ANTIBODY-INDUCED ACUTE FACTOR-X DEFICIENCY - CLINICAL MANIFESTATIONS AND PROPERTIES OF THE ANTIBODY

A patient is described with serious bleeding due to a transient select ive deficiency of factor X. Crossed immunoelectrophoresis of patient's plasma with anti-factor X antibody revealed an abnormal factor X arc suggestive of the presence of plasma factor X/anti-factor X immune com plexes. A similar abnormal arc was obtained on adding the patient's Ig G to normal plasma. Immunoblotting of factor X after reduced SDS-PAGE revealed that the patient's IgG bound to the light chain of intact fac tor X but not Gla-domainless factor X. The patient's IgG inhibited act ivation of factor X by VIIa/tissue factor (TF), by IXa/VIIIa/phospholi pid complex, and by Russell's viper venom. The IgG failed to inhibit t he proteolytic activity of factor Xa towards a chromogenic substrate. However, under reaction conditions of limited factor Xa availability, the IgG could be shown to impair hemostatic functions of factor Xa tha t require the participation of its light chain: activation of prothrom bin by prothrombinase; activation of factor VII bound to TF; and inhib ition of VIIa/TF activity by factor Xa/tissue factor pathway inhibitor complexes. A few earlier patients have been described with transient, selective factor X deficiency and serious bleeding, but in only one w as evidence obtained of an antibody against factor X. It will be of in terest to learn whether use of the techniques described in this report will permit the identification of immunoglobulin with similar binding and functional properties in future patients with this rare syndrome.