The fetal liver counterpart of adult common lymphoid progenitors gives rise to all lymphoid lineages, CD45+CD4+CD3- cells, as well as macrophages

Lymphoid tissue inducer cells: architects of CD4 immune responses in mice and men

In this review, we summarize the current understanding of the multiple functions of the mouse lymphoid tissue inducer (LTi) cells in: (i) the development of organized lymphoid tissue, (ii) the generation and maintenance of CD4-dependent immunity in adult lymphoid tissues; and (iii) the regulation of central tolerance in thymus. By contrast with mouse LTi cells, which have been well described, the human equivalent is only just beginning to be characterized. Human LTi-like cells expressing interleukin (IL)-22 have been identified recently and found to differentiate into natural killer (NK) cells. The relationship of LTi cells to NK cells is discussed in the light of several studies reporting a close relationship in the mouse between LTi cells and transcription factor retinoid-related orphan receptor gammat-dependent IL-22 producing NK cells in the gut. We also outline our data suggesting that these cells are present in adult human lymphoid tissues.

Definitive hematopoietic stem/progenitor cells manifest distinct differentiation output in the zebrafish VDA and PBI

One unique feature of vertebrate definitive hematopoiesis is the ontogenic switching of hematopoietic stem cells from one anatomical compartment or niche to another. In mice, hematopoietic stem cells are believed to originate in the aorta-gonad-mesonephros (AGM), subsequently migrate to the fetal liver (FL) and finally colonize the bone marrow (BM). Yet, the differentiation potential of hematopoietic stem cells within early niches such as the AGM and FL remains incompletely defined. Here, we present in vivo analysis to delineate the differentiation potential of definitive hematopoietic stem/progenitor cells (HSPCs) in the zebrafish AGM and FL analogies, namely the ventral wall of dorsal aorta (VDA) and the posterior blood island (PBI), respectively. Cell fate mapping and analysis of zebrafish runx1(w84x) and vlad tepes (vlt(m651)) mutants revealed that HSPCs in the PBI gave rise to both erythroid and myeloid lineages. However, we surprisingly found that HSPCs in the VDA were not quiescent but were uniquely adapted to generate myeloid but not erythroid lineage cells. We further showed that such distinct differentiation output of HSPCs was, at least in part, ascribed to the different micro-environments present in these two niches. Our results highlight the importance of niche in shaping the differentiation output of developing HSPCs.

Human fetal lymphoid tissue-inducer cells are interleukin 17-producing precursors to RORC+ CD127+ natural killer-like cells

The human body contains over 500 individual lymph nodes, yet the biology of their formation is poorly understood. Here we identify human lymphoid tissue-inducer cells (LTi cells) as lineage-negative RORC+ CD127+ cells with the functional ability to interact with mesenchymal cells through lymphotoxin and tumor necrosis factor. Human LTi cells were committed natural killer (NK) cell precursors that produced interleukin 17 (IL-17) and IL-22. In vitro, LTi cells gave rise to RORC+ CD127+ NK cells that retained the ability to produce IL-17 and IL-22. Postnatally, similar populations of LTi cell-like cells and RORC+ CD127+ NK cells were present in tonsils, and both secreted IL-17 and IL-22 but no interferon-gamma. Our data indicate that lymph node organogenesis is controlled by an NK cell precursor population with adaptive immune features and demonstrate a previously unappreciated link between the innate and adaptive immune systems.

Wnt3a deficiency irreversibly impairs hematopoietic stem cell self-renewal and leads to defects in progenitor cell differentiation

Canonical Wnt signaling has been implicated in various aspects of hematopoiesis. Its role is controversial due to different outcomes between various inducible Wnt-signaling loss-of-function models and also compared with gain-of-function systems. We therefore studied a mouse deficient for a Wnt gene that seemed to play a nonredundant role in hematopoiesis. Mice lacking Wnt3a die prenatally around embryonic day (E) 12.5, allowing fetal hematopoiesis to be studied using in vitro assays and transplantation into irradiated recipient mice. Here we show that Wnt3a deficiency leads to a reduction in the numbers of hematopoietic stem cells (HSCs) and progenitor cells in the fetal liver (FL) and to severely reduced reconstitution capacity as measured in secondary transplantation assays. This deficiency is irreversible and cannot be restored by transplantation into Wnt3a competent mice. The impaired long-term repopulation capacity of Wnt3a(-/-) HSCs could not be explained by altered cell cycle or survival of primitive progenitors. Moreover, Wnt3a deficiency affected myeloid but not B-lymphoid development at the progenitor level, and affected immature thymocyte differentiation. Our results show that Wnt3a signaling not only provides proliferative stimuli, such as for immature thymocytes, but also regulates cell fate decisions of HSC during hematopoiesis.

The use of experimental murine models to assess novel agents of hematopoietic stem and progenitor cell mobilization

The recent explosion in the understanding of the cellular and molecular mechanisms underlying hematopoietic stem and progenitor cell (HSPC) mobilization has facilitated development of novel therapeutic agents, targeted at improving mobilization kinetics as well as HSPC yield. With the development of new agents comes the challenge of choosing efficient and relevant preclinical studies for the testing of the HSPC mobilization efficacy of these agents. This article reviews the use of the mouse as a convenient small animal model of HSPC mobilization and transplantation, and outlines the range of murine assays that can be applied to assess novel HSPC mobilizing agents. Techniques to demonstrate murine HSPC mobilization are discussed, as well as the role of murine assays to confirm human HSPC mobilization, and techniques to investigate the biologic phenotype of HSPC mobilized by these novel agents. Technical aspects regarding mobilization regimens and control arms, and choice of experimental animals are also discussed.

