Antiphospholipid antibodies (aPLA) are associated with thrombophilia and recurrent pregnancy loss. They bind directly to anionic phospholipids or via phospholipid-binding proteins such as beta(2)-glycoprotein 1 (beta(2)GP1). The underlying mechanisms by which aPLA induce a thrombophilic phenotype are not well understood. The present work was done to determine whether antibodies to beta(2)GP1 activate endothelial cells (EC) and whether NFkappaB is involved in this activation. Incubation of EC with these antibodies resulted in a redistribution of NFkappaB from the cytoplasm to the nucleus after a delay of several hours. This was accompanied by an increased expression of tissue factor and of the leukocyte adhesion molecules ICAM-1, VCAM-1 and E-selectin. Inhibition of the nuclear translocation of NFkappaB abolished the response to these antibodies. In comparison to anti-beta(2)GP1 antibodies, incubation of EC with TNF resulted in a more rapid (within 30 minutes) redistribution of NFkappaB and a more pronounced expression of tissue factor and of the leukocyte adhesion molecules. The slower response to the antibodies as compared to TNF suggests that the NFkappaB response to anti-beta(2)GP1 antibodies is indirect. Taken together our results imply that NFkappaB is an essential intermediate in the activation of EC by anti-beta(2)GP1 antibodies.