ANTIPHOSPHOLIPID ANTIBODIES PROMOTE CLOTTING BY INHIBITING THE INACTIVATION OF FACTOR VA.

Matthew D. Linnik, Patricia A. McNeeley, G. Michael Iverson, Edward J. Victoria and David M. Marquis.

La Jolla Pharmaceutical Co., San Diego, CA 92121

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 ß2-glycoprotein 1 (ß2-GPI). ß2-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 ß2-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 ß2-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 ß2-GPI are pathogenic in part because they delay the inactivation of factor Va. IgG from animals immunized with human ß2-GPI also delays the inactivation of factor Va and these data have implications for developing animals models of APS.

Presented at the Keystone Symposia on B Lymphocyte Biology and Disease, Taos, New Mexico, Feb. 8-14, 1999.