Platelets support pulmonary recruitment of neutrophils in abdominal sepsis*:

Rac1 regulates sepsis-induced formation of platelet-derived microparticles and thrombin generation

Dysfunctional coagulation aggravates clinical outcome in patients with sepsis. The aim of this study was to define the role of Rac-1 in the formation of platelet-derived microparticles (PMPs) and thrombin generation (TG) in abdominal sepsis. Male C57BL/6 mice underwent cecal ligation and puncture (CLP). Scanning electron microscopy and flow cytometry were used to quantify PMPs. TG was determined by use of a fluorimetric assay. It was found that CLP increased Rac1 activity in platelets, which was abolished by administration of the Rac1 inhibitor NSC23766. Sepsis-induced TG in vivo was reflected by reduced capacity of plasma from septic animals to generate thrombin ex vivo. Administration of NSC23766 increased peak and total TG in plasma from CLP mice indicating that Rac-1 regulates sepsis-induced formation of thrombin. The number of circulating PMPs was markedly elevated in animals with abdominal sepsis. Treatment with NSC23766 significantly decreased formation of PMPs in septic mice. Platelet activation in vitro caused release of numerous MPs. Notably, NSC23766 abolished PMP formation in activated platelets in vitro. These findings suggest that Rac-1 regulates PMP formation and TG in sepsis and that inhibition of Rac1 activity could be a useful target to inhibit dysfunctional coagulation in abdominal sepsis.

Thrombopoietin as Early Biomarker of Disease Severity in Patients With Acute Pancreatitis

OBJECTIVES

To study the concentrations of thrombopoietin (TPO), a growth factor recently involved in the pathogenesis of experimental acute pancreatitis (AP), and its potential role as an early diagnostic and prognostic biomarker in patients with AP.

METHODS

Thrombopoietin was measured in 44 AP patients, 18 patients with nonpancreatic acute abdominal pain, and 18 healthy volunteers. Acute pancreatitis severity was classified on the basis of the 2012 International Atlanta Symposium on Acute Pancreatitis criteria.

RESULTS

Thrombopoietin levels did not differ between AP patients and control subjects, whereas these were higher in patients with moderately severe or severe AP compared with those with mild AP. Receiver operating characteristic curve analysis of TPO for severe AP diagnosis showed an area under the curve of 0.80. A cutoff value of 31.48 pg/mL showed the highest sensitivity, allowing to rule out severe AP when TPO was lower, whereas TPO higher than 98.23 pg/mL was associated with severe AP with high specificity (93.5%). Furthermore, TPO levels were greater in AP patients developing organ dysfunction or sepsis and in nonsurvivors compared with survivors.

CONCLUSIONS

Our data provide the first evidence for TPO as potential early prognostic biomarker in AP patients. High TPO levels at hospital admission may predict organ dysfunction, sepsis, and fatal outcome in AP patients.

Similar change in platelets and leucocytes 24 h after injury is associated with septic shock a week later

BACKGROUND

Septic shock is a severe complication in polytrauma patients. Early identification of patients at risk can guide future prevention strategies. Platelets (PLTs) and leucocytes presumably play an important role in the post-injury inflammatory response. The role of early changes in PLT and leucocyte counts was investigated in search for the aetiology of the development of septic complications.

METHODS

Polytrauma patients (aged 16-80 years) admitted to the intensive care unit with an expected stay of at least 3 days were included. PLT and leucocyte counts were measured on a daily basis for 14 days.

RESULTS

A total of 41 patients were included, of whom nine (22%) developed septic shock. There was no difference in (New) Injury Severity Score or Acute Physiology and Chronic Health Evaluation II scores between patients who developed septic shock and patients who did not. Three patients died, one of them in septic shock. Patients who developed septic shock during hospital stay had lower PLTs and a slower recovery to normal PLT counts than patients without septic shock. Patients who developed either a decrease in both PLTs and leucocytes or an increase in PLTs and leucocytes in the first 24 h after trauma were more likely to develop septic shock. This correlation was not found in patients who did not develop septic shock.

