Concentrates containing factor IX could improve haemostasis under conditions of thrombocytopenia: studies in an in vitro model

BACKGROUND AND OBJECTIVES

We explored the effect on haemostasis of different factor IX (FIX) concentrates under thrombocytopenic conditions using an in vitro perfusion technique.

MATERIALS AND METHODS

A moderate experimental thrombocytopenia (25 000-30 000 platelets/microl) was induced by means of a filtration procedure in blood anticoagulated with low-molecular-weight heparin. The effects of three different FIX concentrates - a prothrombin complex concentrate (PCC), an intermediate-purity concentrate (FIX/X), and a high-purity concentrate (HPFIX) - on platelet deposition and fibrin formation on subendothelium were assessed at two different shear rates (600/second and 1200/second). Activation of the coagulation system was monitored through assessment of prothrombin activation fragment 1 + 2 (F1 + 2).

RESULTS

Fibrin deposition increased after addition of FIX concentrates, but only showed a significant increase in experiments performed after incubation of PCC at the lower shear rate (600/second) (64.25 +/- 9.61% vs. control 31.22 +/- 8.02%; P < 0.05). Addition of FIX concentrates caused a small increase in the percentage of platelet deposition and area of those aggregates. These differences reached levels of statistical significance in the presence of FIX/X and HPFIX in experiments performed at a shear rate of 600/second. F1 + 2 baseline values in anticoagulated thrombocytopenic blood were 1.15 +/- 0.13 nm and reached levels of 2.49 +/- 0.24 and 3.60 +/- 0.33 nm at shear rates of 600 and 1200/second, respectively. Increments in F1 + 2 observed after addition of different FIX concentrates always remained in the previous ranges.

CONCLUSIONS

Data from the present study provide experimental support favouring the concept that FIX concentrates containing other activated factors could improve haemostasis under conditions of moderate thrombocytopenia.

Uremic medium disturbs the hemostatic balance of cultured human endothelial cells

We have investigated the ability of serum from uremic patients to modify the thrombogenic properties of the endothelium. The effects of uremic medium on the morphology of endothelial cells (ECs), and their resistance to flow was analyzed. The influence of uremic media on the reactivity of the extracellular matrix (ECM) generated by ECs towards normal platelets was evaluated in a parallel-plate perfusion chamber. Exposure of ECs to uremic medium resulted in abnormal cell morphology and signs of an accelerated growth. Detachment of ECs exposed to circulating blood was increased when cells had been grown with media supplemented with uremic serum (21% vs. 14% non exposed). Platelet deposition was significantly elevated on ECMs generated in the presence of uremic media (uremicECMs) (p<0.01 vs. control studies). Effects of uremic serum were not observed at short incubation periods (5 h) but were evident after 24 or 72 h of incubation. Northern blot analysis revealed increased expression of tissue factor (TF) mRNA in ECs exposed to uremic conditions. Immunocytochemical methods detected an augmented expression of TF antigen on uremic ECMs. Incubation of ECMs with an antibody to human tissue factor prevented the increase in platelet deposition observed in uremic ECMs, suggesting that the presence of TF in ECM could be responsible for the enhanced platelet deposition. Results from our study indicate that uremic medium impairs the antithrombotic functions of cultured endothelial cells.

Thrombin facilitates primary platelet adhesion onto vascular surfaces in the absence of plasma adhesive proteins: studies under flow conditions

BACKGROUND AND OBJECTIVE

The effect of local and circulating thrombin on platelet adhesion onto vascular surfaces was explored in the absence of plasma adhesive proteins using flow conditions.

DESIGN AND METHODS

To study the local effects of thrombin, denuded rabbit aorta segments were incubated with thrombin concentrations of 0.001, 0.01 and 0.1 U/mL. To evaluate the effects of circulating thrombin, the same concentrations were added to perfusates consisting of washed platelets and washed red blood cells suspended in a human albumin solution (5%). In some experiments, purified von Willebrand's factor (vWF) (Haemate-P) was added to the perfusates (0. 8 U/mL of vWF, final concentration). A humanized chimeric antibody to the GPIIb-IIIa complex (Reopro) was used to determine the role of this glycoprotein on platelet adhesion under the conditions described. The effect of blocking GPIb was also assessed. Perfusions were carried out at 800 s(-1) for 10 min. The interaction of platelet with the vessel surface was morphometrically evaluated and expressed as percentage of surface coverage (%SC). Changes in the surface expression of the major platelet antigens were also analyzed by flow cytometry.

