Novel anti-bacterial activities of β-defensin 1 in human platelets: suppression of pathogen growth and signaling of neutrophil extracellular trap formation

Recent Advances in the Discovery and Function of Antimicrobial Molecules in Platelets

The conventional function described for platelets is maintaining vascular integrity. Nevertheless, increasing evidence reveals that platelets can additionally play a crucial role in responding against microorganisms. Activated platelets release molecules with antimicrobial activity. This ability was first demonstrated in rabbit serum after coagulation and later in rabbit platelets stimulated with thrombin. Currently, multiple discoveries have allowed the identification and characterization of PMPs (platelet microbicidal proteins) and opened the way to identify kinocidins and CHDPs (cationic host defense peptides) in human platelets. These molecules are endowed with microbicidal activity through different mechanisms that broaden the platelet participation in normal and pathologic conditions. Therefore, this review aims to integrate the currently described platelet molecules with antimicrobial properties by summarizing the pathways towards their identification, characterization, and functional evaluation that have promoted new avenues for studying platelets based on kinocidins and CHDPs secretion.

Antimicrobial Effects of Equine Platelet Lysate

The development of antimicrobial resistant bacteria and the lack of novel antibiotic strategies to combat those bacteria is an ever-present problem in both veterinary and human medicine. The goal of this study is to evaluate platelet lysate (PL) as a biological alternative antimicrobial product. Platelet lysate is an acellular platelet-derived product rich in growth factors and cytokines that is manufactured via plateletpheresis and pooled from donor horses. In the current study, we sought to define the antimicrobial properties of PL on select gram-positive and gram-negative bacteria. Results from an end-point in vitro assay showed that PL did not support bacterial growth, and in fact significantly reduced bacterial content compared to normal growth media. An in vitro assay was then utilized to further determine the effects on bacterial growth dynamics and showed that all strains exhibited a slower growth rate and lower yield in the presence of PL. The specific effects of PL were unique for each bacterial strain: E. coli and P. aeruginosa growth was affected in a concentration-dependent manner, such that higher amounts of PL had a greater effect, while this was not true for S. aureus or E. faecalis. Furthermore, the onset of exponential growth was delayed for E. coli and P. aeruginosa in the presence of PL, which has significant clinical implications for developing a dosing schedule. In conclusion, our findings demonstrate the potential value of PL as a broad-spectrum antimicrobial that would offer an alternative to traditional antibiotics for the treatment of bacterial infection in equine species.

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.

Repurposed drugs block toxin-driven platelet clearance by the hepatic Ashwell-Morell receptor to clear Staphylococcus aureus bacteremia

Staphylococcus aureus (SA) bloodstream infections cause high morbidity and mortality (20 to 30%) despite modern supportive care. In a human bacteremia cohort, we found that development of thrombocytopenia was correlated to increased mortality and increased α-toxin expression by the pathogen. Platelet-derived antibacterial peptides are important in bloodstream defense against SA, but α-toxin decreased platelet viability, induced platelet sialidase to cause desialylation of platelet glycoproteins, and accelerated platelet clearance by the hepatic Ashwell-Morell receptor (AMR). Ticagrelor (Brilinta), a commonly prescribed P2Y12 receptor inhibitor used after myocardial infarction, blocked α-toxin-mediated platelet injury and resulting thrombocytopenia, thereby providing protection from lethal SA infection in a murine intravenous challenge model. Genetic deletion or pharmacological inhibition of AMR stabilized platelet counts and enhanced resistance to SA infection, and the anti-influenza sialidase inhibitor oseltamivir (Tamiflu) provided similar therapeutic benefit. Thus, a "toxin-platelet-AMR" regulatory pathway plays a critical role in the pathogenesis of SA bloodstream infection, and its elucidation provides proof of concept for repurposing two commonly prescribed drugs as adjunctive therapies to improve patient outcomes.

Luminal microvesicles uniquely influence translocating bacteria after SIV infection

Microbial translocation contributes to persistent inflammation in both treated and untreated HIV infection. Although translocation is due in part to a disintegration of the intestinal epithelial barrier, there is a bias towards the translocation of Proteobacteria. We hypothesized that intestinal epithelial microvesicle cargo differs after HIV infection and contributes to biased translocation. We isolated gastrointestinal luminal microvesicles before and after progressive simian immunodeficiency virus (SIV) infection in rhesus macaques and measured miRNA and antimicrobial peptide content. We demonstrate that these microvesicles display decreased miR-28-5p, -484, -584-3p, and -584-5p, and let-7b-3p, as well as increased beta-defensin 1 after SIV infection. We further observed dose-dependent growth sensitivity of commensal Lactobacillus salivarius upon co-culture with isolated microvesicles. Infection-associated microvesicle differences were not mirrored in non-progressively SIV-infected sooty mangabeys. Our findings describe novel alterations of antimicrobial control after progressive SIV infection that influence the growth of translocating bacterial taxa. These studies may lead to the development of novel therapeutics for treating chronic HIV infection, microbial translocation, and inflammation.

Inflammation, Infection and Venous Thromboembolism

The association between inflammation, infection, and venous thrombosis has long been recognized; yet, only in the last decades have we begun to understand the mechanisms through which the immune and coagulation systems interact and reciprocally regulate one another. These interconnected networks mount an effective response to injury and pathogen invasion, but if unregulated can result in pathological thrombosis and organ damage. Neutrophils, monocytes, and platelets interact with each other and the endothelium in host defense and also play critical roles in the formation of venous thromboembolism. This knowledge has advanced our understanding of both human physiology and pathophysiology, as well as identified mechanisms of anticoagulant resistance and novel therapeutic targets for the prevention and treatment of thrombosis. In this review, we discuss the contributions of inflammation and infection to venous thromboembolism.

Inflammatory signaling in dengue-infected platelets requires translation and secretion of nonstructural protein 1

Emerging evidence identifies major contributions of platelets to inflammatory amplification in dengue, but the mechanisms of infection-driven platelet activation are not completely understood. Dengue virus nonstructural protein-1 (DENV NS1) is a viral protein secreted by infected cells with recognized roles in dengue pathogenesis, but it remains unknown whether NS1 contributes to the inflammatory phenotype of infected platelets. This study shows that recombinant DENV NS1 activated platelets toward an inflammatory phenotype that partially reproduced DENV infection. NS1 stimulation induced translocation of α-granules and release of stored factors, but not of newly synthesized interleukin-1β (IL-1β). Even though both NS1 and DENV were able to induce pro-IL-1β synthesis, only DENV infection triggered caspase-1 activation and IL-1β release by platelets. A more complete thromboinflammatory phenotype was achieved by synergistic activation of NS1 with classic platelet agonists, enhancing α-granule translocation and inducing thromboxane A2 synthesis (thrombin and platelet-activating factor), or activating caspase-1 for IL-1β processing and secretion (adenosine triphosphate). Also, platelet activation by NS1 partially depended on toll-like receptor-4 (TLR-4), but not TLR-2/6. Finally, the platelets sustained viral genome translation and replication, but did not support the release of viral progeny to the extracellular milieu, characterizing an abortive viral infection. Although DENV infection was not productive, translation of the DENV genome led to NS1 expression and release by platelets, contributing to the activation of infected platelets through an autocrine loop. These data reveal distinct, new mechanisms for platelet activation in dengue, involving DENV genome translation and NS1-induced platelet activation via platelet TLR4.

