Factor-VIII-related antigen in platelets

A novel approach to laboratory assessment and reporting of platelet von Willebrand factor

The interaction of platelets with von Willebrand factor is essential for primary hemostasis. Concentration and activity of plasma von Willebrand factor are routine parameters in the assessment of hemostasis disorders. In addition to plasma von Willebrand factor, platelet von Willebrand factor, synthesized in megakaryocytes and stored in α-granules of circulating platelets, is known to contribute to primary hemostasis and the microenvironment of thrombus formation. The laboratory assessment of platelet von Willebrand factor however is cumbersome and not widely established as a routine parameter. We here propose a method for laboratory assessment and reporting of platelet von Willebrand factor potentially useful for laboratory routines in specialized laboratories. Our model allows to describe platelet von Willebrand factor as 1. the concentration of platelet von Willebrand factor in whole blood, 2. the amount of platelet von Willebrand factor in a sample with a defined concentration of 1000 platelets/nl, and 3. the concentration of platelet von Willebrand factor in one platelet. According to our results in healthy individuals, the proportion of platelet von Willebrand factor activity is estimated to be about 10% of total von Willebrand factor in human plasma under physiological circumstances. The concentration of platelet von Willebrand factor is estimated to be 0.4 IU/ml in a sample with a defined concentration of 1000 platelets/nl and to be about 42 IU/ml in one platelet (both expressed as VWF:Ag).

Bivalent property of intra-platelet VWF in liver regeneration and HCC recurrence: A prospective multicenter study

BACKGROUND AND AIMS

A striking difference has been observed in structure and functional properties between plasma and platelet von Willebrand factor (VWF). While the existing evidence has revealed a clinical relevance of plasma VWF-Ag in liver regeneration (LR) and different cancers, this study was designed to explore the properties of intra-platelet (IP) and serum VWF-Ag in patients with hepatocellular carcinoma (HCC) undergoing partial hepatectomy.

METHODS

A total of 40 patients undergoing partial hepatectomy were prospectively recruited from 3 institutions. VWF-Ag concentrations were evaluated mainly in serum and platelet extracts. Patients were followed-up for postoperative liver dysfunction and HCC recurrence.

RESULTS

We observed a post-resection increase in the concentration of VWF-Ag in serum and platelet. Patients with postoperative liver dysfunction had substantially reduced serum and IP VWF-Ag concentrations. After a 2-year follow-up, patients with higher post-resection serum and IP VWF-Ag concentrations were found to develop early HCC recurrence. Likewise, IP VWF-Ag was able to independently predict post-resection early HCC recurrence.

CONCLUSION

This multicenter, prospective, pilot study demonstrates a bivalent property of IP VWF in LR and oncological outcome; low preoperative VWF appeared to have a negative association on post-resection liver dysfunction, whereas, patients with higher post-resection VWF-Ag concentrations were found to have early HCC recurrence.

von Willebrand factor sialylation-A critical regulator of biological function

Essentials Von Willebrand Factor (VWF) is extensively glycosylated with serial studies demonstrating that these carbohydrate determinants play critical roles in regulating multiple aspects of VWF biology. Terminal sialic acid residues, expressed on both the N- and O-linked glycans of VWF, regulate VWF functional activity, susceptibility to proteolysis and plasma clearance in vivo. Quantitative and qualitative variations in VWF sialylation have been reported in patients with von Willebrand Disease, as well as in a number of other physiological and pathological states. Further studies are warranted to define the molecular mechanisms through which N- and O-linked sialylation impacts upon the multiple biological activities of VWF. von Willebrand factor (VWF) undergoes complex post-translational modification prior to its secretion into the plasma. Consequently, VWF monomers contain complex N-glycan and O-glycan structures that, together, account for approximately 20% of the final monomeric mass. An increasing body of evidence has confirmed that these carbohydrate determinants play critical roles in regulating multiple aspects of VWF biology. In particular, studies have demonstrated that terminal ABO blood group has an important effect on plasma VWF levels. This effect is interesting, given that only 15% of the N-glycans and 1% of the O-glycans of VWF actually express terminal ABO(H) determinants. In contrast, the vast majority of the N-glycans and O-glycans on human VWF are capped by terminal negatively charged sialic acid residues. Recent data suggest that sialylation significantly regulates VWF functional activity, susceptibility to proteolysis, and clearance, through a number of independent pathways. These findings are of direct clinical relevence, in that quantitative and qualitative variations in VWF sialylation have been described in patients with VWD, as well as in patients with a number of other physiologic and pathologic conditions. Moreover, platelet-derived VWF is significantly hyposialylated as compared with plasma-derived VWF, whereas the recently licensed recombinant VWF therapeutic is hypersialylated. In this review, we examine the evidence supporting the hypothesis that VWF sialylation plays multiple biological roles. In addition, we consider data suggesting that quantitative and qualitative variations in VWF sialylation may play specific roles in the pathogenesis of VWD, and that sialic acid expression on VWF may also differ across a number of other physiologic and pathologic conditions.

