Autoimmune myasthenia gravis (MG) is a muscular disease mediated by autoantibodies, mainly directed against the acetylcholine receptor (AChR). The pathogenic antibodies are especially produced in the thymus, which is often characterized by a hyperplasia with germinal centers. Recent studies demonstrated the overexpression of chemokines and the abnormal development of high endothelial venules (HEV) in the MG thymus. The aim of my thesis was to better understand the mechanisms that lead to thymic hyperplasia in MG by analyzing the role of chemokines in peripheral cell recruitment. We demonstrated that the number of HEVs correlated with the degree of hyperplasia suggesting a direct link between HEVs and peripheral cell recruitment. To define its mechanism of action, we examined which chemokines were expressed on thymic HEVs. We uniquely detected SDF-1 and observed that B cells, myeloid dendritic cells (mDCs), plasmacytoid DCs and monocytes/macrophages that expressed the SDF-1 receptor CXCR4 localized inside and around thymic HEV. In parallel we observed a decreased CXCR4 expression and a decreased number of mDCs and also monocytes in the periphery suggesting their recruitment to the MG thymus. As the MG thymus was recently characterized by the overexpression of CXCL13 in thymic epithelial cells (TECs), we investigated its contribution to thymic hyperplasia. We therefore generated a transgenic mouse model overexpressing in medullary TECs CXCL13 under the control of keratin 5. We demonstrated that transgenic K5-CXCL13 mice specifically overexpressed CXCL13 in the thymus, while no other tested chemokines were upregulated. Preliminary results showed that elevated levels of CXCL13 resulted in an increased number of B cells in the thymus of transgenic mice, which localized preferentially in loose aggregates in medullary areas. We are presently investigating if immunization with purified AChR induces experimental MG with thymic hyperplasia in these mice. Myasthenic mice with a hyperplastic thymus could present a new animal model for MG with a phenotype that is closer to the human disease than the current MG model. As the hyperplastic MG thymus displays the hallmarks of a viral signature, we investigated the effect of pathogen-associated molecules on thymic changes associated with MG. We demonstrated that dsRNA signaling induced by Poly(I:C) specifically triggers the overexpression of α-AChR in human TECs through the release of IFN-I. We also observed that IFN-I was able to upregulate CXCL13 and CCL21, similarly to what is observed in the MG thymus. In addition, Poly(I:C) injections in wildtype mice, but not in IFN-I receptor KO mice, specifically increase thymic expression of α-AChR and, in parallel, CXCL13 and CCL21 expression. In periphery, Poly(I:C) even induced an anti-AChR autoimmune response characterized by a significant production of serum anti-AChR antibodies and a specific proliferation of B cells. Overall the results obtained in the course of my PhD showed that the abnormal development of SDF-1-expressing HEVs and the CXCL13 overexpression play a central role in the recruitment of peripheral cells to the MG thymus. Once these cells have arrived in the inflammatory environment, which is characteristic for MG, they could develop an autoimmune reaction against AChR. New therapeutic molecules that control chemokine expression and angiogenic processes could diminish the development of thymic hyperplasia and avoid thymectomy or the use of corticoids.
