Sonic hedgehog regulates early human thymocyte differentiation by counteracting the IL-7-induced development of CD34+ precursor cells

Hedgehog signalling in CD4+ T helper cell polarisation

CD4+ T cells are critical in orchestrating immune responses against various pathogens and cancer but can also be drivers of autoimmune disease, allergy and pro-tumour responses. Naïve CD4+ T cells polarise into specialised T helper cell subsets with unique effector functions. While the guiding transcription factors and effector molecules of the T helper cell lineages are well understood, the signalling pathways orchestrating the intricate T helper cell polarisation programmes remain poorly understood. Here we review an emerging role of Hedgehog signalling - a classical morphogen signalling pathway - in T helper cell polarisation. Importantly, the Hedgehog pathway is pharmacologically highly tractable and existing clinically-approved Hedgehog inhibitors may prove useful therapeutic modulators of T helper cell-driven immune responses.

Understanding the Roles of the Hedgehog Signaling Pathway during T-Cell Lymphopoiesis and in T-Cell Acute Lymphoblastic Leukemia (T-ALL)

The Hedgehog (HH) signaling network is one of the main regulators of invertebrate and vertebrate embryonic development. Along with other networks, such as NOTCH and WNT, HH signaling specifies both the early patterning and the polarity events as well as the subsequent organ formation via the temporal and spatial regulation of cell proliferation and differentiation. However, aberrant activation of HH signaling has been identified in a broad range of malignant disorders, where it positively influences proliferation, survival, and therapeutic resistance of neoplastic cells. Inhibitors targeting the HH pathway have been tested in preclinical cancer models. The HH pathway is also overactive in other blood malignancies, including T-cell acute lymphoblastic leukemia (T-ALL). This review is intended to summarize our knowledge of the biological roles and pathophysiology of the HH pathway during normal T-cell lymphopoiesis and in T-ALL. In addition, we will discuss potential therapeutic strategies that might expand the clinical usefulness of drugs targeting the HH pathway in T-ALL.

Microenvironmental cues for T-cell acute lymphoblastic leukemia development

Intensive chemotherapy regimens have led to a substantial improvement in the cure rate of patients suffering from T-cell acute lymphoblastic leukemia (T-ALL). Despite this progress, about 15% and 50% of pediatric and adult cases, respectively, show resistance to treatment or relapse with dismal prognosis, calling for further therapeutic investigations. T-ALL is an heterogeneous disease, which presents intrinsic alterations leading to aberrant expression of transcription factors normally involved in hematopoietic stem/progenitor cell development and mutations in genes implicated in the regulation of cell cycle progression, apoptosis, and T-cell development. Gene expression profiling allowed the classification of T-ALL into defined molecular subgroups that mostly reflects the stage of their differentiation arrest. So far this knowledge has not translated into novel, targeted therapy. Recent evidence points to the importance of extrinsic signaling cues in controlling the ability of T-ALL to home, survive, and proliferate, thus offering the perspective of new therapeutic options. This review summarizes the present understanding of the interactions between hematopoietic cells and bone marrow/thymic niches during normal hematopoiesis, describes the main signaling pathways implicated in this dialog, and finally highlights how malignant T cells rely on specific niches to maintain their ability to sustain and propagate leukemia.

Hedgehog Signalling in the Embryonic Mouse Thymus

T cells develop in the thymus, which provides an essential environment for T cell fate specification, and for the differentiation of multipotent progenitor cells into major histocompatibility complex (MHC)-restricted, non-autoreactive T cells. Here we review the role of the Hedgehog signalling pathway in T cell development, thymic epithelial cell (TEC) development, and thymocyte-TEC cross-talk in the embryonic mouse thymus during the last week of gestation.

