A Specialized Few Among Many: Identification of a Novel Lung Epithelial Stem Cell Population

Summary

The landscape of lung epithelial stem cells is getting more nuanced. In this issue of Cell Stem Cell, Kathiriya et al. (2020) describe a novel distal airway epithelial cell population with high regenerative potential.

Main Text

The primary function of the lung is to facilitate gas exchange, and in performing this job, the organ must directly interface with the environment. As a result, the lung is frequently exposed to injurious stimuli (including viral pathogens, e-cigarette vapors, and airborne pollutants, to name a few) that damage the epithelium and challenge it to repeatedly and efficiently repair itself so as to avoid disruption of the lung’s critical gas-exchange function.

Early work in lung epithelial stem cell biology led to the identification and characterization of several important regional epithelial stem cell types that maintain the mouse lung during homeostasis and contribute to lung repair. These include basal cells in the trachea that self-renew and differentiate into luminal secretory and ciliated cells (Rock et al., 2009), Secretoglobin 1a1 (SCGB1A1)-positive club cells in the airway epithelium that self-renew and differentiate into ciliated cells (Rawlins et al., 2009), and Surfactant protein C (SFTPC)-positive type 2 alveolar epithelial cells (AEC2s) that self-renew and differentiate into type 1 alveolar epithelial cells (AEC1s) in the gas-exchanging epithelium in the alveolar space (Barkauskas et al., 2013). However, with time, we have developed a more nuanced understanding of these populations and the field now identifies subsets within the regional populations (Figure 1 ). For example, in the trachea, subsets of basal cells have been shown to contribute to repair following an injury (Pardo-Saganta et al., 2015). In the distal airway epithelium, a few, localized bronchioalveolar stem cells (BASCs) that co-express SFTPC and SCGB1A1 (Liu et al., 2019) and a rare group of p63-expressing cells (Vaughan et al., 2015) both contribute to repair of the airways and alveolar space. Similarly, a subset of Wnt-responsive AEC2s have been shown to be facultative progenitors within the alveolar epithelium, capable of expansion and contributing to a large proportion of alveolar repair following lung injury (Nabhan et al., 2018, Zacharias et al., 2018).

Figure 1.

The Complexity of the Epithelial Stem Cell Landscape in the Lung Is Increasing

Subsets of stem cell populations, including the H2-K1^high distal airway cells identified by Kathiriya et al. (2020), are becoming recognized as major contributors to alveolar epithelial regeneration. Stem cells are shown to be capable of either self-renewal (circular arrow) or differentiation (arched arrows pointing to a different cell type). The relative regenerative contribution of each stem cell population is depicted by the thickness of the corresponding arrows.

The majority of these studies have relied upon lineage tracing, a technique in which investigators can follow the fate of individual cells or populations of cells with a permanent mark. But lineage tracing is only as specific as the cell-type-specific drivers that are used. Through the use of single-cell RNA-sequencing technologies and forward-thinking bioinformatics analysis approaches, our field has begun to uncover even more nuanced subsets of regional epithelial cell stem cells, and some of these populations have been shown to display more robust regenerative potential than the parent population as a whole.

In this issue of Cell Stem Cell, Kathiriya et al. (2020) describe a distinct distal airway epithelial cell population that is a subset of cells lineage traced by Scgb1a1 and that expresses a unique combination of antigen-presenting genes, an activated host defense pathway, and the MHC class I marker H2-K1 (Figure 1). The authors demonstrate the regenerative potential of these H2-K1^high cells through in vitro organoid and in vivo transplantation assays, revealing that this specific subset accounts for the regenerative capacity of the airway epithelium in alveolar repair. In the absence of lineage tracing tools to track this cellular subset after injury, the authors relied upon RNA velocity analysis to suggest that H2-K1^high cells give rise to both AEC2s and AEC1s in the alveolar space as well as ciliated and club cells in the airway. Interestingly, these H2-K1^high cells seem to be heterogeneous themselves, as the clonal organoids derived from individual cells were either basal or alveolar. The factors that contribute to the final fate choice decision in this cell population have not yet been uncovered.

From a stem cell biology standpoint, the lung’s response to epithelial injury appears to be context dependent. While there exist examples in the literature that support the notion that epithelial injury triggers de-differentiation of mature cells (Tata et al., 2013), this work from Kathiriya et al. (2020) supports the notion that lung repair is modulated by expansion of pre-specified or committed progenitors. However, until we have the tools to rigorously lineage-trace, deplete, and/or genetically modify this H2-K1^high cell population at specific times in the context of different levels of injury, we will not have definitive proof that they are permanently programmed to their fate or proof of whether there is natural cycling of Scgb1a1+ airway cells to adopt this H2-K1^high identity and function.

In the context of lung regenerative medicine, this work from Kathiriya et al. (2020) raises two very interesting points. First, the authors suggest these H2-K1^high cells have enhanced susceptibility to direct viral infection, perhaps explaining the abnormal alveolar epithelial repair following severe injury from influenza (Vaughan et al., 2015). Depletion of this highly regenerative cellular subset with concomitant alveolar injury could potentially contribute to the development and pathogenesis of acute respiratory distress syndrome (ARDS) in patients with infection with viruses with significant distal lung epithelial tropism, such as influenza and potentially the novel coronavirus (COVID-19).

The second interesting point is the finding that the H2-K1^high cells begin to express markers of senescence following robust activation. This suggests that the H2-K1^high population has limited expansion and repair capacity in the long term. This could perhaps explain why some individuals develop non-resolving ARDS and why others develop chronic abnormal alveolar remodeling (as in fibrosis). As the authors suggest, the opportunity to transplant “fresh” H2-K1^high cells (assuming a human correlate is identified) into patients’ lungs at times of depletion could represent a novel therapeutic strategy. To move in this direction, there is a clear need to look for and study correlates of the H2-K1^high population in human lung. Our field is only beginning to recognize epithelial stem cell subsets in human lung, and to date, it has been difficult to identify direct correlates of distinct murine lung stem cell subsets in human tissue.

This is an exciting time in lung stem cell biology. As our understanding of epithelial stem cell populations and dynamics increases, so too will our ability to harness lung stem cells for regenerative medicine. This paper marks yet another step along this journey.