STAT5 is required for thymopoiesis in a development stage-specific manner

Development of γδ T cells in the thymus - A human perspective

γδ T cells are increasingly emerging as crucial immune regulators that can take on innate and adaptive roles in the defence against pathogens. Although they arise within the thymus from the same hematopoietic precursors as conventional αβ T cells, the development of γδ T cells is less well understood. In this review, we focus on summarising the current state of knowledge about the cellular and molecular processes involved in the generation of γδ T cells in human.

Skin γδ T Cells and Their Function in Wound Healing

For the skin immune system, γδ T cells are important components, which help in defensing against damage and infection of skin. Compared to the conventional αβ T cells, γδ T cells have their own differentiation, development and activation characteristics. In adult mice, dendritic epidermal T cells (DETCs), Vγ4 and Vγ6 γδ T cells are the main subsets of skin, the coordination and interaction among them play a crucial role in wound repair. To get a clear overview of γδ T cells, this review synopsizes their derivation, development, colonization and activation, and focuses their function in acute and chronic wound healing, as well as the underlining mechanism. The aim of this paper is to provide cues for the study of human epidermal γδ T cells and the potential treatment for skin rehabilitation.

Multi-objective optimization reveals time- and dose-dependent inflammatory cytokine-mediated regulation of human stem cell derived T-cell development

The generation of T-cells from stem cells in vitro could provide an alternative source of cells for immunotherapies. T-cell development from hematopoietic stem and progenitor cells (HSPCs) is tightly regulated through Notch pathway activation by Delta-like (DL) ligands 1 and 4. Other molecules, such as stem cell factor (SCF) and interleukin (IL)-7, play a supportive role in regulating the survival, differentiation, and proliferation of developing T-cells. Numerous other signaling molecules influence T-lineage development in vivo, but little work has been done to understand and optimize their use for T-cell production. Using a defined engineered thymic niche system, we undertook a multi-stage statistical learning-based optimization campaign and identified IL-3 and tumor necrosis factor α (TNFα) as a stage- and dose-specific enhancers of cell proliferation and T-lineage differentiation. We used this information to construct an efficient three-stage process for generating conventional TCRαβ⁺CD8⁺ T-cells expressing a diverse TCR repertoire from blood stem cells. Our work provides new insight into T-cell development and a robust system for generating T-cells to enable clinical therapies for treating cancer and immune disorders.

Epidermal resident γδ T cell development and function in skin

Epidermal resident γδ T cells, or dendritic epidermal T cells (DETCs) in mice, are a unique and conserved population of γδ T cells enriched in the epidermis, where they serve as the regulators of immune responses and sense skin injury. Despite the great advances in the understanding of the development, homeostasis, and function of DETCs in the past decades, the origin and the underlying molecular mechanisms remain elusive. Here, we reviewed the recent research progress on DETCs, including their origin and homeostasis in the skin, especially at transcriptional and epigenetic levels, and discuss the involvement of DETCs in skin diseases.

Get in Touch With Dendritic Epithelial T Cells!

Innate and adaptive immune systems continuously interchange information and orchestrate their immune responses to protect the host. γδT cells play crucial roles, as they incorporate both innate and adaptive immune characteristics. Dendritic epidermal T cells (DETC) are specialized γδT cells, which are uniquely positioned to rapidly respond to skin wounds and infections. Their elongated dendrite morphology allows them to be in continuous contact with multiple neighboring keratinocytes and Langerhans cells. Cellular interactions are fundamental to the formation, activation and maintenance of immune cell functions during steady state and pathology. Recent technological advances, especially in the field of cellular imaging, have contributed greatly to the characterization of complex cellular interactions in a spatiotemporally resolved manner. In this review, we will highlight the often-underappreciated function of DETC and other γδT cells during steady state and an ongoing immune response. More specifically, we discuss how DETC-precursors are shaped in the fetal thymus during embryogenesis as well as how direct cell-cell interactions of DETC with neighboring epidermal cells shape skin homeostasis and effector functions. Furthermore, we will discuss seminal work and recent discoveries made in the γδT cell field, which have highlighted the importance of γδT cells in the skin, both in humans and mice.

STAT5A and STAT5B-Twins with Different Personalities in Hematopoiesis and Leukemia

The transcription factors STAT5A and STAT5B have essential roles in survival and proliferation of hematopoietic cells-which have been considered largely redundant. Mutations of upstream kinases, copy number gains, or activating mutations in STAT5A, or more frequently in STAT5B, cause altered hematopoiesis and cancer. Interfering with their activity by pharmacological intervention is an up-and-coming therapeutic avenue. Precision medicine requests detailed knowledge of STAT5A's and STAT5B's individual functions. Recent evidence highlights the privileged role for STAT5B over STAT5A in normal and malignant hematopoiesis. Here, we provide an overview on their individual functions within the hematopoietic system.

