Reconsideration of macrophage and dendritic cell classification

It is well known that the activation of innate immune cells, especially antigen-presenting cells such as macrophages and dendritic cells, can ameliorate or exacerbate various diseases, including cancer. Currently, the macrophages and dendritic cells are categorized into several groups by their cell surface and intracellular molecules. However, the detailed classification of the differences between macrophages and dendritic cells has still not been established. Here, we summarized and reviewed the previous studies on the classification of macrophages and dendritic cells. In addition, the previous classification of monocytes, macrophages and dendritic cells is discussed based on our findings of macrophage activation, which has both conventional and plasmacytoid dendritic cell phenotype.

Rapid accumulation of CD14+CD11c+ dendritic cells in gut mucosa of celiac disease after in vivo gluten challenge

BACKGROUND

Of antigen-presenting cells (APCs) expressing HLA-DQ molecules in the celiac disease (CD) lesion, CD11c(+) dendritic cells (DCs) co-expressing the monocyte marker CD14 are increased, whereas other DC subsets (CD1c(+) or CD103(+)) and CD163(+)CD11c(-) macrophages are all decreased. It is unclear whether these changes result from chronic inflammation or whether they represent early events in the gluten response. We have addressed this in a model of in vivo gluten challenge.

METHODS

Treated HLA-DQ2(+) CD patients (n = 12) and HLA-DQ2(+) gluten-sensitive control subjects (n = 12) on a gluten-free diet (GFD) were orally challenged with gluten for three days. Duodenal biopsies obtained before and after gluten challenge were subjected to immunohistochemistry. Single cell digests of duodenal biopsies from healthy controls (n = 4), treated CD (n = 3) and untreated CD (n = 3) patients were analyzed by flow cytometry.

RESULTS

In treated CD patients, the gluten challenge increased the density of CD14(+)CD11c(+) DCs, whereas the density of CD103(+)CD11c(+) DCs and CD163(+)CD11c(-) macrophages decreased, and the density of CD1c(+)CD11c(+) DCs remained unchanged. Most CD14(+)CD11c(+) DCs co-expressed CCR2. The density of neutrophils also increased in the challenged mucosa, but in most patients no architectural changes or increase of CD3(+) intraepithelial lymphocytes (IELs) were found. In control tissue no significant changes were observed.

CONCLUSIONS

Rapid accumulation of CD14(+)CD11c(+) DCs is specific to CD and precedes changes in mucosal architecture, indicating that this DC subset may be directly involved in the immunopathology of the disease. The expression of CCR2 and CD14 on the accumulating CD11c(+) DCs indicates that these cells are newly recruited monocytes.

Interleukin 23 production by intestinal CD103(+)CD11b(+) dendritic cells in response to bacterial flagellin enhances mucosal innate immune defense

Microbial penetration of the intestinal epithelial barrier triggers inflammatory responses that include induction of the bactericidal C-type lectin RegIIIγ. Systemic administration of flagellin, a bacterial protein that stimulates Toll-like receptor 5 (TLR5), induces epithelial expression of RegIIIγ and protects mice from intestinal colonization with antibiotic-resistant bacteria. Flagellin-induced RegIIIγ expression is IL-22 dependent, but how TLR signaling leads to IL-22 expression is incompletely defined. By using conditional depletion of lamina propria dendritic cell (LPDC) subsets, we demonstrated that CD103(+)CD11b(+) LPDCs, but not monocyte-derived CD103(-)CD11b(+) LPDCs, expressed high amounts of IL-23 after bacterial flagellin administration and drove IL-22-dependent RegIIIγ production. Maximal expression of IL-23 subunits IL-23p19 and IL-12p40 occurred within 60 min of exposure to flagellin. IL-23 subsequently induced a burst of IL-22 followed by sustained RegIIIγ expression. Thus, CD103(+)CD11b(+) LPDCs, in addition to promoting long-term tolerance to ingested antigens, also rapidly produce IL-23 in response to detection of flagellin in the lamina propria.

