Accelerated airway dendritic cell maturation, trafficking, and elimination in a mouse model of asthma

Catching Our Breath: Updates on the Role of Dendritic Cell Subsets in Asthma

Dendritic cells (DCs), as potent antigen presenting cells, are known to play a central role in the pathophysiology of asthma. The understanding of DC biology has evolved over the years to include multiple subsets of DCs with distinct functions in the initiation and maintenance of asthma. Furthermore, asthma is increasingly recognized as a heterogeneous disease with potentially diverse underlying mechanisms. The goal of this review is to summarize the role of DCs and the various subsets therein in the pathophysiology of asthma and highlight some of the crucial animal models shaping the field today. Potential future avenues of investigation to address existing gaps in knowledge are discussed.

The lymphatic vasculature in lung function and respiratory disease

The lymphatic vasculature maintains tissue homeostasis via fluid drainage in the form of lymph and immune surveillance due to migration of leukocytes through the lymphatics to the draining lymph nodes. Lymphatic endothelial cells (LECs) form the lymphatic vessels and lymph node sinuses and are key players in shaping immune responses and tolerance. In the healthy lung, the vast majority of lymphatic vessels are found along the bronchovascular structures, in the interlobular septa, and in the subpleural space. Previous studies in both mice and humans have shown that the lymphatics are necessary for lung function from the neonatal period through adulthood. Furthermore, changes in the lymphatic vasculature are observed in nearly all respiratory diseases in which they have been analyzed. Recent work has pointed to a causative role for lymphatic dysfunction in the initiation and progression of lung disease, indicating that these vessels may be active players in pathologic processes in the lung. However, the mechanisms by which defects in lung lymphatic function are pathogenic are understudied, leaving many unanswered questions. A more comprehensive understanding of the mechanistic role of morphological, functional, and molecular changes in the lung lymphatic endothelium in respiratory diseases is a promising area of research that is likely to lead to novel therapeutic targets. In this review, we will discuss our current knowledge of the structure and function of the lung lymphatics and the role of these vessels in lung homeostasis and respiratory disease.

Granulocyte-targeted therapies for airway diseases

The average respiration rate for an adult is 12-20 breaths per minute, which constantly exposes the lungs to allergens and harmful particles. As a result, respiratory diseases, which includes asthma, chronic obstructive pulmonary disease (COPD) and acute lower respiratory tract infections (LTRI), are a major cause of death worldwide. Although asthma, COPD and LTRI are distinctly different diseases with separate mechanisms of disease progression, they do share a common feature - airway inflammation with intense recruitment and activation of granulocytes and mast cells. Neutrophils, eosinophils, basophils, and mast cells are crucial players in host defense against pathogens and maintenance of lung homeostasis. Upon contact with harmful particles, part of the pulmonary defense mechanism is to recruit these cells into the airways. Despite their protective nature, overactivation or accumulation of granulocytes and mast cells in the lungs results in unwanted chronic airway inflammation and damage. As such, understanding the bright and the dark side of these leukocytes in lung physiology paves the way for the development of therapies targeting this important mechanism of disease. Here we discuss the role of granulocytes in respiratory diseases and summarize therapeutic strategies focused on granulocyte recruitment and activation in the lungs.

Silencing of CD86 in dendritic cells by small interfering RNA regulates cytokine production in T cells from patients with allergic rhinitis in vitro

The aim of the present study was to investigate the expression and role of the co‑stimulatory molecule T‑lymphocyte activation antigen CD86 (CD86) in dendritic cells (DCs) from the peripheral blood of patients with allergic rhinitis (AR) compared with those from healthy individuals. It was observed that mature DCs from the peripheral blood of patients with AR expressed high levels of the co‑stimulatory molecule CD86, but not CD80, compared with healthy control subjects. CD86 expression levels in DCs decreased significantly following transfection with siRNA in a lentiviral vector. Furthermore, the level of transforming growth factor‑β1 produced by T cells co‑cultured with DCs was significantly increased in the siRNA group, while interleukin (IL)‑4 and IL‑5 production was significantly decreased. The findings of the present study indicated that CD86 may play a pivotal role in the regulatory T cell/type 2 helper T cell imbalance in allergic inflammation.

Lymphatic impairment leads to pulmonary tertiary lymphoid organ formation and alveolar damage

The lung is a specialized barrier organ that must tightly regulate interstitial fluid clearance and prevent infection in order to maintain effective gas exchange. Lymphatic vessels are important for these functions in other organs, but their roles in the lung have not been fully defined. In the present study, we addressed how the lymphatic vasculature participates in lung homeostasis. Studies using mice carrying a lymphatic reporter allele revealeded that, in contrast to other organs, lung lymphatic collecting vessels lack smooth muscle cells entirely, suggesting that forward lymph flow is highly dependent on movement and changes in pressure associated with respiration. Functional studies using CLEC2-deficient mice in which lymph flow is impaired due to loss of lympho-venous hemostasis or using inducible lung-specific ablation of lymphatic endothelial cells in a lung transplant model revealeded that loss of lymphatic function leads to an inflammatory state characterized by the formation of tertiary lymphoid organs (TLOs). In addition, impaired lymphatic flow in mice resulteds in hypoxia and features of lung injury that resemble emphysema. These findings reveal both a lung-specific mechanism of lymphatic physiology and a lung-specific consequence of lymphatic dysfunction that may contribute to chronic lung diseases that arise in association with TLO formation.

Dendritic Cell Trafficking and Function in Rare Lung Diseases

Dendritic cells (DCs) are highly specialized immune cells that capture antigens and then migrate to lymphoid tissue and present antigen to T cells. This critical function of DCs is well defined, and recent studies further demonstrate that DCs are also key regulators of several innate immune responses. Studies focused on the roles of DCs in the pathogenesis of common lung diseases, such as asthma, infection, and cancer, have traditionally driven our mechanistic understanding of pulmonary DC biology. The emerging development of novel DC reagents, techniques, and genetically modified animal models has provided abundant data revealing distinct populations of DCs in the lung, and allow us to examine mechanisms of DC development, migration, and function in pulmonary disease with unprecedented detail. This enhanced understanding of DCs permits the examination of the potential role of DCs in diseases with known or suspected immunological underpinnings. Recent advances in the study of rare lung diseases, including pulmonary Langerhans cell histiocytosis, sarcoidosis, hypersensitivity pneumonitis, and pulmonary fibrosis, reveal expanding potential pathogenic roles for DCs. Here, we provide a review of DC development, trafficking, and effector functions in the lung, and discuss how alterations in these DC pathways contribute to the pathogenesis of rare lung diseases.

Tissue-resident dendritic cells and diseases involving dendritic cell malfunction

Dendritic cells (DCs) control immune responses and are central to the development of immune memory and tolerance. DCs initiate and orchestrate immune responses in a manner that depends on signals they receive from microbes and cellular environment. Although DCs consist mainly of bone marrow-derived and resident populations, a third tissue-derived population resides the spleen and lymph nodes (LNs), different subsets of tissue-derived DCs have been identified in the blood, spleen, lymph nodes, skin, lung, liver, gut and kidney to maintain the tolerance and control immune responses. Tissue-resident DCs express different receptors for microbe-associated molecular patterns (MAMPs) and damage-associated molecular patterns (DAMPs), which were activated to promote the production of pro- or anti-inflammatory cytokines. Malfunction of DCs contributes to diseases such as autoimmunity, allergy, and cancer. It is therefore important to update the knowledge about resident DC subsets and diseases associated with DC malfunction.

Aggravation of Allergic Airway Inflammation by Cigarette Smoke in Mice Is CD44-Dependent

Background

Although epidemiological studies reveal that cigarette smoke (CS) facilitates the development and exacerbation of allergic asthma, these studies offer limited information on the mechanisms involved. The transmembrane glycoprotein CD44 is involved in cell adhesion and acts as a receptor for hyaluronic acid and osteopontin. We aimed to investigate the role of CD44 in a murine model of CS-facilitated allergic airway inflammation.

