In vivo tracking of platelets: circulating degranulated platelets rapidly lose surface P-selectin but continue to circulate and function

Laboratory diagnostics of inherited platelet disorders

Abstract Inherited platelet disorders (IPDs) are the general and common denomination of a broad number of different rare and congenital pathologies affecting platelets. Even if these disorders are characterized by widely heterogeneous clinical presentations, all of them are commonly present as defects in hemostasis. Platelet number and/or function are affected by a wide spectrum of severity. IPDs might be associated with defects in bone marrow megakaryocytopoiesis and, rarely, with somatic defects. Although in the last few years new insights in the genetic bases and pathophysiology of IPDs have greatly improved our knowledge of these disorders, much effort still needs to be made in the field of laboratory diagnosis. This review discusses the laboratory approach for the differential diagnosis of the most common IPDs, suggesting a common multistep flowchart model which starts from the simpler test (platelet count) ending with the more selective and sophisticated analyses.

Platelet transfusion - the new immunology of an old therapy

Platelet transfusion has been a vital therapeutic approach in patients with hematologic malignancies for close to half a century. Randomized trials show that prophylactic platelet transfusions mitigate bleeding in patients with acute myeloid leukemia. However, even with prophylactic transfusions, as many as 75% of patients, experience hemorrhage. While platelet transfusion efficacy is modest, questions and concerns have arisen about the risks of platelet transfusion therapy. The acknowledged serious risks of platelet transfusion include viral transmission, bacterial sepsis, and acute lung injury. Less serious adverse effects include allergic and non-hemolytic febrile reactions. Rare hemolytic reactions have occurred due to a common policy of transfusing without regard to ABO type. In the last decade or so, new concerns have arisen; platelet-derived lipids are implicated in transfusion-related acute lung injury after transfusion. With the recognition that platelets are immune cells came the discoveries that supernatant IL-6, IL-27 sCD40L, and OX40L are closely linked to febrile reactions and sCD40L with acute lung injury. Platelet transfusions are pro-inflammatory, and may be pro-thrombotic. Anti-A and anti-B can bind to incompatible recipient or donor platelets and soluble antigens, impair hemostasis and thus increase bleeding. Finally, stored platelet supernatants contain biological mediators such as VEGF and TGF-β1 that may compromise the host versus tumor response. This is particularly of concern in patients receiving many platelet transfusions, as for acute leukemia. New evidence suggests that removing stored supernatant will improve clinical outcomes. This new view of platelets as pro-inflammatory and immunomodulatory agents suggests that innovative approaches to improving platelet storage and pre-transfusion manipulations to reduce toxicity could substantially improve the efficacy and safety of this long-employed therapy.

Bioprofiling of platelets in medicated patients with depression

BACKGROUND

Changes in platelet bioactivity and aggregation are of interest when studying patients with depression as this could help to explain the statistically observed association of depression and chronic somatic, especially cardiovascular disease. This link could potentially be mediated through serotonergic signaling or immunological changes.

METHODS

38 medicated patients with major depressive disorder (MDD) and 30 mentally healthy controls, both without a diagnosis of cardiovascular disease, were included in this naturalistic study. Demographic and psychometric data were obtained. Platelet aggregability was measured by PFA-100 and bioactive compounds and serotonin levels were quantified in platelet sonicate.

RESULTS

The comparison of patients with controls revealed no changes in platelet aggregability, but significant differences in platelet content of several bioactive compounds. In a second analysis, patients were grouped according to the receptors and transporters influenced by their medication and again compared to controls. A significant effect of MDD was found for platelet content of serotonin, CD40L, interleukin-1β, and platelet factor-4, independent of medication. These markers can thus be classified as sensitive to MDD. The effect of medication on platelet parameters was also evaluated. Platelet content of matrix metalloproteinase-2 and β-thromboglobulin was normalized in MDD patients by medication acting on the serotonin transporter.

LIMITATIONS

Owing to the naturalistic study design, patients were on a variety of different medications and combination therapies. This was accounted for by a novel analysis method.

CONCLUSION

Platelet serotonin levels and content of immunomodulatory compounds are significantly altered in patients with MDD, even if treatment effects are taken into account.

Expression of surface platelet receptors (CD62P and CD41/61) in horses with recurrent airway obstruction (RAO)

Recurrent airway obstruction (RAO) is an allergic disease of horses similar to human asthma, which is characterized by airway inflammation and activation of neutrophils, lymphocytes and platelets. Platelet activation and an increase in circulating platelet-leukocyte aggregates may lead to airway remodeling. The aim of this study was to investigate platelet status in RAO-affected horses based on the platelet morphology and platelet surface expression of CD41/61 and CD62P. Ten RAO-affected horses and ten healthy horses were included in this study. Blood samples were obtained to determine the platelet count (PLT), mean platelet volume (MPV) and platelet large cell ratio (P-LCR). Expression of CD62P and CD41/61 was detected by flow cytometry on activated platelets. The median PLT was significantly reduced in horses with RAO compared to the controls. The MPV and the P-LCR values were significantly higher in RAO horses than controls. Expression of CD41/61 on platelets was increased in RAO horses, while CD62P expression was reduced. This study demonstrated the morphological changes in platelets and expression of platelet surface receptors. Despite the decrease of CD62P expression, the observed increased surface expression of CD41/61 on platelets in horses with RAO may contribute to the formation of platelet aggregates in their respiratory system.

Critical role of platelet glycoprotein ibα in arterial remodeling

OBJECTIVE

Arteriogenesis is strongly dependent on the recruitment of leukocytes, especially monocytes, into the perivascular space of growing collateral vessels. On the basis of previous findings that platelets are central players in inflammatory processes and mediate the recruitment of leukocytes, the aim of this study was to assess the role of platelets in a model of arterial remodeling.

APPROACH AND RESULTS

C57Bl6 wild-type mice, IL4-R/Iba mice lacking the extracellular domain of the glycoprotein Ibα (GPIbα) receptor, and mice treated with antibodies to block GPIbα or deplete circulating platelets were studied in peripheral arteriogenesis. Using a novel model of intravital 2-photon and epifluorescence imaging, we visualized and quantified the interaction of platelets with leukocytes and the vascular endothelium in vivo. We found that transient platelet adhesion to the endothelium of collateral vessels was a major event during arteriogenesis and depended on GPIbα. Furthermore, leukocyte recruitment was obviously affected in animals with defective platelet GPIbα function. In IL4-R/Iba mice, transient and firm leukocyte adhesion to the endothelium of collateral vessels, as well as leukocyte accumulation in the perivascular space, were significantly reduced. Furthermore, we detected platelet-leukocyte aggregates within the circulation, which were significantly reduced in IL4-R/Iba animals. Finally, platelet depletion and loss of GPIbα function resulted in poor reperfusion recovery as determined by laser Doppler imaging.