RESULTS

Incubation of subendothelial surfaces with thrombin enhanced platelet deposition with respect to control levels (increases in SC of 64%, 79% and 86% with 0.001, 0.01 and 0.1 U/mL of thrombin, respectively). Low concentrations of thrombin (0.001 and 0.01 U/mL) incorporated in the perfusates resulted in a similar pro-adhesive effect (increases in SC of 64% and 71%, respectively) while the highest concentration (0.1 U/mL) failed to produce a pro-adhesive effect due to the augmented formation of platelet aggregates with subsequent thrombocytopenia (15+/-1 vs. 160+/-5x10(9) plt/L in the perfusates). Similar results were obtained when VWF was present in the perfusate. Reduction of platelet deposition by blockade of GPIIb-IIIa (to 5.3+/-0.7%) was partially restored by thrombin. Blockade of GPIb prevented platelets from adhering even when thrombin was present (%SC of 2.0+/-0.8%). No significant changes in the distribution of platelet membrane glycoproteins during perfusion experiments were detected.

INTERPRETATION AND CONCLUSIONS

Our results suggest that thrombin facilitates primary platelet adhesion onto vascular surfaces even in the absence of plasma adhesive proteins. This effect seems to be mainly dependent on the GPIb/vWF axis.

Assessment of potential thrombogenicity of coagulation factor IX concentrates in an in vitro model of human thrombogenesis

We have investigated the potential use of perfusion techniques in the evaluation of the thrombogenic profile of factor IX concentrates. Blood from healthy donors was anticoagulated with low molecular weight heparin and incubated with one of the following: (a) diluent (DIL); (b) a prothrombin complex concentrate (PCC); (c) an intermediate-purity concentrate (FIX/X); or (d) a high-purity concentrate (HPFIX). The thrombogenic potential was assessed as: (1) fibrin formation on subendothelium (Baumgartner's perfusion) and (2) prothrombin activation fragment 1+2 (F1+2, nM) determination. The percentage of fibrin deposition on the subendothelium was only significantly increased after incubation with PCC (62.0+/-3.6% vs. DIL 35.0+/-6.1%;p<0.05). None of the FIX concentrates modified platelet interaction versus control blood (DIL: 26.7+/-2.1%). F1+2 baseline values in anticoagulated blood were 0.6+/-0.1 nM. Preperfusion levels of F1+2 reached values of 4.4+/-0.1 nM for PCC and 5.4+/-0.1 nM for FIX/X. After perfusion, F1+2 values were 2.7+/-0.2 nM for DIL, 5.6+/-0.1 nM for PCC and FIX/X, and 3.3+/-0.2 nM for HPFIX. While measurement of F1+2 was influenced by residual contaminants present in the concentrates, the morphometric evaluation of fibrin deposition on perfused vascular surfaces could be more closely related to the net thrombogenic profile of each FIX preparation.

von Willebrand factor contained in factor VIII concentrates of different purities supports platelet adhesion in blood samples from a heterogeneous group of patients with von Willebrand disease

BACKGROUND AND OBJECTIVE

Plasma derived FVIII-VWF concentrates in which the VWF structure is reasonably maintained are recommended as substitutive therapy in VWD. Our aim was to assess platelet deposition and binding to subendothelial structures of VWF present in FVIII concentrates.

DESIGN AND METHODS

Cryoprecipitate (CRY), intermediate-purity (IPC), or high-purity (HPC) FVIII concentrates were added in vitro to citrated blood samples from 11 patients affected by different subtypes of VWD, with the aim of normalizing VWF levels. Measurements of VWF:Ag, ristocetin cofactor (RiCof) activities, FVIII coagulant activity (FVIII:C), and platelet interaction with subendothelium under flow conditions (Baumgartner's perfusion method, computer-assisted morphometry, shear rate 1000 s-1, 10 min, 37 degrees C) were determined. Binding of VWF to the luminal surface of the perfused vessels was assessed by immunofluorescence microscopy. Paired t-test statistics were performed.

RESULTS

Addition of FVIII-VWF preparations raised VWF:Ag from baseline (BSL) values of 0.3 (SD 0.2) to averages of 1.4 (SD 0.5, p < 0.001), 1.2 (SD 0.6, p < 0.001), and 0.4 (SD 0.3) IU mL-1 after CRY, IPC, and HPC, respectively. A positive labeling for VWF was observed by immunofluorescence in vessels perfused with blood containing any of the concentrates. Platelet adhesion of 13.2 (SD 7.6), 22.4 (SD 10.8), 24.8 (SD 7.8, p < 0.03), or 22.5 (SD 4.8)% was measured in BSL, CRY, IPC, or HPC tests, respectively.

