Growth factor independence-1B expression leads to defects in T cell activation, IL-7 receptor alpha expression, and T cell lineage commitment

Linking candidate causal autoimmune variants to T cell networks using genetic and epigenetic screens in primary human T cells

Genetic variants associated with autoimmune diseases are highly enriched within putative cis -regulatory regions of CD4 + T cells, suggesting that they alter disease risk via changes in gene regulation. However, very few genetic variants have been shown to affect T cell gene expression or function. We tested >18,000 autoimmune disease-associated variants for allele-specific expression using massively parallel reporter assays in primary human CD4 + T cells. The 545 expression-modulating variants (emVars) identified greatly enrich for likely causal variants. We provide evidence that many emVars are mediated by common upstream regulatory conduits, and that putative target genes of primary T cell emVars are highly enriched within a lymphocyte activation network. Using bulk and single-cell CRISPR-interference screens, we confirm that emVar-containing T cell cis -regulatory elements modulate both known and novel target genes that regulate T cell proliferation, providing plausible mechanisms by which these variants alter autoimmune disease risk.

Type B thymomas in patients with myasthenia gravis display a distinctive pattern of αβ TCR and IL-7 receptor α expression on CD4+CD8+ thymocytes

Thymoma is closely associated with myasthenia gravis (MG). However, due to the heterogeneity of thymoma and the intricate pathogenesis of MG, it remains unclear why some patients with thymoma develop MG and others do not. In this study, we conducted a comparative phenotype analysis of thymocytes in type B thymomas in patients with MG (MG (+) thymomas) and without MG (MG (-) thymomas) via fluorescence-activated cell sorting (FACS). Our results show that the developmental stages defined by the expression of CD3, CD4, and CD8 were largely maintained in both MG (+) and MG (-) thymomas, with CD4+CD8+ cells constituting the majority of thymocytes in type B thymoma, and no significant difference between this cell population was observed in MG (+) and MG (-) thymomas.We discovered that CD4+CD8+ thymocytes in MG (+) thymomas expressed low levels of αβ TCR and high levels of IL-7 receptor α (IL-7Rα), whereas in MG (-) thymomas, CD4+CD8+ thymocytes exhibited the opposite pattern of αβ TCR and IL-7Rα expression. These results suggest that the positive and negative selection processes of CD4+CD8+ thymocytes might differ between MG (+) thymomas and MG (-) thymomas. The expression of the Helios transcription factor is induced during negative selection and marks a group of T cells that have undergone negative selection and are likely to be deleted due to strong TCR binding with self-peptides/MHC ligands. We observed that the percentage of Helios-positive CD4SP T cells was greater in MG (-) than in MG (+) thymomas. Thus, the differentially regulated selection process of CD4+CD8+ thymocytes, which involves TCR and IL-7/IL-7Rα signaling, is associated with the presence of MG in type B thymomas.

The transcription factors GFI1 and GFI1B as modulators of the innate and acquired immune response

GFI1 and GFI1B are small nuclear proteins of 45 and 37kDa, respectively, that have a simple two-domain structure: The first consists of a group of six c-terminal C₂H₂ zinc finger motifs that are almost identical in sequence and bind to very similar, specific DNA sites. The second is an N-terminal 20 amino acid SNAG domain that can bind to the pocket of the histone demethylase KDM1A (LSD1) near its active site. When bound to DNA, both proteins act as bridging factors that bring LSD1 and associated proteins into the vicinity of methylated substrates, in particular histone H3 or TP53. GFI1 can also bring methyl transferases such as PRMT1 together with its substrates that include the DNA repair proteins MRE11 and 53BP1, thereby enabling their methylation and activation. While GFI1B is expressed almost exclusively in the erythroid and megakaryocytic lineage, GFI1 has clear biological roles in the development and differentiation of lymphoid and myeloid immune cells. GFI1 is required for lymphoid/myeloid and monocyte/granulocyte lineage decision as well as the correct nuclear interpretation of a number of important immune-signaling pathways that are initiated by NOTCH1, interleukins such as IL2, IL4, IL5 or IL7, by the pre TCR or -BCR receptors during early lymphoid differentiation or by T and B cell receptors during activation of lymphoid cells. Myeloid cells also depend on GFI1 at both stages of early differentiation as well as later stages in the process of activation of macrophages through Toll-like receptors in response to pathogen-associated molecular patterns. The knowledge gathered on these factors over the last decades puts GFI1 and GFI1B at the center of many biological processes that are critical for both the innate and acquired immune system.

Epigenetic Regulation of Gfi1 in Endocrine-Related Cancers: a Role Regulating Tumor Growth

Prostate and breast cancer constitute the most common cancers among men and women worldwide. The aging population is one of the main risk factors for prostate and breast cancer development and accumulating studies link aging with epigenetic changes. Growth factor independence-1 (Gfi1) is a transcriptional repressor with an important role in human malignancies, including leukemia, colorectal carcinoma, and lung cancer, but its role in prostate and breast cancer is unknown. We have found that Gfi1 epigenetic silencing is a common event in prostate and breast cancer. Gfi1 re-expression in prostate and breast cancer cell lines displaying Gfi1 epigenetic silencing decreases cell proliferation, reduced colony formation density, and tumor growth in nude mice xenografts. In addition, we found that Gfi1 repress alpha 1-anti-trypsin (AAT) and alpha 1-anti-chymotrypsin (ACT) expression, two genes with important functions in cancer development, suggesting that Gfi1 silencing promotes tumor growth by increasing AAT and ACT expression in our system. Finally, Gfi1 epigenetic silencing could be a promising biomarker for prostate cancer progression because it is associated with shorter disease-free survival. In conclusion, our findings strongly indicate that Gfi1 epigenetic silencing in prostate and breast cancer could be a crucial step in the development of these two-well characterized endocrine related tumors.

Role of GFI1 in Epigenetic Regulation of MDS and AML Pathogenesis: Mechanisms and Therapeutic Implications

Growth factor independence 1 (GFI1) is a DNA binding zinc finger protein, which can mediate transcriptional repression mainly by recruiting histone-modifying enzymes to its target genes. GFI1 plays important roles in hematopoiesis, in particular by regulating both the function of hematopoietic stem- and precursor cells and differentiation along myeloid and lymphoid lineages. In recent years, a number of publications have provided evidence that GFI1 is involved in the pathogenesis of acute myeloid leukemia (AML), its proposed precursor, myelodysplastic syndrome (MDS), and possibly also in the progression from MDS to AML. For instance, expression levels of the GFI1 gene correlate with patient survival and treatment response in both AML and MDS and can influence disease progression and maintenance in experimental animal models. Also, a non-synonymous single nucleotide polymorphism (SNP) of GFI1, GFI1-36N, which encodes a variant GFI1 protein with a decreased efficiency to act as a transcriptional repressor, was found to be a prognostic factor for the development of AML and MDS. Both the GFI1-36N variant as well as reduced expression of the GFI1 gene lead to genome-wide epigenetic changes at sites where GFI1 occupies target gene promoters and enhancers. These epigenetic changes alter the response of leukemic cells to epigenetic drugs such as HDAC- or HAT inhibitors, indicating that GFI1 expression levels and genetic variants of GFI1 are of clinical relevance. Based on these and other findings, specific therapeutic approaches have been proposed to treat AML by targeting some of the epigenetic changes that occur as a consequence of GFI1 expression. Here, we will review the well-known role of Gfi1 as a transcription factor and describe the more recently discovered functions of GFI1 that are independent of DNA binding and how these might affect disease progression and the choice of epigenetic drugs for therapeutic regimens of AML and MDS.

