Toll-like receptor 4 signal transduction in platelets: novel pathways

Toll-like Receptors and Thrombopoiesis

Platelets are the second most abundant blood component after red blood cells and can participate in a variety of physiological and pathological functions. Beyond its traditional role in hemostasis and thrombosis, it also plays an indispensable role in inflammatory diseases. However, thrombocytopenia is a common hematologic problem in the clinic, and it presents a proportional relationship with the fatality of many diseases. Therefore, the prevention and treatment of thrombocytopenia is of great importance. The expression of Toll-like receptors (TLRs) is one of the most relevant characteristics of thrombopoiesis and the platelet inflammatory function. We know that the TLR family is found on the surface or inside almost all cells, where they perform many immune functions. Of those, TLR2 and TLR4 are the main stress-inducing members and play an integral role in inflammatory diseases and platelet production and function. Therefore, the aim of this review is to present and discuss the relationship between platelets, inflammation and the TLR family and extend recent research on the influence of the TLR2 and TLR4 pathways and the regulation of platelet production and function. Reviewing the interaction between TLRs and platelets in inflammation may be a research direction or program for the treatment of thrombocytopenia-related and inflammatory-related diseases.

Effect of antiplatelet agents on Escherichia coli sepsis mechanisms: A review

Despite ever-increasing improvements in the prognosis of sepsis, this condition remains a frequent cause of hospitalization and mortality in Western countries. Sepsis exposes the patient to multiple complications, including thrombotic complications, due to the ability of circulating bacteria to activate platelets. One of the bacteria most frequently implicated in sepsis, Escherichia coli, a Gram-negative bacillus, has been described as being capable of inducing platelet activation during sepsis. However, to date, the mechanisms involved in this activation have not been clearly established, due to their multiple characteristics. Many signaling pathways are thought to be involved. At the same time, reports on the use of antiplatelet agents in sepsis to reduce platelet activation have been published, with variable results. To date, their use in sepsis remains controversial. The aim of this review is to summarize the currently available knowledge on the mechanisms of platelet activation secondary to Escherichia coli sepsis, as well as to provide an update on the effects of antiplatelet agents in these pathological circumstances.

Effect of platelet count and platelet transfusion on fever duration in patients with febrile neutropenia

Platelets play a role in hemostasis, thrombosis, and vascular integrity. They also play a major role in the development of inflammation and the activation of immune responses. They have phagocytic activity, stimulate the secretion of immune modulators, and activate other immune cells, which results in platelet-neutrophil aggregation, platelet-induced neutrophil degranulation, and the formation of neutrophil extracellular traps. Data on 124 febrile neutropenia attacks were retrospectively examined. Patients' age, sex, diagnosis, and relapse history were obtained. The complete blood count levels on the first and last febrile day of the febrile neutropenia attacks, duration of fever, and number, type, and timing of thrombocyte suspension transfusions were recorded. The patients were divided into three groups according to the day of fever when the platelet suspension was administered (1 day, 2-3 days, and >3 days); the duration of fever was compared between the three groups. The fever duration of those who were transfused with platelet suspension on the first day of fever was found to be significantly shorter (p = 0.03 and p < 0.001, respectively). When treating a patient with febrile neutropenia, if thrombocyte suspension transfusion is indicated, transfusing thrombocytes in the first days of fever shortens the fever duration and improves the prognosis of febrile neutropenia attack, supporting the hypothesis that not only neutrophils but also platelets may play a role in fighting against microorganisms.

New insights into the role and therapeutic potential of HSP70 in diabetes

Emerging evidence indicates that HSP70 represents a key mechanism in the pathophysiology of β-cell dysfunction, insulin resistance, and various diabetic complications, including micro- and macro-vascular alterations, as well as impaired hemostasis. Hyperglycemia, a hallmark of both types of diabetes, increases the circulating levels of HSP70 (eHSP70), but there is still divergence about whether diabetes up- or down-regulates the intracellular fraction of this protein (iHSP70). Here, we consider that iHSP70 levels reduce in diabetic arterial structures and that the vascular system is in direct contact with all other systems in the body suggesting that a systemic response might also be happening for iHSP70, which is characterized by decreased levels of HSP70 in the vasculature. Furthermore, although many pathways have been proposed to explain HSP70's functions in diabetes, and organs/tissues/cells-specific variations occur, the membrane-bound receptor of the innate immune system, Toll-like receptor 4, and its downstream signal transduction pathways appear to be a constant, not only when we explore the actions of eHSP70, but also when we assess the contributions of iHSP70. In this review, we focus on discussing the multiple roles of HSP70 across organs/tissues/cells affected by hyperglycemia to further explore the possibility of targeting this protein with pharmacological and non-pharmacological approaches in the context of diabetes.

Associations Among Disseminated Intravascular Coagulation, Thrombocytopenia Cytokines/Chemokines and Genetic Polymorphisms of Toll-Like Receptor 2/4 in Chinese Patients with Sepsis

Objectives(s)

Toll-like receptors (TLRs) on platelets have been extensively studied. Both TLR2 and TLR4 have been shown to augment platelet activation and alter its function from a hemostatic regulator to an immune sentinel. However, few studies have investigated the relationship between genetic polymorphisms in TLR2, TLR4 and platelets. We investigated whether genetic polymorphisms of TLR2 and TLR4 were related to thrombocytopenia and coagulation failure in Chinese patients with sepsis.