Sonic Hedgehog regulates thymic epithelial cell differentiation

Sonic Hedgehog (Shh) is expressed in the thymus, where it regulates T cell development. Here we investigated the influence of Shh on thymic epithelial cell (TEC) development. Components of the Hedgehog (Hh) signalling pathway were expressed by TEC, and use of a Gli Binding Site-green fluorescence protein (GFP) transgenic reporter mouse demonstrated active Hh-dependent transcription in TEC in the foetal and adult thymus. Analysis of Shh-deficient foetal thymus organ cultures (FTOC) showed that Shh is required for normal TEC differentiation. Shh-deficient foetal thymus contained fewer TEC than wild type (WT), the proportion of medullary TEC was reduced relative to cortical TEC, and cell surface expression of MHC Class II molecules was increased on both cortical and medullary TEC populations. In contrast, the Gli3-deficient thymus, which shows increased Hh-dependent transcription in thymic stroma, had increased numbers of TEC, but decreased cell surface expression of MHC Class II molecules on both cortical and medullary TEC. Neutralisation of endogenous Hh proteins in WT FTOC led to a reduction in TEC numbers, and in the proportion of mature Aire-expressing medullary TEC, but an increase in cell surface expression of MHC Class II molecules on medullary TEC. Likewise, conditional deletion of Shh from TEC in the adult thymus resulted in alterations in TEC differentiation and consequent changes in T cell development. TEC numbers, and the proportion of mature Aire-expressing medullary TEC were reduced, and cell surface expression of MHC Class II molecules on medullary TEC was increased. Differentiation of mature CD4 and CD8 single positive thymocytes was increased, demonstrating the regulatory role of Shh production by TEC on T cell development. Treatment of human thymus explants with recombinant Shh or neutralising anti-Shh antibody indicated that the Hedgehog pathway is also involved in regulation of differentiation from DP to mature SP T cells in the human thymus.

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TEC express components of the Hedgehog signalling pathway and transduce it's signals.

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Sonic hedgehog (Shh) is required for normal TEC development.

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Sonic hedgehog particularly influences differentiation to the medullary TEC lineage.

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Shh regulates cell surface MHCII expression on both cortical and medullary TEC.

Hedgehog and Resident Vascular Stem Cell Fate

The Hedgehog pathway is a pivotal morphogenic driver during embryonic development and a key regulator of adult stem cell self-renewal. The discovery of resident multipotent vascular stem cells and adventitial progenitors within the vessel wall has transformed our understanding of the origin of medial and neointimal vascular smooth muscle cells (SMCs) during vessel repair in response to injury, lesion formation, and overall disease progression. This review highlights the importance of components of the Hh and Notch signalling pathways within the medial and adventitial regions of adult vessels, their recapitulation following vascular injury and disease progression, and their putative role in the maintenance and differentiation of resident vascular stem cells to vascular lineages from discrete niches within the vessel wall.

Oligodendrogenesis after cerebral ischemia

Neural stem cells in the subventricular zone (SVZ) of the lateral ventricle of adult rodent brain generate oligodendrocyte progenitor cells (OPCs) that disperse throughout the corpus callosum and striatum where some of OPCs differentiate into mature oligodendrocytes. Studies in animal models of stroke demonstrate that cerebral ischemia induces oligodendrogenesis during brain repair processes. This article will review evidence of stroke-induced proliferation and differentiation of OPCs that are either resident in white matter or are derived from SVZ neural progenitor cells and of therapies that amplify endogenous oligodendrogenesis in ischemic brain.

Intraamniotic lipopolysaccharide exposure changes cell populations and structure of the ovine fetal thymus

RATIONALE

Chorioamnionitis induces preterm delivery and acute involution of the fetal thymus which is associated with postnatal inflammatory disorders. We studied the immune response, cell composition, and architecture of the fetal thymus following intraamniotic lipopolysaccharide (LPS) exposure.

METHODS

Time-mated ewes received an intraamniotic injection of LPS 5, 12, or 24 hours or 2, 4, 8, or 15 days before delivery at 125 days gestational age (term = 150 days).

RESULTS

The LPS exposure resulted in decreased blood lymphocytes within 5 hours and decreased thymic corticomedullary ratio within 24 hours. Thymic interleukin 6 (IL6) and IL17 messenger RNA (mRNA) increased 5-fold 24 hours post-LPS exposure. Increased toll-like receptor 4 (TLR4) mRNA and nuclear factor κB positive cells at 24 hours after LPS delivery demonstrated acute thymic activation. Both TLR4 and IL1 mRNA increased by 5-fold and the number of Foxp3-positive cells (Foxp3+ cells) decreased 15 days after exposure.

CONCLUSION

Intraamniotic LPS exposure caused a proinflammatory response, involution, and a persistent depletion of thymic Foxp3+ cells indicating disturbance of the fetal immune homeostasis.

