Differential transcriptional regulation of individual TCR V beta 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 ge ne rearrangement by the dominance of the TCR beta 3' enhancer, However, thy mocytes of recombination-activating gene-2 (Rag2)-deficient mice allow the transcriptional activity of V beta promoters to be studied before rearrange ment. Here we report that many V beta segments are detectably transcribed i n Rag2(-/-) thymocytes and that there are significant differences in expres sion among different V beta segments. Primer extension and characterization of cDNA clones from SCID thymocytes suggest that these germline V beta tra nscripts 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 mos t distal V beta cluster (V beta2.1, V beta1.1, V beta4.1) are relatively st rongly 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 thym ocyte subsets, with the V beta5.1(L)-8.2(V) spliced transcript expressed ea rliest as well as most strongly overall. Comparison with Rag(+) MHC class I -/- and class II-/- thymocytes confirms that many of these expression diffe rences are leveled by rearrangement and/or by beta selection, before MHC-de pendent selection. However, the expression pattern of V beta2.1 is highly d istinctive and includes cell types apparently outside the T lineage, sugges ting potential acquisition of specialized roles.