Electron Microscopic Study on Acute Thymic Involution Induced by Polyoma Virus Infection

Summary

Acute thymic involution induced by polyoma virus (PV) in AKR mice was studied ultrastructurally. The involuted thymuses were characterized by the presence of abundant nuclear debris derived from thymic lymphocytes (TL). Besides intercellular spaces, TL were included within the cytoplasm of epithelial reticular cells and macrophages. In epithelial cell cytoplasm, TL often contacted the nuclear membrane, and occasionally the limiting membrane, which surrounded these TL, was obscured. PV particles were often detected in epithelial cell nuclei and in phagosomes of macrophages. Their presence in epithelial cell nuclei and the consequent development of epithelial thymoma in adults strongly suggest that epithelial cells in the thymus are susceptible to PV and are transformed just after PV infection at the neonatal period. Their roles in lymphogenesis in earlier life and in regulation of peripheral lymphocytes suggest that TL destruction is caused by the functional alteration of the epithelial reticular cells. PV particles were not around most of the destroyed TL, and direct viral participation in TL destruction was not evident. Peculiar osmiophilic dense structures occasionally observed near the altered TL nuclei were studied morphologically and compared with structures detected in thymuses involuted by cortisone or X irradiation.