Soluble CD40 ligand stimulates CD40-dependent activation of the β2 integrin Mac-1 and protein kinase C zeda (PKCζ) in neutrophils: implications for neutrophil-platelet interactions and neutrophil oxidative burst

Deciphering Abnormal Platelet Subpopulations in COVID-19, Sepsis and Systemic Lupus Erythematosus through Machine Learning and Single-Cell Transcriptomics

This study focuses on understanding the transcriptional heterogeneity of activated platelets and its impact on diseases such as sepsis, COVID-19, and systemic lupus erythematosus (SLE). Recognizing the limited knowledge in this area, our research aims to dissect the complex transcriptional profiles of activated platelets to aid in developing targeted therapies for abnormal and pathogenic platelet subtypes. We analyzed single-cell transcriptional profiles from 47,977 platelets derived from 413 samples of patients with these diseases, utilizing Deep Neural Network (DNN) and eXtreme Gradient Boosting (XGB) to distinguish transcriptomic signatures predictive of fatal or survival outcomes. Our approach included source data annotations and platelet markers, along with SingleR and Seurat for comprehensive profiling. Additionally, we employed Uniform Manifold Approximation and Projection (UMAP) for effective dimensionality reduction and visualization, aiding in the identification of various platelet subtypes and their relation to disease severity and patient outcomes. Our results highlighted distinct platelet subpopulations that correlate with disease severity, revealing that changes in platelet transcription patterns can intensify endotheliopathy, increasing the risk of coagulation in fatal cases. Moreover, these changes may impact lymphocyte function, indicating a more extensive role for platelets in inflammatory and immune responses. This study identifies crucial biomarkers of platelet heterogeneity in serious health conditions, paving the way for innovative therapeutic approaches targeting platelet activation, which could improve patient outcomes in diseases characterized by altered platelet function.

Increased Apolipoprotein A1 Expression Correlates with Tumor-Associated Neutrophils and T Lymphocytes in Upper Tract Urothelial Carcinoma

An increased neutrophil-to-lymphocyte ratio (NLR) is a poor prognostic biomarker in various types of cancer, because it reflects the inhibition of lymphocytes in the circulation and tumors. In urologic cancers, upper tract urothelial carcinoma (UTUC) is known for its aggressive features and lack of T cell infiltration; however, the association between neutrophils and suppressed T lymphocytes in UTUC is largely unknown. In this study, we examined the relationship between UTUC-derived factors and tumor-associated neutrophils or T lymphocytes. The culture supernatant from UTUC tumor tissue modulated neutrophils to inhibit T cell proliferation. Among the dominant factors secreted by UTUC tumor tissue, apolipoprotein A1 (Apo-A1) exhibited a positive correlation with NLR. Moreover, tumor-infiltrating neutrophils were inversely correlated with tumor-infiltrating T cells. Elevated Apo-A1 levels in UTUC were also inversely associated with the population of tumor-infiltrating T cells. Our findings indicate that elevated Apo-A1 expression in UTUC correlates with tumor-associated neutrophils and T cells. This suggests a potential immunomodulatory effect on neutrophils and T cells within the tumor microenvironment, which may represent therapeutic targets for UTUC treatment.

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

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

The roles of immune cells in Behçet's disease

Behçet's disease (BD) is a systemic vasculitis that can affect multiple systems, including the skin, mucous membranes, joints, eyes, gastrointestinal and nervous. However, the pathogenesis of BD remains unclear, and it is believed that immune-inflammatory reactions play a crucial role in its development. Immune cells are a critical component of this process and contribute to the onset and progression of BD. By regulating the function of these immune cells, effective control over the occurrence and development of BD can be achieved, particularly with regards to monocyte activation and aggregation, macrophage differentiation and polarization, as well as T cell subset differentiation. This review provides a brief overview of immune cells and their role in regulating BD progression, which may serve as a theoretical foundation for preventing and treating this disease.

Extracellular vesicles: effectors of transfusion-related acute lung injury

Respiratory transfusion reactions represent some of the most severe adverse reactions related to receiving blood products. Of those, transfusion-related acute lung injury (TRALI) is associated with elevated morbidity and mortality. TRALI is characterized by severe lung injury associated with inflammation, pulmonary neutrophil infiltration, lung barrier leak, and increased interstitial and airspace edema that cause respiratory failure. Presently, there are few means of detecting TRALI beyond clinical definitions based on physical examination and vital signs or preventing/treating TRALI beyond supportive care with oxygen and positive pressure ventilation. Mechanistically, TRALI is thought to be mediated by the culmination of two successive proinflammatory hits, which typically comprise a recipient factor (1st hit-e.g., systemic inflammatory conditions) and a donor factor (2nd hit-e.g., blood products containing pathogenic antibodies or bioactive lipids). An emerging concept in TRALI research is the contribution of extracellular vesicles (EVs) in mediating the first and/or second hit in TRALI. EVs are small, subcellular, membrane-bound vesicles that circulate in donor and recipient blood. Injurious EVs may be released by immune or vascular cells during inflammation, by infectious bacteria, or in blood products during storage, and can target the lung upon systemic dissemination. This review assesses emerging concepts such as how EVs: 1) mediate TRALI, 2) represent targets for therapeutic intervention to prevent or treat TRALI, and 3) serve as biochemical biomarkers facilitating TRALI diagnosis and detection in at-risk patients.

