Flt3+ macrophage precursors commit sequentially to osteoclasts, dendritic cells and microglia

Mouse bone marrow cells were expanded in vitro in the presence of Flt3-ligand (FL), yielding high numbers of non-adherent cells exhibiting immature monocyte characteristics. Cells expanded for 6 days, 8 days, or 11 days (day 6-FL, day 8-FL, and day 11-FL cells, respectively) exhibited constitutive potential towards macrophage differentiation. In contrast, they showed time-dependent potential towards osteoclast, dendritic, and microglia differentiation that was detected in day 6-, day 8-, and day 11-FL cells, in response to M-CSF and receptor activator of NFκB ligand (RANKL), granulocyte-macrophage colony stimulating-factor (GM-CSF) and tumor necrosis factor-α (TNFα), and glial cell-conditioned medium (GCCM), respectively. Analysis of cell proliferation using the vital dye CFSE revealed homogenous growth in FL-stimulated cultures of bone marrow cells, demonstrating that changes in differential potential did not result from sequential outgrowth of specific precursors.We propose that macrophages, osteoclasts, dendritic cells, and microglia may arise from expansion of common progenitors undergoing sequential differentiation commitment. This study also emphasizes differentiation plasticity within the mononuclear phagocyte system. Furthermore, selective massive cell production, as shown here, would greatly facilitate investigation of the clinical potential of dendritic cells and microglia.The mononuclear phagocyte system encompasses a widely distributed family of related cells exhibiting highly specialized functions such as macrophages, osteoclasts, dendritic cells, and microglia. Resident macrophages, found in most organs and connective tissues, serve as professional phagocytes, removing pathogens or apoptotic cells [1]. Microglia represents a unique category of mononuclear phagocytes distributed throughout the central nervous system (CNS) parenchyma in both white and grey matter [2]. Microglial cells share a number of immunological markers with other mononuclear phagocy