Dendritic cell potentials of early lymphoid and myeloid progenitors

Asynchronous RAG-1 expression during B lymphopoiesis

Changes in cell surface markers and patterns of gene expression are commonly used to construct sequences of events in hematopoiesis. However, the order may not be as rigid as once thought and it is unclear which changes represent the best milestones of differentiation. We developed a fate-mapping model where cells with a history of RAG-1 expression are permanently marked by red fluorescence. This approach is valuable for appreciating lymphoid-lineage relationships without need for irradiation and transplantation. Hematopoietic stem cells (HSC) as well as myeloid and dendritic cell progenitors were unlabeled. Also as expected, most previously identified RAG-1(+) early lymphoid progenitors in bone marrow and all lymphoid-affiliated cells were marked. Of particular interest, there was heterogeneity among canonical common lymphoid progenitors (CLP) in bone marrow. Labeled CLP expressed slightly higher levels of IL-7Ralpha, displayed somewhat less c-Kit, and generated CD19(+) lymphocytes faster than the unlabeled CLP. Furthermore, CLP with a history of RAG-1 expression were much less likely to generate dendritic and NK cells. The RAG-1-marked CLP were lineage stable even when exposed to LPS, while unlabeled CLP were redirected to become dendritic cells in response to this TLR4 ligand. These findings indicate that essential events in B lymphopoiesis are not tightly synchronized. Some progenitors with increased probability of becoming lymphocytes express RAG-1 while still part of the lineage marker-negative Sca-1(+)c-Kit(high) (LSK) fraction. Other progenitors first activate this locus after c-Kit levels have diminished and cell surface IL-7 receptors are detectable.

The many face-lifts of CD4 T helper cells

Recent advances in stem cell research have redefined previous concepts of hematopoietic hierarchy, lineage commitment, and cell fate. The immune system is comprised of several well-defined cell lineages of which many exhibit high levels of plasticity or capacity in changing their phenotype. The CD4 T helper cells provide a peculiar example of apparently defined cell subsets, at times described as lineages, but also highly sensitive to tissue environmental cues that may change their fate. The classical Th1/Th2 CD4 T cell differentiation referred to for many years as the main CD4 T cell fate dichotomy and the later additions of CD4 helper T cell variants, such as T helper 17 (Th17) and induced regulatory T cells (iTreg), have added complexity but also doubts on the accuracy of defining CD4 T cell subsets as fixed T cell lineages.

Generation of large numbers of pro-DCs and pre-DCs in vitro

The CD8(+), CD8(-), and plasmacytoid dendritic cell (DC) subtypes develop from progenitors that express surface fms-like tyrosine kinase 3 (Flt3). Recently, two developmentally sequential progenitors have been identified that give rise to these subtypes. This includes a transition from an earlier CD11c(-)MHC-II(-) "pro-DC," which divides and differentiates to give rise to CD11c(+)MHC-II(-) "pre-DC," en route to generating the three CD11c(+)MHC-II(+) DC subtypes - plasmacytoid DCs, CD8(+) DCs, and CD8(-) DCs. In this chapter, we describe the very simple method of generating large numbers of in vitro-derived pro-DCs and pre-DCs. As these precursors are largely DC-restricted, they can be used to either reconstitute a mouse with DCs of desired background, to study the developmental steps in vitro or in vivo, among other purposes.

Progenitor cell origin plays a role in fate choices of mature B cells

B cells, the Ab-producing cells of the immune system, develop from hematopoietic stem cells (HSCs) through well-defined stages during which Ig genes are rearranged to generate a clonal BCR. Signaling through the BCR plays a role in the subsequent cell fate decisions leading to the generation of three distinct types of B cells: B1, marginal zone, and follicular B cells. Common lymphoid progenitors (CLPs) are descended from HSCs, and although recent observations suggest that CLPs may not be physiological T cell precursors, it is generally accepted that CLPs are obligate progenitors for B cells. In addition, a CLP-like progenitor of unknown significance that lacks expression of c-kit (kit(-)CLP) was recently identified in the mouse model. In this study, we show that CLPs, kit(-)CLPs and a population within the lin(-)Sca1(+)kit(+)flt3(-) HSC compartment generate mature B cell types in different proportions: CLPs and kit(-)CLPs show a stronger marginal zone/follicular ratio than lin(-)Sca1(+)kit(+)flt3(-) cells, whereas kit(-)CLPs show a stronger B1 bias than any other progenitor population. Furthermore, expression of Sca1 on B cells depends on their progenitor origin as B cells derived from CLPs and kit(-)CLPs express more Sca1 than those derived from lin(-)Sca1(+)kit(+)flt3(-) cells. These observations indicate a role for progenitor origin in B cell fate choices and suggest the existence of CLP-independent B cell development.

Activated liver dendritic cells generate strong acquired immunity in alpha-galactosylceramide treatment

BACKGROUND/AIMS

Alpha-galactosylceramide (alpha-GalCer) presented by dendritic cells (DCs) activates NKT cells that in turn drive DC maturation. However, the potential of generating acquired immunity of liver DCs in alpha-GalCer treatment remains unclear.

METHODS

We examined the activation of acquired immunity in the alpha-GalCer treatment against liver or spleen tumor and the ability of liver and spleen DCs in the generation of acquired immunity.

