The TCRbeta enhancer is dispensable for the expression of rearranged TCRbeta genes in thymic DN2/DN3 populations but not at later stages

The Cyclin D3 Protein Enforces Monogenic TCRβ Expression by Mediating TCRβ Protein-Signaled Feedback Inhibition of Vβ Recombination

In jawed vertebrates, adaptive immunity depends on the process of V(D)J recombination creating vast numbers of T and B lymphocytes that each expresses unique Ag receptors of uniform specificity. The asynchronous initiation of V-to-(D)J rearrangement between alleles and the resulting protein from one allele signaling feedback inhibition of V recombination on the other allele ensures homogeneous receptor specificity of individual cells. Upon productive Vβ-to-DβJβ rearrangements in noncycling double-negative thymocytes, TCRβ protein signals induction of the cyclin D3 protein to accelerate cell cycle entry, thereby driving proliferative expansion of developing αβ T cells. Through undetermined mechanisms, the inactivation of cyclin D3 in mice causes an increased frequency of αβ T cells that express TCRβ proteins from both alleles, producing lymphocytes of heterogeneous specificities. To determine how cyclin D3 enforces monogenic TCRβ expression, we used our mouse lines with enhanced rearrangement of specific Vβ segments due to replacement of their poor-quality recombination signal sequence (RSS) DNA elements with a better RSS. We show that cyclin D3 inactivation in these mice elevates the frequencies of αβ T cells that display proteins from RSS-augmented Vβ segments on both alleles. By assaying mature αβ T cells, we find that cyclin D3 deficiency increases the levels of Vβ rearrangements that occur within developing thymocytes. Our data demonstrate that a component of the cell cycle machinery mediates TCRβ protein-signaled feedback inhibition in thymocytes to achieve monogenic TCRβ expression and resulting uniform specificity of individual αβ T cells.

Characterization of transcripts emanating from enhancer Eβ of the murine TCRβ locus

Enhancers are well established as critical regulators of gene expression, but the mechanisms underlying the molecular basis of their specificity and activity are only partly understood. One of the most exciting recent observations is the discovery of enhancer RNA (eRNA), a class of noncoding RNAs derived from enhancer regions. Transcription of developmentally regulated enhancers has been observed to be associated with their active state. The nature of transcripts (eRNA) and their functional attributes are diverse and context dependent. The majority of eRNA are nonpolyadenylated and present in low abundance owing to their low stability, and may represent transcriptional noise. However, some eRNAs have been reported to be reasonably long and stable, are enriched in nuclei, exhibit tissue-specific expression and may contribute to enhancer function. Transcription of enhancers has been postulated to mediate enhancer function through either the act of transcription or via the transcribed RNA per se and is a useful feature to be analysed to understand mechanisms underlying enhancer activity. Enhancer Eβ at the murine TCRβ locus has been reported to exhibit enhanced occupancy of RNA polymerase II in developing thymocytes. Here, we investigated the transcriptional potential of Eβ in developing thymocytes and detected overlapping bidirectional transcripts at Eβ ranging between 0.7 and 1.7 kb. These noncoding transcripts are capped, polyadenylated, nuclear and expressed specifically in thymocytes. Delineation of these characteristics is important to further investigate their functional roles in mediating enhancer activity.

Dual-function injectable angiogenic biomaterial for the repair of brain tissue following stroke

Stroke is the primary cause of disability due to the brain's limited ability to regenerate damaged tissue. After stroke, an increased inflammatory and immune response coupled with severely limited angiogenesis and neuronal growth results in a stroke cavity devoid of normal brain tissue. In the adult, therapeutic angiogenic materials have been used to repair ischaemic tissues through the formation of vascular networks. However, whether a therapeutic angiogenic material can regenerate brain tissue and promote neural repair is poorly understood. Here we show that the delivery of an engineered immune-modulating angiogenic biomaterial directly to the stroke cavity promotes tissue formation de novo, and results in axonal networks along thee generated blood vessels. This regenerated tissue produces functional recovery through the established axonal networks. Thus, this biomaterials approach generates a vascularized network of regenerated functional neuronal connections within previously dead tissue and lays the groundwork for the use of angiogenic materials to repair other neurologically diseased tissues.

