Safety and efficacy of damoctocog alfa pegol prophylaxis in patients with severe haemophilia A: Results of an interventional, post-marketing study

INTRODUCTION

Damoctocog alfa pegol (BAY 94-9027, Jivi® ) is an approved extended half-life factor VIII (FVIII) for treatment of previously treated patients with haemophilia A aged ≥12 years. We report the final results of an interventional, post-marketing study of damoctocog alfa pegol prophylaxis in patients with severe haemophilia A.

METHODS

In this open-label, interventional, post-marketing, phase 4 trial (NCT04085458), previously FVIII-treated patients with severe haemophilia A aged ≥18 years received damoctocog alfa pegol for ≥100 exposure days (EDs). Patients initially received 45 IU/kg every 5 days (recommended) or 40 IU/kg twice-weekly. At Visit 3, patients' doses could be increased, or treatment frequency adapted. The primary endpoint was FVIII inhibitor development (titre ≥.6 Bethesda units). Secondary endpoints included anti-polyethylene glycol (PEG) antibody development, treatment-emergent adverse events (AEs) and annualized bleeding rate (ABR).

RESULTS

Overall, 36 patients were enrolled; 32 patients received treatment, of whom, 27 completed the study. No patients developed FVIII inhibitors; three tested transiently positive for low-titre anti-PEG antibodies without clinical relevance. Three patients reported study-drug-related AEs of mild or moderate intensity. Two patients discontinued the study due to AEs. No deaths occurred. Most patients (70%) were treated with E5D/E7D regimens. The median (Q1;Q3) total ABR (N = 30) was 3.0 (.0;9.0) pre-study and 1.8 (.7;5.9) during the study.

CONCLUSION

Damoctocog alfa pegol individualized prophylaxis regimens were well-tolerated with no immunogenicity concerns. ABRs improved following the switch from pre-study prophylaxis to damoctocog alfa pegol prophylaxis. These results support the favourable safety and efficacy profile of damoctocog alfa pegol prophylaxis.

Elevated FVIII levels in hereditary hemorrhagic telangiectasia: Implications for clinical management

Objectives

The objective of this study was twofold: to determine the prevalence of arterial and venous thromboembolic events in the Norwegian Hereditary Hemorrhagic Telangiectasia (HHT) population, and to explore potential factors linked to such events, with particular emphasis on FVIII.

Methods

Patients with an HHT diagnosis attending the Otorhinolaryngology Department at Oslo University Hospital-Rikshospitalet were included consecutively between April 2021 and November 2022. We recorded the participants' medical history with an emphasis on thromboembolic events. Measurements of blood constituents, including FVIII, FIX, vWF, hemoglobin, iron, ferritin, and CRP were performed.

Results

One hundred and thirty-four patients were included in the study. The total prevalence of thromboembolic events among the participants was 23.1%. FVIII levels were high (>150 IU/dL) in the majority of HHT patients (n = 84) (68.3%) and were significantly associated with thromboembolic events (p < .001), as was age. Of the patients with high FVIII levels, 28 (33%) had experienced a thromboembolic event. Furthermore, FVIII levels were measured consecutively in 51 patients and were found to fluctuate above or below 150 IU/dL in 25% of these cases.

Conclusion

Thromboembolic events are highly prevalent in the Norwegian HHT population and are significantly associated with FVIII levels. FVIII levels can fluctuate, and measurements should be repeated in HHT patients to assess the risk of thromboembolic events.

Level of Evidence

4.

Factors associated with physical activity in young people with haemophilia A on prophylaxis

INTRODUCTION

Physical activity (PA) is influenced by numerous factors, and the literature describing why people with haemophilia (PWH) are physically active or not is inconclusive.

AIMS

To investigate factors associated with PA (mean min/day in light (LPA), moderate (MPA), vigorous (VPA) and total PA, and proportion meeting World Health Organization (WHO) weekly moderate-to-vigorous (MVPA) recommendations) among young PWH A.

METHODS

Forty PWH A on prophylaxis from the HemFitbit study were included. PA was measured using Fitbit devices and participant characteristics were collected. Potential factors associated with PA were investigated by univariable linear regression models for continuous PA outcomes, and descriptively for teenagers meeting/not meeting WHO MVPA recommendations only, because all except one adult met PA recommendations.

RESULTS

Mean age (n = 40) was 19.5 years (SD 5.7). Annual bleeding rate was nearly zero and joint scores were low. We found an increase of four min/day in LPA (95% confidence interval (CI) 1-7) per year increase in age. Participants with 'Haemophilia Early Arthropathy Detection with Ultrasound' (HEAD-US) score ≥1 engaged in mean 14 min/day less MPA (95% CI -23.2 to -3.8), and 8 min less VPA (95% CI -15.0 to -0.4) compared to participants with HEAD-US score 0. Teenagers who met PA recommendations had slightly better joint status compared to those who did not meet recommendations.

CONCLUSION

These findings indicate that presence of mild arthropathy does not affect LPA but may have a negative impact on PA of higher intensities. Early start of prophylaxis may be an important determinant of PA.

Physical activity in Norwegian teenagers and young adults with haemophilia A compared to general population peers

INTRODUCTION

Limited evidence exists on objectively measured habitual physical activity (PA) of young people with haemophilia (PWH).

AIMS

To compare different outcomes of objective PA between young PWH A and controls using a commercial activity tracker.

METHODS

We enrolled males aged 13-30 years with moderate and severe haemophilia A, without inhibitors on regular prophylaxis. PA was measured with the activity tracker Fitbit Charge 3 for 12 weeks. Control group data was obtained from ≈60,000 Fitbit users, matched on age, sex and measurement period. PA variables [steps, intensities, volume, activity types, exercise frequencies and proportion meeting the World Health Organization's moderate-to-vigorous PA (MVPA) recommendations] were compared between groups descriptively and using Welch's two-sample t-test and two-sample test of proportions.

RESULTS

Forty PWH A were enrolled (mean age 19.5 years, 50% teenagers, 50% adults, three (7.5%) with moderate and 37 (92.5%) with severe haemophilia). Mean daily steps and minutes MVPA were similar between PWH and controls. PWH spent more time in light PA (mean 227 vs. 192 min/day, P = .033) and exercised more frequently (mean 5.6 vs. 3.9 exercise sessions/week, P < .001). Among teenagers, 40% PWH and 8% controls reached MVPA recommendations, compared to 95% and 100% among adults. The most common type of PA was walking.