Notch signaling regulates the development of a novel type of Thy1-expressing dendritic cell in the thymus

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) required for T-cell activation and are classified into several subtypes by phenotypic and functional characteristics. However, it remains unclear if distinct transcription factors control the development of each DC subpopulation. In this report, we demonstrate that Notch signaling controls the development of a novel DC subtype that expresses Thy1 (Thy1(+) DCs). Overstimulation of bone marrow cells with the Notch ligand Delta-like 1 promoted the development of Thy1(+) DCs. Thy1(+) DCs are characterized as CD11c(+) MHC class II(+) NK1.1(-) B220(-) CD8α(+) , and are present in the thymus but not in the spleen and lymph nodes. Thymic Thy1(+) DCs are able to capture exogenous proteins and delete CD4(+) CD8(+) T cells. Transplantation experiments demonstrated that CD44(+) CD25(-) and CD44(+) CD25(+) thymocytes can differentiate into Thy1(+) DCs. Recombination signal binding protein for immunoglobulin kappa J region (RBP-J) deficiency in lineage-negative bone marrow cells, but not CD11c(+) cells, disrupted Thy1(+) DC development in the thymus. Our data indicate that Notch signaling controls the development of a novel type of Thy1-expressing DC in the thymus that possibly controls negative selection, and indicates that there may be highly regulated, differential transcriptional control of DC development. Furthermore, our findings suggest that Notch signaling regulates T-cell development not only by intrinsically inducing T-cell lineage-specific gene programs, but also by regulating negative selection through Thy1(+) DCs.

Norwalk virus does not replicate in human macrophages or dendritic cells derived from the peripheral blood of susceptible humans

Human noroviruses are difficult to study due to the lack of an efficient in vitro cell culture system or small animal model. Murine norovirus replicates in murine macrophages (MPhi) and dendritic cells (DCs), raising the possibility that human NoVs might replicate in such human cell types. To test this hypothesis, we evaluated DCs and MPhi derived from monocyte subsets and CD11c(+) DCs isolated from peripheral blood mononuclear cells of individuals susceptible to Norwalk virus (NV) infection. These cells were exposed to NV and replication was evaluated by immunofluorescence and by quantitative RT-PCR. A few PBMC-derived DCs expressed NV proteins. However, NV RNA did not increase in any of the cells tested. These results demonstrate that NV does not replicate in human CD11c(+) DCs, monocyte-derived DCs and MPhi, but abortive infection may occur in a few DCs. These results suggest that NV tropism is distinct from that of murine noroviruses.

Norwalk virus does not replicate in human macrophages or dendritic cells derived from the peripheral blood of susceptible humans

Human noroviruses are difficult to study due to the lack of an efficient in vitro cell culture system or small animal model. Murine norovirus replicates in murine macrophages (MPhi) and dendritic cells (DCs), raising the possibility that human NoVs might replicate in such human cell types. To test this hypothesis, we evaluated DCs and MPhi derived from monocyte subsets and CD11c(+) DCs isolated from peripheral blood mononuclear cells of individuals susceptible to Norwalk virus (NV) infection. These cells were exposed to NV and replication was evaluated by immunofluorescence and by quantitative RT-PCR. A few PBMC-derived DCs expressed NV proteins. However, NV RNA did not increase in any of the cells tested. These results demonstrate that NV does not replicate in human CD11c(+) DCs, monocyte-derived DCs and MPhi, but abortive infection may occur in a few DCs. These results suggest that NV tropism is distinct from that of murine noroviruses.

[Identification and enumeration of dendritic cells in peripheral blood and bone marrow of healthy individuals]

The study was aimed to analyse and enumerate the dendritic cells (DC) subsets in peripheral blood and bone marrow (BM) of healthy individuals in China by using 2 panel 4-color flow cytometry (FCM). The percentage and absolute number of Lin-HLA-DR+CD11chiBDCA1+ myeloid DC (mDC) and Lin-HLA-DR+CD123hiBDCA2+ plasmacytoid DC (pDC) were detected in 35 normal BM (NBM) and 29 normal peripheral blood (NPB), the results were compared with the Lin-HLA-DR+CD11chimDC and Lin-HLA-DR+CD123hi pDC obtained by 3-color FCM. The results indicated that both absolute count of DC subset and relative count of pDC in BM were decreased along with increase of age, the absolute count of DC subset in male BM was higher than that in femoral BM (p<0.05). The DC subsets in NBM and NPB were different whatever by 3 or 4-color cytometric analysis, there were more mDCs than pDCs in PB and the ratio of mDC to pDC was 2.70 and 2.31 respectively. In contrast, pDCs predominated in BM, the ratio of mDC to pDC in BM was 0.90 and 0.71 respectively. The quantity of DC subsets significantly correlated to both frequency (mDC r=0.86; pDC r=0.96, p<0.05) and absolute number (mDC r=0.95; pDC r=0.98, p<0.05) between 3 and 4-color cytometric analysis. The quantity of DC subsets in PB and BM were significantly different, counted by 3 and 4-color cytometric analysis except pDC in PB (p<0.001). The quantity of DC subsets were much higher by 3-color than that by 4-color analysis. Since some Lin-HLA-DR+CD11chimDC and Lin-HLA-DR+CD123hi pDC were BDCA1- and BDCA-2dim/- respectively, that more were in BM than in PB. It is concluded that the DC absolute enumeration is correlated with sample type, gender, age and total nucleated cells (p<0.05). 4-color antibody combination may help to identify the real DC subsets in BM. DC subsets in NBM and NPB are different, that more mDC are in PB whereas more pDC in BM.