Transcriptional regulation of the IL-7Rα gene by dexamethasone and IL-7 in primary human CD8 T cells

Interleukin-7 is essential for the development and maintenance of T cells, and the expression of the IL-7 receptor is tightly regulated at every stage of the T cell's lifespan. In mature CD8 T cells, IL-7 plays important roles in cell survival, peripheral homeostasis, and cytolytic function. The IL-7 receptor alpha-chain (CD127) is expressed at high levels on naïve and memory cells, but it is rapidly downregulated upon IL-7 stimulation. In this study, we illustrate the dynamicity of the CD127 promoter and show that it possesses positive as well as negative regulatory sites involved in upregulating and downregulating CD127 expression, respectively. We cloned the CD127 gene promoter and identified key cis-regulatory elements required for CD127 expression in mature resting primary CD8 T cells. The core promoter necessary for efficient basal transcription is contained within the first 262 bp upstream of the TATA box. Additional positive regulatory elements are located between -1200 and -2406 bp, conferring a further 2- to 4-fold enhancement in gene expression. While transcription of the CD127 gene is increased directly through a glucocorticoid response element located between -2255 and -2269 bp upstream of the TATA box, we identified a suppressive region that lies upstream of 1760 bp from the TATA box, which is likely involved in the IL-7-mediated suppression of CD127 transcription. Finally, we illustrated IL-7 does not bias alternative splicing of CD127 transcripts in primary human CD8 T cells.

Genetic risk variants for autoimmune diseases that influence gene expression in thymus

Genome-wide association studies (GWAS) have boosted our knowledge of genetic risk variants in autoimmune diseases (AIDs). Most risk variants are located within or near genes with immunological functions, and the majority is found to be non-coding, pointing towards a regulatory role. In this study, we performed a cis expression quantitative trait locus (eQTL) screen restricted to 353 AID associated risk variants selected from the GWAS catalog to investigate whether these single nucleotide polymorphisms (SNPs) influence gene expression in thymus. Genotypes were obtained by Immunochip (Ichip) and tested against expression of surrounding genes (±1 Mb) in human thymic tissue (n = 42). We identified eight significant eQTLs located within seven genetic regions (FCRL3, RNASET2, C2orf74, NPIPB8, SIRPG, SYS1 and AJ006998.2) where the expression was associated with AID risk SNPs at a study-wide level of significance (P < 2.7 × 10^-5). In NPIPB8 and AJ006998.2, the eQTL signals appeared to be thymus-specific. Furthermore, many AID risk SNPs from GWAS have been subsequently fine-mapped in recent Ichip projects, and fine-mapped AID SNPs overlapped with the thymic eQTLs within RNASET2 and SIRPG Finally, in all the eQTL regions, except C2orf74, SNPs underlying the thymic eQTLs were predicted to interfere with transcription factors important in T cell development. Our study therefore reveals autoimmune risk variants that act as eQTLs in thymus, and suggest that thymic gene regulation may play a functional role at some AID risk loci.

Responsiveness of Developing T Cells to IL-7 Signals Is Sustained by miR-17∼92

miRNAs regulate a large variety of developmental processes including development of the immune system. T cell development is tightly controlled through the interplay of transcriptional programs and cytokine-mediated signals. However, the role of individual miRNAs in this process remains largely elusive. In this study, we demonstrated that hematopoietic cell-specific loss of miR-17∼92, a cluster of six miRNAs implicated in B and T lineage leukemogenesis, resulted in profound defects in T cell development both at the level of prethymic T cell progenitors as well as intrathymically. We identified reduced surface expression of IL-7R and concomitant limited responsiveness to IL-7 signals as a common mechanism resulting in reduced cell survival of common lymphoid progenitors and thymocytes at the double-negative to double-positive transition. In conclusion, we identified miR-17∼92 as a critical modulator of multiple stages of T cell development.

From cytopenia to leukemia: the role of Gfi1 and Gfi1b in blood formation

The DNA-binding zinc finger transcription factors Gfi1 and Gfi1b were discovered more than 20 years ago and are recognized today as major regulators of both early hematopoiesis and hematopoietic stem cells. Both proteins function as transcriptional repressors by recruiting histone-modifying enzymes to promoters and enhancers of target genes. The establishment of Gfi1 and Gfi1b reporter mice made it possible to visualize their cell type-specific expression and to understand their function in hematopoietic lineages. We now know that Gfi1 is primarily important in myeloid and lymphoid differentiation, whereas Gfi1b is crucial for the generation of red blood cells and platelets. Several rare hematologic diseases are associated with acquired or inheritable mutations in the GFI1 and GFI1B genes. Certain patients with severe congenital neutropenia carry mutations in the GFI1 gene that lead to the disruption of the C-terminal zinc finger domains. Other mutations have been found in the GFI1B gene in families with inherited bleeding disorders. In addition, the Gfi1 locus is frequently found to be a proviral integration site in retrovirus-induced lymphomagenesis, and new, emerging data suggest a role of Gfi1 in human leukemia and lymphoma, underlining the role of both factors not only in normal hematopoiesis, but also in a wide spectrum of human blood diseases.

The Corepressor Rcor1 Is Essential for Normal Myeloerythroid Lineage Differentiation

Based on its physical interactions with histone-modifying enzymes, the transcriptional corepressor Rcor1 has been implicated in the epigenetic regulation blood cell development. Previously, we have demonstrated that Rcor1 is essential for the maturation of definitive erythroid cells and fetal survival. To determine the functional role of Rcor1 in steady-state hematopoiesis in the adult, we used a conditional knockout approach. Here, we show that the loss of Rcor1 expression results in the rapid onset of severe anemia due to a complete, cell autonomous block in the maturation of committed erythroid progenitors. By contrast, both the frequency of megakaryocyte progenitors and their capacity to produce platelets were normal. Although the frequency of common lymphoid progenitors and T cells was not altered, B cells were significantly reduced and showed increased apoptosis. However, Rcor1-deficient bone marrow sustained normal levels of B-cells following transplantation, indicating a non-cell autonomous requirement for Rcor1 in B-cell survival. Evaluation of the myelomonocytic lineage revealed an absence of mature neutrophils and a significant increase in the absolute number of monocytic cells. Rcor1-deficient monocytes were less apoptotic and showed ∼100-fold more colony-forming activity than their normal counterparts, but did not give rise to leukemia. Moreover, Rcor1(-/-) monocytes exhibited extensive, cytokine-dependent self-renewal and overexpressed genes associated with hematopoietic stem/progenitor cell expansion including Gata2, Meis1, and Hoxa9. Taken together, these data demonstrate that Rcor1 is essential for the normal differentiation of myeloerythroid progenitors and for appropriately regulating self-renewal activity in the monocyte lineage.

