Effects of donor characteristics and platelet in vitro time and temperature on platelet aggregometry

Shear-Mediated Platelet Microparticles Demonstrate Phenotypic Heterogeneity as to Morphology, Receptor Distribution, and Hemostatic Function

Objective

Implantable cardiovascular therapeutic devices (CTD) including stents, percutaneous heart valves and ventricular assist devices, while lifesaving, impart supraphysiologic shear stress to platelets resulting in thrombotic and bleeding device-related coagulopathy. We previously demonstrated that shear-mediated platelet dysfunction is associated with downregulation of platelet GPIb-IX-V and αIIbβ3 receptors via generation of platelet-derived microparticles (PDMPs). Here, we test the hypothesis that shear-generated PDMPs manifest phenotypical heterogeneity of their morphology and surface expression of platelet receptors, and modulate platelet hemostatic function.

Approach and Results

Human gel-filtered platelets were exposed to continuous shear stress and sonication. Alterations of platelet morphology were visualized using transmission electron microscopy. Surface expression of platelet receptors and PDMP generation were quantified by flow cytometry. Thrombin generation was quantified spectrophotometrically, and platelet aggregation in plasma was measured by optical aggregometry. We demonstrate that platelet exposure to shear stress promotes notable alterations in platelet morphology and ejection of several distinctive types of PDMPs. Shear-mediated microvesiculation is associated with the differential remodeling of platelet receptors with PDMPs expressing significantly higher levels of both adhesion (α _IIb β ₃ , GPIX, PECAM-1, P-selectin, and PSGL-1) and agonist-evoked receptors (P ₂ Y ₁₂ & PAR1). Shear-mediated PDMPs have a bidirectional effect on platelet hemostatic function, promoting thrombin generation and inhibiting platelet aggregation induced by collagen and ADP.

Conclusions

Shear-generated PDMPs demonstrate phenotypic heterogeneity as to morphologic features and defined patterns of surface receptor alteration, and impose a bidirectional effect on platelet hemostatic function. PDMP heterogeneity suggests that a range of mechanisms are operative in the microvesiculation process, contributing to CTD coagulopathy and posing opportunities for therapeutic manipulation.

Current Understanding of the Relationship between Blood Donor Variability and Blood Component Quality

While differences among donors has long challenged meeting quality standards for the production of blood components for transfusion, only recently has the molecular basis for many of these differences become understood. This review article will examine our current understanding of the molecular differences that impact the quality of red blood cells (RBC), platelets, and plasma components. Factors affecting RBC quality include cytoskeletal elements and membrane proteins associated with the oxidative response as well as known enzyme polymorphisms and hemoglobin variants. Donor age and health status may also be important. Platelet quality is impacted by variables that are less well understood, but that include platelet storage sensitive metabolic parameters, responsiveness to agonists accumulating in storage containers and factors affecting the maintenance of pH. An increased understanding of these variables can be used to improve the quality of blood components for transfusion by using donor management algorithms based on a donors individual molecular and genetic profile.

Platelet Dysfunction During Mechanical Circulatory Support: Elevated Shear Stress Promotes Downregulation of αIIbβ3 and GPIb via Microparticle Shedding Decreasing Platelet Aggregability

[Figure: see text].

The MICELI (MICrofluidic, ELectrical, Impedance): Prototyping a Point-of-Care Impedance Platelet Aggregometer

As key cellular elements of hemostasis, platelets represent a primary target for thrombosis and bleeding management. Currently, therapeutic manipulations of platelet function (antithrombotic drugs) and count (platelet transfusion) are performed with limited or no real-time monitoring of the desired outcome at the point-of-care. To address the need, we have designed and fabricated an easy-to-use, accurate, and portable impedance aggregometer called “MICELI” (MICrofluidic, ELectrical, Impedance). It improves on current platelet aggregation technology by decreasing footprint, assay complexity, and time to obtain results. The current study aimed to optimize the MICELI protocol; validate sensitivity to aggregation agonists and key blood parameters, i.e., platelet count and hematocrit; and verify the MICELI operational performance as compared to commercial impedance aggregometry. We demonstrated that the MICELI aggregometer could detect platelet aggregation in 250 μL of whole blood or platelet-rich plasma, stimulated by ADP, TRAP-6, collagen, epinephrine, and calcium ionophore. Using hirudin as blood anticoagulant allowed higher aggregation values. Aggregation values obtained by the MICELI strongly correlated with platelet count and were not affected by hematocrit. The operational performance comparison of the MICELI and the Multiplate^® Analyzer demonstrated strong correlation and similar interdonor distribution of aggregation values obtained between these devices. With the proven reliability of the data obtained by the MICELI aggregometer, it can be further translated into a point-of-care diagnostic device aimed at monitoring platelet function in order to guide pharmacological hemostasis management and platelet transfusions.

