Identification of poly-reactive natural IgM antibody that recognizes late apoptotic cells and promotes phagocytosis of the cells

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Abstract
Natural IgM can recognize apoptotic cells, but the molecular structure and the role in macrophage phagocytosis of apoptotic cells remain unclear. Objectives(1) To examine the binding of previously isolated natural IgM (3B4) to apoptotic cells and its effects on phagocytosis of apoptotic cells. (2) To characterize the molecular structure of 3B4. Methods:3B4 binding to apoptotic thymocytes was examined by flow cytometry. Polyreactivity of 3B4 was assayed by ELISA. PKH26-labeled Macrophages were incubated with PKH67-stained apoptotic cells in the presence of 3B4. Macrophages phagocytosis of apoptotic cell was evaluated by flow cytometry. The DNA segments of 3B VH and VK were sequenced and analyzed. Results:3B4 IgM recognized late apoptotic cells. Polyreactive-recognitions of lysophosphatidylcholine (LPC) as well as some autoantigens were observed in 3B4. Phagocytosis of late apoptotic cells was increased in the presence of 3B4. The VH and VK genes of 3B4 showed a germline gene context, while N-sequences and nucleotide loss were observed in CDR3. Conclusion: 3B4 promotes macrophage phagocytosis of late apoptotic cells in a complement-independent process. 3B4 has a germline configuration and is possibly ligand-selected. Out experiments suggest an independent role of natural IgM as opsonin in clearance of late apoptotic cells.