Platelet Procoagulant Research Articles

Procoagulant platelet activation promotes venous thrombosis

AbstractPlatelets 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.

Hemin-induced platelet activation is regulated by the ACKR3 chemokine surface receptor and has implications for passivation of vulnerable atherosclerotic plaques.

In vulnerable atherosclerotic plaques, intraplaque hemorrhages (IPH) result in hemolysis of red blood cells and release of hemoglobin and free hemin. Hemin activates platelets and leads to thrombosis. Agonism of the inhibitory platelet receptor ACKR3 inhibits hemin-dependent platelet activation and thrombus formation. To characterize the effect of hemin and ACKR3 agonism on isolated human platelets, multi-color flow cytometry and classical experimental setup such as light transmission aggregometry and a flow chamber assay were used. Hemin induces platelet aggregation and ex vivo platelet-dependent thrombus formation on immobilized collagen under a low shear rate of 500 s-1, indicating that free hemin is a strong activator of platelet-dependent thrombosis. Recently, we described that ACKR3 is a prominent inhibitory receptor of platelet activation. Specific ACKR3 agonists but not conventional antiplatelet compounds such as COX-1 inhibitor (indometacin), ADP-receptor blocker (cangrelor), or PAR1 inhibitor (ML161) inhibit both hemin-dependent aggregation and thrombus formation. To further characterize the effect of hemin on platelet subpopulations, we established a multi-color flow cytometry assay. We found that hemin induces procoagulant (CD42bpos/PAC-1neg/AnnexinVpos), aggregatory (CD42bpos/PAC-1pos/AnnexinVneg), and inflammatory (CD42bpos/CXCR4pos/ACKR3pos/AnnexinVpos) platelet subpopulations. Treatment with ACKR3 agonists significantly decreased the formation of procoagulant and ACKR3pos platelets in response to hemin. We conclude that hemin is a strong activator for the formation of procoagulant platelets and thrombus formation which is dependent on the function of ACKR3. Activation of ACKR3 using specific agonists may offer a therapeutic strategy to regulate the vulnerability of atherosclerotic plaques in areas of IPH.

Plant-derived compounds normalize platelet bioenergetics and function in hyperglycemia

BackgroundPolyphenols have been shown to decrease oxidative stress and modulate glycemic response. Nevertheless, their effect on platelet bioenergetics and clot structure in diabetes and hyperglycemia is unknown. ObjectivesTo investigate the effect of polyphenols on human platelet bioenergetics and its subsequent effect on clot structure in normoglycemia vs acute hyperglycemia in vitro. MethodsFour polyphenols (resveratrol, hesperetin, epigallocatechin gallate [EGCG], and quercetin) were selected for initial analysis. Healthy volunteers’ isolated platelets/platelet-rich plasma were treated with 5 or 25 mM glucose to represent normoglycemia and acute hyperglycemia, respectively. Platelet-derived reactive oxygen species (ROS), citrate synthase activity (mitochondrial density), mitochondrial calcium flux, and mitochondrial respiration were performed following exposure to polyphenols (20 µM, 1 hour) to determine their effects on platelet bioenergetics. Procoagulant platelets (annexin V) and fibrin fiber density (Alexa Fluor-488 fibrinogen; Invitrogen) were analyzed by laser scanning confocal microscopy, while clot porosity was determined using platelet-rich plasma following exposure to polyphenols (20 µM, 20 minutes). ResultsAcute hyperglycemia increased ROS, mitochondrial calcium flux, maximal respiration, and procoagulant platelet number. Resveratrol, quercetin, and EGCG reduced platelet ROS in normoglycemic and acute hyperglycemic conditions. Mitochondrial density was decreased by quercetin and EGCG in normoglycemia. Resveratrol and EGCG reduced mitochondrial calcium flux in acute hyperglycemia. Resveratrol also decreased procoagulant platelet number and attenuated oxygen consumption rate in normoglycemia and acute hyperglycemia. No effect of hyperglycemia or polyphenols was observed on fibrin fiber density or clot pore size. ConclusionOur results suggest polyphenols attenuate increased platelet activity stemming from hyperglycemia and may benefit thrombosis-preventative strategies in patients with diabetes.

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.

