A flow cytometric assay using mepacrine for study of uptake and release of platelet dense granule contents

Angiopoietin-like protein 2 inhibits thrombus formation

Acute myocardial infarction is mainly caused by a lack of blood flood in the coronary artery. Angiopoietin-like protein 2 (ANGPTL2) induces platelet activation and thrombus formation in vitro through binding with immunoglobulin-like receptor B, an immunoglobulin superfamily receptor. However, the mechanism by which it regulates platelet function in vivo remains unclear. In this study, we investigated the role of ANGPTL2 during thrombosis in relationship with ST-segment elevation myocardial infarction (STEMI) with spontaneous recanalization (SR). In a cohort of 276 male and female patients, we measured plasma ANGPTL2 protein levels. Using male Angptl2-knockout and wild-type mice, we examined the inhibitory effect of Angptl2 on thrombosis and platelet activation both in vivo and ex vivo. We found that plasma and platelet ANGPTL2 levels were elevated in patients with STEMI with SR compared to those in non-SR (NSR) patients, and was an independent predictor of SR. Angptl2 deficiency accelerated mesenteric artery thrombosis induced by FeCl3 in Angptl2-/- compared to WT animals, promoted platelet granule secretion and aggregation induced by thrombin and collogen while purified ANGPTL2 protein supplementation reversed collagen-induced platelet aggregation. Angptl2 deficiency also increased platelet spreading on immobilized fibrinogen and clot contraction. In collagen-stimulated Angptl2-/- platelets, Src homology region 2 domain-containing phosphatase (Shp)1-Y564 and Shp2-Y580 phosphorylation were attenuated while Src, Syk, and Phospholipase Cγ2 (PLCγ2) phosphorylation increased. Our results demonstrate that ANGPTL2 negatively regulated thrombus formation by activating ITIM which can suppress ITAM signaling pathway. This new knowledge provides a new perspective for designing future antiplatelet aggregation therapies.

Mepacrine Flow Cytometry Assay for the Diagnosis of Platelet δ-granule Defects: Literature Review on Methods-Towards a Shared Detailed Protocol

Accurate assessment of platelet secretion is essential for the diagnosis of inherited or acquired platelet function disorders and more specifically in identifying δ-storage pool disease. Mepacrine, a fluorescent dye, specifically accumulates in platelet δ-granules. The mepacrine flow cytometry (mepacrine FCM) assay has been used for more than half a century in the clinical laboratory as a diagnostic tool for platelet δ-granule disorders. The assay requires a small volume of blood, can be performed in thrombocytopenic patients, provides rapid assessment of δ-granule content and secretion, and, thus, enables differentiation between storage and release defects. There is however a broad heterogeneity in methods, reagents, and equipment used. Lack of standardization and limited data on analytical and clinical performances have led the 2022 ISTH SSC (International Society on Thrombosis and Haemostasis Scientific and Standardization Committee) Subcommittee on Platelet Physiology expert consensus to rate this assay as simple but of uncertain value. Yet, the data used by experts to formulate the recommendations were not discussed and even not mentioned. Guidance for laboratory studies of platelet secretion assay would be very helpful for clinical laboratories and health authorities especially considering the implications of the new In Vitro Diagnostic Regulation in Europe. The purpose of the present work was to review the reported methodologies for the mepacrine FCM assay and to offer an example of detailed protocol. This would help standardization and pave the way for more rigorous comparative studies.

Flow Cytometry and Platelets

Clinical assessment of platelet activation by flow cytometry is useful in the characterization and diagnosis of platelet-specific disorders and as a measure of risk for thrombosis or bleeding. Platelets circulate in a resting, "unactivated" state, but when activated they undergo alterations in surface glycoprotein function and/or expression level, exposure of granule membrane proteins, and exposure of procoagulant phospholipids. Flow cytometry provides the means to detect these changes and, unlike other platelet tests, is appropriate for measuring platelet function in samples from patients with low platelet counts. The present review will focus on flow cytometric tests for platelet activation markers.

Biofilm-dispersed pneumococci induce elevated leukocyte and platelet activation

Introduction

Streptococcus pneumoniae (the pneumococcus) effectively colonizes the human nasopharynx, but can migrate to other host sites, causing infections such as pneumonia and sepsis. Previous studies indicate that pneumococci grown as biofilms have phenotypes of bacteria associated with colonization whereas bacteria released from biofilms in response to changes in the local environment (i.e., dispersed bacteria) represent populations with phenotypes associated with disease. How these niche-adapted populations interact with immune cells upon reaching the vascular compartment has not previously been studied. Here, we investigated neutrophil, monocyte, and platelet activation using ex vivo stimulation of whole blood and platelet-rich plasma with pneumococcal populations representing distinct stages of the infectious process (biofilm bacteria and dispersed bacteria) as well as conventional broth-grown culture (planktonic bacteria).

Methods

Flow cytometry and ELISA were used to assess surface and soluble activation markers for neutrophil and monocyte activation, platelet-neutrophil complex and platelet-monocyte complex formation, and platelet activation and responsiveness.

Results

Overall, we found that biofilm-derived bacteria (biofilm bacteria and dispersed bacteria) induced significant activation of neutrophils, monocytes, and platelets. In contrast, little to no activation was induced by planktonic bacteria. Platelets remained functional after stimulation with bacterial populations and the degree of responsiveness was inversely related to initial activation. Bacterial association with immune cells followed a similar pattern as activation.

Discussion

Differences in activation of and association with immune cells by biofilm-derived populations could be an important consideration for other pathogens that have a biofilm state. Gaining insight into how these bacterial populations interact with the host immune response may reveal immunomodulatory targets to interfere with disease development.

Bleeding Disorder of Unknown Cause: A Diagnosis of Exclusion

Patients with an unexplained mild to moderate bleeding tendency are diagnosed with bleeding disorder of unknown cause (BDUC), a classification reached after ruling out other mild to moderate bleeding disorders (MBD) including von Willebrand disease (VWD), platelet function defects (PFDs), coagulation factor deficiencies (CFDs), and non-hemostatic causes for bleeding. This review outlines our diagnostic approach to BDUC, a diagnosis of exclusion, drawing on current guidelines and insights from the Vienna Bleeding Biobank (VIBB). According to guidelines, we diagnose VWD based on VWF antigen and/or activity levels ≤50 IU/dL, with repeated VWF testing if VWF levels are <80 IU/dL. This has been introduced in our clinical routine after our findings of diagnostically relevant fluctuations of VWF levels in a high proportion of MBD patients. PFDs are identified through repeated abnormalities in light transmission aggregometry (LTA), flow cytometric mepacrine fluorescence, and glycoprotein expression analysis. Nevertheless, we experience diagnostic challenges with regard to reproducibility and unspecific alterations of LTA. For factor (F) VIII and FIX deficiency, a cutoff of 50% is utilized to ensure detection of mild hemophilia A or B. We apply established cutoffs for other rare CFD being aware that these do not clearly reflect the causal role of the bleeding tendency. Investigations into very rare bleeding disorders due to hyperfibrinolysis or increase in natural anticoagulants are limited to cases with a notable family history or distinct bleeding phenotypes considering cost-effectiveness. While the pathogenesis of BDUC remains unknown, further explorations of this intriguing area may reveal new mechanisms and therapeutic targets.

