Human Dendritic Cells Subsets Research Articles

Reduced numbers of mucosal DRint macrophages and increased numbers of CD103+ dendritic cells during anti-TNF-α treatment in patients with Crohn’s disease

Objective Anti-TNF-α treatment constitutes a mainstay in the treatment of Crohn’s disease (CD), but its mechanisms of action are not fully understood. We aimed to investigate the effects of adalimumab, a human monoclonal TNF-α antibody, on macrophage (MQ) and dendritic cell (DC) subsets in mucosal biopsies and peripheral blood. Material and methods Intestinal biopsies and blood samples were obtained from 12 different CD patients both before and 4 weeks after the initiation of the induction of adalimumab treatment. Endoscopic disease activity was estimated by the Simple Endoscopic Score for Crohn’s Disease. Biopsies were obtained from inflamed and non-inflamed areas. The numbers of lamina propria CD14 + DRint and CD14 + DRhi MQs, CD141+, CD141− and CD103+ DCs subsets, and circulating monocytes and DCs were analyzed using flow cytometry. Results At baseline, we observed higher numbers of DRint MQs and lower numbers of CD103+ DCs in inflamed versus non-inflamed mucosa [843 vs. 391/105 lamina propria mononuclear cells (LPMCs) (p < 0.05) and 9 vs. 19 × 105 LPMCs (p = 0.01), respectively]. After four weeks of adalimumab treatment, the numbers of DRint MQs decreased [843 to 379/105 LPMCs (p = 0.03)], whereas the numbers of CD103+ DCs increased [9–20 × 105 LPMCs (p = 0.003)] compared with baseline. In peripheral blood, no alterations were observed in monocyte or DC numbers between baseline and week 4. Conclusions In CD, mucosal inflammation is associated with high numbers of DRint MQs and low numbers of CD103+ DCs. This composition of intestinal myeloid subsets is reversed by anti-TNF-α treatment. These results suggest that DRint MQs play a pivotal role in CD inflammation.

Elevation of HLA-G-expressing DC-10 cells in patients with gastric cancer

DC-10 is a distinct subset of human tolerogenic dendritic cells (DCs) which express high levels of human leukocyte antigen-G (HLA-G). DC-10 could induce adaptive type 1 regulatory T cells through the IL-10 dependent ILT4/HLA-G signaling pathway. However, the significance of DC-10 in malignancies remains unclear. In this study, the frequency and mean fluorescence intensity (MFI) of HLA-G+ DC-10 in the peripheral blood of 124 patients with gastric cancer (GC) and 130 normal controls was analyzed with flow cytometry. Plasma sHLA-G was analyzed with ELISA. Results showed both the percentages of peripheral HLA-G+ DC-10 (median: 0.13% vs 0.01%; p<0.01) and MFI of HLA-G on these cells (median: 310.0 vs 91.5; p<0.01) were dramatically increased in GC patients than in normal controls. The frequency of HLA-G+ DC-10 in GC patients was strongly relative to the tumor grade (p=0.021). sHLA-G levels in GC patients were significantly higher than in healthy controls (median: 85.80U/ml vs 61.20U/ml; p<0.01). There was no significant correlation between the percentage of DC-10 and plasma sHLA-G (p>0.05). However, the increased HLA-G+ DC-10, HLA-G MFI and plasma sHLA-G in patients with gastric cancer could be a diagnostic factor with the area under the ROC curve with 0.947 (p<0.01), 0.882 (p<0.01) and 0.700 (p<0.01) respectively. Given the immune tolerant function of DC-10 could play, the increased DC-10 might play an important role in immune suppression for patients with gastric cancer, while more studies are necessary to illustrate the clinical relevance of DC-10 in cancer patients.

Review of Mouse and Human Dendritic Cell Subsets.

Dendritic cells are specialized antigen-presenting cells that initiate and orient immune responses. Numerous studies in mice and humans have shown that dendritic cells are heterogeneous and comprise several subsets that can be distinguished by their surface phenotype, ontogeny, and molecular signature. This review gives an overview of mouse and human dendritic cell subsets and their defining features and summarizes the current knowledge of dendritic cell subsets' functional specialization in terms of antigen presentation.

In Vitro Generation of Human XCR1(+) Dendritic Cells from CD34(+) Hematopoietic Progenitors.