Canonical and noncanonical Hedgehog pathway in the pathogenesis of multiple myeloma

The Hedgehog (Hh) pathway is required for cell-fate determination during the embryonic life, as well as cell growth and differentiation in the adult organism, where the inappropriate activation has been implicated in several cancers. Here we demonstrate that Hh signaling plays a significant role in growth and survival of multiple myeloma (MM) cells. We observed that CD138(+) MM cells express Hh genes and confirmed Smoothened (Smo)-dependent Hh signaling in MM using a novel synthetic Smo inhibitor, NVP-LDE225 (Novartis), which decreased MM cell viability by inducing specific down-regulation of Gli1 and Ptch1, hallmarks of Hh activity. In addition, we detected a nuclear localization of Gli1 in MM cells, which is completely abrogated by Forskolin, a Gli1-modulating compound, confirming Smo-independent mechanisms leading to Hh activation in MM. Finally, we identified that bone marrow stromal cells are a source of the Shh ligand, although they are resistant to the Hh inhibitor because of defective Smo expression and Ptch1 up-regulation. Further in vitro as well as in vivo studies showed antitumor efficacy of NVP-LDE225 in combination with bortezomib. Altogether, our data demonstrate activation of both canonical and noncanonical Hh pathway in MM, thus providing the rationale for testing Hh inhibitors in clinical trials to improve MM patient outcome.

Ovine fetal thymus response to lipopolysaccharide-induced chorioamnionitis and antenatal corticosteroids

Rationale

Chorioamnionitis is associated with preterm delivery and involution of the fetal thymus. Women at risk of preterm delivery receive antenatal corticosteroids which accelerate fetal lung maturation and improve neonatal outcome. However, the effects of antenatal corticosteroids on the fetal thymus in the settings of chorioamnionitis are largely unknown. We hypothesized that intra-amniotic exposure to lipopolysaccharide (LPS) causes involution of the fetal thymus resulting in persistent effects on thymic structure and cell populations. We also hypothesized that antenatal corticosteroids may modulate the effects of LPS on thymic development.

Methods

Time-mated ewes with singleton fetuses received an intra-amniotic injection of LPS 7 or 14 days before preterm delivery at 120 days gestational age (term = 150 days). LPS and corticosteroid treatment groups received intra-amniotic LPS either preceding or following maternal intra-muscular betamethasone. Gestation matched controls received intra-amniotic and maternal intra-muscular saline. The fetal intra-thoracic thymus was evaluated.

Results

Intra-amniotic LPS decreased the cortico-medullary (C/M) ratio of the thymus and increased Toll-like receptor (TLR) 4 mRNA and CD3 expression indicating involution and activation of the fetal thymus. Increased TLR4 and CD3 expression persisted for 14 days but Foxp3 expression decreased suggesting a change in regulatory T-cells. Sonic hedgehog and bone morphogenetic protein 4 mRNA, which are negative regulators of T-cell development, decreased in response to intra-amniotic LPS. Betamethasone treatment before LPS exposure attenuated some of the LPS-induced thymic responses but increased cleaved caspase-3 expression and decreased the C/M ratio. Betamethasone treatment after LPS exposure did not prevent the LPS-induced thymic changes.

Conclusion

Intra-amniotic exposure to LPS activated the fetal thymus which was accompanied by structural changes. Treatment with antenatal corticosteroids before LPS partially attenuated the LPS-induced effects but increased apoptosis in the fetal thymus. Corticosteroid administration after the inflammatory stimulus did not inhibit the LPS effects on the fetal thymus.

Role of Hedgehog signalling at the transition from double-positive to single-positive thymocyte

In the thymus, developing T cells receive signals that determine lineage choice, specificity, MHC restriction and tolerance to self-antigen. One way in which thymocytes receive instruction is by secretion of Sonic hedgehog (Shh) from thymic epithelial cells. We have previously shown that Hedgehog (Hh) signalling in the thymus decreases the CD4:CD8 single-positive (SP) thymocyte ratio. Here, we present data indicating that double-positive (DP) thymocytes are Hh-responsive and that thymocyte-intrinsic Hh signalling plays a role in modulating the production of CD4(+) (SP4), CD8(+) (SP8) and unconventional T-cell subsets. Repression of physiological Hh signalling in thymocytes altered the proportions of DP and SP4 cells. Thymocyte-intrinsic Hh-dependent transcription also attenuated both the production of mature SP4 and SP8 cells, and the establishment of peripheral T-cell compartments in TCR-transgenic mice. Additionally, stimulation or withdrawal of Hh signals in the WT foetal thymus impaired or enhanced upregulation of the CD4 lineage-specific transcription factor Gata3 respectively. These data together suggest that Hh signalling may play a role in influencing the later stages of thymocyte development.

