T cell recognition of the acetylcholine receptor in myasthenia gravis

A Sensitive Method for Detecting Peptide-specific CD4+ T Cell Responses in Peripheral Blood from Patients with Myasthenia Gravis

Myasthenia gravis (MG) is an autoimmune neurological disorder typified by skeletal muscle fatigue and most often production of autoantibodies against the nicotinic acetylcholine receptor (AChR). The present study was undertaken to assess the extent of AChR-peptide recognition in MG patients using co-culturing (DC:TC) of autologous monocyte-derived dendritic cells (moDCs) and highly enriched CD4⁺ T cells from the blood as compared to the traditional whole peripheral blood mononuclear cell (PBMC) cultures. We found that the DC:TC cultures were highly superior to the PBMC cultures for detection of reactivity toward HLA-DQ/DR-restricted AChR-peptides. In fact, whereas DC:TC cultures identified recognition in all MG patients the PBMC cultures failed to detect responsiveness in around 40% of the patients. Furthermore, reactivity to multiple peptides was evident in DC:TC cultures, while PBMC cultures mostly exhibited reactivity to a single peptide. No healthy control (HC) CD4⁺ T cells responded to the peptides in either culture system. Interestingly, whereas spontaneous production of IFNγ and IL-17 was observed in the DC:TC cultures from MG patients, recall responses to peptides enhanced IL-10 production in 9/13 MG patients, while little increase in IFNγ and IL-17 was seen. HCs did not produce cytokines to peptide stimulations. We conclude that the DC: TC culture system is significantly more sensitive and better identifies the extent of responsiveness in MG patients to AChR-peptides than traditional PBMC cultures.

A study of the factors inducing the development of childhood-onset myasthenia gravis using CDR3 spectratyping analysis of the TCR repertoire

Myasthenia gravis (MG) is an autoimmune disease. AChR-specific autologous helper T (Th) cells are essential to the pathogenesis of MG. Factors correlated with the development of childhood-onset MG are unknown. In longitudinal studies, we found TCR Vbeta 2/5.1/6/7 usage in the development or relapse phases, but not in the remission phase. We also found that TCR Vbeta 8/9/13.1/15/18/20 usage persisted. The polyclonally expanded TCR Vbeta 2/5.1/6/7 by CDR3 spectratyping was found to be associated with the development of disease. These data suggest that in patients with childhood-onset MG, stimuli such as superantigens induced by a preceding infection, which cause development of the polyclonal pattern in TCR Vbeta families, play an important role in the development of the disease.

[Plasma exchange as a therapeutic option in neurological disorders]

Plasma exchange is a therapeutic procedure commonly used in various neurological disorders. Here we review its current role as a treatment option in diseases of the central and peripheral nervous system.

Immunomodulation by a dual altered peptide ligand of autoreactive responses to the acetylcholine receptor of peripheral blood lymphocytes of patients with myasthenia gravis

Myasthenia gravis (MG) is a T cell-dependent, antibody-mediated autoimmune disease. A dual altered peptide ligand (APL) that is composed of the tandemly arranged two single amino acid analogs of two myasthenogenic peptides was demonstrated to downregulate in vitro and in vivo murine MG associated autoreactive responses. Furthermore, treatment with the dual APL ameliorated the clinical manifestations of an established experimental autoimmune MG in mice. This study was undertaken in order to investigate the ability of the dual APL to immunomodulate MG-associated responses of peripheral blood lymphocytes (PBL) of patients with MG to the native autoantigen acetylcholine receptor (AChR). PBL of 22 of 27 patients with MG tested responded by proliferation to torpedo AChR. The proliferative responses of PBL of 21 of 22 responders were significantly inhibited by the dual APL. The inhibition was specific because a control peptide did not inhibit these proliferative responses. The dual APL also downregulated the levels of the secreted pathogenic cytokine IFN-gamma in supernatants of stimulated PBL of 80% of the tested patients. The latter inhibitions correlated with an upregulated production of the immunosuppressive cytokine, tumor growth factor beta. Thus, the results of our study demonstrate that the dual APL is capable of downregulating in vitro autoreactive responses of patients with MG and suggest that this peptide is a potential candidate for a novel specific treatment of patients with MG.

