E protein binding at the enhancer promotes repertoire diversity.

V(D)J recombination of antigen receptor loci is a highly developmentally regulated process. During T lymphocyte development, recombination of the gene occurs in CD4CD8 double positive (DP) thymocytes and requires the enhancer (Eα). E proteins are known regulators of DP thymocyte development and have three identified binding sites in Eα. To understand the contribution of E proteins to Eα function, mutants lacking one or two of the respective binding sites were generated. The double-binding site mutant displayed a partial block at the positive selection stage of αβ T cell development. Further investigation revealed loss of germline transcription within the locus at the Jα array, along with dysregulated primary and impaired secondary Vα-Jα rearrangement. Eα E protein binding increases locus accessibility and regulates TCRα recombination, thus directly promoting repertoire diversity.