Growth factor independent-1 maintains Notch1-dependent transcriptional programming of lymphoid precursors

Growth factor independent 1 (Gfi1) is a transcriptional repressor originally identified as a gene activated in T-cell leukemias induced by Moloney-murine-leukemia virus infection. Notch1 is a transmembrane receptor that is frequently mutated in human T-cell acute lymphoblastic leukemia (T-ALL). Gfi1 is an important factor in the initiation and maintenance of lymphoid leukemias and its deficiency significantly impedes Notch dependent initiation of T-ALL in animal models. Here, we show that immature hematopoietic cells require Gfi1 to competently integrate Notch-activated signaling. Notch1 activation coupled with Gfi1 deficiency early in T-lineage specification leads to a dramatic loss of T-cells, whereas activation in later stages leaves development unaffected. In Gfi1 deficient multipotent precursors, Notch activation induces lethality and is cell autonomous. Further, without Gfi1, multipotent progenitors do not maintain Notch1-activated global expression profiles typical for T-lineage precursors. In agreement with this, we find that both lymphoid-primed multipotent progenitors (LMPP) and early T lineage progenitors (ETP) do not properly form or function in Gfi1^−/− mice. These defects correlate with an inability of Gfi1^−/− progenitors to activate lymphoid genes, including IL7R, Rag1, Flt3 and Notch1. Our data indicate that Gfi1 is required for hematopoietic precursors to withstand Notch1 activation and to maintain Notch1 dependent transcriptional programming to determine early T-lymphoid lineage identity.

Understanding the mechanisms that protect lymphoid cells from transformation is a critical first step in developing therapies against blood cancers. Recently, we demonstrated that the Growth factor independent-1 transcriptional repressor protein is required for cancer development driven by activation of Notch1 signaling. Here, we investigated the mechanisms by which Gfi1 protects lymphoid transformation. Using complex genetic mouse models to delete Gfi1 and activate Notch1, we demonstrate that Gfi1 is required to maintain both the homeostatic levels of Notch1 target genes in normal lymphoid precursors in the bone marrow, as well as to maintain the supraphysiologic levels of Notch1 signaling present in pre-malignant lymphoid progenitors. Consequently, without Gfi1 the pool of premalignant cells available for transformation is depleted. Our data provide additional insight into the multiple mechanisms by which developmental networks may have evolved to protect lymphoid cells from transformation.

GFI1B, EVI5, MYB--additional genes that cooperate with the human BCL6 gene to promote the development of lymphomas

The BCL6 gene, which is expressed in certain B- and T-cell human lymphomas, is involved with chromosomal rearrangements and mutations in a number of these neoplasms. Lymphomagenesis is believed to evolve through a multi-step accumulation of genetic alterations in these tumors. We used retroviral insertional mutagenesis in transgenic mice expressing the human BCL6 transgene in order to identify genes that cooperate with BCL6 during lymphomatous transformation. We previously reported PIM1 as the most frequently recurring cooperating gene in this model. We now report three newly identified cooperating genes-GFI1B, EVI5, and MYB-that we identified in the lymphomas of retroviral-injected BCL6 transgenic mice (but not in retroviral-injected non-transgenic controls); mRNA and protein expression of GFI1B and EVI5 were decreased in the murine tumors, whereas MYB mRNA and protein expression were increased or decreased. These findings correlated with protein expression in human lymphomas, both B- and T-cell. Improved therapy of lymphomas may necessitate the development of combinations of drugs that target the alterations specific to each neoplasm.

CD8 lineage-specific regulation of interleukin-7 receptor expression by the transcriptional repressor Gfi1

Interleukin-7 receptor α (IL-7Rα) is essential for T cell survival and differentiation. Glucocorticoids are potent enhancers of IL-7Rα expression with diverse roles in T cell biology. Here we identify the transcriptional repressor, growth factor independent-1 (Gfi1), as a novel intermediary in glucocorticoid-induced IL-7Rα up-regulation. We found Gfi1 to be a major inhibitory target of dexamethasone by microarray expression profiling of 3B4.15 T-hybridoma cells. Concordantly, retroviral transduction of Gfi1 significantly blunted IL-7Rα up-regulation by dexamethasone. To further assess the role of Gfi1 in vivo, we generated bacterial artificial chromosome (BAC) transgenic mice, in which a modified Il7r locus expresses GFP to report Il7r gene transcription. By introducing this BAC reporter transgene into either Gfi1-deficient or Gfi1-transgenic mice, we document in vivo that IL-7Rα transcription is up-regulated in the absence of Gfi1 and down-regulated when Gfi1 is overexpressed. Strikingly, the in vivo regulatory role of Gfi1 was specific for CD8(+), and not CD4(+) T cells or immature thymocytes. These results identify Gfi1 as a specific transcriptional repressor of the Il7r gene in CD8 T lymphocytes in vivo.

IL-7 signaling and CD127 receptor regulation in the control of T cell homeostasis

After their development in the thymus, mature T cells are maintained in the periphery by two sets of survival signals, namely TCR signals from contact with self-peptide/MHC ligands and the cytokine receptor signals from binding IL-7 and IL-15. These signals cooperate to maximize the utility of finite resources to support a diverse pool of mature T cells. It is becoming increasingly clear that multiple mechanisms exist to regulate expression of IL-7R at the transcriptional and post-translational levels. The interplay between TCR signals and IL-7R signals are also important in regulation of IL-7R expression. This review will focus on regulation of T cell homeostasis by IL-7R signaling, with an emphasis on the cross talk between signals from TCR and IL-7R.

IL-7: the global builder of the innate lymphoid network and beyond, one niche at a time

The development and homeostasis of adaptive and innate lymphocytes is dependent on the stromal cytokine IL-7. The initial priming of immune responses to pathogenic challenges is executed by innate lymphoid cells (ILCs) with programmed capacity to rapidly secrete effector cytokines. How ILCs are controlled by IL-7 in distinct anatomical locale has evolved into a more complex problem as IL-7 receptor is not only expressed on ILCs, but also on surrounding neighbors, including vascular endothelium and mesenchymal cells that compete for limiting IL-7. For the generation of γδ T and B cells IL-7 is required for the production of antigen receptors, and it is likely that IL-7 performs critical function in facilitating ILC effector programming in addition to its regulatory actions on cell survival and proliferation. Most of our current understanding of the highly calibrated regulatory circuits of IL-7 function and IL-7 receptor signaling has derived from studies of adaptive, conventional lymphocytes. Here we highlight recent advances in mapping the gene circuits and cellular interactions that regulate temporospatial activities of IL-7 in diverse macro and micro niches that have direct relevance to deciphering the sphere of impact of IL-7 on ILC differentiation.

