Cell-Surface Programmed Death Ligand-1 Expression Identifies a Sub-Population of Distal Epithelial Cells Enriched in Idiopathic Pulmonary Fibrosis.

Idiopathic lung fibrosis (IPF) is a fatal lung disease characterized by chronic epithelial injury and exhausted repair capacity of the alveolar compartment, associated with the expansion of cells with intermediate alveolar epithelial cell (AT2) characteristics. Using SftpcCreERT2/+: tdTomatoflox/flox mice, we previously identified a lung population of quiescent injury-activated alveolar epithelial progenitors (IAAPs), marked by low expression of the AT2 lineage trace marker tdTomato (Tomlow) and characterized by high levels of Pd-l1 (Cd274) expression. This led us to hypothesize that a population with similar properties exists in the human lung. To that end, we used flow cytometry to characterize the CD274 cell-surface expression in lung epithelial cells isolated from donor and end-stage IPF lungs. The identity and functional behavior of these cells were further characterized by qPCR analysis, in vitro organoid formation, and ex vivo precision-cut lung slices (PCLSs). Our analysis led to the identification of a population of CD274pos cells expressing intermediate levels of SFTPC, which was expanded in IPF lungs. While donor CD274pos cells initiated clone formation, they did not expand significantly in 3D organoids in AT2-supportive conditions. However, an increased number of CD274pos cells was found in cultured PCLS. In conclusion, we demonstrate that, similar to IAAPs in the mouse lung, a population of CD274-expressing cells exists in the normal human lung, and this population is expanded in the IPF lung and in an ex vivo PCLS assay, suggestive of progenitor cell behavior. CD274 function in these cells as a checkpoint inhibitor may be crucial for their progenitor function, suggesting that CD274 inhibition, unless specifically targeted, might further injure the already precarious lung epithelial compartment in IPF.

Identification of a novel subset of alveolar type 2 cells enriched in PD-L1 and expanded following pneumonectomy.

Alveolar type 2 (AT2) cells are heterogeneous cells, with specialised AT2 subpopulations within this lineage exhibiting stem cell properties. However, the existence of quiescent, immature cells within the AT2 lineage that are activated during lung regeneration is unknown.SftpcCreERT2/+;tdTomatoflox/flox mice were used for the labelling of AT2 cells and labelled subpopulations were analysed by flow cytometry, quantitative PCR, assay for transposase-accessible chromatin using sequencing (ATAC-seq), gene arrays, pneumonectomy and culture of precision-cut lung slices. Single-cell RNA-sequencing (scRNA-seq) data from human lungs were analysed.In mice, we detected two distinct AT2 subpopulations, with low tdTomato level (TomLow) and high tdTomato level (TomHigh). TomLow cells express lower levels of the AT2 differentiation markers Fgfr2b and Etv5, while TomHigh, as bona fide mature AT2 cells, show higher levels of Sftpc, Sftpb, Sftpa1, Fgfr2b and Etv5 expression. ATAC-seq analysis indicates that TomLow and TomHigh cells constitute two distinct cell populations, with specific silencing of Sftpc, Rosa26 and cell cycle gene loci in the TomLow population. Upon pneumonectomy, the number of TomLow but not TomHigh cells increases and TomLow cells show upregulated expression of Fgfr2b, Etv5, Sftpc, Ccnd1 and Ccnd2 compared to Sham. TomLow cells overexpress programmed cell death 1 ligand 1 (PD-L1), an immune inhibitory membrane receptor ligand, which is used by flow cytometry to differentially isolate these two subpopulations. In the human lung, data mining of a recent scRNA-seq AT2 data set demonstrates the existence of a PD-L1 Pos population. Therefore, we have identified a novel population of AT2 quiescent, immature progenitor cells in mouse that expand upon pneumonectomy and we have provided evidence for the existence of such cells in human.

Origin and characterization of alpha smooth muscle actin-positive cells during murine lung development.

ACTA2 expression identifies pulmonary airway and vascular smooth muscle cells (SMCs) as well as alveolar myofibroblasts (MYF). Mesenchymal progenitors expressing fibroblast growth factor 10 (Fgf10), Wilms tumor 1 (Wt1), or glioma-associated oncogene 1 (Gli1) contribute to SMC formation from early stages of lung development. However, their respective contribution and specificity to the SMC and/or alveolar MYF lineages remain controversial. In addition, the contribution of mesenchymal cells undergoing active WNT signaling remains unknown. Using Fgf10CreERT2 , Wt1CreERT2 , Gli1CreERT2 , and Axin2CreERT2 inducible driver lines in combination with a tdTomatoflox reporter line, the respective differentiation of each pool of labeled progenitor cells along the SMC and alveolar MYF lineages was quantified. The results revealed that while FGF10+ and WT1+ cells show a minor contribution to the SMC lineage, GLI1+ and AXIN2+ cells significantly contribute to both the SMC and alveolar MYF lineages, but with limited specificity. Lineage tracing using the Acta2-CreERT2 transgenic line showed that ACTA2+ cells labeled at embryonic day (E)11.5 do not expand significantly to give rise to new SMCs at E18.5. However, ACTA2+ cells labeled at E15.5 give rise to the majority (85%-97%) of the SMCs in the lung at E18.5 as well as alveolar MYF progenitors in the lung parenchyma. Fluorescence-activated cell sorting-based isolation of different subpopulations of ACTA2+ lineage-traced cells followed by gene arrays, identified transcriptomic signatures for alveolar MYF progenitors versus airway and vascular SMCs at E18.5. Our results establish a new transcriptional landscape for further experiments addressing the function of signaling pathways in the formation of different subpopulations of ACTA2+ cells. Stem Cells 2017;35:1566-1578.

Fgf10 deficiency is causative for lethality in a mouse model of bronchopulmonary dysplasia.

Inflammation-induced FGF10 protein deficiency is associated with bronchopulmonary dysplasia (BPD), a chronic lung disease of prematurely born infants characterized by arrested alveolar development. So far, experimental evidence for a direct role of FGF10 in lung disease is lacking. Using the hyperoxia-induced neonatal lung injury as a mouse model of BPD, the impact of Fgf10 deficiency in Fgf10+/- versus Fgf10+/+ pups was investigated. In normoxia, no lethality of Fgf10+/+ or Fgf10+/- pups was observed. By contrast, all Fgf10+/- pups died within 8 days of hyperoxic injury, with lethality starting at day 5, whereas Fgf10+/+ pups were all alive. Lungs of pups from the two genotypes were collected on postnatal day 3 following normoxia or hyperoxia exposure for further analysis. In hyperoxia, Fgf10+/- lungs exhibited increased hypoalveolarization. Analysis by FACS of the Fgf10+/- versus control lungs in normoxia revealed a decreased ratio of alveolar epithelial type II (AECII) cells over total Epcam-positive cells. In addition, gene array analysis indicated reduced AECII and increased AECI transcriptome signatures in isolated AECII cells from Fgf10+/- lungs. Such an imbalance in differentiation is also seen in hyperoxia and is associated with reduced mature surfactant protein B and C expression. Attenuation of the activity of Fgfr2b ligands postnatally in the context of hyperoxia also led to increased lethality with decreased surfactant expression. In summary, decreased Fgf10 mRNA levels lead to congenital lung defects, which are compatible with postnatal survival, but which compromise the ability of the lungs to cope with sub-lethal hyperoxic injury. Fgf10 deficiency affects quantitatively and qualitatively the formation of AECII cells. In addition, Fgfr2b ligands are also important for repair after hyperoxia exposure in neonates. Deficient AECII cells could be an additional complication for patients with BPD. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.