We report here the appearance of retrovirus-like particles in cultured thymus cells from patients with myasthenia gravis. Cell from five patients with myasthenia gravis were examined. Histologically three were thymomas and two were thymus hyperplasias. The thymus cells were co-cultured for 7 days with allogenic B cells pretreated with mitomycin C. Retrovirus-like particles were demonstrated by electron microscopy in 4 cases of myasthenia gravis. These particles were detected in thymus epithelial cells. In 4 cases of myasthenia gravis examined, reverse transcriptase activity sedimenting at a density of 1.15-1.17 g/cm3 in sucrose density gradients was eluted in the culture fluids. These results suggest that in myasthenic thymuses, retroviruses that can be induced, following a proliferative stimulus, may be involevd in the genesis of thymic disorders, and in the pathogenesis of myasthenia gravis.
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