Evaluation of von Willebrand factor concentrates by platelet adhesion to collagen using an in vitro flow assay

BACKGROUND

Von Willebrand disease (VWD) results from quantitative or qualitative deficiency of von Willebrand factor (VWF) and is treated using VWF-containing concentrates. Several studies have compared the function of various VWF containing concentrates however this has not been performed using shear based assays.

OBJECTIVES

To compare the platelet-capture potential of 10 commercially available, plasma-derived VWF concentrates under shear conditions.

METHODS

VWF containing concentrates were assessed for VWF:Ag, VWF:CB, VWF:RCo, factor VIII:C ADAMTS13 content, VWF multimeric profile and glycan content using lectin binding assays. Free-thiol content of each concentrate was investigated using MPB binding assays. An in vitro flow assay was used to determine the ability of each concentrate to mediate platelet capture to collagen.

RESULTS

VWF multimeric analysis revealed reduction of high molecular weight (HMW) forms in four of the concentrates (Alphante, Octanate and Haemoctin, and 8Y). The high MW multimer distribution of the remaining six concentrates (Optivate, Wilate, Fandhi, Wilfactin, Haemate P, and Voncento) was similar to the plasma control. Lectin analysis demonstrated that 8Y had increased amount of T-antigen. Although platelet capture after 5 minutes perfusion was similar for all concentrates; Alphante, Octanate, and Haemoctin, demonstrated the lowest levels of platelet capture after 60 seconds of perfusion. Free-thiol content and ADAMTS13 levels varied widely between the concentrates but was not correlated with function.

CONCLUSION

Alphanate, Octanate, and Haemoctin, lacked HMW multimers and had the lowest initial platelet capture levels suggesting that the presence of VWF HMW multimers are required for initial platelet deposition.

von Willebrand factor and inflammation

Von Willebrand factor (VWF) is a plasma glycoprotein best known for its crucial hemostatic role in serving as a molecular bridge linking platelets to subendothelial components following vascular injury. In addition, VWF functions as chaperone for coagulation factor VIII. In pathological settings, VWF is recognized as a risk factor for both arterial and venous thrombosis, as well as a molecular player that directly promotes the thrombotic process. Recent years have seen the emergence of the concept of immuno-thrombosis by which inflammatory cells participate in thrombotic processes. In return, reports about the involvement of hemostatic proteins or cells (such as platelets) in inflammatory responses have become increasingly common, emphasizing the intricate link between hemostasis and inflammation. However, evidence of a link between VWF and inflammation arose much earlier than these recent developments. At first, VWF was considered only as a marker of inflammation in various pathologies, due to its acute release by the activated endothelium. Later on, a more complex role of VWF in inflammation was uncovered, owing to its capacity to direct the biogenesis of specific endothelial organelles, the Weibel-Palade bodies that contain known inflammation players such as P-selectin. Finally, a more direct link between VWF and inflammation has become apparent with the discovery that VWF is able to recruit leukocytes, either via direct leukocyte binding or by recruiting platelets which in turn will attract leukocytes. This review will focus on these different aspects of the connection between VWF and inflammation, with particular emphasis on VWF-leukocyte interactions.

Von Willebrand factor processing

Von Willebrand factor (VWF) is a multimeric glycoprotein essential for primary haemostasis that is produced only in endothelial cells and megakaryocytes. Key to VWF's function in recruitment of platelets to the site of vascular injury is its multimeric structure. The individual steps of VWF multimer biosynthesis rely on distinct posttranslational modifications at specific pH conditions, which are realized by spatial separation of the involved processes to different cell organelles. Production of multimers starts with translocation and modification of the VWF prepropolypeptide in the endoplasmic reticulum to produce dimers primed for glycosylation. In the Golgi apparatus they are further processed to multimers that carry more than 300 complex glycan structures functionalized by sialylation, sulfation and blood group determinants. Of special importance is the sequential formation of disulfide bonds with different functions in structural support of VWF multimers, which are packaged, stored and further processed after secretion. Here, all these processes are being reviewed in detail including background information on the occurring biochemical reactions.

