Preeclampsia is characterised by an exaggerated inflammatory response and maternal endothelial cell activation. Syncytial knots, dead multinucleated fetal cells shed from the placenta in large numbers during all pregnancies, may be phagocytosed by maternal endothelial cells. Our previous studies showed that phagocytosis of necrotic but not apoptotic syncytial knots led to endothelial cell activation. It is known that phagocytosis of apoptotic cells leads to active tolerance of immune responses and in this study we questioned whether phagocytosis of apoptotic syncytial knots leads to suppression of the endothelial cells ability to be activated. Syncytial knots were harvested from 1st trimester placental explants. Monolayers of endothelial cells were pre-treated with apoptotic syncytial knots for 24 h. After washing, the endothelial cells were treated with the endothelial cell activators LPS, PMA, IL-6, or necrotic syncytial knots for 24 h. In some experiments the inhibitor of phagocytosis, cytochalasin D, was added into the cultures along with apoptotic syncytial knots. Endothelial cell-surface ICAM-1 was measured using cell based ELISAs. Expression of ICAM-1 by endothelial cells that had phagocytosed apoptotic syncytial knots prior to treatment with LPS, PMA, IL-6, or necrotic syncytial knots was significantly (P =/<0.003) reduced, compared to control endothelial cells that had not phagocytosed apoptotic syncytial knots. Inhibiting phagocytosis of apoptotic syncytial knots with cytochalasin D abolished this protective effect. Our data suggest phagocytosis of apoptotic syncytial knots results in the suppression of the ability of endothelial cells to be activated by a number of potent chemical activators, as well as by the physiologically relevant activator, necrotic syncytial knots. This work suggests that the release of apoptotic syncytial knots from the placenta during normal pregnancy may be a mechanism by which the fetus attempts to protect the maternal vasculature against activation.
The role of valvular endothelial cell paracrine signaling and matrix elasticity on valvular interstitial cell activation