Neutrophil enhancement of fibrin deposition under flow through platelet-dependent and -independent mechanisms

Neutrophils Mediate Pulmonary Artery Thrombosis In Situ

Pulmonary embolism is a life-threatening condition, which can result in respiratory insufficiency and death. Blood clots occluding branches of the pulmonary artery (PA) are traditionally considered to originate from thrombi in deep veins (usually in legs). However, growing evidence suggests that occlusion of the vessels in the lungs can develop without preceding deep vein thrombosis (DVT). In this work, we used an inferior vena cava (IVC) complete ligation model of DVT in Wistar rats to explore the possibility and mechanisms of PA thrombosis under the conditions where all routes of thrombotic mass migration from peripheral veins are blocked. We demonstrate that rats both with normal and reduced neutrophil counts developed thrombi in the IVC, although, neutropenia caused a substantial decrease in thrombus size and a shift from fresh fibrin toward mature fibrin and connective tissue inside the thrombus. Massive fibrin deposition was found in the PA branches in the majority of DVT rats with normal neutrophil counts, but in none of the neutropenic animals. Neutrophil ablation also abolished macroscopic signs of lung damage. Altogether, the results demonstrate that thrombi in the lung vasculature can form in situ by mechanisms that require local neutrophil recruitment taking place in the DVT setting.

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.

Neutrophil extracellular traps (NETs) infiltrate haematoma and surrounding brain tissue after intracerebral haemorrhage: A post-mortem study

AIMS

Because of their prothrombotic and neuroinflammatory effects, neutrophils and neutrophil extracellular traps (NETs) represent interesting therapeutic targets for spontaneous intracerebral haemorrhage (sICH). We investigated the presence, spatial and temporal distribution of NETs in a human sICH post-mortem study.

METHODS

From 2005 to 2019, all sICH patients who came to autopsy within the first month after stroke were included and grouped according to the timing of death: 72 h, 4-7 days, 8-15 days and >15 days after ICH onset. Paraffin-embedded tissue was extracted from four strategic areas: haematoma, peri-haematomal area, ipsilateral surrounding brain tissue and a control contralateral area. Myeloperoxidase and histone H3 citrulline were immunolabelled to detect neutrophils and NETs respectively.

RESULTS

Neutrophils were present in the brains of the 14 cases (4 men, median age: 78 years) and NETs were found in 7/14 cases. Both neutrophils and NETs were detected within the haematoma but also in the surrounding tissue. The appearance of neutrophils and NETs was time-dependent, following a two-wave pattern: during the first 72 h and between 8 and 15 days after ICH onset. Qualitative examination showed that neutrophils and NETs were mainly located around dense fibrin fibres within the haematoma.

CONCLUSIONS

These observations provide evidence for NETs infiltration in the brain of patients who die from sICH. NETs might interact with early haemostasis within the haematoma core, and with the surrounding neuroinflammatory response. These findings open research perspectives for NETs in the treatment of sICH injuries.

Resolution of Deep Venous Thrombosis: Proposed Immune Paradigms

Venous thromboembolism (VTE) is a pathology encompassing deep vein thrombosis (DVT) and pulmonary embolism (PE) associated with high morbidity and mortality. Because patients often present after a thrombus has already formed, the mechanisms that drive DVT resolution are being investigated in search of treatment. Herein, we review the current literature, including the molecular mechanisms of fibrinolysis and collagenolysis, as well as the critical cellular roles of macrophages, neutrophils, and endothelial cells. We propose two general models for the operation of the immune system in the context of venous thrombosis. In early thrombus resolution, neutrophil influx stabilizes the tissue through NETosis. Meanwhile, macrophages and intact neutrophils recognize the extracellular DNA by the TLR9 receptor and induce fibrosis, a complimentary stabilization method. At later stages of resolution, pro-inflammatory macrophages police the thrombus for pathogens, a role supported by both T-cells and mast cells. Once they verify sterility, these macrophages transform into their pro-resolving phenotype. Endothelial cells both coat the stabilized thrombus, a necessary early step, and can undergo an endothelial-mesenchymal transition, which impedes DVT resolution. Several of these interactions hold promise for future therapy.

Fibrin Matrices as (Injectable) Biomaterials: Formation, Clinical Use, and Molecular Engineering

This review focuses on fibrin, starting from biological mechanisms (its production from fibrinogen and its enzymatic degradation), through its use as a medical device and as a biomaterial, and finally discussing the techniques used to add biological functions and/or improve its mechanical performance through its molecular engineering. Fibrin is a material of biological (human, and even patient's own) origin, injectable, adhesive, and remodellable by cells; further, it is nature's most common choice for an in situ forming, provisional matrix. Its widespread use in the clinic and in research is therefore completely unsurprising. There are, however, areas where its biomedical performance can be improved, namely achieving a better control over mechanical properties (and possibly higher modulus), slowing down degradation or incorporating cell-instructive functions (e.g., controlled delivery of growth factors). The authors here specifically review the efforts made in the last 20 years to achieve these aims via biomimetic reactions or self-assembly, as much via formation of hybrid materials.

The coagulation system helps control infection caused by the ciliate parasite Philasterides dicentrarchi in the turbot Scophthalmus maximus (L.)

Many studies have shown that coagulation systems play an important role in the defence against pathogens in invertebrates and vertebrates. In vertebrates, particularly in mammals, it has been established that the coagulation system participates in the entrapment of pathogens and activation of the early immune response. However, functional studies investigating the importance of the fish coagulation system in host defence against pathogens are scarce. In the present study, injection of turbot (Scopthalamus maximus) with the pathogenic ciliate Philasterides dicentrarchi led to the formation of macroscopic intraperitoneal clots in the fish. The clots contained abundant, immobilized ciliates, many of which were lysed. We demonstrated that the plasma clots immobilize and kill the ciliates in vitro. To test the importance of plasma clotting in ciliate killing, we inhibited the process by adding a tetrapeptide known to inhibit fibrinogen/thrombin clotting in mammals. Plasma tended to kill P. dicentrarchi slightly faster when clotting was inhibited by the tetrapeptide, although the total mortality of ciliates was similar. We also found that kaolin, a particulate activator of the intrinsic pathway in mammals, accelerates plasma clotting in turbot. In addition, PMA-stimulated neutrophils, living ciliates and several ciliate components such as cilia, proteases and DNA also displayed procoagulant activity in vitro. Injection of fish with the ciliates generated the massive release of neutrophils to the peritoneal cavity, with formation of large aggregates in those fish with live ciliates in the peritoneum. We observed, by SEM, numerous fibrin-like fibres in the peritoneal exudate, many of which were associated with peritoneal leukocytes and ciliates. Expression of the CD18/CD11b gene, an integrin associated with cell adhesion and the induction of fibrin formation, was upregulated in the peritoneal leukocytes. In conclusion, the findings of the present study show that P. dicentrarchi induces the formation of plasma clots and that the fish coagulation system may play an important role in immobilizing and killing this parasite.

