Novel assay demonstrates that coronary artery disease patients have heightened procoagulant platelet response

A novel technique to characterize procoagulant platelet formation and evaluate platelet procoagulant tendency in cats by flow cytometry

Introduction

Cardiogenic arterial thromboembolism (CATE) is a life-threatening complication of hypertrophic cardiomyopathy (HCM) with a high mortality rate. As the primary responders in hemostasis, platelets play a crucial role in the progression of CATE. Procoagulant platelets are a subpopulation of activated platelets that facilitate thrombin generation to strengthen thrombus structure. Since their discovery, procoagulant platelets have gained a considerable amount of attention due to their potential role in thromboembolic complications. Uncovering the key phenotypic markers and agonists underlying the procoagulant response in feline platelets may provide diagnostic and therapeutic insights in the treatment of CATE. However, species differences in platelet physiology and the sensitive nature of feline platelets pose some significant challenges in studying procoagulant platelets in cats.

Objectives

To first devise a flow cytometric method to sequentially assess procoagulant platelet markers and to identify agonists that could induce procoagulant platelet phenotypes. Furthermore, a novel scoring system was established to evaluate the procoagulant tendency of platelets in cats.

Methods

Platelets were isolated from eight healthy cats and activated by thrombin in the presence or absence of collagen (COL) and convulxin (CVX). The following markers were measured sequentially by flow cytometry: (1) loss of mitochondrial membrane potential (ΔΨm), (2) phosphatidylserine (PS) externalization, and (3) P-selectin upregulation.

Results

Thrombin in the presence of CVX significantly reduced ΔΨm and induced P-selectin upregulation (p = 0.0078, p = 0.0128, respectively). In addition, thrombin-treated platelets in the presence of COL and CVX augmented PS externalization significantly (p = 0.043, p = 0.0172, respectively). Of the agonists tested, thrombin and CVX resulted in the highest procoagulant tendency score with 75% cats reaching a score ≥2/3. The number of cats with a procoagulant tendency score of 2 or 3 for thrombin and thrombin + COL was 4/8 (50%) and 5/8 (62.5%), respectively.

Discussion

Sequential analysis of procoagulant markers via flow cytometry may be an effective technique for studying procoagulant platelets in cats.

Procoagulant platelet activation promotes venous thrombosis

ABSTRACT

Platelets are key players in cardiovascular disease, and platelet aggregation represents a central pharmacologic target, particularly in secondary prevention. However, inhibition of adenosine diphosphate and thromboxane signaling has low efficacy in preventing venous thromboembolism, necessitating the inhibition of the plasmatic coagulation cascade in this disease entity. Anticoagulation carries a significantly higher risk of bleeding complications, highlighting the need of alternative therapeutic approaches. We hypothesized that procoagulant activation (PA) of platelets promotes venous thrombus formation and that targeting PA could alleviate venous thrombosis. Here, we found elevated levels of procoagulant platelets in the circulation and in thrombi of patients with deep vein thrombosis (DVT) and pulmonary embolism, and in mice developing DVT following inferior vena cava stenosis. Furthermore, we detected PA of recruited platelets within murine venous thrombi and human pulmonary emboli. Mice with platelet-specific deficiency in central pathways of PA-cyclophilin D and transmembrane protein 16F-were more resistant toward low flow-induced venous thrombosis. Finally, we found that a clinically approved carbonic anhydrase inhibitor, methazolamide, reduced platelet procoagulant activity and alleviated murine thrombus formation without affecting trauma-associated hemostasis. These findings identify an essential role of platelet procoagulant function in venous thrombosis and delineate novel pharmacologic strategies targeting platelets in the prevention of venous thromboembolism.

Dissecting the roles of dynamin and clathrin in platelet pinocytosis

Platelets endocytose many molecules from their environment. However, this process of pinocytosis in platelets is poorly understood. Key endocytic regulators such as dynamin, clathrin, CDC42 and Arf6 are expressed in platelets but their roles in pinocytosis is not known. Stimulated platelets form two subpopulations of pro-aggregatory and procoagulant platelets. The effect of stimulation on pinocytosis is also poorly understood. In this study, washed human platelets were treated with a range of endocytosis inhibitors and stimulated using different activators. The rate of pinocytosis was assessed using pHrodo green, a pH-sensitive 10 kDa dextran. In unstimulated platelets, pHrodo fluorescence increased over time and accumulated as intracellular puncta indicating constituently active pinocytosis. Stimulated platelets (both pro-aggregatory and procoagulant) had an elevated pinocytosis rate compared to unstimulated platelets. Dynamin inhibition blocked pinocytosis in unstimulated, pro-aggregatory and procoagulant platelets indicating that most platelet pinocytosis is dynamin dependent. Although pinocytosis was clathrin-independent in unstimulated and procoagulant populations, clathrin partially contributed to pinocytosis in pro-aggregatory platelets.

The Etiology of the Thrombotic Phenomena Involved in the Process of Coronary Artery Disease-What Is the Role of Thrombophilic Genes in the Development of This Pathology?

Cardiovascular diseases, among which includes coronary artery disease, represent one of the most important causes of mortality and morbidity worldwide. Research aimed at determining the risk factors involved recognizes a group of "traditional" risk factors, but also more recent studies identified over 100 "novel" ones which may have a role in the disease. Among the latter is the thrombophilia profile of a patient, a pathology well-established for its involvement in venous thromboembolism, but with less studied implications in arterial thrombosis. This paper reviews the literature, explaining the pathophysiology of the thrombophilia causes associated most with coronary thrombosis events. Results of several studies on the subject, including a meta-analysis with over 60,000 subjects, determined the significant involvement of factor V Leiden, prothrombin G20210A mutation, plasminogen activator inhibitor-1 and antiphospholipid syndrome in the development of coronary artery disease. The mechanisms involved are currently at different stages of research, with some already established and used as therapeutic targets.

Diagnostic value of antibody-induced procoagulant platelets in heparin-induced thrombocytopenia: communication from the ISTH SSC Subcommittee on Platelet Immunology

Heparin-induced thrombocytopenia (HIT) is an immune-mediated prothrombotic disorder characterized by a drop in platelet count and an increased risk of thromboembolic events. The accurate diagnosis of HIT involves clinical assessment and laboratory testing with well-characterized functional tests. Recent research has shown the potential of investigating procoagulant platelet formation induced by HIT antibodies. To successfully implement these assays in clinical laboratories, careful consideration of technical and preanalytical factors is crucial. In this communication from the SSC Platelet Immunology, we provide a consensus from experts on the use of flow cytometry in HIT diagnosis, highlighting the importance of standardized protocols.

