The pre-TCR signal induces transcriptional silencing of the TCRγ locus by reducing the recruitment of STAT5 and Runx to transcriptional enhancers

The mouse TCR gamma locus is positively regulated by the transcription factors STAT5 and Runx. While the locus undergoes frequent rearrangements in T lymphocytes, TCR gamma transcription is repressed in alpha beta T cells. This phenomenon, known as TCR gamma silencing, depends on pre-TCR-induced thymocyte proliferation. The molecular basis for TCR gamma silencing, however, is largely unknown. Here, we show that pre-TCR signaling reduces transcription and histone acetylation of the TCR gamma locus irrespective of V-J rearrangements. We also demonstrate that Runx is recruited to E gamma and HsA enhancer elements of the TCR gamma locus, primarily at the CD4(-)CD8(-) double-negative stage and that Runx binding to these elements decreases at later stages of thymocyte development. Importantly, anti-CD3 antibody treatment decreased IL-7R expression levels, STAT5 phosphorylation and recruitment of STAT5 and Runx to E gamma and HsA elements in RAG2-deficient thymocytes, suggesting that pre-TCR signaling triggers reduced binding of STAT5 and Runx to the enhancer elements. Furthermore, we observed that misexpression of STAT5 or Runx in the CD4(+)CD8(+) double-positive cell line DPK induces TCR gamma gene transcription. Finally, we showed that TCR gamma transcription is induced in alpha beta T cells from Runx3 transgenic mice, suggesting that Runx3 counteracts TCR gamma silencing in alpha beta T cells in vivo. Our results suggest that pre-TCR signaling indirectly inactivates TCR gamma enhancers by reducing recruitment of STAT5 and Runx and imply that this effect is an important step for TCR gamma silencing in alpha beta T cells.