miR449 Protects Airway Regeneration by Controlling AURKA/HDAC6-Mediated Ciliary Disassembly.

Airway mucociliary regeneration and function are key players for airway defense and are impaired in chronic obstructive pulmonary disease (COPD). Using transcriptome analysis in COPD-derived bronchial biopsies, we observed a positive correlation between cilia-related genes and microRNA-449 (miR449). In vitro, miR449 was strongly increased during airway epithelial mucociliary differentiation. In vivo, miR449 was upregulated during recovery from chemical or infective insults. miR0449-/- mice (both alleles are deleted) showed impaired ciliated epithelial regeneration after naphthalene and Haemophilus influenzae exposure, accompanied by more intense inflammation and emphysematous manifestations of COPD. The latter occurred spontaneously in aged miR449-/- mice. We identified Aurora kinase A and its effector target HDAC6 as key mediators in miR449-regulated ciliary homeostasis and epithelial regeneration. Aurora kinase A is downregulated upon miR449 overexpression in vitro and upregulated in miR449-/- mouse lungs. Accordingly, imaging studies showed profoundly altered cilia length and morphology accompanied by reduced mucociliary clearance. Pharmacological inhibition of HDAC6 rescued cilia length and coverage in miR449-/- cells, consistent with its tubulin-deacetylating function. Altogether, our study establishes a link between miR449, ciliary dysfunction, and COPD pathogenesis.

Quantification and role of innate lymphoid cell subsets in Chronic Obstructive Pulmonary Disease.

OBJECTIVES: Innate lymphoid cells (ILCs) secrete cytokines, such as IFN-γ, IL-13 and IL-17, which are linked to chronic obstructive pulmonary disease (COPD). Here, we investigated the role of pulmonary ILCs in COPD pathogenesis. METHODS: Lung ILC subsets in COPD and control subjects were quantified using flow cytometry and associated with clinical parameters. Tissue localisation of ILC and T-cell subsets was determined by immunohistochemistry. Mice were exposed to air or cigarette smoke (CS) for 1, 4 or 24 weeks to investigate whether pulmonary ILC numbers and activation are altered and whether they contribute to CS-induced innate inflammatory responses. RESULTS: Quantification of lung ILC subsets demonstrated that ILC1 frequency in the total ILC population was elevated in COPD and was associated with smoking and severity of respiratory symptoms (COPD Assessment Test [CAT] score). All three ILC subsets localised near lymphoid aggregates in COPD. In the COPD mouse model, CS exposure in C57BL/6J mice increased ILC numbers at all time points, with relative increases in ILC1 in bronchoalveolar lavage (BAL) fluid. Importantly, CS exposure induced increases in neutrophils, monocytes and dendritic cells that remained elevated in Rag2/Il2rg-deficient mice that lack adaptive immune cells and ILCs. However, CS-induced CXCL1, IL-6, TNF-α and IFN-γ levels were reduced by ILC deficiency. CONCLUSION: The ILC1 subset is increased in COPD patients and correlates with smoking and severity of respiratory symptoms. ILCs also increase upon CS exposure in C57BL/6J mice. In the absence of adaptive immunity, ILCs contribute to CS-induced pro-inflammatory mediator release, but are redundant in CS-induced innate inflammation.

Innate lymphoid cells in isocyanate-induced asthma: role of microRNA-155.

BACKGROUND: Occupational asthma, induced by workplace exposures to low molecular weight agents such as toluene 2,4-diisocyanate (TDI), causes a significant burden to patients and society. Little is known about innate lymphoid cells (ILCs) in TDI-induced asthma. A critical regulator of ILC function is microRNA-155, a microRNA associated with asthma. OBJECTIVE: To determine whether TDI exposure modifies the number of ILCs in the lung and whether microRNA-155 contributes to TDI-induced airway inflammation and hyperresponsiveness. METHODS: C57BL/6 wild-type and microRNA-155 knockout mice were sensitised and challenged with TDI or vehicle. Intracellular cytokine expression in ILCs and T-cells was evaluated in bronchoalveolar lavage (BAL) fluid using flow cytometry. Peribronchial eosinophilia and goblet cells were evaluated on lung tissue, and airway hyperresponsiveness was measured using the forced oscillation technique. Putative type 2 ILCs (ILC2) were identified in bronchial biopsies of subjects with TDI-induced occupational asthma using immunohistochemistry. Human bronchial epithelial cells were exposed to TDI or vehicle. RESULTS: TDI-exposed mice had higher numbers of airway goblet cells, BAL eosinophils, CD4+ T-cells and ILCs, with a predominant type 2 response, and tended to have airway hyperresponsiveness. In TDI-exposed microRNA-155 knockout mice, inflammation and airway hyperresponsiveness were attenuated. TDI exposure induced IL-33 expression in human bronchial epithelial cells and in murine lungs, which was microRNA-155 dependent in mice. GATA3+CD3- cells, presumably ILC2, were present in bronchial biopsies. CONCLUSION: TDI exposure is associated with increased numbers of ILCs. The proinflammatory microRNA-155 is crucial in a murine model of TDI asthma, suggesting its involvement in the pathogenesis of occupational asthma due to low molecular weight agents.

