Rapamycin inhibits the proliferation of many mammalian cell types, including lymphocytes, making the compound useful as an immunosuppressant. Rapamycin has also been a useful tool for studying signaling mechanisms regulating cellular proliferation. However, the effects of rapamycin remain poorly understood, and the precise mechanisms of clinical action remain elusive. Previously, we found that, depending on the strength of the signal delivered to the T cell via both the T cell receptor and the costimulatory molecule CD28, CD8؉ T cells are capable of rapamycin-resistant proliferation. Here, we have further elucidated the mechanism of rapamycin-resistant proliferation of human CD8؉ T cells. Under conditions where rapamycin inhibited proliferation, p27 kip1 down-regulation was prevented, whereas under conditions resulting in rapamycin-resistant proliferation, p27 kip1 was down-regulated. Further, T cell receptor/CD28-dependent induction of bcl-x L expression was not inhibited by rapamycin, which correlated with both rapamycin-resistant proliferation and increased cell survival. Moreover, an inhibitor of phosphoinositide 3-kinase activity was able to eliminate rapamycin-resistant proliferation of freshly isolated CD8؉ human cells, strongly suggesting that phosphoinositide 3-kinase activity was required for the rapamycin-resistant proliferation of CD8؉ T cells. The selective immunosuppressive effect of rapamycin in human CD8؉ T cell populations could be predictive of a selective effect allowing cytotoxic responses during microbial infections where there are strong strengths of signals associated with high affinity T cell receptors and strong costimulatory second signals. In contrast, the weaker autoimmune and perhaps allogeneic responses can be selectively inhibited by the actions of rapamycin.Rapamycin is an immunosuppressant that functions by inhibiting entry into the cell cycle. This inhibition of proliferation by rapamycin occurs at a later stage of cellular activation than the inhibition by cyclosporin A and FK506, which inhibit calcineurin or calcineurin-dependent transcriptional activation of lymphokine genes (Ref. 1; reviewed in Ref. 2). It is thought that rapamycin primarily inhibits growth factor signaling rather than growth factor synthesis (3). The potent immunosuppressive activity of rapamycin has led to its wide clinical use in solid organ transplantation (reviewed in Ref. 4). More recent studies demonstrated successful islet cell transplantation in patients with type I diabetes treated with low dose rapamycin in combination with humanized anti-IL-2 1 receptor monoclonal antibody (mAb) and FK506, which prevented autoimmune destruction of islet cells (5). However, the clinical mechanism of action of rapamycin in suppressing immune responses is not well defined.Although it is clear that entry into the cell cycle is generally inhibited by rapamycin, we recently made the surprising observation that increasing the strength of signal delivered to the T cell via both the T cell receptor (TcR) and the costimula...