Isolation of myeloid and plasmacytoid dendritic cells from human bronchoalveolar lavage fluid

Airway Macrophage and Dendritic Cell Subsets in the Resting Human Lung

Dendritic cells (DCs) and macrophages (MΦs) are antigen-presenting phagocytic cells found in many peripheral tissues of the human body, including the blood, lymph nodes, skin, and lung. They are vital to maintaining steady-state respiration in the human lung based on their ability to clear airways while also directing tolerogenic or inflammatory responses based on specific stimuli. Over the past three decades, studies have determined that there are multiple subsets of these two general cell types that exist in the airways and interstitium. Identifying these numerous subsets has proven challenging, especially with the unique microenvironments present in the lung. Cells found in the vasculature are not the same subsets found in the skin or the lung, as demonstrated by surface marker expression. By transcriptional profiling, these subsets show similarities but also major differences. Primary human lung cells and/ or tissues are difficult to acquire, particularly in a healthy condition. Additionally, surface marker screening and transcriptional profiling are continually identifying new DC and MΦ subsets. While the overall field is moving forward, we emphasize that more attention needs to focus on replicating the steady-state microenvironment of the lung to reveal the physiological functions of these subsets.

Allergen-induced Changes in Bone Marrow and Airway Dendritic Cells in Subjects with Asthma

RATIONALE

Dendritic cells (DCs) are antigen-presenting cells essential for the initiation of T-cell responses. Allergen inhalation increases the number of airway DCs and the release of epithelial-derived cytokines, such as IL-33 and thymic stromal lymphopoietin (TSLP), that activate DCs.

OBJECTIVES

To examine the effects of inhaled allergen on bone marrow production of DCs and their trafficking into the airways in subjects with allergic asthma, and to examine IL-33 and TSPL receptor expression on DCs.

METHODS

Bone marrow, peripheral blood, bronchoalveolar lavage (BAL), and bronchial biopsies were obtained before and after inhalation of diluent and allergen from subjects with asthma that develop allergen-induced dual responses. Classical DCs (cDCs) were cultured from bone marrow CD34(+) cells. cDC1s, cDC2s, and plasmacytoid DCs were measured in bone marrow aspirates, peripheral blood, and BAL by flow cytometry, and cDCs were quantified in bronchial biopsies by immunofluorescence staining.

MEASUREMENTS AND MAIN RESULTS

Inhaled allergen increased the number of cDCs grown from bone marrow progenitors, and cDCs and plasmacytoid DCs in bone marrow aspirates 24 hours after allergen. Allergen also increased the expression of the TSLP receptor, but not the IL-33 receptor, on bone marrow DCs. Finally, inhaled allergen increased the percentage of cDC1s and cDC2s in BAL but only cDC2s in bronchial tissues.

CONCLUSIONS

Inhaled allergen increases DCs in bone marrow and trafficking of DCs into the airway, which is associated with the development airway inflammation in subjects with allergic asthma. Inhaled allergen challenge also increases expression of TSLP, but not IL-33, receptors on bone marrow DCs.

Human Lung Mononuclear Phagocytes in Health and Disease

The lungs are vulnerable to attack by respiratory insults such as toxins, allergens, and pathogens, given their continuous exposure to the air we breathe. Our immune system has evolved to provide protection against an array of potential threats without causing collateral damage to the lung tissue. In order to swiftly detect invading pathogens, monocytes, macrophages, and dendritic cells (DCs)-together termed mononuclear phagocytes (MNPs)-line the respiratory tract with the key task of surveying the lung microenvironment in order to discriminate between harmless and harmful antigens and initiate immune responses when necessary. Each cell type excels at specific tasks: monocytes produce large amounts of cytokines, macrophages are highly phagocytic, whereas DCs excel at activating naïve T cells. Extensive studies in murine models have established a division of labor between the different populations of MNPs at steady state and during infection or inflammation. However, a translation of important findings in mice is only beginning to be explored in humans, given the challenge of working with rare cells in inaccessible human tissues. Important progress has been made in recent years on the phenotype and function of human lung MNPs. In addition to a substantial population of alveolar macrophages, three subsets of DCs have been identified in the human airways at steady state. More recently, monocyte-derived cells have also been described in healthy human lungs. Depending on the source of samples, such as lung tissue resections or bronchoalveolar lavage, the specific subsets of MNPs recovered may differ. This review provides an update on existing studies investigating human respiratory MNP populations during health and disease. Often, inflammatory MNPs are found to accumulate in the lungs of patients with pulmonary conditions. In respiratory infections or inflammatory diseases, this may contribute to disease severity, but in cancer patients this may improve clinical outcomes. By expanding on this knowledge, specific lung MNPs may be targeted or modulated in order to attain favorable responses that can improve preventive or treatment strategies against respiratory infections, lung cancer, or lung inflammatory diseases.

Transcriptional Classification and Functional Characterization of Human Airway Macrophage and Dendritic Cell Subsets

The respiratory system is a complex network of many cell types, including subsets of macrophages and dendritic cells that work together to maintain steady-state respiration. Owing to limitations in acquiring cells from healthy human lung, these subsets remain poorly characterized transcriptionally and phenotypically. We set out to systematically identify these subsets in human airways by developing a schema of isolating large numbers of cells by whole-lung bronchoalveolar lavage. Six subsets of phagocytic APC (HLA-DR+) were consistently observed. Aside from alveolar macrophages, subsets of Langerin+, BDCA1-CD14+, BDCA1+CD14+, BDCA1+CD14-, and BDCA1-CD14- cells were identified. These subsets varied in their ability to internalize Escherichia coli, Staphylococcus aureus, and Bacillus anthracis particles. All subsets were more efficient at internalizing S. aureus and B. anthracis compared with E. coli Alveolar macrophages and CD14+ cells were overall more efficient at particle internalization compared with the four other populations. Subsets were further separated into two groups based on their inherent capacities to upregulate surface CD83, CD86, and CCR7 expression levels. Whole-genome transcriptional profiling revealed a clade of "true dendritic cells" consisting of Langerin+, BDCA1+CD14+, and BDCA1+CD14- cells. The dendritic cell clade was distinct from a macrophage/monocyte clade, as supported by higher mRNA expression levels of several dendritic cell-associated genes, including CD1, FLT3, CX3CR1, and CCR6 Each clade, and each member of both clades, was discerned by specific upregulated genes, which can serve as markers for future studies in healthy and diseased states.

