In vivo effects of lipopolysaccharide and TLR4 on platelet production and activity: implications for thrombotic risk

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.

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.

Thrombosis in COVID-19 infection: Role of platelet activation-mediated immunity

Background

Thrombosis plays an important role in the Coronavrus Disease 2019 (COVID-19) infection-related complications such as acute respiratory distress syndrome and myocardial infarction. Multiple factors such as oxygen demand injuries, endothelial cells injury related to infection, and plaque formation.

Main body

Platelets obtained from the patients may have severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA, showing that the increased activation potential recommends platelet can be hyper-activated in severely ill SARS-CoV-2 cases. Platelets contain multiple receptors that interact with specific ligands. Pathogen’s receptors such as Toll-like receptors (TLRs), NOD-like receptor, C-type lectin receptor family, glycoprotein (GP) such as GPαIIbβ3 and GPIbα which allow pathogens to interact with platelets. Platelet TLRs and NOD2 are involved in platelet activation and thrombosis. Accordingly, TLRs are critical receptors that could recognize various endogenous damage-associated molecular patterns and exogenous pathogen-associated molecular patterns (PAMPs). TLRs are considered as important components in the activation of innate immunity response against pathogenic and non-pathogenic components like damaged tissues. TLRs-1,-2,-4,-6,-7 expression on or within platelets has been reported previously. Various PAMPs were indicated to be capable of binding to platelet-TLRs and inducing both the activation and promotion of downstream proinflammatory signaling cascade.

Conclusion

It is possible that the increased TLRs expression and TLR-mediated platelets activation during COVID-19 may enhance vascular and coronary thrombosis. It may be hypothesized using TLRs antagonist and monoclonal antibody against P-selectin, as the marker of leukocyte recruitment and platelet activation, besides viral therapy provide therapeutic advances in fighting against the thrombosis related complications in COVID-19.

The critical roles of histone deacetylase 3 in the pathogenesis of solid organ injury

Histone deacetylase 3 (HDAC3) plays a crucial role in chromatin remodeling, which, in turn, regulates gene transcription. Hence, HDAC3 has been implicated in various diseases, including ischemic injury, fibrosis, neurodegeneration, infections, and inflammatory conditions. In addition, HDAC3 plays vital roles under physiological conditions by regulating circadian rhythms, metabolism, and development. In this review, we summarize the current knowledge of the physiological functions of HDAC3 and its role in organ injury. We also discuss the therapeutic value of HDAC3 in various diseases.

Dengue virus induces interferon-β by activating RNA sensing pathways in megakaryocytes

Activation of innate receptors in megakaryocytes (MKs) may affect the ability to produce functional platelets. Low platelet count is one of the clinical manifestations of dengue virus (DENV) infection. In MKs, the effect of innate receptors during DENV-infection is not well studied. Here we used MEG-01 cells to investigate DENV serotype 2 induced innate receptors in these cells. DENV RNA was estimated by qRT-PCR in the culture supernatant. The expression of innate receptors was determined by western blot and qPCR. DENV infection led to increased expression of RIG-I at 24 hrs post-infection (hpi) and MDA-5 at 48 and 72 hpi (p<0.05). However, no change in the expression of TLR3 at protein level was observed. Activation of MDA-5 resulted in increased expression of IFN-β and ISG-15 in DENV infected MEG-01 cells, which was further confirmed by MDA-5 siRNA treatment. Apart from inducing innate receptors, DENV significantly decreases the expression of CD61, an activation marker of megakaryocyteson MEG-01 cells as observed by flow cytometry analysis (p<0.01). Results from this study confirm that DENV infection activates the type-I interferon in megakaryocytes and may play a significant role in maturation and development.

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.

Systemic Inflammatory Response Syndrome After Surgery: Mechanisms and Protection

The immune system is an evolutionary hallmark of higher organisms that defends the host against invading pathogens and exogenous infections. This defense includes the recruitment of immune cells to the site of infection and the initiation of an inflammatory response to contain and eliminate pathogens. However, an inflammatory response may also be triggered by noninfectious stimuli such as major surgery, and, in case of an overshooting, still not comprehensively understood reaction, lead to tissue destruction and organ dysfunction. Unfortunately, in some cases, the immune system may not effectively distinguish between stimuli elicited by major surgery, which ideally should only require a modest inflammatory response, and those elicited by trauma or pathogenic infection. Surgical procedures thus represent a potential trigger for systemic inflammation that causes the secretion of proinflammatory cytokines, endothelial dysfunction, glycocalyx damage, activation of neutrophils, and ultimately tissue and multisystem organ destruction. In this review, we discuss and summarize currently available mechanistic knowledge on surgery-associated systemic inflammation, demarcation toward other inflammatory complications, and possible therapeutic options. These options depend on uncovering the underlying mechanisms and could include pharmacologic agents, remote ischemic preconditioning protocols, cytokine blockade or clearance, and optimization of surgical procedures, anesthetic regimens, and perioperative inflammatory diagnostic assessment. Currently, a large gap between basic science and clinically confirmed data exists due to a limited evidence base of translational studies. We thus summarize important steps toward the understanding of the precise time- and space-regulated processes in systemic perioperative inflammation.

Reduced miR-26b Expression in Megakaryocytes and Platelets Contributes to Elevated Level of Platelet Activation Status in Sepsis

In sepsis, platelets may become activated via toll-like receptors (TLRs), causing microvascular thrombosis. Megakaryocytes (MKs) also express these receptors; thus, severe infection may modulate thrombopoiesis. To explore the relevance of altered miRNAs in platelet activation upon sepsis, we first investigated sepsis-induced miRNA expression in platelets of septic patients. The effect of abnormal Dicer level on miRNA expression was also evaluated. miRNAs were profiled in septic vs. normal platelets using TaqMan Open Array. We validated platelet miR-26b with its target SELP (P-selectin) mRNA levels and correlated them with clinical outcomes. The impact of sepsis on MK transcriptome was analyzed in MEG-01 cells after lipopolysaccharide (LPS) treatment by RNA-seq. Sepsis-reduced miR-26b was further studied using Dicer1 siRNA and calpain inhibition in MEG-01 cells. Out of 390 platelet miRNAs detected, there were 121 significantly decreased, and 61 upregulated in sepsis vs. controls. Septic platelets showed attenuated miR-26b, which were associated with disease severity and mortality. SELP mRNA level was elevated in sepsis, especially in platelets with increased mean platelet volume, causing higher P-selectin expression. Downregulation of Dicer1 generated lower miR-26b with higher SELP mRNA, while calpeptin restored miR-26b in MEG-01 cells. In conclusion, decreased miR-26b in MKs and platelets contributes to an increased level of platelet activation status in sepsis.

