The transcription factor LRF promotes Integrin β7 expression by and gut homing of CD8αα intraepithelial lymphocyte precursors

TCRαβ+ CD8α+ CD8β− intraepithelial lymphocytes (CD8αα IEL) differentiate from thymic IEL precursors (IELp) and contribute to gut homeostasis. However, their development remains poorly understood. Here we show that the zinc finger transcription factor LRF, encoded by Zbtb7a, is highly expressed in both CD8αα+ IELs and in IELp and serves in a cell-intrinsic manner to promote IEL differentiation. Mice genetically deficient for LRF (Cd4-Cre Zbtb7afl/fl) lack CD8αα+ IELs, contrasting with largely conserved numbers of conventional CD8αβ+ T cells in the thymus and the secondary lymphoid organs. Unlike their wild-type counterparts, LRF-deficient IELp failed to prevent colitis caused by adoptive transfer of CD4+ T cells in T cell-deficient mice. LRF disruption affects neither the development of thymic IELp, nor their in vitro expansion or differentiation in response to IL-15. However, Cd4-Cre Zbtb7afl/fl mice accumulate CD8αα+ T cells in the spleen, unlike wild-type controls, suggesting impaired migration of IELp to the gut. Single-cell RNA sequencing found LRF necessary for the expression of genes characteristic of the most mature IELp, including Itgb7, encoding the β7 subunit of α4β7. Chromatin immunoprecipitation and gene regulatory network analyses both defined Itgb7 as an LRF target. Our study identifies LRF as an essential transcriptional regulator of IELp maturation in the thymus and subsequent migration to the intestinal epithelium.

The histone demethylase Lsd1 regulates multiple repressive gene programs during T cell development

Analysis of the transcriptional profiles of developing thymocytes has shown that T lineage commitment is associated with loss of stem cell and early progenitor gene signatures and the acquisition of T cell gene signatures. Less well understood are the epigenetic alterations that accompany or enable these transcriptional changes. Here, we show that the histone demethylase Lsd1 (Kdm1a) performs a key role in extinguishing stem/progenitor transcriptional programs in addition to key repressive gene programs during thymocyte maturation. Deletion of Lsd1 caused a block in late T cell development and resulted in overexpression of interferon response genes as well as genes regulated by the Gfi1, Bcl6, and, most prominently, Bcl11b transcriptional repressors in CD4+CD8+ thymocytes. Transcriptional overexpression in Lsd1-deficient thymocytes was not always associated with increased H3K4 trimethylation at gene promoters, indicating that Lsd1 indirectly affects the expression of many genes. Together, these results identify a critical function for Lsd1 in the epigenetic regulation of multiple repressive gene signatures during T cell development.

Complex Interplay Between MAZR and Runx3 Regulates the Generation of Cytotoxic T Lymphocyte and Memory T Cells

The BTB zinc finger transcription factor MAZR (also known as PATZ1) controls, partially in synergy with the transcription factor Runx3, the development of CD8 lineage T cells. Here we explored the role of MAZR as well as combined activities of MAZR/Runx3 during cytotoxic T lymphocyte (CTL) and memory CD8+ T cell differentiation. In contrast to the essential role of Runx3 for CTL effector function, the deletion of MAZR had a mild effect on the generation of CTLs in vitro. However, a transcriptome analysis demonstrated that the combined deletion of MAZR and Runx3 resulted in much more widespread downregulation of CTL signature genes compared to single Runx3 deletion, indicating that MAZR partially compensates for loss of Runx3 in CTLs. Moreover, in line with the findings made in vitro, the analysis of CTL responses to LCMV infection revealed that MAZR and Runx3 cooperatively regulate the expression of CD8α, Granzyme B and perforin in vivo. Interestingly, while memory T cell differentiation is severely impaired in Runx3-deficient mice, the deletion of MAZR leads to an enlargement of the long-lived memory subset and also partially restored the differentiation defect caused by loss of Runx3. This indicates distinct functions of MAZR and Runx3 in the generation of memory T cell subsets, which is in contrast to their cooperative roles in CTLs. Together, our study demonstrates complex interplay between MAZR and Runx3 during CTL and memory T cell differentiation, and provides further insight into the molecular mechanisms underlying the establishment of CTL and memory T cell pools.

Aberrant generation and function of natural killer T (NKT) cell in absence of Utx and Jmjd3

Histone H3 lysine 27 trimethylation (H3K27Me3) demethylases Jmjd3 and Utx, the epigenetic regulators, affects transcription and cell differentiation. But their specific roles in invariant natural killer T (NKT) cells development, maturation and function remain to be explored. Using T cell-specific Utx and Jmjd3 double knock out (DKO) mouse model, we identified that Utx and Jmjd3 deletion results in profound impairment in NKT numbers and maturation of both thymic and peripheral immune organs. DKO thymus NKT cells showed dramatic blockade of maturation at stage 0 and stage 1. Defective expression of Slamf1 (CD150) in DKO CD69+DP thymocytes was identified, suggesting an interrupted DP selecting process. Additionally, we found the impaired expression of PLZF, Tbet, ICOS in DKO NKT cells, which is consistent with the defective NKT sublineage development in those mice. Furthermore, DKO iNKT cells showed significantly defective IFN-r, IL-4 and IL-17 production upon activation. Bone marrow chimera experiment confirmed the major defects of NKT cell development, maturation and function in DKO mice are cell autonomous. Altogether, our present study demonstrates critical contribution of Utx and Jmjd3 in NKT cell development, lineage differentiation and function. Utx and Jmjd3 related epigenetic landscape and gene expression network involved in NKT cell development and function regulation are currently under investigation.

