AbstractThe effects of rapamycin (RAP) on cell cycle progression of human T cells stimulated with PHA were examined. Cell cycle analysis showed that the RNA content of cells stimulated with PHA in the presence of RAP was similar to that of control T cells stimulated with PHA for 12–24 hr in the absence of the drug. This level was substantially higher than that seen in cells stimulated in the presence of cyclosporin A (CsA), an immunosuppressant known to block cell cycle progression at an early point in the cycle. However, the point in the cell cycle at which RAP acted appeared to be well before the G1/S transition, which occurs about 30–36 hr after stimulation with PHA. In an attempt to further localize the point in the cell cycle where arrest occurred, a set of key regulatory events leading to the G1/S boundary were examined, including p110Rbphosphorylation, which occurred at least 6 hr prior to DNA synthesis, p34cdc2synthesis, and cyclin A synthesis. In control cultures, p110Rbphosphorylation was detected within 24 hr of PHA stimulation; p34cdc2and cyclin A synthesis were detected within 30 hr. Addition of RAP to the cultures inhibited each of these events. In contrast, early events, including c‐fos, IL‐2, and IL‐4 mRNAs expression, and IL‐2 receptor (p55) expression, were only marginally affected, if at all, in PHA‐stimulated T cells. Furthermore, the inhibition of cell proliferation by RAP could not be overcome by addition of exogenous IL‐2. These results indicate that RAP blocks cell cycle progression of activated T cells after IL‐2/IL‐2 receptor interaction but prior to p110Rbphosphorylation and other key regulatory events signaling G1/S transition. ©