Activation of toll-like receptors 2 and 4 on CD34+ cells increases human megakaryo/thrombopoiesis induced by thrombopoietin

BACKGROUND

Platelet Toll-like receptor (TLR)2/4 are key players in amplifying the host immune response; however, their role in human megakaryo/thrombopoiesis has not yet been defined.

OBJECTIVES

We evaluated whether Pam3CSK4 or lipopolysaccharide (LPS), TLR2/4 ligands respectively, modulate human megakaryocyte development and platelet production.

METHODS

CD34+ cells from human umbilical cord were stimulated with LPS or Pam3CSK4 with or without thrombopoietin (TPO).

RESULTS

CD34+ cells and megakaryocytes express TLR2 and TLR4 at both RNA and protein level; however, direct stimulation of CD34+ cells with LPS or Pam3CSK4 had no effect on cell growth. Interestingly, both TLR ligands markedly increased TPO-induced CD34+ cell proliferation, megakaryocyte number and maturity, proplatelet and platelet production when added at day 0. In contrast, this synergism was not observed when TLR agonists were added 7 days after TPO addition. Interleukin-6 (IL-6) release was observed upon CD34+ or megakaryocyte stimulation with LPS or Pam3CSK4 but not with TPO and this effect was potentiated in combination with TPO. The increased proliferation and IL-6 production induced by TPO + LPS or Pam3CSK4 were suppressed by TLR2/4 or IL-6 neutralizing antibodies, as well as by PI3K/AKT and nuclear factor-κB inhibitors. Additionally, increased proplatelet and platelet production were associated with enhanced nuclear translocation of nuclear factor-E2. Finally, the supernatants of CD34+ cells stimulated with TPO+LPS-induced CFU-M colonies.

CONCLUSIONS

Our data suggest that the activation of TLR2 and TLR4 in CD34+ cells and megakaryocytes in the presence of TPO may contribute to warrant platelet provision during infection episodes by an autocrine IL-6 loop triggered by PI3K/NF-κB axes.

Platelet Inflammatory Response to Stress

Blood platelets play a central hemostatic role, (i) as they repair vascular epithelial damage, and (ii) they play immune defense roles, as they have the capacity to produce and secrete various cytokines, chemokines, and related products. Platelets sense and respond to local dangers (infectious or not). Platelets, therefore, mediate inflammation, express and use receptors to bind infectious pathogen moieties and endogenous ligands, among other components. Platelets contribute to effective pathogen clearance. Damage-associated molecular patterns (DAMPs) are danger signals released during inflammatory stress, such as burns, trauma and infection. Each pathogen is recognized by its specific molecular signature or pathogen-associated molecular pattern (PAMP). Recent data demonstrate that platelets have the capacity to sense external danger signals (DAMPs or PAMPs) differentially through a distinct type of pathogen recognition receptor (such as Toll-like receptors). Platelets regulate the innate immune response to pathogens and/or endogenous molecules, presenting several types of “danger” signals using a complete signalosome. Platelets, therefore, use complex tools to mediate a wide range of functions from danger sensing to tissue repair. Moreover, we noted that the secretory capacity of stored platelets over time and the development of stress lesions by platelets upon collection, processing, and storage are considered stress signals. The key message of this review is the “inflammatory response to stress” function of platelets in an infectious or non-infectious context.

Megakaryocytes as immune cells

Platelets play well-recognized roles in inflammation, but their cell of origin-the megakaryocyte-is not typically considered an immune lineage. Megakaryocytes are large polyploid cells most commonly identified in bone marrow. Egress via sinusoids enables migration to the pulmonary capillary bed, where elaboration of platelets can continue. Beyond receptors involved in hemostasis and thrombosis, megakaryocytes express receptors that confer immune sensing capacity, including TLRs and Fc-γ receptors. They control the proliferation of hematopoietic cells, facilitate neutrophil egress from marrow, possess the capacity to cross-present antigen, and can promote systemic inflammation through microparticles rich in IL-1. Megakaryocytes internalize other hematopoietic lineages, especially neutrophils, in an intriguing cell-in-cell interaction termed emperipolesis. Together, these observations implicate megakaryocytes as direct participants in inflammation and immunity.

Hypermucoviscous Klebsiella pneumoniae infections induce platelet aggregation and apoptosis and inhibit maturation of megakaryocytes

INTRODUCTION

Different Klebsiella pneumoniae strains carry different virulence factors and antibiotic resistance and may cause thrombocytopenia. This study aimed to investigate the effects of different infections caused by K. pneumoniae on platelets.

METHODS

Two hypermucoviscous K. pneumoniae strains and two classic strains were collected from clinical blood culture, and in both groups, there was a carbapenem-resistant strain and a carbapenem-sensitive strain. Mouse infection models were constructed by intraperitoneally injecting different strains, and mice injected with phosphate-buffered saline served as a control. Count, aggregation rate and apoptosis proportion of platelets within 12 h were examined. CD41 expression was measured in bone marrow cells to determine the maturation of megakaryocytes. The concentrations of lipopolysaccharides and related signaling molecules were also measured.

RESULTS

The platelet aggregation rate was much significantly higher in the two hypermucoviscous groups, while it showed no difference in the classic groups compared to the control group. All infections induced apoptosis of platelets, among which the highest apoptosis proportions were observed in infections caused by the hypermucoviscous carbapenem-sensitive strain. In both hypermucoviscous groups the CD41 mean fluorescence intensity was much lower than that in the control group, indicating that the maturation of megakaryocytes in the hypermucoviscous groups was significantly inhibited. Lipopolysaccharides were significantly higher and TLR4/Myd88 and JNK/MAPK pathways were strongly activated in hypermucoviscous groups.

CONCLUSIONS

The results indicate that hypermucoviscous K. pneumoniae can reduce platelet count by several pathways. Although antibiotic resistance is rapidly emerging worldwide, it has little influence on the decrease in platelets.

