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Immunity. 2008 Feb;28(2):231-45. doi: 10.1016/j.immuni.2007.12.013.

Thymocyte proliferation induced by pre-T cell receptor signaling is maintained through polycomb gene product Bmi-1-mediated Cdkn2a repression.

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Department of Immunology, Graduate School of Biomedical Science, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan.


Thymocytes undergo massive proliferation before T cell receptor (TCR) gene rearrangement, ensuring the diversification of the TCR repertoire. Because activated cells are more susceptible to damage, cell-death restraint as well as promotion of cell-cycle progression is considered important for adequate cell growth. Although the molecular mechanism of pre-TCR-induced proliferation has been examined, the mechanisms of protection against cell death during the proliferation phase remain unknown. Here we show that the survival of activated pre-T cells induced by pre-TCR signaling required the Polycomb group (PcG) gene product Bmi-1-mediated repression of Cdkn2A, and that p19Arf expression resulted in thymocyte cell death and inhibited the transition from CD4(-)CD8(-) (DN) to CD4(+)CD8(+) (DP) stage upstream of the transcriptional factor p53 pathway. The expression of Cdkn2A (the gene encoding p19Arf) in immature thymocytes was directly regulated by PcG complex containing Bmi-1 and M33 through the maintenance of local trimethylated histone H3K27. Our results indicate that this epigenetic regulation critically contributes to the survival of the activated pre-T cells, thereby supporting their proliferation during the DN-DP transition.

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