Background: Antiphospholipid antibodies reacting with b2-glycoprotein I (b2GPI) have been associated with recurrent fetal loss and pregnancy complications. Objective: To investigate whether specific mutations in the phospholipid binding site of b2GPI might affect its binding to trophoblast and in turn the anti-b2GPI antibody induced functional effects. Methods: b2GPI adhesion to trophoblast was evaluated as human monoclonal IgM or polyclonal IgG antib2GPI antibody binding to trophoblast monolayers cultured (1) in complete medium; (2) in serum-free medium; (3) after serum starvation in the presence of purified human b2GPI; or (4) in the presence of b2GPI with single or multiple mutations in the amino acid loop Cys 281 -Lys-Asn-Lys-Glu-Lys-Lys-Cys 288 . The effect of anti-b2GPI binding to trophoblast was evaluated as chorionic gonadotropin (hCG) mRNA expression, and protein release by RT-PCR and radioimmunoassay, respectively. Results: b2GPI adhesion to trophoblast and its consequent recognition by the specific antibodies were inversely proportional to the mutation number in the phospholipid binding site. Anti-b2GPI antibodies reduced gonadotropin release, hormone dependent hCG mRNA expression, and protein synthesis in the presence of b2GPI, while the addition of the mutants or the absence of b2GPI had no effect. Conclusions: b2GPI binds to trophoblast in vitro through its fifth domain, as reported for endothelial cells, and can be recognised by anti-b2GPI antibodies; the antibody binding downregulates trophoblast hCG synthesis and secretion. Such a mechanism might contribute to defective placentation in women with fetal loss associated with the antiphospholipid syndrome.

Most of the clinical manifestations of the antiphospholipid syndrome (APS) can be related to thrombotic events; however, placental thrombosis cannot explain all of the pregnancy complications that occur in women with this syndrome. In this regard, it has been hypothesized that antiphospholipid (aPL) antibodies can directly attack trophoblasts, but it is still unclear what pathogenetic mechanisms play a role and which aPL antibodies subpopulations are involved. Although it has been assumed that aPL antibodies are directed against anionic phospholipids (PLs), current advances in the field suggest that antibodies to PL-binding plasma protein such as beta2-glycoprotein-I (beta2-GPI) are the clinically relevant aPL antibodies. It appears that following the attachment of beta2-GPI to PLs, both molecules undergo conformational changes that result in the exposure of cryptic epitopes within the structure of beta2-GPI allowing the subsequent binding of antibodies. aPL antibodies detected by anti-beta2-GPI assays are associated with fetal loss. However, there is still debate on how the antibodies might induce the obstetrical manifestations. The significantly improved outcome of pregnancies treated with heparin has stimulated interest in the drug's mechanisms of action. Several mechanisms could explain its beneficial effects, because in addition to a direct effect of heparin on the coagulation cascade, it might protect pregnancies by reducing the binding of aPL antibodies, reducing inflammation, facilitating implantation and/or inhibiting complement activation. Further investigations are needed to better understand how aPL antibodies induce obstetric complications and to better clarify the functional role of heparin in the human placenta leading to more successful therapeutic options.

Resistin is a novel hormone that is secreted by human adipocytes and mononuclear cells and is probably associated with insulin resistance. Recently, resistin has been postulated to play a role in pregnancy, and resistin gene expression has been observed in placental tissues. However, it is still not known if resistin is able to affect trophoblast functions and development. Therefore, we investigated the hypothesis that resistin might regulate trophoblast production of matrix metalloproteinases (MMPs), the tissue inhibitors of metalloproteinases (TIMPs), trophoblast invasive behavior and the angiogenic processes.In human choriocarcinoma cells (BeWo), resistin (10-100 ng/ml) enhanced both MMP-2 protein and mRNA expression, significantly reduced TIMP-1 and TIMP-2 and increased trophoblast-like cell invasiveness.We analyzed the effect of resistin on an in vitro angiogenesis system for endothelial cells (HUVEC) and we evaluated its ability to modulate the secretion of an angiogenic factor, vascular endothelial growth factor (VEGF). Our data showed that resistin induced VEGF production and we observed that the addition of resistin stimulated endothelial cell tube formation.These findings suggest that resistin might be able to induce BeWo cell invasiveness and to contribute to the control of placental vascular development.

Antiphospholipid antibodies detected by lupus anticoagulant, anticardiolipin or anti-beta2 glycoprotein I assays were associated with fetal loss. Rather than being diagnostic tools only, antiphospholipid antibodies are thought to be pathogenic. The strongest demonstration of their pathogenic role lies in the ability to induce fetal resorptions--the experimental equivalents of the human fetal losses--when passively infused in pregnant naive animals. However, still debated is how the antibodies might induce the obstetrical manifestations. Thrombotic events at the placental levels might be related to endothelial cell activation, inhibition of protein C/S system and fibrinolysis as well as to Annexin V displacement. However, the thrombophilic state apparently cannot explain all the miscarriages and a direct antibody-mediated damage on the trophoblast has been suggested. During differentiation to syncytium, trophoblasts express cell membrane anionic phospholipids that can bind beta2 glycoprotein I, the main cationic phospholipid binding protein recognized by the antiphospholipid antibodies. Adhered beta2-glycoprotein I might be recognized by the antibodies that, once bound, strongly interfere with in vitro trophoblast cell maturation so resulting in a defective placentation. These mechanisms have been suggested to play a role in early fetal loss, while thrombotic events would be responsible for miscarriages late in the pregnancy.

In trophoblast cells exposed to homocysteine (Hcy) we observed cellular apoptosis and the inhibition of trophoblast functions. Because folate and Hcy, linked in the same metabolic pathway, are inversely related, we investigated the role of folic acid in reversing the Hcy effect in human placenta. In primary trophoblast cells we examined the cytosolic release of cytochrome c, both M30 and terminal deoxynucleotidyl transferase-mediated dUDP nick-end labelling (TUNEL) and DNA laddering. Hcy (20 micromol/l) treatment resulted in cytochrome c release from mitochondria to the cytosol, and an increased number of M30-positive trophoblast cells and TUNEL positive nuclei. Furthermore, DNA cleavage in agarose gel and the determination of histone-associated DNA fragments have been investigated. Homocysteine induced DNA fragmentation and significantly reduced hCG secretion. The addition of folic acid (20 nmol/l) resulted in inhibition of the effects of Hcy on human trophoblast. These results suggest a protective role of folic acid in the prevention of trophoblast apoptosis linked to Hcy.