Toll-like receptors in immunity and inflammatory diseases: Past, present, and future

The immune system is a very diverse system of the host that evolved during evolution to cope with various pathogens present in the vicinity of environmental surroundings inhabited by multicellular organisms ranging from achordates to chordates (including humans). For example, cells of immune system express various pattern recognition receptors (PRRs) that detect danger via recognizing specific pathogen-associated molecular patterns (PAMPs) and mount a specific immune response. Toll-like receptors (TLRs) are one of these PRRs expressed by various immune cells. However, they were first discovered in the Drosophila melanogaster (common fruit fly) as genes/proteins important in embryonic development and dorso-ventral body patterning/polarity. Till date, 13 different types of TLRs (TLR1-TLR13) have been discovered and described in mammals since the first discovery of TLR4 in humans in late 1997. This discovery of TLR4 in humans revolutionized the field of innate immunity and thus the immunology and host-pathogen interaction. Since then TLRs are found to be expressed on various immune cells and have been targeted for therapeutic drug development for various infectious and inflammatory diseases including cancer. Even, Single nucleotide polymorphisms (SNPs) among various TLR genes have been identified among the different human population and their association with susceptibility/resistance to certain infections and other inflammatory diseases. Thus, in the present review the current and future importance of TLRs in immunity, their pattern of expression among various immune cells along with TLR based therapeutic approach is reviewed.

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TLRs are first described PRRs that revolutionized the biology of host-pathogen interaction and immune response

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The discovery of different TLRs in humans proved milestone in the field of innate immunity and inflammation

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The pattern of expression of all the TLRs expressed by human immune cells

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An association of various TLR SNPs with different inflammatory diseases

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Currently available drugs or vaccines based on TLRs and their future in drug targeting along with the role in reproduction, and regeneration

Translational Implications of Platelets as Vascular First Responders

Platelets play a vital role in normal hemostasis to stem blood loss at sites of vascular injury by tethering and adhering to sites of injury, recruiting other platelets and blood cells to the developing clot, releasing vasoactive small molecules and proteins, and assembling and activating plasma coagulation proteins in a tightly regulated temporal and spatial manner. In synchrony with specific end products of coagulation, primarily cross-linked fibrin, a stable thrombus quickly forms. Far beyond physiological hemostasis and pathological thrombosis, emerging evidence supports platelets playing a pivotal role in vascular homeostasis, inflammation, cellular repair, regeneration, and wide range of autocrine and paracrine functions. In essence, platelets play both structural and functional roles as reporters, messengers, and active transporters surveying the vasculature for cues of environmental or developmental stimuli and participating as first responders.1 In this review, we will provide a contemporary perspective of platelet physiology, including fundamental, translational, and clinical constructs that apply directly to human health and disease.

Mature murine megakaryocytes present antigen-MHC class I molecules to T cells and transfer them to platelets

Megakaryocytes (MKs) are bone marrow-derived cells that are primarily responsible for generating platelets for the maintenance of hemostasis. Although MK can variably express major histocompatibility complex (MHC) class I and II molecules during their differentiation, little is known whether they can elicit nonhemostatic immune functions such as T-cell activation. Here, we demonstrate that mature CD34- MHC class II- CD41+ MKs can endocytose exogenous ovalbumin (OVA) and proteolytically generate its immunogenic peptide ligand, which is crosspresented on their surface in association with MHC class I molecules. This crosspresentation triggered in vitro and in vivo OVA-specific CD8+ T-cell activation and proliferation. In addition, the OVA-MHC class I complexes were transferred from MK to pro-platelets upon thrombopoiesis in vitro. MK could also present endogenous MK-associated (CD61) peptides to activate CD61-specific CD8+ T cells and mediate immune thrombocytopenia in vivo. These results suggest that, in addition to their hemostatic role, mature MKs can significantly affect antigen-specific CD8+ T-cell responses via antigen presentation and are able to spread this immunogenic information through platelets.

A Randomized Placebo Controlled Trial of Aspirin Effects on Immune Activation in Chronically Human Immunodeficiency Virus-Infected Adults on Virologically Suppressive Antiretroviral Therapy

BACKGROUND

Immune activation persists despite suppressive antiretroviral therapy (ART) in human immunodeficiency virus (HIV) infection and predicts non-Acquired Immune Deficiency Syndrome (AIDS) comorbidities including cardiovascular disease. Activated platelets play a key role in atherothrombosis and inflammation, and platelets are hyperactivated in chronic HIV infection. Aspirin is a potent inhibitor of platelet activation through the cyclooxygenase-1 (COX-1) pathway. We hypothesized that platelet activation contributes to immune activation and that aspirin would reduce immune activation and improve endothelial function in ART-suppressed HIV-infected individuals.

METHODS

In this prospective, double-blind, randomized, placebo-controlled 3-arm trial of 121 HIV-infected participants on suppressive ART for >48 weeks, we evaluated the effects of 12 weeks of daily aspirin 100 mg, aspirin 300 mg, or placebo on soluble and cellular immune activation markers, flow-mediated dilation (FMD) of the brachial artery, and serum thromboxane B₂, a direct readout of platelet COX-1 inhibition.

RESULTS

The 300-mg and 100-mg aspirin arms did not differ from placebo in effects on soluble CD14, interleukin (IL)-6, soluble CD163, D-dimer, T-cell or monocyte activation, or the other immunologic endpoints measured. Endothelial function, as measured by FMD, also was not significantly changed when comparing the 300-mg and 100-mg aspirin arms to placebo.

CONCLUSIONS

Aspirin treatment for 12 weeks does not have a major impact on soluble CD14, IL-6, soluble CD163, D-dimer, T-cell or monocyte activation, or FMD, suggesting that inhibition of COX-1-mediated platelet activation does not significantly improve HIV-related immune activation and endothelial dysfunction. Although future studies are needed to further identify the causes and consequences of platelet activation in ART-treated HIV infection, interventions other than COX-1 inhibition will need to be explored to directly reduce immune activation in treated HIV infection.

Histone deacetylases 3 deletion restrains PM2.5-induced mice lung injury by regulating NF-κB and TGF-β/Smad2/3 signaling pathways

Acute lung injury (ALI) as a serious disease with high mortality has been emphasized as a threat to human health and life. Accumulating studies demonstrated that PM2.5 plays a significant role in metabolic and lung diseases. Histone deacetylases 3 (HDAC3) is an important regulator in control of gene transcription, required in up-regulation of inflammation-related signaling, and has been known as a key hotpot in treating a lot of chronic inflammatory diseases. TGF-β/Smad signaling pathway has been proven to be of significance in fibrosis development. Our results found that PM2.5 induced lung function injury in WT mice with a inflammatory responses through the activation of TGF-β/Smad signaling pathways, resulting in lung injury. Of note, HDAC3-deficient mice after PM2.5 administration further promoted TGF-β/Smad signaling pathways activation. In addition, TLR4, p-NF-κB and p-IκBα indicated that HDAC3 knockout mice have a higher inflammation-related signals expression in lung tissue than WT mice after PM2.5 administration, resulting in pro-inflammatory cytokines releasing. Moreover, in vitro experiment of lung epithelial cells challenged with PM2.5, further indicated that TGF-β/Smad2/3 was involved in fibrosis development, leading to inflammation response. Also, the activation of TLR4/NF-κB could be observed in PM2.5-induced lung epithelial cells, leading to inflammation infiltration. These results indicate a new therapeutic target to protect against lung injury caused by PM2.5.

