Differential transcriptional regulation of individual TCR Vβ segments before gene rearrangement

The promoter sequences of individual murine TCR VP segments are dissimilar, but any functional differences between them are masked after productive gene rearrangement by the dominance of the TCR beta 3' enhancer, However, thymocytes of recombination-activating gene-2 (Rag2)-deficient mice allow the transcriptional activity of V beta promoters to be studied before rearrangement. Here we report that many V beta segments are detectably transcribed in Rag2(-/-) thymocytes and that there are significant differences in expression among different V beta segments. Primer extension and characterization of cDNA clones from SCID thymocytes suggest that these germline V beta transcripts generally use the same start sites as those previously determined in mature T cells. The strength of expression before rearrangement does not correlate with proximity to the known enhancer, because members of the most distal V beta cluster (V beta2.1, V beta1.1, V beta4.1) are relatively strongly expressed and more proximal V beta segments (V beta 14.1, V beta3.1, V beta7.1, V beta6.1) are only weakly expressed. Different V beta segments also show different developmental programs of activation in different thymocyte subsets, with the V beta5.1(L)-8.2(V) spliced transcript expressed earliest as well as most strongly overall. Comparison with Rag(+) MHC class I-/- and class II-/- thymocytes confirms that many of these expression differences are leveled by rearrangement and/or by beta selection, before MHC-dependent selection. However, the expression pattern of V beta2.1 is highly distinctive and includes cell types apparently outside the T lineage, suggesting potential acquisition of specialized roles.