Neutrophil cathepsin G promotes prothrombinase and fibrin formation under flow conditions by activating fibrinogen-adherent platelets

Platelet-leukocyte crosstalk in COVID-19: How might the reciprocal links between thrombotic events and inflammatory state affect treatment strategies and disease prognosis?

Platelet-leukocyte crosstalk is commonly manifested by reciprocal links between thrombosis and inflammation. Platelet thrombus acts as a reactive matrix that recruits leukocytes to the injury site where their massive accumulation, activation and migration promote thrombotic events while triggering inflammatory responses. As a life-threatening condition with the associations between inflammation and thrombosis, COVID-19 presents diffuse alveolar damage due to exaggerated macrophage activity and cytokine storms. These events, together with direct intracellular virus invasion lead to pulmonary vascular endothelialitis, cell membranes disruption, severe endothelial injury, and thrombosis. The developing pre-alveolar thrombus provides a hyper-reactive milieu that recruits circulating leukocytes to the injury site where their activation contributes to thrombus stabilization and thrombosis propagation, primarily through the formation of Neutrophil extracellular trap (NET). NET fragments can also circulate and deposit in further distance where they may disseminate intravascular thrombosis in severe cases of disease. Thrombi may also facilitate leukocytes migration into alveoli where their accumulation and activation exacerbate cytokine storms and tissue damage, further complicating the disease. Based on these mechanisms, whether an effective anti-inflammatory protocol can prevent thrombotic events, or on the other hand; efficient antiplatelet or anticoagulant regimens may be associated with reduced cytokine storms and tissue damage, is now of interests for several ongoing researches. Thus shedding more light on platelet-leukocyte crosstalk, the review presented here discusses the detailed mechanisms by which platelets may contribute to the pathogenesis of COVID-19, especially in severe cases where their interaction with leukocytes can intensify both inflammatory state and thrombosis in a reciprocal manner.

Assessment of Systemic Immune-inflammation Index Levels in Patients with Retinal Vein Occlusion

PURPOSE

To assess the predictive value of inflammatory markers calculated from complete blood counts in patients with retinal vein occlusion (RVO).

METHODS

This was a retrospective cross-sectional study with a total of 56 RVO patients and 56 age- and gender-matched controls involved. All subjects went through a routine ocular examination, and the peripheral venous blood samples were collected to analyze the differences in inflammatory markers between groups.

RESULTS

The systemic immune-inflammation index (SII) and neutrophil-to-lymphocyte ratio (NLR) values were significantly higher in RVO patients than those in the controls (p=0.002, p=0.004, respectively). According to the receiver operating characteristic (ROC) curve analysis, the areas under the curve (AUC) of SII was 0.666. The AUC of NLR was 0.657.

CONCLUSION

As a novel inflammatory indicator, SII is a more promising indicator than NLR and PLR in the prediction of RVO development.

The Efficacy of Platelet-Rich Plasma for Ligament Injuries: A Systematic Review of Basic Science Literature With Protocol Quality Assessment

Background:

Despite the existence of many clinical studies on platelet-rich plasma (PRP) interventions for ligamentous pathology, basic science consensus regarding the indications, mechanisms, and optimal composition of PRP for treating ligament injuries is lacking.

Purpose:

To (1) compare the efficacy of PRP in animal models of ligament injury with placebo and (2) describe the potential variability in PRP preparation using accepted classification systems.

Study Design:

Systematic review.

Methods:

The Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, PubMed, Embase, and Ovid MEDLINE were queried in April 2020 for in vivo and in vitro basic science studies regarding PRP use for ligament injury. Study design, results, PRP composition, and analyzed cellular and molecular markers were extracted, and outcomes relative to control models were documented. Bias was assessed using the SYRCLE risk-of-bias tool.

Results:

Included were 43 articles (31 in vivo and 12 in vitro studies) investigating the anterior cruciate ligament/cranial cruciate ligament (n = 32), medial collateral ligament (n = 6), suspensory ligament (n = 3), patellar ligament (n = 1), and Hock ligament (n = 1). Platelet concentration was reported in 34 studies (77.3%); leukocyte composition, in 12 (27.3%); and red blood cell counts, in 7 (15.9%). With PRP treatment, 5 of 12 in vitro studies demonstrated significant increases in cell viability, 6 of 12 in gene expression, 14 of 32 in vivo studies reported superior ligament repair via histological evaluation, and 13 in vivo studies reported superior mechanical properties. Variability in PRP preparation methods was observed across all articles, and only 1 study reported all necessary information to be classified by the 4 schemes we used to evaluate reporting. Among the in vivo studies, detection and performance bias were consistently high, whereas selection, attrition, reporting, and other biases were consistently low.

Conclusion:

Conflicting data on the cellular and molecular effects of PRP for ligament injuries were observed secondary to the finding that included studies were heterogeneous, limiting interpretation across studies and the ability to draw meaningful conclusions. Clinical trials and any causal relationship between PRP use in ligament injuries and its potential for regeneration and healing should be pursued with caution if based solely on basic science data.