#855 Cyclophilin D in cholesterol crystal-induced acute kidney injury

Abstract Background and Aims Cholesterol crystal embolism (CCE) occurs when cholesterol crystal (CC) clots move from an atherosclerotic plaque in a large artery to a smaller artery, leading to obstruction, ischemia, and organ failure. CCE directly impairs blood flow by provoking a local inflammatory response and triggering arterial thrombus formation, consisting of collagen and fibrin mesh, platelet and neutrophil activation, and extracellular DNA trap formation. Activated platelets form a procoagulant surface that supports adhesion to collagen and fibrin mesh, promoting thrombus growth and the recruitment of immune cells. CCE often targets kidney tissue, leading to thrombotic angiopathy, acute loss of glomerular filtration rate (GFR), and necrosis in tubular cells. Cyclophilin D (CypD) plays a central role in regulating platelet procoagulant activity by triggering calcium bursts and mitochondrial depolarization. While the increased number of procoagulant platelets contributes to thrombosis, CypD-mediated procoagulant pathomechanisms in the kidney during CCE remain unknown. In our study presented here, we hypothesized that CypD deficiency might attenuate procoagulant platelet activity, thereby inhibiting CCE-induced kidney infarction and organ injury. Method 6-week-old wild-type (WT) and CypD-/- mice were administered 10 mg/kg CC through injections into the left kidney arteries in vivo. Kidneys were collected 24 hours later, and kidney function, infarct size, necroinflammation and kidney injury were evaluated using GFR measurements, TTC colorimetric assay, immunohistochemistry, and immunofluorescence microscopy. For in vitro studies, whole blood samples were isolated from WT and CypD-/- mice and perfused on a collagen-coated surface in the presence or absence of CC under arterial shear conditions of 1000s-1. Results After CC injection in mice, CypD deficiency ameliorated immunothrombosis, acute kidney injury, and organ failure. CypD-/- mice were protected from the sudden drop in GFR, i.e., acute kidney injury, which resulted from the reduced number of arteries obstructed by crystal clots. Kidney infarct size consistently decreased in CypD-/- mice. CypD-/- mice displayed reduced neutrophil recruitment to thrombotic microangiopathic lesions, indicating diminished immunothrombosis. Preemptive injection of the CypD inhibitor cyclosporin A could prevent CC-mediated vessel obstruction and infarct size, thereby improving kidney function. Interestingly, WT platelets showed increased platelet adhesion and phosphatidylserine (PS) exposure on a collagen-coated surface in the presence of CC under shear conditions in whole blood, suggesting increased procoagulant activity, which was strongly reduced in CypD-/- platelets. Conclusion During CCE-mediated kidney injury, CypD-mediated platelet activation and consequent procoagulant activity accelerate ischemic tissue necrosis and the recruitment of neutrophils. Inhibition of CypD-mediated signaling could be a potential therapeutic approach to inhibit acute kidney injury in patients with CCE.

Binding of coagulation factor IXa to procoagulant platelets revisited: Low affinity and interactions with other factors

Binding of activated factor IX (fIXa) to the phosphatidylserine-expressing procoagulant platelets is a critical step in blood coagulation, which is necessary for the membrane-dependent intrinsic tenase complex assembly and factor X activation. However, the nature and parameters of the fIXa binding sites on the procoagulant platelet surface remain unclear. We used flow cytometry to elucidate the quantitative details of the fluorescently labeled fIXa binding to gel-filtered activated platelets. FIXa bound to the procoagulant platelet subpopulation only, with the parameters (maximal number of binding sites at 58900 ± 3400, Kd at 1000 ± 170 nM) similar to binding observed with phospholipid vesicles. No specific high-affinity binding sites for fIXa were detected, and binding proceeded similarly for different methods of procoagulant platelet production (thrombin, thrombin receptor activation peptide, collagen-related peptide, their combinations, or calcium ionophore A23187). Factor VIII, known to form a high affinity complex with fIXa, enhanced fIXa binding to platelets. In contrast, only competition effects were observed for factor X, which binds fIXa with much lower affinity. Unexpectedly, fIXa itself, fIX, and prothrombin also dose-dependently enhance fIXa binding at concentrations below 1000 nM, suggesting the formation of membrane-bound fIXa dimers and fIXa-prothrombin complexes on platelets. These findings provide a novel perspective on the fIXa binding site on procoagulant platelets, which does not have any major differences from pure phospholipid-based model membranes, exhibits inherently low affinity (3–5 orders of magnitude below the physiologically relevant fIXa concentration) but is significantly enhanced by its cofactor VIII, and regulated by previously unknown membrane interactions.

Abstract 140: Exploring Protein Kinase C Theta (PKCθ) Signaling In The Generation Of Procoagulant Platelet: Insights From Phosphoproteomics And Systems Analysis

Background: Platelet activation plays a pivotal role in physiological hemostasis and thrombosis. Protein Kinase C theta (PKCθ) isoforms exert significant regulatory influence on platelet activation. While specific PKC family members have well-defined roles in platelet function, the roles of PKCθ remain less explored. Objectives: To determine mechanisms of PKCθ signaling that orchestrate procoagulant platelet generation. Methods: Platelets were prepared from healthy human donors for ex vivo experiments. Platelet signaling and function were studied following stimulation with GPVI agonist crosslinked collagen-related peptide (CRP-XL). To investigate PKCθ inhibition mechanisms in platelets, small molecule inhibitors targeting the PKCθ (CC-90005 and C20) were employed. Results: From phosphoproteomics, we noted prominent phosphorylation on the C-terminus PKCθ at Ser685. Pretreatment of platelets with inhibitors specific PKC isoforms revealed that PKCα/β, PKCθ, and PKCΔ activities determine PKCθ Ser685 phosphorylation. From the PKC Inhibitors Activity Curves, we noted a robust and selective inhibitory impact of CC-90005 on PKCθ, with specificity surpassing that of C20. Parallel platelet function analyses revealed that inhibiting PKCθ had minimal impact on platelet spreading and aggregation. However, PKCθ inhibition in the presence of Ca 2+ upregulated platelet phosphatidylserine (PS) exposure, as measured by flow cytometry. This was accompanied by an increase in platelet procoagulant activity, where PKCθ inhibition reduced the lag time for fibrin formation. Furthermore, fluorescence microscopy also found that PKCθ inhibition significantly augmented the proportion of procoagulant platelets and microparticles. Additionally, compared to the control group, the use of cyclosporin A (CsA) reduced the production of procoagulant platelets, but the addition of CC-90005 or C20 rescued this phenomenon, with signal intensity surpassing that of the control group. Conclusions: Our study proposes a model wherein PKCΔ and PKCα/β activities induce the phosphorylation of PKCθ at S685, influencing mitochondrial calcium signaling and supporting procoagulant platelet generation.