CONCLUSION

A similar change in PLT and leucocyte counts in the first 24 h after trauma is associated with the development of septic shock after a week. This indicates an early interaction between PLTs and leucocytes, which needs further investigation to gain more insight in the aetiology of post-injury septic complications.

Assessment of Platelet Function in Whole Blood by Flow Cytometry

Evaluation of platelet function is important for understanding the physiology of hemostasis and thrombosis and is utilized in clinical practice to diagnose inherited and acquired platelet bleeding disorders. Flow cytometry is a powerful tool for rapid evaluation of multiple functional properties of large number of platelets in whole blood and offers many advantages over other traditional methods. Attention to pre-analytical factors is required to ensure biologically valid and robust results.

Role of platelets in neutrophil extracellular trap (NET) production and tissue injury

In addition to their well-known role as the cellular mediator of thrombosis, numerous studies have identified key roles for platelets during various disease processes. Importantly, platelets play a critical role in the host immune response, directly interacting with, and eliminating pathogens, from the blood stream. In addition to pathogen clearance, platelets also contribute to leukocyte recruitment at sites of infection and inflammation, and modulate leukocyte activity. Platelet interaction with activated neutrophils is a potent inducer of neutrophil extracellular trap (NET). NETs consist of a diffuse, sticky web of extracellular DNA, nuclear and granular proteins, and serve to ensnare and kill pathogens. In addition to catching pathogens, the cytotoxic molecules and proteases on NETs have the potential to inflict significant tissue damage. Additionally, NET components have been suggested to be key activators of infection-induced coagulopathy. These critical roles, at the interface between hemostasis and immunity, highlight the need for balance in the platelet response; too little platelet activity results in bleeding and immune deficit, too much leads to tissue pathogenesis. In this review, we highlight recent advances in our understanding of the role platelets play in inflammation, the link between platelets and NETs and the role platelets play in disease pathogenesis.

Lung injury during LPS-induced inflammation occurs independently of the receptor P2Y1

Disruption of the lung endothelial and epithelial barriers during acute inflammation leads to excessive neutrophil migration. It is likely that activated platelets promote pulmonary recruitment of neutrophils during inflammation, and previous studies have found that anti-platelet therapy and depletion of circulating platelets have lung-protective effects in different models of inflammation. Because ADP signaling is important for platelet activation, I investigated the role of the ADP-receptor P2Y₁, a G protein-coupled receptor expressed on the surface of circulating platelets, during lipopolysaccharide (LPS)-induced inflammation and lung injury in P2Y₁-null and wild-type mice. Systemic inflammation was induced by a single intraperitoneal dose of LPS (3 mg/kg), and the mice were analyzed 24 h posttreatment. The data show that the LPS-induced inflammation levels were comparable in the P2Y₁-null and wild-type mice. Specifically, splenomegaly, counts of circulating platelets and white blood cells (lymphocytes and neutrophils), and assessments of lung injury (tissue architecture and cell infiltration) were similar in the P2Y₁-null and wild-type mice. Based on my results, I conclude that lung injury during LPS-induced inflammation in mice is independent of P2Y₁ signaling. I propose that if a blockade of purinergic signaling in platelets is a potential lung-protective strategy in the treatment of acute inflammation, then it is more likely to be a result of the disruption of the signaling pathway mediated by P2Y₁₂, another G protein-coupled receptor that mediates platelet responses to ADP.

