Tcrd and Tcrg display identical developmental programs that depend on the activity of the enhancers Ed and Eg being "on" in pre-b-selection thymocytes to activate transcription and V(D)J recombination of the unrearranged genes and "off" in post-bselection CD4 + CD8 + double-positive thymocytes to inhibit transcription of the rearranged genes and avoid the expression of TCR dand TCR g-chains in ab T lymphocytes. Ed and Eg activity depends on transcription factor binding to essential Runx and Myb sites and parallels that of Notch signaling. We performed Notch gain-and loss-of-function experiments and found that Notch signaling activates Tcrd and Tcrg transcription by favoring the recruitment of RUNX1 and MYB to the enhancers. Our results suggest that the dissociation of RUNX1 and MYB from Ed and Eg chromatin in double-positive thymocytes, which results in enhancer inactivation, is caused by decreased Notch signaling triggered by pre-TCR signaling, thereby deciphering the molecular mechanism of Tcrd and Tcrg silencing during b-selection. These findings reveal a novel molecular mechanism for gene regulation via Notch signaling through the recruitment of RUNX1 and MYB to enhancer chromatin during thymocyte development.

γδ T cells play important roles in immune responses by rapidly producing large quantities of cytokines. Recently, γδ T cells have been found to be involved in tissue homeostatic regulation, playing roles in thermogenesis, bone regeneration and synaptic plasticity. Nonetheless, the mechanisms involved in γδ T-cell development, especially the regulation of TCRδ gene transcription, have not yet been clarified. Previous studies have established that NOTCH1 signaling plays an important role in the Tcrg and Tcrd germline transcriptional regulation induced by enhancer activation, which is mediated through the recruitment of RUNX1 and MYB. In addition, interleukin-7 signaling has been shown to be required for Tcrg germline transcription, VγJγ rearrangement and γδ T-lymphocyte generation as well as for promoting T-cell survival. In this study, we discovered that interleukin-7 is required for the activation of enhancer-dependent Tcrd germline transcription during thymocyte development. These results indicate that the activation of both Tcrg and Tcrd enhancers during γδ T-cell development in the thymus depends on the same NOTCH1- and interleukin-7-mediated signaling pathways. Understanding the regulation of the Tcrd enhancer during thymocyte development might lead to a better understanding of the enhancer-dependent mechanisms involved in the genomic instability and chromosomal translocations that cause leukemia.

The combined T-cell receptor α and δ locus, Tcra/Tcrd, encodes the TCRα and TCRδ chains of the αβ or γδ T-cell receptors (TCRαβ and TCRγδ), respectively, which define the two distinct T-cell lineages, αβ and γδ T lymphocytes. Like other antigen receptor loci, this locus must recombine its variable (V), diversity (D), and joining (J) gene segments to generate a diverse range of TCR that allow vertebrates to respond to an unlimited number of antigens. The Tcra/Tcrd germline transcription and subsequent V(D)J gene segment rearrangements are strictly regulated by two distant transcriptional enhancers, Eα and Eδ, respectively, during thymocyte development. Once the Tcra locus is productively rearranged, it is assumed Eα remains active for the transcription of the rearranged locus and the expression of the functional TCRα chain in αβ T lymphocytes. However, our recent experiments have shown Eα is significantly inhibited during the final stage of thymocyte development, concomitantly with the expression of the rearranged Tcra locus, and remains inhibited in αβ T lymphocytes. These results imply the existence of an Eα-independent mechanism to activate transcription of the rearranged Tcra locus in αβ T lymphocytes. Interestingly, Eα is essential for the normal expression of the rearranged Tcrd locus in γδ T lymphocytes. In this review, the current knowledge about the regulation of Tcra/Tcrd germline transcription and gene segment rearrangement during thymocyte development and the possible mechanisms for transcription of the rearranged Tcra locus in mature αβ T lymphocytes are discussed. The knowledge of the detailed mechanisms involved in the regulation of transcription at the Tcra/Tcrd locus by distant enhancers is important to understand the cases in which deregulation this process results in disease. Keywords: Transcription; T-cell receptor; V(D)J recombination; Enhancer Abbreviations:A Temporal Control of TCR Gene RearrangementsDuring thymic T-cell development (Figure 1), early T-cell progenitors (ETP) arising from fetal liver or bone marrow enter to the thymus, where they mature progressively through different stages that can be distinguished based on the expression of the CD4 and CD8 surface markers: CD4-CD8-double-negative (DN) thymocytes, immature single-positive (ISP) CD8 + thymocytes, CD4 + CD8 + doublepositive (DP) thymocytes, and CD4 + or CD8 + single-positive (SP) thymocytes [1]. Among the DN thymocyte population, four subpopulations can be further distinguished based on the expression of CD25 and CD44 surface markers: DN1 (CD44 + CD25 -), DN2 (CD44 + CD25 + ), DN3 (CD44 -CD25 + ), and DN4 (CD44 -CD25 -) thymocytes. In addition, two DN3 subpopulations can be distinguished based on the expression of CD27: DN3a (CD27 low ) and DN3b (CD27 high ) thymocytes [2]. Furthermore, two DP thymocyte populations can be distinguished based on the expression of CD71: early DP (eDP) (CD71 + ) and late DP (lDP) (CD71 -) thymocytes [3]. For αβ T-cell development, thymocytes transition from DN1 to SP thymocytes by matu...