Dermal exposure determines the outcome of repeated airway exposure in a long-term chemical-induced asthma-like mouse model.

BACKGROUND: Exposure to diisocyanates is an important cause of occupational asthma (OA) in the industrialized world. Since OA occurs after long-term exposure to diisocyanates, we developed a chronic mouse model of chemical-induced asthma where toluene diisocyanate (TDI) was administered at two different exposure sites. OBJECTIVES: Evaluating the effect of long-term respiratory isocyanate exposure - with or without prior dermal exposure- on sensitization, inflammatory responses and airway hyperreactivity (AHR). METHODS: On days 1 and 8, BALB/c mice were dermally treated (20 µl/ear) with 0.5% 2,4-toluene diisocyanate TDI or the vehicle acetone olive oil (AOO) (3:2). Starting from day 15, mice received intranasal instillations with 0.1% TDI of vehicle five times in a week, for five successive weeks. One day after the last instillation airway hyperreactivity (AHR) to methacholine was assessed, followed by an evaluation of pulmonary inflammation and structural lung changes. Immune-related parameters were assessed in the lungs (BAL and tissue), blood, cervical- and auricular lymph nodes. RESULTS: Mice repeatedly intranasally exposed to TDI showed systemic sensitization and a mixed Th1/Th2 type immune response, without the presence of AHR. However, when mice are first dermally sensitized with TDI, followed by repeated intranasal TDI challenges, this results in a pronounced Th2 response and AHR. CONCLUSION: Dermal exposure to TDI determines airway hyperreactivity after repeated airway exposure to TDI.

IL-33 signalling contributes to pollutant-induced allergic airway inflammation.

BACKGROUND: Clinical and experimental studies have identified a crucial role for IL-33 and its receptor ST2 in allergic asthma. Inhalation of traffic-related pollutants, such as diesel exhaust particles (DEP), facilitates the development of asthma and can cause exacerbations of asthma. However, it is unknown whether IL-33/ST2 signalling contributes to the enhancing effects of air pollutants on allergic airway responses. OBJECTIVE: We aim to investigate the functional role of IL-33/ST2 signalling in DEP-enhanced allergic airway responses, using an established murine model. METHODS: C57BL/6J mice were exposed to saline, DEP alone, house dust mite (HDM) alone or combined DEP+HDM. To inhibit IL-33 signalling, recombinant soluble ST2 (r-sST2) was given prophylactically (ie, during the whole experimental protocol) or therapeutically (ie, at the end of the experimental protocol). Airway hyperresponsiveness and the airway inflammatory responses were assessed in bronchoalveolar lavage fluid (BALF) and lung. RESULTS: Combined exposure to DEP+HDM increased IL-33 and ST2 expression in lung, elevated inflammatory responses and bronchial hyperresponsiveness compared to saline, sole DEP or sole HDM exposure. Prophylactic interference with the IL-33/ST2 signalling pathway impaired the DEP-enhanced allergic airway inflammation in the BALF, whereas effects on lung inflammation and airway hyperresponsiveness were minimal. Treatment with r-sST2 at the end of the experimental protocol did not modulate the DEP-enhanced allergic airway responses. CONCLUSION: Our data suggest that the IL-33/ST2 pathway contributes to the onset of DEP-enhanced allergic airway inflammation.

The IL-33/ST2 axis is crucial in type 2 airway responses induced by Staphylococcus aureus-derived serine protease-like protein D.

