Platelet interaction with bacterial toxins and secreted products

Predictive value of thrombocytopenia for bloodstream infection in patients with sepsis and septic shock

BACKGROUND

Thrombocytopenia is common in patients with sepsis and septic shock.

AIM

To analyse the decrease in the number of platelets for predicting bloodstream infection in patients with sepsis and septic shock in the intensive care unit.

METHODS

A retrospective analysis of patients admitted with sepsis and septic shock in Xingtai People Hospital was revisited. Patient population characteristics and laboratory data were collected for analysis.

RESULTS

The study group consisted of 85 (39%) inpatients with bloodstream infection, and the control group consisted of 133 (61%) with negative results or contamination. The percentage decline in platelet counts (PPCs) in patients positive for pathogens [57.1 (41.3-74.6)] was distinctly higher than that in the control group [18.2 (5.1-43.1)] (P < 0.001), whereas the PPCs were not significantly different among those with gram-positive bacteraemia, gram-negative bacteraemia, and fungal infection. Using receiver operating characteristic curves, the area under the curve of the platelet drop rate was 0.839 (95%CI: 0.783-0.895).

CONCLUSION

The percentage decline in platelet counts is sensitive in predicting bloodstream infection in patients with sepsis and septic shock. However, it cannot identify gram-positive bacteraemia, gram-negative bacteraemia, and fungal infection.

Bacteria-derived extracellular vesicles: endogenous roles, therapeutic potentials and their biomimetics for the treatment and prevention of sepsis

Sepsis is one of the medical conditions with a high mortality rate and lacks specific treatment despite several years of extensive research. Bacterial extracellular vesicles (bEVs) are emerging as a focal target in the pathophysiology and treatment of sepsis. Extracellular vesicles (EVs) derived from pathogenic microorganisms carry pathogenic factors such as carbohydrates, proteins, lipids, nucleic acids, and virulence factors and are regarded as "long-range weapons" to trigger an inflammatory response. In particular, the small size of bEVs can cross the blood-brain and placental barriers that are difficult for pathogens to cross, deliver pathogenic agents to host cells, activate the host immune system, and possibly accelerate the bacterial infection process and subsequent sepsis. Over the years, research into host-derived EVs has increased, leading to breakthroughs in cancer and sepsis treatments. However, related approaches to the role and use of bacterial-derived EVs are still rare in the treatment of sepsis. Herein, this review looked at the dual nature of bEVs in sepsis by highlighting their inherent functions and emphasizing their therapeutic characteristics and potential. Various biomimetics of bEVs for the treatment and prevention of sepsis have also been reviewed. Finally, the latest progress and various obstacles in the clinical application of bEVs have been highlighted.

The Controversial Role of LPS in Platelet Activation In Vitro

Circulating platelets are responsible for hemostasis and thrombosis but are also primary sensors of pathogens and are involved in innate immunity, inflammation, and sepsis. Sepsis is commonly caused by an exaggerated immune response to bacterial, viral, and fungal infections, and leads to severe thrombotic complications. Among others, the endotoxin lipopolysaccharide (LPS) found in the outer membrane of Gram-negative bacteria is the most common trigger of sepsis. Since the discovery of the expression of the LPS receptor TLR4 in platelets, several studies have investigated the ability of LPS to induce platelet activation and to contribute to a prothrombotic phenotype, per se or in combination with plasma proteins and platelet agonists. This issue, however, is still controversial, as different sources, purity, and concentrations of LPS, different platelet-purification protocols, and different methods of analysis have been used in the past two decades, giving contradictory results. This review summarizes and critically analyzes past and recent publications about LPS-induced platelet activation in vitro. A methodological section illustrates the principal platelet preparation protocols and significant differences. The ability of various sources of LPS to elicit platelet activation in terms of aggregation, granule secretion, cytokine release, ROS production, and interaction with leukocytes and NET formation is discussed.

Probiotic Engineering and Targeted Sonoimmuno-Therapy Augmented by STING Agonist

Tumor targeting and effective immunomodulation are of critical significance during tumor treatment by sonodynamic therapy (SDT). Herein, the probiotic engineering of the clinically approved sonosensitizer (hematoporphyrin monomethyl ether (HMME)) is reported onto the probiotic bacterium Bifidobacteria Longum (BiL) for sonosensitive bifidobacterium construction (HMME@BiL cells). Based on the hypoxic tropism feature of the strain, effective tumor-targeted sonodynamic therapeutics can be achieved both in vitro and in vivo. To improve the immunological responses against tumor during sonodynamics, a recently-developed stimulator of interferon genes immune agonist SR717 has been employed to improve the anti-tumor immunity with prominent activities, eradicating both primary and metastatic tumors with high efficiency and satisfied biocompatibility. The present work provides a promising paradigm of microbiotic nanomedicine in a sophisticated sonoimmunotherapeutic strategy against malignant tumors.