CONCLUSIONS

Thus, GPIbα-mediated interactions between platelets and endothelial cells, as well as leukocytes, support innate immune cell recruitment and promote arteriogenesis-establishing platelets as critical players in this process.

The effect of cyanosis on perioperative platelet function as measured by multiple electrode aggregometry and postoperative blood loss in neonates and infants undergoing cardiac surgery

OBJECTIVES

Platelet dysfunction is one of the major haematological disturbances of cardiopulmonary bypass (CPB). In addition, cyanosis is known to cause further coagulation disturbances.

METHODS

We prospectively studied 110 children under 1 year of age for the effects of cyanosis on baseline platelet aggregation, the time course of function on cardiopulmonary bypass, the effect on chest tube drainage (CTD) and the transfusion requirements. Using multiple electrode aggregometry (MULTIPLATE™) with the activators adenosine diphosphate (ADP) and thrombin-related activation peptide (TRAP), platelet aggregation was assessed and examined for predictive value.

RESULTS

Neonates under 30 days of age (n = 51) and infants (n = 59) were separated for analysis. Cyanosis had no significant effect on platelet function during the first 24 h after surgery. Similarly, there was no association to perioperative platelet function, CTD or exposures to blood products. ADP after protamine correlated significantly with the total number of exposures for neonates and infants and CTD at 6 h in the newborn group. Upon intensive care unit admission, ADP values correlated to the total number of exposures to blood products. No other platelet function value was able to clinically predict CTD or subsequent blood transfusion requirements.

CONCLUSIONS

In our study population, we observed no clinically significant effect of cyanosis on baseline and the perioperative course of platelet function, CTD and the number of exposures to blood products. Therefore, children under 1 year of age do not require a different approach with regard to platelet transfusions, independent of cyanosis. Clinically, platelet function was not a reliable predictor of CTD or blood transfusion requirements.

Endothelial C-type natriuretic peptide maintains vascular homeostasis

The endothelium plays a fundamental role in maintaining vascular homeostasis by releasing factors that regulate local blood flow, systemic blood pressure, and the reactivity of leukocytes and platelets. Accordingly, endothelial dysfunction underpins many cardiovascular diseases, including hypertension, myocardial infarction, and stroke. Herein, we evaluated mice with endothelial-specific deletion of Nppc, which encodes C-type natriuretic peptide (CNP), and determined that this mediator is essential for multiple aspects of vascular regulation. Specifically, disruption of CNP leads to endothelial dysfunction, hypertension, atherogenesis, and aneurysm. Moreover, we identified natriuretic peptide receptor-C (NPR-C) as the cognate receptor that primarily underlies CNP-dependent vasoprotective functions and developed small-molecule NPR-C agonists to target this pathway. Administration of NPR-C agonists promotes a vasorelaxation of isolated resistance arteries and a reduction in blood pressure in wild-type animals that is diminished in mice lacking NPR-C. This work provides a mechanistic explanation for genome-wide association studies that have linked the NPR-C (Npr3) locus with hypertension by demonstrating the importance of CNP/NPR-C signaling in preserving vascular homoeostasis. Furthermore, these results suggest that the CNP/NPR-C pathway has potential as a disease-modifying therapeutic target for cardiovascular disorders.

Platelets and their chemokines in atherosclerosis-clinical applications

The concept of platelets as important players in the process of atherogenesis has become increasingly accepted due to accumulating experimental and clinical evidence. Despite the progress in understanding the molecular details of atherosclerosis, particularly by using animal models, the inflammatory and thrombotic roles of activated platelet s especially in the human system remain difficult to dissect, as often only the complications of atherosclerosis, i.e., stroke and myocardial infarction are definable but not the plaque burden. Platelet indices including platelet count and mean platelet volume (MPV) and soluble mediators released by activated platelets are associated with atherosclerosis. The chemokine CXCL4 has multiple atherogenic activities, e.g., altering the differentiation of T cells and macrophages by inhibiting neutrophil and monocyte apoptosis and by increasing the uptake of oxLDL and synergizing with CCL5. CCL5 is released and deposited on endothelium by activated platelets thereby triggering atherogenic monocyte recruitment, which can be attenuated by blocking the corresponding chemokine receptor CCR5. Atheroprotective and plaque stabilizing properties are attributed to CXCL12, which plays an important role in regenerative processes by attracting progenitor cells. Its release from luminal attached platelets accelerates endothelial healing after injury. Platelet surface molecules GPIIb/IIIa, GP1bα, P-selectin, JAM-A and the CD40/CD40L dyade are crucially involved in the interaction with endothelial cells, leukocytes and matrix molecules affecting atherogenesis. Beyond the effects on the arterial inflammatory infiltrate, platelets affect cholesterol metabolism by binding, modifying and endocytosing LDL particles via their scavenger receptors and contribute to the formation of lipid laden macrophages. Current medical therapies for the prevention of atherosclerotic therapies enable the elucidation of mechanisms linking platelets to inflammation and atherosclerosis.

Elevated platelet activation in patients with chronic urticaria: a comparison between aspirin-intolerant and aspirin-tolerant groups

BACKGROUND

Platelets are actively involved in immune inflammatory processes that release inflammatory mediators. Platelet activation has been reported in various inflammatory diseases; however, few studies have described platelet involvement in chronic urticaria (CU).

OBJECTIVE

To investigate platelet-activation markers, namely P2Y12 receptor and P-selectin expression, and soluble P-selectin level in patients with aspirin-intolerant CU (AICU) and aspirin-tolerant CU (ATCU).

METHODS

Forty-eight patients with CU and 25 normal controls were enrolled in this study. Aspirin intolerance in patients with CU was confirmed by an oral provocation test. P2Y12 and P-selectin expressions on platelets were measured using flow cytometry; soluble P-selectin level in plasma was measured by enzyme-linked immunosorbent assay. To study the functional effects of aspirin, platelets were treated with aspirin (2 mmol/L) and the expressions of P2Y12 and P-selectin were compared between the AICU and ATCU groups.

RESULTS

The expression of P2Y12 was significantly higher in patients with CU compared with controls, whereas no significant difference was noted in the expression of P-selectin level. The levels were not significantly different according to urticaria symptom score, symptom control status, and aspirin intolerance. Soluble P-selectin level was significantly higher in the AICU group than in the ATCU group compared with controls. Aspirin did not significantly suppress P2Y12 and P-selectin expressions on platelets in the AICU group, whereas significant suppression was noted in the ATCU group.