Platelets and Thrombotic Antiphospholipid Syndrome

Antiphospholipid antibody syndrome (APS) is an autoimmune disorder characterised by thrombosis and the presence of antiphospholipid antibodies (aPL): lupus anticoagulant and/or IgG/IgM anti-β2-glycoprotein I and anticardiolipin antibodies. APS carries significant morbidity for a relatively young patient population from recurrent thrombosis in any vascular bed (arterial, venous, or microvascular), often despite current standard of care, which is anticoagulation with vitamin K antagonists (VKA). Platelets have established roles in thrombosis at any site, and platelet hyperreactivity is clearly demonstrated in the pathophysiology of APS. Together with excess thrombin generation, platelet activation and aggregation are the common end result of all the pathophysiological pathways leading to thrombosis in APS. However, antiplatelet therapies play little role in APS, reserved as a possible option of low dose aspirin in addition to VKA in arterial or refractory thrombosis. This review outlines the current evidence and mechanisms for excessive platelet activation in APS, how it plays a central role in APS-related thrombosis, what evidence for antiplatelets is available in clinical outcomes studies, and potential future avenues to define how to target platelet hyperreactivity better with minimal impact on haemostasis.

Acute effects of high intensity interval training versus moderate intensity continuous training on haemostasis in patients with coronary artery disease

Exercise training is associated with an acute net increase in coagulation, which may increase the risk of atherothrombosis in coronary artery disease (CAD) patients. We sought to compare the acute haemostatic effects of a bout of moderate-intensity continuous (MICT) and high-intensity interval training (HIIT) in patients with CAD. Patients after a recent myocardial infarction were randomized into a HIIT or MICT session of exercise training on a stationary bike. Blood was sampled at baseline, after the exercise bout and after a one-hour resting period. We measured overall haemostatic potential (OHP), overall coagulation potential (OCP), fibrinogen, D-dimer and von Willebrand factor (vWF) and calculated overall fibrinolytic potential (OFP). Linear mixed models for repeated measures were constructed to assess the treatment effect. A total of 117 patients were included. OCP, OHP, fibrinogen, D-dimer and vWF significantly increased after exercise and returned to baseline after a one-hour rest, OFP decreased after exercise and returned to baseline levels after a one-hour rest. Linear mixed models showed a significant difference between HIIT and MICT in fibrinogen (p 0.043) and D-dimer (p 0.042). Our study has shown that an exercise bout is associated with a transient procoagulant state in patients with CAD, with similar exercise-induced haemostatic changes for HIIT and MICT.

Mitofusin-2 Regulates Platelet Mitochondria and Function

BACKGROUND

Single-nucleotide polymorphisms linked with the rs1474868 T allele (MFN2 [mitofusin-2] T/T) in the human mitochondrial fusion protein MFN2 gene are associated with reduced platelet MFN2 RNA expression and platelet counts. This study investigates the impact of MFN2 on megakaryocyte and platelet biology.

METHODS

Mice with megakaryocyte/platelet deletion of Mfn2 (Mfn2-/- [Mfn2 conditional knockout]) were generated using Pf4-Cre crossed with floxed Mfn2 mice. Human megakaryocytes were generated from cord blood and platelets isolated from healthy subjects genotyped for rs1474868. Ex vivo approaches assessed mitochondrial morphology, function, and platelet activation responses. In vivo measurements included endogenous/transfused platelet life span, tail bleed time, transient middle cerebral artery occlusion, and pulmonary vascular permeability/hemorrhage following lipopolysaccharide-induced acute lung injury.

RESULTS

Mitochondria was more fragmented in megakaryocytes derived from Mfn2-/- mice and from human cord blood with MFN2 T/T genotype compared with control megakaryocytes. Human resting platelets of MFN2 T/T genotype had reduced MFN2 protein, diminished mitochondrial membrane potential, and an increased rate of phosphatidylserine exposure during ex vivo culture. Platelet counts and platelet life span were reduced in Mfn2-/- mice accompanied by an increased rate of phosphatidylserine exposure in resting platelets, especially aged platelets, during ex vivo culture. Mfn2-/- also decreased platelet mitochondrial membrane potential (basal) and activated mitochondrial oxygen consumption rate, reactive oxygen species generation, calcium flux, platelet-neutrophil aggregate formation, and phosphatidylserine exposure following dual agonist activation. Ultimately, Mfn2-/- mice showed prolonged tail bleed times, decreased ischemic stroke infarct size after cerebral ischemia-reperfusion, and exacerbated pulmonary inflammatory hemorrhage following lipopolysaccharide-induced acute lung injury. Analysis of MFN2 SNPs in the iSPAAR study (Identification of SNPs Predisposing to Altered ALI Risk) identified a significant association between MFN2 and 28-day mortality in patients with acute respiratory distress syndrome.

CONCLUSIONS

Mfn2 preserves mitochondrial phenotypes in megakaryocytes and platelets and influences platelet life span, function, and outcomes of stroke and lung injury.

In vitro characterization of rare anti-αIIbβ3 isoantibodies produced by patients with Glanzmann thrombasthenia that severely block fibrinogen binding and generate procoagulant platelets via complement activation

Background

Glanzmann thrombasthenia (GT) is a rare bleeding disorder caused by inherited defects of the platelet αIIbβ3 integrin. Platelet transfusions can be followed by an immune response that can block integrin function by interfering with fibrinogen binding.

Objectives

In this study, we aimed to determine the prevalence of such isoantibodies and better characterize their pathogenic properties.

Methods

Twelve patients with GT were evaluated for anti-αIIbβ3 isoantibodies. Sera from patients with GT with or without anti-αIIbβ3 isoantibodies were then used to study their in vitro effect on platelets from healthy donors. We used several approaches (IgG purification, immunofluorescence staining, and inhibition of signaling pathways) to characterize the pathogenic properties of the anti-αIIbβ3 isoantibodies.

Results

Only 2 samples were able to severely block integrin function. We observed that these 2 sera caused a reduction in platelet size similar to that observed when platelets become procoagulant. Mixing healthy donor platelets with patients' sera or purified IgGs led to microvesiculation, phosphatidylserine exposure, and induction of calcium influx. This was associated with an increase in procoagulant platelets. Pore formation and calcium entry were associated with complement activation, leading to the constitution of a membrane attack complex (MAC) with enhanced complement protein C5b-9 formation. This process was inhibited by the complement 5 inhibitor eculizumab and reduced by polyvalent human immunoglobulins.

Conclusion

Our data suggest that complement activation induced by rare blocking anti-αIIbβ3 isoantibodies may lead to the formation of a MAC with subsequent pore formation, resulting in calcium influx and procoagulant platelet phenotype.