Roles of embryonic and adult lymphoid tissue inducer cells in primary and secondary lymphoid tissues

The nomenclature "embryonic lymphoid tissue inducer (LTi) cell" reflects the fundamental role of the cell in secondary lymphoid tissue organization. In addition, it is equally important in primary lymphoid tissue development as it regulates central tolerance to self-antigens in the thymus. An adult LTi cell constitutively expresses two sets of tumor necrosis factor (TNF) family members, whereas its embryonic counterpart expresses only one. The first set is lymphotoxin (LT)alpha, LTbeta, and TNalpha, which are essential for the secondary lymphoid organogenesis during embryogenesis and for maintaining an organized secondary lymphoid structure during adulthood. The second set is OX40- and CD30-ligands, which are critical for memory T cell generation. Adult LTi cells regulate adaptive immune responses by providing LTbetaR signals to stromal cells to maintain secondary lymphoid tissue structure, and determine adaptive immune responses by providing OX40 and CD30 survival signals to activated T cells in memory T cell generation. Along with the consideration of the roles of embryonic LTi cells in primary and secondary lymphoid tissues, this review highlights the roles of adult LTi cells in secondary lymphoid tissue function.

Development of human lymph nodes and Peyer's patches

In contrast to our understanding of murine lymphoid organogenesis, detailed knowledge on the mechanisms of human lymph node development is virtually lacking. This is mainly due to the obvious difficulties that accompany research using human fetal organs. In this review we will highlight current knowledge on human lymph node and Peyer's patch development and will temporally align observations made in humans with data available from murine studies. In the final paragraphs we will put this knowledge in the context of human malignancies in which interactions between lymphocytes and stroma, resembling those seen in lymphoid organs, are recapitulated.

Development of secondary lymphoid organs

Secondary lymphoid organs develop during embryogenesis or in the first few weeks after birth according to a highly coordinated series of interactions between newly emerging hematopoietic cells and immature mesenchymal or stromal cells. These interactions are orchestrated by homeostatic chemokines, cytokines, and growth factors that attract hematopoietic cells to sites of future lymphoid organ development and promote their survival and differentiation. In turn, lymphotoxin-expressing hematopoietic cells trigger the differentiation of stromal and endothelial cells that make up the scaffolding of secondary lymphoid organs. Lymphotoxin signaling also maintains the expression of adhesion molecules and chemokines that govern the ultimate structure and function of secondary lymphoid organs. Here we describe the current paradigm of secondary lymphoid organ development and discuss the subtle differences in the timing, molecular interactions, and cell types involved in the development of each secondary lymphoid organ.

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.

Ectopic lymphoid-organ development occurs through interleukin 7-mediated enhanced survival of lymphoid-tissue-inducer cells

Development of Peyer's patches and lymph nodes requires the interaction between CD4+ CD3- IL-7Ralpha+ lymphoid-tissue inducer (LTi) and VCAM-1+ organizer cells. Here we showed that by promoting their survival, enhanced expression of interleukin-7 (IL-7) in transgenic mice resulted in accumulation of LTi cells. With increased IL-7 availability, de novo formation of VCAM-1+ Peyer's patch anlagen occurred along the entire fetal gut resulting in a 5-fold increase in Peyer's patch numbers. IL-7 overexpression also led to formation of multiple organized ectopic lymph nodes and cecal patches. After immunization, ectopic lymph nodes developed normal T cell-dependent B cell responses and germinal centers. Mice overexpressing IL-7 but lacking either RORgamma, a factor required for LTi cell generation, or lymphotoxin alpha1beta2 had neither Peyer's patches nor ectopic lymph nodes. Therefore, by controlling LTi cell numbers, IL-7 can regulate the formation of both normal and ectopic lymphoid organs.

Delineation of the earliest lineage commitment steps of haematopoietic stem cells: new developments, controversies and major challenges

PURPOSE OF REVIEW

This review addresses recently reported evidence for alternative cellular pathways for haematopoietic stem cell lineage commitment.

RECENT FINDINGS

Using various approaches, several laboratories suggested the existence of adult as well as foetal multipotent progenitor cells with combined B cell, T cell and granulocyte/macrophage potential, but little or no megakaryocyte/erythroid potential. Compared with haematopoietic stem cells, these multipotent progenitor cells exhibited downregulated transcriptional expression of genes of the megakaryocyte/erythroid lineages and upregulated expression of lymphoid lineage genes. The existence of these lineage-restricted multipotent progenitor cells suggests that the first lineage commitment step of haematopoietic stem cells does not result in strict separation into myelopoiesis and lymphopoiesis, and that there might be alternative pathways for commitment toward different lineage fates. These findings have been questioned by other studies, however. To resolve this controversy and establish the complete road map for haematopoietic lineage commitment, improved tools and more stringent standards for how to identify and characterize lineage fate options of distinct stem and progenitor cells are needed.

SUMMARY

Current and future progress in establishing the complete cellular roadmap for haematopoietic lineage commitment will permit identification and characterization of key regulators of lineage fate decisions in haematopoietic stem cells.

