BASC‐ing in the glow: bronchioalveolar stem cells get their place in the lung

Abstract

Lung epithelial progenitor cell populations fulfill the needs of this complex facultative regenerative organ when exposed to insult and contribute to repair in either the airway or alveolar compartments. The presence of a cell that can populate both epithelia had been proposed previously but has remained elusive. In this issue, Salwig et al (2019) provide compelling, genetic evidence that supports a growing narrative—a rare population of bronchioalveolar stem cells (BASCs) that can contribute to both airway and alveolar epithelium in the distal murine lung.

The process of lung regeneration and repair after injury is complex and involves many cell types, with different progenitor populations responding to different injuries (Leach & Morrisey, 2018). Within the epithelial compartment, there have been multiple identified progenitor cell populations that can respond to injury, and these responses are classically divided into airway epithelial regeneration (basal cells and club cells; Rock et al, 2009, 2011b) and alveolar epithelial regeneration (AT2 cells and AT2 subsets; Barkauskas et al, 2013; Zacharias et al, 2018). Both of these anatomical sites contain progenitor populations, and they have been thought to be governed separately. However, more than a decade ago, a small population of cells at the bronchioalveolar duct junction (BADJ) in the mouse lung was identified and suggested to repopulation both airway and alveolar epithelium after injury (Kim et al, 2005). This cell population—termed bronchioalveolar stem cells (BASCs)—was marked by co‐expression of Scgb1a1 (also called CC10), the canonical club cell marker, and Sftpc, the classic alveolar type II (AT2) cell maker. These Scgb1a1/Sftpc double‐positive cells were initially identified using immunofluorescence approaches, were shown to be a rare population located at the BADJ, and suggested to contribute toward to both airway and alveolar epithelium as well as having a putative role in lung adenocarcinoma (Giangreco et al, 2002; Kim et al, 2005). The presence and role of the BASC have been debated since their initial description, however, with different experimental approaches yielding conflicting results (Rawlins et al, 2009). The concept that the adult lung epithelium is repaired predominately through anatomically restricted cells has been adopted due to a preponderance of experimental evidence (Fig 1).

Figure 1. Bronchioalveolar stem cells (BASCs) contribute to injury‐induced repair of the epithelium in the distal murine lung.

Located at the bronchioalveolar duct junction (BADJ), BASCs are critical for replenishment of AT2 cells and subsets in the alveolar as well as of club and ciliated cells in the airway epithelium postinjury.

Now, two intersectional genetic approaches, Salwig et al (2019) and Liu et al (2019) in another recent report, provide evidence that BASCs are indeed a rare, but distinct population that exists at the BADJ in the mouse lung and can contribute to both airway and alveolar compartments after injury. The well‐designed study by Salwig utilized a split‐intein effector recombination system to label cells expressing both Scgb1a1 and Sftpc in vivo. Through a series of creative genetic approaches, the authors provided a series of elegant assays in which they labeled individual and dual‐positive cells with fluorescent lineage reporters, lineage‐traced these cells after various injuries, and selectively ablated the Scgb1a1/Sftpc double‐positive population to assess its contribution to injury responses. The combined results of these studies provide formidable evidence that a dual‐positive population, while rare, exists and can contribute to airway and alveolar epithelial populations after injury.

Generation of split‐Cre and split‐tTA mouse lines allowed for a series of downstream experiments based on specific labeling of Scgb1a1/Sftpc double‐positive cells. These models allowed for isolation of single‐ and double‐positive epithelial cells, secretory airway cells (club cells), alveolar type 2 cells (AT2), and Scgb1a1/Sftpc double‐positive BASCs. The resulting three‐way RNA‐seq provides a genetic signature for BASCs, highlighting their position transcriptionally between club and AT2 cells, with an expression pattern that contains aspects of both cell types, as well as unique markers. In contrast to earlier work, Sca‐1 was not a distinctive marker for BASCs, highlighting the importance of their genetically defined lineage approach.

While the authors show that there may be a minor BASC contribution to homeostatic turnover in the lung, they demonstrate that BASCs respond differently to an airway injury (naphthalene) versus a predominant alveolar injury (bleomycin), and both responses are distinct from the response of BASCs to influenza. In naphthalene, BASCs contribute to the site of injury by repopulating airway epithelium, giving rise to both club and ciliated cells. Conversely, in bleomycin injury, BASCs give rise predominately to AT2 epithelium. Influenza represented an amalgam of insults and responses, with BASCs contributing to both airway and alveolar epithelium. Despite multiple progenitor populations that contribute to repair in influenza, selective ablation of BASCs through a combinatorial split‐Cre inducible diphtheria toxin model resulted in impaired early epithelial repair of the airway, which recovered at later time points. The BASC ability to differentially contribute to repair based on the type of injury highlights the complex and facultative nature of lung regeneration/repair, as well as the need to understand how different injuries drive differential repair responses.

This work nicely dovetails with the recent intersectional genetics approach from Liu et al (2019), using a Cre/Dre dual recombination system. This work corroborates the description that BASCs are a very rare population that contributes differentially to both airway and alveolar epithelium depending on the type of insult and do so in a clonal matter. Taken together, these studies provide evidence that the field of lung regeneration will need to reckon with the contribution of the BASC to epithelial repair from now on.

The definitive role of the BASC in lung development, disease, and repair, however, is far from a closed case. Both studies rely upon the expression of the two genes, Scgb1a1 and Sftpc, to mark BASCs. In the case of Scgb1a1, this gene is expressed both in club cells and at lower levels in a subset of AT2 cells (Rock et al, 2011a). Therefore, it remains possible that Scgb1a1/Sftpc dual‐positive cells are just a subset of existing club or AT2 cells rather than a distinct cell type. The fact that BASCs are spatially restricted to the BADJ, however, adds support to a distinctive population located in specific niche. Another important issue is that the human lung airways do not terminate in a BADJ, but rather contain a distinct anatomical structure called the respiratory airways. Respiratory airways contain both respiratory bronchioles and alveolar ducts, which allow for continued dispersion of air to the more distal lung along with gas exchange (Weibel et al, 2005). This complex structure is noted in larger mammals and may be related to the need for dispersion of a substantially larger tidal volume (6,000 times greater volume in human than in mouse, for example). Thus, the putative BASC niche, the BADJ, may have no direct anatomical equivalent in the human lung. The human respiratory airways may contain one or more populations of progenitor cells; however, they have yet to be identified and further work will be required to reveal their existence and importance.

The role of BASCs in the regenerative response after injury in the murine lung highlights intriguing questions about how a relatively quiescent organ such as the lung responds to insults. It is increasingly clear there are multiple progenitor populations that can respond to injury of the pulmonary epithelium, and various progenitor cells can contribute to the regeneration of the same epithelial populations. A more integrative and quantitative approach to assess the overall contribution of each of these different populations to the regenerative response is needed. Understanding how the lung chooses which cell populations will be utilized for the response to a particular injury will enhance our comprehension of how the lung responds to various insults and how it balances regenerative repair and maladaptive scaring. Decoding the signals that control this balance will push the field toward the ultimate goal of developing better treatments for human lung injury and disease.

The EMBO Journal (2019) 38: e102344