Transcriptional regulation in the development of CD8ab+ iNKT cells in a humanized mouse model

Invariant NKT (iNKT) cells are innate-like T cells showing potent anti-tumor function in conventional mouse models. In sharp contrast, iNKT cell ligands have had limited efficacy in human anti-tumor clinics, mostly due to the profound differences in the properties and compositions of iNKT cells between human and mice, in particular, the presence of CD8+ iNKT cells only in humans. To build more relevant in vivo models for studying human iNKT cells, we first developed a CD1d-humanized mouse model (hCD1d-KI) with human CD1d knocked in (Proc. Natl. Acad. Aci. USA 110: 2963–8, 2013). To further humanize the mouse model, we introduced human invariant TCRa chain (Va24Ja18) into the hCD1d-KI mice. Interestingly, we observed a substantial subset of iNKT cells expressing CD8ab in the new humanized mouse model (J. Immunol. 195:1459–69, 2015). The CD8ab+ iNKT cells show a strong Th1-biased cytokine response and potent cytotoxicity upon activation. The low binding of iNKT TCRs to human CD1d/lipid complex and high prevalence of Vb7 TCRb among these CD8+ iNKT cells strongly suggested a novel low avidity-based developmental program for the ontogeny of these iNKT cells which includes the suppression of the transcriptional factor, Th-POK. This suppression of Th-POK is essential for the development of CD8ab+ iNKT cells as these iNKT cells can not be detected upon forced expression of Th-POK early in T cell development. Our establishment of the new mouse model extensively humanized in CD1d/iNKT TCR system will greatly facilitate the investigation of in vivo functional properties of human iNKT cells as well as future design and optimization of iNKT cell ligands for diverse immunotherapies.

HMGN proteins modulate chromatin regulatory sites and gene expression during activation of naïve B cells

The activation of naïve B lymphocyte involves rapid and major changes in chromatin organization and gene expression; however, the complete repertoire of nuclear factors affecting these genomic changes is not known. We report that HMGN proteins, which bind to nucleosomes and affect chromatin structure and function, co-localize with, and maintain the intensity of DNase I hypersensitive sites genome wide, in resting but not in activated B cells. Transcription analyses of resting and activated B cells from wild-type and Hmgn^−/− mice, show that loss of HMGNs dampens the magnitude of the transcriptional response and alters the pattern of gene expression during the course of B-cell activation; defense response genes are most affected at the onset of activation. Our study provides insights into the biological function of the ubiquitous HMGN chromatin binding proteins and into epigenetic processes that affect the fidelity of the transcriptional response during the activation of B cell lymphocytes.

Coupling of T cell receptor specificity to natural killer T cell development by bivalent histone H3 methylation

The fidelity of T cell immunity depends greatly on coupling T cell receptor signaling with specific T cell effector functions. Here, we describe a chromatin-based mechanism that enables integration of TCR specificity into definite T cell lineage commitment. Using natural killer T cells (iNKT cell) as a model of a T cell subset that differentiates in response to specific TCR signaling, we identified a key role of histone H3 lysine 27 trimethylation (H3K27me3) in coupling iNKT cell TCR specificity with the generation of iNKT cells. We found that the Zbtb16/PLZF gene promoter that drives iNKT cell differentiation possesses a bivalent chromatin state characterized by the simultaneous presence of negative and positive H3K27me3 and H3K4me3 modifications. Depletion of H3K27me3 at the Zbtb16/PLZF promoter leads to uncoupling of iNKT cell development from TCR specificity and is associated with accumulation of iNKT-like CD4(+) cells that express a non-iNKT cell specific T cell repertoire. In turn, stabilization of H3K27me3 leads to a drastic reduction of the iNKT cell population. Our data suggest that H3K27me3 levels at the bivalent Zbtb16/PLZF gene define a threshold enabling precise coupling of TCR specificity to lineage commitment.

