Whole blood rotation thromboelastometry (ROTEM®) in nine severe factor V deficient patients and evaluation of the role of intraplatelets factor V

Summary. Severe factor V (FV) deficiency (parahaemophilia) is a rare congenital hemorrhagic disorder characterized by very low or undetectable plasma FV levels and bleeding phenotype ranging from mild to severe. We evaluated whole blood (WB) rotation thromboelastometry (ROTEM) in parahaemophilia patients and the contribution of intraplatelets FV, if any, to clot formation. Standard ROTEM^® assays were performed in WB from nine parahaemophilia patients and 50 healthy controls. In addition, platelets poor plasma from one parahaemophilia patient (PPP‐Pt) or normal subjects (PPP‐N) was reconstituted with washed platelets obtained either from one patient with parahaemophilia (Plts‐Pt) or normal subjects (Plts‐N) and ROTEM assays were performed in platelets rich plasma (PRP) samples. There was a prolongation of the WB clotting time (CT) in all assays in patients as compared with controls. However, maximum clot firmness (MCF) was similar in patients and controls. ROTEM in PPP‐Pt showed both a prolongation of CT and a reduction of MCF as compared with PPP‐N. The addition of either Plts‐Pt or Plts‐N to PPP‐Pt resulted in similar increase in MCF and a decrease of CT which was more evident for PPP‐Pt + Plts‐N than PPP‐Pt + Plts‐Pt. In contrast, the addition of Plts‐Pt or Plts‐N to PPP‐N had superimposable effects on both CT and MCF. In parahaemophilia patients, WB ROTEM^® presents mainly with prolongation of CT and no relevant effect on MCF. Residual intraplatelets FV in parahaemophilia contributes significantly to thrombin generation as shown in artificially reconstituted PRP models.     

Biology of human megakaryocyte factor V

To learn more about human megakaryocyte coagulation cofactor V (FV), we studied the expression of this protein in normal bone marrow megakaryocytes and in megakaryocytes cloned from their colony-forming unit in FV-depleted plasma clot cultures. Mouse monoclonal antibodies directed against either the light chain or an activation peptide of human FV and a rabbit polyclonal, monospecific FV antiserum were used as probes for these experiments in conjunction with a variety of immunochemical detection techniques. All morphologically recognizable megakaryocytes were shown to contain FV. The origin of this protein appeared to be both from FV bound to the cell as well as from endogenous FV in the majority of cells examined. The existence of a population of small bone marrow mononuclear cells that simultaneously expressed platelet glycoproteins and FV was also noted. Such cells represented approximately 70% of all small cells positive for platelet glycoproteins. In contrast, only about 40% of megakaryocyte colonies cloned in FV-deficient medium contained cells with immunochemically detectable FV. FV expression was most clearly demonstrated in large cells in the colonies, whereas smaller, presumably less mature cells labeled weakly or not at all. Synthesis of FV by human megakaryocytes was documented using elutriation-enriched cells incubated in 35S-methionine-containing medium. Megakaryocyte lysates and medium conditioned by these cells were subjected to immunoaffinity column purification. Column eluates analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed radioactive bands comigrating with the heavy and light chains of thrombin-activated FV. These studies suggest that human megakaryocytes both bind and synthesize FV. Expression of these traits appears to be related to cell maturation, with binding ability appearing earlier than the ability to synthesize this protein. Finally, although the ability to bind FV appears to be universal among megakaryocytes, our culture data suggest that synthesis may be a restricted, or constitutively expressed property of these cells.     