RESULTS

Administration of alpha-GalCer resulted in generation of p53 peptide-specific cytotoxic T lymphocytes (CTLs) in mice bearing liver CMS4 tumor, aberrantly expressing p53, but not in mice bearing spleen CMS4 tumor. The growth of rechallenged CMS4 subcutaneous tumor was inhibited in alpha-GalCer-treated mice against liver CMS4 tumor, but not in alpha-GalCer-treated mice against CMS4 spleen tumor. The antigen presenting related functions of liver DCs were significantly higher than those of spleen DCs in alpha-GalCer-treated mice. Vaccination of normal mice with p53 peptide pulsed liver DCs isolated from alpha-GalCer treated mice resulted in generation of p53 peptide-specific CTLs, but that with p53 peptide pulsed spleen DCs did not.

CONCLUSIONS

These results demonstrated that alpha-GalCer treatment induced unique immunologic activation of liver DCs in comparison with spleen DCs, which might be favorable to generate liver acquired immunity.

A novel cell-surface protein CSP82 on bone marrow stem cells and a cytosolic phosphoprotein DP58 (ankyrinRD 34B) are involved in promyeloid progenitor induction

The molecular events associated with the development of common myeloid progenitor (CMP) remain largely unknown. This study reports that a novel glycosylphosphatidylinositol (GPI)-anchored lactoferrin CSP82 on uninitiated mouse bone marrow cells (BMC) may be involved in inducing pro-DC from CMP. By peptide mass fingerprinting, CSP82 has been identified as the mouse lactoferrin precursor, but unlike the latter, it occurs as a GPI-linked cell-surface protein. The GPI-linkage was demonstrated on BMC-derived immunoprecipitates and by other techniques. Furthermore, BMC and hematopoietic stem BM cells following incubation with either CSP82 peptide antibody or purified Reagent A yielded CMP-like progenitors (BM4 cells). These progenitors expressed a previously reported cytosolic phosphoprotein DP58 (AnkRD 34B protein). Continued cultivation of BMC in media containing only anti-CSP82 antibody led to DC-like cells, that bore phenotypic and endocytic resemblance with those obtained using GM-CSF. The results suggest that a receptor lactoferrin on BMC may be an important non-cytokine mechanism for early promyeloid progenitor differentiation.

Bacterial signalling overrides cytokine signalling and modifies dendritic cell differentiation

Heterogeneity of dendritic cells (DC) is evident in the gut-associated lymphoid tissue and determined, in part, by incompletely understood local environmental factors. Bacterial signalling is likely to be a dominant influence on precursor cells when recruited to the mucosa. We assessed the influence of commensal bacteria on DC differentiation and function. Murine bone marrow progenitors were exposed to Lactobacillus salivarius, Bifidobacterium breve or Bifidobacterium infantis. Differences in cell surface phenotype and function were assessed. Myeloid differentiation factor 88(-/-) (MyD88) cells were used to determine the influence of Toll-like receptor signalling. While bacterial strains varied in impact, there was a consistent dose-dependent inhibition of DC differentiation with a shift toward a Gr-1(+) CD11b(+) monocyte-like phenotype. A single bacterium on a per cell basis (1 : 1) was sufficient to alter cell phenotype. The effect was only evident in early precursors. Enhanced interleukin-10 production correlated with increased Forkhead box P3 expression and reduced T-cell proliferation. The bacterial effect on DC differentiation was found to be MyD88-dependent. Signalling by enteric commensals through pattern recognition receptors on precursor cells alters DC differentiation and results in cells that are phenotypically monocyte-like and functionally suppressive. This may account for some of the features of mucosal immune tolerance to the microbiota.

The balance between immunity and tolerance: the role of Langerhans cells

Langerhans cells are immature skin-homing dendritic cells that furnish the epidermis with an immune surveillance system, and translate information between the internal and external milieu. Dendritic cells, in particular Langerhans cells, are gaining prominence as one of the potential principal players orchestrating the decision between immunity and tolerance. Langerhans cells capture aberrant self-antigen and pathogen-derived antigen for display to the efferent immune response. Recent evidence suggests redundancy in the antigen-presenting function of Langerhans cells, with dermal dendritic subsets capable of fulfilling an analogous role. There is mounting evidence that Langerhans cells can cross-prime T cells to recognize antigens. Langerhans cells are proposed to stimulate T regulatory cells, and are implicated in the pathogenesis of cutaneous T cell lymphoma.The phenotype of Langerhans cells, which may be tolerogenic or immunogenic, appears to depend on their state of maturity, inciting immunogen and cytokine environment, offering the potential for manipulation in immunotherapy.

Novel function for interleukin-7 in dendritic cell development

Interleukin-7 (IL-7) is crucial for the development of T and B lymphocytes from common lymphoid progenitors (CLPs) and for the maintenance of mature T lymphocytes. Its in vivo role for dendritic cells (DCs) has been poorly defined. Here, we investigated whether IL-7 is important for the development or maintenance of different DC types. Bone marrow-derived DCs expressed the IL-7 receptor (IL-7R) and survived significantly longer in the presence of IL-7. Migratory DCs (migDCs) isolated from lymph nodes also expressed IL-7R. Surprisingly, IL-7R was not required for their maintenance but indirectly for their development. Conventional DCs (cDCs) and plasmacytoid DCs (pDCs) resident in lymph nodes and spleen were IL-7R(-). Using mixed bone marrow chimeras, we observed an intrinsic requirement for IL-7R signals in their development. As the number of CLPs but not myeloid progenitors was reduced in the absence of IL-7 signals, we propose that a large fraction of cDCs and pDCs derives from CLPs and shares not only the lymphoid origin but also the IL-7 requirement with lymphocyte precursors.