Aberrant activation of the hedgehog signaling pathway in malignant hematological neoplasms

The hedgehog (HH) signaling pathway is a highly regulated signaling pathway that is important not only for embryonic development, tissue patterning, and organogenesis but also for tissue repair and the maintenance of stem cells in adult tissues. In the adult hematopoietic system, HH signaling regulates intrathymic T-cell development, and it is one of the survival signals provided by follicular dendritic cells to prevent apoptosis in germinal center B cells. HH signaling is required for primitive hematopoiesis; however, conflicting data have been reported regarding the role of the HH pathway in adult hematopoiesis. Inappropriate activation of the HH signaling pathway occurs in several human cancers, including hematological neoplasms. Emerging data demonstrate abnormal HH pathway activation in chronic lymphocytic leukemia/small lymphocytic lymphoma, plasma cell myeloma, mantle cell lymphoma, diffuse large B-cell lymphoma, ALK-positive anaplastic large cell lymphoma, chronic myelogenous leukemia, and acute leukemias. In these neoplasms, HH signaling promotes proliferation and survival, contributes to the maintenance of cancer stem cells, and enhances tolerance or resistance to chemotherapeutic agents. Here, we review current understanding of HH signaling, its role in the pathobiology of hematological malignancies, and its potential as a therapeutic target to treat malignant hematological neoplasms.

Non-redundant role for the transcription factor Gli1 at multiple stages of thymocyte development

The Hedgehog (Hh) signaling pathway influences multiple stages of murine T-cell development. Hh signaling mediates transcriptional changes by the activity of the Gli family of transcription factors, Gli1, Gli2 and Gli3. Both Gli2 and Gli3 are essential for mouse development and can be processed to function as transcriptional repressors or transcriptional activators, whereas Gli1, itself a transcriptional target of Hh pathway activation, can only function as a transcriptional activator and is not essential for mouse development. Gli1-deficient mice are healthy and appear normal and nonredundant functions for Gli1 have been difficult to identify. Here we show that Gli1 is non-redundant in the regulation of T-cell development in the thymus, at multiple developmental stages. Analysis of Gli1-deficient embryonic mouse thymus shows a role for Gli1 to promote the differentiation of CD4⁻CD8⁻ double negative (DN) thymocytes before pre- TCR signal transduction, and a negative regulatory function after pre-TCR signaling. In addition, introduction of a Class I-restricted transgenic TCR into the adult Gli1-deficient and embryonic Gli2-deficient thymus showed that both Gli1 and Gli2 influence its selection to the CD8 lineage.

The Gli3 transcription factor expressed in the thymus stroma controls thymocyte negative selection via Hedgehog-dependent and -independent mechanisms

The Hedgehog (Hh) responsive transcription factor Gli3 is required for efficient thymocyte development in the fetus. In this study we show that Gli3, not detected in adult thymocytes, is expressed in the murine fetal and adult thymus stroma. PCR array analysis revealed Cxcl9, Rbp1, and Nos2 as novel target genes of Gli3. We show that Gli3 positively regulates the expression of these genes, most likely by suppressing an intermediate repressor. Deletion of autoreactive thymocytes depends on their interactions with the thymus stroma. Repression of the proapoptotic gene Nos2 in Gli3 mutants coincides with reduced apoptosis of double positive thymocytes undergoing negative selection in vitro and in vivo, and the production of autoreactive thymocytes. Taken together these data indicate that Gli3 controls thymocyte apoptosis and negative selection possibly via the regulation of Nos2. Defective Gli3 expression in the thymus stroma also resulted in decreased CD5 expression on mature thymocytes and inappropriate production of MHC class I-selected CD4(+) cells, both consistent with reduced TCR signal strength. Overall our data indicate that Gli3 expressed in the thymus stroma regulates negative selection and TCR signal strength via Hh-dependent and -independent mechanisms, with implications for autoimmunity.