Cellular Nanosponges for Biological Neutralization

Biological neutralization represents a general strategy that deploys therapeutic agents to bind with harmful molecules or infectious pathogens, block their bioactivity, and thus prevent them from causing the diseases. Here, a comprehensive review of using cell-membrane-coated nanoparticles, namely "cellular nanosponges," as host decoys for a wide range of biological neutralization applications is provided. Compared to traditional neutralization strategies, the cellular nanosponges stand out by mimicking susceptible host cells rather than accommodating the structures of the causative agents for the design of therapeutics. As all pathological agents must interact with host cells for bioactivity, nanosponges bypass the diversity of these agents and create function-driven and broad-spectrum neutralization solutions. The review focuses on the recent progress of using this new nanomedicine platform for neutralization against five primary pathological agents, including bacterial toxins, chemical toxicants, inflammatory cytokines, pathological antibodies, and viruses. Existing studies have established cellular nanosponges as versatile tools for biological neutralization. A thorough review of the cellular nanosponge technology is expected to inspire more refined cellular nanosponge designs and unique neutralization applications to address unsolved medical problems.

Autologous platelet-rich plasma effects on Staphylococcus aureus-induced chondrocyte death in an in vitro bovine septic arthritis model

OBJECTIVE

To investigate the chondroprotective effects of autologous platelet-rich plasma (PRP), ampicillin-sulbactam (AmpS), or PRP combined with AmpS (PRP+AmpS) in an in vitro chondrocyte explant model of bovine Staphylococcus aureus-induced septic arthritis.

SAMPLE

Autologous PRP and cartilage explants obtained from 6 healthy, adult, nonlactating Jersey-crossbred cows.

ProcedureS

Autologous PRP was prepared prior to euthanasia using an optimized double centrifugation protocol. Cartilage explants collected from grossly normal stifle joints were incubated in synovial fluid (SF) alone, S aureus-inoculated SF (SA), or SA supplemented with PRP (25% culture medium volume), AmpS (2 mg/mL), or both PRP (25% culture medium volume) and AmpS (2 mg/mL; PRP+AmpS) for 24 hours. The metabolic activity, percentage of dead cells, and glycosaminoglycan content of cartilage explants were measured with a resazurin-based assay, live-dead cell staining, and dimethylmethylene blue assay, respectively. Treatment effects were assessed relative to the findings for cartilage explants incubated in SF alone.

RESULTS

Application of PRP, AmpS, and PRP+AmpS treatments significantly reduced S aureus-induced chondrocyte death (ie, increased metabolic activity and cell viability staining) in cartilage explants, compared with untreated controls. There were no significant differences in chondrocyte death among explants treated with PRP, AmpS, or PRP+AmpS.

CLINICAL RELEVANCE

In this in vitro explant model of S aureus-induced septic arthritis, PRP, AmpS, and PRP+AmpS treatments mitigated chondrocyte death. Additional work to confirm the efficacy of PRP with bacteria commonly associated with clinical septic arthritis in cattle as well as in vivo evaluation is warranted.

What Function Do Platelets Play in Inflammation and Bacterial and Viral Infections?

The article presents the function of platelets in inflammation as well as in bacterial and viral infections, which are the result of their reaction with the endovascular environment, including cells of damaged vascular endothelium and cells of the immune system. This role of platelets is conditioned by biologically active substances present in their granules and in their specific structures - EV (extracellular vesicles).

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.

Inhibition of immunothrombosis does not affect pathogen capture and does not promote bacterial dissemination in a mouse model of sepsis

After infection, neutrophils release neutrophil extracellular traps (NETs), decondensed DNA fibers decorated with both nuclear proteins and proteins derived from intracellular granules. These structures have a fundamental role in the development of immunothrombosis; a physiological process mediated by immune cells and molecules from the coagulation system that facilitates the recognition, containment, and destruction of pathogens. Although NETs and immunothrombi are widely hypothesized to be key host defense responses responsible for limiting bacterial dissemination, their actual role in this process has not been formally assessed within the context of a bloodstream infection. Mice were first treated with LPS to generate inflammation (NETs and immunothrombi) and then bacteria dissemination was analyzed by intravital microscopy and colony-forming units (CFU) assay. Blocking NETs or coagulation by the administration of DNase or Argatroban (thrombin inhibitor), respectively, did not modify the percentage of bacteria capture by Kupffer cells, neutrophils or platelets. Moreover, both inhibitors reduced the number of bacteria in the spleen, without modifying CFUs in the liver or lung. In conclusion, we demonstrate that immunothrombi are not necessary to limit the dissemination of bloodstream bacterial infections.