Mature natural killer cell and lymphoid tissue-inducing cell development requires Id2-mediated suppression of E protein activity

The Id2 transcriptional repressor is essential for development of natural killer (NK) cells, lymphoid tissue–inducing (LTi) cells, and secondary lymphoid tissues. Id2 was proposed to regulate NK and LTi lineage specification from multipotent progenitors through suppression of E proteins. We report that NK cell progenitors are not reduced in the bone marrow (BM) of Id2^−/− mice, demonstrating that Id2 is not essential for NK lineage specification. Rather, Id2 is required for development of mature (m) NK cells. We define the mechanism by which Id2 functions by showing that a reduction in E protein activity, through deletion of E2A, overcomes the need for Id2 in development of BM mNK cells, LTi cells, and secondary lymphoid tissues. However, mNK cells are not restored in the blood or spleen of Id2^−/−E2A^−/− mice, suggesting a role for Id2 in suppression of alternative E proteins after maturation. Interestingly, the few splenic mNK cells in Id2^−/− and Id2^−/−E2A^−/− mice have characteristics of thymus-derived NK cells, which develop in the absence of Id2, implying a differential requirement for Id2 in BM and thymic mNK development. Our findings redefine the essential functions of Id2 in lymphoid development and provide insight into the dynamic regulation of E and Id proteins during this process.

Beta-selection: abundance of TCRbeta-/gammadelta- CD44- CD25- (DN4) cells in the foetal thymus

Expression of TCRbeta and pre-TCR signalling are essential for differentiation of CD4- CD8- double negative (DN) thymocytes to the CD4+ CD8+ double-positive (DP) stage. Thymocyte development in adult Rag1, Rag2 or TCRbetadelta-deficient mice is arrested at the DN3 stage leading to the assumption that pre-TCR signalling and beta-selection occur at, and are obligatory for, the transition from DN3 to DN4. We show that the majority of DN3 and DN4 cells that differentiate during early embryogenesis in wild-type mice do not express intracellular (ic) TCRbeta/gammadelta. These foetal icTCRbeta-/gammadelta- DN4 cells were T lineage as determined by expression of Thy1 and icCD3 and TCRbeta DJ rearrangement. In addition, in the foetal Rag1-/- thymus, a normal percentage of DN4 cells were present. In wild-type mice after hydrocortisone-induced synchronisation of differentiation, the majority of DN4 cells that first emerged did not express icTCRbeta/gammadelta, showing that adult thymocytes can also differentiate to the DN4 stage independently of pre-TCR signalling. Pre-TCR signalling induced expansion in the DN4 population, but lack of TCRbeta/gammadelta expression did not immediately induce apoptosis. Our data demonstrate in vivo differentiation from DN3 to DN4 cell in the absence of TCRbeta/gammadelta expression in the foetal thymus, and after hydrocortisone treatment of adult mice.

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.

Lymphoid Tissue Inducer Cells in Intestinal Immunity

During fetal development, lymphoid tissue inducer cells (LTis) seed the developing lymph node and Peyer's patch anlagen and initiate the formation of both types of lymphoid organs. In the adult, a similar population of cells, termed lymphoid tissue inducer-like cells (LTi-like cells), supports the formation of organized gut-associated lymphoid tissue (GALT) in the intestine, including both isolated lymphoid follicles (ILFs) and cryptopatches (CPs). Both LTi and LTi-like cells require expression of the transcription factor RORgammat for their differentiation and function, and mice lacking RORgammat lack lymph nodes, Peyer's patches, and other organized GALT. In ILFs and cryptopatches, LTi-like cells are in close contact with different populations of intestinal dendritic cells (DCs), including a subpopulation recently shown to extend dendrites and sample luminal microflora. This interaction may allow for communication between the intestinal lumen and the immune cells in the lamina propria, which is necessary for maintaining homeostasis between the commensal microflora and the intestinal immune system. The potential functional implications of the organization of LTi-like cells, DCs, and lymphocytes in the lamina propria are discussed in the context of maintenance of homeostasis and of infectious diseases, particularly HIV infection.

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.

Regulation of intrathymic T-cell development by Lunatic Fringe- Notch1 interactions

Intrathymic Notch1 signaling critically regulates T-lineage specification and commitment as well as T-cell progenitor survival and differentiation. Notch1 activation is continuously required during progression of early CD4/CD8-double-negative thymocytes to the CD4/CD8-double-positive stage. This developmental transition occurs as thymocytes migrate from the corticomedullary junction (CMJ) to the outer subcapsular zone (SCZ) of the thymus. Members of two families of structurally distinct Notch ligands, Delta-like 1 and Jagged-1, are expressed by cortical thymic epithelial cells, but it is not known which ligands are functionally required within the CMJ and SCZ microenvironmental niches. Our laboratory has investigated this question by genetically manipulating thymocyte expression of Lunatic Fringe (L-Fng), a glycosyltransferase that enhances sensitivity of Notch receptors to Delta-like ligands. This approach has revealed that low-threshold intrathymic Notch1 signals instruct multipotent thymus-seeding progenitors to suppress their B-cell potential and choose the T-cell fate. This strategy has also revealed that Delta-like Notch ligands are functionally limiting in both the CMJ and SCZ microenvironmental niches. Finally, we discuss our recent demonstration that L-Fng-mediated competition for Delta-like ligands is an important mechanism for regulating thymus size.

Fetal spleen stroma drives macrophage commitment

The role of the fetal spleen in hematopoeisis remains largely unknown. In this particular environment, we show that hematopoietic stem cells do not proliferate, but that they lose multipotency and differentiate exclusively into mature macrophages. B lymphocytes in the spleen derive from committed B cell precursors that are likely to have immigrated from the fetal liver. We developed fetal spleen stromal cell lines that are unique in their capacity to expand myeloid precursors, resulting in large numbers of mature macrophages. These lines secrete high levels of anti-inflammatory molecules. By phenotype, fetal splenic macrophages are reminiscent of their adult counterparts found in the red pulp. We postulate that F4/80(+) splenic macrophages participate in fetal erythropoiesis, as well as in the formation of the splenic architecture.