α2-Antiplasmin: New Insights and Opportunities for Ischemic Stroke

Thrombotic vascular occlusion is the leading cause of ischemic stroke. High blood levels of α₂-antiplasmin (a2AP), an ultrafast, covalent inhibitor of plasmin, have been linked in humans to increased risk of ischemic stroke and failure of tissue plasminogen activator (tPA) therapy. Consistent with these observations, a2AP neutralizes the therapeutic benefit of tPA therapy in experimental stroke. In addition, a2AP has deleterious, dose-related effects on ischemic brain injury in the absence of therapy. Experimental therapeutic inactivation of a2AP markedly reduces microvascular thrombosis, ischemic brain injury, brain swelling, brain hemorrhage, and death after thromboembolic stroke. These data provide new insights into the critical importance of a2AP in the pathogenesis of ischemic brain injury and suggest that transiently inactivating a2AP may have therapeutic value in ischemic stroke.

Rapid Early Triage by Leukocytosis and the Thrombolysis in Myocardial Infarction (TIMI) Risk Score for ST-Elevation Myocardial Infarction Undergoing Primary Percutaneous Coronary Intervention: An Observational Study

The clinical utility of leukocytosis in risk assessment for ST-elevation myocardial infarction (STEMI) is still unclear. We aim to demonstrate the prognostic value of leukocyte counts independent from traditional risk factors and the TIMI risk score (TRS) for STEMI and to propose a practical model comprising leukocyte count for early triage in STEMI undergoing primary angioplasty. A prospective database (n = 796) of consecutive STEMI cases receiving primary angioplasty at a tertiary medical center was retrospectively analyzed in the period from February 1, 2007 through December 31, 2012. Primary endpoints were 30-day and 1-year mortality. Propensity score-adjusted Cox regression models and subdivision analysis were performed. Leukocytosis group (n = 306) had higher 30-day mortality (5.9% vs 3.1%, P = 0.048) and 1-year mortality (9.2% vs 5.1%, P = 0.022). After adjustment by propensity score and TRS, leukocyte count (per 10/μL) was an independent predictor of 1-year mortality (HR: 1.086, 95% CI: 1.034-1.140, P = 0.001). Subdivision analysis demonstrated the correlation between leukocytosis and higher 1-year mortality within both high and low TRS strata (divided by 4, the median of TRS). Additionally, 24% (191 out of 796) of patients were characterized by nonleukocytosis and TRS < 4, having 0% of mortality rate at 1-year follow-up. In conclusion, leukocyte count is an independent prognostic factor adding incremental value to TRS for STEMI. Nonleukocytosis in conjunction with TRS < 4 identifies a large patient group at extremely low risk and thus provides rapid early triage for STEMI patients undergoing primary PCI. This finding is worth validation in the future.

Regulation of fibrin-mediated tumor cell adhesion to the endothelium using anti-thrombin aptamer

Molecular intervention during transient stages of various metastatic pathways may lead to development of promising therapeutic technologies. One of such involves soluble fibrin (sFn) that has been implicated as a cross-linker between circulating blood or tumor cells and endothelial cell receptors, promoting cell arrest on the endothelium during circulation. sFn generation is a result of thrombin-mediated fibrinogen (Fg) cleavage due to either vascular injuries or a tumor microenvironment. For cancer therapy, thrombin-mediated conversions of Fg to sFn thus serve as potential intervention points to decrease circulating tumor cell adhesion to the endothelium and subsequent metastatic events. The purpose of this work was to investigate the function of an anti-thrombin oligonucleotide aptamer in reducing tumor cell arrest. Both molecular and cellular interactions were examined to demonstrate the binding and inhibitory effects of anti-thrombin aptamer. The results show that the aptamer is capable of inhibiting thrombin-mediated Fg conversion, thereby reducing sFn-mediated tumor cell adhesion in a concentration-dependent manner. Notably, the aptamer is able to bind thrombin under dynamic flow conditions and reduce tumor cell adhesive events at various physiological shear rates. This study further indicates that oligonucleotide aptamers hold great promise as therapeutic regulators of tumor cell adhesion, and consequently, metastatic activity.

White blood cell, hemoglobin and platelet distribution width as short-term prognostic markers in patients with acute myocardial infarction

The aim of this study was to assess the prognostic value of combined use of white blood cell (WBC), hemoglobin (Hb), and platelet distribution width (PDW) in patients with acute myocardial infarction (AMI). This study included 1,332 consecutive patients with AMI. Patients were categorized into complete blood cell (CBC) group 0 (n=346, 26.0%), 1 (n=622, 46.7%), 2 (n=324, 24.3%), and 3 (n=40, 3.0%) according to the sum of the value defined by the cut-off levels of WBC (1, ≥ 14.5 × 10(3)/µL; 0, <14.5 × 10(3)/µL), Hb (1, <12.7 g/dL; 0, ≥ 12.7 g/dL), and PDW (1, ≥ 51.2%; 0: <51.2%). In-hospital death occurred in 59 (4.4%) patients. Patients who died during index hospitalization had higher WBC and PDW and lower Hb. The patients could be stratified for in-hospital mortality according to CBC group; 1.2%, 2.7%, 9.0%, and 22.5% in CBC groups 0, 1, 2, and 3 (P<0.001), respectively. In multivariate logistic regression analysis, CBC group ≥ 2 (odds ratio, 3.604; 95% confidence interval, 1.040-14.484, P=0.043) was an independent predictor for in-hospital death. The prognostic impact of the combined use of CBC markers remained significant over 12 months. In conclusions, combination of WBC, Hb, and PDW, a cheap and simple hematologic marker, is useful in early risk stratification of patients with AMI.

Fever after primary percutaneous coronary intervention in ST-segment elevation myocardial infarction is associated with adverse outcomes

BACKGROUND

Fever is a common finding after primary percutaneous coronary intervention (PPCI) in patients with ST-segment elevation myocardial infarction (STEMI). However, its prognostic value is not validated yet.

OBJECTIVES

This study sought to evaluate the impact of fever after PPCI in STEMI on adverse clinical outcomes.