Enterohemorrhagic Escherichia coli Pathogenesis and the Host Response

Enterohemorrhagic Escherichia coli (EHEC) is a highly pathogenic bacterial strain capable of causing watery or bloody diarrhea, the latter termed hemorrhagic colitis, and hemolytic-uremic syndrome (HUS). HUS is defined as the simultaneous development of non-immune hemolytic anemia, thrombocytopenia, and acute renal failure. The mechanism by which EHEC bacteria colonize and cause severe colitis, followed by renal failure with activated blood cells, as well as neurological symptoms, involves the interaction of bacterial virulence factors and specific pathogen-associated molecular patterns with host cells as well as the host response. The innate immune host response comprises the release of antimicrobial peptides as well as cytokines and chemokines in addition to activation and/or injury to leukocytes, platelets, and erythrocytes and activation of the complement system. Some of the bacterial interactions with the host may be protective in nature, but, when excessive, contribute to extensive tissue injury, inflammation, and thrombosis, effects that may worsen the clinical outcome of EHEC infection. This article describes aspects of the host response occurring during EHEC infection and their effects on specific organs.

Micro-concentration Lipopolysaccharide as a Novel Stimulator of Megakaryocytopoiesis that Synergizes with IL-6 for Platelet Production

Lipopolysaccharide (LPS) induces platelet activation and enhances platelet sensitivity to aggregation, which might alter platelet counts. We found that serial doses of micro-concentration LPS significantly increased the platelet count in mice treated with kanamycin, along with increased expression of IL-6 compared with IL-3 and TPO in megakaryocytes obtained from the mouse bone morrow following LPS administration. Furthermore, LPS at lower levels ranging plus IL-6 effectively stimulated CFU-MK formation and increased CD41 expression and megakaryocyte polyploidization. Meanwhile, there was a sustained rise in the percentage of reticulated platelets in the whole blood in response to low-dosage LPS combined with IL-6. In vivo experiments also demonstrated that the administration of LPS combined with IL-6 substantially enhanced the number of circulating platelets in normal and thrombocytopenic mice. Notably, the optimal LPS concentration in combination with IL-6 might be a novel stimulator of TLR4 and IL-6R expression in Dami cell lines, which initially occurs through TLR4-IL-6R crosstalk and then involves the activation of NF-κB and phosphorylation of p38 MAPK. These data suggest a new paradigm for the regulation of megakaryocytopoiesis and platelet production via a synergistic effect of LPS and IL-6, which has the potential to be used for the design of new therapies.

Nouvelle cuisine: platelets served with inflammation

Platelets are small cellular fragments with the primary physiological role of maintaining hemostasis. In addition to this well-described classical function, it is becoming increasingly clear that platelets have an intimate connection with infection and inflammation. This stems from several platelet characteristics, including their ability to bind infectious agents and secrete many immunomodulatory cytokines and chemokines, as well as their expression of receptors for various immune effector and regulatory functions, such as TLRs, which allow them to sense pathogen-associated molecular patterns. Furthermore, platelets contain RNA that can be nascently translated under different environmental stresses, and they are able to release membrane microparticles that can transport inflammatory cargo to inflammatory cells. Interestingly, acute infections can also result in platelet breakdown and thrombocytopenia. This report highlights these relatively new aspects of platelets and, thus, their nonhemostatic nature in an inflammatory setting.

A review on thromboangiitis obliterans pathophysiology: thrombosis and angiitis, which is to blame?

A century has passed since thromboangiitis obliterans (TAO), or Buerger's disease, was first described, but the etiology remains unclear. It is still uncertain as to whether thrombosis or vascular inflammation is the first event. TAO is an episodic inflammatory and thrombotic-occlusive vascular disease of unknown origin. The involvement of the distal vessels and nerves within the neuro-vascular bundles occurs almost always in legs and occasionally in arms. The cumulative data demonstrate that at the cellular and molecular levels, at least four main components of inflammatory reactions, including endothelial cells, platelets, leukocytes and sensory neurons, might be involved in TAO pathogenesis. The interactions among these cells in an altered microenvironment of small- and medium-sized vessels may also orchestrate the onset of TAO events. In this review, the factors that may promote thrombosis and angiitis are reconsidered at three levels: (1) host characteristics such as male gender and genetic background; (2) probable triggers including cigarette smoking and infectious agents; and (3) environmental factors such as chronic anxiety and mental stress as a consequence of low socioeconomic status. At each level, the interactions among vascular endothelium, platelets, leukocytes and sensory neurons are discussed.

Angptl4 is upregulated under inflammatory conditions in the bone marrow of mice, expands myeloid progenitors, and accelerates reconstitution of platelets after myelosuppressive therapy

BACKGROUND

Upon inflammation, myeloid cell generation in the bone marrow (BM) is broadly enhanced by the action of induced cytokines which are produced locally and at multiple sites throughout the body.

METHODS

Using microarray studies, we found that Angptl4 is upregulated in the BM during systemic inflammation.

RESULTS

Recombinant murine Angptl4 (rmAngptl4) stimulated the proliferation of myeloid colony-forming units (CFUs) in vitro. Upon repeated in vivo injections, rmAngptl4 increased BM progenitor cell frequency and this was paralleled by a relative increase in phenotypically defined granulocyte-macrophage progenitors (GMPs). Furthermore, in vivo treatment with rmAngptl4 resulted in elevated platelet counts in steady-state mice while allowing a significant acceleration of reconstitution of platelets after myelosuppressive therapy. The administration of rmAngptl4 increased the number of CD61(+)CD41(low)-expressing megakaryocytes (MK) in the BM of steady-state and in the spleen of transplanted mice. Furthermore, rmAngptl4 improved the in vitro differentiation of immature MKs from hematopoietic stem and progenitor cells. Mechanistically, using a signal transducer and activator of transcription 3 (STAT3) reporter knockin model, we show that rmAngptl4 induces de novo STAT3 expression in immature MK which could be important for the effective expansion of MKs after myelosuppressive therapy.

CONCLUSION

Whereas the definitive role of Angptl4 in mediating the effects of lipopolysaccharide (LPS) on the BM has to be demonstrated by further studies involving multiple cytokine knockouts, our data suggest that Angptl4 plays a critical role during hematopoietic, especially megakaryopoietic, reconstitution following stem cell transplantation.