Basic Methods

Adult Chinese patients with sepsis in the intensive care unit of a university medical center were monitored for up to 28 days. Thrombocytopenia and disseminated intravascular coagulation (DIC), diagnosed using Japanese Association for Acute Medicine (JAAM) criteria, were observed as the primary outcomes. Single-nucleotide polymorphisms (SNPs) in TLR2 (rs111200466, rs5743708) and TLR4 (rs11536889, rs145801336, rs11536896, rs7869402) in patients with sepsis were detected by polymerase chain reaction. The data were analyzed using chi-square and rank sum tests.

Results

The genotype of TLR2 (rs111200466) (Del/Del) was associated with the initial DIC. The genotype of TLR4 (rs11536889) (C/C&C/G) was associated with initial DIC, DIC onset during hospitalization and platelet counts. Furthermore, both DIC and platelet counts were associated with cytokines and chemokines, especially the IL10.

Conclusion

Our results demonstrate that in Chinese sepsis patients, the rs111200466 SNP in TLR2 and rs11536889 SNP in TLR4 are associated with thrombocytopenia and DIC, with potential effects on the TLR4 pathways of platelets.

Effect of heparanase inhibitor on tissue factor overexpression in platelets and endothelial cells induced by anti-β2-GPI antibodies

BACKGROUND

Anti-phospholipid syndrome (APS) is characterized by arterial and/or venous thrombosis and pregnancy morbidity associated with the presence of "anti-phospholipid antibodies." Thrombosis may be the result of a hypercoagulable state related to activation of endothelial cells and platelets by anti-β2-glycoprotein I (β2-GPI) antibodies. Anti-β2-GPI antibodies induce a proinflammatory and procoagulant phenotype in these cells that, after activation, express tissue factor (TF), the major initiator of the clotting cascade, playing a role in thrombotic manifestations. Moreover, TF expression may also be induced by heparanase, an endo-β-D-glucuronidase, that generates heparan sulfate fragments, regulating inflammatory responses.

OBJECTIVES

In this study we analyzed, in human platelets and endothelial cells, the effect of a new symmetrical 2-aminophenyl-benzazolyl-5-acetate derivative (RDS3337), able to inhibit heparanase activity, on signal transduction pathways leading to TF expression triggered by anti-β2-GPI.

METHODS

Platelets and endothelial cells were incubated with affinity purified anti-β2-GPI after pretreatment with RDS3337. Cell lysates were analyzed for phospho-interleukin-1 receptor-associated kinase 1 (IRAK1), phospho-p65 nuclear factor kappa B (NF-κB) and TF by western blot. In addition, platelet activation and secretion by ATP release dosage were evaluated.

RESULTS

IRAK phosphorylation and consequent NF-κB activation, as well as TF expression triggered by anti-β2-GPI treatment were significantly prevented by previous pretreatment with RDS3337. In the same vein, pretreatment with RDS3337 prevented platelet aggregation and ATP release triggered by anti-β2-GPI antibodies.

CONCLUSION

These findings support the view of heparanase involvement in a prothrombotic state related to APS syndrome, suggesting a novel target to regulate overexpression of procoagulant protein(s).

Platelet Innate Immune Receptors and TLRs: A Double-Edged Sword

Platelets are hematopoietic cells whose main function has for a long time been considered to be the maintenance of vascular integrity. They have an essential role in the hemostatic response, but they also have functional capabilities that go far beyond it. This review will provide an overview of platelet functions. Indeed, stress signals may induce platelet apoptosis through proapoptotis or hemostasis receptors, necrosis, and even autophagy. Platelets also interact with immune cells and modulate immune responses in terms of activation, maturation, recruitment and cytokine secretion. This review will also show that platelets, thanks to their wide range of innate immune receptors, and in particular toll-like receptors, and can be considered sentinels actively participating in the immuno-surveillance of the body. We will discuss the diversity of platelet responses following the engagement of these receptors as well as the signaling pathways involved. Finally, we will show that while platelets contribute significantly, via their TLRs, to immune response and inflammation, these receptors also participate in the pathophysiological processes associated with various pathogens and diseases, including cancer and atherosclerosis.

Development and characterization of a novel, megakaryocyte NF-κB reporter cell line for investigating inflammatory responses

Essentials An easily detectable readout in megakaryocyte cell lines will enhance inflammatory research in these cells. Here, we report the development and characterization of a novel megakaryocyte NF-κB-reporter cell line (Meg-01R). Multiple inflammatory molecules modulate NF-κB activity in Meg-01R cells. Meg-01R cells respond to small molecule inhibitors such as IMD0354 and C87 that are known to inhibit NF-κB activity upon stimulation with TNFα. ABSTRACT: Background Because of the difficulties in acquiring large numbers of megakaryocytes, the impact of inflammatory responses on these cells and their ability to produce fully functional platelets under various pathological conditions has not been investigated in detail. Objectives The primary objective of this study is to develop and functionally characterize a novel megakaryocyte nuclear factor κB (NF-κB) reporter cell line to determine the effects of various inflammatory molecules on megakaryocytes and their signalling pathways. Methods A Meg-01-NF-κB-GFP-Luc (Meg-01R) cell line was developed by inserting a reporter NF-κB-GFP-Luc cassette into normal Meg-01 cells to produce luciferase following activation of NF-κB to enable easy detection of pro-inflammatory and reparative signalling. Results and conclusions Meg-01 and Meg-01R cells have comparable characteristics, including the expression of both GPIbα and integrin β3 . Meg-01R cells responded to various inflammatory molecules as measured by NF-κB-dependent bioluminescence. For example, inflammatory molecules such as tumor necrosis factor-α and Pam3CSK4 increased NF-κB activity, whereas an antimicrobial peptide, LL37, reduced its activity. Meg-01R cells were also found to be sensitive to inhibitors (IMD0354 and C87) of inflammatory pathways. Notably, Meg-01R cells were able to respond to lipopolysaccharide (LPS; non-ultrapure), although it was not able to react to ultrapure LPS because of the lack of sufficient TLR4 molecules on their surface. For the first time, we report the development and characterization of a novel megakaryocyte NF-κB reporter cell line (Meg-01R) as a robust tool to study the inflammatory responses/signalling of megakaryocytes upon stimulation with a broad range of inflammatory molecules that can affect NF-κB activity.