COVID-19 and Venous Thromboembolism: From Pathological Mechanisms to Clinical Management

Coronavirus disease 2019 (COVID-19), which is becoming a global pandemic, is caused by SARS-CoV-2 infection. In COVID-19, thrombotic events occur frequently, mainly venous thromboembolism (VTE), which is closely related to disease severity and clinical prognosis. Compared with historical controls, the occurrence of VTE in hospitalized and critical COVID-19 patients is incredibly high. However, the pathophysiology of thrombosis and the best strategies for thrombosis prevention in COVID-19 remain unclear, thus needing further exploration. Virchow’s triad elements have been proposed as important risk factors for thrombotic diseases. Therefore, the three factors outlined by Virchow can also be applied to the formation of venous thrombosis in the COVID-19 setting. A thorough understanding of the complex interactions in these processes is important in the search for effective treatments for COVID-19. In this work, we focus on the pathological mechanisms of VTE in COVID-19 from the aspects of endothelial dysfunction, hypercoagulability, abnormal blood flow. We also discuss the treatment of VTE as well as the ongoing clinical trials of heparin anticoagulant therapy. In addition, according to the pathophysiological mechanism of COVID-19-associated thrombosis, we extended the range of antithrombotic drugs including antiplatelet drugs, antifibrinolytic drugs, and anti-inflammatory drugs, hoping to find effective drug therapy and improve the prognosis of VTE in COVID-19 patients.

Immunothrombosis in COVID-19: Implications of Neutrophil Extracellular Traps

SARS-CoV-2 is a member of the family of coronaviruses associated with severe outbreaks of respiratory diseases in recent decades and is the causative agent of the COVID-19 pandemic. The recognition by and activation of the innate immune response recruits neutrophils, which, through their different mechanisms of action, form extracellular neutrophil traps, playing a role in infection control and trapping viral, bacterial, and fungal etiological agents. However, in patients with COVID-19, activation at the vascular level, combined with other cells and inflammatory mediators, leads to thrombotic events and disseminated intravascular coagulation, thus leading to a series of clinical manifestations in cerebrovascular, cardiac, pulmonary, and kidney disease while promoting severe disease and mortality. Previous studies of hospitalized patients with COVID-19 have shown that elevated levels of markers specific for NETs, such as free DNA, MPO, and H3Cit, are strongly associated with the total neutrophil count; with acute phase reactants that include CRP, D-dimer, lactate dehydrogenase, and interleukin secretion; and with an increased risk of severe COVID-19. This study analyzed the interactions between NETs and the activation pathways involved in immunothrombotic processes in patients with COVID-19.

Neutrophils can promote clotting via FXI and impact clot structure via neutrophil extracellular traps in a distinctive manner in vitro

Neutrophils and neutrophil extracellular traps (NETs) have been shown to be involved in coagulation. However, the interactions between neutrophils or NETs and fibrin(ogen) in clots, and the mechanisms behind these interactions are not yet fully understood. In this in vitro study, the role of neutrophils or NETs on clot structure, formation and dissolution was studied with a combination of confocal microscopy, turbidity and permeation experiments. Factor (F)XII, FXI and FVII-deficient plasmas were used to investigate which factors may be involved in the procoagulant effects. We found both neutrophils and NETs promote clotting in plasma without the addition of other coagulation triggers, but not in purified fibrinogen, indicating that other factors mediate the interaction. The procoagulant effects of neutrophils and NETs were also observed in FXII- and FVII-deficient plasma. In FXI-deficient plasma, only the procoagulant effects of NETs were observed, but not of neutrophils. NETs increased the density of clots, particularly in the vicinity of the NETs, while neutrophils-induced clots were less stable and more porous. In conclusion, NETs accelerate clotting and contribute to the formation of a denser, more lysis resistant clot architecture. Neutrophils, or their released mediators, may induce clotting in a different manner to NETs, mediated by FXI.

Tissue-level inflammation and ventricular remodeling in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common cardiac condition caused primarily by sarcomeric protein mutations with several distinct phenotypes, ranging from asymmetric septal hypertrophy, either with or without left ventricular outflow tract obstruction, to moderate left ventricular dilation with or without apical aneurysm formation and marked, end-stage dilation with refractory heart failure. Sudden cardiac death can occur at any stage. The phenotypic variability observed in HCM is the end-result of many factors, including pre-load, after-load, wall stress and myocardial ischemia stemming from microvascular dysfunction and thrombosis; however, tissue level inflammation to include leukocyte-derived extracellular traps consisting of chromatin and histones, apoptosis, proliferation of matrix proteins and impaired or dysfunctional regulatory pathways contribute as well. Our current understanding of the pathobiology, developmental stages, transition from hypertrophy to dilation and natural history of HCM with emphasis on the role of tissue-level inflammation in myocardial fibrosis and ventricular remodeling is summarized.

Platelet biology of the rapidly failing lung

Acute respiratory distress syndrome (ARDS) is characterized by a rapid-onset respiratory failure with a mortality rate of approximately 40%. This physiologic inflammatory process is mediated by disruption of the alveolar-vascular interface, leading to pulmonary oedema and impaired oxygen exchange, which often warrants mechanical ventilation to increase survival in the acute setting. One of the least understood aspects of ARDS is the role of the platelets in this process. Platelets, which protect vascular integrity, play a pivotal role in the progression and resolution of ARDS. The recent substantiation of the age-old theory that megakaryocytes are found in the lungs has rejuvenated interest in and raised new questions about the importance of platelets for pulmonary function. In addition to primary haemostasis, platelets provide a myriad of inflammatory functions that are poised to aid the innate immune system. This review focuses on the evidence for regulatory roles of platelets in pulmonary inflammation, with an emphasis on two receptors, CLEC-2 and TLT-1. Studies of these receptors identify novel pathways through which platelets may regulate vascular integrity and inflammation in the lungs, thereby influencing the development of ARDS.