Whole blood platelet aggregation determined by the ROTEM platelet equipment; reference intervals and stability

Point of care testing of residual effect of antiplatelet therapy in trauma patients or during major surgery may result in improved clinical management of significant bleeding. We included 121 healthy individuals (57 females and 64 males, aged 22-65 years) in order to establish reference intervals for platelet aggregation induced by adenosine diphosphate (ADPTEM, 10 µM), arachidonic acid (ARATEM, 0.42 mM) and thrombin activating peptide (TRAPTEM, 36 µM) employing the ROTEM platelet module. Further, the impact of citrate (3.2%) and hirudin (>15 µg/ml) as anticoagulants was evaluated. Finally, we investigated assay stability (15, 30, 60, and 120 min after blood sampling) (n = 8) and between-day variation (n = 5). We report reference intervals for 121 healthy individuals and reference intervals by gender. We observed significantly higher platelet aggregation in females than in males (all P-values < 0.05). No correlation between age and platelet aggregation was observed, except for the parameter TRAPTEM amplitude (A6), in which a decline in A6 was observed with increasing age (P = 0.03). We observed significantly lower levels of platelet aggregation in citrate tubes than in hirudin tubes (all P-values < 0.05), except from TRAPTEM maximum slope, where no significant difference was observed (P = 0.40).The stability was acceptable (≤20% deviation) for up to 120 min for ARATEM in citrate tubes, and up to 60 min for the ADPTEM and TRAPTEM assays in citrate tubes. In hirudin tubes we found ADPTEM and ARATEM assays to be stable for 60 min, while the stability of TRAPTEM in hirudin tubes was found to be stable for 30 min. Using citrate tubes, the between-day variation (mean coefficient of variation, CV) was 19-20% for ADPTEM, 19-26% for TRAPTEM, and 10% for ARATEM, whereas the mean CV was 11-13% for all three assays in hirudin tubes.In conclusion, we established combined and gender-specific reference intervals for three platelet aggregation assays in both citrate- and hirudin tubes. In citrate tubes, the stability of the ROTEM platelet assays was 60-120 min, while the stability in hirudin tubes was 30-60 min. The between-day variation was lowest for samples obtained in hirudin tubes.

Role of Purinergic Receptor Expression and Function for Reduced Responsiveness to Adenosine Diphosphate in Washed Human Platelets

Background

Washing of platelets is an important procedure commonly used for experimental studies, e.g. in cardiovascular research. As a known phenomenon, responsiveness to adenosine diphosphate (ADP) is reduced in washed platelets, although underlying molecular mechanisms—potentially interfering with experimental results—have not been thoroughly studied.

Objectives

Since ADP mediates its effects via three purinergic receptors P2Y1, P2X1 and P2Y12, their surface expression and function were investigated in washed platelets and, for comparison, in platelet-rich-plasma (PRP) at different time points for up to 2 hours after preparation.

Results

In contrast to PRP, flow cytometric analysis of surface expression in washed platelets revealed an increase of all receptors during the first 60 minutes after preparation followed by a significant reduction, which points to an initial preactivation of platelets and consecutive degeneration. The activity of the P2X1 receptor (measured by selectively induced calcium flux) was substantially maintained in both PRP and washed platelets. P2Y12 function (determined by flow cytometry as platelet reactivity index) was partially reduced after platelet washing compared to PRP, but remained stable in course of ongoing storage. However, the function of the P2Y1 receptor (measured by selectively induced calcium flux) continuously declined after preparation of washed platelets.

Conclusion

In conclusion, decreasing ADP responsiveness in washed platelets is particularly caused by impaired activity of the P2Y1 receptor associated with disturbed calcium regulation, which has to be considered in the design of experimental studies addressing ADP mediated platelet function.

