Inflammatory responses in the initiation of lung repair and regeneration: their role in stimulating lung resident stem cells

The lungs are the primary organs for respiration, the process by which carbon dioxide and oxygen are exchanged. The alveolus, which is the site of gas exchange in the lungs, consists of multiple cell types including alveolar epithelial cells, lung capillary endothelial cells and fibroblasts. Because of their complexity, lung parenchymal cells including epithelial lineage have been thought to have a lower rate of cellular turnover in adult lung. However, accumulating observations suggest that the turnover of parenchymal cells in adult lungs is essential for maintaining homeostasis during the steady state as well as for the repair and regeneration after lung injury. After lung injury by harmful pathogens, inflammation occurs to protect the host. Although excessive inflammation damages lung tissue, inflammatory cells are essential for regeneration because they remove harmful pathogens as well as debris derived from apoptotic and necrotic cells. In addition, subsets of inflammatory cells, especially phagocytic monocytes, produce cytokines and growth factors to resolve inflammation and promote tissue regeneration by stimulating tissue-resident stem cells. Recent advances in the biology of lung-resident stem cells, especially those addressing epithelial lineage, have revealed that there are several cellular populations capable of self-renewal that can differentiate into airway and/or alveolar epithelial cells. A part of these populations does not exist in the steady state but emerges after lung injury, suggesting that signals induced by inflammation may play an important role in initiating the proliferation and differentiation of lung stem or progenitor cells. Understanding the interaction between inflammatory responses and tissue-resident stem cells would help elucidate the pathogenesis of inflammatory lung diseases and promote the discovery of new therapeutic targets.