Antiplatelet Effects of Flavonoid Aglycones Are Mediated by Activation of Cyclic Nucleotide-Dependent Protein Kinases.

Flavonoid aglycones are secondary plant metabolites that exhibit a broad spectrum of pharmacological activities, including anti-inflammatory, antioxidant, anticancer, and antiplatelet effects. However, the precise molecular mechanisms underlying their inhibitory effect on platelet activation remain poorly understood. In this study, we applied flow cytometry to analyze the effects of six flavonoid aglycones (luteolin, myricetin, quercetin, eriodictyol, kaempferol, and apigenin) on platelet activation, phosphatidylserine externalization, formation of reactive oxygen species, and intracellular esterase activity. We found that these compounds significantly inhibit thrombin-induced platelet activation and decrease formation of reactive oxygen species in activated platelets. The tested aglycones did not affect platelet viability, apoptosis induction, or procoagulant platelet formation. Notably, luteolin, myricetin, quercetin, and apigenin increased thrombin-induced thromboxane synthase activity, which was analyzed by a spectrofluorimetric method. Our results obtained from Western blot analysis and liquid chromatography-tandem mass spectrometry demonstrated that the antiplatelet properties of the studied phytochemicals are mediated by activation of cyclic nucleotide-dependent signaling pathways. Specifically, we established by using Förster resonance energy transfer that the molecular mechanisms are, at least partly, associated with the inhibition of phosphodiesterases 2 and/or 5. These findings underscore the therapeutic potential of flavonoid aglycones for clinical application as antiplatelet agents.

Procoagulant platelets promote immune evasion in triple-negative breast cancer

AbstractTriple-negative breast cancer (TNBC) is an aggressive tumor entity in which immune checkpoint (IC) molecules are primarily synthesized in the tumor environment. Here, we report that procoagulant platelets bear large amounts of such immunomodulatory factors and that the presence of these cellular blood components in TNBC relates to protumorigenic immune-cell activity and impaired survival. Mechanistically, tumor-released nucleic acids attract platelets to the aberrant tumor microvasculature, where they undergo procoagulant activation, thus delivering specific stimulatory and inhibitory IC molecules. This concomitantly promotes protumorigenic myeloid leukocyte responses and compromises antitumorigenic lymphocyte activity, ultimately supporting tumor growth. Interference with platelet-leukocyte interactions prevented immune cell misguidance and suppressed tumor progression, nearly as effective as systemic IC inhibition. Hence, our data uncover a self-sustaining mechanism of TNBC by using platelets to misdirect immune-cell responses. Targeting this irregular multicellular interplay may represent a novel immunotherapeutic strategy for TNBC without the adverse effects of systemic IC inhibition.

Type 2 Diabetes: Platelets and Long-Term Metabolic Control as Estimated from Glycosylated Haemoglobin (HbA1c).

In type 2 diabetes, platelets are likely affected by impaired long-term glycaemic control, but such pathophysiological links are poorly understood. This study thus compares platelet reactivity (i.e. agonist-evoked platelet reactions) in vitro with glycosylated haemoglobin (HbA1c), a measure commonly used for monitoring long-term metabolic control of type 2 diabetes. Elders with type 2 diabetes (n = 35) were divided according to HbA1c into groups (HbA1c-low and high) consisting of 17 and 18 subjects, respectively. For estimating mitochondria disintegration, a flow cytometer determined mitochondrial transmembrane potentials after whole blood agonist stimulation. The activating agents used were α-thrombin (10 μM) and collagen (0.15 μg/mL). The same apparatus analysed the fibrinogen receptor activity, lysosomal exocytosis (surface lysosomal-associated membrane protein 1), and platelet procoagulant characteristics (membrane-attached annexin V) after stimulation. In type 2 diabetes, after in vitro agonist stimulation, platelet mitochondria injury was higher in the HbA1c-high group. The fibrinogen receptor, lysosomal secretion, and the creation of procoagulant platelets proved to be uninfluenced by HbA1c.

