Lung stem and progenitor cells in tissue homeostasis and disease

Abstract

The mammalian lung is a complex organ containing numerous putative stem/progenitor cell populations that contribute to region-specific tissue homeostasis and repair. In this review, we discuss recent advances in identifying and studying these cell populations in the context of lung homeostasis and disease. Genetically engineered mice now allow for lineage tracing of several lung stem and progenitor cell populations in vivo during different types of lung injury repair. Using specific sets of cell surface markers, these cells can also be isolated from murine and human lung and tested in 3D culture systems and in vivo transplant assays. The pathology of devastating lung diseases, including lung cancers, is likely in part due to dysregulation and dysfunction of lung stem cells. More precise characterization of stem cells with identification of new, unique markers; improvement in isolation and transplant techniques; and further development of functional assays will ultimately lead to new therapies for a host of human lung diseases. In particular, lung cancer biology may be greatly informed by findings in normal lung stem cell biology as evidence suggests that lung cancer is a disease that begins in, and may be driven by, neoplastic lung stem cells.

Keywords: Cell-based therapy; Embryonic stem cells; Lung cancer; Lung cancer stem cells; Lung stem cells; Mesenchymal stromal cells; iPS cells.

Figure 8.1

Cell types in the Lung. The proximal region of the murine respiratory system is lined by a pseudostratified epithelium containing secretory CCSP+ Clara cells, mucus-producing goblet cells, and host-defending FoxJ1/Actub+ ciliated cells. Variant Clara cells are thought to give rise to ciliated and Clara cell lineages after injuries such as naphthalene, and are enriched within the EpCAM^hi/Sca1^lo/Auto-fluorescence^lo cells. At the basal edge of the epithelium are the NGFR+/p63+ basal cells, which are thought to be able to give rise to Clara and ciliated cells during repair and in culture. In the more distal bronchioles, ciliated and Clara cells are interspersed with CGRP+ neuroendocrine cells. Alveolar epithelial type 1 cells (AT1 cells), which express T1α and Aquaporin5, and SPC+ alveolar epithelial type 2 cells (AT2 cells) line the alveolar space where gas exchange takes place. An alveolar progenitor cell has been identified that can give rise to AT2 and AT1 cells after injuries such as bleomycin, and is termed integrin α6β4+. At the brochioalveolar duct junction, a rare cell population termed the brochioalveolar stem cells (BASC) coexpresses both CCSP and SPC, and is enriched in the CD24^lo/Sca1^lo/EpCAM+/integrin α6+ fraction of lung epithelial cells. BASCs are thought to be able to give rise to both Clara and AT2 cell lineages after injury. Alveolar epithelial cells and BASCs are closely associated with mesenchymal cells such as fibroblasts, the extracellular matrix (ECM), and CD31+ endothelial cells. Useful markers for FACS isolation of cell types are indicated.

Figure 8.2

Orthotopic serial transplant assay for lung cancer stem cells. Using genetically engineered mouse models (GEMMs) of lung cancer, tumors with defined genetic characteristics can be induced. These primary tumors can be resected, enzymatically dissociated, and sorted, based on cell surface marker expression. Fluorescence-activated cell sorting (FACS) is typically used to separate tumor cell types. Antibodies against CD31 and CD45 can be used to exclude endothelial cells and hematopoietic cells, respectively, while the remaining tumor cell population can be fractionated using putative cancer stem cell markers such as Sca-1 (x-axis of representative FACS plot shown). FACS-isolated populations can then be introduced into the lungs of immunocompromised mice, such as nude mice, and cancer stem cell activity can be measured by secondary tumor growth. As cancer stem cells are thought to have self-renewal ability, a true cancer stem cell pool should be able to propagate tumors in a serial manner.