Platelets mediate serological memory to neutralize viruses in vitro and in vivo

Platelets contain virus-specific IgGs that potently diminish viral infection in vitro and in vivo. Release of platelet IgG is more efficient at virus neutralization than equal amounts of plasma IgG.

Hemocyte extracellular traps of Manila clam Ruditapes philippinarum: Production characteristics and antibacterial effects

Extracellular traps (ETs) have been found to be an important strategy of mammals to immobilize and kill invading microorganisms. In the present study, we observed the formation of ETs in the hemocytes of marine mollusks Ruditapes philippinarum in response to challenge from bacteria Vibrio anguillarum, and examined the potential factors and signaling pathways underling this process. We detected an increase of reactive oxygen species (ROS) and myeloperoxidase (MPO) production during ETosis, accompanied by significantly up-regulated expression of ROS-related and MPO genes. The suppression of ETs structures by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor (diphenyleneiodonium chloride, DPI) and MPO inhibitor (aminobenzoic acid hydrazide, ABAH) further confirmed the essential roles ROS and MPO played in ETosis. Furthermore, ET production could be inhibited by phosphotidylinsitol-3-kinase (PI3K) inhibitor (LY294002) and extracellular regulated protein kinase (ERK) inhibitor (U0126), suggesting the idea that both the PI3K and ERK pathways were suggested to function during ETosis. In addition, the ETosis process was accompanied by enhancement of glycolysis-related enzymatic activities, e.g., pyruvate kinase (PK) and hexokinase (HK), and over-expression of the glycolysis-related genes, e.g., PK, HK and glucose transport protein (GLUT), indicating high involvement of glycolysis in the ETosis process. Furthermore, our scanning electron microscopy (SEM) observation and antibacterial activities test successfully showed the patterns how clam ETs entrapped and killed the invading V. anguillarum. Taken together, our results revealed that ETosis with bactericidal effect increased ROS, MPO and glycolysis level and carried out in a ROS-, MPO-, PI3K-ERK-dependent manner.

Platelets immune response against Mycobacterium tuberculosis infection

Tuberculosis (TB) is the first cause of death by a single infectious agent. Previous reports have highlighted the presence of platelets within Tb granulomas, albeit the immune-associated platelet response to Mycobacterium tuberculosis (Mtb) has not been deeply studied. Our results showed that platelets are recruited into the granuloma in the late stages of tuberculosis. Furthermore, electron-microscopy studies showed that platelets can internalize Mtb and produce host defense peptides (HDPs), such as RNase 7, HBD2 and hPF-4 that bind to the internalized Mtb. Mtb-infected platelets exhibited higher transcription and secretion of IL-1β and TNF-α, whereas IL-10 and IL-6 protein levels decreased. These results suggest that platelets participate in the immune response against Mtb through HDPs and cytokines production.

Curbing gastrointestinal infections by defensin fragment modifications without harming commensal microbiota

The occurrence and spread of multidrug-resistant pathogens, especially bacteria from the ESKAPE panel, increases the risk to succumb to untreatable infections. We developed a novel antimicrobial peptide, Pam-3, with antibacterial and antibiofilm properties to counter this threat. The peptide is based on an eight-amino acid carboxyl-terminal fragment of human β-defensin 1. Pam-3 exhibited prominent antimicrobial activity against multidrug-resistant ESKAPE pathogens and additionally eradicated already established biofilms in vitro, primarily by disrupting membrane integrity of its target cell. Importantly, prolonged exposure did not result in drug-resistance to Pam-3. In mouse models, Pam-3 selectively reduced acute intestinal Salmonella and established Citrobacter infections, without compromising the core microbiota, hence displaying an added benefit to traditional broad-spectrum antibiotics. In conclusion, our data support the development of defensin-derived antimicrobial agents as a novel approach to fight multidrug-resistant bacteria, where Pam-3 appears as a particularly promising microbiota-preserving candidate.

Platelet lysate for COVID-19 pneumonia-a newer adjunctive therapeutic avenue

The linchpin for COVID-19 pathogenesis is the severe inflammatory process in the respiratory tract wherein the accumulation of excessive cytokines paves the way for a series of systemic hemodynamic alterations and mortality. The mortality rate is higher in individuals with co-morbidities and advancing age. The absence of a specific therapy is responsible for this uncontrolled spread and the significant mortality. This renders potential insight for considering biologics as a plausible option to repair and regenerate the affected lung tissue and pulverize the causative organism. The plausible role of megakaryocytes against invading microbes was not clearly understood. Platelet lysate is an acellular product consisting of regenerative molecules released from a cluster of platelets. It attenuates the changes caused by immune reactions in allogenic utility with the introduction of growth factors, cytokines, and proteins at supraphysiologic levels and thereby serves as a regenerative immunomodulatory agent to combat COVID-19. This platelet lysate can be used in nebulized form for such acute respiratory distress conditions in COVID-19 elderly patients. Platelet lysate may emerge as a pivotal player provided investigations pace up in this context. Here, we discuss how the platelet lysate can plausibly perquisite to relegate COVID-19. Undertaking prospective randomized controlled trials to prove its efficacy is the need of the hour in this pandemic scenario.

Role of Platelets in Detection and Regulation of Infection

Platelets are classically known as essential mediators of hemostasis and thrombosis. However, in recent years, platelets have gained recognition for their inflammatory functions, which modulate the immune response during infectious diseases. Platelets contain various immunoreceptors that enable them to act as sentinels to recognize intravascular pathogens. Upon activation, platelets directly limit pathogen growth through the release of AMPs (antimicrobial proteins) and ensure pathogen clearance through activation of immune cells. However, aberrant platelet activation can lead to inflammation and thrombotic events.