Cytokines, Transcription Factors, and the Initiation of T-Cell Development

Multipotent blood progenitor cells migrate into the thymus and initiate the T-cell differentiation program. T-cell progenitor cells gradually acquire T-cell characteristics while shedding their multipotentiality for alternative fates. This process is supported by extracellular signaling molecules, including Notch ligands and cytokines, provided by the thymic microenvironment. T-cell development is associated with dynamic change of gene regulatory networks of transcription factors, which interact with these environmental signals. Together with Notch or pre-T-cell-receptor (TCR) signaling, cytokines always control proliferation, survival, and differentiation of early T cells, but little is known regarding their cross talk with transcription factors. However, recent results suggest ways that cytokines expressed in distinct intrathymic niches can specifically modulate key transcription factors. This review discusses how stage-specific roles of cytokines and transcription factors can jointly guide development of early T cells.

γδ T cells in homeostasis and host defence of epithelial barrier tissues

Epithelial surfaces line the body and provide a crucial interface between the body and the external environment. Tissue-resident epithelial γδ T cells represent a major T cell population in the epithelial tissues and are ideally positioned to carry out barrier surveillance and aid in tissue homeostasis and repair. In this Review, we focus on the intraepithelial γδ T cell compartment of the two largest epithelial tissues in the body - namely, the epidermis and the intestine - and provide a comprehensive overview of the crucial contributions of intraepithelial γδ T cells to tissue integrity and repair, host homeostasis and protection in the context of the symbiotic relationship with the microbiome and during pathogen clearance. Finally, we describe epithelium-specific butyrophilin-like molecules and briefly review their emerging role in selectively shaping and regulating epidermal and intestinal γδ T cell repertoires.

The MicroRNA miR-191 Supports T Cell Survival Following Common γ Chain Signaling

To ensure lifelong immunocompetency, naïve and memory T cells must be adequately maintained in the peripheral lymphoid tissues. Homeostatic maintenance of T cells is controlled by tonic signaling through T cell antigen receptors and common γ chain cytokine receptors. In this study, we identify the highly expressed microRNA miR-191 as a key regulator of naïve, memory, and regulatory T cell homeostasis. Conditional deletion of miR-191 using LckCre resulted in preferential loss of peripheral CD4⁺ regulatory T cells as well as naïve and memory CD8⁺ T cells. This preferential loss stemmed from reduced survival following deficient cytokine signaling and STAT5 activation. Mechanistically, insulin receptor substrate 1 (Irs1) is a direct target of miR-191, and dysregulated IRS1 expression antagonizes STAT5 activation. Our study identifies a novel role for microRNAs in fine-tuning immune homeostasis and thereby maintaining the lymphocyte reservoir necessary to mount productive immune responses.

An invertebrate signal transducer and activator of transcription 5 (STAT5) ortholog from the disk abalone, Haliotis discus discus: Genomic structure, early developmental expression, and immune responses to bacterial and viral stresses

Signal transducer and activator of transcription (STAT) family members are key signaling molecules that transduce cellular responses from the cell membrane to the nucleus upon Janus kinase (JAK) activation. Although seven STAT members have been reported in mammals, very limited information on STAT genes in molluscans is available. In this study, we identified and characterized a STAT paralog that is homologous to STAT5 from the disk abalone, Haliotis discus discus, and designated as AbSTAT5. Comparison of the deduced amino acid sequence for AbSTAT5 (790 amino acids) with other counterparts revealed conserved residues important for functions and typical domain regions, including the N-terminal domain, coiled-coil domain, DNA-binding domain, linker domain, and Src homology 2 (SH2) domains as mammalian counterparts. Analysis of STAT phylogeny revealed that AbSTAT5 was clustered with the molluscan subgroup in STAT5 clade with distinct evolution. According to the genomic structure of AbSTAT5, the coding sequence was distributed into 20 exons with 19 introns. Immunologically essential transcription factor-binding sites, such as GATA-1, HNF, SP1, C/EBP, Oct-1, AP1, c-Jun, and Sox-2, were predicted at the 5'-proximal region of AbSTAT5. Expression of AbSTAT5 mRNA was detected in different stages of embryonic development and observed at considerably higher levels in the morula and late veliger stages. Tissue-specific expressional studies revealed that the highest level of AbSTAT5 transcripts was detected in hemocytes, followed by gill tissues. Temporal expressions of AbSTAT5 were analyzed upon live bacterial (Vibrio parahemolyticus and Listeria monocytogenes), viral (viral hemorrhagic septicemia virus), and pathogen-associated molecular pattern (lipopolysaccharides and Poly I:C) stimulations, and significant elevations indicated immune modulation. These results suggest that AbSTAT5 may be involved in maintaining innate immune responses from developmental to adult stages in the disk abalone. Further, this study provides a basis for structural and functional exploration of STAT members in the invertebrate JAK/STAT signaling pathway.