The microRNA biogenesis machinery modulates lineage commitment during αβ T cell development

Differentiation of CD4(+) helper and CD8(+) cytotoxic αβ T cells from CD4(+)CD8(+) thymocytes involves upregulation of lineage-specifying transcription factors and transcriptional silencing of CD8 or CD4 coreceptors, respectively, in MHC class II or I (MHCII or I)-restricted thymocytes. In this study, we demonstrate that inactivation of the Dicer RNA endonuclease in murine thymocytes impairs initiation of Cd4 and Cd8 silencing, leading to development of positively selected MHCI- and MHCII-restricted mature CD4(+)CD8(+) thymocytes. Expression of the antiapoptotic BCL2 protein or inactivation of the p53 proapoptotic protein rescues these thymocytes from apoptosis, increasing their frequency and permitting accumulation of CD4(+)CD8(+) αβ T cells in the periphery. Dicer-deficient MHCI-restricted αβ T cells fail to normally silence Cd4 and display impaired induction of the CD8 lineage-specifying transcription factor Runx3, whereas Dicer-deficient MHCII-restricted αβ T cells show impaired Cd8 silencing and impaired induction of the CD4 lineage-specifying transcription factor Thpok. Finally, we show that the Drosha RNA endonuclease, which functions upstream of Dicer in microRNA biogenesis, also regulates Cd4 and Cd8 silencing. Our data demonstrate a previously dismissed function for the microRNA biogenesis machinery in regulating expression of lineage-specifying transcription factors and silencing of Cd4 and Cd8 during αβ T cell differentiation.

Transcriptional and epigenetic networks of helper T and innate lymphoid cells

The discovery of the specification of CD4(+) helper T cells to discrete effector 'lineages' represented a watershed event in conceptualizing mechanisms of host defense and immunoregulation. However, our appreciation for the actual complexity of helper T-cell subsets continues unabated. Just as the Sami language of Scandinavia has 1000 different words for reindeer, immunologists recognize the range of fates available for a CD4(+) T cell is numerous and may be underestimated. Added to the crowded scene for helper T-cell subsets is the continuously growing family of innate lymphoid cells (ILCs), endowed with common effector responses and the previously defined 'master regulators' for CD4(+) helper T-cell subsets are also shared by ILC subsets. Within the context of this extraordinary complexity are concomitant advances in the understanding of transcriptomes and epigenomes. So what do terms like 'lineage commitment' and helper T-cell 'specification' mean in the early 21st century? How do we put all of this together in a coherent conceptual framework? It would be arrogant to assume that we have a sophisticated enough understanding to seriously answer these questions. Instead, we review the current status of the flexibility of helper T-cell responses in relation to their genetic regulatory networks and epigenetic landscapes. Recent data have provided major surprises as to what master regulators can or cannot do, how they interact with other transcription factors and impact global genome-wide changes, and how all these factors come together to influence helper cell function.

Coreceptor gene imprinting governs thymocyte lineage fate

Double-positive (CD4⁺CD8⁺) thymocytes differentiate into CD4⁺ helper T cells and CD8⁺ cytotoxic T cells. A knock-in approach replacing CD8-coding sequences with CD4 cDNA shows that it is the expression kinetics of CD8, and not the identity of the coreceptor, that governs thymocyte-lineage fate.

Immature thymocytes are bipotential cells that are signalled during positive selection to become either helper- or cytotoxic-lineage T cells. By tracking expression of lineage determining transcription factors during positive selection, we now report that the Cd8 coreceptor gene locus co-opts any coreceptor protein encoded within it to induce thymocytes to express the cytotoxic-lineage factor Runx3 and to adopt the cytotoxic-lineage fate, findings we refer to as ‘coreceptor gene imprinting'. Specifically, encoding CD4 proteins in the endogenous Cd8 gene locus caused major histocompatibility complex class II-specific thymocytes to express Runx3 during positive selection and to differentiate into CD4⁺ cytotoxic-lineage T cells. Our findings further indicate that coreceptor gene imprinting derives from the dynamic regulation of specific cis Cd8 gene enhancer elements by positive selection signals in the thymus. Thus, for coreceptor-dependent thymocytes, lineage fate is determined by Cd4 and Cd8 coreceptor gene loci and not by the specificity of T-cell antigen receptor/coreceptor signalling. This study identifies coreceptor gene imprinting as a critical determinant of lineage fate determination in the thymus.

GATA3 controls Foxp3⁺ regulatory T cell fate during inflammation in mice

Tregs not only keep immune responses to autoantigens in check, but also restrain those directed toward pathogens and the commensal microbiota. Control of peripheral immune homeostasis by Tregs relies on their capacity to accumulate at inflamed sites and appropriately adapt to their local environment. To date, the factors involved in the control of these aspects of Treg physiology remain poorly understood. Here, we show that the canonical Th2 transcription factor GATA3 is selectively expressed in Tregs residing in barrier sites including the gastrointestinal tract and the skin. GATA3 expression in both murine and human Tregs was induced upon TCR and IL-2 stimulation. Although GATA3 was not required to sustain Treg homeostasis and function at steady state, GATA3 played a cardinal role in Treg physiology during inflammation. Indeed, the intrinsic expression of GATA3 by Tregs was required for their ability to accumulate at inflamed sites and to maintain high levels of Foxp3 expression in various polarized or inflammatory settings. Furthermore, our data indicate that GATA3 limits Treg polarization toward an effector T cell phenotype and acquisition of effector cytokines in inflamed tissues. Overall, our work reveals what we believe to be a new facet in the complex role of GATA3 in T cells and highlights what may be a fundamental role in controlling Treg physiology during inflammation.