NET-(works) in arterial and venous thrombo-occlusive diseases

Formation of Neutrophil Extracellular Traps (NETosis), accompanied by the release of extracellular decondensed chromatin and pro-inflammatory as well as pro-thrombotic factors, is a pivotal element in the development and progression of thrombo-occlusive diseases. While the process of NETosis is based on complex intracellular signalling mechanisms, it impacts a wide variety of cells including platelets, leukocytes and endothelial cells. Consequently, although initially mainly associated with venous thromboembolism, NETs also affect and mediate atherothrombosis and its acute complications in the coronary, cerebral and peripheral arterial vasculature. In this context, besides deep vein thrombosis and pulmonary embolism, NETs in atherosclerosis and especially its acute complications such as myocardial infarction and ischemic stroke gained a lot of attention in the cardiovascular research field in the last decade. Thus, since the effect of NETosis on platelets and thrombosis in general is extensively discussed in other review articles, this review focusses on the translational and clinical relevance of NETosis research in cardiovascular thrombo-occlusive diseases. Consequently, after a brief summary of the neutrophil physiology and the cellular and molecular mechanisms underlying NETosis are presented, the role of NETosis in atherosclerotic and venous thrombo-occlusive diseases in chronic and acute settings are discussed. Finally, potential prevention and treatment strategies of NET-associated thrombo-occlusive diseases are considered.

Platelet-neutrophil interaction in COVID-19 and vaccine-induced thrombotic thrombocytopenia

Coronavirus disease 2019 (COVID-19) is known to commonly induce a thrombotic diathesis, particularly in severely affected individuals. So far, this COVID-19-associated coagulopathy (CAC) has been partially explained by hyperactivated platelets as well as by the prothrombotic effects of neutrophil extracellular traps (NETs) released from neutrophils. However, precise insight into the bidirectional relationship between platelets and neutrophils in the pathophysiology of CAC still lags behind. Vaccine-induced thrombotic thrombocytopenia (VITT) is a rare autoimmune disorder caused by auto-antibody formation in response to immunization with adenoviral vector vaccines. VITT is associated with life-threatening thromboembolic events and thus, high fatality rates. Our concept of the thrombophilia observed in VITT is relatively new, hence a better understanding could help in the management of such patients with the potential to also prevent VITT. In this review we aim to summarize the current knowledge on platelet-neutrophil interplay in COVID-19 and VITT.

Blood Platelets in Infection: The Multiple Roles of the Platelet Signalling Machinery

Platelets are classically recognized for their important role in hemostasis and thrombosis but they are also involved in many other physiological and pathophysiological processes, including infection. Platelets are among the first cells recruited to sites of inflammation and infection and they exert their antimicrobial response actively cooperating with the immune system. This review aims to summarize the current knowledge on platelet receptor interaction with different types of pathogens and the consequent modulations of innate and adaptive immune responses.

Immunothrombosis and the Role of Platelets in Venous Thromboembolic Diseases

Venous thromboembolism (VTE) is the third leading cardiovascular cause of death and is conventionally treated with anticoagulants that directly antagonize coagulation. However, recent data have demonstrated that also platelets play a crucial role in VTE pathophysiology. In the current review, we outline how platelets are involved during all stages of experimental venous thrombosis. Platelets mediate initiation of the disease by attaching to the vessel wall upon which they mediate leukocyte recruitment. This process is referred to as immunothrombosis, and within this novel concept inflammatory cells such as leukocytes and platelets directly drive the progression of VTE. In addition to their involvement in immunothrombosis, activated platelets can directly drive venous thrombosis by supporting coagulation and secreting procoagulant factors. Furthermore, fibrinolysis and vessel resolution are (partly) mediated by platelets. Finally, we summarize how conventional antiplatelet therapy can prevent experimental venous thrombosis and impacts (recurrent) VTE in humans.

Cold storage alters the immune characteristics of platelets and potentiates bacterial-induced aggregation

BACKGROUND AND OBJECTIVES

Cold-stored platelets are currently under clinical evaluation and have been approved for limited clinical use in the United States. Most studies have focused on the haemostatic functionality of cold-stored platelets; however, limited information is available examining changes to their immune function.

MATERIALS AND METHODS

Two buffy-coat-derived platelet components were combined and split into two treatment arms: room temperature (RT)-stored (20-24°C) or refrigerated (cold-stored, 2-6°C). The concentration of select soluble factors was measured in the supernatant using commercial ELISA kits. The abundance of surface receptors associated with immunological function was assessed by flow cytometry. Platelet aggregation was assessed in response to Escherichia coli and Staphylococcus aureus, in the presence and absence of RGDS (blocks active conformation of integrin α₂ β₃ ).

RESULTS

Cold-stored platelet components contained a lower supernatant concentration of C3a, RANTES and PF4. The abundance of surface-bound P-selectin and integrin α₂ β₃ in the activated conformation increased during cold storage. In comparison, the abundance of CD86, CD44, ICAM-2, CD40, TLR1, TLR2, TLR4, TLR3, TLR7 and TLR9 was lower on the surface membrane of cold-stored platelets compared to RT-stored components. Cold-stored platelets exhibited an increased responsiveness to E. coli- and S. aureus-induced aggregation compared to RT-stored platelets. Inhibition of the active conformation of integrin α₂ β₃ using RGDS reduced the potentiation of bacterial-induced aggregation in cold-stored platelets.

CONCLUSION

Our data highlight that cold storage changes the in vitro immune characteristics of platelets, including their sensitivity to bacterial-induced aggregation. Changes in these immune characteristics may have clinical implications post transfusion.