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.

Platelets and infections - complex interactions with bacteria

Platelets can be considered sentinels of vascular system due to their high number in the circulation and to the range of functional immunoreceptors they express. Platelets express a wide range of potential bacterial receptors, including complement receptors, FcγRII, Toll-like receptors but also integrins conventionally described in the hemostatic response, such as GPIIb–IIIa or GPIb. Bacteria bind these receptors either directly, or indirectly via fibrinogen, fibronectin, the first complement C1q, the von Willebrand Factor, etc. The fate of platelet-bound bacteria is questioned. Several studies reported the ability of activated platelets to internalize bacteria such as Staphylococcus aureus or Porphyromonas gingivalis, though there is no clue on what happens thereafter. Are they sheltered from the immune system in the cytoplasm of platelets or are they lysed? Indeed, while the presence of phagolysosome has not been demonstrated in platelets, they contain antimicrobial peptides that were shown to be efficient on S. aureus. Besides, the fact that bacteria can bind to platelets via receptors involved in hemostasis suggests that they may induce aggregation; this has indeed been described for Streptococcus sanguinis, S. epidermidis, or C. pneumoniae. On the other hand, platelets are able to display an inflammatory response to an infectious triggering. We, and others, have shown that platelet release soluble immunomodulatory factors upon stimulation by bacterial components. Moreover, interactions between bacteria and platelets are not limited to only these two partners. Indeed, platelets are also essential for the formation of neutrophil extracellular traps by neutrophils, resulting in bacterial clearance by trapping bacteria and concentrating antibacterial factors but in enhancing thrombosis. In conclusion, the platelet–bacteria interplay is a complex game; its fine analysis is complicated by the fact that the inflammatory component adds to the aggregation response.

Platelets promote tumour metastasis via interaction between TLR4 and tumour cell-released high-mobility group box1 protein

Increasing evidence suggests that TLR4 expression by tumour cells promotes tumour progression, but it is unclear whether TLR4 is involved in metastasis. Here we show that TLR4 deficiency significantly diminishes experimental lung metastasis without affecting primary tumour growth. Bone marrow transplantation experiment and application of antiplatelet agents in mice demonstrate that TLR4 on platelets plays an important role in metastasis. TLR4 is critical for platelet-tumour cell interaction in vitro. Furthermore, high-mobility group box1 (HMGB1) neutralization attenuates platelet-tumour cell interaction in vitro and metastasis in vivo in a TLR4-dependent manner, indicating that tumour cell-released HMGB1 is the key factor that interacts with TLR4 on platelets and mediates platelet-tumour cell interaction, which promotes metastasis. These findings demonstrate a mechanism by which platelets promote tumour cell metastasis and suggest TLR4, and its endogenous ligand HMGB1 as targets for antimetastatic therapies.

Cardioprotective effect of carvedilol: inhibition of apoptosis in H9c2 cardiomyocytes via the TLR4/NF-κB pathway following ischemia/reperfusion injury

Carvedilol is a non-selective β-blocker used in the treatment of cardiovascular disease, including myocardial ischemia. The aim of the present study was to investigate the molecular mechanisms underlying the effects of carvedilol on simulated ischemia/reperfusion (SI/R)-induced cardiomyocyte apoptosis in vitro. H9c2 cardiomyocytes were incubated with either a vehicle or an ischemic buffer during hypoxia followed by reoxygenation with or without carvedilol. In two additional groups, toll-like receptor 4 (TLR4) and nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) were inhibited by a TLR4 antibody and pyrrolidine dithiocarbamate, respectively. The results revealed that carvedilol markedly decreased SI/R-induced apoptosis in a concentration-dependent manner, as demonstrated by flow-cytometric analysis. This effect was shown to be associated with an increase in the B-cell lymphoma 2 (Bcl-2)/Bcl-2-associated X (Bax) protein ratio and concurrent reductions in the expression levels of TLR4 and NF-κB. These results suggest that carvedilol provides significant cardioprotection against SI/R-induced injury in H9c2 cardiomyocytes, an effect likely to be mediated through the TLR4/NF-κB signaling pathway.

The role of calpain-myosin 9-Rab7b pathway in mediating the expression of Toll-like receptor 4 in platelets: a novel mechanism involved in α-granules trafficking

Toll-like receptors (TLRs) plays a critical role in innate immunity. In 2004, Aslam R. and Shiraki R. first determined that murine and human platelets express functional TLRs. Additionally, Andonegui G. demonstrated that platelets express TLR4, which contributes to thrombocytopenia. However, the underlying mechanisms of TLR4 expression by platelets have been rarely explored until now. The aim of this study was to identify the mechanism of TLR4 expression underlying thrombin treatment. The human washed platelets were used in this study. According to flowcytometry and western blot analysis, the surface levels of TLR4 were significantly enhanced in thrombin-activated human platelets and decreased by TMB-8, calpeptin, and U73122, but not Y27632 (a Rho-associated protein kinase ROCK inhibitor) indicating that thrombin-mediated TLR4 expression was modulated by PAR/PLC pathway, calcium and calpain. Co-immunoprecipitation (co-IP) assay demonstrated that the interaction between TLR4 and myosin-9 (a substrate of calpain) was regulated by calpain; cleavage of myosin-9 enhanced TLR4 expression in thrombin treated platelets. Transmission electron microscope data indicated that human platelets used α-granules to control TLR4 expression; the co-IP experiment suggested that myosin-9 did not coordinate with Rab7b to negatively regulate TLR4 trafficking in thrombin treated platelets. In summary, phospholipase Cγ-calpain-myosin 9-Rab7b axis was responsible for the mechanism underlying the regulation of TLR4 containing α-granules trafficking in thrombin-stimulated platelets, which was involved in coagulation.

Aspirin attenuates platelet activation and immune activation in HIV-1-infected subjects on antiretroviral therapy: a pilot study

BACKGROUND

Mechanisms for increased cardiovascular risk in HIV-1-infected adults are incompletely understood, but platelet activation and immune activation leading to a prothrombotic state have been proposed as significant contributors. Aspirin has antiplatelet and immunomodulatory properties. We explored whether 1 week of low-dose aspirin attenuates platelet activation and immune activation in HIV-1-infected and virologically suppressed adults on antiretroviral therapy.

METHODS

Platelet activation and immune activation were measured in HIV-1-infected subjects virologically suppressed on antiretroviral therapy and controls before and after 1 week of low-dose aspirin.

