Differentiation of CD45‑/CD31+ lung side population cells into endothelial and smooth muscle cells in vitro.

Side population (SP) cells are a small subpopulation of cells found in many mammalian tissues and organs, identified by their capacity to efflux Hoechst 33342 dye. They are enriched for stem/progenitor cell activity. SP cells isolated from the adult mouse lung can be separated into a CD45+ subset (bone marrow‑derived) and a CD45‑ subset that can be subdivided into CD31‑ and CD31+ subpopulations. CD45‑/CD31‑ lung SP (LSP) cells are known to be mesenchymal stem cells. However, CD45‑/CD31+ LSP cells are not fully characterized. In the present study, it was found that CD45‑/CD31+ LSP cells were able to form colonies. Based on the expression of vascular endothelial growth factor receptor 2 (VEGFR2), these cells were separated into VEGFR2‑ and VEGFR2+ cells. The CD45‑/CD31+/VEGFR2‑ LSP cells expressed genes characteristic of smooth muscle and endothelial progenitors, and were able to differentiate into smooth muscle and endothelial cells in vitro. The CD45‑/CD31+/VEGFR2+ LSP cells expressed genes characteristic of endothelial progenitors and gave rise to endothelial cells, although not smooth muscle, in vitro. The data demonstrate that CD45‑/CD31+/VEGFR2‑ LSP cells differentiated into CD45‑/CD31+/VEGFR2+ LSP cells and then endothelial cells, indicating that CD45‑/CD31+/VEGFR2+ LSP cells are likely to be derived from CD45‑/CD31+/VEGFR2‑ LSP cells. Taken together, the results suggest that CD45‑/CD31+ LSP cells can be separated into CD45‑/CD31+/VEGFR2‑ LSP cells, which may be progenitors of endothelial and smooth muscle, whereas CD45‑/CD31+/VEGFR2+ LSP cells may serve as late commitment endothelial progenitors in the adult mouse lung.