Liver ischaemia-reperfusion injury: a new understanding of the role of innate immunity

Liver ischaemia-reperfusion injury (LIRI), a local sterile inflammatory response driven by innate immunity, is one of the primary causes of early organ dysfunction and failure after liver transplantation. Cellular damage resulting from LIRI is an important risk factor not only for graft dysfunction but also for acute and even chronic rejection and exacerbates the shortage of donor organs for life-saving liver transplantation. Hepatocytes, liver sinusoidal endothelial cells and Kupffer cells, along with extrahepatic monocyte-derived macrophages, neutrophils and platelets, are all involved in LIRI. However, the mechanisms underlying the responses of these cells in the acute phase of LIRI and how these responses are orchestrated to control and resolve inflammation and achieve homeostatic tissue repair are not well understood. Technological advances allow the tracking of cells to better appreciate the role of hepatic macrophages and platelets (such as their origin and immunomodulatory and tissue-remodelling functions) and hepatic neutrophils (such as their selective recruitment, anti-inflammatory and tissue-repairing functions, and formation of extracellular traps and reverse migration) in LIRI. In this Review, we summarize the role of macrophages, platelets and neutrophils in LIRI, highlight unanswered questions, and discuss prospects for innovative therapeutic regimens against LIRI in transplant recipients.

Platelets, Bacterial Adhesins and the Pneumococcus

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

Staphylococcus aureus and Neutrophil Extracellular Traps: The Master Manipulator Meets Its Match in Immunothrombosis

Over the past 10 years, neutrophil extracellular traps (NETs) have become widely accepted as an integral player in immunothrombosis, due to their complex interplay with both pathogens and components of the coagulation system. While the release of NETs is an attempt by neutrophils to trap pathogens and constrain infections, NETs can have bystander effects on the host by inducing uncontrolled thrombosis, inflammation, and tissue damage. From an evolutionary perspective, pathogens have adapted to bypass the host innate immune response. Staphylococcus aureus (S. aureus), in particular, proficiently overcomes NET formation using several virulence factors. Here we review mechanisms of NET formation and how these are intertwined with platelet activation, the release of endothelial von Willebrand factor, and the activation of the coagulation system. We discuss the unique ability of S. aureus to modulate NET formation and alter released NETs, which helps S. aureus to escape from the host's defense mechanisms. We then discuss how platelets and the coagulation system could play a role in NET formation in S. aureus-induced infective endocarditis, and we explain how targeting these complex cellular interactions could reveal novel therapies to treat this disease and other immunothrombotic disorders.

Transcriptome Profiling Reveals CD73 and Age-Driven Changes in Neutrophil Responses against Streptococcus pneumoniae

Neutrophils are required for host resistance against Streptococcus pneumoniae, but their function declines with age. We previously found that CD73, an enzyme required for antimicrobial activity, is downregulated in neutrophils (also known as polymorphonuclear leukocytes [PMNs]) from aged mice. This study explored transcriptional changes in neutrophils induced by S. pneumoniae to identify pathways controlled by CD73 and dysregulated with age. Pure bone marrow-derived neutrophils isolated from wild-type (WT) young and old and CD73 knockout (CD73KO) young mice were mock challenged or infected with S. pneumoniae ex vivo. RNA sequencing (RNA-Seq) was performed to identify differentially expressed genes (DEGs). We found that infection triggered distinct global transcriptional changes across hosts that were strongest in CD73KO neutrophils. Surprisingly, there were more downregulated than upregulated genes in all groups upon infection. Downregulated DEGs indicated a dampening of immune responses in old and CD73KO hosts. Further analysis revealed that CD73KO neutrophils expressed higher numbers of long noncoding RNAs (lncRNAs) than those in WT controls. Predicted network analysis indicated that CD73KO-specific lncRNAs control several signaling pathways. We found that genes in the c-Jun N-terminal kinase (JNK)-mitogen-activated protein kinase (MAPK) pathway were upregulated upon infection in CD73KO mice and in WT old mice, but not in WT young mice. This corresponded to functional differences, as phosphorylation of the downstream AP-1 transcription factor component c-Jun was significantly higher in neutrophils from infected CD73KO mice and old mice. Importantly, inhibition of JNK/AP-1 rescued the ability of these neutrophils to kill S. pneumoniae. Together, our findings revealed that the ability of neutrophils to modify their gene expression to better adapt to bacterial infection is in part regulated by CD73 and declines with age.

CD40L modulates transcriptional signatures of neutrophils in the bone marrow associated with development and trafficking

Neutrophils are produced in the BM in a process called granulopoiesis, in which progenitor cells sequentially develop into mature neutrophils. During the developmental process, which is finely regulated by distinct transcription factors, neutrophils acquire the ability to exit the BM, properly distribute throughout the body, and migrate to infection sites. Previous studies have demonstrated that CD40 ligand (CD40L) influences hematopoiesis and granulopoiesis. Here, we investigate the effect of CD40L on neutrophil development and trafficking by performing functional and transcriptome analyses. We found that CD40L signaling plays an essential role in the early stages of neutrophil generation and development in the BM. Moreover, CD40L modulates transcriptional signatures, indicating that this molecule enables neutrophils to traffic throughout the body and to migrate in response to inflammatory signals. Thus, our study provides insights into the complex relationships between CD40L signaling and granulopoiesis, and it suggests a potentially novel and nonredundant role of CD40L signaling in neutrophil development and function.

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.