Sonic hedgehog expands diaphyseal trabecular bone altering bone marrow niche and lymphocyte compartment

Bone marrow contains distinct microenvironments that regulate hematopoietic stem cells (HSCs). The endosteal HSC niche includes osteoblasts, mineral, and extracellular matrix proteins that interact through various molecular signals to control HSCs. Sonic hedgehog (Shh) is a morphogen involved in the regulation of skeletal development and hematopoiesis, but the effects of Shh on bone in relation to the HSC niche are not well understood. We demonstrate that systemic overexpression of Shh in mice increases osteoblast number with the resultant formation of new trabeculae in the femoral diaphysis. Suggestive of a functional change in the hematopoietic niche, numbers of Lin(-) Sca-1(+) c-Kit(+) cells with hematopoietic progenitor function expand, although cells with in vivo repopulating capacity in the wild-type environment do not increase. Instead, Shh mediates a decrease in number of bone marrow lymphocytes accompanied by a decreased expression of stromal-derived growth factor 1 (SDF-1) and a decrease in Flk2-expressing Lin(-) Sca-1(+) c-Kit(+) cells, indicating a modulation of early lymphopoiesis. This is caused by a microenvironment-induced mechanism as Shh treatment of bone marrow recipients, but not donors, results in a dramatic depletion of lymphocytes. Together, these data suggest that Shh mediates alterations in the bone marrow hematopoietic niche affecting the early lymphoid differentiation.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

Sonic hedgehog negatively regulates pre-TCR-induced differentiation by a Gli2-dependent mechanism

Hedgehog signaling regulates differentiation, survival, and proliferation of the earliest double-negative (DN) thymocytes, but its importance at later stages of T-cell development is controversial. Here we use loss- and gain-of-function mouse models to show that Shh, by signaling directly to the developing thymocyte, is a negative regulator of pre-TCR-induced differentiation from DN to double-positive (DP) cell. When hedgehog signaling was reduced, in the Shh(-/-) and Gli2(-/-) thymus, or by T lineage-specific transgenic expression of a transcriptional-repressor form of Gli2 (Gli2DeltaC(2)), differentiation to DP cell after pre-TCR signal transduction was increased. In contrast, when Hh signaling was constitutively activated in thymocytes, by transgenic expression of a constitutive transcriptional-activator form of Gli2 (Gli2DeltaN(2)), the production of DP cells was decreased. Gene expression profiling showed that physiologic Hh signaling in thymocytes maintains expression of the transcription factor FoxA2 on pre-TCR signal transduction.

Indian hedgehog (Ihh) both promotes and restricts thymocyte differentiation

We show that Indian Hedgehog (Ihh) regulates T-cell development and homeostasis in both fetal and adult thymus, controlling thymocyte number. Fetal Ihh(-/-) thymi had reduced differentiation to double-positive (DP) cell and reduced cell numbers compared with wild-type littermates. Surprisingly, fetal Ihh(+/-) thymi had increased thymocyte numbers and proportion of DP cells relative to wild type, indicating that Ihh also negatively regulates thymocyte development. In vitro treatment of thymus explants with exogenous recombinant Hedgehog protein promoted thymocyte development in Ihh(-/-) thymi but inhibited thymocyte development in Ihh(+/-), confirming both positive and negative regulatory functions of Ihh. Analysis of Rag(-/-)Ihh(+/-) thymi showed that Ihh promotes T-cell development before pre-T-cell receptor (pre-TCR) signaling, but negatively regulates T-cell development only after pre-TCR signaling has taken place. We show that Ihh is most highly expressed by the DP population and that Ihh produced by DP cells feeds back to negatively regulate the differentiation and proliferation of their double-negative progenitors. Thus, differentiation from double-negative to DP cell, and hence the size of the DP population, is dependent on the concentration of Ihh in the thymus. Analysis of Ihh conditional knockout and heterozygote adult mice showed that Ihh also influences thymocyte number in the adult.

Association of serum interleukin-7 levels with the development of acute graft-versus-host disease

PURPOSE

Morbidity from acute graft-versus-host disease (GVHD) limits the success of allogeneic hematopoietic stem-cell transplantation (HSCT) to treat malignancy. Interleukin-7 (IL-7), the principal homeostatic cytokine for T cells, is required for acute GVHD in murine models. In contrast to inflammatory cytokines (eg, IL-2, tumor necrosis factor alpha), IL-7 has not been studied extensively in the clinical transplant setting relative to its relationship with acute GVHD.