E proteins and the regulation of early lymphocyte development

Lymphopoiesis generates mature B, T, and NK lymphocytes from hematopoietic stem cells via a series of increasingly restricted developmental intermediates. The transcriptional networks that regulate these fate choices are composed of both common and lineage-specific components, which combine to create a cellular context that informs the developmental response to external signals. E proteins are an important factor during lymphopoiesis, and E2A in particular is required for normal T- and B-cell development. Although the other E proteins, HEB and E2-2, are expressed during lymphopoiesis and can compensate for some of E2A's activity, E2A proteins have non-redundant functions during early T-cell development and at multiple checkpoints throughout B lymphopoiesis. More recently, a role for E2A has been demonstrated in the generation of lymphoid-primed multipotent progenitors and shown to favor their specification toward lymphoid over myeloid lineages. This review summarizes both our current understanding of the wide-ranging functions of E proteins during the development of adaptive lymphocytes and the novel functions of E2A in orchestrating a lymphoid-biased cellular context in early multipotent progenitors.

Transcription factor Foxp1 exerts essential cell-intrinsic regulation of the quiescence of naive T cells

The molecular mechanisms that underlie T cell quiescence are poorly understood. Here we report that mature naive CD8(+) T cells lacking the transcription factor Foxp1 gained effector phenotype and function and proliferated directly in response to interleukin 7 (IL-7) in vitro. Foxp1 repressed expression of the IL-7 receptor α-chain (IL-7Rα) by antagonizing Foxo1 and negatively regulated signaling by the kinases MEK and Erk. Acute deletion of Foxp1 induced naive T cells to gain an effector phenotype and proliferate in lympho-replete mice. Foxp1-deficient naive CD8(+) T cells proliferated even in lymphopenic mice deficient in major histocompatibility complex class I. Our results demonstrate that Foxp1 exerts essential cell-intrinsic regulation of naive T cell quiescence, providing direct evidence that lymphocyte quiescence is achieved through actively maintained mechanisms that include transcriptional regulation.

Transcriptional deregulation of oncogenic myocyte enhancer factor 2C in T-cell acute lymphoblastic leukemia

Myocyte enhancer factor 2C (MEF2C) encodes a transcription factor which is ectopically expressed in T-cell acute lymphoblastic leukemia (T-ALL) cell lines, deregulated directly by ectopically expressed homeodomain protein NKX2-5 or by loss of promoter regions via del(5)(q14). Here, we analyzed the MEF2C 5'-region, thus identifying potential regulatory binding sites for GFI1B, basic helix-loop-helix proteins, STAT5, and HOXA9/HOXA10. Chromatin immunoprecipitation and overexpression analyses demonstrated direct activation by GFI1B and LYL1 and inhibition by STAT5. HOXA9/HOXA10 activated expression of NMYC which in turn mediated MEF2C repression, indicating an indirect mode of regulation via NMYC interactor (NMI) and STAT5. Lacking comma: Chromosomal deletion of the STAT5 binding site in LOUCY cells reduced protein levels of STAT5 in some MEF2C-positve T-ALL cell lines, and the presence of inhibitory IL7-JAK-STAT5 signaling highlighted the repressive impact of this factor in MEF2C regulation. Taken together, our results indicate that the expression of MEF2C in T-ALL cells is principally deregulated via activating leukemic transcription factors GFI1B or NKX2-5 and by escaping inhibitory developmental STAT5 signaling.

From the cradle to the grave: activities of GATA-3 throughout T-cell development and differentiation

GATA family transcription factors play multiple vital roles in hematopoiesis in many cell lineages, and in particular, T cells require GATA-3 for execution of several developmental steps. Transcriptional activation of the Gata3 gene is observed throughout T-cell development and differentiation in a stage-specific fashion. GATA-3 has been described as a master regulator of T-helper 2 (Th2) cell differentiation in mature CD4(+) T cells. During T-cell development in the thymus, its roles in the CD4 versus CD8 lineage choice and at the β-selection checkpoint are the best characterized. In contrast, its importance prior to β-selection has been obscured both by the developmental heterogeneity of double negative (DN) 1 thymocytes and the paucity of early T-lineage progenitors (ETPs), a subpopulation of DN1 cells that contains the most immature thymic progenitors that retain potent T-lineage developmental potential. By examining multiple lines of in vivo evidence procured through the analysis of Gata3 mutant mice, we have recently demonstrated that GATA-3 is additionally required at the earliest stage of thymopoiesis for the development of the ETP population. Here, we review the characterized functions of GATA-3 at each stage of T-cell development and discuss hypothetical molecular pathways that mediate these functions.

Coordination of IL-7 receptor and T-cell receptor signaling by cell-division cycle 42 in T-cell homeostasis

T-cell homeostasis is essential for normal functioning of the immune system. IL-7 receptor (IL-7R) and T-cell receptor (TCR) signaling are pivotal for T-cell homeostatic regulation. The detailed mechanisms regulating T-cell homeostasis and how IL-7R and TCR signaling are coordinated are largely unknown. Here we demonstrate that T cell-specific deletion of cell-division cycle 42 (Cdc42) GTPase causes a profound loss of mature T cells. Deletion of Cdc42 leads to a markedly increased expression of growth factor independence-1 (Gfi-1) and represses expression of IL-7Rα. In the absence of Cdc42, aberrant ERK1/2 MAP kinase activity results in enhanced, TCR-mediated T-cell proliferation. In vivo reconstitution of effector-binding-defective Cdc42 mutants and the effector p21 protein-activated kinase 1 (PAK1) into Cdc42-deficient T cells showed that PAK1 is both necessary and sufficient for Cdc42-regulated T-cell homeostasis. Thus, T-cell homeostasis is maintained through a concerted regulation of Gfi-1-IL-7R-controlled cytokine responsiveness and ERK-mediated TCR signaling strength by the Cdc42-PAK1 signaling axis.

Active immune response protects Stat6VT transgenic mice from developing a lymphoproliferative disorder

Stat6 is a transcription factor that regulates important cellular processes such as proliferation, differentiation, and survival through mediating IL-4 and IL-13 signaling. Importantly, increasing evidence indicates of a role for Stat6 in lymphoproliferative disorders. Mice expressing a constitutively active form of Stat6 (Stat6VT) primarily in T lymphocytes were generated, and it has been recently described that a small percentage (approximately 5%) of these mice develop a spontaneous lymphoproliferative disorder (LPD) resulting in dramatic splenomegaly and altered splenic cell populations. Here, we report that Stat6VT mice housed in a non-pathogen-free environment have an increased incidence (37%) of the LPD. Additionally, examination of the expression of Stat6-regulated genes known to have roles in tumorigenesis demonstrated that there appears to be no one genetic alteration common to lymphocytes from Stat6VT/LPD mice. Interestingly, however, uniform exposure to antigen via immunization resulted in complete abrogation of the LPD in Stat6VT mice.

