Airway basal stem cells: a perspective on their roles in epithelial homeostasis and remodeling

Abstract

The small airways of the human lung undergo pathological changes in pulmonary disorders, such as chronic obstructive pulmonary disease (COPD), asthma, bronchiolitis obliterans and cystic fibrosis. These clinical problems impose huge personal and societal healthcare burdens. The changes, termed 'pathological airway remodeling', affect the epithelium, the underlying mesenchyme and the reciprocal trophic interactions that occur between these tissues. Most of the normal human airway is lined by a pseudostratified epithelium of ciliated cells, secretory cells and 6-30% basal cells, the proportion of which varies along the proximal-distal axis. Epithelial abnormalities range from hypoplasia (failure to differentiate) to basal- and goblet-cell hyperplasia, squamous- and goblet-cell metaplasia, dysplasia and malignant transformation. Mesenchymal alterations include thickening of the basal lamina, smooth muscle hyperplasia, fibrosis and inflammatory cell accumulation. Paradoxically, given the prevalence and importance of airway remodeling in lung disease, its etiology is poorly understood. This is due, in part, to a lack of basic knowledge of the mechanisms that regulate the differentiation, maintenance and repair of the airway epithelium. Specifically, little is known about the proliferation and differentiation of basal cells, a multipotent stem cell population of the pseudostratified airway epithelium. This Perspective summarizes what we know, and what we need to know, about airway basal cells to evaluate their contributions to normal and abnormal airway remodeling. We contend that exploiting well-described model systems using both human airway epithelial cells and the pseudostratified epithelium of the genetically tractable mouse trachea will enable crucial discoveries regarding the pathogenesis of airway disease.

Figures

Fig. 1.

Schematic comparison of the structure and epithelial organization of rodent and human lungs. Left panel: mouse lung. The trachea, ∼1.5 mm internal diameter, is lined by a pseudostratified epithelium with about 55% ciliated cells, 30% BCs, secretory cells and sparse neuroendocrine cells. In the rat (but not apparently in the mouse), there are more ciliated cells in the epithelium overlying intercartilage segments versus cartilage (Toskala et al., 2005). The submucosal glands are typically restricted to the four most proximal intercartilage regions. In the mouse, there are ∼six to eight generations of intralobar branches (intrapulmonary airways), which have a stereotypical branching pattern (Metzger et al., 2008). In these airways, the epithelium is simple and columnar, and is made up of ∼48% ciliated cells and the remainder of secretory and neuroendocrine cells. Most of the secretory cells have electron-dense cytoplasmic granules and domed apical surfaces that project into the lumen and express high levels of the secretoglobin SCGB1A1. They are therefore defined as Clara cells (Mercer et al., 1994). There are no BCs and, in laboratory mice, few goblet cells in these intrapulmonary airways. Smooth muscle (yellow lines) surrounds the airways, but there are no cartilage plates. The terminal bronchioles leading into the bronchio-alveolar duct have fewer ciliated cells (∼26%) compared with more proximal airways. Right panel: human lung. The average human trachea has an internal diameter of ∼12 mm. There are more generations of intrapulmonary branches than in the mouse, and cartilage plates and smooth muscle surround the intrapulmonary airways deep into the lung. A pseudostratified epithelium with ∼30% BCs, 30% ciliated and 30% secretory cells lines these airways. The latter are predominantly goblet cells with a few Clara cells (Mercer et al., 1994). The respiratory bronchioles are lined by a simple cuboidal epithelium. The precise identity and gene expression profiles of these cells are poorly understood but they do not seem to express SCGB1A1 [for a detailed description see ten Have-Opbroek et al. (ten Have-Opbroek et al., 1991)]. Scale bars: 25 μm.

Fig. 2.