Molecular basis and therapeutic implications of CD40/CD40L immune checkpoint

The CD40 receptor and its ligand CD40L is one of the most critical molecular pairs of the stimulatory immune checkpoints. Both CD40 and CD40L have a membrane form and a soluble form generated by proteolytic cleavage or alternative splicing. CD40 and CD40L are widely expressed in various types of cells, among which B cells and myeloid cells constitutively express high levels of CD40, and T cells and platelets express high levels of CD40L upon activation. CD40L self-assembles into functional trimers which induce CD40 trimerization and downstream signaling. The canonical CD40/CD40L signaling is mediated by recruitment of TRAFs and NF-κB activation, which is supplemented by signal pathways such as PI3K/AKT, MAPKs and JAK3/STATs. CD40/CD40L immune checkpoint leads to activation of both innate and adaptive immune cells via two-way signaling. CD40/CD40L interaction also participates in regulating thrombosis, tissue inflammation, hematopoiesis and tumor cell fate. Because of its essential role in immune activation, CD40/CD40L interaction has been regarded as an attractive immunotherapy target. In recent years, significant advance has been made in CD40/CD40L-targeted therapy. Various types of agents, including agonistic/antagonistic monoclonal antibodies, cellular vaccines, adenoviral vectors and protein antagonist, have been developed and evaluated in early-stage clinical trials for treating malignancies, autoimmune diseases and allograft rejection. In general, these agents have demonstrated favorable safety and some of them show promising clinical efficacy. The mechanisms of benefits include immune cell activation and tumor cell lysis/apoptosis in malignancies, or immune cell inactivation in autoimmune diseases and allograft rejection. This review provides a comprehensive overview of the structure, processing, cellular expression pattern, signaling and effector function of CD40/CD40L checkpoint molecules. In addition, we summarize the progress, targeted diseases and outcomes of current ongoing and completed clinical trials of CD40/CD40L-targeted therapy.

Immunothrombosis and thromboinflammation in host defense and disease

Platelets are increasingly being recognized for playing roles beyond thrombosis and hemostasis. Today we know that they mediate inflammation by direct interactions with innate immune cells or secretion of cytokines/chemokines. Here we review their interactions with neutrophils and monocytes/macrophages in infection and sepsis, stroke, myocardial infarction and venous thromboembolism. We discuss new roles for platelet surface receptors like GPVI or GPIb and also look at platelet contributions to the formation of neutrophil extracellular traps (NETs) as well as to deep vein thrombosis during infection, e.g. in COVID-19 patients.

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.

Evidence of CD40L/CD40 pathway involvement in experimental transfusion-related acute lung injury

Platelet transfusions can cause adverse reactions in their recipients, including transfusion-related acute lung injury (TRALI). The pathophysiology of TRALI depends on a number of signaling pathways and the inflammatory role played by blood platelets remains controversial. Platelets are important in inflammation, particularly via the immunomodulator complex CD40/CD40L. We studied the specific function of the CD40/CD40L interaction in regulating an experimental TRALI Two-hit model. A mouse model of immune TRALI was triggered by injection of LPS and an anti-MHC I antibody, and the effect of injection of a neutralizing anti-CD40L antibody before induction of TRALI investigated. The characteristics of TRALI were decreased body temperature, pulmonary lesions, and immune cell infiltration into the alveolar space. Pulmonary infiltration was evaluated by blood counts of specific immune cells and their detection in lung sections. Inhibition of the CD40/CD40L immunomodulator interaction significantly reduced communication between immune and/or endothelial cells and the development of pulmonary edema. Hence, our results indicate that targeting of the CD40/CD40L interaction could be an important method to prevent TRALI. While considering that our work concerned a mouse model, we postulate that improvement of the conditions under which platelet concentrates are prepared/stored would assist in alleviating the risk of TRALI.

Targeting the CD40-CD40L pathway in autoimmune diseases: Humoral immunity and beyond

CD40 is a TNF receptor superfamily member expressed on both immune and non-immune cells. Interactions between B cell-expressed CD40 and its binding partner, CD40L, predominantly expressed on activated CD4+ T cells, play a critical role in promoting germinal center formation and the production of class-switched antibodies. Non-hematopoietic cells expressing CD40 can also engage CD40L and trigger a pro-inflammatory response. This article will highlight what is known about the biology of the CD40-CD40L axis in humans and describe the potential contribution of CD40 signaling on both hematopoietic and non-hematopoietic cells to autoimmune disease pathogenesis. Additionally, novel therapeutic approaches to target this pathway, currently being evaluated in clinical trials, are discussed.

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

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

Neutrophils: New insights and open questions

Neutrophils are the first line of defense against bacteria and fungi and help combat parasites and viruses. They are necessary for mammalian life, and their failure to recover after myeloablation is fatal. Neutrophils are short-lived, effective killing machines. Their life span is significantly extended under infectious and inflammatory conditions. Neutrophils take their cues directly from the infectious organism, from tissue macrophages and other elements of the immune system. Here, we review how neutrophils traffic to sites of infection or tissue injury, how they trap and kill bacteria, how they shape innate and adaptive immune responses, and the pathophysiology of monogenic neutrophil disorders.

The role of inflammation in the development of epilepsy

Epilepsy, a neurological disease characterized by recurrent seizures, is often associated with a history of previous lesions in the nervous system. Impaired regulation of the activation and resolution of inflammatory cells and molecules in the injured neuronal tissue is a critical factor to the development of epilepsy. However, it is still unclear as to how that unbalanced regulation of inflammation contributes to epilepsy. Therefore, one of the goals in epilepsy research is to identify and elucidate the interconnected inflammatory pathways in systemic and neurological disorders that may further develop epilepsy progression. In this paper, inflammatory molecules, in neurological and systemic disorders (rheumatoid arthritis, Crohn’s, Type I Diabetes, etc.) that could contribute to epilepsy development, are reviewed.

Understanding the neurobiology of inflammation in epileptogenesis will contribute to the development of new biomarkers for better screening of patients at risk for epilepsy and new therapeutic targets for both prophylaxis and treatment of epilepsy.