Dendritic cells (DCs) are a heterogeneous population of professional antigen-presenting cells which play a key role in orchestrating immune defenses. Most of the information gained on human DC biology was derived from studies conducted with DCs generated in vitro from peripheral blood CD14(+) monocytes (MoDCs) or from CD34(+) hematopoietic progenitors. Recent advances in the field revealed that these types of in vitro-derived DCs strikingly differ from the DC subsets that are naturally present in human lymphoid organs, in terms of global gene expression, of specialization in the sensing of different types of danger signals, and of the ability to polarize T lymphocytes toward different functions. Major efforts are being made to better characterize the biology and the functions of lymphoid organ-resident DC subsets in humans, as an essential step for designing innovative DC-based vaccines against infections or cancers. However, this line of research is hampered by the low frequency of certain DC subsets in most tissues, their fragility, and the complexity of the procedures necessary for their purification. Hence, there is a need for robust procedures allowing large-scale in vitro generation of human DC subsets, under conditions allowing their genetic or pharmacological manipulation, to decipher their functions and their molecular regulation. Human CD141(+)CLEC9A(+)XCR1(+) DCs constitute a very interesting DC subset for the design of immunotherapeutic treatments against infections by intracellular pathogens or against cancer, because these cells resemble mouse professional cross-presenting CD8α(+)Clec9a(+)Xcr1(+) DCs. Human XCR1(+) DCs have indeed been reported by several teams to be more efficient than other human DC subsets for cross-presentation, in particular of cell-associated antigens but also of soluble antigens especially when delivered into late endosomes or lysosomes. However, human XCR1(+) DCs are the rarest and perhaps the most fragile of the human DC subsets and hence the most difficult to study ex vivo. Here, we describe a protocol allowing simultaneous in vitro generation of human MoDCs and XCR1(+) DCs, which will undoubtedly be extremely useful to better characterize the functional specialization of human XCR1(+) DCs and to identify its molecular bases.

A Comparative Study of the T Cell Stimulatory and Polarizing Capacity of Human Primary Blood Dendritic Cell Subsets.

Dendritic cells (DCs) are central players of immune responses; they become activated upon infection or inflammation and migrate to lymph nodes, where they can initiate an antigen-specific immune response by activating naive T cells. Two major types of naturally occurring DCs circulate in peripheral blood, namely, myeloid and plasmacytoid DCs (pDCs). Myeloid DCs (mDCs) can be subdivided based on the expression of either CD1c or CD141. These human DC subsets differ in surface marker expression, Toll-like receptor (TLR) repertoire, and transcriptional profile, suggesting functional differences between them. Here, we directly compared the capacity of human blood mDCs and pDCs to activate and polarize CD4+ T cells. CD141+ mDCs show an overall more mature phenotype over CD1c+ mDC and pDCs; they produce less IL-10 and more IL-12 than CD1c+ mDCs. Despite these differences, all subsets can induce the production of IFN-γ in naive CD4+ T cells. CD1c+ and CD141+ mDCs especially induce a strong T helper 1 profile. Importantly, naive CD4+ T cells are not polarized towards regulatory T cells by any subset. These findings further establish all three human blood DCs—despite their differences—as promising candidates for immunostimulatory effectors in cancer immunotherapy.

Generation of Humanized Mice for Analysis of Human Dendritic Cells.

Transplantation of human CD34(+) hematopoietic stem and progenitor cells into severe immunocompromised newborn mice allows the development of a human hemato-lymphoid system (HHLS) including dendritic cells (DCs) in vivo. Therefore, it can be a powerful tool to study human DC subsets, residing in different lymphoid and nonlymphoid organs. We have recently generated novel mouse strains called human cytokine knock-in mice in which human versions of several cytokines are knocked into Rag2(-/-)γC(-/-) strains. In addition, human SIRPα, which is a critical factor to prevent donor cell to be eliminated by host macrophages, is expressed as transgene. These mice efficiently support human myeloid cell development and, indeed, allow the analysis of three major subsets of human DC lineages, plasmacytoid DCs and CD1c(+) and CD141(+) classical DCs. Moreover, these strains also support cytokine-mobilized peripheral blood CD34(+) cell engraftment and subsequent DC development. Here we describe our standard methods to characterize DCs developed in human cytokine knock-in mice.

Dectin-1 in the control of Th2-type T cell responses.