PATIENTS AND METHODS

We evaluated the association of serum IL-7 levels with acute GVHD in 31 patients who were uniformly treated in a prospective clinical trial with reduced-intensity allogeneic HSCT from human leukocyte antigen-identical siblings. GVHD prophylaxis consisted of cyclosporine and methotrexate. Serum IL-7 levels and lymphocyte populations were determined at enrollment, the day of transplantation before the allograft infusion, and at specified intervals through 12 months post-transplantation.

RESULTS

As expected, IL-7 levels were inversely correlated with T-cell populations (P < .00001). Acute GVHD was significantly associated with higher IL-7 levels at day +7 (P = .01) and day +14 (P = .00003) post-transplantation as well as with the allograft CD34(+) cell dose (P = .01). IL-7 levels at day +14 also correlated with the severity of acute GVHD (P < .0001). In logistic regression models, these factors were highly sensitive (up to 86%) and specific (100%) for classifying whether patients developed acute GVHD.

CONCLUSION

These data support preclinical observations that IL-7 plays a critical role in inducing acute GVHD and provide a rational basis for novel approaches to prevent and treat acute GVHD through modulation of the IL-7 pathway.

The Sonic Hedgehog Pathway Mediates Carbamylated Erythropoietin-enhanced Proliferation and Differentiation of Adult Neural Progenitor Cells

Carbamylated erythropoietin (CEPO), a well characterized erythropoietin (EPO) derivative, does not bind to the classical EPO receptor and does not stimulate erythropoiesis. Using neural progenitor cells derived from the subventricular zone of the adult mouse, we investigated the effect of CEPO on neurogenesis and the associated signaling pathways in vitro. We found that CEPO significantly increased neural progenitor cell proliferation and promoted neural progenitor cell differentiation into neurons, which was associated with up-regulation of Sonic hedgehog (Shh), its receptor ptc, and mammalian achaete-scute homolog 1 (Mash1), a pro-neuron basic helix-loop-helix protein transcription factor. Blockage of the Shh signaling pathway with a pharmacological inhibitor, cyclopamine, abolished the CEPO-induced neurogenesis. Attenuation of endogenous Mash1 expression by short-interfering RNA blocked CEPO-promoted neuronal differentiation. In addition, recombinant mouse Shh up-regulated Mash1 expression in neural progenitor cells. These results demonstrate that the Shh signaling pathway mediates CEPO-enhanced neurogenesis and Mash1 is a downstream target of the Shh signaling pathway that regulates CEPO-enhanced neuronal differentiation.

Adenoviral vector-mediated transfer of the Indian hedgehog gene modulates lymphomyelopoiesis in vivo

Indian hedgehog (Ihh) plays an essential role in angiogenesis, hematogenesis, and epiphysis formation during embryogenesis. In the present study, we injected an adenoviral vector (Adv) carrying the mock-control (Adv-control) or Ihh (Adv-Ihh) gene into severe combined immunodeficiency (SCID) or BALB/c mice to evaluate the effects of lhh on the regulation of postnatal hematopoiesis in vivo. After the i.v. injection of Adv-Ihh, the expression of vector-derived Ihh mRNA was detected in the liver. Four weeks after administration of Adv-Ihh to SCID mice, we observed an increase in the number of c-Kit+ cells and clonogenic cells per 10(5) mononuclear cells in the bone marrow compared with Adv-control-administered mice. Moreover, after administration of Adv-Ihh to BALB/c mice, the number of splenic B220+IgM(low)CD23(int)CD21(int) B lymphocytes and CD4+ T lymphocytes was strongly increased. Furthermore, the number of thymic double-negative (DN)2, DN3, CD8+ immature single-positive, and CD4+/CD8- cells was significantly elevated relative to the number in mice that received the control Adv vector. Our results suggest that enhanced signaling by Ihh can modulate the proliferation and differentiation of splenic B lymphocytes and thymic T lymphocytes during bone marrow hematopoiesis in vivo. Thus, modulation of the hedgehog signaling pathway may provide a therapeutic strategy to stimulate lymphomyelopoiesis in vivo.

Sonic hedgehog signalling in T-cell development and activation

The production of mature functional T cells in the thymus requires signals from the thymic epithelium. Here, we review recent experiments showing that one way in which the epithelium controls the production of mature T cells is by the secretion of sonic hedgehog (SHH). We consider the increasing evidence that SHH-induced signalling is not only important for the differentiation and proliferation of early thymocyte progenitors, but also for modulating T-cell receptor signalling during repertoire selection, with implications for positive selection, CD4 versus CD8 lineage commitment, and clonal deletion of autoreactive cells. We also review the influence of hedgehog signalling in peripheral T-cell activation.