Methods

Wild type (WT) and CD44 knock-out (KO) mice were exposed simultaneously to house dust mite (HDM) extract and CS. Inflammatory cells, hyaluronic acid (HA) and osteopontin (OPN) levels were measured in bronchoalveolar lavage fluid (BALF). Proinflammatory mediators, goblet cell metaplasia and peribronchial eosinophilia were assessed in lung tissue. T-helper (Th) 1, Th2 and Th17 cytokine production was evaluated in mediastinal lymph node cultures.

Results

In WT mice, combined HDM/CS exposure increased the number of inflammatory cells and the levels of HA and OPN in BALF and Th2 cytokine production in mediastinal lymph nodes compared to control groups exposed to phosphate buffered saline (PBS)/CS, HDM/Air or PBS/Air. Furthermore, HDM/CS exposure significantly increased goblet cell metaplasia, peribronchial eosinophilia and inflammatory mediators in the lung. CD44 KO mice exposed to HDM/CS had significantly fewer inflammatory cells in BALF, an attenuated Th2 cytokine production, as well as decreased goblet cells and peribronchial eosinophils compared to WT mice. In contrast, the levels of inflammatory mediators were similar or higher than in WT mice.

Conclusion

We demonstrate for the first time that the aggravation of pulmonary inflammation upon combined exposure to allergen and an environmental pollutant is CD44-dependent. Data from this murine model of concomitant exposure to CS and HDM might be of importance for smoking allergic asthmatics.

Mesenchymal stem cells abrogate experimental asthma by altering dendritic cell function

Mesenchymal stem cells (MSCs) have been investigated in the treatment of numerous autoimmune diseases. However, the immune properties of MSCs on the development of asthma have remained to be fully elucidated. Airway dendritic cells (DCs) have an important role in the pathogenesis of allergic asthma, and disrupting their function may be a novel therapeutic approach. The present study used a mouse model of asthma to demonstrate that transplantation of MSCs suppressed features of asthma by targeting the function of lung myeloid DCs. MSCs suppressed the maturation and migration of lung DCs to the mediastinal lymph nodes, and thereby reducing the allergen-specific T helper type 2 (Th2) response in the nodes. In addition, MSC-treated DCs were less potent in activating naive and effector Th2 cells and the capacity of producing chemokine (C-C motif) ligand 17 (CCL17) and CCL22, which are chemokines attracting Th2 cells, to the airways was reduced. These results supported that MSCs may be used as a potential treatment for asthma.

Small interfering RNA against CD86 during allergen challenge blocks experimental allergic asthma

Background

CD86-CD28 interaction has been suggested as the principal costimulatory pathway for the activation and differentiation of naïve T cells in allergic inflammation. However, it remains uncertain whether this pathway also has an essential role in the effector phase. We sought to determine the contribution of CD86 on dendritic cells in the reactivation of allergen-specific Th2 cells.

Methods

We investigated the effects of the downregulation of CD86 by short interfering RNAs (siRNAs) on Th2 cytokine production in the effector phase in vitro and on asthma phenotypes in ovalbumin (OVA)-sensitized and -challenged mice.

Results

Treatment of bone marrow-derived dendritic cells (BMDCs) with CD86 siRNA attenuated LPS-induced upregulation of CD86. CD86 siRNA treatment impaired BMDCs’ ability to activate OVA-specific Th2 cells. Intratracheal administration of CD86 siRNA during OVA challenge downregulated CD86 expression in the airway mucosa. CD86 siRNA treatment ameliorated OVA-induced airway eosinophilia, airway hyperresponsiveness, and the elevations of OVA-specific IgE in the sera and IL-5, IL-13, and CCL17 in the bronchoalveolar lavage fluid, but not the goblet cell hyperplasia.

Conclusion

These results suggest that local administration of CD86 siRNA during the effector phase ameliorates lines of asthma phenotypes. Targeting airway dendritic cells with siRNA suppresses airway inflammation and hyperresponsiveness in an experimental model of allergic asthma.

Downregulation of key early events in the mobilization of antigen-bearing dendritic cells by leukocyte immunoglobulin-like Receptor B4 in a mouse model of allergic pulmonary inflammation

Leukocyte Immunoglobulin-like Receptor B4 (LILRB4) null mice have an exacerbated T helper cell type 2 (Th2) immune response and pulmonary inflammation compared with Lilrb4(+/+) animals when sensitized intranasally with ovalbumin (OVA) and low-dose lipopolysaccharide (LPS) followed by challenge with OVA. Moreover, OVA-challenged Lilrb4(-/-) mice exhibit greater migration of antigen (Ag)-bearing dendritic cells (DCs) to lymph nodes and accumulation of interleukin 4- and interleukin 5-producing lymph node lymphocytes. The main objective of this study was to determine how the absence of LILRB4 leads to a greater number of DCs in the lymph nodes of Ag-challenged mice and increased lung Th2 inflammation. Mice were sensitized intranasally with PBS alone or containing OVA and LPS; additional cohorts were subsequently challenged with OVA. Expression of chemokine (C-C motif) ligand 21 (CCL21) in the lung was assessed immunohistologically. OVA ingestion and expression of LILRB4 and chemokine (C-C motif) receptor 7 (CCR7) were quantified by flow cytometry. Inhalation of OVA and LPS induced upregulation of LILRB4 selectively on lung Ag-bearing DCs. After sensitization and challenge, the lung lymphatic vessels of Lilrb4(-/-) mice expressed more CCL21, a chemokine that directs the migration of DCs from peripheral tissue to draining lymph nodes, compared with Lilrb4(+/+) mice. In addition, lung DCs of challenged Lilrb4(-/-) mice expressed more CCR7, the CCL21 receptor. The lungs of challenged Lilrb4(-/-) mice also contained significantly greater numbers of CD4+ cells expressing interleukin-4 or interleukin-5, consistent with the greater number of Ag-bearing DCs and Th2 cells in lymph nodes and the attendant exacerbated Th2 lung pathology. Our data establish a new mechanism by which LILRB4 can downregulate the development of pathologic allergic inflammation: reduced upregulation of key molecules needed for DC migration leading to decreases in Th2 cells in lymph nodes and their target tissue.

Aeroallergen challenge promotes dendritic cell proliferation in the airways

Aeroallergen provocation induces the rapid accumulation of CD11c(+)MHC class II (MHC II)(+) dendritic cells (DCs) in the lungs, which is driven by an increased recruitment of blood-derived DC precursors. Recent data show, however, that well-differentiated DCs proliferate in situ in various tissues. This may also contribute to their allergen-induced expansion; therefore, we studied DC proliferation in the airways of mice in the steady state and after local aeroallergen provocation. Confocal whole-mount microscopy was used to visualize proliferating DCs in different microanatomical compartments of the lung. We demonstrate that in the steady state, CD11c(+)MHC II(+) DCs proliferate in both the epithelial and subepithelial layers of the airway mucosa as well as in the lung parenchyma. A 1-h pulse of the nucleotide 5-ethynyl-2'-deoxyuridine was sufficient to label 5% of DCs in both layers of the airway mucosa. On the level of whole-lung tissue, 3-5% of both CD11b(+) and CD11b(-) DC populations and 0.3% of CD11c(+)MHC II(low) lung macrophages incorporated 5-ethynyl-2'-deoxyuridine. Aeroallergen provocation caused a 3-fold increase in the frequency of locally proliferating DCs in the airway mucosa. This increase in mucosal DC proliferation was later followed by an elevation in the number of DCs. The recruitment of monocyte-derived inflammatory DCs contributed to the increasing number of DCs in the lung parenchyma, but not in the airway mucosa. We conclude that local proliferation significantly contributes to airway DC homeostasis in the steady state and that it is the major mechanism underlying the expansion of the mucosal epithelial/subepithelial DC network in allergic inflammation.

