Regulation of T cell receptor β allelic exclusion at a level beyond accessibility

Allelic exclusion of V-beta-to- DJ(beta) recombination depends on asynchronous rearrangement of alleles of the gene encoding T cell receptor beta in double-negative thymocytes and feedback inhibition that is maintained in double-positive thymocytes. Feedback is thought to be enforced through downregulation of V-beta accessibility. In an attempt to override this negative regulation, we introduced the enhancer of the gene encoding T cell receptor alpha into the V-beta gene cluster downstream of Vbeta(12). In double-negative thymocytes, the introduced enhancer had no measurable effect on accessibility, but V(beta)12 rearrangement was stimulated and V(beta)12 allelic exclusion was partially subverted. In contrast, double-positive thymocytes showed increased V-beta transcription and accessibility, but feedback inhibition of V-beta-to- DJ(beta) recombination remained intact. Our results indicate additional regulatory constraints on V-beta-to-DJ(beta) recombination that operate beyond the accessibility barrier.