The Role of Thymomas in the Development of Myasthenia Gravis

Thymic pathology occurs in 80-90% of myasthenia gravis patients. Significant associations between different thymic alterations and clinical findings are discussed. To highlight peculiarities in thymoma-associated myasthenia gravis, we briefly review myasthenia gravis associated with thymic lymphofollicular hyperplasia (TFH) and thymic atrophy.

T Cells and Cytokines in the Pathogenesis of Acquired Myasthenia Gravis

Although the symptoms of myasthenia gravis (MG) and experimental MG (EAMG) are caused by autoantibodies, CD4(+) T cells specific for the target antigen, the nicotinic acetylcholine receptor, and the cytokines they secrete, have an important role in these diseases. CD4(+) T cells have a pathogenic role, by permitting and facilitating the synthesis of high-affinity anti-AChR antibodies. Th1 CD4(+) cells are especially important because they drive the synthesis of anti-AChR complement-fixing IgG subclasses. Binding of those antibodies to the muscle AChR at the neuromuscular junction will trigger the complement-mediated destruction of the postsynaptic membrane. Thus, IL-12, a crucial cytokine for differentiation of Th1 cells, is necessary for development of EAMG. Th2 cells secrete different cytokines, with different effects on the pathogenesis of EAMG. Among them, IL-10, which is a potent growth and differentiation factor for B cells, facilitates the development of EAMG. In contrast, IL-4 appears to be involved in the differentiation of AChR-specific regulatory CD4(+) T cells, which can prevent the development of EAMG and its progression to a self-maintaining, chronic autoimmune disease. Studies on the AChR-specific CD4(+) cells commonly present in the blood of MG patients support a crucial role of CD4(+) T cells in the development of MG. Circumstantial evidence supports a pathogenic role of IL-10 also in human MG. On the other hand, there is no direct or circumstantial evidence yet indicating a role of IL-4 in the modulatory or immunosuppressive circuits in MG.

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

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

Autoreactive T cells to the P3A+ isoform of AChR alpha subunit in myasthenia gravis

In vitro T cell proliferative response to an alternative splicing variant of acetylcholine receptor alpha subunit (AChR alpha) with the P3A exon-encoded region was examined in peripheral blood samples from 28 myasthenia gravis (MG) patients and 14 healthy donors using recombinant fragments and synthetic peptides. T cells responsive to the P3A region-specific sequences were detected in five MG patients, all of whom were late-onset disease with thymoma, but in none of healthy donors. These autoreactive T cells may be involved in the pathogenic process in a subset of MG patients.

Split tolerance in a novel transgenic model of autoimmune myasthenia gravis

Because it is one of the few autoimmune disorders in which the target autoantigen has been definitively identified, myasthenia gravis (MG) provides a unique opportunity for testing basic concepts of immune tolerance. In most MG patients, Abs against the acetylcholine receptors (AChR) at the neuromuscular junction can be readily identified and have been directly shown to cause muscle weakness. T cells have also been implicated and appear to play a role in regulating the pathogenic B cells. A murine MG model, generated by immunizing mice with heterologous AChR from the electric fish Torpedo californica, has been used extensively. In these animals, Abs cross-react with murine AChR; however, the T cells do not. Thus, to study tolerance to AChR, a transgenic mouse model was generated in which the immunodominant Torpedo AChR (T-AChR) alpha subunit is expressed in appropriate tissues. Upon immunization, these mice showed greatly reduced T cell responses to T-AChR and the immunodominant alpha-chain peptide. Limiting dilution assays suggest the likely mechanism of tolerance is deletion or anergy. Despite this tolerance, immunization with intact T-AChR induced anti-AChR Abs, including Abs against the alpha subunit, and the incidence of MG-like symptoms was similar to that of wild-type animals. Furthermore, evidence suggests that this B cell response to the alpha-chain receives help from T cells directed against the other AChR polypeptides (beta, gamma, or delta). This model offers a novel opportunity to elucidate mechanisms of tolerance regulation to muscle AChR and to clarify the role of T cells in MG.

