Invasive Streptococcus pneumoniae trigger platelet activation via Toll-like receptor 2

Platelet aggregation responses to Salmonella Typhimurium are determined by host anti-Salmonella antibody levels

Invasive non-typhoidal Salmonella infections are responsible for >75 000 deaths/year and >500 000 cases/year globally. Seventy-five percent of these cases occur in Sub-Saharan Africa, an increasing number of which are from multi-drug resistant strains. Interactions between bacteria and platelets can lead to thrombus formation, which can be beneficial for control of infection (immunothrombosis), or harmful through uncontrolled inflammation and organ damage (thromboinflammation). It is unknown whether Salmonella Typhimurium can activate human platelets. To assess this, light transmission aggregometry was used to measure platelet activation by two different Salmonella Typhimurium strains in 26 healthy donors in platelet-rich plasma and washed platelets. In platelet-rich plasma, but not in washed platelets, Salmonella Typhimurium activated platelets in a donor- and strain-dependent manner mediated through the low affinity immune receptor FcγRIIA and the feedback agonists, ADP and thromboxane A2. Plasma swap studies between strong and weak responders demonstrated a plasma component was responsible for the variation between donors. Depletion of anti-Salmonella antibodies from plasma abolished Salmonella-induced platelet aggregation responses, and addition of polyclonal anti-Salmonella antibody allowed aggregation in washed platelets. Correlating levels of anti-Salmonella total IgG or the IgG1, IgG2, IgG3 and IgG4 subclasses to platelet responses revealed total IgG levels, rather than levels of individual subclasses, positively correlated with maximum platelet aggregation results, and negatively with lag times. Overall, we show that anti-Salmonella IgG antibodies are responsible for donor variation in platelet aggregation responses to Salmonella and mediate this activity through FcγRIIA.

Biofilm-dispersed pneumococci induce elevated leukocyte and platelet activation

Introduction

Streptococcus pneumoniae (the pneumococcus) effectively colonizes the human nasopharynx, but can migrate to other host sites, causing infections such as pneumonia and sepsis. Previous studies indicate that pneumococci grown as biofilms have phenotypes of bacteria associated with colonization whereas bacteria released from biofilms in response to changes in the local environment (i.e., dispersed bacteria) represent populations with phenotypes associated with disease. How these niche-adapted populations interact with immune cells upon reaching the vascular compartment has not previously been studied. Here, we investigated neutrophil, monocyte, and platelet activation using ex vivo stimulation of whole blood and platelet-rich plasma with pneumococcal populations representing distinct stages of the infectious process (biofilm bacteria and dispersed bacteria) as well as conventional broth-grown culture (planktonic bacteria).

Methods

Flow cytometry and ELISA were used to assess surface and soluble activation markers for neutrophil and monocyte activation, platelet-neutrophil complex and platelet-monocyte complex formation, and platelet activation and responsiveness.

Results

Overall, we found that biofilm-derived bacteria (biofilm bacteria and dispersed bacteria) induced significant activation of neutrophils, monocytes, and platelets. In contrast, little to no activation was induced by planktonic bacteria. Platelets remained functional after stimulation with bacterial populations and the degree of responsiveness was inversely related to initial activation. Bacterial association with immune cells followed a similar pattern as activation.

Discussion

Differences in activation of and association with immune cells by biofilm-derived populations could be an important consideration for other pathogens that have a biofilm state. Gaining insight into how these bacterial populations interact with the host immune response may reveal immunomodulatory targets to interfere with disease development.

Neutrophil Extracellular DNA Traps in Response to Infection or Inflammation, and the Roles of Platelet Interactions

Neutrophils present the host's first line of defense against bacterial infections. These immune effector cells are mobilized rapidly to destroy invading pathogens by (a) reactive oxygen species (ROS)-mediated oxidative bursts and (b) via phagocytosis. In addition, their antimicrobial service is capped via a distinct cell death mechanism, by the release of their own decondensed nuclear DNA, supplemented with a variety of embedded proteins and enzymes. The extracellular DNA meshwork ensnares the pathogenic bacteria and neutralizes them. Such neutrophil extracellular DNA traps (NETs) have the potential to trigger a hemostatic response to pathogenic infections. The web-like chromatin serves as a prothrombotic scaffold for platelet adhesion and activation. What is less obvious is that platelets can also be involved during the initial release of NETs, forming heterotypic interactions with neutrophils and facilitating their responses to pathogens. Together, the platelet and neutrophil responses can effectively localize an infection until it is cleared. However, not all microbial infections are easily cleared. Certain pathogenic organisms may trigger dysregulated platelet-neutrophil interactions, with a potential to subsequently propagate thromboinflammatory processes. These may also include the release of some NETs. Therefore, in order to make rational intervention easier, further elucidation of platelet, neutrophil, and pathogen interactions is still needed.

An Update on Toll-like Receptor 2, Its Function and Dimerization in Pro- and Anti-Inflammatory Processes

While a certain level of inflammation is critical for humans to survive infection and injury, a prolonged inflammatory response can have fatal consequences. Pattern recognition Toll-like receptors (TLRs) are key players in the initiation of an inflammatory process. TLR2 is one of the most studied pattern recognition receptors (PRRs) and is known to form heterodimers with either TLR1, TLR4, TLR6, and TLR10, allowing it to recognize a wide range of pathogens. Although a large number of studies have been conducted over the past decades, there are still many unanswered questions regarding TLR2 mechanisms in health and disease. In this review, we provide an up-to-date overview of TLR2, including its homo- and heterodimers. Furthermore, we will discuss the pro- and anti-inflammatory properties of TLR2 and recent findings in prominent TLR2-associated infectious and neurodegenerative diseases.

Sepsis - it is all about the platelets

Sepsis is accompanied by thrombocytopenia and the severity of the thrombocytopenia is associated with mortality. This thrombocytopenia is characteristic of disseminated intravascular coagulation (DIC), the sepsis-associated coagulopathy. Many of the pathogens, both bacterial and viral, that cause sepsis also directly activate platelets, which suggests that pathogen-induced platelet activation leads to systemic thrombosis and drives the multi-organ failure of DIC. In this paper we review the mechanisms of platelet activation by pathogens and the evidence for a role for anti-platelet agents in the management of sepsis.

Endothelial dysfunction and immunothrombosis in sepsis

Sepsis is a life-threatening clinical syndrome characterized by multiorgan dysfunction caused by a dysregulated or over-reactive host response to infection. During sepsis, the coagulation cascade is triggered by activated cells of the innate immune system, such as neutrophils and monocytes, resulting in clot formation mainly in the microcirculation, a process known as immunothrombosis. Although this process aims to protect the host through inhibition of the pathogen’s dissemination and survival, endothelial dysfunction and microthrombotic complications can rapidly lead to multiple organ dysfunction. The development of treatments targeting endothelial innate immune responses and immunothrombosis could be of great significance for reducing morbidity and mortality in patients with sepsis. Medications modifying cell-specific immune responses or inhibiting platelet–endothelial interaction or platelet activation have been proposed. Herein, we discuss the underlying mechanisms of organ-specific endothelial dysfunction and immunothrombosis in sepsis and its complications, while highlighting the recent advances in the development of new therapeutic approaches aiming at improving the short- or long-term prognosis in sepsis.