RESULTS

Compared with control subjects, HIV-1-infected subjects had increased platelet activation, as measured by spontaneous platelet aggregation and aggregation in response to adenosine diphosphate, collagen, and arachidonic acid. After aspirin therapy, percent aggregation decreased similarly in both HIV-1-infected and control subjects to all platelet agonists tested except aggregation in response to arachidonic acid, which remained elevated in the HIV-1-infected group. HIV-1-infected subjects exhibited increased markers of T-cell activation (CD38 and HLA-DR) and monocyte activation (sCD14), which decreased after 1 week of aspirin therapy. Moreover, leukocyte responses to Toll-like receptor stimulation were enhanced after 1 week of aspirin therapy. In vitro studies showed that HIV-1 plasma could activate healthy platelets, which in turn activated monocytes, implicating a direct role for activated platelets in immune activation.

CONCLUSIONS

Our data demonstrate that heightened platelet activation and immune activation in treated HIV-1 disease are attenuated by 1 week of aspirin therapy. Aspirin should be further studied for its antithrombotic and immunomodulatory benefits in treated HIV-1 disease.

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.

Platelet activation suppresses HIV-1 infection of T cells

Background

Platelets, anucleate cell fragments abundant in human blood, can capture HIV-1 and platelet counts have been associated with viral load and disease progression. However, the impact of platelets on HIV-1 infection of T cells is unclear.

Results

We found that platelets suppress HIV-1 spread in co-cultured T cells in a concentration-dependent manner. Platelets containing granules inhibited HIV-1 spread in T cells more efficiently than degranulated platelets, indicating that the granule content might exert antiviral activity. Indeed, supernatants from activated and thus degranulated platelets suppressed HIV-1 infection. Infection was inhibited at the stage of host cell entry and inhibition was independent of the viral strain or coreceptor tropism. In contrast, blockade of HIV-2 and SIV entry was less efficient. The chemokine CXCL4, a major component of platelet granules, blocked HIV-1 entry and neutralization of CXCL4 in platelet supernatants largely abrogated their anti-HIV-1 activity.

Conclusions

Release of CXCL4 by activated platelets inhibits HIV-1 infection of adjacent T cells at the stage of virus entry. The inhibitory activity of platelet-derived CXCL4 suggests a role of platelets in the defense against infection by HIV-1 and potentially other pathogens.

Platelet-derived Toll-like receptor 4 (Tlr-4) is sufficient to promote microvascular thrombosis in endotoxemia

Endotoxin (lipopolysaccharide, LPS) produced by gram-negative bacteria initiates a host of pro-inflammatory effects through Toll-like receptor 4 (TLR-4). We reported previously that LPS enhances microvascular thrombosis in cremaster venules of wild-type mice, but had no effect in mice deficient in TLR-4. Since TLR-4 is expressed on various cell types, the cellular origin of TLR-4 responsible for the LPS-enhanced thrombosis remains undetermined. Platelets are known to express functional TLR-4. Platelet-derived TLR-4 has been suggested to mediate various inflammatory responses in endotoxemia, including production of tumor necrosis factor alpha (TNF-α) and interleukin-1 beta (IL-1β), two cytokines reported to enhance microvascular thrombosis. We determined whether platelet-derived TLR-4 was sufficient to mediate the enhanced thrombosis induced by endotoxin and whether these responses were accompanied by systemic increases in TNF-α and IL-1β. We isolated platelets from wild-type mice and transfused them into either of two strains of TLR-4-deficient mice (C57BL/10ScN and B6.B10ScN-TLR-4^lps-del/Jth). The mice were then injected with LPS or saline, and the kinetics of thrombosis were studied 4 hours later. Transfusion of wild-type platelets restored responsiveness to LPS in TLR-4-deficient mice with regards to microvascular thrombosis but not to plasma levels of TNF-α or IL-1β. The accelerated rates of microvascular thrombosis induced by platelet transfusions were specific to TLR-4, since isolation and transfusion of platelets derived from TLR-4-deficient donors did not restore responsiveness to LPS. These studies demonstrate that platelet-derived TLR-4 is sufficient to promote microvascular thrombosis in endotoxemia, independent of systemic increases in TNF-α or IL-1β.

Microvesicles at the crossroads between infection and cardiovascular diseases

Observational and experimental studies continue to support the association of infection and infection-stimulated inflammation with development of cardiovascular disease (CVD) including atherosclerosis and thrombosis. Microvesicles (MV) are heterogeneous populations of sealed membrane-derived vesicles shed into circulation by activated mammalian cells and/or pathogenic microbes that may represent an interface between bacterial/microbial infection and increased risk of CVD. This review evaluates how MV act to modulate and intersect immunological and inflammatory responses to infection with particular attention to progression of CVD. Although infection-related stimuli provoke release of MV from blood and vascular cells, MV express phosphatidylserine and other procoagulant factors on their surface, which initiate and amplify blood coagulation. In addition, MV mediate cell-cell adhesion, which may stimulate production of pro-inflammatory cytokines in vascular cells, which in turn aggravate progression of CVD and propagate atherothrombosis. MV transfer membrane receptors, RNA and proteins among cells, and present auto-antigens from their cells of origin to proximal or remote target cells. Because MV harbor cell surface proteins and contain cytoplasmic components of the parent cell, they mediate biological messages and play a pivotal role in the crossroad between infection-stimulated inflammation and CVDs.

Leukocyte- and platelet-derived microvesicle interactions following in vitro and in vivo activation of toll-like receptor 4 by lipopolysaccharide

Background

Pro-coagulant membrane microvesicles (MV) derived from platelets and leukocytes are shed into the circulation following receptor-mediated activation, cell-cell interaction, and apoptosis. Platelets are sentinel markers of toll-like receptor 4 (TLR4) activation. Experiments were designed to evaluate the time course and mechanism of direct interactions between platelets and leukocytes following acute activation of TLR4 by bacterial lipopolysaccharide (LPS).

Methodology/Principal Findings

Blood from age-matched male and female wild type (WT) and TLR4 gene deleted (dTLR4) mice was incubated with ultra-pure E. coli LPS (500 ng/ml) for up to one hour. At designated periods, leukocyte antigen positive platelets, platelet antigen positive leukocytes and cell-derived MV were quantified by flow cytometry. Numbers of platelet- or leukocyte-derived MV did not increase within one hour following in vitro exposure of blood to LPS. However, with LPS stimulation numbers of platelets staining positive for both platelet- and leukocyte-specific antigens increased in blood derived from WT but not dTLR4 mice. This effect was blocked by inhibition of TLR4 signaling mediated by My88 and TRIF. Seven days after a single intravenous injection of LPS (500 ng/mouse or 20 ng/gm body wt) to WT mice, none of the platelets stained for leukocyte antigen. However, granulocytes, monocytes and apoptotic bodies stained positive for platelet antigens.