CONCLUSION

This cohort of young PWH A on prophylactic treatment had PA levels comparable to controls. Still, a considerable proportion of teenagers did not meet the recommended weekly volume of MVPA, and we encourage clinicians to have a particular focus on promoting PA for this group.

Comparison of free-living physical activity measurements between ActiGraph GT3X-BT and Fitbit Charge 3 in young people with haemophilia

INTRODUCTION

Measurement of physical activity (PA) using commercial activity trackers such as Fitbit devices has become increasingly popular, also for people with haemophilia (PWH). The accuracy of the Fitbit model Charge 3 has not yet been examined.

AIMS

To compare the Fitbit Charge 3 against the research-grade accelerometer ActiGraph GT3X-BT in measuring average daily steps and minutes spent in different PA intensities.

METHODS

Twenty-four young PWH wore a wrist-worn Fitbit Charge 3 and hip-worn ActiGraph GT3X-BT simultaneously for seven consecutive days in free-living conditions. Correlation of and differences between the devices for daily averages of PA parameters were assessed using Pearson's correlation coefficient and paired t-test, respectively. Agreement between devices was assessed using Bland-Altman plots.

RESULTS

Twenty participants (mean age 21.8) were included in the analyses. We found moderate to high correlations between Fitbit and ActiGraph measured daily averages for all PA variables, but statistically significant differences between devices for all variables except daily minutes of moderate PA. Fitbit overestimated average daily steps, minutes of light, vigorous and moderate-to-vigorous PA. Bland-Altman plots showed a measurement bias between devices for all parameters with increasing overestimation by the Fitbit for higher volumes of PA.

CONCLUSION

The Fitbit Charge 3 overestimated steps and minutes of light, moderate and moderate-to-vigorous PA as compared to the ActiGraph GT3X-BT, and this bias increased with PA volume. The Fitbit should therefore be used with caution in research, and we advise users of the device to be cognizant of this overestimation.

Surgical outcomes in patients with haemophilia A or B receiving extended half-life recombinant factor VIII and IX Fc fusion proteins: Real-world experience in the Nordic countries

Introduction

Perioperative dosing recommendations vary across Nordic haemophilia treatment centres (HTCs) for extended half‐life (EHL) factor concentrates in haemophilia A/B (HA/HB) patients.

Aim

To summarise Nordic real‐world surgical experiences with EHL recombinant factor VIII/IX Fc (rFVIIIFc/rFIXFc) fusion proteins using retrospective data from clinical records at four HTCs in Finland, Sweden and Norway.

Methods

Factor dosing and surgical outcomes were recorded from HA/HB patients who underwent surgery and were treated with rFVIIIFc/rFIXFc. Perioperative factor dosing regimens were clinician‐determined based on local practises.

Results

Twenty five surgeries were performed on 20 patients, all covered by bolus injections except one minor HA surgery; eight minor surgeries were in paediatric patients. Median preoperative rFVIIIFc dose for major HA surgeries (n = 8) was 48 IU/kg (range: 35–57), with total consumption up to Day 14 of 427 IU/kg (196–568). For the two major HB surgeries (in one patient), preoperative rFIXFc doses were 50 IU/kg and 20 IU/kg; total consumption up to Day 14 was 130 IU/kg and 40 IU/kg. Median preoperative rFVIIIFc/rFIXFc bolus doses for minor HA (n = 10) and HB (n = 4) surgeries were 50 IU/kg (24–79) and 47 IU/kg (40–71), with total consumption up to Day 5 of 138 IU/kg (49–404) and 100 IU/kg (43–125), respectively. Intraoperative and postoperative haemostatic responses were rated as at least good/excellent for 24/25 surgeries, with bleeding episodes reported in only three surgeries.

Conclusion

Nordic real‐world experiences suggest that EHL products can be used safely and effectively for peri‐operative haemostasis. Further research is required to develop local dosing guidelines for optimised treatment schedules.

Thrombosis and thrombocytopenia after HPV vaccination

BACKGROUND

Vaccine-induced immune thrombotic thrombocytopenia (VITT) has so far only been reported after adenovirus vector severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines.

OBJECTIVE

We report findings in a 25-year-old woman who presented with thrombocytopenia, venous thrombosis, elevated D-dimer levels, and high levels of platelet-activating antibodies to platelet factor 4-polyanion complexes 10 days after Gardasil 9 vaccination for human papillomavirus (HPV). The patient exhibited clinical and laboratory features in line with the recently defined VITT syndrome, described after adenoviral vector vaccination to prevent coronavirus disease 2019.

CONCLUSION

We report a case of VITT following HPV vaccination. This should raise awareness of the possibility of VITT also occurring after other vaccines, not exclusively adenoviral vector-based SARS-CoV-2 vaccines.

Bone mineral density in haemophilia - a multicentre study evaluating the impact of different replacement regimens

AIM

The aim of this study was to investigate if prophylactic treatment in severe haemophilia impact on bone mineral densisty (BMD) in adults with haemophilia A/B.

METHODS

Subjects with haemophilia (n = 120) underwent bone-density measurement and clinical data was collected. BMD in subjects with severe haemophilia on high-dose prophylaxis (n = 41) was compared to BMD in subjects with mild haemophilia (n = 33) and to severe haemophilia treated with intermediate-dose prophylaxis (n = 32) or on-demand replacement therapy (n = 14).

RESULTS

Subjects with severe haemophilia on high-dose prophylaxis showed BMD at total hip comparable to subjects with mild haemophilia (median BMD 955.8 and 977.4 mg/cm² (P = .17), respectively). No difference in BMD was found related to type of prophylactic regimen (median BMD 955.8 and 942.4 mg/cm² , in high-dose and intermediate dose groups, respectively; P = .70). Subjects with severe disease treated on-demand had significantly lower BMD compared to subjects on a high-dose prophylactic regimen (median BMD 771.8 and 955.8 mg/cm² (P = .001), respectively). BMD decreased significantly with age, regardless of severity of haemophilia disease. In a multivariate analysis, adjusted for disease status and age, type of prophylactic regimen was not significantly associated with osteoporosis development.

CONCLUSION

We show that BMD differs in persons with severe haemophilia on propylaxis as compared to those treated on-demand, but that type of prophylactic regimen does not reflect on BMD. The difference between treatment groups was mainly explained by an age difference between groups. However, patients on prophylaxis displayed a high degree of normal BMD not far from mild haemophilia at comparative age.