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.

Local and systemic effects of microbiome‐derived metabolites

Commensal microbes form distinct ecosystems within their mammalian hosts, collectively termed microbiomes. These indigenous microbial communities broadly expand the genomic and functional repertoire of their host and contribute to the formation of a “meta‐organism.” Importantly, microbiomes exert numerous biochemical reactions synthesizing or modifying multiple bioactive small molecules termed metabolites, which impact their host's physiology in a variety of contexts. Identifying and understanding molecular mechanisms of metabolite–host interactions, and how their disrupted signaling can contribute to diseases, may enable their therapeutic application, a modality termed “postbiotic” therapy. In this review, we highlight key examples of effects of bioactive microbe‐associated metabolites on local, systemic, and immune environments, and discuss how these may impact mammalian physiology and associated disorders. We outline the challenges and perspectives in understanding the potential activity and function of this plethora of microbially associated small molecules as well as possibilities to harness them toward the promotion of personalized precision therapeutic interventions.

Platelets and Escherichia coli: A Complex Interaction

Apart from their involvement in hemostasis, platelets have been recognized for their contribution to inflammation and defense against microbial agents. The interaction between platelets and bacteria has been well studied in the model of Staphylococcus and Streptococcus but little described in Gram-negative bacteria, especially Escherichia coli. Being involved in the hemolytic uremic syndrome as well as sepsis, it is important to study the mechanisms of interaction between platelets and E. coli. Results of the published studies are heterogeneous. It appears that some strains interact with platelets through the toll-like receptor-4 (TLR-4) and others through the Fc gamma glycoprotein. E. coli mainly uses lipopolysaccharide (LPS) to activate platelets and cause the release of antibacterial molecules, but this is not the case for all strains. In this review, we describe the different mechanisms developed in previous studies, focusing on this heterogeneity of responses that may depend on several factors; mainly, the strain studied, the structure of the LPS and the platelet form used in the studies. We can hypothesize that the structure of O-antigen and an eventual resistance to antibiotics might explain this difference.

Exaggerated inflammatory response to bacterial products in decompensated cirrhotic patients is orchestrated by interferons IL-6 and IL-8

Cirrhosis-associated immune dysfunction (CAID) contributes to disease progression and organ failure development. We interrogated immune system function in nonseptic compensated and decompensated cirrhotic patients using the TruCulture whole blood stimulation system, a novel technique that allows a more accurate representation than traditional methods, such as peripheral blood mononuclear cell culture, of the immune response in vivo. Thirty cirrhotics (21 decompensated and 9 compensated) and seven healthy controls (HCs) were recruited. Whole blood was drawn directly into three TruCulture tubes [unstimulated to preloaded with heat-killed Escherichia coli 0111:B4 (HKEB) or lipopolysaccharide (LPS)] and incubated in dry heat blocks at 37°C for 24 h. Cytokine analysis of the supernatant was performed by multiplex assay. Cirrhotic patients exhibited a robust proinflammatory response to HKEB compared with HCs, with increased production of interferon-γ-induced protein 10 (IP-10) and IFN-λ1, and to LPS, with increased production of IFN-λ1. Decompensated patients demonstrated an augmented immune response compared with compensated patients, orchestrated by an increase in type I, II, and III interferons, and higher levels of IL-1β, IL-6, and IL-8 post-LPS stimulation. IL-1β, TNF-α, and IP-10 post-HKEB stimulation and IP-10 post-LPS stimulation negatively correlated with biochemical markers of liver disease severity and liver disease severity scores. Cirrhotic patients exposed to bacterial products exhibit an exaggerated inflammatory response orchestrated by IFNs, IL-6, and IL-8. Poststimulation levels of a number of proinflammatory cytokines negatively correlate with markers of liver disease severity raising the possibility that the switch to an immunodeficient phenotype in CAID may commence earlier in the course of advanced liver disease. NEW & NOTEWORTHY Decompensated cirrhotic patients, compared with compensated patients, exhibit a greater exaggerated inflammatory response to bacterial products orchestrated by interferons, IL-6, and IL-8. Postbacterial product stimulation levels of a number of pro-inflammatory cytokines negatively correlate with liver disease severity biomarkers and liver disease severity scores raising the possibility that the switch to an immunodeficient phenotype in cirrhosis-associated immune dysfunction may commence earlier in the course of advanced liver disease.

Platelets, Bacterial Adhesins and the Pneumococcus

Systemic infections with pathogenic or facultative pathogenic bacteria are associated with activation and aggregation of platelets leading to thrombocytopenia and activation of the clotting system. Bacterial proteins leading to platelet activation and aggregation have been identified, and while platelet receptors are recognized, induced signal transduction cascades are still often unknown. In addition to proteinaceous adhesins, pathogenic bacteria such as Staphylococcus aureus and Streptococcus pneumoniae also produce toxins such as pneumolysin and alpha-hemolysin. They bind to cellular receptors or form pores, which can result in disturbance of physiological functions of platelets. Here, we discuss the bacteria-platelet interplay in the context of adhesin–receptor interactions and platelet-activating bacterial proteins, with a main emphasis on S. aureus and S. pneumoniae. More importantly, we summarize recent findings of how S. aureus toxins and the pore-forming toxin pneumolysin of S. pneumoniae interfere with platelet function. Finally, the relevance of platelet dysfunction due to killing by toxins and potential treatment interventions protecting platelets against cell death are summarized.

Atypical Roles of the Chemokine Receptor ACKR3/CXCR7 in Platelet Pathophysiology

The manifold actions of the pro-inflammatory and regenerative chemokine CXCL12/SDF-1α are executed through the canonical GProteinCoupledReceptor CXCR4, and the non-canonical ACKR3/CXCR7. Platelets express CXCR4, ACKR3/CXCR7, and are a vital source of CXCL12/SDF-1α themselves. In recent years, a regulatory impact of the CXCL12-CXCR4-CXCR7 axis on platelet biogenesis, i.e., megakaryopoiesis, thrombotic and thrombo-inflammatory actions have been revealed through experimental and clinical studies. Platelet surface expression of ACKR3/CXCR7 is significantly enhanced following myocardial infarction (MI) in acute coronary syndrome (ACS) patients, and is also associated with improved functional recovery and prognosis. The therapeutic implications of ACKR3/CXCR7 in myocardial regeneration and improved recovery following an ischemic episode, are well documented. Cardiomyocytes, cardiac-fibroblasts, endothelial lining of the blood vessels perfusing the heart, besides infiltrating platelets and monocytes, all express ACKR3/CXCR7. This review recapitulates ligand induced differential trafficking of platelet CXCR4-ACKR3/CXCR7 affecting their surface availability, and in regulating thrombo-inflammatory platelet functions and survival through CXCR4 or ACKR3/CXCR7. It emphasizes the pro-thrombotic influence of CXCL12/SDF-1α exerted through CXCR4, as opposed to the anti-thrombotic impact of ACKR3/CXCR7. Offering an innovative translational perspective, this review also discusses the advantages and challenges of utilizing ACKR3/CXCR7 as a potential anti-thrombotic strategy in platelet-associated cardiovascular disorders, particularly in coronary artery disease (CAD) patients post-MI.