INTERPRETATION AND CONCLUSIONS

Our observations support the hypothesis above the mechanisms involved in the beneficial effects of commercial concentrates in von Willebrand disease: the VWF in these concentrates has functional capacity to bind to subendothelium and to support platelet adhesion.

Hemostasis in patients with severe von Willebrand disease improves after normal platelet transfusion and normalizes with further correction of the plasma defect

BACKGROUND

A defective hemostatic effect of plasma concentrate infusion in patients with severe von Willebrand disease (vWD) has been ascribed to the absence of platelet von Willebrand factor (vWF) STUDY DESIGN AND METHODS: The role of platelet vWF in hemostasis of severe vWD was investigated. A plateletpheresis unit (4-5 x 10(11) platelets) from a normal compatible donor was transfused before any cryoprecipitate infusion to three type 3 vWD patients and to one patient with severe type 1 vWD with low levels of platelet vWF who required replacement therapy for bleeding episodes. Autologous platelets were transfused to one of the patients with type 3 vWD.

RESULTS

Partial corrections of bleeding times (14-17 min vs. baseline >30 min) were observed in all patients after the transfusion of normal platelets. During cryoprecipitate infusion, bleeding times were normalized (<6 min), and bleeding episodes stopped when plasma levels of vWF activity ranged from 14 to 18 U per dL. Platelet interactions with the subendothelium increased in parallel with the correction of bleeding times. These results indicate that if approximately 20 percent of the total number of platelets have normal vWF antigen and if plasma vWF levels are at least 14 U per dL, then bleeding times will normalize and mucosal hemorrhages will stop. Transfusion of autologous platelets in one patient with type 3 vWD did not modify bleeding times or platelet adhesion on the subendothelium.

CONCLUSION

The hemostatic effect of normal platelets in type 3 vWD seems to be related to the platelet vWF in the transfused platelets.

Uraemic medium causes endothelial cell dysfunction characterized by an alteration of the properties of its subendothelial matrix

Uraemic patients suffer from haemorrhagic disorders and accelerated atherosclerosis. To evaluate the possible role of the vessel wall in these haemostatic alterations associated with uraemia, we investigated the effect of a uraemic milieu on human endothelial cell (EC) cultures and the reactivity of the extracellular matrices (ECM) generated by these cells towards platelets. EC cultures were exposed to a pool of sera (20% in the culture medium) obtained either from uraemic patients or from normal donors, and the following parameters were evaluated: (1) EC viability (trypan blue exclusion test); (2) von Willebrand factor (vWF) levels in supernatants and associated with ECM; (3) the reactivity of EC and EC-derived ECM towards platelets, measured 'ex vivo' under flow conditions (5 min, wall shear rate 800 s-1); and (4) ultrastructure of the ECM. The viability of EC cultured in the presence of uraemic sera was similar to controls. Platelet interaction with ECM generated by EC exposed to uraemic sera was significantly reduced (P < 0.05). This decrease was mainly related to a reduction in platelet adhesion (9.8 +/- 1.9% vs 16.7 +/- 1.8% in controls, P < 0.02). VWF levels in supernatants and associated with ECM were similar to controls. Ultrastructural analysis of the ECM generated by EC exposed to uraemic sera revealed a deficient matrix. An increased removal of EC was observed in experiments in which EC cultured in the presence of uraemic sera were perfused with citrated blood. These results indicate that a uraemic milieu induces quantitative and qualitative changes in the vascular subendothelium, characterized by a less intrincate network of fibrils, as well as a decreased attachment of EC and reduced thrombogenicity to the ECM. These changes may represent another mechanism which contributes to the haemostatic dysfunction observed in uraemic patients.

[Expression of von Willebrand's disease in endothelial cord cell from a type 1 patient. Discrepancies between clinical manifestations and laboratory tests]

PURPOSE

The reactivity of subendothelium generated by endothelial cells attained from the umbilical cord of a newborn girl from a patient with type I von Willebrand's disease was evaluated.

MATERIAL AND METHODS

Platelet adhesiveness was assessed by means of "ex vivo" blood perfusion systems (shearing coefficients from 300 to 1,300 s-1), on umbilical artery subendothelium and over the extracellular matrix (ECM) generated by cultured endothelial cells attained from the umbilical cord of a newborn baby from a type I von Willebrand mother.

RESULTS

By means of an ELISA technique, a significant reduction of both the vWF secreted into the culture (p < 0.05) and of that associated with ECM (p < 0.05) was noticed. The two subendothelial surfaces examined supported a lesser extent of platelet adhesion than the control surfaces from healthy individuals in the perfusion tests at 1,300 s-1 (p < 0.05). The experimental results did not agree with the lack of clinical manifestations and laboratory abnormalities in the propositus at age 3.

