Factor VIII related antigen in platelets of patients with Von Willebrand's disease

Alloantibodies in von Willebrand disease

The development of alloantibodies against von Willebrand factor (VWF) represents a rare but serious complication of treatment of von Willebrand disease (VWD), occurring in ~5% to 10% of type 3 VWD patients. Affected patients can present with a range of symptoms, including lack or loss of hemostatic response to infused VWF concentrates up to anaphylactic reactions in rare cases. It is classically reported in multitransfused patients and occurs most frequently in patients with partial or complete VWF gene deletions. A positive family history of anti-VWF antibodies also appears to be a risk factor. There is a lack of standardization of laboratory methods for antibody identification and characterization. Issues of variability in laboratory approaches as well as the rarity of the complication act as a barrier to future studies. Recombinant factor VIII as well as bypassing agents and immune tolerance have been reported as effective treatments; however, aside from case reports, little exists in the literature to guide management. The imminent clinical availability of recombinant VWF has prompted a resurgence of interest in this area. Additional study is warranted to address the deficiencies in our understanding of this treatment complication.

Platelet von Willebrand factor: comparison with plasma von Willebrand factor

Platelet von Willebrand factor (vWf) was compared to its plasma counterpart. The platelet vWf was different from plasma vWf in that the multimeric organization was different, larger multimers were present, and the ratio of vWf activity to antigen was higher. Platelet and plasma vWf were similar in their antigenic reactivity in the electroimmunoassay and by liquid phase radioimmunoassay. The amount of vWf activity in large platelets was significantly higher than in normal platelets while the antigen content, although somewhat higher, was not significant. These studies show differences between normal platelet and plasma vWf, and also suggest that platelet size must be considered when platelet vWf is measured in disease states.

The identification and functional significance of factor VIII components on normal platelets

Factor VIII-related antigen (VIII R:Ag) has previously been identified on human platelets by fluorescein-labelled heterologous antibodies to factor VIII. However, it is not known whether these antibodies identify only VIII R:Ag or whether antigens associated with factor VIII coagulant activity (VIII:C Ag) are also present. The current studies utilize specific antibodies and their Fab' fragments to evaluate the presence of both VIII R:Ag and VIII:C Ag on human platelets and to study the functional role of the platelet-associated VIII R:Ag on ristocetin-induced platelet agglutination. Immunofluorescent studies demonstrate the presence of VIII R:Ag, but do not identify VIII:C Ag on platelets. The functional studies reveal that after incubation with either of the Fab' fragments, platelets retain their reactivity in the ristocetin cofactor assay. These results indicate that this population of VIII R:Ag is not essential for the ristocetin aggregation reaction and that no antigenic determinants for factor VIII coagulant activity (VIII:C) are detectable on human platelets by immunofluorescence.