Cell-based therapy for neonatal lung diseases

Alveolar stem cells

It is now well appreciated that alveolar type 2 epithelial cells (AEC2) are the stem cells for the alveoli. Not only are AEC2 capable of self-renewal but they also replenish the alveolar type 1 epithelial cells (AEC1) population under both steady state and injury conditions. In 3D culture, AEC2 form alveolospheres containing cells which express both AEC2 and AEC1 markers. Subsets of AEC2, however, have variable regenerative potential. In whole lung and primary cultures of adult rat AEC2, a hyperoxia-resistant subpopulation of telomerase-positive AEC2 expands in response to injury. In contrast, a subpopulation of AEC2 with high surface levels of telomerase but low E-cadherin are more proliferative and less likely to undergo hyperoxic damage as compared to AEC2 which express high levels of E-cadherin. More recent evidence suggests that a rare population of AEC2 that express Axin2 ⁺ are the main alveolar progenitor cells of the distal lung. Axin2 ⁺ AEC2 are in close proximity to fibroblasts expressing Wnt ligands. During severe lung injury, Axin2 ⁺ AEC2 proliferate in response to Wnt ligand and transiently expand the stem cell pool for repair. On the other hand, in the absence of Wnt ligand, AEC2 differentiate into AEC1.

Other rare populations of undifferentiated basally located distal airway stem cells expressing cytokeratin 5, transcription factor 63, and cytokeratin 14 have been also identified as potential alveolar progenitors. These cells originate from SOX2 ⁺ airway progenitors, are inactive under steady-state conditions but proliferate and migrate into the alveoli following severe influenza-induced lung injury, differentiating into both airway and alveolar lineages. Moreover, in an ovine chorioamnionitis model, acute intrauterine inflammation reduces lung P63 ⁺ alveolar progenitor cell population. The role of P63 ⁺ alveolar progenitor cells in neonatal lung disease, however, remains poorly understood as these cells do not give rise to AEC.

Rare populations of epithelial cells located at the bronchoalveolar duct junction, so-called bronchoalveolar stem cells (BASCS), are another candidate alveolar progenitor population. These cells express both surfactant protein C (SP-C) and secretoglobulin ( Scgb1a1), are resistant to naphthalene, and proliferate rapidly after bleomycin-induced lung injury. Although BASCs are unchanged in hyperoxia-induced lung injury, BASCs are able to give rise to AEC2 cells following bleomycin-induced lung injury. ^, In specialized culture media, BASCs give rise to cells which express pro–SP-C, aquaporin 5, and Scgb1a1, suggesting that these cells could give rise to cells in the conducting as well as gas exchange portion of the lung. More research is however needed to elucidate the regenerative capacity of these alveolar progenitor cells in BPD, their interactions with cells in the lung vasculature and mesenchymal compartment, and the molecular mechanisms that drive these alveolar progenitors toward quiescence or activation.

Lung mesenchymal stem cells

Emerging evidence suggests that the fetal lung is enriched with MSCs. Lung MSCs are similar to bone marrow (BM)–derived MSCs in their adherence to plastic, expression of cell surface markers such as CD105, CD90, and CD73, and their capacity to differentiate into osteoblasts, chondroblasts, and adipocytes under appropriate in vitro conditions. However, compared with BM-MSCs, lung MSCs seem to be constitutively more prone to epithelial differentiation and have a distinct pattern of Hox gene expression implicated in lung development. Lung MSCs produce proangiogenic and epithelial cell growth factors and appear to be directly involved in regulating the growth and function of endothelial and epithelial cells. ^, Lung MSCs are decreased in newborn rodents exposed to hyperoxia ^, and supraphysiologic oxygen levels alter the morphology and secretome of lung MSCs. Unlike steady-state conditions, lung MSCs exposed to oxygen secrete less proangiogenic factors and have increased expression of myofibroblast markers. Intriguingly, increased MSCs in the tracheal aspirate of preterm infants predict an increased risk for BPD by more than 25-fold. Moreover, MSCs isolated from the lungs of preterm infants differentiate into myofibroblasts under the influence of the profibrotic factor, transforming growth factor-β. Conceivably, lung microenvironmental cues may influence MSC behavior and function, potentially driving them toward a more dysfunctional fibrotic phenotype.