Autoreactivity to AChR in the thymus of MG patientsMyasthenia Gravis (MG) is a disease caused by anti-acetylcholine receptor (AChR) autoantibodies. The thymus is clearly involved in the pathogenesis of MG: 50 percent of patients show thymic hyperplasia and 15 to 20 percent a thymoma; thymectomy is an efficient treatment for many patients. Our hypothesis is that the onset of MG disease with thymic hyperplasia is due to a coincidence of several components: the presence of the autoantigen, predisposition genes (namely MHC and cytokine genes) and a triggering event. In this favorable genetic situation, the triggering event could induce over activation of the immune system. Since AChR is present in the thymus and is highly immunogenic, and could be over expressed after the activation of the immune system, it could be efficiently presented. These events would lead to the formation of germinal centers and to production of anti-AChR antibodies inside the thymus. The long-term objectives of this application are to devise new approaches for treating MG disease. We must first understand the mechanisms underlying the induction of the anti-AChR autoimmune response in the thymus of MG patients.
The specific aims are to identify molecular and cellular components of the thymus that may be involved in inducing the anti-AChR autoimmune response and to create new models of thymic hyperplasia based on the data obtained. Several questions will be addressed: 1) Is the pattern of AChR expression normal in the thymic tissue of MG patients? Is AChR up regulated by mediators present in the thymus of MG patients? 2) Is the immune system dysregulated? Are the number and distribution of regulatory cells normal? 3) What are the phenotypic and functional characteristics of the autoreactive T cell clones isolated from the MG thymuses? 4) Where are the autoreactive B cells located in the thymus? and what are their frequencies? 5) What is the cascade of events leading to thymic hyperplasia? How to create an experimental model with thymic hyperplasia? The first 2 objectives will help to define the accurate state of the abnormalities in thymic hyperplasia; the objectives 3 and 4 will characterize the T and B cells involved in the pathogenesis and will be very helpful for defining new therapeutics. Finally if the components relevant for thymic hyperplasia are well defined, we should be able to create a new experimental model in which the pathophysiological role of these components will be directly evidenced and where the cascade of events leading to the thymic anti-AChR antibody production could be easily manipulated. Our project is original because we aim to study physiopathological mechanisms directly at the effector site using the relevant material. Patients with MG undergo thymectomy in our hospital. A large collection of pathological human tissue is available and represents an outstanding research material. Several methods will be used: quantitative autoradiography, quantitative RT-PCR, microarrays, transfection, digital image analysis, microdissection, flow cytometry, T cell clones, in situ hybridization, ELISA, alpha-bungarotoxin binding assays, SCID mice.
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