Premorbid aspirin use is not associated with lower mortality in older inpatients with SARS-CoV-2 pneumonia

Platelet aggregation has been associated with COVID-19 pathogenesis. In older patients hospitalized for SARS-CoV-2 pneumonia, we aimed to investigate the association between aspirin use before admission and the risk of in-hospital all-cause mortality. We performed a retrospective international cohort study in five COVID-19 geriatric units in France and Switzerland. Among 1,357 consecutive hospitalized patients aged 75 or older and testing positive for SARS-CoV-2, we included 1,072 with radiologically confirmed pneumonia. To adjust for confounders, a propensity score for treatment was created, and stabilized inverse probability of treatment weighting (SIPTW) was applied. To assess the association between aspirin use and in-hospital 30-day mortality, SIPTW-adjusted Kaplan–Meier and Cox proportional hazards regression analyses were performed. Of the 1047 patients with SARS-CoV-2 pneumonia and median age 86 years, 301 (28.7%) were taking aspirin treatment before admission. One hundred forty-seven (34.3%) patients who had taken aspirin died in hospital within 1 month vs 118 patients (30.7%) without aspirin. After SIPTW, aspirin treatment was not significantly associated with lower mortality (adjusted hazard ratio: 1.10 [0.81–1.49], P = .52). Moreover, patients on aspirin had a longer hospital stay and were more frequently transferred to the intensive care unit. In a large multicenter cohort of older inpatients with SARS-CoV-2 pneumonia, aspirin use before admission did not appear to be associated with an improved prognosis.

Detecting Critical Functional Ingredients Group and Mechanism of Xuebijing Injection in Treating Sepsis

Sepsis is a systemic inflammatory reaction caused by various infectious or noninfectious factors, which can lead to shock, multiple organ dysfunction syndrome, and death. It is one of the common complications and a main cause of death in critically ill patients. At present, the treatments of sepsis are mainly focused on the controlling of inflammatory response and reduction of various organ function damage, including anti-infection, hormones, mechanical ventilation, nutritional support, and traditional Chinese medicine (TCM). Among them, Xuebijing injection (XBJI) is an important derivative of TCM, which is widely used in clinical research. However, the molecular mechanism of XBJI on sepsis is still not clear. The mechanism of treatment of "bacteria, poison and inflammation" and the effects of multi-ingredient, multi-target, and multi-pathway have still not been clarified. For solving this issue, we designed a new systems pharmacology strategy which combines target genes of XBJI and the pathogenetic genes of sepsis to construct functional response space (FRS). The key response proteins in the FRS were determined by using a novel node importance calculation method and were condensed by a dynamic programming strategy to conduct the critical functional ingredients group (CFIG). The results showed that enriched pathways of key response proteins selected from FRS could cover 95.83% of the enriched pathways of reference targets, which were defined as the intersections of ingredient targets and pathogenetic genes. The targets of the optimized CFIG with 60 ingredients could be enriched into 182 pathways which covered 81.58% of 152 pathways of 1,606 pathogenetic genes. The prediction of CFIG targets showed that the CFIG of XBJI could affect sepsis synergistically through genes such as TAK1, TNF-α, IL-1β, and MEK1 in the pathways of MAPK, NF-κB, PI3K-AKT, Toll-like receptor, and tumor necrosis factor signaling. Finally, the effects of apigenin, baicalein, and luteolin were evaluated by in vitro experiments and were proved to be effective in reducing the production of intracellular reactive oxygen species in lipopolysaccharide-stimulated RAW264.7 cells, significantly. These results indicate that the novel integrative model can promote reliability and accuracy on depicting the CFIGs in XBJI and figure out a methodological coordinate for simplicity, mechanism analysis, and secondary development of formulas in TCM.

Neutrophil-Derived Extracellular Vesicles Activate Platelets after Pneumolysin Exposure

Pneumolysin (PLY) is a pore-forming toxin of Streptococcus pneumoniae that contributes substantially to the inflammatory processes underlying pneumococcal pneumonia and lung injury. Host responses against S. pneumoniae are regulated in part by neutrophils and platelets, both individually and in cooperative interaction. Previous studies have shown that PLY can target both neutrophils and platelets, however, the mechanisms by which PLY directly affects these cells and alters their interactions are not completely understood. In this study, we characterize the effects of PLY on neutrophils and platelets and explore the mechanisms by which PLY may induce neutrophil–platelet interactions. In vitro studies demonstrated that PLY causes the formation of neutrophil extracellular traps (NETs) and the release of extracellular vesicles (EVs) from both human and murine neutrophils. In vivo, neutrophil EV (nEV) levels were increased in mice infected with S. pneumoniae. In platelets, treatment with PLY induced the cell surface expression of P-selectin (CD62P) and binding to annexin V and caused a significant release of platelet EVs (pl-EVs). Moreover, PLY-induced nEVs but not NETs promoted platelet activation. The pretreatment of nEVs with proteinase K inhibited platelet activation, indicating that the surface proteins of nEVs play a role in this process. Our findings demonstrate that PLY activates neutrophils and platelets to release EVs and support an important role for neutrophil EVs in modulating platelet functions in pneumococcal infections.

Platelets: a potential role in chronic respiratory diseases?

Platelets are small anucleate cells known for their role in haemostasis and thrombosis. In recent years, an increasing number of observations have suggested that platelets are also immune cells and key modulators of immunity. They express different receptors and molecules that allow them to respond to pathogens, and to interact with other immune cells. Platelets were linked to the pathogenesis of some inflammatory disorders including respiratory diseases such as asthma and idiopathic pulmonary fibrosis. Here, we discuss the involvement of platelets in different immune responses, and we focus on their potential role in various chronic lung diseases.

In addition to their essential role in haemostasis and thrombosis, platelets are strong modulators of different immune responses, and could be involved in the physiopathology of several chronic airway diseaseshttps://bit.ly/3cB6Xnj

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.

Targeting SARS-CoV-2-Platelet Interactions in COVID-19 and Vaccine-Related Thrombosis

It is clear that COVID-19 is more than a pneumonia and is associated with a coagulopathy and multi-organ failure. While the use of anti-coagulants does reduce the incidence of pulmonary emboli, it does not help with survival. This suggests that the coagulopathy is more likely to be platelet-driven rather than thrombin-driven. There is significant evidence to suggest that SARS-CoV-2 virions directly interact with platelets to trigger activation leading to thrombocytopenia and thrombosis. I propose a model of multiple interactions between SARS-CoV-2 and platelets that has many similarities to that with Staphylococcus aureus and Dengue virus. As platelet activation and thrombosis are major factors in poor prognosis, therapeutics that target the platelet-SARS-CoV-2 interaction have potential in treating COVID-19 and other virus infections.