Dysregulation of platelet serotonin, 14-3-3, and GPIX in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the leading cause of post-neonatal infant mortality, but the underlying cause(s) are unclear. A subset of SIDS infants has abnormalities in the neurotransmitter, serotonin (5-hydroxytryptamine [5-HT]) and the adaptor molecule, 14-3-3 pathways in regions of the brain involved in gasping, response to hypoxia, and arousal. To evaluate our hypothesis that SIDS is, at least in part, a multi-organ dysregulation of 5-HT, we examined whether blood platelets, which have 5-HT and 14-3-3 signaling pathways similar to brain neurons, are abnormal in SIDS. We also studied platelet surface glycoprotein IX (GPIX), a cell adhesion receptor which is physically linked to 14-3-3. In infants dying of SIDS compared to infants dying of known causes, we found significantly higher intra-platelet 5-HT and 14-3-3 and lower platelet surface GPIX. Serum and plasma 5-HT were also elevated in SIDS compared to controls. The presence in SIDS of both platelet and brainstem 5-HT and 14-3-3 abnormalities suggests a global dysregulation of these pathways and the potential for platelets to be used as a model system to study 5-HT and 14-3-3 interactions in SIDS. Platelet and serum biomarkers may aid in the forensic determination of SIDS and have the potential to be predictive of SIDS risk in living infants.

Serotonin secretion by blood platelets: accuracy of high-performance liquid chromatography-electrochemical technique compared with the isotopic test and use in a clinical laboratory

Background

Mild secretion defects are the most frequent and challenging blood platelet disorders to diagnose. Most δ-granule secretion tests lack validation, are not quantitative, or have unreliable response to weak platelet agonists.

Objectives

To compare platelet serotonin secretion by HPLC-electrochemical detection technique (HPLC-ECD) with the reference isotopic test (3H-5-HT), evaluating its performance in clinical laboratories.

Methods

The assay validation followed STARD-2015 recommendations. HPLC-ECD measured the nonsecreted serotonin remaining in platelet pellets after aggregation, comparing it with the reference 3H-5-HT assay. We studied subjects with inherited and aspirin-induced blood platelet disorders and assessed the HPLC-ECD operation for routine clinical diagnosis.

Results

Calibration curves were linear (R2 = 0.997), with SD for residuals of 3.91% and analytical sensitivity of 5ng/mL. Intra- and interassay imprecision bias ranged between -8.5% and 2.1% and -9% and 3.1%, respectively. Serotonin recovery and stability were >95%, and the variability range of measurements was -5.5% to 4.6%. Statistical differences detected between tests were biologically irrelevant, with bias of 1.48% (SD, 8.43) and CI agreement of -18% to 15%. Both assays distinctly detected platelet secretion induced by 10 μM epinephrine and 4 μmM adenosine diphosphate. However, HPLC-ECD is quantitative and more sensitive to low serotonin content in blood platelets. Reference cutoffs for each agonist were determined in 87 subjects. Initially, the HPLC-ECD requires relatively expensive equipment and trained operators but has remarkably cheap running costs and a turn-around time of 24-36 hours. We have used this diagnostic tool routinely for >8 years.

Conclusion

HPLC-ECD assay for platelet serotonin secretion is highly accurate, has advantages over the reference 3H-5-HT test, and is suitable as a clinical laboratory technique.

Loss of zinc transporters ZIP1 and ZIP3 augments platelet reactivity in response to thrombin and accelerates thrombus formation in vivo

Zinc (Zn2+) is considered as important mediator of immune cell function, thrombosis and haemostasis. However, our understanding of the transport mechanisms that regulate Zn2+ homeostasis in platelets is limited. Zn2+ transporters, ZIPs and ZnTs, are widely expressed in eukaryotic cells. Using mice globally lacking ZIP1 and ZIP3 (ZIP1/3 DKO), our aim was to explore the potential role of these Zn2+ transporters in maintaining platelet Zn2+ homeostasis and in the regulation of platelet function. While ICP-MS measurements indicated unaltered overall Zn2+ concentrations in platelets of ZIP1/3 DKO mice, we observed a significantly increased content of FluoZin3-stainable free Zn2+, which, however, appears to be released less efficiently upon thrombin-stimulated platelet activation. On the functional level, ZIP1/3 DKO platelets exhibited a hyperactive response towards threshold concentrations of G protein-coupled receptor (GPCR) agonists, while immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptor agonist signalling was unaffected. This resulted in enhanced platelet aggregation towards thrombin, bigger thrombus volume under flow ex vivo and faster in vivo thrombus formation in ZIP1/3 DKO mice. Molecularly, augmented GPCR responses were accompanied by enhanced Ca2+ and PKC, CamKII and ERK1/2 signalling. The current study thereby identifies ZIP1 and ZIP3 as important regulators for the maintenance of platelet Zn2+ homeostasis and function.

Screening and diagnosis of inherited platelet disorders

Inherited platelet disorders are important conditions that often manifest with bleeding. These disorders have heterogeneous underlying pathologies. Some are syndromic disorders with non-blood phenotypic features, and others are associated with an increased predisposition to developing myelodysplasia and leukemia. Platelet disorders can present with thrombocytopenia, defects in platelet function, or both. As the underlying pathogenesis of inherited thrombocytopenias and platelet function disorders are quite diverse, their evaluation requires a thorough clinical assessment and specialized diagnostic tests, that often challenge diagnostic laboratories. At present, many of the commonly encountered, non-syndromic platelet disorders do not have a defined molecular cause. Nonetheless, significant progress has been made over the past few decades to improve the diagnostic evaluation of inherited platelet disorders, from the assessment of the bleeding history to improved standardization of light transmission aggregometry, which remains a "gold standard" test of platelet function. Some platelet disorder test findings are highly predictive of a bleeding disorder and some show association to symptoms of prolonged bleeding, surgical bleeding, and wound healing problems. Multiple assays can be required to diagnose common and rare platelet disorders, each requiring control of preanalytical, analytical, and post-analytical variables. The laboratory investigations of platelet disorders include evaluations of platelet counts, size, and morphology by light microscopy; assessments for aggregation defects; tests for dense granule deficiency; analyses of granule constituents and their release; platelet protein analysis by immunofluorescent staining or flow cytometry; tests of platelet procoagulant function; evaluations of platelet ultrastructure; high-throughput sequencing and other molecular diagnostic tests. The focus of this article is to review current methods for the diagnostic assessment of platelet function, with a focus on contemporary, best diagnostic laboratory practices, and relationships between clinical and laboratory findings.

