Developmental Regulation of  VDJ Recombination By the Core Fragment of the T Cell Receptor α Enhancer

The role of T cell receptor α enhancer (E(α)) cis-acting elements in the developmental regulation of VDJ recombination at the TCR α/δ locus was examined in transgenic mice containing variants of a minilocus VDJ recombination substrate. We demonstrate that the 116-bp Tα1,2 core enhancer fragment of the 1.4-kb E(α) is sufficient to activate the enhancer-dependent step of minilocus rearrangement, and that within Tα1,2, intact binding sites for TCF/LEF and Ets family transcription factors are essential. Although minilocus rearrangement under the control of the 1.4-kb E(α) initiates at fetal day 16.5 and is strictly limited to αβ T cells, we find that rearrangement under the control of Tα1,2 initiates slightly earlier during ontogeny and occurs in both γδ and αβ T cells. We conclude that the core fragment of E(α) can establish accessibility to the recombinase in developing thymocytes in vivo in a fashion that is dependent on the binding of TCF/LEF and Ets family transcription factors, but that these and other factors that bind to the E(α) core cannot account for the precise developmental onset of accessibility that is provided by the intact E(α). Rather, our data suggests a critical role for factors that bind E(α) outside of the core Tα1,2 region in establishing the precise developmental onset of TCR α rearrangement in vivo.