Heparin prevents antiphospholipid antibody–induced fetal loss by inhibiting complement activation

The antiphospholipid syndrome (APS) is defined by thrombosis and recurrent pregnancy loss in the presence of antiphospholipid (aPL) antibodies and is generally treated with anticoagulation therapy. Because complement activation is essential and causative in aPL antibody–induced fetal injury, we hypothesized that heparin protects pregnant APS patients from complications through inhibition of complement. Treatment with heparin (unfractionated or low molecular weight) prevented complement activation in vivo and in vitro and protected mice from pregnancy complications induced by aPL antibodies. Neither fondaparinux nor hirudin, other anticoagulants, inhibited the generation of complement split products or prevented pregnancy loss, demonstrating that anticoagulation therapy is insufficient protection against APS-associated miscarriage. Our data indicate that heparins prevent obstetrical complications in women with APS because they block activation of complement induced by aPL antibodies targeted to decidual tissues, rather than by their anticoagulant effects.