Porphyromonas gingivalis lipopolysaccharide activates platelet Cdc42 and promotes platelet spreading and thrombosis

BACKGROUND

Periodontitis confers an increased risk for cardiovascular diseases, including thrombosis. However, the molecular mechanisms that potentially link periodontitis with thrombosis are undefined. Here we test the hypothesis that Gram-negative periodontal infection promotes pathological platelet activation and amplifies shape change. We focus specifically on lipopolysaccharide (LPS) signaling to platelets.

METHODS

Platelets were isolated from blood samples and allowed to spread on coverslips in the presence or absence of LPS purified from the periodontal pathogen Porphyromonas gingivalis. Platelets were fixed and stained with Alexa-488-phalloidin to label the actin cytoskeleton. The degree of platelet spreading and shape change was quantified by confocal microscopy. In a translational pilot study, blood samples were obtained from human subjects exhibiting generalized severe periodontitis (SP) or healthy periodontium (HP). Rotational thromboelastometry was used to quantify the rate of clot formation via the intrinsic coagulation pathway.

RESULTS

LPS-treated platelets exhibited significantly (P < 0.05) greater spreading and higher numbers of actin-rich filopodia (cell extensions) than controls. We also found that LPS stimulation of platelets promoted the activation of Cdc42, the small GTPase responsible for filopodia formation. Exposure of whole blood samples to LPS significantly (P < 0.05) reduced clotting times. Blood from SP patients clotted significantly (P < 0.05) more rapidly and exhibited shorter partial thromboplastin times compared with HP controls.

CONCLUSIONS

This is the first study to suggest a mechanism by which LPS stimulation drives Cdc42 activation and platelet spreading. These data are consistent with the notion that periodontitis promotes accelerated clot formation and an increased risk of thrombosis.

Understanding Platelets in Infectious and Allergic Lung Diseases

Emerging evidence suggests that platelets, cytoplasmic fragments derived from megakaryocytes, can no longer be considered just as mediators in hemostasis and coagulation processes, but as key modulators of immunity. Platelets have received increasing attention as the emergence of new methodologies has allowed the characterization of their components and functions in the immune continuum. Platelet activation in infectious and allergic lung diseases has been well documented and associated with bacterial infections reproduced in several animal models of pulmonary bacterial infections. Direct interactions between platelets and bacteria have been associated with increased pulmonary platelet accumulation, whereas bacterial-derived toxins have also been reported to modulate platelet function. Recently, platelets have been found extravascular in the lungs of patients with asthma, and in animal models of allergic lung inflammation. Their ability to interact with immune and endothelial cells and secrete immune mediators makes them one attractive target for biomarker identification that will help characterize their contribution to lung diseases. Here, we present an original review of the last advances in the platelet field with a focus on the contribution of platelets to respiratory infections and allergic-mediated diseases.

Impact of Escherichia coli K12 and O18:K1 on human platelets: Differential effects on platelet activation, RNAs and proteins

Blood platelets can interact with bacteria, possibly leading to platelet activation, cytokine and microparticle release and immune signalling. Besides, bacteria can also affect the platelet RNA content. We investigated the impact of non-pathogenic K12 and pathogenic O18:K1 Escherichia (E.) coli strains on platelet activation, RNA expression patterns, and selected proteins. Depending on bacteria concentration, contact of platelets with E. coli K12 lead to an increase of P-selectin (24–51.3%), CD63 (15.9–24.3%), PAC-1 (3.8–14.9%) and bound fibrinogen (22.4–39%) on the surface. E. coli O18:K1 did not affect these markers. Sequencing analysis of total RNA showed that E. coli K12 caused a significant concentration change of 103 spliced mRNAs, of which 74 decreased. For the RNAs of HMBS (logFC = +5.73), ATP2C1 (logFC = −3.13) and LRCH4 (logFC = −4.07) changes were detectable by thromboSeq and Tuxedo pipelines. By Western blot we observed the conversion of HMBS protein from a 47 kDA to 40 kDa product by E. coli K12, O18:K1 and by purified lipopolysaccharide. While ATP2C1 protein was released from platelets, E. coli either reduced the secretion or broke down the released protein making it undetectable by antibodies. Our results demonstrate that different E. coli strains influence activation, RNA and protein levels differently which may affect platelet-bacteria crosstalk.