Schematic diagram of the mouse trachea showing BCs and their dynamic niche. (A) Schematic representation of BCs in the proximal mouse trachea, in which submucosal glands are present. The surface epithelium consists of basal, ciliated and secretory cells in roughly equal proportions, with relatively few neuroendocrine cells. In laboratory mice, mucus-producing goblet cells are largely restricted to the submucosal glands. Immune cells, including dendritic cells, reside transiently within the epithelium. (B) Whole-mount immunohistochemistry of a mouse trachea. Staining for CD31 (also known as PECAM-1; red) shows dense vascularization of the trachea, especially dorsally and in the intercartilage regions ventrally. These blood vessels are from the systemic circulation. Staining for calcitonin gene-related peptide (CGRP; green) reveals nerves. (C) Immunohistochemistry of a section of mouse trachea showing that most BCs express cytokeratin 5 (KRT5, green), whereas only a subset co-expresses KRT14 (red). DAPI stains nuclei (blue). (B,C) Outlines of cartilage rings are in dotted lines. (D) Higher magnification of boxed region in C. Scale bars: 600 μm (B); 20 μm (C).

Fig. 3.

BCs in normal and diseased human small airways. (A–C) Sections of a normal human airway (∼2 mm diameter) from a non-smoker that were obtained after lobectomy for a non-lung-cancer metastasis. BCs of the normal pseudostratified airway epithelium are identified by morphology and proximity to the basal lamina in a hematoxylin and eosin (H&E) stained section (A) or by expression of the transcription factor TRP63 (B) and NGFR (C) in adjacent sections. Presence of SCGB1A1 in B marks Clara secretory cells. (D) H&E stained section of an airway from a smoker with COPD. The boxed region shows relatively normal epithelium transitioning into squamous metaplasia. (E–G) High magnification of sections adjacent to the boxed region in D showing expansion of TRP63+ (E, red), KRT5+ (F, red) and KRT14+ (G, red) cells in regions of squamous metaplasia. Note that only rare BCs in normal airway are stained by a mouse monoclonal antibody against KRT14 (arrowhead in G). (H–J) Regions of mucous hyperplasia and squamous metaplasia in an airway of a smoker with COPD stained with H&E (H), Alcian Blue (I) and anti-TRP63 (J, red). Note the sharp transition from squamous metaplasia to mucous hyperplasia. (K,L) Small airway from an organ donor with moderate asthma stained with Alcian-Blue–periodic-acid–Schiff and anti-TRP63 (red, L). (M) Airway from a patient with post-transplant bronchiolitis obliterans syndrome stained with H&E, anti-TRP63 (red, N) and anti-KRT5 (green, N). Nuclei were stained with DAPI (blue) in fluorescent images. Scale bars: 25 μm (A–C,J–N); 100 μm (D–I).

Fig. 4.

Schematic model of how paracrine signals from the niche influence BC behaviors. A model for how signals from the niche – which includes neighboring epithelial cells, vasculature, nerves, stroma and immune cells – influence BCs and their progeny. In pathological conditions, including bacterial infection and inflammation, epithelial cells and the surrounding stroma produce factors that might modulate BC behaviors. Evidence from other stem cell systems suggests that expression levels of Notch signaling components (which are known to regulate the balance of ciliated and secretory cells in human and embryonic mouse airways) can be modulated by paracrine signals, including cytokines such as IL-6. How these signals are transduced to affect cell biology (e.g. polarity, attachment), proliferation and cell fate decisions, and how they contribute to airway remodeling, such as squamous and mucous metaplasia, are not known.

Fig. 5.

Tentative working model for the self-renewal and differentiation of basal stem cells in mouse and human airways. The upper panels schematize a normal human pseudostratified mucociliary epithelium (left), goblet cell (mucous) metaplasia (center) and squamous metaplasia (right). In the mouse trachea there are more Clara than goblet cells and BCs are mostly discontinuous. Note that only a subset of BCs in the normal airway expresses both KRT5 and KRT14. The lower panel illustrates probable lineage relationships in the adult pseudostratified epithelium. The relationships between Trp63+ BCs that differ in cytokeratin expression and other properties is not yet clear. As a population (within broken circle), BCs self-renew over the long term and generate ciliated, secretory cells (including SCGB1A1+ Clara cells) and goblet cells. We predict the existence of a multipotent early progenitor that also proliferates. SCGB1A1+ cells can proliferate and give rise to ciliated cells (Rawlins et al., 2009) and, based on studies in the mouse intrapulmonary airways, reversibly give rise to goblet cells (Chen et al., 2009). The origin of squamous cells that express involucrin is not clear.