METHODS

Five hundred fourteen consecutive patients who underwent PPCI due to STEMI were enrolled. Body temperature (BT) was checked every 6 h for 5 days after PPCI. Patients were divided into two groups according to the highest quartile of peak BT; peak BT≤37.6 °C (control group) and peak BT>37.6 °C (fever group). Rates of 1-year major adverse cardiovascular events (MACE; death, myocardial infarction, any revascularization) were compared.

RESULTS

The prevalence of fever group (peak BT>37.6 °C) was 24.7% (127/514). White blood cell count, highly sensitive C-reactive protein and serum cardiac troponin I level were higher in fever group than control group (12,162±4199/μL vs. 10,614±3773/μL, p<0.001; 22.9±49.4 mg/L vs. 7.4±2.5 mg/L, p=0.001, 16.7±36.9 ng/dl vs. 8.70±26.2 ng/dl, p=0.027, respectively). The frequency of a history of previous myocardial infarction and left ventricular ejection fraction was lower in fever group (0.0% vs. 4.7%, p=0.010; 47±8 % vs. 49±9 %, p=0.002, respectively). There was no significant difference in angiographic characteristics between 2 groups. 1-year MACE rates were higher in fever group (11.0% vs. 4.7%, p=0.010). Multivariate analysis revealed fever (OR 2.358, 95% CI 1.113-4.998, p=0.025), diabetes mellitus as risk factor (2.227, 1.031-4.812, 0.042), and left anterior descending artery as infarct related artery (2.443, 1.114-5.361, 0.026) as independent predictors for 1-year MACE.

CONCLUSIONS

Fever after PPCI in patients with STEMI is frequently developed and it can predict adverse clinical outcome.

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.

Platelets as cellular effectors of inflammation in vascular diseases

Platelets are chief effector cells in hemostasis. In addition, they are multifaceted inflammatory cells with functions that span the continuum from innate immune responses to adaptive immunity. Activated platelets have key thromboinflammatory activities in a variety of vascular disorders and vasculopathies. Recently identified inflammatory and immune activities provide insights into the biology of these versatile blood cells that are directly relevant to human vascular diseases.

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.

Association among leukocyte count, mortality, and bleeding in patients with non-ST-segment elevation acute coronary syndromes (from the Acute Catheterization and Urgent Intervention Triage StrategY [ACUITY] trial)

Although inflammation is involved in the pathogenesis of acute coronary syndromes, the extent of inflammation is not routinely assessed, and its prognostic implications in patients with non-ST-segment elevation acute coronary syndrome have not been investigated in depth. We analyzed the prognostic implications of an elevated white blood cell count (WBCc) in patients with moderate and high-risk non-ST-segment elevation acute coronary syndrome undergoing an early invasive strategy in the large-scale Acute Catheterization and Urgent Intervention Triage StrategY trial. The WBCc at admission was available for 13,678 of 13,819 patients (98.9%). The patients in the upper tertile of the WBCc had an increased risk of 30-day major bleeding, 1-year mortality, and definite/probable stent thrombosis compared to those in the mid or lower tertiles. On multivariate analysis, the WBCc was an independent predictor of 30-day major bleeding and 1-year cardiac, noncardiac, and all-cause mortality. The association between the WBCc and cardiac mortality was present in multiple prespecified subgroups, with no significant interaction between the WBCc and age, gender, diabetes, smoking, renal dysfunction, elevated baseline biomarkers, antithrombotic therapy, revascularization, and Thrombolysis In Myocardial Infarction risk score. The WBCc remained an independent predictor of mortality after adjusting for bleeding, C-reactive protein level, and angiographic variables, including left ventricular ejection fraction, Thrombolysis In Myocardial Infarction flow, and number of diseased vessels. The WBCc significantly improved the prognostic accuracy of the Thrombolysis In Myocardial Infarction risk score, with a net reclassification improvement of 11% (p <0.0001). In conclusion, in patients with moderate- and high-risk non-ST-segment elevation acute coronary syndrome, an elevated admission WBCc was an independent predictor of 30-day major bleeding, and 1-year cardiac, noncardiac, and all-cause mortality.

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.

Biomarkers for the diagnosis of deep vein thrombosis

Venous thromboembolic disease (VTE) remains a significant source of morbidity and mortality. As non-specific subjective complaints and a paucity of objective clinical examination findings complicate the diagnosis of both deep venous thrombosis (DVT) and pulmonary embolism, diagnostic modalities remain essential. Compression ultrasound remains the gold standard for DVT diagnosis. Reliable imaging is not always available making a serologic diagnosis, or biomarker, highly desirable. While D-dimer, a highly sensitive biomarker, is useful for excluding acute VTE, it lacks the specificity necessary for diagnostic confirmation. As such, ongoing research efforts target and support the utility of alternative plasma biomarkers to aid in the diagnosis of VTE including selectins, microparticles, IL-10 and other inflammatory markers. These molecular markers may also predict recurrence risk, guide length and modality of treatment, and predict which thrombi will resolve spontaneously or recanalize, thus potentially identifying patients who would benefit from more aggressive therapies than standard anticoagulation.

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.

Platelets: versatile effector cells in hemostasis, inflammation, and the immune continuum

Platelets are chief effector cells in hemostasis. In addition, however, their specializations include activities and intercellular interactions that make them key effectors in inflammation and in the continuum of innate and adaptive immunity. This review focuses on the immune features of human platelets and platelets from experimental animals and on interactions between inflammatory, immune, and hemostatic activities of these anucleate but complex and versatile cells. The experimental findings and evidence for physiologic immune functions include previously unrecognized biologic characteristics of platelets and are paralleled by new evidence for unique roles of platelets in inflammatory, immune, and thrombotic diseases.