Platelet gene expression and function in patients with COVID-19

There is an urgent need to understand the pathogenesis of coronavirus disease 2019 (COVID-19). In particular, thrombotic complications in patients with COVID-19 are common and contribute to organ failure and mortality. Patients with severe COVID-19 present with hemostatic abnormalities that mimic disseminated intravascular coagulopathy associated with sepsis, with the major difference being increased risk of thrombosis rather than bleeding. However, whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection alters platelet function to contribute to the pathophysiology of COVID-19 remains unknown. In this study, we report altered platelet gene expression and functional responses in patients infected with SARS-CoV-2. RNA sequencing demonstrated distinct changes in the gene-expression profile of circulating platelets of COVID-19 patients. Pathway analysis revealed differential gene-expression changes in pathways associated with protein ubiquitination, antigen presentation, and mitochondrial dysfunction. The receptor for SARS-CoV-2 binding, angiotensin-converting enzyme 2 (ACE2), was not detected by messenger RNA (mRNA) or protein in platelets. Surprisingly, mRNA from the SARS-CoV-2 N1 gene was detected in platelets from 2 of 25 COVID-19 patients, suggesting that platelets may take-up SARS-COV-2 mRNA independent of ACE2. Resting platelets from COVID-19 patients had increased P-selectin expression basally and upon activation. Circulating platelet-neutrophil, -monocyte, and -T-cell aggregates were all significantly elevated in COVID-19 patients compared with healthy donors. Furthermore, platelets from COVID-19 patients aggregated faster and showed increased spreading on both fibrinogen and collagen. The increase in platelet activation and aggregation could partially be attributed to increased MAPK pathway activation and thromboxane generation. These findings demonstrate that SARS-CoV-2 infection is associated with platelet hyperreactivity, which may contribute to COVID-19 pathophysiology.

Evaluation of Platelet Responses in Transfusion-Related Acute Lung Injury (TRALI)

Platelets are versatile cells which are capable of eliciting nonhemostatic immune functions, especially under inflammatory conditions. Depending on the specific setting, platelets may be either protective or pathogenic in acute lung injury and acute respiratory distress syndrome (ARDS). Their role in transfusion-related acute lung injury (TRALI) is less well defined; however, it has been hypothesized that recipient platelets and transfused platelets both play a pathogenic role in TRALI. Overall, despite conflicting findings, it appears that recipient platelets may play a pathogenic role in antibody-mediated TRALI; however, their contribution appears to be limited. It is imperative to first validate the involvement of recipient platelets by standardizing the animal models, methods, reagents, and readouts for lung injury and taking the animal housing environment into consideration. For the involvement of transfused platelets in TRALI, it appears that predominantly lipids such as ceramide in stored platelets are able to induce TRALI in animal models. These studies will also need to be validated, and moreover, the platelet-derived lipid-mediated mechanisms leading to TRALI will need to be investigated.

The Immune Nature of Platelets Revisited

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

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

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

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

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

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

Platelets and Their Role in the Pathogenesis of Cardiovascular Events in Patients With Community-Acquired Pneumonia

Community-acquired pneumonia (CAP) remains an important cause of morbidity and mortality throughout the world with much recent and ongoing research focused on the occurrence of cardiovascular events (CVEs) during the infection, which are associated with adverse short-term and long-term survival. Much of the research directed at unraveling the pathogenesis of these events has been undertaken in the settings of experimental and clinical CAP caused by the dangerous, bacterial respiratory pathogen, Streptococcus pneumoniae (pneumococcus), which remains the most common bacterial cause of CAP. Studies of this type have revealed that although platelets play an important role in host defense against infection, there is also increasing recognition that hyperactivation of these cells contributes to a pro-inflammatory, prothrombotic systemic milieu that contributes to the etiology of CVEs. In the case of the pneumococcus, platelet-driven myocardial damage and dysfunction is exacerbated by the direct cardiotoxic actions of pneumolysin, a major pore-forming toxin of this pathogen, which also acts as potent activator of platelets. This review is focused on the role of platelets in host defense against infection, including pneumococcal infection in particular, and reviews the current literature describing the potential mechanisms by which platelet activation contributes to cardiovascular complications in CAP. This is preceded by an evaluation of the burden of pneumococcal infection in CAP, the clinical features and putative pathogenic mechanisms of the CVE, and concludes with an evaluation of the potential utility of the anti-platelet activity of macrolides and various adjunctive therapies.

Platelets Promote Macrophage Polarization toward Pro-inflammatory Phenotype and Increase Survival of Septic Mice

We investigated the contribution of human platelets to macrophage effector properties in the presence of lipopolysaccharide (LPS), as well as the beneficial effects and time frame for platelet transfusion in septic animals. Our results show that platelets sequester both pro-(TNF-α/IL-6) and anti-(IL-10) inflammatory cytokines released by monocytes. Low LPS concentrations (0.01 ng/mL) induced M2 macrophage polarization by decreasing CD64 and augmenting CD206 and CD163 expression; yet, the presence of platelets skewed monocytes toward type 1 macrophage (M1) phenotype in a cell-contact-dependent manner by the glycoprotein Ib (GPIb)-CD11b axis. Accordingly, platelet-licensed macrophages showed increased TNF-α levels, bacterial phagocytic activity, and a reduced healing capability. Platelet transfusion increased inducible nitric oxide synthase (iNOS)⁺ macrophages, improving bacterial clearance and survival rates in septic mice up to 6 h post-infection, an effect that was abolished by CD11b and GPIb blockade. Our results demonstrate that platelets orchestrate macrophage effector responses, improving the clinical outcome of sepsis in a narrow but relevant time frame.

Innate immune receptors in platelets and platelet-leukocyte interactions

Platelets are chief cells in hemostasis. Apart from their hemostatic roles, platelets are major inflammatory effector cells that can influence both innate and adaptive immune responses. Activated platelets have thromboinflammatory functions linking hemostatic and immune responses in several physiological and pathological conditions. Among many ways in which platelets exert these functions, platelet expression of pattern recognition receptors (PRRs), including TLR, Nod-like receptor, and C-type lectin receptor families, plays major roles in sensing and responding to pathogen-associated or damage-associated molecular patterns (PAMPs and DAMPs, respectively). In this review, an increasing body of evidence is compiled showing the participation of platelet innate immune receptors, including PRRs, in infectious diseases, sterile inflammation, and cancer. How platelet recognition of endogenous DAMPs participates in sterile inflammatory diseases and thrombosis is discussed. In addition, platelet recognition of both PAMPs and DAMPs initiates platelet-mediated inflammation and vascular thrombosis in infectious diseases, including viral, bacterial, and parasite infections. The study also focuses on the involvement of innate immune receptors in platelet activation during cancer, and their contribution to tumor microenvironment development and metastasis. Finally, how innate immune receptors participate in platelet communication with leukocytes, modulating leukocyte-mediated inflammation and immune functions, is highlighted. These cell communication processes, including platelet-induced release of neutrophil extracellular traps, platelet Ag presentation to T-cells and platelet modulation of monocyte cytokine secretion are discussed in the context of infectious and sterile diseases of major concern in human health, including cardiovascular diseases, dengue, HIV infection, sepsis, and cancer.