New insights into the roles of dendritic cells in intestinal immunity and tolerance

Dendritic cells (DCs) play a critical key role in the initiation of immune responses to pathogens. Paradoxically, they also prevent potentially damaging immune responses being directed against the multitude of harmless antigens, to which the body is exposed daily. These roles are particularly important in the intestine, where only a single layer of epithelial cells provides a barrier against billions of commensal microorganisms, pathogens, and food antigens, over a huge surface area. In the intestine, therefore, DCs are required to perform their dual roles very efficiently to protect the body from the dual threats of invading pathogens and unwanted inflammatory reactions. In this review, we first describe the biology of DCs and their interactions with other cells types, paying particular attention to intestinal DCs. We, then, examine the ways in which this biology may become misdirected, resulting in inflammatory bowel disease. Finally, we discuss how DCs potentiate immune responses against viral, bacterial, parasitic infections, and their importance in the pathogenesis of prion diseases. We, therefore, provide an overview of the complex cellular interactions that affect intestinal DCs and control the balance between immunity and tolerance.

Dendritic cell homeostasis

Dendritic cells (DCs) are a heterogeneous fraction of rare hematopoietic cells that coevolved with the formation of the adaptive immune system. DCs efficiently process and present antigen, move from sites of antigen uptake to sites of cellular interactions, and are critical in the initiation of immune responses as well as in the maintenance of self-tolerance. DCs are distributed throughout the body and are enriched in lymphoid organs and environmental contact sites. Steady-state DC half-lives account for days to up to a few weeks, and they need to be replaced via proliferating hematopoietic progenitors, monocytes, or tissue resident cells. In this review, we integrate recent knowledge on DC progenitors, cytokines, and transcription factor usage to an emerging concept of in vivo DC homeostasis in steady-state and inflammatory conditions. We furthermore highlight how knowledge of these maintenance mechanisms might impact on understanding of DC malignancies as well as posttransplant immune reactions and their respective therapies.

The origins of the identification and isolation of hematopoietic stem cells, and their capability to induce donor-specific transplantation tolerance and treat autoimmune diseases

Advances in the understanding of the cells of the hematopoietic system have provided a rich basis for improving clinical hematopoietic cell transplants; finding and using proteins and molecules to amplify or suppress particular blood cell types; understanding the stepwise progression of preleukemic stages leading first to chronic myeloid disorders, then the emergence of acute blastic leukemias; and treating malignant and nonmalignant diseases with cell subsets. As a result of intense scientific investigation, hematopoietic stem cells (HSCs) have been isolated and their key functional characteristics revealed-self-renewal and multilineage differentiation. These characteristics are now found to be present in all tissue/organ stem cell studies, and even in the analysis of pluripotent embryonic, nuclear transfer, and induced pluripotent stem cells. Studies on HSC have identified hematopoiesis as one of the best systems for studying developmental cell lineages and as the best for understanding molecular changes in cell fate decision-making and for finding preclinical and clinical platforms for tissue and organ replacement, regeneration, and oncogenesis. Here we review the steps, from our viewpoint, that led to HSC isolation and its importance in self-nonself immune recognition.

The steady-state development of splenic dendritic cells

Dendritic cells (DCs) are a heterogenous population of cells that can be grouped into the conventional DCs (cDCs) and plasmacytoid DCs (pDCs), or interferon-producing cells. pDCs are thought to develop in the bone marrow and migrate to the periphery as mature cells. In contrast, cDC precursors are thought to migrate to the periphery, where they further differentiate into cDCs. In the case of migratory cDCs, these precursors are thought to be monocytes, whereas resident cDCs derive from a different precursor. Recent activity on this subject has shed some light on the precursors that differentiate into resident cDCs and pDCs, but often with conflicting findings. Here, we review some of these findings and discuss some of the outstanding issues in the field.

Reductive isolation from bone marrow and blood implicates common lymphoid progenitors as the major source of thymopoiesis

Ongoing thymopoiesis requires continual seeding from progenitors that reside within the bone marrow (BM), but the identity of the most proximate prethymocytes has remained controversial. Here we take a comprehensive approach to prospectively identify the major source of thymocyte progenitors that reside within the BM and blood, and find that all thymocyte progenitor activity resides within a rare Flk2(+)CD27(+) population. The BM Flk2(+)CD27(+) subset is predominantly composed of common lymphoid progenitors (CLPs) and multipotent progenitors. Of these 2 populations, only CLPs reconstitute thymopoiesis rapidly after intravenous injection. In contrast, multipotent progenitor-derived cells reconstitute the thymus with delayed kinetics only after they have reseeded the BM, self-renewed, and generated CLPs. These results identify CLPs as the major source of thymocyte progenitors within the BM.

Effects of chronic ethanol feeding on murine dendritic cell numbers, turnover rate, and dendropoiesis

BACKGROUND

Chronic alcoholics have increased susceptibility to and severity of infection, which are likely to be a result of impaired immune defense mechanisms. The contribution of dendritic cells (DC) to these immune defense changes is not well understood. Alterations in DC numbers, dendropoiesis, and lifespan have not been specifically studied in vivo in chronic ethanol (EtOH) exposure models. As DC play an essential role in initiating immune responses, alterations in these DC characteristics would help explain changes observed in adaptive immune responses.