Sequential binding of αVβ3 and ICAM-1 determines fibrin-mediated melanoma capture and stable adhesion to CD11b/CD18 on neutrophils

Fibrin (Fn) deposition defines several type 1 immune responses, including delayed-type hypersensitivity and autoimmunity in which polymorphonuclear leukocytes (PMNs) are involved. Fn monomer and fibrinogen are multivalent ligands for a variety of cell receptors during cell adhesion. These cell receptors provide critical linkage among thrombosis, inflammation, and cancer metastasis under venous flow conditions. However, the mechanisms of Fn-mediated interactions among immune cells and circulating tumor cells remain elusive. By using a cone-plate viscometer shear assay and dual-color flow cytometry, we demonstrated that soluble fibrinogen and Fn had different abilities to enhance heterotypic aggregation between PMNs and Lu1205 melanoma cells in a shear flow, regulated by thrombin levels. In addition, the involvement of integrin α(v)β(3), ICAM-1, and CD11b/CD18 (Mac-1) in fibrin(ogen)-mediated melanoma-PMN aggregations was explored. Kinetic studies provided evidence that ICAM-1 mediated initial capture of melanoma cells by PMNs, whereas α(v)β(3) played a role in sustained adhesion of the two cell types at a shear rate of 62.5 s(-1). Quantitative analysis of the melanoma-PMN interactions conducted by a parallel-plate flow chamber assay further revealed that at a shear rate of 20 s(-1), α(v)β(3) had enough contact time to form bonds with Mac-1 via Fn, which could not otherwise occur at a shear rate higher than 62.5 s(-1). Our studies have captured a novel finding that leukocytes could be recruited to tumor cells via thrombin-mediated Fn formation within a tumor microenvironment, and α(v)β(3) and ICAM-1 may participate in multistep fibrin(ogen)-mediated melanoma cell adhesion within the circulation.

Proteomics of microparticles after deep venous thrombosis

BACKGROUND

Microparticles (MP) are submicron size membrane vesicles released from activated cells that are associated with thrombosis and inflammation. MP present diverse biological expressions that may be linked to a unique subset of proteins derived from their origin cells.

METHODS

To identify these proteins, plasma samples were taken from 9 patients with deep venous thrombosis (DVT) documented by duplex ultrasound, 9 with leg pain but negative for DVT by duplex, and 6 healthy controls without a history of thrombosis, for fold variation. MP were extracted from platelet-poor plasma, digested separately with trypsin and tagged using iTRAQ reagents. The digests were subjected to 2-D LC separation followed by MALDI tandem mass spectrometry. Peak lists were generated and searched against all human sequences. For protein identification, a minimum of two peptides at 95% confidence was required. Later, iTRAQ ratios were generated comparing relative protein levels of DVT patients to baseline. The proteomic analysis was performed twice for each blood sample. Proteins were considered elevated or depressed if the iTRAQ ratio (R) deviated by 20% change from normal and a p-value less than 0.05.

RESULTS

Two proteins (Galectin-3 Binding Protein, [Gal3BP], R=1.76 and Alpha-2 macroglobulin [A2M] R=1.57) were differentially expressed on DVT patients. Nine proteins were depleted including fibrinogen beta and gamma chain precursors (R=0.65).

CONCLUSIONS

These proteins influence thrombosis through inflammation, cell shedding, inhibition of fibrinolysis and hemostatic plug formation. Further studies are needed to confirm the mechanistic role of these proteins in the pathogenesis of venous thrombosis in humans.

Microparticle surface protein are associated with experimental venous thrombosis: a preliminary study

Microparticles are small membrane vesicles released from activated cells and are associated with thrombosis and inflammation. Microparticle contain a unique subset of surface protein derived form the parent cell and may be responsible for their diverse biological functions. To identify these proteins, juvenile baboons (Papio anubis, n = 4) underwent iliac vein thrombosis with 6-hour balloon occlusion. Plasma samples were taken at baselines and at 2 days postthrombosis for microparticle analysis. Microparticles were extracted from platelet-poor plasma, digest separately with trypsin and tagged using isobaric tagging for relative and absolute quantitation reagents. The digests were subjected to 2-dimensional liquid chromatographic separation followed by matrix-assisted laser desorption/ionization tandem mass spectrometry. Peak lists were generated and searched against all primate sequences. For protein identity, a minimum of 2 peptides at 95% confidence interval was required. Later, isobaric tagging for relative and absolute quantitation ratios were generated comparing relative protein level of day 2 to baseline. The proteomic analysis was performed twice for each blood sample, totaling 8 experiments. Proteins were considered elevated of depressed if the isobaris tagging for relative and absolute quantitation ratio deviated by 20% changes from normal and a P value less than .05. Significantly, 7 proteins were differentially expressed on day 2 compared to baseline, and appeared in at least 3 animals and regulated in at least 4 experiment. Among these 7 proteins, upregulated proteins include various forms of fibrinogen and alpha-1-antichymotrypsin and downregulated proteins include immunoglobulins. These proteins influence thrombosis and inflammation through hemostatic plug formation (fibrinogen), inhibiting neutrophil adhesion (alpha-1-antichymoptrypsin), and immunoregulation (immunoglobulins). Further studies are needed to confirm the mechanistic role of these proteins in the pathogenesis of venous thrombosis.

Complement activation triggered by chondroitin sulfate released by thrombin receptor-activated platelets

BACKGROUND

Chondroitin sulfate (CS) is a glycosaminoglycan released by activated platelets.

OBJECTIVE

Here we test the hypothesis that CS released by activated platelets can trigger complement activation in the fluid phase.

METHODS AND RESULTS

Thrombin receptor-activating peptide (TRAP)-6 was used to activate platelets in platelet-rich plasma and blood, anticoagulated with the thrombin inhibitor lepirudin. TRAP activation induced fluid-phase complement activation, as reflected by the generation of C3a and sC5b-9, which could be attenuated by the C3 inhibitor compstatin. Chondroitinase ABC treatment of supernatants from activated platelets totally inhibited the activation, indicating that platelet-derived CS had initiated the complement activation. Furthermore, addition of purified CS to plasma strongly triggered complement activation. C1q was identified as the recognition molecule, as it bound directly to CS, and CS-triggered complement activation could be restored in C1q-depleted serum by adding purified C1q. TRAP activation of whole blood increased the expression of CD11b on leukocytes and generation of leukocyte-platelet complexes. It was demonstrated that these leukocyte functions were dependent on C3 activation and signaling via C5a, as this expression could be inhibited by compstatin and by a C5aR antagonist.

CONCLUSIONS

We conclude that platelets trigger complement activation in the fluid phase by releasing CS, which leads to inflammatory signals mediated by C5a.

The Effect of Negative Pressure Therapy on the Femoral Vein Blood Flow and Wall Structure

Negative pressure therapy has been recently used for managing lymphatic or infective groin complications. The aim of this study was to investigate any possible association between application of negative pressure therapy in the groin area and deep-vein thrombosis. Acute surgical wounds were created at the inguinal areas in 7 pigs. Different negative pressures ranging from −50 to −200 mmHg were applied directly over the femoral vessels, and blood flow alterations were studied using a Doppler ultrasound. Femoral vein specimens were also removed for histological examination after 12 hours of therapy. It has been demonstrated that negative pressure therapy does not significantly alter the baseline lower limb venous return. Histology demonstrated several changes, which are associated with vein thrombogenesis. The hemodynamic and pathological findings still leave a potential for thrombogenic effects of negative pressure therapy and warrant care to protect the femoral veins, with the use of thrombosis prophylaxis measures.