Altered glycosylation of platelet-derived von Willebrand factor confers resistance to ADAMTS13 proteolysis

Platelet-VWF exists as a distinct natural glycoform. Platelet-VWF is resistant to ADAMTS13 proteolysis.

Role of platelets in the development of atherosclerotic disease and possible interference with platelet inhibitor drugs

During the last two decades, significant advances have been made in the understanding of atherosclerotic disease. The pathogenesis of atherosclerosis appears to depend on a precise sequence of critical events based on the interaction of blood elements and lipids with the arterial wall. The major critical events and their sequence appears to be as follows: hemodynamic stress and endothelial injury; arterial wall-platelet interaction; smooth muscle cell proliferation; lipid entry and accumulation; significant arterial narrowing with fibrosis and development of thrombi; and complications in the form of calcification, ulceration, aneurysm, acute thrombotic occlusion and embolization. This sequence of critical events starts at a young age and in all geographic racial groups. Their evolution into advanced symptomatic lesions takes many years and varies in incidence and extent among different geographic and ethnic groups. It appears that in promoting and accelerating this process into the advanced stage of the disease, the presence at a young age of the so-called risk factors of atherosclerotic disease is most important. The recent advances in the understanding of the atherosclerotic process will be highlighted in this chapter with particular attention being focused on the role of platelets and thrombosis in the development of the disease and the possible role of platelet inhibitor drugs on the prevention of coronary atherosclerotic disease.

Apical sorting of ADAMTS13 in vascular endothelial cells and Madin-Darby canine kidney cells depends on the CUB domains and their association with lipid rafts

ADAMTS13 biosynthesis appeared to occur mainly in hepatic stellate cells, but detection of ADAMTS13 mRNA in many other tissues suggests that vascular endothelium may also produce ADAMTS13. We showed that ADAMTS13 mRNA and protein were detectable in human umbilical vein endothelial cells, aortic endothelial cells, and endothelium-derived cell line (ECV304). ADAMTS13 in cell lysate or serum-free conditioned medium cleaved von Willebrand factor (VWF) specifically. ADAMTS13 and VWF were localized to the distinct compartments of endothelial cells. Moreover, ADAMTS13 was preferentially sorted into apical domain of ECV304 and Madin-Darby canine kidney (MDCK) cells. Apical sorting of ADAMTS13 depended on the CUB domains and their association with lipid rafts. A mutation in the second CUB domain of ADAMTS13 (4143-4144insA), naturally occurring in patients with inherited thrombotic thrombocytopenic purpura, resulted in a significant reduction of ADAMTS13 secretion and a reversal of its polarity in MDCK cells. These data demonstrated that ADAMTS13 is synthesized and secreted from endothelial cells; the apically secreted ADAMTS13 from endothelial cells may contribute significantly to plasma ADAMTS13 proteases. The data also suggest a critical role of the CUB domains and a novel cargo-selective mechanism for apical sorting of a soluble ADAMTS protease in polarized cells.

Platelet-derived VWF-cleaving metalloprotease ADAMTS-13

The adhesion ligand von Willebrand factor (VWF) is synthesized and stored in vascular endothelial cells and megakaryocytes/platelets. As in endothelial cells, platelet VWF also contains ultra-large (UL) multimers that are hyperactive in aggregating platelets. ULVWF in platelet lysates of thrombin-stimulated platelets was only detected in the presence of EDTA, suggesting that ULVWF is cleaved by a divalent cation-dependent protease. A recent study shows that platelets contain the VWF-cleaving metalloprotease ADAMTS-13, but its activity remains unknown. In this study, we show that platelet lysates cleave endothelial cell-derived ULVWF under static and flow conditions. This activity is inhibited by EDTA and by an ADAMTS-13 antibody from the plasma of a patient with acquired TTP. ADAMTS-13 was detected in platelet lysates and on the platelet surface by four antibodies that bind to different domains of the metalloprotease. Expression of ADAMTS-13 on the platelet surface increases significantly upon platelet activation by the thrombin receptor-activating peptide, but not by ADP. These results demonstrate that platelets contain functionally active ADAMTS-13. This intrinsic activity may be physiologically important to prevent the sudden release of hyperactive ULVWF from platelets and serves as the second pool of ADAMTS-13 to encounter the increase in ULVWF release from endothelial cells.

Prevalence, morphology, and topography of blood vessels in herniated disc tissue. A comparative immunocytochemical study

STUDY DESIGN

Ninety disc herniations removed during surgery were studied by immunocytochemistry, using two different endothelial cell markers, to study the prevalence, morphology, and topography of blood vessels in disc herniations.

