RANKL contributes to lung tissue repair via promoting type II epithelial cell proliferation

Higher levels of receptor activator for NF-κβ ligand (RANKL) are found in serum of emphysema patients. RANKL is a well-known stimulator of bone tissue degradation, possibly explaining the association of COPD with osteoporosis. However, RANKL is also reported to be involved in epithelial cell regeneration in breast and thymus. Given RANKL is produced directly in lung tissue, we hypothesized a role for RANKL in lung epithelial cell regeneration. The aim of this study was to elucidate the role of RANKL in lung epithelial repair. Mouse soluble RANKL (sRANKL) treatment of murine precision-cut lung slices resulted in a higher number of proliferating cells compared to untreated controls. We also found that sRANKL stimulated proliferation of type II alveolar (A549 cells) but not airway (16HBE) epithelial cells. Using an organoid model of epithelial development by co-culturing primary EpCAM+ cells with fibroblasts, we found higher numbers of alveolar organoids in cultures derived from murine epithelial cells upon sRANKL treatment compared to control. Importantly, this effect was similar in RANKL-treated organoids derived from epithelial cells isolated from lung tissue of COPD patients. The effect of sRANKL was abrogated upon addition of osteoprotegerin, the soluble, inhibitory, receptor for RANKL. In conclusion, we found that sRANKL promotes type II alveolar epithelial cell proliferation and may therefore contribute to lung tissue repair. Our data suggest that the higher levels of RANKL found in emphysema, may reflect an attempt at epithelial repair by the lung to counteract the lung tissue destruction that characterizes emphysema.