House dust mite allergic airway inflammation facilitates neosensitization to inhaled allergen in mice

BACKGROUND

The mechanism by which many monosensitized allergic individuals progress to polysensitization over time remains to be elucidated. Mouse models have contributed greatly to the understanding of sensitization to inhaled allergens in healthy airways but hardly any studies have addressed sensitization during established allergy. We hypothesized that an allergic inflammatory milieu might facilitate sensitization to inhaled allergens by the presence of mature dendritic cells (DCs) and IL-4.

METHODS

Mice with house dust mite (HDM)-induced allergic airway inflammation received a single intratracheal dose of ovalbumin (OVA), 2 days after the last HDM exposure. Ten days later, sensitization was assessed by rechallenge with OVA. We evaluated the following factors for their importance in neosensitization: (1) maturation and recruitment of DCs to the airways, (2) dependency on DCs using CD11cDTR conditional knockout mice, (3) presence of ongoing airway inflammation by comparing sensitization at day 2 and day 14 after the last HDM exposure and (4) dependency on IL-4 by treatment with blocking antibodies.

RESULTS

House dust mite -induced inflammation facilitated neosensitization to OVA. HDM-induced inflammation increased the number of airway DCs with a mature phenotype but a DC reduction of 93% did not inhibit sensitization. Neosensitization to OVA was dependent on ongoing inflammation and in particular on IL-4.

CONCLUSIONS

These findings show that HDM-induced allergic airway inflammation facilitates neosensitization to a second inhaled allergen in an IL-4-dependent manner and provide insight into the underlying mechanism of the frequently observed progression to polysensitization in HDM-monosensitized individuals.

Inhibition of histamine H1 receptor activity modulates proinflammatory cytokine production of dendritic cells through c-Rel activity

BACKGROUND

Histamine exerts diverse effects on immune regulation through four types of histamine receptors (HRs). Among them, type 1 receptor (H1R) plays an important role in allergic inflammation. Dendritic cells (DCs), which express at least three types of HRs, are professional antigen-presenting cells controlling the development of allergic inflammation. However, the molecular mechanisms involved in H1R-mediated NF-ĸB signaling of DCs remain poorly defined.

METHODS

Bone-marrow (BM)-derived DCs (BM-DCs) were treated with H1R inverse agonists to interrupt basal H1R-mediated signaling. The crosstalk of H1R-mediated signaling and the NF-ĸB pathway was examined by NF-ĸB cellular activity using a luciferase reporter assay, NF-ĸB subunit analysis using Western blotting and TNF-α promoter activity using chromatin immunoprecipitation.

RESULTS

Blockage of H1R signaling by inverse agonists significantly inhibited TNF-α and IL-6 production of BM-DCs. H1R-specific agonists were able to enhance TNF-α production, but this overexpression was significantly inhibited by NF-ĸB inhibitor. The H1R inverse agonist ketotifen also suppressed cellular NF-ĸB activity, suggesting crosstalk between H1R and NF-ĸB signaling in DCs. After comprehensive analysis of NF-ĸB subunits, c-Rel protein expression was significantly down-regulated in ketotifen-treated BM-DCs, which led to inhibition of the promoter activity of TNF-α. Finally, adoptive transfer of the ketotifen-treated BM-DCs did not induce significant allergic airway inflammation compared to that of control cells in vivo.

CONCLUSIONS

Our results suggest that c-Rel controls H1R-mediated proinflammatory cytokine production in DCs. This study provides a potential mechanism of H1R-mediated signaling and NF-ĸB pathway crosstalk in allergic inflammation.

Inert 50-nm polystyrene nanoparticles that modify pulmonary dendritic cell function and inhibit allergic airway inflammation

Nanoparticles are being developed for diverse biomedical applications, but there is concern about their potential to promote inflammation, particularly in the lung. Although a variety of ambient, anthropogenic and man-made nanoparticles can promote lung inflammation, little is known about the long-term immunomodulatory effects of inert noninflammatory nanoparticles. We previously showed polystyrene 50-nm nanoparticles coated with the neutral amino acid glycine (PS50G nanoparticles) are not inflammatory and are taken up preferentially by dendritic cells (DCs) in the periphery. We tested the effects of such nanoparticles on pulmonary DC function and the development of acute allergic airway inflammation. Surprisingly, exposure to PS50G nanoparticles did not exacerbate but instead inhibited key features of allergic airway inflammation including lung airway and parenchymal inflammation, airway epithelial mucus production, and serum allergen-specific IgE and allergen-specific Th2 cytokines in the lung-draining lymph node (LN) after allergen challenge 1 mo later. PS50G nanoparticles themselves did not induce lung oxidative stress or cardiac or lung inflammation. Mechanistically, PS50G nanoparticles did not impair peripheral allergen sensitization but exerted their effect at the lung allergen challenge phase by inhibiting expansion of CD11c(+)MHCII(hi) DCs in the lung and draining LN and allergen-laden CD11b(hi)MHCII(hi) DCs in the lung after allergen challenge. PS50G nanoparticles further suppressed the ability of CD11b(hi) DCs in the draining LN of allergen-challenged mice to induce proliferation of OVA-specific CD4(+) T cells. The discovery that a defined type of nanoparticle can inhibit, rather than promote, lung inflammation via modulation of DC function opens the door to the discovery of other nanoparticle types with exciting beneficial properties.

Eosinophils regulate dendritic cells and Th2 pulmonary immune responses following allergen provocation

Reports have recently suggested that eosinophils have the potential to modulate allergen-dependent pulmonary immune responses. The studies presented expand these reports demonstrating in the mouse that eosinophils are required for the allergen-dependent Th2 pulmonary immune responses mediated by dendritic cells (DCs) and T lymphocytes. Specifically, the recruitment of peripheral eosinophils to the pulmonary lymphatic compartment(s) was required for the accumulation of myeloid DCs in draining lymph nodes and, in turn, Ag-specific T effector cell production. These effects on DCs and Ag-specific T cells did not require MHC class II expression on eosinophils, suggesting that these granulocytes have an accessory role as opposed to direct T cell stimulation. The data also showed that eosinophils uniquely suppress the DC-mediated production of Th17 and, to smaller degree, Th1 responses. The cumulative effect of these eosinophil-dependent immune mechanisms is to promote the Th2 polarization characteristic of the pulmonary microenvironment after allergen challenge.

Elf3 regulates allergic airway inflammation by controlling dendritic cell-driven T cell differentiation

Elf3 belongs to the Ets family of transcription factors and has been implicated in inflammation. Elf3 is highly expressed in the lungs, and Elf3(-/-) mice are impaired in IL-6 production after intranasal LPS exposure. To identify the role of Elf3 in Th17-driven pulmonary inflammation, we have performed epicutaneous sensitization of Elf3(-/-) mice with OVA followed by airway OVA challenge and have identified Elf3(-/-) mice to be impaired in induction of Th17 response, attributable to impairment of IL-6 production by dendritic cells (DCs). However, increased serum levels of OVA-specific IgG1 and IgE were observed, pointing toward an exaggerated Th2 response. To study Th2 response, we performed i.p. sensitization of Elf3(-/-) mice with OVA and confirmed loss of Elf3 to result in an aggravated Th2 response, characterized by increased generation of IL-4-producing T cells, increased levels of OVA-specific IgE and IgG1 Ab titers, and increased serum levels of Th2 cytokines, together with extensive inflammation and mucus production in airways. Elf3(-/-) DCs were impaired in priming Th1 differentiation, which, in turn, promoted Th2 differentiation. This was mediated by the ability of Elf3(-/-) DCs to undergo hypermaturation but secrete significantly lower levels of IL-12 in response to inflammatory stimuli. The impairment of IL-12 production was due to impairment of IL-12p40 gene induction in Elf3(-/-) DCs in response to inflammatory stimuli. Taken together, our study identifies a novel function of Elf3 in regulating allergic airway inflammation by regulating DC-driven Th1, Th2, and Th17 differentiation.

