CD45+/CD11b+ monocytes are required for mesenchymal stem cell proliferation In Vitro

Mesenchymal stem cells (MSCs) have been used to treat patients with incurable diseases. However, the low yield of MSCs even after long periods of ex vivo culture remains a serious problem. Several attempts were made to increase cell expansion rate of MSCs in cultures. In this study, we examined the stimulatory effect of monocytes on the proliferation and colony forming efficiency of MSCs by co-culturing CD29+/CD45-MSCs with CD45+/ CD11b+ monocytes. In presence of CD45+/CD11b+ monocytes, CD29+/CD45-MSCs proliferated rapidly to form large and compact colonies within 10 days. In the absence of CD45+/CD11b+ monocytes, CD29+/CD45-MSC colony formation was much delayed, indicating that the presence of CD45+/CD11b+ monocytes is required for CD29+/ CD45-MSC proliferation and stemness in vitro. Similar growth-stimulating effect on MSC culture was also found when conditioned medium from CD45+/CD11b+ monocyte cultures was supplied instead of CD45+/CD11b+ monocytes. These results suggest that secretory molecules of CD45+/CD11b+ monocytes may mainly mediate the enhanced growth of CD29+/CD45-MSCs. Collectively, this study reveals that co-culture of CD29+/CD45-MSCs with CD45+/CD11b+ monocytes stimulate MSC growth via paracrine factors, which could improve colony forming capacity of MSC. Furthermore, conditioned medium of CD45+/CD11b+ monocytes may also be developed for cell culture supplement for MSC cell therapeutics.