Obesity shifts house dust mite-induced airway cellular infiltration from eosinophils to macrophages: effects of glucocorticoid treatment

Abstract

Although classically characterized by chronic airway inflammation with eosinophil infiltration, asthma is a complex and multifactorial condition with numerous clinical phenotypes. Epidemiological studies strongly support the link between obesity and asthma and suggest that obesity precedes and promotes asthma development, increases asthma severity, and reduces steroid responsivity. Using a house dust mite (HDM) model of airway hyperresponsiveness in C57BL/6 mice, we examined the effects of diet-induced obesity on allergic airway inflammation and its treatment with dexamethasone. When compared to lean mice treated with HDM, obese-HDM mice had reduced plasma adiponectin, an anti-inflammatory adipokine, lower eosinophil and higher macrophage infiltration into the lungs and bronchoalveolar lavage (BAL) fluid, increased expression of total, M1, and M2 macrophage markers in the lungs, and enhanced Th2 and non-Th2 cytokine expression in the lungs. While Th2-associated responses in obese-HDM mice were suppressed by systemic dexamethasone, several Th2-independent responses, including total and M1 macrophage markers in the lungs, and lung CXC-motif ligand 1 (CXCL1) levels, were not improved following dexamethasone treatment. Thus, HDM combined with obesity promotes mixed localized inflammatory responses (e.g., M1, M2, Th1, and Th2) and shifts the cellular infiltration from eosinophils to macrophages, which are less sensitive to dexamethasone regulation. Because obese asthmatics exhibit more severe symptoms, lack a predominance of Th2 biomarkers, and are predicted to experience more steroid resistance when compared to lean asthmatics, this model could be used to study blunted steroid responses in obese-HDM mice and to define the macrophages found in the lungs.

Keywords: Airway inflammation; Asthma; Diet-induced obesity; Steroid resistance.

Figure 1. High fat diet induces obesity in C57BL/6 mice irrespective of HDM and Dex treatments

The average weight gain (in grams, g) over the course of the study among lean and obese mice (A) and final average weights among lean and obese groups (±house dust mite, HDM, ±dexamethasone, Dex) (B). ***=p<0.001, comparing high fat-fed obese vs. lean mice subgroups (±HDM, ±Dex).

Figure 2. Obesity reduces HDM-induced plasma adiponectin levels in obese mice

Plasma adiponectin levels in lean and obese mice (±house dust mite, HDM, ±dexamethasone, Dex) were determined by ELISA. *=p<0.05 compared to obese-HDM mice.

Figure 3. Obesity shifts the cellular infiltrate into lung tissue from eosinophils to macrophages following HDM: Dex does not suppress macrophage accumulation

Representative H&E sections of lungs obtained from lean and obese mice (±house dust mite, HDM, ± dexamethasone, Dex) (A). Yellow arrows indicate areas of inflammation. Enumeration of Congo Red⁺ eosinophils per high power field (HPF) in lung tissues obtained from lean and obese mice following (±HDM, ±Dex) (B) and assessment of plasma IgE concentrations (C). Enumeration of F4/80⁺ lung macrophages (MФ) per HPF in lung tissue sections (D). *=p<0.05, **=p<0.01; ***=p<0.001, as indicated.

Figure 4. BAL fluid shows increased macrophage accumulation in obese-HDM mice: No effect of Dex

The total number of cells (A) and number of eosinophils (B) and macrophages (MФ) (C) found in the BAL fluid of lean and obese mice following house dust mite (HDM) administration in the presence and absence of dexamethasone (Dex). *=p<0.05 and **=p<0.01, as indicated.

Figure 5. Obesity differentially regulates gene expression markers for macrophages in the lungs following HDM (±Dex)

The effect of house dust mite (HDM) administration (± dexamethasone, Dex) on the expression of genes indicative of total (F480 mRNA, A), as well as M1 (Nos2 mRNA, B) and M2 (Arg1 mRNA, C and Fizz1 mRNA, D) macrophages in lung tissues obtained from lean and obese mice. Data are shown as relative mRNA expression with the untreated lean control group set to 1. *=p<0.05, **=p<0.01; ***=p<0.001, as indicated.

Figure 6. HDM increases IL-4 and IL-5 levels in BAL fluid obtained from obese mice: Reversed by Dex

IL-2 (A), IL-4 (B), and IL-5 (C) protein levels in bronchoalveolar lavage (BAL) fluids in lean and obese mice following house dust mite (HDM) ±dexamethasone (Dex). *=p<0.05 and **=p<0.01, as indicated.

Figure 7. CXCL1 protein levels in the lungs of obese and lean asthmatics

The effect of house dust mite (HDM) (±dexamethasone, Dex) on CXCL1 protein levels in the BAL fluids obtained from lean and obese mice were determined by ELISA. Data are shown as pg CXCL1 per gram (g) lung tissue. *=p<0.05 and **=p<0.01, as indicated.

Figure 8. Glucocorticoid receptor mRNA expression in the lungs of lean and obese mice

The effect of house dust mite (HDM) (±dexamethasone, Dex) on Gra (A) and Grb (B) mRNA expression in the lungs of lean and obese mice were determined by qPCR. Data are shown as relative mRNA expression with the untreated lean control group set to 1.