L-selectin shear thresholding modulates leukocyte secondary capture

Transient homotypic adhesions between flowing leukocytes and those previously adherent on the vessel wall has been proposed to amplify the accumulation of leukocytes at sites of inflammation. While adhesion of leukocytes to the vessel wall (primary capture) is mediated primarily by P-selectin on the endothelium and P-selectin Glycoprotein Ligand-1 (PSGL-1) on the leukocyte, the homotypic interactions leading to downstream leukocyte adhesion (secondary capture) are mediated primarily by reciprocal interactions between PSGL-1 and L-selectin on apposing leukocytes. One consequence of leukocyte secondary capture events are the formation of strings of adherent leukocytes as each recently captured leukocyte in turn captures another one flowing over its surface. Interestingly, PSGL-1-L-selectin interactions also mediate leukocyte hydrodynamic shear thresholding, whereby leukocyte rolling on purified L-selectin ligands such as PSGL-1 is maximized at a wall shear stress of approximately 1 dyne/cm(2) and minimized at both higher and lower flow rates. Using a novel quantitative method, we analyzed leukocyte string formation in vitro and found that hydrodynamic shear thresholding precluded secondary capture at low shear stresses yet amplified it at high shear stresses. Addition of the L-selectin mAb DREG-56 strongly inhibited leukocyte string formation, suggesting adhesion contributed significantly to hydrodynamic interactions in secondary capture processes. Taken together, the data suggest that secondary capture is modulated by the shear thresholding property of L-selectin. L-selectin mediated shear thresholding may therefore play a significant role in the regulation of leukocyte secondary capture in addition to recently described hydrodynamic recruitment mechanisms.

Hemodynamic regulation of inflammation at the endothelial-neutrophil interface

Arterial shear stress can regulate endothelial phenotype. The potential for anti-inflammatory effects of shear stress on TNFalpha-activated endothelium was tested in assays of cytokine expression and neutrophil adhesion. In cultured human aortic endothelial cells (HAEC), arterial shear stress of 10 dyne/cm(2) blocked by >80% the induction by 5 ng/mL TNFalpha of interleukin-8 (IL-8) and IL-6 secretion (50 and 90% reduction, respectively, in the presence of nitric oxide synthase antagonism with 200 microM nitro-L-arginine methylester, L-NAME). Exposure of TNFalpha-stimulated HAEC to arterial shear stress for 5 h also reduced by 60% (p < 0.001) the conversion of neutrophil rolling to firm arrest in a venous flow assay conducted at 1 dyne/cm(2). Also, neutrophil rolling lengths at 1 dyne/cm(2) were longer when TNFalpha-stimulated HAEC were presheared for 5 h at arterial stresses. In experiments with a synthetic promoter that provides luciferase induction to detect cis interactions of glucocorticoid receptor (GR) and NFkappaB, shear stress caused a marked 40-fold induction of luciferase in TNFalpha-treated cells, suggesting a role for GR pathways in the anti-inflammatory actions of fluid shear stress. Hemodynamic force exerts anti-inflammatory effects on cytokine-activated endothelium by attenuation of cytokine expression and neutrophil firm arrest.

Coagulation and fibrinolytic protein kinetics in cardiopulmonary bypass

The development of Cardiopulmonary Bypass (CPB) catopulted the field of cardiothoracic surgery into a new dimension--one that changed the lives of individuals with congenital and acquired heart disease worldwide. Despite its contributions, CPB has clear limitations and creates unique challenges for clinicians and patients alike, stemming from profound hemostatic pertubations and accompanying risk for bleeding and possibly thrombotic complications.

Matrix protein microarrays for spatially and compositionally controlled microspot thrombosis under laminar flow

Microarraying allows the spatial and compositional control of surfaces, typically for the purpose of binding reactions. Collagen and/or von Willebrand Factor (vWF) in 5% glycerol was contact printed onto glass slides to create defined microspots (176-microm diameter) of adsorbed protein without sample dehydration. The arrays were mounted on flow chambers allowing video microscopy during perfusion (wall shear rate of 100-500 s(-1)) of recalcified corn trypsin inhibitor-treated whole blood or platelet rich plasma and subsequent array scanning via anti-GPIbalpha and anti-fibrin(ogen) immunofluorescence. To mimic the subendothelial matrix, vWF was microarrayed over sonicated type I collagen microspots. For whole blood perfusion (500 s(-1), 10 min) over collagen, vWF, and collagen/vWF microspots, the amount of platelet deposition on the collagen/vWF spots was approximately 2 times greater in comparison to the collagen spots and approximately 18 times greater in comparison to the vWF spots. The amount of fibrin(ogen) deposition on the collagen/vWF spots was approximately 2 times greater in comparison to the collagen spots and approximately 4 times greater in comparison to the vWF spots. This protocol allowed for highly uniform (CV = 18%) and precisely located thrombus formation at a density of >or=400 spots/cm(2). Microarrays are ideal for the combinatorial assembly of adhesive and procoagulant proteins to study thrombosis as well as to study axial and lateral transport effects between discrete microspots of distinct composition.

Increased level of cardiorespiratory fitness blunts the inflammatory response in metabolic syndrome

OBJECTIVE

The presence of metabolic syndrome is associated with a higher degree of inflammation. We sought to assess whether the higher levels of cardiorespiratory fitness attenuate the levels of inflammation in people with metabolic syndrome.

RESEARCH DESIGN AND METHODS

We studied 449 consecutive asymptomatic men (47+/-7 years) who underwent a maximal treadmill exercise test according to the Bruce protocol. Cardiorespiratory fitness was divided into tertiles based on metabolic equivalents (METs). White blood cells (WBC) (x10(9) cells/L) count was used as marker of inflammation.

RESULTS

In our study population, 23% of the participants had the metabolic syndrome. The WBC count increased (p < 0.0001 for trend) with increasing number of risk factors for metabolic syndrome; however there was an inverse relationship (p < 0.0001 for trend) with increasing tertiles of fitness (6.47 cells x 10(9) cells/L for lowest tertile and 5.7 x 10(9) cells/L for highest tertile). Multiple linear regression analyses demonstrated that as compared to individuals with no MS risk factor, the WBC count remained significantly higher in men with metabolic syndrome in first tertile (regression coefficient: 1.2, 95% CI 0.4-2.0, p = 0.003) and second tertile (regression coefficient: 0.61, 95% CI 0.4-2.0, p = 0.02) of cardiorespiratory fitness, respectively. However, in the highest tertile of fitness no increase in level of WBC count was observed with increasing metabolic syndrome risk factors.