Unveiling combinatorial regulation through the combination of ChIP information and in silico cis-regulatory module detection

Computationally retrieving biologically relevant cis-regulatory modules (CRMs) is not straightforward. Because of the large number of candidates and the imperfection of the screening methods, many spurious CRMs are detected that are as high scoring as the biologically true ones. Using ChIP-information allows not only to reduce the regions in which the binding sites of the assayed transcription factor (TF) should be located, but also allows restricting the valid CRMs to those that contain the assayed TF (here referred to as applying CRM detection in a query-based mode). In this study, we show that exploiting ChIP-information in a query-based way makes in silico CRM detection a much more feasible endeavor. To be able to handle the large datasets, the query-based setting and other specificities proper to CRM detection on ChIP-Seq based data, we developed a novel powerful CRM detection method 'CPModule'. By applying it on a well-studied ChIP-Seq data set involved in self-renewal of mouse embryonic stem cells, we demonstrate how our tool can recover combinatorial regulation of five known TFs that are key in the self-renewal of mouse embryonic stem cells. Additionally, we make a number of new predictions on combinatorial regulation of these five key TFs with other TFs documented in TRANSFAC.

JunD/AP-1-mediated gene expression promotes lymphocyte growth dependent on interleukin-7 signal transduction

Interleukin-7 (IL-7) is an essential cytokine for lymphocyte growth that has the potential for promoting immune reconstitution. This feature makes IL-7 an ideal candidate for therapeutic development. As with other cytokines, signaling through the IL-7 receptor induces the JAK/STAT pathway. However, the broad scope of IL-7 regulatory targets likely necessitates the use of other signaling components whose identities remain poorly defined. To this end, we used an IL-7 dependent T-cell line to examine how expression of the glycolytic enzyme, Hexokinase II (HXKII) was regulated by IL-7 in a STAT5-independent manner. Our studies revealed that IL-7 promoted the activity of JNK (Jun N-terminal Kinase), and that JNK, in turn, drove the expression of JunD, a component of the Activating Protein 1 (AP-1) transcription factors. Gel shifts showed that the AP-1 complex induced by IL-7 contained JunD but not c-Fos or c-Jun. Inhibition of JNK/JunD blocked glucose uptake and HXKII gene expression, indicating that this pathway was responsible for promoting HXKII expression. Because others had shown that JunD was a negative regulator of cell growth, we performed a bioinformatics analysis to uncover possible JunD-regulated gene targets. Our search revealed that JunD could control the expression of proteins involved in signal transduction, cell survival and metabolism. One of these growth promoters was the oncogene, Pim-1. Pim-1 is an IL-7-induced protein that was inhibited when the activities of JNK or JunD were blocked, showing that in IL-7 dependent T-cells JunD can promote positive signals transduced through Pim-1. This was confirmed when the IL-7-induced proliferation of CD8 T-cells was impaired upon JunD inhibition. These results show that engagement of the IL-7 receptor drives a signal that is more complex than the JAK/STAT pathway, activating JNK and JunD to induce rapid growth stimulation through the expression of metabolic and signaling factors like HXKII and Pim-1.

HIV infection of thymocytes inhibits IL-7 activity without altering CD127 expression

BACKGROUND

Thymic function is altered in HIV infection and characterized by dysregulation of the thymic epithelial network, reduced thymic output and ultimately an impaired naïve T-cell pool. The IL-7/IL-7 receptor (IL-7R) signalling pathway is critical for the maturation and differentiation of thymocytes. HIV infection is associated with a decrease in IL-7Rα (CD127) expression and impaired CD127 signalling in circulating CD8+ T-cells; however, little is known about the effect of HIV on CD127 expression and IL-7 activity in the thymus. Therefore, the effect of in vitro HIV infection on CD127 expression and IL-7-mediated function in thymocytes was investigated.

FINDINGS

In vitro HIV infection of thymocytes did not affect CD127 expression on either total thymocytes or on single positive CD4 or single positive CD8 subsets. However, HIV infection resulted in a decrease in the level of IL-7-induced STAT-5 phosphorylation and Bcl-2 expression in unfractionated thymocytes.

CONCLUSION

These findings indicate that HIV infection alters IL-7 responsiveness of thymocytes by a mechanism other than CD127 downregulation and potentially explain the disruption in thymopoiesis observed in HIV infection.

Functions of skin-resident γδ T cells

The murine epidermis contains resident T cells that express a canonical γδ TCR and arise from fetal thymic precursors. These cells are termed dendritic epidermal T cells (DETC) and use a TCR that is restricted to the skin in adult animals. DETC produce low levels of cytokines and growth factors that contribute to epidermal homeostasis. Upon activation, DETC can secrete large amounts of inflammatory molecules which participate in the communication between DETC, neighboring keratinocytes and langerhans cells. Chemokines produced by DETC may recruit inflammatory cells to the epidermis. In addition, cell-cell mediated immune responses also appear important for epidermal-T cell communication. Information is provided which supports a crucial role for DETC in inflammation, wound healing, and tumor surveillance.

