ANTIPHOSPHOLIPID IMMUNOGLOBULIN-G ANTIBODIES REDUCE ANNEXIN-V LEVELS ON SYNCYTIOTROPHOBLAST APICAL MEMBRANES AND IN CULTURE MEDIA OF PLACENTAL VILLI

OBJECTIVES: The mechanism by which antiphospholipid antibodies are ass ociated with pregnancy loss and thromboembolism has not been establish ed. We previously showed that annexin-V, a phospholipid-binding protei n with potent anticoagulant activity, is present on the apical membran es or the syncytiotrophoblasts that line placental villi and that this protein is reduced, by immunohistochemistry, on placentas of patients with antiphospholipid antibodies. We therefore investigated whether a nnexin-V in apical membranes of placental villi is quantitatively redu ced by antiphospholipid antibody immunoglobulin G. STUDY DESIGN: Place ntas were obtained from an index patient with an antiphospholipid synd rome with intrauterine growth restriction and from a patient with an u ncomplicated pregnancy who were both delivered by cesarean section. Ap ical villous membranes were isolated and annexin-V levels were measure d by enzyme-linked immunosorbent assay. We then studied the effects of antiphospholipid immunoglobulin G on placental villous apical annexin -V in vitro. Antiphospholipid immunoglobulin G was isolated from the s era of five different patients with antiphospholipid antibody syndrome along with five paired control immunoglobulin Ga. Short-term cultures were established from normal placental villi and were exposed to the antibodies, after which isolated apical membranes and culture media we re immunoassayed for annexin-V levels. RESULTS: Measurements of apical membrane-associated annexin-V from the antiphospholipid placenta show ed significantly less apical membrane-associated annexin-V than did th e normal placenta (mean +/- SEM: 4.9 +/- 0.4 mu g/gm villi for antipho spholipid placenta vs 10.2 +/- 0.6 mu g/gm villi for control, p < 0.00 1, n = 4). Exposure of placental villous cultures to five different an tiphospholipid immunoglobulin Gs for 24 hours resulted in significant reduction of the levels of apical membrane annexin-V (mean +/- SEM: 3. 9 +/- 0.3 mu g/gm villi) compared with paired controls (5.1 +/- 0.3 mu g/gm villi, p = 0.02). Villi incubated with the different antiphospho lipid immunoglobulin Gs had significantly less annexin-V in conditione d media (mean +/- SEM: 45.1 +/- 4.9 ng/gm villi) compared with the pai red normal immunoglobulin G control levels (72.6 +/- 11.4 ng/gm villi, p = 0.03). CONCLUSIONS: Antiphospholipid immunoglobulin G reduces the levels of syncytiotrophoblast apical membrane-associated annexin-V in placental villi and the release of annexin-V into surrounding media. Reduction of this anticoagulant protein at the maternal-fetal interfac e may account for the pregnancy loss observed in patients with antipho spholipid syndrome. Short-term culture of placental villi may offer an in vitro model to further study the mechanism of this effect of antip hospholipid antibodies.