Relationship between anti-phospholipid and anti-endothelial cell antibodies III: β2 glycoprotein I mediates the antibody binding to endothelial membranes and induces the expression of adhesion molecules

Objective: To investigate the role of antibodies reacting with β2 glycoprotein I (β2GPI) in the anti-endothelial cell binding activity present in sera from patients with the anti-phospholipid syndrome. Methods: Sera positive for anti-phospholipid, anti-endothelial and anti-β2 GPI antibodies were studied for their binding activity on endothelial monolayers cultured in the presence or absence of media containing bovine serum as a source of β2GPI. Anti-endothelial activity was also evaluated on endothelial cells cultured without serum and supplemented with exogenous human purified β2GPI. Affinity purified anti-β2 GPI antibodies were investigated under the same experimental conditions. Finally, the effect of the incubation of these affinity purified fractions on the expression of adhesion molecules (ELAM-1) was studied. Results: The reactivity of the sera decreased on endothelial cells incubated in serum-free medium, while endothelial cell binding was restored in a dose dependent manner after the addition of exogenous purified human β2 GPI. Affinity purified anti-β2 GPI antibodies obtained from the same sera retained their endothelial cell binding and were able to activate endothelial cells by inducing the ex novo surface expression of adhesion molecules (ELAM-1). Conclusions: These findings indicate that the close association between anti-endothelial and anti-phospholipid antibodies is sustained by antibodies which recognize β2GPI adhering to the endothelial cells, and can promote their activation