OBJECTIVES

To increase the specific localization of even very small blood vessels present in disc herniations by using specific antibodies to endothelial cells; to study blood vessels comparatively with two different endothelial cell antibodies, comparing their prevalence; and to study blood vessel morphology and topographic relationships of blood vessels to other tissue elements, particularly disc cells.

SUMMARY OF BACKGROUND DATA

In many previous macroscopic studies and in studies using conventional histologic methodology, blood vessels have been observed in degenerated and injured intervertebral discs. In a smaller patient sample, the authors previously observed blood vessels in approximately 80% of disc herniations by immunocytochemistry, the blood vessels co-localizing with macrophage cells. Many of these blood vessels are the product of very active neovascularization after disc tissue injury. The presence of such blood vessels has not, however, been studied in greater detail or in larger patient samples. Immunocytochemistry offers superior visualization and more specific localization and was thus used in the present study.

METHODS

Thin frozen sections from 90 disc herniations were immunostained in parallel with von Willebrand factor and Ulex europaeus antibodies, both of which localize endothelial cells specifically. Indirect immunocytochemistry by avidin-biotin-peroxidase complex or alkaline phosphatase-antialkaline phosphatase were used for immunolocalization. Blood vessels were classified as being: +, abundant: (+), very few; or +, totally absent.

RESULTS

The prevalence of blood vessels in disc herniations was found in 82 of 90 (91%) disc herniations with von Willebrand factor antibody and in 75 of 90 (83%) disc herniations with Ulex europaeus antibody. In 59 disc herniations (66%), blood vessels were observed with both antibodies in parallel, whereas they were observed with neither antibody in only six of 90 disc herniations. Furthermore, the ratio of abundant to very few blood vessels was 73:9 with von Willebrand factor antibody and 63:12 with Ulex europaeus antibody, further supporting the abundance of blood vessels in disc herniations. Blood vessels were most prevalent in sequestrated discs, but they were also observed in six of eight protrusions. Dense blood vessel networks were observed to penetrate the disc tissue, and blood vessels were also present in areas of inflammatory cell infiltration. Topographically, blood vessels were, on several occasions and with both antibodies, seen to pass close by or to surround disc cells.

CONCLUSIONS

By immunocytochemistry with endothelial cell markers, blood vessels can be observed to be numerous, and their prevalence in herniated discs is very high, presumably as a result of a very intense neovascularization process after the disc injury. A close apposition to disc cells may suggest attempts to increase the nutrition of these cells and will influence the metabolism of the cells.

Immunohistochemistry in diagnostic dermatopathology

Immunopathology continues to be important in diagnostic dermatopathology. Immunopathology is an invaluable tool for assessing the tissue of origin or direction of differentiation of cells. In some cases this can result in a more precise diagnosis. This article reviews the role of immunopathology in determining the biologic behavior of hematolymphoid infiltrates. It explores the methodology of immunoperoxidase, discusses the most commonly used antibody reagents, and presents a series of diagnostic dilemmas in which immunopathology can be useful. In each case a strategy is established that maximizes the likelihood of making a definitive diagnosis.

Pathogenesis, clinical picture and treatment of von Willebrand's disease

Von Willebrand's disease is probably the most common congenital bleeding disorder, with a prevalence close to 1% in some epidemiological studies. The disease is caused by a quantitative deficiency or a qualitative defect of the von Willebrand factor, which is a multimeric glycoprotein consisting of subunits of 2050 amino acids. The size of multimers ranges from approximately 500 kDa to 20 MDa. Each subunit consist of repeated domain structures. Several functional domains have been identified which can bind such structures as platelet receptors glycoprotein Ib or IIb/IIIA, heparin, collagen or factor VIII. The von Willebrand factor has two main functions in haemostasis, to promote normal platelet adhesion and to be a carrier protein for factor VIII. Von Willebrand's disease is divided into three major types and several subtypes depending on the quantity and quality of the von Willebrand factor in plasma and platelets. A new classification has recently been proposed. Typical symptoms are mucosal bleeding, easy bruising and increased bleeding tendency in connection with tooth extractions and other invasive procedures. Severe cases may have joint bleeding and other haemophilia-like bleeding. Desmopressin is the treatment of choice in mild cases, whereas more severe cases need treatment with factor VIII concentrates.