Inverse association of repressor growth factor independent-1 with CD8 T cell interleukin (IL)-7 receptor [alpha] expression and limited signal transducers and activators of transcription signaling in response to IL-7 among [gamma]-chain cytokines in HIV patients

BACKGROUND

CD8 T lymphocytes from chronically infected HIV-positive patients degenerate into a preapoptotic state and exhibit impaired functionality. Particularly in viremic patients, this was associated with an increased proportion of interleukin-7 receptor-alpha low-expressing (IL-7Ralpha(low)) effector-like CD8 T cells. As cytokine signaling through signal transducers and activators of transcription (STAT) is essential for cellular function, we hypothesized that activation of this pathway may be impaired in these cells.

OBJECTIVES

To evaluate cytokine-induced STAT activation in IL-7Ralpha(low) and IL-7Ralpha(high) CD8 T cells from chronically infected HIV-positive patients and investigate the potential molecular mechanism involved in the reduced IL-7Ralpha expression.

METHODS

CD8 T cells from HIV-positive patients on and off antiretroviral therapy were assayed respectively for STAT activation, cytokine receptor, and transcription factor expression by flow cytometry and real-time PCR.

RESULTS

IL-7 stimulation failed to activate STAT5 in a substantial proportion of patient CD8 T cells. This correlated with reduced IL-7Ralpha mRNA and surface protein expression. Interestingly, IL-7Ralpha(low) cells appeared to be fully capable of recruiting the STAT pathway in response to IL-2, IL-4, IL-10, and IL-15. mRNA expression suggested a potential role for growth factor independent (Gfi)-1 as an IL-7Ralpha transcriptional repressor, but not that of other transcriptional regulators studied, including Gfi-1B and GA-binding protein alpha. Programmed death-1 inhibitory receptor, though upregulated in CD8 T cells from HIV-positive patients, appeared unrelated to IL-7Ralpha expression and STAT signaling capacity.

Egr2 is required for Bcl-2 induction during positive selection

The repertoire of TCR specificities is established by a selection process in the thymus, during which precursor survival and maturation is dictated by the nature of the TCR signals. The differences in signals that determine whether precursors will survive and mature or be induced to die remain poorly understood. Among the molecular effectors involved in executing the differentiation process initiated by TCR-ligand interactions is a family of Zn-finger transcription factors termed early growth response genes (Egr). Indeed, ablation of the Egr1 gene impairs ligand-induced maturation (positive selection) but not ligand-induced deletion (negative selection). The partial impairment of positive selection by Egr1 deficiency is not enhanced by simultaneous deletion of another Egr family member, Egr3. Accordingly, we asked whether this results from compensation by another family member, Egr2. In this manuscript, we demonstrate that deletion of Egr2 impairs positive selection of both CD4 and CD8 single-positive thymocytes. Interestingly, many of the genes involved in positive selection and T cell differentiation are up-regulated normally in the Egr2-deficient thymocytes. However, Bcl-2 up-regulation is not sustained during late stages of positive selection. This defect is at least partially responsible for the developmental blockade in Egr2-deficient thymocytes, as enforced expression of Bcl-2 rescues T cell development in Egr2(-/-) thymocytes. Taken together, these data suggest that Egr2 plays a central role in the up-regulation of the survival molecule Bcl-2 during positive selection.

Formation of IL-7Ralphahigh and IL-7Ralphalow CD8 T cells during infection is regulated by the opposing functions of GABPalpha and Gfi-1

IL-7 is essential for the survival of naive and memory T cells, and IL-7 receptor alpha-chain (IL-7Ralpha) expression is dynamically regulated in activated CD8 T cells during acute viral and bacterial infections. Most virus-specific CD8 T cells become IL-7Ralpha(low) and are relatively short-lived, but some escape IL-7Ralpha repression (referred to as IL-7Ralpha(high) memory precursor effector cells) and preferentially enter the memory CD8 T cell pool. How antiviral effector CD8 T cells regulate IL-7Ralpha expression in an "on and off" fashion remains to be characterized. During lymphocytic choriomeningitis virus infection, we found that opposing actions of the transcription factors GABPalpha (GA binding protein alpha) and Gfi-1 (growth factor independence 1) control IL-7Ralpha expression in effector CD8 T cells. Specifically, GABPalpha was required for IL-7Ralpha expression in memory precursor effector cells, and this correlated with hyperacetylation of the Il7ra promoter. In contrast, Gfi-1 was required for stable IL-7Ralpha repression in effector CD8 T cells and acted by antagonizing GABPalpha binding and recruiting histone deacetylase 1, which deacetylated the Il7ra promoter. Thus, Il7ra promoter acetylation and activity was dependent on the reciprocal binding of GABPalpha and Gfi-1, and these data provide a biochemical mechanism for the generation of stable IL-7Ralpha(high) and IL-7Ralpha(low) states in virus-specific effector CD8 T cells.

Regulatory factors for initial T lymphocyte lineage specification

PURPOSE OF REVIEW

Initiation of T lymphocyte development depends on balanced regulatory inputs from multiple essential transcription factors. This review highlights contributions of E2A, hematopoietic transcription factor PU.1, growth factor independence (Gfi)-1, T cell factor (TCF)-1, and Runx factors and their interactions with the Notch pathway to promote T cell development.

RECENT FINDINGS

E2A and Runx family factors have been implicated in establishing competent precursors in which Notch signaling can induce the T cell program. An early role was also indicated for PU.1. Later PU.1 activities are antagonistic to pro-T cell factors, however, including E proteins, Myb, Gfi-1, and TCF-1. Diversion to a non-T lineage can be promoted by PU.1, CCAAT/enhancer binding protein, or even GATA and TCF, but these diversion mechanisms are blocked by Notch signaling. An emergent gene network summarizes the cross-regulatory relationships among these factors.

SUMMARY

Entry into the T-cell pathway is controlled by a dynamic balance among essential regulatory factors that depend on Notch signaling not only to trigger initiation of the T-cell program but also to maintain the lineage fidelity of their collective action.

Transcriptional repressor Gfi1 integrates cytokine-receptor signals controlling B-cell differentiation

Hematopoietic stem cell differentiation is specified by cytokines and transcription factors, but the mechanisms controlling instructive and permissive signalling networks are poorly understood. We provide evidence that CLP1-dependent IL7-receptor mediated B cell differentiation is critically controlled by the transcriptional repressor Gfi1. Gfi1-deficient progenitor B cells show global defects in IL7Rα-dependent signal cascades. Consequently, IL7-dependent trophic, proliferative and differentiation-inducing responses of progenitor B cells are perturbed. Gfi1 directly regulates expression levels of IL7Rα and indirectly controls STAT5 signalling via expression of SOCS3. Thus, Gfi1 selectively specifies IL7-dependent development of B cells from CLP1 progenitors, providing clues to the transcriptional networks integrating cytokine signals and lymphoid differentiation.