Determinants of lymphoid-myeloid lineage diversification

In recent years, investigators have made great progress in delineating developmental pathways of several lymphoid and myeloid lineages and in identifying transcription factors that establish and maintain their fate. However, the developmental branching points between these two large cell compartments are still controversial, and little is known about how their diversification is induced. Here, we give an overview of determinants that play a role at lymphoid-myeloid junctures, in particular transcription factors and cytokine receptors. Experiments showing that myeloid lineages can be reversibly reprogrammed into one another by transcription factor network perturbations are used to highlight key principles of lineage commitment. We also discuss experiments showing that lymphoid-to-myeloid but not myeloid-to-lymphoid conversions can be induced by the enforced expression of a single transcription factor. We close by proposing that this asymmetry is related to a higher complexity of transcription factor networks in lymphoid cells compared with myeloid cells, and we suggest that this feature must be considered when searching for mechanisms by which hematopoietic stem cells become committed to lymphoid lineages.

Failure of transdifferentiation of adult hematopoietic stem cells into neurons

Previous studies of bone marrow-derived stem cell transdifferentiation into neurons have not involved purified cell populations and determined their exact phenotype prior to differentiation. The present study investigates whether highly purified mouse adult hematopoietic stem cells (HSCs), characterized by lineage marker depletion and expression of the cell surface markers Sca1 and c-Kit (Lin(-) Sca1(+) c-Kit(+) [LSK]), can be stimulated to adopt a neuronal fate. When the HSC(LSK) cells were cultured in vitro in neuronal differentiation medium supplemented with retinoic acid, 50% of the cells expressed the neural progenitor marker nestin and no cells had become postmitotic. Electrophysiological recordings on neuron-like cells showed that these cells were incapable of generating action potentials. When the HSC(LSK) cells either were grown in vitro together with neural precursor cells or were transplanted into the striatum or cerebellum of wild-type mouse, they either differentiated into Iba1-immunopositive macrophage/microglia or died. In conclusion, we demonstrate that adult HSC(LSK) cells do not have the capacity to leave the hematopoietic lineage and differentiate into neurons.

Primitive human hematopoietic cells give rise to differentially specified daughter cells upon their initial cell division

It is often predicted that stem cells divide asymmetrically, creating a daughter cell that maintains the stem-cell capacity, and 1 daughter cell committed to differentiation. While asymmetric stem-cell divisions have been proven to occur in model organisms (eg, in Drosophila), it remains illusive whether primitive hematopoietic cells in mammals actually can divide asymmetrically. In our experiments we have challenged this question and analyzed the developmental capacity of separated offspring of primitive human hematopoietic cells at a single-cell level. We show for the first time that the vast majority of the most primitive, in vitro–detectable human hematopoietic cells give rise to daughter cells adopting different cell fates; 1 inheriting the developmental capacity of the mother cell, and 1 becoming more specified. In contrast, approximately half of the committed progenitor cells studied gave rise to daughter cells, both of which adopted the cell fate of their mother. Although our data are compatible with the model of asymmetric cell division, other mechanisms of cell fate specification are discussed. In addition, we describe a novel human hematopoietic progenitor cell that has the capacity to form natural killer (NK) cells as well as macrophages, but not cells of other myeloid lineages.

Retinoid-related Orphan Receptors (RORs): Roles in Cellular Differentiation and Development

Retinoid-related orphan receptors RORalpha, -beta, and -gamma are transcription factors belonging to the steroid hormone receptor superfamily. During embryonic development RORs are expressed in a spatial and temporal manner and are critical in the regulation of cellular differentiation and the development of several tissues. RORalpha plays a key role in the development of the cerebellum particularly in the regulation of the maturation and survival of Purkinje cells. In RORalpha-deficient mice, the reduced production of sonic hedgehog by these cells appears to be the major cause of the decreased proliferation of granule cell precursors and the observed cerebellar atrophy. RORalpha has been implicated in the regulation of a number of other physiological processes, including bone formation. RORbeta expression is largely restricted to several regions of the brain, the retina, and pineal gland. Mice deficient in RORbeta develop retinal degeneration that results in blindness. RORgamma is essential for lymph node organogenesis. In the intestine RORgamma is required for the formation of several other lymphoid tissues: Peyer's patches, cryptopatches, and isolated lymphoid follicles. RORgamma plays a key role in the generation of lymphoid tissue inducer (LTi) cells that are essential for the development of these lymphoid tissues. In addition, RORgamma is a critical regulator of thymopoiesis. It controls the differentiation of immature single-positive thymocytes into double-positive thymocytes and promotes the survival of double-positive thymocytes by inducing the expression of the anti-apoptotic gene Bcl-X(L). Interestingly, all three ROR receptors appear to play a role in the control of circadian rhythms. RORalpha positively regulates the expression of Bmal1, a transcription factor that is critical in the control of the circadian clock. This review intends to provide an overview of the current status of the functions RORs have in these biological processes.

