Effect of acyclovir in vitro on lymphopoiesis in the foetal rat thymus

The thymic lobes of 17-day-old rat foetuses were cultured for 7 days with the addition of acyclovir (ACV) at concentrations of 3-300 mum. The effect of the nucleoside analogue on lymphopoiesis in the organ culture was measured by isolation and quantification of the cultured thymocytes. Starting from 6.9 +/- 1.6 x 10(4) lymphatic cells per thymic lobe at the beginning of the culture period, this value increased in untreated control cultures to 98 +/- 22.5 x 10(4) thymocytes per lobe at the end of the period. Concentrations as low as 10 mum-ACV caused a significant (P < 0.05) reduction in the number of lymphatic cells to 59.7 +/- 4.5 x 10(4) thymocytes per lobe compared with the untreated controls. Increasing concentrations of acyclovir led to a dose-dependent inhibition of cell proliferation and to an apparent cytotoxicity. Light- and electron-microscopic investigations revealed a selective effect on lymphatic cells, whereas the morphology of the thymic epithelium was only slightly altered.