Interleukin-7 receptor expression: intelligent design

Interleukin-7 (IL-7) is produced by stromal cells in lymphoid tissues and is required for the development of T cells and for their persistence in the periphery. Unlike many other cytokines that act on lymphocytes, IL-7 production by stromal cells is not substantially affected by extrinsic stimuli. So, the amount of available IL-7 protein is thought to be regulated by the rate that it is scavenged by T cells. As we review here, there is mounting evidence indicating that the amount of IL-7 receptor expressed on a cell not only determines how vigorously the cell responds to IL-7, but it can also determine how efficiently the cell consumes IL-7 and, therefore, affect the supply of this limiting resource in the niche.

Growth factor independent 1B (Gfi1b) is an E2A target gene that modulates Gata3 in T-cell lymphomas

The E2A transcription factors are required for normal T lymphopoiesis and to prevent T-lymphocyte progenitor transformation. Ectopic expression of E2A proteins in E2A-deficient lymphomas results in growth arrest and apoptosis, indicating that these cells remain responsive to the targets of E2A. Here we identify the transcriptional repressor growth factor independent 1B (Gfi1b) as a target of E2A that promotes growth arrest and apoptosis in lymphomas. Gfi1b expression in primary T-lymphocyte progenitors is dependent on E2A and excess Gfi1b prevents the outgrowth of T lymphocyte progenitors in vitro. Gfi1b represses expression of Gata3, a transcription factor whose appropriate regulation is required for survival of lymphomas and T-lymphocyte progenitors. We also show that ectopic expression of Gata3 in lymphomas promotes expression of Gfi1b, indicating that these proteins may function in an autoregulatory loop that maintains appropriate levels of Gata3. Therefore, we propose that E2A proteins prevent lymphoma cell expansion, at least in part through regulation of Gfi1b and modulation of Gata3 expression.

Gfi1b:green fluorescent protein knock-in mice reveal a dynamic expression pattern of Gfi1b during hematopoiesis that is largely complementary to Gfi1

Gfi1b and Gfi1 are 37- and 55-kDa transcriptional repressors that share common features such as a 20-amino acid (aa) N-terminal SNAG domain, a nonconserved intermediary domain, and 6 highly conserved C-terminal zinc fingers. Both gene loci are under autoregulatory and cross-regulatory feedback control. We have generated a reporter mouse strain by inserting the cDNA for green fluorescent protein (GFP) into the Gfi1b gene locus which allowed us to follow Gfi1b expression during hematopoiesis and lymphopoiesis by measuring green fluorescence. We found highly dynamic expression patterns of Gfi1b in erythroid cells, megakaryocytes, and their progenitor cells (MEPS) where Gfi1 is not detected. Vice versa, Gfi1b could not be found in granulocytes, activated macrophages, or their granulomonocytic precursors (GMPs) or in mature naive or activated lymphocytes where Gfi1 is expressed, suggesting a complementary regulation of both loci during hematopoiesis. However, Gfi1b was found to be up-regulated in early stages of B-cell and in a subset of early T-cell development, where Gfi1 is also present, suggesting that cross-regulation of both loci exists but is cell-type specific.

The growth factor independence-1 transcription factor: new functions and new insights

The growth factor independence-1 (Gfi1) transcription factor is required for proper development of neuroendocrine cells, sensory neurons, and blood. Patients with mutations in Gfi1 exhibit severe congenital neutropenia (SCN) or non-immune chronic idiopathic neutropenia of adults. Gfi1 was initially described as an oncoprotein that mediates tumor progression in a mouse model of leukemia; however, recent data suggest that Gfi1 may act as either an oncogene or an anti-proliferative tumor suppressor gene depending on the cell type. Here we review the latest literature on Gfi1, and emphasize its role in the hematopoietic, sensory and neuroendocrine systems.

IL-7R alpha gene expression is inversely correlated with cell cycle progression in IL-7-stimulated T lymphocytes

IL-7 plays a major role in T lymphocyte homeostasis and has been proposed as an immune adjuvant for lymphopenic patients. This prospect is based, at least in part, on the short-term expansion of peripheral T cells in rIL7-treated mice and primates. Nevertheless, in vivo, following initial increases in T cell proliferation and numbers, lymphocytes return to a quiescent state. As the bases for this cell cycle exit have not yet been elucidated, it is important to assess the long-term biological effects of IL-7 on quiescent human T lymphocyte subsets. In this study, we find that IL-7-stimulated CD4+ naive lymphocytes enter into cell cycle with significantly delayed kinetics as compared with the memory population. Importantly though, these lymphocytes exit from the cell cycle despite the continuous replenishment of rIL-7. This response is distinct in memory and naive CD4+ lymphocytes with memory cells starting to exit from cycle by day 10 vs day 18 for naive cells. Return to quiescence is associated with a cessation in IL-7R signaling as demonstrated by an abrogation of STAT-5 phosphorylation, despite an up-regulation of surface IL-7Ralpha. Indeed, an initial 10-fold decrease in IL-7Ralpha mRNA levels is followed by increased IL-7Ralpha expression in naive as well as memory T cells, with kinetics paralleling cell cycle exit. Altogether, our data demonstrate that IL-7 promotes the extended survival of both naive and memory CD4+ T cells, whereas cycling of these two subsets is distinct and transient. Thus, IL-7 therapy should be designed to allow optimal responsiveness of naive and memory T cell subsets.

Zinc-finger transcription factor Gfi-1: versatile regulator of lymphocytes, neutrophils and hematopoietic stem cells

PURPOSE OF REVIEW

Gfi-1, originally identified as a transcriptional repressor in lymphoid cells, is now recognized to have essential, independent functions in neutrophil maturation and hematopoietic stem cell biology. Here we review recent studies pertaining to Gfi-1 and its cell context specific functions in hematopoietic development.

RECENT FINDINGS

Progress in mapping the precise timing of requirements for myeloid transcription factors during hematopoietic development reveals a more refined picture of their sequence of action. We contrast Gfi-1's role in neutrophil development with PU.1, C/EBPalpha, and C/EBPepsilon. Gfi-1 has been found to be a major regulator of adult hematopoietic stem cells. It is required for restricting the proliferation of hematopoietic stem cells and maintaining their functional integrity. We discuss its role and compare its function with that of other regulators recently implicated in the biology of hematopoietic stem cells.

SUMMARY

Considerable progress has been made in understanding Gfi-1's context-sensitive roles at defined stages of hematopoietic differentiation.