Dendritic cells (DCs) are major antigen-presenting cells (APCs) that can induce and control host immune responses. DCs express pattern recognition receptors (PRRs), which can translate external and internal triggers into different types of T cell responses. The types of CD4+ T cell responses elicited by DCs (e.g., Th1, Th2, Th17, Th21, Th22 and regulatory T cells (Tregs)) are associated with either host immunity or inflammatory diseases, including allergic diseases and autoimmune diseases. In particular, the pathogenic functions of Th2-type T cells in allergic immune disorders have been well documented, although Th2-type T cell responses are crucial for immunity against certain parasite infections. Recent evidence also indicates that the inflammatory Th2 signatures in cancers, including breast and pancreatic cancers, are highly associated with poor clinical outcomes in patients. It is thus important to find cellular/molecular targets expressed in DCs that control such inflammatory Th2-type T cell responses. In a recent paper published in The Journal of Immunology, we demonstrated that Dectin-1 expressed on the two major human DC subsets, myeloid DCs (mDCs) and plasmacytoid DCs (pDCs), has opposing roles in the control of Th2-type CD4+ T cell responses. Dectin-1 expressed on mDCs decreases Th2-type CD4+ T cell responses, while Dectin-1 expressed on pDCs favors Th2-type CD4+ T cell responses. This finding expands our understanding of the roles of DCs and Dectin-1 expressed on DCs in the pathogenesis of Th2-associated diseases and in host immunity to microbial infections and cancers.

Primary Human Blood Dendritic Cells for Cancer Immunotherapy-Tailoring the Immune Response by Dendritic Cell Maturation.

Dendritic cell (DC)-based cancer vaccines hold the great promise of tipping the balance from tolerance of the tumor to rejection. In the last two decades, we have gained tremendous knowledge about DC-based cancer vaccines. The maturation of DCs has proven indispensable to induce immunogenic T cell responses. We review the insights gained from the development of maturation cocktails in monocyte derived DC-based trials. More recently, we have also gained insights into the functional specialization of primary human blood DC subsets. In peripheral human blood, we can distinguish at least three primary DC subsets, namely CD1c+ and CD141+ myeloid DCs and plasmacytoid DCs. We reflect the current knowledge on maturation and T helper polarization by these blood DC subsets in the context of DC-based cancer vaccines. The maturation stimulus in combination with the DC subset will determine the type of T cell response that is induced. First trials with these natural DCs underline their excellent in vivo functioning and mark them as promising tools for future vaccination strategies.

Immunity to Pathogens Taught by Specialized Human Dendritic Cell Subsets

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) that have a key role in immune responses because they bridge the innate and adaptive arms of the immune system. They mature upon recognition of pathogens and upregulate MHC molecules and costimulatory receptors to activate antigen-specific CD4+ and CD8+ T cells. It is now well established that DCs are not a homogeneous population but are composed of different subsets with specialized functions in immune responses to specific pathogens. Upon viral infections, plasmacytoid DCs (pDCs) rapidly produce large amounts of IFN-α, which has potent antiviral functions and activates several other immune cells. However, pDCs are not particularly potent APCs and induce the tolerogenic cytokine IL-10 in CD4+ T cells. In contrast, myeloid DCs (mDCs) are very potent APCs and possess the unique capacity to prime naive T cells and consequently to initiate a primary adaptive immune response. Different subsets of mDCs with specialized functions have been identified. In mice, CD8α+ mDCs capture antigenic material from necrotic cells, secrete high levels of IL-12, and prime Th1 and cytotoxic T-cell responses to control intracellular pathogens. Conversely, CD8α− mDCs preferentially prime CD4+ T cells and promote Th2 or Th17 differentiation. BDCA-3+ mDC2 are the human homologue of CD8α+ mDCs, since they share the expression of several key molecules, the capacity to cross-present antigens to CD8+ T-cells and to produce IFN-λ. However, although several features of the DC network are conserved between humans and mice, the expression of several toll-like receptors as well as the production of cytokines that regulate T-cell differentiation are different. Intriguingly, recent data suggest specific roles for human DC subsets in immune responses against individual pathogens. The biology of human DC subsets holds the promise to be exploitable in translational medicine, in particular for the development of vaccines against persistent infections or cancer.