RUNX transcription factor-mediated association of Cd4 and Cd8 enables coordinate gene regulation

T cell fate is associated with mutually exclusive expression of CD4 or CD8 in helper and cytotoxic T cells, respectively. How expression of one locus is temporally coordinated with repression of the other has been a long-standing enigma, though we know RUNX transcription factors activate the Cd8 locus, silence the Cd4 locus, and repress the Zbtb7b locus (encoding the transcription factor ThPOK), which is required for CD4 expression. Here we found that nuclear organization was altered by interplay among members of this transcription factor circuitry: RUNX binding mediated association of Cd4 and Cd8 whereas ThPOK binding kept the loci apart. Moreover, targeted deletions within Cd4 modulated CD8 expression and pericentromeric repositioning of Cd8. Communication between Cd4 and Cd8 thus appears to enable long-range epigenetic regulation to ensure that expression of one excludes the other in mature CD4 or CD8 single-positive (SP) cells.

Two distinct functions for the BTB domain of the CD4-differentiating factor Thpok (85.20)

Thpok is a zinc-finger (ZF) transcription factor essential for CD4 T cell development. Thpok promotes CD4 T helper lineage commitment, in part through the repression of genes necessary for CD8 differentiation. A member of the BTB-ZF subclass of ZF transcription factors, Thpok contains two conserved motifs: a carboxy-terminal ZF DNA binding domain, and an amino-terminal BTB motif. Although the BTB domain is required for Thpok to promote CD4 lineage differentiation, its contribution to Thpok function is not clear. In vitro analyses of the BTB domain of the prototype family member, Bcl6 (B cell lymphoma 6) revealed a role of the BTB domain in oligomerization and transcriptional repression. In the present study, we examined whether the Thpok BTB domain simply serves to promote dimerization of Thpok molecules, or whether it also serves to recruit other transcriptional regulators. We generated chimeric Thpok proteins by replacing the BTB domain of Thpok with that of other family members, or with a heterologous dimerization domain. All chimeric molecules containing the BTB domains tested blocked CD8 lineage specific gene expression; in contrast, the chimeric molecule carrying a heterologous dimerization domain was only partially functional. These results indicate that the BTB domain of Thpok not only promotes dimerization but it also serves to control transcription, presumably through interactions with transcriptional corepressors.

Ets1 is required for CD4 shut down during CD8 T cell differentiation in the thymus (46.11)

During the latest steps of thymocyte differentiation, T cell precursors that have rearranged TCRbeta and TCRalpha genes and express both CD4 and CD8 (double positive, DP) are selected into MHC II-restricted CD4 T cells, or MHC I-restricted CD8 T cells. Several transcription factors regulate this developmental transition: Gata3 and Zbtb7b promote the generation of CD4 cells, whereas Runx3 is important for CD8 T cell development by promoting the cessation of CD4 expression. The transcription factor Ets1 was reported to contribute to CD8 cell development, but its function in this process remains unclear. The present study has tested the hypothesis that Ets1 participates in CD8 cell differentiation by enabling Runx3 function. Expression of the P14 MHC I-restricted transgenic TCR in Ets1-deficient mice resulted in the generation of a population of CD8 lineage cells that failed to downregulate CD4, suggesting that Ets1 is required for the down-regulation of CD4. Transient transfection analyses failed to reveal any Ets1-Runx3 cooperation on CD4 expression. Rather, we found that Ets1 is necessary for the proper expression of Runx3 during CD8 thymocyte differentiation, and enforced expression of Runx3 restores CD4 shut-down in Ets1-/- MHC I-restricted thymocytes. These findings identify Ets1 as a key player during CD8-lineage differentiation and show that it acts at least in part by promoting Runx3 expression.

'Coreceptor tuning': cytokine signals transcriptionally tailor CD8 coreceptor expression to the self-specificity of the TCR

T cell immunity requires the long-term survival of T cells that are capable of recognizing self antigens but are not overtly autoreactive. How this balance is achieved remains incompletely understood. Here we identify a homeostatic mechanism that transcriptionally tailors CD8 coreceptor expression in individual CD8+ T cells to the self-specificity of their clonotypic T cell receptor (TCR). 'Coreceptor tuning' results from interplay between cytokine and TCR signals, such that signals from interleukin 7 and other common gamma-chain cytokines transcriptionally increase CD8 expression and thereby promote TCR engagement of self ligands, whereas TCR signals impair common gamma-chain cytokine signaling and thereby decrease CD8 expression. This dynamic interplay induces individual CD8+ T cells to express CD8 in quantities appropriate for the self-specificity of their TCR, promoting the engagement of self ligands, yet avoiding autoreactivity.

