AbstractThe effect of cyclosporin A (CsA) on mouse thymocytes was investigatedin vitro. Ultrastructural examination and DNA electrophoresis of 24hr-CsA-treated thymocytes showed chromatin condensation, cellular shrinkage and nuclear fragmentation in oligonucleosomal fragments. DNA labeling of CsA-treated-thymocytes with propidium iodide (PI) showed an increase in the number of apoptotic nuclei compared to untreated thymocytes. Furthermore, flow cytometric analysis using monoclonal antibodies (mAbs) specific to particular thymocyte subsets showed that CsA induces a decrease in the relative number of immature double positive (DP) CD4+CD8+ thymocytes in direct proportion to the concentration of CsA. No significant changes were observed in mature single positive (SP) CD4+CD8-and CD8+CD4-cells. Moreover, the cell viability of CsA-treated thymocytes was decreased. These results suggest that CsA induces the programmed cell death of thymocytesin vitro. Taken together with our previous study on thein vivoeffect of CsA on mouse thymus and thymocytes, the present study confirms that, in addition to its effect on the thymic epithelium, CsA acts directly on thymocytes by inducing their programmed cell death. We postulate that immature DP thymocytes are the most likely targets of CsA.