Platelet Function in Viral Immunity and SARS-CoV-2 Infection

Platelets, as nonnucleated blood components, are classically recognized for their pivotal role in hemostasis. In recent years, however, accumulating evidence points to a nonhemostatic role for platelets, as active participants in the inflammatory and immune responses to microbial organisms in infectious diseases. This stems from the ability of activated platelets to secrete a plethora of immunomodulatory cytokines and chemokines, as well as directly interplaying with viral receptors. While much attention has been given to the role of the cytokine storm in the severity of the coronavirus disease 2019 (COVID-19), less is known about the contribution of platelets to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Here, we give a brief overview on the platelet contribution to antiviral immunity and response during SARS-CoV-2 infection.

Troponin Elevation in Older Patients with Acute Pneumonia: Frequency and Prognostic Value

Cardiovascular (CV) events are particularly frequent after acute pneumonia (AP) in the elderly. We aimed to assess whether cardiac troponin I, a specific biomarker of myocardial injury, independently predicts CV events and death after AP in older inpatients. Among 214 consecutive patients with AP aged ≥75 years admitted to a university hospital, 171 with a cardiac troponin I sample in the 72 h following diagnosis of AP were included, and 71 (42%) were found to have myocardial injury (troponin > 100 ng/L). Patients with and without myocardial injury were similar in terms of age, gender and comorbidities, but those with myocardial injury had more severe clinical presentation (median (interquartile range) Pneumonia Severity Index: 60 (40-95) vs. 45 (30-70), p = 0.003). Myocardial injury was strongly associated with in-hospital myocardial infarction (25% vs. 0%, p < 0.001), CV mortality (11 vs. 1%, p = 0.003) and all-cause mortality (34 vs. 13%, p = 0.002). After adjustment for confounders, myocardial injury remained a strong predictive factor of in-hospital mortality (odds ratio (95% confidence interval): 3.32 (1.42-7.73), p = 0.005) but not one-year mortality (1.61 (0.77-3.35), p = 0.2). Cardiac troponin I elevation, a specific biomarker of myocardial injury, was found in nearly half of an unselected cohort of older inpatients with AP and was associated with a threefold risk of in-hospital death.

Revisiting Platelets and Toll-Like Receptors (TLRs): At the Interface of Vascular Immunity and Thrombosis

While platelet function has traditionally been described in the context of maintaining vascular integrity, recent evidence suggests that platelets can modulate inflammation in a much more sophisticated and nuanced manner than previously thought. Some aspects of this expanded repertoire of platelet function are mediated via expression of Toll-like receptors (TLRs). TLRs are a family of pattern recognition receptors that recognize pathogen-associated and damage-associated molecular patterns. Activation of these receptors is crucial for orchestrating and sustaining the inflammatory response to both types of danger signals. The TLR family consists of 10 known receptors, and there is at least some evidence that each of these are expressed on or within human platelets. This review presents the literature on TLR-mediated platelet activation for each of these receptors, and the existing understanding of platelet-TLR immune modulation. This review also highlights unresolved methodological issues that potentially contribute to some of the discrepancies within the literature, and we also suggest several recommendations to overcome these issues. Current understanding of TLR-mediated platelet responses in influenza, sepsis, transfusion-related injury and cardiovascular disease are discussed, and key outstanding research questions are highlighted. In summary, we provide a resource—a “researcher’s toolkit”—for undertaking further research in the field of platelet-TLR biology.

Crosstalk Between Platelets and Microbial Pathogens

Platelets, small anucleate cells circulating in the blood, are critical mediators in haemostasis and thrombosis. Interestingly, recent studies demonstrated that platelets contain both pro-inflammatory and anti-inflammatory molecules, equipping platelets with immunoregulatory function in both innate and adaptive immunity. In the context of infectious diseases, platelets are involved in early detection of invading microorganisms and are actively recruited to sites of infection. Platelets exert their effects on microbial pathogens either by direct binding to eliminate or restrict dissemination, or by shaping the subsequent host immune response. Reciprocally, many invading microbial pathogens can directly or indirectly target host platelets, altering platelet count or/and function. In addition, microbial pathogens can impact the host auto- and alloimmune responses to platelet antigens in several immune-mediated diseases, such as immune thrombocytopenia, and fetal and neonatal alloimmune thrombocytopenia. In this review, we discuss the mechanisms that contribute to the bidirectional interactions between platelets and various microbial pathogens, and how these interactions hold relevant implications in the pathogenesis of many infectious diseases. The knowledge obtained from "well-studied" microbes may also help us understand the pathogenesis of emerging microbes, such as SARS-CoV-2 coronavirus.

A Champion of Host Defense: A Generic Large-Scale Cause for Platelet Dysfunction and Depletion in Infection

Thrombocytopenia is commonly associated with sepsis and infections, which in turn are characterized by a profound immune reaction to the invading pathogen. Platelets are one of the cellular entities that exert considerable immune, antibacterial, and antiviral actions, and are therefore active participants in the host response. Platelets are sensitive to surrounding inflammatory stimuli and contribute to the immune response by multiple mechanisms, including endowing the endothelium with a proinflammatory phenotype, enhancing and amplifying leukocyte recruitment and inflammation, promoting the effector functions of immune cells, and ensuring an optimal adaptive immune response. During infection, pathogens and their products influence the platelet response and can even be toxic. However, platelets are able to sense and engage bacteria and viruses to assist in their removal and destruction. Platelets greatly contribute to host defense by multiple mechanisms, including forming immune complexes and aggregates, shedding their granular content, and internalizing pathogens and subsequently being marked for removal. These processes, and the nature of platelet function in general, cause the platelet to be irreversibly consumed in the execution of its duty. An exaggerated systemic inflammatory response to infection can drive platelet dysfunction, where platelets are inappropriately activated and face immunological destruction. While thrombocytopenia may arise by condition-specific mechanisms that cause an imbalance between platelet production and removal, this review evaluates a generic large-scale mechanism for platelet depletion as a repercussion of its involvement at the nexus of responses to infection.

Platelets in chronic liver disease, from bench to bedside

In the last decade, numerous studies revealed physiologic and pathophysiologic roles of platelets beyond haemostasis, a process to prevent and stop bleeding. These include the activation of the immune system and the promotion of inflammation, infection and cancer. Hence, the emerging view on the role of platelets has shifted - platelets are now seen as alert "sentinels" of the immune compartment, rather than passive bystanders. Herein, we review well-established and newly discovered features of platelets that define their natural role in maintaining blood haemostasis, but also their functional relationship with other cells of the immune system. We focus on recent studies underlining functional involvement of platelets in chronic liver diseases and cancer, as well as the effects of anti-platelet therapy in these contexts. Finally, we illustrate the potential of platelets as possible diagnostic and therapeutic tools in liver disease based on recently developed methodologies.

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.

Early Host Interactions That Drive the Dysregulated Response in Sepsis

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. While many individual cells and systems in the body are involved in driving the excessive and sometimes sustained host response, pathogen engagement with endothelial cells and platelets early in sepsis progression, are believed to be key. Significant progress has been made in establishing key molecular interactions between platelets and pathogens and endothelial cells and pathogens. This review will explore the growing number of compensatory connections between bacteria and viruses with platelets and endothelial cells and how a better understanding of these interactions are informing the field of potential novel ways to treat the dysregulated host response during sepsis.