Heterogeneity of Flt3-expressing multipotent progenitors in mouse bone marrow

Mechanisms of lymphoid and myeloid lineage choice by hemopoietic stem cells remain unclear. In this study we show that the multipotent progenitor (MPP) population, which is immediately downstream of hemopoietic stem cells, is heterogeneous and can be subdivided in terms of VCAM-1 expression. VCAM-1(+) MPPs were fully capable of differentiating into both lymphoid and myeloid lineages. In contrast, VCAM-1(-) MPPs gave rise to lymphocytes predominately in vivo. T and B cell development from VCAM-1(-) MPPs was 1 wk faster than that from VCAM-1(+) MPPs. Furthermore, VCAM-1(+) MPPs gave rise to common myeloid progenitors and VCAM-1(-) MPPs in vivo, indicating that VCAM-1(-) MPPs are progenies of VCAM-1(+) MPPs. VCAM-1(-) MPPs, in turn, developed into lymphoid lineage-restricted common lymphoid progenitors. These results establish a hierarchy of developmental relationship between MPP subsets and lymphoid and myeloid progenitors. In addition, VCAM-1(+) MPPs may represent the branching point between the lymphoid and myeloid lineages.

Expression of rearranged TCRgamma genes in natural killer cells suggests a minor thymus-dependent pathway of lineage commitment

Natural killer (NK) cells are thought to develop from common lymphoid progenitors in the bone marrow. However, immature thymocytes also retain NK potential. Currently, the contribution of the thymus-dependent pathway in normal steady-state NK-cell development is unknown. Here, we show that TCRgamma genes are rearranged in approximately 5% of neonatal and 1% of adult mouse splenic NK cells, and similar levels are detected in NK cells from TCRbeta,delta double-knockout mice, excluding the possibility of T-cell contamination. NK-cell TCRgamma gene rearrangement is thymus dependent because this rearrangement is undetectable in nude mouse NK cells. These results change the current view of NK-cell development and show that a subset of NK cells develops from immature thymocytes that have rearranged TCRgamma genes.

Unraveling the origin of lymphocyte progenitors

Lymphocytes are ultimately derived from hematopoietic stem cells, however the intervening steps that give rise to lymphocyte progenitors are still under intense investigation. In particular, whether a common lymphocyte progenitor serves exclusively as a lymphoid precursor, or whether this cell retains limited myeloid progenitor potential are addressed by Rolink and colleagues. This issue is highlighted by their identification within the bone marrow of early progenitors with lymphoid and myeloid developmental potential or EPLMs. How these cells fit into our current understanding of lymphocyte development, and how this new subset helps to further refine our view of the common lymphocyte progenitor are discussed.

Helix-loop-helix proteins in lymphocyte lineage determination

The cells of the lymphoid system develop from multipotent hematopoietic stem cells through a series of intermediate progenitors with progressively restricted developmental options. Commitment to a given lymphoid lineage appears to be controlled by numerous transcriptional regulatory proteins that activate lineage-specific gene expression programs and extinguish expression of lineage-inappropriate genes. In this review I discuss the function of transcription factors belonging to the helix-loop-helix protein family in the control of lymphoid cell fate decisions. A model of lymphocyte lineage determination based on the antagonistic activity of transcriptional activating and repressing helix-loop-helix proteins is presented.

Hematopoietic stem and progenitor cells: clinical and preclinical regeneration of the hematolymphoid system

A vast literature exists on the biology of blood formation and regeneration under experimental and clinical conditions. The field of hematopoiesis was recently advanced by the capacity to purify to homogeneity primitive hematopoietic stem and progenitor cells. Isolation of cells at defined maturational stages has enhanced the understanding of the fundamental nature of stem cells, including how cell fate decisions are made, and this understanding is relevant to the development of other normal as well as malignant tissues. This review updates the basic biology of hematopoietic stem cells (HSC) and progenitors, the evolving use of purified HSC as grafts for clinical hematopoietic cell transplantation (HCT) including immune tolerance induction, and the application of HSC biology to other stem cell fields.

Fate and function of lymphoid tissue inducer cells

The discovery that Peyer's patch and lymph node development is regulated by the collaboration between fetal hematopoietic cells and mesenchymal cells has thrown new light on our understanding of the mechanisms underlying the formation of lymphoid organs. Lymphoid tissue inducer cells trigger a coordinated series of events leading to cell clustering and changes in gene expression and differentiation. Nevertheless, many questions regarding the origin, recruitment and fate of the inducer cells and cellular crosstalk with neighboring cells remain unanswered.

PU.1 regulates the commitment of adult hematopoietic progenitors and restricts granulopoiesis

Although the transcription factor PU.1 is essential for fetal lymphomyelopoiesis, we unexpectedly found that elimination of the gene in adult mice allowed disturbed hematopoiesis, dominated by granulocyte production. Impaired production of lymphocytes was evident in PU.1-deficient bone marrow (BM), but myelocytes and clonogenic granulocytic progenitors that are responsive to granulocyte colony-stimulating factor or interleukin-3 increased dramatically. No identifiable common lymphoid or myeloid progenitor populations were discernable by flow cytometry; however, clonogenic assays suggested an overall increased frequency of blast colony-forming cells and BM chimeras revealed existence of long-term self-renewing PU.1-deficient cells that required PU.1 for lymphoid, but not granulocyte, generation. PU.1 deletion in granulocyte-macrophage progenitors, but not in common myeloid progenitors, resulted in excess granulocyte production; this suggested specific roles of PU.1 at different stages of myeloid development. These findings emphasize the distinct nature of adult hematopoiesis and reveal that PU.1 regulates the specification of the multipotent lymphoid and myeloid compartments and restrains, rather than promotes, granulopoiesis.

