Evidence for a lipofibroblast-to-Cthrc1pos myofibroblast reversible switch during the development and resolution of lung fibrosis in old mice
Idiopathic pulmonary fibrosis (IPF) is frequently diagnosed in elderly men. IPF is characterized by irreversible accumulation of excessive extracellular matrix components by activated myofibroblasts (aMYFs) leading to lung failure. Lineage tracing of alpha smooth muscle actin-positive (ACTA2+) cells in the context of bleomycin-induced pulmonary fibrosis in young mice has shown that activated myofibroblasts (aMYFs) are heterogeneous population. However, aMYFs heterogeneity and as well as their fate during fibrosis resolution in aged mice is still studied. Tg(Acta2-CreERT2), tdTomatoflox, 52-56 week-old mice were used to identify Acta2+ cells following bleomycin treatment (Bleo-Tam condition). At the peak of fibrosis formation (day 14) as well as during resolution (days 30 and 60), the origin and fate of Acta2+ cells converging on the aMYF/Cthrc1+ myofibroblast lineage were examined using scRNA-seq.  Our findings demonstrate that at the peak of fibrosis day 14, aMYF/Cthrc1+ cells significantly recruited. Four subclusters within aMYF/Cthrc1+ exhibit distinct pro-fibrotic vs pro-alveologenic characteristics. During fibrosis formation and resolution, alveolar fibroblasts with a strong LIF-like signature are primarily responsible for the origin and fate of aMYFs. Finally, our findings indicate that a LIF-to-aMYF reversible switch is observed during the development and resolution of fibrosis in both young and old mice. Overall design: Tg(Acta2-CreERT2), tdTomatoflox, 52-56 weeks old mice were used to identify Acta2+ cells following bleomycin treatment (Bleo-Tam condition), at the peak of fibrosis formation (day 14) as well as during resolution (days 30 and 60).