Platelet-Neutrophil Interplay: Insights Into Neutrophil Extracellular Trap (NET)-Driven Coagulation in Infection

Well established for their central role in hemostasis, platelets have increasingly been appreciated as immune cells in recent years. This emerging role should not come as a surprise as the central immune cells of invertebrates, hemocytes, are able to phagocytose, secrete soluble mediators and promote coagulation of hemolymph, blurring the line between immunity and hemostasis. The undeniable evolutionary link between coagulation and immunity becomes even clearer as the role of platelets in inflammation is better understood. Platelets exert a range of immune-related functions, many of which involve an intimate interplay with leukocytes. Platelets promote leukocyte recruitment via endothelial activation and can serve as “landing pads” for leukocytes, facilitating cellular adhesion in vascular beds devoid of classic adhesion molecules. Moreover, platelets enhance leukocyte function both through direct interactions and through release of soluble mediators. Among neutrophil-platelets interactions, the modulation of neutrophil extracellular traps (NETs) is of great interest. Platelets have been shown to induce NET formation; and, in turn, NET components further regulate platelet and neutrophil function. While NETs have been shown to ensnare and kill pathogens, they also initiate coagulation via thrombin activation. In fact, increased NET formation has been associated with hypercoagulability in septic patients as well as in chronic vascular disorders. This review will delve into current knowledge of platelet-neutrophil interactions, with a focus on NET-driven coagulation, in the context of infectious diseases. A better understanding of these mechanisms will shed a light on the therapeutic potential of uncoupling immunity and coagulation through targeting of NETs.

Lactobacillus casei CRL 431 improves endothelial and platelet functionality in a pneumococcal infection model

Streptococcus pneumoniae is able to activate coagulation and induce platelet aggregation, both of which are typical responses to systemic inflammation. The interactions between inflammation and coagulation and between soluble adhesion molecules and endothelial cells are important in the pathogenesis of an unbalanced haemostatic system. Therefore, an exaggerated and/or insufficiently controlled haemostatic activity may appreciably contribute to the severity of the disease. The aim of the present study was to evaluate the effect of the oral administration of Lactobacillus casei CRL 431 on platelet and endothelial activation mechanisms in a respiratory pneumococcal infection model in mice. S. pneumoniae induced an increase in platelet counts and enhanced the expression of P-selectin in control group, with higher endothelial activation in lung shown by the increase in von Willebrand factor (vWF) and vascular cell adhesion molecule 1 (VCAM-1) expression. Also, infection induced a decrease in CXCR-4 leukocytes, increased expression in annexinV and cell death at the pulmonary level and decreased antithrombin levels in bronchoalveolar lavage. In contrast, L. casei mice restored platelet counts, favoured faster P-selectin expression, lower vWF levels and VCAM-1 expression than control group. Also, L. casei induced higher levels of annexinV expression and lower cell death in the lung. Moreover, it was able to modulate antithrombin levels within the normal range, which would indicate lower coagulation activation and a protective effect locally exerted by L. casei. In this work, the ability of L. casei to favourably modulate platelet and endothelial functionality during a pulmonary infection with S. pneumoniae was demonstrated. Our findings offer a promising perspective for the use of this probiotic strain in the prevention of thrombotic complications associated with pneumococcal pneumonia, especially in at-risk patients. In addition, the use of L. casei would provide novel alternatives for the prevention and treatment of thrombosis associated with various diseases.

Contribution of Human Thrombospondin-1 to the Pathogenesis of Gram-Positive Bacteria

A successful colonization of different compartments of the human host requires multifactorial contacts between bacterial surface proteins and host factors. Extracellular matrix proteins and matricellular proteins such as thrombospondin-1 play a pivotal role as adhesive substrates to ensure a strong interaction with pathobionts like the Gram-positive Streptococcus pneumoniae and Staphylococcus aureus. The human glycoprotein thrombospondin-1 is a component of the extracellular matrix and is highly abundant in the bloodstream during bacteremia. Human platelets secrete thrombospondin-1, which is then acquired by invading pathogens to facilitate colonization and immune evasion. Gram-positive bacteria express a broad spectrum of surface-exposed proteins, some of which also recognize thrombospondin-1. This review highlights the importance of thrombospondin-1 as an adhesion substrate to facilitate colonization, and we summarize the variety of thrombospondin-1-binding proteins of S. pneumoniae and S. aureus.

Host-pathogen interactions and prognosis of critically ill immunocompetent patients with pneumococcal pneumonia: the nationwide prospective observational STREPTOGENE study

PURPOSE

To assess the relative importance of host and bacterial factors associated with hospital mortality in patients admitted to the intensive care unit (ICU) for pneumococcal community-acquired pneumonia (PCAP).

METHODS

Immunocompetent Caucasian ICU patients with PCAP documented by cultures and/or pneumococcal urinary antigen (UAg Sp) test were included in this multicenter prospective study between 2008 and 2012. All pneumococcal strains were serotyped. Logistic regression analyses were performed to identify risk factors for hospital mortality.

RESULTS

Of the 614 patients, 278 (45%) had septic shock, 270 (44%) had bacteremia, 307 (50%) required mechanical ventilation at admission, and 161 (26%) had a diagnosis based only on the UAg Sp test. No strains were penicillin-resistant, but 23% had decreased susceptibility. Of the 36 serotypes identified, 7 accounted for 72% of the isolates, with different distributions according to age. Although antibiotics were consistently appropriate and were started within 6 h after admission in 454 (74%) patients, 116 (18.9%) patients died. Independent predictors of hospital mortality in the adjusted analysis were platelets ≤ 100 × 10⁹/L (OR, 7.7; 95% CI, 2.8-21.1), McCabe score ≥ 2 (4.58; 1.61-13), age > 65 years (2.92; 1.49-5.74), lactates > 4 mmol/L (2.41; 1.27-4.56), male gender and septic shock (2.23; 1.30-3.83 for each), invasive mechanical ventilation (1.78; 1-3.19), and bilateral pneumonia (1.59; 1.02-2.47). Women with platelets ≤ 100 × 10⁹/L had the highest mortality risk (adjusted OR, 7.7; 2.8-21).

CONCLUSIONS

In critically ill patients with PCAP, age, gender, and organ failures at ICU admission were more strongly associated with hospital mortality than were comorbidities. Neither pneumococcal serotype nor antibiotic regimen was associated with hospital mortality.