From blood coagulation to innate and adaptive immunity: the role of platelets in the physiology and pathology of autoimmune disorders

Thrombosis and cardiovascular complications are common manifestations of a variety of pathological conditions, including infections and chronic inflammatory diseases. Hence, there is great interest in determining the hitherto unforeseen immune role of the main blood coagulation executor-the platelet. Platelets store and release a plethora of immunoactive molecules, generate microparticles, and interact with cells classically belonging to the immune system. The observed effects of platelet involvement in immune processes, especially in autoimmune diseases, are conflicting-from inciting inflammation to mediating its resolution. An in-depth understanding of the role of platelets in inflammation and immunity could open new therapeutic pathways for patients with autoimmune disorders. This review aims to summarize the current knowledge on the role of platelets in the patomechanisms of autoimmune disorders and suggests directions for future research.

Macrophage extracellular trap formation promoted by platelet activation is a key mediator of rhabdomyolysis-induced acute kidney injury

Rhabdomyolysis is a serious syndrome caused by skeletal muscle injury and the subsequent release of breakdown products from damaged muscle cells into systemic circulation. The muscle damage most often results from strenuous exercise, muscle hypoxia, medications, or drug abuse and can lead to life-threatening complications, such as acute kidney injury (AKI). Rhabdomyolysis and the AKI complication can also occur during crush syndrome, an emergency condition that commonly occurs in victims of natural disasters, such as earthquakes, and man-made disasters, such as wars and terrorism. Myoglobin released from damaged muscle is believed to trigger renal dysfunction in this form of AKI. Recently, macrophages were implicated in the disease pathogenesis of rhabdomyolysis-induced AKI, but the precise molecular mechanism remains unclear. In the present study, we show that macrophages released extracellular traps (ETs) comprising DNA fibers and granule proteins in a mouse model of rhabdomyolysis. Heme-activated platelets released from necrotic muscle cells during rhabdomyolysis enhanced the production of macrophage extracellular traps (METs) through increasing intracellular reactive oxygen species generation and histone citrullination. Here we report, for the first time to our knowledge, this unanticipated role for METs and platelets as a sensor of myoglobin-derived heme in rhabdomyolysis-induced AKI. This previously unknown mechanism might be targeted for treatment of the disease. Finally, we found a new therapeutic tool for prevention of AKI after rhabdomyolysis, which might rescue some sufferers of this pathology.

Revealing pathway cross-talk related to diabetes mellitus by Monte Carlo Cross-Validation analysis

Objective

To explore potential functional biomarkers in diabetes mellitus (DM) by utilizing gene pathway cross-talk.

Methods

Firstly, potential disrupted pathways that were enriched by differentially expressed genes (DEGs) were identified based on biological pathways downloaded from the Ingenuity Pathways Analysis (IPA) database. In addition, we quantified the pathway crosstalk for each pair of pathways based on Discriminating Score (DS). Random forest (RF) classification was then employed to find the top 10 pairs of pathways with a high area under the curve (AUC) value between DM samples versus normal samples based on 10-fold cross-validation. Finally, a Monte Carlo Cross-Validation was applied to demonstrate the identified pairs of pathways by a mutual information analysis.

Results

A total of 247 DEGs in normal and disease samples were identified. Based on the F-test, 50 disrupted pathways were obtained with false discovery rate (FDR) < 0.01. Simultaneously, after calculating the DS, the top 10 pairs of pathways were selected based on a higher AUC value as measured by RF classification. From the Monte Carlo Cross-Validation, we considered the top 10 pairs of pathways with higher AUC values ranked for all 50 bootstraps as the most frequently detected ones.

Conclusion

The pairs of pathways identified in our study might be key regulators in DM.

Soluble CD40 Ligand and Oxidative Response Are Reciprocally Stimulated during Shiga Toxin-Associated Hemolytic Uremic Syndrome

Shiga toxin (Stx), produced by Escherichia coli, is the main pathogenic factor of diarrhea-associated hemolytic uremic syndrome (HUS), which is characterized by the obstruction of renal microvasculature by platelet-fibrin thrombi. It is well known that the oxidative imbalance generated by Stx induces platelet activation, contributing to thrombus formation. Moreover, activated platelets release soluble CD40 ligand (sCD40L), which in turn contributes to oxidative imbalance, triggering the release of reactive oxidative species (ROS) on various cellular types. The aim of this work was to determine if the interaction between the oxidative response and platelet-derived sCD40L, as consequence of Stx-induced endothelium damage, participates in the pathogenic mechanism during HUS. Activated human glomerular endothelial cells (HGEC) by Stx2 induced platelets to adhere to them. Although platelet adhesion did not contribute to endothelial damage, high levels of sCD40L were released to the medium. The release of sCD40L by activated platelets was inhibited by antioxidant treatment. Furthermore, we found increased levels of sCD40L in plasma from HUS patients, which were also able to trigger the respiratory burst in monocytes in a sCD40L-dependent manner. Thus, we concluded that platelet-derived sCD40L and the oxidative response are reciprocally stimulated during Stx2-associated HUS. This process may contribute to the evolution of glomerular occlusion and the microangiopathic lesions.

Soluble CD40L is associated with increased oxidative burst and neutrophil extracellular trap release in Behçet's disease

Background

Studies have suggested that soluble factors in plasma from patients with active (aBD) and inactive (iBD) Behçet’s disease (BD) stimulate neutrophil function. Soluble CD40 ligand (sCD40L) is an important mediator of inflammation in BD. Its expression and effect on neutrophil oxidative burst and neutrophil extracellular trap (NET) release have not been characterized. In this study, we sought to investigate the role of plasma and the CD40L pathway on NET release and the oxidative burst profile in patients with aBD and iBD.