Platelets in Pulmonary Immune Responses and Inflammatory Lung Diseases

Platelets are essential for physiological hemostasis and are central in pathological thrombosis. These are their traditional and best known activities in health and disease. In addition, however, platelets have specializations that broaden their functional repertoire considerably. These functional capabilities, some of which are recently discovered, include the ability to sense and respond to infectious and immune signals and to act as inflammatory effector cells. Human platelets and platelets from mice and other experimental animals can link the innate and adaptive limbs of the immune system and act across the immune continuum, often also linking immune and hemostatic functions. Traditional and newly recognized facets of the biology of platelets are relevant to defensive, physiological immune responses of the lungs and to inflammatory lung diseases. The emerging view of platelets as blood cells that are much more diverse and versatile than previously thought further predicts that additional features of the biology of platelets and of megakaryocytes, the precursors of platelets, will be discovered and that some of these will also influence pulmonary immune defenses and inflammatory injury.

P2Y12 Receptor Modulates Sepsis-Induced Inflammation

OBJECTIVE

Platelets modulate hemostasis and immune responses via interactions with immune cells through secretion of immunemodulators and cell-cell interactions. The P2Y12 receptor mediates ADP-induced aggregation and secretion in platelets.

APPROACH AND RESULTS

Using a mouse model of intra-abdominal sepsis and acute lung injury, we investigated the role of the P2Y12 receptor in neutrophil migration and lung inflammation in P2Y12 null mice and in mice pretreated with the P2Y12 antagonist clopidogrel. Our data show a decrease in circulating white blood cells and a decrease in platelet activation and platelet-leukocyte interactions in treated mice compared with untreated mice. Additionally, lung injury and platelet sequestration were diminished in clopidogrel-treated mice compared with their untreated septic littermates. Similar results were observed in P2Y12 null mice: platelet activation and platelet-leukocyte aggregates were decreased in septic P2Y12 null mice compared with wild-type mice. P2Y12 null mice were refractory to lung injury compared with wild-type mice. Finally, to evaluate P2Y12-independent effects of clopidogrel, we pretreated P2Y12 null mice. Interestingly, the number of circulating neutrophils was reduced in treated septic P2Y12 null mice, suggesting neutrophils as a target for clopidogrel pleiotropic effects. No difference was observed in P2Y1 null mice during sepsis, indicating that the P2Y12 receptor is responsible for the effects.

CONCLUSIONS

P2Y12 null mice are refractory to sepsis-induced lung injury, suggesting a key role for activated platelets and the P2Y12 receptor during sepsis.

Platelet-Derived CCL5 Regulates CXC Chemokine Formation and Neutrophil Recruitment in Acute Experimental Colitis

Accumulating data suggest that platelets not only regulate thrombosis and haemostasis but also inflammatory processes. Platelets contain numerous potent pro-inflammatory compounds, including the chemokines CCL5 and CXCL4, although their role in acute colitis remains elusive. The aim of this study is to examine the role of platelets and platelet-derived chemokines in acute colitis. Acute colitis is induced in female Balb/c mice by administration of 5% dextran sodium sulfate (DSS) for 5 days. Animals receive a platelet-depleting, anti-CCL5, anti-CXCL4, or a control antibody prior to DSS challenge. Colonic tissue is collected for quantification of myeloperoxidase (MPO) activity, CXCL5, CXCL2, interleukin-6 (IL-6), and CCL5 levels as well as morphological analyses. Platelet depletion reduce tissue damage and clinical disease activity index in DSS-exposed animals. Platelet depletion not only reduces levels of CXCL2 and CXCL5 but also levels of CCL5 in the inflamed colon. Immunoneutralization of CCL5 but not CXCL4 reduces tissue damage, CXC chemokine expression, and neutrophil recruitment in DSS-treated animals. These findings show that platelets play a key role in acute colitis by regulating CXC chemokine generation, neutrophil infiltration, and tissue damage in the colon. Moreover, our results suggest that platelet-derived CCL5 is an important link between platelet activation and neutrophil recruitment in acute colitis.

