A comparison of five different rabbit antisera to factor VIII and the demonstration of a factor VIII related antigen in normal and von Willebrand's disease platelets

Platelet von Willebrand factor

von Willebrand factor (vWF) circulates in the blood in two distinct compartments. One, plasma vWF, is synthesized and released from endothelial cells; the second, synthesized by megakaryocytes, circulates in platelets primarily stored in the alpha granules. Recent experimental and clinical studies of von Willebrand's disease (vWD) indicate that platelet vWF plays an important role in the bleeding time determination and the degree of clinical bleeding in vWD. Platelet vWF is released from the platelet alpha granules by various agonists and then rebinds to the glycoprotein IIb/IIIa complex. Fibrinogen or monoclonal antibodies against this complex inhibit 60 to 70% of the expression of platelet vWF. Aspirin inhibits 80% of the adenosine diphosphate-induced platelet vWF surface expression, and the platelet vWF surface expression that is not inhibited by aspirin can be almost totally inhibited by disruption of the platelet cytoskeleton. Platelet vWF may, in part, be expressed in the open canalicular system prebound to a receptor. Transfusion studies have shown that correction of the bleeding time in severe vWD requires both plasma and platelet vWF. On the basis of numerous studies, we hypothesize that platelet vWF plays an important role in platelet interaction with the subendothelial surfaces under conditions of high shear stress. After platelet contact, platelet vWF is released, binds to the glycoprotein IIb/IIIa complex, and forms a bridge between the subendothelial surface and the platelet, which initiates and supports platelet spreading. Platelet vWF also acts as an intercellular bridge between platelets and thereby promotes platelet aggregation. This process is important not only in the initial steps of hemostasis but also in the process of thrombosis.

Assessing the specificity of anti-canine-factor VIII-related antigen

Antibodies to canine factor VIII-related antigen should be monospecific to accurately study this protein's location in tissue and its structure. Because no uniformly accepted standard for monospecificity exists for anti-factor VIII-related antigen, we have compared the sensitivities of three common evaluation techniques--immunodiffusion, immunoelectrophoresis, and crossed immunoelectrophoresis. Unabsorbed rabbit anti-canine factor VIII-related antigen appeared monospecific against whole plasma by all three methods; however, multiple specificities were recognized against a plasma cryoconcentrate. These results demonstrate the insensitivity of the techniques under certain conditions, which can lead to erroneous interpretation of data.

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.

Acquired factor VIII:C inhibitor (IgG) and positive direct Coombs' test (IgM) in a patient with lung carcinoma. Clinical course and immunochemical studies

A 73-year-old man with lung squamous-cell carcinoma simultaneously developed a factor VIII:C inhibitor and a transient positive direct Coombs' test. The factor VIII:C inhibitor was characterized as a mixture of IgG1, IgG2, IgG4 with kappa and lambda light chains. The Coombs' test was IgM-specific.

TREATMENT

blood transfusions (packed red cells), cyclophosphamide and prednisone. Bleeding subsided without factor VIII-replacement therapy. Before death, in spite of the tumour-mass progression, factor VIII:C was normal.

The effect of ticlopidine on human endothelial cells in culture

The effect of ticlopidine at various concentrations (150, 30, 6 microM), has been studied on cultured endothelial cells derived from human umbilical cord vein. Ticlopidine affects the initial attachment of endothelial cells to artificial substrata and has an inhibitory effect on endothelial cell growth rate which correlates to the concentration of the chemical in the culture medium. These effects are related to a marked reduction of intra- and extracellular fibronectin as evidentiated by immunofluorescence. The drug seems to interfere with the formation of fibronectin filaments from intracellular granules. The reduction of fibronectin availability could affect platelet adhesion to subendothelium as well as endothelial cell repair, and subsequently influence the bleeding time. The inhibition of cell proliferation and its possible effect on the thickness of the vessel wall should be considered as additional mechanisms of action for this substance.