Pharmacokinetics and pharmacodynamics of a recombinant fusion protein linking activated coagulation factor VII with human albumin (rVIIa-FP) in patients with congenital FVII deficiency

Objectives: Recombinant fusion protein linking activated factor VIIa to human albumin (rVIIa-FP) is a therapeutic option designed to prevent and treat bleeding events in patients with congenital FVII deficiency with reduced infusion frequency compared to current FVII treatments. This study characterized the pharmacokinetics (PK) and pharmacodynamics (PD) of rVIIa-FP.Methods: A phase I multicenter, randomized, open-label, parallel-arm, single-dose study (NCT02470871) was conducted in nine patients with severe congenital FVII deficiency. Patients received their routine FVII product (30 IU/kg plasma-derived FVII [pdFVII] or 25 μg/kg recombinant activated FVII (rFVIIa) [eptacog alfa]), and were then randomly assigned to receive 100 or 300 μg/kg of rVIIa-FP. Blood samples for PK and PD assessments were drawn up to 48 hr after administration. FVIIa activity was determined using a one-stage clotting assay. PD parameters were derived from thrombin generation testing, using the Nijmegen hemostasis assay.Results: rVIIa-FP showed improved PK compared to rFVIIa, with 2- to 3-fold longer t_1/2 and 4- to 8-fold lower clearance. Analysis of PD data showed a sustained suppression of lag time below 4.5 min (upper limit of healthy people) for rVIIa-FP compared to rFVIIa. AUEC and EC_max were similar across the two dose groups of rVIIa-FP and rFVIIa.Discussion: rVIIa-FP was well tolerated in patients with congenital FVII deficiency, showed a longer half-life and lower clearance compared to rFVIIa, and lag time remaining within healthy ranges for ≥8 hr.Conclusion: These results warrant further investigation into the efficacy of rVIIa-FP to control and prevent bleeding in patients with FVII deficiency.

Stimulated Glanzmann’s Thrombasthenia Platelets Produce Microvesicles

The mechanism of formation of platelet-derived microvesicles remains controversial.

The aim of the present work was to study the formation of microvesicles in view of a possible involvement of the GPIIb-IIIa complex, and of exposure of negatively charged phospholipids as procoagulant material on the platelet surface. This was studied in blood from three Glanzmann’s thrombasthenia patients lacking GPIIb-IIIa and healthy blood donors. MAb FN52 against CD9 which activates the complement system and produces microvesicles due to a membrane permeabilization, ADP (9.37 μM), and the thrombin receptor agonist peptide SFLLRN (100 μM) that activates platelets via G-proteins were used as inducers. In a series of experiments platelets were also preincubated with PGE1 (20 μM). The number of liberated microvesicles, as per cent of the total number of particles (including platelets), was measured using flow cytometry with FITC conjugated antibodies against GPIIIa or GPIb. Activation of GPIIb-IIIa was detected as binding of PAC-1, and exposure of aminophospholipids as binding of annexin V. With normal donors, activation of the complement system induced a reversible PAC-1 binding during shape change. A massive binding of annexin V was seen during shape change as an irreversible process, as well as formation of large numbers of microvesicles (60.6 ±2.7%) which continued after reversal of the PAC-1 binding. Preincubation with PGE1 did not prevent binding of annexin V, nor formation of microvesicles (49.5 ± 2.7%), but abolished shape change and PAC-1 binding after complement activation. Thrombasthenic platelets behaved like normal platelets after activation of complement except for lack of PAC-1 binding (also with regard to the effect of PGE1 and microvesicle formation). Stimulation of normal platelets with 100 μM SFLLRN gave 16.3 ± 1.2% microvesicles, and strong PAC-1 and annexin V binding. After preincubation with PGE1 neither PAC-1 nor annexin V binding, nor any significant amount of microvesicles could be detected. SFLLRN activation of the thrombasthenic platelets produced a small but significant number of microvesicles (6.4 ± 0.8%). Incubation of thrombasthenic platelets with SFLLRN after preincubation with PGE1, gave results identical to those of normal platelets. ADP activation of normal platelets gave PAC-1 binding, but no significant annexin V labelling, nor production of microvesicles. Thus, different inducers of the shedding of microvesicles seem to act by different mechanisms. For all inducers there was a strong correlation between the exposure of procoagulant surface and formation of microvesicles, suggesting that the mechanism of microvesicle formation is linked to the exposure of aminophospholipids. The results also show that the GPIIb-IIIa complex is not required for formation of microvesicles after activation of the complement system, but seems to be of importance, but not absolutely required, after stimulation with SFLLRN.

The Difference Between Platelet and Plasma FXIII Used to Study the Mechanism of Platelet Microvesicle Formation

The formation of microvesicles from platelets was induced either by activation of the complement system by a monoclonal antibody to CD9, or by incubation of platelets with the calcium ionophore A23187. A filter technique to isolate the microvesicles without plasma contamination is described. The microvesicles contained FXIIIa2 from the platelet cytoplasm which shows that these particles contain significant amounts of intracellular material. This was shown by the use of crossed immunoelectrophoresis with rabbit antibodies to total human platelet proteins in the second dimension gel and polyclonal antibodies against the a- and b-subunit of FXIII in the intermediate gel. The FXIIIa2 in the microvesicle was found to be functional as an enzyme. To prove this, it was shown that FXIII in its immunoprecipitate arc could catalyze the incorporation of monodansylcadaverine into casein as identified by fluorescence of this arc in ultraviolet light. The observation that the plasma form of FXIII (FXIIIa2b2) was absent from the microvesicles collected by the filtration technique, whereas it was present in platelet fragments obtained by mechanical disruption by ultrasonication, indicates that the activation-dependent microvesicles are formed by a true budding process with the inclusion of intracellular, but not extracellular material.