Platelet biology and function: plaque erosion vs. rupture

The leading cause of heart disease in developed countries is coronary atherosclerosis, which is not simply a result of ageing but a chronic inflammatory process that can lead to acute clinical events upon atherosclerotic plaque rupture or erosion and arterial thrombus formation. The composition and location of atherosclerotic plaques determine the phenotype of the lesion and whether it is more likely to rupture or to erode. Although plaque rupture and erosion both initiate platelet activation on the exposed vascular surface, the contribution of platelets to thrombus formation differs between the two phenotypes. In this review, plaque phenotype is discussed in relation to thrombus composition, and an overview of important mediators (haemodynamics, matrix components, and soluble factors) in plaque-induced platelet activation is given. As thrombus formation on disrupted plaques does not necessarily result in complete vessel occlusion, plaque healing can occur. Therefore, the latest findings on plaque healing and the potential role of platelets in this process are summarized. Finally, the clinical need for more effective antithrombotic agents is highlighted.

Overexpression of facilitative glucose transporter-3 and membrane procoagulation in maternal platelets of preeclamptic pregnancy

BACKGROUND

Preeclampsia (PE) is a hypertensive disorder during pregnancy that results in significant adverse maternal and neonatal outcomes. Platelet activation is present in PE and contributes to the thrombo-hemorrhagic states of the disorder. However, the mechanisms that initiate and/or sustain platelet activation in PE are ill-defined.

OBJECTIVES

We aimed to characterise this mechanism and the procoagulant potentials of platelets in PE.

METHODS

In this quantitative observational study, we analyzed platelet procoagulant membrane dynamics in patients with PE (n = 21) compared with age-matched normotensive pregnancies (n = 20), gestational hypertension (n = 10), and non-pregnant female controls (n = 19). We analyzed fluorescently labeled indicators of platelet activation, bioenergetics, and procoagulation (phosphatidylserine exposure and thrombin generation), coupled with high-resolution imaging and thrombelastography. We then validated our findings using flow cytometry, immunoassays, classical pharmacology, and convolutional neural network analysis.

RESULTS

PE platelets showed significant ultra-structural remodeling, are more extensively preactivated than in healthy pregnancies and can circulate as microaggregates. Preactivated platelets of PE externalized phosphatidylserine and thrombin formed on the platelet membranes. Platelets' expression of facilitative glucose transporter-1 increased in all pregnant groups. However, PE platelets additionally overexpress glucose transporter-3 to enhance glucose uptake and sustain activation and secretion events. Although preeclampsia platelets exposed to subendothelial collagen showed incremental activation, the absolute hemostatic response to collagen was diminished, and likely contributed to greater blood loss perioperatively.

CONCLUSIONS

We revealed 2 bioenergetic mediators in the mechanism of sustained platelet procoagulation in preeclampsia. Although glucose transporter-1 and glucose transporter-3 remain elusive antiprocoagulant targets, they may be sensitive monitors of PE onset and progression.

Platelets from patients with myeloproliferative neoplasms have increased numbers of mitochondria that are hypersensitive to depolarization by thrombin

Thrombosis is one of the cardinal manifestations of myeloproliferative neoplasms (MPN). The mechanisms leading to a prothrombotic state in MPN are complex and remain poorly understood. Platelet mitochondria play a role in platelet activation, but their number and function have not been extensively explored in MPN to date. We observed an increased number of mitochondria in platelets from MPN patients compared with healthy donors. MPN patients had an increased proportion of dysfunctional platelet mitochondria. The fraction of platelets with depolarized mitochondria at rest was increased in essential thrombocythemia (ET) patients and the mitochondria were hypersensitive to depolarization following thrombin agonist stimulation. Live microscopy showed a stochastic process in which a higher proportion of individual ET platelets underwent mitochondrial depolarization and after a shorter agonist exposure compared to healthy donors. Depolarization was immediately followed by ballooning of the platelet membrane, which is a feature of procoagulant platelets. We also noted that the mitochondria of MPN patients were on average located nearer the platelet surface and we observed extrusion of mitochondria from the platelet surface as microparticles. These data implicate platelet mitochondria in a number of prothrombotic phenomena. Further studies are warranted to assess whether these findings correlate with clinical thrombotic events.

Small-Molecule Cyclophilin Inhibitors Potently Reduce Platelet Procoagulant Activity

Procoagulant platelets are associated with an increased risk for thrombosis. Procoagulant platelet formation is mediated via Cyclophilin D (CypD) mediated opening of the mitochondrial permeability transition pore. Inhibiting CypD activity could therefore be an interesting approach to limiting thrombosis. In this study, we investigated the potential of two novel, non-immunosuppressive, non-peptidic small-molecule cyclophilin inhibitors (SMCypIs) to limit thrombosis in vitro, in comparison with the cyclophilin inhibitor and immunosuppressant Cyclosporin A (CsA). Both cyclophilin inhibitors significantly decreased procoagulant platelet formation upon dual-agonist stimulation, shown by a decreased phosphatidylserine (PS) exposure, as well as a reduction in the loss of mitochondrial membrane potential. Furthermore, the SMCypIs potently reduced procoagulant platelet-dependent clotting time, as well as fibrin formation under flow, comparable to CsA. No effect was observed on agonist-induced platelet activation measured by P-selectin expression, as well as CypA-mediated integrin α_IIbβ₃ activation. Importantly, whereas CsA increased Adenosine 5'-diphosphate (ADP)-induced platelet aggregation, this was unaffected in the presence of the SMCypIs. We here demonstrate specific cyclophilin inhibition does not affect normal platelet function, while a clear reduction in procoagulant platelets is observed. Reducing platelet procoagulant activity by inhibiting cyclophilins with SMCypIs forms a promising strategy to limit thrombosis.

Discriminating young platelets on human leukocyte antigen-I expression highlights their extremely high reactivity potential

Background

The platelet population is heterogeneous, with different subsets that differ on the basis of their function and reactivity. An intrinsic factor participating in this difference of reactivity could be the platelet age. The lack of relevant tools allowing a formal identification of young platelets prevents so far to draw solid conclusions regarding platelet reactivity. We recently reported that human leukocyte antigen-I (HLA-I) molecules are more expressed on human young platelets.

Objectives

The aim of this study was to assess platelet reactivity according to their age based on HLA-I expression level.

Methods

Platelet activation was assessed by flow cytometry (FC) for different platelet subsets based on their HLA-I expression. These populations were further cell sorted and their intrinsic properties were determined by FC and electron microscopy (EM). Statistical analyses were performed with GraphPad Prism 5.02 software using two-way ANOVA followed by a Tukey post hoc test.

Results

HLA-I expression level allowed the identification of 3 platelet subpopulations regarding to their age (HLA low, dim, and high). HLA-I was reliable to guide platelet cell sorting and highlighted the features of young platelets in the HLA-I^high population. In response to different soluble agonists, HLA-I^high platelets were the most reactive subset as shown by the level of P-selectin secretion and fibrinogen binding assessed by flow cytometry. Moreover, the highest capacity of HLA-I^high platelets to simultaneously express annexin-V and von Willebrand factor or activated αIIbβ3 after coactivation with TRAP and CRP indicated that the procoagulant feature of platelets was age-related.

