STAT1-regulated lung MDSC-like cells produce IL-10 and efferocytose apoptotic neutrophils with relevance in resolution of bacterial pneumonia

Bacterial pneumonia remains a significant burden worldwide. Although an inflammatory response in the lung is required to fight the causative agent, persistent tissue-resident neutrophils in non-resolving pneumonia can induce collateral tissue damage and precipitate acute lung injury. However, little is known about mechanisms orchestrated in the lung tissue that remove apoptotic neutrophils to restore tissue homeostasis. In mice infected with Klebsiella pneumoniae, a bacterium commonly associated with hospital-acquired pneumonia, we show that interleukin (IL)-10 is essential for resolution of lung inflammation and recovery of mice after infection. Although IL-10(-/-) mice cleared bacteria, they displayed increased morbidity with progressive weight loss and persistent lung inflammation in the later phase after infection. A source of tissue IL-10 was found to be resident CD11b(+)Gr1(int)F4/80(+) cells resembling myeloid-derived suppressor cells (MDSCs) that appeared with a delayed kinetics after infection. These cells efficiently efferocytosed apoptotic neutrophils, which was aided by IL-10. The lung neutrophil burden was attenuated in infected signal transducer and activator of transcription 1 (STAT1)(-/-) mice with concomitant increase in the frequency of the MDSC-like cells and lung IL-10 levels. Thus, inhibiting STAT1 in combination with antibiotics may be a novel therapeutic strategy to address inefficient resolution of bacterial pneumonia.

Early infection with respiratory syncytial virus impairs regulatory T cell function and increases susceptibility to allergic asthma

Immune tolerance is instituted early in life, during which time regulatory T (T(reg)) cells have an important role. Recurrent infections with respiratory syncytial virus (RSV) in early life increase the risk for asthma in adult life. Repeated infection of infant mice tolerized to ovalbumin (OVA) through their mother's milk with RSV induced allergic airway disease in response to OVA sensitization and challenge, including airway inflammation, hyper-reactivity and higher OVA-specific IgE, as compared to uninfected tolerized control mice. Virus infection induced GATA-3 expression and T helper type 2 (T(H)2) cytokine production in forkhead box P3 (FOXP3)(+) T(reg) cells and compromised the suppressive function of pulmonary T(reg) cells in a manner that was dependent on interleukin-4 receptor α (IL-4Rα) expression in the host. Thus, by promoting a T(H)2-type inflammatory response in the lung, RSV induced a T(H)2-like effector phenotype in T(reg) cells and attenuated tolerance to an unrelated antigen (allergen). Our findings highlight a mechanism by which viral infection targets a host-protective mechanism in early life and increases susceptibility to allergic disease.

LPS-induced CD11b+Gr1intF4/80+ myeloid cells suppress allergic airway inflammation (55.3)

The bacterial product lipopolysaccharide (LPS) has been associated with protection from allergic diseases such as asthma in humans. However, differential effects of LPS based on the dose have been noted in the mouse models of allergic asthma. Our recent work has described the ability of LPS to increase the frequency of CD11b+Gr1intF4/80+ cells in the lung tissue of mice in a dose-dependent fashion that is dependent on TLR4 and MyD88. Although the Gr1int cells are readily detectable in the lung tissue of LPS-treated mice, they are not detected in the lung-draining lymph nodes or in the airway lumen which causes selective enrichment of these cells over dendritic cells (DCs) in the tissue. The Gr1int cells were found to blunt the ability of lung DCs to upregulate GATA-3 or to promote STAT5 activation in primed Th2 cells, both transcription factors having critical roles in Th2 effector function. Suppression of Th2 effector function was reversed by anti-interleukin-10 or inhibition of arginase 1. Adoptive transfer of LPS-induced Gr1int cells suppressed allergen-induced airway inflammation suggesting that these myeloid cells may have regulatory functions in allergic asthma. Thus, a complete understanding of the generation and regulation of the Gr1int cells would provide new avenues to either promote or delete these cells for disease-specific immunoregulation.