CONCLUSIONS

These findings suggest that the endothelial cells cultured from the tissues of a newborn baby from a type I von Willebrand woman expressed less vWF, at the same time confirming the importance of vWF bound to connective tissue in sustaining platelet adhesion. The data attained here illustrate the complex model of inheritance of moderate von Willebrand's disease, along with the technical limitations of cell culture studies. Such observations must be borne in mind whenever any experimental approach is performed with material attained from ill-differentiated tissues such as the umbilical cord vein.

Dipyridamole induces changes in the thrombogenic properties of extracellular matrix generated by endothelial cells in culture

Dipyridamole (DIP) is a drug widely used as an antiplatelet agent, which also has effects on endothelial cells. In this study, the effects of treating confluent endothelial cell monolayers (EC) with DIP on EC viability (trypan blue exclusion test) and metabolic activity (3H-thymidine incorporation) were examined. Platelet reactivity of the extracellular matrix (ECM) produced by untreated and DIP-treated ECs was determined morphometrically by a perfusion technique. Levels of ECM-associated von Willebrand factor (vWF) and fibronectin (FN) were also quantified (ELISA). The present results indicate that treatment of EC with 10 microM DIP did not reduce EC viability but that the incorporation of labelled nucleotides was significantly decreased (p less than 0.01). Platelet deposition onto the ECM generated by DIP-treated cells, perfused at a shear rate of 1300 sec-1, differed significantly with respect to controls (p less than 0.05), and platelet adhesion was also reduced (25% less, p less than 0.05). This effect was shear rate dependent, as no differences were noted when the ECMs were perfused at 300 sec-1 shear rate. Levels of VWF and FN associated with ECM remained unchanged with respect to controls. These results suggest that treatment with DIP alters EC metabolic activity, which in turn, influences the reactivity of the ECM generated by treated cells.

Platelet adhesion to exposed endothelial cell extracellular matrixes is influenced by the method of preparation

The relative thrombogenicity of extracellular matrixes (ECMs) produced by cultured human umbilical endothelial cells (ECs) was studied under flow conditions. ECMs were prepared using a number of physical and chemical methods, and their reactivity toward platelets was morphometrically evaluated. von Willebrand factor (vWF), fibronectin (FN), and 13-hydroxy-9-cis,11-trans-octadecadienoic acid (13-HODE) were also determined. We found that platelet adhesion to ECMs differed significantly, both quantitatively and qualitatively, with the method of ECM preparation. Mechanically prepared ECM exposed a less thrombogenic surface compared with ECM prepared by chemical methods (platelet-covered surface of 20% and 50%, respectively). Evaluation of the ECM components vWF, FN, and 13-HODE showed significant changes, both in their concentrations and distribution patterns, depending on the method of ECM preparation. The decrease measured in the levels of ECM-associated vWF (from 108 to 9.2 ng/10(4) cells) and the minor changes observed in the distribution pattern of subendothelial FN did not appear to be sufficient to explain the altered platelet adhesion observed in our model. This suggests that the amount of 13-HODE probably associated to the remaining ECs present in the mechanically exposed ECM could be one factor that specifically contributed to the nonthrombogenic state of these preparations. We conclude that the degree of ECM reactivity toward platelets is dependent on the method of ECM preparation and that this is related to the removal of specific EC/ECM components that modulate their thromboresistant/thrombogenic properties. This fact should be taken into account when ECMs produced by cultured ECs are used in platelet adhesion studies.

Comparison between human umbilical artery and rabbit abdominal aorta as substrata for platelet adhesion and platelet thrombus formation under flow conditions

Rabbit abdominal aortas and human umbilical arteries are currently used as substrata for the study of platelet adhesion and aggregate formation under flow conditions. Using immunohistochemical and ultrastructural methods, we have analyzed both vessel surfaces. The reactivity towards platelets of the subendothelium (SE) exposed on these vessels after mechanical or enzymatic digestion (alpha-chymotrypsin) was morphometrically quantified and the nature of the interaction studied in the electron microscope. After mechanical damage, the ultrastructural study of rabbit aortas showed a clearly defined internal elastic lamina (IEL). In contrast, umbilical vessels lacked a consistent IEL and masses of amorphous material often located deeper in the media were the main constitutents of the SE. Immunohistochemical labeling of the von Willebrand factor bound to both types of vessel differed considerably. Quantification of platelet interactions after perfusion of citrated blood showed qualitative differences between mechanically damaged rabbit or human vessels. Enzymatic digestion produced a more thrombogenic surface on rabbit aortas (p less than 0.01 vs. nondigested), but decreased their reactivity towards platelets on umbilical arteries (p less than 0.01 vs. nondigested). The ultrastructural study of the interacting platelets revealed that aggregates, when present, were found on the extracellular matrix underlying endothelial cells of rabbit aortas, but interacting with fibrillar structures probably derived from cell elements of the media in the case of umbilical arteries. These findings indicate that rabbit aortas and umbilical arteries possess structural characteristics that result in different thrombogenic properties with respect to circulating platelets.