Microvesicles Bind Soluble Fibrinogen, Adhere to Immobilized Fibrinogen and Coaggregate with Platelets

In the present study we have investigated whether platelet derived microvesicles can bind soluble fibrinogen, bind to immobilized fibrinogen, and coaggregate with platelets. Flow cytometry was used for studies on binding of soluble fibrinogen and coaggregation, whereas ELISA wells were used to study binding of microvesicles to immobilized fibrinogen. Biotinylated microvesicles produced by stimulation with A23187, thrombin or SFLLRN of platelets which had been surface-labelled with biotin, were used both for the coaggregation experiments and for the binding studies with immobilized fibrinogen. Unlabelled microvesicles and biotinylated fibrinogen were employed when studying binding of soluble fibrinogen to the microvesicles. For the flow cytometry, the biotinylated proteins were reacted with avidin or streptavidin which was PE-conjugated, whereas the same substances were conjugated with alkaline phosphatase for the ELISA studies. The microvesicles formed after stimulation of platelets by SFLLRN or A23187 clearly bound the soluble, biotinylated fibrinogen. Moreover, isolated biotinylated microvesicles added to washed platelets prior to activation, were associated to the microaggregates that formed after stimulation. A significant binding of biotinylated microvesicles to immobilized fibrinogen could also be detected. The binding of micro-vesicles to soluble and immobilized fibrinogen and association to platelets was clearly specific and at least partly dependent on the GPIIb-IIIa complex, as all of these phenomena could be prevented or reduced by addition of the c7E3 Fab which blocks the activated form of this receptor complex. From these in vitro results it is clear that microvesicles can bind to immobilized fibrinogen, bind soluble fibrinogen and are able to coaggregate with platelets. It may be speculated that these results also reflect a haemostatic role of microvesicles in vivo.

Continuous infusion of coagulation factor concentrates during intensive treatment

In clinical management of bleeds and surgical procedures in patients suffering from bleeding disorders either repetitive bolus injections (BI) or continuous infusion (CI) can be used for coagulation factor replacement. Continuous infusion seems to be an attractive route of administration and may be considered if replacement therapy is required for more than 3 days. The strongest argument favouring continuous infusion is its superiority in providing the patient with a safe and constant level of the deficient coagulation factor by balancing input with clearance. Furthermore, several studies have shown that coagulation factor consumption may be reduced by CI compared to repetitive bolus injections (BI) since unnecessary peaks of factor level are avoided. Concerns have been raised whether continuous infusion of coagulation concentrates is associated with an increased risk of developing inhibitors. However, available data have so far not shown an increased risk for inhibitor development in severe haemophilia patients with more than 50 exposure days of coagulation factor concentrates. Further, previously reported complications when using CI such as phlebitis at the infusion site and pump failure are nowadays very seldom seen when small amounts of heparin are added to the infusion bag, and increased quality of the pumps are available. Over the last decades, numerous reports have confirmed CI to be a safe and effective mode of coagulation factor replacement even in the most challenging surgical procedures, such as total joint arthroplasties.

Evaluation of the utility of the ISTH-BAT in haemophilia carriers: a multinational study

INTRODUCTION

There has been increasing recognition in recent years that female carriers of haemophilia manifest abnormal bleeding; however, data on the use of bleeding assessment tools in this population are lacking.

AIM

Our objective was to validate the ISTH-BAT in haemophilia carriers to describe bleeding symptoms and allow for comparisons with factor levels and other patient groups.

METHODS

This was a prospective, observational, cross-sectional study performed by members of Global Emerging HEmostasis Panel (GEHEP). Unselected consecutive haemophilia carriers were recruited and a CRF and the ISTH-BAT were completed by study personnel.

RESULTS

A total of 168 haemophilia carriers were enrolled: 155 haemophilia A and 13 haemophilia B. The mean age was 40 years (range: 20-82). Carriers had higher mean bleeding scores (BS) compared with age-matched controls (n = 46; 5.7 vs. 1.43; P < 0.0001) and Type 3 VWD OC (n = 32; 3.0; P = 0.009), but lower BS compared with women with Type 1 VWD (n = 83; 8.7; P < 0.0001). Fifteen carriers reported haemarthrosis, and of those six had normal FVIII/FIX levels. There was a significant but weak negative correlation between BS and factor level (Spearman's r²  = -0.36, P < 0.001).

CONCLUSION

Our results show that haemophilia carriers experience abnormal bleeding, including haemarthrosis. Overall, BS in women with Type 1 VWD > haemophilia carriers > Type 3 VWD OC > controls. Understanding the performance of the ISTH-BAT in this population is a critical step in future research aimed at investigating the underlying pathophysiology of abnormal bleeding, with the ultimate goal of optimizing treatment.

[Allogeneic stem-cell transplantation in adults 1985-2012: results and development]

BACKGROUND

Allogeneic stem cell transplantation (ASCT) has been a treatment option for patients with serious diseases of the blood and haematopoietic organs in Norway since 1985. Such treatment is potentially curative for selected patients who have a relatively short predicted survival with other treatment modalities. This article summarises the experience and results from ASCT at Oslo University Hospital Rikshospitalet.

MATERIAL AND METHOD

The study included all of the 734 adult patients who had undergone allogeneic stem cell transplantation at the Department of Haematology, Rikshospitalet, later Oslo University Hospital Rikshospitalet, from November 1985 to October 2012.

RESULTS

At the time of analysis, altogether 384 patients were alive, and the five and ten-year survival rates were 54% and 48% respectively. The median follow-up time was six years. A total of 339 patients (46%) had developed acute graft-versus-host disease (GvHD), and 250 (73%) of these had GvHD ≥ grade II. Altogether 280 out of 602 patients who lived ≥ 100 days after the transplantation (46.5%) developed chronic GvHD. The most frequent causes of death included recurrence of the initial disease in 116 patients (33.1 %), multi organ failure after transplantation in 88 patients (25.4%), infections in 54 patients (16%) and GvHD in 33 patients (9.4%).

INTERPRETATION

ASCT is a treatment option with a curative potential for patients with serious haematological diseases when other forms of treatment provide few prospects for recovery. The total survival rate in our study is in accordance with international results for the same time period, and the indications have consistently been in line with what is accepted internationally.

Glycoprotein IIb-IIIa on platelet-derived microparticles, and microparticle structures studied by electron microscopy, confocal laser microscopy and crossed radio-immunoelectrophoresis

Shedding of microparticles from the platelet surface is usually associated with exposure of platelet procoagulant activity. Platelet-derived microparticles have been detected in blood in various disease states. In vitro, platelet stimulation with a number of different agonists results in formation of microparticles. In the present study, microparticles induced by platelet stimulation by calcium ionophore or by membrane incorporation of the terminal complement complex C5b-9 were studied using electron microscopy, confocal laser microscopy, flow cytometry and radio-immunoelectrophoresis. When studied by electron microscopy, microparticle morphology was found to be dependent upon the induction method. Platelet stimulation with the calcium ionophore resulted in smaller, more homogeneous and electron dense microparticles than those induced by insertion of the terminal complement complex. With flow cytometry and confocal laser immunofluorescence microscopy, microparticle GPIIb-IIIa was demonstrated using a FITC-conjugated antibody to GPIIIa. Surface-bound GPIIb-IIIa was demonstrated on the microparticles by immunoelectron microscopy. Crossed immunoelectrophoresis of detergent-solubilized microparticles visualized a very prominent GPIIb-IIIa immunoprecipitate arc, and binding of [(125)1]fibrinogen to microparticle GPIIb-IIIa was demonstrated by radio-immunoelectrophoresis. This suggests that the activated GPIIb-IIIa complex is preserved intact during the shedding of microparticles from the platelet surface.