Activation of Paired-homeobox gene PITX1 by del(5)(q31) in T-cell acute lymphoblastic leukemia

In T-cell acute lymphoblasic leukemia (T-ALL), neoplastic chromosomal rearrangements are known to deregulate members of the homeobox gene families NKL and HOXA. Here, analysis of T-ALL cell lines and primary cells identified aberrant expression of a third homeobox gene group, the Paired (PRD) class. LOUCY cells revealed chromosomal deletion at 5q31, which targets the downstream regulatory region of the PRD homeobox gene PITX1, removing a STAT1 binding site. STAT1 mediates repressive interleukin 2 (IL2)-STAT1 signaling, implicating IL2 pathway avoidance as a possible activation mechanism. Among primary T-ALL samples, 2/22 (9%) aberrantly expressed PITX1, highlighting the importance of this gene. Forced expression of PITX1 in JURKAT cells and subsequent target gene analysis prompted deregulation of genes involved in T-cell development including HES1, JUN, NKX3-1, RUNX1, RUNX2, and TRIB2. Taken together, our data show leukemic activation of PITX1, a novice PRD-class homeobox gene in a subset of early-staged T-ALL, which may promote leukemogenesis by inhibiting T-cell development.

The role of STAT5 in the development, function, and transformation of B and T lymphocytes

The transcription factor signal transducer and activator of transcription 5 (STAT5) is activated by a number of cytokine and growth hormone receptors and plays a key role in the development and function of many organ systems. In this review, we focus on recent discoveries about the role of STAT5 in the development and function of B and T lymphocytes. Of particular interest is the growing appreciation for the function of STAT5 as a transcriptional repressor. Finally, we discuss recent discoveries about the role of STAT5 in transformation of B and T lymphocytes.

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.

Disorderly conduct in gammadelta versus alphabeta T cell lineage commitment

The mechanism of T cell precursor commitment to the gammadelta or alphabeta T cell lineage remains unclear. While TCR signal strength has emerged as a key factor in lineage commitment based on TCR transgenic models, the entire TCR repertoire may not possess the same discriminatory power. A counterbalance to the TCR as the lineage determinant is the pre-existing heterogeneity in gene expression among precursors, which suggests that single precursors are unlikely to respond homogeneously to a given instructive signal.

Mechanics of T cell receptor gene rearrangement

The four T cell receptor genes (Tcra, Tcrb, Tcrg, Tcrd) are assembled by V(D)J recombination according to distinct programs during intrathymic T cell development. These programs depend on genetic factors, including gene segment order and recombination signal sequences. They also depend on epigenetic factors. Regulated changes in chromatin structure, directed by enhancers and promoter, can modify the availability of recombination signal sequences to the RAG recombinase. Regulated changes in locus conformation may control the synapsis of distant recombination signal sequences, and regulated changes in subnuclear positioning may influence locus recombination events by unknown mechanisms. Together these influences may explain the ordered activation and inactivation of T cell receptor locus recombination events and the phenomenon of Tcrb allelic exclusion.

Molecular mechanisms that control mouse and human TCR-alphabeta and TCR-gammadelta T cell development

Following specification of hematopoietic precursor cells into the T cell lineage, several developmental options remain available to the immature thymocytes. The paradigm is that the outcome of the T cell receptor rearrangements and the corresponding T cell receptor signaling events will be predominant to determine the first of these choices: the alphabeta versus gammadelta T cell pathways. Here, we review the thymus-derived environmental signals, the transcriptional mediators, and other molecular mechanisms that are also involved in this decision in both the mouse and human. We discuss the differences in cellular events between the alphabeta and gammadelta developmental pathways and try to correlate these with a corresponding complexity of the molecular mechanisms that support them.

New insights into the roles of Stat5a/b and Stat3 in T cell development and differentiation

T cell development and differentiation is carefully orchestrated by a series of cytokines. The importance of STAT family proteins in mediating signals by these cytokines is well-known, but new information on the role of STATs in novel aspects of T cell function and T cell subsets continues to accumulate. Recent studies have placed Stat5a/b and Stat3 center stage in T cell development and differentiation. Stat5a/b are indispensable in T regulatory (Treg) cell development and maintenance, and negatively regulate T helper 17 (Th17) cell differentiation. Conversely, Stat3 is essential for Th17 differentiation and inhibits Treg cells. The balance of Treg and Th17 cells is thought to be critical in maintaining immune tolerance, while preserving effective host defense. Therefore, Stat5a/b and Stat3 are emerging to be key players in T cell differentiation and homeostasis.