Factor V deficiency: a concise review

Summary. Factor V (FV; proaccelerin or labile factor) is the plasma cofactor for the prothrombinase complex that activates prothrombin to thrombin. FV deficiency can be caused by mutations in the FV gene or in genes encoding components of a putative cargo receptor that transports FV (and factor VIII) from the endoplasmic reticulum to the Golgi. Because FV is present in platelet α‐granules as well as in plasma, low FV levels are also seen in disorders of platelet granules. Additionally, acquired FV deficiencies can occur in the setting of rheumatologic disorders, malignancies, and antibiotic use and, most frequently, with the use of topical bovine thrombin. FV levels have limited correlation with the risk of bleeding, but overall, FV‐deficient patients appear to have a less severe phenotype than patients with haemophilia A or B. The most commonly reported symptoms are bleeding from mucosal surfaces and postoperative haemorrhage. However, haemarthroses and intramuscular and intracranial haemorrhages can also occur. Because no FV‐specific concentrate is available, fresh frozen plasma remains the mainstay of treatment. Antifibrinolytics can also provide benefit, especially for mucosal bleeding. In refractory cases, or for patients with inhibitors, prothrombin complex concentrates, recombinant activated FVIIa, and platelet transfusions have been successfully used. Some patients with inhibitors may also require immunosuppression.     

Biosynthetic origin and functional significance of murine platelet factor V

Factor V (FV), a central regulatory protein in hemostasis, is distributed into distinct plasma and platelet compartments. Although platelet FV is highly concentrated within the platelet alpha-granule, previous analysis of human bone marrow and liver transplant recipients has demonstrated that platelet FV in these individuals originates entirely from the uptake of plasma FV. In order to examine further the biosynthetic origins of the platelet and plasma FV pools, we performed bone marrow transplantations of Fv-null (Fv-/-) fetal liver cells (FLCs) into wild-type mice. Fractionation of whole blood from control mice demonstrated that approximately 14% of total blood FV activity is platelet-associated. Mice that received transplants of Fv-null FLCs displayed a high degree of engraftment and appeared grossly normal, with no evidence for spontaneous hemorrhage. Although total FV levels in Fv-null FLC recipients were only mildly decreased, the FV activity within the platelet compartment was reduced to less than 1% of that in normal mice. We conclude that the murine platelet FV compartment is derived exclusively from primary biosynthesis within cells of marrow origin, presumably megakaryocytes, and that an intact platelet FV pool is not required for protection from spontaneous hemorrhage or bleeding following minor trauma.     

Endocytosis of fibrinogen into hamster megakaryocyte alpha granules is dependent on a dimeric gamma A configuration

Two species of fibrinogen that differ only in the structure of their gamma chains, gamma A and gamma', are present in normal plasma. Fibrinogen stored in platelet alpha granules does not contain gamma' chains. Because platelet fibrinogen was recently shown to be derived exclusively by receptor-mediated endocytosis from plasma and not by endogenous megakaryocyte synthesis, we postulated that the gamma' fibrinogen present in plasma is not endocytosed by megakaryocytes and platelets. We tested this hypothesis by studying endocytosis of peak 1 (containing two gamma A chains) and peak 2 (containing one gamma A and one gamma' chain) fractions of human fibrinogen obtained from diethyl aminoethyl (DEAE) cellulose chromatography in an in vivo hamster model. When 10 mg of biotinylated, unfractionated, or peak 1 fibrinogen was injected intravenously, each protein was endocytosed into megakaryocytes and platelets within 24 hours. In contrast, equivalent doses of biotinylated peak 2 fibrinogen and bovine serum albumin were barely detectable within megakaryocytes and platelets. We conclude that gamma' fibrinogen is not endocytosed and incorporated into megakaryocytes and platelet alpha granules. Furthermore, a dimeric gamma A-chain configuration is required for receptor-mediated endocytosis of fibrinogen into these organelles.     

Incorporation of a circulating protein into megakaryocyte and platelet granules.

