Epithelial stem cell-mediated development of the human respiratory mucosa in SCID mice

We have developed an in vivo assay for progenitor cells of the human tracheobronchial epithelium relying on the transplantation of human prenatal respiratory tissues into severe combined immunodeficiency mice. Engrafted embryonic or fetal open tracheobronchial rudiments are rapidly closed at each end by a neoformed membrane that we named the operculum. After 2-4 weeks, differentiated human respiratory epithelium covers both the native airway matrix and the new operculum. Human epithelial cells dissociated from either emerging embryonic lung primordia or mature xenografts were seeded in host human airway grafts, of which native epithelium had been eliminated by several cycles of freezing and thawing. All grafts seeded with donor epithelial cells and implanted back into SCID mice recovered a surface mucociliary epithelium expressing expected markers and secreting mucus. Spontaneous epithelium regrowth was never observed in control unseeded, denuded grafts. In some experiments, donor epithelial cells and host denuded airway were sex-mismatched and the donor origin of newly formed epithelial structures was confirmed by sex chromosome detection. After two rounds of seeding and reimplantation, a normal epithelium was observed to line the 3rd generation operculum. These observations substantiate a functional assay for human candidate airway epithelium stem cells.