Mite allergen decreases DC-SIGN expression and modulates human dendritic cell differentiation and function in allergic asthma

House dust mites (HDMs) induce allergic asthma in sensitized individuals, although how HDMs activate immature mucosal dendritic cells (DCs) to render the T helper cell type 2 (Th2)-mediated immune response is unclear. In this study, our results showed a significant calcium-dependent lectin binding of Dermatophagoides pteronyssinus (Der p) extracts to DC-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), the C-type lectin receptors (CLRs) of DCs. Moreover, monocyte-derived DCs (MDDCs) of Der p-sensitized asthmatics (AS) exhibited decreased expression of DC-SIGN, increased endocytosis, and impaired differentiation of DC precursors. The Der p-induced downregulation of DC-SIGN expression in the differentiation of immature MDDCs may be because of the internalization of Der p-DC-SIGN complex. MDDCs of AS produced more interleukin (IL)-6 and less IL-12p70 cytokines when stimulated with lipopolysaccharide (LPS) or Der p than those of nonallergic controls (NC). In the co-culture experiments, MDDCs pretreated with Der p induced GATA-3 expression and IL-4 cytokine productions in naive CD4(+) T cells. These effects of Der p on the differentiation and function of MDDCs could be partially blocked by anti-DC-SIGN antibodies. In conclusion, our results suggest a critical step of allergen sensitization that involves CLRs in the innate immune response of DCs, which may provide a therapeutic or preventive potential for allergic asthma.

Mouse lung CD103+ and CD11bhigh dendritic cells preferentially induce distinct CD4+ T-cell responses

Mouse lung dendritic cells (LDCs) have been recently shown to contain two major subpopulations: CD103(+) CD11b(low or negative) (CD103(+) LDCs) and CD103(-) CD11b(high) LDCs (CD11b(high) LDCs). Although several studies have demonstrated functional differences between them, it is unclear whether the subpopulations induce distinct T helper (Th) cell responses. The present study was conducted to examine whether CD103(+) and CD11b(high) LDCs preferentially generate different Th responses. Naive DO11.10 CD4(+) T cells were primed with CD103(+) or CD11b(high) LDCs obtained from normal BALB/c mice. The primed CD4(+) T cells were restimulated, and their cytokine secretions were assessed. The expression of intracellular cytokines and the mRNA levels of chemokine receptors were also measured. We found that the CD4(+) T cells primed with CD103(+) LDCs secreted significantly larger amounts of IFN-γ and IL-17A, whereas those primed with CD11b(high) LDCs released significantly higher levels of IL-4, IL-6, and IL-10. Intracellular cytokine assay showed that CD103(+) LDCs induced greater frequencies of CD4(+) T cells producing IFN-γ and IL-17A, whereas CD11b(high) LDCs were more efficient at inducing CD4(+) T cells producing IL-4 and IL-10. The mRNA levels of CXCR3 and CCR5, which are expressed preferentially in Th1 cells, were significantly higher in CD4(+) T cells primed with CD103(+) LDCs. The mRNA levels of CXCR4 and CCR4, which are expressed primarily in Th2 cells, were significantly greater in those primed with CD11b(high) LDCs. These data suggest that mouse CD103(+) LDCs predominantly elicit Th1 and Th17 responses, whereas CD11b(high) LDCs primarily provoke a Th2 response under the steady state.

Persistent activation of dendritic cells after resolution of allergic airway inflammation breaks tolerance to inhaled allergens in mice

RATIONALE

Polysensitization of patients who are allergic is a common feature. The underlying immunologic mechanism is not clear. The maturation status of dendritic cells (DCs) is considered to be important for priming naive T cells in the draining lymph nodes. We hypothesized that chronic airway inflammation can induce an enhanced maturation of airway DCs and facilitate subsequent priming to neoallergens.

OBJECTIVES

To investigate whether chronic airway inflammation could induce an altered activation of airway DCs in mice and whether this influences the development of allergic sensitization.

METHODS

Balb/c mice were repeatedly challenged with DCs to induce a chronic airway inflammation. We evaluated (1) the induction of the main characteristic features of human asthma including persistent remodeling, (2) the maturation status of airway DCs 1 month after inflammation resolved, (3) whether this influences tolerance to inhaled neoallergen, and (4) what type of T helper response would be induced by DCs.

MEASUREMENTS AND MAIN RESULTS

Airway DCs displayed a mature phenotype after complete resolution of airway eosinophilia. Inhalation of a neoallergen without any adjuvant was able to induce airway inflammation in postinflammation lungs but not in control lungs. One month after inflammation, airway DCs were able to induce Th2 polarization in naive T cells consistent with the up-regulation of the Th2 skewing molecules Ym1/2 and OX-40L compared with DCs of control airways.

CONCLUSIONS

This study provides evidence that sustained maturation of DCs after resolution of Th2-mediated inflammation can contribute to polysensitization.

Distinct roles of vascular endothelial growth factor receptor-1- and receptor-2-mediated signaling in T cell priming and Th17 polarization to lipopolysaccharide-containing allergens in the lung

Vascular endothelial growth factor (VEGF) is a key mediator in the development of airway immune dysfunction to inhaled allergens. However, the exact role of its receptors-mediated signaling is controversial. In this study, we evaluated the role of VEGF receptor (VEGFR)-1- and VEGFR-2-mediated signaling in T cell priming and polarization in the context of inhalation of LPS-containing allergens. A murine asthma model of mixed Th1 and Th17 cell responses was generated using intranasal sensitization with LPS-containing allergens. Pharmacologic intervention was performed during sensitization. In vivo production of VEGF and Th1- and Th17-polarizing cytokines (IL-12p70 and IL-6, respectively) were upregulated by airway exposure to LPS. Pharmacological intervention with a VEGFR-2-neutralizing Ab (anti-Flk1 mAb) abolished the production of IL-6 (but not IL-12p70) and the subsequent development of allergen-specific Th17 cell response. On the other hand, blocking VEGFR-1 signaling with a VEGFR-1 antagonist (anti-Flt1 hexapeptide) did not affect the production of IL-12p70 and IL-6. However, blocking VEGFR-1 signaling resulted in T cell tolerance rather than priming, mainly by inhibiting the maturation of lung dendritic cells, and their migration into lung-draining lymph nodes. These results suggest that T cell priming to LPS-containing allergens depends on VEGFR-1-mediated signaling, and the subsequent Th17 polarization depends on VEGFR-2 signaling.

Role of dendritic cells in the lung: in vitro models, animal models and human studies

Dendritic cells (DCs) are the most potent antigen-presenting cells in the human lung and are now recognized as crucial initiators of immune responses in general. They are arranged as sentinels in a dense surveillance network inside and below the epithelium of the airways and alveoli, where they are ideally situated to sample inhaled antigen. DCs are known to play a pivotal role in maintaining the balance between tolerance and active immune response in the respiratory system. It is no surprise that the lungs became a main focus of DC-related investigations as this organ provides a large interface for interactions of inhaled antigens with the human body. During recent years there has been a constantly growing body of lung DC-related publications that draw their data from in vitro models, animal models and human studies. This review focuses on the biology and functions of different DC populations in the lung and highlights the advantages and drawbacks of different models with which to study the role of lung DCs. Furthermore, we present a number of up-to-date visualization techniques to characterize DC-related cell interactions in vitro and/or in vivo.