Procoagulant activity of bovine neutrophils incubated with conditioned media or extracellular vesicles from Histophilus somni stimulated bovine brain endothelial cells

Histophilus somni is a Gram negative coccobacillus that causes respiratory, reproductive and central nervous system disease in cattle. The hallmark of H. somni infection is diffuse vasculitis and intravascular thrombosis that can lead to an acute central nervous system disease known as thrombotic meningoencephalitis (TME). Because neutrophils are major players in the pathophysiology of septic meningitis, we sought to determine their role in H. somni-induced fibrin clot formation in vitro. Bovine brain endothelial cells (TBBE cells) were exposed to H. somni cells at a 1:25 ratio, respectively. Conditioned media (CM) were collected after a 6 h incubation at 37 °C with 5% CO₂, and then incubated with bovine peripheral blood polymorphonuclear neutrophils (PMNs). Following incubation, fibrin clot formation and tissue factor activity were assessed by a re-calcified plasma clotting assay. We found greater tissue factor activity in cell lysates and CM from H. somni-stimulated TBBE cells than unstimulated control TBBE cells. In addition, PMNs exposed to CM or extracellular vesicles from H. somni-stimulated TBBE cells expressed von Willenbrand factor, exhibited increased fibrin clot formation, and displayed greater tissue factor activity than PMNs exposed to CM or extracellular vesicles from unstimulated control TBBE cells. These results suggest that bovine PMNs might acquire extracellular vesicles from endothelial cells that leads to thrombus formation in bovine brain microvasculature and contribute to the process that characterizes TME.

Neutrophil α-defensins promote thrombosis in vivo by altering fibrin formation, structure, and stability

Inflammation and thrombosis are integrated, mutually reinforcing processes, but the interregulatory mechanisms are incompletely defined. Here, we examined the contribution of α-defensins (α-defs), antimicrobial proteins released from activated human neutrophils, on clot formation in vitro and in vivo. Activation of the intrinsic pathway of coagulation stimulates release of α-defs from neutrophils. α-Defs accelerate fibrin polymerization, increase fiber density and branching, incorporate into nascent fibrin clots, and impede fibrinolysis in vitro. Transgenic mice (Def++) expressing human α-Def-1 developed larger, occlusive, neutrophil-rich clots after partial inferior vena cava (IVC) ligation than those that formed in wild-type (WT) mice. IVC thrombi extracted from Def++ mice were composed of a fibrin meshwork that was denser and contained a higher proportion of tightly packed compressed polyhedral erythrocytes than those that developed in WT mice. Def++ mice were resistant to thromboprophylaxis with heparin. Inhibiting activation of the intrinsic pathway of coagulation, bone marrow transplantation from WT mice or provision of colchicine to Def++ mice to inhibit neutrophil degranulation decreased plasma levels of α-defs, caused a phenotypic reversion characterized by smaller thrombi comparable to those formed in WT mice, and restored responsiveness to heparin. These data identify α-defs as a potentially important and tractable link between innate immunity and thrombosis.

Role of platelets in neutrophil extracellular trap (NET) production and tissue injury

In addition to their well-known role as the cellular mediator of thrombosis, numerous studies have identified key roles for platelets during various disease processes. Importantly, platelets play a critical role in the host immune response, directly interacting with, and eliminating pathogens, from the blood stream. In addition to pathogen clearance, platelets also contribute to leukocyte recruitment at sites of infection and inflammation, and modulate leukocyte activity. Platelet interaction with activated neutrophils is a potent inducer of neutrophil extracellular trap (NET). NETs consist of a diffuse, sticky web of extracellular DNA, nuclear and granular proteins, and serve to ensnare and kill pathogens. In addition to catching pathogens, the cytotoxic molecules and proteases on NETs have the potential to inflict significant tissue damage. Additionally, NET components have been suggested to be key activators of infection-induced coagulopathy. These critical roles, at the interface between hemostasis and immunity, highlight the need for balance in the platelet response; too little platelet activity results in bleeding and immune deficit, too much leads to tissue pathogenesis. In this review, we highlight recent advances in our understanding of the role platelets play in inflammation, the link between platelets and NETs and the role platelets play in disease pathogenesis.

Functional characterization of tissue factor in von Willebrand factor-dependent thrombus formation under whole blood flow conditions

Von Willebrand factor (VWF) plays an important role in mediating platelet adhesion and aggregation under high shear rate conditions. Such platelet aggregates are strengthened by fibrin-network formation triggered by tissue factor (TF). However, little is known about the role of TF in VWF-dependent thrombus formation under blood flow conditions. We evaluated TF in thrombus formation on immobilized VWF under whole blood flow conditions in an in vitro perfusion chamber system. Surface-immobilized TF amplified intra-thrombus fibrin generation significantly under both low and high shear flow conditions, while TF in sample blood showed no appreciable effects. Furthermore, immobilized TF enhanced VWF-dependent platelet adhesion and aggregation significantly under high shear rates. Neutrophil cathepsin G and elastase increased significantly intra-thrombus fibrin deposition on immobilized VWF-TF complex, suggesting the involvement of leukocyte inflammatory responses in VWF/TF-dependent mural thrombogenesis under these flow conditions. These results reveal a functional link between VWF and TF under whole blood flow conditions, in which surface-immobilized TF and VWF mutually contribute to mural thrombus formation, which is essential for normal hemostasis. By contrast, TF circulating in blood may be involved in systemic hypercoagulability, as seen in sepsis caused by severe microbial infection, in which neutrophil inflammatory responses may be active.