CONCLUSION

Our findings suggest that in people with metabolic syndrome an increased level of physical fitness might exert its beneficial effect via attenuating inflammation.

Leukocyte adhesion and thrombosis

PURPOSE OF THE REVIEW

The consequences of arterial thrombosis such as myocardial infarction, stroke and peripheral vascular occlusion are the leading causes of morbidity and mortality. A high leukocyte count and an elevation in inflammatory markers are identified as significant risk factors for thrombosis. Leukocytes form the front line in defense against infection and are the first cells arriving at the site of inflammation. This review summarizes the cellular and molecular mechanisms by which adherent leukocytes can induce a prothrombotic state.

RECENT FINDINGS

Circulating tissue factor has been recognized as a potential prothrombotic factor initiating thrombosis after vascular injury. The tissue factor is present on microvesicles originated from activated leukocytes. Leukocytes generate tissue factor containing microvesicles following stimulation with cytokines and following platelet adhesion via P-selectin. Additionally, activated leukocytes release several mediators, such as cathepsin G and elastase, which can activate both the coagulation cascade and platelets. Furthermore, new roles for leukocytes have been identified in vascular injury in sickle cell anemia, in vascular occlusion following the rupture of atherosclerotic plaque, and in thrombotic complications of myeloproliferative diseases.

SUMMARY

Leukocyte adhesion to endothelium and platelets plays an important role in the activation of the coagulation cascade. An excessive activation of leukocytes during the inflammatory process may induce a systemic procoagulant state. Elucidation of critical steps in activation of coagulation by leukocytes may offer a new therapeutic target for antithrombotic therapy based on blocking leukocyte adhesion.

Long-term anticoagulant therapy in patients with coronary artery disease

Secondary prevention of coronary events in coronary artery disease (CAD) patients with aspirin is generally accepted because of ease of administration, predictable safety, and proven efficacy. The use of long-term anticoagulant therapy with heparins, vitamin-K antagonists (VKAs), or thrombin inhibitors is, however, more controversial. During the last 40 years, several trials have been conducted in order to evaluate the role of anticoagulant therapy in patients with CAD as a protection against subsequent death and thrombo-embolic complications. The conducted trials are heterogeneous in many ways, concerning comparative medications, patient populations, endpoints and follow-up, which makes a standardized recommendation on the basis of these studies difficult. This review is an overview of the largest and best studies on this topic and discusses the scientific background for a possible use of VKA or an alternative anticoagulant treatment in CAD patients, looking at both the beneficial effects and the risk of bleeding.

Effect of the novel direct factor Xa inhibitor DX-9065a on thrombin generation and inhibition among patients with stable atherosclerotic coronary artery disease

INTRODUCTION

Thrombin, a pluripotential effector enzyme with prothrombotic, proinflammatory, and mitogenic properties, plays a pivotal role in the pathobiology and clinical expression of atherothrombotic coronary artery disease. Existing anticoagulant drugs have not been shown to attenuate thrombin generation or activity consistently. We sought to investigate the effect of DX-9065a on thrombin generation and inhibition in patients with stable CAD. DX-9065a is a small-molecule, synthetic, direct inhibitor of factor Xa.

MATERIALS AND METHODS

Peripheral venous blood samples were collected serially during and after administration of either placebo or 1 of 4 weight-adjusted regimens of DX-9065a, in 73 patients with stable CAD participating in the XaNADU-1B study.

RESULTS AND CONCLUSIONS

At baseline, the median (25th, 75th) prothrombin activation fragment 1.2 (F1.2) level was 2.56 (2.05, 3.20) nmol/L, and the median d-dimer level was 0.26 (0.19, 0.38) microg FEU/L. There were significant relationships between measured plasma DX-9065a concentrations and both F1.2 (4.9% decrease for each doubling of DX-9065a) (P<0.0001) and d-dimer (5.5% decrease for each doubling of DX-9065a) (P=0.001). F1.2 was suppressed (below baseline) at 96 h after administration of DX-9065a. Coronary thrombotic events did not occur during or after study drug administration. DX-9065a, the first in a class of small-molecule, direct, selective and reversible factor Xa inhibitors, reduces thrombin generation and fibrin formation among patients with stable CAD. The effect is concentration-dependent and persists for at least 96 h following drug cessation, without biochemical or clinical evidence of rebound.

Neutrophil-bead collision assay: pharmacologically induced changes in membrane mechanics regulate the PSGL-1/P-selectin adhesion lifetime

Visualization of flowing neutrophils colliding with adherent 1-mum-diameter beads presenting P-selectin allowed the simultaneous measurement of collision efficiency (epsilon), membrane tethering fraction (f), membrane tether growth dynamics, and PSGL-1/P-selectin binding lifetime. For 1391 collisions analyzed over venous wall shear rates from 25 to 200 s(-1), epsilon decreased from 0.17 to 0.004, whereas f increased from 0.15 to 0.70, and the average projected membrane tether length, L(tether)(m), increased from 0.35 mum to approximately 2.0 mum over this shear range. At all shear rates tested, adhesive collisions lacking membrane tethers had average bond lifetimes less than those observed for collisions with tethers. For adhesive collisions that failed to form membrane tethers, the regressed Bell parameters (consistent with single bond Monte Carlo simulation) were zero-stress off-rate, k(off)(0) = 0.56 s(-1) and reactive compliance, r = 0.10 nm, similar to published atomic force microscopy (AFM) measurements. For all adhesion events (+/- tethers), the bond lifetime distributions were more similar to those obtained by rolling assay and best simulated by Monte Carlo with the above Bell parameters and an average of 1.48 bonds (n = 1 bond (67%), n = 2 (22%), and n = 3-5 (11%)). For collisions at 100 s(-1), pretreatment of neutrophils with actin depolymerizing agents, latrunculin or cytochalasin D, had no effect on epsilon, but increased L(tether)(m) by 1.74- or 2.65-fold and prolonged the average tether lifetime by 1.41- or 1.65-fold, respectively. Jasplakinolide, an actin polymerizing agent known to cause blebbing, yielded results similar to the depolymerizing agents. Conversely, cholesterol-depletion with methyl-beta-cyclodextrin or formaldehyde fixation had no effect on epsilon, but reduced L(tether)(m) by 66% or 97% and reduced the average tether lifetime by 30% or 42%, respectively. The neutrophil-bead collision assay combines advantages of atomic force microscopy (small contact zone), aggregometry (discrete interactions), micropipette manipulation (tether visualization), and rolling assays (physiologic flow loading). Membrane tether growth can be enhanced or reduced pharmacologically with consequent effects on PSGL-1/P-selectin lifetimes.

