Transgenic Expression of the p16INK4a Cyclin-Dependent Kinase Inhibitor Leads to Enhanced Apoptosis and Differentiation Arrest of CD4−CD8− Immature Thymocytes

Abstract

In the thymus, T cell development proceeds by successive steps of differentiation, expansion, and selection. Control of thymocyte proliferation is critical to insure the full function of the immune system and to prevent T cells from transformation. Deletion of the cell cycle inhibitor p16INK4a is frequently observed in human T cell neoplasias and, in mice, gene targeted inactivation of the Ink4a locus enhances thymocyte expansion and predisposes mutant animal to tumorigenesis. Here, we investigate the mechanism by which p16Ink4a controls thymocyte development by analyzing transgenic mice expressing the human p16INK4a into the T cell lineage. We show that forced expression of p16INK4a in thymocytes blocked T cell differentiation at the early CD4−CD8−CD3−CD25+ stage without significantly affecting the development of γδ T cells. Pre-TCR function was mimicked by the induction of CD3 signaling in thymocytes of recombinase activating gene (RAG)-2-deficient mice (RAG-2−/−). Upon anti-CD3ε treatment in vivo, p16INK4a-expressing RAG-2−/− thymocytes were not rescued from apoptosis, nor could they differentiate. Our data demonstrate that expression of p16INK4a prevents the pre-TCR-mediated expansion and/or survival of differentiating thymocytes.

The tumor suppressor p16Ink4a regulates T lymphocyte survival

“…Similar differences between cell types have been observed in response to the cell cycle inhibitor p21 Cip1/Waf1 , that typically induces growth arrest in fibroblasts and other adherent cells, but promotes apoptosis in T lymphocytes when induced in response to FasL (Hingorani et al, 2000). Further supporting a link between p16 expression and cell death in lymphoid cells, p16 can interfere with Abl-mediated transformation of primary B cells by enhancing apoptosis (Sachs et al, 2004) and transgenic p16 expression prevents expansion and survival of DN immature thymocytes (Lagresle et al, 2002). In contrast, Ink4a À/À mouse embryo fibroblasts have increased sensitivity to UV-induced apoptosis (Al-Mohanna et al, 2004), reinforcing the idea that p16 expression in these cells is related to irreversible growth arrest rather than cell death.…”

Thymomegaly, Microsplenia, and Defective Homeostatic Proliferation of Peripheral Lymphocytes in p51-Ets1 Isoform-Specific Null Mice

“…Ϫ stage and reduced proliferation and function of lymphocytes (30,56). Gene-targeted mice deficient in either p16…”

MYB IS Required for B-Selection During Thymopoises

“…The cdk inhibitors p15, p16, and p17 inhibit cyclin D/cdk complexes and p27 and p21 inhibit all other cyclin/cdk complexes [10]. Expression of a p16 Ink4α transgene or a p27 Kip1 transgene in thymocytes blocked differentiation at the DN3 stage [91,92]. We performed one experiment where we detected an increase in p27 levels in DN3 thymocytes in the absence of c-Myb.…”

“…Expression of a p16 Ink4α transgene or a p27 Kip1 transgene in thymocytes blocked differentiation at the DN3 stage [91,92]. Furthermore, expression of a p16 Ink4α transgene in thymocytes, also resulted in apoptosis in thymocytes attempting to undergo β-selection [91] demonstrating that β-selection proliferation and survival signals may not be exclusive.…”

“…Signals generated by the pre-TCR drive a series of events referred to as β-selection and provide cues for survival, proliferation, allelic exclusion at the Tcrb locus, down regulation of CD25, and differentiation to the . Proliferation during β-selection is also regulated by cdk inhibitors p16 and p27 and expression of a p16 Ink4α transgene or a p27 Kip1 transgene in thymocytes blocked differentiation at the DN3 stage [91,92]. Moreover, expression of p27 protein decreases as cells transit from the DN3E to DN3L stage suggesting pre-TCR control over expression of p27 [2].…”