INDUCTION OF APOPTOSIS IN MOUSE THYMOCYTES BY CYCLOSPORINE-A - IN-VIVO STUDY

We investigated the in vivo effect of cyclosporin A (CsA) on mouse thy mus and thymocytes. Administration Of CsA (10 mg/kg of body weight) wa s found to induce a marked reduction in the size, weight and consisten cy of the thymus. These modifications were associated with thymic reti culo-epithelial cells (TREC) and thymocyte damage. Some of the damaged thymocytes displayed characteristic of cells undergoing apoptosis. Ul trastructural study of thymocytes and thymic tissue, as well as DNA el ectrophoresis of thymocytes, showed chromatin condensation, cellular s hrinkage, and nuclear fragmentation in oligonucleosomal fragments. DNA labeling with propidium iodide (PI) of thymocytes from CsA treated mi ce cultured for 24 hrs showed an increased number of apoptotic nuclei. Furthermore, flow cytometric analysis using monoclonal antibodies (mA bs) specific for thymocyte subsets confirmed that CsA induces a large decrease in the relative number of mature single positive (SP) CD4+CD8 - and CD8+CD4- thymocytes expressing high densities of CD3 and T cell receptor ab (TCR alpha beta) surface molecules, but also a decrease in the absolute number of the other thymocyte subsets. These results sug gest that CsA causes macroscopic and ultrastructural modifications of the thymus, associated with an active process of cell death in mouse t hymocytes in vivo. In line with these results we formulate a hypothesi s concerning the stage of T-cell development at which CsA induces apop tosis.