To determine whether or not proteins circulating in plasma can be incorporated into megakaryocytes and platelets, horseradish peroxidase (HRP) was injected intravenously into guinea pigs and these cells were examined for its uptake by electron microscopy and cytochemistry. Enriched samples of megakaryocytes enabled ultrastructural analysis of large numbers of these rare cells. In megakaryocytes, 50% of alpha granules contained HRP between 75 min and 7 hr after injection. At 24 hr, 25% of the megakaryocyte granules were peroxidase-positive, less were positive by 48 hr, and there were none at 4 days. Thus, the findings demonstrate that a circulating protein can be endocytosed by megakaryocytes and rapidly packaged into alpha granules. Platelet granules also contain HRP by 7 hr after injection, and they can secrete it in response to thrombin. Unfortunately, our present studies do not allow us to distinguish between direct endocytosis by the platelet and/or shedding of new platelets from recently labeled megakaryocytes. It is concluded that while some alpha granule proteins are synthesized by megakaryocytes, others may be acquired from plasma by endocytosis. In addition to providing evidence that some of the proteins of alpha granules may be of exogenous origin, this study has allowed the definition of a pathway whereby plasma proteins may be temporarily sequestered in megakaryocytes before reentering the circulation in platelets.     

Low plasma levels of tissue factor pathway inhibitor in patients with congenital factor V deficiency

Severe factor V (FV) deficiency is associated with mild to severe bleeding diathesis, but many patients with FV levels lower than 1% bleed less than anticipated. We used calibrated automated thrombography to screen patients with severe FV deficiency for protective procoagulant defects. Thrombin generation in FV-deficient plasma was only measurable at high tissue factor concentrations. Upon reconstitution of FV-deficient plasma with purified FV, thrombin generation increased steeply with FV concentration, reaching a plateau at approximately 10% FV. FV-deficient plasma reconstituted with 100% FV generated severalfold more thrombin than normal plasma, especially at low tissue factor concentrations (1.36 pM) or in the presence of activated protein C, suggesting reduced tissue factor pathway inhibitor (TFPI) levels in FV-deficient plasma. Plasma TFPI antigen and activity levels were indeed lower (P < .001) in FV-deficient patients (n = 11; 4.0 +/- 1.0 ng/mL free TFPI) than in controls (n = 20; 11.5 +/- 4.8 ng/mL), while persons with partial FV deficiency had inter-mediate levels (n = 16; 7.9 +/- 2.5 ng/mL). FV immunodepletion experiments in normal plasma and surface plasmon resonance analysis provided evidence for the existence of a FV/TFPI complex, possibly affecting TFPI stability/clearance in vivo. Low TFPI levels decreased the FV requirement for minimal thrombin generation in FV-deficient plasma to less than 1% and might therefore protect FV-deficient patients from severe bleeding.     

Covalent crosslinking of human coagulation factor V by activated factor XIII from guinea pig megakaryocytes and human plasma

Coagulation factor V (FV) has been shown to be synthesized in both the liver and megakaryocytes. We now present evidence that FV can be covalently crosslinked by an enzyme originating from megakaryocytes to form polymeric multimers of factor V. The guinea pig megakaryocyte enzyme appears to be factor XIIIa since the FV-crosslinking activity (1) had an absolute requirement for Ca++, (2) was completely inhibited by iodoacetamide, 5,5'-dithiobis- (2-nitrobenzoic acid), p- chloromercuribenzene sulfonic acid, and N-ethylmaleimide, all known alkylators of the thiol group at the active site of the factor XIIIa, (3) was blocked by known pseudoamine donor substrates of factor XIIIa including dansylcadaverine and putrescine, and (4) could be directly demonstrated in the guinea pig megakaryocyte lysate by a specific activity staining procedure. No tranglutaminase was detected in guinea pig megakaryocytes in contrast to red cells and liver. A similar pattern of covalent crosslinking of human FV by purified activated human plasma factor XIII was also demonstrated. Analysis of the crosslinked products of FV formed by the guinea pig enzyme by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) indicates the formation of intermediate as well as higher molecular weight polymers, suggesting that the crosslinking is a stepwise polymerization process.     