A novel GATA1 variant p.G229D causing the defect of procoagulant platelet formation

IntroductionGATA1 is one of the master transcription factors in hematopoietic lineages development which is crucial for megakaryocytic differentiation and maturation. Previous studies have shown that distinct GATA1 variants are associated with varying severities of macrothrombocytopenia and platelet dysfunction. ObjectiveTo determine the underlying pathological mechanisms of a novel GATA1 variant (c. 686G > A, p. G229D) in a patient with recurrent traumatic muscle hematomas. MethodsComprehensive phenotypic analysis of the patient platelets was performed. Procoagulant platelet formation and function were detected using flow cytometry assay and thrombin generation test (TGT), respectively. The ANO6 expression was measured by qPCR and western blot. The intracellular supramaximal calcium flux was detected by Fluo-5N fluorescent assay. ResultsThe patient displayed mild macrothrombocytopenia with defects of platelet granules, aggregation, and integrin αIIbβ3 activation. The percentage of the procoagulant platelet formation of the patient upon the stimulation of thrombin plus collagen was lower than that of the healthy controls (40.9 % vs 49.0 % ± 5.1 %). The patient platelets exhibited a marked reduction of thrombin generation in platelet rich plasma TGT compared to the healthy controls (peak value: ∼70 % of the healthy controls; the endogenous thrombin potential: ∼40 % of the healthy controls). The expression of ANO6 and intracellular calcium flux were impaired, which together with abnormal granules of the patient platelets might contribute to defect of procoagulant platelet function. ConclusionsThe G229D variant could lead to a novel platelet phenotype characterized by defective procoagulant platelet formation and function, which extended the range of GATA1 variants associated platelet disorders.

Symmetrical peripheral gangrene: potential mechanisms and therapeutic approaches in severe COVID-19

Symmetrical peripheral gangrene is a rare condition that is characterized by ischemic damage and tissue death (gangrene) in the extremities. Recent reports have shed light on SPG in patients with severe COVID-19. This condition presents with symmetrical cyanosis of the extremities and common COVID-19 symptoms and what the most frightening is within a few days, cutaneous necrosis occurred and patients died. Skin biopsy results have shown the presence of microthrombi in small vessels. The formation of SPG in COVID-19 patients results from immunothrombosis, endothelial dysfunction, and procoagulant platelets, leading to a hypercoagulation state and microvascular thrombosis. Thrombotic microangiopathy, shock, disseminated intravascular coagulation, and anticoagulant depletion promote the development of SPG in COVID-19. At the early stage, SPG patients with COVID-19 exhibit similar clinical manifestations. TMA causes early damage to microvasculature in SPG, and the shock state further exacerbates the ischemic injury due to local hypo-perfusion. The disturbed procoagulant-anticoagulant balance caused by DIC and anticoagulant depletion, combined with the pre-ischemic state brought on by TMA and shock, leads to the rapid formation of extensive microthrombi in the late stage of COVID-19 associated SPG. This review will delve into the clinical features, possible mechanisms, and potential therapeutic managements for COVID-19 associated SPG.

A Mouse Model of the PAR4 Sequence Variant P310L Shows Decreased Platelet Reactivity and Thrombosis

Protease activated receptors 1 and 4 (PAR1, PAR4) on platelets mediate thrombin signaling and are vital for thrombus development. Hypercoagulation and thrombin generation are major risk factors for thrombosis. Sustained thrombin signaling through PAR4 promotes platelet activation, phosphatidylserine exposure, and subsequent thrombin generation. A single-nucleotide polymorphism (SNP, rs2227376) leads to an amino acid change in extracellular loop 3 (ECL3) of PAR4, which decreases PAR4 activation (PAR4-P310L). This SNP is also associated with a significantly lower risk for venous thromboembolism (VTE) in a GWAS meta-analysis. We aim to determine the mechanism by which PAR4-P310L (rs2227376) impacts platelet function and reduces the risk of VTE using mice with a homologous mutation (PAR4-P322L). Ex vivo platelet function was determined using flow cytometry and aggregometry. Arterial and venous thrombosis was examined using the ferric-chloride induced carotid artery injury and inferior vena cava (IVC) stasis models, respectively. Platelets from heterozygous (PAR4-322P/L) and homozygous (PAR4-322L/L) mice had decreased thrombin-mediated aggregation and decreased PAR4 reactivity compared to wild types. These mice also had a prolonged time to occlusion in arterial thrombosis, with 55% of PAR4-322P/L mice unable to develop stable thrombi within 30 minutes. In venous thrombosis, wild-type mice developed clotted IVCs that averaged 31 mg after 48 hours, while global PAR4-knockout mice averaged 21 mg (p<0.01), PAR4-322P/L 23 mg (p<0.01), and PAR4-322L/L 28 mg. Since platelet procoagulant activity is known to drive venous thrombosis, we used lactadherin to show that PAR4-P322L platelets express less phosphatidylserine in response to thrombin (5 nM) and convulxin (1 nM). Platelet-rich plasma from PAR4-P322L mice also generated less thrombin compared to wild-types, and whole blood from mice lacking PAR4 showed delayed clot formation in rotational thromboelastometry. Altogether, this suggests a direct impact of PAR4 signaling on platelet-mediated thrombin generation. Our new mouse model shows that the polymorphism in ECL3, PAR4-322L, decreases platelet reactivity and thrombosis. We also show PAR4-322L leads to less procoagulant platelets and decreases the endogenous thrombin potential. This decreased ability to generate thrombin offers a mechanism for PAR4's role in VTE. Our findings highlight a key role of PAR4 signaling in platelet activation and procoagulant activity in thrombosis.

