Allergen-induced accumulation of airway dendritic cells is supported by an increase in CD31(hi)Ly-6C(neg) bone marrow precursors in a mouse model of asthma

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.

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.

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.

Interactions between epithelial cells and dendritic cells in airway immune responses: lessons from allergic airway disease

Micro-organisms constantly invade the human body and may form a threat to our health. Traditionally, concepts of defence mechanisms have included a protective outer layer of epithelia and a vigilant immune system searching for areas where the integrity of the outer layer may be compromised. Instead of considering these elements as two independent mechanisms, we should be treating them as a single integrated system. This review will present and discuss the role of local immune-competent cells and local epithelia in the recognition of potential pathogens and how the interaction between the two components may affect the initiation of the airway immune response. A concept emerges where airway mucosal dendritic cells act as integrators of both immunostimulatory and immunosuppressive signals that act within actively-involved mucosal tissue.

Local blockade of TSLP receptor alleviated allergic disease by regulating airway dendritic cells

Thymic stromal lymphopoietin (TSLP) emerges as a central mediator of T helper cell (Th)2-dominant allergic diseases. However, the role of TSLP receptor (TSLPR) in allergen-induced Th2 priming, and the effects of TSLP signaling blocking on the development of asthma remain unclear. Here we showed that allergen challenge caused a rapid accumulation of TSLP in the airways of asthmatic mice, correlating well with eosinophils counts and interleukin (IL)-5 productions. When TSLP signaling was blocked by intratracheal administration of anti-TSLPR antibody before sensitization, eosinophilic airway inflammation, goblet cell hyperplasia and Th2 cytokines productions were significantly reduced. The alleviating effects of TSLPR blocking were achieved by inhibition of maturation and migration of airway dendritic cells (DCs), as well as their abilities of initiating CD4+T cell responses. Thus, local application of anti-TSLPR prevented Th2-mediated airway inflammation, at least partly, by regulating DCs function, which might be exploited to develop novel treatments for asthma.

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.

Clearance of influenza virus from the lung depends on migratory langerin+CD11b- but not plasmacytoid dendritic cells

Although dendritic cells (DCs) play an important role in mediating protection against influenza virus, the precise role of lung DC subsets, such as CD11b- and CD11b+ conventional DCs or plasmacytoid DCs (pDCs), in different lung compartments is currently unknown. Early after intranasal infection, tracheal CD11b-CD11chi DCs migrated to the mediastinal lymph nodes (MLNs), acquiring co-stimulatory molecules in the process. This emigration from the lung was followed by an accumulation of CD11b+CD11chi DCs in the trachea and lung interstitium. In the MLNs, the CD11b+ DCs contained abundant viral nucleoprotein (NP), but these cells failed to present antigen to CD4 or CD8 T cells, whereas resident CD11b-CD8+ DCs presented to CD8 cells, and migratory CD11b-CD8- DCs presented to CD4 and CD8 T cells. When lung CD11chi DCs and macrophages or langerin+CD11b-CD11chi DCs were depleted using either CD11c-diphtheria toxin receptor (DTR) or langerin-DTR mice, the development of virus-specific CD8+ T cells was severely delayed, which correlated with increased clinical severity and a delayed viral clearance. 120G8+ CD11cint pDCs also accumulated in the lung and LNs carrying viral NP, but in their absence, there was no effect on viral clearance or clinical severity. Rather, in pDC-depleted mice, there was a reduction in antiviral antibody production after lung clearance of the virus. This suggests that multiple DCs are endowed with different tasks in mediating protection against influenza virus.

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.

The role of oxidative stress in ambient particulate matter-induced lung diseases and its implications in the toxicity of engineered nanoparticles

Ambient particulate matter (PM) is an environmental factor that has been associated with increased respiratory morbidity and mortality. The major effect of ambient PM on the pulmonary system is the exacerbation of inflammation, especially in susceptible people. One of the mechanisms by which ambient PM exerts its proinflammatory effects is the generation of oxidative stress by its chemical compounds and metals. Cellular responses to PM-induced oxidative stress include activation of antioxidant defense, inflammation, and toxicity. The proinflammatory effect of PM in the lung is characterized by increased cytokine/chemokine production and adhesion molecule expression. Moreover, there is evidence that ambient PM can act as an adjuvant for allergic sensitization, which raises the possibility that long-term PM exposure may lead to increased prevalence of asthma. In addition to ambient PM, rapid expansion of nanotechnology has introduced the potential that engineered nanoparticles (NP) may also become airborne and may contribute to pulmonary diseases by novel mechanisms that could include oxidant injury. Currently, little is known about the potential adverse health effects of these particles. In this communication, the mechanisms by which particulate pollutants, including ambient PM and engineered NP, exert their adverse effects through the generation of oxidative stress and the impacts of oxidant injury in the respiratory tract will be reviewed. The importance of cellular antioxidant and detoxification pathways in protecting against particle-induced lung damage will also be discussed.

Dendritic cells and epithelial cells: linking innate and adaptive immunity in asthma

Dendritic cells (DCs) are generally held responsible for initiating and maintaining allergic T helper 2 (T(H)2)-cell responses to inhaled allergens in asthma. Although the epithelium was initially considered to function solely as a physical barrier, it is now seen as a central player in the T(H)2-cell sensitization process by influencing the function of DCs. Clinically relevant allergens, as well as known environmental and genetic risk factors for allergy and asthma, often interfere directly or indirectly with the innate immune functions of airway epithelial cells and DCs. A better understanding of these interactions, ascertained from human and animal studies, might lead to better prevention and treatment of asthma.

Lung dendritic cells are stimulated by ultrafine particles and play a key role in particle adjuvant activity

BACKGROUND

The adjuvant activity of air pollution particles on allergic airway sensitization is well known, but the cellular mechanisms underlying this adjuvant potential are not clear.

OBJECTIVE

We sough to study the role of dendritic cells and the costimulatory molecules CD80 and CD86 in the adjuvant activity of ultrafine carbon black particles (CBP).

METHODS

The proliferation of CFSE-labeled DO11.10 CD4 cells was studied after intranasal exposure to particles and ovalbumin (OVA). Next the frequency of myeloid dendritic cells (mDCs) and plasmacytoid dendritic cells and their expression of CD80 and CD86 were studied in the peribronchial lymph nodes (PBLNs). The expression of costimulatory molecules was also studied on bone marrow-derived mDCs after exposure to CBPs in vitro, and the importance of costimulation in CBP adjuvant activity was assessed by using CD80/CD86-deficient mice or cytotoxic T lymphocyte-associated antigen 4 (CTLA4)-Ig in vivo.

