Dysregulated expression of pre-Tα reveals the opposite effects of pre-TCR at successive stages of T cell development

The pre-TCR complex (TCRβ-pre-TCRα chain (pTα)), first expressed in a fraction of CD8^-4^-CD44^-25⁺ (DN3) cells, is believed to facilitate or enable an efficient transition from the CD8^-4^- double-negative (DN) to the CD⁸⁺4⁺ double-positive (DP) developmental stage. Subsequent to pre-TCR expression, DN3 thymocytes receive survival, proliferation, and differentiation signals, although it is still unclear which of these outcomes are directly induced by the pre-TCR. To address this issue, we generated mice bearing a range of ptα transgene copy number under the transcriptional control of the p56^lck proximal promoter. All lines exhibited increased DN3 cycling, accelerated DN3/4 transition, and improved DN4 survival. However, the high copy number lines also showed a selective reduction in thymic cellularity due to increased apoptosis of DP thymocytes, which could be reversed by the ectopic expression of Bcl-2. Our results suggest that transgenic ptα likely caused apoptosis of DP thymocytes due to competitive decrease in surface TCRαβ formation. These results highlight the critical importance of precise temporal and stoichiometric regulation of pre-TCR and TCR component expression.