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.

The Platelet Concentrates Therapy: From the Biased Past to the Anticipated Future

The ultimate goal of research on platelet concentrates (PCs) is to develop a more predictable PC therapy. Because platelet-rich plasma (PRP), a representative PC, was identified as a possible therapeutic agent for bone augmentation in the field of oral surgery, PRP and its derivative, platelet-rich fibrin (PRF), have been increasingly applied in a regenerative medicine. However, a rise in the rate of recurrence (e.g., in tendon and ligament injuries) and adverse (or nonsignificant) clinical outcomes associated with PC therapy have raised fundamental questions regarding the validity of the therapy. Thus, rigorous evidence obtained from large, high-quality randomized controlled trials must be presented to the concerned regulatory authorities of individual countries or regions. For the approval of the regulatory authorities, clinicians and research investigators should understand the real nature of PCs and PC therapy (i.e., adjuvant therapy), standardize protocols of preparation (e.g., choice of centrifuges and tubes) and clinical application (e.g., evaluation of recipient conditions), design bias-minimized randomized clinical trials, and recognize superfluous brand competitions that delay sound progress. In this review, we retrospect the recent past of PC research, reconfirm our ultimate goals, and discuss what will need to be done in future.

Statins potentiate the antibacterial effect of platelets on Staphylococcus aureus

Platelets have largely demonstrated their implication in anti-infectious immunity. This effect is ensured by the secreted molecules stored mostly in platelet alpha granules. Previous studies have reported that Staphylococcus aureus showed sensitivity to this antibacterial effect of platelets. Statins, for their part, have shown a modulating effect on platelet activation. Furthermore, several studies have reported a protective effect of statins in staphylococcal endocarditis. The aim of this study was to investigate the influence of statins on the antibacterial effect of washed platelets. Blood samples were collected from healthy donors (n = 35). PRP was prepared according to the ISTH recommendation. Bacteria were incubated for four hours with untreated-washed platelets, or rather treated by statins and/or GPIIbIIIa antagonists. In order to evaluate the antibacterial effect, the platelet-bacteria mix was spread on the blood agar to count the number of colonies after 18 hours of incubation. Measurement of CD 41 and CD62P expression by flow cytometry was performed to evaluate the effect of statin on bacterial-induced platelet activation. Statins have shown a potentiation of the antibacterial effect of washed platelets (p < .01 for Atorvastatin and Rosuvastatin and p < .001 for Fluvastatin vs untreated washed platelets condition). This effect of statins was dose-dependent and was more significant at 20 μM. The addition of Fluvastatin to platelet-bacterial mix significantly increased the expression of platelet CD41 and CD62P (p < .05 and p < .01 vs resting washed platelets, respectively). Tirofiban, GPIIbIIIa antagonist, reversed the antibacterial effect of washed platelets and suppressed the potentiating effect of statins. Our study demonstrated that statins potentiate the anti-staphylococcal effect of washed platelets. This result may explain the beneficial effect of statins on Staphylococcus aureus infective endocarditis. Further studies are therefore required to explain this effect at the molecular level and to assess its impact in vivo.

Activated platelets kill Staphylococcus aureus, but not Streptococcus pneumoniae-The role of FcγRIIa and platelet factor 4/heparinantibodies

BACKGROUND

Heparin induced thrombocytopenia (HIT) is likely a misdirected bacterial host defense mechanism. Platelet factor 4 (PF4) binds to polyanions on bacterial surfaces exposing neo-epitopes to which HIT antibodies bind. Platelets are activated by the resulting immune complexes via FcγRIIA, release bactericidal substances, and kill Gram-negative Escherichia coli.

OBJECTIVES

To assess the role of PF4, anti-PF4/H antibodies and FcγRIIa in killing of Gram-positive bacteria by platelets.

METHODS

Binding of PF4 to protein-A deficient Staphylococcus aureus (SA113Δspa) and non-encapsulated Streptococcus pneumoniae (D39Δcps) and its conformational change were assessed by flow cytometry using monoclonal (KKO,5B9) and patient derived anti-PF4/H antibodies. Killing of bacteria was quantified by counting colony forming units (cfu) after incubation with platelets or platelet releasate. Using flow cytometry, platelet activation (CD62P-expression, PAC-1 binding) and phosphatidylserine (PS)-exposure were analyzed.

RESULTS

Monoclonal and patient-derived anti-PF4/H antibodies bound in the presence of PF4 to both S. aureus and S. pneumoniae (1.6-fold increased fluorescence signal for human anti-PF4/H antibodies to 24.0-fold increase for KKO). Staphylococcus aureus (5.5 × 104 cfu/mL) was efficiently killed by platelets (2.7 × 104 cfu/mL) or their releasate (2.9 × 104 cfu/mL). Killing was not further enhanced by PF4 or anti-PF4/H antibodies. Blocking FcγRIIa had no impact on killing of S. aureus by platelets. In contrast, S. pneumoniae was not killed by platelets or releasate. Instead, after incubation with pneumococci platelets were unresponsive to TRAP-6 stimulation and exposed high levels of PS.

CONCLUSIONS

Anti-PF4/H antibodies seem to have only a minor role for direct killing of Gram-positive bacteria by platelets. Staphylococcus aureus is killed by platelets or platelet releasate. In contrast, S. pneumoniae affects platelet viability.

LPS-induced Lung Platelet Recruitment Occurs Independently from Neutrophils, PSGL-1, and P-Selectin

Platelets are recruited to inflammatory foci and contribute to host defense and inflammatory responses. Compared with platelet recruitment in hemostasis and thrombosis, the mechanisms of platelet recruitment in inflammation and host defense are poorly understood. Neutrophil recruitment to lung airspaces after inhalation of bacterial LPS requires platelets and PSGL-1 in mice. Given this association between platelets and neutrophils, we investigated whether recruitment of platelets to lungs of mice after LPS inhalation was dependent on PSGL-1, P-selectin, or interaction with neutrophils. BALB/c mice were administered intranasal LPS (O55:B5, 5 mg/kg) and, 48 hours later, lungs were collected and platelets and neutrophils quantified in tissue sections by immunohistochemistry. The effects of functional blocking antibody treatments targeting the platelet-neutrophil adhesion molecules, P-selectin or PSGL-1, or treatment with a neutrophil-depleting antibody targeting Ly6G, were tested on the extent of LPS-induced lung platelet recruitment. Separately in Pf4-Cre × mTmG mice, two-photon intravital microscopy was used to image platelet adhesion in live lungs. Inhalation of LPS caused both platelet and neutrophil recruitment to the lung vasculature. However, decreasing lung neutrophil recruitment by blocking PSGL-1, P-selectin, or depleting blood neutrophils had no effect on lung platelet recruitment. Lung intravital imaging revealed increased adhesion of platelets in the lung microvasculature which was not associated with thrombus formation. In conclusion, platelet recruitment to lungs in response to LPS occurs through mechanisms distinct from those mediating neutrophil recruitment, or the occurrence of pulmonary emboli.