Gfi1 and Gfi1b act equivalently in haematopoiesis, but have distinct, non-overlapping functions in inner ear development

Gfi1 is a transcriptional repressor essential for haematopoiesis and inner ear development. It shares with its paralogue Gfi1b an amino-terminal SNAG repressor domain and six carboxy-terminal zinc-finger motifs, but differs from Gfi1b in sequences separating these domains. Here, we describe two knock-in mouse models, in which the N-terminal SNAG repressor domain was mutated or in which the Gfi1 coding region was replaced by Gfi1b. Mouse mutants without an intact SNAG domain show the full phenotype of Gfi1 null mice. However, Gfi1:Gfi1b knock-in mice show almost normal pre-T-cell and neutrophil development, but lack properly formed inner ear hair cells. Hence, our findings show that an intact SNAG domain is essential for all functions of Gfi1 and that Gfi1b can replace Gfi1 functionally in haematopoiesis but, surprisingly, not in inner ear hair cell development, demonstrating that Gfi1 and Gfi1b have equivalent and domain-dependent, cell type-specific functions.

The many faces of IL-7: from lymphopoiesis to peripheral T cell maintenance

IL-7 is well known as a lymphopoietic cytokine, but recent studies have also identified a critical role for IL-7 in peripheral T cell homeostasis. IL-7 is well poised to serve as a homeostatic cytokine because it is produced by resting stromal cells, the IL-7R is present on most T cells, and IL-7 down-regulates its own receptor. These features allow IL-7 to signal large numbers of resting T cells and to be efficiently used when supplies are limiting. Consistent with this, in normal hosts, IL-7 is required for survival of naive T cell populations, and IL-7 contributes to homeostatic cycling of naive and memory cells. In addition, lymphopenic hosts accumulate increased levels of IL-7, and the supranormal levels are largely responsible for inducing homeostatic peripheral expansion in response to lymphopenia. Thus, IL-7 plays critical and nonredundant roles in both T cell lymphopoiesis and in maintaining and restoring peripheral T cell homeostasis.

The transcriptional repressor Gfi1 controls STAT3-dependent dendritic cell development and function

The mechanisms controlling the differentiation of dendritic cells (DCs) remain largely unknown. Using a transcriptional profiling approach, we identified Gfi1 as a novel critical transcription factor in DC differentiation. Gfi1-/- mice showed a global reduction of myeloid and lymphoid DCs in all lymphoid organs whereas epidermal Langerhans cells were enhanced in number. In vivo, Gfi1-/- DCs showed striking phenotypic and functional alterations such as defective maturation and increased cytokine production. In vitro, Gfi1-/- hematopoietic progenitor cells were unable to develop into DCs. Instead, they differentiated into macrophages, suggesting that Gfi1 is a critical modulator of DC versus macrophage development. Analysis of hematopoietic chimeras and retrovirus-reconstituted hematopoietic progenitor cells established a cell autonomous and nonredundant role for Gfi1 in DC development. The developmental defect of Gfi1-/- progenitor cells was associated with decreased STAT3 activation. In conclusion, we have identified Gfi1 as a critical transcription factor that controls DC versus macrophage development and dissociates DC maturation and activation.

Receptor signals and nuclear events in CD4 and CD8 T cell lineage commitment

MHC specificity in positive selection is a major determinant in the CD4/CD8 T cell lineage decision. Previous studies support the view that quantitative differences in T cell receptor (TCR) signaling in immature CD4+CD8+ double positive thymocytes leads to an instructive bias in CD4/CD8 T cell lineage commitment that must be re-inforced in subsequent selection steps to ensure that MHC-restricted antigen recognition is linked to appropriate effector functions in mature T cells. Recent work has further defined the TCR signaling pathways involved in this process, but a major effort has been made to identify transcription factors and other regulators of CD4 and CD8 T cell lineage commitment. Methods and screens for detecting changes in gene expression, associated with TCR signaling in positive selection and lineage determination, are starting to provide a better understanding of these complex developmental processes.

Molecular genetics of T cell development

T cell development is guided by a complex set of transcription factors that act recursively, in different combinations, at each of the developmental choice points from T-lineage specification to peripheral T cell specialization. This review describes the modes of action of the major T-lineage-defining transcription factors and the signal pathways that activate them during intrathymic differentiation from pluripotent precursors. Roles of Notch and its effector RBPSuh (CSL), GATA-3, E2A/HEB and Id proteins, c-Myb, TCF-1, and members of the Runx, Ets, and Ikaros families are critical. Less known transcription factors that are newly recognized as being required for T cell development at particular checkpoints are also described. The transcriptional regulation of T cell development is contrasted with that of B cell development, in terms of their different degrees of overlap with the stem-cell program and the different roles of key transcription factors in gene regulatory networks leading to lineage commitment.

The zinc finger transcription factor Growth factor independence 1 (Gfi1)

Growth Factor Independence 1 (Gfi1) is a transcriptional repressor with a molecular weight between 47 and 55 kDa The protein has six C-terminal C2H2-type zinc-finger domains and a characteristic stretch of 20 amino acids, called the SNAG-domain, at its N-terminus. Expression of Gfi1 ranges from the hematopoietic and lymphoid system, to sensory epithelia, lung and parts of the CNS. Gene knock-out studies revealed that Gfi1 is essential for the development of granulocytes and plays a role in T-cell differentiation and macrophage-dependent cytokine production, indicating that this protein shares responsibility for different lines of defense against pathogens. Also, Gfi1 is required for the proper development of inner ear hair cells illustrated by ataxia and deafness in knock-out mice. While hereditary hearing loss has so far not been associated with Gfi1 malfunction, crippling mutations in the Gfi1 gene have been reported in patients suffering from neutropenia suggesting an important role of Gfi1 in this human disease.

Direct transcriptional repression of the genes encoding the zinc-finger proteins Gfi1b and Gfi1 by Gfi1b

Gfi1b is a 37 kDa transcriptional repressor with six zinc-finger domains that is differentially expressed during hemato- and lymphopoiesis. We show here that transcription from the Gfi1b gene locus is silenced in the spleen but not in the bone marrow of transgenic mice that constitutively express Gfi1b under the control of the pan-hematopoietic vav promoter. Sequence analysis of the Gfi1b promoter showed the presence of potential Gfi1/Gfi1b-binding sites close to the mRNA start site. The expression of reporter gene constructs containing the Gfi1b core promoter appended to the luciferase gene were strongly repressed in the presence of exogenous Gfi1b. Moreover, analysis of combinatorial mutant mice that carry the vav-Gfi1b transgene and a green fluorescent protein-tagged Gfi1 gene locus demonstrated that the Gfi1 gene can be repressed by Gfi1b. Direct binding of Gfi1b and Gfi1 to the potential binding sites in the Gfi1b promoter could be demonstrated by gel-shift analyses in vitro. Chromatin-immunoprecipitation experiments showed that both the Gfi1b and the Gfi1 promoter are indeed occupied by Gfi1b in vivo. Hence, we conclude from our data that Gfi1b can auto-repress its own expression, but, in addition, is also able to cross-repress expression of the Gfi1 gene most likely in a cell type specific manner.

