c-Kit⁺ Cells in Adult Salivary Glands do not Function as Tissue Stem Cells

A rare population of salivary gland cells isolated based on c-Kit immunoreactivity are thought to represent tissue stem cells since they exhibit the most robust proliferative and differentiation capacity ex vivo. Despite their high promise for cell-based therapies aimed at restoring salivary function, the precise location and in vivo function of c-Kit⁺ stem cells remain unclear. Here, by combining immunostaining with c-KitCre^ERT2-based genetic labeling and lineage tracing in the adult mouse salivary glands, we show that c-Kit is expressed in a relatively large and heterogeneous cell population that consists mostly of differentiated cells. Moreover, c-Kit does not mark ductal stem cells that are known to express cytokeratin K14. Tracking the fate of in vivo-labeled c-Kit⁺ or that of K14⁺ cells in spheroid cultures reveals a limited proliferative potential for c-Kit⁺ cells and identifies K14⁺ cells as the major source of salispheres in these cultures. Long-term in vivo lineage tracing studies indicate that although c-Kit marks at least two discrete ductal cell lineages, c-Kit⁺ cells do not contribute to the normal maintenance of any other cell lineages. Our results indicate that c-Kit is not a reliable marker for salivary gland stem cells, which has important implications for salivary gland regenerative therapies.