Cystic fibrosis transmembrane conductance regulator dysfunction in platelets drives lung hyperinflammation

Cystic fibrosis (CF) lung disease is characterized by an inflammatory response that can lead to terminal respiratory failure. The cystic fibrosis transmembrane conductance regulator (CFTR) is mutated in CF, and we hypothesized that dysfunctional CFTR in platelets, which are key participants in immune responses, is a central determinant of CF inflammation. We found that deletion of CFTR in platelets produced exaggerated acute lung inflammation and platelet activation after intratracheal LPS or Pseudomonas aeruginosa challenge. CFTR loss of function in mouse or human platelets resulted in agonist-induced hyperactivation and increased calcium entry into platelets. Inhibition of the transient receptor potential cation channel 6 (TRPC6) reduced platelet activation and calcium flux, and reduced lung injury in CF mice after intratracheal LPS or Pseudomonas aeruginosa challenge. CF subjects receiving CFTR modulator therapy showed partial restoration of CFTR function in platelets, which may be a convenient approach to monitoring biological responses to CFTR modulators. We conclude that CFTR dysfunction in platelets produces aberrant TRPC6-dependent platelet activation, which is a major driver of CF lung inflammation and impaired bacterial clearance. Platelets and TRPC6 are what we believe to be novel therapeutic targets in the treatment of CF lung disease.

Differential Abilities of Mammalian Cathelicidins to Inhibit Bacterial Biofilm Formation and Promote Multifaceted Immune Functions of Neutrophils

Mammalian cathelicidins act as the potent microbicidal molecules for controlling bacterial infection, and are considered promising alternatives to traditional antibiotics. Their ability to modulate host immune responses, as well as their bactericidal activities, is essential for therapeutic interventions. In this study, we compared the bactericidal activities, antibiofilm activities and immune-modulatory properties of cathelicidins BMAP-27, BMAP-34, mCRAMP, and LL-37, and evaluated the therapeutic efficacy of the combination of BMAP-27 and LL-37 using a mouse pulmonary infection model. Our results showed that all of the four cathelicidins effectively killed bacteria via rapid induction of membrane permeabilization, and BMAP-27 exhibited the most excellent bactericidal activity against diverse bacterial pathogens. BMAP-27, mCRAMP, and LL-37 effectively inhibited biofilm formation, while BMAP-34, mCRAMP and LL-37 exerted immunomodulatory functions with varying degrees of efficacy by stimulating the chemotaxis of neutrophils, inducing the production of reactive oxygen species, and facilitating the formation of neutrophil extracellular traps. Of note, the combination of BMAP-27 and LL-37 effectively enhanced the clearance of Pseudomonas aeruginosa and reduced the organ injury in vivo. Together, these findings highlight that identifying the appropriate synergistic combination of mammalian cathelicidins with different beneficial properties may be an effective strategy against bacterial infection.

Platelet Activation and Endothelial Cell Dysfunction

In sepsis, coagulation is activated and there is an increased risk of developing a consumptive coagulopathy with attendant increase in mortality. The processes that regulate hemostasis evolved as a component of the inflammatory response to infection. Many points of interaction occur on the endothelial cell surface linking the 2 cell types in the initiation and regulation of hemostasis and inflammation. Consequently, inflammation stimulates both platelets and endothelial cells in ways that affect both hemostasis and the immune response. Platelets are also prime drivers of the inflammatory response. This article discusses the pathways wherein inflammation regulates platelet and endothelial cell function.

The dual role of platelet-innate immune cell interactions in thrombo-inflammation

Beyond their role in hemostasis and thrombosis, platelets are increasingly recognized as key regulators of the inflammatory response under sterile and infectious conditions. Both platelet receptors and secretion are critical for these functions and contribute to their interaction with the endothelium and innate immune system. Platelet-leukocyte interactions are increased in thrombo-inflammatory diseases and are sensitive biomarkers for platelet activation and targets for the development of new therapies. The crosstalk between platelets and innate immune cells promotes thrombosis, inflammation, and tissue damage. However, recent studies have shown that these interactions also regulate the resolution of inflammation, tissue repair, and wound healing. Many of the platelet and leukocyte receptors involved in these bidirectional interactions are not selective for a subset of immune cells. However, specific heterotypic interactions occur in different vascular beds and inflammatory conditions, raising the possibility of disease- and organ-specific pathways of intervention. In this review, we highlight and discuss prominent and emerging interrelationships between platelets and innate immune cells and their dual role in the regulation of the inflammatory response in sterile and infectious thrombo-inflammatory diseases. A better understanding of the functional relevance of these interactions in different vascular beds may provide opportunities for successful therapeutic interventions to regulate the development, progression, and chronicity of various pathological processes.

The pro-tumor effect and the anti-tumor effect of neutrophils extracellular traps

Significant advances in our understanding of neutrophil biology were made in the past several years. A newly discovered mechanism was discovered, the formation of neutrophils extracellular traps (NETs). The structure of NETs is composed of the DNA strand and neutrophil granule proteins. NETs were found to have an association with tumor progression. This review highlights the latest knowledge about the controversial effect on tumors of NETs. Pro-tumor and anti-tumor effects are described respectively. The probable mechanisms of the anti-tumor effect are related to its direct killing of cancer cells or stimulation of the immune system to fight against the tumor. The pro-tumor effect has a correlation with matrix metalloproteinase 9 (MMP-9), cathepsin G, and neutrophil elastase (NE). Moreover, the structure of the NETs makes it able to catch the circulating tumor cells, which could lead to metastasis. This review summarizes our knowledge about the proven roles of NETs in the progression of cancer with particular focus on the components of the NETs, and considers NETs as a potential target for cancer therapy.

