ANNEXINS I AND II VERSUS β2-GLYCOPROTEIN 1 AS BRIDGING MOLECULES IN THE PHAGOCYTOSIS OF APOPTOTIC CELLS
Open Access
- Author:
- Fan, Xiaoxuan
- Graduate Program:
- Biochemistry, Microbiology, and Molecular Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 21, 2006
- Committee Members:
- Robert Allen Schlegel, Committee Chair/Co-Chair
Andrew Thomas Henderson, Committee Member
Avery August, Committee Member
Andrea Marie Mastro, Committee Member
Mary J Kennett, Committee Member - Keywords:
- β2-glycoprotein 1
annexins
phosphatidylserine
phagocytosis
T lymphocytes - Abstract:
- Phosphatidylserine (PS) has been found on apoptotic cells and macrophages. PS on both cells is required for phagocytosis. It was asked whether bivalent annexins, which bind PS in a Ca2+-dependent manner, could bridge the target cell and macrophage. Monoclonal antibodies (mAbs) against annexins I or II bound to 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. Pretreatment of macrophages with mAbs to annexins I or II inhibited phagocytosis of apoptotic targets. Pretreatment of apoptotic thymocytes had no effect, while pretreating Jurkat cells with anti-annexin I or removing annexin I with EGTA was inhibitory. EGTA could not remove annexins from macrophages, suggesting that annexins bind to a receptor rather than PS on macrophages. Annexin bridging is vectorial, since annexin is bound to PS molecules on targets but not on macrophages, suggesting annexins serve as both ligand and receptor in promoting phagocytosis. Whether exogenous annexins could bind to apoptotic targets and enhance phagocytosis was also tested. Purified annexins I and II were found to bind to apoptotic thymocytes. In their presence, phagocytosis was inhibited or unaffected depending on the concentration of annexins, suggesting that exogenous annexins can mask PS on apoptotic thymocytes required for phagocytosis. Phagocytosis of apoptotic Jurkat cells, which have both endogenous annexin I and free PS on their surface, was also inhibited by exogenous annexins I, II, or V, suggesting that free PS is essential for recognition and phagocytosis, even if annexin I is already present on the target cell surface. The PS-binding protein, β2-glycoprotein 1 (β2GP1), has also been suggested as a bridging molecule in the phagocytosis of apoptotic thymocytes. β2GP1 in human serum was found to bind apoptotic thymocytes and apoptotic Jurkat cells, and this binding was independent of the presence of annexin I on the apoptotic targets. Surprisingly, β2GP1 was also found to be present normally on the surface of macrophages. However, neither β2GP1 on apoptotic cells or on macrophages appears to play a role in phagocytosis of apoptotic target cells.