Thrombocytopenia is associated with a dysregulated host response in critically ill sepsis patients

Preclinical studies have suggested that platelets influence the host response during sepsis. We sought to assess the association of admission thrombocytopenia with the presentation, outcome, and host response in patients with sepsis. Nine hundred thirty-one consecutive sepsis patients were stratified according to platelet counts (very low <50 × 10(9)/L, intermediate-low 50 × 10(9) to 99 × 10(9)/L, low 100 × 10(9) to 149 × 10(9)/L, or normal 150 × 10(9) to 399 × 10(9)/L) on admission to the intensive care unit. Sepsis patients with platelet counts <50 × 10(9)/L and 50 × 10(9) to 99 × 10(9)/L presented with higher Acute Physiology and Chronic Health Evaluation scores and more shock. Both levels of thrombocytopenia were independently associated with increased 30-day mortality (hazard ratios with 95% confidence intervals 2.00 [1.32-3.05] and 1.72 [1.22-2.44], respectively). To account for baseline differences besides platelet counts, propensity matching was performed, after which the association between thrombocytopenia and the host response was tested, as evaluated by measuring 17 plasma biomarkers indicative of activation and/or dysregulation of pathways implicated in sepsis pathogenesis and by whole genome blood leukocyte expression profiling. In the propensity matched cohort, platelet counts < 50 × 10(9)/L were associated with increased cytokine levels and enhanced endothelial cell activation. All thrombocytopenic groups showed evidence of impaired vascular integrity, whereas coagulation activation was similar between groups. Blood microarray analysis revealed a distinct gene expression pattern in sepsis patients with <50 × 10(9)/L platelets, showing reduced signaling in leukocyte adhesion and diapedesis and increased complement signaling. These data show that admission thrombocytopenia is associated with enhanced mortality and a more disturbed host response during sepsis independent of disease severity, thereby providing clinical validity to animal studies on the role of platelets in severe infection.

Platelet-neutrophil complex formation-a detailed in vitro analysis of murine and human blood samples

Platelets form complexes with neutrophils during inflammatory processes. These aggregates migrate into affected tissues and also circulate within the organism. Several studies have evaluated platelet-neutrophil complexes as a marker of cardiovascular diseases in human and mouse. Although multiple publications have reported platelet-neutrophil complex counts, we noticed that different methods were used to analyze platelet-neutrophil complex formation, resulting in significant differences, even in baseline values. We established a protocol for platelet-neutrophil complex measurement with flow cytometry in murine and human whole blood samples. In vitro platelet-neutrophil complex formation was stimulated with ADP or PMA. We tested the effect of different sample preparation steps and cytometer settings on platelet-neutrophil complex detection and noticed false-positive counts with increasing acquisition speed. Platelet-neutrophil complex formation depends on platelet P-selectin expression, and antibody blocking of P-selectin consequently prevented ADP-induced platelet-neutrophil complex formation. These findings may help generating more comparable data among different research groups that examine platelet-neutrophil complexes as a marker for cardiovascular disease and novel therapeutic interventions.

Therapeutic implications of innate immune system in acute pancreatitis

INTRODUCTION

Acute pancreatitis (AP) is an inflammatory disorder of the pancreas encompassing a cascade of cellular and molecular events. It starts from premature activation of zymogens with the involvement of innate immune system to a potential systemic inflammatory response and multiple organ failure. Leukocytes are the major cell population that participate in the propagation of the disease. Current understanding of the course of AP is still far from complete, limiting treatment options mostly to conservative supportive care. Emerging evidence has pointed to modulation of the immune system for strategic therapeutic development, by mitigating the inflammatory response and severity of AP. In the current review, we have focused on the role of innate immunity in the condition and highlighted therapeutics targeting it for treatment of this challenging disease.

AREAS COVERED

The current review has aimed to elaborate in-depth understanding of specific roles of innate immune cells, derived mediators and inflammatory pathways that are involved in AP. Summarizing the recent therapeutics and approaches applied experimentally that target immune responses to attenuate AP.

