[Effect of cyclosporin on the thymus and spleen in rats]

Cyclosporin is a potent immunosuppressive agent. When cyclosporin is administered at a dose of 5 mg/kg/day i.p. for 2 weeks and 8 weeks, 15 mg/kg/day i.p. for 2 weeks, and 30 mg/kg/day i.p. for 1 week to normal Fischer rats, all thymuses were atrophic and their weight and number of thymocytes decreased dose dependently. However, no change was observed in spleen size. Histologically, thymic medulla was severely atrophic and the cellularity of thymic cortex, and splenic periarterial sheath and splenic marginal zone were decreased. Thymic cortex was more atrophic in the high dose group. All components recovered to near normal morphology 2 to 3 weeks after drug withdrawal, but recovery of splenic change was delayed in many rats. A reduction of T lymphocytes, particularly, helper T lymphocytes was detected by avidin-biotin peroxidase complex method using the monoclonal antibodies, W3/13, W3/25, OX8 and OX6. Many of the rats administered cyclosporin developed infection. Because of these changes in the immune response, precaution must be taken to prevent infection.