The Immune Nature of Platelets Revisited

Platelets are the primary cellular mediators of hemostasis and this function firmly acquaints them with a variety of inflammatory processes. For example, platelets can act as circulating sentinels by expressing Toll-like receptors (TLR) that bind pathogens and this allows platelets to effectively kill them or present them to cells of the immune system. Furthermore, activated platelets secrete and express many pro- and anti-inflammatory molecules that attract and capture circulating leukocytes and direct them to inflamed tissues. In addition, platelets can directly influence adaptive immune responses via secretion of, for example, CD40 and CD40L molecules. Platelets are also the source of most of the microvesicles in the circulation and these miniscule elements further enhance the platelet’s ability to communicate with the immune system. More recently, it has been demonstrated that platelets and their parent cells, the megakaryocytes (MK), can also uptake, process and present both foreign and self-antigens to CD8+ T-cells conferring on them the ability to directly alter adaptive immune responses. This review will highlight several of the non-hemostatic attributes of platelets that clearly and rightfully place them as integral players in immune reactions.

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Platelets can act as circulating sentinels by expressing pathogen-associated molecular pattern receptors that bind pathogens and induce their killing and elimination.

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Activated platelets secrete and express a multitude of pro- and anti-inflammatory molecules that attract and capture circulating leukocytes and direct them to inflamed tissues.

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Platelets express and secrete many critical immunoregulatory molecules that significantly affect both innate and adaptive immune responses.

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Platelets are the primary source of microparticles in the circulation and these augment the platelet’s ability to communicate with the immune system.

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Platelets and megakaryocytes can act as antigen presenting cells and present both foreign- and self-peptides to T-cells.

Effect of ultrapure lipopolysaccharides derived from diverse bacterial species on the modulation of platelet activation

Platelets are small circulating blood cells that play essential roles in the maintenance of haemostasis via blood clotting. However, they also play critical roles in the regulation of innate immune responses. Inflammatory receptors, specifically Toll-like receptor (TLR)-4, have been reported to modify platelet reactivity. A plethora of studies have reported controversial functions of TLR4 in the modulation of platelet function using various chemotypes and preparations of its ligand, lipopolysaccharide (LPS). The method of preparation of LPS may explain these discrepancies however this is not fully understood. Hence, to determine the impact of LPS on platelet activation, we used ultrapure preparations of LPS from Escherichia coli (LPS_EC), Salmonella minnesota (LPS_SM), and Rhodobacter sphaeroides (LPS_RS) and examined their actions under diverse experimental conditions in human platelets. LPS_EC did not affect platelet activation markers such as inside-out signalling to integrin α_IIbβ₃ or P-selectin exposure upon agonist-induced activation in platelet-rich plasma or whole blood whereas LPS_SM and LPS_RS inhibited platelet activation under specific conditions at supraphysiological concentrations. Overall, our data demonstrate that platelet activation is not largely influenced by any of the ultrapure LPS chemotypes used in this study on their own except under certain conditions.

Inflammation in acute coronary syndrome: Expression of TLR2 mRNA is increased in platelets of patients with ACS

Background

Platelets are key components in atherogenesis and determine the course of its clinical sequelae acute coronary syndrome (ACS). Components of the innate immune system—the superfamily of TLR receptors–are present in platelets and represent a link between atherothrombosis and inflammation. We hypothesize that alteration in platelet TLR mRNA expression is a result of inflammation driving coronary atherosclerosis and may represent an alternative platelet activation pathway in ACS.

TLR2-, TLR4- and TLR9- mRNA-expression was determined in ACS patients and compared to patients with invasive exclusion of atherosclerotic lesions of coronary arteries.

Methods

A total of fifty-four patients were enrolled in this clinical retrospective cohort single centre study. Total RNA from sepharose-filtered highly purified platelets was isolated using acid guanidinium thiocyanate-phenol-chloroform extraction and transcribed to cDNA using a first strand cDNA synthesis kit. To determine absolute copy numbers of TLR2, TLR4 and TLR9 we used plasmid based quantitative PCR with normalisation to an internal control.

Results

We found that mRNA expression levels of TLR2 but not TLR 4 and 9 are up-regulated in platelets of patients with ACS when compared to patients without coronary atherosclerosis.

Conclusion

Our results suggest elevated TLR2 mRNA expression in platelets as a biomarker reflecting the underlying inflammation in ACS and possibly severity of coronary atherosclerosis. Platelet TLR2 may represent a link between inflammation and atherothrombosis in ACS.