The distinct effects of aspirin on platelet aggregation induced by infectious bacteria

Bacteria induce platelet aggregation triggered by several mechanisms. The goal of this work was to characterize platelet aggregates induced by different bacterial strains and to quantify the effect of aspirin treatment using aggregation tests, as well as a novel approach based on confocal analysis. Blood samples were obtained from either healthy donors (n = 27) or patients treated with long-term aspirin (n = 15). The bacterial species included were Staphylococcus aureus, Enterococcus faecalis, and Streptococcus sanguinis. The different aggregate's ultrastructures depending on the bacterial strain were analyzed using Scanning electron microscopy. Quantification of the size of the platelet aggregates, their mean number as well as the bacterial impregnation within the aggregates was performed using confocal laser scanning light microscopy. Light Transmission Aggregometry was also performed. Our results reported distinct characteristics of platelet aggregates depending on the bacterial strain. Using confocal analysis, we have shown that aspirin significantly reduced platelet aggregation induced by S. aureus (p = .003) and E. faecalis (p = .006) with no effect in the case of S. sanguinis (p = .529). The results of the aggregometry were concordant with those of the confocal technique in the case of S. aureus and S. sanguinis. Interestingly, aggregation induced by E. faecalis was detected only with confocal analysis. In conclusion, our confocal scanning microscopy allowed a detailed study of the platelet aggregation induced by bacteria. We showed that aspirin acts on bacterial-induced platelet aggregation depending on the species. These results are in favor of the use of aspirin considering the species and the bacterial strain involved.

Platelet Proteasome Activity and Metabolism Is Upregulated during Bacterial Sepsis

Dysregulation of platelet function can contribute to the disease progression in sepsis. The proteasome represents a critical and vital element of cellular protein metabolism in platelets and its proteolytic activity has been associated with platelet function. However, the role of the platelet proteasome as well as its response to infection under conditions of sepsis have not been studied so far. We measured platelet proteasome activity by fluorescent substrates, degradation of poly-ubiquitinated proteins and cleavage of the proteasome substrate Talin-1 in the presence of living E. coli strains and in platelets isolated from sepsis patients. Upregulation of the proteasome activator PA28 (PSME1) was assessed by quantitative real-time PCR in platelets from sepsis patients. We show that co-incubation of platelets with living E. coli (UTI89) results in increased degradation of poly-ubiquitinated proteins and cleavage of Talin-1 by the proteasome. Proteasome activity and cleavage of Talin-1 was significantly increased in α-hemolysin (HlyA)-positive E. coli strains. Supporting these findings, proteasome activity was also increased in platelets of patients with sepsis. Finally, the proteasome activator PA28 (PSME1) was upregulated in this group of patients. In this study we demonstrate for the first time that the proteasome in platelets is activated in the septic milieu.

Aspirin Effect on Staphylococcus aureus-Platelet Interactions During Infectious Endocarditis

Infectious endocarditis (IE) is a rare disease associated with high mortality and morbidity rate. The platelet-bacterial interaction presents the cornerstone of the development of endocardial vegetation. The epidemiology of IE has undergone profound changes between the last and the new decade, with Staphylococcus aureus becoming the main incriminated species. Despite improvements in antibiotic and surgical therapies, embolic disorders remain highly associated with IE that can be fatal. Antiplatelet drugs have been widely proposed to overcome embolic events associated with IE. This proposal has been supported by numerous in vitro, experimental, and clinical studies. However, other studies have yielded conflicting results. In this review, we focus on the effect of aspirin on the genesis of S. aureus endocarditic vegetation, as well as on the management of embolic and hemorrhagic events related to it, starting by its influence on the platelet-bacteria interaction.

Platelets and Intravascular Immunity: Guardians of the Vascular Space During Bloodstream Infections and Sepsis

Despite their humble origins as anuclear fragments of megakaryocytes, platelets have emerged as versatile mediators of thrombosis and immunity. The diverse spectrum of platelet functions are on full display during the host response to severe infection and sepsis, with platelets taking center-stage in the intravascular immune response to blood-borne pathogens. Platelets are endowed with a comprehensive armamentarium of pathogen detection systems that enable them to function as sentinels in the bloodstream for rapid identification of microbial invasion. Through both autonomous anti-microbial effector functions and collaborations with other innate immune cells, platelets orchestrate a complex intravascular immune defense system that protects against bacterial dissemination. As with any powerful immune defense system, dysregulation of platelet-mediated intravascular immunity can lead to profound collateral damage to host cells and tissues, resulting in sepsis-associated organ dysfunction. In this article, the cellular and molecular contributions of platelets to intravascular immune defenses in sepsis will be reviewed, including the roles of platelets in surveillance of the microcirculation and elicitation of protective anti-bacterial responses. Mechanisms of platelet-mediated thromboinflammatory organ dysfunction will be explored, with linkages to clinical biomarkers of platelet homeostasis that aid in the diagnosis and prognostication of human sepsis. Lastly, we discuss novel therapeutic opportunities that take advantage of our evolving understanding of platelets and intravascular immunity in severe infection.

Platelets in Host Defense: Experimental and Clinical Insights

Platelets are central players in thrombosis and hemostasis but are increasingly recognized as key components of the immune system. They shape ensuing immune responses by recruiting leukocytes, and support the development of adaptive immunity. Recent data shed new light on the complex role of platelets in immunity. Here, we summarize experimental and clinical data on the role of platelets in host defense against bacteria. Platelets bind, contain, and kill bacteria directly; however, platelet proinflammatory effector functions and cross-talk with the coagulation system, can also result in damage to the host (e.g., acute lung injury and sepsis). Novel clinical insights support this dichotomy: platelet inhibition/thrombocytopenia can be either harmful or protective, depending on pathophysiological context. Clinical studies are currently addressing this aspect in greater depth.

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.

The Era of Thromboinflammation: Platelets Are Dynamic Sensors and Effector Cells During Infectious Diseases

Platelets are anucleate cells produced by megakaryocytes. In recent years, a robust body of literature supports the evolving role of platelets as key sentinel and effector cells in infectious diseases, especially critical in bridging hemostatic, inflammatory, and immune continuums. Upon intravascular pathogen invasion, platelets can directly sense viral, parasitic, and bacterial infections through pattern recognition receptors and integrin receptors or pathogen: immunoglobulin complexes through Fc and complement receptors—although our understanding of these interactions remains incomplete. Constantly scanning for areas of injury or inflammation as they circulate in the vasculature, platelets also indirectly respond to pathogen invasion through interactions with leukocytes and the endothelium. Following antigen recognition, platelets often become activated. Through a diverse repertoire of mechanisms, activated platelets can directly sequester or kill pathogens, or facilitate pathogen clearance by activating macrophages and neutrophils, promoting neutrophil extracellular traps (NETs) formation, forming platelet aggregates and microthrombi. At times, however, platelet activation may also be injurious to the host, exacerbating inflammation and promoting endothelial damage and thrombosis. There are many gaps in our understandings of the role of platelets in infectious diseases. However, with the emergence of advanced technologies, our knowledge is increasing. In the current review, we mainly discuss these evolving roles of platelets under four different infectious pathogen infections, of which are dengue, malaria, Esterichia coli (E. coli) and staphylococcus aureus S. aureus, highlighting the complex interplay of these processes with hemostatic and thrombotic pathways.