Do neutrophil extracellular traps contribute to the heightened risk of thrombosis in inflammatory diseases?

Thrombotic events, both arterial and venous, are a major health concern worldwide. Further, autoimmune diseases, such as systemic lupus erythematosus, anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis, and antiphospholipid syndrome, predispose to thrombosis, and thereby push the risk for these morbid events even higher. In recent years, neutrophils have been identified as important players in both arterial and venous thrombosis. Specifically, chromatin-based structures called neutrophil extracellular traps (NETs) play a key role in activating the coagulation cascade, recruiting platelets, and serving as scaffolding upon which the thrombus can be assembled. At the same time, neutrophils and NETs are emerging as important mediators of pathogenic inflammation in the aforementioned autoimmune diseases. Here, we first review the general role of NETs in thrombosis. We then posit that exaggerated NET release contributes to the prothrombotic diatheses of systemic lupus erythematosus, ANCA-associated vasculitis, and antiphospholipid syndrome.

Prognostic Value of the Percentage of Neutrophils on Admission in Patients with ST-elevated Myocardial Infarction Undergoing Primary Percutaneous Coronary Intervention

BACKGROUND AND AIMS

An elevated neutrophil count or neutrophil/lymphocyte ratio on admission has been reported to be an independent predictor of adverse cardiac events in patients with acute coronary syndrome (ACS). The relationship between the percentage of neutrophils (N%) at the time of admission and the long-term outcomes in patients with ST-segment elevated myocardial infarction (STEMI) who have undergone primary percutaneous coronary intervention (PCI) remains unclear. The aim of this study was to investigate the usefulness of the admission N% in predicting long-term mortality in patients with STEMI who were undergoing primary PCI.

METHODS

We evaluated 701 consecutive patients admitted to nine medical institutions in northwest China within 24 h after symptom onset from January 1, 2009-December 31, 2011. Using a receiver-operating characteristic analysis, N% ≥82.1% was the best predictor of long-term mortality. Patients were divided into two groups according to this criterion. Mean follow-up time was 39.03 months.

RESULTS

The long-term all-cause mortality rate was significantly higher in patients with a high N% level (7.17 vs. 3.11%, p = 0.015) as was the rate of cardiac mortality (6.48 vs. 2.59%, p = 0.013). In a multivariate logistic analysis, a high N% level was an independent predictor of long-term all-cause mortality (odds ratio 2.59, 95% confidence interval 1.21-5.53, p = 0.002) and long-term cardiac mortality (odds ratio 2.79, 95% confidence interval 1.24-6.28, p = 0.013).

CONCLUSIONS

A high N% level on admission is an independent predictor of long-term mortality in STEMI patients undergoing primary PCI.

White blood cell counts, leukocyte ratios, and eosinophils as inflammatory markers in patients with coronary artery disease

Inflammation is a key feature of atherosclerosis and its clinical manifestations. The leukocyte count has emerged as a marker of inflammation that is widely available in clinical practice. Since inflammation plays a key role in atherosclerosis and its end results, discovering new biomarkers of inflammation becomes important in order to help diagnostic accuracy and provide prognostic information about coronary cardiac disease. In acute coronary syndromes and percutaneous coronary intervention, elevated levels of almost all subtypes of white blood cell counts, including eosinophils, monocytes, neutrophils, and lymphocytes, and neutrophil-lymphocyte ratio and eosinophil-leukocyte ratio constitute independent predictors of adverse outcomes. Eosinophil count and eosinophil-leukocyte ratio, in particular, emerge as novel biomarkers for risk stratification in patients with coronary artery disease. Since the presence of eosinophils denotes hypersensitivity inflammation and hypersensitivity associated with Kounis syndrome, this reality is essential for elucidating the etiology of inflammation in order to consider predictive and preventive measures and to apply the appropriate therapeutic methods.

Relation between white blood cell count and final infarct size in patients with ST-segment elevation acute myocardial infarction undergoing primary percutaneous coronary intervention (from the INFUSE AMI trial)

Although it has been shown that elevated white blood cell count (WBCc) on presentation is associated with an increased risk of cardiac mortality in patients with ST-segment elevation myocardial infarction (STEMI), the responsible mechanisms are unknown. We therefore sought to investigate whether elevated WBCc is associated with increased infarct size measured with cardiac magnetic resonance imaging 30 days after primary percutaneous coronary intervention in the Intracoronary Abciximab and Aspiration Thrombectomy in Patients With Large Anterior Myocardial Infarction trial. INFUSE AMI randomized patients with STEMI and proximal or mid-left anterior descending coronary artery occlusion to bolus intracoronary abciximab versus no abciximab and to manual aspiration versus no aspiration. WBCc at hospital admission was available in 407 of 452 randomized patients. Patients were stratified according to tertiles of WBCc. At 30 days, a significant stepwise increase in infarct size (percentage of total left ventricular mass) was apparent across tertiles of increasing WBCc (median [interquartile range] for tertiles I vs II vs III = 11.2% [3.8% to 19.6%] vs 17.5% [0.5% to 22.9%] vs 19.1% [13.7 to 26.0], respectively, p <0.0001). Absolute infarct mass in grams and abnormal wall motion score were also significantly increased across tertiles of WBC. By multivariate linear regression analysis, WBCc was an independent predictor of infarct size along with intracoronary abciximab randomization, age, time from symptom onset to first device, proximal left anterior descending location, and baseline TIMI flow of 0/1. In conclusion, in patients with anterior wall STEMI, an elevated admission WBCc is a powerful independent predictor of infarct size measured with cardiac magnetic resonance imaging 30 days after primary percutaneous coronary intervention.