Impact of Recombinant Factor VIII and Platelet Interaction on Platelet Functionality and Hemophilia a Treatment

Background: Platelets play an important role throughout the various stages of hemostasis. After vascular injury, platelets become activated in a pro-aggregatory state, and subsequently a subpopulation undergoes a phenotype shift to a pro-coagulant state. Pro-coagulant platelets bind to factor VIII (FVIII) and ensure efficient localization of FVIII at the site of injury. It is unclear whether modifications in recombinant FVIII (rFVIII) products impact FVIII-platelet binding and subsequent platelet signaling. Aims: To examine binding of different rFVIII products to platelets in vitro, and the impact of rFVIII on platelet phenotype and intracellular signaling. Methods: Platelet activation: Platelets isolated from healthy donors were activated with thrombin and cross-linked collagen-related peptide (CRP-XL). Platelet-FVIII binding: Activated platelets were incubated with simoctocog alfa, efmoroctocog alfa, rurioctocog alfa pegol or damoctocog alfa pegol. Binding was quantified by flow cytometry either by immunostaining with an anti-FVIII antibody conjugated to Alexa fluor (AF)647 or direct measurement of rFVIII concentrates labelled with AF647. Phenotype shift: During the shift to a pro-coagulant state, phosphatidylserine (PS) becomes exposed on the platelet membranes, with the extent of PS exposure correlating with the level of FVIII binding. To assess PS exposure, activated platelets were incubated with Annexin V-BV421, a molecular probe with high affinity for PS. Inhibition of integrin αIIbβ3 signaling: Activated platelets were incubated with the integrin αIIbβ3 inhibitor antibody 10E5. Results: Binding to pro-coagulant platelets was significantly higher with simoctocog alfa than with efmoroctocog alfa, rurioctocog alfa pegol or damoctocog alfa pegol irrespective of immunostaining method (p<0.05; Figure 1 shows detection with anti-FVIII antibody conjugated with AF647).To validate the variability in rFVIII-platelet interactions observed, we performed a dynamic platelet-binding assay with the two FVIII products exhibiting the strongest binding (simoctocog alfa and efmoroctocog alfa). The results of the dynamic assay confirmed the results of the static experiments (Figure 1). Exposure of activated platelets to simoctocog alfa and efmoroctocog alfa resulted in increased PS exposure, with a greater effect observed with simoctocog alfa, consistent with the results of the binding experiments. Immunostaining experiments revealed co-localization of simoctocog alfa and clusters of integrin αIIbβ3 in pro-aggregatory platelets. Inhibition of integrin αIIbβ3 by 10E5 decreased binding of simoctocog alfa to platelets and subsequent PS exposure in a dose-dependent manner (Figure 2). Analysis of platelet binding and phenotype shift in patients with hemophilia A are ongoing. Conclusion: Simoctocog alfa demonstrated higher binding to activated platelets in vitro compared with efmoroctocog alfa, rurioctocog alfa pegol or damoctocog alfa pegol, resulting in an increased phenotype shift of platelets from the pro-aggregatory to the pro-coagulant state. The increased binding of simoctocog alfa was associated with a phenotypic shift in platelets as evidenced by increased exposure of PS on the platelet membranes. The binding of simoctocog alfa to platelets was disrupted when integrin αIIbβ3 activation was inhibited, suggesting a role of integrin αIIbβ3 signaling following binding of FVIII to platelets. Variations in platelet binding and signaling between different rFVIIIs might impact their efficacy for the prevention of bleeds. Further experiments are ongoing to assess the impact of FVIII on the release of von Willebrand factor and thrombin generation by activated platelets, and to repeat the FVIII-binding experiments using platelets from healthy donors vs hemophilia A patients.

Complement Activation Is Increased in Antiphospholipid Syndrome-Related Ischemic Stroke with Higher Risk Features