Novel tumor necrosis factor-knockout mice that lack Peyer's patches

We generated a novel tumor necrosis factor (TNF) null mutation using Cre-loxP technology. Mice homozygous for this mutation differ from their "conventional" counterparts; in particular, they completely lack Peyer's patches (PP) but retain all lymph nodes. Our analysis of these novel TNF-knockout mice supports the previously disputed notion of the involvement of TNF-TNFR1 signaling in PP organogenesis. Availability of TNF-knockout strains both with and without PP enables more definitive studies concerning the roles of TNF and PP in various immune functions and disease conditions. Here, we report that systemic ablation of TNF, but not the presence of PP per se, is critical for protection against intestinal Listeria infection in mice.

Identification of Flt3+ lympho-myeloid stem cells lacking erythro-megakaryocytic potential a revised road map for adult blood lineage commitment

All blood cell lineages derive from a common hematopoietic stem cell (HSC). The current model implicates that the first lineage commitment step of adult pluripotent HSCs results in a strict separation into common lymphoid and common myeloid precursors. We present evidence for a population of cells which, although sustaining a high proliferative and combined lympho-myeloid differentiation potential, have lost the ability to adopt erythroid and megakaryocyte lineage fates. Cells in the Lin-Sca-1+c-kit+ HSC compartment coexpressing high levels of the tyrosine kinase receptor Flt3 sustain granulocyte, monocyte, and B and T cell potentials but in contrast to Lin-Sca-1+c-kit+Flt3- HSCs fail to produce significant erythroid and megakaryocytic progeny. This distinct lineage restriction site is accompanied by downregulation of genes for regulators of erythroid and megakaryocyte development. In agreement with representing a lymphoid primed progenitor, Lin-Sca-1+c-kit+CD34+Flt3+ cells display upregulated IL-7 receptor gene expression. Based on these observations, we propose a revised road map for adult blood lineage development.

Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent "priming" of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

Visualizing PU.1 activity during hematopoiesis

OBJECTIVE

PU.1 is a critical transcription factor for hematopoietic development that is required for the early differentiation of myeloid, erythroid, and B lineage cells. To gain a better insight into PU.1 function, we performed a comprehensive analysis of PU.1 gene activity in the hematopoietic system, using a green fluorescent protein reporter mouse line.

METHODS

We used flow cytometry to analyze green fluorescent protein (GFP) expression, along with various cell surface markers, in heterozygote mice that harbor a GFP reporter knocked into exon1 of the PU.1 gene. Phenotypic and functional properties of GFP+ and GFP- precursors were studied.

RESULTS

We show that PU.1 is dynamically and heterogeneously expressed in many hematopoietic lineages, from the stem cell stage to terminally differentiated cells, suggesting that PU.1 is not only important in early differentiation events but also may play a role in mature hematopoietic cell function. Further, examination of GFP+ vs GFP- populations shows that differentiation, but not commitment, to the myeloid lineage requires PU.1. In contrast, B cell commitment is associated with low levels of PU.1 expression.

CONCLUSION

Our study provides a detailed visualization of PU.1 gene activity in hematopoietic cells, and shows that highly dynamic regulation of PU.1 accompanies cell fate decisions during hematopoiesis.

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.

Inducible lymphoid tissues in the adult gut: recapitulation of a fetal developmental pathway?

The intestinal immune system faces an extraordinary challenge from the large numbers of commensal bacteria and potential pathogens that are restrained by only a single layer of epithelial cells. Here, I discuss evidence that the intestinal immune system develops an extensive network of inducible, reversible lymphoid tissues that contributes to the vital equilibrium between the gut and the bacterial flora. I propose that this network is induced by cryptopatches, which are small clusters of dendritic cells and lymphoid cells that are identical to fetal inducers of lymph-node and Peyer's-patch development.

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.

Interleukin-7 is necessary to maintain the B cell potential in common lymphoid progenitors

Interleukin-7 (IL-7) promotes survival and expansion of lymphoid precursors. We show here that, in addition, IL-7 has a fundamental role, as early as the stage of the multipotent (B/T/NK) common lymphoid progenitor (CLP), in maintaining the B cell differentiation program open. CLPs generated in the absence of IL-7 have normal T/NK differentiation potential, but severely impaired B potential. Accordingly, CLPs from IL-7–deficient mice express lower amounts of early B cell factor (EBF) and Pax5 than wild-type CLPs, but similar amounts of GATA-3. Importantly, induced overexpression of EBF is sufficient to restore the B potential in these cells. These results indicate that IL-7 directs commitment of CLPs by modulating EBF expression. This is the first example of a cytokine influencing lymphoid lineage commitment in multipotent progenitors and highlights the relevance of the expression of a functional IL-7 receptor at the CLP stage.

Thymic Regulation--Hidden in Plain Sight

The thymus is the nursery where T cell precursors mature into T cells of the and lineages. In a Perspective, Rothenberg discusses intriguing findings that reveal a new level of regulation in the thymus (Silva-Santos et al.). Apparently, double-positive T cells that are destined to mature into T cells directly regulate the development of the T-cell lineage through the production of lymphotoxin and the action of a transcription factor called RORt.

Interleukin-7 receptor alpha (IL-7Ralpha) deficiency: cellular and molecular bases. Analysis of clinical, immunological, and molecular features in 16 novel patients

Analysis of gene-targeted mice and patients with severe combined immunodeficiency due to mutations of the alpha chain of the interleukin-7 receptor (IL-7Ralpha) has shown important differences between mice and humans in the role played by IL-7 in lymphoid development. More recently, it has been shown that IL-7Ralpha is also shared by the receptor for another cytokine, thymic stromal lymphopoietin (TSLP). In this review, we discuss recent advances in IL-7- and TSLP-mediated signaling. We also report on the clinical and immunological features of 16 novel patients with IL-7Ralpha deficiency and discuss the results of hematopoietic stem cell transplantation.