BACKGROUND: Chronic airway inflammatory diseases, such as chronic rhinosinusitis with nasal polyps and asthma, show increased nasal Staphylococcus aureus colonization. Staphylococcus aureus-derived serine protease-like protein (Spl) D and other closely related proteases secreted by S aureus have recently been identified as inducers of allergic asthma in human subjects and mice, but their mechanism of action is largely unknown. OBJECTIVE: We investigated the role of recombinant SplD in driving TH2-biased responses and IgE formation in a murine model of allergic asthma. METHODS: Allergic asthma was induced in C57BL/6 J wild-type mice, Toll-like receptor (TLR) 4 knockout (Tlr4-/-) mice, and recombination-activating gene (Rag2) knockout (Rag2-/-) mice by means of repeated intratracheal applications of SplD. Inflammatory parameters in the airways were assessed by means of flow cytometry, ELISA, Luminex, and immunohistochemistry. Serum SplD-specific IgE levels were analyzed by using ELISA. RESULTS: We observed that repeated intratracheal exposure to SplD led to IL-33 and eotaxin production, eosinophilia, bronchial hyperreactivity, and goblet cell hyperplasia in the airways. Blocking IL-33 activity with a soluble ST2 receptor significantly decreased the numbers of eosinophils, IL-13+ type 2 innate lymphoid cells and IL-13+CD4+ T cells and IL-5 and IL-13 production by lymph node cells but had no effect on IgE production. SplD-induced airway inflammation and IgE production were largely dependent on the presence of the functional adaptive immune system and independent of TLR4 signaling. CONCLUSION: The S aureus-derived protein SplD is a potent allergen of S aureus and induces a TH2-biased inflammatory response in the airways in an IL-33-dependent but TRL4-independent manner. The soluble ST2 receptor could be an efficient strategy to interfere with SplD-induced TH2 inflammation but does not prevent the allergic sensitization.

Role of tumor necrosis factor-α and its receptors in diesel exhaust particle-induced pulmonary inflammation.

Inhalation of diesel exhaust particles (DEP) induces an inflammatory reaction in the lung. However, the underlying mechanisms remain to be elucidated. Tumor necrosis factor alpha (TNF-α) is a pro-inflammatory cytokine that operates by binding to tumor necrosis factor receptor 1 (TNFR1) and tumor necrosis factor receptor 2 (TNFR2). The role of TNF-α signaling and the importance of either TNFR1 or TNFR2 in the DEP-induced inflammatory response has not yet been elucidated. TNF-α knockout (KO), TNFR1 KO, TNFR2 KO, TNFR1/TNFR2 double KO (TNFR-DKO) and wild type (WT) mice were intratracheally exposed to saline or DEP. Pro-inflammatory cells and cytokines were assessed in the bronchoalveolar lavage fluid (BALF). Exposure to DEP induced a dose-dependent inflammation in the BALF in WT mice. In addition, levels of TNF-α and its soluble receptors were increased upon exposure to DEP. The DEP-induced inflammation in the BALF was decreased in TNF-α KO, TNFR-DKO and TNFR2 KO mice. In contrast, the inflammatory response in the BALF of DEP-exposed TNFR1 KO mice was largely comparable with WT controls. In conclusion, these data provide evidence for a regulatory role of TNF-α in DEP-induced pulmonary inflammation and identify TNFR2 as the most important receptor in mediating these inflammatory effects.

Dysregulation of type 2 innate lymphoid cells and TH2 cells impairs pollutant-induced allergic airway responses.

