Early TCR Expression and Aberrant T Cell Development in Mice with Endogenous Prerearranged T Cell Receptor Genes

The factors that regulate the rate of production of T cells by the thymus remain incompletely defined. To test whether generation of functional T cell receptors limits the rate of thymic T cell export, we made use of a line of mice, LN3αβ, that have endogenously prerearranged TCR genes. The prerearranged TCR genes were expressed abnormally early in hemopoietic development, indicating that RAG-mediated recombination, rather than transcription factor expression, is the key determinant of the initiation of robust TCR transcription. Thymic T cell export rates were similar between wild-type (wt) and LN3αβ mice, indicating that T cell maturation rates in these mice are determined by factors other than TCR gene rearrangement. In competitive bone marrow chimeras, however, LN3αβ thymocytes were out-competed by wt cells and failed to develop beyond the double-negative 4 stage. Furthermore, wt progenitors transplanted intrathymically into LN3αβ mice proliferated excessively, suggesting that increased proliferative signals in the LN3αβ thymus compensate for faulty T cell development driven by early TCR expression.