Methods

Neutrophils and peripheral blood mononuclear cells (PBMCs) were obtained from patients with aBD (n = 30), patients with iBD (n = 31), and healthy control subjects (HCs; n = 30). sCD40L plasma concentration was determined in individual samples. A pool of plasma for each group was created. In some experiments, plasma pools were treated with recombinant CD40 (rhCD40-muIg) for sCD40L blockade. NET release and H₂O₂/O₂⁻ production were determined after stimulation with phorbol 12-myristate 13-acetate, sCD40L, or plasma pool. Flow cytometric analysis was performed to evaluate the expression of (1) CD40, Mac-1, and phosphorylated NF-κB p65 on neutrophils and monocytes and (2) CD40L on activated T cells and platelets. CD40L gene expression in PBMCs was determined by qRT-PCR.

Results

sCD40L plasma levels were significantly higher in patients with iBD (median 17,234, range 2346–19,279 pg/ml) and patients with aBD (median 18,289, range 413–19,883 pg/ml) than in HCs (median 47.5, range 33.7–26.7 pg/ml; p < 0.001). NET release was constitutively increased in BD compared with HC. NET release and H₂O₂/O₂⁻ were higher after stimulation with sCD40L or BD plasma and decreased after sCD40L blockade. Mac-1 expression was constitutively increased in neutrophils of patients with aBD (88.7 ± 13.2% of cells) and patients with iBD (89.2 ± 20.1% of cells) compared with HC (27.1 ± 18.8% of cells; p < 0.01). CD40 expression on phagocytes and CD40L expression on platelets were similar in the three groups. PBMCs as well as nonactivated and activated CD4⁺ T cells from patients with BD showed higher CD40L expression.

Conclusions

Plasma from patients with aBD exerts a stimulus on NET release and oxidative burst, probably induced by sCD40L.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1443-5) contains supplementary material, which is available to authorized users.

CD40L and Its Receptors in Atherothrombosis-An Update

CD40L (CD154), a member of the tumor necrosis factor superfamily, is a co-stimulatory molecule that was first discovered on activated T cells. Beyond its fundamental role in adaptive immunity-ligation of CD40L to its receptor CD40 is a prerequisite for B cell activation and antibody production-evidence from more than two decades has expanded our understanding of CD40L as a powerful modulator of inflammatory pathways. Although inhibition of CD40L with neutralizing antibodies has induced life-threatening side effects in clinical trials, the discovery of cell-specific effects and novel receptors with distinct functional consequences has opened a new path for therapies that specifically target detrimental properties of CD40L. Here, we carefully evaluate the signaling network of CD40L by gene enrichment analysis and its cell-specific expression, and thoroughly discuss its role in cardiovascular pathologies with a specific emphasis on atherosclerotic and thrombotic disease.

Thymosin Beta-4 Is Elevated in Women With Heart Failure With Preserved Ejection Fraction

BACKGROUND

Thymosin beta-4 (TB4) is an X-linked gene product with cardioprotective properties. Little is known about plasma concentration of TB4 in heart failure (HF), and its relationship with other cardiovascular biomarkers. We sought to evaluate circulating TB4 in HF patients with preserved (HFpEF) or reduced (HFrEF) ejection fraction compared to non-HF controls.

METHODS AND RESULTS

TB4 was measured using a liquid chromatography and mass spectrometry assay in age- and sex-matched HFpEF (n=219), HFrEF (n=219) patients, and controls (n=219) from a prospective nationwide study. Additionally, a 92-marker multiplex proximity extension assay was measured to identify biomarker covariates. Compared with controls, plasma TB4 was elevated in HFpEF (985 [421-1723] ng/mL versus 1401 [720-2379] ng/mL, P<0.001), but not in HFrEF (1106 [556-1955] ng/mL, P=0.642). Stratifying by sex, only women (1623 [1040-2625] ng/mL versus 942 [386-1891] ng/mL, P<0.001), but not men (1238.5 [586-1967] ng/mL versus 1004 [451-1538] ng/mL, P=1.0), had significantly elevated TB4 in the setting of HFpEF. Adjusted for New York Heart Association class, N-terminal pro B-type natriuretic peptide, age, and myocardial infarction, hazard ratio to all-cause mortality is significantly higher in women with elevated TB4 (1.668, P=0.036), but not in men (0.791, P=0.456) with HF. TB4 is strongly correlated with a cluster of 7 markers from the proximity extension assay panel, which are either X-linked, regulated by sex hormones, or involved with NF-κB signaling.

CONCLUSIONS

We show that plasma TB4 is elevated in women with HFpEF and has prognostic information. Because TB4 can preserve EF in animal studies of cardiac injury, the relation of endogenous, circulating TB4 to X chromosome biology and differential outcomes in female heart disease warrants further study.

Anti-Inflammation of Natural Components from Medicinal Plants at Low Concentrations in Brain via Inhibiting Neutrophil Infiltration after Stroke

Inflammation after stroke consists of activation of microglia/astrocytes in situ and infiltration of blood-borne leukocytes, resulting in brain damage and neurological deficits. Mounting data demonstrated that most natural components from medicinal plants had anti-inflammatory effects after ischemic stroke through inhibiting activation of resident microglia/astrocytes within ischemic area. However, it is speculated that this classical activity cannot account for the anti-inflammatory function of these natural components in the cerebral parenchyma, where they are detected at very low concentrations due to their poor membrane permeability and slight leakage of BBB. Could these drugs exert anti-inflammatory effects peripherally without being delivered across the BBB? Factually, ameliorating blood-borne neutrophil recruitment in peripheral circulatory system has been proved to reduce ischemic damage and improve outcomes. Thus, it is concluded that if drugs could achieve effective concentrations in the cerebral parenchyma, they can function via crippling resident microglia/astrocytes activation and inhibiting neutrophil infiltration, whereas the latter will be dominating when these drugs localize in the brain at a low concentration. In this review, the availability of some natural components crossing the BBB in stroke will be discussed, and how these drugs lead to improvements in stroke through inhibition of neutrophil rolling, adhesion, and transmigration will be illustrated.