Nordic Haemophilia Council's practical guidelines on diagnosis and management of von Willebrand disease

Von Willebrand disease (VWD) is the most common inherited bleeding disorder characterized by spontaneous or tissue injury-related, mostly mucocutaneous, bleeding events. VWD affects both males and females and is caused by quantitative or qualitative deficiency of Von Willebrand factor. The diagnostic procedure is complicated because VWD is highly heterogeneous, and differential diagnosis from platelet disorders may be challenging. Moreover, these defects may even coexist, impacting the bleeding phenotype. Mild and moderate VWD can be difficult to distinguish from the normal population, and VWD subtyping may also be problematic. This article summarizes the guidelines of the Nordic Haemophilia Council (NHC), which are intended to serve as a practical tool and provide the standards for diagnosing and treating VWD patients. The complete Nordic Guidelines on VWD are available at the NHC Web site (http://nordhemophilia.org).

Recovery, survival, and function of transfused platelets and detection of platelet engraftment after allogeneic stem cell transplantation

BACKGROUND

Recovery and survival of transfused platelets (PLTs) are usually assessed by radioisotope labeling methods for evaluation of transfusion efficacy and new progress in the processing of PLT concentrates. Alternative, nonradioactive methods are warranted.

STUDY DESIGN AND METHODS

A multicolor flow cytometry method was developed for simultaneous studies of recovery, survival, and function of transfused PLTs. Eight consecutive patients undergoing allogeneic stem cell transplantation (TX) were transfused with apheresis PLTs of nonself human leukocyte antigen (HLA) Class I types, and HLA Class I discrepancy between donor and recipient was used to identify transfused PLTs. Hematologic status and HLA Class I surface expression were analyzed immediately before transfusion, 1 and 6 hours after transfusion, and daily during the subsequent week. PLT activation was assessed by surface expression of CD63, CD62P, or CD42a, before and after stimulation with thrombin receptor agonist peptide.

RESULTS

PLT recovery was 43, 41, and 31% for fresh (5-72 hr old) and 30, 27, and 17% for stored (73-148 hr old) PLTs, after 1, 6, and 15 to 28 hours, respectively. Survival of fresh versus stored PLTs were 160 and 105 hours, respectively. Spontaneous PLT activation and residual activation potential were almost equal for fresh and stored PLTs. PLT engraftment was detected between Day 7 and Day 9, which was significantly earlier than first sign of neutrophil engraftment (Days 11-19; p=0.01).

CONCLUSION

Flow cytometry is an attractive alternative to radiolabeling of PLTs for simultaneous studies of survival, recovery, and function of transfused PLTs and early detection of PLT engraftment after allogeneic stem cell TX.

rFVIIa administered by continuous infusion during surgery in patients with severe congenital FVII deficiency

The use of recombinant FVIIa (rFVIIa) to control bleed in individuals with FVII deficiency has been proven to be effective. The main problems associated with its use are that it requires frequent bolus injections to counteract its short half-life and high cost. Our study aimed to evaluate whether any advantage could be gained by providing rFVIIa by continuous infusion during surgery with regard to haemostatic efficacy, safety and cost. The prospective study included 10 patients with severe FVII deficiency, who underwent 25 surgical procedures (13 major and 12 minor procedures) and were treated with rFVIIa administered by continuous infusion. Tranexamic acid was given concomitantly every 8 h. Prothrombin time, FVII:C assay and thrombin generation assay were used to monitor the treatment. The mean total dose given was 10 mg during a major surgery and 4.4 mg during a minor surgery for a mean treatment duration of 7.5 and 4.0 days respectively. This corresponds to a reduction of 70-90% in drug usage and medication cost compared with bolus injections. Except for one major perioperative bleeding, excellent haemostasis was achieved in all procedures. One patient developed a transient inhibitory activity. None of these events affected the postoperative course or prolonged the hospital stay. Our study demonstrated that continuous infusion of rFVIIa during surgery is safe, effective and highly cost effective.

Non-genetic risk factors and the development of inhibitors in haemophilia: a comprehensive review and consensus report

SUMMARY

The development of inhibitors to the infused factor in patients with haemophilia is a serious clinical problem. Recent evidence suggests that alongside the strong genetic contribution to inhibitor formation, there are a number of non-genetic factors--perceived by the immune system as danger signals--which promote formation of inhibitors. This study provides a comprehensive review of clinical studies relating to these factors and also presents a survey of opinion concerning their importance and clinical influence, conducted among the members of the European Haemophilia Treatment Standardisation Board (EHTSB). Taken together, this information highlights the lack of robust data concerning the influence of several non-genetic risk factors on inhibitor development, and an urgent need for prospective, well-conducted studies that adhere to recommendations made by the European Medicines Agency (EMEA) for studying inhibitors. Based on current literature, the EHTSB formulated consensus recommendations. It is desirable to minimize intensive treatment wherever possible, given the clinical situation. Prophylaxis should be offered to all children, although we still need to determine optimal dosing with respect to inhibitor development, and age for starting treatment. Vaccinations should be given subcutaneously and concomitant factor concentrate infusions avoided. According to the board, there is no evidence in the literature supporting suggestions that the type of concentrate influences inhibitor risk; but all patients should be monitored during their first exposures. Furthermore, there is no evidence to support an association between pregnancy-related issues, breast feeding and treatment-related factors (e.g. route of administration, or use of blood components) and inhibitor development.

Catheter-directed thrombolysis vs. anticoagulant therapy alone in deep vein thrombosis: results of an open randomized, controlled trial reporting on short-term patency

BACKGROUND

Approximately one in four patients with acute proximal deep vein thrombosis (DVT) given anticoagulation and compression therapy develop post-thrombotic syndrome (PTS). Accelerated removal of thrombus by thrombolytic agents may increase patency and prevent PTS.

OBJECTIVES

To assess short-term efficacy of additional catheter-directed thrombolysis (CDT) compared with standard treatment alone.

PATIENTS AND METHODS

Open, multicenter, randomized, controlled trial. Patients (18-75 years) with iliofemoral DVT and symptoms < 21 days were randomized to receive additional CDT or standard treatment alone. After 6 months, iliofemoral patency was investigated using duplex ultrasound and air-plethysmography assessed by an investigator blinded to previous treatment.

