Monoclonal antibodies against oxidized low-density lipoprotein bind to apoptotic cells and inhibit their phagocytosis by elicited macrophages: Evidence that oxidation-specific epitopes mediate macrophage recognition

Apoptosis is recognized as important for normal cellular homeostasis in mul ticellular organisms. Although there have been great advances in our knowle dge of the molecular events regulating apoptosis, much less is known about the receptors on phagocytes responsible for apoptotic cell recognition and phagocytosis or the ligands on apoptotic cells mediating such recognition. The observations that apoptotic cells are under increased oxidative stress and that oxidized low-density lipoprotein (OxLDL) competes with apoptotic c ells for macrophage binding suggested the hypothesis that both OxLDL and ap optotic cells share oxidatively modified moieties on their surfaces that se rve as ligands for macrophage recognition. To test this hypothesis, we used murine monoclonal autoantibodies that bind to oxidation-specific epitopes on OxLDL. in particular, antibodies EO6 and EO3 recognize oxidized phosphol ipids, including l-palmitoyl 2-(5-oxovaleroyl) phosphatidylcholine (POVPC), and antibodies EO12 and EO14 recognize malondialdehydelysine, as in malond ialdehyde-LDL. Using FAGS analysis, we demonstrated that each of these EO a ntibodies bound to apoptotic cells but not to normal cells, whereas control IgM antibodies did not. Confocal microscopy demonstrated cell-surface expr ession of the oxidation-specific epitopes on apoptotic cells. Furthermore, each of these antibodies inhibited the phagocytosis of apoptotic cells by e licited peritoneal macrophages, as did OxLDL. In addition, an adduct of POV PC with BSA also effectively prevented phagocytosis. These data demonstrate that apoptotic cells express oxidation-specific epitopes-including oxidize d phospholipids-on their cell surface, and that these serve as ligands for recognition and phagocytosis by elicited macrophages.