Immunohistological comparison of platelet factor 4 (PF4), fibronectin (Fn) and factor VIII related antigen (VIIIR:Ag) in human platelet granules

Immunofluorescence microscopy was used to study the localization of platelet factor 4 (PF4), fibronectin (Fn) and factor VIII related antigen in washed normal platelets and those from patients with severe von Willebrand's disease (vWd). Platelets were prepared by an improved multi-unit modification of the Sayk chamber. This facilitated the preparation of samples with minimum disruption of platelets and readily permitted the demonstration of intact platelet granules by the immunological techniques used. The antigens were demonstrated by treating the same preparations sequentially with appropriate heterologous antisera and species specific fluorescein or rhodamine conjugated antisera. Comparison of the different antigens in identical platelets indicated that Fn and VIIIR:Ag were localized to the same granules as PF4 and the results were thus consistent with their presence in alpha granules. Fn and PF4, but not VIIIR:Ag, were present in platelet granules of patients with severe vWd. The antigens were always detected in the same granules, and major sub-populations of differently stained granules were not observed. The methods were applied to investigate normal platelets aggregated with collagen. Fn and VIIIR:Ag were detected in platelet granules after aggregation although the granules themselves were possibly differently distributed in these samples compared with the non-aggregated platelets. The localization of PF4 could not be reliably assessed in aggregated platelets by these methods. The techniques may be useful in localizing platelet antigens and studying release during aggregation.

Production of antibodies to human factor VIII in the ascitic fluid of C57BL/6J mice

A method is described for the production of a murine antiserum to human factor VIII, and the quantitation of factor VIII related antigen using this antiserum. C57BL/6J mice were injected intraperitoneally with a mixture of factor VIII and complete Freund's adjuvant. Ascitic fluid developed after repeated innoculations for seven to eight weeks, and was subsequently tapped. Using this antiserum, factor VIII related antigen was quantitated by immunoelectrophoresis and enzyme immunoassay. This method provides a simple and economical way of producing an antiserum to factor VIII, and may also be applicable to other proteins available in submiligram quantities.

Structure-function of the factor VIII complex studied with an immobilized heteroantisera to VIII:C

An antibody was raised in rabbits to the small active fragment of human factor VIII, obtained by Ca2+ dissociation of a human factor VIII preparation made from a multidonor plasma pool. After absorption, the antibody neutralized the factor VIII coagulant activity of normal human plasma, but did not precipitate with any plasma or plasma fractions or neutralize von Willebrand factor (vwf) activity as measured by ristocetin aggregation of fixed washed platelets. Immune beads were prepared by CNBr binding of the partially purified rabbit antibody to 1% agarose beads. Non-immune beads were prepared with IgG fractions obtained from the rabbits before immunization and used throughout as a control. The amount of factor VIII coagulant activity (VII:C) removed from plasma by immune beads was time-dependent and proportional to the amount of beads used, but all of the VIII:C could not be readily removed. Removal of VIII:C by immune beads parallelled removal of factor VIII:antigen, but less VWF activity was removed. Immune beads could be blocked or saturated by treatment with large amounts of normal plasma, but not by von Willebrand disease plasma and only by some haemophilic plasma.

The separation of Willebrand factor from factor VIII-related antigen

The three activities associated with factor VIII--coagulant (VIII:C), antigenic (VIIIR:Ag), and platelet agglutinating or Willebrand factor (VIIIR:WF)--have been separated by sequential antibody affinity chromatography, utilizing a rabbit antibody to factor VIII and a spontaneous human antibody to VIII:C. Normal plasma differentially lost its factor VIII-related antigen following passage over the rabbit antibody column. Subsequent passage of the VIIIR:Ag-depleted plasma over the human antibody column resulted in the loss of VIII:C activity, with retention of the Willebrand factor activity, antigen being partially recovered from the heterologous antibody column. These experiments demonstrate that it is possible to separate two of the factor VIII activities, VIIIR:Ag and VIIIR:WF, which are usually regarded as properties of a single molecule.