Crosstalk between Platelet and Bacteria: A Therapeutic Prospect

Platelets are typically recognized for their roles in the maintenance of hemostasis and vascular wall repair to reduce blood loss. Beyond hemostasis, platelets also play a critical role in pathophysiological conditions like atherosclerosis, stroke, thrombosis, and infections. During infection, platelets interact directly and indirectly with bacteria through a wide range of cellular and molecular mechanisms. Platelet surface receptors such as GPIbα, FcγRIIA, GPIIbIIIa, and TLRs, etc. facilitate direct interaction with bacterial cells. Besides, the indirect interaction between platelet and bacteria involves host plasma proteins such as von Willebrand Factor (vWF), fibronectin, IgG, and fibrinogen. Bacterial cells induce platelet activation, aggregation, and thrombus formation in the microvasculature. The activated platelets induce the Neutrophil Extracellular Traps (NETs) formation, which further contribute to thrombosis. Thus, platelets are extensively anticipated as vital immune modulator cells during infection, which may further lead to cardiovascular complications. In this review, we cover the interaction mechanisms between platelets and bacteria that may lead to the development of thrombotic disorders. Platelet receptors and other host molecules involved in such interactions can be used to develop new therapeutic strategies to combat against infection-induced cardiovascular complications. In addition, we highlight other receptor and enzyme targets that may further reduce infection-induced platelet activation and various pathological conditions.

Cell Type-Specific Roles of NF-κB Linking Inflammation and Thrombosis

The transcription factor NF-κB is a central mediator of inflammation with multiple links to thrombotic processes. In this review, we focus on the role of NF-κB signaling in cell types within the vasculature and the circulation that are involved in thrombo-inflammatory processes. All these cells express NF-κB, which mediates important functions in cellular interactions, cell survival and differentiation, as well as expression of cytokines, chemokines, and coagulation factors. Even platelets, as anucleated cells, contain NF-κB family members and their corresponding signaling molecules, which are involved in platelet activation, as well as secondary feedback circuits. The response of endothelial cells to inflammation and NF-κB activation is characterized by the induction of adhesion molecules promoting binding and transmigration of leukocytes, while simultaneously increasing their thrombogenic potential. Paracrine signaling from endothelial cells activates NF-κB in vascular smooth muscle cells and causes a phenotypic switch to a “synthetic” state associated with a decrease in contractile proteins. Monocytes react to inflammatory situations with enforced expression of tissue factor and after differentiation to macrophages with altered polarization. Neutrophils respond with an extension of their life span—and upon full activation they can expel their DNA thereby forming so-called neutrophil extracellular traps (NETs), which exert antibacterial functions, but also induce a strong coagulatory response. This may cause formation of microthrombi that are important for the immobilization of pathogens, a process designated as immunothrombosis. However, deregulation of the complex cellular links between inflammation and thrombosis by unrestrained NET formation or the loss of the endothelial layer due to mechanical rupture or erosion can result in rapid activation and aggregation of platelets and the manifestation of thrombo-inflammatory diseases. Sepsis is an important example of such a disorder caused by a dysregulated host response to infection finally leading to severe coagulopathies. NF-κB is critically involved in these pathophysiological processes as it induces both inflammatory and thrombotic responses.

Toll-Like Receptor 4 Signalling and Its Impact on Platelet Function, Thrombosis, and Haemostasis

Platelets are anucleated blood cells that participate in a wide range of physiological and pathological functions. Their major role is mediating haemostasis and thrombosis. In addition to these classic functions, platelets have emerged as important players in the innate immune system. In particular, they interact with leukocytes, secrete pro- and anti-inflammatory factors, and express a wide range of inflammatory receptors including Toll-like receptors (TLRs), for example, Toll-like receptor 4 (TLR4). TLR4, which is the most extensively studied TLR in nucleated cells, recognises lipopolysaccharides (LPS) that are compounds of the outer surface of Gram-negative bacteria. Unlike other TLRs, TLR4 is able to signal through both the MyD88-dependent and MyD88-independent signalling pathways. Notably, despite both pathways culminating in the activation of transcription factors, TLR4 has a prominent functional impact on platelet activity, haemostasis, and thrombosis. In this review, we summarise the current knowledge on TLR4 signalling in platelets, critically discuss its impact on platelet function, and highlight the open questions in this area.

Review manuscript: Mechanisms of platelet activation by the pneumococcus and the role of platelets in community-acquired pneumonia

There is increasing recognition of the involvement of platelets in orchestrating inflammatory responses, driving the activation of neutrophils, monocytes and vascular endothelium, which, if poorly controlled, may lead to microvascular dysfunction. Importantly, hyperreactivity of platelets has been implicated in the pathogenesis of myocardial injury and the associated particularly high prevalence of acute cardiovascular events in patients with severe community-acquired pneumonia (CAP), of which Streptococcus pneumoniae (pneumococcus) is the most commonly encountered aetiologic agent. In this context, it is noteworthy that a number of studies have documented various mechanisms by which the pneumococcus may directly promote platelet aggregation and activation. The major contributors to platelet activation include several different types of pneumococcal adhesin, the pore-forming toxin, pneumolysin, and possibly pathogen-derived hydrogen peroxide, which collectively represent a major focus of the current review. This is followed by an overview of the limited experimental studies together with a larger series of clinical studies mainly focused on all-cause CAP, which have provided evidence in support of associations between alterations in circulating platelet counts, most commonly thrombocytopenia, and a poor clinical outcome. The final section of the review covers, albeit briefly, systemic biomarkers of platelet activation which may have prognostic potential.