Conclusions/Significance

Within one hour of exposure to LPS, leukocytes exchange surface antigens with platelets through TLR4 activation. In vivo, leukocyte expression of platelet antigen is retained after a single exposure to LPS following turn over of the platelet pool. Acute expression of leukocyte antigen on platelets within one hour of exposure to LPS and the sustained expression of platelet antigen on leukocytes following a single acute exposure to LPS in vivo explains, in part, associations of platelets and leukocytes in response to bacterial infection and changes in thrombotic propensity of the blood.

Platelets as immune mediators: their role in host defense responses and sepsis

Platelets occupy a central role at the interface between thrombosis and inflammation. At sites of vascular damage, adherent platelets physically and functionally interact with circulating leukocytes. Activated platelets release soluble factors into circulation that may have local and systemic effects on blood and vascular cells. Platelets can also interact with a wide variety of microbial pathogens. Emerging evidence from animal models suggests that platelets may participate in a wide variety of processes involving tissue injury, immune responses and repair that underlie diverse diseases such as atherosclerosis, autoimmune disorders, inflammatory lung and bowel disorders, host-defense responses and sepsis. In this review, we summarize the general mechanisms by which platelets may contribute to immune function, and then discuss evidence for their role in host defense responses and sepsis from preclinical and clinical studies.

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.

Platelets and innate immunity

Although platelets are best known as primary mediators of hemostasis, this function intimately associates them with inflammatory processes, and it has been increasingly recognized that platelets play an active role in both innate and adaptive immunity. For example, platelet adhesive interactions with leukocytes and endothelial cells via P-selectin can lead to several pro-inflammatory events, including leukocyte rolling and activation, production of cytokine cascades, and recruitment of the leukocytes to sites of tissue damage. Superimposed on this, platelets express immunologically-related molecules such as CD40L and Toll-like receptors that have been shown to functionally modulate innate immunity. Furthermore, platelets themselves can interact with microorganisms, and several viruses have been shown to cross-react immunologically with platelet antigens. This review discusses the central role that platelets play in inflammation, linking them with varied pathological conditions, such as atherosclerosis, sepsis, and immune thrombocytopenic purpura, and suggests that platelets also act as primary mediators of our innate defences.

Aggregation and microparticle production through toll-like receptor 4 activation in platelets from recently menopausal women

Bacterial infection may increase risk for thrombosis and atherosclerosis. Human platelets express toll-like receptor 4 (TLR4), the receptor for gram-negative bacterial lipopolysaccharide (LPS). Experiments were designed to evaluate direct, acute effects of TLR4 activation on aggregation, secretion, and generation of prothrombogenic microparticles in vitro on platelets derived from healthy women at risk for development of cardiovascular disease because of their hormonal status. Platelet-rich plasma from recently menopausal women was incubated with ultrapure Escherichia coli LPS in the absence or presence of antibodies that neutralize the human TLR4. Incubating platelets with LPS (100 ng/mL) for 5 minutes decreased aggregation and dense granule adenosine triphosphate secretion induced by thrombin receptor agonist peptide (TRAP) but not by adenosine diphosphate or collagen. The antibody to TLR4 blocked this effect of LPS. TLR4 activation increased phosphorylation of p38 mitogen-activated protein kinase and decreased production of prothrombotic phosphatidylserine and P-selectin-positive microparticles in response to TRAP. Therefore, acute, direct activation of TLR4 reduces platelet reactivity to TRAP stimulation in vitro. Increased thrombotic and cardiovascular risk with bacterial infection most likely reflects the sum of TLR4 activation on other blood and vascular cells to release proinflammatory cytokines/chemokines, which indirectly affect platelet reactivity.

The role of inflammation in regulating platelet production and function: Toll-like receptors in platelets and megakaryocytes

Platelets have been extensively studied as hemostatic regulators, stopping uncontrolled flow of blood from an injured vessel and allowing for repair. However, multiple studies have shown that platelets can interact with bacterial proteins, particularly seen during sepsis and inflammation. Immune cells recognize pathogens through Toll-like Receptors (TLRs). These same receptors allow platelets to recognize bacterial proteins and regulate platelet immunity and function. This review examines the TLRs expressed on platelets and megakaryocytes and how these receptors affect the function of these cells. Through TLRs, platelets go beyond hemostatic regulation and play a pivotal role in inflammation and infection.

Innate immune signals in atherosclerosis

Atherosclerosis is a chronic disease characterised by lipid retention and inflammation in the arterial intima. Innate immune mechanisms are central to atherogenesis, involving activation of pattern-recognition receptors (PRRs) and induction of inflammatory processes. In a complex tissue, such as the atherosclerotic lesion, innate signals can originate from several sources and promote atherogenesis through ligation of PRRs. The receptors recognise conserved molecular patterns on pathogens and endogenous products of tissue injury and inflammation. Activation of PRRs might affect several aspects of atherosclerosis by acting on lesion resident cells. Scavenger receptors mediate antigen uptake and clearance of lipoproteins, thereby promoting foam cell formation. Signalling receptors, such as Toll-like receptors (TLRs), lead to induction of pro-inflammatory cytokines and antigen-specific immune responses. In this review we describe the innate mechanisms present in the plaque. We focus on TLRs, their cross-talk with other PRRs, and how their signalling cascades influence inflammation within the atherosclerotic lesion.

Innate immunity and toll-like receptor antagonists: a potential role in the treatment of cardiovascular diseases

Toll-like receptors (TLRs) are germline-encoded receptors that recognize various pathogen-associated molecular patterns (PAMPs). They are key components of the innate immunity which are activated in response to pathogens as well as non-pathogenic components of damaged tissues. TLR agonists have been developed to treat allergies, cancers, and chronic infections by upregulating the innate immune system. TLR antagonists may be used to treat a number of inflammatory conditions, such as rheumatoid arthritis and systemic lupus erythematosus. Recent research also has shown that TLRs are involved in the pathogenesis of atherosclerosis, thrombosis, myocardial remodeling, ischemic/reperfusion injury, and valvular disease. This article reviews the current experimental and clinical evidence for the role of TLRs in the cardiovascular system, and examines the mechanisms by which TLR antagonists could potentially be used in targeted therapy.

Circulating microparticles and endogenous estrogen in newly menopausal women

BACKGROUND

Estrogen modulates antithrombotic characteristics of the vascular endothelium and the interaction of blood elements with the vascular surface. A marker of these modulatory activities is formation of cell-specific microparticles. This study examined the relationship between blood-borne microparticles and endogenous estrogen at menopause.