Purification and characterization of human platelet von Willebrand factor

Platelet von Willebrand factor (vWf) was purified from human platelet concentrates. The multimeric structure of the purified platelet vWf was similar to that observed in the initial platelet lysate, and, like the platelet lysate, the purified platelet vWf contained higher molecular weight multimers than plasma vWf. The apparent molecular weight of the reduced platelet vWf subunit was similar to the plasma vWf subunit. The N-terminal amino acid of the purified platelet and plasma vWf was blocked. In concentration dependent binding to botrocetin- or ristocetin-stimulated platelets, 125I-plasma vWf bound with a higher affinity than platelet. The ristocetin cofactor activity per mg of purified plasma vWf was 5-fold greater than the platelet vWf activity. Platelet and plasma vWf bound to collagen with similar affinities; however, platelet vWf bound to thrombin-stimulated platelets and to heparin with a higher affinity than plasma vWf. The differences in the binding affinity(s) of plasma and platelet vWf to platelet GPIb and GPIIb/IIIa and extracellular matrix proteins may reflect different roles for plasma and platelet vWf in the initial stages of haemostasis and thrombosis.

Desmopressin (DDAVP) and hemostasis

Desmopressin is a widely used hemostatic drug. It is a synthetic analogue of the natural hormone vasopressin, but, in contrast to vasopressin, it has no pressor activity. The effect is immediate, with two- to sixfold increases in the plasma concentrations of coagulation factor VIII, on Willebrand factor, and tissue plasminogen activator, and increases in platelet adhesiveness of comparable magnitude. Desmopressin is used in patients with mild hemophilia A, von Willebrand's disease, congenital platelet dysfunction, or acquired platelet dysfunction due to uremia or intake of such drugs as aspirin. It may also be used to reduce surgical blood loss in patients without known bleeding diathesis. Optimal hemostatic effect is achieved with a dosage of 0.3 micrograms/kg given intravenously. Other routes of administration are subcutaneous injection or intranasal spray. The latter proved to be efficient for home treatment of patients with bleeding disorders.

Alpha granule proteins in type I von Willebrand's disease

Platelet von Willebrand factor (vWf) is located in the alpha granules. Individuals with type I von Willebrand's disease (vWd) with prolonged bleeding times are best discriminated from those who have normal bleeding times by the normal level of platelet vWf ristocetin cofactor activity (vWf activity) and, to a lesser extent, by their platelet vWf antigen content. We have studied the content of adhesive proteins and platelet factor-4 (PF-4), and beta-thromboglobulin (beta TG) in the platelet alpha granules of types I and III vWd patients to determine if other alterations in alpha granule contents of proteins occur in vWd. We found that type I vWd patients with prolonged or normal bleeding times could not be differentiated on the basis of their platelet levels of beta TG, PF-4, fibronectin, or fibrinogen. The levels of the alpha granule constituents in the type I vWd patient were similar to normal except for the platelet fibrinogen concentration. Patients with type I vWd, regardless of the level of platelet vWf activity of antigen, had increased levels of platelet fibrinogen. The patients with type III vWd who had undetectable levels of platelet and plasma vWf also had increased levels of platelet fibrinogen. In our study we could not attribute the variation in the platelet vWf activity and antigen in type I vWd to the size of the alpha granule pool as determined by the measurement of other alpha granule proteins. The mechanism(s) of increased platelet fibrinogen in these vWd patients is at present unknown.

Stability of canine factor VIII activity and von Willebrand factor antigen concentration in vitro

The in vitro stability of canine factor VIII activity, von Willebrand factor antigen concentration and the ratio of these two factors was studied. Samples were stored for up to 48 hours, either as plasma or as whole blood, at 4 degrees, 20 degrees and 37 degrees C. Factor VIII activity was generally stable in both plasma and whole blood samples for up to 48 hours at 4 degrees or 20 degrees C. The concentration of von Willebrand factor antigen was more stable in samples stored as plasma than whole blood, and for a shorter time than factor VIII activity. Consequently, the stability of the ratio of these two factors was relatively poor in vitro.

von Willebrand factor and the blood vessel wall

The role of von Willebrand factor (vWF) in mediating platelet adhesion has been established with the use of in vitro perfusion systems. vWF binds to the subendothelium, changes in conformation, and is then able to interact with glycoprotein Ib. vWF deposited in the subendothelium is responsible for up to 40% of normal adhesion. The action of vWF is seen at high shear rates. It also acts at low shear rates, but other factors can assume its role. Binding of vWF to the subendothelium is not through the established collagen binding sites but involves a new domain on the vWF molecule. Collagen VI has been implicated as the vessel wall component involved. Deposition of vWF in the subendothelium is determined by growth conditions and activation state of the endothelial cell. Subendothelial vWF in von Willebrand's disease may support platelet adhesion in situations in which plasma vWF of the same patient does not.

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.

