ENHANCER-DEPENDENT AND ENHANCER-INDEPENDENT STEPS IN THE REARRANGEMENT OF A HUMAN T-CELL RECEPTOR DELTA TRANSGENE

The rearrangement and expression of T cell receptor (TCR) gene segment s occurs in a highly ordered fashion during thymic ontogeny of T lymph ocytes. To study the regulation of gene rearrangement within the TCR a lpha/delta locus, we generated transgenic mice that carry a germline h uman TCR delta minilocus that includes V delta 1, V delta 2, D delta 3 , J delta 1, J delta 3, and C delta segments, and either contains or l acks the TCR delta enhancer. We found that the enhancer-positive const ruct rearranges stepwise, first V to D, and then V-D to J. Construct V -D rearrangement mimics a unique property of the endogenous TCR delta locus. V-D-J rearrangement is T cell specific, but is equivalent in al pha/beta and gamma/delta T lymphocytes. Thus, either there is no commi tment to the alpha/beta and gamma/delta T cell lineages before TCR del ta gene rearrangement, or if precommitment occurs, it does not operate directly on TCR delta gene cis-acting regulatory elements to control TCR delta gene rearrangement. Enhancer-negative mice display normal V to D rearrangement, but not V-D to J rearrangement. Thus, the V-D to J step is controlled by the enhancer, but the V to D step is controlled by separate elements. The enhancer apparently controls access to J de lta 1 but not D delta 3, suggesting that a boundary between two indepe ndently regulated domains of the minilocus lies between these elements . Within the endogenous TCR alpha/delta locus, this boundary may repre sent the 5' end of a chromatin regulatory domain that is opened by the TCR delta enhancer during T cell development. The position of this bo undary may explain the unique propensity of the TCR delta locus to und ergo early V to D rearrangement. Our results indicate that the TCR del ta enhancer performs a crucial targeting function to regulate TCR delt a gene rearrangement during T cell development.