Intra- and inter-allelic ordering of T cell receptor β chain gene assembly

Allelic exclusion at the TCR beta locus mandates that gene assembly be regulated in a manner that permits feedback inhibition of further complete TCR beta rearrangements upon pre-TCR expression. Here we show that assembly of TCR beta chain genes from V beta, D beta and J beta gene segments is intra-allelically ordered, proceeding primarily through DJ beta, and not VD beta, intermediates. This ensures that V beta to DJ beta rearrangement, which can be feedback inhibited, is the final step in the assembly process. A newly assembled VDJ beta rearrangement must be tested to determine if it is in-frame before V beta to DJ beta rearrangement is permitted on the alternate allele. This inter-allelic ordering may occur through a general inefficiency of V beta to DJ beta rearrangement and/or through static differences in accessibility of the two TCR beta alleles. However, we find that within the regulatory context of allelic exclusion, V beta to DJ beta rearrangement proceeds to completion on both alleles. Furthermore, all possible VDJ beta rearrangements are not completed on one allele before V beta to DJ beta rearrangement is initiated on the alternate allele. Together, these data support a dynamic model of inter-allelic accessibility that permits the ordered and efficient assembly of complete variable region genes on both TCR beta alleles during T cell development.