Immunology

The concept of forbidden foods that should not be eaten goes back to the Garden of Eden and apart from its religious meanings it may also have foreshadowed the concept of foods that can provoke adverse reactions. Thus we could say that allergic diseases have plagued mankind since the beginning of life on earth. The prophet Job was affected by a condition that following the rare symptoms described by the Holy Bible might be identified as a severe form of atopic dermatitis (AD). The earliest record of an apparently allergic reaction is 2621 B.C., when death from stinging insects was first described by hieroglyphics carved into the walls of the tomb of Pharaoh Menes depicting his death following the sting of a wasp. In 79 A.D., the death of the Roman admiral Pliny the Elder was ascribed to the SO₂-rich gases emanating from the eruption of Mount Vesuvius. Hippocrates (460–377 B.C.) was probably the first to describe how cow’s milk (CM) could cause gastric upset and hives, proposing dietetic measures including both treatment and prevention for CM allergy.

Stat5 is essential for early B cell development but not for B cell maturation and function

The two closely related Stat5 (Stat5A and Stat5B) proteins are activated by a broad spectrum of cytokines. However, with the complication of the involvement of Stat5A/5B in stem cell function, the role of Stat5A/5B in the development and function of lymphocytes, especially B cells, is not fully understood. In this study, we demonstrated that Stat5A/5B(-/-) fetal liver cells had severe diminution of B cell progenitors but clearly had myeloid progenitors. Consistently, the mutant fetal liver cells could give rise to hemopoietic progenitors and myeloid cells but not B cells beyond pro-B cell progenitors in lethally irradiated wild-type or Jak3(-/-) mice. Deletion of Stat5A/5B in vitro directly impaired IL-7-mediated B cell expansion. Of note, reintroduction of Stat5A back into Stat5A/5B(-/-) fetal liver cells restored their abilities to develop B cells. Importantly, CD19-Cre-mediated deletion of Stat5A/5B in the B cell compartment specifically impaired early B cell development but not late B cell maturation. Moreover, the B cell-specific deletion of Stat5A/5B did not impair splenic B cell survival, proliferation, and Ig production. Taken together, these data demonstrate that Stat5A/5B directly control IL-7-mediated early B cell development but are not required for B cell maturation and Ig production.

Nonredundant roles for Stat5a/b in directly regulating Foxp3

Stats (signal transducers and activators of transcription) regulate multiple aspects of T-cell fate. T regulatory (Treg) cells are a critical subset that limits immune responses, but the relative importance of Stat5a/b versus Stat3 for Treg cell development has been contentious. We observed that peripheral CD25(+)CD4(+) T cells were reduced in Stat5(DeltaN) mice; however, the levels of Foxp3, a transcription factor that is critical for Treg cells, were normal in splenic CD4(+) T cells even though they were reduced in the thymus. In contrast, complete deletion of Stat5a/b (Stat5(-/-)) resulted in dramatic reduction in CD25- or Foxp3-expressing CD4(+) T cells. An intrinsic requirement was demonstrated by reduction of Stat5a/b in CD4-expressing cells and by stem cell transplantation using Stat5(-/-) fetal liver cells. Stat5a/b were also required for optimal induction of Foxp3 in vitro and bound directly to the Foxp3 gene. Reduction of Stat3 in T cells did not reduce the numbers of Treg cells in the thymus or spleen; however, Stat3 was required for IL-6-dependent down-regulation of Foxp3. Therefore, we conclude that Stat5a/b have an essential, nonredundant role in regulating Treg cells, and that Stat3 and Stat5a/b appear to have opposing roles in the regulation of Foxp3.

Cutting edge: Decreased accumulation and regulatory function of CD4+ CD25(high) T cells in human STAT5b deficiency

We show that STAT5b is important for the in vivo accumulation of CD4+ CD25(high) T cells with regulatory cell function. A patient homozygous for a missense A630P STAT5b mutation displayed immune dysregulation and decreased numbers of CD4+ CD25(high) T cells. STAT5b(A630P/A630P) CD4+ CD25(high) T cells had low expression of forkhead box P3 and an impaired ability to suppress the proliferation of or to kill CD4+ CD25- T cells. Expression of CD25, a component of the high-affinity IL-2R, was also reduced in response to IL-2 or after in vitro propagation. The impact of the STAT5b mutation was selective in that IL-2-mediated up-regulation of the common gamma-chain cytokine receptor and perforin, and activation-induced expressions of CD154 and IFN-gamma were normal. These results indicate that STAT5b propagates an important IL-2-mediated signal for the in vivo accumulation of functional regulatory T cells.