RESULTS

Our data show that CBPs plus OVA caused proliferation of DO11.10 CD4 cells and high levels of cytokine production in the PBLNs. Furthermore, the combined CBP plus OVA exposure increased the number of mDCs and expression of costimulatory molecules in the PBLNs. In addition, CBPs upregulated the expression of CD80/CD86 molecules on dendritic cells in vitro, which are necessary for the particle adjuvant effects in vivo.

CONCLUSION

Together this study shows the importance of dendritic cells and costimulation in particle adjuvant activity. Furthermore, we show for the first time that CBPs can also directly induce maturation of dendritic cells.

Lung CD11c+ cells from mice deficient in Epstein-Barr virus-induced gene 3 (EBI-3) prevent airway hyper-responsiveness in experimental asthma

Epstein-Barr virus-induced gene (EBI)-3 codes for a soluble type 1 cytokine receptor homologous to the p40 subunit of IL-12 that is expressed by antigen-presenting cells following activation. Here, we analyzed the functional role of EBI-3 in a murine model of asthma associated with airway hyper-responsiveness (AHR) in ovalbumin-sensitized mice. Upon allergen challenge, EBI-3-/- mice showed less severe AHR, decreased numbers and degranulation of eosinophils and a significantly reduced number of VCAM-1+ cells in the lungs as compared to wild-type littermates. We thus analyzed lung CD11c+ cells before and after allergen challenge in these mice and found that before allergen challenge, lung CD11c+ cells isolated from EBI-3-/- mice express markers of a more plasmacytoid phenotype without releasing IFN-alpha as compared to those from wild-type littermates. Moreover, allergen challenge induced the development of myeloid CD11c+ cells in the lungs of EBI-3-/- mice, which released increased amounts of IL-10 and IL-12 while not expressing IFN-alpha. Finally, inhibition of EBI-3 expression in lung DC could prevent AHR in adoptive transfer studies by suppressing mediator release of effector cells into the airways. These results indicate a novel role for EBI-3 in controlling local immune responses in the lungs in experimental asthma.

Spatial interactions between dendritic cells and sensory nerves in allergic airway inflammation

Neuroimmune interactions play a critical role in the pathogenesis of asthma. Symptoms like wheezing and cough have been attributed to neural dysregulation, whereas sensitization and the induction of allergic inflammation have been linked with the activity of dendritic cells. Neuropeptides were previously shown to control dendritic cell function in vitro, suggesting interactions between dendritic cells and sensory nerves. Here we characterized the anatomical basis of the interactions between dendritic cells and nerves in the airways of mice and monitored the changes during allergic inflammation. Airway microdissection, whole-mount immunohistology, and confocal microscopy were used for the three-dimensional quantitative mapping of airway nerves and dendritic cells along the main axial pathway of nonsensitized versus ovalbumin-sensitized and -challenged CD11c-enhanced yellow fluorescent protein (CD11c-EYFP) transgenic mice. CD11c-EYFP-positive airway mucosal dendritic cells were contacted by calcitonin gene-related peptide-immunoreactive sensory fibers and their co-localization increased in allergic inflammation. Moreover, protein gene product 9.5-positive neuroepithelial bodies and airway ganglia were associated with dendritic cells. In human airways, human leukocyte antigen DR-positive mucosal dendritic cells were found in the close proximity of sensory nerves and neuroepithelial cells. These results provide morphologic evidence of the interactions between dendritic cells and the neural network of the airways at multiple anatomical sites.

The role of innate immunity in asthma: leads and lessons from mouse models

According to the Hygiene Hypothesis, respiratory infections should protect individuals from allergic diseases including asthma, but epidemiologic data on the role of infections or exposure to microbial compounds in asthma are contradictory. Meanwhile, a number of murine models of airway sensitization are available facilitating the elucidation of pathways involved in asthma pathogenesis. Such studies have linked antigen presentation by activated pulmonary dendritic cells (DCs) with airway sensitization. Toll-like receptors (TLRs), which play a major role in innate immunity by sensing various microbial compounds, are expressed on DCs, as well as on mast cells (MCs). Activation of TLRs by administration of specific bacterial ligands, in particular lipopolysaccharide, can augment airway sensitization in mice, and there is evidence that this process involves TLR-dependent activation of DCs. Intriguingly, viral infection has been shown to increase airway inflammation in a murine asthma model via activation of DCs as well. TLR-4-dependent stimulation of MCs may also play a role in allergic sensitization in mice, and in vitro studies in murine cells show that ligation of TLRs expressed on MCs enhances degranulation. Therefore, evidence obtained from studies on mice indicates that innate immune responses may promote, rather than protect from, the development as well as the exacerbation of asthma.

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.

Dendritic cell subsets and immune regulation in the lung

The lung is continuously exposed to inhaled particles, microbes and harmless antigens to which either immunity or tolerance is induced. Dendritic cells are mainly recognized for their extraordinary capacity to induce a primary immune response in the lung. Recent evidence suggests that particular subsets of DCs are essential in the decision between immunity or tolerance. Moreover, DCs play an essential role during secondary immune responses in the lung, where they control the inflammatory reaction. These novel concepts are of particular interest in understanding the pathogenesis of asthma, a disorder of aberrant immune reactivity to inhaled harmless allergens.

Inhaled iloprost suppresses the cardinal features of asthma via inhibition of airway dendritic cell function

Inhalation of iloprost, a stable prostacyclin (PGI(2)) analog, is a well-accepted and safe treatment for pulmonary arterial hypertension. Although iloprost mainly acts as a vasodilator by binding to the I prostanoid (IP) receptor, recent evidence suggests that signaling via this receptor also has antiinflammatory effects through unclear mechanisms. Here we show in a murine model of asthma that iloprost inhalation suppressed the cardinal features of asthma when given during the priming or challenge phase. As a mechanism of action, iloprost interfered with the function of lung myeloid DCs, critical antigen-presenting cells of the airways. Iloprost treatment inhibited the maturation and migration of lung DCs to the mediastinal LNs, thereby abolishing the induction of an allergen-specific Th2 response in these nodes. The effect of iloprost was DC autonomous, as iloprost-treated DCs no longer induced Th2 differentiation from naive T cells or boosted effector cytokine production in primed Th2 cells. These data should pave the way for a clinical effectiveness study using inhaled iloprost for the treatment of asthma.