Hematopoiesis and thymic apoptosis are not affected by the loss of Cdk2

Cell cycle regulation is essential for proper homeostasis of hematopoietic cells. Cdk2 is a major regulator of S phase entry, is activated by mitogenic cytokines, and has been suggested to be involved in antigen-induced apoptosis of T lymphocytes. The role of Cdk2 in hematopoietic cells and apoptosis in vivo has not yet been addressed. To determine whether Cdk2 plays a role in these cells, we performed multiple analyses of bone marrow cells, thymocytes, and splenocytes from Cdk2 knockout mice. We found that Cdk2 is not required in vivo to induce apoptosis in lymphocytes, a result that differs from previous pharmacological in vitro studies. Furthermore, thymocyte maturation was not affected by the lack of Cdk2. We then analyzed the hematopoietic stem cell compartment and found similar proportions of stem cells and progenitors in Cdk2(-)(/)(-) and wild-type animals. Knockouts of Cdk2 inhibitors (p21, p27) affect stem cell renewal, but a competitive graft experiment indicated that renewal and multilineage differentiation are normal in the absence of Cdk2. Finally, we stimulated T lymphocytes or macrophages to induce proliferation and observed normal reactivation of Cdk2(-)(/)(-) quiescent cells. Our results indicate that Cdk2 is not required for proliferation and differentiation of hematopoietic cells in vivo, although in vitro analyses consider Cdk2 to be a major player in proliferation and apoptosis in these cells and a potential target for therapy.

Unraveling a revealing paradox: Why major histocompatibility complex I-signaled thymocytes "paradoxically" appear as CD4+8lo transitional cells during positive selection of CD8+ T cells

The mechanism by which T cell receptor specificity determines the outcome of the CD4/CD8 lineage decision in the thymus is not known. An important clue is the fact that major histocompatibility complex (MHC)-I-signaled thymocytes paradoxically appear as CD4+8lo transitional cells during their differentiation into CD8+ T cells. Lineage commitment is generally thought to occur at the CD4+8+ (double positive) stage of differentiation and to result in silencing of the opposite coreceptor gene. From this perspective, the appearance of MHC-I-signaled thymocytes as CD4+8lo cells would be due to effects on CD8 surface protein expression, not CD8 gene expression. But contrary to this perspective, this study demonstrates that MHC-I-signaled thymocytes appear as CD4+8lo cells because of transient down-regulation of CD8 gene expression, not because of changes in CD8 surface protein expression or distribution. This study also demonstrates that initial cessation of CD8 gene expression in MHC-I-signaled thymocytes is not necessarily indicative of commitment to the CD4+ T cell lineage, as such thymocytes retain the potential to differentiate into CD8+ T cells. These results challenge classical concepts of lineage commitment but fulfill predictions of the kinetic signaling model.

CD5-mediated inhibition of TCR signaling during intrathymic selection and development does not require the CD5 extracellular domain

CD5 functions as a negative regulator of TCR signaling during intrathymic T cell development, but it is not known if this negative regulatory function requires CD5 engagement of an extracellular ligand. The present study has specifically examined the role of the CD5 extracellular domain in T cell development by introducing into CD5-/- mice a chimeric CD5 molecule in which the extracellular domain of CD5 is replaced with the extracellular domain of human IL-2R p55 (Tac) for which no ligand exists in the mouse. We now report that CD5 mediated down-regulation of TCR signaling during thymocyte development does not require the CD5 extracellular domain and, consequently, does not involve CD5 binding of an extracellular ligand in the thymus.

Strength of signaling by CD4 and CD8 coreceptor tails determines the number but not the lineage direction of positively selected thymocytes

The present study has assessed the impact of the intracellular domains of CD4 and CD8 on positive selection and lineage direction of MHC class I-restricted thymocytes. Contrary to current presumption, we found that the CD4 tail promotes the generation of both CD4+ and CD8+ T cells without preference for the CD4+ T cell lineage. We also found that the identity of the coreceptor tail and hence the strength of coreceptor signaling determine the number of thymocytes undergoing positive selection but not their ultimate CD4/CD8 phenotype. These findings demonstrate that the strength of coreceptor signaling has a significant quantitative but not qualitative impact on positive selection and provide a simple explanation for the greater numbers of CD4+ than CD8+ T cells selected in the normal thymus.

Role of CD8beta domains in CD8 coreceptor function: importance for MHC I binding, signaling, and positive selection of CD8+ T cells in the thymus

The contribution of the CD8beta subunit to CD8 coreceptor function is poorly understood. We now demonstrate that the CD8beta extracellular domain increases the avidity of CD8 binding to MHC I, and that the intracellular domain of CD8beta enhances association with two intracellular molecules required for TCR signal transduction, Lck and LAT. By assessing CD8+ T cell differentiation in CD8beta-deficient mice reconstituted with various transgenic CD8beta chimeric molecules, we also demonstrate that the intracellular and extracellular domains of CD8beta can contribute independently to CD8+ T cell development, but that both CD8beta domains together are most efficient. Thus, this study identifies the molecular functions of the CD8beta intracellular and extracellular domains and documents their contributions to CD8+ T cell development.

Signals involved in CD4/CD8 lineage commitment: current concepts and potential mechanisms

A competent immune system requires that mature T cells express TCR and CD4/CD8 coreceptors with matching MHC specificities. Such matching of TCR and coreceptor specificity is induced in the thymus at the CD4(+)8(+)stage of development and is referred to as lineage commitment. The process by which immature CD4(+)8(+)thymocytes are signaled to undergo lineage commitment continues to be the subject of intense investigation and discussion. Here, we review the major models by which lineage commitment is thought to occur and discuss the experimental results on which they were based.