Diagnosis of Platelet Function Disorders: A Challenge for Laboratories

In patients with normal plasmatic coagulation and bleeding tendency, platelet function defect can be assumed. Congenital platelet function defects are rare. Much more commonly they are acquired. The clinical bleeding tendency of platelet function defects is heterogeneous, which makes diagnostic approaches difficult. During the years, a large variety of tests for morphological phenotyping and functional analysis have been developed. The diagnosis of platelet function defects is based on standardized bleeding assessment tools followed by a profound morphological evaluation of the platelets. Platelet function assays like light transmission aggregation, luminoaggregometry, and impedance aggregometry followed by flow cytometry are commonly used to establish the diagnosis in these patients. Nevertheless, despite great efforts, standardization of these tests is poor and in most cases, quality control is lacking. In addition, these tests are still limited to specialized laboratories. This review summarizes the approaches to morphologic phenotyping and platelet testing in patients with suspected platelet dysfunction, beginning with a standardized bleeding score and ending with flow cytometry testing. The diagnosis of a functional defect requires a good collaboration between the laboratory and the clinician.

Early platelet dysfunction in patients receiving extracorporeal membrane oxygenation is associated with mortality

Extracorporeal membrane oxygenation (ECMO) is used for patients with cardiopulmonary failure and is associated with severe bleeding and poor outcome. Platelet dysfunction may be a contributing factor. The aim of this prospective observational study was to characterize platelet dysfunction and its relation to outcome in ECMO patients. Blood was sampled from thirty ECMO patients at three timepoints. Expression of CD62P, CD63, activated GPIIb/IIIa, GPVI, GPIbα and formation platelet-leukocyte aggregates (PLA) were analyzed at rest and in response to stimulation. Delta granule storage-pool deficiency and secretion defects were also investigated. Fifteen healthy volunteers and ten patients with coronary artery disease served as controls. Results were also compared between survivors and non-survivors. Compared to controls, expression of platelet surface markers, delta granule secretion and formation of PLA was reduced, particularly in response to stimulation. Baseline CD63 expression was higher and activated GPIIb/IIIa expression in response to stimulation was lower in non-survivors on day 1 of ECMO. Logistic regression analysis revealed that these markers were associated with mortality. In conclusion, platelets from ECMO patients are severely dysfunctional predisposing patients to bleeding complications and poor outcome. Platelet dysfunction on day 1 of ECMO detected by the platelet surface markers CD63 and activated GPIIb/IIIa is associated with mortality. CD63 and activated GPIIb/IIIa may therefore serve as novel prognostic biomarkers, but future studies are required to determine their true potential.

Functional Assessment of Platelet Dense Granule ATP Release

OBJECTIVES

This study was undertaken to explore the feasibility of assessing platelet dense granule release in response to platelet stimuli, using less than 1 mL of whole blood (WB).

METHODS

Optimization of the luciferin-luciferase (LL) assay for ATP release, together with additional modifications, was applied to 1:10 diluted WB.

RESULTS

LL assay optimization using nonstirred 1:10 diluted WB resulted in dense granule ATP release in response to thrombin receptor-activating peptide (TRAP) of similar magnitude to that observed using stirred platelet-rich plasma. Stirring of the 1:10 diluted WB restored collagen-induced dense granule secretion. Addition of lyophilized, formalin-fixed platelets, together with stirring, restored dense granule secretion responsiveness to ADP. TRAP, ADP, and collagen all stimulated ATP release in 1:10 diluted WB under the optimized conditions of this study at levels close to those observed using platelet-rich plasma. Blood sample reconstitution experiments offer hope that this assay may prove robust down to WB platelet counts as low as 50 × 103/μL.

CONCLUSIONS

Platelet dense granule release in response to a number of classic stimuli, including ADP, was accomplished from less than 1 mL WB with minimal specimen processing, using widely available reagents and instrumentation.

A zinc transporter, transmembrane protein 163, is critical for the biogenesis of platelet dense granules

Lysosome-related organelles (LROs) are a category of secretory organelles enriched with ions such as calcium, which are maintained by ion transporters or channels. Homeostasis of these ions is important for LRO biogenesis and secretion. Hermansky-Pudlak syndrome (HPS) is a recessive disorder with defects in multiple LROs, typically platelet dense granules (DGs) and melanosomes. However, the underlying mechanism of DG deficiency is largely unknown. Using quantitative proteomics, we identified a previously unreported platelet zinc transporter, transmembrane protein 163 (TMEM163), which was significantly reduced in BLOC-1 (Dtnbp1sdy and Pldnpa)-, BLOC-2 (Hps6ru)-, or AP-3 (Ap3b1pe)-deficient mice and HPS patients (HPS2, HPS3, HPS5, HPS6, or HPS9). We observed similar platelet DG defects and higher intracellular zinc accumulation in platelets of mice deficient in either TMEM163 or dysbindin (a BLOC-1 subunit). In addition, we discovered that BLOC-1 was required for the trafficking of TMEM163 to perinuclear DG and late endosome marker-positive compartments (likely DG precursors) in MEG-01 cells. Our results suggest that TMEM163 is critical for DG biogenesis and that BLOC-1 is required for the trafficking of TMEM163 to putative DG precursors. These new findings suggest that loss of TMEM163 function results in disruption of intracellular zinc homeostasis and provide insights into the pathogenesis of HPS or platelet storage pool deficiency.

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.

Laboratory Techniques Used to Diagnose Constitutional Platelet Dysfunction

Platelets play a major role in primary hemostasis, where activated platelets form plugs to stop hemorrhaging in response to vessel injuries. Defects in any step of the platelet activation process can cause a variety of platelet dysfunction conditions associated with bleeding. To make an accurate diagnosis, constitutional platelet dysfunction (CPDF) should be considered once von Willebrand disease and drug intake are ruled out. CPDF may be associated with thrombocytopenia or a genetic syndrome. CPDF diagnosis is complex, as no single test enables the analysis of all aspects of platelet function. Furthermore, the available tests lack standardization, and repeat tests must be performed in specialized laboratories especially for mild and moderate forms of the disease. In this review, we provide an overview of the laboratory tests used to diagnose CPDF, with a focus on light transmission platelet aggregation (LTA), flow cytometry (FC), and granules assessment. Global tests, mainly represented by LTA, are often initially performed to investigate the consequences of platelet activation on platelet aggregation in a single step. Global test results should be confirmed by additional analytical tests. FC represents an accurate, simple, and reliable test to analyze abnormalities in platelet receptors, and granule content and release. This technique may also be used to investigate platelet function by comparing resting- and activated-state platelet populations. Assessment of granule content and release also requires additional specialized analytical tests. High-throughput sequencing has become increasingly useful to diagnose CPDF. Advanced tests or external research laboratory techniques may also be beneficial in some cases.