METHODS

Mice received 20% EtOH (w/v) in the drinking water for up to 28 weeks, with mouse chow ad libitum. In EtOH-fed and water control mice, DC were enumerated by flow cytometry. The effect of EtOH on DC precursor numbers was determined by differentiation in vitro in the presence of granulocyte-macrophage colony-stimulating factor and interleukin-4, and the effect of an EtOH environment on untreated DC differentiation was measured following bone marrow transfer to irradiated hosts. DC turnover rate was also examined by bromodeoxyuridine incorporation and loss.

RESULTS

The percentage and absolute numbers of DC were decreased in spleen and increased in thymus beginning as early as 4 weeks of EtOH feeding. In addition, the overall cellularity of spleen and thymus were altered by this regimen. However, chronic EtOH consumption did not adversely affect DC precursor numbers, differentiation abilities, or turnover rates.

CONCLUSIONS

Decreased splenic DC numbers observed following chronic murine EtOH consumption are not because of altered DC precursor numbers or differentiation, nor increased DC turnover rate. Similarly, increased thymic DC numbers are not the result of alterations in DC precursor differentiation or turnover rate. Compartment size plays a role in determining splenic and thymic DC numbers following chronic EtOH feeding. EtOH-induced alterations in total DC numbers provide several mechanisms to partially explain why chronic alcoholics have increased susceptibility to infections.

Space-time considerations for hematopoietic stem cell transplantation

The mammalian blood system contains a multitude of distinct mature cell lineages adapted to serving diverse functional roles. Mutations that abrogate the development or function of one or more of these lineages can lead to profound adverse consequences, such as immunodeficiency, autoimmunity, or anemia. Replacement of hematopoietic stem cells (HSC) that carry such mutations with HSC from a healthy donor can reverse such disorders, but because the risks associated with the procedure are often more serious than the blood disorders themselves, bone marrow transplantation is generally not used to treat a number of relatively common inherited blood diseases. Aside from a number of other problems, risks associated with cytoreductive treatments that create "space" for donor HSC, and the slow kinetics with which immune competence is restored following transplantation hamper progress. This review will focus on how recent studies using experimental model systems may direct future efforts to implement routine use of HSC transplantation to cure inherited blood disorders.

Distinct functional capacities of mouse thymic and splenic dendritic cell populations

Dendritic cells (DC) are antigen-presenting cells that activate naive T cells. Murine DC are a heterogeneous population and can be subdivided into distinct subsets with different immune regulatory functions, namely the conventional DC (cDC), which include the CD8(+)Sirpalpha(-) and CD8(-)Sirpalpha(+) DC, and the plasmacytoid DC (pDC). In this study, the phenotype and function of DC subsets in both the thymus and spleen were compared. Significant differences between the thymic and splenic DC were observed in the expression of genes encoding chemokine receptors (CCRs), toll-like receptors (TLRs) and chemokines. Thymic DC expressed high levels of genes encoding a unique set of chemokines (CCL17 and CCL22) known to be important for T-cell development. Moreover, the capacity of the DC from the two organs to produce IL-6, IFN-alpha and IL-12p70 in response to the TLR 9 agonist CpG differed markedly, indicating intrinsic functional differences between subsets with similar surface phenotype. These results indicate that the microenvironment is an important factor that contributes to the functional specification of DC subsets in different lymphoid tissues.

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.

Notch signaling in differentiation and function of dendritic cells

Hematopoietic stem cells give rise to multiple lineages of cells. This process is governed by a tightly controlled signaling network regulated by cytokines and a direct cell-cell contact. Notch signaling represents one of the major pathways activated during direct interaction between hematopoietic progenitor cells and bone marrow stroma. A critical role of Notch signaling in differentiation of T- and B-lymphocytes has now been established. Until recently, the role of Notch signaling in the development of myeloid cells and particular dendritic cells remained unclear. In this review, we discuss recent exciting findings that shed light on the critical role of Notch in differentiation and the function of dendritic cells and its impact on immune responses.

The TNF receptor and Ig superfamily members form an integrated signaling circuit controlling dendritic cell homeostasis

Dendritic cells (DC) constitute the most potent antigen presenting cells of the immune system, playing a key role bridging innate and adaptive immune responses. Specialized DC subsets differ depending on their origin, tissue location and the influence of trophic factors, the latter remain to be fully understood. Myeloid-associated lymphotoxin-beta receptor (LTbetaR) signaling is required for the local proliferation of lymphoid tissue DC. This review focuses on the LTbetaR signaling cascade as a crucial positive trophic signal in the homeostasis of DC subsets. The noncanonical coreceptor pathway comprised of the immunoglobulin (Ig) superfamily member, B and T lymphocyte attenuator (BTLA) and TNFR superfamily member, herpesvirus entry mediator (HVEM) counter regulates the trophic signaling by LTbetaR. Together both pathways form an integrated signaling circuit achieving homeostasis of DC subsets.