Platelet Depletion Impairs Host Defense to Pulmonary Infection with Pseudomonas aeruginosa in Mice

Platelets have been implicated in pulmonary inflammatory cell recruitment after exposure to allergic and nonallergic stimuli, but little is known about the role of platelets in response to pulmonary infection with Pseudomonas aeruginosa. In this study, we have investigated the impact of the experimental depletion of circulating platelets on a range of inflammatory and bacterial parameters, and their subsequent impact on mortality in a murine model of pulmonary infection with P. aeruginosa. P. aeruginosa infection in mice induced a mild, but significant, state of peripheral thrombocytopenia in addition to pulmonary platelet accumulation. Increased platelet activation was detected in infected mice through increased levels of the platelet-derived mediators, platelet factor-4 and β-thromboglobulin, in BAL fluid and blood plasma. In mice depleted of circulating platelets, pulmonary neutrophil recruitment was significantly reduced 24 hours after infection, whereas the incidence of systemic dissemination of bacteria was significantly increased compared with non-platelet-depleted control mice. Furthermore, mortality rates were increased in bacterial-infected mice depleted of circulating platelets. This work demonstrates a role for platelets in the host response toward a gram-negative bacterial respiratory infection.

Determining Platelet Activation and Aggregation in Response to Bacteria

Many pathogenic bacteria have been reported to interact with human platelets to mediate platelet activation and aggregation. The importance of these interactions to the immune response or pathogenesis of bacterial infection has not been clarified. It may therefore be valuable to assess platelet responses mediated by diverse strains of bacteria. Here, I describe a method to study platelet integrin activation and granule release using flow cytometry, and a complementary method to study platelet aggregation using a dedicated platelet aggregometer. The combination of these methods represents a rapid and cost-effective strategy to provide mechanistic insight on the type of platelet response mediated by the bacteria.

Platelet-borne complement proteins and their role in platelet-bacteria interactions

Essentials Platelets play an important role in pathogen recognition. Platelets contain several complement factors and can interact with E. coli. Platelet's complement protein C3 differs from plasmatic C3 in its electrophoretic mobility. Upon contact with bacteria, platelets are activated and can enhance complement activation.

SUMMARY

Background The role of platelets in immune defense is increasingly being recognized. Platelets bind complement proteins from plasma, initiate complement activation, and interact with bacteria. However, the contribution of platelets to complement-mediated defense against bacterial infections is not known in detail. Objectives To assess platelet interactions with Escherichia coli strains, and evaluate the contributions of platelet complement proteins to host defense. Methods We studied the cell-cell interactions of a pathogenic and a non-pathogenic E. coli strain with platelet concentrates, washed platelets and manually isolated platelets by flow cytometry and ELISA. The presence of complement proteins and complement RNA in megakaryocytes and platelets was analyzed by PCR, RT-PCR, confocal microscopy, and western blotting. Results Incubation with E. coli leads to platelet activation, as indicated by the expression of CD62P and CD63 on the platelet surface. RNA and protein analyses show that megakaryocytes and platelets contain complement C3, and that platelet C3 migrates differently on polyacrylamide gels than plasmatic C3. Activation of platelets by bacteria leads to translocation of C3 to the cell surface. This translocation is not induced by thrombin receptor activating peptide or lipopolysaccharide. Interaction of platelets with E. coli occurs even in the absence of plasma proteins, and is independent of platelet toll-like receptor 4 and α2b β3 (glycoprotein IIbIIIa). Conclusion Platelets contain a specific form of C3. Importantly, they can modulate immune defense against bacteria by enhancing plasmatic complement activation.

Antimicrobial properties of platelet-rich preparations. A systematic review of the current pre-clinical evidence

In recent years autologous platelet concentrates (APCs) have become popular in several medicine fields, representing a valuable adjunct to regenerative surgical procedures. Beneficial effects in the control of postsurgical discomfort and infection have also been frequently reported, suggesting that APC may possess anti-inflammatory and antimicrobial properties. The aim of the present review was to summarize the current evidence regarding the antimicrobial effects of platelet concentrates, investigated by in vitro and animal studies. This review was conducted following a systematic approach. An electronic search was performed on MEDLINE, EMBASE and Scopus databases using appropriate search terms, without language or time restrictions. Preclinical studies assessing the antimicrobial activity of APC were included and divided according to the experimental design. Twenty in vitro studies and four animal studies, investigating APC effects on a broad range of microorganisms, were included. In in vitro studies APC reduced the growth of microorganisms during the first hours of incubation, while they could not completely break down the microbial load. In fact, over time a recovery of bacterial growth was always observed, suggesting that APCs display a bacteriostatic rather than a microbicidal activity. All animal studies showed that APC administered by local injections were able to reduce the infection caused by different microorganisms, although to a lesser extent compared to antibiotics. In conclusion, although the exact action mechanisms of interaction with microbial pathogens need further investigation, platelet concentrates proved to have antimicrobial properties, and therefore could represent a useful natural substance for controlling postoperative infections at surgical sites.