p21 Cip1 and p27 Kip1 Act in Synergy to Alter the Sensitivity of Naive T Cells to TGF-β-Mediated G1 Arrest through Modulation of IL-2 Responsiveness

Abstract

Induction of G1 arrest by TGF-β correlates with the regulation of p21Cip1 and p27Kip1, members of the Cip/Kip family of cyclin-dependent kinase inhibitors (cki). However, no definitive evidence exists that these proteins play a causal role in TGF-β1-induced growth arrest in lymphocytes. In this report we show the suppression of cell cycle progression by TGF-β is diminished in T cells from mice deficient for both p21Cip1 and p27Kip1 (double-knockout (DKO)) only when activated under conditions of optimal costimulation. Although there is an IL-2-dependent enhanced proliferation of CD8+ T cells from DKO mice, TGF-β is able to maximally suppress the proliferation of DKO T cells when activated under conditions of low costimulatory strength. We also show that the induction of p15Ink4b in T cells stimulated in the presence of TGF-β is not essential, as TGF-β also efficiently suppressed proliferation of T cells from p15Ink4b−/− mice. Finally, although these cki are dispensable for the suppression of T cell proliferation by TGF-β, we now describe a Smad3-dependent down-regulation of cdk4, suggesting a potential mechanism underlying to resistance of Smad3−/− T cells to the induction of growth arrest by TGF-β. In summary, the growth suppressive effects of TGF-β in naive T cells are a function of the strength of costimulation, and alterations in the expression of cki modify the sensitivity to TGF-β by lowering thresholds for a maximal mitogenic response.