Differences in circulating dendritic cell subtypes in pregnant women, cord blood and healthy adult women

Different subtypes of dendritic cells (DC) influence the differentiation of naíve T lymphocytes into T helper type 1 (Th1) and Th2 effector cells. We evaluated the percentages of DC subtypes in peripheral blood from pregnant women (maternal blood) and their cord blood compared to the peripheral blood of healthy non pregnant women (control). Circulating DC were identified by flow cytometry as lineage (CD3, CD14, CD16, CD19, CD20, and CD56)-negative and HLA-DR-positive cells. Subtypes of DC were further characterized as myeloid DC (CD11c(+)/CD123(+/-)), lymphoid DC (CD11c(-)/CD123(+++)) and less differentiated DC (CD11c(-)/CD123(+/-)). The frequency of DC out of all nucleated cells was significantly lower in maternal blood than in control (P<0.001). The ratio of myeloid DC/lymphoid DC was significantly higher in maternal blood than in control (P<0.01). HLA-DR expressions of myeloid DC as mean fluorescence intensity (MFI) were significantly less in maternal blood and in cord blood than in control (P<0.001, respectively). The DC differentiation factors, TNF-alpha and GM-CSF, released from mononuclear cells after lipopolysaccharide stimulation were significantly lower in maternal blood than in control (P<0.01). The distribution of DC subtypes was different in maternal and cord blood from those of non-pregnant women. Their role during pregnancy remains to be determined.

[Classification and functional study of peripheral blood dendritic cells in patients with coronary artery disease with different atherosclerotic plaques]

OBJECTIVE

To study the quantitative and functional changes of peripheral blood dendritic cells (DCs) and their subsets in the leukocyte population in patients with coronary artery disease (CHD) with different coronary artery plaques and explore the relation between DCs and coronary plaque development.

METHODS

Thirty CHD patients were divided into SAP (10 cases), UAP (10 cases) and ACS (10 cases) groups, with another 10 patients having negative result in coronary angiography as the control group. Intravascular ultrasound (IVUS) was performed to identify the nature of the plaques. The percentage and absolute number of peripheral blood DCs and DC subsets were measured by flow cytometry. The functional status of the DCs was analyzed by enzyme-linked immunosorbent assay (ELISA) and flow cytometry.

RESULTS

In the SAP group, IVUS found stable plaques in 8 cases and unstable plaques in 2 cases; in UAP group, 7 patients had unstable plaques, 2 had stable plaques, and 1 had plaque rupture. Plaque rupture, unstable plaques and stable plaques were found in 6, 3 and 1 patients in ACS group, respectively. In comparison with patients with stable plaques, those with unstable plaques had significantly increased percentages and number of DCs, mDCs and mDC1 (P<0.05), while the mDC2s and pDCs showed no obvious difference between them (P>0.05). The percentages and number of DCs, mDCs, mDC1s and pDCs were significantly decreased in patients with ruptured plaques (P<0.05). In peripheral blood monouclear cells cultured for 7 days, the CD83 expression was significantly higher in unstable and rupture plaque groups than in stable plaque group, and no significant difference was found between stable plaque group and the control group (P>0.05). In unstable and rupture plaque groups, co-culture with 2x10(5)/ml DCs evoked strong proliferation of the T cells in comparison with the stable plaque group, but no difference was found between the stable plaque and the control groups (P>0.05). Significantly higher levels of interleukin-2 and interferon-alpha were detected in the supernatant of the mixed lymphocyte reaction in unstable and ruptured plaque groups than in stable plaque and control groups, without obvious difference between the latter two groups.

CONCLUSION

The percentage and absolute number of peripheral blood DCs and their functional status suggest the alterations of the coronary artery plaques in CHD patients.

