β-2-glycoprotein 1-dependent macrophage uptake of apoptotic cells: Binding to lipoprotein receptor-related protein receptor family members

The recognition and removal of apoptotic cells is critical to development, tissue homeostasis, and the resolution of inflammation. Many studies have shown that phagocytosis is regulated by signaling mechanisms that involve distinct ligand-receptor interactions that drive the engulfment of apoptotic cells. Studies from our laboratory have shown that the plasma protein β-2-glycoprotein 1 (β2GP1), a member of the short consensus repeat superfamily, binds phosphatidylserine-containing vesicles and apoptotic cells and promotes their bridging and subsequent engulfment by phagocytes. The phagocyte receptor for the protein/apoptotic cell complex, however, is unknown. Here we report that a member of the low density lipoprotein receptor-related protein family on phagocytes binds and facilitates engulfment of β2GP1-phosphatidylserine and β2GP1-apoptotic cell complexes. Using recombinant β2GP1, we also show that β2GP1-dependent uptake is mediated by bridging of the target cell to the phagocyte through the protein C- and N-terminal domains, respectively.