ID: 221: “Super-pDCs”: A novel highly potent, CD141+ human blood plasmacytoid dendritic cell subtype

Dendritic cells(DCs) are a heterogeneous population that regulate both innate and adaptive immunity. Human blood DCs are broadly characterized as pDC(BDCA2+), mDC1(BDCA1+), and mDC2(BDCA3+(CD141)). While much work has focused on the functional role of CD141hi mDCs, we investigated the maturation potential and functional significance of a novel BDCA3int subtype of classically-defined pDCs. Using flow cytometry, we identified four human blood DC subsets: BDCA2+/3−, BDCA2+/3int, BDCA2−/3int, and BDCA2−/3hi. Upon stimulation with TLR9 agonists CpG-A or HSV-1, the BDCA2+/3int DCs had the highest frequency of IFN- α + and TNF- α + production, while mDC2 were non-responsive. Although CD141+ DC produce IFN- λ in response to pI:pC, both pDC populations were the highest responders to HSV-1. Virus-stimulated BDCA2+/3int DCs became more activated and mature with higher expression of co-stimulatory and migratory receptors than the BDCA2+/3− cells. IRF7, the master regulator of IFN production in pDCs, was constitutively expressed by both pDC but not BDCA2-DCs. The BDCA2+/3int DCs were more potent than BDCA2+/3− cells at the preferential acquisition of cellular material from HSV-infected Raji cells. AMNIS ImageStream analysis of morphology showed no difference in circularity and size between the BDCA2+/3int and BDCA2+/3-DCs, but the mDC2 were larger and less circular than the pDC subsets. These data indicate that BDCA2+/3int DCs exhibit major classical pDC functions, but are phenotypically and functionally distinct from BDCA2−/3int and BDCA2−/3hi DCs. Together, these results describe a novel, high functioning CD141+ pDC subset and demonstrates CD141 is not unique to mDC2.

Type I IFN-mediated synergistic activation of mouse and human DC subsets by TLR agonists.

Novel approaches of dendritic cell (DC) based cancer immunotherapy aim at harnessing the unique attributes of different DC subsets. Classical monocyte-derived DC vaccines are currently being replaced by either applying primary DCs or specifically targeting antigens and adjuvants to these subsets in vivo. Appropriate DC activation in both strategies is essential for optimal effect. For this purpose TLR agonists are favorable adjuvant choices, with TLR7 triggering being essential for inducing strong Th1 responses. However, mouse CD8α(+) DCs, considered to be the major cross-presenting subset, lack TLR7 expression. Interestingly, this DC subset can respond to TLR7 ligand upon concurrent TLR3 triggering. Nevertheless, the mechanism underlying this synergy remains obscure. We now show that TLR3 ligation results in the production of IFN-α, which rapidly induces the expression of TLR7, resulting in synergistic activation. Moreover, we demonstrate that this mechanism conversely holds for plasmacytoid DCs that respond to TLR3 ligation when TLR7 pathway is mobilized. We further demonstrate that this mechanism of sharpening DC senses is also conserved in human BDCA1(+) DCs and plasmacytoid DCs. These findings have important implications for future clinical trials as it suggests that combinations of TLR ligands should be applied irrespective of initial TLR expression profiles on natural DC subsets for optimal stimulation.

Galectin-1 Regulates Tissue Exit of Specific Dendritic Cell Populations

During inflammation, dendritic cells emigrate from inflamed tissue across the lymphatic endothelium into the lymphatic vasculature and travel to regional lymph nodes to initiate immune responses. However, the processes that regulate dendritic cell tissue egress and migration across the lymphatic endothelium are not well defined. The mammalian lectin galectin-1 is highly expressed by vascular endothelial cells in inflamed tissue and has been shown to regulate immune cell tissue entry into inflamed tissue. Here, we show that galectin-1 is also highly expressed by human lymphatic endothelial cells, and deposition of galectin-1 in extracellular matrix selectively regulates migration of specific human dendritic cell subsets. The presence of galectin-1 inhibits migration of immunogenic dendritic cells through the extracellular matrix and across lymphatic endothelial cells, but it has no effect on migration of tolerogenic dendritic cells. The major galectin-1 counter-receptor on both dendritic cell populations is the cell surface mucin CD43; differential core 2 O-glycosylation of CD43 between immunogenic dendritic cells and tolerogenic dendritic cells appears to contribute to the differential effect of galectin-1 on migration. Binding of galectin-1 to immunogenic dendritic cells reduces phosphorylation and activity of the protein-tyrosine kinase Pyk2, an effect that may also contribute to reduced migration of this subset. In a murine lymphedema model, galectin-1(-/-) animals had increased numbers of migratory dendritic cells in draining lymph nodes, specifically dendritic cells with an immunogenic phenotype. These findings define a novel role for galectin-1 in inhibiting tissue emigration of immunogenic, but not tolerogenic, dendritic cells, providing an additional mechanism by which galectin-1 can dampen immune responses.