Canine von Willebrand's disease. A heterogeneous group of bleeding disorders

The term "von Willebrand's disease," refers to a group of inherited bleeding disorders, all of which are caused by a deficiency of the multimeric plasma glycoprotein, von Willebrand factor. The various forms of canine von Willebrand's disease can be categorized into one of three major types: in type I canine von Willebrand's disease, all sizes of von Willebrand factor multimers can be detected in the plasma; in type II canine von Willebrand's disease, only the smaller von Willebrand factor multimers are found in the plasma (larger multimers are absent); and in type III canine von Willebrand's disease, von Willebrand factor is completely absent from the plasma or present in only trace amounts. Von Willebrand's disease is common in dogs, but some forms of the disease are so mild that they are of questionable clinical significance.

Grey platelet syndrome: studies on platelet alpha-granules, lysosomes and defective response to thrombin

The platelets of a young man with the grey platelet syndrome were severely depleted of all seven alpha-granule proteins assayed as well as partially deficient in alpha-mannosidase and alpha-fucosidase; four other lysosomal enzymes were present in normal concentrations. Total platelet 5-hydroxytryptamine (5HT) and adenine nucleotides were normal, and 14C-5HT uptake reached normal levels only slightly more slowly than a control. Aggregation and dense body secretion occurred normally in response to ADP, adrenaline, collagen, PAF-acether, sodium arachidonate, A23187, Ionomycin, TPA and U44069, but were very delayed in response to thrombin. The increase in cytosolic free calcium in response to thrombin was very slow and much reduced in amplitude, whether in the presence or absence of extracellular Ca2+. These defects in response to thrombin were not corrected by the separate addition of purified alpha-granule proteins or by a whole releasate from normal platelets. It is suggested that these platelets, in addition to their alpha-granule deficiency, may have a specific defect of thrombin receptor-mediated activation of phospholipase C.

Antiheparin activity in human endothelial cells

Human endothelial cells possess antiheparin activity that neutralizes the anticoagulant action of heparin as measured by different tests of the clotting system. The antiheparin activity appears to be associated with an acid-soluble basic protein present in the particulate fraction of the endothelial cell cytoplasm. This finding might have some relevance in the maintenance of hemostasis. Furthermore, it might also have a pharmacological role in terms of resistance to exogenously infused heparin in patients with thromboembolic disorders.

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.

Haemostatic alterations in malaria correlate to parasitaemia

Fibrin(ogen) degradation products, platelet counts, antithrombin III, and the components of the Factor VIII complex were studied in a total of 80 patients with Plasmodium falciparum, Plasmodium vivax or Plasmodium ovale infections. The haemostatic findings were correlated to the numbers of parasitized erythrocytes and to each other. The results indicate that haemostatic changes in malaria correlate with the degree of parasitaemia. Evidence for moderate hyperfibrinolysis was found in patients with high P. falciparum parasitaemias only. Thrombocytopenia closely corresponded to parasitaemia and to von Willebrand factor levels, but appeared not to be linked to a consumption of coagulation factors. It was concluded that thrombocytopenia in malaria is not indicative of disseminated intravascular coagulation (DIC) but may relate to endothelial damage.

Aortic atherosclerosis in pigs with heterozygous von Willebrand disease. Comparison with homozygous von Willebrand and normal pigs

We have reported that pigs with severe homozygous von Willebrand disease (vWd) are resistant to spontaneous and high fat, high cholesterol, diet-induced atherosclerosis. In this study we report the quantitation of aortic atherosclerotic plaques in three groups of pigs fed with a high fat, high cholesterol (2%) diet from age 3 to 9 months. Nine normal pigs (normal factor VIII antigen, VIII R:AG, and ristocetin co-factor, VIII:RWF) had a mean of 21% atherosclerotic involvement of the distal aortic surface and a 4.5% mean involvement of the entire aorta. Five homozygous vWd pigs (undetected VIII R:AG and VIII:RWF) had a mean of 4.2% atherosclerotic involvement of the distal aortic surface and 1.2% involvement of the entire aorta (p less than 0.01, rank sum test). Five heterozygous vWd pigs (approximately 35% VIII R:AG and VIII:RWF) had a mean of 25% atherosclerotic involvement of the distal aortic surface and 6% involvement of the entire aorta; the results were not significantly different from those in the normal pigs. We concluded that resistance to atherosclerosis is not found in animals with moderate reduction of VIII R:AG and VIII:RWF. This may have implications for humans, since in human vWd both factors are almost always present.