Lung dendritic cell migration

Dendritic cells (DCs) are crucial in regulating the immune response by bridging innate and adaptive immunity. DCs are constantly migrating from the blood to the lungs and from the lungs to the draining lymph nodes. How DCs populate the lung in the absence of inflammation and how they are recruited there during inflammation remain unclear. Since DCs play a central role in immune responses, both under steady-state and inflammatory conditions, detailed characterization of their migratory behavior may be essential for the development of future therapeutic strategies.

Local application of FTY720 to the lung abrogates experimental asthma by altering dendritic cell function

Airway DCs play a crucial role in the pathogenesis of allergic asthma, and interfering with their function could constitute a novel form of therapy. The sphingosine 1-phosphate receptor agonist FTY720 is an oral immunosuppressant that retains lymphocytes in lymph nodes and spleen, thus preventing lymphocyte migration to inflammatory sites. The accompanying lymphopenia could be a serious side effect that would preclude the use of FTY720 as an antiasthmatic drug. Here we show in a murine asthma model that local application of FTY720 via inhalation prior to or during ongoing allergen challenge suppresses Th2-dependent eosinophilic airway inflammation and bronchial hyperresponsiveness without causing lymphopenia and T cell retention in the lymph nodes. Effectiveness of local treatment was achieved by inhibition of the migration of lung DCs to the mediastinal lymph nodes, which in turn inhibited the formation of allergen-specific Th2 cells in lymph nodes. Also, FTY720-treated DCs were intrinsically less potent in activating naive and effector Th2 cells due to a reduced capacity to form stable interactions with T cells and thus to form an immunological synapse. These data support the concept that targeting the function of airway DCs with locally acting drugs is a powerful new strategy in the treatment of asthma.

Local CD11c+MHC Class II−Precursors Generate Lung Dendritic Cells during Respiratory Viral Infection, but Are Depleted in the Process

Increases in numbers of lung dendritic cells (DC) observed during respiratory viral infections are assumed to be due to recruitment from bone marrow precursors. No local production has been demonstrated. In this study, we isolated defined populations of murine lung cells based on CD11c and MHC class II (MHC II) expression. After culture for 12 days with GM-CSF, we analyzed cell numbers, DC surface markers, and Ag-presenting capacity. Only CD11c+ MHC II- cells from naive mice proliferated, yielding myeloid DC, which induced Ag-specific proliferation of naive T cells. After respiratory syncytial virus (RSV) infection, numbers of pulmonary CD11c+ MHC II- precursor cells were significantly reduced and DC could not be generated. Moreover, RSV infection prevented subsequent in vivo expansion of pulmonary DC in response to influenza infection or LPS treatment. These results provide direct evidence of local generation of fully functional myeloid DC in the lung from CD11c+ MHC II(-) precursor cells that are depleted by RSV infection, leading to an inability to expand lung DC numbers in response to subsequent viral infection or exposure to bacterial products. This depletion of local DC precursors in respiratory viral infections may be important in explaining complex interactions between multiple and intercurrent pulmonary infections.

Intranasal delivery of the cytoplasmic domain of CTLA-4 using a novel protein transduction domain prevents allergic inflammation

CTLA-4 is a negative regulator of T-cell activation, and its inhibitory effects can be accomplished either by competition with CD28 or by transmitting negative signals through its intracellular domain. To utilize the cytoplasmic domain of CTLA-4 to suppress allergic inflammation, we fused it to a novel protein-transduction domain in the human transcriptional factor Hph-1. Transduction efficiency was verified in vitro and in vivo after ocular, intranasal and intradermal administration. After transduction into T cells, the Hph-1-ctCTLA-4 fusion protein inhibited the production of interleukin (IL)-2, and downregulated CD69 and CD25. Intranasal administration of Hph-1-ctCTLA-4 resulted in markedly reduced infiltration of inflammatory cells, secretion of T helper type 2 (T(H)2) cytokines, serum IgE levels and airway hyper-responsiveness in a mouse model of allergic airway inflammation. These results indicated that Hph-1-ctCTLA-4 constitutes an effective immunosuppressive protein drug for potential use in the treatment of allergic asthma, via nasal administration.

Emerging roles of DP and CRTH2 in allergic inflammation

The lipid mediator prostaglandin D(2) (PGD(2)) has long been implicated in various inflammatory diseases including asthma. PGD(2) elicits biological responses by activating two seven-transmembrane (7TM) G-protein-coupled receptors, the D-prostanoid receptor DP and the chemoattractant receptor homologous-molecule expressed on T-helper-type-2 cells (CRTH2), which are linked to different signaling pathways. Understanding how immune cells integrate and coordinate signals that are triggered by the same ligand is crucial for the development of novel anti-inflammatory therapies. Here, we examine the roles of DP and CRTH2 in the orchestration of complex inflammatory processes, and discuss their importance as emerging targets for the treatment of asthma and inflammatory diseases.

Fas-ligand-expressing adenovirus-transfected dendritic cells decrease allergen-specific T cells and airway inflammation in a murine model of asthma

T cells expressing a type-2 T helper profile of cytokines (Th2 cells) have been demonstrated to play an important role in the initiation and progression of allergic asthma, and it is well known that Fas ligand (FasL) induces apoptosis when bound to its receptor, Fas. In the present study, we examined the possibility of modulating asthma manifestations by dendritic cells (DCs) genetically engineered to express FasL (DC-FasL), which could deliver a death signal to T cells in an antigen-specific manner. The delivery of DC-FasL into ovalbumin (OVA)-immunized allergic mice decreased the airway hyper-responsiveness (AHR). Moreover, we established a mouse model of airway inflammation by using an adoptive transfer of Th2 cells derived from ovalbumin T cell receptor transgenic mice to study the effect of DC-FasL on airway reactivity. The administration of DC-FasL in Th2-cell-induced allergic mice had significantly decreased AHR, airway inflammation, and IL-4, IL-5 and IL-13 production. Furthermore, the numbers of OVA-specific T cells were decreased in the lung of mice receiving DC-FasL. These results demonstrate that FasL-expressing dendritic cells might be applied for the modulation of allergic responses.

