The Tcra/Tcrd locus undergoes V-Dδ-Jδ rearrangement in CD4-CD8- thymocytes to form the TCRδ chain of the γδ TCR and V-Jα rearrangement in CD4+CD8+ thymocytes to form the TCRα chain of the αβ TCR. Most V segments in the locus participate in V-Jα rearrangement, but only a small and partially overlapping subset participates in V-Dδ-Jδ rearrangement. What determines any particular Tcra/Tcrd locus V gene segment as a Vδ, a Vα, or both is currently unknown. We tested the hypothesis that V segment usage is specified by V segment promoter-dependent chromatin accessibility in developing thymocytes. TRAV15/DV6 family V gene segments contribute to both the Tcrd and Tcra repertoires, whereas TRAV12 family V gene segments contribute almost exclusively to the Tcra repertoire. To understand whether the TRAV15/DV6 promoter region specifies TRAV15/DV6 as a Vδ, we used gene targeting to replace the promoter region of a TRAV12 family member with one from a TRAV15/DV6 family member. The TRAV15/DV6 promoter region conferred increased germline transcription and histone modifications to TRAV12 in DN thymocytes and caused a substantial increase in usage of TRAV12 in Tcrd recombination events. Our results demonstrate that usage of TRAV15/DV6 family V gene segments for Tcrd recombination in DN thymocytes is regulated at least in part by intrinsic features of TRAV15/DV6 promoters, and argue that Tcra/Tcrd locus Vδ gene segments are defined by their local chromatin accessibility in CD4-CD8- thymocytes.


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