Malignant transformation initiated by Mll-AF9: gene dosage and critical target cells

The pathways by which oncogenes, such as MLL-AF9, initiate transformation and leukemia in humans and mice are incompletely defined. In a study of target cells and oncogene dosage, we found that Mll-AF9, when under endogenous regulatory control, efficiently transformed LSK (Lin(-)Sca1(+)c-kit(+)) stem cells, while committed granulocyte-monocyte progenitors (GMPs) were transformation resistant and did not cause leukemia. Mll-AF9 was expressed at higher levels in hematopoietic stem (HSC) than GMP cells. Mll-AF9 gene dosage effects were directly shown in experiments where GMPs were efficiently transformed by the high dosage of Mll-AF9 resulting from retroviral transduction. Mll-AF9 upregulated expression of 192 genes in both LSK and progenitor cells, but to higher levels in LSKs than in committed myeloid progenitors.

Estradiol acts directly on bone marrow myeloid progenitors to differentially regulate GM-CSF or Flt3 ligand-mediated dendritic cell differentiation

Estrogen receptor (ER) ligands modulate hemopoiesis and immunity in the normal state, during autoimmunity, and after infection or trauma. Dendritic cells (DC) are critical for initiation of innate and adaptive immune responses. We demonstrate, using cytokine-driven culture models of DC differentiation, that 17-beta-estradiol exerts opposing effects on differentiation mediated by GM-CSF and Flt3 ligand, the two cytokines that regulate DC differentiation in vivo. We also show that estradiol acts on the same highly purified Flt3+ myeloid progenitors (MP) to differentially regulate the DC differentiation in each model. In GM-CSF-supplemented cultures initiated from MP, physiological amounts of estradiol promoted differentiation of Langerhans-like DC. Conversely, in Flt3 ligand-supplemented cultures initiated from the same MP, estradiol inhibited cell survival in a dose-dependent manner, thereby decreasing the yield of plasmacytoid and conventional myeloid and lymphoid DC. Experiments with bone marrow cells from ER-deficient mice and the ER antagonist ICI182,780 showed that estradiol acted primarily via ERalpha to regulate DC differentiation. Thus, depending on the cytokine environment, pathways of ER signaling and cytokine receptor signaling can differentially interact in the same Flt3+ MP to regulate DC development. Because the Flt3 ligand-mediated differentiation pathway is important during homeostasis, and GM-CSF-mediated pathways are increased by inflammation, our data suggest that endogenous or pharmacological ER ligands may differentially affect DC development during homeostasis and disease, with consequent effects on DC-mediated immunity.

The inhibitory HVEM-BTLA pathway counter regulates lymphotoxin receptor signaling to achieve homeostasis of dendritic cells

Proliferation of dendritic cells (DC) in the spleen is regulated by positive growth signals through the lymphotoxin (LT)-beta receptor; however, the countering inhibitory signals that achieve homeostatic control are unresolved. Mice deficient in LTalpha, LTbeta, LTbetaR, and the NFkappaB inducing kinase show a specific loss of CD8- DC subsets. In contrast, the CD8alpha- DC subsets were overpopulated in mice deficient in the herpesvirus entry mediator (HVEM) or B and T lymphocyte attenuator (BTLA). HVEM- and BTLA-deficient DC subsets displayed a specific growth advantage in repopulating the spleen in competitive replacement bone marrow chimeric mice. Expression of HVEM and BTLA were required in DC and in the surrounding microenvironment, although DC expression of LTbetaR was necessary to maintain homeostasis. Moreover, enforced activation of the LTbetaR with an agonist Ab drove expansion of CD8alpha- DC subsets, overriding regulation by the HVEM-BTLA pathway. These results indicate the HVEM-BTLA pathway provides an inhibitory checkpoint for DC homeostasis in lymphoid tissue. Together, the LTbetaR and HVEM-BTLA pathways form an integrated signaling network regulating DC homeostasis.

Regulation of dendritic cell differentiation and function by estrogen receptor ligands

Estrogen receptor (ER) ligands can modulate innate and adaptive immunity and hematopoiesis, which may explain the clear sex differences in immune responses during autoimmunity, infection or trauma. Dendritic cells (DC) are antigen presenting cells important for initiation of innate and adaptive immunity, as well as immune tolerance. DC progenitors and terminally differentiated DC express ER, indicating the ER ligands may regulate DC at multiple developmental and functional stages. Although there are profound differences in innate immunity between males and females or upon systemic imposition of sex hormones, studies are just beginning to link these differences to DC. Our and others studies demonstrate that estradiol and other ER ligands regulate the homeostasis of bone marrow myeloid and lymphoid progenitors of DC, as well as DC differentiation mediated by GM-CSF and Flt3 Ligand. Since DC have a brief lifespan, these data suggest that relatively short exposures to ER ligands in vivo will alter DC numbers and intrinsic functional capacity related to their developmental state. Studies in diverse experimental models also show that agonist and antagonist ER ligands modulate DC activation and production of inflammatory mediators. These findings have implications for human health and disease since they suggest that both DC development and functional capacity will be responsive to the physiological, pharmacological and environmental ER ligands to which an individual is exposed in vivo.

An in vitro method to study the effects of hematopoietic regulators during immune and blood cell development

In adults, hematopoiesis occurs in bone marrow (BM) through a complex process with differentiation of hematopoietic stem cells (HSCs) to immune and blood cells. Human HSCs and their progenitors express CD34. Methods on hematopoietic regulation are presented to show the effects of the chemokine, stromal-derived growth factor (SDF)-1I and the neuropeptide, substance P (SP). SDF-1I production in BM stroma causes interactions with HSCs, thereby retaining the HSCs in regions close to the endosteum, at low oxygen. Small changes in SDF-1I levels stimulate HSC functions through direct and indirect mechanisms. The indirect method occurs by SP production, which stimulates CD34+ cells, supported by ligand-binding studies, long-term culture-initiating cell assays for HSC functions, and clonogenic assays for myeloid progenitors. These methods can be applied to study other hematopoietic regulators.