Platelet immunophenotyping in health and inherited bleeding disorders, a review and practical hints

Inherited platelet function disorders are rare hemorrhagic diseases. The gold standard for their exploration is optical aggregometry; however, investigations by flow cytometry (FCM) are being increasingly used. In this review, the physiology of platelets is first recalled, setting the stage for the compartments of platelets that can be apprehended by specific and appropriate labeling. As this requires some pre-analytical precautions and specific analytical settings, a second part focuses on these characteristic aspects, based on literature and on the authors' experience in the field, for qualitative or quantitative explorations. Membrane labeling with antibodies to CD42a or CD41, respectively, useful to assess the genetic-related defects of Glanzmann thrombocytopenia and Bernard Soulier syndrome are then described. Platelet degranulation disorders are detailed in the next section, as they can be explored, upon platelet activation, by measuring the expression of surface P-Selectin (CD62P) or CD63. Mepacrin uptake and release after activation is another test allowing to explore the function of dense granules. Finally, the flip-flop anomaly related to Scott syndrome is depicted. Tables summarizing possible FCM assays, and characteristic histograms are provided as reference for flow laboratories interested in developing platelet exploration.

A Morphometric Analysis of Platelet Dense Granules of Patients with Unexplained Bleeding: A New Entity of Delta-Microgranular Storage Pool Deficiency

One thousand and eighty patients, having prolonged bleeding times, frequent epistaxis, menorrhagia or easy bruising or other bleeding manifestations, and excluding those with von Willebrand's disease, were evaluated for platelet dense granule deficiency. The mean diameter of platelet dense granules was determined for all patients using image analysis. Four hundred and ninety-nine had "classic" dense (delta) granule storage pool deficiency (δ-SPD). Five hundred and eighty-one individuals (53.8%) were found to have a normal mean number of dense granules, but for some of these patients, the dense granules were smaller than for the controls. Of the patients having a normal number of dense granules, 165 (28.4%) were found to have significantly smaller granules than the platelets obtained from the control subjects. Their average granule diameter was 123.35 ± 0.86 nm, that is more than three standard deviations below the mean of the control data. Total δ-granule storage pool volumes (TDGV)/platelet were calculated using these measurements. Individuals with δ-SPD had half the number of granules (2.25 ± 0.04 DG/PL) and storage pool volume (3.88 ± 1.06 × 106 nm3) when compared to our control data (4.64 ± 0.11 DG/PL; 10.79 × 106 nm3 ± 0.42). Individuals having a bleeding history but a normal average of small dense granules had a calculated storage pool volume statistically different than controls and essentially the same storage pool volume as patients with δ-SPD. We have identified a sub-classification of δ-SPD that we have defined as micro-granular storage pool deficiency (δ-MGSPD).

Targeted inhibition of thrombin attenuates murine neonatal necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is an inflammatory bowel necrosis of premature infants and an orphan disease with no specific treatment. Most patients with confirmed NEC develop moderate-severe thrombocytopenia requiring one or more platelet transfusions. Here we used our neonatal murine model of NEC-related thrombocytopenia to investigate mechanisms of platelet depletion associated with this disease [K. Namachivayam, K. MohanKumar, L. Garg, B. A. Torres, A. Maheshwari, Pediatr. Res. 81, 817-824 (2017)]. In this model, enteral administration of immunogen trinitrobenzene sulfonate (TNBS) in 10-d-old mouse pups produces an acute necrotizing ileocolitis resembling human NEC within 24 h, and these mice developed thrombocytopenia at 12 to 15 h. We hypothesized that platelet activation and depletion occur during intestinal injury following exposure to bacterial products translocated across the damaged mucosa. Surprisingly, platelet activation began in our model 3 h after TNBS administration, antedating mucosal injury or endotoxinemia. Platelet activation was triggered by thrombin, which, in turn, was activated by tissue factor released from intestinal macrophages. Compared to adults, neonatal platelets showed enhanced sensitivity to thrombin due to higher expression of several downstream signaling mediators and the deficiency of endogenous thrombin antagonists. The expression of tissue factor in intestinal macrophages was also unique to the neonate. Targeted inhibition of thrombin by a nanomedicine-based approach was protective without increasing interstitial hemorrhages in the inflamed bowel or other organs. In support of these data, we detected increased circulating tissue factor and thrombin-antithrombin complexes in patients with NEC. Our findings show that platelet activation is an important pathophysiological event and a potential therapeutic target in NEC.

A Case of Chronic Thrombocytopenia in a 17-Year-Old Female

Storage pool deficiency (SPD) is a group of rare platelet disorders that result from deficiencies in α-granules, δ-granules, or both. One type of α-SPD is gray platelet syndrome (GPS), caused by mutations in the neurobeachin-like 2 (NBEAL2) gene that results in a bleeding diathesis, thrombocytopenia, splenomegaly, and progressive myelofibrosis. Due to the lack of α-granules, platelets have a gray and degranulated appearance by light microscopy. However, definitive diagnosis of GPS requires confirmation of α-granule deficiency by electron microscopy. Treatment is nonspecific, with the conservative utilization of platelet transfusions being the most important form of therapy. We present a case of a 17-year-old female with a past medical history of thrombocytopenia, first identified at the age of five. Her clinical symptomatology included chronic fatigue, gingival bleeding, bruising, menorrhagia, and leg pain. This report will discuss both the clinical and the pathophysiologic aspects of this rare platelet disorder.

Generation of procoagulant collagen- and thrombin-activated platelets in platelet concentrates derived from buffy coat: the role of processing, pathogen inactivation, and storage

BACKGROUND

Collagen- and thrombin-activated (COAT) platelets (PLTs), generated by dual-agonist stimulation with collagen and thrombin (THR), enhance THR generation at the site of vessel wall injury. There is evidence that higher amounts of procoagulant COAT PLTs are associated with stroke, while a decreased ability to generate them is associated with bleeding diathesis. Our aim was to study PLT functions, particularly the ability to generate COAT PLTs, in PLT concentrates (PCs) from buffy coat. Thus, we investigated the effect of processing, pathogen inactivation treatment (amotosalen-UVA), and PC storage.

STUDY DESIGN AND METHODS

Two PCs from five donors each were pooled and split in two bags; one of them was pathogen inactivated and the other one was left untreated (n = 5). Flow cytometric analyses were performed immediately after PC preparation (Day 1) and thereafter on Days 2, 5, 7, and 9 in treated and untreated PCs to measure the reactivity of PLTs (CD62P and PAC-1), the content and secretion of dense granule after stimulation with different agonists, and the percentage of COAT PLTs after dual stimulation with convulxin (agonist of the collagen receptor GPVI) and THR.

RESULTS

Preparation of PCs resulted in a significant decrease of COAT PLTs and in an impaired response to adenosine 5'-diphosphate sodium (ADP). Storage further decreased ADP response. Minor differences were observed between untreated or amotosalen-UVA-treated PCs.

CONCLUSION

Preparation of PCs from buffy coats decreased the ability to generate COAT PLTs and impaired PLT response to ADP.

Synthesis of thieno[2,3-c]acridine and furo[2,3-c]acridine derivatives via an iodocyclization reaction and their fluorescence properties and DFT mechanistic studies

In this paper, we report the novel synthesis of thieno[2,3-c]acridine and furo[2,3-c]acridine derivatives via intramolecular iodocyclization reaction. The thieno[2,3-c]acridine derivatives exhibited blue fluorescence in hexane.