TNF-α produced by inflammatory DCs as a molecular switch in Th17/Th2 and neutrophilic/eosinophilic responses during persistent fungal infection (103.2)

Aspergillus fumigatus is associated with allergic bronchopulmonary aspergillosis, a pulmonary allergic disorder, in which neutrophilia predicts a poor outcome. In our efforts to recapitulate fungus-induced airway neutrophilia in the allergic setting, we found that similar exposure to the fungus resulted in a neutrophil-dominated response in BALB/c mice but an eosinophil-biased response in C57BL/6 mice. By performing a comparative study, we identified TNF-α as the key mediator that skewed the response towards neutrophilia but away from eosinophilia. Between the two strains, TNF-αhigh CD11b+ Ly6C+ DCs were more abundant in the lungs of WT BALB/c mice which was compromised in MyD88-/-, dectin-1-/- and TNF-α-/- mice. As compared to TNF-αhigh Balb/c-DCs, TNF-αlow C57BL/6-DCs contained more NF-B p50 homodimers as strong repressors of TNF-α transcription. Lower TNF-α accounted for lower TLR2 levels on C57BL/6 DCs impairing collaboration between TLR2 and Dectin-1 in augmenting TNF-α via a positive feedback loop. Functionally, in Balb/c mice, the high level of TNF-α enhanced Th17 responses and in collaboration with IL-17A upregulated the neutrophil chemoattractants KC and MIP-2. Conversely, the lungs of C57BL/6 or TNF-α-/- mice showed higher IL-5 levels and therefore an eosinophil-rich response. Our study for the first time highlights a central role of TNF-α as a molecular switch that regulates the neutrophil/eosinophil balance in airway inflammation.

The role of Gr1int interstitial cells in lung host defense against gram-negative bacterial infection. (162.8)

We have recently identified a CD11b+Gr1intF4/80+ (Gr1int) regulatory, non-migratory, cell type in the lung that is able to suppress a Th2 effector response via secretion of IL-10, arginase 1 and nitric oxide (NO). These regulatory cells also secrete IL-6 and GM-CSF, but low levels of IL-12. Our studies show that although Gr1int cells are present in the lung of naïve mice, their frequency increases following LPS exposure in a TLR4/MyD88 dependent fashion. The rapid appearance of these cells in response to a TLR ligand, combined with their non-migratory nature and their anatomical location in the lung interstitium, suggests a unique function during host defense against bacterial infections. Much is already known about the role of alveolar macrophages and neutrophils in host defense against Klebsiella Pneumoniae, however little is known about host defense mechanisms in the lung tissue/interstitium. For the first time, we have examined the role of Gr1int interstitial cells in defense against K. Pneumoniae. We show that the Gr1int cell population increases following exposure to K. Pneumoniae, are highly phagocytic, and co-localize with K. Pneumoniae, suggesting phagocytic uptake in vivo. Furthermore, Gr1int secrete high levels of NO, previously shown to be critical for effective innate immunity against K. Pneumoniae.

Respiratory Syncytial Virus infection induces alternatively activated macrophages in the lung (158.3)

Asthma is a chronic inflammatory disease of the airways characterized by a heightened response to normally harmless antigens. Respiratory Syncytial Virus (RSV) infections in children have been associated with predisposition to asthma in later life. However, the underlying molecular mechanisms are not well understood. RSV infection was recently shown to induce Th2 type cytokines such as IL-13 within 6 days after infection. Given this early induction of IL-13, we hypothesized that the source of RSV-induced IL-13 is a cell of the innate immune system. Our ongoing studies show rapid induction of alternatively activated macrophages (AAMs) in the lung following RSV infection. RSV induces expression of Arginase I (Arg I), a characteristic marker of AAMs and stimulates secretion of the Th2 cytokines IL-5 and IL-13 from the alveolar macrophages as early as 48 hours after infection. This was found to continue on day 5 after infection, being detectable in the lung tissue as well. These observations suggest that RSV, directly or indirectly, alters gene expression in alveolar macrophages very early after infection, which we hypothesize plays an important role in promoting an allergic immune response in the infected lungs.