Acetylsalicylic acid differentially limits the activation and expression of cell death markers in human platelets exposed to Staphylococcus aureus strains

Beyond their hemostatic functions, platelets alter their inflammatory response according to the bacterial stimulus. Staphylococcus aureus is associated with exacerbated inflammation and thrombocytopenia, which is associated with poor prognosis during sepsis. Acetylsalicylic acid and statins prevent platelet aggregation and decrease the mortality rate during sepsis. Therefore, we assessed whether these two molecules could reduce in vitro platelet activation and the inflammatory response to S. aureus. Platelets were exposed to clinical strains of S. aureus in the presence or absence of acetylsalicylic acid or fluvastatin. Platelet activation, aggregation, and release of soluble sCD62P, sCD40 Ligand, RANTES and GROα were assessed. Platelet cell death was evaluated by analyzing the mitochondrial membrane potential, phosphatidylserine exposure, platelet microparticle release and caspase-3 activation. All S. aureus strains induced platelet activation but not aggregation and decreased the platelet count, the expression of cell death markers and the release of RANTES and GROα. Acetylsalicylic acid but not fluvastatin limited platelet activation and inflammatory factor release and restored the platelet count by protecting platelets from Staphylococcus-induced expression of cell death markers. This study demonstrates that acetylsalicylic acid limits S. aureus-induced effects on platelets by reducing cell death, revealing new strategies to reduce the platelet contribution to bacteremia-associated inflammation.

NF-κB Links TLR2 and PAR1 to Soluble Immunomodulator Factor Secretion in Human Platelets

The primary toll-like receptor (TLR)-mediated immune cell response pathway common for all TLRs is MyD88-dependent activation of NF-κB, a seminal transcription factor for many chemokines and cytokines. Remarkably, anucleate platelets express the NF-κB machinery, whose role in platelets remains poorly understood. Here, we investigated the contribution of NF-κB in the release of cytokines and serotonin by human platelets, following selective stimulation of TLR2 and protease activated receptor 1 (PAR1), a classical and non-classical pattern-recognition receptor, respectively, able to participate to the innate immune system. We discovered that platelet PAR1 activation drives the process of NF-κB phosphorylation, in contrast to TLR2 activation, which induces a slower phosphorylation process. Conversely, platelet PAR1 and TLR2 activation induces similar ERK1/2, p38, and AKT phosphorylation. Moreover, we found that engagement of platelet TLR2 with its ligand, Pam3CSK4, significantly increases the release of sCD62P, RANTES, and sCD40L; this effect was attenuated by incubating platelets with a blocking anti-TLR2 antibody. This effect appeared selective since no modulation of serotonin secretion was observed following platelet TLR2 activation. Platelet release of sCD62P, RANTES, and sCD40L following TLR2 or PAR1 triggering was abolished in the presence of the NF-κB inhibitor Bay11-7082, while serotonin release following PAR1 activation was significantly decreased. These new findings support the concept that NF-κB is an important player in platelet immunoregulations and functions.

Transfusion as an Inflammation Hit: Knowns and Unknowns

Transfusion of blood cell components is frequent in the therapeutic arsenal; it is globally safe or even very safe. At present, residual clinical manifestations are principally inflammatory in nature. If some rare clinical hazards manifest as acute inflammation symptoms of various origin, most of them linked with conflicting and undesirable biological material accompanying the therapeutic component (infectious pathogen, pathogenic antibody, unwanted antigen, or allergen), the general feature is subtler and less visible, and essentially consists of alloimmunization or febrile non-hemolytic transfusion reaction. The present essay aims to present updates in hematology and immunology that help understand how, when, and why subclinical inflammation underlies alloimmunization and circumstances characteristic of red blood cells and – even more frequently – platelets that contribute inflammatory mediators. Modern transfusion medicine makes sustained efforts to limit such inflammatory hazards; efforts can be successful only if one has a clear view of each element’s role.

Toll-like receptor 9 expression and activation in acute coronary syndrome patients on dual anti-platelet therapy

INTRODUCTION

The Toll-like receptor 9 (TLR9) pathway can activate platelets but its role in acute coronary syndromes (ACS) is unknown. This study examined TLR9 expression and platelet activation in response to ODN2006, a TLR9 agonist, in healthy subjects and in ACS subjects treated with dual anti-platelet therapy (DAPT).

MATERIALS AND METHODS

TLR9 expression was examined in both resting and thrombin receptor activator peptide (TRAP)-activated platelets (1 and 10μM) from healthy and ACS subjects by flow cytometry. In both cohorts, ODN2006-mediated platelet activation (5μM) was examined in whole blood (WB) and platelet-rich plasma (PRP) using cell-surface CD62p and CD63 expression by flow cytometry.

RESULTS

Baseline TLR9 expression was significantly greater in ACS subjects compared to healthy subjects (p<0.01). Following TRAP activation, TLR9 expression increased dose-dependently in healthy subjects. However, no difference in TLR9 expression was seen in ACS platelets following TRAP activation. ODN2006 treatment resulted in significant increases in cell-surface expression of CD62p and CD63 in both WB (all p<0.001) and PRP (all p<0.001) in comparison to unstimulated platelets in healthy subjects. Despite DAPT, ODN2006 treatment produced significant increases in both activation markers in the ACS cohort across WB and PRP (all p<0.0001). Elevated baseline expression of TLR9 in ACS platelets may indicate increased sensitivity to TLR9 agonists and contribute to increased platelet activation in these patients. Furthermore, ODN2006 stimulation can activate platelets in ACS subjects despite treatment with DAPT.

CONCLUSION

This study demonstrates TLR9 expression and activation to be of potential therapeutic importance in ASC patients.