Annexin A1 Is a Physiological Modulator of Neutrophil Maturation and Recirculation Acting on the CXCR4/CXCL12 Pathway

Neutrophil production and traffic in the body compartments is finely controlled, and the strong evidences support the role of CXCL12/CXCR4 pathway on neutrophil trafficking to and from the bone marrow (BM). We recently showed that the glucocorticoid-regulated protein, Annexin A1 (AnxA1) modulates neutrophil homeostasis and here we address the effects of AnxA1 on steady-state neutrophil maturation and trafficking. For this purpose, AnxA1(-/-) and Balb/C wild-type mice (WT) were donors of BM granulocytes and mesenchymal stem cells and blood neutrophils. In vivo treatments with the pharmacological AnxA1 mimetic peptide (Ac2-26) or the formyl peptide receptor (FPR) antagonist (Boc-2) were used to elucidate the pathway of AnxA1 action, and with the cytosolic glucocorticoid antagonist receptor RU 38486. Accelerated maturation of BM granulocytes was detected in AnxA1(-/-) and Boc2-treated WT mice, and was reversed by treatment with Ac2-26 in AnxA1(-/-) mice. AnxA1 and FPR2 were constitutively expressed in bone marrow granulocytes, and their expressions were reduced by treatment with RU38486. Higher numbers of CXCR4(+) neutrophils were detected in the circulation of AnxA1(-/-) or Boc2-treated WT mice, and values were rescued in Ac2-26-treated AnxA1(-/-) mice. Although circulating neutrophils of AnxA1(-/-) animals were CXCR4(+) , they presented reduced CXCL12-induced chemotaxis. Moreover, levels of CXCL12 were reduced in the bone marrow perfusate and in the mesenchymal stem cell supernatant from AnxA1(-/-) mice, and in vivo and in vitro CXCL12 expression was re-established after Ac2-26 treatment. Collectively, these data highlight AnxA1 as a novel determinant of neutrophil maturation and the mechanisms behind blood neutrophil homing to BM via the CXCL12/CXCR4 pathway. J. Cell. Physiol. 231: 2418-2427, 2016. © 2016 Wiley Periodicals, Inc.

Neutrophil's weapons in atherosclerosis

Neutrophils are important components of immunity associated with inflammatory responses against a broad spectrum of pathogens. These cells could be rapidly activated by proinflammatory stimuli and migrate to the inflamed and infected sites where they release a variety of cytotoxic molecules with antimicrobial activity. Neutrophil antibacterial factors include extracellular proteases, redox enzymes, antimicrobial peptides, and small bioactive molecules. In resting neutrophils, these factors are stored in granules and released upon activation during degranulation. These factors could be also secreted in a neutrophil-derived microparticle-dependent fashion. Neutrophils exhibit a unique property to produce neutrophil extracellular traps (NETs) composed of decondensed chromatin and granular proteins to catch and kill bacteria. Neutrophil-released factors are efficient in inactivation and elimination of pathogens through oxidation-dependent or independent damage of bacterial cells, inactivation and neutralization of virulence factors and other mechanisms. However, in chronic atherosclerosis-associated inflammation, protective function of neutrophils could be impaired and misdirected against own cells. This could lead to deleterious effects and progressive vascular injury. In atherogenesis, a pathogenic role of neutrophils could be especially seen in early stages associated with endothelial dysfunction and induction of vascular inflammation and in late atherosclerosis associated with plaque rupture and atherothrombosis. Assuming a prominent impact of neutrophils in cardiovascular pathology, developing therapeutic strategies targeting neutrophil-specific antigens could have a promising clinical potential.

Withaferin A suppresses the up-regulation of acetyl-coA carboxylase 1 and skin tumor formation in a skin carcinogenesis mouse model

Withaferin A (WA), a natural product derived from Withania somnifera, has been used in traditional oriental medicines to treat neurological disorders. Recent studies have demonstrated that this compound may have a potential for cancer treatment and a clinical trial has been launched to test WA in treating melanoma. Herein, WA's chemopreventive potential was tested in a chemically-induced skin carcinogenesis mouse model. Pathological examinations revealed that WA significantly suppressed skin tumor formation. Morphological observations of the skin tissues suggest that WA suppressed cell proliferation rather than inducing apoptosis during skin carcinogenesis. Antibody Micro array analysis demonstrated that WA blocked carcinogen-induced up-regulation of acetyl-CoA carboxylase 1 (ACC1), which was further confirmed in a skin cell transformation model. Overexpression of ACC1 promoted whereas knockdown of ACC1 suppressed anchorage-independent growth and oncogene activation of transformable skin cells. Further studies demonstrated that WA inhibited tumor promotor-induced ACC1 gene transcription by suppressing the activation of activator protein 1. In melanoma cells, WA was also able to suppress the expression levels of ACC1. Finally, results using human skin cancer tissues confirmed the up-regulation of ACC1 in tumors than adjacent normal tissues. In summary, our results suggest that withaferin A may have a potential in chemoprevention and ACC1 may serve as a critical target of WA. © 2015 Wiley Periodicals, Inc.