Diagnose angeborener Störungen der Thrombozytenfunktion

Angeborene Störungen der Thrombozytenfunktion sind eine heterogene Gruppe von Erkrankungen, die oft erst bei Auftreten von Blutungen erkannt werden. Im klinischen Bereich haben sich nur wenige Methoden zur Diagnose und Klassifizierung von angeborenen Thrombozytenfunktionsstörungen bewährt. Für eine rationelle Diagnostik ist ein stufenweises Vorgehen empfehlenswert. Anamnese und klinische Untersuchung sind Grundvoraussetzungen. Das von-Willebrand-Syndrom und andere plasmatische Gerinnungsstörungen sollten vor einer spezifischen Thrombozytenfunktionsdiagnostik immer ausgeschlossen werden. Die Bestimmung von Zahl, Größe, Volumen (MPV) und Morphologie der Thrombozyten erlauben Rückschlüsse auf die zu Grunde liegende Störung.

Die PFA-100®-Verschlusszeit eignet sich als Screening zum Ausschluss schwerer Thrombozytenfunktionsstörungen. Die Aggrego metrie ermöglicht die Untersuchung zahlreicher Aspekte der Thrombozytenfunktion. Die Durchflusszytometrie ist zur Diagnose von Thrombasthenie Glanzmann, Bernard-Soulier- Syndrom und Freisetzungsstörungen geeignet. Molekulargenetische Untersuchungen können die Verdachtsdiagnose bestätigen oder zum Nachweis nicht beschriebener Defekte verwendet werden. Hier wird die ungekürzte Version der inter -disziplinären Leitlinie* präsentiert.

A review of platelet secretion assays for the diagnosis of inherited platelet secretion disorders

Measurement of platelet granule release to detect inherited platelet secretion disorders (IPSDs) is essential for the evaluation of patients with abnormal bleeding and is necessary to distinguish which granule sub-types are affected and whether there is abnormal granule bio-synthesis or secretion. The radioactive serotonin incorporation and release assay, described before 1970, is still considered the “gold standard” test to assess platelet δ-granule release, although is unsuitable for clinical diagnostic laboratories. Luciferin-based assays, such as lumiaggregometry, are the most widely performed alternatives, although these methods do not distinguish defects in δ-granule biosyn-thesis from defects in secretion. Platelet α-granule release is commonly evaluated using flow cytometry by measuring surface exposure of P-selectin after platelet activation. However, this assay has poor sensitivity for some α-granule disorders. Only few studies have been published with more recently developed assays and no critical reviews on these methods are available. In this review, we describe the rationale for developing robust and accurate laboratory tests of platelet granule release and describe the characteristics of the currently available tests. We identify an unmet need for further systematic evaluation of new assays and for standardisation of methodologies for clinical diagnostic laboratories.

Hypoxia impairs agonist-induced integrin αIIbβ3 activation and platelet aggregation

Under ischemic conditions, tissues are exposed to hypoxia. Although human physiology, to a certain extent, can adapt to hypoxic conditions, the impact of low oxygen levels on platelet function is unresolved. Therefore, we explored how reduction of atmospheric oxygen levels to 1% might affect agonist-induced aggregation and static adhesion of isolated human platelets. We uncovered that isolated, washed human platelets exposed to hypoxic conditions show reduced thrombin receptor-activating peptide-6 (TRAP-6) and convulxin-induced aggregation. Of note, this hypoxia-triggered effect was not observed in platelet-rich plasma. Independent of the agonist used (TRAP-6, ADP), activation of the platelet fibrinogen receptor integrin α_IIbβ₃ (GPIIbIIIa, CD41/CD61) was strongly reduced at 1% and 8% oxygen. The difference in agonist-induced integrin α_IIbβ₃ activation was apparent within 5 minutes of stimulation. Following hypoxia, re-oxygenation resulted in the recovery of integrin α_IIbβ₃ activation. Importantly, platelet secretion was not impaired by hypoxia. Static adhesion experiments revealed decreased platelet deposition to fibrinogen coatings, but not to collagen or vitronectin coatings, indicating that specifically the function of the integrin subunit α_IIb is impaired by exposure of platelets to reduced oxygen levels. Our results reveal an unexpected effect of oxygen deprivation on platelet aggregation mediated by the fibrinogen receptor integrin α_IIbβ₃.

Dysregulation of PLDN (pallidin) is a mechanism for platelet dense granule deficiency in RUNX1 haplodeficiency

Essentials Platelet dense granule (DG) deficiency is a major abnormality in RUNX1 haplodeficiency patients. The molecular mechanisms leading to the platelet DG deficiency are unknown. Platelet expression of PLDN (BLOC1S6, pallidin), involved in DG biogenesis, is regulated by RUNX1. Downregulation of PLDN is a mechanism for DG deficiency in RUNX1 haplodeficiency.

SUMMARY

Background Inherited RUNX1 haplodeficiency is associated with thrombocytopenia and platelet dysfunction. Dense granule (DG) deficiency has been reported in patients with RUNX1 haplodeficiency, but the molecular mechanisms are unknown. Platelet mRNA expression profiling in a patient previously reported by us with a RUNX1 mutation and platelet dysfunction showed decreased expression of PLDN (BLOC1S6), which encodes pallidin, a subunit of biogenesis of lysosome-related organelles complex-1 (BLOC-1) involved in DG biogenesis. PLDN mutations in the pallid mouse and Hermansky-Pudlak syndrome-9 are associated with platelet DG deficiency. Objectives We postulated that PLDN is a RUNX1 target, and that its decreased expression leads to platelet DG deficiency in RUNX1 haplodeficiency. Results Platelet pallidin and DG levels were decreased in our patient. This was also observed in two siblings from a different family with a RUNX1 mutation. Chromatin immunoprecipitation and electrophoretic mobility shift assays with phorbol ester-treated human erythroleukemia (HEL) cells showed RUNX1 binding to RUNX1 consensus sites in the PLDN1 5' upstream region. In luciferase reporter studies, mutation of RUNX1 sites in the PLDN promoter reduced activity. RUNX1 overexpression enhanced and RUNX1 downregulation decreased PLDN1 promoter activity and protein expression. RUNX1 downregulation resulted in impaired handling of mepacrine and mislocalization of the DG marker CD63 in HEL cells, indicating impaired DG formation, recapitulating findings on PLDN downregulation. Conclusions These studies provide the first evidence that PLDN is a direct target of RUNX1 and that its dysregulation is a mechanism for platelet DG deficiency associated with RUNX1 haplodeficiency.

Assessment of Platelet Function in Whole Blood by Flow Cytometry

Evaluation of platelet function is important for understanding the physiology of hemostasis and thrombosis and is utilized in clinical practice to diagnose inherited and acquired platelet bleeding disorders. Flow cytometry is a powerful tool for rapid evaluation of multiple functional properties of large number of platelets in whole blood and offers many advantages over other traditional methods. Attention to pre-analytical factors is required to ensure biologically valid and robust results.