An improved ontological representation of dendritic cells as a paradigm for all cell types

BACKGROUND

Recent increases in the volume and diversity of life science data and information and an increasing emphasis on data sharing and interoperability have resulted in the creation of a large number of biological ontologies, including the Cell Ontology (CL), designed to provide a standardized representation of cell types for data annotation. Ontologies have been shown to have significant benefits for computational analyses of large data sets and for automated reasoning applications, leading to organized attempts to improve the structure and formal rigor of ontologies to better support computation. Currently, the CL employs multiple is_a relations, defining cell types in terms of histological, functional, and lineage properties, and the majority of definitions are written with sufficient generality to hold across multiple species. This approach limits the CL's utility for computation and for cross-species data integration.

RESULTS

To enhance the CL's utility for computational analyses, we developed a method for the ontological representation of cells and applied this method to develop a dendritic cell ontology (DC-CL). DC-CL subtypes are delineated on the basis of surface protein expression, systematically including both species-general and species-specific types and optimizing DC-CL for the analysis of flow cytometry data. We avoid multiple uses of is_a by linking DC-CL terms to terms in other ontologies via additional, formally defined relations such as has_function.

CONCLUSION

This approach brings benefits in the form of increased accuracy, support for reasoning, and interoperability with other ontology resources. Accordingly, we propose our method as a general strategy for the ontological representation of cells. DC-CL is available from http://www.obofoundry.org.

Human dendritic cell subsets for vaccination

Protective immunity results from the interplay of antigen (Ag)-nonspecific innate immunity and Ag-specific adaptive immunity. The cells and molecules of the innate system employ non-clonal recognition pathways such as lectins and TLRs. B and T lymphocytes of the adaptive immune system employ clonal receptors recognizing Ag or peptides in a highly specific manner. An essential link between innate and adaptive immunity is provided by dendritic cells (DCs). As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology that shows the complexity of this cell system. Thus, presentation of antigen by immature (non-activated) DCs leads to tolerance, whereas mature, antigen-loaded DCs are geared towards the launching of antigen-specific immunity. Furthermore, DCs are composed of multiple subsets with distinct functions at the interface of the innate and adaptive immunity. Our increased understanding of DC pathophysiology will permit their rational manipulation for therapy such as vaccination to improve immunity.

Big tumor regression induced by GM-CSF gene-modified 3LL tumor cells via facilitating DC maturation and deviation toward CD11c+CD8alpha+ subset

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a powerful immune-stimulating factor that helps to generate a systemic, strong, and long-lasting immune response. However, whether the transduction of GM-CSF to tumor cell results in tumor regression and optimizes local immune microenvironment remains to be investigated. In this study, using an experimental murine tumor model, we demonstrated that the in vivo growth of 3LL tumor cells modified with the GM-CSF gene (3LL-GM) was inhibited even when the tumor diameter was over 7 mm (big tumor), and mice inoculated with GM-CSF gene-modified 3LL cells survived over 90 days, whereas mice inoculated with control parental 3LL cells and 3LL cells transduced with control vector all succumbed to the tumor by day 17 after tumor inoculation. Further analysis showed that targeted expression of GM-CSF in 3LL tumor cells markedly enhanced the systemic antitumor effect, including specific lymphocytes proliferation, cytotoxicity against 3LL tumor, and increased production of IFN-gamma. GM-CSF gene-modified 3LL cells significantly protected the mice from the parental 3LL tumor challenge. More importantly, the percentage of dendritic cells (DCs) in tumor site was greatly increased and the DCs differentiated into CD11c(+)CD8alpha(+) cells, which were reported to be able to benefit the induction of CD8(+) cytotoxic T lymphocytes (CTLs) that contribute to tumor regression. Our research indicated that GM-CSF could optimize the immune microenvironment in the tumor site, which provides a potent approach for immunotherapy of tumors.

Plasmacytoid dendritic cells and the regulation of immunoglobulin heavy chain class switching

By substituting the heavy chain constant region of IgM and IgD with that of IgG, IgA or IgE, immunoglobulin class switching endows antibodies with novel effector functions that enhance the ability of the immune system to effectively clear invading pathogens. Plasmacytoid dendritic cells critically link innate immunity with adaptive immunity by producing massive amounts of type 1 IFN in response to viruses. We have recently found that type 1 IFN triggers class switching by inducing myeloid dendritic cells to upregulate the expression of BAFF and APRIL, two powerful B cell-activating molecules. In this paper, we propose that IFN-producing plasmacytoid dendritic cells modulate class switching by activating B cells through both T cell-dependent and T cell-independent pathways. A better understanding of these pathways may facilitate the development of novel antiviral vaccine strategies and aid in identifying new therapies for antibody-mediated autoimmune disorders, such as lupus.