Targeting neutrophils in ischemic stroke: translational insights from experimental studies

Neutrophils have key roles in ischemic brain injury, thrombosis, and atherosclerosis. As such, neutrophils are of great interest as targets to treat and prevent ischemic stroke. After stroke, neutrophils respond rapidly promoting blood-brain barrier disruption, cerebral edema, and brain injury. A surge of neutrophil-derived reactive oxygen species, proteases, and cytokines are released as neutrophils interact with cerebral endothelium. Neutrophils also are linked to the major processes that cause ischemic stroke, thrombosis, and atherosclerosis. Thrombosis is promoted through interactions with platelets, clotting factors, and release of prothrombotic molecules. In atherosclerosis, neutrophils promote plaque formation and rupture by generating oxidized-low density lipoprotein, enhancing monocyte infiltration, and degrading the fibrous cap. In experimental studies targeting neutrophils can improve stroke. However, early human studies have been met with challenges, and suggest that selective targeting of neutrophils may be required. Several properties of neutrophil are beneficial and thus may important to preserve in patients with stroke including antimicrobial, antiinflammatory, and neuroprotective functions.

New frontiers for platelet CD154

The role of platelets extends beyond hemostasis. The pivotal role of platelets in inflammation has shed new light on the natural history of conditions associated with acute or chronic inflammation. Beyond the preservation of vascular integrity, platelets are essential to tissue homeostasis and platelet-derived products are already used in the clinics. Unanticipated was the role of platelets in the adaptative immune response, allowing a renewed conceptual approach of auto-immune diseases. Platelets are also important players in cancer growth and dissemination. Platelets fulfill most of their functions through the expression of still incompletely characterized membrane-bound or soluble mediators. Among them, CD154 holds a peculiar position, as platelets represent a major source of CD154 and as CD154 contributes to most of these new platelet attributes. Here, we provide an overview of some of the new frontiers that the study of platelet CD154 is opening, in inflammation, tissue homeostasis, immune response, hematopoiesis and cancer.

Platelet-neutrophil interactions under thromboinflammatory conditions

Platelets primarily mediate hemostasis and thrombosis, whereas leukocytes are responsible for immune responses. Since platelets interact with leukocytes at the site of vascular injury, thrombosis and vascular inflammation are closely intertwined and occur consecutively. Recent studies using real-time imaging technology demonstrated that platelet-neutrophil interactions on the activated endothelium are an important determinant of microvascular occlusion during thromboinflammatory disease in which inflammation is coupled to thrombosis. Although the major receptors and counter receptors have been identified, it remains poorly understood how heterotypic platelet-neutrophil interactions are regulated under disease conditions. This review discusses our current understanding of the regulatory mechanisms of platelet-neutrophil interactions in thromboinflammatory disease.

Platelets: essential components of the immune system

Platelets are anucleate cell fragments known for their central role in coagulation and vascular integrity. However, it is becoming increasingly clear that platelets contribute to diverse immunological processes extending beyond the traditional view of platelets as fragmentary mediators of hemostasis and thrombosis. There is recent evidence that platelets participate in: 1) intervention against microbial threats; 2) recruitment and promotion of innate effector cell functions; 3) modulating antigen presentation; and 4) enhancement of adaptive immune responses. In this way, platelets should be viewed as the underappreciated orchestrator of the immune system. This review will discuss recent and historical evidence regarding how platelets influence both innate and adaptive immune responses.

Diminished nitric oxide generation from neutrophils suppresses platelet activation in chronic renal failure

Chronic renal failure (CRF) is a complex clinical condition associated with accelerated atherosclerosis and thrombosis leading to cardiovascular events. The aim of this study was to investigate in detail the NO pathway in neutrophils obtained from hemodialysis patients and its association with platelet function and oxidative status. Fifteen CRF patients on hemodialysis and fifteen controls were included in this study. Laboratory and experimental evaluations were performed after hemodialysis in CRF patients. We evaluated L-[³H] arginine transport, NO synthase (NOS) activity, amino acid concentration in neutrophils, and expressions of NOS isoforms and p47(phox) by western blotting. Platelet aggregation was analyzed in the presence or absence of neutrophils. Oxidative status was measured through glutathione peroxidase, catalase activities, protein oxidation, lipid peroxidation, and DNA/RNA oxidation in serum. Basal NOS activity (pmol/10⁶ cells/min) was impaired in CRF patients on hemodialysis (0.33 ± 0.17) compared to controls (0.65 ± 0.12), whereas the expression of NOS isoforms remained unaltered. L-Arginine transport into neutrophils was similar in CRF patients on hemodialysis and controls. In addition, intracellular concentration of L-arginine was increased fourfold in the patient group. Systemic oxidative stress markers were not affected by CRF. On the other hand, NADPH oxidase subunit p47(phox) in neutrophils was overexpressed in CRF. In the presence of neutrophils, there was a reduction time-dependent in platelet aggregation in both groups with no difference between them. This data suggest that reduced basal generation of NO by neutrophils in CRF patients on hemodialysis occurs independently of L-arginine bioavailability and is able to suppress platelet activation.

Platelets in inflammation and immunity

The paradigm of platelets as mere mediators of hemostasis has long since been replaced by a dual role: hemostasis and inflammation. Now recognized as key players in innate and adaptive immune responses, platelets have the capacity to interact with almost all known immune cells. These platelet-immune cell interactions represent a hallmark of immunity, as they can potently enhance immune cell functions and, in some cases, even constitute a prerequisite for host defense mechanisms such as NETosis. In addition, recent studies have revealed a new role for platelets in immunity: They are ubiquitous sentinels and rapid first-line immune responders, as platelet-pathogen interactions within the vasculature appear to precede all other host defense mechanisms. Here, we discuss recent advances in our understanding of platelets as inflammatory cells, and provide an exemplary review of their role in acute inflammation.