Platelet adenosine diphosphate inhibition in trauma patients by thromboelastography correlates with paradoxical increase in platelet dense granule content by flow cytometry

BACKGROUND

The mechanism of platelet dysfunction in acute traumatic coagulopathy is unknown. Traumatic brain injury is hypothesized as a cause, while some investigators presume platelets become "exhausted." We hypothesized that platelet hyperstimulation and consumption resulting from trauma leads to decreased platelet function secondary to depletion of platelet granules.

METHODS

Twenty-five trauma patients were divided into traumatic brain injury and no traumatic brain injury groups. Healthy volunteers served as controls. All had thromboelastography with platelet mapping and flow cytometric assays of mepacrine performed. Mepacrine uptake in unstimulated platelets was used for quantification of platelet content of dense granules.

RESULTS

Twelve patients with traumatic brain injury and 13 patients without traumatic brain injury were enrolled. Twenty-one trauma patients showed adenosine diphosphate inhibition (>30%) on thromboelastography with platelet mapping compared with the healthy volunteers who served as controls (P < .01). Mepacrine assay showed a difference in mean fluorescent intensity for all trauma patients of 4,259 ± 1,341 compared with controls of 3,143 ± 709 (P = .044), correlating with greater quantities of dense granules. Neither adenosine diphosphate inhibition nor average difference in mean fluorescent intensity between traumatic brain injury and no traumatic brain injury groups were significant (P = .2).

CONCLUSION

Trauma patients maintain their dense granule, contradicting the theory of platelet granule exhaustion as the etiology for platelet dysfunction in traumatic brain injury.

Inherited platelet function disorders. Diagnostic approach and management

Inherited platelet function disorders (IPFDs) make up a significant proportion of congenital bleeding diatheses, but they remain poorly understood and often difficult to diagnose. Therefore, a rational diagnostic approach, based on a standardized sequence of laboratory tests, with consecutive steps of increasing level of complexity, plays a crucial role in the diagnosis of most IPFDs. In this review we discuss a diagnostic approach through platelet phenotyping and genotyping and we give an overview of the options for the management of bleeding in these disorders and an account of the few systematic studies on the bleeding risk associated with invasive procedures and its treatment.

Safety and efficacy of cryopreserved autologous platelet concentrates in HLA-alloimmunized patients with hematologic malignancies

BACKGROUND

Curative chemotherapy approaches in patients with malignancies and platelet (PLT) transfusion refractoriness due to alloimmunization may be hampered by the lack of suitable PLT donors. For these patients, transfusion of cryopreserved autologous PLTs is an option, but is time- and resource-consuming. We aimed at further simplifying this process.

STUDY DESIGN AND METHODS

A retrospective single-center analysis was conducted on the transfusion of cryopreserved autologous PLTs in nine female alloimmunized, PLT transfusion-refractory patients treated for acute leukemia (n = 8) and non-Hodgkin's lymphoma (n = 1). No additional processing was used before transfusion, and most notably, washing and centrifugation steps were omitted. Clinical efficacy and safety, as well as a flow cytometric assessment of structural and functional PLT changes, were analyzed.

RESULTS

A total of 40 autologous PLT concentrates were thawed at bedside and transfused a median of 32 (range, 9 to 994) days after cryopreservation. No major bleeds and no severe dimethyl sulfoxide toxicity were observed. The median PLT count increments did not differ 1 and 18 to 24 hours after transfusion and reached 6 × 10⁹ /L (interquartile range [IQR], 3 × 10⁹ -7.5 × 10⁹ /L) and 6 × 10⁹ /L (IQR, 2.5 × 10⁹ -9.5 × 10⁹ /L), respectively. Cryopreservation resulted in partial activation of one-third of the PLTs. In vitro stimulation with strong agonists induced additional full activation of cryopreserved PLTs: median, 55% (IQR, 42%-60%) after thrombin and 39% (IQR, 36%-39%) after convulxin.

CONCLUSION

The transfusion of cryopreserved autologous PLTs is feasible and safe. Despite the cryopreservation process, PLT functionality is partially maintained.

Full activation of mouse platelets requires ADP secretion regulated by SERCA3 ATPase-dependent calcium stores

The role of the sarco-endoplasmic reticulum calcium (Ca(2+)) adenosine triphosphatase (ATPase) 3 (SERCA3) in platelet physiology remains poorly understood. Here, we show that SERCA3 knockout (SERCA3(-/-)) mice exhibit prolonged tail bleeding time and rebleeding. Thrombus formation was delayed both in arteries and venules in an in vivo ferric chloride-induced thrombosis model. Defective platelet adhesion and thrombus growth over collagen was confirmed in vitro. Adenosine 5'-diphosphate (ADP) removal by apyrase diminished adhesion and thrombus growth of control platelets to the level of SERCA3(-/-) platelets. Aggregation, dense granule secretion, and Ca(2+) mobilization of SERCA3(-/-) platelets induced by low collagen or low thrombin concentration were weaker than controls. Accordingly, SERCA3(-/-) platelets exhibited a partial defect in total stored Ca(2+) and in Ca(2+) store reuptake following thrombin stimulation. Importantly ADP, but not serotonin, rescued aggregation, secretion, and Ca(2+) mobilization in SERCA3(-/-) platelets, suggesting specificity. Dense granules appeared normal upon electron microscopy, mepacrine staining, and total serotonin content, ruling out a dense granule defect. ADP induced normal platelet aggregation, excluding a defect in ADP activation pathways. The SERCA3-specific inhibitor 2,5-di-(tert-butyl)-1,4-benzohydroquinone diminished both Ca(2+) mobilization and secretion of control platelets, as opposed to the SERCA2b inhibitor thapsigargin. This confirmed the specific role of catalytically active SERCA3 in ADP secretion. Accordingly, SERCA3-dependent Ca(2+) stores appeared depleted in SERCA3(-/-) platelets. Finally, αIIbβ3 integrin blockade did not affect SERCA3-dependent secretion, therefore proving independent of αIIbβ3 engagement. Altogether, these results show that SERCA3-dependent Ca(2+) stores control a specific ADP secretion pathway required for full platelet secretion induced by agonists at low concentration and independent of αIIbβ3.

Haematological spectrum and genotype-phenotype correlations in nine unrelated families with RUNX1 mutations from the French network on inherited platelet disorders

Background

Less than 50 patients with FPD/AML (OMIM 601309) have been reported as of today and there may an underestimation. The purpose of this study was to describe the natural history, the haematological features and the genotype-phenotype correlations of this entity in order to, first, screen it better and earlier, before leukaemia occurrence and secondly to optimize appropriate monitoring and treatment, in particular when familial stem cell transplantation is considered.

Methods

We have investigated 41 carriers of RUNX1 alteration belonging to nine unrelated French families with FPD/AML and two syndromic patients, registered in the French network on rare platelet disorders from 2005 to 2015.