Farm exposures are associated with lower percentage of circulating myeloid dendritic cell subtype 2 at age 6

BACKGROUND

Early life farm exposures have been shown to decrease the risk of allergic diseases. Dendritic cells (DCs) may mediate asthma-protective effect of farm exposures as they play an important role in the development of immunity and tolerance. Our aim was to investigate whether the numbers and phenotypes of circulating DCs at age 6 are associated with farming, asthma, and atopy in a selected sample of French and Finnish children from the PASTURE study.

METHODS

We studied 82 farm and 86 nonfarm children with and without asthma. Using flow cytometry, BDCA1+ CD11c+ myeloid DC1s (mDC1), BDCA3+(high) mDC2s and BDCA2+ plasmacytoid DCs (pDCs) were identified and expressions of CD86, immunoglobulin-like transcript 3 (ILT3) and ILT4 were analyzed. Questionnaires were used to assess prenatal and lifetime patterns of farm exposures and to define asthma. Atopic sensitization was defined by specific IgE measurements.

RESULTS

The percentage of mDC2 cells was lower in farm children (0.033 ± 0.001) than in nonfarm children (0.042 ± 0.001; P = 0.008). Similar associations were found between mDC2 percentage and prenatal (P = 0.02) and lifetime exposure to farm milk (P = 0.03) and stables (P = 0.003), but these associations were not independent from farming. Asthma was positively associated with ILT4 + mDCs (P = 0.04) and negatively with CD86 + pDCs (P = 0.048) but only in nonfarm children.

CONCLUSIONS

Inverse association between farm exposure and mDC2 percentage suggest that this DC subset may play a role in farm-related immunoregulation.

Allergen-induced airway responses

Environmental allergens are an important cause of asthma and can contribute to loss of asthma control and exacerbations. Allergen inhalation challenge has been a useful clinical model to examine the mechanisms of allergen-induced airway responses and inflammation. Allergen bronchoconstrictor responses are the early response, which reaches a maximum within 30 min and resolves by 1-3 h, and late responses, when bronchoconstriction recurs after 3-4 h and reaches a maximum over 6-12 h. Late responses are followed by an increase in airway hyperresponsiveness. These responses occur when IgE on mast cells is cross-linked by an allergen, causing degranulation and the release of histamine, neutral proteases and chemotactic factors, and the production of newly formed mediators, such as cysteinyl leukotrienes and prostaglandin D2. Allergen-induced airway inflammation consists of an increase in airway eosinophils, basophils and, less consistently, neutrophils. These responses are mediated by the trafficking and activation of myeloid dendritic cells into the airways, probably as a result of the release of epithelial cell-derived thymic stromal lymphopoietin, and the release of pro-inflammatory cytokines from type 2 helper T-cells. Allergen inhalation challenge has also been a widely used model to study potential new therapies for asthma and has an excellent negative predictive value for this purpose.

BDCA3(+)CLEC9A(+) human dendritic cell function and development

Dendritic cells (DC) are the most potent antigen presenting cells (APC). They comprise a family of different subsets and play an essential role in the induction and regulation of immune responses. Recently, gene expression profiling identified BDCA3(+)CLEC9A(+) DC as a separate human DC subset. This subset was identified in blood, where they represent the smallest population of human DC, as well as in lymphoid and peripheral tissues. This review summarizes the phenotypic, functional and developmental characteristics of BDCA3(+)CLEC9A(+) DC in relation to their mouse equivalents CD8α(+) DC and CD103(+) DC and other human DC subsets. Apart from being potent antigen presenting cells, their specialized functional capacities compared to other human DC subsets, indicate that these BDCA3(+)CLEC9A(+) DC are of major importance in the induction of anti-viral and anti-tumor immunity. Further characterization of their functional properties, developmental pathways and underlying molecular mechanisms may identify target molecules to fully exploit the immune modulatory function of BDCA3(+)CLEC9A(+) DC and potential use of these cells in immunotherapy.

Myeloid dendritic cells type 2 after allergen inhalation in asthmatic subjects

BACKGROUND

Dendritic cells (DCs) are professional antigen-presenting cells that mediate the response to inhaled allergen. A major division in DC ontogeny exists between myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). A subtype of mDC expressing thrombomodulin, termed myeloid DCs type 2 (mDC2s), has been identified in both the circulation and lung and has recently been suggested to have a role in allergic asthma.

OBJECTIVE

To investigate changes in circulating and sputum mDC2s after allergen inhalation in subjects with asthma.

METHODS

Peripheral blood and induced sputum were obtained before and 3, 7, and 24 h after inhalation of diluent and allergen from allergic asthmatic subjects who develop both allergen-induced early- and late-phase responses. mDC2s were measured by flow cytometry. Soluble BDCA-3 (thrombomodulin) was measured in sputum by ELISA.

RESULTS

The number of sputum mDC2s significantly increased 24 h after allergen challenge compared with diluent. The expression of BDCA-3 on sputum mDCs also increased, albeit non-significantly, at 7 and 24 h after allergen. Soluble BDCA-3 in sputum and the number of circulating mDC2s were not different between allergen and diluent.

CONCLUSIONS AND CLINICAL RELEVANCE

Myeloid DCs type 2 (mDC2s) increase in the sputum of subjects with asthma after allergen challenge, suggesting this subtype of mDC is involved in the regulation of allergen responses in the lung.

Elevated presence of myeloid dendritic cells in nasal polyps of patients with chronic rhinosinusitis

BACKGROUND

Although chronic rhinosinusitis with nasal polyps (CRSwNP) is characterized by Th2 inflammation, the mechanism underlying the onset and amplification of this inflammation has not been fully elucidated. Dendritic cells (DCs) are major antigen-presenting cells, central inducers of adaptive immunity and critical regulators of many inflammatory diseases. However, the presence of DCs in CRS, especially in nasal polyps (NPs), has not been extensively studied.

OBJECTIVE

The objective of this study was to characterize DC subsets in CRS.

METHODS

We used real-time PCR to assess the expression of mRNA for markers of myeloid DCs (mDCs; CD1c), plasmacytoid DCs (pDCs; CD303) and Langerhans cells (LCs; CD1a, CD207) in uncinate tissue (UT) from controls and patients with CRS as well as in NP. We assayed the presence of DCs by immunohistochemistry and flow cytometry.