RESULTS

One hundred and three patients (64 men, mean age 52 years) were allocated additional CDT (n = 50) or standard treatment alone (n = 53). After CDT, grade III (complete) lysis was achieved in 24 and grade II (50%-90%) lysis in 20 patients. One patient suffered major bleeding and two had clinically relevant bleeding related to the CDT procedure. After 6 months, iliofemoral patency was found in 32 (64.0%) in the CDT group vs. 19 (35.8%) controls, corresponding to an absolute risk reduction (RR) of 28.2% (95% CI: 9.7%-46.7%; P = 0.004). Venous obstruction was found in 10 (20.0%) in the CDT group vs. 26 (49.1%) controls; absolute RR 29.1% (95% CI: 20.0%-38.0%; P = 0.004). Femoral venous insufficiency did not differ between the two groups.

CONCLUSIONS

After 6 months, additional CDT increased iliofemoral patency from 36% to 64%. The ongoing long-term follow-up of this study will document whether patency is related to improved functional outcome.

[Allogeneic stem cell transplantation in adults with acute lymphoblastic leukaemia]

BACKGROUND

The success rate for chemotherapy of adults with acute lymphoblastic leukaemia in Norway compares favourably with that in international reports, but improvements are still needed. Allogeneic stem cell transplantation is an option for patients up to 60 years and may contribute to improving the outcome for these patients.

MATERIAL AND METHODS

Allogen stem cell transplantation was performed in 61 high-risk patients (38 men and 23 women) with acute lymphoblastic leukaemia at Rikshospitalet between 1985 and 2005. 19 patients were transplanted in first remission and 42 at a later stage of the disease.

RESULTS

At the end of 2006, 26 patients (43%) were alive; 21 (35%) in complete remission and 5 with relapse. Median survival time was 1.5 years. Relapse was the most important cause of treatment failure (38%), but transplantation-related mortality (25%) was also a substantial problem. Estimated 5-year actuarial leukemia-free survival was 35 %.

INTERPRETATION

Our results are in line with international reports on the results of allogen stem cell transplantation in high-risk acute lymphoblastic leukaemia. This treatment offers cure for patients with an otherwise dismal prognosis. A larger number of patients should be offered such treatment during the first remission than what was the case in the 20-year period this study took place.

Acquired haemophilia: management of bleeds and immune therapy to eradicate autoantibodies

Acquired haemophilia is a rare, but often severe bleeding disorder caused by autoantibodies against a coagulation factor, usually factor VIII (FVIII). Between 1997 and 2004 we observed 14 patients (mean age of 78 years) with acquired haemophilia. The aim of the present study was to investigate the effect of activated prothrombin complex concentrate (aPCC) for bleeds and the response to corticosteroids and cyclophosphamide to eradicate the offending autoantibodies. The most common clinical presentations were severe profuse bruising (12) and haematuria (5). Ten patients were classified as idiopathic. At the time of diagnosis all patients had a very low FVIII level, and one patient also showed factor IX < 1%. High levels of antibodies to FVIII varying from 10 to 1340 Bethesda units (BU) and prolonged activated partial thromboplastin time were disclosed in all patients. Eight severe bleeds were treated with aPCC (FEIBA) at a dosage of 70 IU kg(-1) every 8 h until haemostasis. Ten patients received corticosteroids and cyclophosphamide as immunomodulatory therapy. Effective haemostasis was achieved in all bleeds after aPCC. Ten of 11 patients responded either completely or partially to the immunomodulatory regime within 6 months. Five patients achieved complete response (CR) whereas partial responses were seen in five patients. The anti-CD20 monoclonal antibody rituximab was given to two patients in conventional doses and a CR was seen in one patient. aPCC is effective in treating acute bleeds in patients with acquired haemophilia with high inhibitor levels. The combination of oral corticosteroids and cyclophosphamide seems to be effective to eradicate the inhibitor.

Shear-induced platelet activation and platelet microparticle formation in native human blood

Shear-induced platelet activation and platelet microparticle formation are triggered in native human blood by high arterial shear or by a sudden increase in shear as introduced by a stenosis with potential consequences for collagen-induced platelet thrombus formation. Blood was drawn from healthy volunteers and directly perfused ex vivo over various well-defined eccentric stenoses. Shear-induced platelet activation was determined by using flow cytometry to assess: 1) GPIIb-IIIa activation by fluorescein isothiocyanate (FITC)-labeled Mab PAC-1; and 2) translocation of membrane aminophospholipids (procoagulant activity) by FITC-labeled Annexin V. Microparticle formation was measured by flow cytometry and FITC-labeled Mab Y2/51 directed against GPIIIa. Significant platelet activation and platelet microparticle formation were elicited when the wall shear rate reached 10,500 sec-1 for a period of 0.075 sec. Prolonged exposure to or a rapid increase in shear further enhanced activation and microparticle formation. Shear-induced platelet activation was associated with significantly increased collagen-induced platelet thrombus formation that was insensitive to aspirin ingestion. Exposure of native blood to very high shear thus activates platelets to express GPIIb-IIIa, renders the platelet membrane procoagulant and stimulates microparticle formation. These responses are associated with enhanced collagen-induced thrombus formation by prostaglandin-independent mechanisms.

Microvesicles bind soluble fibrinogen, adhere to immobilized fibrinogen and coaggregate with platelets

In the present study we have investigated whether platelet derived microvesicles can bind soluble fibrinogen, bind to immobilized fibrinogen, and coaggregate with platelets. Flow cytometry was used for studies on binding of soluble fibrinogen and coaggregation, whereas ELISA wells were used to study binding of microvesicles to immobilized fibrinogen. Biotinylated microvesicles produced by stimulation with A23187, thrombin or SFLLRN of platelets which had been surface-labelled with biotin, were used both for the coaggregation experiments and for the binding studies with immobilized fibrinogen. Unlabelled microvesicles and biotinylated fibrinogen were employed when studying binding of soluble fibrinogen to the microvesicles. For the flow cytometry, the biotinylated proteins were reacted with avidin or streptavidin which was PE-conjugated, whereas the same substances were conjugated with alkaline phosphatase for the ELISA studies. The microvesicles formed after stimulation of platelets by SFLLRN or A23187 clearly bound the soluble, biotinylated fibrinogen. Moreover, isolated biotinylated microvesicles added to washed platelets prior to activation, were associated to the microaggregates that formed after stimulation. A significant binding of biotinylated microvesicles to immobilized fibrinogen could also be detected. The binding of microvesicles to soluble and immobilized fibrinogen and association to platelets was clearly specific and at least partly dependent on the GPIIb-IIIa complex, as all of these phenomena could be prevented or reduced by addition of the c7E3 Fab which blocks the activated form of this receptor complex. From these in vitro results it is clear that microvesicles can bind to immobilized fibrinogen, bind soluble fibrinogen and are able to coaggregate with platelets. It may be speculated that these results also reflect a haemostatic role of microvesicles in vivo.