EXPERT OPINION

The current state of knowledge on AP, limitations of presently available therapeutic approaches and the promise of therapeutic implications of innate immune system in AP are discussed.

Platelet secretion of CXCL4 is Rac1-dependent and regulates neutrophil infiltration and tissue damage in septic lung damage

BACKGROUND AND PURPOSE

Platelets are potent regulators of neutrophil accumulation in septic lung damage. We hypothesized that platelet-derived CXCL4 might support pulmonary neutrophilia in a murine model of abdominal sepsis.

EXPERIMENTAL APPROACH

Polymicrobial sepsis was triggered by coecal ligation and puncture (CLP) in C57BL/6 mice. Platelet secretion of CXCL4 was studied by using confocal microscopy. Plasma and lung levels of CXCL4, CXCL1 and CXCL2 were determined by elisa. Flow cytometry was used to examine surface expression of Mac-1 on neutrophils.

KEY RESULTS

CLP increased CXCL4 levels in plasma, and platelet depletion reduced plasma levels of CXCL4 in septic animals. Rac1 inhibitor NSC23766 decreased the CLP-enhanced CXCL4 in plasma by 77%. NSC23766 also abolished PAR4 agonist-induced secretion of CXCL4 from isolated platelets. Inhibition of CXCL4 reduced CLP-evoked neutrophil recruitment, oedema formation and tissue damage in the lung. However, immunoneutralization of CXCL4 had no effect on CLP-induced expression of Mac-1 on neutrophils. Targeting CXCL4 attenuated plasma and lung levels of CXCL1 and CXCL2 in septic mice. CXCL4 had no effect on neutrophil chemotaxis in vitro, indicating it has an indirect effect on pulmonary neutrophilia. Intratracheal CXCL4 enhanced infiltration of neutrophils and formation of CXCL2 in the lung. CXCR2 antagonist SB225002 markedly reduced CXCL4-provoked neutrophil accumulation in the lung. CXCL4 caused secretion of CXCL2 from isolated alveolar macrophages.

CONCLUSIONS AND IMPLICATIONS

Rac1 controls platelet secretion of CXCL4 and CXCL4 is a potent stimulator of neutrophil accumulation in septic lungs via generation of CXCL2 in alveolar macrophages. Platelet-derived CXCL4 plays an important role in lung inflammation and tissue damage in polymicrobial sepsis.

Aspirin as a potential treatment in sepsis or acute respiratory distress syndrome

Sepsis is a common condition that is associated with significant morbidity, mortality and health-care cost. Pulmonary and non-pulmonary sepsis are common causes of the acute respiratory distress syndrome (ARDS). The mortality from ARDS remains high despite protective lung ventilation, and currently there are no specific pharmacotherapies to treat sepsis or ARDS. Sepsis and ARDS are characterised by activation of the inflammatory cascade. Although there is much focus on the study of the dysregulated inflammation and its suppression, the associated activation of the haemostatic system has been largely ignored until recently. There has been extensive interest in the role that platelet activation can have in the inflammatory response through induction, aggregation and activation of leucocytes and other platelets. Aspirin can modulate multiple pathogenic mechanisms implicated in the development of multiple organ dysfunction in sepsis and ARDS. This review will discuss the role of the platelet, the mechanisms of action of aspirin in sepsis and ARDS, and aspirin as a potential therapy in treating sepsis and ARDS.

Purinergic Receptors in Thrombosis and Inflammation

Under various pathological conditions, including thrombosis and inflammation, extracellular nucleotide levels may increase because of both active release and passive leakage from damaged or dying cells. Once in the extracellular compartment, nucleotides interact with plasma membrane receptors belonging to the P2 purinergic family, which are expressed by virtually all circulating blood cells and in most blood vessels. In this review, we focus on the specific role of the 3 platelet P2 receptors P2Y1, P2Y12, and P2X1 in hemostasis and arterial thrombosis. Beyond platelets, these 3 receptors, along with the P2Y2, P2Y6, and P2X7 receptors, constitute the main P2 receptors mediating the proinflammatory effects of nucleotides, which play important roles in various functions of circulating blood cells and cells of the vessel wall. Each of these P2 receptor subtypes specifically contributes to chronic or acute vascular inflammation and related diseases, such as atherosclerosis, restenosis, endotoxemia, and sepsis. The potential for therapeutic targeting of these P2 receptor subtypes is also discussed.