Distinct requirement of Runx complexes for TCRβ enhancer activation at distinct developmental stages

A TCRβ enhancer, known as the Eβ enhancer, plays a critical role in V(D)J recombination and transcription of the Tcrb gene. However, the coordinated action of trans-acting factors in the activation of Eβ during T cell development remains uncharacterized. Here, we characterized the roles of Runx complexes in the regulation of the Eβ function. A single mutation at one of the two Runx binding motifs within the Eβ severely impaired Tcrb activation at the initiation phase in immature thymocytes. However, TCRβ expression level in mature thymocytes that developed under such a single Runx site mutation was similar to that of the control. In contrast, mutations at two Runx motifs eliminated Eβ activity, demonstrating that Runx complex binding is essential to initiate Eβ activation. In cells expressing Tcrb harboring rearranged V(D)J structure, Runx complexes are dispensable to maintain TCRβ expression, whereas Eβ itself is continuously required for TCRβ expression. These findings imply that Runx complexes are essential for Eβ activation at the initiation phase, but are not necessary for maintaining Eβ activity at later developmental stages. Collectively, our results indicate that the requirements of trans-acting factor for Eβ activity are differentially regulated, depending on the developmental stage and cellular activation status.

Transcriptional regulation and spatial organisation of the human AML1/RUNX1 gene

The transcription factor RUNX1 is a key regulator of haematopoiesis in vertebrates. In humans, the 260-kb long gene coding for this transcription factor is located on chromosome 21. This gene is transcribed from two alternative promoters that are commonly referred to as the distal and the proximal promoters. In model experiments, these two promoters were found to be active in cells of different lineages, although RUNX1 is preferentially expressed in haematopoietic cells. In the present study, we attempted to identify the regulatory elements that could guide tissue-specific expression of the RUNX1 gene. Two such regulatory elements were found within the RUNX1 gene. One of these elements, located within intron 1, is a haematopoietic-specific enhancer. The second regulatory element, located within intron 5.2, contributes to the formation of an active chromatin hub, which integrates the above-mentioned enhancer and the P1 and P2 promoters.

Differential regulation of proximal and distal Vbeta segments upstream of a functional VDJbeta1 rearrangement upon beta-selection

Developmental stage-specific regulation of transcriptional accessibility helps control V(D)J recombination. Vβ segments on unrearranged TCRβ alleles are accessible in CD4(-)/CD8(-) (double-negative [DN]) thymocytes, when they recombine, and inaccessible in CD4(+)/CD8(+) (double-positive [DP]) thymocytes, when they do not rearrange. Downregulation of Vβ accessibility on unrearranged alleles is linked with Lat-dependent β-selection signals that inhibit Vβ rearrangement, stimulate Ccnd3-driven proliferation, and promote DN-to-DP differentiation. Transcription and recombination of Vβs on VDJβ-rearranged alleles in DN cells has not been studied; Vβs upstream of functional VDJβ rearrangements have been found to remain accessible, yet not recombine, in DP cells. To elucidate contributions of β-selection signals in regulating Vβ transcription and recombination on VDJβ-rearranged alleles, we analyzed wild-type, Ccnd3(-/-), and Lat(-/-) mice containing a preassembled functional Vβ1DJCβ1 (Vβ1(NT)) gene. Vβ10 segments located just upstream of this VDJCβ1 gene were the predominant germline Vβs that rearranged in Vβ1(NT/NT) and Vβ1(NT/NT)Ccnd3(-/-) thymocytes, whereas Vβ4 and Vβ16 segments located further upstream rearranged at similar levels as Vβ10 in Vβ1(NT/NT)Lat(-/-) DN cells. We previously showed that Vβ4 and Vβ16, but not Vβ10, are transcribed on Vβ1(NT) alleles in DP thymocytes; we now demonstrate that Vβ4, Vβ16, and Vβ10 are transcribed at similar levels in Vβ1(NT/NT)Lat(-/-) DN cells. These observations indicate that suppression of Vβ rearrangements is not dependent on Ccnd3-driven proliferation, and DN residence can influence the repertoire of Vβs that recombine on alleles containing an assembled VDJCβ1 gene. Our findings also reveal that β-selection can differentially silence rearrangement of germline Vβ segments located proximal and distal to functional VDJβ genes.