Fibrinogen gamma-chain mRNA is not detected in human megakaryocytes

Human megakaryocytes and platelets contain counterparts of several plasma proteins. The origin of most of these alpha-granule proteins is unclear. Fibrinogen represents one of those molecules, being essential in hemostasis, thrombosis, and platelet aggregation. To study whether fibrinogen is endocytosed by megakaryocytes and packaged into alpha- granules or newly synthesized by these cells, we established a highly sensitive nested primer polymerase chain reaction for the detection of human fibrinogen gamma-chain mRNA. In enriched megakaryocyte fractions, as well as fluorescence-activated cell sorter-purified megakaryocytes from bone marrow samples of healthy volunteers, no fibrinogen gamma- chain mRNA could be detected, despite the presence of the corresponding fibrinogen gamma-chain DNA. We conclude that fibrinogen gamma-chain mRNA, as detectable by our amplification system, is missing in megakaryocytes. This finding suggests that fibrinogen might be acquired from plasma by endocytosis and sequestered in alpha-granules before reentering the circulation after platelet activation.     

Coinheritance of Factor V (FV) Leiden enhances thrombin formation and is associated with a mild bleeding phenotype in patients homozygous for the FVII 9726+5G>A (FVII Lazio) mutation

We investigated the role of thrombophilic mutations as possible modifiers of the clinical phenotype in severe factor VII (FVII) deficiency. Among 7 patients homozygous for a cross-reacting material-negative (CRM-) FVII defect (9726+5G>A, FVII Lazio), the only asymptomatic individual carried FV Leiden. Differential modulation of FVII levels by intragenic polymorphisms was excluded by a FVII to factor X (FX) gene haplotype analysis. The coagulation efficiency in the FV Leiden carrier and a noncarrier was evaluated by measuring FXa, FVa, and thrombin generation after extrinsic activation of plasma in the absence and presence of activated protein C (APC). In both patients coagulation factor activation was much slower and resulted in significantly lower amounts of FXa and thrombin than in a normal control. However, more FXa and thrombin were formed in the plasma of the patient carrying FV Leiden than in the noncarrier, especially in the presence of APC. These results were confirmed in FV-FVII doubly deficient plasma reconstituted with purified normal FV or FV Leiden. The difference in thrombin generation between plasmas reconstituted with normal FV or FV Leiden gradually decreased at increasing FVII concentration. We conclude that coinheritance of FV Leiden increases thrombin formation and can improve the clinical phenotype in patients with severe FVII deficiency.     

Megakaryocytopoiesis in vitro of patients with essential thrombocythaemia: effect of plasma and serum on megakaryocyte colony formation

To clarify the mechanism of increased numbers of megakaryocytes in patients with essential thrombocythaemia (ET), we studied in vitro megakaryocytopoiesis in ET and other myeloproliferative disorders, using a megakaryocytic colony assay in methylcellulose containing plasma or serum and medium conditioned by phytohaemagglutinin (PHA) stimulated leucocytes (PHA-LCM). Megakaryocytic colony formation was supported well by heparinized or citrated plasma and citrated serum which was harvested after clot formation of citrated plasma. Whole serum was inhibitory for megakaryocytic colony growth. The addition of platelet releasates and partially purified platelet derived growth factor (PDGF) resulted in a decrease in the number of megakaryocytic colonies. These findings suggested that platelet-derived factor(s) in serum was inhibitory to megakaryocytic colony formation. ET plasma supported the megakaryocytic colony formation by normal or ET bone marrow cells better than normal plasma. Moreover, in ET bone marrow cells, spontaneous megakaryocytic colonies were formed in the absence of PHA-LCM. Increased megakaryocytopoiesis in ET may be ascribed to (i) increased megakaryocyte-colony stimulating activity (Meg-CSA) in plasma and (ii) increased sensitivity to Meg-CSA or autonomous proliferation of megakaryocytic progenitor cells.     

