Figure 1. Identification of an Axin2⁺ alveolar epithelial progenitor (AEP) in the adult lung that regenerates a substantial percentage of the alveolar epithelium.

(A) Schematic of Axin2^creERT2:TdT:R26R^EYFP mice. EYFP is detected by an anti-GFP antibody. Lineage tracing experimental design is as indicated. (B–D) Axin2 marks a subset of AT2 cells. Unmarked = white arrowheads. AEP-marked = yellow arrowheads. D shows orthogonal view of C. (E) Hopx⁺ AT1 cells are not marked by EYFP. (F) Approximately 20% of AT2 cells express Axin2. (G–H) Epithelial Wnt responsiveness is stable for up to 9 months. The majority of the AEP lineage remains Axin2^TdTomato+, while some AEP progeny lose Axin2^TdTomato+ expression. Very few Sftpc⁺/Axin2⁻ cells gain Axin2^TdTomato+ expression. Red arrow indicates an Axin2⁺ mesenchymal cell. (I) Influenza-induced lung injury results in regionalized alveolar damage: minimal (Zone 1), mild (Zone 2), severe (Zone 3), or complete (Zone 4). (J–L) AEP-generated Sftpc⁺ cells (J–K) and Hopx⁺ AT1 cells (L) expand in Zones 2 and 3. (M) Ki67⁺ AEPs preferentially re-enter the cell cycle in areas of regeneration. (N) AEPs can self-renew (YFP⁺/RFP⁺) while regenerating a significant number of AT2 cells (YFP⁺/RFP⁻), but very few non-AEP cells acquire Axin2 expression (YFP⁻/RFP⁺). (O) A region of regenerated lung epithelium near a persistent Krt5⁺ pod. Black line shows border of Krt5⁺ pod. Yellow dotted line indicates region of regeneration. (P–Q) A large number of new AEP-derived AT1 and AT2 cells are found within 3 alveolar units (regenerated Zone 3) of Krt5⁺ pods. N=5 (M,N), N=6 (F–H, O–P), or N=10 (others) animals from 2 (G–H, O–P) or 3 (others) individual experiments. Statistics are representative of all biological replicates. Plots are centered on mean with bars indicating SD. *=p<0.05, **=p<0.01, ***=p<0.001, ****=p<0.0001 by two-tailed T-test (E, P–Q) or ANOVA with adjustment for multiple comparison testing (others). Scale bars: B=100μm, C–E, G, J, O=50μm.