An Apoptosis-Associated Mammary Protein Deficiency Leads to Enhanced Production of IgM Antibodies against Multiple Damage-Associated Molecules

Milk fat globule epidermal growth factor 8 (MFG-E8) is a protein that binds to apoptotic cells by recognizing phosphatidylserine and enhances the engulfment of apoptotic cells by macrophages. Many apoptotic cells are left unengulfed in the germinal centers of the spleen in the MFG-E8-deficient (MFG-E8^−/−) mice, and these mice develop an autoimmune disease resembling human systemic lupus erythematosus. We found that the MFG-E8 deficiency was accompanied by the increased production of immunoglobulins. Further Western blot and ELISA analyses validated the increase in the IgM levels in the MFG-E8^−/− mice. It was also revealed that the sera from the MFG-E8^−/− mice cross-reacted with oxidation-specific epitopes generated upon incubation of serum albumin with the peroxidized lipids. Among the modified proteins with several unsaturated aldehydes of chain lengths varying from three to nine carbons, the MFG-E8^−/− mice sera exclusively cross-reacted with the protein-bound 4-oxo-2-nonenal (ONE), a highly reactive aldehyde originating from the peroxidation of ω6 polyunsaturated fatty acids. In addition, the IgM monoclonal antibodies (mAbs) that selectively cross-reacted with the ONE-modified proteins were generated from the MFG-E8^−/− mice. A subset of the ONE-specific IgM mAbs significantly recognized the late apoptotic and necrotic cells and enhanced the phagocytosis by macrophages. These data demonstrate that the impairment of the phagocytic clearance of apoptotic cells through MFG-E8 can lead to the generation of natural antibodies, which may play a critical role in removing multiple damage-associated molecules, including oxidation-specific epitopes and late apoptotic/necrotic cells.