Lung regeneration with rat fetal lung implantation and promotion of alveolar stem cell differentiation by corticosteroids.

Introduction: The lung is a difficult organ to regenerate, and the development of functional lungs has still not been achieved. In this study, we investigated lung regeneration using a rat fetal lung tissue-implanted model. This study aimed to evaluate the functioning of the implanted fetal lung tissue and investigate the graft differentiation and maturation mechanism, focusing on alveolar stem cells. Methods: Fetal lung tissue fragments were obtained from Lewis rats on day 17 and implanted into adult lungs. Animals were divided into the following three groups: group 1, injection into the adult left lung parenchyma; group 2, injection with post-caval lobectomy; and group 3, injection with post-caval lobectomy and corticosteroid administration. Computed tomography was performed on weeks 1, 2, 4, and 8. The presence of alveolar pore, CD31 expression, and bipotential progenitor cell (podoplanin+/surfactant protein C+) localization were histologically evaluated. MiRNA expression was comprehensively compared among the three groups. Results: The grafts comprised type I and type II alveolar cells connected to the recipient lungs with alveolar pores and capillary networks in the interstitial tissue. The alveolar space was the largest and the computed tomography value was the lowest in the grafts of the corticosteroid-administered group. The number of bipotential progenitor cells was the lowest in the corticosteroid administration group on day 7. Moreover, microRNA-487-3p, 374-5p, and 20b-5p expression was changed by more than 2-fold between the post-caval lobectomy and corticosteroid administration groups. Conclusions: Implanted fetal lung tissues established airway and capillary communication with the recipient lungs, and corticosteroids accelerated their maturation by promoting the differentiation of progenitor cells. The study findings provide new insights into lung regeneration research.