The role of platelets in inflammation

There is growing recognition of the critical role of platelets in inflammation and immune responses. Recent studies have indicated that antiplatelet medications may reduce mortality from infections and sepsis, which suggests possible clinical relevance of modifying platelet responses to inflammation. Platelets release numerous inflammatory mediators that have no known role in haemostasis. Many of these mediators modify leukocyte and endothelial responses to a range of different inflammatory stimuli. Additionally, platelets form aggregates with leukocytes and form bridges between leukocytes and endothelium, largely mediated by platelet P-selectin. Through their interactions with monocytes, neutrophils, lymphocytes and the endothelium, platelets are therefore important coordinators of inflammation and both innate and adaptive immune responses.

Platelets and Their Interactions with Other Immune Cells

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

Rac1-dependent secretion of platelet-derived CCL5 regulates neutrophil recruitment via activation of alveolar macrophages in septic lung injury

Accumulating evidence suggest that platelets play an important role in regulating neutrophil recruitment in septic lung injury. Herein, we hypothesized that platelet-derived CCL5 might facilitate sepsis-induced neutrophil accumulation in the lung. Abdominal sepsis was induced by CLP in C57BL/6 mice. CLP increased plasma levels of CCL5. Platelet depletion and treatment with the Rac1 inhibitor NSC23766 markedly reduced CCL5 in the plasma of septic mice. Moreover, Rac1 inhibition completely inhibited proteasePAR4-induced secretion of CCL5 in isolated platelets. Immunoneutralization of CCL5 decreased CLP-induced neutrophil infiltration, edema formation, and tissue injury in the lung. However, inhibition of CCL5 function had no effect on CLP-induced expression of Mac-1 on neutrophils. The blocking of CCL5 decreased plasma and lung levels of CXCL1 and CXCL2 in septic animals. CCL5 had no effect on neutrophil chemotaxis in vitro, suggesting an indirect effect of CCL5 on neutrophil recruitment. Intratracheal challenge with CCL5 increased accumulation of neutrophils and formation of CXCL2 in the lung. Administration of the CXCR2 antagonist SB225002 abolished CCL5-induced pulmonary recruitment of neutrophils. Isolated alveolar macrophages expressed significant levels of the CCL5 receptors CCR1 and CCR5. In addition, CCL5 triggered significant secretion of CXCL2 from isolated alveolar macrophages. Notably, intratracheal administration of clodronate not only depleted mice of alveolar macrophages but also abolished CCL5-induced formation of CXCL2 in the lung. Taken together, our findings suggest that Rac1 regulates platelet secretion of CCL5 and that CCL5 is a potent inducer of neutrophil recruitment in septic lung injury via formation of CXCL2 in alveolar macrophages.

Thrombocytopenia impairs host defense during murine Streptococcus pneumoniae pneumonia

OBJECTIVES

Streptococcus pneumoniae is the most common causative pathogen in community-acquired pneumonia. In patients, thrombocytopenia is correlated with an adverse outcome of pneumonia. Platelets can modulate the host response to infection in several ways, that is, by facilitating clot formation, production of antimicrobial proteins, and interaction with neutrophils. We studied the effect of thrombocytopenia during murine pneumococcal pneumonia.

DESIGN

Animal study.

SETTING

University research laboratory.

SUBJECTS

Mice.