RESULTS

Compared to UT from control subjects (n = 15) and patients with CRS without NP (CRSsNP) (n = 16) and CRSwNP (n = 17), mRNAs for CD1a and CD1c were significantly elevated in NPs (n = 29). In contrast, CD207 mRNA was not elevated in NPs. Immunohistochemistry showed that CD1c(+) cells but not CD303(+) cells were significantly elevated in NPs compared to control subjects or patients with CRSsNP. Flow cytometric analysis showed that CD1a(+) cells in NPs might be a subset of mDC1s and that CD45(+) CD19(-) CD1c(+) CD11c(+) CD141(-) CD303(-) HLA-DR(+) mDC1s and CD45(+) CD19(-) CD11c(+) CD1c(-) CD141(high) HLA-DR(+) mDC2s were significantly elevated in NPs compared to UT from controls and CRSsNP, but CD45(+) CD11c(-) CD303(+) HLA-DR(+) pDCs were only elevated in NPs compared to control UT.

CONCLUSION AND CLINICAL RELEVANCE

Myeloid DCs are elevated in CRSwNP, especially in NPs. Myeloid DCs thus may indirectly contribute to the inflammation observed in CRSwNP.

Tolerogenic signaling by pulmonary CD1c+ dendritic cells induces regulatory T cells in patients with chronic obstructive pulmonary disease by IL-27/IL-10/inducible costimulator ligand

BACKGROUND

Increased mortality rates in patients with chronic obstructive pulmonary disease (COPD) are largely due to severe infectious exacerbations. Impaired respiratory immunity is linked to the enhanced susceptibility to infections. Dendritic cells (DCs) direct host immune responses toward immunity or tolerance. Pulmonary CD1c(+) DCs elicit robust antiviral immune responses in healthy subjects. Nevertheless, their functional specialization in patients with COPD remains unexplored.

OBJECTIVE

We sought to better understand the mechanisms that suppress respiratory immunity in patients with COPD by examining the immunostimulatory and tolerogenic properties of pulmonary CD1c(+) DCs.

METHODS

We analyzed the expression of costimulatory and tolerogenic molecules by pulmonary CD1c(+) DCs from patients with COPD (CD1c(+)DCCOPD) and former smokers without COPD. We isolated lung CD1c(+) DCs and determined their ability to stimulate allogeneic T-cell responses. The suppressive effects of lung CD1c(+) DCs and CD1c(+) DC-primed T cells on mixed leukocyte reactions were examined. An experimental human model of COPD exacerbation was used to investigate the levels of critical immunosuppressive molecules in vivo.

RESULTS

CD1c(+) DCs from patients with COPD hinder T-cell effector functions and favor the generation of suppressive IL-10-secreting CD4(+) T cells that function through IL-10 and TGF-β. IL-27, IL-10, and inducible T-cell costimulator ligand signaling are essential for CD1c(+)DCCOPD-mediated differentiation of IL-10-producing suppressive T cells. Exposure of lung CD1c(+) DCs from nonobstructed subjects to lungs of patients with COPD confers tolerogenic properties. IL-27 and IL-10 levels are increased in the lung microenvironment on rhinovirus-induced COPD exacerbation in vivo.

CONCLUSION

We identify a novel tolerogenic circuit encompassing suppressive CD1c(+) DCs and regulatory T cells in patients with COPD that might be implicated in impaired respiratory immunity and further highlight IL-10 and IL-27 as potent therapeutic targets.

Myeloid dendritic cells type 2 in allergic asthma

BACKGROUND

Myeloid dendritic cells type 2 (mDC2s) are a new subtype of DCs identified in both the circulation and the lung and suggested to have a role in allergic asthma.

METHODS

Circulating mDC2s were enumerated in 19 healthy, 18 atopic nonasthmatic, 18 mild atopic asthmatic, and 16 moderate/severe atopic asthmatic subjects using flow cytometry.

RESULTS

The number of circulating mDC2s was significantly lower in atopic subjects compared with healthy controls and in asthmatic subjects compared with nonasthmatic subjects. There was a trend toward lower levels of circulating mDC2s with increasing allergy and asthma severity. The largest differences were seen in moderate/severe atopic asthmatics being 430.78 ± 48.91/ml compared with healthy controls being 767.05 ± 101.64/ml (P < 0.05).

CONCLUSIONS

Circulating mDC2s are lower in atopic and asthmatic subjects, which suggests that these cells efflux from the blood into the airways in patients with allergic disease.

Pathological consequence of misguided dendritic cell differentiation in histiocytic diseases

Histiocytic disorders represent a group of complex pathologies characterized by the accumulation of histiocytes, an old term for tissue-resident macrophages and dendritic cells. Langerhans cell histiocytosis is the most frequent of histiocytosis in humans and has been thought to arise from the abnormal accumulation of epidermal dendritic cells called Langerhans cells. In this chapter, we discuss the origin and differentiation of Langerhans cells and dendritic cells and present accumulated evidence that suggests that Langerhans cell histiocytosis does not result from abnormal Langerhans cell homeostasis but rather is a consequence of misguided differentiation programs of myeloid dendritic cell precursors. We propose reclassification of Langerhans cell histiocytosis, juvenile xanthogranuloma, and Erdheim-Chester disease as inflammatory myeloid neoplasias.

Human tissues contain CD141hi cross-presenting dendritic cells with functional homology to mouse CD103+ nonlymphoid dendritic cells

Dendritic cell (DC)-mediated cross-presentation of exogenous antigens acquired in the periphery is critical for the initiation of CD8⁺ T cell responses. Several DC subsets are described in human tissues but migratory cross-presenting DCs have not been isolated, despite their potential importance in immunity to pathogens, vaccines, and tumors and tolerance to self. Here, we identified a CD141^hi DC present in human interstitial dermis, liver, and lung that was distinct from the majority of CD1c⁺ and CD14⁺ tissue DCs and superior at cross-presenting soluble antigens. Cutaneous CD141^hi DCs were closely related to blood CD141⁺ DCs, and migratory counterparts were found among skin-draining lymph node DCs. Comparative transcriptomic analysis with mouse showed tissue DC subsets to be conserved between species and permitted close alignment of human and mouse DC subsets. These studies inform the rational design of targeted immunotherapies and facilitate translation of mouse functional DC biology to the human setting.

