The very low density lipoprotein receptor attenuates house dust mite-induced airway inflammation by suppressing dendritic cell-mediated adaptive immune responses.

The very low density lipoprotein receptor (VLDLR) is a member of the low-density lipoprotein receptor family that binds multiple ligands and plays a key role in brain development. Although the VLDLR mediates pleiotropic biological processes, only a limited amount of information is available regarding its role in adaptive immunity. In this study, we identify an important role for the VLDLR in attenuating house dust mite (HDM)-induced airway inflammation in experimental murine asthma. We show that HDM-challenged Vldlr(-/-) mice have augmented eosinophilic and lymphocytic airway inflammation with increases in Th2 cytokines, C-C chemokines, IgE production, and mucous cell metaplasia. A genome-wide analysis of the lung transcriptome identified that mRNA levels of CD209e (DC-SIGNR4), a murine homolog of DC-SIGN, were increased in the lungs of HDM-challenged Vldlr(-/-) mice, which suggested that the VLDLR might modify dendritic cell (DC) function. Consistent with this, VLDLR expression by human monocyte-derived DCs was increased by HDM stimulation. In addition, 55% of peripheral blood CD11c(+) DCs from individuals with allergy expressed VLDLR under basal conditions. Lastly, the adoptive transfer of HDM-pulsed, CD11c(+) bone marrow-derived DCs (BMDCs) from Vldlr(-/-) mice to the airways of wild type recipient mice induced augmented eosinophilic and lymphocytic airway inflammation upon HDM challenge with increases in Th2 cytokines, C-C chemokines, IgE production, and mucous cell metaplasia, as compared with the adoptive transfer of HDM-pulsed, CD11c(+) BMDCs from wild type mice. Collectively, these results identify a novel role for the VLDLR as a negative regulator of DC-mediated adaptive immune responses in HDM-induced allergic airway inflammation.

Rapamycin attenuates airway hyperreactivity, goblet cells, and IgE in experimental allergic asthma.

The mammalian target of rapamycin (mTOR) signaling pathway integrates environmental cues, promotes cell growth/differentiation, and regulates immune responses. Although inhibition of mTOR with rapamycin has potent immunosuppressive activity, mixed effects have been reported in OVA-induced models of allergic asthma. We investigated the impact of two rapamycin treatment protocols on the major characteristics of allergic asthma induced by the clinically relevant allergen, house dust mite (HDM). In protocol 1, BALB/c mice were exposed to 10 intranasal HDM doses over a period of 24 d and treated with rapamycin simultaneously during the sensitization/exposure period. In protocol 2, rapamycin was administered after the mice had been sensitized to HDM (i.p. injection) and prior to initiation of two intranasal HDM challenges over 4 d. Airway hyperreactivity (AHR), IgE, inflammatory cells, cytokines, leukotrienes, goblet cells, and activated T cells were assessed. In protocol 1, rapamycin blocked HDM-induced increases in AHR, inflammatory cell counts, and IgE, as well as attenuated goblet cell metaplasia. In protocol 2, rapamycin blocked increases in AHR, IgE, and T cell activation and reduced goblet cell metaplasia, but it had no effect on inflammatory cell counts. Increases in IL-13 and leukotrienes were also blocked by rapamycin, although increases in IL-4 were unaffected. These data demonstrated that rapamycin can inhibit cardinal features of allergic asthma, including increases in AHR, IgE, and goblet cells, most likely as a result of its ability to reduce the production of two key mediators of asthma: IL-13 and leukotrienes. These findings highlight the importance of the mTOR pathway in allergic airway disease.

Epithelial EGF receptor signaling mediates airway hyperreactivity and remodeling in a mouse model of chronic asthma.