Conclusion

The young HLA-I^high population is the most reactive and prone to become procoagulant. These results open up new perspectives to investigate deeply the role of young and old platelets.

Whole Blood Procoagulant Platelet Flow Cytometry Protocol for Heparin-Induced Thrombocytopenia (HIT) and Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT) Testing

Heparin-induced thrombocytopenia (HIT) is a well-characterized, iatrogenic complication of heparin anticoagulation with significant morbidity. In contrast, vaccine-induced immune thrombotic thrombocytopenia (VITT) is a recently recognized severe prothrombotic complication of adenoviral vaccines, including the ChAdOx1 nCoV-19 (Vaxzevria, AstraZeneca) and Ad26.COV2.S (Janssen, Johnson & Johnson) vaccines against COVID-19. The diagnosis of HIT and VITT involve laboratory testing for antiplatelet antibodies by immunoassays followed by confirmation by functional assays to detect platelet-activating antibodies. Functional assays are critical to detect pathological antibodies due to the varying sensitivity and specificity of immunoassays. This chapter presents a protocol for a novel whole blood flow cytometry-based assay to detect procoagulant platelets in healthy donor blood in response to plasma from patients suspected of HIT or VITT. A method to identify suitable healthy donors for HIT and VITT testing is also described.

The Correlation between the Triglyceride-Glucose Index and Coagulation Markers in Patients with Recent Acute Myocardial Infarction

Background

Metabolic abnormalities and hypercoagulability seem to have an important predictive role in patients with coronary artery disease (CAD). The triglyceride-glucose (TyG) index has emerged as a good marker for metabolic syndrome with predictive value for cardiovascular events. Overall haemostatic potential (OHP) is a reliable global haemostatic essay to identify hypercoagulability in CAD patients. The aim of our study was therefore to evaluate a possible correlation between the TyG index and haemostatic derangements in patients with CAD.

Methods

Consecutive patients referred for the first follow-up visit after acute myocardial infarction between December 1, 2018, and March 31, 2020, and did not meet exclusion criteria were included. We determined OHP, overall coagulation potential (OCP), overall fibrinolytic potential (OFP), fibrinogen, D-dimer, and von Willebrand factor from peripheral blood samples. The TyG index was calculated with the previously described and validated formula. Linear regression models were constructed for the multivariate analysis.

Results

A total of 117 patients (mean age 56 ± 10 years, 20% women) were included. A correlation was found between TyG index and OCP (r = 0.229, p = 0.026), TyG index and OHP (r = 0.202, p = 0.050), and TyG index and fibrinogen (r = 0.271, p = 0.005). In the multivariate model which accounted for sex, age, and BMI, the correlation between TyG index and OCP (R² 0.108; ANOVA for regression p = 0.035; beta 2.08 [0.79-4.01], p = 0.042) and between TyG index and fibrinogen (R² 0.11; ANOVA for regression p = 0.015; beta 0.35 [0.08-0.62], p = 0.012) emerged as statistically significant.

Conclusion

The TyG index, a marker of metabolic syndrome, has a strong correlation with a hypercoagulability state in CAD, as determined by the OCP and higher fibrinogen levels. Our findings suggest that metabolic syndrome may be an important driver of atherothrombotic risk in patients with CAD.

Procoagulant platelets: novel players in thromboinflammation

Platelets play a key role in maintaining hemostasis. However, dysregulated platelet activation can lead to pathological thrombosis or bleeding. Once a platelet gets activated, it will either become an aggregatory platelet or eventually a procoagulant platelet with both types playing distinct roles in thrombosis and hemostasis. Although aggregatory platelets have been extensively studied, procoagulant platelets have only recently come into the spotlight. Procoagulant platelets are a subpopulation of highly activated platelets that express phosphatidylserine and P-selectin on their surface, allowing for coagulation factors to bind and thrombin to be generated. In recent years, novel roles for procoagulant platelets have been identified and they have increasingly been implicated in thromboinflammatory diseases. Here, we provide an up-to-date review on the mechanisms resulting in the formation of procoagulant platelets and how they contribute to hemostasis, thrombosis, and thromboinflammation.

Update on platelet procoagulant mechanisms in health and in bleeding disorders

Platelet procoagulant mechanisms are emerging to be complex and important to achieving haemostasis. The mechanisms include the release of procoagulant molecules from platelet storage granules, and strong agonist-induced expression of procoagulant phospholipids on the outer platelet membrane for tenase and prothrombinase assembly. The release of dense granule polyphosphate is important to platelet procoagulant function as it promotes the activation of factors XII, XI and V, inhibits tissue factor pathway inhibitor and fibrinolysis, and strengthens fibrin clots. Platelet procoagulant function also involves the release of partially activated factor V from platelets. Scott syndrome has provided important insights on the mechanisms that regulate procoagulant phospholipids expression on the external platelet membrane, which require strong agonist stimulation that increase cystolic calcium levels, mitochondrial calcium uptake, the loss of flippase function and activation of the transmembrane scramblase protein anoctamin 6. There have been advances in the methods used to directly and indirectly assess platelet procoagulant function in health and disease. Assessments of thrombin generation with platelet rich plasma samples has provided new insights on how platelet procoagulant function is altered in inherited platelet disorders, and how platelets influence the bleeding phenotype of a number of severe coagulation factor deficiencies. Several therapies, including desmopressin and recombinant factor VIIa, improve thrombin generation by platelets. There is growing interest in targeting platelet procoagulant function for therapeutic benefit. This review highlights recent advances in our understanding of platelet-dependent procoagulant mechanisms in health and in bleeding disorders.

A novel flow cytometry procoagulant assay for diagnosis of vaccine-induced immune thrombotic thrombocytopenia

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a severe prothrombotic complication of adenoviral vaccines, including the ChAdOx1 nCoV-19 (Vaxzevria) vaccine. The putative mechanism involves formation of pathological anti-platelet factor 4 (PF4) antibodies that activate platelets via the low-affinity immunoglobulin G receptor FcγRIIa to drive thrombosis and thrombocytopenia. Functional assays are important for VITT diagnosis, as not all detectable anti-PF4 antibodies are pathogenic, and immunoassays have varying sensitivity. Combination of ligand binding of G protein-coupled receptors (protease-activated receptor-1) and immunoreceptor tyrosine-based activation motif-linked receptors (FcγRIIa) synergistically induce procoagulant platelet formation, which supports thrombin generation. Here, we describe a flow cytometry-based procoagulant platelet assay using cell death marker GSAO and P-selectin to diagnose VITT by exposing donor whole blood to patient plasma in the presence of a protease-activated receptor-1 agonist. Consecutive patients triaged for confirmatory functional VITT testing after screening using PF4/heparin ELISA were evaluated. In a development cohort of 47 patients with suspected VITT, plasma from ELISA-positive patients (n = 23), but not healthy donors (n = 32) or individuals exposed to the ChAdOx1 nCov-19 vaccine without VITT (n = 24), significantly increased the procoagulant platelet response. In a validation cohort of 99 VITT patients identified according to clinicopathologic adjudication, procoagulant flow cytometry identified 93% of VITT cases, including ELISA-negative and serotonin release assay-negative patients. The in vitro effect of intravenous immunoglobulin (IVIg) and fondaparinux trended with the clinical response seen in patients. Induction of FcγRIIa-dependent procoagulant response by patient plasma, suppressible by heparin and IVIg, is highly indicative of VITT, resulting in a sensitive and specific assay that has been adopted as part of a national diagnostic algorithm to identify vaccinated patients with platelet-activating antibodies.

