116. TROPHOBLAST ANTIPHOSPHOLIPID ANTIBODY INTERNALISATION BY A β₂ GLYCOPROTEIN I-ANIONIC PHOSPHOLIPID-MEGALIN COMPLEX

Abstract

Women with antiphospholipid antibodies (aPL) are at an increased risk of preeclampsia, recurrent miscarriage, stillbirth and intrauterine growth restriction. Antiphospholipid antibodies may predispose to these pathologies by damaging the placenta, although exactly how is not understood. Recently, a novel pathogenic mechanism was suggested by work which showed that aPL are specifically internalised by placental trophoblasts where they caused aberrant trophoblast death. Internalisation may occur via an endocytic receptor called megalin in a process that seems to involve at least one of the two components of the antigen for aPL, the anionic phospholipid-binding protein β₂ glycoprotein I (β₂GPI). However, whether internalisation is also dependent upon anionic phospholipids is unknown. Identifying the receptor pathway responsible for aPL internalisation may provide insight into the pathogenesis of aPL in the placenta. To investigate the process of aPL internalisation, first trimester placental explants were cultured with fluorescently-labeled monoclonal aPL, or a control antibody and/or β₂GPI or acetylated β₂GPI, which can not bind anionic phospholipids. The explants were then sectioned and the localisation of the aPL, β₂GPI, or acetylated β₂GPI was determined by confocal microscopy. The localisation of megalin expression in placental explants was determined by immunohistochemistry. Megalin was expressed throughout the syncytiotrophoblast but more strongly in some regions. After an overnight incubation, both aPL and β₂GPI, but not control antibodies were co-localised in the cytoplasm of the syncytiotrophoblast. Acetylated β₂GPI was not internalised and partially blocked aPL uptake. These results suggest that aPL are internalised into the synctiotrophoblast by a receptor-dependent mechanism involving β₂GPI, anionic phospholipids and megalin. This work forms the first step to understanding how aPL are internalised by trophoblasts. Further investigation of this mechanism and the subsequent intracellular effects of aPL may lead to a new therapeutic strategy for aPL-positive pregnant women by preventing the pathogenic effect of aPL on the placenta.