IL-7-induced proliferation of recent thymic emigrants requires activation of the PI3K pathway

The IL-7 cytokine promotes the survival of a diverse T-cell pool, thereby ensuring an efficient immune response. Moreover, IL-7 induces the proliferation of recent thymic emigrants (RTEs) in neonates. Here, we demonstrate that the survival and proliferative effects of IL-7 on human RTEs can be distinguished on the basis of dose as well as duration of IL-7 administration. A dose of 0.1 ng/mL IL-7 is sufficient to promote viability, whereas cell-cycle entry is observed only at doses higher than 1 ng/mL. Moreover, a short 1-hour exposure to high-dose IL-7 (10 ng/mL) induces long-term survival but continuous IL-7 exposure is necessary for optimal cell-cycle entry and proliferation. We find that distinct signaling intermediates are activated under conditions of IL-7-induced survival and proliferation; STAT5 tyrosine phosphorylation does not correlate with proliferation, whereas up-regulation of the glucose transporter Glut-1 as well as increased glucose uptake are markers of IL-7-induced cell cycle entry. Glut-1 is directly regulated by PI3K and, indeed, inhibiting PI3K activity abrogates IL-7-induced proliferation. Our finding that the survival and proliferation of RTEs are differentially modulated by the dose and kinetics of exogenous IL-7 has important implications for the clinical use of this cytokine.

Molecular Networks Orchestrating GALT Development

During evolution, the development of secondary lymphoid organs has evolved as a strategy to promote adaptive immune responses at sites of antigen sequestration. Mesenteric lymph nodes (LNs) and Peyer's patches (PPs) are localized in proximity to mucosal surfaces, and their development is coordinated by a series of temporally and spatially regulated molecular events involving the collaboration between hematopoietic, mesenchymal, and, for PPs, epithelial cells. Transcriptional control of cellular differentiation, production of cytokines as well as adhesion molecules are mandatory for organogenesis, recruitment of mature leukocytes, and lymphoid tissue organization. Similar to fetal and neonatal organogenesis, lymphoid tissue neoformation can occur in adult individuals at sites of chronic stimulation via cytokines and TNF-family member molecules. These molecules represent new therapeutic targets to manipulate the microenvironment during autoimmune diseases.

Progression of regulatory gene expression states in fetal and adult pro-T-cell development

Precursors entering the T-cell developmental pathway traverse a progression of states characterized by distinctive patterns of gene expression. Of particular interest are regulatory genes, which ultimately control the dwell time of cells in each state and establish the mechanisms that propel them forward to subsequent states. Under particular genetic and developmental circumstances, the transitions between these states occur with different timing, and environmental feedbacks may shift the steady-state accumulations of cells in each state. The fetal transit through pro-T-cell stages is faster than in the adult and subject to somewhat different genetic requirements. To explore causes of such variation, this review presents previously unpublished data on differentiation gene activation in pro-T cells of pre-T-cell receptor-deficient mutant mice and a quantitative comparison of the profiles of transcription factor gene expression in pro-T-cell subsets of fetal and adult wildtype mice. Against a background of consistent gene expression, several regulatory genes show marked differences between fetal and adult expression profiles, including those encoding two basic helix-loop-helix antagonist Id factors, the Ets family factor SpiB and the Notch target gene Deltex1. The results also reveal global differences in regulatory alterations triggered by the first T-cell receptor-dependent selection events in fetal and adult thymopoiesis.

A role for STAT5A/B in protection of peripheral T-lymphocytes from postactivation apoptosis: Insights from gene expression profiling

Activation of the transcription factors STAT5A and STAT5B by JAK1 and JAK3 tyrosine kinases is a key event in downstream signaling of gammac (common gamma chain)-family cytokine receptors in lymphoid cells. STAT5A/B-deficiency in mice causes, among other consequences, a reduced size and altered composition of the peripheral T-cell pool with predominance of an activated or memory-like population (CD4(+)/CD44(high)/CD62L(low)) and a proliferative deficiency following antigenic stimulation and subsequent IL-2 treatment. To further elucidate the critical function of STAT5A/B in homeostasis and activation of murine naïve peripheral T-lymphocytes, we have analyzed global gene expression of STAT5A/B-deficient versus wild-type splenic T-cells by profiling with high-density oligonucleotide arrays (Affymetrix). Comparison of (1) basal gene expression of untreated peripheral STAT5A/B-deficient and control T-cells and (2) immediate early gene induction upon in vitro stimulation of either population with IL-2 has revealed differential expression of a broad range of genes potentially contributing to the defects of STAT5A/B deficient T-cells. In the context of enhanced apoptotic rates of STAT5A/B(-/-)-T-cells in vivo and upon TCR-stimulation in culture our data suggest a role for STAT5 in post-activation survival beyond regulation of antiapoptotic Bcl-2 proteins and hence provide new insights into the nature of the late proliferative block in the T-cell compartment caused by STAT5-deficiency.

