Evidence that FTY720 induces rat thymocyte apoptosis.

FTY720, a novel immunomodulator with the potential to improve immunosuppressive therapy after organ transplantation, is currently under clinical investigation. FTY720 drastically decreases blood lymphocytes, especially T cells, accelerating lymphocyte homing to secondary lymphoid organs. However, its immunosuppressive effects remain unknown. We investigated these effects in rat thymocytes. Rats were intramuscularly injected with 10mg/kg/day FTY720 or saline for 7days. Thymuses were removed on days 0, 1, 3, 5, 7 and 14 after treatment. Three-color analysis was performed with a flow cytofluorometer. Apoptotic nuclei in the tissue sections were identified by TUNEL. Genomic DNA was then extracted and samples were electrophoresed on 2.0% agarose gel. FTY720 reduced the total number of thymocytes and, with time, significantly reduced the percentage of CD4+8+ TCRalphabeta(negative/low) thymocytes. Light microscopy of thymuses of FTY720-treated rats revealed obvious reductions in the size of the cortical region. TUNEL analysis showed that FTY720 induced thymocyte apoptosis in the cortical region. Furthermore, DNA fragmentation was observed in thymocytes treated with FTY720, indicating thymocyte apoptosis. FTY720 reduced the number of CD4+8+ thymocytes before TCRalphabeta expression resulting in impaired thymocyte differentiation and maturation. This might be an immunosuppressive effect of FTY720.