Gender, race and diet affect platelet function tests in normal subjects, contributing to a high rate of abnormal results

To assess sources of variability in platelet function tests in normal subjects, 64 healthy young adults were tested on 2-6 occasions at 2 week intervals using four methods: platelet aggregation (AGG) in platelet-rich plasma (PRP) in the Bio/Data PAP-4 Aggregometer (BD) and Chrono-Log Lumi-Aggregometer (CL); and AGG in whole blood (WB) in the CL and Multiplate Platelet Function Analyser (MP), with ATP release (REL) in CL-PRP and CL-WB. Food and medication exposures were recorded prospectively for 2 weeks prior to each blood draw. At least one AGG abnormality was seen in 21% of 81 drug-free specimens with CL-PRP, 15% with CL-WB, 13% with BD-PRP and 6% with MP-WB, increasing with inclusion of REL to 28% for CL-PRP and 30% for CL-WB. Epinephrine AGG and REL were significantly reduced in males (P < 0·0001). Ristocetin AGG and collagen and thrombin REL were significantly reduced in Blacks (P < 0·0001). One-third of specimens drawn following flavonoid-rich food exposures had aberrant results, compared to 8·5% of specimens without such exposures (P = 0·0035). PRP tests had less intra-individual variation than WB tests. Gender, race, diet and test system affected results of platelet function testing in healthy subjects, suggesting caution when interpreting the results of platelet function testing in patients.

Estrogen, inflammation, and platelet phenotype

BACKGROUND

Although exogenous estrogenic therapies increase the risk of thrombosis, the effects of estrogen on formed elements of blood are uncertain.

OBJECTIVE

This article examines the genomic and nongenomic actions of estrogen on platelet phenotype that may contribute to increased thrombotic risk.

METHODS

To determine aggregation, secretion, protein expression, and thrombin generation, platelets were collected from experimental animals of varying hormonal status and from women enrolled in the Kronos Early Estrogen Prevention Study.

RESULTS

Estrogen receptor beta predominates in circulating platelets. Estrogenic treatment in ovariectomized animals decreased platelet aggregation and adenosine triphosphate (ATP) secretion. However, acute exposure to 17beta-estradiol did not reverse decreases in platelet ATP secretion invoked by lipopolysaccharide. Thrombin generation was positively correlated to the number of circulating microvesicles expressing phosphatidylserine.

CONCLUSION

Assessing the effect of estrogen treatments on blood platelets may lead to new ways of identifying women at risk for adverse thrombotic events with such therapies.

Effects of autologous platelet concentrate reinfusion after open heart surgery in patients with congenital heart disease

BACKGROUND

Plasma pheresis and reinfusion of platelet-rich plasma has not been shown to reduce blood loss in cardiac patients. Recently, freshly prepared autologous platelet concentrates (PC) can be made from patient's blood and has a higher concentration than conventional platelet rich plasma. In this study, the effects of autologous PC reinfusion were examined after open heart surgery in patients with congenital heart disease.

METHODS

Eight patients with noncyanotic congenital heart disease, who underwent open heart surgery and reinfusion of autologous PC, were classified as the PC group. Eight other patients with noncyanotic congenital heart disease, who underwent only open heart surgery, were defined as the control group. Ages ranged from 2 to 24 years and were not significantly different between the two groups (9.3 +/- 5.1 years in the PC group and 12.6 +/- 7.9 years in the control group, p = 0.33). In the PC group, blood was collected from the femoral vein through a 6F catheter introducer; 9 to 20 U (13.0 +/- 5.4 U, 0.42 +/- 0.22 U/kg) of autologous PC were prepared and were reinfused after protamine administration. The time course of platelet counts was examined until postoperative day 7. Aggregation responses to adenosine diphosphate; (4 micromol/L and 8 micromol/L), collagen (1 micromol/L and 5 micromol/L), and epinephrine (5 micromol/L and 10 micromol/L) were evaluated after induction of anesthesia (individual references), after protamine administration, at the end of the operation; these responses were shown as recovery ratios.

RESULTS

Blood loss during surgery in the PC group was significantly less than in the control group (4.8 +/- 3.0 mL/kg versus 7.8 +/- 1.7 mL/kg, p = 0.044). Similarly blood loss on postoperative day 1 in the PC group was significantly less than in the control group (3.6 +/- 1.2 mL/kg versus 7.2 +/- 3.1 mL/kg, p = 0.013). The platelet counts in the PC group were larger than those in the control group until postoperative day 5, after reinfusion of prepared autologous PC. The recovery ratios of the aggregation responses to adenosine diphosphate, collagen, and epinephrine after protamine administration were not significantly different between the two groups. However, recovery in the PC group after reinfusion of the prepared autologous PC was greater than in the control group.

CONCLUSIONS

Reinfusion of the freshly prepared autologous PC was followed by good aggregation responses and low blood loss in patients with noncyanotic congenital heart disease after open heart surgery. This procedure may be useful in pediatric open heart surgery without blood transfusion or with little administration of homologous blood products.