Desialylation of Platelets by Pneumococcal Neuraminidase A Induces ADP-Dependent Platelet Hyperreactivity

Platelets are increasingly recognized to play a role in the complications of Streptococcus pneumoniae infections. S. pneumoniae expresses neuraminidases, which may alter glycans on the platelet surface. In the present study, we investigated the capability of pneumococcal neuraminidase A (NanA) to remove sialic acid (desialylation) from the platelet surface, the consequences for the platelet activation status and reactivity, and the ability of neuraminidase inhibitors to prevent these effects. Our results show that soluble NanA induces platelet desialylation. Whereas desialylation itself did not induce platelet activation (P-selectin expression and platelet fibrinogen binding), platelets became hyperreactive to ex vivo stimulation by ADP and cross-linked collagen-related peptide (CRP-XL). Platelet aggregation with leukocytes also increased. These processes were dependent on the ADP pathway, as inhibitors of the pathway (apyrase and ticagrelor) abrogated platelet hyperreactivity. Inhibition of NanA-induced platelet desialylation by neuraminidase inhibitors (e.g., oseltamivir acid) also prevented the platelet effects of NanA. Collectively, our findings show that soluble NanA can desialylate platelets, leading to platelet hyperreactivity, which can be prevented by neuraminidase inhibitors.

Start a fire, kill the bug: The role of platelets in inflammation and infection

Platelets are the main players in thrombosis and hemostasis; however they also play important roles during inflammation and infection. Through their surface receptors, platelets can directly interact with pathogens and immune cells. Platelets form complexes with neutrophils to modulate their capacities to produce reactive oxygen species or form neutrophil extracellular traps. Furthermore, they release microbicidal factors and cytokines that kill pathogens and influence the immune response, respectively. Platelets also maintain the vascular integrity during inflammation by a mechanism that is different from classical platelet activation. In this review we summarize the current knowledge about how platelets interact with the innate immune system during inflammation and infection and highlight recent advances in the field.

Thrombocytopenia in bacteraemia and association with bacterial species

Thrombocytopenia is common in patients with invasive bacterial infections. Bacteria can activate platelets, but it is unclear if this affects platelet count. The aim of this study was to examine whether bacteraemia with Staphylococcus aureus, which readily activate human platelets, was more likely to be complicated by thrombocytopenia than bacteraemia with Escherichia coli or Streptococcus pneumoniae with different abilities to activate platelets.We compared information from 600 adult patients with community-acquired bacteraemia with S. aureus (n = 140), E. coli (n = 420) and S. pneumoniae (n = 40) in Southern Sweden, 2012, linking information on positive blood cultures from microbiological databases and medical charts. The proportion of patients with thrombocytopenia (platelet count <150 × 109/ml) was calculated. Logistic regression was used to estimate the odds ratios (OR) for thrombocytopenia according to bacterial species adjusted for confounders.The proportion of thrombocytopenia was 29% in S. aureus, 28% in E. coli and 20% in S. pneumonia bacteraemia (P = 0.50), corresponding to an OR of 1.2 (95% confidence interval 0.7-1.9) for thrombocytopenia for S. aureus as compared with E. coli or S. pneumoniae, adjusted for confounders.This study indicates that platelet activation by bacteria is not a major causative mechanism in sepsis-associated thrombocytopenia.

Platelets Promote Brucella abortus Monocyte Invasion by Establishing Complexes With Monocytes

Brucellosis is an infectious disease elicited by bacteria of the genus Brucella. Platelets have been extensively described as mediators of hemostasis and responsible for maintaining vascular integrity. Nevertheless, they have been recently involved in the modulation of innate and adaptive immune responses. Although many interactions have been described between Brucella abortus and monocytes/macrophages, the role of platelets during monocyte/macrophage infection by these bacteria remained unknown. The aim of this study was to investigate the role of platelets in the immune response against B. abortus. We first focused on the possible interactions between B. abortus and platelets. Bacteria were able to directly interact with platelets. Moreover, this interaction triggered platelet activation, measured as fibrinogen binding and P-selectin expression. We further investigated whether platelets were involved in Brucella-mediated monocyte/macrophage early infection. The presence of platelets promoted the invasion of monocytes/macrophages by B. abortus. Moreover, platelets established complexes with infected monocytes/macrophages as a result of a carrier function elicited by platelets. We also evaluated the ability of platelets to modulate functional aspects of monocytes in the context of the infection. The presence of platelets during monocyte infection enhanced IL-1β, TNF-α, IL-8, and MCP-1 secretion while it inhibited the secretion of IL-10. At the same time, platelets increased the expression of CD54 (ICAM-1) and CD40. Furthermore, we showed that soluble factors released by B. abortus-activated platelets, such as soluble CD40L, platelet factor 4, platelet-activating factor, and thromboxane A₂, were involved in CD54 induction. Overall, our results indicate that platelets can directly sense and react to B. abortus presence and modulate B. abortus-mediated infection of monocytes/macrophages increasing their pro-inflammatory capacity, which could promote the resolution of the infection.

Platelet toll-like receptors are crucial sensors of infectious danger moieties

In addition to their haemostatic role and function in the repair of damaged vascular epithelium, platelets play a defensive role in innate immunity, having the capacity to produce and secrete various anti-infectious factors, as well as cytokines, chemokines and related products, to interact with other immune cells to modulate immune responses to pathogens. Thus, it is now widely acknowledged that platelets participate in inflammatory processes and infection resolution, most notably by expressing and using receptors to bind infectious pathogen moieties and contributing to pathogen clearance. The ability of platelets to sense external danger signals relates to the expression of certain innate immunity receptors, such as toll-like receptors (TLRs), and the activation of efficient cell signalling machinery. TLR engagement triggers platelet response, which results in adapted degranulation according to: the type of TLR engaged, the nature of the ligand and the milieu; together, the TLR-mediated event and other signalling events may be followed by aggregation. Platelets thus use complex tools to mediate a whole range of functions upon sensing danger. By linking the inflammatory and haemostatic platelet response to infection, TLRs play a central role. The extent of the inflammatory response to pathogen clearance is still a debatable issue and is discussed in this short review.

Strains of Group B streptococci from septic patients induce platelet activation via Toll-like Receptor 2

Group B Streptococcus (GBS) causes life-threatening bacterial sepsis, especially in newborns and pregnant women. Patients suffering from sepsis often display low platelet counts, characterized by thrombocytopenia, because of platelet activation. In the present study, the roles of six GBS strains from septic patients in platelet aggregation, as well as the underlying mechanisms, were investigated. Incubation of platelets with three of the strains induced platelet aggregation, increased the secretion of cellular adhesin molecule CD62P and activation of GPIIb/IIIa. Furthermore, the GBS strains that induced platelet activation also caused an increase in the expression of Toll-like receptor (TLR) 2 in platelets. Pre-incubation of platelets with anti-TLR2 monoclonal antibody, but not anti-TLR4 monoclonal antibody, inhibited these functional responses induced by GBS. TLR2 stimulation also activated the phosphoinositide 3-kinase (PI3-K)/Akt signalling pathway in platelets, and inhibition of PI3-K significantly reduced GBS-induced platelet responses. Our results indicate that three of the GBS strains from the septic patients can trigger platelet activation by interacting with platelets, which involves the elevation of platelet TLR2 expression.

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.