Dendritic cells in asthma: a function beyond sensitization

Allergic asthma is one of the most common chronic diseases in western society, characterized by variable airway obstruction, mucus hypersecretion and infiltration of the airway wall with T-helper type 2 (Th2) cells, eosinophils and mast cells. If we are to devise new causal therapies for this disease, it is important to elucidate how Th2 cells are activated and respond to intrinsically harmless allergens. Dendritic cells (DCs) are the most important antigen-presenting cells in the lung and are mainly recognized for their exceptional potential to generate a primary immune response and sensitization to aeroallergens. Much less attention has been paid to the role of DCs in established inflammation. Based on functional studies in a murine model for asthma, in this review article, we propose that DCs are essential for generating allergen-specific effector Th2 responses in ongoing inflammation in sensitized mice. A better understanding of the role of DCs in the maintenance of the inflammatory response to allergens in asthma should lead to new therapeutic approaches intervening at the top of the inflammatory cascade.

CCR2 and CCR6, but not endothelial selectins, mediate the accumulation of immature dendritic cells within the lungs of mice in response to particulate antigen

Dendritic cells (DC) migrate from sites of inflammation to lymph nodes to initiate primary immune responses, but the molecular mechanisms by which DC are replenished in the lungs during ongoing pulmonary inflammation are unknown. To address this question, we analyzed the secondary pulmonary immune response of Ag-primed mice to intratracheal challenge with the particulate T cell-dependent Ag sheep erythrocytes (SRBC). We studied wild-type C57BL/6 mice and syngeneic gene-targeted mice lacking either both endothelial selectins (CD62E and CD62P), or the chemokine receptors CCR2 or CCR6. DC, defined as non-autofluorescent, MHC class II(+)CD11c(mod) cells, were detected in blood, enzyme-digested minced lung, and bronchoalveolar lavage fluid using flow cytometry and immunohistology. Compared with control mice, Ag challenge increased the frequency and absolute numbers of DC, peaking at day 1 in peripheral blood (6.5-fold increase in frequency), day 3 in lung mince (20-fold increase in total DC), and day 4 in bronchoalveolar lavage fluid (55-fold increase in total DC). Most lung DC expressed CD11c, CD11b, and low levels of MHC class II, CD40, CD80, and CD86, consistent with an immature myeloid phenotype. DC accumulation depended in part upon CCR2 and CCR6, but not endothelial selectins. Thus, during lung inflammation, immature myeloid DC from the bloodstream replace emigrating immature DC and transiently increase total intrapulmonary APC numbers. Early DC recruitment depends in part on CCR2 to traverse vascular endothelium, plus CCR6 to traverse alveolar epithelium. The recruitment of circulating immature DC represents a potential therapeutic step at which to modulate immunological lung diseases.

Respiratory viral infections and asthma pathogenesis: a critical role for dendritic cells?

BACKGROUND

Respiratory viral infections can influence the course of asthma at different time points. Severe respiratory viral infections during early age are associated with a higher prevalence of asthma in later childhood. In established asthma, viral infections are a frequent cause of asthma exacerbation.

OBJECTIVES

The present review focuses on epidemiological and experimental animal data that can illuminate the mechanisms by which viral infections can lead to sensitization to antigen, and exacerbate ongoing allergic airway inflammation and focuses on the role played by dendritic cells (DCs).

RESULTS

In experimental rodent models of asthma, respiratory viral infection at the time of a first inhaled antigen exposure is described to induce Th2 sensitization and to enhance the allergic response to a second encounter with the same antigen. Virus infections can modulate airway dendritic cell function by upregulation of costimulatory molecule expression, enhanced recruitment, and by inducing an inflammatory environment, all leading to an enhanced antigen presentation and possibly changing the normal tolerogenic response to inhaled antigen into an immunogenic response. In established asthma, respiratory viral infections attract several inflammatory cells, alter receptor expression on airway smooth muscle and modulate neuroimmune mechanisms, possibly leading to exacerbation of disease.

CONCLUSIONS

Animal data suggest that the link between respiratory viral infections and increased asthma is causally related, the viral infection acting on the immune and structural cells to enhance antigen presentation and inflammatory cell recruitment.

Ultrafine carbon black particles cause early airway inflammation and have adjuvant activity in a mouse allergic airway disease model

To gain more insight into the mechanisms of particulate matter (PM)-induced adjuvant activity, we studied the kinetics of airway toxicity/inflammation and allergic sensitization to ovalbumin (OVA) in response to ultrafine carbon black particles (CBP). Mice were exposed intranasally to OVA alone or in combination with different concentrations of CBP. Airway toxicity and inflammation were assessed at days 4 and 8. Immune adjuvant effects were studied in the lung draining peribronchial lymph nodes (PBLN) at day 8. Antigen-specific IgE was measured at days 21 and 28, whereas allergic airway inflammation was studied after OVA challenges (day 28). Results show that a total dose of 200 microg CBP per mouse, but not 20 microg or 2 microg, induced immediate airway inflammation. This 200 microg CBP was the only dose that had immune adjuvant activity, by inducing enlargement of the PBLN and increasing OVA-specific production of Th2 cytokines (IL-4, IL-5, and IL-10). The immune adjuvant activity of 200 microg CBP dosing was further examined. Whereas increased OVA-specific IgE levels in serum on day 21 confirms systemic sensitization, this was further supported by allergic airway inflammation after challenges with OVA. Our data show a link between early airway toxicity and adjuvant effects of CBP. In addition, results indicate that local cytokine production early after exposure to CBP is predictive of allergic airway inflammation. In addition this model appears suitable for studying the role of airway toxicity, inflammation and other mechanisms of particle adjuvant activity, and predicting the adjuvant potential of different particles.

The beta2 integrin CD11c distinguishes a subset of cytotoxic pulmonary T cells with potent antiviral effects in vitro and in vivo

BACKGROUND

The integrin CD11c is known as a marker for dendritic cells and has recently been described on T cells following lymphotropic choriomeningitis virus infection, a systemic infection affecting a multitude of organs. Here, we characterise CD11c bearing T cells in a murine model of localised pulmonary infection with respiratory syncytial virus (RSV).

METHODS

Mice were infected intranasally with RSV and expression of beta2 integrins and T lymphocyte activation markers were monitored by flow cytometry. On day 8 post RSV infection CD11c+ CD8+ and CD11c- CD8+ T cells were assessed for cytokine production, cytotoxic activity and migration. Expression of CD11c mRNA in CD8+ T cells was assessed by quantitative PCR.