BACKGROUND: Although the prominent role of TH2 cells in type 2 immune responses is well established, the newly identified type 2 innate lymphoid cells (ILC2s) can also contribute to orchestration of allergic responses. Several experimental and epidemiologic studies have provided evidence that allergen-induced airway responses can be further enhanced on exposure to environmental pollutants, such as diesel exhaust particles (DEPs). However, the components and pathways responsible remain incompletely known. OBJECTIVE: We sought to investigate the relative contribution of ILC2 and adaptive TH2 cell responses in a murine model of DEP-enhanced allergic airway inflammation. METHODS: Wild-type, Gata-3+/nlslacZ (Gata-3-haploinsufficient), RAR-related orphan receptor α (RORα)fl/flIL7RCre (ILC2-deficient), and recombination-activating gene (Rag) 2-/- mice were challenged with saline, DEPs, or house dust mite (HDM) or DEP+HDM. Airway hyperresponsiveness, as well as inflammation, and intracellular cytokine expression in ILC2s and TH2 cells in the bronchoalveolar lavage fluid and lung tissue were assessed. RESULTS: Concomitant DEP+HDM exposure significantly enhanced allergic airway inflammation, as characterized by increased airway eosinophilia, goblet cell metaplasia, accumulation of ILC2s and TH2 cells, type 2 cytokine production, and airway hyperresponsiveness compared with sole DEPs or HDM. Reduced Gata-3 expression decreased the number of functional ILC2s and TH2 cells in DEP+HDM-exposed mice, resulting in an impaired DEP-enhanced allergic airway inflammation. Interestingly, although the DEP-enhanced allergic inflammation was marginally reduced in ILC2-deficient mice that received combined DEP+HDM, it was abolished in DEP+HDM-exposed Rag2-/- mice. CONCLUSION: These data indicate that dysregulation of ILC2s and TH2 cells attenuates DEP-enhanced allergic airway inflammation. In addition, a crucial role for the adaptive immune system was shown on concomitant DEP+HDM exposure.

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.

Characterization and Quantification of Innate Lymphoid Cell Subsets in Human Lung.

BACKGROUND: Innate lymphoid cells (ILC) are a new family of innate immune cells that have emerged as important regulators of tissue homeostasis and inflammation. However, limited data are available concerning the relative abundance and characteristics of ILC in the human lung. METHODS: The aim of this study was to characterize and enumerate the different ILC subsets in human lung by multi-color flow cytometry. RESULTS: Within the CD45+ Lin- CD127+ pulmonary ILC population, we identified group 1 (ILC1), group 2 (ILC2) and group 3 (ILC3) innate lymphoid cells using specific surface markers (i.e. IL12Rß2, CRTH2 and CD117 respectively) and key transcription factors (i.e. T-bet, GATA-3 and RORγT respectively). Based on the presence of NKp44, ILC3 were further subdivided in natural cytotoxicity receptor (NCR)+ and NCR- ILC3. In addition, we demonstrated the production of signature cytokines IFN-γ, IL-5, IL-17A, IL-22 and GM-CSF in the pulmonary ILC population. Interestingly, we observed a tendency to a higher frequency of NCR- ILC3 in lungs of patients with chronic obstructive pulmonary disease (COPD) compared with controls. CONCLUSIONS: We show that the three main ILC subsets are present in human lung. Importantly, the relative abundance of ILC subsets tended to change in COPD patients in comparison to control individuals.

Pro- and Anti-Inflammatory Role of ChemR23 Signaling in Pollutant-Induced Inflammatory Lung Responses.

Inhalation of traffic-related particulate matter (e.g., diesel exhaust particles [DEPs]) is associated with acute inflammatory responses in the lung, and it promotes the development and aggravation of allergic airway diseases. We previously demonstrated that exposure to DEP was associated with increased recruitment and maturation of monocytes and conventional dendritic cells (DCs), resulting in TH2 polarization. Monocytes and immature DCs express the G-protein coupled receptor chemR23, which binds the chemoattractant chemerin. Using chemR23 knockout (KO) and corresponding wild-type (WT) mice, we determined the role of chemR23 signaling in response to acute exposure to DEPs and in response to DEP-enhanced house dust mite (HDM)-induced allergic airway inflammation. Exposure to DEP alone, as well as combined exposure to DEP plus HDM, elevated the levels of chemerin in the bronchoalveolar lavage fluid of WT mice. In response to acute exposure to DEPs, monocytes and monocyte-derived DCs accumulated in the lungs of WT mice, but this response was significantly attenuated in chemR23 KO mice. Concomitant exposure to DEP plus HDM resulted in allergic airway inflammation with increased eosinophilia, goblet cell metaplasia, and TH2 cytokine production in WT mice, which was further enhanced in chemR23 KO mice. In conclusion, we demonstrated an opposing role for chemR23 signaling depending on the context of DEP-induced inflammation. The chemR23 axis showed proinflammatory properties in a model of DEP-induced acute lung inflammation, in contrast to anti-inflammatory effects in a model of DEP-enhanced allergic airway inflammation.

