Differential Transcriptional Regulation of Individual TCR V Segments Before Gene Rearrangement

Abstract

The promoter sequences of individual murine TCR Vβ segments are dissimilar, but any functional differences between them are masked after productive gene rearrangement by the dominance of the TCRβ 3′ enhancer. However, thymocytes of recombination-activating gene-2 (Rag2)-deficient mice allow the transcriptional activity of Vβ promoters to be studied before rearrangement. Here we report that many Vβ segments are detectably transcribed in Rag2^(−/−) thymocytes and that there are significant differences in expression among different Vβ segments. Primer extension and characterization of cDNA clones from SCID thymocytes suggest that these germline Vβ 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 Vβ cluster (Vβ2.1, Vβ1.1, Vβ4.1) are relatively strongly expressed and more proximal Vβ segments (Vβ14.1, Vβ3.1, Vβ7.1, Vβ6.1) are only weakly expressed. Different Vβ segments also show different developmental programs of activation in different thymocyte subsets, with the Vβ5.1(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 β selection, before MHC-dependent selection. However, the expression pattern of Vβ2.1 is highly distinctive and includes cell types apparently outside the T lineage, suggesting potential acquisition of specialized roles.

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