RESULTS

Following RSV infection CD11c+ CD8+ T cells were detectable in the lung from day 4 onwards and accounted for 45.9 +/- 4.8% of CD8+ T cells on day 8 post infection, while only few such cells were present in mediastinal lymph nodes, spleen and blood. While CD11c was virtually absent from CD8+ T cells in the absence of RSV infection, its mRNA was expressed in CD8+ T cells of both naïve and RSV infected mice. CD11c+, but not CD11c-, CD8+ T cells showed signs of recent activation, including up-regulation of CD11a and expression of CD11b and CD69 and were recruited preferentially to the lung. In addition, CD11c+ CD8+ T cells were the major subset responsible for IFNgamma production, induction of target cell apoptosis in vitro and reduction of viral titres in vivo.

CONCLUSION

CD11c is a useful marker for detection and isolation of pulmonary antiviral cytotoxic T cells following RSV infection. It identifies a subset of activated, virus-specific, cytotoxic T cells that exhibit potent antiviral effects in vivo.

In vivo depletion of lung CD11c+ dendritic cells during allergen challenge abrogates the characteristic features of asthma

Although dendritic cells (DCs) play an important role in sensitization to inhaled allergens, their function in ongoing T helper (Th)2 cell-mediated eosinophilic airway inflammation underlying bronchial asthma is currently unknown. Here, we show in an ovalbumin (OVA)-driven murine asthma model that airway DCs acquire a mature phenotype and interact with CD4(+) T cells within sites of peribronchial and perivascular inflammation. To study whether DCs contributed to inflammation, we depleted DCs from the airways of CD11c-diphtheria toxin (DT) receptor transgenic mice during the OVA aerosol challenge. Airway administration of DT depleted CD11c(+) DCs and alveolar macrophages and abolished the characteristic features of asthma, including eosinophilic inflammation, goblet cell hyperplasia, and bronchial hyperreactivity. In the absence of CD11c(+) cells, endogenous or adoptively transferred CD4(+) Th2 cells did not produce interleukin (IL)-4, IL-5, and IL-13 in response to OVA aerosol. In CD11c-depleted mice, eosinophilic inflammation and Th2 cytokine secretion were restored by adoptive transfer of CD11c(+) DCs, but not alveolar macrophages. These findings identify lung DCs as key proinflammatory cells that are necessary and sufficient for Th2 cell stimulation during ongoing airway inflammation.

Immunotherapy of Murine Malignant Mesothelioma Using Tumor Lysate–pulsed Dendritic Cells

RATIONALE

Exploiting the immunostimulatory capacities of dendritic cells holds great promise for cancer immunotherapy. Currently, dendritic cell-based immunotherapy is evaluated clinically in a number of malignancies, including melanoma and urogenital and lung cancer, showing variable but promising results.

OBJECTIVE

To evaluate if pulsed dendritic cells induce protective immunity against malignant mesothelioma in a mouse model.

METHODS

Malignant mesothelioma was induced in mice by intraperitoneal injection of the AB1 mesothelioma cell line, leading to death within 28 days. For immunotherapy, dendritic cells were pulsed overnight either with AB1 tumor cell line lysate, AB1-derived exosomes, or ex vivo AB1 tumor lysate, and injected either before (Days -14 and -7) at the day of (Day 0) or after (Days +1 and +8) tumor implantation.

MAIN RESULTS

Mice receiving tumor lysate-pulsed dendritic cells before tumor implantation demonstrated protective antitumor immunity with prolonged survival (> 3 months) and even resisted secondary tumor challenge. Tumor protection was associated with strong tumor-specific cytotoxic T-lymphocyte responses. Adoptive transfer of splenocytes or purified CD8+ T lymphocytes transferred tumor protection to unimmunized mice in vivo. When given after tumor implantation in a therapeutic setting, pulsed dendritic cells prevented mesothelioma outgrowth. With higher tumor load and delayed administration after tumor implantation, dendritic cells were no longer effective.

CONCLUSIONS

We demonstrate in this murine model that immunotherapy using pulsed dendritic cells may emerge as a powerful tool to control mesothelioma outgrowth. In the future, immunotherapy using dendritic cells could be used as adjuvant to control local recurrence after multimodality treatment for malignant mesothelioma.

Pulmonary dendritic cells

Dendritic cells (DCs) are leukocytes that are emerging as chief orchestrators of immune responses. The crucial task of DCs is the continuous surveillance of antigen-exposed sites throughout the body, and their unique responsibility is to decide whether to present sampled antigen in an immunogenic or tolerogenic way. Any misstep can either lead to a flawed immune defense or to allergy, even autoimmunity. It comes as no surprise that the lungs become increasingly the subject of DC-related investigations, as they represent a vast interface between the body and the outer world. This constitutes an enormous challenge for the immune system: "firing up" immune responses inappropriately could have devastating results for the fragile gas exchange structures. Evidence accumulates that DCs play a pivotal role in maintaining the delicate balance between tolerance and active immune response in our respiratory system. The exponentially growing body of DC-related publications is a big challenge. This article aims to provide researchers and clinicians with an up-to-date view on DC biology and its relevance to pulmonary medicine. A developing trend in the field of DCs is the shift from fundamental immunologic research toward exciting clinical insights and applications. For the pulmonary clinician, this heralds the dawn of promising therapies in various domains such as infections, allergy, and cancer.

Differential capacity of CD8+ alpha or CD8- alpha dendritic cell subsets to prime for eosinophilic airway inflammation in the T-helper type 2-prone milieu of the lung

BACKGROUND

Different subsets of dendritic cells (DCs), identified in mouse spleen by their differential expression of CD8 alpha, can induce different T-helper (Th) responses after systemic administration. CD8 alpha(-) DCs have been shown to preferentially induce Th type 2 (Th2) responses whereas CD8 alpha(+) DCs induce Th1 responses.

OBJECTIVE

To study if these DC subsets can still induce different Th responses in the Th2-prone milieu of the lung and differentially prime for eosinophilic airway inflammation, typical of asthma.

METHODS

Donor mice first received daily Flt3L injections to expand DC numbers. Purified CD8 alpha(+) or CD8 alpha(-) splenic DCs were pulsed with ovalbumin (OVA) or phosphate-buffered saline and injected intratracheally into recipient mice in which carboxyfluorescein diacetate succinimidyl ester-labelled OVA-specific T cell receptor transgenic T cells had been injected intravenously 2 days earlier. T cell proliferation and cytokine production of Ag-specific T cells were evaluated in the mediastinal lymph nodes (MLNs) 4 days later. The capacity of both subsets of DCs, to prime for eosinophilic airway inflammation was determined by challenging the mice with OVA aerosol 10 days later.

