Cell-surface PD-L1 expression identifies a sub-population of distal epithelial cells enriched in idiopathic pulmonary fibrosis

Abstract

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 (PCLS). 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 the 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.

FGF10 Therapeutic Administration Promotes Mobilization of Injury-Activated Alveolar Progenitors in a Mouse Fibrosis Model

“…How IAAPs responded to FGFR2b signaling and escaped Fgfr2b deletion remains to be further investigated. Our most recent study further identified the human equivalent of the IAAPs, which were amplified in IPF patients [ 27 ]. These findings are consistent with our initial description of this newly identified AT2 subpopulation as becoming activated and proliferative in the context of PNX-induced injury [ 22 ].…”

“…A puzzling question related to the behavior of IAAPs in end-stage IPF is their lack of contribution to the mature AT2 lineage. These cells are indeed amplified in IPF but seem to be stalled in their differentiation status [ 27 ]. One possibility explaining this failure to differentiate could be linked to the high inflammatory status of these lungs.…”

“…However, a first difficulty to carry out this experiment deals with the IAAPs themselves. We have previously isolated IAAPs from non-injured lungs [ 22 ] and from lungs displaying deletion of Fgfr2b in AT2s [ 27 ]. The deletion of Fgfr2b in AT2s leads to a situation similar to what we showed in the context of bleomycin treatment: we observed a significant drop in Tom High (mature) AT2s and an increase in IAAPs.…”

“…Recently, we have identified the equivalent of the mouse IAAPs in the human lungs [ 27 ]. These cells were amplified in the context of end-stage IPF, concomitant with a significant decrease in the number of mature AT2s.…”

“…These cells were amplified in the context of end-stage IPF, concomitant with a significant decrease in the number of mature AT2s. In addition, an in vitro culture of precision cut lung slides from donor lungs demonstrated the emergence of HTII-280 Neg PD-L1 Pos cells (likely the IAAPs) and HTII-280 Pos PD-L1 Pos cells (IAAPs differentiating into AT2s) [ 27 ]. We also reported recently that in mice, IAAPs are activated upon injury, upregulate Fgfr2b expression and proliferate.…”