Platelets in Pulmonary Immune Responses and Inflammatory Lung Diseases

Platelets are essential for physiological hemostasis and are central in pathological thrombosis. These are their traditional and best known activities in health and disease. In addition, however, platelets have specializations that broaden their functional repertoire considerably. These functional capabilities, some of which are recently discovered, include the ability to sense and respond to infectious and immune signals and to act as inflammatory effector cells. Human platelets and platelets from mice and other experimental animals can link the innate and adaptive limbs of the immune system and act across the immune continuum, often also linking immune and hemostatic functions. Traditional and newly recognized facets of the biology of platelets are relevant to defensive, physiological immune responses of the lungs and to inflammatory lung diseases. The emerging view of platelets as blood cells that are much more diverse and versatile than previously thought further predicts that additional features of the biology of platelets and of megakaryocytes, the precursors of platelets, will be discovered and that some of these will also influence pulmonary immune defenses and inflammatory injury.

HMGB1: a novel protein that induced platelets active and aggregation via Toll-like receptor-4, NF-κB and cGMP dependent mechanisms

BACKGROUND

Thrombotic diseases are a group of prevalent and life-threatening diseases. Selective inhibition of pathological thrombosis holds the key to treat variety of thrombotic diseases. The pathological thrombosis can be induced by either tissue necrosis and deregulated inflammation. HMGB1, as an important proinflammatory cytokine and a late mediator, also involves on thrombosis disease. However, the underlying mechanisms are not fully understood.

METHODS

Immunofluorescence, ELISA assay, Platelet Aggregation, Thromboelastogram (TEG) analyzes. Flow cytometric analysis and Western blot analysis were used to investigated the role of HMGB1 in platelet aggregation and obtained following observations.

RESULTS

By doing so, we obtained the following observations: i) Highly purified HMGB1 recombinant protein induces platelet aggregation and secretion in a dose-dependent manner in the presence of serum. ii) Low concentration of extracellular HMGB1 could synergistically promote subthreshold concentration of collagen or thrombin induced platelet aggregation. iii) Extracellular HMGB1 promoted platelet aggregation in a platelet-expressed GPIIb/IIIa-dependent manner. iv) We proposed that extracellular HMGB1 seems to promote the phosphorylation of GPIIb/IIIa and subsequent platelet aggregation via TLR4/NF-κB and cGMP pathway.

CONCLUSIONS

In this study, we provide evidence for the hypothesis that HMGB1 interact with platelet might play an important role in the haemostasis and thrombotic diseases. Our research might be provide an interesting avenue for the treatment of thrombotic diseases in the future.

Expression and functionality of Toll-like receptor 3 in the megakaryocytic lineage

BACKGROUND

In addition to their key role in hemostasis, platelets and megakaryocytes regulate immune and inflammatory responses, in part through their expression of Toll-like receptors (TLRs). Among the TLRs, TLR3 recognizes dsRNA associated with viral infection. Thrombocytopenia is a frequent complication of viral infection. However, the expression and functionality of TLR3 in megakaryocytes and platelets is not yet well understood.

OBJECTIVE

To study the expression and functionality of TLR3 in the megakaryocytic lineage.

METHODS AND RESULTS

RT-PCR, flow cytometric and immunofluorescence assays showed that TLR3 is expressed in CD34(+) cells, megakaryocytes, and platelets. Immunoblotting assays showed that stimulation of megakaryocytes with two synthetic agonists of TLR3, Poly(I:C) and Poly(A:U), activated the nuclear factor-κB (NF-κB), phosphoinositide 3-kinase (PI3K)/Akt, extracellular signal-related kinase (ERK)1/2 and p38 pathways. TLR3-megakaryocyte activation resulted in reduced platelet production in vitro and interferon-β release through the PI3K-Akt and NF-κB signaling pathways. TLR3 ligands potentiated the aggregation mediated by classic platelet agonists. This effect was also observed for ATP release, but not for P-selectin or CD40L membrane exposure, indicating that TLR3 activation was not involved in α-granule release. In addition, TLR3 agonists induced activation of the NF-κB, PI3K-Akt and ERK1/2 pathways in platelets. Reductions in platelet production and platelet fibrinogen binding mediated by Poly(I:C) or Poly(A:U) were prevented by the presence of an inhibitor of the TLR3-dsRNA complex.

CONCLUSIONS

Our findings indicate that functional TLR3 is expressed in CD34(+) cells, megakaryocytes, and platelets, and suggest a potential role for this receptor in the megakaryopoiesis/thrombopoiesis alterations that occur in viral infections.

The Inflammatory Role of Platelets via Their TLRs and Siglec Receptors

Platelets are non-nucleated cells that play central roles in the processes of hemostasis, innate immunity, and inflammation; however, several reports show that these distinct functions are more closely linked than initially thought. Platelets express numerous receptors and contain hundreds of secretory products. These receptors and secretory products are instrumental to the platelet functional responses. The capacity of platelets to secrete copious amounts of cytokines, chemokines, and related molecules appears intimately related to the role of the platelet in inflammation. Platelets exhibit non-self-infectious danger detection molecules on their surfaces, including those belonging to the “toll-like receptor” family, as well as pathogen sensors of other natures (Ig- or complement receptors, etc.). These receptors permit platelets to both bind infectious agents and deliver differential signals leading to the secretion of cytokines/chemokines, under the control of specific intracellular regulatory pathways. In contrast, dysfunctional receptors or dysregulation of the intracellular pathway may increase the susceptibility to pathological inflammation. Physiological vs. pathological inflammation is tightly controlled by the sensors of danger expressed in resting, as well as in activated, platelets. These sensors, referred to as pathogen recognition receptors, primarily sense danger signals termed pathogen associated molecular patterns. As platelets are found in inflamed tissues and are involved in auto-immune disorders, it is possible that they can also be stimulated by internal pathogens. In such cases, platelets can also sense danger signals using damage associated molecular patterns (DAMPs). Some of the most significant DAMP family members are the alarmins, to which the Siglec family of molecules belongs. This review examines the role of platelets in anti-infection immunity via their TLRs and Siglec receptors.

