Phagocytosis of apoptotic trophoblastic debris protects endothelial cells against activation

During normal pregnancy trophoblastic debris is shed from the placenta into the maternal blood and endothelial cells may contribute to the phagocytosis of this material. Many researchers believe the majority of this trophoblastic material is apoptotic in normal pregnancy. Previously we demonstrated that phagocytosis of necrotic, but not apoptotic trophoblastic debris induced endothelial cell activation. In macrophages, phagocytosis of necrotic cell bodies leads to inflammation but phagocytosis of apoptotic bodies actively induces tolerogenic immune responses. We undertook this study to determine whether phagocytosis of apoptotic trophoblastic debris had a "tolerogenic" effect on endothelial cells analogous to their effect in macrophages. Apoptotic or necrotic trophoblastic debris was obtained from placental explants and endothelial cell activation was examined by quantifying, cell surface ICAM-1 expression using ELISAs, or monocyte adhesion. The response of endothelial cells to the activating stimuli of necrotic trophoblastic debris, interleukin-6 (IL-6), Lipopolysaccharide (LPS) or phorbol mysterate acetate (PMA) was reduced in endothelial cells that had phagocytosed apoptotic trophoblastic debris. This protective effect was short-lived being not apparent 24 h after removal of the trophoblastic debris. This work demonstrates that the ability of the endothelial cells to respond to a variety of activating stimuli is reduced by prior phagocytosis of apoptotic trophoblast debris. This might explain why endothelial cells are not activated by the small numbers of necrotic trophoblastic debris that may be found in normal pregnancy. This phenomenon may also contribute to the maternal vascular adaptation that occurs in normal pregnancy.