Tissue-specific activity of the gammac chain gene promoter depends upon an Ets binding site and is regulated by GA-binding protein

The gammac chain is a subunit of multiple cytokine receptors (interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15), the expression of which is restricted to hematopoietic lineages. A defect in gammac leads to the X-linked severe combined immunodeficiency characterized by a block in T cell differentiation. In order to better characterize the human gammac promoter and define the minimal tissue-specific promoter region, progressive 5'-deletion constructs of a segment extending 1053 base pairs upstream of the major transcription start site were generated and tested for promoter activity in various hematopoietic and nonhematopoietic cell types. The -1053/+34 construct allowed promoter activity only in cells of hematopoietic origin, and tissue specificity was conserved in all other constructs tested. The region downstream of -90 appeared critical for basal promoter activity. It contains two potential Ets binding sites conserved in the murine gammac promoter gene, one of which was found essential for functional promoter activity as determined by mutational analysis. The functional Ets binding site was found to bind Ets family proteins, principally GA-binding protein and Elf-1 and could be transactivated by GABPalpha and -beta synergistically. These results indicate that, as already reported for the IL2Rbeta promoter, GA-binding protein is an essential component of gammac basal promoter activity. Although GABP expression is not restricted to the hematopoietic lineage, its interaction with other specific factors may contribute to the tissue-specific expression of the gammac gene.

Identification of human granzyme B promoter regulatory elements interacting with activated T-cell-specific proteins: implication of Ikaros and CBF binding sites in promoter activation

Granzyme B serine protease is found in the granules of activated cytotoxic T cells and in natural and lymphokine-activated killer cells. This protease plays a critical role in the rapid induction of target cell DNA fragmentation. The DNA regulatory elements that are responsible for the specificity of granzyme B gene transcription in activated T-cells reside between nt -148 and +60 (relative to the transcription start point at +1) of the human granzyme B gene promoter. This region contains binding sites for the transcription factors Ikaros, CBF, Ets, and AP-1. Mutational analysis of the human granzyme B promoter reveals that the Ikaros binding site (-143 to -114) and the AP-1/CBF binding site (-103 to -77) are essential for the activation of transcription in phytohemagglutinin-activated peripheral blood lymphocytes, whereas mutation of the Ets binding site does not affect promoter activity in these cells.

Myb and Ets related transcription factors are required for activity of the human lck type I promoter

The lck gene, which encodes a lymphoid-specific Src family tyrosine kinase, is transcribed from two promoters that are differentially utilized during T cell development. We have shown previously that the human lck type I promoter, which is preferentially expressed in immature thymocytes, requires a binding site (-97 to -90) for the Ets family of transcription factors for its activity in Jurkat T leukemia cells. Three putative Myb binding sites (-86 to -82, -77 to -72 and -59 to -54) were analysed for their ability to activate the lck type I promoter. In vitro assays demonstrated specific binding of purified, bacterially expressed c-Myb DNA binding domain to the Myb (-59 to -54) site. Transient transfection assays using the site-directed mutants of the lck type I promoter in Jurkat cells revealed that mutation of the Myb (-59 to -54) site abolished transcriptional activity. In transiently transfected HeLa cells, the lck type I promoter was activated by co-transfection with a vector that expresses c-Myb. This c-Myb dependent activation required the presence of intact Myb and Ets binding sites, indicating that the expressed c-Myb functions with endogenous Ets related transcription factors to activate the lck type I promoter. This effect was further enhanced by co-transfection with vectors that express either Ets1 or Ets2. These results demonstrate that Myb and Ets related transcription factors synergistically activate the human lck type I promoter.

DNA-binding and transcriptional activation properties of the EWS-FLI-1 fusion protein resulting from the t(11;22) translocation in Ewing sarcoma

The 5' half of the EWS gene has recently been described to be fused to the 3' regions of genes encoding the DNA-binding domain of several transcriptional regulators, including ATF1, FLI-1, and ERG, in several human tumors. The most frequent occurrence of this situation results from the t(11;22)(q24;q12) chromosome translocation specific for Ewing sarcoma (ES) and related tumors which joins EWS sequences to the 3' half of FLI-1, which encodes a member of the Ets family of transcriptional regulators. We show here that this chimeric gene encodes an EWS-FLI-1 nuclear protein which binds DNA with the same sequence specificity as the wild-type parental FLI-1 protein. We further show that EWS-FLI-1 is an efficient sequence-specific transcriptional activator of model promoters containing FLI-1 (Ets)-binding sites, a property which is strictly dependent on the presence of its EWS domain. Comparison of the properties of the N-terminal activation domain of FLI-1 to those of the EWS domain of the fusion protein indicates that EWS-FLI-1 has altered transcriptional activation properties compared with FLI-1. These results suggest that EWS-FLI-1 contributes to the transformed phenotype of ES tumor cells by inducing the deregulated and/or unscheduled activation of genes normally responsive to FLI-1 or to other close members of the Ets family. ES and related tumors are characterized by an elevated level of c-myc expression. We show that EWS-FLI-1 is a transactivator of the c-myc promoter, suggesting that upregulation of c-myc expression is under control of EWS-FLI-1.