Role of B Cell-Activating Factor in Chronic Obstructive Pulmonary Disease.

RATIONALE: B cell-activating factor (BAFF) plays a major role in activation of B cells and in adaptive humoral immune responses. In chronic obstructive pulmonary disease (COPD), lymphoid follicles have been associated with disease severity, and overexpression of BAFF has been demonstrated within lymphoid follicles of patients with severe COPD. OBJECTIVES: To investigate expression and localization of BAFF in the lungs of patients with COPD and to study the role of BAFF in COPD by antagonizing BAFF in a mouse model of chronic cigarette smoke (CS) exposure. METHODS: We quantified and localized BAFF expression in lungs of never-smokers, smokers without COPD, and patients with COPD and in lungs of air- or CS-exposed mice by reverse-transcriptase polymerase chain reaction, ELISA, immunohistochemistry, and confocal imaging. Next, to investigate the role of BAFF in COPD, we antagonized BAFF by prophylactic or therapeutic administration of a soluble fusion protein of the BAFF-receptor, BAFFR-Fc, in mice exposed to air or CS for 24 weeks and evaluated several hallmarks of COPD and polarization of lung macrophages. MEASUREMENTS AND MAIN RESULTS: BAFF expression was significantly increased in lungs of patients with COPD and CS-exposed mice. BAFF staining in lymphoid follicles was observed around B cells, CD4(+) cells, dendritic cells, follicular dendritic cells, and fibroblastic reticular cells. Prophylactic and therapeutic administration of BAFFR-Fc in mice reduced pulmonary B-cell numbers and prevented CS-induced formation of lymphoid follicles and increases in immunoglobulin levels. Interestingly, prophylactic BAFFR-Fc administration significantly attenuated pulmonary inflammation and destruction of alveolar walls. Moreover, antagonizing BAFF altered the phenotype of alveolar and interstitial macrophages. CONCLUSIONS: BAFF is significantly increased in lungs of patients with COPD and is present around both immune and stromal cells within lymphoid follicles. Antagonizing BAFF in CS-exposed mice attenuates pulmonary inflammation and alveolar destruction.

Inflammasomes in respiratory disease: from bench to bedside.

The respiratory tract of human subjects is constantly exposed to harmful microbes and air pollutants. The immune system responds to these offenders to protect the host, but an unbalanced inflammatory response itself may promote tissue damage and ultimately lead to acute and chronic respiratory diseases. Deregulated inflammasome activation is emerging as a key modulator of respiratory infections and pathologic airway inflammation in patients with asthma, COPD, and pulmonary fibrosis. Assembly of these intracellular danger sensors in cells of the respiratory mucosa and alveolar compartment triggers a proinflammatory cell death mode termed pyroptosis and leads to secretion of bioactive IL-1ß and IL-18. Here, we summarize and review the inflammasome and its downstream effectors as therapeutic targets for the treatment of respiratory diseases.

Monocyte-derived dendritic cell recruitment and allergic T(H)2 responses after exposure to diesel particles are CCR2 dependent.

BACKGROUND: The inhalation of diesel exhaust particles (DEPs) is associated with increased sensitization toward inhaled allergens. Dendritic cells (DCs) are important mediators in immune regulation. We previously showed that the inhalation of DEPs increased the accumulation of DCs in the lung and enhanced the T(H)2 response in the mediastinal lymph node. OBJECTIVE: We hypothesized that CC chemokine receptors CCR2, CCR5, and CCR6 critically mediate the DC recruitment upon exposure to DEPs and that these CC chemokine receptors are important in the DEP-induced T(H)2 response. METHODS: We exposed CCR2 knockout, CCR5 knockout, CCR6 knockout, and wild-type mice to DEPs and examined the pulmonary monocyte and DC accumulation. By an adoptive transfer experiment, we assessed the direct involvement of CCR2 and CCR6 in the recruitment of blood monocytes toward the lung upon exposure to DEPs. We also examined the T(H)2 cytokine production in the mediastinal lymph nodes of DEP-exposed CCR2 knockout and CCR6 knockout mice. RESULTS: We observed that the DEP-induced monocyte and monocyte-derived DC recruitment was completely abolished in CCR2 knockout mice. CCR6 knockout mice also showed impaired monocyte recruitment upon exposure to DEPs. In contrast, monocyte and DC recruitment was comparable between DEP-exposed wild-type and CCR5 knockout mice. The impaired monocyte-derived DC recruitment in DEP-exposed CCR2 knockout, not CCR6 knockout, mice resulted in an abolished T(H)2 response in the mediastinal lymph node. CONCLUSION: These data suggest that monocyte-derived DCs, recruited in a CCR2-dependent manner, are critical in inducing T(H)2 responses upon inhalation of DEPs.

