Lineage-negative progenitors mobilize to regenerate lung epithelium after major injury

Abstract

Broadly, tissue regeneration is achieved in two ways: by proliferation of common differentiated cells and/or by deployment of specialized stem/progenitor cells. Which of these pathways applies is both organ and injury-specific1–4. Current paradigms in the lung posit that epithelial repair can be attributed to cells expressing mature lineage markers5–8. In contrast we here define the regenerative role of previously uncharacterized, rare lineage-negative epithelial stem/progenitor (LNEPs) cells present within normal distal lung. Quiescent LNEPs activate a ΔNp63/cytokeratin 5 (Krt5+) remodeling program after influenza or bleomycin injury. Activated cells proliferate and migrate widely to occupy heavily injured areas depleted of mature lineages, whereupon they differentiate toward mature epithelium. Lineage tracing revealed scant contribution of pre-existing mature epithelial cells in such repair, whereas orthotopic transplantation of LNEPs, isolated by a definitive surface profile identified through single cell sequencing, directly demonstrated the proliferative capacity and multipotency of this population. LNEPs require Notch signaling to activate the ΔNp63/Krt5+ program whereas subsequent Notch blockade promotes an alveolar cell fate. Persistent Notch signaling post-injury led to parenchymal micro-honeycombing, indicative of failed regeneration. Lungs from fibrosis patients show analogous honeycomb cysts with evidence of hyperactive Notch signaling. Our findings indicate distinct stem/progenitor cell pools repopulate injured tissue depending on the extent of injury, and the outcomes of regeneration or fibrosis may ride in part on the dynamics of LNEP Notch signaling.

Plasticity in the lung: making and breaking cell identity

“…Indeed, different regions of the mouse airway may model different aspects of human cell biology and disease to varying degrees, despite the histological similarity of murine tracheal and human airway epithelium. Recent studies in mice have also demonstrated the presence of small numbers of basal-like progenitor cells in the distal airways, even at steady state (Kumar et al, 2011;Vaughan et al, 2015). Remarkably, the number of such cells is increased in the small airways and alveoli following severe injury caused by H1N1 influenza injury or bleomycin (Vaughan et al, 2015;Zuo et al, 2015).…”

“…Recent studies in mice have also demonstrated the presence of small numbers of basal-like progenitor cells in the distal airways, even at steady state (Kumar et al, 2011;Vaughan et al, 2015). Remarkably, the number of such cells is increased in the small airways and alveoli following severe injury caused by H1N1 influenza injury or bleomycin (Vaughan et al, 2015;Zuo et al, 2015). Finally, it is likely that each of the cell types of the airway epithelium have additional functions beyond that Anatomical and cellular differences between murine and human lung.…”

“…However, this study was also called into question due to the caveat that tamoxifen-induced labeling might have persisted into the time period in which pod cells differentiated into basal and secretory cells. Thus, instead of demonstrating that pods cells were derived from secretory or preexisting basal-like cells, it has been suggested that these authors simply labeled secretory and basal cells that had already differentiated from pod cells (Ray et al, 2016;Vaughan et al, 2015).…”

Epigenetic and Transcriptomic Regulation of Lung Repair during Recovery from Influenza Infection

“…The inability of these cells to differentiate leads to the honeycomb-like structures. 29 Recently, it was shown that these Krt5 pods are not observed during lung injury induced by less virulent X31 influenza in mice, suggesting that the severity of lung injury may dictate the cellular repair process. 31 It is possible that the persistent ER stress observed herein may modulate the ability of the progenitor cells to differentiate and to form alveolar structure.…”

Lung organoids: current uses and future promise

“…Basal cells expressing detectable levels of Trp63 and Krt5 are only present in trachea and main stem bronchi. Lineage-negative epithelial progenitors (LNEPs) have been proposed for the distal airways (Vaughan et al, 2015), which in the mouse are known as bronchioles as they lack associated cartilage. There is evidence that the Club cell population is heterogeneous, with a few cells in the bronchioalveolar duct junction (BADJ) and alveoli (marked with an asterisk) co-expressing Scbg1a1 and Sftpc (Kim et al, 2005;Rawlins et al, 2009).…”