Enhanced activation of platelets with abnormal release of RANTES in human immunodeficiency virus type 1 infection

Besides their role in hemostasis, platelets are involved in inflammatory and immunological processes, and we hypothesize that platelet activation may play an immunopathogenetic role in HIV-1 infection. Blood was drawn from 15 controls and 20 HIV-1-infected patients with normal platelet counts, classified into groups of non-AIDS and AIDS. Platelet activation was detected using flow cytometry with mAbs against the release markers P-selectin and CD63, mAb against GPIb, and the probe annexin V detecting surface exposure of aminophospholipids. The amount of microvesicles was measured using mAb against GPIIIa. Compared to controls, blood samples from HIV-1-infected patients showed significantly enhanced levels of microvesicles and activated platelets as detected by their exposure of P-selectin, CD63, and aminophospholipids, as well as reduction in GPIb expression. Increased expression of P-selectin and amounts of microvesicles were most pronounced in advanced clinical and immunological disease. When studying the effect of HIV-1 protease inhibitor therapy (indinavir) on platelet activation, we found that concomitant with a profound decrease in plasma viral load, there was a near normalization of several of the parameters reflecting enhanced platelet activation. Finally, we demonstrated that platelets may be an important source of the chemokine RANTES in HIV-1-infected patients. Although both unstimulated and SFLLRN-stimulated platelets from asymptomatic patients had enhanced release of RANTES, platelets from AIDS patients were characterized by markedly enhanced spontaneous, but decreased SFLLRN-stimulated release of this chemokine. Taken together, these results, which demonstrate for the first time increased platelet activation in HIV-1-infected patients with normal platelet counts, may represent a previously unrecognized immunopathogenic factor in HIV-1 infection.

Shear-induced platelet activation and platelet microparticle formation at blood flow conditions as in arteries with a severe stenosis

In the present study, we investigated whether high arterial shear stresses at various exposure times or a sudden increase in shear stress introduced by a stenosis affect platelet activation and platelet microparticle formation in native human blood. We used a parallel-plate perfusion chamber device through which nonanticoagulated human blood was drawn (10 mL/min) by a pump directly from an antecubital vein through the flow channel of a perfusion chamber at wall shear rates of 420, 2600, and 10500 s-1. In another set of experiments, an eccentric stenosis was introduced into the flow channel. Wall shear rates of 2600 or 10500 s-1 at the stenosis apex were maintained at the same flow rate. The wall shear rate upstream and downstream of these stenoses was 420 s-1. A shear rate of 420 s-1 is within the range of those encountered in healthy small coronary arteries, whereas those of 2600 and 10500 s-1 are representative for vessels with various degrees of stenotic lesions. The blood was exposed to these shear rates for periods varying from 0.075 to 3.045 seconds. Platelet activation was assessed as activated glycoprotein (GP) IIb/IIIa by FITC-labeled monoclonal antibody (MAb) PAC-1 and aminophospholipid translocation by FITC-labeled annexin V. Microparticle formation was quantified by FITC-labeled MAb Y2/51 directed against GP IIIa. Significant platelet activation and formation of microparticles were observed at 10500 s-1 only (P < .008). This shear-induced platelet activation and microparticle formation were enhanced by introduction of a thrombus-promoting surface consisting of type III human collagen fibrils. Introduction of the most severe stenosis at 10500 s-1 further increased platelet activation (P < .017). The collagen-induced thrombus formation increased the platelet thrombus volume at 10500 s-1 from 16.5 to 33.8 microns3/microns2 (P < .003) on the stenosis apex when the most severe stenosis was used. A correlation (P < .0001) between platelet thrombus volume and platelet microparticle formation was observed in the presence of the eccentric stenoses. Apparently, high shear stress (315 dynes/cm2 at 10500 s-1), as encountered in severe atherosclerotic arteries, activated platelets and triggered platelet microparticle formation. In contrast, no significant platelet activation or formation of platelet microparticles was observed at physiological shear (420 s-1) or at the shear condition simulating shear in arteries with a less severe stenosis (2600 s-1). The data imply that platelets are activated and form microparticles in native blood at very high shear stresses. These events are potentiated by prolonged exposure to the high shear or by a sudden change of increasing shear due to the stenosis. The latter situation apparently enhances platelet thrombus formation at the stenosis.

Stimulated Glanzmann's thrombasthenia platelets produced microvesicles. Microvesiculation correlates better to exposure of procoagulant surface than to activation of GPIIb-IIIa

The mechanism of formation of platelet-derived microvesicles remains controversial. The aim of the present work was to study the formation of microvesicles in view of a possible involvement of the GPIIb-IIIa complex, and of exposure of negatively charged phospholipids as procoagulant material on the platelet surface. This was studied in blood from three Glanzmann's thrombasthenia patients lacking GPIIb-IIIa and healthy blood donors. MAb FN52 against CD9 which activates the complement system and produces microvesicles due to a membrane permeabilization, ADP (9.37 microM), and the thrombin receptor agonist peptide SFLLRN (100 microM) that activates platelets via G-proteins were used as inducers. In a series of experiments platelets were also preincubated with PGE1 (20 microM). The number of liberated microvesicles, as per cent of the total number of particles (including platelets), was measured using flow cytometry with FITC conjugated antibodies against GPIIIa or GPIb. Activation of GPIIb-IIIa was detected as binding of PAC-1, and exposure of aminophospholipids as binding of annexin V. With normal donors, activation of the complement system induced a reversible PAC-1 binding during shape change. A massive binding of annexin V was seen during shape change as an irreversible process, as well as formation of large numbers of microvesicles (60.6 +/- 2.7%) which continued after reversal of the PAC-1 binding. Preincubation with PGE1 did not prevent binding of annexin V, nor formation of microvesicles (49.5 +/- 2.7%), but abolished shape change and PAC-1 binding after complement activation. Thrombasthenic platelets behaved like normal platelets after activation of complement except for lack of PAC-1 binding (also with regard to the effect of PGE1 and microvesicle formation). Stimulation of normal platelets with 100 microM SFLLRN gave 16.3 +/- 1.2% microvesicles, and strong PAC-1 and annexin V binding. After preincubation with PGE1 neither PAC-1 nor annexin V binding, nor any significant amount of microvesicles could be detected. SFLLRN activation of the thrombasthenic platelets produced a small but significant number of microvesicles (6.4 +/- 0.8%). Incubation of thrombasthenic platelets with SFLLRN after preincubation with PGE1, gave results identical to those of normal platelets. ADP activation of normal platelets gave PAC-1 binding, but no significant annexin V labelling, nor production of microvesicles. Thus, different inducers of the shedding of microvesicles seem to act by different mechanisms. For all inducers there was a strong correlation between the exposure of procoagulant surface and formation of microvesicles, suggesting that the mechanism of microvesicle formation is linked to the exposure of aminophospholipids. The results also show that the GPIIb-IIIa complex is not required for formation of microvesicles after activation of the complement system, but seems to be of importance, but not absolutely required, after stimulation with SFLLRN.

