synthetic peptides corresponding to the human gamma subunit sequence. Each line had an individual epitope repertoire, but two 20-residue sequence regions were recognized by three lines of different HLA haplotype. Most T epitope sequences were highly diverged between the gamma and the other AChR subunits, confirming the specificity of the T cells for embryonic AChR. These T cells may have been sensitized against AChR expressed by a tissue other than innervated skeletal muscle, possibly the thymus, which expresses an embryonic muscle AChR-like protein, containing a gamma subunit. Several sequence segments forming T epitopes are similar to regions of microbial and/or mammalian proteins unrelated to the AChR. These findings are consistent with the possibility that T cell cross-reactivity between unrelated proteins ("molecular mimicry"), proposed as a cause of autoimmune responses, is not a rare event.

T-cell epitopes on the human acetylcholine receptor alpha-subunit residues 10-84 in myasthenia gravis

In myasthenia gravis the production of anti-acetylcholine receptor antibodies is modulated by acetylcholine receptor-specific T cells. Most B- and T-cell epitopes are located on the alpha-subunit of the receptor. In order to map the fine specificity of the antigen-specific T cells in myasthenia gravis, T-cell stimulation in response to 70 hexapeptides was studied in 24 patients and 24 healthy individuals. The hexapeptides overlapped with one amino acid and represented residues 10-84 of the NH2-terminal part of the alpha-subunit of the receptor. The IFN-gamma secretion from single T cells was used to detect T-cell stimulation. A significant difference in the T-cell response to several of the peptides was found between patients and healthy controls. The majority of the hexapeptides induced T-cell stimulation in at least one of the patients. Peptide-induced T-cell stimulation was evident in all but one of the patients. The results indicate that different epitopes and multiple T-cell clones are involved in the T-cell recognition of the acetylcholine receptor.

Precise epitope mapping of monoclonal antibodies to the cytoplasmic side of the acetylcholine receptor alpha subunit. Dissecting a potentially myasthenogenic epitope

The epitopes for twelve monoclonal antibodies against the cytoplasmic side of the acetylcholine receptor (AChR) alpha subunit were precisely mapped using over 300 continuously overlapping synthetic peptides attached on poly(ethylene) rods. mAb cross-reactive between Torpedo and human AChR generally bound to the homologous peptides from both species. Epitopes 4-10-residues long were identified. One mAb could bind to either arm on both sides of a beta-turn structure. Five mAb bound to a very-immunogenic cytoplasmic epitope on alpha 373-380 (VICE-alpha). Three of the mAb against VICE-alpha were earlier found to cross-react with non-AChR protein(s), present in thymomas from myasthenia gravis patients but absent in thymomas from non-myasthenics. Since VICE-alpha has a potentially crucial pathogenic role, the antigenic role of each residue within it was subsequently studied by 55 analogues, most having single amino acid substitutions. All the mAb against VICE-alpha bound similarly but not identically to the analogues, thus explaining their known binding heterogeneity. Lys373 proved indispensable for mAb binding. Ile376, Glu377, Gly378 and Lys380 were quite critical, while Ser374, Ala375 and Val379 seemed rather inactive. These data should prove instructive in searches for VICE-alpha-like epitopes carrying autoantigens with potential involvement in myasthenia gravis and should further expand the applications of the anti-(AChR) mAb in AChR studies.

T cell responses to human recombinant acetylcholine receptor-alpha subunit in myasthenia gravis and controls

Antibodies against the nicotinic acetylcholine receptor (AChR) of the neuromuscular junction are detectable in most patients with myasthenia gravis (MG) and assumed to participate in the destruction of the AChR, thereby, causing the characteristics signs and symptoms of the disease. The extent and importance of T cell responses to AChR and its subunits in MG are still unsettled. We have now examined T cell reactivities using human recombinant AChR-alpha subunit as antigen. Upon recognition of appropriate antigen in an MHC-class II-restricted fashion, memory T cells secrete interferon-gamma (IFN-gamma). Adopting this principle in an immunospot assay we found that 73% of MG patients had recombinant human AChR-alpha subunit-reactive T cells at a median value of 1 per 56,000 blood mononuclear cells, while only 27% of the MG patients responded to the alpha subunit in a conventional lymphocyte proliferation assay. This compares with even lower numbers of AChR-reactive T cells and 14% positivity in the proliferation assay among control subjects. The T cell responses to the control antigens purified protein derivative and myelin basic protein did not differ between MG and controls, underlining the specificity of an augmented T cell reactivity to AChR-alpha subunit in MG. Alpha Subunit-specific T cell lines and clones propagated from patients with MG and healthy controls yielded a high proportion of alpha subunit-reactive T cells in the IFN-gamma immunospot assay. Their appearance was inhibited by the addition of monoclonal anti-MHC class II antibodies, demonstrating that an MHC-restricted T cell response was measured. Our data underline that the AChR-alpha subunit is a major T cell autoantigen in MG.

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.