Selective increase in blood dendritic cell antigen-3-positive dendritic cells in bronchoalveolar lavage fluid in allergic patients

Dendritic cells (DCs) are specific antigen-presenting cells that play critical roles in the initiation and polarization of immune responses. DCs residing in the lungs might be detected in the bronchoalveolar lavage fluid (BALF). We analysed DC compartment in the peripheral blood and BALF of patients with allergy and in controls. Plasmacytoid and four distinct subsets of myeloid DCs [characterized by the expression of blood dendritic cell antigen (BDCA)-1+ and -3+ and CD16 positivity or negativity] were detected in both tested compartments. We further evaluated the expression of C-type lectins [mannose receptor (MR), dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) and dendritic and epithelial cells (DEC)-205] relevant to the pathogenesis of asthma. Interestingly, we found a selective increase in the frequency of myeloid DC-expressing BDCA-3 and MR particularly in BALF from allergic patients. Specific and highly statistically significant increase in BDCA-3+ and/or MR+ DCs brings a novel characteristic to BAL analysis in allergic patients.

Resident CD141 (BDCA3)+ dendritic cells in human skin produce IL-10 and induce regulatory T cells that suppress skin inflammation

Human skin-resident IL-10⁺ regulatory dendritic cells induce T reg cells that suppress allogeneic skin graft inflammation.

Human skin immune homeostasis, and its regulation by specialized subsets of tissue-residing immune sentinels, is poorly understood. In this study, we identify an immunoregulatory tissue-resident dendritic cell (DC) in the dermis of human skin that is characterized by surface expression of CD141, CD14, and constitutive IL-10 secretion (CD141⁺ DDCs). CD141⁺ DDCs possess lymph node migratory capacity, induce T cell hyporesponsiveness, cross-present self-antigens to autoreactive T cells, and induce potent regulatory T cells that inhibit skin inflammation. Vitamin D₃ (VitD3) promotes certain phenotypic and functional properties of tissue-resident CD141⁺ DDCs from human blood DCs. These CD141⁺ DDC-like cells can be generated in vitro and, once transferred in vivo, have the capacity to inhibit xeno-graft versus host disease and tumor alloimmunity. These findings suggest that CD141⁺ DDCs play an essential role in the maintenance of skin homeostasis and in the regulation of both systemic and tumor alloimmunity. Finally, VitD3-induced CD141⁺ DDC-like cells have potential clinical use for their capacity to induce immune tolerance.

Efficient sensing of avian influenza viruses by porcine plasmacytoid dendritic cells

H5N1 influenza A virus (IAV) infections in human remain rare events but have been associated with severe disease and a higher mortality rate compared to infections with seasonal strains. An excessive release of pro-inflammatory cytokine together with a greater virus dissemination potential have been proposed to explain the high virulence observed in human and other mammalian and avian species. Among the cells involved in the cytokine storm, plasmacytoid dendritic cells (pDC) could play an important role considering their unique capacity to secrete massive amounts of type I interferon (IFN). Considering the role of IFN as a major component of antiviral responses as well as in priming inflammatory responses, we aimed to characterize the induction of IFN-α release upon infection with IAV originating from various avian and mammalian species in a comparative way. In our porcine pDC model, we showed that the viral components triggering IFN responses related to the ability to hemagglutinate, although virosomes devoid of viral RNA were non-stimulatory. Heat-treatment at 65 °C but not chemical inactivation destroyed the ability of IAV to stimulate pDC. All IAV tested induced IFN-α but at different levels and showed different dose-dependencies. H5 and H7 subtypes, in particular H5N1, stimulated pDC at lower doses when compared to mammalian IAV. At high viral doses, IFN-α levels reached by some mammalian IAV surpassed those induced by avian isolates. Although sialic acid-dependent entry was demonstrated, the α-2,3 or α-2,6 binding specificity alone did not explain the differences observed. Furthermore, we were unable to identify a clear role of the hemagglutinin, as the IFN-α doses-response profiles did not clearly differ when viruses with all genes of identical avian origin but different HA were compared. This was found with IAV bearing an HA derived from either a low, a high pathogenic H5N1, or a human H3. Stimulation of pDC was associated with pDC depletion within the cultures. Taken together and considering the efficient sensing of H5N1 at low dose, pDC on one side may play a role in the cytokine storm observed during severe disease, on the other hand could participate in early antiviral responses limiting virus replication.

Chimerism of dendritic cell subsets in peripheral blood after lung transplantation

BACKGROUND

Passenger leukocytes of donor origin are transferred to the patient resulting in circulatory microchimerism after lung transplantation (LTx). This chimeric state has been shown to occur in the total leukocyte fraction as well as unseparated peripheral blood mononuclear cells (PBMCs). In this study we determined the microchimerism levels of B cells, monocytes, natural killer (NK) and T cells and dendritic cell (DC) subsets (mDC1, mDC2 and pDC) during the first year after lung transplantation.

METHODS

To identify circulating donor cells, 11 donor-patient combinations were selected, which were mismatched for HLA-B8. Analysis consisted of flow cytometry on a minimum of 1 million PBMCs taken monthly up to 1 year after LTx.

RESULTS

Levels of microchimerism were found to be stable after LTx for all cell types investigated, although for NK+T cells an above-baseline chimerism of donor cells from the donor lung was observed in the first month after transplantation. Circulating PBMCs consisted of, on average, 0.002%, 1.7%, 0.03% and 0.001% of B cells, monocytes, NK+T cells and DCs, respectively, indicating that overall levels of microchimerism differed between the cell types investigated. In 2 patients no B-cell chimerism and in 1 patient no DC chimerism could be detected. Cell types and DC subsets of recipient origin were normally distributed. Conversely, monocytes, B cells and DCs of donor origin were increased and donor NK+T cells were decreased in number, compared with the recipient ratios. Analysis of circulating recipient DCs showed a normal distribution of mDC1s (70%), mDC2s (5%) and pDCs (25%). However, circulating donor DCs consisted of 80%, 20% and <1% of DC subsets mDC1, MDC2 and pDC, indicating that donor plasmacytoid dendritic cells were not detectable in the circulation.

CONCLUSIONS

In the first year after lung transplantation a stable microchimerism was detected for all cell types investigated. However, donor pDCs were consistently absent in all samples investigated, which may be linked with graft rejection often observed after LTx.

Harnessing dendritic cells in inflammatory skin diseases

The skin immune system harbors a complex network of dendritic cells (DCs). Recent studies highlight a diverse functional specialization of skin DC subsets. In addition to generating cellular and humoral immunity against pathogens, skin DCs are involved in tolerogenic mechanisms to ensure the maintenance of immune homeostasis, as well as in pathogenesis of chronic inflammation in the skin when excessive immune responses are initiated and unrestrained. Harnessing DCs by directly targeting DC-derived molecules or selectively modulate DC subsets is a convincing strategy to tackle inflammatory skin diseases. In this review we discuss recent advances underlining the functional specialization of skin DCs and discuss the potential implication for future DC-based therapeutic strategies.