Resolution of cell-mediated airways diseases

"Inflammation resolution" has of late become a topical research area. Activation of resolution phase mechanisms, involving select post-transcriptional regulons, transcription factors, 'autacoids', and cell phenotypes, is now considered to resolve inflammatory diseases. Critical to this discourse on resolution is the elimination of inflammatory cells through apoptosis and phagocytosis. For major inflammatory diseases such as asthma and COPD we propose an alternative path to apoptosis for cell elimination. We argue that transepithelial migration of airway wall leukocytes, followed by mucociliary clearance, efficiently and non-injuriously eliminates pro-inflammatory cells from diseased airway tissues. First, it seems clear that numerous infiltrated granulocytes and lymphocytes can be speedily transmitted into the airway lumen without harming the epithelial barrier. Then there are a wide range of 'unexpected' findings demonstrating that clinical improvement of asthma and COPD is not only associated with decreasing numbers of airway wall inflammatory cells but also with increasing numbers of these cells in the airway lumen. Finally, effects of inhibition of transepithelial migration support the present hypothesis. Airway inflammatory processes have thus been much aggravated when transepithelial exit of leukocytes has been inhibited. In conclusion, the present hypothesis highlights risks involved in drug-induced inhibition of transepithelial migration of airway wall leukocytes. It helps interpretation of common airway lumen data, and suggests approaches to treat cell-mediated airway inflammation.

Alarming dendritic cells for allergic sensitization

Allergic patients mount a Th2 response to common allergens, like house dust mite (HDM), pollens, molds and animal dander. Most inhaled antigens are immunologically inert, however if these antigens are accompanied by microbial or endogenous danger patterns (alarmins), they can be recognized by inflammatory cells. Dendritic cells are the most potent antigen presenting cells, which express a wide variety of receptors on their cell surface, recognizing these microbial patterns, damage induced molecules and cytokines. Dendritic cells become reporters of the microenvironment if exposed to the allergen, subsequently migrating to the draining lymph nodes where they activate naïve T lymphocytes. Dendritic cells could also be indirectly activated by epithelial cells, which express various receptors and secrete a variety of cytokines early after allergen exposure. Upon HDM exposure these cells secrete chemokines to attract monocytes and immature dendritic cells, and GM-CSF, TSLP and IL-33 to activate dendritic cells, mast cells and basophils. Danger signals which alert dendritic cells and epithelial cells comprise many proteins and molecules, contributing to an enhanced immune response to inhaled allergens. This review focuses on the role of dendritic cells and alarmins in the sensitization to inhaled allergens in allergic asthma.

Double-stranded RNA exacerbates pulmonary allergic reaction through TLR3: implication of airway epithelium and dendritic cells

Respiratory viral infections have been implicated in exacerbations of allergic asthma, characterized by a Th2-biased immune response. Respiratory viruses target airway epithelial cells and dendritic cells (DCs). Their activation is, at least in part, mediated by the TLR3-dependent recognition of virus-derived dsRNA. To elucidate the role of epithelial cells and DCs and the implication of TLR3/Toll/IL-1R domain-containing adaptor-inducing IFN-beta (TRIF) pathway, we developed a mouse model of lung allergic exacerbation. The effect of intranasal administration of dsRNA in OVA-sensitized wild-type mice and TRIF(-/-) mice was evaluated on airway hyperresponsiveness and pulmonary inflammation. Our data demonstrated that treatment with dsRNA significantly increased the airway hyperresponsiveness, the lung inflammation, and the OVA-specific Th2 response. This was associated with an infiltrate of eosinophils, myeloid DCs, and T lymphocytes. TRIF activation was required for the development of dsRNA-induced exacerbation of the allergic reaction. Intratracheal transfer of IL-4/dsRNA/OVA-pretreated DCs also triggered exacerbation of the allergic reaction, whereas cells primed with dsRNA/OVA had a more limited effect. dsRNA-induced production of CCL20 by airway epithelium was associated with DC recruitment. In vivo and in vitro treatment with dsRNA amplified airway epithelial production of the pro-Th2 chemokines CCL11 and CCL17, their secretion being enhanced by Th2 cytokines. In conclusion, dsRNA derived from respiratory viruses trigger exacerbation of the pulmonary allergic reaction through TLR3/TRIF-dependent pathway. Moreover, Th2 cytokines participate in this process by modulating the response of airway epithelium and DCs to dsRNA.

Interactions between innate and adaptive immunity in asthma pathogenesis: new perspectives from studies on acute exacerbations

Asthma is a complex multigenic disease. The most frequently encountered form is atopic asthma, which is at its highest prevalence during childhood/young adulthood, and this represents the main focus of this review. The primary risk factor for atopic asthma is sensitization to perennial aeroallergens resulting from a failure to generate protective immunologic tolerance. This tolerance process is orchestrated by airway mucosal dendritic cells and normally results in programming of regulatory T cells, which inhibit activation of the T(H)2 memory cells that, among other activities, drive IgE production and prime the effector populations responsible for IgE-mediated tissue damage. Emerging evidence highlights the complexity of this process, in particular the iterative nature of the underlying interactions between innate and adaptive immune mechanisms in which virtually every signal emanating from one cellular compartment provokes an answering response from the other. To further complicate this picture, the local mesenchyme can also interpose signals to fine tune immune responses to optimally meet local microenvironmental needs. Perturbation of the balance between these interlinked innate and adaptive immune pathways is increasingly believed to be the basis for disease expression, and in the specific case of atopic asthma, the prototypic example of this (discussed below) is acute exacerbations triggered by viral infections.

Methods to study pulmonary dendritic cell migration

Dendritic cell migration from the airway to the lymph nodes is a key event in the development of airway immunity during infection, allergy, and vaccination. With judicial selection of materials, there are two approaches to study dendritic cell migration to the mediastinal lymph nodes without the induction of inflammation: airway administration of fluorescent OVA and latex beads. Our protocol describes how to label and track pulmonary dendritic cells from the airway and lung to the mediastinal lymph nodes and reveals how to avoid pitfalls and suboptimal assays.

Inhibition of Th2 adaptive immune responses and pulmonary inflammation by leukocyte Ig-like receptor B4 on dendritic cells

We previously established that the inhibitory receptor LILRB4 mitigates LPS-induced, neutrophil-dependent pathologic effector mechanisms in inflammation. We now report that LILRB4 on dendritic cells (DCs) counterregulates development of an adaptive Th2 immune response and ensuing inflammation in a model of allergic pulmonary inflammation, initiated by inhalation sensitization with OVA and LPS followed by airway challenge with OVA. We found that Lilrb4(-/-) mice had significantly exacerbated eosinophilic pulmonary inflammation, as assessed in bronchoalveolar lavage and lung tissue, as well as elevated levels of OVA-specific IgE and Th2 cytokines produced by OVA-restimulated lymph node cells. LILRB4 was preferentially expressed on MHC class II(high)CD86(high) OVA-bearing DCs in lung-draining lymph nodes after sensitization or challenge. Moreover, the lymph nodes of Lilrb4(-/-) mice had significantly more of these mature DCs after challenge with OVA, which was accompanied by significantly more IL-4-producing lymphocytes, compared with Lilrb4(+/+) mice. Sensitization of naive Lilrb4(+/+) mice by transfer of OVA-LPS-pulsed Lilrb4(-/-) bone marrow-derived DCs was sufficient to confer exacerbated allergic lung pathology upon challenge with OVA, compared with mice that received Lilrb4(+/+) bone marrow-derived DCs. Our findings establish that maturation and migration of pulmonary DCs to lymph nodes in response to Ag and an innate immune stimulus is associated with upregulated expression of LILRB4. In addition, this receptor attenuates the number of these mature DCs and attendant IL-4-producing lymphocytes in the lymph nodes, and accordingly, the ability of DCs to elicit pathologic Th2 pulmonary inflammation.