Phenotypic and functional characterization of mature dendritic cells from pediatric cancer patients

BACKGROUND

Dendritic cells (DCs) are the most potent antigen-presenting cells of the immune system. Clinical trials have demonstrated that mature DCs loaded with tumor-associated antigens can induce tumor-specific immune responses. Theoretically, pediatric patients are excellent candidates for immunotherapy since their immune system is more potent compared to adults. We studied whether sufficient amounts of mature monocyte-derived DCs can be cultured from peripheral blood of pediatric cancer patients.

PROCEDURE

DCs from 15 pediatric patients with an untreated primary tumor were cultured from monocytes and matured with clinical grade cytokines. Phenotype and function were tested with flow cytometry, mixed lymphocyte reaction (MLR), and an in vitro migration assay. DCs of children with a solid tumor were compared with monocyte-derived DCs from age-related non-malignant controls.

RESULTS

Ex vivo-generated monocyte-derived DCs from pediatric patients can be generated in numbers sufficient for DC vaccination trials. Upon cytokine stimulation the DCs highly upregulate the expression of the maturation markers CD80, CD83, and CD86. The mature DCs are six times more potent in inducing T cell proliferation compared to immature DCs. Furthermore, mature DCs, but not immature DCs, express the chemokine receptor CCR7 and have the capacity to migrate in vitro.

CONCLUSIONS

These data indicate that mature DCs can be generated ex vivo to further optimize DC-vaccination trials in pediatric cancer patients.

Dendritic cell type determines the mechanism of bystander suppression by adaptive T regulatory cells specific for the minor antigen HA-1

One hallmark of acquired tolerance is bystander suppression, a process whereby Ag-specific (adaptive) T regulatory cells (TR) inhibit the T effector cell response both to specific Ag and to a colocalized third-party Ag. Using peripheral blood T cells from recipients of HLA-identical kidney transplants as responders in the trans vivo-delayed type hypersensitivity assay, we found that dendritic cells (DC), but not monocyte APCs, could mediate bystander suppression of EBV-specific recall response. When HA-1(H) peptide was added to mixtures of plasmacytoid DC (pDC) and T cells, bystander suppression of the response to a colocalized recall Ag occurred primarily via indolamine-2,3-dioxygenase (IDO) production. Similarly, addition of HA-1(H) peptide to cocultures of T cells and pDC, but not myeloid DC (mDC), induced IDO activity in vitro. When mDC presented HA-1(H) peptide to Ag-specific CD8+ TR, cytokine release (TGF-beta, IL-10, or both) was the primary mode of bystander suppression. Bystander suppression via mDC was reversed not only by Ab to TGF-beta and its receptor on T cells, but also by Ab to thrombospondin-1. EBV addition did not induce IDO or thrombospondin-1 in T-DC cocultures, suggesting that these DC products are not induced by T effector cells, but only by TR cells. These results shed light upon the mechanism of bystander suppression by donor Ag-specific TR in patients with organ transplant tolerance and underscores the distinct and critical roles of mDC and pDCs in this phenomenon.

Differential activation of human monocyte-derived and plasmacytoid dendritic cells by West Nile virus generated in different host cells

Dendritic cells (DCs) play a central role in innate immunity and antiviral responses. In this study, we investigated the production of alpha interferon (IFN-alpha) and inducible chemokines by human monocyte-derived dendritic cells (mDCs) and plasmacytoid dendritic cells (pDCs) infected with West Nile virus (WNV), an emergent pathogen whose infection can lead to severe cases of encephalitis in the elderly, children, and immunocompromised individuals. Our experiments demonstrated that WNV grown in mammalian cells (WNV(Vero)) was a potent inducer of IFN-alpha secretion in pDCs and, to a lesser degree, in mDCs. The ability of WNV(Vero) to induce IFN-alpha in pDCs did not require viral replication and was prevented by the treatment of cells with bafilomycin A1 and chloroquine, suggesting that it was dependent on endosomal Toll-like receptor recognition. On the other hand, IFN-alpha production in mDCs required viral replication and was associated with the nuclear translocation of IRF3 and viral antigen expression. Strikingly, pDCs failed to produce IFN-alpha when stimulated with WNV grown in mosquito cells (WNV(C7/10)), while mDCs responded similarly to WNV(Vero) or WNV(C7/10). Moreover, the IFN-dependent chemokine IP-10 was produced in substantial amounts by pDCs in response to WNV(Vero) but not WNV(C7/10), while interleukin-8 was produced in greater amounts by mDCs infected with WNV(C7/10) than in those infected with WNV(Vero). These findings suggest that cell-specific mechanisms of WNV recognition leading to the production of type I IFN and inflammatory chemokines by DCs may contribute to both the innate immune response and disease pathogenesis in human infections.