A single amino-acid substitution in the Ets domain alters core DNA binding specificity of Ets1 to that of the related transcription factors Elf1 and E74

Ets proteins form a family of sequence specific DNA binding proteins which bind DNA through a 85 aminoacids conserved domain, the Ets domain, whose sequence is unrelated to any other characterized DNA binding domain. Unlike all other known Ets proteins, which bind specific DNA sequences centered over either GGAA or GGAT core motifs, E74 and Elf1 selectively bind to GGAA corecontaining sites. Elf1 and E74 differ from other Ets proteins in three residues located in an otherwise highly conserved region of the Ets domain, referred to as conserved region III (CRIII). We show that a restricted selectivity for GGAA core-containing sites could be conferred to Ets1 upon changing a single lysine residue within CRIII to the threonine found in Elf1 and E74 at this position. Conversely, the reciprocal mutation in Elf1 confers to this protein the ability to bind to GGAT core containing EBS. This, together with the fact that mutation of two invariant arginine residues in CRIII abolishes DNA binding, indicates that CRIII plays a key role in Ets domain recognition of the GGAA/T core motif and lead us to discuss a model of Ets proteins--core motif interaction.

Synergistic activation of the HTLV1 LTR Ets-responsive region by transcription factors Ets1 and Sp1

Ets1 is the prototype of a family of transcriptional activators whose activity depends on the binding to specific DNA sequences characterized by an invariant GGA core sequence. We have previously demonstrated that transcriptional activation by Ets1 of the long terminal repeat (LTR) of human T cell lymphotropic virus type 1 is strictly dependent on the binding of Ets1 to two sites, ERE-A and ERE-B, localized in a 44 bp long Ets-responsive region (ERR1). We report here that the activity of ERR1 as an efficient Ets1 response element in HeLa cells also depends on the integrity of an Sp1 binding site localized immediately upstream of ERE-A. The response to Ets1 of an element restricted to the SP1/ERE-A binding sites is also strictly dependent on both the Ets1 and Sp1 binding sites. In vitro, Sp1 and Ets1 are shown to cooperate to form a ternary complex with the SP1/ERE-A element. Reconstitution experiments in Drosophila melanogaster Schneider cells show that Ets1 and Sp1 act synergistically to activate transcription from either the ERR1 or the SP1/ERE-A elements and that synergy requires the binding of both Sp1 and Ets1 to their cognate sites. SP1/ERE-A elements are found in the enhancer/promoter region of several cellular genes, suggesting that synergy between Ets1 and Sp1 is not restricted to the ERR1 region of the HTLV1 LTR. These results strengthen the notion that Ets1 as well as other members of the Ets family usually function as components of larger transcription complexes to regulate the activity of a variety of viral and cellular genes.

Characterization of Spi-B, a transcription factor related to the putative oncoprotein Spi-1/PU.1

We have cloned a human cDNA from a new gene, spi-B, on the basis of its homology with the DNA-binding domain of the Spi-1/PU.1 putative oncogene product. spi-B codes for a protein of 262 amino acids presenting 43% overall identity with Spi-1. Its highly basic carboxy-terminal region exhibits 34% sequence identity with the DNA-binding domain of the Ets-1 protein. We showed that the Spi-B protein is able to bind the purine-rich sequence (PU box) recognized by Spi-1/PU.1 and to activate transcription of a reporter plasmid containing PU boxes. Chromosome in situ hybridization allowed us to map spi-B to the 19q13.3-19q13.4 region of the human genome. spi-B, like spi-1, was found to be expressed in various murine and human hematopoietic cell lines except T lymphoid cell lines.

Analysis of the DNA binding and transcriptional activation properties of the Ets1 oncoprotein

The c-ets1 gene product (Ets1) is the prototype of a family of sequence-specific transcriptional activators which have been implicated in various developmental processes and in the response of cells to a variety of extracellular stimuli. We report here a structure-function analysis of the DNA binding and transcriptional activation properties of Ets1. The minimal region required for specific DNA binding is located at the carboxy-terminus of Ets1, a domain highly conserved in all known members of the Ets family. Transcriptional activation by Ets1 in mammalian cells requires an additional domain of 110 amino acids characterized by a high content of acidic residues and localized in the amino-terminal half of the protein. This domain also functions as a transcriptional activation domain in yeast cells when linked to the heterologous DNA binding domain of Gal4. In contrast to its conservation in Ets1 proteins across vertebrate species, this activation domain is not conserved in other members of the Ets family. These results indicate that an important level of specificity between different members of the Ets family may reside in the differential interactions of their respective activation domains with distinct general transcription factors or different associated coactivators.

Myb protein binds to multiple sites in the human T cell lymphotropic virus type 1 long terminal repeat and transactivates LTR-mediated expression

The members of the c-myb proto-oncogene family encode sequence-specific transcriptional activators. In T cells, expression of c-myb and the related B-myb gene is induced following mitogenic stimulation. Using a purified recombinant protein, we report here that the human T cell lymphotropic virus type 1 (HTLV-1) LTR contains six specific binding sites for Myb. We also show that HTLV-1 LTR chloramphenicol acetyl transferase reporter plasmids are specifically transactivated by c-Myb. These data suggest a role for members of the Myb family as a link between transcriptional activation of the HTLV-1 LTR and T cell activation events.

