Exit of the pre-TCR from the ER/cis-Golgi is necessary for signaling differentiation, proliferation, and allelic exclusion in immature thymocytes

A major issue is whether surface expression of the pre-TCR is necessary for signaling the development of immature thymocytes. To address this question, we generated transgenic mice expressing a TCR beta chain that had a strong endoplasmic reticulum (ER) retrieval signal (TCR beta(ER)) and that was expressed intracellularly but failed to reach the cell surface. In TCR beta(ER) transgenic mice, there was a failure of allelic exclusion. Also, the transgene failed to rescue the developmental defects observed in TCR beta-null mice. In contrast, TCR beta transgenes with a mutant ER retrieval sequence or lacking this sequence signaled efficient allelic exclusion and suppressed the TCR beta(-/-) defect. These data show that exit of the pre-TCR from the ER/cis-Golgi is required for progression through the double-negative thymocyte checkpoint.