Duality of Enhancer Functioning Mode Revealed in a Reduced TCRβ Gene Enhancer Knockin Mouse Model

The TCRP gene enhancer (E beta) commands TCR beta gene expression through the lifespan of T lymphocytes. Genetic and molecular studies have implied that in early thymocytes, E beta directs chromatin opening over the D beta-J beta-C beta domains and triggers initial D beta-J beta recombination. In mature T cells, E beta is required for expression of the assembled TCRP gene. Whether these separate activities rely on distinct E beta regulatory sequences and involve differing modes of activation is unclear. Using gene targeting in mouse embryonic stem cells, we replaced E beta by a conserved core fragment (E beta 169). We found that E beta 169-carrying alleles were capable of sustaining beta gene expression and the development of mature T cells in homozygous knockin mice. Surprisingly, these procedures and underlying molecular transactions were affected to a wide range of degrees depending on the developmental stage. Early thymocytes barely achieved D beta-J beta germline transcription and recombination. In contrast, T cells displayed substantial though heterogeneous levels of VDJ-rearranged TCR beta gene expression. Our results have implications regarding enhancer function in cells of the adaptive immune system and, potentially, TCR beta gene recombination and allelic exclusion. The Journal of Immunology, 2009, 183: 7939-7948.