Pathogenic role of anti-beta 2 glycoprotein I antibodies in antiphospholipid-associated fetal loss: characterisation of beta 2 glycoprotein I binding to trophoblast cells and functional effects of anti-beta 2 glycoprotein I antibodies in vitro

Background: Antiphospholipid antibodies reacting with s2-glycoprotein I (s2GPI) have been associated with recurrent fetal loss and pregnancy complications. Objective: To investigate whether specific mutations in the phospholipid binding site of s2GPI might affect its binding to trophoblast and in turn the anti-s2GPI antibody induced functional effects. Methods: s2GPI adhesion to trophoblast was evaluated as human monoclonal IgM or polyclonal IgG anti-s2GPI 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 s2GPI; or (4) in the presence of s2GPI with single or multiple mutations in the amino acid loop Cys281-Lys-Asn-Lys-Glu-Lys-Lys-Cys288. The effect of anti-s2GPI binding to trophoblast was evaluated as chorionic gonadotropin (hCG) mRNA expression, and protein release by RT-PCR and radioimmunoassay, respectively. Results: s2GPI adhesion to trophoblast and its consequent recognition by the specific antibodies were inversely proportional to the mutation number in the phospholipid binding site. Anti-s2GPI antibodies reduced gonadotropin release, hormone dependent hCG mRNA expression, and protein synthesis in the presence of s2GPI, while the addition of the mutants or the absence of s2GPI had no effect. Conclusions: s2GPI binds to trophoblast in vitro through its fifth domain, as reported for endothelial cells, and can be recognised by anti-s2GPI 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.