Constitutive STAT5 activation specifically cooperates with the loss of p53 function in B-cell lymphomagenesis

Signal transducers and activator of transcription 5 (STAT5) A and B are transcriptional regulators that play a central role in cytokine signaling in the hematopoietic lineage and which are frequently activated in a persistent manner in human leukemia/lymphoma, as assessed by their constitutive tyrosine phosphorylation and DNA-binding activity. To study the intrinsic oncogenic properties of persistent STAT5 activation, we generated transgenic mice in which a constitutively activated point mutant of STAT5A, STAT5A(S711F), was expressed at physiological level in their lymphoid compartment. In this model, persistent STAT5 activation is weakly oncogenic, leading to the late emergence of clonal B-cell lymphoma/leukemia at a low incidence. In contrast, STAT5(S711F) was found to cooperate with the loss of function of the p53 tumor suppressor gene to both accelerate disease onset and to skew the large tumor spectrum that normally characterize p53-deficient mice to strongly favor B-cell lymphoma/leukemia. The emergence of STAT5A(S711F)-induced B-cell tumors is associated with the activation of STAT5 tyrosine phosphorylation and DNA-binding activity, indicating that activation of STAT5 oncogenic properties in transgenic STAT5A (TgSTAT5A) (S711F) mice involves the deregulation of STAT5 phosphorylation dynamics.

Clarifying the role of Stat5 in lymphoid development and Abelson-induced transformation

The Stat5 transcription factors Stat5a and Stat5b have been implicated in lymphoid development and transformation. Most studies have employed Stat5a/b-deficient mice where gene targeting disrupted the first protein-coding exon, resulting in the expression of N-terminally truncated forms of Stat5a/b (Stat5a/b(DeltaN/DeltaN) mice). We have now reanalyzed lymphoid development in Stat5a/b(null/null) mice having a complete deletion of the Stat5a/b gene locus. The few surviving Stat5a/b(null/null) mice lacked CD8(+) T lymphocytes. A massive reduction of CD8(+) T cells was also found in Stat5a/b(fl/fl) lck-cre transgenic animals. While gammadelta T-cell receptor-positive (gammadeltaTCR(+)) cells were expressed at normal levels in Stat5a/b(DeltaN/DeltaN) mice, they were completely absent in Stat5a/b(null/null) animals. Moreover, B-cell maturation was abrogated at the pre-pro-B-cell stage in Stat5a/b(null/null) mice, whereas Stat5a/b(DeltaN/DeltaN) B-lymphoid cells developed to the early pro-B-cell stage. In vitro assays using fetal liver-cell cultures confirmed this observation. Most strikingly, Stat5a/b(null/null) cells were resistant to transformation and leukemia development induced by Abelson oncogenes, whereas Stat5a/b(DeltaN/DeltaN)-derived cells readily transformed. These findings show distinct lymphoid defects for Stat5a/b(DeltaN/DeltaN) and Stat5a/b(null/null) mice and define a novel functional role for the N-termini of Stat5a/b in B-lymphoid transformation.

Stat5a/b are essential for normal lymphoid development and differentiation

Cytokines that use the common gamma chain gammac are critical for lymphoid development and function. Mutations of the IL-7 receptor, gammac, or its associated kinase, Jak3, are the major cause of human severe combined immunodeficiency. Although activated by IL-7, Stat5a/b (Stat, signal transducer and activator of transcription) have been thought to play limited roles in lymphoid development. However, we now show that mice completely deficient in Stat5a/b have severely impaired lymphoid development and differentiation. Absence of Stat5 also abrogates T cell receptor gamma rearrangement and survival of peripheral CD8(+) T cells. Thus, deficiency of Stat5 results in severe combined immunodeficiency, similar in many respects to deficiency of IL-7R, gammac, and Jak3.

Interleukin 15 controls the generation of the restricted T cell receptor repertoire of gamma delta intestinal intraepithelial lymphocytes

The gammadelta T cells are prevalent in the mucosal epithelia and are postulated to act as 'sentries' for maintaining tissue integrity. What these gammadelta T cells recognize is poorly defined, but given the restricted T cell receptor (TCR) repertoire, the idea that they are selected by self antigens of low complexity has been widely disseminated. Here we present data showing that the generation of the restricted TCR variable gamma-region gene repertoire of intestinal intraepithelial lymphocytes was regulated by interleukin 15, which induced local chromatin modifications specific for the variable gamma-region gene segment and enhanced accessibility conducive to subsequent targeted gene rearrangement. This cytokine-directed tissue-specific TCR repertoire formation probably reflects distinct TCR repertoire selection criteria for gammadelta and alphabeta T cell lineages adopted for different antigen-recognition strategies.

Cell biology of IL-7, a key lymphotrophin

IL-7 is essential for the development and survival of T lymphocytes. This review is primarily from the perspective of the cell biology of the responding T cell. Beginning with IL-7 receptor structure and regulation, the major signaling pathways appear to be via PI3K and Stat5, although the requirement for either has yet to be verified by published knockout experiments. The proliferation pathway induced by IL-7 differs from conventional growth factors and is primarily through posttranslational regulation of p27, a Cdk inhibitor, and Cdc25a, a Cdk-activating phosphatase. The survival function of IL-7 is largely through maintaining a favorable balance of bcl-2 family members including Bcl-2 itself and Mcl-1 on the positive side, and Bax, Bad and Bim on the negative side. There are also some remarkable metabolic effects of IL-7 withdrawal. Studies of IL-7 receptor signaling have yet to turn up unique pathways, despite the unique requirement for IL-7 in T cell biology. There remain significant questions regarding IL-7 production and the major producing cells have yet to be fully characterized.