Expression profiling of immature thymocytes revealed a novel homeobox gene that regulates double-negative thymocyte development

Intrathymic development of CD4/CD8 double-negative (DN) thymocytes can be tracked by well-defined chronological subsets of thymocytes, and is an ideal target of gene expression profiling analysis to clarify the genetic basis of mature T cell production, by which differentiation of immature thymocytes is investigated in terms of gene expression profiles. In this study, we show that development of murine DN thymocytes is predominantly regulated by largely repressive rather than inductive activities of transcriptions, where lineage-promiscuous gene expression in immature thymocytes is down-regulated during their differentiation. Functional mapping of genes showing common temporal expression profiles implicates previously uncharacterized gene regulations that may be relevant to early thymocytes development. A small minority of genes is transiently expressed in the CD44(low)CD25(+) subset of DN thymocytes, from which we identified a novel homeobox gene, Duxl, whose expression is up-regulated by Runx1. Duxl promotes the transition from CD44(high)CD25(+) to CD44(low)CD25(+) in DN thymocytes, while constitutive expression of Duxl inhibits expression of TCR beta-chains and leads to impaired beta selection and greatly reduced production of CD4/CD8 double-positive thymocytes, indicating its critical roles in DN thymocyte development.

Type I interferon regulates pDC maturation and Ly49Q expression

Ly49Q is expressed on peripheral mouse plasmacytoid dendritic cells (pDC). Immature Ly49Q-negative pDC precursors acquire Ly49Q in the bone marrow and then migrate into the periphery. While searching for molecules that regulate pDC maturation, we found that type I interferon (IFN) inhibited Ly49Q acquisition in vitro. Infections that induce type I IFN production by cells other than pDC (a condition mimicked by poly(I:C) injection in vivo) increase the prevalence of Ly49Q(-) pDC in the bone marrow and peripheral lymphoid organs in wild-type but not IFN-alpha/beta receptor knockout BALB/c mice. Moreover, in vivo exposure to type I IFN causes some Ly49Q(-), but not Ly49Q(+), pDC to convert to conventional DC, defined as B220(-) CD11c(+) CD11b(+) cells. These data suggest that type I IFN regulates pDC development and affects their distribution in the body.

Development of plasmacytoid and conventional dendritic cell subtypes from single precursor cells derived in vitro and in vivo

The development of functionally specialized subtypes of dendritic cells (DCs) can be modeled through the culture of bone marrow with the ligand for the cytokine receptor Flt3. Such cultures produce DCs resembling spleen plasmacytoid DCs (pDCs), CD8(+) conventional DCs (cDCs) and CD8(-) cDCs. Here we isolated two sequential DC-committed precursor cells from such cultures: dividing 'pro-DCs', which gave rise to transitional 'pre-DCs' en route to differentiating into the three distinct DC subtypes (pDCs, CD8(+) cDCs and CD8(-) cDCs). We also isolated an in vivo equivalent of the DC-committed pro-DC precursor cell, which also gave rise to the three DC subtypes. Clonal analysis of the progeny of individual pro-DC precursors demonstrated that some pro-DC precursors gave rise to all three DC subtypes, some produced cDCs but not pDCs, and some were fully committed to a single DC subtype. Thus, commitment to particular DC subtypes begins mainly at this pro-DC stage.

Identification of clonogenic common Flt3+M-CSFR+ plasmacytoid and conventional dendritic cell progenitors in mouse bone marrow

Lymphoid tissue plasmacytoid and conventional dendritic cells (DCs) are continuously regenerated from hematopoietic stem cells. The cytokine dependence and biology of plasmacytoid and conventional DCs suggest that regeneration might proceed through common DC-restricted developmental intermediates. By selecting for cytokine receptor expression relevant to DC development, we identify here highly cycling Lin(-)c-Kit(int)Flt3(+)M-CSFR(+) cells with a distinct gene-expression profile in mouse bone marrow that, on a clonal level in vitro and as a population both in vitro and in vivo, efficiently generated plasmacytoid and conventional DCs but no other lineages, which increased in number after in vivo injection of the cytokine Flt3 ligand. These clonogenic common DC progenitors thus define a cytokine-regulated DC developmental pathway that ensures the supply of various DC populations.

Tachykinins and Hematopoiesis

Originally discovered in the 1930s, tachykinins have been a subject of renewed interest. Antagonists to the tachykinin receptors have shown potential in the treatment of a variety of maladies including neurodegenerative disorders, heart disease, pain perception and malignancies. Tachykinins have been the subject of intense studies due to their impact on hematopoiesis that has significant effects on endothelial tissue and vascular conditions. Hematopoiesis relies on a relatively small subset of bone marrow-resident hematopoietic stem cells. This review discusses the network developed by cytokines and the tachykinins to regulate hematopoiesis. An understanding of tachykinin effect on normal hematopoietic functions and their involvement in hematological disorders could lead to new treatments for bone marrow disorders such as fibrosis, leukemia and anemia.