Membrane-bound IL-15 stimulation on peripheral blood natural kiler progenitors leads to the generation of an adherent subset co-expressing dendritic cells and natural kiler functional markers

Human peripheral blood natural killer progenitors represent a flexible, heterogeneous population whose phenotype and function are controlled by their membrane-bound IL-15. Indeed, reciprocal membrane-bond IL-15 trans-presentation commits these cells into NK differentiation, while membrane-bound IL-15 stimulation with its soluble ligand (sIL-15Rα) triggers a reverse signal (pERK1/2 and pFAK) that modifies the developmental program of at least two subsets of PB-NKPs. This treatment generates: i) the expansion of an immature NK subset growing in suspension; ii) the appearance of an unprecedented adherent non-proliferative subset with a dendritic morphology co-expressing marker, cytokines and functions typical of myeloid dendritic cells (CD1a(+)/BDCA1(+)/IL-12(+)) and NK cells (CD3-/NKp46(+)/ CD56(+)/IFNγ(+)). The generation of these putative NK/DCs is associated to the rapid inhibition of negative regulators of myelopoiesis (the transcription factors STAT6 and GATA-3) followed by the transient upregulation of inducers of myeloid development, such as the transcription factors (PU.1, GATA-1) and the anti-apoptotic molecule (MCL-1).

Characterization of human DNGR-1+ BDCA3+ leukocytes as putative equivalents of mouse CD8alpha+ dendritic cells

In mouse, a subset of dendritic cells (DCs) known as CD8α⁺ DCs has emerged as an important player in the regulation of T cell responses and a promising target in vaccination strategies. However, translation into clinical protocols has been hampered by the failure to identify CD8α⁺ DCs in humans. Here, we characterize a population of human DCs that expresses DNGR-1 (CLEC9A) and high levels of BDCA3 and resembles mouse CD8α⁺ DCs in phenotype and function. We describe the presence of such cells in the spleens of humans and humanized mice and report on a protocol to generate them in vitro. Like mouse CD8α⁺ DCs, human DNGR-1⁺ BDCA3^hi DCs express Necl2, CD207, BATF3, IRF8, and TLR3, but not CD11b, IRF4, TLR7, or (unlike CD8α⁺ DCs) TLR9. DNGR-1⁺ BDCA3^hi DCs respond to poly I:C and agonists of TLR8, but not of TLR7, and produce interleukin (IL)-12 when given innate and T cell–derived signals. Notably, DNGR-1⁺ BDCA3⁺ DCs from in vitro cultures efficiently internalize material from dead cells and can cross-present exogenous antigens to CD8⁺ T cells upon treatment with poly I:C. The characterization of human DNGR-1⁺ BDCA3^hi DCs and the ability to grow them in vitro opens the door for exploiting this subset in immunotherapy.

Influenza-induced production of interferon-alpha is defective in geriatric individuals

BACKGROUND

The majority of deaths (90%) attributed to influenza are in person's age 65 or older. Little is known about whether defects in innate immune responses in geriatric individuals contribute to their susceptibility to influenza.

OBJECTIVE

Our aim was to analyze interferon-alpha (IFN-alpha) production in peripheral blood mononuclear cells (PBMCs) isolated from young and geriatric adult donors, stimulated with influenza A or Toll-like receptor (TLR) ligands. IFN-alpha is a signature anti-viral cytokine that also shapes humoral and cell-mediated immune responses.

RESULTS

Geriatric PBMCs produced significantly less IFN-alpha in response to live or inactivated influenza (a TLR7 ligand) but responded normally to CpG ODN (TLR9 ligand) and Guardiquimod (TLR7 ligand). All three ligands activate plasmacytoid dendritic cells (pDCs). While there was a modest decline in pDC frequency in older individuals, there was no defect in uptake of influenza by geriatric pDCs.

DISCUSSION AND CONCLUSION

Influenza-induced production of IFN-alpha was defective in geriatric PBMCs by a mechanism that was independent of reduced pDC frequency or viability, defects in uptake of influenza, inability to secrete IFN-alpha, or defects in TLR7 signaling.

Selective accumulation of langerhans-type dendritic cells in small airways of patients with COPD

Background

Dendritic cells (DC) linking innate and adaptive immune responses are present in human lungs, but the characterization of different subsets and their role in COPD pathogenesis remain to be elucidated. The aim of this study is to characterize and quantify pulmonary myeloid DC subsets in small airways of current and ex-smokers with or without COPD.

Methods

Myeloid DC were characterized using flowcytometry on single cell suspensions of digested human lung tissue. Immunohistochemical staining for langerin, BDCA-1, CD1a and DC-SIGN was performed on surgical resection specimens from 85 patients. Expression of factors inducing Langerhans-type DC (LDC) differentiation was evaluated by RT-PCR on total lung RNA.

Results

Two segregated subsets of tissue resident pulmonary myeloid DC were identified in single cell suspensions by flowcytometry: the langerin+ LDC and the DC-SIGN+ interstitial-type DC (intDC). LDC partially expressed the markers CD1a and BDCA-1, which are also present on their known blood precursors. In contrast, intDC did not express langerin, CD1a or BDCA-1, but were more closely related to monocytes.

Quantification of DC in the small airways by immunohistochemistry revealed a higher number of LDC in current smokers without COPD and in COPD patients compared to never smokers and ex-smokers without COPD. Importantly, there was no difference in the number of LDC between current and ex-smoking COPD patients.

In contrast, the number of intDC did not differ between study groups. Interestingly, the number of BDCA-1+ DC was significantly lower in COPD patients compared to never smokers and further decreased with the severity of the disease. In addition, the accumulation of LDC in the small airways significantly correlated with the expression of the LDC inducing differentiation factor activin-A.

Conclusions

Myeloid DC differentiation is altered in small airways of current smokers and COPD patients resulting in a selective accumulation of the LDC subset which correlates with the pulmonary expression of the LDC-inducing differentiation factor activin-A. This study identified the LDC subset as an interesting focus for future research in COPD pathogenesis.

A novel method for isolating dendritic cells from human bronchoalveolar lavage fluid

BACKGROUND

Dendritic cells (DCs) play a pivotal role in linking the innate and adaptive immune response and have been implicated in a variety of pulmonary diseases. Currently, studies on the role of DCs are limited by difficulties in isolating DCs from the lung. Surgical lung specimens are not readily available and purification of DCs from digested lung tissue is likely to induce phenotypical and functional changes. DCs obtained from the alveolar spaces are thought to represent the local microenvironment and can be obtained using minimally invasive techniques. We developed a novel method of isolating DCs from bronchoalveolar lavage (BAL) fluid.