Phage-Derived Protein Induces Increased Platelet Activation and Is Associated with Mortality in Patients with Invasive Pneumococcal Disease

To improve our understanding about the severity of invasive pneumococcal disease (IPD), we investigated the association between the genotype of Streptococcus pneumoniae and disease outcomes for 349 bacteremic patients. A pneumococcal genome-wide association study (GWAS) demonstrated a strong correlation between 30-day mortality and the presence of the phage-derived gene pblB, encoding a platelet-binding protein whose effects on platelet activation were previously unknown. Platelets are increasingly recognized as key players of the innate immune system, and in sepsis, excessive platelet activation contributes to microvascular obstruction, tissue hypoperfusion, and finally multiorgan failure, leading to mortality. Our in vitro studies revealed that pblB expression was induced by fluoroquinolones but not by the beta-lactam antibiotic penicillin G. Subsequently, we determined pblB induction and platelet activation by incubating whole blood with the wild type or a pblB knockout mutant in the presence or absence of antibiotics commonly administered to our patient cohort. pblB-dependent enhancement of platelet activation, as measured by increased expression of the α-granule protein P-selectin, the binding of fibrinogen to the activated αIIbβ3 receptor, and the formation of platelet-monocyte complex occurred irrespective of antibiotic exposure. In conclusion, the presence of pblB on the pneumococcal chromosome potentially leads to increased mortality in patients with an invasive S. pneumoniae infection, which may be explained by enhanced platelet activation. This study highlights the clinical utility of a bacterial GWAS, followed by functional characterization, to identify bacterial factors involved in disease severity.

The exact mechanisms causing mortality in invasive pneumococcal disease (IPD) patients are not completely understood. We examined 349 patients with IPD and found in a bacterial genome-wide association study (GWAS) that the presence of the phage-derived gene pblB was associated with mortality in the first 30 days after hospitalization. Although pblB has been extensively studied in Streptococcus mitis, its consequence for the interaction between platelets and Streptococcus pneumoniae is largely unknown. Platelets are important in immunity and inflammation, and excessive platelet activation contributes to microvascular obstruction and multiorgan failure, leading to mortality. We therefore developed this study to assess whether the expression of pblB might increase the risk of death for IPD patients through its effect on enhanced platelet activation. This study also shows the value of integrating extensive bacterial genomics and clinical data in predicting and understanding pathogen virulence, which in turn will help to improve prognosis and therapy.

Platelets Mediate Host Defense against Staphylococcus aureus through Direct Bactericidal Activity and by Enhancing Macrophage Activities

Platelets are the chief effector cells in hemostasis. However, recent evidence suggests they have multiple roles in host defense against infection. Reports by us and others showed that platelets functionally contribute to protection against Staphylococcus aureus infection. In the current study, the capacity of mouse platelets to participate in host defense against S. aureus infection was determined by assessing two possibilities. First, we determined the ability of platelets to kill S. aureus directly; and, second, we tested the possibility that platelets enhance macrophage phagocytosis and intracellular killing of S. aureus In this study we report evidence in support of both mechanisms. Platelets effectively killed two different strains of S. aureus. A clinical isolate of methicillin-resistant S. aureus was killed by platelets (>40% killing in 2 h) in a thrombin-dependent manner whereas a methicillin-sensitive strain was killed to equal extent but did not require thrombin. Interestingly, thrombin-stimulated platelets also significantly enhanced peritoneal macrophage phagocytosis of both methicillin-resistant S. aureus and methicillin-sensitive S. aureus by >70%, and restricted intracellular growth by >40%. Enhancement of macrophage anti-S. aureus activities is independent of contact with platelets but is mediated through releasable products, namely IL-1β. These data confirm our hypothesis that platelets participate in host defense against S. aureus both through direct killing of S. aureus and enhancing the antimicrobial function of macrophages in protection against S. aureus infection.

Platelets and infection

The primary function of platelets is to patrol the vasculature and seal vessel breaches to limit blood loss. However, it is becoming increasingly clear that they also contribute to pathophysiological conditions like thrombosis, atherosclerosis, stroke and infection. Severe sepsis is a devastating disease that claims hundreds of thousands of lives every year in North America and is a major burden to the public health system. Platelet surface receptors like GPIb, αIIbβ3, TLR2 and TLR4 are involved in direct platelet-bacteria interactions. Plasma proteins like fibrinogen and vWF enable indirect interactions. Furthermore, platelet granules contain a plethora of proteins that modulate the immune response as well as microbicidal agents which can directly lyse bacteria. Bacterial toxins are potent platelet activators and can cause intravascular platelet aggregation. Platelets contribute to the antibacterial response of the host involving Kupffer cells, neutrophils and the complement system. In this review we summarize the current knowledge about platelet-bacteria interactions and highlight recent advances in the field.

Effects of Suilysin on Streptococcus suis-Induced Platelet Aggregation

Blood platelets play important roles during pathological thrombocytopenia in streptococcal toxic shock syndrome (STSS). Streptococcus suis (S. suis) an emerging human pathogen, can cause STSS similarly to S. pyogenes. However, S. suis interactions with platelets are poorly understood. Here, we found that suilysin (SLY), different from other bacterial cholesterol-dependent cytolysins (CDCs), was the sole stimulus that induced platelet aggregation. Furthermore, the inside-out activation of GPIIb/IIIa of platelets mediated SLY-induced platelet aggregation. This process was triggered by Ca²⁺ influx that depend on the pore forming on platelets by SLY. Additionally, although SLY induced α-granule release occurred via the MLCK-dependent pathway, PLC-β-IP3/DAG-MLCK and Rho-ROCK-MLCK signaling were not involved in SLY-induced platelet aggregation. Interestingly, the pore dependent Ca²⁺ influx was also found to participate in the induction of platelet aggregation with pneumolysin (PLY) and streptolysin O (SLO), two other CDCs. It is possible that the CDC-mediated platelet aggregation we observed in S. suis is a similar response mechanism to that used by a wide range of bacteria. These findings might lead to the discovery of potential therapeutic targets for S. suis-associated STSS.

Toll-Like Receptor Signalling Is Not Involved in Platelet Response to Streptococcus pneumoniae In Vitro or In Vivo

Streptococcus (S.) pneumoniae strains vary considerably in their ability to cause invasive disease in humans, which is at least in part determined by the capsular serotype. Platelets have been implicated as sentinel cells in the circulation for host defence. One of their utensils for this function is the expression of Toll-like receptors (TLRs). We here aimed to investigate platelet response to S. pneumoniae and a role for TLRs herein. Platelets were stimulated using four serotypes of S. pneumonia including an unencapsulated mutant strain. In vitro aggregation and flow cytometry assays were performed using blood of healthy volunteers, or blood of TLR knock out and WT mice. For in vivo pneumonia experiments, platelet specific Myd88 knockout (Plt-Myd88^-/-) mice were used. We found that platelet aggregation was induced by unencapsulated S. pneumoniae only. Whole blood incubation with all S. pneumoniae serotypes tested resulted in platelet degranulation and platelet-leukocyte complex formation. Platelet activation was TLR independent, as responses were not inhibited by TLR blocking antibodies, not induced by TLR agonists and were equally induced in wild-type and Tlr2^-/-, Tlr4^-/-, Tlr2/4^-/-, Tlr9^-/- and Myd88^-/- blood. Plt-Myd88^-/- and control mice displayed no differences in bacterial clearance or immune response to pneumonia by unencapsulated S. pneumoniae. In conclusion, S. pneumoniae activates platelets through a TLR-independent mechanism that is impeded by the bacterial capsule. Additionally, platelet MyD88-dependent TLR signalling is not involved in host defence to unencapsulated S. pneumoniae in vivo.