Platelet Subtypes in Inflammatory Settings

In addition to their essential role in hemostasis and thrombosis, platelets also modulate inflammatory reactions and immune responses. This is achieved by specialized surface receptors as well as secretory products including inflammatory mediators and cytokines. Platelets can support and facilitate the recruitment of leukocytes into inflamed tissue. The various properties of platelet function make it less surprising that circulating platelets are different within one individual. Platelets have different physical properties leading to distinct subtypes of platelets based either on their function (procoagulant, aggregatory, secretory) or their age (reticulated/immature, non-reticulated/mature). To understand the significance of platelet phenotypic variation, qualitatively distinguishable platelet phenotypes should be studied in a variety of physiological and pathological circumstances. The advancement in proteomics instrumentation and tools (such as mass spectrometry-driven approaches) improved the ability to perform studies beyond that of foundational work. Despite the wealth of knowledge around molecular processes in platelets, knowledge gaps in understanding platelet phenotypes in health and disease exist. In this review, we report an overview of the role of platelet subpopulations in inflammation and a selection of tools for investigating the role of platelet subpopulations in inflammation.

Measurement of procoagulant platelets provides mechanistic insight and diagnostic potential in heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a prothrombotic, immune-mediated adverse drug reaction associated with high rates of thrombosis-related morbidity and mortality caused by FcγRIIa-activating pathogenic antibodies to PF4-heparin. Procoagulant platelets are a platelet subset that promote thrombin generation, are clinically relevant in prothrombotic diseases, and are formed when platelet G-protein-coupled receptor (GPCR) and ITAM-linked receptors are co-stimulated.

OBJECTIVES

We examined the procoagulant platelet response of healthy donors to platelet agonists in the presence of HIT plasma and determined the contribution of FcγRIIa.

PATIENTS/METHODS

Our previously established flow cytometry-based procoagulant platelet assay was modified to incorporate plasma samples, performed using FcγRIIa-responsive donor platelets. Plasma samples were serotonin-release assay-confirmed HIT (HIT+), or negative on HIT screening.

RESULTS

In response to GPCR stimulation, only HIT+ plasma produced a heparin-dependent sensitization that required active FcγRIIa. As a potential diagnostic tool, the procoagulant platelet assay achieved 98% accuracy in identifying clinically verified HIT when performed blinded to the diagnoses of a validation cohort. Samples inducing a higher procoagulant platelet response were more likely from patients with thrombotic complications. Thrombin stimulation markedly increased the procoagulant platelet response with HIT+ plasma that was heparin independent and only partially reversed by FcγRIIa blockade, possibly reflecting ongoing thrombotic risk after heparin cessation.

CONCLUSIONS

We demonstrate that HIT plasma together with platelet agonists increased the procoagulant platelet proportions, which may contribute to thrombotic risk in HIT. Targeting procoagulant platelet activation may represent a novel treatment strategy. This assay may be a rapid, clinically relevant functional assay for accurately detecting pathological HIT antibodies.

Mechanisms Underlying Dichotomous Procoagulant COAT Platelet Generation-A Conceptual Review Summarizing Current Knowledge

Procoagulant platelets are a subtype of activated platelets that sustains thrombin generation in order to consolidate the clot and stop bleeding. This aspect of platelet activation is gaining more and more recognition and interest. In fact, next to aggregating platelets, procoagulant platelets are key regulators of thrombus formation. Imbalance of both subpopulations can lead to undesired thrombotic or bleeding events. COAT platelets derive from a common pro-aggregatory phenotype in cells capable of accumulating enough cytosolic calcium to trigger specific pathways that mediate the loss of their aggregating properties and the development of new adhesive and procoagulant characteristics. Complex cascades of signaling events are involved and this may explain why an inter-individual variability exists in procoagulant potential. Nowadays, we know the key agonists and mediators underlying the generation of a procoagulant platelet response. However, we still lack insight into the actual mechanisms controlling this dichotomous pattern (i.e., procoagulant versus aggregating phenotype). In this review, we describe the phenotypic characteristics of procoagulant COAT platelets, we detail the current knowledge on the mechanisms of the procoagulant response, and discuss possible drivers of this dichotomous diversification, in particular addressing the impact of the platelet environment during in vivo thrombus formation.

Platelet procoagulant potential is reduced in platelet concentrates ex vivo but appears restored following transfusion

BACKGROUND

The procoagulant profile of platelet concentrates (PCs) following transfusion has been difficult to evaluate due to lack of specific markers. This study aimed to characterize procoagulant platelets in PCs and the effect of transfusion.

STUDY DESIGN AND METHODS

Buffy coat-derived PCs from 12 donors were pooled, split, then stored conventionally, cold (2-6°C) or cryopreserved (-80°C). Procoagulant platelet profiles were assessed by flow cytometry (GSAO⁺ /P-selectin⁺ ), lactadherin-binding, and calibrated automated thrombogram, during storage, unstimulated, or after thrombin and collagen stimulation and compared with blood from healthy volunteers. Platelet activation (P-selectin) and procoagulant platelet formation potential were measured (flow cytometry) in patients receiving clinically indicated conventional PC transfusion.

RESULTS

Independent of significant increases with storage, procoagulant platelet proportions with and without agonist stimulation were significantly blunted in conventionally stored PCs (stimulated day 5 conventional PC 4.2 ± 1.3%, healthy volunteer blood 11.1 ± 2.9%; p < .0001). Cryopreserved PCs contained the highest proportion of procoagulant platelets (unstimulated: cryopreserved 25.6 ± 1.8% vs. day 5 conventional 0.5 ± 0.1% vs. day 14 cold-stored 5.8 ± 1.0%, p < .0001), but demonstrated minimal increase with agonist. Transfusion of PCs was associated with an increase in procoagulant platelets (2.2 ± 1.4% vs. 0.6 ± 0.2%; p = .004) and reversal of the blunted agonist response (15.8 ± 5.9% vs. 4.0 ± 1.6%; p < .0001). Procoagulant responses post-transfusion were significantly higher than healthy controls, suggesting a priming effect. The P-selectin agonist response was not restored upon transfusion (79.4 ± 13.9% vs. 82.0 ± 2.5%).