Cellular fibronectin containing extra domain A promotes arterial thrombosis in mice through platelet Toll-like receptor 4

Cellular fibronectin containing extra domain A (Fn-EDA+), which is produced in response to tissue injury in several disease states, has prothrombotic activity and is known to interact with Toll-like-receptor 4 (TLR4). The underlying mechanism and cell types involved in mediating the prothrombotic effect of Fn-EDA+ still remain unknown. Using intravital microscopy, we evaluated susceptibility to carotid artery thrombosis after FeCl3-induced injury in mice expressing Fn lacking EDA (Fn-EDA(-/-) mice) or Fn containing EDA (Fn-EDA(+/+) mice). Fn-EDA(-/-) mice exhibited prolonged times to first thrombus formation and complete occlusion and a significant decrease in the rate of thrombus growth (P < .05 vs Fn-EDA(+/+) mice). Genetic deletion of TLR4 reversed the accelerated thrombosis in Fn-EDA(+/+) mice (P < .05) but had no effect in Fn-EDA(-/-) mice. Bone marrow transplantation experiments revealed that TLR4 expressed on hematopoietic cells contributes to accelerated thrombosis in Fn-EDA(+/+) mice. In vitro studies showed that cellular Fn-EDA+ interacts with platelet TLR4 and promotes agonist-induced platelet aggregation. Finally, Fn-EDA(+/+) mice specifically lacking platelet TLR4 exhibited prolonged times to first thrombus formation and complete occlusion (P < .05 vs Fn-EDA(+/+) mice containing platelet TLR4). We conclude that platelet TLR4 contributes to the prothrombotic effect of cellular Fn-EDA+, suggesting another link between thrombosis and innate immunity.

Human platelets express Toll-like receptor 3 and respond to poly I:C

Platelets functions in hemostasis have been widely studied. Currently, growing evidence shows that platelets have also a role in the immune innate response. Recently, protein expression of Toll-like receptors (TLR's) 2, 4, 7, 8, and 9, and the presence of TLRs 1 and 6 mRNA in human platelets was described. Up to now the functionality of TLR-2, 4 and 9 in human platelets has been demonstrated. Due to the relevance of TLRs functions to PAMPS (pathogen-associated molecular patterns) recognizing, we evaluated the presence of TLR3 in human platelets founding low percentages of platelets expressing surface or intracellular TLR3 protein. The activation with thrombin induced an increase in the percentage of platelets expressing surface TLR3 and higher levels of TLR3 expression in the whole population. Human platelets responded to poly I:C by increasing [Ca(2+)]i, the percentages of cells expressing TLR4 and CD62P, and by releasing CXCL4 and IL-1β in comparison to unstimulated platelets. These results demonstrate that human platelets express TLR3 and are capable of responding to poly I:C, suggesting that these cells might influence the immune innate response when detecting viral dsRNA.

Platelet glycoprotein Ib-IX as a regulator of systemic inflammation

OBJECTIVE

The platelet glycoprotein Ib-IX (GP Ib-IX) receptor is a well-characterized adhesion receptor supporting hemostasis and thrombosis via interactions with von Willebrand factor. We examine the GP Ib-IX/von Willebrand factor axis in murine polymicrobial sepsis, as modeled by cecal ligation and puncture (CLP).

APPROACH AND RESULTS

Genetic absence of the GP Ib-IX ligand, von Willebrand factor, prolongs survival after CLP, but absence of the receptor, GP Ib-IX, does not. Because absence of either von Willebrand factor or GP Ib-IX significantly impairs hemostasis and thrombosis, we sought to define additional GP Ib-IX-dependent pathways impacting survival in the CLP model. We document that the absence of GP Ib-IX leads to reduced platelet-neutrophil and platelet-monocyte interactions. Twenty-four hours after CLP, absence of GP Ib-IX coincides with an alteration in cytokine levels, such as tumor necrosis factor-α secreted by monocytes, and increased macrophage-1 antigen expression by neutrophils.

CONCLUSIONS

In contrast to the well-characterized proinflammatory properties of platelets, we describe in the CLP model an anti-inflammatory property associated with platelet GP Ib-IX. Thus, a single platelet receptor displays a dual modulatory role in both the thrombotic and inflammatory pathways associated with polymicrobial sepsis. In sharing leucine-rich motifs with toll-like receptors, platelet GP Ib-IX can be considered a multifunctional participant in hemostasis, thrombosis, and the inflammatory cascade. The results highlight a dynamic role for platelets in systemic inflammation and add to the complex pathophysiologic events that occur during the dysregulated coagulation and inflammation associated with sepsis.

Paths to Understand and to Limit Inflammation in Transfusions

Transfusion is generally very safe. However, in a small number of cases, it leads to mild and even serious - life-threatening - hazards (most often after platelet component transfusion). In cases where no preventable accident happens (that exclude infection, chain error, malpractice, antigen-antibody conflict), the symptomatology resembles inflammation. The present essay reports on the causes of inflammation in transfusion and in which way one may intervene on the different steps of the process -from donor to patient - to limit such incidents/accidents.