Results

Five missense, one non-sense, three frameshift mutations and two large deletions involving several genes including RUNX1 were evidenced. The history of familial leukaemia was suggestive of FPD/AML in seven pedigrees, whereas an autosomal dominant pattern of lifelong thrombocytopenia was the clinical presentation of two. Additional syndromic features characterized two large sporadic deletions. Bleeding tendency was mild and thrombocytopenia moderate (>50 x10⁹/L), with normal platelet volume. A functional platelet defect consistent with a δ-granule release defect was found in ten patients regardless of the type of RUNX1 alteration. The incidence of haematological malignancies was higher when the mutated RUNX1 allele was likely to cause a dominant negative effect (19/34) in comparison with loss of function alleles (3/9). A normal platelet count does not rule out the diagnosis of FPD/AML, since the platelet count was found normal for three mutated subjects, a feature that has a direct impact in the search for a related donor in case of allogeneic haematopoietic stem cell transplantation.

Conclusions

Platelet dysfunction suggestive of defective δ-granule release could be of values for the diagnosis of FPD/AML particularly when the clinical presentation is an autosomal dominant thrombocytopenia with normal platelet size in the absence of familial malignancies. The genotype-phenotype correlations might be helpful in genetic counselling and appropriate optimal therapeutic management.

The ABCC4 membrane transporter modulates platelet aggregation

Controlling the activation of platelets is a key strategy to mitigate cardiovascular disease. Previous studies have suggested that the ATP-binding cassette (ABC) transporter, ABCC4, functions in platelet-dense granules. Using plasma membrane biotinylation and super-resolution microscopy, we demonstrate that ABCC4 is primarily expressed on the plasma membrane of both mouse and human platelets. Platelets lacking ABCC4 have unchanged dense-granule function, number, and volume, but harbor a selective impairment in collagen-induced aggregation. Accordingly, Abcc4 knockout (KO) platelet attachment to a collagen substratum was also faulty and associated with elevated intracellular cyclic AMP (cAMP) and reduced plasma membrane localization of the major collagen receptor, GPVI. In the ferric-chloride vasculature injury model, Abcc4 KO mice exhibited markedly impaired thrombus formation. The attenuation of platelet aggregation by the phosphodiesterase inhibitor EHNA (a non-ABCC4 substrate), when combined with Abcc4 deficiency, illustrated a crucial functional interaction between phosphodiesterases and ABCC4. This was extended in vivo where EHNA dramatically prolonged the bleeding time, but only in Abcc4 KO mice. Further, we demonstrated in human platelets that ABCC4 inhibition, when coupled with phosphodiesterase inhibition, strongly impaired platelet aggregation. These findings have important clinical implications because they directly highlight an important relationship between ABCC4 transporter function and phosphodiesterases in accounting for the cAMP-directed activity of antithrombotic agents.

Platelet function recovery following exposure to triple anti-platelet inhibitors using an in vitro transfusion model

INTRODUCTION

Dual anti-platelet therapy (DAPT) with aspirin and a P2Y12 antagonist is standard of care to reduce risk of thrombosis, but does not directly target thrombin-dependent platelet activation. Therefore, PAR-1 antagonist addition to DAPT (i.e., triple anti-platelet therapy; TAPT) may improve the efficacy of treatment, though at the expense of an increase in bleeding risk. Using an in vitro transfusion model, we evaluated if platelet function loss associated with TAPT can be remedied by the addition of drug-naïve platelets.

METHODS

To mimic TAPT, platelet-rich plasma (PRP) prepared from consented DAPT patients (DPRP) was incubated with a vorapaxar at therapeutic plasma levels (TPRP). To simulate platelet transfusions, TPRP was mixed with increasing proportions of drug-naïve PRP (NPRP). Platelet function recovery was assessed by light transmission aggregometry (LTA), aggregate morphology, and P-selectin expression.

RESULTS

LTA results demonstrated that 20% NPRP was required to restore the ADP aggregation response in TPRP to the response observed in DPRP and 40% NPRP recovered aggregation to >65%. Higher NPRP fractions (60%) were required to restore the platelet reactivity using TRAP-6 (SFLLRN) or arachidonic acid (AA). PAR-4 aggregation was unaffected by platelet antagonists. A decrease in single, free platelets and incorporation of mepacrine-labeled naïve platelets into aggregates occurred with increasing NPRP portions. Upon agonist activation, the surface density and percent of P-selectin positive platelets increased linearly upon addition of NPRP.

CONCLUSION

This in vitro model demonstrated that administration of drug-naïve platelets can be a useful strategy for reversing overall platelet inhibition observed with TAPT.

Current status and future prospects for platelet function testing in the diagnosis of inherited bleeding disorders

Platelets play a crucial role in haemostasis by preventing bleeding at the site of vascular injury. Several defects in platelet morphology and function have been identified and described over the years. Although a range of methodologies is available to assess platelet function, a significant proportion of subjects with bleeding symptoms and normal coagulation parameters still appear to have normal results on platelet function testing. This might suggest that the reason for bleeding is multifactorial and is due to a combination of several minor defects in platelet function and/or other parts of the haemostatic system or might indicate that the currently available platelet function tests do not provide optimal diagnostic power. This review will summarize the established platelet function tests used for diagnosing inherited platelet abnormalities in adults and children, and discuss the newly developed methodologies as well as unmet challenges and potential areas for further improvement in this field.

Granule-mediated release of sphingosine-1-phosphate by activated platelets

Sphingosine-1-phosphate (S1P) is an intracellularly generated bioactive lipid essential for development, vascular integrity, and immunity. These functions are mediated by S1P-selective cell surface G-protein coupled receptors. S1P signaling therefore requires extracellular release of this lipid. Several cell types release S1P and evidence for both plasma membrane transporter-mediated and vesicle-dependent secretion has been presented. Platelets are an important source of S1P and can release it in response to agonists generated at sites of vascular injury. S1P release from agonist-stimulated platelets was measured in the presence of a carrier molecule (albumin) using HPLC-MS/MS. The kinetics and agonist-dependence of S1P release were similar to that of other granule cargo e.g. platelet factor IV (PF4). Agonist-stimulated S1P release was defective in platelets from Unc13d(Jinx) (Munc13-4 null) mice demonstrating a critical role for regulated membrane fusion in this process. Consistent with this observation, platelets efficiently converted fluorescent NBD-sphingosine to its phosphorylated derivative which accumulated in granules. Fractionation of platelet organelles revealed the presence of S1P in both the plasma membrane and in α-granules. Resting platelets contained a second pool of constitutively releasable S1P that was more rapidly labeled by exogenously added sphingosine. Our studies indicate that platelets contain two pools of S1P that are released extracellularly: a readily-exchangeable, metabolically active pool of S1P, perhaps in the plasma membrane, and a granular pool that requires platelet activation and regulated exocytosis for release.