Ontogeny and regulation of IL-7-expressing thymic epithelial cells

Epithelial cells in the thymus produce IL-7, an essential cytokine that promotes the survival, differentiation, and proliferation of thymocytes. We identified IL-7-expressing thymic epithelial cells (TECs) throughout ontogeny and in the adult mouse thymus by in situ hybridization analysis. IL-7 expression is initiated in the thymic fated domain of the early primordium by embryonic day 11.5 and is expressed in a Foxn1-independent pathway. Marked changes occur in the localization and regulation of IL-7-expressing TECs during development. IL-7-expressing TECs are present throughout the early thymic rudiment. In contrast, a major population of IL-7-expressing TECs is localized to the medulla in the adult thymus. Using mouse strains in which thymocyte development is arrested at various stages, we show that fetal and postnatal thymi differ in the frequency and localization of IL-7-expressing TECs. Whereas IL-7 expression is initiated independently of hemopoietic-derived signals during thymic organogenesis, thymocyte-derived signals play an essential role in regulating IL-7 expression in the adult TEC compartment. Moreover, different thymocyte subsets regulate the expression of IL-7 and keratin 5 in adult cortical epithelium, suggesting that despite phenotypic similarities, the cortical TEC compartments of wild-type and RAG-1(-/-) mice are developmentally and functionally distinct.

Intratracheal administration of liposomal clodronate accelerates alveolar macrophage reconstitution following fetal liver transplantation

To facilitate study of alveolar macrophages in vivo, we developed a method to rapidly and efficiently replace resident alveolar macrophages with macrophages of a different (donor) genotype. Chimeric mice were generated by lethal irradiation followed by fetal liver transplantation (FLT) using green fluorescent protein (GFP) transgenic reporter mice as donors. Kinetics of peripheral blood monocyte (PBM) and alveolar macrophage reconstitution was determined 4 and 10 weeks post-FLT by quantifying the percentage of GFP+ cells. To enhance the recruitment of donor monocytes into the lung after FLT, mice were treated with intratracheal administration of liposomal clodronate to deplete host alveolar macrophages at 6 weeks post-FLT. PBM reconstitution occurred by 4 weeks after FLT (85.7+/-1.6% of CD11b+/Gr-1+ monocytes were GFP+), and minimal alveolar macrophage repopulation was observed (9.5% GFP+). By 10 weeks following FLT, 48% of alveolar macrophages were GFP+ by immunostaining of macrophages on lung tissue sections, and 55.1 +/- 1.6% of lung lavage macrophages were GFP+ by fluorescein-activated cell sorter analysis. Clodronate treatment resulted in a significant increase in GFP+ alveolar macrophages 10 weeks after FLT. By immunostaining, 90% of macrophages were GFP+ on lung tissue sections and 87.5 +/- 1.1% GFP+ in lung lavage (compared with GFP-transgenic controls). The ability of newly recruited alveolar macrophages to clear Pseudomonas aeruginosa and activate nuclear factor-kappaB in response to Eschericia coli lipopolysaccharide demonstrated normal macrophage function. Optimizing this methodology provides an important tool for the study of specific genes and their contribution to alveolar macrophage function in vivo.

Haemopoietic progenitors in the adult mouse omentum: permanent production of B lymphocytes and monocytes

The coelome-associated lympho-myeloid tissues, including the omentum, are derived from early embryo haemopoietic tissue of the splanchnopleura, and produce B lymphocytes and macrophages. They are reactive in pathologies involving coelomic cavities, in which they can expand in situ the cells of inflammatory infiltrates. We have addressed the question of the role of the adult omentum in permanent basal production of early lymphopoietic progenitors (pro-B/pre-B cells), through characterisation of omentum cells ex vivo, and study of their in vitro differentiation. We have shown that the murine omentum produces early haemopoietic progenitors throughout life, including B-cell progenitors prior to the Ig gene recombination expressing RAG-1 and lambda5, as well as macrophages. Their production is stroma-dependent. The omentum stroma can supply in vitro the cytokines (SDF-1alpha, Flt3 ligand and IL-7) and the molecular environment required for generation of these two cell lineages. Omentum haemopoietic progenitors are similar to those observed in foetal blood cell production, rather than to progenitors found in the adult haemopoietic tissue in the bone marrow--in terms of phenotype expression and differentiation capacity. We conclude that a primitive pattern of haemopoiesis observed in the early embryo is permanently preserved and functional in the adult omentum, providing production of cells engaged in nonspecific protection of abdominal intestinal tissue and of the coelomic cavity.

Presumptive lymph node organizers are differentially represented in developing mesenteric and peripheral nodes

During murine embryogenesis, the formation of Peyer's patches (PPs) is initiated by CD45(+)CD4(+)CD3(-) lymphoid tissue inducers that trigger adhesion molecule expression and specific chemokine production from an organizing stromal cell population through ligation of the lymphotoxin-beta receptor. However, the steps involved in the development of lymph nodes (LNs) are less clear than those of PPs, and the characteristics of the organizing cells within the LN anlagen have yet to be documented. In this study, we show for the first time that the early anlage is bordered by an endothelial layer that retains a mixed lymphatic and blood vascular phenotype up to embryonic day 16.5. This in turn encompasses CD45(+)CD4(+)CD3(-) cells interspersed with ICAM-1/VCAM-1/mucosal addressin cell adhesion molecule-1, lymphotoxin-beta receptor-positive, chemokine-producing cells analogous to the organizing population previously observed in PPs. Moreover, these LN organizers also express the TNF family member, TRANCE. Lastly, we show that the ICAM-1/VCAM-1/mucosal addressin cell adhesion molecule-1 cells present in peripheral and mesenteric LN form two discrete populations expressing either intermediate or high levels of these adhesion molecules but that the former population is specifically reduced in PLN. These findings provide a possible explanation for the well-known differences in developmental requirements for nodes at peripheral or mesenteric locations.

