Macrophage surface expression of annexins I and II in the phagocytosis of apoptotic lymphocytes

When cells undergo apoptosis, or programmed cell death, they expose phosphatidylserine (PS) on their surface. Macrophages that efficiently phagocytose apoptotic cells also express PS on their surface, although at a lower level. The PS exposed on both cells is required for phagocytosis, because uptake is inhibited by masking PS on either cell with annexin V, a PS-binding protein. The inhibition is not additive, suggesting that the exposed PS molecules on the two cells participate in a common process. We asked whether this dual requirement reflects bridging of the target cell and macrophage by bivalent, PS-binding annexins. Monoclonal antibodies (mAbs) against annexins I or II stained a variety of live phagocytes. Apoptotic Jurkat T lymphocytes and human peripheral T lymphocytes, but not apoptotic thymocytes, were stained by anti-annexin I but not II. Phagocytosis of apoptotic targets was inhibited by mAbs to annexins I or II, or by pretreatment of macrophages with the same mAbs. Pretreatment of apoptotic thymocytes had no effect, whereas pretreating Jurkat cells with anti-annexin I or removing annexin I with EGTA was inhibitory. Annexin bridging is vectorial, because annexin is bound to PS molecules on targets but not on macrophages, suggesting annexins serve as both ligand and receptor in promoting phagocytosis.