Anti-CTLA-4 antibody treatment triggers determinant spreading and enhances murine myasthenia gravis

CTLA-4 appears to be a negative regulator of T cell activation and is implicated in T cell-mediated autoimmune diseases. Experimental autoimmune myasthenia gravis (EAMG), induced by immunization of C57BL/6 mice with acetylcholine receptor (AChR) in adjuvant, is an autoantibody-mediated disease model for human myasthenia gravis (MG). The production of anti-AChR Abs in MG and EAMG is T cell dependent. In the present study, we demonstrate that anti-CTLA-4 Ab treatment enhances T cell responses to AChR, increases anti-AChR Ab production, and provokes a rapid onset and severe EAMG. To address possible mechanisms underlying the enhanced autoreactive T cell responses after anti-CTLA-4 Ab treatment, mice were immunized with the immunodominant peptide alpha(146-162) representing an extracellular sequence of the ACHR: Anti-CTLA-4 Ab, but not control Ab, treatment subsequent to peptide immunization results in clinical EAMG with diversification of the autoantibody repertoire as well as enhanced T cell proliferation against not only the immunizing alpha(146-162) peptide, but also against other subdominant epitopes. Thus, treatment with anti-CTLA-4 Ab appears to induce determinant spreading, diversify the autoantibody repertoire, and enhance B cell-mediated autoimmune disease in this murine model of MG.

T cell recognition of muscle acetylcholine receptor in ocular myasthenia gravis

We examined the proliferative response of blood CD4(+) cells to muscle acetylcholine receptor (AChR) subunits and the epitope repertoire of the epsilon and gamma subunits, in ocular myasthenia gravis (oMG) patients and healthy subjects. oMG patients seldom recognized all subunits. The frequency and intensity of recognition was the same for all subunits, irrespective of the disease duration. The responses in oMG were lower than in generalized myasthenia gravis. Healthy subjects had frequent, low responses to one or more subunits. oMG patients recognized several epitopes on the gamma and epsilon subunits, that partially overlapped those recognized in gMG. The subunits and epitopes recognized by individual oMG patients changed over time. Thus, oMG patients have minimal and unstable sensitization of anti-AChR CD4(+) cells, in agreement with their low and inconsistent synthesis of anti-AChR antibody.

Acetylcholine receptors and myasthenia

Much progress has been made in the 26 years since initial studies of the first purified acetylcholine receptors (AChRs) led to the discovery that an antibody-mediated autoimmune response to AChRs causes the muscular weakness and fatigability characteristic of myasthenia gravis (MG) and its animal model, experimental autoimmune myasthenia gravis (EAMG). Now, the structure of muscle AChRs is much better known. Monoclonal antibodies to muscle AChRs, developed as model autoantibodies for studies of EAMG, were used for initial purifications of neuronal AChRs, and now many homologous subunits of neuronal nicotinic AChRs have been cloned. There is a basic understanding of the pathological mechanisms by which autoantibodies to AChRs impair neuromuscular transmission. Immunodiagnostic assays for MG are used routinely. Nonspecific approaches to immunosuppressive therapy have been refined. However, fundamental mysteries remain regarding what initiates and sustains the autoimmune response to muscle AChRs and how to specifically suppress this autoimmune response using a practical therapy. Many rare congenital myasthenic syndromes have been elegantly shown to result from mutations in muscle AChRs. These studies have provided insights into AChR structure and function as well as into the pathological mechanisms of these diseases. Evidence has been found for autoimmune responses even to some central nervous system neurotransmitter receptors, but only one neuronal AChR has so far been implicated in an autoimmune disease. Thus far, only two neuronal AChR mutations have been found to be associated with a rare form of epilepsy, but many more neuronal AChR mutations will probably be found to be associated with disease in the years ahead.