Bone morphogenetic protein-2/4 signalling pathway components are expressed in the human thymus and inhibit early T-cell development

T-cell differentiation is driven by a complex network of signals mainly derived from the thymic epithelium. In this study we demonstrate in the human thymus that cortical epithelial cells produce bone morphogenetic protein 2 (BMP2) and BMP4 and that both thymocytes and thymic epithelium express all the molecular machinery required for a response to these proteins. BMP receptors, BMPRIA and BMPRII, are mainly expressed by cortical thymocytes while BMPRIB is expressed in the majority of the human thymocytes. Some thymic epithelial cells from cortical and medullary areas express BMP receptors, being also cell targets for in vivo BMP2/4 signalling. The treatment with BMP4 of chimeric human-mouse fetal thymic organ cultures seeded with CD34+ human thymic progenitors results in reduced cell recovery and inhibition of the differentiation of human thymocytes from CD4- CD8- to CD4+ CD8+ cell stages. These results support a role for BMP2/4 signalling in human T-cell differentiation.

Activation of the Hedgehog signaling pathway in T-lineage cells inhibits TCR repertoire selection in the thymus and peripheral T-cell activation

TCR signal strength is involved in many cell fate decisions in the T-cell lineage. Here, we show that transcriptional events induced by Hedgehog (Hh) signaling reduced TCR signal strength in mice. Activation of Hh signaling in thymocytes in vivo by expression of a transgenic transcriptional-activator form of Gli2 (Gli2DeltaN(2)) changed the outcome of TCR ligation at many stages of thymocyte development, allowing self-reactive cells to escape clonal deletion; reducing transgenic TCR-mediated positive selection; reducing the ratio of CD4/CD8 single-positive (SP) cells; and reducing cell surface CD5 expression. In contrast, in the Shh(-/-) thymus the ratio of CD4/CD8 cells and both positive and negative selection of a transgenic TCR were increased, demonstrating that Shh does indeed influence TCR repertoire selection and the transition from double-positive (DP) to SP cell in a physiological situation. In peripheral T cells, Gli2DeltaN(2) expression attenuated T-cell activation and proliferation, by a mechanism upstream of ERK phosphorylation.

Techniques: new pharmacological perspectives for the leptin receptor

The function of leptin, initially confined to its role in energy homeostasis and obesity, has now expanded to the regulation of reproduction, glucose homeostasis, bone formation, wound healing and the immune system. Both stimulation and inhibition of the molecular target of leptin, the leptin receptor (LR), might find applications in disease treatment. Recent advances in the understanding of LR activation mechanisms have led to the design of LR antagonists. Several assays have been developed for the screening and evaluation of LR ligands. Both the extracellular and the intracellular domains of the LR are potential drug targets. The bioluminescence resonance energy transfer technique can be used to screen for compounds that target the extracellular part of the LR, and we propose that the novel reverse mammalian protein-protein interaction trap technique can be used to screen compounds that affect intracellular aspects of LR signalling. These assays can be easily adapted to other pharmacologically relevant receptors.

Hedgehog signaling controls thymocyte progenitor homeostasis and differentiation in the thymus

Commitment of hematopoietic progenitors to the T cell lineage requires the integration of multiple signaling pathways. Evidence has suggested involvement of hedgehog (Hh) signaling in T cell differentiation through its signal transducer smoothened (Smo). However, the precise function of the Hh pathway remains controversial, mainly because T cell-specific in vivo genetic models have not been used. Using pre-T cell-specific, mature T cell-specific and poly(I).poly(C)-inducible deletions of Smo and antagonists of Smo signaling, we report here that Hh is an essential positive regulator of T cell progenitor differentiation. Furthermore, we localize Hh function to a stage preceding pre-T cell receptor signaling, connect Smo signaling to the activity of the Gli1 and Gli2 transcription factors and demonstrate that Hh affects regulators of thymocyte survival and proliferation.