Increases in the epidermal growth factor receptor (EGFR) have been associated with the severity of airway thickening in chronic asthmatic subjects, and EGFR signaling is induced by asthma-related cytokines and inflammation. The goal of this study was to determine the role of EGFR signaling in a chronic allergic model of asthma and specifically in epithelial cells, which are increasingly recognized as playing an important role in asthma. EGFR activation was assessed in mice treated with intranasal house dust mite (HDM) for 3 wk. EGFR signaling was inhibited in mice treated with HDM for 6 wk, by using either the drug erlotinib or a genetic approach that utilizes transgenic mice expressing a mutant dominant negative epidermal growth factor receptor in the lung epithelium (EGFR-M mice). Airway hyperreactivity (AHR) was assessed by use of a flexiVent system after increasing doses of nebulized methacholine. Airway smooth muscle (ASM) thickening was measured by morphometric analysis. Sensitization to HDM (IgG and IgE), inflammatory cells, and goblet cell changes were also assessed. Increased EGFR activation was detected in HDM-treated mice, including in bronchiolar epithelial cells. In mice exposed to HDM for 6 wk, AHR and ASM thickening were reduced after erlotinib treatment and in EGFR-M mice. Sensitization to HDM and inflammatory cell counts were similar in all groups, except neutrophil counts, which were lower in the EGFR-M mice. Goblet cell metaplasia with HDM treatment was reduced by erlotinib, but not in EGFR-M transgenic mice. This study demonstrates that EGFR signaling, especially in the airway epithelium, plays an important role in mediating AHR and remodeling in a chronic allergic asthma model.

Early growth response-1 suppresses epidermal growth factor receptor-mediated airway hyperresponsiveness and lung remodeling in mice.

Transforming growth factor (TGF)-alpha and its receptor, the epidermal growth factor receptor, are induced after lung injury and are associated with remodeling in chronic pulmonary diseases, such as pulmonary fibrosis and asthma. Expression of TGF-alpha in the lungs of adult mice causes fibrosis, pleural thickening, and pulmonary hypertension, in addition to increased expression of a transcription factor, early growth response-1 (Egr-1). Egr-1 was increased in airway smooth muscle (ASM) and the vascular adventitia in the lungs of mice conditionally expressing TGF-alpha in airway epithelium (Clara cell secretory protein-rtTA(+/-)/[tetO](7)-TGF-alpha(+/-)). The goal of this study was to determine the role of Egr-1 in TGF-alpha-induced lung disease. To accomplish this, TGF-alpha-transgenic mice were crossed to Egr-1 knockout (Egr-1(ko/ko)) mice. The lack of Egr-1 markedly increased the severity of TGF-alpha-induced pulmonary disease, dramatically enhancing airway muscularization, increasing pulmonary fibrosis, and causing greater airway hyperresponsiveness to methacholine. Smooth muscle hyperplasia, not hypertrophy, caused the ASM thickening in the absence of Egr-1. No detectable increases in pulmonary inflammation were found. In addition to the airway remodeling disease, vascular remodeling and pulmonary hypertension were also more severe in Egr-1(ko/ko) mice. Thus, Egr-1 acts to suppress epidermal growth factor receptor-mediated airway and vascular muscularization, fibrosis, and airway hyperresponsiveness in the absence of inflammation. This provides a unique model to study the processes causing pulmonary fibrosis and ASM thickening without the complicating effects of inflammation.

Differential effects of rapamycin and dexamethasone in mouse models of established allergic asthma.

The mammalian target of rapamycin (mTOR) plays an important role in cell growth/differentiation, integrating environmental cues, and regulating immune responses. Our lab previously demonstrated that inhibition of mTOR with rapamycin prevented house dust mite (HDM)-induced allergic asthma in mice. Here, we utilized two treatment protocols to investigate whether rapamycin, compared to the steroid, dexamethasone, could inhibit allergic responses during the later stages of the disease process, namely allergen re-exposure and/or during progression of chronic allergic disease. In protocol 1, BALB/c mice were sensitized to HDM (three i.p. injections) and administered two intranasal HDM exposures. After 6 weeks of rest/recovery, mice were re-exposed to HDM while being treated with rapamycin or dexamethasone. In protocol 2, mice were exposed to HDM for 3 or 6 weeks and treated with rapamycin or dexamethasone during weeks 4-6. Characteristic features of allergic asthma, including IgE, goblet cells, airway hyperreactivity (AHR), inflammatory cells, cytokines/chemokines, and T cell responses were assessed. In protocol 1, both rapamycin and dexamethasone suppressed goblet cells and total CD4(+) T cells including activated, effector, and regulatory T cells in the lung tissue, with no effect on AHR or total inflammatory cell numbers in the bronchoalveolar lavage fluid. Rapamycin also suppressed IgE, although IL-4 and eotaxin 1 levels were augmented. In protocol 2, both drugs suppressed total CD4(+) T cells, including activated, effector, and regulatory T cells and IgE levels. IL-4, eotaxin, and inflammatory cell numbers were increased after rapamycin and no effect on AHR was observed. Dexamethasone suppressed inflammatory cell numbers, especially eosinophils, but had limited effects on AHR. We conclude that while mTOR signaling is critical during the early phases of allergic asthma, its role is much more limited once disease is established.

