Mac-1 and F4/80 Surface Markers Are Present on Murine Hematopoietic Stem Cells.

Transplanted bone marrow donor cells with tissue specific phenotypes have been found in the brain, liver, heart, skin, lung, kidney, and gut of transplanted humans and mice. Such observations have led to the controversial hypothesis that hematopoietic stem cells (HSC) might be intrinsically plastic, and through transdifferentiation or fusion lead to the repair of damaged tissues throughout the body. Alternately, it is suggested that fusion of macrophages to the recipient cells may explain this phenomenon. We have shown recently that purified HSC are the cells responsible for GFP positive donor-derived muscle fibers in the recipient mice post bone marrow transplantation. However, further studies sorting for macrophage markers Mac-1 and F4/80 also resulted in donor-derived muscle fibers in the host. To address this discrepancy, we investigated subpopulations of Mac-1 and F4/80 positive cells, in the presence or absence of stem cell markers (Sca-1 and C-kit). We demonstrate that only the subpopulations of Mac-1 and F4/80 positive cells harboring stem cell markers, Sca-1 or c-kit, were capable of contributing to the regenerating muscle post transplantation. Furthermore, these same subpopulations demonstrated single cell High Proliferative Potential (HPP) (6–26%) in a 7 factor cytokine cocktail, compared to the Mac-1 or F4/80 cells with no stem cell markers (0%). Additionally, they demonstrated long-term engraftment in all three lineages at 1-year (average chimerism of 55% versus 0% in stem cell marker negative groups). These subpopulations were also evaluated for morphology using Hematoxylin/Eosin (H/E), Wright-Giemsa, and Nonspecific Esterase staining. In the Mac-1 and F4/80 positive groups, those negative for stem cell markers resembled differentiated cells of the myeloid origin (macrophages, granulocytes), while those with positive stem cell markers demonstrated stem cell characteristics. We did not observe any engraftability, donor-derived muscle fibers, or HPP potential for CD14 or cfms positive cells coexpressing stem cell markers, indicating that these markers are more appropriate for identifying macrophages. In conclusion, our studies demonstrate that both Mac-1 and F4/80 surface markers are present on HSC and therefore caution must be taken in the interpretation of data using these macrophage markers. It is reasonable to believe that the use of Mac-1 and/or F4/80 surface markers in a lineage depletion process may result in the loss of a subpopulation of stem cells, and other markers such as CD14 or c-fms may be more appropriate for eliminating differentiated macrophages.