A Novel Approach to Thymic Rejuvenation in the Aged

With advancing age, the mammalian thymus undergoes involution, a progressive loss of architectural integrity and lymphoid cellularity that results in reduced T lymphopoiesis. Thymic involution also is associated with extreme malnutrition and states of immune deficiency, such as active HIV infection, after chemotherapy, or during pregnancy. Immune recovery appears to require restoration of normal thymopoiesis. Although several means are known to increase thymic cellularity in the aged, including systemic administration of hormones, androgen ablation, and thymic tissue transplantation, each suffers from specific limitations that prevent widespread application. This paper presents a novel approach to rejuvenate T cell differentiation in the aged that employs intrathymic implantation of engineered stromal cells. Two different proteins have been examined for their impact on thymopoiesis after delivery by somatic cell implantation. Intrathymic injection of IL-7-producing stromal cells enhances the earliest specification steps of T cell development, resulting in the increased representation of pro-T cells in the aged thymus. In contrast, increasing the intrathymic levels of sonic hedgehog diminishes this aspect of T cell poiesis.

Loss of the serine/threonine kinase fused results in postnatal growth defects and lethality due to progressive hydrocephalus

The Drosophila Fused (Fu) kinase is an integral component of the Hedgehog (Hh) pathway that helps promote Hh-dependent gene transcription. Vertebrate homologues of Fu function in the Hh pathway in vitro, suggesting that Fu is evolutionarily conserved. We have generated fused (stk36) knockout mice to address the in vivo function of the mouse Fu (mFu) homologue. fused knockouts develop normally, being born in Mendelian ratios, but fail to thrive within 2 weeks, displaying profound growth retardation with communicating hydrocephalus and early mortality. The fused gene is expressed highly in ependymal cells and the choroid plexus, tissues involved in the production and circulation of cerebral spinal fluid (CSF), suggesting that loss of mFu disrupts CSF homeostasis. Similarly, fused is highly expressed in the nasal epithelium, where fused knockouts display bilateral suppurative rhinitis. No obvious defects were observed in the development of organs where Hh signaling is required (limbs, face, bones, etc.). Specification of neuronal cell fates by Hh in the neural tube was normal in fused knockouts, and induction of Hh target genes in numerous tissues is not affected by the loss of mFu. Furthermore, stimulation of fused knockout cerebellar granule cells to proliferate with Sonic Hh revealed no defect in Hh signal transmission. These results show that the mFu homologue is not required for Hh signaling during embryonic development but is required for proper postnatal development, possibly by regulating the CSF homeostasis or ciliary function.

The transcription factor Gli3 regulates differentiation of fetal CD4- CD8- double-negative thymocytes

Glioblastoma 3 (Gli3) is a transcription factor involved in patterning and oncogenesis. Here, we demonstrate a role for Gli3 in thymocyte development. Gli3 is differentially expressed in fetal CD4- CD8- double-negative (DN) thymocytes and is most highly expressed at the CD44+ CD25- DN (DN1) and CD44- CD25- (DN4) stages of development but was not detected in adult thymocytes. Analysis of null mutants showed that Gli3 is involved at the transitions from DN1 to CD44+ CD25+ DN (DN2) cell and from DN to CD4+ CD8+ double-positive (DP) cell. Gli3 is required for differentiation from DN to DP thymocyte, after pre-T-cell receptor (TCR) signaling but is not necessary for pre-TCR-induced proliferation or survival. The effect of Gli3 was dose dependent, suggesting its direct involvement in the transcriptional regulation of genes controlling T-cell differentiation during fetal development.

Sonic hedgehog is produced by follicular dendritic cells and protects germinal center B cells from apoptosis

The Hedgehog (Hh) signaling pathway is involved in the development of many tissues during embryogenesis, but has also been described to function in adult self-renewing tissues. In the immune system, Sonic Hedgehog (Shh) regulates intrathymic T cell development and modulates the effector functions of peripheral CD4(+) T cells. In this study we investigate whether Shh signaling is involved in peripheral B cell differentiation in mice. Shh is produced by follicular dendritic cells, mainly in germinal centers (GCs), and GC B cells express both components of the Hh receptor, Patched and Smoothened. Blockade of the Hh signaling pathway reduces the survival, and consequently the proliferation and Ab secretion, of GC B cells. Furthermore, Shh rescues GC B cells from apoptosis induced by Fas ligation. Taken together, our data suggest that Shh is one of the survival signals provided by follicular dendritic cells to prevent apoptosis in GC B cells.