Expression of milk fat globule epidermal growth factor 8 in immature dendritic cells for engulfment of apoptotic cells

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Abstract
Milk fat globule epidermal growth factor 8 (MFG-E8) is a protein that stimulates the engulfment of apoptotic cells by phagocytes. Here, we show that mouse immature dendritic cells (DC) generated in vitro by culturing bone marrow progenitors in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF), and Langerhans cells present in the skins, expressed MFG-E8. Bone marrow-derived macrophages generated by M-CSF did not express MFG-E8. MFG-E8 expressed in immature DC was found to be secreted as exosomes. The expression of MFG-E8 was significantly suppressed when the immature DC were induced to mature by treating them with lipopolysaccharides. This expression of MFG-E8 was well correlated with the ability of the cells to engulf apoptotic cells. That is,immature DC phagocytosed apoptotic cells more efficiently than did mature DC or bone marrow-derived macrophages. The ability of immature DC to engulf apoptotic cells was severely reduced when the immature DC were prepared from MFG-E8-deficient mice. These results indicated that MFG-E8 plays an essential role in the engulfment of apoptotic cells by bone marrow-derived immature DC.