Pathogenic mechanisms of allergic inflammation: atopic asthma as a paradigm

Prospective studies tracking birth cohorts over periods of years indicate that the seeds for atopic asthma in adulthood are sewn during early life. The key events involve programming of functional phenotypes within the immune and respiratory systems which determine long-term responsiveness to ubiquitous environmental stimuli, particularly respiratory viruses and aeroallergens. A crucial component of asthma pathogenesis is early sensitization to aeroallergens stemming from a failure of mucosal tolerance mechanisms during the preschool years, which is associated with delayed postnatal maturation of a range of adaptive and innate immune functions. These maturational defects also increase risk for severe respiratory infections, and the combination of sensitization and infections maximizes risk for early development of the persistent asthma phenotype. Interactions between immunoinflammatory pathways stimulated by these agents also sustain the disease in later life as major triggers of asthma exacerbations. Recent studies on the nature of these interactions suggest the operation of an infection-associated lung:bone marrow axis involving upregulation of FcERlalpha on myeloid precursor populations prior to their migration to the airways, thus amplifying local inflammation via IgE-mediated recruitment of bystander atopic effector mechanisms. The key participants in the disease process are airway mucosal dendritic cells and adjacent epithelial cells, and transiting CD4(+) effector and regulatory T-cell populations, and increasingly detailed characterization of their roles at different stages of pathogenesis is opening up novel possibilities for therapeutic control of asthma. Of particular interest is the application of genomics-based approaches to drug target identification in cell populations of interest, exemplified by recent findings discussed below relating to the gene network(s) triggered by activation of Th2-memory cells from atopics.

Vascular endothelial growth factor is a key mediator in the development of T cell priming and its polarization to type 1 and type 17 T helper cells in the airways

Chronic inflammatory airway diseases including asthma are characterized by immune dysfunction to inhaled allergens. Our previous studies demonstrated that T cell priming to inhaled allergens requires LPS, which is ubiquitously present in household dust allergens. In this study, we evaluated the role of vascular endothelial growth factor (VEGF) in the development of T cell priming and its polarization to Th1 or Th17 cells when exposed to LPS-contaminated allergens. An asthma mouse model was induced by airway sensitization with LPS-contaminated allergens and then challenged with allergens alone. Therapeutic intervention was performed during allergen sensitization. The present study showed that lung inflammation induced by sensitization with LPS-contaminated allergens was decreased in mice with homozygous disruption of the IL-17 gene; in addition, allergen-specific Th17 immune response was abolished in IL-6 knockout mice. Meanwhile, in vivo production of VEGF was up-regulated by airway exposure of LPS. In addition, airway sensitization of allergen plus recombinant VEGF induced both type 1 and type 17 Th cell (Th1 and Th17) responses. Th1 and Th17 responses induced by airway sensitization with LPS-contaminated allergens were blocked by treatment with a pan-VEGF receptor (VEGFR; VEGFR-1 plus VEGFR-2) inhibitor during sensitization. These effects were accompanied by inhibition of the production of Th1 and Th17 polarizing cytokines, IL-12p70 and IL-6, respectively. These findings indicate that VEGF produced by LPS plays a key role in activation of naive T cells and subsequent polarization to Th1 and Th17 cells.

Concomitant inhalation of cigarette smoke and aerosolized protein activates airway dendritic cells and induces allergic airway inflammation in a TLR-independent way

Cigarette smoking is associated with the development of allergic asthma. In mice, exposure to cigarette smoke sensitizes the airways toward coinhaled OVA, leading to OVA-specific allergic inflammation. Pulmonary dendritic cells (DCs) are professional APCs involved in immunosurveillance and implicated in the induction of allergic responses in lung. We investigated the effects of smoking on some of the key features of pulmonary DC biology, including trafficking dynamics and cellular activation status in different lung compartments. We found that cigarette smoke inhalation greatly amplified DC-mediated transport of inhaled Ags to mediastinal lymph nodes, a finding supported by the up-regulation of CCR7 on airway DCs. Pulmonary plasmacytoid DCs, which have been involved in inhalational tolerance, were reduced in number after smoke exposure. In addition, combined exposure to cigarette smoke and OVA aerosol increased surface expression of MHC class II, CD86, and PDL2 on airway DCs, while ICOSL was strongly down-regulated. Although inhaled endotoxins, which are also present in cigarette smoke, have been shown to act as DC activators and Th2-skewing sensitizers, TLR4-deficient and MyD88 knockout mice did not show impaired eosinophilic airway inflammation after concomitant exposure to cigarette smoke and OVA. From these data, we conclude that cigarette smoke activates the pulmonary DC network in a pattern that favors allergic airway sensitization toward coinhaled inert protein. The TLR independency of this phenomenon suggests that alternative immunological adjuvants are present in cigarette smoke.

Targeted NF-kappaB inhibition of asthmatic serum-mediated human monocyte-derived dendritic cell differentiation in a transendothelial trafficking model

Transendothelial trafficking model mimics in vivo differentiation of monocytes into dendritic cells (DC). The serum from patients with systemic lupus erythematosus promotes the differentiation of monocytes into mature DC. We have shown that selective inhibition of NF-kappaB by adenoviral gene transfer of a novel mutated IkappaBalpha (AdIkappaBalphaM) in DC contributes to T cell tolerance. Here we demonstrated for the first time that asthmatic serum facilitated human monocyte-derived DC (MDDC) maturation associated with increased NF-kappaB activation in this model. Furthermore, selective blockade of NF-kappaB by AdIkappaBalphaM in MDDC led to increased apoptosis, and decreased levels of CD80, CD83, CD86, and IL-12 p70 but not IL-10 in asthmatic serum-stimulated MDDC, accompanied by reduced proliferation of T cells. These results suggest that AdIkappaBalphaM-transferred MDDC are at a more immature stage which is beneficial to augment the immune tolerance in asthma.

Role of dendritic cells and alveolar macrophages in regulating early host defense against pulmonary infection with Cryptococcus neoformans

Successful pulmonary clearance of the encapsulated yeast Cryptococcus neoformans requires a T1 adaptive immune response. This response takes up to 3 weeks to fully develop. The role of the initial, innate immune response against the organism is uncertain. In this study, an established model of diphtheria toxin-mediated depletion of resident pulmonary dendritic cells (DC) and alveolar macrophages (AM) was used to assess the contribution of these cells to the initial host response against cryptococcal infection. The results demonstrate that depletion of DC and AM one day prior to infection results in rapid clinical deterioration and death of mice within 6 days postinfection; this effect was not observed in infected groups of control mice not depleted of DC and AM. Depletion did not alter the microbial burden or total leukocyte recruitment in the lung. Mortality (in mice depleted of DC and AM) was associated with increased neutrophil and B-cell accumulation accompanied by histopathologic evidence of suppurative neutrophilic bronchopneumonia, cyst formation, and alveolar damage. Collectively, these data define an important role for DC and AM in regulating the initial innate immune response following pulmonary infection with C. neoformans. These findings provide important insight into the cellular mechanisms which coordinate early host defense against an invasive fungal pathogen in the lung.

Between a cough and a wheeze: dendritic cells at the nexus of tobacco smoke-induced allergic airway sensitization

Exposure to cigarette smoke represents a major risk factor for the development of asthma. Enhanced sensitization toward allergens has been observed in humans and laboratory animals exposed to cigarette smoke. Pulmonary dendritic cells (DCs) are crucially involved in sensitization toward allergens and play an important role in the development of T helper (Th)2-mediated allergic airway inflammation. We propose the concept that aberrant DC activation forms the basis for the deviation of the lung's default tolerogenic response toward allergic inflammation when harmless antigens are concomittantly inhaled with tobacco smoke. This review will summarize evidence suggesting that tobacco smoke can achieve this effect by providing numerous triggers of innate immunity, which can profoundly modulate airway DC biology. Tobacco smoke can affect the airway DC network either directly or indirectly by causing the release of DC-targeted mediators from the pulmonary tissue environment, resulting in the induction of a Th2-oriented pathological immune response. A thorough knowledge of the molecular pathways involved may open the door to novel approaches in the treatment of asthma.

An imbalance in C/EBPs and increased mitochondrial activity in asthmatic airway smooth muscle cells: novel targets in asthma therapy?