CONCLUSION

Storage blunts the procoagulant platelet response to agonist stimulation in PCs. Despite this, conventionally stored PCs have high procoagulant potential following transfusion, with a discordant, persistent reduction in P-selectin response.

The Provocative Roles of Platelets in Liver Disease and Cancer

Both platelets and the liver play important roles in the processes of coagulation and innate immunity. Platelet responses at the site of an injury are rapid; their immediate activation and structural changes minimize the loss of blood. The majority of coagulation proteins are produced by the liver—a multifunctional organ that also plays a critical role in many processes: removal of toxins and metabolism of fats, proteins, carbohydrates, and drugs. Chronic inflammation, trauma, or other causes of irreversible damage to the liver can dysregulate these pathways leading to organ and systemic abnormalities. In some cases, platelet-to-lymphocyte ratios can also be a predictor of disease outcome. An example is cirrhosis, which increases the risk of bleeding and prothrombotic events followed by activation of platelets. Along with a triggered coagulation cascade, the platelets increase the risk of pro-thrombotic events and contribute to cancer progression and metastasis. This progression and the resulting tissue destruction is physiologically comparable to a persistent, chronic wound. Various cancers, including colorectal cancer, have been associated with increased thrombocytosis, platelet activation, platelet-storage granule release, and thrombosis; anti-platelet agents can reduce cancer risk and progression. However, in cancer patients with pre-existing liver disease who are undergoing chemotherapy, the risk of thrombotic events becomes challenging to manage due to their inherent risk for bleeding. Chemotherapy, also known to induce damage to the liver, further increases the frequency of thrombotic events. Depending on individual patient risks, these factors acting together can disrupt the fragile balance between pro- and anti-coagulant processes, heightening liver thrombogenesis, and possibly providing a niche for circulating tumor cells to adhere to—thus promoting both liver metastasis and cancer-cell survival following treatment (that is, with minimal residual disease in the liver).

Building platelet phenotypes: diaphanous-related formin 1 (DIAPH1)-related disorder

Variants of the Diaphanous-Related Formin 1 (DIAPH-1) gene have recently been reported causing inherited macrothrombocytopenia. The essential/"diagnostic" characteristics associated with the disorder are emerging; however, robust and complete criteria are not established. Here, we report the first cases of DIAPH1-related disorder in Australia caused by the autosomal dominant gain-of-function DIAPH1 R1213X variant formed by truncation of the protein within the diaphanous auto-regulatory domain (DAD) with loss of regulatory motifs responsible for autoinhibitory interactions within the DIAPH1 protein. We affirm phenotypic changes induced by the DIAPH1 R1213X variant to include macrothrombocytopenia, early-onset progressive sensorineural hearing loss, and mild asymptomatic neutropenia. High-resolution microscopy confirms perturbations of cytoskeletal dynamics caused by the DIAPH1 variant and we extend the repertoire of changes generated by this variant to include alteration of procoagulant platelet formation and possible dental anomalies.

Colchicine inhibits ROS generation in response to glycoprotein VI stimulation

Colchicine inhibits coronary and cerebrovascular events in patients with coronary artery disease (CAD), and although known to have anti-inflammatory properties, its mechanisms of action are incompletely understood. In this study, we investigated the effects of colchicine on platelet activation with a particular focus on its effects on activation via the collagen glycoprotein (GP)VI receptor, P2Y₁₂ receptor, and procoagulant platelet formation. Therapeutic concentrations of colchicine in vitro (equivalent to plasma levels) significantly decreased platelet aggregation in whole blood and in platelet rich plasma in response to collagen (multiplate aggregometry) and reduced reactive oxygen species (ROS) generation (H₂DCF-DA, flow cytometry) in response to GPVI stimulation with collagen related peptide-XL (CRP-XL, GPVI specific agonist). Other platelet activation pathways including P-selectin expression, GPIIb/IIIa conformational change and procoagulant platelet formation (GSAO⁺/CD62P⁺) (flow cytometry) were inhibited with higher concentrations of colchicine known to inhibit microtubule depolymerization. Pathway specific mechanisms of action of colchicine on platelets, including modulation of the GPVI receptor pathway at low concentrations, may contribute to its protective role in CAD.

Procoagulant platelets: Generation, characteristics, and therapeutic target

Platelets play a pivotal role in hemostasis. Activated platelets are classified into two groups, according to their agonist response: aggregating and procoagulant platelets. Aggregating platelets consist of activated integrin αIIbβ3 and stretch out pseudopods to further attract platelets to the site of injury by connecting with fibrinogen. They mainly gather in the core of the thrombus and perform a secretory function, such as releasing adenosine diphosphate (ADP). Procoagulant platelets promote the formation of thrombin and fibrin by interacting with coagulation factors and can thus be considered as the connector between primary and secondary hemostasis. In addition to their functions in blood coagulation, procoagulant platelets play a proinflammatory role by releasing platelet microparticles and inorganic polyphosphate. Considering these important functions of procoagulant platelets, this subpopulation warrants detailed study to analyze their potential in preventing human diseases. This review summarizes the generation and important characteristics of procoagulant platelets, as well as their potential for preventing the adverse effects associated with current antiplatelet therapies.

High-Dose Epinephrine Enhances Platelet Aggregation at the Expense of Procoagulant Activity

Platelet activation is characterized by shape change, granule secretion, activation of fibrinogen receptor (glycoprotein IIb/IIIa) sustaining platelet aggregation, and externalization of negatively charged aminophospholipids contributing to platelet procoagulant activity. Epinephrine (EPI) alone is a weak platelet activator. However, it is able to potentiate platelet activation initiated by other agonists. In this work, we investigated the role of EPI in the generation of procoagulant platelets. Human platelets were activated with convulxin (CVX), thrombin (THR) or protease-activated receptor (PAR) agonists, EPI, and combination thereof. Platelet aggregation was assessed by light transmission aggregometry or with PAC-1 binding by flow cytometry. Procoagulant collagen-and-THR (COAT) platelets, induced by combined activation with CVX-and-THR, were visualized by flow cytometry as Annexin-V-positive and PAC-1-negative platelets. Cytosolic calcium fluxes were monitored by flow cytometry using Fluo-3 indicator. EPI increased platelet aggregation induced by all agonist combinations tested. On the other hand, EPI dose-dependently reduced the formation of procoagulant COAT platelets generated by combined CVX-and-THR activation. We observed a decreased Annexin-V-positivity and increased binding of PAC-1 with the triple activation (CVX + THR + EPI) compared with CVX + THR. Calcium mobilization with triple activation was decreased with the higher EPI dose (1,000 µM) compared with CVX + THR calcium kinetics. In conclusion, when platelets are activated with CVX-and-THR, the addition of increasing concentrations of EPI (triple stimulation) modulates platelet response reducing cytosolic calcium mobilization, decreasing procoagulant activity, and enhancing platelet aggregation.