CONCLUSION

These findings suggest that increased platelet activation contributes to skin inflammation in patients with AICU and those with ATCU. The functional difference of platelets in response to aspirin may contribute to persistent skin inflammation in patients with AICU.

Biomarkers predictive of venous thromboembolism in patients with newly diagnosed high-grade gliomas

BACKGROUND

High-grade gliomas (HGGs) are among the most prothrombotic of malignancies.

METHODS

We performed a prospective study to investigate 11 potential biomarkers for prediction of venous thromboembolism (VTE) in newly diagnosed HGG patients who had undergone a neurosurgical intervention. In addition, we tested 2 VTE risk assessment models (RAMs). The strongest predictors of VTE, which were identified by statistical forward selection, were used for the first RAM. The parameters used for the second RAM were both predictive of VTE and available in routine clinical practice.

RESULTS

One hundred forty-one HGG patients were included in this study, and 24 (17%) of them developed VTE during follow-up. An association with the risk of future VTE was found for the following parameters: leukocyte count, platelet count, sP-selectin, prothrombin-fragment 1 + 2, FVIII activity, and D-dimer. The first RAM included low platelet count (<25th percentile of the study population) and elevated sP-selectin (≥75th percentile). The cumulative VTE probability after 12 months was 9.7% for score 0 (n = 76), 18.9% for score 1 (n = 59), and 83.3% for score 2 (n = 6). The second RAM included low platelet count (<25th percentile), elevated leukocyte count, and elevated D-dimer (≥75th percentile). The probability of VTE was 3.3% for score 0 (n = 63), 23.0% for score 1 (n = 53), and 37.7% for score 2 (n = 22) or score 3 (n = 3).

CONCLUSIONS

We identified biomarkers suitable for assessing the VTE risk in newly diagnosed HGG patients. The application of 2 RAMs allowed identification of patients at high risk of developing VTE. We could also define patients at low risk of VTE, who would most probably not benefit from extended primary thromboprophylaxis.

Outer membrane vesicles alter inflammation and coagulation mediators

INTRODUCTION

Outer membrane vesicles (OMVs) were previously shown to be capable of initiating the inflammatory response seen in the transition of an infection to sepsis. However, another tenet of sepsis is the development of a hypercoagulable state and the role of OMVs in the development of this hypercoagulability has not been evaluated. The objective of this study was to evaluate the ability of OMVs to elicit endothelial mediators of coagulation and inflammation and induce platelet activation.

METHODS

Human umbilical vein endothelial cells (HUVECs) were incubated with OMVs and were analyzed for the expression of tissue factor (TF), thrombomodulin, and the adhesion molecules P-selectin and E-selectin. Supernatants of OMV-treated HUVECs were mixed with whole blood and assessed for prothrombotic monocyte-platelet aggregates (MPA).

RESULTS

OMVs induce significantly increased expression of TF, E-selectin, and P-selectin, whereas, the expression of thrombomodulin by HUVECs is significantly decreased (P < 0.05). The lipopolysaccharide inhibitor clearly inhibited the expression of E-selectin following incubation with OMVs, although its impact on TF and thrombomodulin expression was nominal. Incubation of whole blood with supernatant from HUVECs exposed to OVMs resulted in increased MPAs.

CONCLUSIONS

This study demonstrates that, at the cellular level, OMVs from pathogenic bacteria play a complex role in endothelial activation. Although OMV-bound lipopolysaccharide modulates inflammatory proteins, including E-selectin, it has a negligible effect on the tested coagulation mediators. Additionally, endothelial activation by OMVs facilitates platelet activation as indicated by increased MPAs. By influencing the inflammatory and coagulation cascades, OMVs may contribute to the hypercoagulable response seen in sepsis.

Blood-banking techniques for plateletpheresis in swine

During the past several years, trauma resuscitation in human patients has evolved from decreased use of crystalloids to increased use of blood products. Of high interest is the role of platelets in trauma resuscitation. Because conducting prehos- pital resuscitation in human trauma patients is very difficult, swine are often the animal model of choice for such studies because their coagulation and hemodynamic systems are similar to those in humans. However, consistent production of sufficient swine platelets for such studies has not previously been achieved. We developed a method for producing swine platelets by using standard human techniques and equipment. We assessed pH, pO2, pCO2, lactate, thromboelastography, and platelet aggregation over 5 d of storage to determine whether the swine platelet product met the American Association of Blood Banks (AABB) standards for transfusion. Swine platelets met AABB standards at 24 h but not at later time points. In addition, we fluorescently labeled nonautologous platelets and then measured their percentage recovery over 5 h (the time used in subsequent experimental studies) when transfused into a recipient pig. We showed that 80% of the platelets stored for 24 h remained in the circulation and increased the recipient pigs' thromboelastographic responses, indicating that the platelets were viable and active. Therefore, swine platelets stored for 24 h by using standard human products met the AABB criteria and were functional.

Long-term outcomes of rheohaemapheresis in the treatment of dry form of age-related macular degeneration

Purpose. Determining long-term effects of rheohaemapheresis on the dry form of age-related macular degeneration. Methods. This study evaluates 19 patients, average age of 67.6 years, treated with rheohaemapheresis and 18 patients, average age of 72.8 years, comprising the control group. Minimum follow up period was 3.5 years. Each treated patient received a series of 8 sessions of rheohaemapheresis of 1.5 plasma volumes within 10 weeks. We measured the drusenoid pigment epithelium detachment (DPED), best-corrected visual acuity (BCVA), electroretinography (ERG), and rheological parameters. Results. In the treatment group, the baseline BCVA was 0.74 (0.36-1.0) 95% CI and BCVA after 3.5 years was 0.79 (0.41-1.0) 95% CI (P = 0.726). In the control group, the baseline BCVA was 0.71 (0.15-1.0) 95% CI and BCVA after 3.5 years decreased to 0.7 (0.32-0.87) 95% CI (P = 0.031). Baseline DPED was 6.78 ± 3.79 mm(2); after 3.5 years, it decreased to 4.13 ± 3.84 mm(2) (P < 0.001). In the control group, the baseline DPED was 4.09 ± 3.48 mm(2); after 3.5 years, it increased to 6.69 ± 4.2 mm(2) (P = 0.001). We noted increasing levels of positive wave peaking at 50 milliseconds (P50) after treatment (P = 0.022) and a stable amplitude of photopic responses of treated patients. Conclusion. Over the long term, rheohaemapheresis reduced the DPED, improved the function of photoreceptors, and prevented the decline of BCVA.