METHODS

After removal of macrophages, the remaining BAL cells were stained with a lineage mix (CD3-, CD14-, CD16-, CD19-, CD56-FITC), CD11c and HLA-DR and sorted with a FACS ARIA. DAPI was used as a dead-live marker. mDCs were low autofluorescent, lineage mix negative, CD11c+ and HLA-DR+ cells. pDCs were CD11c(-) but CD123+. Morphological assessment of sorted mDCs and pDCs was performed. Sorted mDCs were tested in a mixed leukocyte reaction (MLR) with naive CD4+ T cells and evaluated for T cell differentiation and cytokine production. With confocal microscopy DC-T cell interaction was assessed.

RESULTS

Using our sorting strategy, mDCs and pDCs, with a high purity upon FACS analysis of the sorted fraction, were obtained. These cells showed the morphological characteristics of DCs. Most importantly, mDCs were able to induce T cell proliferation and differentiation in a MLR, and interact with T cells as assessed by confocal microscopy. These results indicate the presence of functional DCs. Freezing and thawing of the BAL cells did not affect phenotype or T cell stimulatory capacity of the isolated DCs.

CONCLUSION

Using a novel sorting strategy, functional mDCs can be isolated from BAL fluid, enabling a detailed study in pulmonary disease.

Lung dendritic cell expression of maturation molecules increases with worsening chronic obstructive pulmonary disease

RATIONALE

Dendritic cells (DCs) have not been well studied in chronic obstructive pulmonary disease (COPD), yet their integral role in activating and differentiating T cells makes them potential participants in COPD pathogenesis.

OBJECTIVES

To determine the expression of maturation molecules by individual DC subsets in relationship to COPD stage and to expression of the acute activation marker CD69 by lung CD4(+) T cells.

METHODS

We nonenzymatically released lung leukocytes from human surgical specimens (n = 42) and used flow cytometry to identify three DC subsets (mDC1, mDC2, and pDC) and to measure their expression of three costimulatory molecules (CD40, CD80 and CD86) and of CD83, the definitive marker of DC maturation. Spearman nonparametric correlation analysis was used to identify significant correlations between expression of DC maturation molecules and COPD severity.

MEASUREMENTS AND MAIN RESULTS

Expression of CD40 by mDC1 and mDC2 and of CD86 by mDC2 was high regardless of GOLD stage, but CD80 and CD83 on these two DC subsets increased with disease progression. pDC also showed significant increases in expression of CD40 and CD80. Expression of all but one of the DC molecules that increased with COPD severity also correlated with CD69 expression on lung CD4(+) T cells from the same patients, with the exception of CD83 on mDC2.

CONCLUSIONS

This cross-sectional study implies that COPD progression is associated with significant increases in costimulatory molecule expression by multiple lung DC subsets. Interactions with lung DCs may contribute to the immunophenotype of CD4(+) T cells in advanced COPD. Clinical trial registered with www.clinicaltrials.gov (NCT00281229).

Expression of blood dendritic cell antigens (BDCAs) by CD1a+ human pulmonary cells

BACKGROUND

Myeloid and plasmacytoid dendritic cell (DC) subsets have been recently identified in the human lung based on their differential expression of Blood DC Antigens 1-3 (BDCAs). We investigated the expression of these antigens by isolated human pulmonary CD1a(+) DCs, namely Langerhan's cells.

METHODS

Using an in vitro cell culture system we successfully isolated a population of relatively pure (>70%) CD1a(+) cells from human lung tissue (n=5 subject samples) and stained these with antibodies against the myeloid DC markers BDCA1 (CD1c) and BDCA3 (CD303), the plasmacytoid DC marker BDCA2 (CD141), the Langerhan's cell marker Langerin and the maturation marker CD83.

RESULTS

Among different subject samples, the isolated CD1a(+) cells showed variable expression of Langerin, BDCAs and CD83. Interestingly, in two subject samples, which contained >70% CD83(+) mature CD1a(+) cells, >50% of the cells were positive for all of the BDCAs.

CONCLUSIONS

We conclude that isolated pulmonary CD1a(+) DCs in vitro have the capacity to express both myeloid and plasmacytoid BDCA markers and that rather than subset restriction in pulmonary DCs, a significant degree of flexibility/plasticity can be induced, albeit experimentally.

Allergen-enhanced thrombomodulin (blood dendritic cell antigen 3, CD141) expression on dendritic cells is associated with a TH2-skewed immune response

BACKGROUND

Dendritic cells (DCs) are important in allergic diseases such as asthma, although little is known regarding the mechanisms by which DCs induce T(H)2-polarized responses in atopic individuals. It has been suggested that intrinsic properties of allergens can directly stimulate T(H)2 polarizing functions of DCs, but little is known of the underlying mechanisms.

OBJECTIVE

To identify novel genes expressed by house dust mite (HDM) allergen-exposed DCs.

METHODS

We screened for allergen-induced gene expression by microarray, and validated differentially expressed genes at the mRNA and protein levels.

RESULTS

Thrombomodulin (CD141, blood dendritic cell antigen 3) expression by microarray was higher on HDM-stimulated DCs from atopic (relative to nonatopic) individuals. These findings were confirmed at both the mRNA and protein levels in an independent group. Purified thrombomodulin(+) DCs induced a strongly T(H)2-polarized cytokine response by allergen-specific T cells compared with DCs lacking thrombomodulin. In vivo, thrombomodulin(+) circulating DCs were significantly more frequent in subjects with HDM allergy and asthma, compared with control subjects. Furthermore, thrombomodulin expression in blood leukocytes was higher in children with acute asthma than at convalescence 6 weeks later.

CONCLUSION

Thrombomodulin expression on DCs may be involved in the pathogenesis of atopy and asthma.