The asthma prevalence was increasing over the past two decades worldwide. Allergic asthma, caused by inhaled allergens of different origin or by food, is mediated by inflammatory mechanisms. The action of non-allergic asthma, induced by cold air, humidity, temperature or exercise, is not well understood. Asthma affects up to 15% of the population and is treated with anti-inflammatory and muscle relaxing drugs which allow symptom control. Asthma was first defined as a malfunction of the airway smooth muscle, later as an imbalanced immune response of the lung. Recent studies placed the airway smooth muscle again into the focus. Here we summarize the molecular biological basis of the deregulated function of the human airway smooth muscle cell as a cause or important contributor to the pathology of asthma. In the asthmatic human airway smooth muscle cells, there is: (i) a deregulation of cell differentiation due to low levels of maturation-regulating transcription factors such as CCAAT/enhancer binding proteins and peroxisome proliferator-activated receptors, thereby reducing the cells threshold to proliferate and to secrete pro-inflammatory cytokines under certain conditions; (ii) a higher basal energy turnover that is due to increased number and activity of mitochondria; and (iii) a modified feedback mechanism between cells and the extracellular matrix they are embedded in. All these cellular pathologies are linked to each other and to the innate immune response of the lung, but the sequence of events is unclear and needs further investigation. However, these findings may present the basis for the development of novel curative asthma drugs.

Dendritic cell-nerve clusters are sites of T cell proliferation in allergic airway inflammation

Interactions between T cells and dendritic cells in the airway mucosa precede secondary immune responses to inhaled antigen. The purpose of this study was to identify the anatomical locations where dendritic cell-T cell interactions occur, resulting in T cells activation by dendritic cells. In a mouse model of allergic airway inflammation, we applied whole-mount immunohistology and confocal microscopy to visualize dendritic cells and T cells together with nerves, epithelium, and smooth muscle in three dimensions. Proliferating T cells were identified by the detection of the incorporation of the nucleotide analogue 5-ethynyl-2'-deoxyuridine into the DNA. We developed a novel quantification method that enabled the accurate determination of cell-cell contacts in a semi-automated fashion. Dendritic cell-T cell interactions occurred beneath the smooth muscle layer, but not in the epithelium. Approximately 10% of the dendritic cells were contacted by nerves, and up to 4% of T cells formed clusters with these dendritic cells. T cells that were clustered with nerve-contacting dendritic cells proliferated only in the airways of mice with allergic inflammation but not in the airways of negative controls. Taken together, these results suggest that during the secondary immune response, sensory nerves influence dendritic cell-driven T cell activation in the airway mucosa.

Lung dendritic cells: targets for therapy in allergic disease

Dendritic cells (DCs) are crucial in determining the functional outcome of allergen encounter in the lung. Antigen presentation by myeloid DCs leads to Th2 sensitization typical of allergic disease, whereas antigen presentation by plasmacytoid DCs serves to dampen inflammation. It is increasingly clear that DCs have an antigen presenting function beyond sensitisation. DCs therefore constitute a novel target for the development of antiallergic therapy aimed at the origin of the inflammatory cascade. A careful study of DC biology and of the receptors expressed by lung DCs has provided a framework for the discovery of novel antiallergic compounds.

A new mechanism for inhalational priming: IL-4 bypasses innate immune signals

Signaling via innate immune mechanisms is considered pivotal for T cell-mediated responses to inhaled Ags. Furthermore, Th2 cells specific for one inhaled Ag can facilitate priming of naive T cells to unrelated new inhaled Ags, a process we call "Th2 collateral priming". Interestingly, our previous studies showed that collateral priming is independent of signals via the innate immune system but depends on IL-4 secretion by CD4(+) T cells. We thus hypothesized that IL-4 can bypass the need for signals via the innate immune system, considered essential for pulmonary priming. Indeed, we were able to show that IL-4 bypasses the requirement for TLR4- and MyD88-mediated signaling for responses to new allergens. Furthermore, we characterized the mechanisms by which IL-4 primes for new inhaled allergens: "IL-4-dependent pulmonary priming" relies on IL-4 receptor expression on hematopoietic cells and structural cells. Transfer experiments indicate that within the hematopoietic compartment both T cells and dendritic cells need to express the IL-4 receptor. Finally, we were able to show that IL-4 induces recruitment and maturation of myeloid dendritic cells in vivo and increases T cell recruitment to the draining lymph nodes. Our findings bring new mechanistic knowledge to the phenomenon of polysensitization and primary sensitization in asthma.

Innate immune mechanism in allergic asthma

Allergic asthma is a chronic inflammatory disorder of the airways characterized by eosinophilic inflammation, airway hyperresponsiveness, and mucus hypersecretion. Adaptive, antigen-dependent immunity is critical for asthma pathogenesis. Allergic asthma may involve adaptive and innate, antigen-independent immune responses. This review discusses the current evidence that associates innate immunity with allergic asthma pathogenesis. In particular, we focus on the role of innate immune cells (eg, bronchial epithelial cells, alveolar macrophages, and dendritic cells) and molecules (Toll-like and nucleotide-binding oligomerization domain-like receptors) in modifying allergic immune responses.

Division of labor between dendritic cell subsets of the lung

Dendritic cells (DCs) are a heterogenous population of antigen-presenting cells, of which conventional DCs and plasmacytoid DCs are the main subsets. Like DC subsets in the central lymphoid organs, DC subsets in the lungs exert specific functions that can be associated with distinct expression of endocytic receptors, cell-surface molecules, and anatomical location within the lung. In recent years, DC populations are increasingly split up into a seemingly endless number of defined sub-populations. We argue that this is not a "stamp-collecting" activity but essential for a deeper understanding of the immune response to pathogens like respiratory viruses or tolerance to harmless antigens. In homeostatic conditions, a fine-tuned balance exists between the various functions of lung DC subsets, which is necessary for maintaining immune homeostasis in the lung. However, infectious or inflammatory conditions can profoundly alter the functions of steady-state DC subsets and recruit inflammatory type DCs to the lung. This might be important for clearing the inflicting pathogenic stimulus, but could at the same time also be involved in causing immune pathology.

Leukotriene B4 receptor 1 expression on dendritic cells is required for the development of Th2 responses and allergen-induced airway hyperresponsiveness

Dendritic cells (DC) are important APCs that control allergen-induced airway responses by interacting directly with T cells. Leukotriene B(4) (LTB(4)), interacting with its high-affinity receptor, LTB(4) receptor 1 (BLT1), is known to attract and activate leukocytes during inflammation. We have previously shown that BLT1 expression on Ag-primed T cells is required for the development of airway hyperresponsiveness (AHR; Miyahara et al. 2005. Am. J. Respir. Crit. Care Med. 172: 161-167). However, the role for the LTB(4)-BLT1 pathway in DC function in allergen-induced airway responses has not been defined. Bone marrow-derived DCs (BMDC) were generated. Naive BALB/c mice received OVA-pulsed BLT1-deficient (BLT1(-/-)) BMDCs or wild-type BMDCs intratracheally and were then challenged with OVA for 3 days. Airway responses were monitored 48 h after the last allergen challenge. BLT1(-/-) BMDCs showed normal maturation judged from surface expression of CD markers. Compared with recipients of wild-type BMDCs, mice that received BLT1(-/-) BMDCs developed significantly lower AHR to inhaled methacholine, lower goblet cell metaplasia, and eosinophilic infiltration in the airways and decreased levels of Th2 type cytokines in the bronchoalveolar lavage fluid. Migration of BLT1(-/-) BMDCs into peribronchial lymph nodes was significantly impaired compared with BLT1(+/+) BMDCs after intratracheal instillation. These data suggest that BLT1 expression on DCs is required for migration of DCs to regional lymph nodes as well as in the development of AHR and airway inflammation.