T cell homeostasis in tolerance and immunity

The size of the peripheral T cell pool is remarkably stable throughout life, reflecting precise regulation of cellular survival, proliferation, and apoptosis. Homeostatic proliferation refers to the process by which T cells spontaneously proliferate in a lymphopenic host. The critical signals driving this expansion are "space," contact with self-major histocompatibility complex (MHC)/peptide complexes, and cytokine stimulation. A number of studies have delineated an association between T cell lymphopenia, compensatory homeostatic expansion, and the development of diverse autoimmune syndromes. In the nonobese diabetic mouse model of type 1 diabetes, lymphopenia-induced homeostatic expansion fuels the generation of islet-specific T cells. Excess interleukin-21 facilitates T cell cycling but limited survival, resulting in recurrent stimulation of T cells specific for self-peptide/MHC complexes. Indeed, data from several experimental models of autoimmunity indicate that a full T cell compartment restrains homeostatic expansion of self-reactive cells that could otherwise dominate the repertoire. This review describes the mechanisms that govern T cell homeostatic expansion and outlines the evidence that lymphopenia presents a risk for development of autoimmune disease.

Haploinsufficiency identifies STAT5 as a modifier of IL-7-induced lymphomas

The requirement for receptor components and the signalling effector, signal transducer and activator of transcription (STAT) 5A/5B, was assessed genetically in a lymphoma development model induced by interleukin-7 (IL-7). This growth factor for T- and B-cell progenitors and mature lymphocytes activates survival and proliferative pathways including Bcl-2, phosphatidylinositol-3 kinase and STAT5. Overexpression of IL-7 in vivo causes early mortality from lymphoma development. Mice overexpressing IL-7 that were heterozygous for the IL-7Ralpha subunit showed improved survival compared to wild-type mice. In addition, STAT5A/5B+/- compound heterozygous mice with one targeted allele each of STAT5A and STAT5B showed striking amelioration of IL-7-induced mortality and disease development. STAT5A/5B+/- compound heterozygous mice were otherwise normal in stem cell and lymphocyte development and cellularity. Lower STAT5 protein levels accompanied the reduction in STAT5A/5B copy number, which suggests that STAT5 haploinsufficiency is a modifier of IL-7 signal strength.

Cutting Edge: Stat5 Mediates the IL-7-Induced Accessibility of a Representative D-Distal VH Gene

During B cell development, discrete domains within the Igh locus are activated independently for recombination. The D-distal V(H) genes are uniquely dependent on IL-7R signaling, which is thought to establish local chromatin accessibility through an unknown mechanism. To dissect this mechanism, we used a murine B cell line that responds to IL-7 by specifically inducing accessibility of a representative D-distal gene (A1) but not a D-proximal gene (V11). We demonstrate that IL-7-activated Stat5 is recruited rapidly to the A1 gene, with a concomitant increase in germline transcription and H4 acetylation. Furthermore, retroviral transduction of dominant negative or constitutively active Stat5 demonstrated that Stat5 activation is both necessary and sufficient for the IL-7-induced A1 germline transcription. Lastly, as with all known Stat5 target genes, A1 germline transcription requires a deacetylase activity. These results demonstrate that in response to IL-7 signaling, Stat5 is recruited to the D-distal A1 gene and induces accessibility.

Physical and Functional Interactions between STAP-2/BKS and STAT5

Signal-transducing adaptor protein family of proteins (STAPs), which currently contains two members, are proposed to be adaptor molecules because of their pleckstrin homology (PH) and Src-homology 2 (SH2)-like domains. STAP-1 has been shown to interact with STAT5 and the tyrosine kinase Tec. With regard to STAP-2/BKS functions, immunoprecipitation experiments and intracellular stainings revealed STAP-2/BKS binds STAT5 in several types of cells. Mutational studies revealed that the PH- and SH2-like domains of STAP-2/BKS interacted with the C-terminal region of STAT5. STAP-2/BKS and STAT5 were found to constitutively co-localize in the cytoplasm of resting cells, but STAP-2/BKS was found to dissociate upon STAT5 phosphorylation, suggesting a role in regulating signaling of STAT5. The physiological role of these interactions is not fully understood, but in studies of overexpression of STAP-2/BKS, cytokine-induced tyrosine phosphorylation and transcriptional activation of STAT5 was diminished. In addition, thymocytes from STAP-2/BKS-deficient mice showed the enhanced interleukin-2-dependent cell growth. Taken together, STAP-2/BKS is an additional modulator of STAT5-mediated signaling.