No effect of clopidogrel activity or cessation on vascular function or markers of inflammation

The platelet adenosine diphosphate (ADP)-receptor blocker clopidogrel is effective in reducing the rate of thrombosis in cardiovascular disease, but may also have nonplatelet activity. However, there is variability in the suppression of platelet function in individuals, leading to the concept of clopidogrel resistance, that is, reduced platelet-suppressing activity. We tested the hypothesis that some of the beneficial effect of clopidogrel may be due to the variable activity of this drug on the vascular system (assessed by plasma markers von Willebrand factor and soluble E-selectin, and functional arterial pulse wave velocity) and inflammation (C-reactive protein and interleukin-6) while 32 patients with coronary artery disease taking 75 mg clopidogrel daily, and again 2 weeks after cessation of clopidogrel therapy. Platelet responsiveness to clopidogrel was assessed by the phosphorylation of intracellular regulatory protein-vasodilator-stimulated phosphoprotein method and aggregometry to ADP. Response to aspirin was assessed using arachidonic acid (AA), and soluble P-selectin and PAC-1 were also measured. While on clopidogrel, there were no relationships between any vascular or inflammatory index and the response to clopidogrel. After stopping clopidogrel, there were no differences in platelet aggregation to AA, or the expression of P-selectin or PAC-1 at rest, or after stimulation by AA, but platelet responses to ADP all increased (p < 0.01). Although soluble P-selectin increased when clopidogrel was stopped (p = 0.006), there were no changes in plasma markers or vascular function. We conclude that 75 mg/day clopidogrel has no effect of markers of vascular function or inflammation.

A review of the mechanisms and effectiveness of dietary polyphenols in reducing oxidative stress and thrombotic risk

Dietary sources of polyphenols, which are derivatives and/or isomers of flavones, isoflavones, flavonols, catechins and phenolic acids, possess antioxidant properties and therefore might be important in preventing oxidative-stress-induced platelet activation and attenuating adverse haemostatic function. Free radicals, including reactive oxygen and nitrogen species, promote oxidative stress, leading to platelet hyperactivation and the risk of thrombosis. The consumption of antioxidant/polyphenol rich foods might therefore impart anti-thrombotic and cardiovascular protective effects via their inhibition of platelet hyperactivation or aggregation. Most commonly-used anti-platelet drugs such as aspirin block the cyclooxygenase (COX)-1 pathway of platelet activation, similar to the action of antioxidants with respect to neutralising hydrogen peroxide (H2 O2 ), with a similar effect on thromboxane production via the COX-1 pathway. Polyphenols also target various additional platelet activation pathways (e.g. by blocking platelet-ADP, collagen receptors); thus alleviating fibrinogen binding to platelet surface (GPIIb-IIIa) receptors, reducing further platelet recruitment for aggregation and inhibiting platelet degranulation. As a result of the ability of polyphenols to target additional pathways of platelet activation, they may have the potential to substitute or complement currently used anti-platelet drugs in sedentary, obese, pre-diabetic or diabetic populations who can be resistant or sensitive to pharmacological anti-platelet therapy.

Comparison of platelet ultrastructure and elastic properties in thrombo-embolic ischemic stroke and smoking using atomic force and scanning electron microscopy

Thrombo-embolic ischemic stroke is a serious and debilitating disease, and it remains the second most common cause of death worldwide. Tobacco smoke exposure continues to be responsible for preventable deaths around the world, and is a major risk factor for stroke. Platelets play a fundamental role in clotting, and their pathophysiological functioning is present in smokers and stroke patients, resulting in a pro-thrombotic state. In the current manuscript, atomic force and scanning electron microscopy were used to compare the platelets of smokers, stroke patients and healthy individuals. Results showed that the elastic modulus of stroke platelets is decreased by up to 40%, whereas there is an elasticity decrease of up to 20% in smokers' platelets. This indicates a biophysical alteration of the platelets. Ultrastructurally, both the stroke patients and smokers' platelets are more activated than the healthy control group, with prominent cytoskeletal rearrangement involved; but to a more severe extent in the stroke group than in the smokers. Importantly, this is a confirmation of the extent of smoking as risk factor for stroke. We conclude by suggesting that the combined AFM and SEM analyses of platelets might give valuable information about the disease status of patients. Efficacy of treatment regimes on the integrity, cell shape, roughness and health status of platelets may be tracked, as this cell's health status is crucial in the over-activated coagulation system of conditions like stroke.

Pathogen reduction treatment alters the immunomodulatory capacity of buffy coat-derived platelet concentrates

BACKGROUND

Storage of platelet concentrates (PCs) after Mirasol pathogen reduction technology (PRT) treatment changes platelet (PLT) surface marker expression and secretion of immunomodulatory factors. Given that PLTs are known to participate in immune function, PRT may alter the way PLTs interact with the immune cells of a recipient upon transfusion. As such, the aim of this study was to assess the effects of PRT treatment on the functional ability of PLTs to interact with peripheral blood mononuclear cells (PBMNCs).

STUDY DESIGN AND METHODS

Buffy coat-derived PCs were pooled and split to obtain matched pairs. One unit was treated using the Mirasol PRT system, while the control PC remained untreated. After 5 days of storage, either the PLTs or the PLT supernatants from the PCs were cocultured with PBMNCs, with or without lipopolysaccharide (LPS). The immunomodulatory factors secreted into culture medium after coculture were examined.

RESULTS

PRT-treated PLTs and PLT supernatant significantly increased the interleukin (IL)-8 concentration, which was manifested only in the presence of LPS. Conversely, PRT-treated PLTs secreted less soluble P-selectin (sCD62P) upon coculture with PBMNCs.

CONCLUSION

PRT-treatment induced differential secretion of IL-8 and sCD62P during coculture, which may be attributed to either bioactive substances present in PLT supernatant or as a result of cell-cell interactions.

First report of elevated monocyte-platelet aggregates in healthy children

Platelets are subcellular fragments which circulate in blood and have well established roles in thrombosis and haemostasis in adults. Upon activation, platelets undergo granule exocytosis and express P-Selectin on the cell membrane which binds a ligand on monocytes, leading to monocyte-platelet aggregation. Elevated circulating monocyte-platelet aggregates in adults are linked to atherothrombosis, but have not been investigated in children where thrombosis is less common. This study aimed to measure monocyte-platelet aggregate formation in children using whole blood flow cytometry. Monocyte-platelet aggregates as well as activation and granule exocytosis of platelets were measured in healthy adults (n = 15, median age 28 years) and healthy children (n = 28, median age 7 years). Monocyte-platelet aggregates in healthy children were elevated compared to healthy adults (37.8±4.4% vs 15.5±1.9% respectively, p<0.01). However, this was not accompanied by any difference in platelet activation (PAC-1 binding 6.8±1.5% vs 6.3±2.0% respectively, p = ns) or granule exocytosis (P-selectin expression 4.4±0.5% vs 3.1±0.5% respectively, p = ns). Despite comparable numbers of platelets bound per monocyte (GPIb MFI 117.3±13.7 vs 130.9±28.6 respectively, p = ns), surface P-selectin expression per platelet-bound monocyte was lower in children compared to adults. We therefore provide the first data of elevated monocyte-platelet aggregates in healthy children.

