Summary
A patient is described with serious bleeding due to a transient selective 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 complexes. A similar abnormal arc was obtained on adding the
patient’s IgG to normal plasma. Immunoblotting of factor X after reduced SDS-PAGE
revealed that the patient’s IgG bound to the light chain of intact factor X but not
Gla-domainless factor X. The patient’s IgG inhibited activation of factor X by Vila/tissue
factor (TF), by IXa/VIIIa/phospholipid complex, and by Russell’s viper venom. The
IgG failed to inhibit the 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 that require the participation
of its light chain: activation of prothrombin by prothrombinase; activation of factor
VII bound to TF; and inhibition 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 was evidence obtained of
an antibody against factor X. It will be of interest 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.
