TCRβ enhancer activation occurs in some but not all cells with T cell lineage developmental potential

Previous studies have shown that murine bone marrow contains a fraction of CD3−/B220−/Thy1lo cells that have pre T cell activity following adoptive transfer and produce sterile transcripts of the T cell receptor β chain gene. The relationship between progenitors and TCRβ transcription has not been examined. Transgenic mice were generated that express green fluorescent protein under the control of the TCRβ enhancer (Eβ). Phenotypic analysis of the founders revealed faithful expression of GFP in populations that express TCRβ transcripts. Examination of the bone marrow showed two populations, CD3−/B220−/Thy1− and CD3−/B220−/Thy1lo, which were GFP+. Both populations were analyzed for their developmental potential following intrathymic transfer into recipient mice. Surprisingly, the GFP+/CD3−/B220−/Thy1lo cells failed to reconstitute; however, the GFP+/CD3−/B220−/Thy1− cells exhibited thymic repopulation. These data demonstrate that Eβ is active pre-thymically; however, pre-thymic transcription of the TCRβ chain gene is neither required for T cell development, nor is it limited to pre T cells.