CCR2 mediates conventional dendritic cell recruitment and the formation of bronchovascular mononuclear cell infiltrates in the lungs of mice infected with Cryptococcus neoformans

Pulmonary clearance of the encapsulated yeast Cryptococcus neoformans requires the development of T1-type immunity. CCR2-deficient mice infected with C. neoformans develop a non-protective T2 immune response and persistent infection. The mechanisms responsible for this aberrant response are unknown. The objective of this study was to define the number, phenotype, and microanatomic location of dendritic cells (DC) residing within the lung of CCR2+/+ or CCR2-/- mice throughout a time course following infection with C. neoformans. Results demonstrate the CCR2-mediated recruitment of conventional DC expressing modest amounts of costimulatory molecules. DC recruitment was preceded by the up-regulation in the lung of the CCR2 ligands CCL2 and CCL7. Colocalization of numerous DC and CD4+ T cells within bronchovascular infiltrates coincided with increased expression of IL-12 and IFN-gamma. By contrast, in the absence of CCR2, DC recruitment was markedly impaired, bronchovascular infiltrates were diminished, and mice developed features of T2 responses, including bronchovascular collagen deposition and IL-4 production. Our results demonstrate that CCR2 is required for the recruitment of large numbers of conventional DC to bronchovascular infiltrates in mice mounting a T1 immune response against a fungal pathogen. These findings shed new insight into the mechanism(s) by which DC recruitment alters T cell polarization in response to an infectious challenge within the lung.

Optimization of methods to study pulmonary dendritic cell migration reveals distinct capacities of DC subsets to acquire soluble versus particulate antigen

Dendritic cell migration from the airway to lymph nodes is a key event in the development of airway immunity during infection, allergy, and vaccination. To identify the best approaches to investigate DC migration to lung-draining lymph nodes, we directly compared three methods previously used to track DC migration: airway administration of fluorescent OVA, latex beads, or carboxyfluorescein succinimidyl ester (CFSE). We show that two of the methods employed in optimal conditions-administration of fluorescent OVA or latex particles-have broadly relevant utility in studies of pulmonary DC migration, both in the presence and absence of inflammatory mediators. However, CFSE was of limited value because it induced a robust airway inflammatory response upon instillation. Unexpectedly, antigen-loaded tracers with distinct physical properties differently affected the populations that acquired the tracers and the overall T cell response. Specifically, soluble OVA and OVA formulated as a particulate after conjugation to latex beads were acquired in different proportions in vivo by the two characterized subsets of pulmonary DCs: CD11b(hi)CD103(-) and CD11b(lo)CD103(+)langerin(+) DC populations. Consequently, and in line with recent studies that these two subsets of DCs respectively activate CD4(+) and CD8(+) lymphocyte populations, the physical nature of the antigen delivery vehicle strongly influenced the degree of CD4(+) versus CD8(+) OVA-specific T cell activation. This finding suggests that changes in the physical presentation of the same antigen delivered to the airway during natural immune responses or vaccinations may markedly affect the character of the T cell response that ensues.

Impact of boostering for the strength of asthma parameters and dendritic cell numbers in a C57BL/6 model of allergic airway inflammation

BACKGROUND

Murine models assist in elucidating the pathogenesis of allergic asthma and evaluation of new therapeutic strategies. We aimed to assess the requirement of boostering needed in the BL/6 murine asthma model and its influence on DC populations in lungs and bronchial lymph nodes.

METHODS AND RESULTS

Two injections of OVA+alum - one sensitization and one booster - followed by two aerosol challenges were sufficient to induce a distinct asthma-like inflammation in BL/6 mice, including significant increased immunoglobulin (IgE) level, influx of eosinophils in the airway lumen, and evident histopathology. Using this protocol, CD11chighMHC-II+ DC counts in lungs and lymph nodes doubled with no changes of CD8+ DC in the lungs but increase in lung-draining lymph nodes.

CONCLUSIONS

Given the site-specific changes of dendritic cell (DC) subpopulations during allergic asthma we propose a distinct regulation of antigen transport and antigen presentation in the murine asthma model.

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.

Allergic airways disease develops after an increase in allergen capture and processing in the airway mucosa

Airway mucosal dendritic cells (AMDC) and other airway APCs continuously sample inhaled Ags and regulate the nature of any resulting T cell-mediated immune response. Although immunity develops to harmful pathogens, tolerance arises to nonpathogenic Ags in healthy individuals. This homeostasis is thought to be disrupted in allergic respiratory disorders such as allergic asthma, such that a potentially damaging Th2-biased, CD4(+) T cell-mediated inflammatory response develops against intrinsically nonpathogenic allergens. Using a mouse model of experimental allergic airways disease (EAAD), we have investigated the functional changes occurring in AMDC and other airway APC populations during disease onset. Onset of EAAD was characterized by early and transient activation of airway CD4(+) T cells coinciding with up-regulation of CD40 expression exclusively on CD11b(-) AMDC. Concurrent enhanced allergen uptake and processing occurred within all airway APC populations, including B cells, macrophages, and both CD11b(+) and CD11b(-) AMDC subsets. Immune serum transfer into naive animals recapitulated the enhanced allergen uptake observed in airway APC populations and mediated activation of naive allergen-specific, airway CD4(+) T cells following inhaled allergen challenge. These data suggest that the onset of EAAD is initiated by enhanced allergen capture and processing by a number of airway APC populations and that allergen-specific Igs play a role in the conversion of normally quiescent AMDC subsets into those capable of inducing airway CD4(+) T cell activation.

Innate immune control of pulmonary dendritic cell trafficking

Dendritic cells (DC) are potent antigen-presenting cells that are essential for initiating adaptive immune responses. Residing within the airway mucosa, pulmonary DC continually sample the antigenic content of inhaled air and migrate to draining lymph nodes, where they present these antigens to naive T cells. The migratory patterns of pulmonary DC are highly dependent upon inflammatory conditions in the lung. Under steady-state, or non-inflammatory, conditions, pulmonary DC undergo slow but constitutive migration to draining lymph nodes, where they remain for several days and confer antigen-specific tolerance. With the onset of pulmonary inflammation, airway DC trafficking increases dramatically, and these cells rapidly accumulate within draining lymph nodes. However, within a few days, the number of airway-derived DC in lymph nodes stabilizes or declines, even in the face of ongoing pulmonary inflammation. Here, we summarize current understanding of the molecular and cellular mechanisms underlying pulmonary DC trafficking to the lymph node and the recruitment of DC precurors to the lung. It is hoped that an improved understanding of these mechanisms will lead to novel DC-mediated therapeutic strategies to treat immune-related pulmonary disease.

Dendritic Cells and Macrophages Form a Transepithelial Network against Foreign Particulate Antigens

Fine particles (0.1-2.5 microm in diameter) may cause increased pulmonary morbidity and mortality. We demonstrate with a cell culture model of the human epithelial airway wall that dendritic cells extend processes between epithelial cells through the tight junctions to collect particles in the "luminal space" and to transport them through cytoplasmic processes between epithelial cells across the epithelium or to transmigrate through the epithelium to take up particles on the epithelial surface. Furthermore, dendritic cells interacted with particle-loaded macrophages on top of the epithelium and with other dendritic cells within or beneath the epithelium to take over particles. By comparing the cellular interplay of dendritic cells and macrophages across epithelial monolayers of different transepithelial electrical resistance, we found that more dendritic cells were involved in particle uptake in A549 cultures showing a low transepithelial electrical resistance compared with dendritic cells in16HBE14o cultures showing a high transepithelial electrical resistance 10 min (23.9% versus 9.5%) and 4 h (42.1% versus 14.6%) after particle exposition. In contrast, the macrophages in A549 co-cultures showed a significantly lower involvement in particle uptake compared with 16HBE14o co-cultures 10 min (12.8% versus 42.8%) and 4 h (57.4% versus 82.7%) after particle exposition. Hence we postulate that the epithelial integrity influences the particle uptake by dendritic cells, and that these two cell types collaborate as sentinels against foreign particulate antigen by building a transepithelial interacting cellular network.