Phenotypic homozygous activated protein C resistance associated with compound heterozygosity for Arg506Gln (factor V Leiden) and His1299Arg substitutions in factor V

Two patients from two unrelated families with a history of thrombosis showed severe plasma activated protein C (APC) resistance. However, genotypic analysis demonstrated that the patients were heterozygous for factor V (FV) Leiden mutation. Coagulation studies revealed that FV clotting activity and antigen were similarly reduced at about 50% of normal in the patients. One brother of propositus A also showed the same abnormalities. Genetic analysis showed that, in addition to FV Leiden mutation in exon 10 of the FV gene (G1691A), these patients had a transition in exon 13 of the FV gene (A4070G; R2 allele) predicting His1299Arg substitution in the mature FV. Study by RT-PCR of platelet FV mRNA indicated that the mRNA produced by the FV gene, marked by the R2 allele, was reduced in amount in both pseudohomozygous patients of family A. The R2 allele has previously been demonstrated to be significantly associated with plasma FV deficiency in the Italian population. The presence of FV deficiency did not protect the propositi from thrombosis. These data confirm that genotypic analysis is mandatory in patients with phenotypic severe APC resistance before these patients are definitely classified as homozygotes for FV Leiden and that further genotypic analysis is advisable.     

Endocytosis of fibrinogen into megakaryocyte and platelet alpha- granules is mediated by alpha IIb beta 3 (glycoprotein IIb-IIIa) [published erratum appears in Blood 1993 Nov 1;82(9):2936]

Recently, we showed that platelet alpha-granule fibrinogen is derived entirely from endocytic uptake and not from megakaryocyte synthesis, as previously thought. In this present report, we identify the receptor that mediates endocytosis. We have found that barbourin, a unique disintegrin that is a specific antagonist of alpha IIb beta 3, inhibits the endocytic uptake of fibrinogen into alpha-granules. Continuous intravenous infusion of barbourin (200 micrograms/h) into guinea pigs blocked collagen-induced platelet aggregation as well as endocytosis of biotinylated fibrinogen into megakaryocytes; however, endocytosis of biotinylated albumin by megakaryocytes was not affected. Thus, we have shown that endocytosis of fibrinogen into megakaryocyte and platelet alpha-granules is receptor-mediated, and that alpha IIb beta 3 is the primary receptor.     

Suppression of in vitro megakaryocyte production by antiplatelet autoantibodies from adult patients with chronic ITP

Chronic immune thrombocytopenic purpura (ITP) is manifested by autoantibody-induced platelet destruction. Platelet turnover studies suggest that autoantibody may also affect platelet production. To evaluate this, we studied the effect of plasma from adult patients with chronic ITP on in vitro megakaryocyte production. CD34(+) cells, obtained from healthy donors, were cultured in medium containing PEG-rHuMGDF and 10% plasma from either ITP patients or healthy subjects. Cultures containing plasma from 12 of 18 ITP patients showed a significant decrease (26%-95%) in megakaryocyte production when compared with control cultures. Positive ITP plasmas not only reduced the total number of megakaryocytes produced during the culture period but also inhibited megakaryocyte maturation, resulting in fewer 4N, 8N, and 16N cells. The role of antibody in this suppression is supported by 2 factors: (1) immunoglobulin G (IgG) from ITP patients inhibited megakaryocyte production when compared with control IgG; and (2) adsorption of autoantibody, using immobilized antigen, resulted in significantly less inhibition of megakaryocyte production when compared with unadsorbed plasma. These results show that plasma autoantibody from some adult patients with ITP inhibits in vitro megakaryocyte production, suggesting that a similar effect may occur in vivo.     

Expression of platelet glycoprotein Ib by cultured human megakaryocytes: ultrastructural localization and biosynthesis

Glycoprotein Ib (GPIb), the receptor for von Willebrand factor, is a two-chain member constituent of the platelet/megakaryocytic lineage. Studies on its expression have been hampered by the difficulties in obtaining purified megakaryocytes in a sufficient number. We report a suspension liquid culture procedure that allowed isolation of more than 1 x 10(6) megakaryocytes with a purity ranging from 3% to 88% from the blood of patients with chronic myeloid leukemia, from fetal liver or from normal human bone marrow. GPIb was detected on the plasma membrane of all maturing megakaryocytes and also of promegakaryoblasts devoid of demarcation membranes. GPIb was detected on demarcation membranes of maturing megakaryocytes but was absent from all other organelles, including alpha granules. Biosynthesis of 35S-methionine labeled megakaryocytes showed that GPIb with similar electrophoretic mobility to the platelet molecule was synthesized and that it was also composed of two chains, since its molecular weight shifted in reducing conditions from 170 Kd to 145 Kd. The beta chain remained undetectable after methionine metabolic labeling, but it was immunoprecipitated after 3H-leucine metabolic labeling, confirming that this subunit is devoid of methionine. GPIb was associated with GPIX, as it is in platelets, since anti-GPIb antibodies coprecipitated a 17 Kd polypeptide.     

