We compared T-cell proliferative responses to acetylcholine receptor (AChR) and to purified protein derivative (PPD) (of tuberculin) of hyperplastic thymus, thymoma, and blood cells from patients with myasthenia gravis (MG). Hyperplastic MG thymus cells gave significantly higher and more consistent responses to AChR than parallel cultures of autologous blood cells, whereas responses to PPD showed an opposite trend. Thus there was a preferential localization of AChR-reactive T cells in the hyperplastic MG thymus. Furthermore, there was a strong correlation between blood and thymus cell responses to PPD (but not to AChR), arguing that the hyperplastic MG thymus contains a sample of sensitized peripheral T cells. By contrast, both AChR-and PPD-responsive T cells were almost undetectable in thymus from nonmyasthenic patients, which is evidently much less receptive to circulating T cells. Cells from MG thymomas showed the highest stimulations by AChR but did not consistently react to PPD. However, the uninvolved thymus adjacent to these thymomas behaved almost identically to the hyperplastic samples described above. Our interpretation is that AChR-specific T cells are initially sensitized in the MG thymoma but are selectively trapped in the hyperplastic thymus after being primed elsewhere.
Sommer N, Willcox N, Harcourt GC, Newsom-Davis J. Myasthenic thymus and thymoma are selectively enriched in acetylcholine receptor-reactive T cells. Ann Neurol 1990;28:3 12-319
The thymoma in MG is a neoplasm of subcapsular and cortical epithelial cells that typically generates enormous numbers of cortical thymocytes in a disorganized environment 117, 181. It also contains many mature (CD3+) T cells that might develop directly from these thymocytes or could be invading from outside. Around 30% of patients with thymoma develop MG and some have autoimmune bone marrow dys-From the