Escherichia coli induces platelet aggregation in an FcγRIIa-dependent manner

BACKGROUND

The discovery of pathogen-recognition receptors such as Toll-like receptors on platelets has led to the emergence of the concept of platelets as important components of the host response to infection. Escherichia coli (E. coli)-mediated sepsis is a serious illness characterized by the occurrence of thrombocytopenia. Whereas there has been a wealth of research on platelet activation by Gram-positive bacteria, little is known about the mechanisms associated with Gram-negative bacteria-induced platelet activation with Gram-negative bacteria.

OBJECTIVES

To determine the mechanisms by which Gram-negative E. coli induces platelet aggregation.

METHODS

Induction of platelet aggregation with E. coli strain O157:H7 was tested in platelet-rich plasma (PRP), washed platelets, and serum depleted of complement factors. Platelet inhibitors (against αII b β3 , glycoprotein Ibα and FcγRIIa) were used. Platelet thromboxane synthesis was analyzed after E. coli stimulation. Cell binding assays were used to assess the ability of E. coli to support platelet adhesion. Trypsinization was used to determine the role of E. coli surface proteins.

RESULTS AND CONCLUSION

E. coli-induced aggregation in PRP was donor-dependent. E. coli O157:H7 induced aggregation with a lag time of 6.9 ± 1.3 min in an αII b β3 -dependent and FcγRIIa-dependent manner. Furthermore, this interaction was enhanced by the presence of complement, and was dependent on thromboxane synthesis. These results show E. coli to be a potent inducer of platelet aggregation.

Human platelet activation by Escherichia coli: roles for FcγRIIA and integrin αIIbβ3

Gram-negative Escherichia coli cause diseases such as sepsis and hemolytic uremic syndrome in which thrombotic disorders can be found. Direct platelet–bacterium interactions might contribute to some of these conditions; however, mechanisms of human platelet activation by E. coli leading to thrombus formation are poorly understood. While the IgG receptor FcγRIIA has a key role in platelet response to various Gram-positive species, its role in activation to Gram-negative bacteria is poorly defined. This study aimed to investigate the molecular mechanisms of human platelet activation by E. coli, including the potential role of FcγRIIA. Using light-transmission aggregometry, measurements of ATP release and tyrosine-phosphorylation, we investigated the ability of two E. coli clinical isolates to activate platelets in plasma, in the presence or absence of specific receptors and signaling inhibitors. Aggregation assays with washed platelets supplemented with IgGs were performed to evaluate the requirement of this plasma component in activation. We found a critical role for the immune receptor FcγRIIA, αIIbβ3, and Src and Syk tyrosine kinases in platelet activation in response to E. coli. IgG and αIIbβ3 engagement was required for FcγRIIA activation. Moreover, feedback mediators adenosine 5’-diphosphate (ADP) and thromboxane A₂ (TxA₂) were essential for platelet aggregation. These findings suggest that human platelet responses to E. coli isolates are similar to those induced by Gram-positive organisms. Our observations support the existence of a central FcγRIIA-mediated pathway by which human platelets respond to both Gram-negative and Gram-positive bacteria.

The role of platelet MyD88 in host response during gram-negative sepsis

BACKGROUND

Beside their role in hemostasis, platelets serve as sentinel cells in host defense during infection. In sepsis, platelets have been implicated in both beneficial (antibacterial) and detrimental responses (thrombosis and organ damage). Toll-like receptors and their common adaptor, myeloid differentiation factor 88 (MyD88), are essential for pathogen recognition and protective immunity. Platelets express functional Toll-like receptors and MyD88, which participate in platelet responsiveness to bacterial agonists.

OBJECTIVE

Considering the pivotal involvement of platelets and MyD88 in the host response to bacteria, we studied the role of platelet MyD88 in gram-negative sepsis using intravenous and airway infections with the common human sepsis pathogen Klebsiella pneumoniae.

METHODS

Platelet-specific Myd88(-/-) mice were generated by crossing mice with a conditional Myd88 flox allele with mice expressing Cre recombinase controlled by the platelet factor 4 promoter. In a reverse approach, full Myd88(-/-) mice were transfused with wild-type platelets.

RESULTS

In both settings, platelet MyD88 did not impact on bacterial growth or dissemination. In addition, platelet MyD88 did not influence hallmark sepsis responses such as thrombocytopenia, coagulation or endothelial activation, or distant organ injury. Platelet MyD88 played no role in lung pathology during pneumonia-derived sepsis.

CONCLUSION

Despite known literature, platelet MyD88-dependent TLR signaling does not contribute to the host response during gram-negative sepsis.

Platelets and Their Interactions with Other Immune Cells

Platelets are anucleate blood cells, long known to be critically involved in hemostasis and thrombosis. In addition to their role in blood clots, increasing evidence reveals significant roles for platelets in inflammation and immunity. However, the notion that platelets represent immune cells is not broadly recognized in the field of Physiology. This article reviews the role of platelets in inflammation and immune responses, and highlights their interactions with other immune cells, including examples of major functional consequences of these interactions.

Human platelet IgG Fc receptor FcγRIIA in immunity and thrombosis

Beyond their prominent role in hemostasis and thrombosis, platelets are increasingly recognized as having immunologic functions. Supporting this, human platelets express FcγRIIA (CD32a), a low-affinity Fc receptor (FcR) for the constant region of IgG that recognizes immune complexes (ICs) and IgG-opsonized cells with high avidity. In leukocytes, FcγRIIA engagement initiates strong effector functions that are key for immune and inflammatory responses, including cytokine release, antibody-dependent cell-mediated killing of pathogens, and internalization of ICs. However, the physiologic relevance of platelet-expressed FcγRIIA has received little attention in previous reviews on FcRs. This article summarizes and discusses the available information on human platelet FcγRIIA. The importance of this receptor in heparin-induced thrombocytopenia, a prothrombotic adverse drug effect, is well documented. However, studies demonstrating platelet activation by IgG-opsonized bacteria point to the physiologic relevance of platelet FcγRIIA in immunity. In this context, platelet activation and secretion may facilitate both a direct antimicrobial function of platelets and crosstalk with other immune cells. Additionally, a role for platelet FcγRIIA in IgG-independent hemostasis and physiologic thrombosis, by means of amplifying integrin αII b β3 outside-in signaling, has also been proposed. Nonetheless, the thrombotic complications found in some infective and autoimmune diseases may result from unbalanced FcγRIIA-mediated platelet aggregation. Moreover, FcγRIIA is not expressed in mice, and thrombocytopenia and/or thrombotic events found after drug administration can only be recapitulated by the use of human FcγRIIA-transgenic mice. Altogether, the available data support a functional role for platelet FcγRIIA in health and disease, and emphasize the need for further investigation of this receptor.