INDUCTION OF APOPTOSIS IN MOUSE THYMOCYTES BY CYCLOSPORINE-A - AN IN-VITRO STUDY

The effect of cyclosporin A (CsA) on mouse thymocytes was investigated in vitro. Ultrastructural examination and DNA electrophoresis of 24hr -CsA-treated thymocytes showed chromatin condensation, cellular shrink age and nuclear fragmentation in oligonucleosomal fragments. DNA label ing of CsA-treated-thymocytes with propidium iodide (PI) showed an inc rease in the number of apoptotic nuclei compared to untreated thymocyt es. Furthermore, flow cytometric analysis using monoclonal antibodies (mAbs) specific to particular thymocyte subsets showed that CsA induce s a decrease in the relative number of immature double positive (DP) C D4+CD8+ thymocytes in direct proportion to the concentration of CsA. N o 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 p rogrammed cell death of thymocytes in vitro. Taken together with our p revious study on the in vivo effect of CsA on mouse thymus and thymocy tes, 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 th e most likely targets of CsA.