Rapid host defense against Aspergillus fumigatus involves alveolar macrophages with a predominance of alternatively activated phenotype

The ubiquitous fungus Aspergillus fumigatus is associated with chronic diseases such as invasive pulmonary aspergillosis in immunosuppressed patients and allergic bronchopulmonary aspergillosis (ABPA) in patients with cystic fibrosis or severe asthma. Because of constant exposure to this fungus, it is critical for the host to exercise an immediate and decisive immune response to clear fungal spores to ward off disease. In this study, we observed that rapidly after infection by A. fumigatus, alveolar macrophages predominantly express Arginase 1 (Arg1), a key marker of alternatively activated macrophages (AAMs). The macrophages were also found to express Ym1 and CD206 that are also expressed by AAMs but not NOS2, which is expressed by classically activated macrophages. The expression of Arg1 was reduced in the absence of the known signaling axis, IL-4Rα/STAT6, for AAM development. While both Dectin-1 and TLR expressed on the cell surface have been shown to sense A. fumigatus, fungus-induced Arg1 expression in CD11c⁺ alveolar macrophages was not dependent on either Dectin-1 or the adaptor MyD88 that mediates intracellular signaling by most TLRs. Alveolar macrophages from WT mice efficiently phagocytosed fungal conidia, but those from mice deficient in Dectin-1 showed impaired fungal uptake. Depletion of macrophages with clodronate-filled liposomes increased fungal burden in infected mice. Collectively, our studies suggest that alveolar macrophages, which predominantly acquire an AAM phenotype following A. fumigatus infection, have a protective role in defense against this fungus.

TLR4/MyD88-induced CD11b+Gr-1 int F4/80+ non-migratory myeloid cells suppress Th2 effector function in the lung

In humans, environmental exposure to a high dose of lipopolysaccharide (LPS) protects from allergic asthma, the immunological underpinnings of which are not well understood. In mice, exposure to a high LPS dose blunted house dust mite-induced airway eosinophilia and T-helper 2 (Th2) cytokine production. Although adoptively transferred Th2 cells induced allergic airway inflammation in control mice, they were unable to do so in LPS-exposed mice. LPS promoted the development of a CD11b(+)Gr1(int)F4/80(+) lung-resident cell resembling myeloid-derived suppressor cells in a Toll-like receptor 4 and myeloid differentiation factor 88 (MyD88)-dependent manner that suppressed lung dendritic cell (DC)-mediated reactivation of primed Th2 cells. LPS effects switched from suppressive to stimulatory in MyD88(-/-) mice. Suppression of Th2 effector function was reversed by anti-interleukin-10 (IL-10) or inhibition of arginase 1. Lineage(neg) bone marrow progenitor cells could be induced by LPS to develop into CD11b(+)Gr1(int)F4/80(+)cells both in vivo and in vitro that when adoptively transferred suppressed allergen-induced airway inflammation in recipient mice. These data suggest that CD11b(+)Gr1(int)F4/80(+) cells contribute to the protective effects of LPS in allergic asthma by tempering Th2 effector function in the tissue.

Inhibition and role of let-7d in idiopathic pulmonary fibrosis

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and usually lethal fibrotic lung disease characterized by profound changes in epithelial cell phenotype and fibroblast proliferation.

OBJECTIVES

To determine changes in expression and role of microRNAs in IPF.

METHODS

RNA from 10 control and 10 IPF tissues was hybridized on Agilent microRNA microarrays and results were confirmed by quantitative real-time polymerase chain reaction and in situ hybridization. SMAD3 binding to the let-7d promoter was confirmed by chromatin immunoprecipitation, electrophoretic mobility shift assay, luciferase assays, and reduced expression of let-7d in response to transforming growth factor-beta. HMGA2, a let-7d target, was localized by immunohistochemistry. In mice, let-7d was inhibited by intratracheal administration of a let-7d antagomir and its effects were determined by immunohistochemistry, immunofluorescence, quantitative real-time polymerase chain reaction, and morphometry.

