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Molecular and Cellular Biology, September 2004, p. 7313-7323, Vol. 24, No. 17
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.17.7313-7323.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Hyperresponse to T-Cell Receptor Signaling and Apoptosis of Id1 Transgenic Thymocytes

Immunobiology and Cancer Program, Oklahoma Medical Research Foundation,¹ Department of Microbiology and Immunology,² Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma³

Received 23 January 2004/ Returned for modification 12 March 2004/ Accepted 10 June 2004

The basic helix-loop-helix transcription factors, E2A and HEB, play important roles in T-cell development at multiple checkpoints. Expression of their inhibitor, Id1, abolishes the function of both transcription factors in a dose-dependent manner. The Id1 transgenic thymus is characterized by an accumulation of CD4^– CD8^– CD44⁺ CD25^– thymocytes, a dramatic reduction of CD4⁺ CD8⁺ thymocytes, and an abundance of apoptotic cells. Here we show that these apoptotic cells carry functional T-cell receptors (TCRs), suggesting that apoptosis occurs during T-cell maturation. In contrast, viable Id1 transgenic CD4 single positive T cells exhibit costimulation-independent proliferation upon treatment with anti-CD3 antibody, probably due to a hyperresponse to TCR signaling. Furthermore, Id1 expression causes apoptosis of CD4 and CD8 double- or single-positive thymocytes in HY- or AND-TCR transgenic mice under conditions that normally support positive selection. Collectively, these results suggest that E2A and HEB proteins are crucial for controlling the threshold for TCR signaling, and Id1 expression lowers the threshold, resulting in apoptosis of developing thymocytes.

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