Neutrophil string formation: hydrodynamic thresholding and cellular deformation during cell collisions

Neutrophils unexpectedly display flow-enhanced adhesion (hydrodynamic thresholding) to L-selectin in rolling or aggregation assays. We report that the primary collision efficiency (epsilon) of flowing neutrophils with preadhered neutrophils on intercellular adhesion molecule-1 (ICAM-1) or fibrinogen also displayed a maximum of epsilon approximately 0.4-0.45 at a wall shear rate of 100 s(-1), an example of thresholding. Primary collision lifetime with no detectable bonding decreased from 130 to 10 ms as wall shear rate increased from 30 to 300 s(-1), whereas collision lifetimes with bonding decreased from 300 to 100 ms over this shear range using preadhered neutrophils on ICAM-1, with similar results for fibrinogen. Antibodies against L-selectin, but not against CD11a, CD11b, or CD18, reduced epsilon at 100 s(-1) by >85%. High resolution imaging detected large scale deformation of the flowing neutrophil during the collision at 100 s(-1) with the apparent contact area increasing up to approximately 40 microm(2). We observed the formation of long linear string assemblies of neutrophils downstream of neutrophils preadhered to ICAM-1, but not fibrinogen, with a maximum in string formation at 100 s(-1). Secondary capture events to the ICAM-1 or fibrinogen coated surfaces after primary collisions were infrequent and short lived, typically lasting from 500 to 3500 ms. Between 5 and 20% of neutrophil interactions with ICAM-1 substrate converted to firm arrest (>3500 ms) and greatly exceeded that observed for fibrinogen, thus defining the root cause of poor string formation on fibrinogen at all shear rates. Additionally, neutrophils mobilized calcium after incorporation into strings. Static adhesion also caused calcium mobilization, as did the subsequent onset of flow. To our knowledge, this is the first report of 1). hydrodynamic thresholding in neutrophil string formation; 2). string formation on ICAM-1 but not on fibrinogen; 3). large cellular deformation due to collisions at a venous shear rate; and 4), mechanosensing through neutrophil beta(2)-integrin/adhesion. The increased contact area during deformation was likely responsible for the hydrodynamic threshold observed in the primary collision efficiency since no increase in primary collision lifetime was detected as shear forces were increased (for either surface coating).

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.

Platelet ADP receptors contribute to the initiation of intravascular coagulation

While the adenosine 5'-diphosphate (ADP) pathway is known to enhance thrombus formation by recruiting platelets and leukocytes to the primary layer of collagen-adhering platelets, its role for the initiation of coagulation has not been revealed. Ex vivo inhibition of the P2Y12 ADP receptor by clopidogrel administration diminished the rapid exposure of tissue factor (TF), the major initiator of coagulation, in conjugates of platelets with leukocytes established by the contact of whole blood with fibrillar collagen. Under in vitro conditions, the P2Y12 and P2Y1 ADP receptors were both found to be implicated in the exposure of TF in collagen-activated whole blood. Immunoelectron-microscopy revealed that collagen elicited the release of TF from its storage pools within the platelets. Functional activation of the intravascular TF was reduced by inhibition of the ADP receptors, partially due to the disruption of the platelet-neutrophil adhesions. Injection of collagen into the venous system of mice increased the number of thrombin-antithrombin complexes, indicative for the formation of thrombin in vivo. In P2Y1-deficient mice, the ability of collagen to enhance the generation of thrombin was impaired. In conclusion, the platelet ADP pathway supports the initiation of intravascular coagulation, which is likely to contribute to the concomitant formation of fibrin at the site of the growing thrombus.

P-selectin and leukocyte microparticles are associated with venous thrombogenesis

OBJECTIVES

P-selectin inhibition has been found to limit venous thrombosis. We hypothesize that elevated levels of P-selectin will amplify thrombosis, mediated by procoagulant microparticles (MPs).

METHODS

Male mice (Mus musculus, n659), 20 to 25 grams, underwent IVC ligation to induce thrombosis. Groups consisted of wild type (WT) C57BL/6 controls, mice with high circulating levels of soluble P-selectin (CT), P-selectin gene-interrupted knockout mice (PKO), and E- and P-selectin gene-interrupted mice (EPKO). Additional groups were used to evaluate the ability of a P-sel antagonist (rPSGL-Ig) and an antibody directed against PSGL-1 to downregulate the effects of P-sel in CT mice and WT mice administered soluble P-sel at time of thrombosis. Animals were sacrificed on days 2 and 6 after IVC ligation. Thrombus mass (TM), vein wall morphometrics, and serum leukocyte/platelet microparticles (MPs) were evaluated by means of double-stained fluorescence-activated cell scanning analysis, and soluble P- and E-sel protein determination by ELISA.

RESULTS

At days 2 and 6 in phase I of the experiment, significant differences (P <.01) in TM were noted between groups, with CT animals having the largest thrombi (50% and 57% increase in TM compared to WT at days 2 and 6) while EPKO mice had the smallest thrombi. Statistically, greater levels of neutrophils and total inflammatory cells were noted in the vein walls of CT animals at day 2 compared with WT and PKO animals. A significant difference was noted between CT and EPKO for neutrophils, monocytes, and total inflammatory cells, also at day 2. At day 6, the only statistically significant difference was found for monocytes, with a higher number in the CT animals than in WT animals. The evaluation of MPs revealed that the CT mice had a mixed leukocyte (MAC-1) and platelet (CD41) MP population that was also present in WT and PKO mice on day 2 and day 6. EPKO mice revealed a primarily platelet-derived MP population. Of interest, the CT mice with the highest TM showed a high amount of mean channel fluorescence for MAC-1 (phycoerythrin) antibody, indicative of leukocyte MPs. CT mice revealed statistically higher levels of soluble P-selectin at days 2 and 6. In phase 2, an antibody directed against PSGL-1 was more effective than rPSGL-Ig in decreasing TM and limiting leukocyte-derived MP fluorescence.