Aberrant dendritic cell differentiation initiated by theMll-Eenfusion gene does not require leukemic transformation

Dendritic cells (DCs), as specialized APCs, play a key role in the induction of anti-tumor immunity. They originate from bone marrow (BM) progenitors, which are frequently the targets of chromosomal translocations leading to development of leukemia. Aberrant DC differentiation and functions have been observed and are widely reported in patients with leukemia. It is not clear, however, whether such defects are a direct effect of a leukemic fusion gene or simply an outcome of the clinical disease. In this study, we demonstrate for the first time that knockin of the Mll-Een fusion gene can affect myeloid DC differentiation and functions directly, independent of the leukemic disease activities. We showed that the Mll-Een-expressing BM cells [enhanced green fluorescent protein+ (EGFP+)] from leukemic and nonleukemic mice had similarly impaired DC differentiation capacities with functional abnormalities. In contrast, BM cells without Mll-Een expression (EGFP(-)) showed normal DC differentiation and functions. A reduction in the frequency of CD11c+ DCs was also observed within the EGFP+ population in spleen and lymph nodes, and these cells were dysfunctional. Taken together, our findings suggest that the Mll-Een fusion gene can affect myeloid DC differentiation directly and functions in a cell-autonomous manner, where fully leukemic transformation of the hematopoietic progenitors is not required exclusively. Therefore, the study provides evidence for a direct causal relationship between leukemic gene fusion and abnormal DC differentiation, possibly contributing to the development of leukemia.

Dendritic cells: ontogeny

Dendritic cells (DC) play key rolls in various aspects of immunity. The functions of DC depend on the subsets as well as their location or activation status. Understanding developmental lineages, precursors and inducing factors for various DC subsets would help their clinical application, but despite extensive efforts, the precise ontogeny of various DC, remain unclear and complex. Because of their many functional similarities to macrophages, DC were originally thought to be of myeloid-lineage, an idea supported by many in vitro studies where monocytes or GM-CSF (a key myeloid growth factor) has been extensively used for generating DC. However, there has been considerable evidence which suggests the existence of lymphoid-lineage DC. After the confusion of myeloid-/lymphoid-DC concept regarding DC surface markers, we have now reached a consensus that each DC subset can differentiate through both myeloid- and lymphoid-lineages. The identification of committed populations (such as common myeloid- and lymphoid progenitors) as precursors for every DC subsets and findings from various knockout (KO) mice that have selected lymphoid- or myeloid-lineage deficiency appear to indicate flexibility of DC development rather than their lineage restriction. Why is DC development so flexible unlike other hematopoitic cells? It might be because there is developmental redundancy to maintain such important populations in any occasions, or such developmental flexibility would be advantageous for DC to be able to differentiate from any "available" precursors in situ irrespective of their lineages. This review will cover ontogeny of conventional (CD8 +/- DC) DC, plasmacytoid DC and skin Langerhans cells, and recently-identified many Pre-DC (immediate DC precursor) populations, in addition to monocytes and plasmacytoid DC, will also be discussed.

Immunoregulatory dendritic cells to prevent and reverse new-onset Type 1 diabetes mellitus

Herein, the authors provide an overview of where dendritic cells lie in the immunopathology of autoimmune Type 1 diabetes mellitus and how dendritic cell-based therapy may be usefully translated to treat and reverse the disease. The immunopathology of Type 1 diabetes mellitus offers a number of windows at which immunotherapy can be applied to delay, stop and even reverse the autoimmune processes, especially in light of the recent antibody-based accomplishment of improvement in residual beta-cell mass function. As in almost all cell-specific inflammatory processes, dendritic cells are central regulators of diabetes onset and progression. This realisation, along with accumulating data confirming a role for dendritic cells in maintaining and inducing tolerance in multiple therapeutic settings, has prompted a line of investigation to identify the most effective embodiments of dendritic cells for diabetes immunotherapy.

Myeloid lineage commitment from the hematopoietic stem cell

Prospective isolation of hematopoietic stem and progenitor cells has identified the lineal relationships among all blood-cell types and has allowed their developmental mechanisms to be assayed at the single-cell level. These isolated cell populations are used to elucidate the molecular mechanism of lineage fate decision and of its plasticity directly by stage-specific enforcement or repression of lineage-instructive signaling in purified cells. With an emphasis on the myeloid lineage, this review summarizes current concepts and controversies regarding adult murine hematopoietic development and discusses the potential mechanisms, operated by single or by multiple transcription factors, of myeloid lineage fate decision.

Development of dendritic-cell lineages

Dendritic cells (DCs) are a heterogenous population of bone-marrow-derived immune cells. Although all DCs share a common ability to process and present antigen to naive T cells for the initiation of an immune response, they differ in surface markers, migratory patterns, localization, and cytokine production. DCs were originally considered to be myeloid cells, but recent findings have demonstrated that DCs can develop not only from myeloid- but also from lymphoid-committed progenitors. The common feature of the progenitors capable of developing into DCs is the surface expression of Flt3 receptor. The development of different populations of DCs is differentially regulated by various transcription factors and cytokines. This review summarizes the recent advances made in the field of DC development.