Multimeric analysis of von Willebrand factor in megakaryocytes

Von Willebrand factor (vWF) is a glycoprotein that appears to play a major role in subserving the adhesion of platelets to subendothelium during hemostasis. Endothelial cells have been shown capable of synthesizing and releasing this large, multimeric glycoprotein that normally circulates in the plasma in association with the factor VIII coagulant molecule. Megakaryocytes, the precursor cells of blood platelets, also appear to possess vWF biosynthetic capacity, since cultured guinea pig megakaryocytes have been shown to produce a polypeptide precipitable by antibody to vWF. We now report a study of the multimeric structure of vWF in the megakaryocyte, as well as a quantitative comparison of megakaryocyte vWF with that of platelets and plasma in the guinea pig. Multimeric analysis on SDS agarose gels employing 125I-emu anti-human vWF revealed striking homology between human and guinea pig vWF. Platelets and megakaryocytes from the same guinea pigs contained vWF of highly comparable multimeric composition. Moreover, megakaryocytes and platelets both contained a subset of very high molecular weight multimers not present in plasma. Quantitation of vWF in megakaryocytes and platelets was achieved with a radioimmunoassay performed on detergent (NP-40) lysates of washed cells. These measurements showed that megakaryocytes and platelets contain 0.079 and 0.069 U of vWF per mg protein, respectively. The results of these studies suggest that megakaryocytes represent the primary, if in fact not sole, source of platelet vWF.

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.

Measurement of human endothelial cells in whole blood

To evaluate the usefulness of measuring circulating endothelial cells as an in vivo indicator for vascular injury, studies were performed in vitro using cultured human endothelial cells and Factor VIII-related antigen (VIIIR:Ag) as the specific cell marker. Cultured human endothelial cells mixed with normal whole blood were processed by Ficoll-Hypaque gradient sedimentation to remove red cells and granulocytes, followed by pellet formation of the mononuclear supernatant fraction to separate the bulk of platelets. Cell films, prepared from a measured volume of the final cell suspension, were stained with fluorescent antibody against VIII R:Ag, and positively stained cells were counted under fluorescence microscopy. The final recovery of endothelial cells was 64.8 +/- 19.3% (n = 41), and the minimal detectable concentration of endothelial cells was about 0.02 cells/microliter whole blood. No endothelial cells were detected in the blood of normal individuals.

Biosynthesis of the subunits of factor VIIIR by bovine aortic endothelial cells

The biosynthesis of the subunit of factor VIIIR was studied in bovine aortic endothelial cells by the techniques of immunoprecipitation and NaDodSO4/polyacrylamide gel electrophoresis. It was determined that the subunit is first produced as a Mr 240,000 glycoprotein precursor, which appears to undergo proteolytic cleavage at or about the time of secretion into the medium with a resultant change in apparent Mr to 225,000, the size of the mature subunit found in plasma. The Mr 240,000 species was detected within 10 min of the start of labeling of cells, but factor VIIIR was not detected in the culture medium until approximately equal to 50 min. Treatment of the cells with tunicamycin resulted in a decrease in the apparent Mr of both species but did not grossly inhibit processing of precursor to product or the secretion of the latter. Thus, much or all N-linked glycosylation of factor VIIIR is not essential for these steps. Accumulation of factor VIIIR in the medium continued over a 24-hr period of cell labeling with [35S]-methionine, without significant net intracellular accumulation of the precursor, suggesting that a large storage pool of factor VIIIR is not present in endothelial cells under these conditions.

Malignant hemangioendothelioma of the thyroid and factor VIII-related antigen

Thirty-six malignant hemangioendotheliomas of the thyroid were examined immunohistochemically using antibody probes to factor VIII-related antigen in order to reevaluate the histogenesis of this neoplasia. The 36 cases were reclassified according to their light microscopic features without prior knowledge of the immunohistochemical results. Three different tumor types were discerned: Group I: classical hemangioendotheliomas (20 cases); Group II: borderline cases between malignant hemangioendotheliomas and anaplastic carcinomas (14 cases) and Group III: anaplastic carcinomas with hemangio-endotheliomatous features (2 cases). Factor VIII-related antigen could be demonstrated in 12 (60%) tumors of group I, 3 (21%) tumors of group II and in neither tumor of group III. Five control cases with the typical histological picture of anaplastic carcinoma of the thyroid were negative for factor VIII-related antigen. The results of our study suggest that at least part of the tumors termed as malignant hemangioendotheliomas are in fact derived from endothelial cells.