Introduction: Antiphospholipid syndrome (APS) patients with arterial thrombosis, mostly ischemic stroke, have concerningly high rates of recurrent thrombosis despite antithrombotic treatment, with over 20% thrombosis recurrence within two years. Complement activation is increased in thrombotic APS patients, although its role has not been fully investigated in APS-related stroke. This study aimed to evaluate the degree of complement activation in APS-related ischemic stroke, transient ischemic attack (TIA) or other ischemic brain injury patients and whether complement activation correlated with APS severity and platelet hyperreactivity as a potential mechanism for increased thrombotic risk. Method: Patients with confirmed APS by Sydney Criteria and imaging-proven ischemic stroke, TIA or other ischemic brain injury were recruited, subcategorized as high-risk APS (HR-APS, n=30) and lower-risk APS (LR-APS, n=41). HR-APS included triple antiphospholipid antibody (aPL) positive, anticoagulant refractory or both arterial and venous thromboembolism patients. LR-APS included patients with a single thrombotic event who were single or double aPL positive. Ischemic stroke patients without aPL (n=40) and healthy donors (n=40) were utilized as controls. Patients with active cancer, inflammation, infection or use of hydroxychloroquine, immunosuppression and heparins were excluded. Complement activation markers were measured as Bb fragment (Bb), C3a-desArg (C3a), C5a-des-Arg (C5a), and SC5b-9 using ELISA (MicroVue kits, Quidel Corp, Pathway Diagnostics Ltd). Complement-mediated cell killing was measured by the modified Ham (mHam) test, which determines the degree of cell lysis after adding patient serum to a paroxysmal nocturnal hemoglobinuria cell line (PIGA-null TF-1 cells) lacking cell surface complement regulators. A positive result was considered 20% or greater cell killing. Flow cytometry was used to evaluate whether complement activation correlated with platelet hyperreactivity. For this, plasma from mHam positive patients was added to healthy donor whole blood and procoagulant platelet response was measured as the absolute change in proportion of platelet events that were positive for annexin V and P-selectin following stimulation with thrombin 2U/mL. Results: There was no significant difference in median age between the patient cohorts, ranging from 49 to 56 years, although the healthy controls were younger, with median age 36 (range 25-73, p<0.001). Circulating complement activation markers were all significantly increased in the APS cohorts compared to healthy controls (p<0.01). The most prominent increase was observed in C3a levels: HR-APS median (interquartile range - IQR) 215 ng/mL (137.1-297.4), LR-APS 290.8 ng/mL (120.3-450.3) and healthy controls 62 ng/mL (27.9-121.1)(p=0.004 and p<0.0001 compared to healthy controls, respectively); and Bb fragment levels: HR-APS 1.2 µg/mL (IQR 1.0-1.4), LR-APS 1.3 µg/mL (IQR 1.0-1.5) and healthy controls 0.8 µg/mL (IQR 0.7-0.9)(both p<0.0001 compared to healthy controls). However, only C3a, SC5b-9 and Bb fragment levels in LR-APS remained significantly elevated when compared to stroke controls (p=0.0009, p=0.01 and p=0.03, respectively). There were no differences between HR-APS and LR-APS, nor between stroke controls and healthy controls. Complement-mediated cell killing was also significantly higher in APS patients overall, median (IQR) proportion of non-viable cells 18% (IQR 12-33), compared to stroke controls: 10% (IQR 8-15, p<0.0001) and healthy controls: 12.5% (IQR 6.3-17, p=0.0009). Notably, HR-APS had significantly higher proportion of positive mHam tests (60%), compared to LR-APS: 36.6%, stroke controls: 12.5%, and healthy controls: 17.5% (p<0.0001, Figure 1A). Importantly, HR-APS patients with positive mHam results (n=5) induced a substantially higher increase in procoagulant platelets, median 17.2% (IQR 11.2-28.6), whereas healthy controls (n=4) only resulted in median change of -0.7% (IQR -7.0-1.4, p=0.02) (Figure 1B). Conclusion: Preliminary data suggest that APS-related ischemic stroke, TIA or ischemic brain injury are associated with increased, functionally important, complement activation, more prominent in higher risk disease. Complement-mediated platelet hyperreactivity may have a contributory role that remains to be established.

Platelet Subpopulations and Distinct Platelet Function Defects after Hematopoietic Stem Cell Transplantation in Mice and Humans

Allogeneic hematopoietic stem cell transplantation (HSCT) is often accompanied by bleeding complications due to delayed platelet engraftment. Despite having prolonged thrombocytopenia, patients can also develop thrombotic events. The mechanisms leading to HSCT-related hemorrhage and thrombosis, and especially platelet activity in these patients are yet unknown. Our study aimed to decipher post HSCT megakaryocyte (MK) engraftment and platelet function in a murine model of bone marrow transplantation (BMT) and in patients undergoing HSCT. C57BL/6 mice were lethally irradiated with 10 Gy total body irradiation prior to BMT. Radiation exposure led to massive BM remodeling, including extracellular matrix degradation, vasodilation and aberrant pericyte morphology, accompanied by extensive ectopic platelet release into the BM cavity, which might contribute to thrombocytopenia. We used Actb dsRed reporter mice as BM donors allowing us to track all donor-derived (dsRed +) cells after BMT. Femurs were removed after 14 days and subjected to confocal laser immunofluorescence microscopy using lineage-specific markers. While we found an overall homogenous engraftment pattern for red blood cells and most leukocyte sub-lineages, the engraftment pattern for MKs and Ly6G + cells was characterized by long triangular clusters at the bone cortex. We manipulated BM engraftment by transplanting different numbers of graft cells. Within 2 weeks after BMT with 6 x 10 6 BM cells per mouse, a complete chimerism was achieved for both MKs in BM and platelets in peripheral blood. These newly formed platelets had an increased platelet size, while the surface expression levels (CD41/CD42/GPVI) were decreased by half, implying a massive reduction in receptor density. Flow cytometry analyses of agonist-stimulated platelets revealed a severely impaired granule release and integrin activation that was confirmed by aggregometry. When only 1 x 10 6 BM cells per mouse were transplanted, we found an extended mixed MK chimerism after BMT. During the first 28 days, we detected both dsRed - (recipient-derived) and dsRed + (donor-derived) MKs in the recipient BM and, concomitantly, dsRed - and dsRed + platelets in peripheral blood. Unexpectedly, we observed opposing agonist-induced platelet reactivity with hyporeactive dsRed - platelets, while dsRed + platelets were hyperreactive, indicating that 2 BM MK subpopulations generate 2 functionally distinct platelet subpopulations. Intriguingly, we found an upregulation of annexin V binding on dsRed -, but not on dsRed + resting platelets, implying that the myeloablative conditions preceding HSCT result in increased phosphatidylserine (PS) exposure, a key sign of pro-coagulant platelets. In vitro thrombus formation was assessed under flow using a whole blood collagen-coated microfluidic device. While surface coverage was mildly increased, thrombus volume remained comparable to controls. dsRed + platelets exhibited a distinct incorporation pattern by preferentially attaching to the edges of growing thrombi, indicating that dsRed - and dsRed + platelets may differ in thrombus initiation and growth. While platelet count and other blood cell parameters are routinely monitored in HSCT patients, the functional capacity of graft-derived platelets has not yet been systematically assessed. We thus performed extended platelet function analyses in 13 adult patients (aged 24-73 years) undergoing allogeneic HSCT for malignant leukemic diseases (mostly acute myeloid leukemia) and detected a massive platelet hyporeactivity for up to 100 days observation time after HSCT. Integrin activation was severely impaired upon stimulation of PAR-1 or GPVI. In contrast to mice, α granule release, monitored by P-selectin neo-exposure, was only slightly reduced compared to healthy controls, suggesting a decoupling of integrin activation and P-selectin exposure. In line with our findings in mice, we detected a pronounced upregulation in the proportion of pro-coagulant PS-positive platelets with a reduced mitochondrial membrane potential. Our data demonstrate that a mixed chimerism after HSCT results in two co-existing platelet subpopulations with opposing response profiles. This unexpected finding provides the first experimental evidence why bleedings and thromboembolic events can both occur after HSCT and hints toward pro-coagulant platelets as a potential therapeutic target.