Cytokine functions in the formative stages of a lymphocyte's life

Five core cytokines that control lymphocyte differentiation and maintenance have been identified and studied in depth. IL-7 sits at the apex of this cytokine hierarchy in terms of functional significance during lymphocyte development. The IL-7-dominant phase of lymphopoiesis is preceded by the actions of c-Kit ligand (also called stem cell factor; SCF) and fetal liver kinase 2 ligand (Flk-2L); the function of both of these cytokines is essential for the maintenance and development of the progenitor compartment of multiple lineages. IL-7 activity is complemented by two cytokines whose receptors share components of the IL-7 receptor: thymic stromal lymphopoietin (TSLP) and IL-15. The influences of these core cytokines on precursor lymphocyte subsets overlap during development and are often synergistic. Recent studies are beginning to uncover the molecular mechanisms of these interrelated core cytokine functions.

Growth factor independence-1 is expressed in primary human neuroendocrine lung carcinomas and mediates the differentiation of murine pulmonary neuroendocrine cells

Human small cell lung cancers might be derived from pulmonary cells with a neuroendocrine phenotype. They are driven to proliferate by autocrine and paracrine neuropeptide growth factor stimulation. The molecular basis of the neuroendocrine phenotype of lung carcinomas is relatively unknown. The Achaete-Scute Homologue-1 (ASH1) transcription factor is critically required for the formation of pulmonary neuroendocrine cells and is a marker for human small cell lung cancers. The Drosophila orthologues of ASH1 (Achaete and Scute) and the growth factor independence-1 (GFI1) oncoprotein (Senseless) genetically interact to inhibit Notch signaling and specify fly sensory organ development. Here, we show that GFI1, as with ASH1, is expressed in neuroendocrine lung cancer cell lines and that GFI1 in lung cancer cell lines functions as a DNA-binding transcriptional repressor protein. Forced expression of GFI1 potentiates tumor formation of small-cell lung carcinoma cells. In primary human lung cancer specimens, GFI1 expression strongly correlates with expression of ASH1, the neuroendocrine growth factor gastrin-releasing peptide, and neuroendocrine markers synaptophysin and chromogranin A (P < 0.0000001). GFI1 colocalizes with chromogranin A and calcitonin-gene-related peptide in embryonic and adult murine pulmonary neuroendocrine cells. In addition, mice with a mutation in GFI1 display abnormal development of pulmonary neuroendocrine cells, indicating that GFI1 is important for neuroendocrine differentiation.

Suppression of IL7Ralpha transcription by IL-7 and other prosurvival cytokines: a novel mechanism for maximizing IL-7-dependent T cell survival

Survival of naive T cells is dependent upon IL-7, which is present in vivo in limiting amounts with the result that naive T cells must compete for IL-7-mediated survival signals. It would seem imperative during T cell homeostasis that limiting IL-7 be shared by the greatest possible number of T cells. We now describe a novel regulatory mechanism that specifically suppresses IL7Ralpha transcription in response to IL-7 and other prosurvival cytokines (IL-2, IL-4, IL-6, and IL-15). Consequently, IL7R expression is reduced on T cells that have received cytokine-mediated survival signals so they do not compete with unsignaled T cells for remaining IL-7. Interestingly, cytokine-mediated suppression of IL7Ralpha transcription involves different molecular mechanisms in CD4+ and CD8+ T cells, as CD8+ T cells utilize the transcriptional repressor GFI1 while CD4+ T cells do not. We suggest that this homeostatic regulatory mechanism promotes survival of the maximum possible number of T cells for the amount of IL-7 available.

GATA-1 and NF-Y cooperate to mediate erythroid-specific transcription of Gfi-1B gene

Expression of Gfi (growth factor-independence)-1B, a Gfi-1-related transcriptional repressor, is restricted to erythroid lineage cells and is essential for erythropoiesis. We have determined the transcription start site of the human Gfi-1B gene and located its first non-coding exon approximately 7.82 kb upstream of the first coding exon. The genomic sequence preceding this first non-coding exon has been identified to be its erythroid-specific promoter region in K562 cells. Using gel-shift and chromatin immunoprecipitation (ChIP) assays, we have demonstrated that NF-Y and GATA-1 directly participate in transcriptional activation of the Gfi-1B gene in K562 cells. Ectopic expression of GATA-1 markedly stimulates the activity of the Gfi-1B promoter in a non-erythroid cell line U937. Interestingly, our results have indicated that this GATA-1-mediated trans-activation is dependent on NF-Y binding to the CCAAT site. Here we conclude that functional cooperation between GATA-1 and NF-Y contributes to erythroid-specific transcriptional activation of Gfi-1B promoter.

Targeted transcriptional repression of Gfi1 by GFI1 and GFI1B in lymphoid cells

Growth factor independence-1 (GFI1) and GFI1B are closely related, yet differentially expressed transcriptional repressors with nearly identical DNA binding domains. GFI1 is upregulated in the earliest thymocyte precursors, while GFI1B expression is restricted to T lymphopoiesis stages coincident with activation. Transgenic expression of GFI1 potentiates T-cell activation, while forced GFI1B expression decreases activation. Both mice and humans with mutant Gfi1 display lymphoid abnormalities. Here we describe autoregulation of Gfi1 in primary mouse thymocytes and a human T-cell line. GFI1 binding to cis-element sequences conserved between rat, mouse and human Gfi1 mediates direct and potent transcriptional repression. In addition, dramatic regulation of Gfi1 can also be mediated by GFI1B. These data provide the first example of a gene directly targeted by GFI1 and GFI1B. Moreover, they support a role for auto- and trans-regulation of Gfi1 by GFI1 and GFI1B in maintaining the normal expression patterns of Gfi1, and suggest that GFI1B may indirectly affect T-cell activation through repression of Gfi1.

Gfi-1 oncoproteins in hematopoiesis

The family of Gfi-1 zinc finger transcriptional repressor oncoproteins consists of Gfi-1 and Gfi-1B. Recent gene targeting experiments and mutational screening in humans have revealed an essential role for Gfi-1 and Gfi-1B in hematopoiesis. Mice lacking Gfi-1 are unexpectedly neutropenic. Neutrophil differentiation is abolished and T lymphocyte differentiation is partially blocked in these mice. Heterozygous germline mutations of Gfi-1 causes severe congenital neutropenia (SCN) in humans. Ela2, whose germline mutation is the major contributor to hereditary neutropenias, is repressed in vivo by Gfi-1. Gfi-1B disruption is embryonic lethal due to a block of erythropoiesis. Gfi-1B is required for both erythroid and megakaryocyte development. The ongoing identification of repressed target genes and interacting transcriptional cofactors is helping to unravel the central contributions of these two hematopoietic factors.

Intranuclear staining of proteins in heterogeneous cell populations and verification of nuclear localization by flow cytometric analysis

We describe techniques to detect nuclear transcription factors in thymocyte subsets using flow cytometry. We have adapted a method that minimizes autofluorescence of fixed cells, thereby allowing the detection of proteins expressed at low levels. An accompanying method in which the cytoplasm is removed from stained cells allows the confirmation of nuclear localization. These methods combine to provide a sensitive alternative approach for detecting nuclear proteins within heterogeneous cell populations.