Supplementary materials

Deficiency of surfactant protein-C (SPC) increases susceptibility to lung infections and injury, and suppressed expression of SPC has been associated with the severity of acute respiratory distress syndrome (ARDS). Alveolar type 2 epithelial cells (AT2) are critical for maintenance and repair of the lung. However, the role of the SPC in the regulation of AT2 cell lineage and the underlying mechanisms are not completely understood. This study aimed to investigate the mechanisms by which SPC regulates AT2 lineages. Sftpc^−/− mice were used to model the SPC deficiency in ARDS patients. We utilized three-dimensional (3D) organoids to compare AT2 lineage characteristics between wild type (WT) and sftpc^−/− mice by analyzing AT2 proliferation, AT1 differentiation and CD74 expression, using colony-formation assay, RT-qPCR, immunofluorescence, flow cytometry, and immunoblots. The results showed that sftpc^-/- mice demonstrated a reduced AT2 cell population. H1N1-infected sftpc^−/− mice demonstrated reduced AT2 proliferation and AT1 differentiation. RT-qPCR revealed downregulated expression, while Western blot indicated elevated levels of CD74 protein in AT2 cells of sftpc^−/− mice. Colony-forming efficiency was significantly attenuated in AT2 cells isolated from sftpc^−/− mice compared to the WT controls. Podoplanin (PDPN, a marker of AT1 cells) expression and transient cell count significantly increased in sftpc^−/−organoids. Moreover, siRNA-mediated gene silencing of CD74 in AT2 cells significantly increased AT2 proliferation and AT1 differentiation in sftpc^−/− organoids. This study suggests that SPC regulates AT2 lineage <i>in vitro</i> and <i>in vivo</i>. The SPC might influence AT2 lineage during the lung epithelium repair by activating signaling pathways involving CD74 receptor.