Detection of biotinylated proteins in crossed immunoelectrophoresis gels: studies on platelet membrane receptors and microparticles

Biotinylation can be used as an alternative for surface labeling of cell membrane proteins. The use of the water soluble N-hydroxysulfosuccinimide (NHSS)-biotin or the more lipophilic N-hydroxysuccinimide (NHS)-biotin reagent has been investigated in the present study labeling two central receptor complexes on the platelet surface, i.e. the glycoprotein (GP) Ib-IX and the GP IIb-IIIa complexes involved in platelet adhesion and aggregation. Lack of labeling of the intracellularly located albumin was used as a negative control. The labeling has been studied using crossed immunoelectrophoresis in the PhastSystem format after extraction of the labeled cells in Triton X-100, and it is shown that, using enzyme-conjugated avidin and chromogenic substrates, the biotinylated proteins can be visualized directly in the dried electrophoresis gel without the need for a transfer to a blotting membrane as is used after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Suitable conditions for biotinylation and for visualization in the crossed immunoelectrophoresis gels are described. Further, surface-biotinylation of platelets was used to observe shedding of microparticles as a consequence of formation of the complement membrane attack complex. For this purpose the formation and composition of the biotinylated microparticles were observed by flow cytometry and crossed immunoelectrophoresis.

Platelet-derived microvesicles and activated platelets express factor Xa activity

Activated platelets and platelet-derived microvesicles demonstrate procoagulant properties. It is known that following stimulation, negatively charged phospholipids and factor Va become located on their surfaces. The aim of this study was to see whether activated platelets and platelet-derived microvesicles also expressed some factor Xa activity on their surfaces in a system where factor Xa did not come from external sources. In order to study this question, flow cytometry, as well as the use of a chromogenic substrate to factor Xa and a clotting assay in a factor X depleted plasma, were applied. A prothrombinase assay was also applied using prothrombin, CaCl2 and a chromogenic substrate to thrombin. The platelets were gel-filtered or washed, suspended in Tris-buffered saline, and activated by calcium ionophore A23187 or the thrombin receptor agonist peptide SFLLRN. Microvesicles and activated platelets were separated by centrifugation. Flow cytometry using a monoclonal antibody against factor Xa demonstrated the presence of factor Xa on the surface of the activated platelets. In addition, platelet-derived microvesicles and activated platelets demonstrated factor Xa activity on their surfaces detected directly by splitting of the chromogenic substrate to factor Xa, or by the prothrombinase assay. The thrombin generation in the last assay could be inhibited by a selective factor Xa inhibitor (recombinant tick anticoagulant peptide (rTAP)), soybean trypsin inhibitor, and antithrombin III plus LMW-heparin, all inhibiting at the factor Xa level, as well as by leupeptin which also inhibited the thrombin-chromogenic substrate interaction as such.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of platelet-derived microvesicles in blood from patients with activated coagulation and fibrinolysis using a filtration technique and western blotting

Platelet vesiculation in vitro is correlated to platelet activation. It was therefore considered of interest to see if microvesicles (MV) are present in blood in clinical situations associated with platelet activation in vivo. Patients with both activated coagulation and fibrinolysis, implying that thrombin has been generated, suit such a purpose. Thus, the aim of this study was to investigate whether microvesicles could be detected in patients with activated coagulation and fibrinolysis, as diagnosed by the presence of soluble fibrin (positive ethanol gelation tests) and positive tests for fibrin degradation products (FDP). Platelet-rich plasma was prepared from citrated blood from patients (n = 22) and healthy controls (n = 32) matched as to age and sex. The intact platelets were removed from plasma by centrifugation. Any MV present were isolated from the platelet-free plasma by a filtration procedure, washed, solubilized in Triton X-100 and subjected to SDS-PAGE with Western blotting using a MAb against GPIIb alpha as an indicator of the presence of microvesicles. All of the 22 patients showed the presence of microvesicles detectable by the content of GPIIb alpha, whereas this could be observed in only 4 out of the 32 normal controls and then in small or trace amounts only. The presence of microvesicles among cell-derived material in the plasma of two of the patients was also confirmed by electron microscopy. To the best of our knowledge this is the first report on the presence of microvesicles in plasma from patients with both activated coagulation and fibrinolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

The difference between platelet and plasma FXIII used to study the mechanism of platelet microvesicle formation

The formation of microvesicles from platelets was induced either by activation of the complement system by a monoclonal antibody to CD9, or by incubation of platelets with the calcium ionophore A23187. A filter technique to isolate the microvesicles without plasma contamination is described. The microvesicles contained FXIIIa2 from the platelet cytoplasm which shows that these particles contain significant amounts of intracellular material. This was shown by the use of crossed immunoelectrophoresis with rabbit antibodies to total human platelet proteins in the second dimension gel and polyclonal antibodies against the a- and b-subunit of FXIII in the intermediate gel. The FXIIIa2 in the microvesicle was found to be functional as an enzyme. To prove this, it was shown that FXIII in its immunoprecipitate arc could catalyze the incorporation of monodansylcadaverine into casein as identified by fluorescence of this arc in ultraviolet light. The observation that the plasma form of FXIII (FXIIIa2b2) was absent from the microvesicles collected by the filtration technique, whereas it was present in platelet fragments obtained by mechanical disruption by ultrasonication, indicates that the activation-dependent microvesicles are formed by a true budding process with the inclusion of intracellular, but not extracellular material.