Protamine-stabilized RNA as an ex vivo stimulant of primary human dendritic cell subsets.

Dendritic cells (DCs) are key in connecting innate and adaptive immunity. Their potential in inducing specific immune responses has made them interesting targets for immunotherapeutic approaches. Our research group was the first to exploit the naturally occurring myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) in therapeutic vaccination trials against melanoma. To develop primary DC subsets as an optimal vaccine, the identification of a clinically applicable adjuvant activating both subsets is required. Although the expression of pathogen recognition receptors differs distinctly between the DC subsets, both pDCs and mDCs can respond to single-stranded RNA (ssRNA) via Toll-like receptors 7 and 8, respectively. Since ssRNA is easily degraded by RNases, we stabilized anionic RNA by complexing it with the positively charged protein protamine. This leads to the formation of protamine–RNA complexes with varying features depending on ionic content. We subsequently investigated the immunostimulatory effect of complexes that formed various salt concentrations on purified DC subsets. Both mDCs and pDCs upregulated maturation markers and produced pro-inflammatory cytokines in a dose-dependent way to the protamine–RNA complexes. This was dependent on endosomal acidification and correlated partly with the uptake of protamine–RNA complexes. Furthermore, both DC subsets induced T cell proliferation and IFN gamma secretion in a beneficial ratio to IL-10. These results indicate that protamine–RNA complexes can be used to stimulate human mDC and pDC ex vivo for use in immunotherapeutic settings.Electronic supplementary materialThe online version of this article (doi:10.1007/s00262-015-1746-9) contains supplementary material, which is available to authorized users.

Human dendritic cell subsets and function in health and disease.

The method of choice for the development of new vaccines is to target distinct dendritic cell subsets with antigen in vivo and to harness their function in situ to enhance cell-mediated immunity or induce tolerance to specific antigens. The innate functions of dendritic cells themselves may also be targeted by inhibitors or activators that would target a specific function such as interferon production, potentially important in autoimmune disease and chronic viral infections. Importantly targeting dendritic cells requires detailed knowledge of both the surface phenotype and function of each dendritic cell subset, including how they may respond to different types of vaccine adjuvants, their ability to produce soluble mediators and to process and present antigens and induce priming of naïve T cells. This review summarizes our knowledge of the functional attributes of the human dendritic cell subsets in the steady state and upon activation and their roles in human disease.

Protection of human myeloid dendritic cell subsets against influenza A virus infection is differentially regulated upon TLR stimulation.

The proinflammatory microenvironment in the respiratory airway induces maturation of both resident and infiltrating dendritic cells (DCs) upon influenza A virus (IAV) infection. This results in upregulation of antiviral pathways as well as modulation of endocytic processes, which affect the susceptibility of DCs to IAV infection. Therefore, it is highly relevant to understand how IAV interacts with and infects mature DCs. To investigate how different subsets of human myeloid DCs (MDCs) involved in tissue inflammation are affected by inflammatory stimulation during IAV infection, we stimulated primary blood MDCs and inflammatory monocyte-derived DCs (MDDCs) with TLR ligands, resulting in maturation. Interestingly, MDDCs but not MDCs were protected against IAV infection after LPS (TLR4) stimulation. In contrast, stimulation with TLR7/8 ligand protected MDCs but not MDDCs from IAV infection. The reduced susceptibility to IAV infection correlated with induction of type I IFNs. We found that differential expression of TLR4, TRIF, and MyD88 in the two MDC subsets regulated the ability of the cells to enter an antiviral state upon maturation. This difference was functionally confirmed using small interfering RNA and inhibitors. Our data show that different human MDC subsets may play distinct roles during IAV infection, as their capacity to induce type I IFNs is dependent on TLR-specific maturation, resulting in differential susceptibility to IAV infection.

Enterovirus-infected β-cells induce distinct response patterns in BDCA1+ and BDCA3+ human dendritic cells.

