Opposing Roles for the Cyclin-Dependent Kinase Inhibitor p27kip1in the Control of CD4+ T Cell Proliferation and Effector Function

Abstract

Cell division drives T cell clonal expansion and differentiation, and is the result of concerted signaling from Ag, costimulatory, and growth factor receptors. How these mitogenic signals are coupled to the cell cycle machinery in primary T cells is not clear. We have focused on the role of p27kip1, a major cyclin-dependent kinase binding protein expressed by CD4+ T cells. Our studies using p27kip1 gene dosage demonstrate that early after activation, p27kip1 acts to promote, rather than inhibit, G1 to S phase progression within the first division cycle. However, throughout subsequent cell divisions p27kip1 behaves as a negative regulator, directly establishing the threshold amount of growth factor signaling required to support continued cell division. During this phase, signals from CD28 and IL-2R cooperate with the TCR to “tune” this threshold by inducing the degradation of p27kip1 protein, and we show that agents that block these pathways require elevated p27kip1 levels for their full antiproliferative activity. Finally, we show that p27kip1 opposes the development of CD4+ T cell effector function, and is required for the full development of anergy in response to a tolerizing stimulus. Our results suggest that p27kip1 plays a complex and important role in the regulation of cell division and effector function in primary CD4+ T cells.

The Cyclin-Dependent Kinase Inhibitor p27kip1 Is Required for Transplantation Tolerance Induced by Costimulatory Blockade

“…2B, Ⅺ). These results are consistent with in vitro studies showing that p27 kip1 sets a costimulatory threshold for T cell proliferation (15,24), and suggest that alloreactive lymphocytes that lack p27 kip1 can expand in vivo independently of costimulation. T cell differentiation and proinflammatory cytokine gene expression are linked to the cell cycle (28 -30); therefore, we investigated whether the CDK inhibitor p27 kip1 regulates the generation of effector cells during alloimmune vs tolerant responses in vivo.…”

“…We found little difference between the expansion of wild-type and p27 kip1 -deficient T cells with no costimulatory blockade (Fig. 4A), most likely because wild-type T cells efficiently down-regulate p27 kip1 when strongly stimulated (15). Under conditions of combined costimulatory blockade, there was ϳ1-log decrease in the recovery of alloreactive cells as compared with the control Ig-treated hosts.…”

“…T cell function is critically dependent on costimulatory interactions between APCs and T cells (1). p27 kip1 is expressed at high levels in resting T cells, and is degraded upon activation by signals from CD28 and IL-2R (4,15,35,36). Down-modulation of p27 kip1 is required for efficient T cell cycle entry (5,6,12,15,37), and in the absence of this CDK inhibitor, T cells exhibit increased CDK2-mediated phosphorylation of Rb (16) and undergo a significant degree of costimulation-independent cell division (15,24).…”

“…p27 kip1 is important for the normal development of most mammalian tissues (16), and regulates the capacity of naive T lymphocytes to proliferate to mitogenic stimulation in vitro, particularly under conditions in which costimulation or growth factors are limiting (15,24,25). Therefore, we examined whether costimulation-independent graft rejection by p27 kip1Ϫ/Ϫ mice was characterized by an augmented alloimmune response in the periphery.…”

“…Genetic elimination of p27 kip1 , in contrast, has been shown to lead to resistance to in vitro anergy induction, further confirming its importance in regulating T cell effector function (12,15). Despite mounting evidence that p27 kip1 serves to set the threshold for costimulation and is an important mediator of anergy, it is unclear whether in vivo tolerance requires p27 kip1 .…”