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

The pre-TCR complex (TCR beta -pre-TCR alpha chain (pT alpha)), first expre ssed in a fraction of CD8(-)4(-)CD44(-)25(+) (DN3) cells, is believed to fa cilitate or enable an efficient transition from the CD8(-)4(-) double-negat ive (DN) to the CD8(+)4(+) double-positive (DP) developmental stage. Subseq uent to pre-TCR expression, DN3 thymocytes receive survival, proliferation, and differentiation signals, although it is still unclear which of these o utcomes are directly induced by the pre-TCR. To address this issue, we gene rated mice bearing a range of pT alpha transgene copy number under the tran scriptional control of the p56(lck) proximal promoter. All lines exhibited increased DN3 cycling, accelerated DN3/4 transition, and improved DN4 survi val. However, the high copy number lines also showed a selective reduction in thymic cellularity due to increased apoptosis of DP thymocytes, which co uld be reversed by the ectopic expression of Bcl-2. Our results suggest tha t transgenic pT alpha likely caused apoptosis of DP thymocytes due to compe titive decrease in surface TCR alpha beta formation. These results highligh t the critical importance of precise temporal and stoichiometric regulation of pre-TCR and TCR component expression.