Thrombocytopathies: Not Just Aggregation Defects-The Clinical Relevance of Procoagulant Platelets

Platelets are active key players in haemostasis. Qualitative platelet dysfunctions result in thrombocytopathies variously characterized by defects of their adhesive and procoagulant activation endpoints. In this review, we summarize the traditional platelet defects in adhesion, secretion, and aggregation. In addition, we review the current knowledge about procoagulant platelets, focusing on their role in bleeding or thrombotic pathologies and their pharmaceutical modulation. Procoagulant activity is an important feature of platelet activation, which should be specifically evaluated during the investigation of a suspected thrombocytopathy.

Procoagulant platelets: Laboratory detection and clinical significance

Platelets play a critical role in both haemostasis and thrombosis, and it is now evident that not all platelets behave the same when they are called to action. A functionally distinct subpopulation of platelets forms in response to maximal agonist stimulation: the procoagulant platelet. This platelet subpopulation is defined by its ability to expose phosphatidylserine on its surface, allowing for coagulation factor complexes to form and generate bursts of thrombin and fibrin to stabilize platelet clots. Reduced levels of procoagulant platelets have been linked to bleeding in Scott's syndrome and haemophilia A patients, and elevated levels have been demonstrated in many thrombotic disorders, including identifying patients at higher risk for stroke recurrence. One obstacle for incorporating an assay for measuring procoagulant platelets into clinical management algorithms is the lack of consensus on the exact definition and markers for this subpopulation. This review will outline the biological characteristics of procoagulant platelets and the laboratory assays currently used to identify them in research settings. It will summarize the findings of clinical research demonstrating the relevance of measuring the procoagulant platelet levels in patients and will discuss how an appropriate assay can be used to elucidate the mechanism behind the formation of this subpopulation, facilitating novel drug discovery to improve upon current outcomes in cardiovascular and other thrombotic disorders.

The role of bone marrow-derived cells in venous thromboembolism

Venous thrombosis is a life-threatening condition with high morbidity and mortality. Abnormal functioning of different cells in the blood is an integral part of its pathogenesis. In this review, we describe the contribution of bone marrow-derived cells to the development of this debilitating disease. We present both epidemiological and clinical data demonstrating involvement of various cell types in venous thrombosis, and discuss potential mechanisms underlying these effects. Modern concepts including recently discovered new paradigms in thrombosis, such as neutrophil extracellular traps, mast cells, and polyphosphate, are summarized.

Novel antiplatelet targets in the treatment of acute coronary syndromes

Acute coronary syndromes (ACS) are a global cause of mortality and morbidity that affect millions of lives worldwide. Following atherosclerotic plaque rupture, platelet activation and aggregation are the two major elements that initiate thrombus formation inside a coronary artery, which can obstruct blood flow and cause myocardial ischemia; ergo, antiplatelet therapy forms a major part of the treatment strategy for ACS. Patients with ACS routinely receive dual antiplatelet therapy (DAPT), which consists of aspirin and a platelet P2Y₁₂ inhibitor to both treat and prevent atherothrombosis. Use of platelet glycoprotein (GP) IIb/IIIa inhibitors is now limited due to the risk of severe bleeding and thrombocytopenia. Thus, administration of GPIIb/IIIa inhibitors is generally restricted to bail out thrombotic events associated with PCI. Furthermore, current antiplatelet medications mainly rely on thromboxane A₂ and P2Y₁₂ inhibition, which have broad-acting effects on platelets and are known to cause bleeding, which especially limits the long-term use of these agents. In addition, not all ACS patients treated with current antiplatelet treatments are protected from recurrence of arterial thrombosis, since many platelet mechanisms and activation pathways remain uninhibited by current antiplatelet therapy. Pharmacological antagonism of novel targets involved in platelet function could shape future antiplatelet therapies that could ultimately lead to more effective or safer therapeutic approaches. In this article, we focus on inhibitors of promising targets that have not yet been introduced into clinical practice, including inhibitors of GPVI, protease-activated receptor (PAR)-4, GPIb, 5-hydroxytryptamine receptor subtype 2A (5-HT_2A), protein disulfide isomerase, P-selectin and phosphoinositide 3-kinase β.

Failure of platelet function analyser 200 to demonstrate clinical clopidogrel resistance in a patient undergoing intracranial vascular stenting

A patient undergoes intracranial stent insertion for stent-assisted coiling of a basilar tip aneurysm and left middle cerebral artery aneurysm. A flow diverting stent is also placed across an anterior communicating artery aneurysm. Prior to the procedure, the patient takes dual antiplatelet medications, being aspirin and clopidogrel. Because of the concern regarding in-stent thrombus and thromboembolic complications related to intracranial stenting and the high rate of clopidogrel resistance, preoperative platelet function testing (PFT) was undertaken to ensure platelet inhibition. In this case, PFT was performed on a platelet function analyser which demonstrated platelet inhibition. Ten days following the procedure, the patient represented with thromboembolic stroke. Repeat PFT performed with whole blood impedance aggregometry and despite full medication compliance demonstrated clopidogrel resistance. Clopidogrel was then ceased and prasugrel commenced. This case demonstrates the importance of appropriate platelet inhibition in patients with intracranial stents and the controversy surrounding PFT.

Procoagulant Phosphatidylserine-Exposing Platelets in vitro and in vivo

The physiological heterogeneity of platelets leads to diverse responses and the formation of discrete subpopulations upon platelet stimulation. Procoagulant platelets are an example of such subpopulations, a key characteristic of which is exposure either of the anionic aminophospholipid phosphatidylserine (PS) or of tissue factor on the activated platelet surface. This review focuses on the former, in which PS exposure on a subpopulation of platelets facilitates assembly of the intrinsic tenase and prothrombinase complexes, thereby accelerating thrombin generation on the activated platelet surface, contributing importantly to the hemostatic process. Mechanisms involved in platelet PS exposure, and accompanying events, induced by physiologically relevant agonists are considered then contrasted with PS exposure resulting from intrinsic pathway-mediated apoptosis in platelets. Pathologies of PS exposure, both inherited and acquired, are described. A consideration of platelet PS exposure as an antithrombotic target concludes the review.