[Variations in the thrombogenic properties of the extracellular matrix produced by endothelial cells in consecutive passages]

Cultured endothelial cells are currently used as a model for the study of the interaction of platelets with the vascular wall under flow conditions. Little is known about the influence of cell-culture conditions on the preservation of the original characteristics of primary tissues. In our study, the reactivity of platelets towards extracellular matrices produced by endothelial cells from different passage numbers was morphometrically evaluated. The presence and distribution of the extracellular von Willebrand factor was analyzed by immunofluorescent techniques. Morphology of endothelial cells in culture varied dramatically along the culture and polymorphism increased clearly at fourth or fifth passages. Extracellular matrix thrombogenicity decreased significantly from the fifth passage number. Simultaneously, von Willebrand factor distribution in ECM changed its typical pattern. Our results confirm that the extracellular matrix produced by human umbilical vein derived-endothelial cells constitutes a good model for the study of platelet adhesion onto vascular subendothelium. The limited viability of ECs and the variation in the thrombogenic properties of ECM formed after multiple passages must be taken into account when these cells or their products are used in experimental research.

A rapid embedding procedure for the study of platelet interactions with extracellular matrices in a flowing system. Effect of aspirin on platelet activity

The cultured endothelial cell (EC) is currently used as a model for the study of the interaction of platelets with the vascular wall. Described is a method for rapid quantitative and qualitative evaluation of platelet interactions with extracellular matrices (ECM) produced by human cultured ECs growing on plastic coverslips. Morphometric calculations can be performed on the same perfused coverslips. A very good correlation (r = 0.96) was found between results of a morphometric method en face and those obtained from analysis of cross sections of the perfused coverslips. A shear rate-dependent increase on platelet deposition onto ECMs was observed with both morphometric procedures. The method is sensitive enough to detect drug-related changes of platelet function. An impairment of the interaction of platelets with the ECM was observed when blood obtained from healthy volunteers who took 500 mg aspirin/day for five days was perfused. Aspirin showed a marked effect, decreasing platelet spreading onto the subendothelium (p less than 0.05). The embedding method described benefits from the use of plastic coverslips that are easily detected from the glycol methacrylate compound used for the embedding procedure. Quantitative analysis en face (covered surface) and qualitative evaluation of platelet interactions (contact, adhesive and aggregated platelets) in cross sections are performed on the same coverslip. This embedding procedure provides a useful tool for the study not only of platelet interactions with ECMs but also for the investigation of interactions of blood elements with other cultured cells.

Differential localization of von Willebrand factor, fibronectin and 13-HODE in human endothelial cell cultures

Von Willebrand factor (vWF), fibronectin (FN) and 13-hydroxy-octadecadienoic acid (13-HODE) are known to influence the regulation of the adhesive properties of vascular surfaces. In the present study vWF, FN and 13-HODE were comparatively localized in endothelial cells (EC) and in the extracellular matrix (ECM) produced by EC. An indirect immunofluorescent technique was applied to coverslips containing human EC cultures previously fixed and permeabilized following different procedures: A. Alcohol/acetone; B. Paraformaldehyde alone and C. Paraformaldehyde followed by Triton X-100. vWF was observed inside EC (A), on the ECM produced by EC (B) or in EC and ECM (C) depending on the fixation procedures used. FN was mainly localized in the ECM despite the fixation procedures employed. FN was only seen in relation to cell bodies after strong permeabilization (A). Under our experimental conditions 13-HODE was never found in ECM. This latter antigen was observed randomly dispersed in those preparations fixed with alcohol/acetone, indicating that it is probably extracted by this fixative. 13-HODE was detected in granular shaped structures in EC after permeabilization with detergent (C). These results suggest that the cellular localization of vWF and FN is compatible with an adhesive role related to the abluminal side of ECs. 13-HODE was readily observed after mild permeabilization. This finding would be morphologically consistent with its contribution to the regulation of the vessel wall thromboresistance.