Expanding the Pool of Stem Cell Therapy for Lung Growth and Repair
The lung’s regenerative capacity resides within long-lived stem cells that can divide, self-renew, and differentiate to repair injured tissue or cell loss and maintain normal homeostasis. The ability to enhance endogenous stem cell capacity to regenerate lung tissue is the key to the treatment of a multitude of debilitating lung diseases such as bronchopulmonary dysplasia (BPD), idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, pulmonary arterial hypertension, and other acute and chronic ailments of the lung. The challenge lies in identifying the progenitors of a tissue and, in the case of the lung, understanding the complex interactions lung progenitors have with the unique environment of an air–liquid interphase, proximal and distal airways, the intricate vascular tree, and the innate immune response. Indeed, many different lung stem/progenitor cells have been described, and their identity and role in lung regeneration continue to be debated.
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In the past decade, endothelial progenitor cells (EPCs) have been isolated from the lungs of animals and shown to have vasculogenic activity. Vasculogenesis was thought to occur only in the yolk sac of the developing embryo, but this dogma was challenged in 1997 by Asahara and coworkers1 who first reported the isolation and characterization of putative endothelial progenitor cells from human peripheral blood, showing they can differentiate into mature endothelial cells and be incorporated in the vessels of animal models of disease. Lung vascular development is closely linked with and may drive lung growth and airway development through the release of endothelial-derived angiogenic factors that induce the proliferation of epithelial progenitor cells to support lung alveolization.2 Hence, lung EPCs may play a critical role not only in normal lung development, but also in lung injury and repair to restore endothelial cell function and maintain homeostasis. By extension, the inability of lung EPCs to …