Platelet-associated coagulation factors: immunological detection and the effect of calcium

Normal platelets were examined by conventional coagulation methods and specific immunological techniques for the presence of platelet-associated coagulation factors. Platelets washed in buffer containing calcium chloride gave different results form those washed in the absence of calcium. Factor XI was not detected in washed platelets by any technique. Factor II and factor X were removed from platelets by washing in calcium-free buffer but not by calcium-containing buffer. Procoagulant factor VIII was readily removed or inactivated by washing in the presence or absence of calcium, but factor VIII related antigen was consistently demonstrated after multiple washes in either buffer. Factor V activity and antigen was detected in platelet suspensions after multiple washes although the presence of calcium in the washing buffer gave higher factor V assays and stronger immunological reactions than when calcium-free buffer was used. The results are consistent with the presence of calcium dependent binding sites in platelets which may have a role in the enhancement of factor V and phospholipid reactions.

Immunological studies in combined factor V and factor VIII deficiency

Plasma samples from patients with inherited combined factor V and factor VIII deficiency were examined by immunological methods for the presence of factor V and Factor VIII-related antigens. A factor V-related antigen was consistently demonstrated in all plasma samples by inhibitor neutralization assay using a non-precipitating rabbit antibody. Factor VIII-related antigens were detected by inhibitor neutralization using human antibody and by electroimmunoassay using a precipitating rabbit antibody. The amounts of factor V and factor VIII-related antigens present in the patient's samples were similar to those found in normal human plasma. The findings confirm the presence of normal levels of factor VIII-related antigen in the plasma of these patients and suggest that inactive antigenic determinants of procoagulant factor V and procoagulant factor VIII are also present. The results are consistent with the possibility that a common precursor of porcoagulant factor V and factor VIII is defective in these patients.

Quantitative determination of factor-VIII protein by two-stage gel electrophoresis

A new method for the quantification of factor-VIII protein in plasma is described which utilizes only commercially available equipment and material. The procedure involves separation of factor VII from other plasma proteins by a two-stage sodium dodecyl sulphate gel electrophoretic technique and quantification of the protein by scanning densitometry. The plasma from normal individuals, haemophiliacs, obligate carriers of haemophilia and patients with von Willebrand's disease have been examined by this technique. In general, the data support the earlier findings obtained by quantitative immunoelectrophoresis. In addition, some abnormalities not detected by immunoelectrophoresis are observed by the gel electrophoresis technique.

Presence of factor VIII-related antigen in blood platelets of patients with Von Willebrand's disease

VON WILLEBRAND'S disease (VWD) is an autosomally inherited disorder characterised by low factor VIII activity (antihaemophilic factor, AHF), prolonged bleeding time, reduced retention of platelets in a glass bead column and abnormal distocetin-induced platelet aggregation. The prolonged bleeding time in VWD has been attributed to the absence of a plasma factor, the von Willebrand factor (VWF), as shown by a correction of the bleeding time after infusion of normal and haemophilic plasmas¹. Addition of purified factor VIII in vitro specifically corrects the abnormal platelet retention and ristocetin aggregation in VWD^2–5, whereas transfusion of similar material into dogs with VWD also corrects the prolonged bleeding time (B.N.B., W. J. Dodds, J. A. van Mourik, J.J.S and W. P. Webster, unpublished). This led to the suggestion that factor VIII is closely related if not identical to VWF, although dissociation of factor VIII procoagulant activity from factor VIII-related antigen (F VIII-RA) is observed in certain conditions^6,7. The plasma concentration of F VIII-RA is usually reduced in VWD suggesting a reduced synthesis of factor VIII (VWF)⁸. In contrast to this we now report the presence of normal concentrations of F VIII-RA in platelets of patients with VWD. This F VIII-RA supported aggregation induced by ristocetin in a washed platelet system⁹, a property of factor VIII which has been attributed to VWF activity.