The flavonols quercetin and 3',4'-dihydroxyflavonol reduce platelet function and delay thrombus formation in a model of type 1 diabetes

Diabetes is associated with increased cardiovascular risk. We have recently shown that the naturally occurring flavonol quercetin (Que) or the synthetic flavonol 3',4'-dihydroxyflavonol (DiOHF) inhibits platelet function and delays thrombus formation in healthy mice. Therefore, the aim of this study was to investigate the effect of Que or DiOHF treatment on platelet function and ferric chloride-induced carotid artery thrombosis in a mouse model of type 1 diabetes. Diabetic mice treated with Que or DiOHF maintained blood flow at a significantly higher level than untreated diabetic mice at the end of the recording period. In addition, treatment with Que or DiOHF significantly reduced diabetes-induced platelet hyper-aggregability in response to platelet agonist stimulation. Furthermore, treatment with Que or DiOHF significantly inhibited dense, but not alpha, granule exocytosis in diabetic and control mice. Our demonstration that flavonols delay thrombus formation in diabetes suggests a potential clinical role for these compounds in anti-platelet therapy.

The effect of desmopressin on platelet function: a selective enhancement of procoagulant COAT platelets in patients with primary platelet function defects

DDAVP is the drug of choice for mild hemophilia A and von Willebrand disease and (by unclear mechanisms) for platelet function disorders. In vivo DDAVP selectively and markedly enhances the ability to form procoagulant platelets by enhancing intracellular Na+ and Ca2+ fluxes.

Decreased generation of procoagulant platelets detected by flow cytometric analysis in patients with bleeding diathesis

BACKGROUND

A clinically relevant bleeding diathesis is a frequent diagnostic challenge, which sometimes remains unexplained despite extensive investigations. The aim of our work was to evaluate the diagnostic utility of functional platelet testing by flow cytometry in this context.

METHODS

In case of negative results after standard laboratory workup, flow cytometric analysis (FCA) of platelet function was done. We performed analysis of surface glycoproteins Ibα, IIb, IIIa; P-selectin expression and PAC-1 binding after graded doses of ADP, collagen, and thrombin; content/secretion of dense granules; and ability to generate procoagulant platelets.

RESULTS

Of 437 patients investigated with standard tests between January 2007 and December 2011, we identified 67 (15.3%) with high bleeding scores and nondiagnostic standard laboratory workup including platelet aggregation studies. Among these patients, FCA revealed some potentially causative platelet defects: decreased dense granule content/secretion (n = 13); decreased α-granule secretion induced by ADP (n = 10), convulxin (n = 4), or thrombin (n = 3); decreased fibrinogen receptor activation induced by ADP (n = 11), convulxin (n = 11), or thrombin (n = 8); and decreased generation of COAT platelets, that is, highly procoagulant platelets induced by simultaneous activation with collagen and thrombin (n = 16).

CONCLUSION

Our work confirms that storage pool defects are frequent in patients with a bleeding diathesis and normal coagulation and platelet aggregations studies. Additionally, FCA is able to identify discrete platelet activation defects. In particular, we show for the first time that a relevant proportion of these patients has an isolated impaired ability to generate COAT platelets--a conceptually new defect in platelet procoagulant activity, which is missed by conventional laboratory workup.

Platelet function testing: state of the art

Platelet function testing has evolved from crude tests, such as the bleeding time, to tests that permit a relatively sophisticated evaluation of platelet activity. Nonetheless, these tests are hampered by lack of specificity and sensitivity, and poor standardization of methods and techniques. The bleeding time, which has long been a staple of hemostasis testing, has been dropped from the test menu at many laboratories. In its place, tests such as the Platelet Function Analyzer-100 are increasingly used to screen patients with possible bleeding disorders. Older tests, such as platelet aggregometry and lumiaggregometry, are still used frequently because they provide insight into receptor, signaling pathway and granule release mechanisms. Flow cytometry is available in some specialized laboratories and allows for quantitative and qualitative assessment of some platelet functions, although the expense of testing is often prohibitive. Finally, the wider availability of platelet function testing has stimulated interest and demand for monitoring the effect of platelet inhibitory drugs, such as aspirin and clopidogrel. As platelet function pathways become better understood, the demand for these type of monitoring tests is likely to increase.

Testing platelet function

Platelet function tests have been traditionally used to aid in the diagnosis and management of patients with bleeding problems. Given the role of platelets in atherothrombosis, several dedicated platelet function instruments are now available that are simple to use and can be used as point-of-care assays. These can provide rapid assessment of platelet function within whole blood without the requirement of sample processing. Some tests can be used to monitor antiplatelet therapy and assess risk of bleeding and thrombosis, although current guidelines advise against this. This article discusses the potential utility of tests/instruments that are available.

The Utility of Platelet and Coagulation Testing of Antithrombotics: Fusing Science with Patient Care

[Table: see text] There is an increasing need for the standardization of platelet function and coagulation testing for the assessment of antithrombotic therapies. Investigators continue to strive to identify ideal laboratory testing and monitoring procedures for acquired and inherited platelet function defects as well as for evaluating patient status when treated with existing or emerging antithrombotics. These therapies are used primarily in the treatment of ischemic complications. In patients receiving antithrombotic therapy, the balance between hemostasis and thrombosis is a challenge as there is an ongoing risk for bleeding when patients are receiving antiplatelet agents or anticoagulants to lessen their risk for secondary thrombotic events. There are several diverse tests for monitoring anticoagulant therapy; however, as new agents are developed, more specific tests will be required to directly assess these agents in relationship to overall coagulation status. Research in the platelet biology field is ongoing to provide point-of-care methodologies for the assessment of platelet reactivity in terms of both bleeding and thrombosis risk. Currently there are no instruments that reliably assess the risk of bleeding. The challenges that routinely faced are the complexity of physiology, the need for standardization of platelet testing methodology, and the necessity for appropriate interpretation of the test results.

Flow cytometry in hematological disorders

Flow cytometry with its rapidly increasing applications is being used essentially in all fields of diagnostic medicine. In hematological disorders it is most commonly used in diagnosis, characterization, prognostication and even selecting target therapy of acute leukemia and to some extent lymphomas. It is increasingly finding place in other fields of hematology i.e., non-malignant disorders of all blood cell types including RBCs and platelets along with leukocytes. In this review the authors have discussed some of these applications with an emphasis on disorders specific to pediatric patients.

Determination of ATP and ADP Secretion from Human and Mouse Platelets by an HPLC Assay

BACKGROUND

Secretion of ADP and ATP is an essential prerequisite for platelet aggregation. Impaired nucleotide secretion can cause aggregation defects and increased bleeding risk. Quantitative determination of platelet nucleotide content and exocytosis is thus of importance for the characterization and diagnosis of bleeding phenotypes. For transgenic animal models with hemostatic defects analysis of potential secretion defects is as well imperative.

METHODS

Supernatants of washed platelets and platelet-rich plasma were analyzed by HPLC for ADP and ATP concentration. Calibration of the HPLC data was accomplished with an internal standard compensating for loss of analyte, detection sensitivity, and interference of the biomatrix.

RESULTS

HPLC analysis of nucleotide secretion was carried out with human and mouse platelets. Detection limits were determined for washed platelet and platelet-rich plasma samples. In the physiological concentration range linearity with respect to the peak area is maintained.

CONCLUSION

The method combines reasonable sensitivity with robustness. The internal standard ensures reliable quantification of nucleotide concentrations even in presence of otherwise interfering substances. The low sample consumption renders possible the application to analysis of small samples like in mouse experiments.