Epithelial hyperplasia and metaplasia are common features of inflammatory and neoplastic disease, but the basis for the altered epithelial phenotype is often uncertain. Here we show that long-term ciliated cell hyperplasia coincides with mucous (goblet) cell metaplasia after respiratory viral clearance in mouse airways. This chronic switch in epithelial behavior exhibits genetic susceptibility and depends on persistent activation of EGFR signaling to PI3K that prevents apoptosis of ciliated cells and on IL-13 signaling that promotes transdifferentiation of ciliated to goblet cells. Thus, EGFR blockade (using an irreversible EGFR kinase inhibitor designated EKB-569) prevents virus-induced increases in ciliated and goblet cells whereas IL-13 blockade (using s-IL-13Rα2-Fc) exacerbates ciliated cell hyperplasia but still inhibits goblet cell metaplasia. The distinct effects of EGFR and IL-13 inhibitors after viral reprogramming suggest that these combined therapeutic strategies may also correct epithelial architecture in the setting of airway inflammatory disorders characterized by a similar pattern of chronic EGFR activation, IL-13 expression, and ciliated-to-goblet cell metaplasia. IntroductionEpithelial cell hyperplasia and metaplasia are common consequences of inflammation and may be associated with protective as well as pathogenic outcomes. In the lung, airway epithelial remodeling can be life threatening, since mucous cell metaplasia is the foundation for hypersecretion that can obstruct the airway lumen. Despite the critical nature of this process, little is known about how mucous cell metaplasia develops in the setting of acute or chronic inflammatory disease. Particularly, little is known about the mechanism for what is likely the most common cause of mucous cell metaplasia in the lung, i.e., respiratory viral infection, since previous work has focused on bacterial, allergic, and carcinogenic stimuli. Perhaps because of the paucity of mechanistic information, no effective and specific pharmacologic treatment is currently available to treat epithelial cell metaplasia in general or mucous cell metaplasia in particular.In this context, recent work on mucous cell metaplasia has often focused on signaling pathways initiated by activation of the IL-13 receptor (IL-13R) and EGFR (also designated ErbB1 and HER1). The experimental role of IL-13R was established when a decoy receptor for IL-13 (soluble IL-13Rα2-Fc) was found to inhibit allergen-induced mucous (goblet) cell formation in mice (1, 2). These reports have been followed by evidence that IL-13 can directly drive mucin gene expression in airway epithelial cells cultured under physiologic conditions and in vivo (3-6). Moreover, IL-13 is often overexpressed in the setting of mucous cell metaplasia in asthma

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Unmethylated CpG motifs in bacterial DNA or synthetic oligodeoxynucleotides (ODN) potently stimulate the innate immune system, and they are recognized by Toll-like receptor 9 (TLR9), which is expressed by monocytes/macrophages, dendritic cells, and B cells. However, it is unknown whether alveolar macrophages (AMs) express functional TLR9. To clarify this, we analyzed mRNA expressions of TLRs in murine AMs by real-time polymerase chain reaction, and compared with those in other tissue macrophages and lung antigen-presenting cells. In addition, we determined the sensitivity of these cell populations to CpG-ODN. Interestingly, TLR9 mRNA was almost absent in AMs, but highly expressed in bone marrow-derived macrophages and peritoneal macrophages, whereas TLR2 and TLR4 were present in all macrophage populations. Consistent with the receptor expression, AMs showed no sensitivity to CpG-ODN, whereas other macrophage populations secreted tumor necrosis factor alpha, interleukin 12 p40, and interleukin 6, and enhanced expression of CD40, CD80, and CD86, in response to CpG-ODN. Lung dendritic cells and B cells highly expressed TLR9 mRNA and responded to CpG-ODN. These results indicate selective loss of TLR9 expression in AMs with no sensitivity to CpG-ODN, suggesting that dendritic cells and B cells play a role in the immune response against bacterial DNA in the lung.

Dendritic cells (DCs) are potent antigen-presenting cells (APCs); they are considered to be the most important APC in the lung. Recently, the number of DCs in the large airways was demonstrated to increase in patients with atopic asthma, leading to the concept that DCs play an important role in airway inflammation. However, little is known about the distribution of lung DCs in the small airways under other pathological conditions. The aim of the present study was to examine the distribution of DCs in the bronchiolar tissues in patients with diffuse panbronchiolitis (DPB), which is a chronic inflammatory disorder of the airways histologically characterized by peribronchiolitis. We investigated the distribution of DCs in the bronchiolar tissues of the lungs in 11 patients with DPB and 7 control subjects with normal lungs using immunohistochemical methods. Marked increases in the number of CD1a(+), CD1c(+), and CD83(+) DCs were found in both the bronchiolar epithelium and submucosal tissues of patients with DPB, compared with control subjects with normal lungs. The most striking increase occurred in the number of DCs expressing CD83, a marker of mature DCs, in the submucosal tissues of patients with DPB. The increases of these positive cells in patients with DPB were more marked in the submucosal tissues than in the epithelium. The bronchiolar epithelial cells in patients with DPB strongly expressed GM-CSF protein, which is an important cytokine for the differentiation and function of DCs, suggesting that the increased local production of GM-CSF may be responsible for the accumulation and differentiation of DCs in the bronchiolar tissues of patients with DPB. These results suggest that increased DCs in the bronchiolar tissues, together with their phenotypical maturation, may play an important role in the mucosal immune response in patients with DPB through their potent antigen-presenting function.

The p38 MAPK pathway is involved in IL-13-induced mucous cell metaplasia and MUC5AC mRNA regulation in mTEC. In addition, p38 MAPK phosphorylation may require STAT6-dependent de novo protein synthesis induced by IL-13.