Cathepsin G-dependent modulation of platelet thrombus formation in vivo by blood neutrophils

Neutrophils are consistently associated with arterial thrombotic morbidity in human clinical studies but the causal basis for this association is unclear. We tested the hypothesis that neutrophils modulate platelet activation and thrombus formation in vivo in a cathepsin G-dependent manner. Neutrophils enhanced aggregation of human platelets in vitro in dose-dependent fashion and this effect was diminished by pharmacologic inhibition of cathepsin G activity and knockdown of cathepsin G expression. Tail bleeding time in the mouse was prolonged by a cathepsin G inhibitor and in cathepsin G knockout mice, and formation of neutrophil-platelet conjugates in blood that was shed from transected tails was reduced in the absence of cathepsin G. Bleeding time was highly correlated with blood neutrophil count in wildtype but not cathepsin G deficient mice. In the presence of elevated blood neutrophil counts, the anti-thrombotic effect of cathepsin G inhibition was greater than that of aspirin and additive to it when administered in combination. Both pharmacologic inhibition of cathepsin G and its congenital absence prolonged the time for platelet thrombus to form in ferric chloride-injured mouse mesenteric arterioles. In a vaso-occlusive model of ischemic stroke, inhibition of cathepsin G and its congenital absence improved cerebral blood flow, reduced histologic brain injury, and improved neurobehavioral outcome. These experiments demonstrate that neutrophil cathepsin G is a physiologic modulator of platelet thrombus formation in vivo and has potential as a target for novel anti-thrombotic therapies.

Neutrophil and platelet complexes and their relevance to neutrophil recruitment and activation

The manifestation of platelet 'satallitism' around neutrophils in whole blood is a long acknowledged phenomenon [1]. Circulating platelet-neutrophil complexes (PNC) occur in a diverse range of inflammatory disorders and infections that affect numerous organs of the body. Animal models have revealed that the formation of PNC is required for the recruitment of neutrophils to inflamed tissue, since platelets 'prime' neutrophils for efficient adhesion to vascular endothelium via the up-regulation of integrins and enhanced responsiveness to chemokines (Fig. 1). Perhaps surprisingly, the surface contact between platelets and neutrophils additionally enhances other neutrophil functions, such as chemotaxis that is required for migration into tissues, trans-cellular production of eicosanoids, phagocytosis and trapping of pathogens, increased respiratory burst leading to the production of reactive oxygen species (ROS), and modulation of neutrophil apoptosis (Fig. 1). Platelet P-selectin appears to have a particular role in enhancing the majority of these activities, and the influence of platelet P-selectin is not therefore confined to the initial rolling events in the process of neutrophil extravasation.

Microfluidics and coagulation biology

The study of blood ex vivo can occur in closed or open systems, with or without flow. Microfluidic devices, which constrain fluids to a small (typically submillimeter) scale, facilitate analysis of platelet function, coagulation biology, cellular biorheology, adhesion dynamics, and pharmacology and, as a result, can be an invaluable tool for clinical diagnostics. An experimental session can accommodate hundreds to thousands of unique clotting, or thrombotic, events. Using microfluidics, thrombotic events can be studied on defined surfaces of biopolymers, matrix proteins, and tissue factor, under constant flow rate or constant pressure drop conditions. Distinct shear rates can be generated on a device using a single perfusion pump. Microfluidics facilitated both the determination of intraluminal thrombus permeability and the discovery that platelet contractility can be activated by a sudden decrease in flow. Microfluidic devices are ideal for multicolor imaging of platelets, fibrin, and phosphatidylserine and provide a human blood analog to mouse injury models. Overall, microfluidic advances offer many opportunities for research, drug testing under relevant hemodynamic conditions, and clinical diagnostics.

Baseline white blood cell count is an independent predictor of long-term cardiovascular mortality in patients with non-ST-segment elevation acute coronary syndrome, but it does not improve the risk classification of the GRACE score

OBJECTIVES

To investigate the prognostic significance of baseline white blood cell count (WBCc) in non-ST-segment elevation acute coronary syndrome (NSTE-ACS) and its additive predictive value beyond the Global Registry of Acute Coronary Events (GRACE) score.

METHODS

We included 1,315 consecutive NSTE-ACS patients. Patients were divided in quartiles according to the WBCc (cells per 1 mm(3)) i.e. Q1 <6,850, Q2 = 6,850-8,539, Q3 = 8,540-10,857 and Q4 ≥10,858. The study end point was 3-year cardiovascular death (CVD).

RESULTS

The median age of the study population was 76 years. Overall, 335 patients (25.5%) died with 211 of these (16%) suffering from CVD. Patients in Q4 showed a higher cumulative probability of CVD compared to patients in Q1-Q3. On multivariable analysis, patients in Q4 were at higher risk of CVD [hazard ratio (HR) = 1.47, 95% confidence interval (CI) 1.09-1.98, p = 0.011]. WBCc as a continuous variable was also independently associated with the study end point (HR = 1.043; 95% CI 1.02-1.07; p = 0.001). However, the incorporation of WBCc into the GRACE score did not improve either prediction of risk (C-index = 0.796 for GRACE score with or without WBCc) or classification of risk [relative integrated discrimination improvement = 0.0154, 95% CI) -0.029 to 0.0618; continuous net reclassification improvement = -0.0676, 95% CI -0.2149-0.0738).