Sequence-specific interaction of the Ets1 protein with the long terminal repeat of the human T-lymphotropic virus type I

We recently demonstrated that members of the c-ets proto-oncogene family, Ets1 and Ets2, are sequence-specific transcriptional activators of the human T-lymphotropic virus type I (HTLV-I) long terminal repeat (LTR). We now report that the HTLV-I LTR contains two distinct Ets1-responsive regions, ERR-1 and ERR-2. Expression of Ets1 with reporter plasmids containing ERR-1 or ERR-2 upstream of a basal promoter resulted in an increase in transcriptional activity. By gel mobility shift assay, the interaction of Ets1 with the downstream ERR-1-binding region was found to be more stable than its interaction with the upstream ERR-2 region. By DNase I footprint, gel mobility shift, and methylation interference analyses, ERR-1 was found to contain two Ets1 binding sites, ERE-A and ERE-B. A recombinant Ets1 protein was found to bind with higher affinity to ERE-A than to ERE-B. Binding of Ets1 to these sites appears to result in a specific and sequential protection of a 37-nucleotide sequence of the HTLV-I LTR from -154 to -118. In view of the high-level expression of Ets1 in lymphoid cells, the c-ets proto-oncogenes encode transcription factors which could play an important role in both basal and Tax1-mediated HTLV-I transcription.

The product of the c-ets-1 proto-oncogene and the related Ets2 protein act as transcriptional activators of the long terminal repeat of human T cell leukemia virus HTLV-1

The c-ets-1 proto-oncogene and the related c-ets-2 gene encode related nuclear chromatin-associated proteins which bind DNA in vitro. To investigate the possibility that Ets1 and Ets2 are transcriptional activators, we analyzed the ability of these proteins to trans-activate promoter/enhancer sequences in transient co-transfection experiments. A CAT construct driven by the long terminal repeat of the human T cell leukemia virus, HTLV-1 was found to be trans-activated by both Ets1 and Ets2 in NIH3T3 and HeLa cells. The increased levels of CAT activity were paralleled by increased levels of correctly initiated CAT mRNA. Mutant Ets1 proteins unable to accumulate in the nucleus were found to be inactive. An ets-responsive sequence between positions -117 and -160 of the LTR was identified by analyses of a series of 5' deletion mutants of the HTLV-1 LTR and of dimerized versions of specific motifs of the LTR enhancer region. Using a gel shift binding assay, Ets1 was found to bind specifically to an oligonucleotide corresponding to region -117 to -160. This sequence, which also contributes to Tax1 responsiveness of the HTLV-1 LTR, is characterized by the presence of four repeats of a pentanucleotide sequence of the type CC(T/A)CC. Competition experiments show that integrity of repeats 1 and 4 is important for Ets1 binding. These results show that Ets1 and Ets2 are sequence-specific transcriptional activators. In view of the high level expression of Ets1 in lymphoid cells, Ets1 could be part of the transcription complex which mediates the response to Tax1 and the control of HTLV-1 replication. More generally, Ets1 and Ets2 could regulate transcription of cellular genes.

Identification of a Ets1 variant protein unaffected in its chromatin and in vitro DNA binding capacities by T cell antigen receptor triggering and intracellular calcium rises

We previously showed that thymocytes express high levels of c-ets-1 protein (Ets1) that can be rapidly phosphorylated following mitogenic stimulation using lectins. We demonstrate here that T cell receptor (TCR) specific stimulation with monoclonal antibodies of mature CD8+ or CD4+ T cells also results in the rapid phosphorylation of Ets1, reinforcing the hypothesis of a possible role for Ets1 in T cell activation. In addition to the major Ets1 product (mu-p63c-ets-1), we identify in mouse thymocytes and mature T cells a distinct 52 Kd Ets1 related protein (mu-p52c-ets-1). In contrast to the major Ets1 protein, mu-p52c-ets-1 is poorly phosphorylated in unstimulated cells. Furthermore, mitogenic stimulation of thymocytes and T cells failed to induce in mu-p52c-ets-1 the Ca2(+)-dependent phosphorylation events which are known to drastically affect the migration of the major Ets1 protein in SDS polyacrylamide gels. Mu-p52c-ets-1, like mu-p63c-ets-1, is a nuclear-chromatin associated protein which exhibits DNA binding activity in vitro. However, in contrast to the major Ets1 protein, the association of mu-p53c-ets-1 with chromatin and its ability to bind to DNA in vitro are unaffected by activation stimuli resulting in an increase in [Ca2+]i. Finally, we present indications suggesting that mu-p52c-ets-1 might be the murine equivalent of the translation product of an alternatively c-ets-1 spliced mRNA described in human cells by others.

Thyroid hormone action and the erbA oncogene family

In this review, we discuss the biological action and biochemical function of the v-erbA oncogene product, and the role of c-erbA proto-oncogene products as thyroid hormone receptors, as related to the molecular structure and function of the nuclear hormone receptors at large.