RESULTS

CD8 alpha(-) DCs migrated to the MLN and induced a vigorous proliferative T cell response accompanied by high-level production of IL-4, IL-5, IL-10 and also IFN-gamma during the primary response and during challenge with aerosol, leading to eosinophilic airway inflammation. In the absence of migration to the MLN, CD8 alpha(+) DCs still induced a proliferative response with identical levels of IFN-gamma but reduced Th2 cytokines compared with CD8 alpha(-) DCs, which led to weak eosinophilic airway inflammation upon OVA aerosol challenge. Unpulsed DCs did not induce proliferation or cytokine production in Ag-specific T cells.

CONCLUSION

CD8 alpha(-) DCs are superior compared with CD8 alpha(+) DCs in inducing Th2 responses and eosinophilic airway inflammation in the Th2-prone environment of the lung.

The interplay of dendritic cells, Th2 cells and regulatory T cells in asthma

Dendritic cells are essential for Th2 differentiation of naive CD4+ T cells in response to aeroallergens, and in recent years it has been well established that these cells play a pivotal role in the initiation phase of allergic asthma. Dendritic cells are also crucial for maintaining eosinophilic airway inflammation by controlling the recruitment and activation of primed Th2 cells in the lung. A picture is emerging wherein the balance of pathogenic Th2 cells and regulatory T cells is tuned by dendritic cells not only at the initiation but also at the effector stage of the allergic immune response.

Dendritic cells and the regulation of the allergic immune response

Studies in mouse models of asthma have revealed a critical role for airway dendritic cells in the induction of Th2 sensitization to inhaled allergens. Under some conditions, subsets of dendritic cells can also induce tolerance or Th1 responses to the same allergens, depending on the context in which the antigen is seen. This article discusses various aspects of DC biology as it relates to allergic sensitization and also provides a summary of the recent evidence that dendritic cells function beyond sensitization.

The role of dendritic cells in asthma

PURPOSE OF REVIEW

The central importance of respiratory tract dendritic cells in the regulation of adaptive immune responses to inhaled antigens is now well established. Dendritic cells are not merely a conduit for the transfer of antigen to regional lymph nodes, but rather function as a sophisticated information transfer system linking the airway micro-environment to the adaptive immune system. Evidence from both animal models and clinical studies points to a critical role for dendritic cells in both allergic sensitization and the pathogenesis of chronic airway inflammation.

RECENT FINDINGS

This article reviews recent information on the distribution and function of dendritic cells in healthy individuals, the responsiveness of these cells to external stimuli, and the factors regulating their activation and turnover within the lung. Animal models of allergic airway inflammation continue to shed new light on the role of lung dendritic cells in T helper 1/T helper 2 switching, and the ability of these cells to direct regulatory T-cell development and immune tolerance. Recent studies have further characterized circulating dendritic cell populations, highlighting important functional differences between dendritic cells from atopic and nonatopic individuals, and have delineated the involvement of these cells in the late phase response to inhaled allergen.

SUMMARY

Because of the immunoregulatory properties of dendritic cells, the future is likely to see a concerted effort to further define the role that these cells play in allergic sensitization, as a basis for the development of new treatments for asthma and other atopic disorders.

Essential role of lung plasmacytoid dendritic cells in preventing asthmatic reactions to harmless inhaled antigen

Tolerance is the usual outcome of inhalation of harmless antigen, yet T helper (Th) type 2 cell sensitization to inhaled allergens induced by dendritic cells (DCs) is common in atopic asthma. Here, we show that both myeloid (m) and plasmacytoid (p) DCs take up inhaled antigen in the lung and present it in an immunogenic or tolerogenic form to draining node T cells. Strikingly, depletion of pDCs during inhalation of normally inert antigen led to immunoglobulin E sensitization, airway eosinophilia, goblet cell hyperplasia, and Th2 cell cytokine production, cardinal features of asthma. Furthermore, adoptive transfer of pDCs before sensitization prevented disease in a mouse asthma model. On a functional level, pDCs did not induce T cell division but suppressed the generation of effector T cells induced by mDCs. These studies show that pDCs provide intrinsic protection against inflammatory responses to harmless antigen. Therapies exploiting pDC function might be clinically effective in preventing the development of asthma.

Essential role of dendritic cell CD80/CD86 costimulation in the induction, but not reactivation, of TH2 effector responses in a mouse model of asthma

BACKGROUND

Airway dendritic cells (DCs) are crucial for the generation of TH2 cells from naive T cells during sensitization and for reactivation of primed TH2 cells on allergen challenge in mouse models of asthma. It is unknown whether CD80/CD86 costimulation is necessary during both phases of the response because primed T cells rely less on costimulatory molecules compared with naive T cells.

OBJECTIVE

We sought to study the contribution of CD80/CD86 costimulatory molecules on DCs during sensitization or challenge in a mouse model of asthma.

METHODS

Naive BALB/c mice received an intratracheal injection of ovalbumin (OVA)-pulsed DCs obtained from the bone marrow of wild-type (WT) or CD80/CD86-/- mice and were subsequently challenged with OVA aerosol to address the role of costimulation during sensitization. OVA-sensitized mice received OVA-pulsed WT or CD80/CD86-/- DCs without OVA aerosol to address the role of costimulation during challenge.

RESULTS

WT DCs induced the proliferation and effector TH2 differentiation of naive OVA-specific T cells, whereas CD80/CD86-/- DCs induced only proliferation. Not surprisingly, WT DCs but not CD80/CD86-/- DCs induced sensitization to OVA in naive mice. In contrast, in OVA-sensitized mice intratracheal injection of CD80/CD86-/- OVA-pulsed DCs led to eosinophilic airway inflammation, goblet cell hyperplasia, and effector TH2 cytokine production that was not different from that seen after injection with WT OVA-DCs, even when the inducible costimulator ICOS was blocked or cytotoxic T lymphocyte-associated antigen 4 immunoglobulin was given.

CONCLUSION

CD80/CD86 costimulation on DCs is only necessary during priming of naive T cells into TH2 cells but not during restimulation of previously primed TH2 cells in the challenge phase.

