Rapamycin-induced inhibition of p34^cdc2 kinase activation is associated with G₁/S-phase growth arrest in T lymphocytes

The macrolide rapamycin (RAP) is a potent inhibitor of interleukin-2 (IL-2)-induced T-cell proliferation. Current models suggest that RAP, when completed to its intracellular receptor, FK506-binding protein, interferes with an IL-2 receptor-coupled signaling pathway required for cell-cycle progression from G₁- to S-phase. Here we show that RAP treatment inhibits the growth of an IL-2-dependent cytotoxic T-cell line, CTLL-2, in late G₁-phase, just prior to entry of the cells into S-phase. In contrast, RAP-treated CTLL-2 cells retained the ability to respond to IL-2 with enhanced cytolytic activity, indicating that RAP was not a general suppressant of cellular responsiveness to IL-2. Subsequent studies revealed that IL-2 stimulation triggered a delayed activation of the p34^cdc2 kinase, the timing of which correlated with the G₁- to S-phase transition. The IL-2-dependent increase in p34^cdc2 kinase activity was blocked by RAP. The RAP sensitivity of the p34^cdc2 activation mechanism implicates this signaling pathway in the control of S-phase commitment in IL-2-stimulated T-cells.