Endothelium injury and inflammatory state during abdominal aortic aneurysm surgery: scrutinizing the very early and minute injurious effects using endothelial markers - a pilot study

INTRODUCTION

One of the most severe complications of repair surgery for abdominal aortic aneurysms (AAA) is acute kidney injury (AKI). Acute kidney injury is an inflammatory process whose pathogenesis involves endothelial cells (EC). The aim of this study was to assess the dynamics of endothelium injury markers measured during elective AAA surgery which might confirm the inflammatory character of AKI.

MATERIAL AND METHODS

The study group consisted of 14 patients with AAA. We measured plasma soluble forms of intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), E-selectin, P-selectin as well as the levels of von Willebrand factor (vWF) before, during (including intra-abdominal vein levels before and after aortic clamp removal) and within 2 days after surgery.

RESULTS

We have found a biphasic response of ICAM-1, VCAM-1 and P-selectin with an initial fall and subsequent rise. However, only VCAM-1 changes were significant compared to its baseline value. The maximum decrease of VCAM-1 was observed in the renal vein 5 min after aortic clamp removal (335.42 ±129.63 ng/ml vs. 488.90 ±169.80 ng/ml baseline value, p < 0.05), and the highest rise 48 h after aortic clamp removal (721.46 ±333.99 vs. baseline, p < 0.05).

CONCLUSIONS

Vascular cell adhesion molecule-1 turned out to be the most sensitive indicator of EC injury and inflammatory status after AAA surgery. During AAA surgery, soluble forms of P-selectin, ICAM-1 and VCAM-1 demonstrate a biphasic response with an initial fall and subsequent rise. These soluble forms could have a modulatory effect on the development of inflammation.

Biocompatibility assessment of a long-term wearable artificial pump-lung in sheep

The purpose of this study was to assess the biocompatibility of a newly developed long-term wearable artificial pump-lung (APL) in a clinically relevant ovine animal model. The wearable APL device was implanted in five sheep through left thoracotomy. The device was connected between the right atrium and pulmonary artery and evaluated for 30 days. Three sheep were used as the sham control. Platelet activation was assessed by measuring platelet surface P-selectin (CD62P) expression with flow cytometry and plasma soluble P-selectin with an enzyme-linked immunosorbent assay. Thrombotic deposition on the device components and hollow fiber membranes were analyzed with digital imaging and scanning electron microscopy. Surface P-selectin of the APL and sham groups changed significantly over the study period, but without significant differences between the two groups. Soluble P-selectin for the two groups peaked in the first 24 h after the surgery. Soluble P-selectin of the APL group remained slightly elevated over the study period compared to the presurgical baseline value and was slightly higher compared to that of the sham group. Plasma free hemoglobin remained in the normal ranges in all the animals. In spite of the surgery-related alteration in laboratory tests and elevation of platelet activation status, the APL devices in all the animals functioned normally (oxygen transfer and blood pumping) during the 30-day study period. The device flow path and membrane surface were free of gross thrombus. Electron microscopy images showed only scattered thrombi on the fibers (membrane surface and weft). In summary, the APL exhibited excellent biocompatibility. Two forms of platelet activation, surgery-related and device-induced, in the animals implanted with the wearable APL were observed. The limited device-induced platelet activation did not cause gross thrombosis and impair the long-term device performance.

New horizons in platelets flow cytometry

Platelet flow cytometry is an emerging tool in diagnostic and therapeutic hematology. It is eminently suited to study the expression of platelet surface receptors both qualitatively as well as quantitatively. It can serve as a useful marker for the documentation of in vivo platelet activation, and thus, fore-warn the risk of thromboembolism in patients with diabetes mellitus, coronary syndromes, peripheral vascular diseases, and pre-eclampsia. This technique can also be extended to study and compare the effect of various antiplatelet drugs on the level of activation of platelets and to establish any dose-effect relationship of these drugs. Topographical localization of platelet granules and study of platelet-platelet and platelet-leukocyte interaction is also possible by this procedure. All these parameters serve as pointers towards the presence of activated platelets in the circulation with its thromboembolic consequences. This is a simple reliable and cost effective technique which has a wide application in the diagnosis of various inherited and acquired platelet disorders. Study of platelet cluster of differentiation (CD) markers in various inherited disorders i.e. Bernard Soulier's disease, von Willebrand disease, Glanzman's disease, and Grey platelet syndrome may help categories the molecular lesions in these oft under-studied disorders.

Selective triggering of platelet apoptosis, platelet activation or both

Anucleate platelets perform two fundamental processes, activation and apoptosis. We elaborated an approach for selective and concurrent stimulation of platelet apoptosis and/or activation, processes important in haemostasis and platelet clearance. Human platelets were treated with BH3 mimetic ABT-737, thrombin, calcium ionophore A23187 and matched diluents. Apoptosis was determined as mitochondrial inner membrane potential (ΔΨm) depolarization and activation as P-selectin exposure. At optimal treatment conditions (90-180 min, 37°C), ABT-737 predominantly induced apoptosis, when 77-81% platelets undergo only ΔΨm depolarization. The ABT-737 impact on ΔΨm depolarization is strongly time- and temperature-dependent, and much higher at 37°C than at room temperature. In contrast, when platelets were treated with thrombin for 15-90 min at either temperature, activation-only was predominantly (79-85%) induced, whereas A23187 triggers both apoptosis and activation (73-81%) when platelets were treated for 15-60 min at 37°C or 15-90 min at room temperature. These data demonstrate that, depending on the triggering stimulus, platelets predominantly undergo ΔΨm depolarization-only, P-selectin exposure-only, or both responses, indicating that platelet apoptosis and activation are different phenomena driven by different mechanisms. The described model provides a basis for studying differential pharmacological manipulation of platelet apoptosis and activation and their role in haemostasis, thrombosis and platelet clearance.

Standardization of CD62P measurement: results of an international comparative study

BACKGROUND AND OBJECTIVES

Despite long being a mainstay in describing platelet activation via degranulation, interlaboratory variation remains an issue in measurement of membrane CD62P by flow cytometry. Our objective was to identify actions that may minimize this variation.

