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(American Journal of Pathology. 2005;167:129-139.)
© 2005 American Society for Investigative Pathology

Increased Toll-Like Receptor 4 Expression in Thymus of Myasthenic Patients with Thymitis and Thymic Involution

Pia Bernasconi*, Massimo Barberis{dagger}, Fulvio Baggi*, Laura Passerini*, Maria Cannone{dagger}, Elisa Arnoldi*, Lorenzo Novellino{ddagger}, Ferdinando Cornelio* and Renato Mantegazza*

From the Department of Neurology IV,* Istituto Nazionale Neurologico "Carlo Besta," Milan; MultiLab,{dagger} Gruppo MultiMedica, Milan; and Ospedali Riuniti,{ddagger} Bergamo, Italy

Thymic abnormalities are present in ~80% of myasthenia gravis (MG) patients, and the thymus seems to be the main site of autosensitization to the acetylcholine receptor. In view of findings that the innate immune system can generate an autoimmune response, we studied the expression of Toll-like receptors (TLRs) 2 to 5, key components of innate immunity signaling pathways, in 37 thymuses from patients with autoimmune MG. TLR4 mRNA levels were significantly greater in thymitis (hyperplasia with diffuse B-cell infiltration) and involuted thymus than in germinal center hyperplasia and thymoma. By immunohistochemistry and confocal microscopy, cells positive for TLR4 protein were rarely detected in thymoma. However, in thymitis TLR4 protein was mostly found on epitheliomorphic (cytokeratin-positive) cells located in close association with clusters of acetylcholine receptor-positive myoid cells in thymic medulla and also at the borders between cortical and medullary areas. B cells were never TLR4-positive. TLR4 protein was also present in remnant tissue of involuted thymus. This is the first finding of a possible link between innate immunity and MG. We speculate that in a subgroup of MG patients, an exogenous or endogenous danger signal may activate the innate immune system and give rise to TLR4-mediated mechanisms contributing to autoimmunity.






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Copyright © 2005 by the American Society for Investigative Pathology.