Effect of antimicrobial peptides HNP-1 and hBD-1 on Staphylococcus aureus strains in vitro and in vivo

The aims of this study were: (i) To investigate the activity of recombinant AMPs HNP-1 and hBD-1 in combination with cefotaxime against Staphylococcus aureus strains (MSSA and MRSA) in vitro using checkerboard method; (ii) To investigate the activity of HNP-1 and hBD-1 encapsulated in silicon nanoparticles (niosomes) in the treatment of MRSA-infected wound in rats. For this S. aureus strains (MSSA and MRSA) were isolated from patients with diabetic foot infection. Cefotaxime, recombinant HNP-1 and hBD-1 (in all possible combinations with each other) were used for testing by the checkerboard method. Two niosomal topical gels with HNP-1/hBD-1 were prepared to treat MRSA-infected wounds in rats. Gels were administered once a day, the control group-without treatment. Wound healing rate was calculated on the 4th, 9th and 16th days of the experiment and compared using one-way ANOVA with Bonferroni correction. MIC of HNP-1 for MSSA and MRSA was the same-1 mg/L. MIC of hBD-1 for MSSA and MRSA was also the same-0.5 mg/L. Topical gels with niosomal HNP-1 (or hBD-1) showed a significantly faster wound healing in comparison with the control. The data obtained open up prospects for use of AMPs encapsulated in silica nanoparticles for the development of new antibiotics.

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.

Circulating Platelets as Mediators of Immunity, Inflammation, and Thrombosis

Platelets, non-nucleated blood components first described over 130 years ago, are recognized as the primary cell regulating hemostasis and thrombosis. The vascular importance of platelets has been attributed to their essential role in thrombosis, mediating myocardial infarction, stroke, and venous thromboembolism. Increasing knowledge on the platelets' role in the vasculature has led to many advances in understanding not only how platelets interact with the vessel wall but also how they convey changes in the environment to other circulating cells. In addition to their well-described hemostatic function, platelets are active participants in the immune response to microbial organisms and foreign substances. Although incompletely understood, the immune role of platelets is a delicate balance between its pathogenic response and its regulation of thrombotic and hemostatic functions. Platelets mediate complex vascular homeostasis via specific receptors and granule release, RNA transfer, and mitochondrial secretion that subsequently regulates hemostasis and thrombosis, infection, and innate and adaptive immunity.

Human platelets and megakaryocytes express defensin alpha 1

Platelets are anucleate cells that have a role in several innate immune functions, including the secretion of proteins with antimicrobial activity. Several studies have demonstrated the ability of platelets to secrete thrombin-induced platelet microbicidal proteins and antimicrobial peptides, like hBD-1. However, the expression and secretion of defensins of the alpha family by platelets have not been fully elucidated. The aim of this study was to characterize the expression of defensin alpha 1 (DEFA1) in human platelets and megakaryocytes. Our data indicate that DEFA1 mRNA and protein are present in peripheral blood platelets and in the megakaryoblastic leukemia cell line (MEG-01). DEFA1 co-localize with α-granules of platelets and MEG-01 cells, and was also detected in cytoplasm of MEG-01 cells. The assay of our in vitro model of platelet-like particles (PLPs) revealed that MEG-01 cells could transfer DEFA1 mRNA to their differentiated PLPs. Furthermore, platelets secreted DEFA1 into the culture medium when activated with thrombin, adenosine diphosphate, and lipopolysaccharide; meanwhile, MEG-01 cells secreted DEFA1 when activated with thrombopoietin. Platelet's secreted DEFA1 can rebind to platelet's surface and have antibacterial activity against the gram-negative bacteria Escherichia coli. In summary, our data indicate that both, human platelets and megakaryocytes, can express and secrete DEFA1. These results suggest a new role of platelets and megakaryocytes in the innate immune response.

Human megakaryocytes possess intrinsic antiviral immunity through regulated induction of IFITM3

Evolving evidence indicates that platelets and megakaryocytes (MKs) have unexpected activities in inflammation and infection; whether viral infections upregulate biologically active, antiviral immune genes in platelets and MKs is unknown, however. We examined antiviral immune genes in these cells in dengue and influenza infections, viruses that are global public health threats. Using complementary biochemical, pharmacological, and genetic approaches, we examined the regulation and function of interferon-induced transmembrane protein 3 (IFITM3), an antiviral immune effector gene not previously studied in human platelets and MKs. IFITM3 was markedly upregulated in platelets isolated from patients during clinical influenza and dengue virus (DENV) infections. Lower IFITM3 expression in platelets correlated with increased illness severity and mortality in patients. Administering a live, attenuated DENV vaccine to healthy subjects significantly increased platelet IFITM3 expression. Infecting human MKs with DENV selectively increased type I interferons and IFITM3. Overexpression of IFITM3 in MKs was sufficient to prevent DENV infection. In naturally occurring, genetic loss-of-function studies, MKs from healthy subjects harboring a homozygous mutation in IFITM3 (rs12252-C, a common single-nucleotide polymorphism in areas of the world where DENV is endemic) were significantly more susceptible to DENV infection. DENV-induced MK secretion of interferons prevented infection of bystander MKs and hematopoietic stem cells. Thus, viral infections upregulate IFITM3 in human platelets and MKs, and IFITM3 expression is associated with adverse clinical outcomes. These observations establish, for the first time, that human MKs possess antiviral functions, preventing DENV infection of MKs and hematopoietic stem cells after local immune signaling.

Early Blood Biomarkers to Improve Sepsis/Bacteremia Diagnostics in Pediatric Emergency Settings

Background: Sepsis is the leading cause of death in children worldwide. Early recognition and treatment are essential for preventing progression to lethal outcomes. CRP and Complete Blood Count (CBC) are the initial preferred tests to distinguish between bacterial and viral infections. Specific early diagnostic markers are still missing. Aim: To investigate diagnostic value of Neutrophil-Lymphocyte Ratio (NLR), Mean Platelet Volume (MPV) and Platelet-MPV ratio (PLT/MPV) to distinguish sepsis/bacteremia and viral infection. Methods: We conducted a retrospective data analysis of case records of 115 children from 1 month to 5 years of age. All cases were divided into two groups-sepsis/bacteremia (n = 68) and viral (n = 47) patients, and further subdivided according to the time of arrival into early or late (≤12 or 12-48 h post the onset of fever, respectively). Analysis of CBC and CRP results was performed. NLR and PLT/MPV were calculated. Results: Sepsis/bacteremia group demonstrated higher absolute platelets count (370.15 ± 134.65 × 10⁸/L versus 288.91 ± 107.14 × 10⁸/L; p = 0.001), NLR (2.69 ± 2.03 versus 1.83 ± 1.70; p = 0.006), and PLT/MPV (41.42 ± 15.86 versus 33.45 ± 17.97; p = 0.001). PLT/MPV was increased in early arrival sepsis/bacteremia infants (42.70 ± 8.57 versus 31.01 ± 8.21; p = 0.008). NLR and MPV were significantly lower in infants (≤12 months) with viral infection on late arrival (1.16 ± 1.06 versus 1.90 ± 1.25, p = 0.025 for NLR and 8.94 ± 0.95fl versus 9.44 ± 0.85fl, p = 0.046 for MPV). Conclusion: Together with standard blood biomarkers, such as CRP, neutrophils, or platelets count, PLT/MPV is a promising biomarker for clinical practice to help discriminate between viral disease or sepsis/bacteremia in all children, especially in early onset of symptoms. NLR and MPV could support exclusion of sepsis/bacteremia in late arrival cases.