METHODS

Platelet activation and plasma microparticles were characterized from women being screened (n = 146) for the Kronos Early Estrogen Prevention Study. Women were grouped according to serum estrogen (< 20 pg/ml; low estrogen, n = 21 or > 40 pg/ml; high estrogen, n = 11).

RESULTS

Age, body mass index, blood pressure and blood chemistries were the same in both groups. No woman was hypertensive, diabetic or a current smoker. Platelet counts, basal and activated expression of P-selectin on platelet membranes were the same, but activated expression of glycoprotein IIb/IIIa was greater in the high-estrogen group. Numbers of endothelium-, platelet-, monocyte- and granulocyte-derived microparticles were greater in the low-estrogen group. Of the total numbers of microparticles, those positive for phosphatidylserine and tissue factor were also greater in the low-estrogen group.

CONCLUSION

These results suggest that, with declines in endogenous estrogen at menopause, numbers of procoagulant microparticles increase and thus may provide a means to explore mechanisms for cardiovascular risk development in newly menopausal women.

[Platelet immunology and the immune response]

Platelets exert not only hemostatic activities, but also pro-inflammatory effects. Platelet-linked inflammation seems essentially related to their capacity of secreting cytokines, chemokines and related molecules. This activity is important in terms of concentration of secreted products. This secretory function confers to platelets a regulatory role in immunity. Besides, platelets do exhibit non-self infectious danger detection molecules on their surfaces, belonging in particular to the "Toll-like receptor family"; through this property, platelets can bind infectious agents but also deliver differential signals for the secretion of cytokines and chemokines. Platelets, which are non-nucleated cells deprived of nuclear DNA, possess however some cellular machinery which permits intracellular signalling and even the production of RNA transcripts for certain cytokines. Last, platelets express variant surface determinants of hemostatic molecules (referred to as HPA antigens) along with HLA class I variant molecules, the function of which on platelets is still unknown. An intriguing question is to reconcile those diverse properties and to understand whether the pro-inflammatory secretory process can affect the immunogenicity of transfused, allogeneic, platelet components.

Stimulation of Toll-like receptor 2 in human platelets induces a thromboinflammatory response through activation of phosphoinositide 3-kinase

Cells of the innate immune system use Toll-like receptors (TLRs) to initiate the proinflammatory response to microbial infection. Recent studies have shown acute infections are associated with a transient increase in the risk of vascular thrombotic events. Although platelets play a central role in acute thrombosis and accumulating evidence demonstrates their role in inflammation and innate immunity, investigations into the expression and functionality of platelet TLRs have been limited. In the present study, we demonstrate that human platelets express TLR2, TLR1, and TLR6. Incubation of isolated platelets with Pam(3)CSK4, a synthetic TLR2/TLR1 agonist, directly induced platelet aggregation and adhesion to collagen. These functional responses were inhibited in TLR2-deficient mice and, in human platelets, by pretreatment with TLR2-blocking antibody. Stimulation of platelet TLR2 also increased P-selectin surface expression, activation of integrin alpha(IIb)beta(3), generation of reactive oxygen species, and, in human whole blood, formation of platelet-neutrophil heterotypic aggregates. TLR2 stimulation also activated the phosphoinositide 3-kinase (PI3-K)/Akt signaling pathway in platelets, and inhibition of PI3-K significantly reduced Pam(3)CSK4-induced platelet responses. In vivo challenge with live Porphyromonas gingivalis, a Gram-negative pathogenic bacterium that uses TLR2 for innate immune signaling, also induced significant formation of platelet-neutrophil aggregates in wild-type but not TLR2-deficient mice. Together, these data provide the first demonstration that human platelets express functional TLR2 capable of recognizing bacterial components and activating the platelet thrombotic and/or inflammatory pathways. This work substantiates the role of platelets in the immune and inflammatory response and suggests a mechanism by which bacteria could directly activate platelets.

[Platelets "Toll-like receptor" engagement stimulates the release of immunomodulating molecules]

Platelets are nonnucleated cellular elements that play a role in the process of haemostasis, and also in various ways in innate immunity and in inflammation. Platelets also contain numerous secretory products and can exert critical roles in several aspects of haemostasis. In addition, they house and secrete a variety of cytokines, chemokines and associated molecules which behave as ligands for receptors/counterparts displayed by endothelial cells lining tissue vessels and most leukocyte subsets. These latter studies show that platelets have an important role in innate as well as adaptive immunity; thus platelets can take part in an immune directive response. Moreover, platelets display receptors for several types of cytokines/chemokines along with FcgammaRII receptors. Finally, platelets not only express a variety of Toll-like receptors, with recently identified functions or not as-yet fully identified, but have also been demonstrated to express the key tandem pair of inflammatory and antigen presentation molecules (CD40 and CD40-ligand/CD154), this latter function making them the major purveyors of soluble CD40L in the plasma. It appears that platelets may be regarded as one of the neglected components of immune cell regulators, and platelets contribute to some interesting aspects in bridging innate and adaptive immunity. We propose that platelets discriminate danger signals and adapt the subsequent responses, with polarized cytokine secretion. Platelets may recognize several types of infectious pathogens and limit microbial colonization by sequestering these pathogens and releasing immunomodulatory factors. This review allows us to re-explore indications that platelets exert direct anti-infection immunity and we will present experimentally-driven arguments in favour of a role of platelet TLR in regulating certain immune activities.

Estrogen, inflammation, and platelet phenotype

BACKGROUND

Although exogenous estrogenic therapies increase the risk of thrombosis, the effects of estrogen on formed elements of blood are uncertain.

OBJECTIVE

This article examines the genomic and nongenomic actions of estrogen on platelet phenotype that may contribute to increased thrombotic risk.

METHODS

To determine aggregation, secretion, protein expression, and thrombin generation, platelets were collected from experimental animals of varying hormonal status and from women enrolled in the Kronos Early Estrogen Prevention Study.

RESULTS

Estrogen receptor beta predominates in circulating platelets. Estrogenic treatment in ovariectomized animals decreased platelet aggregation and adenosine triphosphate (ATP) secretion. However, acute exposure to 17beta-estradiol did not reverse decreases in platelet ATP secretion invoked by lipopolysaccharide. Thrombin generation was positively correlated to the number of circulating microvesicles expressing phosphatidylserine.

CONCLUSION

Assessing the effect of estrogen treatments on blood platelets may lead to new ways of identifying women at risk for adverse thrombotic events with such therapies.