SURFACE ANNEXIN II ON PLACENTAL MEMBRANES OF THE FETOMATERNAL INTERFACE

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 ha s arisen whether annexin II is exposed on the surface of syncytiotroph oblast cells thus enabling it to take part in the transport of IgG acr oss the maternal barrier. METHOD: Syncytiotrophoblast microvillous pla sma membranes were analyzed by flow cytometry for annexin II as well a s established surface molecules. Fresh, fixed placental tissue was pre incubated with antibodies to annexin II or known trophoblast surface m olecules, and analyzed by confocal laser scanning microscopy. RESULTS: Annexin II and its subunit pll were expressed on the surface of the s yncytiotrophoblast microvillous plasma membranes as were other establi shed surface proteins (CD46, CD59, placental alkaline phosphatase), us ing both flow cytometry and confocal microscopy. Annexin was not detec ted on the surface of viable cultured trophoblast cells. CONCLUSION: A nnexin II is exposed on the surface of syncytiotrophoblast cells as a heterotetramer together with its light chain pll. It is exposed to mat ernal blood and may be instrumental in IgG transport across the placen tal barrier by binding