The purification of megapoietin: a physiological regulator of megakaryocyte growth and platelet production.

The circulating blood platelet is produced by the bone marrow megakaryocyte. In response to a decrease in the platelet count, megakaryocytes increase in number and ploidy. Although this feedback loop has long been thought to be mediated by a circulating hematopoietic factor, no such factor has been purified. Using a model of thrombocytopenia in sheep, we have identified an active substance called megapoietin, which stimulated an increase in the number and ploidy of megakaryocytes in bone marrow culture. Circulating levels of this factor could be quantified with this assay and were found to be inversely proportional to the platelet count of the sheep. Levels increased from < 0.26 pM in normal sheep to 25-40 pM in thrombocytopenic sheep. From large amounts of thrombocytopenic sheep plasma we have purified a 31,200-Da protein and found that it retained the ability to stimulate both megakaryocyte number and ploidy in vitro. Injection of partially purified megapoietin into rats stimulated a 24% increase in megakaryocyte number and a 60% increase in mean ploidy as well as a 77% increase in the platelet count. Sheep platelets bound megapoietin and the amount of platelets required to eliminate half the activity in vitro was close to the amount associated with this same level of activity in vivo. We believe that megapoietin is the physiologically relevant mediator of megakaryocyte growth and platelet production. Moreover, our data suggest that the level of megapoietin is directly determined by the ability of platelets to remove megapoietin from the circulation.     

Cyclooxygenase-2 expression is induced during human megakaryopoiesis and characterizes newly formed platelets

Cyclooxygenase (COX)-1 or -2 and prostaglandin (PG) synthases catalyze the formation of various PGs and thromboxane (TX) A(2). We have investigated the expression and activity of COX-1 and -2 during human megakaryocytopoiesis. We analyzed megakaryocytes from bone marrow biopsies and derived from thrombopoietin-treated CD34(+) hemopoietic progenitor cells in culture. Platelets were obtained from healthy donors and patients with high platelet regeneration because of immune thrombocytopenia or peripheral blood stem cell transplantation. By immunocytochemistry, COX-1 was observed in CD34(+) cells and in megakaryocytes at each stage of maturation, whereas COX-2 was induced after 6 days of culture, and remained detectable in mature megakaryocytes. CD34(+) cells synthesized more PGE(2) than TXB(2) (214 +/- 50 vs. 30 +/- 10 pg/10(6) cells), whereas the reverse was true in mature megakaryocytes (TXB(2) 8,440 +/- 2,500 vs. PGE(2) 906 +/- 161 pg/10(6) cells). By immunostaining, COX-2 was observed in <10% of circulating platelets from healthy controls, whereas up to 60% of COX-2-positive platelets were found in patients. A selective COX-2 inhibitor reduced platelet production of both PGE(2) and TXB(2) to a significantly greater extent in patients than in healthy subjects. Finally, we found that COX-2 and the inducible PGE-synthase were coexpressed in mature megakaryocytes and in platelets. We conclude that both COX-isoforms contribute to prostanoid formation during human megakaryocytopoiesis and that COX-2-derived PGE(2) and TXA(2) may play an unrecognized role in inflammatory and hemostatic responses in clinical syndromes associated with high platelet turnover.     