Dendritic cells retrovirally overexpressing IL-12 induce strong Th1 responses to inhaled antigen in the lung but fail to revert established Th2 sensitization

It has been postulated that low-level interleukin (IL)-12 production of antigen-presenting cells is associated with the risk of developing atopic asthma. To study the relationship between IL-12 production capacity of dendritic cells (DCs) and development of T helper type 2 (Th2) responses in the lung, we genetically engineered DCs to constutively overexpress bioactive IL-12. Retrovirally mediated overexpression of IL-12 in DCs strongly polarized naive ovalbumin (OVA)-specific CD4+ T cells toward Th1 effector cells in vitro. After intratracheal injection, OVA-pulsed IL-12-overexpressing DCs failed to induce Th2 responses in vivo and no longer primed mice for Th2-dependent eosinophilic airway inflammation upon OVA aerosol challenge, readily observed in mice immunized with sham-transfected, OVA-pulsed DCs. Analysis of a panel of cytokines and chemokines in the lung demonstrated that the lack of Th2 sensitization was accompanied by increased production of the Th1 cytokine interferon-gamma (IFN-gamma), chemokines induced by IFN-gamma, and the immunoregulatory cytokine IL-10. When Th2 priming was induced using OVA/alum prior to intratracheal DC administration, DCs constitutively expressing IL-12 were no longer capable of preventing eosinophilic airway inflammation and even enhanced it. These data show directly that high-level expression of IL-12 in DCs prevents the development of Th2 sensitization. Enhancing IL-12 production in DCs should be seen as a primary prevention strategy for atopic disorders. Enhancing IL-12 production in DCs is less likely to be of benefit in already Th2-sensitized individuals.

Complex role of the IL-4 receptor alpha in a murine model of airway inflammation: expression of the IL-4 receptor alpha on nonlymphoid cells of bone marrow origin contributes to severity of inflammation

Recent studies have suggested the IL-4Ralpha expressed on lung epithelium is necessary for TH2-mediated goblet cell differentiation and mucus hypersecretion in a murine model of allergic lung disease. However, the IL-4Ralpha is expressed on numerous cell types that could contribute to the overall pathology and severity of asthma. The relative role of the receptor on these cells has not yet been conclusively delineated. To dissect the contribution of IL-4Ralpha in the development of pulmonary allergic responses, we generated murine radiation bone marrow (BM) chimeras. BM from IL-4Ralpha(+) or IL-4Ralpha(-) mice was transferred into recipient mice that expressed or lacked IL-4Ralpha. In the absence of IL-4Ralpha in recipient mice, there was no goblet cell metaplasia or mucus hypersecretion in response to OVA, even in the presence of TH2 cells and substantial eosinophilic infiltration. More importantly, we found that expression of the IL-4Ralpha on a nonlymphoid, MHC class II(+), BM-derived cell type contributes to the severity of inflammation and mucus production. These results suggest that IL-4 and IL-13 contribute to the development of allergic inflammation by stimulating a complex interaction between IL-4Ralpha(+) cell types of both bone marrow and non-bone marrow origin.

A rapid flow cytometric method for determining the cellular composition of bronchoalveolar lavage fluid cells in mouse models of asthma

Mouse models of allergic asthma are increasingly used to study the immunopathology of this complex disorder. The degree and type of airway inflammation is often studied by determination of differential cell counts on cytospins of bronchoalveolar lavage fluid (BALF) cells stained with May-Grünwald Giemsa, in which the separation of eosinophils (eos) from neutrophils (neutro) and of monocytes (mono) from activated T cells can be quite problematic. In this study, we compared differential cell counts based on morphological criteria on May-Grünwald Giemsa stained cytospins with a newly developed flow cytometric method. BAL fluid cells were identified based on forward and side scatter characteristics (FSC and SSC), autofluorescence of macrophages, and simultaneous one-step staining with antibodies for T cells (CD3-Cy-Chrome), B cells (B220-Cy-Chrome), eosinophils (CCR3-PE), and dendritic cells (DCs) (MHCII-FITC, CD11c-APC). The validity of this flow cytometric determination was tested by morphological analysis of flow-sorted cellular subsets. In an animal model of ovalbumin-induced asthma, this new method correlated very well with the differential counts based on cytospins. Flow cytometric determination of the cellular composition of BAL fluid in mouse models of asthma is a rapid and easy method that can replace differential cell counts based on morphology.

Activation of peroxisome proliferator-activated receptor-gamma in dendritic cells inhibits the development of eosinophilic airway inflammation in a mouse model of asthma

Peroxisome proliferator-activated receptors (PPARs) are activated by an array of polyunsaturated fatty acid derivatives, oxidized fatty acids, and phospholipids and are proposed to be important modulators of immune and inflammatory responses. Recently, we showed that activation of PPAR-gamma alters the maturation process of dendritic cells (DCs), the most potent antigen-presenting cells. In the present report, we investigated the possibility that, by targeting DCs, PPAR-gamma activation may be involved in the regulation of the pulmonary immune response to allergens. Using a model of sensitization, based on the intratracheal transfer of ovalbumin (OVA)-pulsed DCs, we show that rosiglitazone, a selective PPAR-gamma agonist, reduces the proliferation of Ag-specific T cells in the draining mediastinal lymph nodes but, surprisingly enough, dramatically increases the production of the immunoregulatory cytokine interleukin (IL)-10 by T cells, as compared to control mice sensitized with OVA-pulsed DCs. After aerosol challenge, the recruitment of eosinophils in the bronchoalveolar lavage fluids was strongly reduced compared to control mice. Finally, T cells from the mediastinal lymph nodes produced higher amounts of IL-10 and interferon-gamma. Inhibition of IL-10 activity with anti-IL-10R antibodies partly restored the inflammation. The specificity of the phenomenon was confirmed by treating OVA-pulsed DCs with ciglitazone, another PPAR-gamma agonist, and by using GW9662, a PPAR-gamma antagonist. Our data suggest that PPAR-gamma activation prevents induction of Th2-dependent eosinophilic airway inflammation and might contribute to immune homeostasis in the lung.

