528. THE INTERNALIZATION OF ANTIPHOSPHOLIPID ANTIBODIES INTO TROPHOBLASTS CORRELATES WITH THE EXPRESSION OF MEGALIN

Abstract

Antiphospholipid antibodies (aPL) are autoantibodies that increase the risk of preeclampsia nine fold. We have recently shown aPL increase in the number of syncytial knots shed from placental explants and also change the trophoblast death process towards necrosis. Shedding of necrotic syncytial knots is thought to contribute to the pathogenesis of preeclampsia. Antiphospholipid antibodies but not control antibodies, are internalised into the syncytiotrophoblast suggesting a specific mechanism for internalisation of the aPL. Megalin is known to be an endocytic receptor for the antigen of aPL. We believe that the internalisation of aPL into the syncytiotrophoblast is required to for aPL to affect trophoblast shedding and in this study began to investigate the hypothesis that megalin mediates aPL-internalisation. Monoclonal aPL, IIC5 or ID2, were incubated with monolayers of the trophoblast cell lines, Jar, Jeg 3 or BeWo, or first trimester placental explants for 24 hours. Internalisation of aPL into trophoblasts was determined by fluorescent immuno-staining as was the expression of megalin using an antimegalin antibody (Sigma). Experiments were repeated at least three times. Confocal microscopy demonstrated that the syncytiotrophoblast of explants and BeWo cells, but not Jar or Jeg3 cells, internalised the aPL. Despite treating explants and BeWos with the same amount of aPL the level of aPL internalised by the syncytiotrophoblast of explants was greater than the level internalised by BeWos. Megalin was expressed strongly by the syncytiotrophoblast and weakly by BeWos but was not expressed by Jars or Jeg3 cells. The internalization of aPL into syncytiotrophoblasts may play an important role in regulating trophoblast death leading to aberrant shedding of syncytial knots. This study provides preliminary evidence that megalin expression correlates with the ability of trophoblasts to internalize aPL suggesting it may be the receptor that mediates aPL internalization into trophoblasts making this pathway a potential therapeutic target.