The immunosuppressive agent cyclosporin A (CsA), which interferes with signal transduction pathways leading to cytokine gene transcription in activated T cells. was investigated regarding its ability to induce apoptosis in T cells undergoing cell cycle progression and activation. In Jurkat and peripheral CD4+ T cells, CsA was found to markedly induce apoptosis at the G0 phase of the cell cycle. Susceptibility to CsA-induced apoptosis progressively decreased during cell cycle progression to the S and G2/M phase, and subsequent T cell receptor-and mitogen-mediated activation totally abrogated CsA-induced apoptosis. Because CsA is an inhibitor of the chymotryptic peptidase activity of the proteasome, susceptibility to apoptosis induced by the proteasome inhibitor lactacystin was investigated under the same conditions. A progressive increase of the susceptibility of T cells to lactacystin-induced apoptosis during cell cycle progression and activation was demonstrated. Intracellular protein levels of the cyclin-dependent kinase inhibitor p27(Kip1) deceased from the G0 to G2/M phase and from the cycling to the activation state, but remained unchanged during the induction of apoptosis by CsA and lactacystin, suggesting a role of p27(Kip) in the regulation of susceptibility to apoptosis during cell cycle progression and activation. Inhibition of CsA- but not lactacytin-induced apoptosis by overexpression of Bcl-2 in Jurkat T cells revealed that CsA and proteasome inhibitors activate different apoptotic pathways, while both CsA- and lactacystin-induced apoptosis were found to be dependent on caspase activation and independent of the FasL/Fas system. The results show that T cells can progressively regulate their susceptibility to apoptosis during cell cycle progression and activation in a stimulus-dependent manner, and suggest that lactacystin, but not CsA, is able to deplete activated T cells by apoptosis, a mechanism deemed necessary for the induction of allograft tolerance. (C) 2003 Elsevier Inc. All rights reserved.