MEASUREMENTS AND MAIN RESULTS

Eighteen microRNAs including let-7d were significantly decreased in IPF. Transforming growth factor-beta down-regulated let-7d expression, and SMAD3 binding to the let-7d promoter was demonstrated. Inhibition of let-7d caused increases in mesenchymal markers N-cadherin-2, vimentin, and alpha-smooth muscle actin (ACTA2) as well as HMGA2 in multiple epithelial cell lines. let-7d was significantly reduced in IPF lungs and the number of epithelial cells expressing let-7d correlated with pulmonary functions. HMGA2 was increased in alveolar epithelial cells of IPF lungs. let-7d inhibition in vivo caused alveolar septal thickening and increases in collagen, ACTA2, and S100A4 expression in SFTPC (pulmonary-associated surfactant protein C) expressing alveolar epithelial cells.

CONCLUSIONS

Our results indicate a role for microRNAs in IPF. The down-regulation of let-7d in IPF and the profibrotic effects of this down-regulation in vitro and in vivo suggest a key regulatory role for this microRNA in preventing lung fibrosis. Clinical trial registered with www.clinicaltrials.gov (NCT 00258544).

Identification, phenotype, migration, and function of c-kit-expressing dendritic cells in vivo (91.12)

We previously reported c-kit induction on total lung DCs by in vitro treatment with Th2/Th17- (ovalbumin plus cholera toxin; OVA/CT) but not Th1-inducing (CpG) factors. Furthermore, we showed that c-kit plays a contributory role to Th2, and in particular to Th17, immune responses. Therefore, we sought to determine what, if any DC subset(s) upregulate c-kit after in vivo treatment. A basal level of c-kit expression was found on CD11c-positive, low autofluorescent DCs expressing CD11b/CX3CR1, which was upregulated following intratracheal administration of OVA/CT. Surprisingly, OVA/CpG also upregulated c-kit on CD11b/CX3CR1 DCs in vivo to an extent even greater than OVA/CT. DC expression of c-kit was highest where MHC Class II expression was also highest (i.e. following OVA/CpG treatment) suggesting that it may be linked to the activation state. However, c-kit on DCs migrating to the lung-draining lymph nodes (LNs) was markedly higher on cells from OVA/CT- versus OVA/CpG-treated animals. LN resident DCs also expressed c-kit, but it was not different between naïve and treated animals. Importantly, c-kit does appear to have functional consequences since c-kit-deficient DCs induce less IL-17 production in particular from naïve T cells, possibly due to their own decreased production of IL-6. These data suggest that c-kit expression, and possibly interaction with its ligand, stem cell factor, is important for migration to draining LNs and for skewing naïve T cell priming by DCs.

Distinct signaling pathways during innate and adaptive immune responses to Aspergillus fumigatus induce alternatively activated macrophages expressing Th2 cytokines (37.37)

Macrophages adopt an ‘alternative phenotype’ in a Th2-mediated inflammatory environment. Although identified in a wide variety of disease conditions, alternatively activated macrophages (AAMs) have been functionally best studied in helminth infections. The heterogeneity of functions performed by AAMs is attributed to the diverse molecules they express. Since infection with the fungus, Aspergillus fumigatus induces a chronic allergic response in patients with cystic fibrosis and asthma, we investigated the participation of AAMs in the process. We have found that Aspergillus infection promotes early recruitment of AAMs to the lung and the cells possess features of both AAMs and Th2 cells. The AAMs produce Th2 cytokines, which is independent of Dectin-1 signaling. Interestingly, unlike in helminth infections, both AAM recruitment and Th2 cytokine production were found to be independent of IL-4Rα and STAT6. Besides being part of the innate immune response to A.fumigatus, AAMs were also induced during adaptive immune response to the fungus. However, at this later time, AAM recruitment and their typical features were abolished in the absence of IL-4Rα and STAT6. Collectively, our studies show that different signaling pathways regulate the development of AAMs in the innate and adaptive arms of the immune response to A. fumigatus. Being early producers of Th2 cytokines, the presence of AAMs would be deleterious to the host unless regulated efficiently.

