Stimulatory effects of cyclosporin A on human and mouse thymic epithelial cells.

When injected daily into normal young or aging mice, cyclosporin A (CsA) induces a significant stimulation of thymic hormone (thymulin) secretion, as measured by the peripheral level of the hormone and the number of thymulin-producing cells in the thymus. This stimulation is dose dependent and reversible after the end of treatment. Similar findings have been made in primary cultures of human thymic epithelial cells in which CsA increases the percentage of thymulin-producing cells evaluated by immunofluorescence, as well as the amount of hormone released into the supernatants. No effect of CsA is observed on the expression of class II major histocompatibility complex antigens. CsA also increases the proliferation of cultured human thymic epithelial cells, as assessed by bromodeoxyuridine and [3H]thymidine incorporation. The present study strongly suggests that CsA can stimulate the proliferation and endocrine function of thymic epithelial cells. Its action on the immune system is thus not restricted to lymphoid cells and might be partially mediated by its effect on the thymic epithelium.