MATERIALS AND METHODS

Sixteen laboratories participated in an international comparative study. Two sets of platelet samples were prepared in one laboratory. Set 1 was stained and fixed; set 2 was fixed and required staining at participating laboratories. A single-staining method was used, and platelet populations were selected based on forward scatter/side scatter characteristics. Calibration beads were used to standardize measurement across different instruments.

RESULTS

There was a large discrepancy in reported CD62P values among study sites [interlaboratory coefficient of variance (CV): 36-78%]. When electronic data were re-analysed by a single analyst using a consistent gating strategy and a stable reference point, variation decreased markedly (CV < 12%), indicating a problem with isotype control samples, possibly related to sample fixation or shipment.

CONCLUSION

Consensus regarding gating strategies and use of a reliable reference point would greatly improve agreement in interlaboratory CD62P measurement.

Treatment of buffy coat platelets in platelet additive solution with the mirasol(®) pathogen reduction technology system

BACKGROUND

The Mirasol pathogen reduction technology (PRT) system uses riboflavin and ultraviolet light and is currently approved and used in Europe for the treatment of platelets and plasma. Mirasol treatment is intended to reduce the infectious pathogen load and to inactivate leukocytes in blood products. Our objective was to evaluate buffy coat platelet concentrates (BCPCs) prepared with platelet additive solution (PAS) and treated with the Mirasol system and to examine the effects on platelet cell quality during storage.

METHODS

26 BCPCs were prepared and split, creating 13 paired control and test units. The test units were treated with the Mirasol system and the platelet quality was assessed in all units over 7 days of storage.

RESULTS

All products met the incoming specifications for Mirasol treatment, and the pH of all Mirasol-treated BCPCs in PAS met the requirements of the Council of Europe guidelines throughout storage. Analysis of lactate production and glucose consumption rates, CD62p expression and cytokines indicates enhanced cellular metabolism in treated platelets, but the levels were within previously published ranges.

CONCLUSION

While Mirasol-treated BCPCs in PAS had increased metabolism and activation compared to controls, the results indicate that these units can be stored for 7 days with acceptable cell quality.

Increased Formation of Monocyte-Platelet Aggregates in Ischemic Heart Failure

Background—

Cross-talk between monocytes and platelets is reflected by the formation of monocyte-platelet aggregates (MPAs). It is not known whether MPAs are affected in heart failure (HF), and we examined differences in patients with acute HF (AHF), stable HF (SHF), stable coronary artery disease (CAD) without HF, and healthy controls (HCs).

Methods and Results—

MPAs were analyzed by flow cytometry for the 3 monocyte subsets (CD14++CD16-CCR2+ [Mon1], CD14++CD16+CCR2+ [Mon2] and CD14+CD16++CCR2– [Mon3]) in patients with AHF (n=51), SHF (n=42), stable CAD (n=44), and HCs (n=40). Counts of total MPA and MPAs associated with Mon1 and Mon2 were significantly higher in AHF compared with SHF, CAD, and HCs ( P <0.001 for all). The proportion of Mon1 aggregated with platelets was increased in AHF compared with SHF ( P =0.033), CAD ( P <0.001), and HCs ( P <0.001). A higher percentage of Mon3 aggregated with platelets was also seen in AHF compared with SHF ( P =0.012) and HCs ( P <0.001) but not compared with CAD ( P =0.647). MPAs associated with Mon2 were significantly lower in patients who experienced adverse clinical outcomes of death or rehospitalization compared with those who remained free of events ( P =0.03). Mon2 count remained an independent negative predictor of combined death and rehospitalization after adjustment for age, left ventricular ejection fraction, creatinine, and brain natriuretic peptide (hazard ratio, 0.58 [95% CI, 0.34–0.98]; P =0.043).

Conclusions—

MPA formation in patients with both acute and stable HF is increased and seems to be confined to monocytes from Mon1 and Mon2 subsets. MPAs associated with Mon2 seem to be negatively predictive of a worse prognosis in AHF.

Flow Cytometric Investigation of Classical and Alternative Platelet Activation Markers

Platelets show a substantial role in the maintenance of vascular integrity when these cells after a rapid activation adhere to the vessel wall lesion, aggregate with other platelets and leukocytes resulting in an arterial thrombosis. Analysis of in vivo platelet activation at an early time point is crucial in the detection of developing thrombotic events. In addition, the forecast of future complications as well as the evaluation of the efficacy of anti- platelet medication are also essential in a large group of patients. Changes in the levels of platelet receptors or alteration in other surface properties due to intra- and extracellular responses to a stimulus can be measurable primarily by flow cytometry with specific antibodies via the assessment of classical and alternative platelet activation markers. Some of these biomarkers have been already used in routine laboratory settings in many cases, while others still stand in the phase of research applications. Deficiency in platelet receptors is also accessible with this technique for the diagnosis of certain bleeding disorders. We here describe the most important types of platelet activation markers, and give an overview how the levels of these markers are altered in different diseases.

Platelet physiology

Platelets are cell fragments which circulate in blood. They are of pivotal importance in blood clot formation, affecting thrombosis and haemostasis. By rapidly altering the activation and expression of surface receptors, platelets are able to quickly undergo structural and phenotypic changes in response to stimulation, such as collagen exposure on injured vascular endothelium. This response to stimulation allows platelets to become adhesive, aggregate to form a thrombus, and release a variety of mediators affecting coagulation, inflammation, and chemotaxis at the site of injury. Therefore, in addition to their critical role in thrombosis and haemostasis, platelets also play a role in immunity, inflammation, wound healing, haematologic malignancies, and metabolic disorders. The role of platelets in disease, particularly in atherothrombosis, is increasingly the focus of current research and antiplatelet therapy plays a significant role in the prevention and treatment of atherothrombotic and inflammatory diseases.

Levels of soluble platelet endothelial cell adhesion molecule-1 and P-selectin are decreased in children with autism spectrum disorder

BACKGROUND

Although the etiopathology of autism spectrum disorder (ASD) is not clear, there is increasing evidence that dysfunction in the immune system affects many children with ASD. Findings of immune dysfunction in ASD include increases in inflammatory cytokines, chemokines, and microglial activity in brain tissue and cerebrospinal fluid, as well as abnormal peripheral immune cell function.

METHODS

Adhesion molecules, such as platelet endothelial adhesion molecule-1 (PECAM-1), intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), P-selectin, and L-selectin, function to facilitate leukocyte transendothelial migration. We assessed concentrations of soluble adhesion molecules, sPECAM-1, sICAM-1, sVCAM-1, sP-selectin, and sL-selectin in the plasma of 49 participants with ASD and 31 typically developing controls of the same age, all of whom were enrolled as part of the Autism Phenome Project. Behavioral assessment, the levels of soluble adhesion molecules, and head circumference were compared in the same subjects.