Hypothermia-induced platelet aggregation in heparinized flowing human blood: identification of a high responder subpopulation

Cold-induced platelet aggregation (CIPA) in PRP has previously been documented in connection with platelet preservation (4-15 degrees C). This report describes hypothermia-induced platelet aggregation (HIPA) in whole blood and at temperatures used in open-heart surgery (24-32 degrees C). HIPA (specifically, the formation of occlusive aggregates) was studied in human whole blood. Fresh heparinized (1.5 U/ml) human blood was cooled and maintained at target temperatures (15, 20, 24, 28, 32, or 37 degrees C) as it flowed (1 ml/min) through 75-cm long 1/32 inches internal diameter polymer conduit. The formation of aggregates in the tubing was verified using optical video microscopy and was quantified by a light-scattering method and a constant-pressure filtration method. Donors were tested at least twice at each target temperature and were classified into three separate response regimes (Low, Medium, and High) on the basis of the number of aggregates and the duration of their appearance. The screening of 121 donors (average age 22.3 +/- 4.3 years) for HIPA at 24 degrees C (the temperature of maximum response) indicated 14% High Responders, 18% Medium Responders, and 68% Low Responders. HIPA was inhibited by EDTA, citrate, PGE1, and Tirofiban, but not by aspirin, and it was enhanced by elevated heparin levels. HIPA was consistently noted in the blood of a subpopulation of donors, and the associated platelet aggregates in the blood of High Responders were rigid and occlusive. It is postulated that such aggregates may contribute to cognitive dysfunction noted in patients undergoing hypothermic open-heart surgery, and that postulus is being investigated.

Measurement of human platelet microaggregates by a new method: ultrasonic interferometry

We have adapted the ultrasonic interferometry technique (Echo-Cell), which was initially designed to study red blood cell aggregation and agglutination, to the detection of human platelet microaggregates. The experimental parameter chosen was the slope of the signal over the first 5 minutes of sedimentation. We compared our new method with the conventional aggregometry for the measurement of aggregates after thrombin-, collagen-, and epinephrine-induced platelet activation. Under these conditions we demonstrated the particular sensibility of the present method in detecting small platelet aggregates induced in the first phase of aggregation and formed by low concentrations of agonists. Furthermore, as an illustration of this method, we showed an inhibition of the formation of thrombin-induced platelet aggregates in a concentration-dependent manner by the well known antagonist arginine-glycine-aspartic acid-serine with a median inhibitory concentration of 0.4 micromol/L, which is 30 times lower than the median inhibitory concentration found by aggregometry.

Do patients with the heparin-induced thrombocytopenia syndrome have heparin-specific antibodies?

PURPOSE

Patients with the heparin-induced thrombocytopenia syndrome (HIT) have heparin-associated antibodies (HAb+), which, in the presence of heparin, are responsible for platelet activation and aggregation. This study addressed the questions: (1) are the antibodies specific for heparin; and (2) how do the antibodies cause platelet aggregation?

METHODS

Plasmas from 79 patients with HIT were divided into seven plasma samples: HAb+ plasma sample 1 (24 pooled plasmas); HAb+ plasma sample 2 (50 pooled plasmas); and HAb+ plasma samples 3 through 7 (individual plasmas). Normal patient plasmas were used as controls (HAb-).

RESULTS

All seven HAb+ plasma samples caused platelet aggregation (PLA) in the presence of heparin and formed a precipitation line with heparin in gel immunodiffusion plates (HAb- plasmas did neither). The HAb+ plasma samples reacted with heparin, as determined by immunoprecipitation in sodium dodecylsulfate-polyacrylamide gel, with the production of a band at 50 kd (no band with HAb- plasmas). The plasma samples 1 and 2 were passed over heparin sepharose beads three times; the unabsorbed plasmas produced 3+ PLA, the first effluent produced 2+ PLA, and the second and third effluents produced no PLA. The heparin sepharose beads stained 3+, 2+, and 1+, after the respective passages, with fluorescein-labeled goat sera containing anti-human immunoglobulin G antibody. HAb+ plasma samples were digested with pepsin to separate the F(ab')2 fragments from the Fc fragments. The F(ab')2 fragments reacted with heparin as determined by immunoprecipitation in sodium dodecylsulfate-polyacrylamide gel with the production of a band at 25 kd, but did not cause PLA in the presence of heparin.

CONCLUSION

Patients with HIT have heparin-specific antibodies that react with heparin in a classic F(ab')2 reaction and require the Fc fragment for platelet aggregation.