Dendritic cell neoplasms: an overview

Dendritic cell neoplasms are rare tumors that are being recognized with increasing frequency. They were previously classified as lymphomas, sarcomas, or histiocytic neoplasms. The World Health Organization (WHO) classifies dendritic cell neoplasms into five groups: Langerhans' cell histiocytosis, Langerhans' cell sarcoma, Interdigitating dendritic cell sarcoma/tumor, Follicular dendritic cell sarcoma/tumor, and Dendritic cell sarcoma, not specified otherwise (Jaffe, World Health Organization classification of tumors 2001; 273-289). Recently, Pileri et al. provided a comprehensive immunohistochemical classification of histiocytic and dendritic cell tumors (Pileri et al., Histopathology 2002;59:161-167). In this article, a concise overview regarding the pathological, clinical, and therapeutic aspects of follicular dendritic, interdigitating dendritic, and Langerhans' cell tumors is presented.

Myeloid and plasmacytoid dendritic cells are susceptible to recombinant adenovirus vectors and stimulate polyfunctional memory T cell responses

Although replication-incompetent recombinant adenovirus (rAd) type 5 is a potent vaccine vector for stimulating T and B cell responses, high seroprevalence of adenovirus type 5 (Ad5) within human populations may limit its clinical utility. Therefore, alternative adenovirus serotypes have been studied as vaccine vectors. In this study, we characterized the ability of rAd5 and rAd35 to infect and induce maturation of human CD11c(+) myeloid dendritic cells (MDCs) and CD123(+) plasmacytoid dendritic cells (PDCs), and their ability to stimulate Ag-specific T cells. Both MDCs and PDCs were found to express the primary receptor for Ad35 (CD46) but not Ad5 (coxsackie-adenovirus receptor; CAR). Both dendritic cell (DC) subsets were also more susceptible to rAd35 than to rAd5. MDCs were more susceptible to both rAd35 and rAd5 than were PDCs. Whereas rAd35 used CD46 for entry into DCs, entry of rAd5 may be through a CAR-independent pathway. Exposure to rAd35 but not rAd5 induced high levels of IFN-alpha in PDCs and phenotypic differentiation in both DC subsets. MDCs and PDCs exposed to either rAd5 or rAd35 encoding for CMV pp65 were able to present pp65 and activate CMV-specific memory CD8(+) and CD4(+) T cells in a dose-dependent manner, but MDCs stimulated the highest frequencies of pp65-specific T cells. Responding T cells expressed multiple functions including degranulation (CD107a surface mobilization) and production of IFN-gamma, IL-2, TNF-alpha, and MIP-1beta. Thus, the ability of rAd35 to naturally target important DC subsets, induce their maturation, and appropriately present Ag to T cells may herald greater in vivo immunogenicity than has been observed with rAd5.

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.

Dendritic cells: nature and classification

Dendritic cells (DCs) are antigen (Ag)-presenting cells (APCs) characterized by a unique capacity to stimulate naive T cells and initiate primary immune responses. Recent studies suggest that DCs also play critical roles in the induction of central and peripheral immunological tolerance, regulate the types of T cell immune responses, and function as sentinels in innate immunity against microbes. The diverse functions of DCs in immune regulation depend on the heterogeneity of DC subsets and their functional plasticity. Here we review recent progress in our understanding of the nature and classification of DCs.

The dynamics of dendritic cell: mediated innate immune regulation

After taking up pathogen-derived antigens, dendritic cells (DCs) leave peripheral organs and migrate into sentinel lymph nodes via afferent lymphatic vessels. During this process, they undergo maturation and produce proinflammatory cytokines, which leads to efficient antigen (Ag) presentation and activation of the innate and acquired immune systems. Recent evidence indicates that DC subsets cooperate to activate the innate immune system. It is becoming clear that the total DC population is composed of a network of DC subsets with distinct functions that are critical for sensing pathogens and orchestrating immune responses.