Airway eosinophils accumulate in the mediastinal lymph nodes but lack antigen-presenting potential for naive T cells

Asthma is characterized by infiltration of the airway wall with eosinophils. Although eosinophils are considered to be effector cells, recent studies have reported their ability to activate primed Th2 cells. In this study, we investigated whether eosinophils are capable of presenting Ag to unprimed T cells in draining lymph nodes (DLN) of the lung and compared this capacity with professional dendritic cells (DC). During development of eosinophilic airway inflammation in OVA-sensitized and challenged mice, CCR3(+) eosinophils accumulated in the DLN. To study their function, eosinophils were isolated from the bronchoalveolar lavage fluid of mice by sorting on CCR3(+)B220(-)CD3(-)CD11c(dim) low autofluorescent cells, avoiding contamination with other APCs, and were intratracheally injected into mice that previously received CFSE-labeled OVA TCR-transgenic T cells. Eosinophils did not induce divisions of T cells in the DLN, whereas DC induced on average 3.7 divisions in 45.7% of T cells. To circumvent the need for Ag processing or migration in vivo, eosinophils were pulsed with OVA peptide and were still not able to induce T cell priming in vitro, whereas DC induced vigorous proliferation. This lack of Ag-presenting ability was explained by the very weak expression of MHC class II on fresh eosinophils, despite expression of the costimulatory molecules CD80 and ICAM-1. This investigation does not support any role for airway eosinophils as APCs to naive T cells, despite their migration to the DLN at times of allergen exposure. DC are clearly superior in activating T cells in the DLN of the lung.

Lipopolysaccharide-induced suppression of airway Th2 responses does not require IL-12 production by dendritic cells

The prevalence of atopic asthma, a Th2-dependent disease, is reaching epidemic proportions partly due to improved hygiene in industrialized countries. There is an inverse correlation between the level of environmental endotoxin exposure and the prevalence of atopic sensitization. As dendritic cells (DC) have been implicated in causing sensitization to inhaled Ag, we studied the effect of endotoxin on Th2 development induced by bone marrow DC in vitro and by intratracheal injection in vivo, with particular emphasis on the role played by the polarizing cytokine IL-12. Bone marrow-derived DC stimulated with Escherichia coli O26:B6 LPS produced IL-12p70 for a limited period of time, after which production became refractory to further stimulation with CD40 ligand, a phenomenon previously called "exhaustion." The level of IL-12 production of DC did not correlate with Th1 development, as exhausted OVA-pulsed DC were still capable of shifting the cytokine pattern of responding OVA-specific Th cells toward Th1 in vitro and in vivo. When mice were first immunized by intratracheal injection of OVA-DC and subsequently challenged with OVA aerosol, prior in vitro stimulation of DC with LPS reduced the development of airway eosinophilia and Th2 cytokine production. Most surprisingly, the capacity of LPS to reduce Th2-dependent eosinophilic airway inflammation was IL-12-independent altogether, as IL-12p40 knockout DC had a similar reduced capacity to prime for Th2 responses. These results suggest that LPS reduces sensitization to inhaled Ag by reducing DC-driven Th2 development, but that IL-12 is not necessary for this effect.

Prostaglandin D2 inhibits airway dendritic cell migration and function in steady state conditions by selective activation of the D prostanoid receptor 1

PGD(2) is the major mediator released by mast cells during allergic responses, and it acts through two different receptors, the D prostanoid receptor 1 (DP1) and DP2, also known as CRTH2. Recently, it has been shown that PGD(2) inhibits the migration of epidermal Langerhans cells to the skin draining lymph nodes (LNs) and affects the subsequent cutaneous inflammatory reaction. However, the role of PGD(2) in the pulmonary immune response remains unclear. Here, we show that the intratracheal instillation of FITC-OVA together with PGD(2) inhibits the migration of FITC(+) lung DC to draining LNs. This process is mimicked by the DP1 agonist BW245C, but not by the DP2 agonist DK-PGD(2). The ligation of DP1 inhibits the migration of FITC-OVA(+) DCs only temporarily, but still inhibits the proliferation of adoptively transferred, OVA-specific, CFSE-labeled, naive T cells in draining LNs. These T cells produced lower amounts of the T cell cytokines IL-4, IL-10, and IFN-gamma compared with T cells from mice that received FITC-OVA alone. Taken together, our data suggest that the activation of DP receptor by PGD(2) may represent a pathway to control airway DC migration and to limit the activation of T cells in the LNs under steady state conditions, possibly contributing to homeostasis in the lung.

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

Pulmonary dendritic cells (DC) can induce both tolerogenic as well as inflammatory immune responses in the lung. Conversely, little is known about the impact of ongoing airway inflammation on pulmonary DC biology. In noninflammatory conditions, expression of T cell costimulatory molecules on mouse airway DCs is low and only upregulated after homing into draining thoracic lymph nodes. In this study, we reveal that ongoing allergic airway inflammation induces a premature upregulation of the T cell costimulatory molecules CD40, B7-2 and intercellular adhesion molecule 1 on DCs still present in the airways. In contrast, high surface expression of inducible costimulator ligand, involved in respiratory tolerance induction is restricted to DCs from noninflamed lungs. In addition, during inflammation the migratory flux of allergen-transporting airway DCs toward draining thoracic nodes increases both in amplitude as well as in speed. Remarkably, migratory DCs from inflamed airways are short-lived in the draining lymph nodes, a finding that is temporally associated with a marked loss of the antiapoptotic protein Bcl-2 in these cells. This study demonstrates the profound effects of ongoing allergen-driven airway inflammation on the dynamics of pulmonary DC physiology, a knowledge that could be exploited in the development of novel DC-based immunotherapies.

Matrix metalloproteinase-9-mediated dendritic cell recruitment into the airways is a critical step in a mouse model of asthma

Dendritic cells (DCs) appear to be strategically implicated in allergic diseases, including asthma. Matrix metalloproteinase (MMP)-9 mediates transmigration of inflammatory leukocytes across basement membranes. This study investigated the role of MMP-9 in airway DC trafficking during allergen-induced airway inflammation. MMP-9 gene deletion affected the trafficking of pulmonary DCs in a specific way: only the inflammatory transmigration of DCs into the airway lumen was impaired, whereas DC-mediated transport of airway Ag to the thoracic lymph nodes remained unaffected. In parallel, the local production of the Th2-attracting chemokine CC chemokine ligand 17/thymus and activation-regulated chemokine, which was highly concentrated in purified lung DCs, fell short in the airways of allergen-exposed MMP-9(-/-) mice. This was accompanied by markedly reduced peribronchial eosinophilic infiltrates and impaired allergen-specific IgE production. We conclude that the specific absence of MMP-9 activity inhibits the development of allergic airway inflammation by impairing the recruitment of DCs into the airways and the local production of DC-derived proallergic chemokines.