Surface Annexin II on Placental Membranes of the Fetomaternal Interface

Abstract

PROBLEM: The phospholipidbinding membrane protein annexin II has been demonstrated to possess FcR activity for IgG and has been localized to the outer part of the syncytiotrophoblast cell layer. The question has arisen whether annexin II is exposed on the surface of syncytiotrophoblast cells thus enabling it to take part in the transport of IgG across the maternal barrier. METHOD: Syncytiotrophoblast microvillous plasma membranes were analyzed by flow cytometry for annexin II as well as established surface molecules. Fresh, fixed placental tissue was preincubated with antibodies to annexin II or known trophoblast surface molecules, and analyzed by confocal laser scanning microscopy. RESULTS: Annexin II and its subunit p11 were expressed on the surface of the syncytiotrophoblast microvillous plasma membranes as were other established surface proteins (CD46, CD59, placental alkaline phosphatase), using both flow cytometry and confocal microscopy. Annexin was not detected on the surface of viable cultured trophoblast cells. CONCLUSION: Annexin II is exposed on the surface of syncytiotrophoblast cells as a heterotetramer together with its light chain p11. It is exposed to maternal blood and may be instrumental in IgG transport across the placental barrier by binding