p27kip1 functions as an anergy factor inhibiting interleukin 2 transcription and clonal expansion of alloreactive human and mouse helper T lymphocytes.

Although recent in vitro studies have begun to decipher the molecular events that characterize the anergic state, their in vivo biologic relevance and potential clinical importance remain unclear. Here, using anergic human T-cell clones and tolerant alloreactive mouse T cells that do not induce graft-versus-host disease, we show that p27kip1 cyclin-dependent kinase inhibitor is an essential regulator responsible for the blockade of clonal expansion of anergic T cells in vitro and in vivo. Moreover, in anergic cells, p27kip1 associates with the c-Jun co-activator JAB1, resulting in defective transactivation of AP-1 and interleukin 2 transcription. Therefore, pharmacological agents that upregulate the expression of or prevent the degradation of p27kip1 during antigen recognition should be part of new therapeutic strategies to induce antigen-specific T-cell unresponsiveness.

CD28 costimulation mediates T cell expansion via IL-2-independent and IL-2-dependent regulation of cell cycle progression.

In the presence of TCR ligation by Ag, CD28 pathway mediates the most potent costimulatory signal for T cell activation, cytokine secretion, and T cell expansion. Although CD28 costimulation promotes T cell expansion due to IL-2 secretion and subsequent signaling via the IL-2 receptor, recent studies indicate that the dramatic T cell expansion mediated through the unopposed CD28 stimulation in CTLA4-deficient mice is IL-2 independent. Therefore, we sought to dissect the effects of CD28 and IL-2 receptor pathways on cell cycle progression and determine the molecular mechanisms by which the CD28 pathway regulates T cell expansion. Here we show that CD28 costimulation directly regulates T cell cycle entry and progression through the G1 phase in an IL-2-independent manner resulting in activation of cyclin D2-associated cdk4/cdk6 and cyclin E-associated cdk2. Subsequent progression into the S phase is mediated via both IL-2-dependent and IL-2-independent mechanisms and, although in the absence of IL-2 the majority of T cells are arrested at the G1/S transition, a significant fraction of them progresses into the S phase. The key regulatory mechanism for the activation of cyclin-cdk complexes and cell cycle progression is the down-regulation of p27kip1 cdk inhibitor, which is mediated at the posttranscriptional level by its ubiquitin-dependent degradation in the proteasome pathway. Therefore, CD28 costimulation mediates T cell expansion in an IL-2-independent and IL-2 dependent manner and regulates cell cycle progression at two distinct points: at the early G1 phase and at the G1/S transition.