Circulating Factor IX Antigen-Inhibitor Complexes in Hemophilia B^- Following Infusion of a Factor IX Concentrate

A persistent low titer IgG factor IX inhibitor (0.8–2.3u/ml) has been previously described in an 11 year old boy with severe hemophilia B^-. Since the disappearance of factor IX antigen (IX_AGN) following Konyne(R) infusion was markedly delayed ( 60 hr) compared to factor IX clotting activity (T 8 hr) (Blood 48:977, 1976), a search for circulating factor IX antigen-inhibitor complexes was undertaken. The inhibitor binds firmly to staphlococcal protein A – Sepharose 4B (SPA-Sepharose) but may be eluted in the IgG 1, 2, and 4 fraction with 3M NaSCN. SPA-Sepharose chromatography of post-infusion plasma (IX_AGN 2.7u) from the patient with the inhibitor showed IX_AGN (0.5u) in the IgG 1, 2, and 4 fraction, whereas IX_AGN was not found in that fraction of chromatographed samples of post infusion plasma from non-inhibitor patients with hemophilia B^- or B⁺. Using a highly specific antibody to factor IX, two dimensional Immunoelectrophoresis of purified factor IX or normal plasma showed a single symmetrical fast moving peak. When an equal volume of inhibitor plasma was added to either factor IX or normal plasma a second, slower moving component was also seen. Two dimensional Immunoelectrophoresis of plasma obtained from the patient 45 minutes after infusion showed both the slow and fast components. The presence of IX_AGN in the IgG 1, 2, and 4 peak after SPA-Sepharose chromatography and the demonstration of 2 components on two dimensional Immunoelectrophoresis indicates the presence of circulating factor IX antigen-inhibitor complexes in the plasma of this patient following the infusion of a factor IX concentrate.