CONCLUSIONS

This study demonstrates that high circulating levels of P-selectin are associated with increased thrombosis, whereas a lack of P-selectin and E-selectin is associated with a lessening of thrombosis. Additionally, leukocyte MPs are associated with venous thrombus formation. These data suggest the importance of selectins to venous thrombogenesis and show that P-selectin and leukocyte-derived MPs should be good targets to limit venous thrombus formation.

The coagulation system contributes to synergistic liver injury from exposure to monocrotaline and bacterial lipopolysaccharide

Coexposure to a noninjurious dose of bacterial lipopolysaccharide (LPS; 7.4 x 106 EU/kg) and a nontoxic dose of the food-borne toxin monocrotaline (MCT; 100 mg/kg) leads to synergistic hepatotoxicity in Sprague-Dawley rats. Inflammatory factors, such as Kupffer cells (KCs), tumor necrosis factor-alpha (TNF)-alpha, and neutrophils (polymorphonuclear leukocytes; PMNs), are critical to the pathogenesis. Inasmuch as activation of the coagulation system and sinusoidal endothelial cell (SEC) injury precede hepatic parenchymal cell (HPC) injury, and since fibrin deposition occurs within liver lesions, the coagulation system might be a critical component of injury. In this study, this hypothesis is tested, and the interdependence of the coagulation system and inflammatory factors is explored. Administration of the anticoagulants heparin or warfarin to MCT/LPS-cotreated animals attenuated HPC and SEC injury. Morphometric analysis revealed that anticoagulant treatment significantly reduced the area of centrilobular and midzonal lesions. Heparin treatment also reduced fibrin deposition in these regions. Furthermore, anticoagulant treatment decreased hepatic PMN accumulation but did not affect plasma TNF-alpha concentration. Neither KC inactivation nor TNF-alpha depletion prevented activation of the coagulation system. PMN depletion, however, prevented coagulation system activation, suggesting that PMNs are needed for this response. These results provide evidence that the coagulation system and its interplay with PMNs are important in the pathogenesis of MCT/LPS-induced liver injury.

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

Human neutrophil proteases cathepsin G and elastase can directly alter platelet function and/or participate in coagulation cascade reactions on the platelet or neutrophil surface to enhance fibrin formation. The clotting of recalcified platelet-free plasma (PFP) or platelet-rich plasma (PRP) supplemented with corn trypsin inhibitor (to shut down contact activation) was studied in well-plates or flow assays. Inhibitors of cathepsin G or elastase significantly delayed the burst time (t(50)) of thrombin generation in neutrophil-supplemented PRP from 49 min to 59 and 77 min, respectively, in well-plate assays as well as reduced neutrophil-promoted fibrin deposition on fibrinogen-adherent platelets under flow conditions. In flow assays, purified cathepsin G was a far more potent activator of platelet-dependent coagulation than elastase. Anti-tissue factor had no effect on neutrophil protease-enhanced thrombin formation in PRP. The addition of cathepsin G (425 nm) or convulxin (10 nm) to PRP dramatically reduced the t(50) of thrombin generation from 53 min to 17 or 23 min, respectively. In contrast, the addition of elastase to PRP left the t(50) unaltered. Whereas perfusion of PFP (gamma(w) = 62.5 s(-1)) over fibrinogen-adherent platelets did not result in fibrin formation until 50 min, massive fibrin could be observed on cathepsin G-treated platelets even at 35 min. Cathepsin G addition to corn trypsin inhibitor-treated PFP produced little thrombin unless anionic phospholipid was present. However, further activation inhibition studies indicated that cathepsin G enhances fibrin deposition under flow conditions by elevating the activation state of fibrinogen-adherent platelets rather than by cleaving coagulation factors.

Cell-cell interactions: leukocyte-endothelial interactions

Interactions of leukocytes with endothelial cells are early events in acute and chronic inflammation, immune surveillance of tissues, and wound defense and repair. In contrast with their requisite roles in host defense, dysregulated leukocyte-endothelial interactions mediate inflammatory tissue injury, thrombosis, and other pathologic sequelae. Recent observations also identify dysregulated leukocyte-endothelial interactions in neoplasia and sickle cell vasculopathy. Leukocyte interactions with inflamed endothelial cells are mediated by selectins, signaling molecules that include lipids and chemokines, integrins and their ligands, and junctional molecules. They provide multiple checkpoints for regulation in physiologic inflammation and hemostasis and for dysregulation in pathologic syndromes. Neutrophil-endothelial encounters illustrate a multistep paradigm for inflammatory cell-cell interactions and provide the basis for multiple variations on the central themes.

Adhesion of normal erythrocytes at depressed venous shear rates to activated neutrophils, activated platelets, and fibrin polymerized from plasma

Deep vein thrombosis (DVT) is a low flow pathology often prevented by vascular compression to increase blood movement. We report new heterotypic adhesive interactions of normal erythrocytes operative at low wall shear rates (gamma(w)) below 100 s(-1). Adhesion at gamma(w) = 50 s(-1) of washed red blood cells (RBCs) to fibrinogen-adherent platelets was 4-fold less (P <.005) than to collagen-adherent platelets (279 +/- 105 RBC/mm(2)). This glycoprotein VI (GPVI)-triggered adhesion was antagonized (> 80% reduction) by soluble fibrinogen (3 mg/mL) and ethylenediaminetetraacetic acid (EDTA). RBC-platelet adhesion was reduced in half by antibodies against CD36 or GPIb, but not by antibodies against GPIIb/IIIa, von Willebrand factor (VWF), thrombospondin (TSP), P-selectin, beta(1), alpha(v), or CD47. Adhesion of washed RBCs to fibrinogen-adherent neutrophils was increased 6-fold in the presence of 20 microM N-formyl-Met-Leu-Phe to a level of 67 RBCs per 100 neutrophils after 5 minutes at 50 s(-1). RBC-neutrophil adhesion was diminished by anti-CD11b (76%), anti-RBC Landsteiner-Wiener (LW) (ICAM4; 40%), or by EDTA (> 80%), but not by soluble fibrinogen or antibodies against CD11a, CD11c, CD36, TSP, beta(1), alpha(v), or CD47. RBC adhesion to activated platelets and activated neutrophils was prevented by wall shear stress above 1 dyne/cm(2) (at 100 s(-1)). Whereas washed RBCs did not adhere to fibrin formed from purified fibrinogen, adhesion was marked when pure fibrin was precoated with TSP or when RBCs were perfused over fibrin formed from recalcified plasma. Endothelial activation and unusually low flow may be a setting prone to receptor-mediated RBC adhesion to adherent neutrophils (or platelets/fibrin), all of which may contribute to DVT.