Arteriosclerosis in normal and von Willebrand pigs: long-term prospective study and aortic transplantation study

In a long-term prospective study, five normal control pigs and five pigs with homozygous von Willebrand's disease received a nonfatty diet from age 3 months to 4 years; then the aortas were analyzed. The fibrous arteriosclerotic plaques in the distal abdominal aortic region involved an average of 28% of the surface area in control pigs and only 7% of the surface area in pigs with von Willebrand's disease (P less than 0.01). In a subsequent study of 3-month-old pigs, the distal abdominal aortic segments from nine normal pigs were cross-transplanted with segments from nine other normal pigs (control study), and aortic segments from four normal pigs were transplanted into four host pigs with von Willebrand's disease (exchange study). All pigs received a 2% cholesterol diet for up to 6 months; then the transplanted aortic segments were analyzed. The donor normal aortic segments in the host normal pigs developed arteriosclerosis that involved an average of 20% of the surface; the endothelial fluorescent pattern of von Willebrand factor was identified. In contrast, the donor normal aortic segments in the host pigs with von Willebrand's disease had arteriosclerosis that involved an average of only 4% of the surface (P less than 0.01); the endothelial cell von Willebrand factor was not identified. The long-term prospective study indicates that pigs with von Willebrand's disease are resistant to the development of spontaneous arteriosclerosis. The aortic transplantation data are compatible with the hypothesis that the absence of von Willebrand factor in pigs with von Willebrand's disease may cause impairment of platelet-arterial wall interaction and resistance to arteriosclerosis.

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.

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.

Quinine- and quinidine-dependent antiplatelet antibodies. Requirement of factor VIII-related antigen for platelet damage and for in vitro transformation of lymphocytes from patients with drug-induced thrombocytopenia

The requirement of Factor VIII-related antigen (VIIIR:Ag) for platelet damage by quinine-and quinidine-dependent antibodies was studied in platelet-rich plasma (PRP) of four patients with severe von Willebrand's disease (vWd) (Factor VIII deficiency). Platelet factor 3 availability, platelet aggregation, and release of [(14)C]serotonin from labeled vWd-PRP by drug-dependent antibodies were significantly reduced in comparison with PRP from normal controls. Addition of purified VIIIR:Ag restored levels of platelet damage to that of normal PRP. In vWd-PRP, platelet damage by two human antiplatelet sera, not dependent on drugs, and by a rabbit antiplatelet serum did not differ from that in normal PRP. PRP from patients deficient in Factor VIII coagulant activity, Factor IX, or Factors II, VII, IX, and X behaved like normal PRP. The role of VIIIR:Ag in forming antigen able to transform lymphocytes of patients who had recovered from drug-induced thrombocytopenia was investigated by measuring incorporation of [methyl-(3)H]thymidine into DNA. When lymphocytes were cultured for 7 d, significantly less transformation occurred in response to platelets and the drug in the presence of vWd sera than in normal sera or sera deficient only in Factor VIII coagulant activity or Factor IX. Addition of purified VIIIR:Ag to vWd sera restored transformation to that obtained in normal sera. Nonspecific lymphocyte transformation by pokeweed mitogen was not affected by VIIIR:Ag. Thus VIIIR:Ag is involved both in platelet damage by drug-dependent antibodies and in the interaction between platelet and drug which produces an antigen able to transform sensitized lymphocytes.

Involvement of fibronectin, Von Willebrand factor, and fibrinogen in platelet interaction with solid substrata

The proteins fibronectin (FN), Von Willebrand factor (VWF), and fibrinogen are believed to play a role in platelet function. They are distributed between the plasma and the platelet pool in the resting state and undergo redistribution upon platelet activation. We have studied their expression on the surface of the platelet and their mobilization following platelet binding to substrata. For the purpose of studying protein expression on the surface of intact platelets either adherent to a substratum or in suspension, the enzyme-linked immunosorbent assay (ELISA) was elaborated and modified. Using this technique as well as immunofluorescence, we found that antiserum raised against carefully washed human platelets recognized FN, VWF, and fibrinogen as well as platelet surfaces. However, specific antisera against these three proteins failed to bind to the surface of unactivated gel-filtered platelets. When gel-filtered platelets were exposed to plastic or fibrillar collagen, they adhered and spread. Such platelets did bind antibodies against FN, VWF, and fibrinogen, Moreover, when the adherent platelets were incubated with FN or with VWF in the absence of ristocetin, they bound these proteins in a concentration-dependent fashion. The patterns of the bound proteins were not similar, suggesting a different spatial distribution of binding sites. These findings indicate that platelet activation by adhesion to substrata mobilize both endogenous and exogenous pools of these proteins, thereby making them surface associated and probable participants in further binding properties of the activated platelet.