Platelet function beyond hemostasis and thrombosis

PURPOSE OF REVIEW

The platelet paradigm that is well established in hemostasis and thrombosis can be extended to other disease states. A consideration for some major health issues, such as inflammation, cancer, infection, and neuroscience, and how platelet function impacts the pathophysiology of each clinical situation is provided.

RECENT FINDINGS

Decades of research and knowledge of platelet function exist and the same is true for inflammation and cancer. The literature is full of platelet biology overlapping into other, nonthrombotic disease states. However, major gaps exist that prevent a complete mechanistic understanding of platelet function in these other diseases. Although much of the overlap provides antidotal relationships, future studies will likely uncover novel pathophysiological pathways that are highly relevant to human diseases.

SUMMARY

Recent findings in four major disease areas, inflammation, cancer, infection, and neuroscience, are described, with current literature linking the disease to platelet function. The availability of antiplatelet therapies, such as aspirin, exists and future consideration can be given as to whether antiplatelet therapy is potentially beneficial or harmful as the mechanisms of platelet involvement are better defined.

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.

Engagement of platelet toll-like receptor 9 by novel endogenous ligands promotes platelet hyperreactivity and thrombosis

RATIONALE

A prothrombotic state and increased platelet reactivity are common in pathophysiological conditions associated with oxidative stress and infections. Such conditions are associated with an appearance of altered-self ligands in circulation that can be recognized by Toll-like receptors (TLRs). Platelets express a number of TLRs, including TLR9; however, the role of TLR in platelet function and thrombosis is poorly understood.

OBJECTIVE

To investigate the biological activities of carboxy(alkylpyrrole) protein adducts, an altered-self ligand generated in oxidative stress, on platelet function and thrombosis.

METHODS AND RESULTS

In this study we show that carboxy(alkylpyrrole) protein adducts represent novel unconventional ligands for TLR9. Furthermore, using human and murine platelets, we demonstrate that carboxy(alkylpyrrole) protein adducts promote platelet activation, granule secretion, and aggregation in vitro and thrombosis in vivo via the TLR9/MyD88 pathway. Platelet activation by TLR9 ligands induces IRAK1 and AKT phosphorylation, and it is Src kinase-dependent. Physiological platelet agonists act synergistically with TLR9 ligands by inducing TLR9 expression on the platelet surface.

CONCLUSIONS

Our study demonstrates that platelet TLR9 is a functional platelet receptor that links oxidative stress, innate immunity, and thrombosis.

Human platelets can discriminate between various bacterial LPS isoforms via TLR4 signaling and differential cytokine secretion

Platelets are currently acknowledged as cells of innate immunity and inflammation and play a complex role in sepsis. We examined whether different types of LPS have different effects on the release of soluble signaling/effective molecules from platelets. We used platelet-rich plasma from healthy volunteers and LPS from two strains of gram-negative bacteria with disparate LPS structures. We combined LPS-stimulated platelet supernatants with reporter cells and measured the PBMC cytokine secretion profiles. Upon stimulation of platelets with both Escherichia coli O111 and Salmonella minnesota LPS, the platelet LPS::TLR4 interaction activated pathways to trigger the production of a large number of molecules. The different platelet supernatants caused differential PBMC secretion of IL-6, TNFα, and IL-8. Our data demonstrate that platelets have the capacity to sense external signals differentially through a single type of pathogen recognition receptor and adjust the innate immune response appropriately for pathogens exhibiting different types of 'danger' signals.

Platelets and cytokines: How and why?

For patients with platelet deficiencies, platelet components are therapeutic products for which there is no substitute. However, transfusion complications are more frequent with this labile blood product than with others. This is attributable to products secreted by the platelets themselves, including a variety of cytokines, chemokines, and biological response modifiers, some of which are secreted in large quantities following platelet activation. Why platelets are activated and prone to releasing these molecules during certain inflammatory and innate immune responses is not yet fully understood, but it could be due to several parameters including incompatibilities between blood donors and recipients, the process of platelet preparation and preservation, and the ability of the donor's immune system to sense danger presented by external stimuli during the blood donation process. This review presents our current knowledge of how the platelets that constitute the platelet component for transfusion are sources of cytokines and biological response modifiers and discusses methods to improve the quality of blood transfusion products and safety for patients.

[Blood platelets and biological response to 'danger' signals and subsequent inflammation: towards a new paradigm?]

Blood platelets are cellular elements of primary haemostasis. During the last decade research on platelets has been subsequently based on this paradigm, with separate observations on issues such as the ability for platelets to bind infectious agents or even engulf them, to drop in counts in case of evolving infectious processes, etc. More recently, novel work has set up bases for novel functions for platelets, as members of functional immune cells, principally in innate immunity but capable of influencing adaptive immunity. Platelets are thus essential to haemostasis and to inflammation, questioning their essential functionality and the set up of a novel paradigm: could platelets be tissue-repairing cells? Such an assumption would open an entire new field of investigations. The present "State of the Art" essay attempts to discuss the main arguments on this.

Pathogen sensing, subsequent signalling, and signalosome in human platelets

Beyond haemostasis, platelets exert a potent role in innate immunity and particularly in its inflammatory arm. The extent of this action remains however debatable, despite clear - and old - evidence of a link between platelets and infection. Platelets can sense infectious pathogens by pathogen recognition receptors and they can even discriminate between various types of infectious signatures. In reply, they can shape their capacity to respond by activating a signalosome and by producing different profiles of pro-inflammatory cytokines and related products. The links between pathogen sensing, signalosome activation and protein production, and their finely tuned regulation are still under investigation since platelets lack a nucleus and thus, canonical molecular biology and genomics apparati.