TNF-α and Th17 collaborate to induce airway neutrophilia during persistent fungal infection in a MyD88 and Dectin-1-dependent fashion (40.23)

Aspergillus fumigatus is a fungus that induces allergic bronchopulmonary aspergillosis (ABPA), a disease exhibiting both airway eosinophilia and neutrophilia. The disease is acquired through the inhalation of spores, which are cleared efficiently in healthy humans but not in patients with cystic fibrosis or asthma. The impaired clearance and sustained inflammation contribute to the pathogenesis of ABPA. Neutrophils are important in host defense, but uncontrolled neutrophilia causes lung pathology in ABPA. We have developed an animal model of ABPA that mimics persistent fungal stimulation in ABPA. Interestingly, the model produced different outcomes in BALB/c and C57BL/6 mice with high neutrophilia in the former but low neutrophilia and high eosinophilia in the latter, which provided us with an opportunity to study the mechanisms underlying excessive airway neutrophilia. Our results show that IL-17A, which has been associated with neutrophilia in many models, was induced at similar levels in both strains. However, the BALB/c mice expressed high levels of TNF-α and collaboration between IL-17A and TNF-α was essential in inducing the high neutrophilia in BALB/c mice. Our investigations in MyD88-/- and Dectin-1-/- mice show an important contribution of both pathways in inducing high TNF-α and IL-17A responses in BALB/c mice. Taken together, we identify a novel mechanism involving the cooperation of TNF-α and Th17 for the regulation of airway neutrophilia in Aspergillus infection.

TLR4/MyD88-induced non-migratory myeloid cells suppress Th2 effector function in the lung (91.15)

Bacterial lipopolysaccharide (LPS) is immunostimulatory at a low dose but immunosuppressive at a high dose in the respiratory tract as observed in animal studies. Relevant to LPS-mediated immunosuppression in the lungs of animals is the protective effect of high doses of LPS in allergic diseases such as asthma in humans. Our study investigated the impact of a high dose of LPS on cells of the innate immune system in the lung to elucidate the basis of LPS-mediated immunosuppression. A high LPS dose blunted house dust mite-induced airway eosinophilia and Th2 cytokines production. Conversely, adoptively transferred Th2 cells were unable to mount allergic airway inflammation in LPS-treated lungs. The high LPS dose promoted the development of a lung-resident myeloid cell type in a TLR4- and MyD88-dependent fashion that suppressed lung DC-mediated reactivation of primed Th2 cells. Adoptive transfer of these myeloid cells suppressed allergen-induced airway inflammation. These data suggest that LPS-induced non-migratory myeloid cells might contribute to the protective effects of LPS in allergic airways disease by dampening Th2 effector functions in the lung. RO1 HL060207 (P.R.), RO1 HL077430 (A.R.), RO1 HL084932 (A.R. and P.R.) and AHA 0865379D (M.A.)

A critical role for c-kit on dendritic cells in regulating in T helper cell differentiation and allergic asthma (79.3)

