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

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