RESULTS

Levels of sPECAM-1 and sP-selectin were significantly reduced in the ASD group compared to typically developing controls (p < .02). Soluble PECAM-1 levels were negatively associated with repetitive behavior and abnormal brain growth in children with ASD (p = .03).

CONCLUSIONS

Because adhesion molecules modulate the permeability and signaling at the blood-brain barrier as well as leukocyte infiltration into the central nervous system, the current data suggest a role for these molecules in the complex pathophysiology of ASD.

Increased level of platelet P-selectin in nonarteritic anterior ischemic optic neuropathy

PURPOSE

P-selectin receptor is expressed in platelets and endothelial cells in a cell-activation-dependent manner. Platelet P-selectin (CD62) levels may become elevated in a number of vasoocclusive diseases, including arteriosclerosis, atherothrombosis, and diabetes mellitus (DM). Nonarteritic anterior ischemic optic neuropathy (NAION) is associated with a sudden loss of vision due to the vascular insufficiency of ciliary arteries supplying the optic nerve. In this study, our aim was to investigate the presence of increased platelet reactivity in the development of NAION.

METHODS

Twenty-one NAION patients, 39 healthy control subjects, and 44 patients suffering from diabetes mellitus (DM) were examined in our case-control, pilot study. Platelet activation was investigated by flow cytometric analysis of the mean fluorescence intensity (MFI) of CD62 on platelets. These results were compared among the different study groups.

RESULTS

NAION patients showed considerably although not significantly (p = 0.2017) higher P-selectin MFI values (71.98 ± 40.30) versus healthy subjects (55.48 ± 20.95), insulin-dependent DM patients (50.02 ± 13.08), and non-insulin-dependent DM subjects (54.72 ± 24.74). However, logistic regression analysis resulted in a statistically significant adjusted effect on the odds of NAION when CD62 MFI values were logarithmically transformed (OR: 3.86, 95 % CI: 1.10 to 13.53, p = 0.0346).

CONCLUSION

Elevated platelet CD62 positivity may be related to NAION, suggesting a possible role of enlarged platelet activity in the generation of this type of ischemic optic neuropathy.

Platelet function in health and disease: from molecular mechanisms, redox considerations to novel therapeutic opportunities

Increased oxidative stress appears to be of fundamental importance in the pathogenesis and development of several disease processes. Indeed, it is well known that reactive oxygen species (ROS) exert critical regulatory functions within the vascular wall, and it is, therefore, plausible that platelets represent a relevant target for their action. Platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, ROS may participate in the regulation of platelet activation. It is already established that enhanced ROS release from the vascular wall can indirectly affect platelet activity by scavenging nitric oxide (NO), thereby decreasing the antiplatelet properties of endothelium. On the other hand, recent data suggest that platelets themselves generate ROS, which may evoke pro-thrombotic responses, triggering many biological processes participating in atherosclerosis initiation, progression, and complication. That oxidative stress may alter platelet function is conceivable when considering that antioxidants play a role in the prevention of cardiovascular disease, although the precise mechanism accounting for changes attributable to antioxidants in atherosclerosis remains unknown. It is possible that the effects of antioxidants may be a consequence of their enhancing or promoting the antiplatelet effects of NO derived from both endothelial cells and platelets. This review focuses on current knowledge regarding ROS-dependent regulation of platelet function in health and disease, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Lamin A/C mutation is independently associated with an increased risk of arterial and venous thromboembolic complications

BACKGROUND

Lamin A/C (LMNA) mutation carriers suffer from a variety of clinical phenotypes, including dilated cardiomyopathy (DCM). Although it has been suggested that carriers are at risk for thromboembolic complications, it is unknown whether this risk is higher than can be expected from the underlying cardiac abnormalities. The purpose of this study was to determine whether a LMNA mutation is associated with an increased risk of thromboembolic complications.

METHODS

We compared a cohort of 76 LMNA mutation carriers with a cohort of 224 idiopathic DCM patients without a LMNA mutation, with respect to the prevalence of arterial and venous thromboembolic complications. Furthermore, we carried out a case-control study to explore whether a prothrombotic phenotype was present in LMNA mutation carriers without DCM or atrial tachyarrhythmias (n=14) and compared this with mutation negative relatives (n=13).

RESULTS

The prevalence of thromboembolic complications was higher in the cohort of LMNA mutation carriers than in DCM patients (22 vs 11%; p<0.05), after respectively mean follow-up of 42 ± 12 and 49 ± 12 years. After adjustment for possible confounders, including atrial tachyarrhythmias and left ventricular ejection fraction, LMNA mutation carriership was independently associated with an increased risk of thromboembolic complications (HR 4.8, 95% CI: 2.2-10.6). The results of the case-control study suggested a prothrombotic phenotype in LMNA mutation carriers, as reflected by an altered platelet function and increased thrombin generation.

CONCLUSIONS

LMNA mutation is independently associated with an increased risk of arterial and venous thromboembolic complications. Laboratory research in LMNA mutation carriers without severe cardiac abnormalities suggests a prothrombotic phenotype.

Biomarkers of platelet activation in acute coronary syndromes

The most convincing evidence for the participation of platelets in arterial thrombosis in humans comes from studies of platelet activation in patients with acute coronary syndromes (ACS) and from trials of antiplatelet drugs. Both strongly support the concept that repeated episodes of platelet activation over the thrombogenic surface of a vulnerable plaque may contribute to the risk of death from coronary causes. However, the relation of in vivo platelet activation and adverse clinical events to results of platelet function tests remains largely unknown. A valuable marker of in vivo platelet activation should be specific, unaltered by pre-analytical artefacts and reproducibly measured by easily performed methods. This article describes current biomarkers of platelet activation in ACS, reviews their advantages and disadvantages, discusses their potential pitfalls, and demonstrates emerging data supporting the positive clinical implications of monitoring in vivo platelet activation in the setting of ACS.

Platelet-Derived Microparticles and Platelet Function Profile in Children With Congenital Heart Disease

Platelet microparticles (PMPs) and function profile in children with congenital heart disease (CHD) have not been widely explored. We investigated platelet aggregation, flow cytometric platelet surface receptors (P-selectin and glycoprotein (GP) IIb/IIIa) and PMPs in 23 children with cyanotic CHD (CCHD), 30 children with acyanotic CHD (ACHD) and 30 healthy controls correlating these variables to hematological and coagulation parameters including von Willebrand factor antigen (vWF Ag) as a marker of endothelial dysfunction. Hemoglobin, hematocrit (HCT), d-dimer, and vWF Ag were significantly higher in CCHD than ACHD group. Platelet MPs and P-selectin expression were increased in patients than controls, particularly in CCHD and positively correlated to HCT, d-dimer, and vWF Ag while platelet count, aggregation, and GP IIb/IIIa expression were decreased in CCHD compared with ACHD group and negatively correlated to HCT. The overproduction of PMPs and platelet activation with suppressed aggregation may be implicated in the pathogenesis of coagulation/hemostatic abnormalities in children with CCHD.