CONCLUSIONS

WBCc was an independent predictor of 3-year CVD in patients with NSTE-ACS. However, it did not add prognostic information beyond the GRACE score.

Platelet-leukocyte crosstalk: Linking proinflammatory responses to procoagulant state

Platelet activation is known to be associated with the release of a vast array of chemokines and proinflammatory lipids which induce pleiotropic effects on a wide variety of tissues and cells, including leukocytes. During thrombosis, the recruitment of leukocytes to activated platelets is considered an important step which not only links thrombosis to inflammatory responses but may also enhance procoagulant state. This phenomenon is highly regulated and influenced by precise mutual interactions between the cells at site of vascular injury and thrombi formation. Platelet-leukocyte interaction involves a variety of mediators including adhesion molecules, chemokines and chemoattractant molecules, shed proteins, various proinflammatory lipids and other materials. The current review addresses the detailed mechanisms underlying platelet-leukocyte crosstalk. This includes their adhesive interactions, transcellular metabolisms, induced tissue factor activity and neutrophil extracellular traps formation as well as the impacts of these phenomena in modulation of the proinflammatory and procoagulant functions in a reciprocal manner that enhances the physiological responses.

Relipidated tissue factor linked to collagen surfaces potentiates platelet adhesion and fibrin formation in a microfluidic model of vessel injury

Microfluidic devices allow for the controlled perfusion of human or mouse blood over defined prothrombotic surfaces at venous and arterial shear rates. To mimic in vivo injuries such a plaque rupture, the need exists to link lipidated tissue factor (TF) to surface-bound collagen fibers. Recombinant TF was relipidated in liposomes of phosphatidylserine/phosphatidylcholine/biotin-linked phosphatidylethanolamine (20:79:1 PS/PC/bPE molar ratio). Collagen was patterned in a 250-μm-wide stripe and labeled with biotinylated anticollagen antibody which was then bound with streptavidin, allowing the subsequent capture of the TF liposomes. To verify and detect the TF liposome-collagen assembly, individual molecular complexes of TF-factor VIIa on collagen were visualized using the proximity ligation assay (PLA) to produce discretely localized fluorescent events that were strictly dependent on the presence of factor VIIa and primary antibodies against TF or factor VIIa. Perfusion for 450 s (wall shear rate, 200 s(-1)) of corn trypsin inhibitor (CTI, a factor XIIa inhibitor) treated whole blood over the stripe of TF-collagen enhanced platelet adhesion by 30 ± 8% (p < 0.001) and produced measurable fibrin (>50-fold increase) as compared to surfaces lacking TF. PS/PC/bPE liposomes lacking TF resulted in no enhancement of platelet deposition. Essentially no fibrin was formed during perfusion over collagen surfaces or collagen surfaces with liposomes lacking TF despite the robust platelet deposition, indicating a lack of kinetically significant platelet-borne tissue factor in healthy donor blood. This study demonstrates a reliable approach to link functionally active TF to collagen for microfluidic thrombosis studies.

Differential expression of chemokine and matrix re-modelling genes is associated with contrasting schistosome-induced hepatopathology in murine models

The pathological outcomes of schistosomiasis are largely dependent on the molecular and cellular mechanisms of the host immune response. In this study, we investigated the contribution of variations in host gene expression to the contrasting hepatic pathology observed between two inbred mouse strains following Schistosoma japonicum infection. Whole genome microarray analysis was employed in conjunction with histological and immunohistochemical analysis to define and compare the hepatic gene expression profiles and cellular composition associated with the hepatopathology observed in S. japonicum-infected BALB/c and CBA mice. We show that the transcriptional profiles differ significantly between the two mouse strains with high statistical confidence. We identified specific genes correlating with the more severe pathology associated with CBA mice, as well as genes which may confer the milder degree of pathology associated with BALB/c mice. In BALB/c mice, neutrophil genes exhibited striking increases in expression, which coincided with the significantly greater accumulation of neutrophils at granulomatous regions seen in histological sections of hepatic tissue. In contrast, up-regulated expression of the eosinophil chemokine CCL24 in CBA mice paralleled the cellular influx of eosinophils to the hepatic granulomas. Additionally, there was greater down-regulation of genes involved in metabolic processes in CBA mice, reflecting the more pronounced hepatic damage in these mice. Profibrotic genes showed similar levels of expression in both mouse strains, as did genes associated with Th1 and Th2 responses. However, imbalances in expression of matrix metalloproteinases (e.g. MMP12, MMP13) and tissue inhibitors of metalloproteinases (TIMP1) may contribute to the contrasting pathology observed in the two strains. Overall, these results provide a more complete picture of the molecular and cellular mechanisms which govern the pathological outcome of hepatic schistosomiasis. This improved understanding of the immunopathogenesis in the murine model schistosomiasis provides the basis for a better appreciation of the complexities associated with chronic human schistosomiasis.

Schistosomiasis is a significant cause of morbidity and mortality in the tropical world although its true burden has been historically underestimated. Millions of people currently endure severe pathology as a result of schistosome infections, although some individuals appear to be less susceptible to infection despite constant parasite exposure. A similar range of disease susceptibility is evident in different strains of inbred mice infected with schistosomes, thereby mirroring the clinical situation. Granuloma formation in the liver of both humans and mice is a characteristic manifestation of chronic schistosomiasis, and is largely controlled by gene signalling pathways. Certain genes expressed in particular cohorts of mice and humans may be associated with the development of severe pathology, or may confer a protective phenotype. This murine study highlights some key molecular aspects of chronic schistosomiasis which may be responsible for the development of both mild and severe pathology, and provides a bench mark for studying the mechanisms of schistosome-induced disease in humans.