Enteroviruses often cause mild disease, yet are also linked to development of autoimmune diabetes. Dendritic cells (DCs) shape both innate and adaptive immune responses, including anti-viral responses. How different human DC subsets shape anti-viral responses, whether they have complementary or overlapping functions and how this relates to autoimmune responses is largely unknown. We used enterovirus-infected β-cells and freshly isolated human myeloid DC (mDC) subsets as a model for autoimmune type 1 diabetes. Our data show that both the BDCA1+ and BDCA3+ mDC subsets engulf mock- as well as virus-infected β-cells, albeit BDCA1+ mDCs are more efficient. Uptake of enterovirus-infected, but not mock-infected cells, activated both DC subsets as indicated by the induction of co-stimulatory molecules and secretion of type I and type III interferons. Both subsets produced similar amounts of interferon-α, yet the BDCA3+ DC were superior in IFN-λ production. The BDCA1+ mDCs more strongly upregulated PD-L1, and were superior in IL-12 and IL-10 production as compared to the BDCA3+ DC. Despite lack of IL-12 production by the BDCA3+ DC, both BDCA1+ and BDCA3+ DCs activated T cells in allogeneic mixed lymphocyte reaction towards a Th1-type reactivity while suppressing Th2-associated cytokines.

The Known Unknowns of the Human Dendritic Cell Network

Dendritic cells (DCs) initiate and orient immune responses and comprise several subsets that display distinct phenotypes and properties. Most of our knowledge of DC subsets biology is based on mouse studies. In the past few years, the alignment of the human DC network with the mouse DC network has been the focus of much attention. Although comparative phenotypic and transcriptomic analysis have shown a high level of homology between mouse and human DC subsets, significant differences in phenotype and function have also been evidenced. Here, we review recent advances in our understanding of the human DC network and discuss some remaining gaps and future challenges of the human DC field.

The real thing: how to make human DC subsets.

![Figure][1] Insight from Frederic Geissmann The developmental relationship between monocytes, dendritic cells (DCs), and macrophages has been well defined in mice, but human DC development is less well understood and has been hampered by the lack of a suitable culture system. Now, two

Accumulation of tolerogenic human 6-sulfo LacNAc dendritic cells in renal cell carcinoma is associated with poor prognosis

Dendritic cells (DCs) essentially contribute to the induction and regulation of innate and adaptive immunity. Based on these important properties, DCs may profoundly influence tumor progression in patients. However, little is known about the role of distinct human DC subsets in primary tumors and their impact on clinical outcome. In the present study, we investigated the characteristics of human 6-sulfo LacNAc (slan) DCs in clear cell renal cell carcinoma (ccRCC). slanDCs have been shown to display various tumor-directed properties and to accumulate in tumor-draining lymph nodes from patients. When evaluating 263 ccRCC and 227 tumor-free tissue samples, we found increased frequencies of slanDCs in ccRCC tissues compared to tumor-free tissues. slanDCs were also detectable in the majority of 24 metastatic lymph nodes and 67 distant metastases from ccRCC patients. Remarkably, a higher density of slanDCs was significantly associated with a reduced progression-free, tumor-specific or overall survival of ccRCC patients. Tumor-infiltrating slanDCs displayed an immature phenotype expressing interleukin-10. ccRCC cells efficiently impaired slanDC-induced T-cell proliferation and programming as well as natural killer (NK) cell activation. In conclusion, these findings indicate that higher slanDC numbers in ccRCC tissues are associated with poor prognosis. The induction of a tolerogenic phenotype in slanDCs leading to an insufficient activation of innate and adaptive antitumor immunity may represent a novel immune escape mechanism of ccRCC. These observations may have implications for the design of therapeutic strategies that harness tumor-directed functional properties of DCs against ccRCC.

Restricted dendritic cell and monocyte progenitors in human cord blood and bone marrow.

In mice, two restricted dendritic cell (DC) progenitors, macrophage/dendritic progenitors (MDPs) and common dendritic progenitors (CDPs), demonstrate increasing commitment to the DC lineage, as they sequentially lose granulocyte and monocyte potential, respectively. Identifying these progenitors has enabled us to understand the role of DCs and monocytes in immunity and tolerance in mice. In humans, however, restricted monocyte and DC progenitors remain unknown. Progress in studying human DC development has been hampered by lack of an in vitro culture system that recapitulates in vivo DC hematopoiesis. Here we report a culture system that supports development of CD34(+) hematopoietic stem cell progenitors into the three major human DC subsets, monocytes, granulocytes, and NK and B cells. Using this culture system, we defined the pathway for human DC development and revealed the sequential origin of human DCs from increasingly restricted progenitors: a human granulocyte-monocyte-DC progenitor (hGMDP) that develops into a human monocyte-dendritic progenitor (hMDP), which in turn develops into monocytes, and a human CDP (hCDP) that is restricted to produce the three major DC subsets. The phenotype of the DC progenitors partially overlaps with granulocyte-macrophage progenitors (GMPs). These progenitors reside in human cord blood and bone marrow but not in the blood or lymphoid tissues.