Biosynthesis and secretion of functional protein S by a human megakaryoblastic cell line (MEG-01)

A human megakaryoblastic cell line (MEG-01) was investigated for the presence of protein S in culture medium and cell lysates using a specific enzyme-linked immunoassay (ELISA) and a functional assay. When 5 X 10(5) MEG-01 cells/mL was subcultured in RPMI 1640 medium with 10% fetal calf serum (FCS), the concentration of protein S antigen in the culture medium increased progressively with time from less than 8 ng/mL on day 0 to 105.6 +/- 6.0 ng/mL on day 13. Vitamin K2(1 microgram/mL) increased the production of functional protein S, whereas warfarin (1 microgram/mL) profoundly decreased the quantity and the specific activity of secreted protein S. By an indirect immunofluorescent technique, protein S antigen was detected in both MEG-01 cells and human bone marrow megakaryocytes. Immunoblot analysis of culture medium revealed two distinct bands (mol wt 84,000 and 78,000) that are identical to the doublets of purified plasma protein S. De novo synthesis of protein S was demonstrated by the presence of specific immunoprecipitable radioactivity in the medium after 5 hours of labeling of the cells with [35S]-methionine as a 84,000 mol wt protein. Plasma protein S levels of nine patients with severe aplastic anemia were not significantly different from those of normal controls. These results suggest that megakaryocytes produce functional protein S and contain the enzymes required for the carboxylation of selected glutamic acid residues, and that protein S synthesized by megakaryocytes does not represent a main source of plasma protein S.     

The relationship of the properties of antihemophilic factor (factor VIII) that support ristocetin-induced platelet agglutination (factor VIIIR:RC) and platelet retention by glass beads as demonstrated by a monoclonal antibody

A monoclonal antibody to human antihemophilic factor (AHF, factor VIII) was derived from BALB/c mouse spleen cells fused with P3x63Ag8 mouse plasmacytoma cells. This antibody, harvested from culture medium or ascites fluid, reacted with purified AHF and with plasmas with normal subjects or classic hemophiliacs, as measured by enzyme-linked immunosorbent assay (ELISA), but not with plasmas from patients with severe von Willebrand's disease. The antibody possessed only IgG, heavy chains and kappa light chains. It blocked ristocetin-induced platelet agglutination and, to a lesser degree, platelet retention by glass bead columns, but it did not inhibit the procoagulant activity of AHF significantly. An amount of rabbit antiserum against AHF that provided equivalent inhibition of ristocetin-induced platelet agglutination inhibited glass bead retention much more effectively than the mouse monoclonal antibody. This difference was exaggerated in studies of the corresponding Fab fragments. These data suggest that the site or sites on the AHF complex molecule that are associated with ristocetin-induced platelet agglutination differ quantitatively or qualitatively from those associated with enhancement of platelet retention by glass beads. ELISA titers of immunoreactive AHF, using the monoclonal antibody, were closely correlated to those using rabbit antiserum against AHF in normal, hemophilic, and most von Willebrand's disease plasma.     

Molecular characterization of 3 factor V mutations, R2174L, V1813M, and a 5-bp deletion, that cause factor V deficiency

We identified 3 mutations in the factor V (FV) gene (F5) associated with FV deficiency in 3 unrelated Japanese patients. Patient 1 had severe bleeding symptoms (plasma FV activity, <1%; FV antigen, 9%) and was a compound heterozygote for a novel 5-bp deletion in exon 22 and the V1813M mutation. Patient 2 had moderate bleeding symptoms (plasma FV activity, <1%; FV antigen, 4%) and was homozygous for the V1813M mutation. Patient 3 had very mild symptoms (plasma FV activity, 1%; FV antigen, 5%) and was homozygous for the novel R2174L mutation. A study of recombinant protein expression revealed that the FV coagulant-specific activities in conditioned media for the FV-R2174L and FV-V1813M mutants were reduced to approximately 40% and 28% of wild-type FV, respectively. The amounts of FV-R2174L protein and messenger RNA in the platelets of patient 3 were similar to those of healthy subjects; however, the amount of FV-V1813M protein in patient 2 was decreased. Our data suggest that the severity of the bleeding tendency in patients with FV deficiency is correlated not only with plasma FV activity but also with the amount of FV protein in the platelets.