Prevention of autoimmune attack by targeting specific T-cell receptors in a severe combined immunodeficiency mouse model of myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease targeting the skeletal muscle acetylcholine receptor. We have previously demonstrated a selection bias of CD4+ T cells expressing the Vbeta5.1 T-cell receptor gene in the thymus of HLA-DR3 patients with MG. To evaluate the pathogenicity of these cells, severe combined immunodeficiency mice engrafted with MG thymic lymphocytes were treated with anti-Vbeta5.1 antibody. Signs of pathogenicity (eg, acetylcholine receptor loss and complement deposits at the muscle end plates of chimeric mice) were prevented in anti-Vbeta5.1-treated severe combined immunodeficiency chimeras. Pathogenicity was mediated by autoantibodies against acetylcholine receptor. Thymic cells depleted of Vbeta5.1-positive cells in vitro before cell transfer were nonpathogenic, indicating that Vbeta5.1-positive cells are involved in the production of pathogenic autoantibodies. Acetylcholine receptor loss was prevented by Vbeta5.1 targeting in HLA-DR3 patients only, demonstrating specificity for HLA-DR3-peptide complexes. The action of the anti-Vbeta5.1 antibody involved both the in vivo depletion of Vbeta5.1-expressing cells and an increase in the interferon-gamma/interleukin-4 ratio, pointing to an immune deviation-based mechanism. This demonstration that a selective and specific T-helper cell population is involved in controlling pathogenic autoantibodies in MG holds promise for the treatment of MG.

Detection of antibodies directed against the cytoplasmic region of the human acetylcholine receptor in sera from myasthenia gravis patients

The nicotinic acetylcholine receptor (AChR) is the autoantigen in the human autoimmune disease myasthenia gravis (MG). Anti-AChR antibodies in MG sera bind mainly to conformational epitopes, therefore the determination of their specificities requires the use of native AChR. Antibody competition studies suggest that most MG antibodies are directed against the extracellular part of the molecule, whereas antibodies directed against the cytoplasmic region of the AChR have not been detected. To determine whether even small quantities of such antibodies exist in MG sera, we performed competition experiments based on the inhibition by MG sera of the binding of MoAbs to the human AChR, rather than inhibition by MoAbs of the binding of MG sera performed earlier. When MoAbs directed against cytoplasmic epitopes on the alpha or beta subunits (alpha 373-380 and beta 354-360) were used as test MoAbs, 17% or 9% of MG sera inhibited the binding of the anti-alpha or anti-beta subunit MoAbs, respectively, by > or = 50%. Non-specific inhibition was excluded. These results suggest the presence, in several MG sera, of antibodies directed against cytoplasmic regions of the AChR; yet these antibodies seemed to represent a relatively small proportion of the total anti-AChR antibodies. The corresponding epitopes may be involved in the inducing mechanisms in certain MG cases, and knowledge of the presence of such antibodies may be useful in understanding the autoimmune mechanism involved in MG.

TCR-Vbeta usage in the thymus and blood of myasthenia gravis patients

In myasthenia gravis (MG) the muscle acetylcholine receptor (AChR) is the target of an autoimmune response. The anti-AChR response may originate in the thymus, which is abnormal in most MG patients and contains anti-AChR T and B cells. Microbial superantigens (sAg) may trigger autoimmune responses and in this study we sought clues as to whether sAg play a role in the pathogenesis of MG. We investigated the frequency of use of the different TCR Vbeta families by the thymus and blood T cells in MG patients and in control subjects, using a multi-primer PCR assay. Identical TCR-Vbeta usage was found in the thymi of MG patients and controls, except Vbeta2, which showed a small increase in MG patients' thymi. Blood T cells of MG patients used Vbeta4, Vbeta6, Vbeta15, Vbeta16 and Vbeta24 significantly more than those of the controls. Vbeta4 and Vbeta6 are the gene families most frequently used by anti-AChR CD4(+) cells in MG patients. Blood T cells from MG patients used Vbeta12, Vbeta14, Vbeta17 and Vbeta18 significantly less than controls. MG patients used Vbeta4 and Vbeta6 significantly more in the blood than in the thymus, while the opposite occurred for Vbeta7, Vbeta12 and Vbeta14. Controls used Vbeta17 more and Vbeta24 less in the blood than in the thymus. The preferential expansion of Vbeta4 and Vbeta6 in MG patients might reflect the immunodominance of certain AChR epitopes, or the action of a sAg outside the thymus. The minimal differences in the TCR-Vbeta usage in the blood and thymus of control subjects might be due to expansion of T cell clones specific for common antigens. Identical Vbeta usage in the thymi of MG patients and controls does not support an important role of the thymus as the location of anti-AChR sensitization when MG is clinically evident. The differences observed in the Vbeta usage in blood and thymi of MG patients are likely to be due to preferential Vbeta usage by the anti-AChR T cells in the blood.