Analysis of cytokine secretion from human plasmacytoid dendritic cells infected with H5N1 or low-pathogenicity influenza viruses

Mechanisms underlying the virulence of H5N1 influenza viruses in humans are poorly understood, though evidence of hyperinflammation and systemic viral replication has been reported. Plasmacytoid dendritic cells (PDCs), a major source of type I interferon, potentially affect host defense against influenza viruses. To analyze how influenza virus infection alters PDC function, we measured cytokine secretion from primary human PDCs infected with high- or low-pathogenicity influenza viruses. IFN-alpha responses induced by H5N1 viruses were several-fold higher than those induced by low-pathogenicity strains; differences in the secretion of the proinflammatory cytokines TNF-alpha and IP-10 were less pronounced, in contrast with findings from human macrophage studies. Reassortant viruses bearing H5N1-derived NS genes did not elicit enhanced IFN-alpha secretion by PDCs; thus, other H5N1 gene(s) are responsible for the heightened response. Their central role in the induction of an effective antiviral immune response and the finding that they respond differently to influenza viruses of different pathogenicities suggest that PDCs may play a role in the hypercytokinemia associated with H5N1 infection in humans.

Dendritic cells in chronic obstructive pulmonary disease: new players in an old game

Dendritic cells (DCs) are professional antigen-presenting cells responsible for immune homeostasis. In the lung's responses to tissue damage or infection, they initiate and orchestrate innate and adaptive immunity. There are immature and mature states and at least three phenotypic and functional subsets. DCs circulate in the blood and localize to mucosal surfaces in immature form where they act as sentinels, sampling constituents of the external environment that breach the epithelium. With internalization of antigen, they are activated, mature, and migrate to draining lymph nodes to induce the proliferation and regulate the balance of Th1/Th2 T cells or to induce a state of tolerance, the last dependent on maturation status, extent of cell surface costimulatory molecule expression, and cytokine release. Cigarette smoke has modulatory effects varying with species, dose, the location examined within the lung, and the marker or technique used to identify DCs. Healthy smokers (and smokers with asthma) have reduced numbers of large airway mature DCs. In chronic obstructive pulmonary disease, the number of immature DCs is increased in small airways, whereas in smokers with chronic obstructive pulmonary disease, the total number of DCs appears to be reduced in large airways. We hypothesize that the long-term effects of cigarette smoke include reduction of DC maturation and function, changes that favor repeated infection, increased exacerbation frequency, and the altered (CD8(+) T-cell predominant) pattern of inflammation associated with this progressive chronic disease.

Endotoxin augments myeloid dendritic cell influx into the airways in patients with allergic asthma

RATIONALE

Epidemiologic studies have shown that exacerbation of asthma is modulated by environmental endotoxin. High levels of endotoxin are associated with asthma symptoms and the current use of asthma medication. However, the underlying mechanisms by which endotoxin modulates asthma are not completely understood.

OBJECTIVES

The aim of the study was to test whether endotoxin enhances the response of individuals with allergic asthma to allergen, and to determine if this interaction is associated with increased numbers of antigen-presenting cells in the airways.

METHODS

Seventeen subjects with mild allergic asthma underwent segmental challenge with allergen, endotoxin, and the combination of both in three different lung segments via bronchoscopy. The cellular influx including monocytes, myeloid dendritic cells (mDCs), and plasmacytoid dendritic cells (pDCs), as well as the level of cytokines, were assessed in bronchoalveolar lavage fluid obtained 24 hours after segmental challenge. Monocytes, mDCs, and pDCs were isolated and their capacity to induce T cell proliferation was determined.

MEASUREMENTS AND MAIN RESULTS

Endotoxin enhanced the cellular response to allergen. The combination of allergen and endotoxin resulted in increased numbers of total cells, lymphocytes, neutrophils, eosinophils, monocytes, and mDCs, as well as increased levels of lipopolysaccharide-binding protein, IL-1alpha, IL-6, and tumor necrosis factor-alpha in the bronchoalveolar lavage fluid compared with allergen alone. Isolated mDCs but not pDCs induced a strong T cell proliferation in vitro.

CONCLUSIONS

Endotoxin augments the allergic inflammation in the lungs of individuals with asthma, and induces an enhanced influx of monocytes and functionally active antigen-presenting mDCs into the respiratory tract.

BDCA-1+, BDCA-2+ and BDCA-3+ dendritic cells in early human pregnancy decidua

Dendritic cells (DCs) can acquire unique features or phenotypes in different tissue microenvironments and decide whether immunity or tolerance develops. DCs observed within the decidua have been implicated in pregnancy maintenance. However, the precise distribution of decidual DC subsets and their phenotypic characteristics are largely unknown. Using flow cytometry, we identified three DC subsets in normal human first-trimester decidua: BDCA-1+ CD19- CD14(-) myeloid DC type 1 (MDC1), BDCA-3+ CD14- myeloid DC type 2 (MDC2) and BDCA-2+ CD123+ plasmacytoid DC (PDC). The percentage of MDC1 to mononuclear cells in the decidua was similar to that in the peripheral blood controls. The percentage of MDC2 in the decidua was significantly higher than that in the peripheral blood controls, whereas the percentage of PDC was significantly lower. Both MDC1 and MDC2 subsets expressed human leucocyte antigen D-related, CD86 and CD80 at low levels, suggesting a characteristic of immature myeloid DCs. Immunoglobulin-like transcript 3, suggested to be involved in immune tolerance induction, was also expressed on decidual MDC1 and MDC2 subsets. In addition, as gestational age increased from 6 to 9 weeks, the numbers of MDC1 decreased but MDC2 increased significantly. This is the first study to demonstrate the presence of three previously unidentified BDCA-1+, BDCA-3+ and BDCA-2+ DC subsets in human decidua, these decidual DCs might play important role in the maintenance of pregnancy.

Silica suppresses Toll-like receptor ligand-induced dendritic cell activation

Inhalation of silica, without evidence of silicosis, is believed to predispose individuals to bacterial infections and impair respiratory immune functions. Silica may alter the sensitivity of antigen-presenting cells (APCs), such as macrophages and dendritic cells (DCs), to other types of infection; however, the exact nature of these exchanges remains uncertain. The purpose of the present study is to characterize the effect of silica exposure on innate pulmonary defense mechanisms following Toll-like receptor (TLR) ligand-induced activation using DCs as a model APC and determine whether these signals act in synergy or opposition to one another. Using C57Bl/6 mice, pattern recognition receptor expression on DCs was examined in vitro and in vivo using flow cytometry, and the activation state of pulmonary and granulocyte-macrophage colony-stimulating factor-derived DCs was assessed in response to silica in combination with TLR ligands (lipopolysaccharide, cytosine-phosphate-guanine, or polyinosinic:polycytidylic acid) using flow cytometry and measurement of cytokine production. In this study, silica attenuated TLR ligand-dependent DC activation with regards to accessory molecule expression as well as nitric oxide and inflammatory cytokine production. Furthermore, silica's ability to modulate TLR ligand-dependent DC activation did not appear to be dependent on the class A scavenger receptors. Taken together, silica's ability to alter susceptibility to infection may be due to impaired inflammatory responses and reduced antibacterial activity.

