Bronchial epithelial cells produce IL-5: Implications for local immune responses in the airways

Abstract

IL-5 is a pleiotropic cytokine that promotes eosinophil differentiation and survival. While naïve bronchial epithelial cells (BEC) produce low levels of IL-5, the role of BEC-derived IL-5 in allergic airway inflammation is unknown. We now show that BEC, isolated from mice with OVA-induced allergic airway disease (AAD), produced elevated levels of IL-5 mRNA and protein as compared to BEC from naïve mice. To determine the contribution of BEC-derived IL-5 to effector responses in the airways, IL-5 deficient bone marrow chimeric mice were generated in which IL-5 expression was restricted to stromal (e.g. BEC) or hematopoietic cells. When subjected to AAD, IL-5 produced by BECs contributed to mucous metaplasia, airway eosinophilia, and OVA-specific IgA levels. Thus, IL-5 production by BEC can impact the microenvironment of the lung, modifying pathologic and protective immune responses in the airways.

Introduction

Epithelial cells form a continuous lining in the airways, providing a protective barrier between the external and internal environment. Comprised of ciliated cells, goblet cells, and basal cells, the epithelium functions in preventing the passage of airborne irritants, allergens, and pathogens into the lung. In addition to providing a barrier function, the epithelium possesses the ability to generate a wide range of mediators that can modulate inflammatory responses, either helping to maintain homeostasis or enhance inflammation. Among the cytokines and chemokines produced by airway epithelial cells are several key factors that influence pathologies associated with asthma. For example, IL-13, a mediator in goblet cell metaplasia and airway remodeling, is elevated in patients with asthma [1], [2]. Goblet cell metaplasia with increased mucus plugging is markedly increased in fatal asthma [3], [4], [5]. IL-13 also contributes to eosinophil recruitment through eotaxin-1- and IL-5-dependent pathways [6], [7]. In addition, recent studies show that IL-13 release by epithelial cells occurs in rapid response to injury, facilitating repair and repopulation of the damaged area [8]. Granulocyte–macrophage colony stimulating factor (GM-CSF) is produced by the airway epithelium and plays a role in prolonging the survival of eosinophils [9], [10]. Elevated levels of GM-CSF are present in the bronchoalveolar lavage fluid (BALF) collected from individuals with asthma [11]. The airway epithelium is a major source of eotaxin, an eosinophil specific chemokine that recruits eosinophils from the blood into the lung and elevated concentrations of eotaxin are found in the BALF and sputum from atopic asthmatic individuals [12], [13], [14]. Finally, increased expression of IL-5 has been reported in BALF from individuals with asthma and correlates with increased asthma severity [15], [16], [17]. IL-5 is a pleiotropic cytokine, regulating eosinophil proliferation, differentiation, and recruitment in parasitic diseases and allergic asthma [18], [19], [20], [21], [22]. While eosinophils are not present in all individuals with asthma, the eosinophilic phenotype is associated with an increased risk of exacerbations and higher risk of fatal outcomes in individuals with severe disease [15].

In the ovalbumin (OVA)-induced murine model of allergic airway disease (AAD), several different approaches have been taken to establish a pivotal role for IL-5 in the development of airway eosinophilia. Anti-IL-5 antibody treatment inhibits eosinophilic lung inflammation, but has no effect on IgE levels or Th2 cell function [23]. IL-5 null mice fail to display increased levels of eosinophils during the acute [22] or chronic stages of disease [24], [25]. Together, these results define a role for IL-5 in the generation of airway eosinophilia. Eosinophils are associated with the development of lung dysfunction and to better understand the role of eosinophils in AAD, two separate laboratories generated an eosinophil null mouse. Characterization of C57BL/6 PHIL mice by Lee et al. [26] shows that eosinophils are integral to mucus accumulation and airway hyperreactivity. In contrast, Humbles et al. [27] observe structural changes associated with airway remodeling in Balb/c ΔdblGATA mice, deficient in eosinophils, but no differences in mucus hypersecretion or airway hyperreactivity are noted. The disparity in these outcomes likely involves strain differences specific to the mice [28]. Recent studies further define the role of eosinophils in AAD, showing they are critical in the recruitment of local T effector cells to the lung, by modulating chemokine production [28], [29].

IL-5 may also influence other aspects of asthma as demonstrated by studies of IL-5 transgenic mice. Here, constitutive expression of IL-5 in the lung, in the absence of antigen challenge, results in mucous metaplasia, expansion of bronchus-associated lymphoid tissue (BALT), enhanced collagen deposition, and extensive airway eosinophilia [30]. Thus, an activated airway epithelium can dictate changes in the local inflammatory milieu that may result in exacerbation or resolution of disease. In this study, we isolated bronchial epithelial cells (BEC) from mice with AAD and characterized their production of Th2 cytokines, focusing on IL-5. To investigate the function of BEC-derived IL-5, a series of bone marrow (BM) chimeric mice were generated where IL-5 production was limited to stromal cells (e.g. BEC) or hematopoietic cells. This study provides evidence that IL-5 production by airway epithelial cells during AAD can influence local immune responses and induce pathologic changes in the lung.