Mitochondrial Fission Protein Drp1 Regulates Platelet Function

Background: Mitochondria in platelets contribute to platelet function and are altered by diseases that involve dysregulated hemostasis or thrombosis. Mitochondrial integrity, governed by its size, number and distribution is modulated by fusion and fission proteins. Pharmacological inhibition of the mitochondrial fission protein Dynamin-related protein 1 (Drp1) has been shown to inhibit granule release in human platelets and impair thrombus formation in a murine model of thrombosis. However, the mitochondrial roles for Drp1 in platelet function are still largely unknown. Objective: To determine the mitochondrial roles of Drp1 in regulating platelet function. Methods: PF4-Cre mediated MK/platelet specific Drp1 null mice (Drp1 -/-) were used. Platelet morphology in Drp1 -/- and wild type (WT) platelets was analyzed by confocal microscopy. Procoagulant platelet formation (PS +ve platelets) after thrombin and the GPVI agonist convulxin (THR+CVX) stimulation was measured using Annexin V staining. Mitochondrial potential (ΔΨm) and mass at baseline and after stimulation was measured using TMRM and mitotracker green. Washed platelet activation in WT and Drp1 -/- platelets in response to convulxin was measured by flow cytometry. Platelet cytoplasmic calcium transients were measured using FURA-2AM after stimulation with THR+CVX. Phosphorylation of Drp1 (pDrp1 S616) in washed WT platelets in response to dual agonist THR+CVX was measured by western blotting. Results: Drp1 -/- platelets were bigger in size than WT platelets as measured by microscopy and had increased mitochondrial mass as determined by flow cytometry. Absence of Drp1 reduced activation of the αIIb/β3 fibrinogen binding receptor (P=0.035; n=5-6 per group) and reduced alpha granule release (P=0.025; n=5-6 per group) in response to CVX.Dual agonist stimulation resulted in reduced PS +ve platelets in Drp1 -/- compared to WT platelets (P=0.0035; n=7 per group). Since depolarization precedes PS exposure, we measured ΔΨm using TMRM in resting and dual agonist stimulated WT and Drp1 -/- platelets. No difference in ΔΨm was observed in resting Drp1 -/- vs WT platelets whereas, stimulated Drp1 -/- platelets exhibited significantly higher ΔΨm (P=0.038; n=4 per group) than stimulated WT platelets. Interestingly, in response to dual agonists, Drp1 -/- platelets also showed a significant increase (P<0.001) in cytoplasmic calcium transients than WT platelets. Western blot analyses of washed WT platelets stimulated with dual agonist showed phosphorylation of Drp1 at S616 (S616), indicating Drp1 activation. Conclusion: These findings highlight a role for Drp1 in murine platelet mitochondrial function, GPVI activation, and procoagulant platelet formation.

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

BackgroundGlanzmann 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. ObjectivesIn this study, we aimed to determine the prevalence of such isoantibodies and better characterize their pathogenic properties. MethodsTwelve 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. ResultsOnly 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. ConclusionOur 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.