Dendritic cells infiltrating human non-small cell lung cancer are blocked at immature stage

The efficacy of immune response to control human cancer remains controversial. It is particularly debated whether and to what extent the capacity of tumor-infiltrating dendritic cells (DC) to drive immunization can be turned off by transformed cells, leading to tumor-specific tolerance rather than immunization. To address this issue, we have characterized the DC isolated from human non-small cell lung cancer (NSCLC). These biopsy specimens contained CD11c(high) myeloid DC (mDC), but also CD11c(-) plasmacytoid DC (pDC) and a third DC subset expressing intermediate level of CD11c. Compared with peripheral blood, CD11c(high) tumor-infiltrating DC (TIDC) displayed a "semi-mature" phenotype, and TLR4 or TLR8 stimulation drove them to mature partially and to secrete limited amounts of cytokines. In contrast, most tumor-infiltrating pDC were immature but underwent partial maturation after TLR7 activation, whereas TLR9 ligation triggered low secretion of IFN-alpha. CD11c(int) mDC represented approximately 25% of total DC in tumoral and peritumoral tissues and expressed low levels of costimulatory molecules contrasting with high levels of the immunoinhibitory molecule B7-H1. Finally, the poor APC function of total TIDC even after TLR stimulation and the migratory response of both tumor-infiltrating mDC and pDC toward CCL21 and SDF-1 in vitro suggested their ability to compromise the tumor-specific immune response in draining lymph nodes in vivo. Further studies will be required to establish the specific role of the three TIDC subsets in tumor immunity and to draw conclusions for the design of therapeutic strategies.

Cigarette smoking alters bronchial mucosal immunity in asthma

RATIONALE

Cigarette smoking worsens asthma and is associated with reduced response to corticosteroid therapy. As cigarette smoke is known to have immunomodulatory effects, we hypothesized that one mechanism by which smoking mediates its adverse effect is by reduction of the numbers of bronchial mucosal dendritic cells (DCs), which control B-cell growth and T-cell responses.

OBJECTIVES

We set out to sample the bronchial mucosa in smoking and never-smoking patients with asthma and to count DCs, B cells, and cells expressing genes for two key T-lymphocyte regulatory cytokines.

METHODS

Twenty-one never-smoker patients with asthma (6 steroid naive), 24 smoker patients with asthma (9 steroid naive), and 10 healthy never-smokers (control subjects) were recruited and their endobronchial biopsy samples were immunostained for detection of mature DCs (CD83(+)), Langerhans cells (CD1a(+)), B lymphocytes (CD20(+)), and helper T-cell type 1 (IFN-gamma) and helper T-cell type 2 (IL-4) cytokine-expressing cells.

MEASUREMENTS AND MAIN RESULTS

The number (per square millimeter) of CD83(+) mature DCs was significantly lower in smoker patients with asthma (median [range]: 37 [0, 131]) in comparison with never-smoker steroid-naive and steroid-treated patients with asthma (76 [24, 464]; p = 0.006) or control subjects (85 [40, 294]; p = 0.004). Moreover, B cells were fewer in smoker (26 [4, 234]) versus never-smoker steroid-naive and steroid-treated patients with asthma (45 [10, 447]; p = 0.01) and in smoker steroid-naive patients with asthma (23 [4, 111]) versus control subjects (34 [10, 130]; p = 0.05). The number of cells expressing IFN-gamma showed a trend toward fewer in smoker (70 [6, 24]) versus never-smoker steroid-naive patients with asthma (144 [44, 323]; p = 0.10).

CONCLUSIONS

There are important and statistically significant differences in the number of CD83(+) mature DCs and B cells in the large airways of smokers with asthma. We speculate that their reductions may render patients with asthma less responsive to corticosteroids and more susceptible to infection.

Dendritic cell subsets in human bronchoalveolar lavage fluid after segmental allergen challenge

BACKGROUND

Dendritic cells control pulmonary immune reactions. Characteristics of dendritic cells in human bronchoalveolar lavage fluid (BALF) after allergen challenge are unknown.

METHODS

7 patients with allergic asthma (median 23 years, range 19-25 years) underwent segmental challenge and were lavaged 10 min and 24 h after challenge. Dendritic cell subsets and surface markers in BALF and in peripheral blood were analysed using four-colour flow cytometry.

RESULTS

Plasmacytoid dendritic cells (pDCs, median 0.06%, range 0.01-0.08%) and myeloid dendritic cells (mDCs, median 0.47%, range 0.27-0.87%) were detectable in BALF from control segments. CD1a-positive dendritic cells in BALF were identified as a subpopulation of mDCs. Both pDCs (median 0.56%, range 0.09-1.83%) and mDCs (median 1.82%, range 0.95-2.29%) increased significantly in BALF 24 h (p = 0.018 compared with the control segments for pDCs and mDCs), but not 10 min, after allergen challenge. The percentage increase in pDCs was higher than that of mDCs after allergen challenge, as reflected by an enhanced pDC:mDC ratio after allergen challenge. In peripheral blood, there was a significant decrease in mDCs (p = 0.038) and a trend to a decrease in pDCs (p = 0.068) 24 h after allergen challenge. Analysis of dendritic cell surface molecules showed that after allergen challenge, BALF dendritic cells have a less mature phenotype compared with BALF dendritic cells from control segments.

CONCLUSION

Using a comprehensive strategy to analyse dendritic cell subsets in human BALF, we have shown for the first time that both myeloid and plasmacytoid dendritic cells accumulate in the airway lumen after allergen challenge in patients with asthma.

HIV interactions with dendritic cells: has our focus been too narrow?

Although few in number, dendritic cells (DCs) are heterogeneous, ubiquitous, and are crucial for protection against pathogens. In this review, the different DC subpopulations have been described and aspects of DC biology are discussed. DCs are important, not only in the pathogenesis of HIV, but also in the generation of anti-HIV immune responses. This review describes the roles that DC are thought to play in HIV pathogenesis, including uptake and transport of virus. We have also discussed the effects that the virus exerts on DCs such as infection and dysfunction. Then we proceed to focus on DC subsets in different organs and show how widespread the effects of HIV are on DC populations. It is clear that the small number of studies on tissue-derived DCs limits current research into the pathogenesis of HIV.