Factor VIII (antihaemophilic factor) in tissues detected by antibody neutralization

Molecular pathology and immunology of factor VIII (hemophilia A and factor VIII inhibitors)

Factor VIII is a large procoagulant glycoprotein that circulates in plasma in a noncovalent complex with von Willebrand factor. It is essential for the efficient cleavage of coagulation factor X by factor IXa, and its absence causes a severe bleeding disorder. Plasma factor VIII is reduced from the normal range of approximately 100 to 200 ng/ml in patients with the hereditary coagulation defect, hemophilia A, as well as in patients who develop autoantibodies that inactivate factor VIII. The understanding of factor VIII structure has been enhanced by recent studies that have characterized the X chromosome gene responsible for its synthesis, and preliminary information is now available about specific genetic defects. The basis for antibody formation in approximately 15 per cent of repeatedly transfused hemophilic patients is less clear at this time, however, for these individuals appear to have a variety of genetic defects that are not characteristically different from the patients who do not develop inhibitors. Although the antibodies cause a serious problem for affected individuals, they have been very useful in characterizing normal factor VIII and nonfunctional factor VIII-like protein that is found in the plasmas of 10 per cent of patients with mild hemophilia. Moreover, they are very useful reagents that can be used for immunoassay of factor VIII that has been modified in ways that have destroyed its procoagulant function.

Distribution of factor VIII mRNA and antigen in human liver and other tissues

The cellular site of synthesis of factor VIII (FVIII:C; anti-haemophilic factor) has long been sought. Previous studies suggested the liver as a major site of synthesis, but extrahepatic sources such as spleen and lung have been implicated. Using an immunoradiometric assay (IRMA), we recently localized factor VIII antigen (FVIII:Ag, formerly FVIII:CAg), to whole perfused guinea pig liver and spleen, and to isolated hepatocytes, with lesser or trace amounts in other tissues. Using an immunohistological technique, Stel et al. detected FVIII:Ag in normal human liver sinusoidal endothelial cells, while Exner et al. detected FVIII:Ag by IRMA in extracts of human lymph nodes, lung, liver and spleen. The localization of antigen in tissues does not, however, distinguish sites of factor VIII synthesis from those of storage, and such experiments are subject to misinterpretation due to entrapment of plasma factor VIII in tissues. The recent cloning of the human factor VIII gene provides hybridization probes for the detection of factor VIII messenger RNA in cells, thus directly determining sites of synthesis. During complementary DNA cloning, we detected factor VIII mRNA in liver, and it has been localized by others in liver and placenta and in liver and kidney. In the present study, we detected factor VIII mRNA in isolated human hepatocytes, in spleen and in numerous tissues including lymph nodes and kidney, but not in white blood cells or cultured endothelial cells. We also found that the factor VIII, factor VII, factor IX and protein C antigens in liver are predominantly localized in hepatocytes, while very little von Willebrand factor antigen (vWF:Ag, formerly FVIIIR Ag) is detectable in this organ.

Heterogeneity of human procoagulant factor VIII (VIIIC) antibodies in their reaction with factor VIII related antigen (VIIIRAG)

Human inhibitors to procoagulant factor VIII (VIIIC) were tested for possible reactivity with factor VIII related antigen (VIIIRAg) by a modified VIIIRAg immunoradiometric assay (IRMA). Inhibitor IgG's were screened for anti-VIIRAg by competition with I125 labelled VIIIRAg antibodies for common antigenic determinants using either an VIIIRAg concentrate from which factor VIII coagulant antigen (VIIICAg) had been dissociated, or normal plasma (containing VIIIRAg and VIIICAg) as an antigen source. There was no evidence of VIIIRAg antibodies in the five haemophilic VIIIC inhibitor IgG's tested but low titre VIIIRAg antibodies were detected in one spontaneous VIIIC inhibitor IgG.

Measurement of factor VIII CAg by immunoradiometric assay in human tissue extracts

Samples of normal tissues taken from three patients within 24 hours of death were physically disrupted, washed and extracted. Centrifuged supernatants were tested for factor VIII coagulant antigen (VIII CAg) and factor VIII related antigen (VIII RAg) by radioimmunoassays using homologous and heterologous antisera respectively. VIII RAg was readily washed out by isotonic saline whereas VIII CAg required 1.5 M sodium chloride for optimal extraction from tissue homogenates. Highest levels of VIII CAg were detected in some lymph nodes followed by lung, liver and spleen. There was rapid loss of VIII CAg from most tissues in the absence of proteolytic inhibitors.

An immunoradiometric assay for procoagulant factor VIII antigen: results in haemophilia, von Willebrand's disease and fetal plasma and serum

An immunoradiometric assay (IRMA) has been developed based on the inhibitor which arose in a polytransfused severe haemophiliac. The two-site IRMA measures antigens closely associated with the procoagulant parts of the factor VIII complex, which are termed FVIIC antigens or FVIIICAG. FVIIICAG was present in normal plasma and also, at a slightly lower concentration, in normal serum. In 37 patients with haemophilia A, 36 had FVIIICAG levels of less than 10% of the normal plasma pool. In patients with von Willebrand's disease the levels of FVIIIC and FVIIICAG were in good agreement, both before and after treatment with cryoprecipitate or DDAVP. FVIIICAG was relatively stable in plasma at 37 degrees C and could also be detected in cord and fetal serum. The assay is of potential value for detecting reduced levels of factor VIII, for carrier detection and for the prenatal diagnosis of haemophilia.

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.

Factor VIII coagulant activity and factor VIII-related antigen released from isolated perfused human spleens

Eight human spleens were perfused for up to 65 h at normothermia and the coagulant Factor VIII activity measured in the perfusate. In addition, in three experiments Factor VIII-related antigen was determined in the perfusate. Although the spleens were pathologically enlarged and the normal structure involved by different diseases, all spleens released Factor VIII coagulant activity and Factor VIII-related antigen. On average the total amount of Factor VIII coagulant activity released was equivalent to that of 3.5 l of human plasma.

The immunological localization of factor V in human tissue

Antibody to factor V was produced by immunizing rabbits with purified factor V from human plasma. Various tissues were examined for the presence of factor-V antigen using this antiserum. It was consistently demonstrated in homogenates of liver and spleen by means of an antibody (coagulation inhibitor) neutralization technique. The antigen was further localized histologically by the indirect fluorescent antiglobulin technique. It was present on the endothelium of normal blood vessels in all organs examined. In the liver it was detected in hepatic parenchymal cells and a distinctive pattern of fluorescence in the spleen suggested that it was being detected on platelets. Results were negative in all other tissues examined. The findings confirm the presence of factor V in hepatic parenchymal cells and support the suggestion that endothelial coagulation factors may play a role in haemostasis and thrombosis.

Antihemophilic factor antigen. Localization in endothelial cells by immunofluorescent microscopy

The tissue localization of antihemophilic factor (AHF, Factor VIII) has been determined by immunofluorescent studies using monospecific rabbit antibody to human AHF. Specific staining demonstrating AHF antigens has been identified in endothelial cells of a wide range of human tissues. The staining pattern was observed in endothelial cells of arteries, capillaries, and veins as well as the cells lining hepatic and splenic sinusoids. Specific fluorescence was limited to these endothelial cells in sections of kidney, liver, spleen, lymph node, cardiac and smooth muscle, thyroid, umbilical cord, and skin. Absorption studies established that the staining was specific for cells in which there were proteins that had AHF antigens. The demonstration of fluorescence within the cytoplasm of endothelial cells suggests that these cells synthesize proteins that have AHF antigens.