Circulating Platelets in Thrombotic Antiphospholipid Syndrome Display High Procoagulant Activity and Surface Complement Deposition: Correlation with Thrombotic Risk

Background Antiphospholipid syndrome (APS) is characterized by circulating antiphospholipid antibodies (aPL) accompanied by arterial and/or venous thrombosis, and/or recurrent pregnancy loss. It commonly affects young women in whom it dramatically increases their risk of myocardial infarction, ischemic stroke, deep vein thrombosis and pulmonary embolism. Although the thrombotic risk associated with aPL is established, the underlying mechanisms are still incompletely defined and there are no definitive predictive biomarkers. As a result the management of APS involves indefinite anticoagulation. Increased activation of vascular cells, including platelets is considered to underlie the pathogenesis of APS, and may be mediated in some cases by complement. There is a considerable crosstalk between platelets and complement at various levels during thrombosis. First, activated complement proteins stimulate platelets through their cognate receptors on the platelet surface, and genetic deficiency of complement proteins in mice leads to decreased platelet responsiveness. Second, platelets secrete complement proteins into the medium upon activation. Third, agonist-stimulated platelets contribute to activation of both the classical and alternative pathways of the complement cascade. Hence, we sought to investigate the association between the complement-platelet axis and the prothrombotic pathology observed in APS. Methods Venous blood was drawn from patients with APS and healthy controls, and washed human platelets were prepared by differential centrifugation. Platelets either remained unstimulated or were treated with thrombin (0.1, 0.25, 0.5 U/ml) for 5 min and then labelled with FITC-PAC1, PE-anti-CD62P antibody, Alexa Fluor 488-anti-C4d conjugate, PE-Annexin V, Mitotracker Red, MitoSOX Red, Fluo4-AM, Rhod2-AM, Cell Event Green Caspase 3/7 detection reagent and FITC-IETD-FMK for 30 min in the dark, and then analyzed by flow cytometry to measure integrin activation, P-selectin expression (α-granule secretion), complement C4d binding, phosphatidylserine (PS) exposure, mitochondrial membrane potential, mitochondrial ROS, cytosolic calcium, mitochondrial calcium, caspase 3/7 activity (apoptosis) and caspase 8 activity (extrinsic apoptosis pathway), respectively. Platelet markers were correlated with complement binding and clinical characteristics of APS patients including number of thrombotic events and circulating levels of aPL. Results Thrombin-induced platelet integrin activation and P-selectin expression were identical in APS patients and healthy controls. However, platelets isolated from APS patients had significantly higher procoagulant activity (PS exposure) as well as complement C4d binding in both unstimulated and thrombin-stimulated states compared to healthy controls. There was a strong positive correlation between platelet PS exposure and C4d binding in APS patients, but not healthy individuals, suggesting a role for complement activation in formation of procoagulant platelets in APS. A subset of APS patients with high platelet procoagulant activity (PS exposure > mean + 2SD observed in healthy individuals) had lower mitochondrial membrane potential and increased mitochondrial calcium transients compared to healthy individuals, suggesting mitochondrial permeability transition pore (mPTP)-driven PS exposure. Another subset of APS patients with PS exposing platelets showed caspase activation indicating apoptosis. Platelet PS exposure and C4d binding correlated strongly with the number of thrombotic events as well as levels of circulating aPL, which identifies complement-induced procoagulant platelets as predictors and possible underlying mechanisms for thrombosis in APS. Conclusion Our preliminary studies suggest that PS-exposing platelets are generated in APS by multiple mechanisms including agonist-induced mPTP formation and apoptosis-associated caspase activation. Platelet PS exposure in APS is positively correlated with C4d binding, suggesting a role for complement activation in driving platelet procoagulant activity. Complement activation and PS exposure on platelets are strong predictors of thrombotic risk in APS and could serve as promising targets for novel antithrombotic agents in APS management.

Heparin-activated procoagulant platelet assay: a flow cytometry-based functional test for heparin-induced thrombocytopenia

BackgroundFunctional platelet activation assays are required for the diagnosis of heparin-induced thrombocytopenia (HIT). Due to their sophisticated methodology, they are only available in reference centers. ObjectivesTo evaluate the diagnostic accuracy of the flow cytometry-based heparin-activated procoagulant platelet (HAPP) assay in the laboratory diagnosis of HIT. MethodsProcoagulant platelets (PCP), defined by the expression of phosphatidylserine and CD62-P, were evaluated by flow cytometry in platelet-rich plasma from healthy donors after incubation with patient sera in the absence and presence of heparin. A sample was considered positive in HAPP assay, if the following 3 criteria were met: 1) the percentage of PCPs was ≥10.3% after incubation with 0.2 IU/mL heparin, 2) the fold increase in presence of 0.2 IU/mL heparin compared with buffer was ≥1.5, and 3) 100 IU/mL of heparin resulted in ≥50% inhibition of PCP. HAPP assay was validated in a prospective cohort (n = 202) of consecutive specimens submitted to our laboratory for serologic diagnosis of HIT. Heparin-induced platelet activation (HIPA) assay was used as the reference standard. ResultsHIT-positive sera induced PCPs in the presence of 0.2 IU/mL heparin, which was inhibited with 100 IU/mL of heparin. In the prospective validation cohort, there were 15 HIPA+ and 187 HIPA- sera. HAPP was positive in 20 samples in this cohort. Using optimized cut-offs, HAPP assay had a sensitivity of 93.3% and specificity of 96.8%. ConclusionHAPP assay is promising as a simple and reliable functional assay for HIT; however, further studies are needed to confirm our results in larger cohorts.