Dendritic cells are integral in instructing the differentiation of T helper cells into Th1, Th2 and Th17 subsets and IL-6 is a key regulator of CD4 T cell differentiation .In this study, we investigated pathways in DCs that promote IL-6 production. We show exposure of DCs to house dust mite (HDM) or the mucosal adjuvant cholera toxin (CT) promotes cell surface expression of c-Kit and its ligand, stem cell factor (SCF), on DCs. This dual upregulation of c-Kit and SCF results in sustained signaling downstream of c-Kit, thereby promoting IL-6 secretion. Intranasal administration of antigen into c-Kit-mutant mice or neutralization of IL-6 in cultures from the lung-draining lymph nodes of immunized wild-type mice blunted the Th2 and Th7 responses. DCs lacking functional c-Kit or those unable to express membrane-bound SCF secreted lower amounts of IL-6 in response to HDM or CT. Expression of the Th2-Notch ligand Jagged-2 was also reduced in DCs from c-Kit-mutant mice in response to CT or HDM and the Th1-inducing adjuvant, CpG oligodeoxynucleotide, did not promote either c-Kit or Jagged-2 expression. DCs expressing nonfunctional c-Kit were unable to induce a robust Th2 or Th17 response and elicited diminished allergic airway inflammation when adoptively transferred into WT mice. DCs generated from mice expressing a catalytically inactive form of the p110delta subunit of phosphatidylinositol-3 (PI3) kinase secreted lower amounts of IL-6 upon stimulation with cholera toxin. Taken together, these results reveal the importance of the c-Kit-PI3 kinase-IL-6 signaling axis in DCs in the regulation of T cell responses.

Distinct responses of lung and spleen dendritic cells to the TLR9 agonist CpG oligodeoxynucleotide

Dendritic cells (DCs) sense various components of invading pathogens via pattern recognition receptors such as TLRs. CpG oligodeoxynucleotides (ODNs), which mimic bacterial DNA, inhibit allergic airways disease and promote responses in the spleen to bacterial components. Because many TLR agonists are currently being tested for potential therapeutic effects, it is important to characterize the expression and function of TLRs in different tissues. We show that both myeloid and plasmacytoid DCs in the spleen express TLR9, the receptor for CpG ODNs, but lung DCs show no detectable expression in either subset. TLR4 expression in contrast was detected on both lung and spleen DCs. LPS was superior to CpG ODN in increasing the allostimulatory potential of lung DCs and their expression of CD40. However, both agonists efficiently stimulated spleen DCs. CpG ODNs administered to mice efficiently inhibited Th2 cytokine production both in the lung draining lymph node and in the spleen. Surprisingly, inhibition of Th2 cytokine production was evident despite high levels of expression of GATA-3 and additional transcription factors that regulate Th2 responses. Although in the spleen CpG ODNs induced IL-6, a key cytokine induced via TLR9-MyD88 signaling, no IL-6 was detectable in lung LN cells. These studies show for the first time that lung DCs lack TLR9 expression, but, despite this deficiency, CpG ODNs induce potent inhibitory effects on Th2 cytokine production in the lung without inducing expression of the proinflammatory cytokine, IL-6, which has been linked to chronic diseases in the lung and the gut.

Inducible expression of keratinocyte growth factor (KGF) in mice inhibits lung epithelial cell death induced by hyperoxia

Oxidant-induced injury to the lung is associated with extensive damage to the lung epithelium. Instillation of keratinocyte growth factor (KGF) in the lungs of animals protects animals from oxidant-induced injury but the mechanism of protection is not well understood. An inherent problem in studying KGF function in vivo has been that constitutive overexpression of KGF in the lung causes embryonic lethality with extensive pulmonary malformation. Here we report the development of a stringently regulated, tetracycline-inducible, lung-specific transgenic system that allows regulated expression of KGF in the lung without causing developmental abnormalities from leaky KGF expression. By using this system, we show that exposure of KGF-expressing mice to hyperoxia protects the lung epithelium but not the endothelium from cell death in accordance with the selective expression of KGF receptor on epithelial and not on endothelial cells. Investigations of KGF-induced cell survival pathways revealed KGF-induced activation of the multifunctional pro-survival Akt signaling axis both in vitro and in vivo. Inhibition of KGF-induced Akt activation by a dominant-negative mutant of Akt blocked the KGF-mediated protection of epithelial cells exposed to hyperoxia. KGF-induced Akt activation may play an important role in inhibiting lung alveolar cell death thereby preserving the lung architecture and function during oxidative stress.