Inflammatory stress and idiosyncratic hepatotoxicity: hints from animal models

Adverse drug reactions (ADRs) present a serious human health problem. They are major contributors to hospitalization and mortality throughout the world (Lazarou et al., 1998; Pirmohamed et al., 2004). A small fraction (less than 5%) of ADRs can be classified as "idiosyncratic." Idiosyncratic ADRs (IADRs) are caused by drugs with diverse pharmacological effects and occur at various times during drug therapy. Although IADRs affect a number of organs, liver toxicity occurs frequently and is the primary focus of this review. Because of the inconsistency of clinical data and the lack of experimental animal models, how IADRs arise is largely undefined. Generation of toxic drug metabolites and induction of specific immunity are frequently cited as causes of IADRs, but definitive evidence supporting either mechanism is lacking for most drugs. Among the more recent hypotheses for causation of IADRs is that inflammatory stress induced by exogenous or endogenous inflammagens is a susceptibility factor. In this review, we give a brief overview of idiosyncratic hepatotoxicity and the inflammatory response induced by bacterial lipopolysaccharide. We discuss the inflammatory stress hypothesis and use as examples two drugs that have caused IADRs in human patients: ranitidine and diclofenac. The review focuses on experimental animal models that support the inflammatory stress hypothesis and on the mechanisms of hepatotoxic response in these models. The need for design of epidemiological studies and the potential for implementation of inflammation interaction studies in preclinical toxicity screening are also discussed briefly.

Fibrin Affinity of Erythrocyte-Coupled Tissue-Type Plasminogen Activators Endures Hemodynamic Forces and Enhances Fibrinolysis in Vivo

Plasminogen activators (PAs; e.g., tissue-type, tPA) coupled to red blood cells (RBCs) display in vivo features useful for thromboprophylaxis: prolonged circulation, minimal extravasation, and preferential lysis of nascent versus preexisting clots. Yet, factors controlling the activity of RBC-bound PAs in vivo are not defined and may not mirror the profile of soluble PAs. We tested the role of RBC/PA binding to fibrin in fibrinolysis. RBC/tPA and RBC/tPA variant with low fibrin affinity (rPA) bound to and lysed plasminogen-containing fibrin clots in vitro comparably. In contrast, when coinjected in mice with fibrin emboli lodging in pulmonary vasculature, only RBC/tPA accumulated in lungs, which resulted in a more extensive fibrinolysis versus RBC/rPA (p < 0.01). Reconciling this apparent divergence between in vitro and in vivo behaviors, RBC/tPA, but not RBC/rPA perfused over fibrin in vitro at physiological shear stress bound to fibrin clots and caused greater fibrinolysis versus RBC/rPA (p < 0.001). These results indicate that because of high fibrin affinity, RBC/tPA binding to clots endures hemodynamic stress, which enhances fibrinolysis. Behavior of RBC/PAs under hemodynamic pressure is an important predictor of their performance in vivo.

High Throughput Substrate Specificity Profiling of Serine and Cysteine Proteases Using Solution-phase Fluorogenic Peptide Microarrays

Proteases regulate numerous biological processes with a degree of specificity often dictated by the amino acid sequence of the substrate cleavage site. To map protease/substrate interactions, a 722-member library of fluorogenic protease substrates of the general format Ac-Ala-X-X-(Arg/Lys)-coumarin was synthesized (X=all natural amino acids except cysteine) and microarrayed with fluorescent calibration standards in glycerol nanodroplets on glass slides. Specificities of 13 serine proteases (activated protein C, plasma kallikrein, factor VIIa, factor IXabeta, factor XIa and factor alpha XIIa, activated complement C1s, C1r, and D, tryptase, trypsin, subtilisin Carlsberg, and cathepsin G) and 11 papain-like cysteine proteases (cathepsin B, H, K, L, S, and V, rhodesain, papain, chymopapain, ficin, and stem bromelain) were obtained from 103,968 separate microarray fluorogenic reactions (722 substrates x 24 different proteases x 6 replicates). This is the first comprehensive study to report the substrate specificity of rhodesain, a papain-like cysteine protease expressed by Trypanasoma brucei rhodesiense, a parasitic protozoa responsible for causing sleeping sickness. Rhodesain displayed a strong P2 preference for Leu, Val, Phe, and Tyr in both the P1=Lys and Arg libraries. Solution-phase microarrays facilitate protease/substrate specificity profiling in a rapid manner with minimal peptide library or enzyme usage.

Silica Particles Enhance Peripheral Thrombosis

RATIONALE

Inflammation and thrombosis are related via interactions between leukocytes, platelets, the vasculature, and the coagulation system. However, the mechanisms behind these interactions remain poorly understood.

OBJECTIVES

We have investigated the effects of the well known pulmonary inflammation induced by silica for the development of peripheral thrombogenicity in a hamster model of thrombosis. In addition, the consequences of pulmonary macrophage and circulating monocyte and neutrophil depletion on the thrombogenicity were investigated.

METHODS

Silica particles (2-200 mug/hamster) were intratracheally instilled, and experimental thrombosis in photochemically induced femoral vein lesions was assessed 24 hours later, in association with cellular infiltration in the lung.