Dual actions of group B Streptococcus capsular sialic acid provide resistance to platelet-mediated antimicrobial killing

Circulating platelets have important functions in thrombosis and in modulating immune and inflammatory responses. However, the role of platelets in innate immunity to bacterial infection is largely unexplored. While human platelets rapidly kill Staphylococcus aureus, we found the neonatal pathogen group B Streptococcus (GBS) to be remarkably resistant to platelet killing. GBS possesses a capsule polysaccharide (CPS) with terminal α2,3-linked sialic acid (Sia) residues that mimic a common epitope present on the human cell surface glycocalyx. A GBS mutant deficient in CPS Sia was more efficiently killed by human platelets, thrombin-activated platelet releasate, and synthetic platelet-associated antimicrobial peptides. GBS Sia is known to bind inhibitory Sia-recognizing Ig superfamily lectins (Siglecs) to block neutrophil and macrophage activation. We show that human platelets also express high levels of inhibitory Siglec-9 on their surface, and that GBS can engage this receptor in a Sia-dependent manner to suppress platelet activation. In a mouse i.v. infection model, antibody-mediated platelet depletion increased susceptibility to platelet-sensitive S. aureus but did not alter susceptibility to platelet-resistant GBS. Elimination of murine inhibitory Siglec-E partially reversed platelet suppression in response to GBS infection. We conclude that GBS Sia has dual roles in counteracting platelet antimicrobial immunity: conferring intrinsic resistance to platelet-derived antimicrobial components and inhibiting platelet activation through engagement of inhibitory Siglecs. We report a bacterial virulence factor for evasion of platelet-mediated innate immunity.

Proteolytic Degradation of reduced Human Beta Defensin 1 generates a Novel Antibiotic Octapeptide

Microbial resistance against clinical used antibiotics is on the rise. Accordingly, there is a high demand for new innovative antimicrobial strategies. The host-defense peptide human beta-defensin 1 (hBD-1) is produced continuously by epithelial cells and exhibits compelling antimicrobial activity after reduction of its disulphide bridges. Here we report that proteolysis of reduced hBD-1 by gastrointestinal proteases as well as human duodenal secretions produces an eight-amino acid carboxy-terminal fragment. The generated octapeptide retains antibiotic activity, yet with distinct characteristics differing from the full-length peptide. We modified the octapeptide by stabilizing its termini and by using non-natural D-amino acids. The native and modified peptide variants showed antibiotic activity against pathogenic as well as antibiotic-resistant microorganisms, including E. coli, P. aeruginosa and C. albicans. Moreover, in an in vitro C. albicans infection model the tested peptides demonstrated effective amelioration of C. albicans infection without showing cytotoxity on human cells. In summary, protease degradation of hBD-1 provides a yet unknown mechanism to broaden antimicrobial host defense, which could be used to develop defensin-derived therapeutic applications.

Platelets inhibit apoptotic lung epithelial cell death and protect mice against infection-induced lung injury

Thrombocytopenia is associated with worse outcomes in patients with acute respiratory distress syndrome, which is most commonly caused by infection and marked by alveolar-capillary barrier disruption. However, the mechanisms by which platelets protect the lung alveolar-capillary barrier during infectious injury remain unclear. We found that natively thrombocytopenic Mpl ^-/- mice deficient in the thrombopoietin receptor sustain severe lung injury marked by alveolar barrier disruption and hemorrhagic pneumonia with early mortality following acute intrapulmonary Pseudomonas aeruginosa (PA) infection; barrier disruption was attenuated by platelet reconstitution. Although PA infection was associated with a brisk neutrophil influx, depletion of airspace neutrophils failed to substantially mitigate PA-triggered alveolar barrier disruption in Mpl ^-/- mice. Rather, PA cell-free supernatant was sufficient to induce lung epithelial cell apoptosis in vitro and in vivo and alveolar barrier disruption in both platelet-depleted mice and Mpl ^-/- mice in vivo. Cell-free supernatant from PA with genetic deletion of the type 2 secretion system, but not the type 3 secretion system, mitigated lung epithelial cell death in vitro and lung injury in Mpl ^-/- mice. Moreover, platelet releasates reduced poly (ADP ribose) polymerase cleavage and lung injury in Mpl ^-/- mice, and boiling of platelet releasates, but not apyrase treatment, abrogated PA supernatant-induced lung epithelial cell cytotoxicity in vitro. These findings indicate that while neutrophil airspace influx does not potentiate infectious lung injury in the thrombocytopenic host, platelets and their factors protect against severe pulmonary complications from pathogen-secreted virulence factors that promote host cell death even in the absence of overt infection.

Coevolution of the coagulation and immune systems

BACKGROUND

Higher organisms rely on the coagulation and immune systems to fight disease-causing pathogens and other foreign invaders in the body. Coagulation has an important role as a barrier against foreign bodies, including bacteria, viruses, and protozoa. The protective responses associated with the coagulation and immune systems can protect the host organism from a wide range of pathogens, such as viruses, parasites, fungi, and even bacteria.

AIM

The purpose of this paper was to review available research on the evolution of the coagulation and immune systems.

MATERIALS AND METHODS

The study analyzed evidence from studies that have examined the coagulation and immune systems in the context of evolutionary processes. The articles used in the review were identified from the PsycINFO, CIHAHL, PubMed, Web of Science, and CIHAHL databases.

RESULTS

Studies have shown that both the coagulation system and the early immune system originated from the same initial system in early organisms. Some researchers argue that hemocytes from lower organisms are the common link from which the immune system and coagulation system developed.

DISCUSSION AND CONCLUSION

Simple organisms have hemocytes that can carry out both immune response and coagulation processes. Evolution led to the separation of these processes in higher organisms. Furthermore, this divergence resulted in the emergence of thrombocytes and plasmatic coagulation subsystems. These observations explain why there is some form of overlap between immunity and hemostasis, even in advanced organisms such as vertebrates. Several phenomena in clinical medicine related to coagulation and immunity can be explained by this overlap and are consistent with the hypothesis of the coevolution of coagulation and the immune system.