Résumé :
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Myasthenia Gravis (MG) with anti-acetylcholine receptor antibodies is commonly associated with thymic hyperplasia characterized by the presence of germinal centers containing B cells producing pathogenic antibodies.Our analysis of hyperplastic thymuses from MG patients revealed active angiogenic processes characterized by an increased expression of the vascular marker CD31. By immunohistochemistry, we observed an abnormal presence of numerous high endothelial venules (HEVs). We investigated which chemokines were expressed on the endothelial luminal surface of HEVs and among all the chemokines tested, we only detected the presence of SDF1. SDF1 is known to be expressed by TECs (thymic epithelial cells), but we did not observe an increased expression of SDF1 in TEC cultures from MG patients as compared to non-MG. SDF1 on HEVs could thus be produced directly by endothelial cells or could be transcytosed from TECs to HEVs.SDF1 is involved in the trafficking of monocytes and lymphocytes. We studied the expression of its receptor, CXCR4, but did not observe differences in CXCR4 expression between MG and non-MG thymus. In the periphery, CXCR4 expression did not vary on B and T cells, but surprisingly, decreased on myeloid dendritic cells from MG patients as compared to non-MG patients. The presence of SDF1 on thymic HEVs suggests its involvement in MG thymic hyperplasia. Whether the decreased CXCR4 expression on monocytes reflects an active recruitment of these cells to the thymus deserves further investigation.
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