Regulatory T cells and Hassall's corpuscles in myasthenia gravis patients thymus

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Myasthenia gravis (MG) is a neurological autoimmune disease caused by autoantibodies against components of the neuromuscular junction. The thymus is implicated as a site that triggers autoimmunity in MG. The medulla of the thymus provides central self-tolerance in terms of the deletion of autoreactive T cells and the generation of regulatory T cells (Treg). However, our previous study showed that the cellularity of Treg in the thymus and the circulation was not diminished in MG patients. Hassall’s corpuscles (HCs) are assumed to represent the terminal differentiated stage of thymic epithelial cells (mTECs). We examined the HCs in the thymus. Multicolor immunohistofluorescence analysis of involucrin, CD4, and CD8 was performed in the thymic sections obtained from MG(+) (n=27) and MG (-) patients (n=24). HCs were identified to be involucrin (+) concentric cellular clusters. The number of HCs per medullary area was significantly elevated in the thymus of MG patients who exhibited the thymic hyperplasia. CCL21 expression was also increased in the thymus of those patients. We speculate that altered differentiation of mTECs may be associated with thymic hyperplasia and the onset of MG.