Our results suggest that the human lung possesses a pool of c-kit–positive cells that have the fundamental properties of stem cells: they are self-renewing, clonogenic, and multipotent in vitro and in vivo. The ability of human lung stem cells to create human bronchioles, alveoli, and pulmonary vessels in the mouse provides evidence in favor of the crucial role that human lung stem cells may have in lung homeostasis and tissue regeneration after injury. These observations challenge the suggestion that the lung is an organ lacking a hierarchical cellular organization21
regulated by a compartment of resident stem cells.13,22–26
Our findings do not rule out or challenge the notion that basal epithelial cells, bronchoalveolar stem cells, Clara cells, side population cells, and type II alveolar epithelial cells are involved in the epithelial-cell response to inflammation or injury caused by aromatic hydrocarbons.6,10,21,27
Similarly, the presence of a human lung stem cell does not rule out the possibility that mature cells within the adult lung dedifferentiate or reprogram themselves to form a committed progeny, a phenomenon shown to be operative in other organs.28–30
For restoration of damaged tissue to occur, cells must be capable of the coordinated formation of both distal airways and distal pulmonary vasculature. Unipotent progenitors with distinct differentiation potential would have to be simultaneously activated to form functionally competent gas-exchange units. This limitation is shared by bronchoalveolar stem cells, Clara cells, side population cells, and type II alveolar epithelial cells, since they generate only type I and type II pneumocytes3,4,6–9,27
or stromal cells.10
Similarly, bone marrow cells expressing Clara-cell secretory protein transdifferentiate and acquire the phenotype of epithelial-cell lineages31
but lack the ability to replace and integrate the other components of the gas-exchange unit.
Our results indicate that human lung stem cells give rise to different populations of epithelial cells of endodermal origin and to pulmonary vessels derived from the mesoderm.32
However, human lung stem cells express four genes — NANOG, OCT3/4, SOX2,
— that constitute the network of transcription factors that governs the pluripotency of human embryonic stem cells.33
The gene products have been found in c-kit–positive cells of the fetal human lung, strengthening the notion that adult human lung stem cells retain characteristics typical of stem cells residing in the developing organ. These transcription factors also promote reprogramming of adult somatic cells into pluripotent stem cells.16
Our study was not designed to improve the respiratory function of animals with pulmonary failure by means of cell implantation. The objective was to identify a class of human lung stem cells that had the ability to form distal airways and vessels in a mouse model of lung injury and to document that the newly formed structures of human origin replaced, in part, the damaged parenchyma and were connected with the vasculature and respiratory system of the recipient mouse organ. These findings, together with the results of our in vitro studies, provide evidence of a resident multipotent stem cell in the human lung. In addition, the in vivo demonstration that resident human lung stem cells divide both symmetrically and asymmetrically, do not express any markers typical of hematopoietic cells, and reconstitute lung tissue where hematopoietic stem cells fail to do so argues against the bone marrow origin of this stem-cell class.
In conclusion, we provide several lines of evidence suggesting the existence of human lung stem cells. Clonal human lung stem cells divided asymmetrically and generated bronchioles, alveoli, and pulmonary vessels of various dimensions, including capillaries, in vivo in a mouse model. Furthermore, human lung stem cells obtained from regenerated lung tissue were able to self-renew and create lung parenchyma in vivo in another mouse with lung damage. The immuno-histochemical identification of newly regenerated pulmonary structures is strengthened by the recognition of human sex chromosomes and human transcripts of epithelial and vascular genes within the regenerated mouse lung.13,25,34
The human X chromosome was detected in regenerated type II alveolar epithelial cells, suggesting the capacity of the human lung stem cells to terminally differentiate in vivo into functional, highly specialized cells that produce and secrete SP-C, a critical determinant of alveolar function and lung performance. Thus, c-kit–positive human lung stem cells show self-renewal, clonogenicity, and multipotentiality.