BACKGROUND: As a fatal interstitial lung disease, idiopathic pulmonary fibrosis (IPF) was characterized by the insidious proliferation of extracellular matrix (ECM)-producing mesenchymal cells. Recent studies have demonstrated that lung resident mesenchymal/stromal cells (LR-MSC) are the source of myofibroblasts. Endoplasmic reticulum (ER) stress is prominent in IPF lung. This study sought to investigate the effects of ER stress on the behavior of LR-MSC during pulmonary fibrosis. METHODS: ER stress and myofibroblast differentiation of LR-MSC in patients with IPF were evaluated. Primary mouse LR-MSC was harvested and used in vitro for testing the effects of ER stress and C/EBP homologous protein (CHOP) on LR-MSC. Adoptive transplantation of LR-MSC to bleomycin-induced pulmonary fibrosis was done to test the in vivo behavior of LR-MSC and its influence on pulmonary fibrosis. RESULTS: We found that myofibroblast differentiation of LR-MSC is associated with ER stress in IPF and bleomycin-induced mouse fibrotic lung. Tunicamycin-induced ER stress impairs the paracrine, migration, and reparative function of mouse LR-MSC to injured type 2 alveolar epithelial cells MLE-12. Overexpression of the ER stress responder C/EBP homologous protein (CHOP) facilitates the TGFß1-induced myofibroblast transformation of LR-MSC via boosting the TGFß/SMAD signaling pathway. CHOP knockdown facilitates engraftment and inhibits the myofibroblast transformation of LR-MSC during bleomycin-induced pulmonary fibrosis, thus promoting the efficacy of adopted LR-MSC in alleviating pulmonary fibrosis. CONCLUSION: Our work revealed a novel role that ER stress involved in pulmonary fibrosis by influencing the fate of LR-MSC and transformed to "crime factor" myofibroblast, during which CHOP acts as the key modulator. These results indicate that pharmacies targeting CHOP or therapies based on CHOP knockdown LR-MSC may be promising ways to treat pulmonary fibrosis.