Upregulation of CD40 ligand and enhanced monocyte-platelet aggregate formation are associated with worse clinical outcome after ischaemic stroke

The white blood cell count and mean platelet volume determined shortly after the symptom onset are known as independent predictors for clinical outcome after stroke. In the present study we sought to evaluate the prognostic value of platelet-derived inflammatory biomarkers measured prospectively after an ischaemic event. Using five-colour flow cytometry, the platelet surface expression of CD40L, CD62P and subpopulations of leukocyte-platelet aggregates were assessed in 93 stroke patients on the first (V(0)), 10th (V(1)) and 90th (V(2)) day after stroke, and once in 65 disease controls. The clinical outcome was evaluated using the Scandinavian Stroke Scale (SSS) and modified Rankin Scale (mRS) at the same time points as blood sampling and 24 months after the event. Patients with either CD40L surface expression or the percentage of monocyte-platelet aggregates (M-plt) in the third tertile (T3) at V0 had a significantly lower score on the SSS at V(1). Patients with the percentage M-plt at V(0) higher than the median value of M-plt in controls were at increased risk of SSS < 40 at V(1) (odds ratio: 2.6; 95% confidence interval [CI]: 1.4 - 8.7; p=0.006). Patients with the percentage of M-plt in T3 at V(0) showed progressive decline in survival (hazard ratio [HR]: 1.6; 95% CI: 1.1-1.9; p=0.02) and a significantly higher number of recurrent vascular events (HR: 2.64; 95% CI: 1.3-3.2; p=0.02) when compared to the first tertile. In conclusion, increased levels of M-plt could be a predictive marker for both early outcome and long-term prognosis while increased CD40L was correlated with worse clinical outcome.

Platelet activation, P-selectin, and eosinophil β1-integrin activation in asthma

RATIONALE

Eosinophil β1-integrin activation correlates inversely with FEV1 and directly with eosinophil-bound P-selectin in subjects with nonsevere allergic asthma.

OBJECTIVES

Determine the relationships between β1-integrin activation and pulmonary function or eosinophil-bound P-selectin in subjects with asthma of varying severity and discern the source of eosinophil-bound P-selectin.

METHODS

Blood was assayed by flow cytometry for P-selectin and activated β1-integrin on eosinophils and platelets. Plasma was analyzed with ELISA for soluble P-selectin, platelet factor 4, and thrombospondin-1.

MEASUREMENTS AND MAIN RESULTS

Activated β1-integrin correlated with eosinophil-bound P-selectin among all subjects with asthma even though activated β1-integrin was higher in subjects with nonsevere asthma than severe asthma. Activated β1-integrin correlated inversely with FEV1 corrected for FVC only in younger subjects with nonsevere asthma. Paradoxically, platelet surface P-selectin, a platelet activation marker, was low in subjects with severe asthma, whereas plasma platelet factor 4, a second platelet activation marker, was high. Correlations indicated that P-selectin-positive platelets complexed to eosinophils are the major source of the eosinophil-bound P-selectin associated with β1-integrin activation. After whole-lung antigen challenge of subjects with nonsevere asthma, a model of asthma exacerbation known to cause platelet activation, circulating eosinophils bearing P-selectin and activated β1-integrin disappeared.

CONCLUSIONS

The relationship between eosinophil β1-integrin activation and pulmonary function was replicated only for younger subjects with nonsevere asthma. However, we infer that platelet activation and binding of activated platelets to eosinophils followed by P-selectin-mediated eosinophil β1-integrin activation occur in both nonsevere and severe asthma with rapid movement of platelet-eosinophil complexes into the lung in more severe disease.

Platelet life span and apoptosis

Like many nucleated mammalian cells, the life and death of the anucleate platelet is regulated by Bcl-2 family proteins. Platelets depend on Bcl-x(L) for survival. Bcl-x(L) maintains platelet viability by restraining the killer protein Bak. When Bak is unleashed, it triggers classical intrinsic apoptosis by causing mitochondrial damage. The latter leads to caspase activation and phosphatidylserine (PS) exposure. Platelet apoptosis can be blocked by caspase inhibitors, or by genetic deletion of Bak and its close relative Bax. Perturbations in the platelet apoptosis program lead to changes in platelet life span in vivo. Here, we describe methods to determine platelet life span, enumerate young platelets, and measure hallmarks of platelet apoptosis, such as PS exposure, caspase activation, and mitochondrial dysfunction.

Whole blood flow cytometry measurements of in vivo platelet activation in critically-Ill patients are influenced by variability in blood sampling techniques

INTRODUCTION

Flow cytometry is often used to measure in vivo platelet activation in critically-ill patients. Variability in blood sampling techniques, which may confound these measurements, remains poorly characterized.

MATERIALS AND METHODS

Platelet activation was measured by flow cytometry performed on arterial and venous blood from 116 critically-ill patients. We determined how variability in vascular sampling site, processing times, and platelet counts influenced levels of platelet-monocyte aggregates (PMA), PAC-1 binding (for glycoprotein (GP) IIbIIIa), and P-selectin (P-SEL) expression.

RESULTS

Levels of PMA, but not PAC-1 binding or P-SEL expression, were significantly affected by variability in vascular sampling site. Average PMA levels were approximately 60% higher in whole blood drawn from an arterial vessel compared to venous blood (16.2±1.8% vs. 10.7±1.2%, p<0.05). Levels of PMA in both arterial and venous blood increased significantly during ex vivo processing delays (1.7% increase for every 10 minute delay, p<0.05). In contrast, PAC-1 binding and P-SEL expression were unaffected by processing delays. Levels of PMA, but not PAC-1 binding or P-SEL expression, were correlated with platelet count quartiles (9.4±1.6% for the lowest quartile versus 15.4±1.6% for the highest quartile, p<0.05).

CONCLUSIONS

In critically-ill patients, variability in vascular sampling site, processing times, and platelet counts influence levels of PMA, but not PAC-1 binding or P-SEL expression. These data demonstrate the need for rigorous adherence to blood sampling protocols, particularly when levels of PMA, which are most sensitive to variations in blood collection, are measured for detection of in vivo platelet activation.