Development of CD4+Macrophages from Intrathymic T Cell Progenitors Is Induced by Thymic Epithelial Cells

It was recently demonstrated that there are CD4(+) macrophages, which exhibit strong phagocytic activity, in the thymus. They are suggested to play an important role for the elimination of apoptotic thymocytes. However, the origin and nature of CD4(+) macrophages in the thymus remain unexplored. In this study, we describe that the most immature intrathymic progenitors (CD25(-)/CD44(+)/FcR(+)) give rise to CD4(+) macrophages by oncostatin M-responsive thymic epithelial cells (ORTEC) in an IL-7-dependent manner. Neither conditioned medium of ORTEC nor a mixture of cytokines induced CD4(+) macrophages, and oncostatin M receptor was not expressed in thymocytes, suggesting that the development of CD4(+) macrophages from the immature thymocytes requires a direct interaction with ORTEC. These results collectively suggest that the development of CD4(+) macrophages from the intrathymic T cell progenitors is induced by thymic epithelial cells.

Role of chemokines in the development of secondary and tertiary lymphoid tissues

Organogenesis of lymph nodes and Peyer's patches is initiated during embryonic development and depends on the correct expression of a wide variety of molecules. Essential for this process is the paracrine triggering of stromal cells by CD45+CD4+CD3- cells. Hereto CD45+CD4+CD3- cells and stromal cells need to be spatially positioned in close proximity to each other. Expression of adhesion molecules and chemokines is thought to be essential for this process. During adult life, similar processes might also be at the basis of development of organized tertiary lymphoid structures often seen in inflammatory lesions.

Thymic origin of intestinal alphabeta T cells revealed by fate mapping of RORgammat+ cells

Intestinal intraepithelial T lymphocytes (IELs) are likely to play a key role in host mucosal immunity and, unlike other T cells, have been proposed to differentiate from local precursors rather than from thymocytes. We show here that IELs expressing the alphabeta T cell receptor are derived from precursors that express RORgammat, an orphan nuclear hormone receptor detected only in immature CD4+CD8+ thymocytes, fetal lymphoid tissue-inducer (LTi) cells, and LTi-like cells in cryptopatches within the adult intestinal lamina propria. Using cell fate mapping, we found that all intestinal alphabeta T cells are progeny of CD4+CD8+ thymocytes, indicating that the adult intestine is not a significant site for alphabeta T cell development. Our results suggest that intestinal RORgammat+ cells are local organizers of mucosal lymphoid tissue.

Characterization of a novel bipotent hematopoietic progenitor population in normal and osteopetrotic mice

Several reports indicate that osteoclasts and B-lymphocytes share a common progenitor. This study focuses on the characterization of this bipotent progenitor from the bone marrow of the osteopetrotic oc/oc mouse, where the bipotent progenitor population is amplified, and of normal mice.

INTRODUCTION

Osteoclasts have a myelomonocytic origin, but they can also arise in vitro from pro-B-cells, suggesting that a subset of normal pro-B-cells is uncommitted and may reorient into the myeloid lineage representing a B-lymphoid/osteoclastic progenitor. The aim of this study was to characterize this progenitor population.

MATERIALS AND METHODS

The osteopetrotic oc/oc mouse was used as a choice model because it displays an increased number of both osteoclasts and pro-B-cells in the bone marrow. Our results have been confirmed in normal littermates. Bone marrow cells from these animals were analyzed by flow cytometry. After sorting, the cells were cultured under different conditions to assess their differentiation capacity.

RESULTS

Pro-B-cells from oc/oc and normal mice include an unusual biphenotypic population expressing markers from the B-lymphoid (CD19, CD43, CD5) and the myeloid (F4/80) lineages. This population also expresses progenitor markers (CD34 and Flt3) and is uncommitted. After sorting from the oc/oc bone marrow, this population is able to differentiate in vitro into osteoclast-like cells in the presence of RANKL and macrophage colony-stimulating factor (M-CSF), into dendritic-like cells in the presence of granulocyte/macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-4, and TNFalpha, and into immature B-cells when seeded onto ST2 cells in the presence of IL-7.

CONCLUSION

Our results show the existence of a novel bipotent biphenotypic hematopoietic progenitor population present in the bone marrow that has retained the capacity to differentiate into myeloid and B-lymphoid cells.

Transcriptional control of B cell development and function

The generation, development, maturation and selection of mammalian B lymphocytes is a complex process that is initiated in the embryo and proceeds throughout life to provide the organism an essential part of the immune system it requires to cope with pathogens. Transcriptional regulation of this highly complex series of events is a major control mechanism, although control is also exerted on all other layers, including splicing, translation and protein stability. This review summarizes our current understanding of transcriptional control of the well-studied murine B cell development, which bears strong similarity to its human counterpart. Animal and cell models with loss of function (gene "knock outs") or gain of function (often transgenes) have significantly contributed to our knowledge about the role of specific transcription factors during B lymphopoiesis. In particular, a large number of different transcriptional regulators have been linked to distinct stages of the life of B lymphocytes such as: differentiation in the bone marrow, migration to the peripheral organs and antigen-induced activation.