The developmental program of human dendritic cells is operated independently of conventional myeloid and lymphoid pathways

Two distinct dendritic cell (DC) subsets, conventional DCs (cDCs) and plasmacytoid DCs (pDCs), have been shown to develop via either the myeloid or the lymphoid pathway in murine hematopoiesis. Lineage-specific phenotypes or functions of "myeloid" and "lymphoid" DCs, however, still remain elusive. Furthermore, such analysis has been particularly difficult in humans, due to lack of an assay system appropriate for the analysis of human stem and progenitor cell differentiation. Here, using a highly efficient xenotransplantation model, we extensively analyze the origin and the molecular signature of human DCs. Purified human common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs) were intravenously transplanted into nonobese diabetic-severe combined immunodeficiency (NOD-scid)/IL2rgamma(null) newborn mice. CMPs and CLPs displayed significant expansion in the xenogeneic host, and human cDC and pDC progeny were isolatable. Strikingly, each human DC subset possessed indistinguishable expression patterns of surface phenotype and gene transcripts regardless of their CMP or CLP origin, even at the genome-wide level. Thus, cDC and pDC normally develop after cells have committed to the myeloid or the lymphoid lineage in human hematopoiesis, while their transcriptional signatures are well preserved irrespective of their lineage origin. We propose that human DCs use unique and flexible developmental programs that cannot be categorized into the conventional myeloid or lymphoid pathway.

Gata1 regulates dendritic-cell development and survival

Dendritic cells are key initiators and regulators of the immune response. Dendritic cell commitment and function require orchestrated regulation of transcription. Gata1 is a transcription factor expressed in several hematopoietic lineages. However, Gata1 function has not been explored in the monocytic or dendritic cell compartment. Here, we show that Gata1 is expressed in myeloid and plasmacytoid dendritic cells and that Gata1 ablation affects the survival of dendritic cells. Furthermore, lipopolysaccharide (LPS) stimulation of dendritic cells prompts Gata1 up-regulation, which is accompanied by increased levels of BclX and Ifng. Our findings show that Gata1 is a transcriptional regulator of dendritic cell differentiation and suggest that Gata1 is involved in the dendritic cell and macrophage lineage separation.

Exploiting dendritic cells for active immunotherapy of cancer and chronic infections

Dendritic cells (DCs) are important antigen-presenting cells (APCs) that can prime naive T cells and control adaptive immune responses with respect to magnitude, memory and self-tolerance. Understanding the biology of these cells is central to the development of new generation immunotherapies for cancer and chronic infections. This review presents a brief overview of DC biology and of the preparation and use of DC-based vaccines.

Heterogeneity of thymic dendritic cells

Thymus is the site of generation and selection of T-lymphocytes. It also contains phenotypically and functionally distinct dendritic cell (DC) populations, including conventional DC (cDC) and plasmacytoid DC (pDC). Thymic cDC are heterogeneous and contain two subsets: a major subset derived from the precursors within thymus, and a minor subset presumably of extrathymic origin. Increasing evidence suggest that thymic cDC can cross-present self-antigens to developing thymocytes and play an important role in thymocyte negative selection and central tolerance induction. Thymic pDC can produce type-I interferon upon appropriate activation. However, their role in a steady state thymus is currently unclear.

Flt3 in regulation of type I interferon-producing cell and dendritic cell development

Flt3-ligand is a nonredundant cytokine in type I interferon-producing cell (IPC) and dendritic cell (DC) development. We demonstrated that IPC and DC differentiation potential is confined to Flt3(+)-hematopoietic progenitor cells, that Flt3-ligand drives development along both lymphoid and myeloid developmental pathways from Flt3(+)-progenitors to Flt3(+)-IPCs and -DCs, and that in vivo pharmacologic inhibition of Flt3-signaling leads to disruption of IPC and DC development in spite of consecutive Flt3-ligand upregulation in treated animals. We here summarize our recent findings that overexpression of human Flt3 in Flt3(-) and Flt3(+) hematopoietic progenitors rescues and enhances their IPC and DC differentiation potential, respectively. Based on these data, we propose an instructive, demand-regulated, cytokine-driven IPC and DC regeneration model, where high Flt3-ligand levels initiate a self-sustaining, Flt3-STAT3 and -PU.1-mediated IPC and DC differentiation program in Flt3(+)-hematopoietic progenitor cells.

Interferon-producing killer dendritic cells (IKDCs) arise via a unique differentiation pathway from primitive c-kitHiCD62L+ lymphoid progenitors

Interferon-producing killer dendritic cells (IKDCs) have only recently been described and they share some properties with plasmacytoid dendritic cells (pDCs). We now show that they can arise from some of the same progenitors. However, IKDCs expressed little or no RAG-1, Spi-B, or TLR9, but responded to the TLR9 agonist CpG ODN by production of IFNgamma. The RAG-1(-)pDC2 subset was more similar to IKDCs than RAG-1(+) pDC1s with respect to IFNgamma production. The Id-2 transcriptional inhibitor was essential for production of IKDCs and natural killer (NK) cells, but not pDCs. IKDCs developed from lymphoid progenitors in culture but, unlike pDCs, were not affected by Notch receptor ligation. While IKDCs could be made from estrogen-sensitive progenitors, they may have a slow turnover because their numbers did not rapidly decline in hormone-treated mice. Four categories of progenitors were compared for IKDC-producing ability in transplantation assays. Of these, Lin(-)Sca-1(+)c-Kit(Hi)Thy1.1(-)L-selectin(+) lymphoid progenitors (LSPs) were the best source. While NK cells resemble IKDCs in several respects, they develop from different progenitors. These observations suggest that IKDCs may arise from a unique differentiation pathway, and one that diverges early from those responsible for NK cells, pDCs, and T and B cells.