MEASUREMENTS AND MAIN RESULTS

Intratracheally instilled silica particles (20 and 200 mug/hamster) triggered pulmonary inflammation, together with stimulation of peripheral platelet-rich thrombus formation. Both the selective depletion of lung macrophages by intratracheal administration of clodronate liposomes and the combined depletion of circulating monocytes and neutrophils by intraperitoneal injection of cyclophosphamide significantly reduced silica-induced influx of macrophages and neutrophils in bronchoalveolar lavage, and reduced peripheral thrombogenicity. Silica-induced lung inflammation was accompanied by increased neutrophil elastase levels in bronchoalveolar lavage and in plasma. Specific neutrophil elastase inhibition in the lung did not affect lung inflammation but reduced peripheral thrombogenicity.

CONCLUSION

These findings uncover pulmonary macrophage-neutrophil cross-talk releasing neutrophil elastase into the blood circulation. Elastase, triggering activation of circulating platelets, may then predispose platelets to initiate thrombotic events on mildly damaged vasculature.

Leukocytosis and ischemic vascular disease morbidity and mortality: is it time to intervene?

The association between leukocytosis and increased morbidity and mortality of ischemic vascular disease has been observed for more than half a century, and recent studies in >350,000 patients confirm the robustness of the association and the dramatically higher relative and absolute acute and chronic mortality rates in patients with high versus low leukocyte counts. Although there is reason to believe that the association is not causal (that is, that leukocytosis is simply a marker of inflammation), there is also reason to believe that the leukocytosis directly enhances acute thrombosis and chronic atherosclerosis. Leukocytosis also is associated with poor prognosis and vaso-occlusive events in patients with sickle cell disease, and experimental data suggest a direct role for leukocytes in microvascular obstruction. The only way to test whether leukocytes contribute directly to poor outcome in ischemic cardiovascular disease is to assess the effect of modifying leukocyte function or number. Because selective blockade of leukocyte integrin alphaMbeta2 and P-selectin have thus far been disappointing as therapeutic strategies in human cardiovascular and cerebrovascular disease, I discuss the potential risks and benefits of short-term treatment with hydroxyurea to decrease the leukocyte count in select populations of patients at the highest risk of short-term death.

Factor VIIa-mediated tenase function on activated platelets under flow

BACKGROUND

Tissue factor (TF) and/or active factor (F)VIIa may be stored inside resting platelets.

OBJECTIVES

The objective of this study was to examine if platelets, following activation of GPVI, could support tenase and prothrombinase activity without any exogenously added tissue factor.

METHODS

Thrombin (IIa) formation on gel-filtered platelets with added factors or the clotting of platelet-free plasma (PFP) or platelet-rich plasma (PRP) supplemented with corn trypsin inhibitor (CTI) (to inhibit factor XIIa) was studied in well plate assays with a fluorogenic thrombin substrate or in flow assays by fibrin visualization.

RESULTS

Pretreatment of convulxin (CVX)-stimulated, fibrinogen-adherent, gel-filtered platelets with anti-TF, anti-FVII/VIIa, or 1 nm PPACK [inhibitor of FVIIa, factor XIa and factor (F)IIa] delayed fibrin deposition on platelets perfused with PFP/CTI at 62.5 s(-1). Anti-TF or anti-FVII/VIIa also attenuated thrombin generation in plate assays using recalcified PRP/CTI treated with CVX. Anti-TF or anti-FVII/VIIa (but not inhibited factor IXa) delayed the burst in thrombin production by gel-filtered platelets suspended in prothrombin and CVX by 14 min and 40 min, respectively. Anti-FVII/VIIa completely eliminated thrombin generation on fibrinogen-adherent, gel-filtered platelets pretreated with 10 micro m PPACK and 10 micro m EGR-CK [inhibitor of factor (F)Xa], rinsed, and then supplemented with CVX, prothrombin, and FX. Addition of anionic phospholipid to PFP/CTI or to a mixture of prothrombin, FX, and recVIIa was not sufficient to generate detectable tenase activity. Lastly, isolated, unactivated neutrophils suspended in FX, FII and recVIIa supported a very low level of thrombin generation sensitive to antagonism of P-selectin, CD18, and TF.

CONCLUSIONS

Activated platelets supported tenase and prothrombinase activity by elevating the function or level of FVIIa and exposing active FVIIa or FVIIa-cofactor(s), distinct from anionic lipid, that may be, in part, TF.

A novel mouse-driven ex vivo flow chamber for the study of leukocyte and platelet function

Various in vitro and in vivo techniques exist for study of the microcirculation. Whereas in vivo systems impress with their physiological fidelity, in vitro systems excel in the amount of reduction that can be achieved. Here we introduce the autoperfused ex vivo flow chamber designed to study murine leukocytes and platelets under well-defined hemodynamic conditions. In our model, the murine heart continuously drives the blood flow through the chamber, providing a wide range of physiological shear rates. We used a balance of force approach to quantify the prevailing forces at the chamber walls. Numerical simulations show the flow characteristics in the chamber based on a shear-thinning fluid model. We demonstrate specific rolling of wild-type leukocytes on immobilized P-selectin, abolished by a blocking MAb. When uncoated, the surfaces having a constant shear rate supported individual platelet rolling, whereas on areas showing a rapid drop in shear platelets interacted in previously unreported grapelike conglomerates, suggesting an influence of shear rate on the type of platelet interaction. In summary, the ex vivo chamber amounts to an external vessel connecting the arterial and venous systems of a live mouse. This method combines the strengths of existing in vivo and in vitro systems in the study of leukocyte and platelet function.