Experimental myasthenia gravis in congenic mice. Sequence mapping and H-2 restriction of T helper epitopes on the alpha subunits of Torpedo californica and murine acetylcholine receptors

Immunization of mice with nicotinic acetylcholine receptor from Torpedo electric organ (TAChR) causes a disease similar to human myasthenia gravis (experimental autoimmune myasthenia gravis, EAMG). Susceptibility to EAMG correlates with the H-2 haplotype. In this study we used overlapping synthetic peptide corresponding to the complete sequences of the alpha subunits from TAChR and murine muscle AChR (MAChR) to map T helper epitopes in congenic murine strains of different H-2 haplotype. C57BL/6 and BALB/B mice (highly susceptible to EAMG) and BALB/c and CB17 mice (less susceptible to EAMG), immunized with TAChR, developed similar anti-TAChR antibody titers and L3T4+ (T helper) cell sensitization. Different sequence segments of the TAChR alpha subunit were recognized by L3T4+ cells from strains of H-2b and H-2d haplotype. The sequence segments recognized by the H-2d strains have the highest predicted propensity to form amphipatic alpha helices, while those recognized by the H-2b strains do not. We investigated whether in EAMG T helper cells cross-react with autologous AChR sequences, and a true break of the tolerance occurs. Overlapping synthetic peptides, corresponding to the complete sequence of MAChR alpha subunit, were used to test L3T4+ cell from mice immunized with TAChR. L3T4+ cell strains did not cross-react with any murine peptide sequence, while L3T4+ cells from H-2d mice were strongly stimulated by the peptide sequence Ma alpha 304-322, which is very similar to the homologous Torpedo peptide.

Abnormal T-cell expansion and V-gene usage in myasthenia gravis patients

To determine the extent of V-gene heterogeneity of blood T lymphocytes in patients suffering from Myasthenia Gravis (MG), we used eight recently available monoclonal antibodies (MoAb), directed against different V alpha and V beta gene products of the variable part of the T-cell receptor (TCR), covering approximately 25% of the alpha/beta T cells in normal peripheral blood (PBL) of healthy individuals. Using a two-colour immunofluorescence method, we could calculate the expression of alpha/beta V segments within the two major T-cell subsets, CD4-/CD8+ and CD4+/CD8- lymphocytes. Twenty-seven per cent (4/15) of the MG patients had T cells showing signs of abnormal expansion. Furthermore, among these expanded T cells, a restricted V beta 12 gene expansion could be seen, in three out of four patients. No correlation between TCR V-gene usage and HLA haplotypes (HLA-A, -B, -DR and -DQ) could be seen. Our data suggest that the majority of MG patients have abnormally expanded T-cell clones. The relevance of these findings is discussed.

Acetylcholine receptor-reactive T and B cells in myasthenia gravis and controls

Myasthenia gravis (MG) is strongly associated with antibodies to acetylcholine receptor (AChR), whereas the extent of T cell involvement is not settled. The number of cells secreting interferon-gamma (IFN-gamma) in response to AChR during 48 h culture of blood mononuclear cells (PBL) may reflect AChR-reactive T cells. Using an immunospot assay, we detected such cells in 23 of 30 patients with MG at a mean number of 1 per 33.333 PBL. AChR-reactive T cells were also found in patients with other neurological diseases (OND) and in healthy subjects but at lower frequencies and numbers. The T cell response to purified protein derivative and to PHA, and also to two major myelin proteins (basic protein and proteolipid protein) did not differ between MG and the two control groups, underlining the specificity of an augmented T cell reactivity to AChR in MG. Evaluation of the B cell response by enumerating anti-AChR IgG antibody secreting cells revealed such cells in 27 of 28 patients with MG at a mean value of 1 per 14,085 PBL. Cells secreting anti-AChR antibodies of the IgA and IgM isotypes were also detected in MG, but less frequently, at lower numbers, and only in conjunction with IgG antibody secreting cells. Anti-AChR antibody secreting cells were also found among patient with OND and in healthy controls, but at lower frequencies and numbers. These data confirm that AChR is a major target for autoimmune response in MG.

Central nervous system myelin: structure, function, and pathology

Multiple sclerosis (MS) and a number of related distinctive diseases are characterized by the active degradation of central nervous system (CNS) myelin, an axonal sheath comprised essentially of proteins and lipids. These demyelinating diseases appear to arise from complex interactions of genetic, immunological, infective, and biochemical mechanisms. While circumstances of MS etiology remain hypothetical, one persistent theme involves recognition by the immune system of myelin-specific antigens derived from myelin basic protein (MBP), the most abundant extrinsic myelin membrane protein, and/or another equally susceptible myelin protein or lipid component. Knowledge of the biochemical and physical-chemical properties of myelin proteins and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to the understanding of how and why these antigens become selected during the development of MS. This review focuses on current understanding of the molecular basis underlying demyelinating disease as it may relate to the impact of the various protein and lipid components on myelin morphology; the precise molecular architecture of this membrane as dictated by protein-lipid and lipid-lipid interactions; and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.