A double negative thymocyte specific enhancer augments Notch1 signaling to direct early T cell progenitor expansion, lineage restriction and ?-selection.

T cell differentiation requires Notch1 signaling. Here we show that an enhancer upstream of Notch1 active in double-negative (DN) mouse thymocytes is responsible for raising Notch1 signaling intra-thymically. This enhancer is required to expand multipotent progenitors intra-thymically while delaying early differentiation until lineage restrictions are established. Early thymic progenitors lacking the enhancer show accelerated differentiation through the DN stages and increased frequency of B-, ILC-, and NK-cell differentiation. Transcription regulators for T cell lineage restriction and commitment are expressed normally, but ILC- and NK-cell gene expression persists after T cell lineage commitment and TCRb V-DJ recombination, Cd3 expression and b-selection are impaired. This Notch1 enhancer is inactive in double-positive (DP) thymocytes. Its aberrant reactivation at this stage in Ikaros mutants is required for leukemogenesis. Thus, the DN-specific Notch1 enhancer harnesses the regulatory architecture of DN and DP thymocytes to achieve carefully orchestrated changes in Notch1 signaling required for early lineage restrictions and normal T cell differentiation. Overall design: 3 ChIP-seq samples