Antigen receptor allelic exclusion: an update and reappraisal

Most lymphocytes express cell surface Ag receptor chains from single alleles of distinct Ig or TCR loci. Since the identification of Ag receptor allelic exclusion, the importance of this process and the precise molecular mechanisms by which it is achieved have remained enigmatic. This brief review summarizes current knowledge of the extent to which Ig and TCR loci are subject to allelic exclusion. Recent progress in studying and defining mechanistic steps and molecules that may control the monoallelic initiation and subsequent inhibition of V-to-(D)-J recombination is outlined using the mouse TCRβ locus as a model with frequent comparisons to the mouse IgH and Igκ loci. Potential consequences of defects in mechanisms that control Ag receptor allelic exclusion and a reappraisal of the physiologic relevance of this immunologic process also are discussed.

The role of the Runx transcription factors in thymocyte differentiation and in homeostasis of naive T cells

Members of the Runx family of transcriptional regulators are required for the appropriate expression of CD4 and CD8 at discrete stages of T cell development. The roles of these factors in other aspects of T cell development are unknown. We used a strategy to conditionally inactivate the genes encoding Runx1 or Runx3 at different stages of thymocyte development, demonstrating that Runx1 regulates the transitions of developing thymocytes from the CD4⁻CD8⁻ double-negative stage to the CD4⁺CD8⁺ double-positive (DP) stage and from the DP stage to the mature single-positive stage. Runx1 and Runx3 deficiencies caused marked reductions in mature thymocytes and T cells of the CD4⁺ helper and CD8⁺ cytotoxic T cell lineages, respectively. Runx1-deficient CD4⁺ T cells had markedly reduced expression of the interleukin 7 receptor and exhibited shorter survival. In addition, inactivation of both Runx1 and Runx3 at the DP stages resulted in a severe block in development of CD8⁺ mature thymocytes. These results indicate that Runx proteins have important roles at multiple stages of T cell development and in the homeostasis of mature T cells.

BAC to immunology--bacterial artificial chromosome-mediated transgenesis for targeting of immune cells

Thirty years after the first transgenic mouse was produced, a plethora of genetic tools has been developed to study immune cells in vivo. A powerful development is the bacterial artificial chromosome (BAC) transgenic approach, combining advantages of both conventional transgenic and knock-in gene-targeting strategies. In immunology the potential of BAC transgenic technology has yet to be fully harvested and, combined with a variety of elegant genetic tools, it will allow the analysis of complex immunological processes in vivo. In this short review, we discuss the applications of BACs in immunology, such as identification of regulatory regions, expression and cell-fate mapping, cell ablation, conditional mutations and the generation of humanized mice.

At the crossroads: diverse roles of early thymocyte transcriptional regulators

Transcriptional regulation of T-cell development involves successive interactions between complexes of transcriptional regulators and their binding sites within the regulatory regions of each gene. The regulatory modules that control expression of T-lineage genes frequently include binding sites for a core set of regulators that set the T-cell-specific background for signal-dependent control, including GATA-3, Notch/CSL, c-myb, TCF-1, Ikaros, HEB/E2A, Ets, and Runx factors. Additional regulators in early thymocytes include PU.1, Id-2, SCL, Spi-B, Erg, Gfi-1, and Gli. Many of these factors are involved in simultaneous regulation of non-T-lineage genes, T-lineage genes, and genes involved in cell cycle control, apoptosis, or survival. Potential and known interactions between early thymic transcription factors such as GATA-3, SCL, PU.1, Erg, and Spi-B are explored. Regulatory modules involved in the expression of several critical T-lineage genes are described, and models are presented for shifting occupancy of the DNA-binding sites in the regulatory modules of pre-Talpha, T-cell receptor beta (TCRbeta), recombinase activating genes 1 and 2 (Rag-1/2), and CD4 during T-cell development. Finally, evidence is presented that c-kit, Erg, Hes-1, and HEBAlt are expressed differently in Rag-2(-/-) thymocytes versus normal early thymocytes, which provide insight into potential regulatory interactions that occur during normal T-cell development.