Platelet necrosis mediates ischemic stroke outcome in mice

Dysregulated platelet functions contribute to the development and progression of ischemic stroke. Utilizing mice with a platelet-specific deletion of cyclophilin D (CypD), a mediator of necrosis, we found that platelet necrosis regulates tissue damage and outcomes during ischemic stroke in vivo. Mice with loss of CypD in platelets (CypDplt-/-mice) exhibited significantly enhanced cerebral blood flow, improved neurological and motor functions, and reduced ischemic stroke infarct volume after cerebral ischemia-reperfusion injury. These effects were attributable, at least in part, to platelet-neutrophil interactions. Twenty-four hours after stroke, significantly more circulating platelet-neutrophil aggregates (PNAs) were found in CypDplt+/+ mice. Underscoring the role of platelet necrosis in PNA formation, we observed a significant number of phosphatidylserine (PS)+ platelets in PNAs in CypDplt+/+ mice. In contrast, significantly fewer platelets in PNAs were PS+ in CypDplt-/- counterparts. Accordingly, mice with CypD-deficient platelets had fewer neutrophils and PNAs recruited to their brain following stroke relative to wild-type counterparts. Neutrophil depletion in wild-type mice conferred protection from ischemic stroke to a similar degree as observed in mice with CypD-deficient platelets. Neutrophil depletion in CypDplt-/- mice did not further reduce infarct size. Transmission electron microscopy of ex vivo-formed PNAs revealed a propensity of necrotic platelets to interact with neutrophils. These results suggest that necrotic platelets interact with neutrophils to exacerbate brain injury during ischemic stroke. Because inhibiting platelet necrosis does not compromise hemostasis, targeting platelet CypD may be a potential therapeutic strategy to limit brain damage following ischemic stroke.

Platelet biology and functions: new concepts and clinical perspectives

Platelets - blood cells continuously produced from megakaryocytes mainly in the bone marrow - are implicated not only in haemostasis and arterial thrombosis, but also in other physiological and pathophysiological processes. This Review describes current evidence for the heterogeneity in platelet structure, age, and activation properties, with consequences for a diversity of platelet functions. Signalling processes of platelet populations involved in thrombus formation with ongoing coagulation are well understood. Genetic approaches have provided information on multiple genes related to normal haemostasis, such as those encoding receptors and signalling or secretory proteins, that determine platelet count and/or responsiveness. As highly responsive and secretory cells, platelets can alter the environment through the release of growth factors, chemokines, coagulant factors, RNA species, and extracellular vesicles. Conversely, platelets will also adapt to their environment. In disease states, platelets can be positively primed to reach a pre-activated condition. At the inflamed vessel wall, platelets interact with leukocytes and the coagulation system, interactions mediating thromboinflammation. With current antiplatelet therapies invariably causing bleeding as an undesired adverse effect, novel therapies can be more beneficial if directed against specific platelet responses, populations, interactions, or priming conditions. On the basis of these novel concepts and processes, we discuss several initiatives to target platelets therapeutically.

Using PAR4 Inhibition as an Anti-Thrombotic Approach: Why, How, and When?

Protease-activated receptors (PARs) are a family of four GPCRs with a variety of cellular functions, yet the only advanced clinical endeavours to target these receptors for therapeutic gain to date relates to the impairment of platelet function for anti-thrombotic therapy. The only approved PAR antagonist is the PAR1 inhibitor, vorapaxar—the sole anti-platelet drug against a new target approved in the past 20 years. However, there are two PARs on human platelets, PAR1 and PAR4, and more recent efforts have focused on the development of the first PAR4 antagonists, with first-in-class agents recently beginning clinical trial. Here, we review the rationale for this approach, outline the various modes of PAR4 inhibition, and speculate on the specific therapeutic potential of targeting PAR4 for the prevention of thrombotic conditions.

Thrombin Generation and Cancer: Contributors and Consequences

The high occurrence of cancer-associated thrombosis is associated with elevated thrombin generation. Tumour cells increase the potential for thrombin generation both directly, through the expression and release of procoagulant factors, and indirectly, through signals that activate other cell types (including platelets, leukocytes and erythrocytes). Furthermore, cancer treatments can worsen these effects. Coagulation factors, including tissue factor, and inhibitors of coagulation are altered and extracellular vesicles (EVs), which can promote and support thrombin generation, are released by tumour and other cells. Some phosphatidylserine-expressing platelet subsets and platelet-derived EVs provide the surface required for the assembly of coagulation factors essential for thrombin generation in vivo. This review will explore the causes of increased thrombin production in cancer, and the availability and utility of tests and biomarkers. Increased thrombin production not only increases blood coagulation, but also promotes tumour growth and metastasis and as a consequence, thrombin and its contributors present opportunities for treatment of cancer-associated thrombosis and cancer itself.

Flow Cytometry Assessment of Procoagulant Platelets Using a Dithiol-Reactive Probe

Flow cytometry assessment of platelets using the combination of GSAO [4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid], a dithiol-reactive probe, and P-selectin, a platelet activation marker, is a novel and powerful assay in the identification and quantification of the procoagulant subpopulation of platelets that has the capacity to support thrombin generation. In this chapter, we provide the flow cytometry protocols aimed at the study of procoagulant platelets under resting and agonist-stimulated conditions in whole blood and washed platelets of both human and murine (mouse) samples.

Novel assay demonstrates that coronary artery disease patients have heightened procoagulant platelet response

Essentials Procoagulant platelets can be detected using GSAO in human whole blood. Stable coronary artery disease is associated with a heightened procoagulant platelet response. Agonist-induced procoagulant platelet response is not inhibited by aspirin alone. Collagen plus thrombin induced procoagulant platelet response is partially resistant to clopidogrel.

SUMMARY

Background Procoagulant platelets are a subset of highly activated platelets with a critical role in thrombin generation. Evaluation of their clinical utility in thrombotic disorders, such as coronary artery disease (CAD), has been thwarted by the lack of a sensitive and specific whole blood assay. Objectives We developed a novel assay, utilizing the cell death marker, GSAO [(4-(N-(S-glutathionylacetyl)amino)phenylarsonous acid], and the platelet activation marker, P-selectin, to identify procoagulant platelets in whole blood by flow cytometry. Patients/Methods Using this assay, we characterized the procoagulant platelet population in healthy controls and a cohort of patients undergoing elective coronary angiography. Results In patients with CAD, compared with patients without CAD, there was a heightened procoagulant platelet response to thrombin (25.2% vs. 12.2%), adenosine diphosphate (ADP) (7.8% vs. 2.7%) and thrombin plus collagen (27.2% vs. 18.3%). The heightened procoagulant platelet potential in CAD patients was not associated with other markers of platelet function, including aggregation, dense granule release and activation of α2b β3 integrin. Although dual antiplatelet therapy (DAPT) was associated with partial suppression of procoagulant platelets, this inhibitory effect on a patient level could not be predicted by aggregation response to ADP and was not fully suppressed by clopidogrel. Conclusions We report for the first time that procoagulant platelets can be efficiently detected in a few microliters of whole blood using the cell death marker, GSAO, and the platelet activation marker, P-selectin. A heightened procoagulant platelet response may provide insight into the thrombotic risk of CAD and help identify a novel target for antiplatelet therapies in CAD.