NLRP3/caspase-1-independent IL-1beta production mediates diesel exhaust particle-induced pulmonary inflammation.

Inhalation of diesel exhaust particles (DEP) induces an inflammatory reaction in the lung; however, the mechanisms are largely unclear. IL-1ß/IL-1RI signaling is crucial in several lung inflammatory responses. Typically, caspase-1 is activated within the NLRP3 inflammasome that recognizes several damage-associated molecular patterns, which results in cleavage of pro-IL-1ß into mature IL-1ß. In this study, we hypothesized that the NLRP3/caspase-1/IL-1ß pathway is critical in DEP-induced lung inflammation. Upon DEP exposure, IL-1RI knockout mice had reduced pulmonary inflammation compared with wild-type mice. Similarly, treatment with rIL-1R antagonist (anakinra) and IL-1ß neutralization impaired the DEP-induced lung inflammatory response. Upon DEP exposure, NLRP3 and caspase-1 knockout mice, however, showed similar IL-1ß levels and comparable pulmonary inflammation compared with wild-type mice. In conclusion, these data show that the DEP-induced pulmonary inflammation acts through the IL-1ß/IL-1RI axis. In addition, DEP initiates inflammation independent of the classical NLRP3/caspase-1 pathway, suggesting that other proteases might be involved.

Mouse models to unravel the role of inhaled pollutants on allergic sensitization and airway inflammation.

Air pollutant exposure has been linked to a rise in wheezing illnesses. Clinical data highlight that exposure to mainstream tobacco smoke (MS) and environmental tobacco smoke (ETS) as well as exposure to diesel exhaust particles (DEP) could promote allergic sensitization or aggravate symptoms of asthma, suggesting a role for these inhaled pollutants in the pathogenesis of asthma. Mouse models are a valuable tool to study the potential effects of these pollutants in the pathogenesis of asthma, with the opportunity to investigate their impact during processes leading to sensitization, acute inflammation and chronic disease. Mice allow us to perform mechanistic studies and to evaluate the importance of specific cell types in asthma pathogenesis. In this review, the major clinical effects of tobacco smoke and diesel exhaust exposure regarding to asthma development and progression are described. Clinical data are compared with findings from murine models of asthma and inhalable pollutant exposure. Moreover, the potential mechanisms by which both pollutants could aggravate asthma are discussed.

Diesel exhaust particles stimulate adaptive immunity by acting on pulmonary dendritic cells.

Particulate matter, such as diesel exhaust particles (DEPs), modulate adaptive immune responses in the lung; however, their mechanism of action remains largely unclear. Pulmonary dendritic cells (DCs) are crucial mediators in regulating immune responses. We hypothesized that the immunomodulatory effects of DEPs are caused by alteration of DC function. To test this, we instilled mice with DEPs and examined the pulmonary DC recruitment and maturation, their migration to the mediastinal lymph node (MLN), and the subsequent T cell response. We demonstrated that exposure to DEPs increased DC numbers in the bronchoalveolar lavage and the lungs and that DEPs increased the maturation status of these DCs. DEP exposure also enhanced the DC migration to the MLN. Moreover, we showed that DEPs themselves were transported to the MLN in a CCR7- and DC-dependent manner. This resulted in an enhanced T cell recruitment and effector differentiation in the MLN. These data suggest that DEP inhalation modulates immune responses in the lung via stimulation of DC function.

The role of immune tolerance in asthma pathogenesis.

Immune-mediated tolerance encompasses a wide spectrum of mechanisms that can prevent unnecessary and potentially harmful inflammatory responses. An increasing number of scientific publications provide proof for the concept that an impairment of immune-tolerance mechanisms might be causally related to the development of unwanted Th2-driven, allergen-induced airway diseases. In this review, we discuss immune tolerance and the evidence supporting its role in asthma pathogenesis.