ANTIPHOSPHOLIPID ANTIBODIES AGAINST DOMAIN 1 OF b2-GPI PROMOTE CLOTTING BY INHIBITING THE INACTIVATION OF FACTOR VA.

MD Linnik, PA McNeeley, GM Iverson, EJ Victoria, D Marquis.

La Jolla Pharmaceutical Co., San Diego, CA 92121 USA

Antiphospholipid syndrome (APS) is a hypercoagulative disorder that has been implicated in stroke, deep vein thrombosis, thrombocytopenia and recurrent fetal loss. APS is characterized by the presence of pathogenic autoantibodies directed against the circulating protein b2-glycoprotein 1 (b2-GPI). b2-GPI is composed of 5 short consensus repeat domains and recent studies from our laboratories have demonstrated that the binding epitope(s) for the pathogenic antibodies in APS are localized to domain 1 of b2-GPI. Recent data has suggested that APS antibodies contribute to hypercoagulation by interfering with the inactivation of factor Va by the activated protein C complex. The current study examined affinity purified antibodies from 10 APS patients for their ability to influence the inactivation of factor Va. All 10 patient antibodies demonstrated a delay in factor Va inactivation relative to normal IgG controls. Furthermore, the relative effect of these antibodies on factor Va inactivation could be segregated by titrating the antibodies to no effect concentrations. In a parallel study, mice, goats and rabbits were immunized with recombinant human b2-GPI and the IgG fraction from these animals were analyzed in the factor Va assay. The IgG fraction from each of the immunized species also caused a delay in factor Va inactivation relative to IgG from normal animals. These results indicate that APS antibodies directed against domain 1 of b2-GPI are pathogenic in part because they delay the inactivation of factor Va. IgG from animals immunized with human b2-GPI also delays the inactivation of factor Va and this data has implications for developing animal models of APS.

Presented at the 2nd International Congress on Autoimmunity, March 7-12, 1999, Tel Aviv, Israel.