PD-L1 mediates lung fibroblast to myofibroblast transition through Smad3 and β-catenin signaling pathways

Programmed death ligand-1 (PD-L1) is an immune checkpoint protein that has been linked with idiopathic pulmonary fibrosis (IPF) and fibroblast to myofibroblast transition (FMT). However, it remains largely unclear how PD-L1 mediates this process. We found significantly increased PD-L1 in the lungs of idiopathic pulmonary fibrosis patients and mice with pulmonary fibrosis induced by bleomycin and TGF-beta. In primary human lung fibroblasts (HLFs), TGF-beta induced PD-L1 expression that is dependent on both Smad3 and p38 pathways. PD-L1 knockdown using siRNA significantly attenuated TGF-beta-induced expression of myofibroblast markers alpha-SMA, collagen-1, and fibronectin in normal and IPF HLFs. Further, we found that PD-L1 interacts with Smad3, and TGF-beta induces their interaction. Interestingly, PD-L1 knockdown reduced alpha-SMA reporter activity induced by TGF-beta in HLFs, suggesting that PD-L1 might act as a co-factor of Smad3 to promote target gene expression. TGF-beta treatment also phosphorylates GSK3 beta and upregulates beta-catenin protein levels. Inhibiting beta-catenin signaling with the pharmaceutical inhibitor ICG001 significantly attenuated TGF-beta-induced FMT. PD-L1 knockdown also attenuated TGF-beta-induced GSK3 beta phosphorylation/inhibition and beta-catenin upregulation, implicating GSK3 beta/beta-catenin signaling in PD-L1-mediated FMT. Collectively, our findings demonstrate that fibroblast PD-L1 may promote pulmonary fibrosis through both Smad3 and beta-catenin signaling and may represent a novel interventional target for IPF.

Automated summary

PD-L1 may promote pulmonary fibrosis through both Smad3 and beta-catenin signaling, and may represent a novel interventional target for IPF.