Chronic allergic inflammation causes vascular remodeling and pulmonary hypertension in BMPR2 hypomorph and wild-type mice.

Loss-of-function mutations in the bone morphogenetic protein receptor type 2 (BMPR2) gene have been identified in patients with heritable pulmonary arterial hypertension (PAH); however, disease penetrance is low, suggesting additional factors play a role. Inflammation is associated with PAH and vascular remodeling, but whether allergic inflammation triggers vascular remodeling in individuals with BMPR2 mutations is unknown. Our goal was to determine if chronic allergic inflammation would induce more severe vascular remodeling and PAH in mice with reduced BMPR-II signaling. Groups of Bmpr2 hypomorph and wild-type (WT) Balb/c/Byj mice were exposed to house dust mite (HDM) allergen, intranasally for 7 or 20 weeks to generate a model of chronic inflammation. HDM exposure induced similar inflammatory cell counts in all groups compared to controls. Muscularization of pulmonary arterioles and arterial wall thickness were increased after 7 weeks HDM, more severe at 20 weeks, but similar in both groups. Right ventricular systolic pressure (RVSP) was measured by direct cardiac catheterization to assess PAH. RVSP was similarly increased in both HDM exposed groups after 20 weeks compared to controls, but not after 7 weeks. Airway hyperreactivity (AHR) to methacholine was also assessed and interestingly, at 20 weeks, was more severe in HDM exposed Bmpr2 hypomorph mice versus WT. We conclude that chronic allergic inflammation caused PAH and while the severity was mild and similar between WT and Bmpr2 hypomorph mice, AHR was enhanced with reduced BMPR-II signaling. These data suggest that vascular remodeling and PAH resulting from chronic allergic inflammation occurs independently of BMPR-II pathway alterations.

Rapamycin decreases airway remodeling and hyperreactivity in a transgenic model of noninflammatory lung disease.

Airway hyperreactivity (AHR) and remodeling are cardinal features of asthma and chronic obstructive pulmonary disease. New therapeutic targets are needed as some patients are refractory to current therapies and develop progressive airway remodeling and worsening AHR. The mammalian target of rapamycin (mTOR) is a key regulator of cellular proliferation and survival. Treatment with the mTOR inhibitor rapamycin inhibits inflammation and AHR in allergic asthma models, but it is unclear if rapamycin can directly inhibit airway remodeling and AHR, or whether its therapeutic effects are entirely mediated through immunosuppression. To address this question, we utilized transforming growth factor-α (TGF-α) transgenic mice null for the transcription factor early growth response-1 (Egr-1) (TGF-α Tg/Egr-1(ko/ko) mice). These mice develop airway smooth muscle thickening and AHR in the absence of altered lung inflammation, as previously reported. In this study, TGF-α Tg/Egr-1(ko/ko) mice lost body weight and developed severe AHR after 3 wk of lung-specific TGF-α induction. Rapamycin treatment prevented body weight loss, airway wall thickening, abnormal lung mechanics, and increases in airway resistance to methacholine after 3 wk of TGF-α induction. Increases in tissue damping and airway elastance were also attenuated in transgenic mice treated with rapamycin. TGF-α/Egr-1(ko/ko) mice on doxycycline for 8 wk developed severe airway remodeling. Immunostaining for α-smooth muscle actin and morphometric analysis showed that rapamycin treatment prevented airway smooth muscle thickening around small airways. Pentachrome staining, assessments of lung collagen and fibronectin mRNA levels, indicated that rapamycin also attenuated fibrotic pathways induced by TGF-α expression for 8 wk. Thus rapamycin reduced airway remodeling and AHR, demonstrating an important role for mTOR signaling in TGF-α-induced/EGF receptor-mediated reactive airway disease.