INTERVENTIONS

Pneumonia was induced by intranasal inoculation of S. pneumoniae. Platelets were depleted by anti-mouse thrombocyte serum; controls received nonimmunogenic serum. In separate studies, mice were treated with the platelet P2Y12 receptor inhibitor clopidogrel or placebo.

MEASUREMENTS AND MAIN RESULTS

Thrombocytopenic mice (platelet counts < 1% of uninfected controls) showed a reduced survival during pneumococcal pneumonia (27% vs 75% among controls; p = 0.003), which was associated with higher bacterial loads in lungs, spleen, and blood. Thrombocytopenic mice showed enhanced coagulation activation (thrombin-antithrombin complexes) in plasma. Proinflammatory cytokine levels were higher in plasma but not in lungs of thrombocytopenic mice. Although clopidogrel treatment strongly prolonged the bleeding time, it did not impact on bacterial loads during pneumococcal pneumonia.

CONCLUSIONS

Platelets play a protective role during pneumococcal pneumonia independent of their aggregation.

Platelets in inflammation and infection

Although platelets are traditionally recognized for their central role in hemostasis, many lines of research clearly demonstrate these rather ubiquitous blood components are potent immune modulators and effectors. Platelets have been shown to directly recognize, sequester and kill pathogens, to activated and recruit leukocytes to sites of infection and inflammation, and to modulate leukocyte behavior, enhancing their ability to phagocytose and kill pathogens and inducing unique effector functions, such as the production of Neutrophil Extracellular Traps (NETs). This multifaceted response to infection and inflammation is due, in part, to the huge array of soluble mediators and cell surface molecules expressed by platelets. From their earliest origins as primordial hemocytes in invertebrates to their current form as megakaryocyte-derived cytoplasts, platelets have evolved to be one of the key regulators of host intravascular immunity and inflammation. In this review, we present the diverse roles platelets play in immunity and inflammation associated with autoimmune diseases and infection. Additionally, we highlight recent advances in our understanding of platelet behavior made possible through the use of advanced imaging techniques that allow us to visualize platelets and their interactions, in real-time, within the intact blood vessels of a living host.

The Inflammatory Role of Platelets via Their TLRs and Siglec Receptors

Platelets are non-nucleated cells that play central roles in the processes of hemostasis, innate immunity, and inflammation; however, several reports show that these distinct functions are more closely linked than initially thought. Platelets express numerous receptors and contain hundreds of secretory products. These receptors and secretory products are instrumental to the platelet functional responses. The capacity of platelets to secrete copious amounts of cytokines, chemokines, and related molecules appears intimately related to the role of the platelet in inflammation. Platelets exhibit non-self-infectious danger detection molecules on their surfaces, including those belonging to the “toll-like receptor” family, as well as pathogen sensors of other natures (Ig- or complement receptors, etc.). These receptors permit platelets to both bind infectious agents and deliver differential signals leading to the secretion of cytokines/chemokines, under the control of specific intracellular regulatory pathways. In contrast, dysfunctional receptors or dysregulation of the intracellular pathway may increase the susceptibility to pathological inflammation. Physiological vs. pathological inflammation is tightly controlled by the sensors of danger expressed in resting, as well as in activated, platelets. These sensors, referred to as pathogen recognition receptors, primarily sense danger signals termed pathogen associated molecular patterns. As platelets are found in inflamed tissues and are involved in auto-immune disorders, it is possible that they can also be stimulated by internal pathogens. In such cases, platelets can also sense danger signals using damage associated molecular patterns (DAMPs). Some of the most significant DAMP family members are the alarmins, to which the Siglec family of molecules belongs. This review examines the role of platelets in anti-infection immunity via their TLRs and Siglec receptors.

Platelets and infections - complex interactions with bacteria

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

The role of P2Y₁₂ receptor and activated platelets during inflammation

Platelets play an important role not only during thrombosis, but also in modulating immune responses through their interaction with immune cells and by releasing inflammatory mediators upon activation. The P2Y12 receptor is a Gi-coupled receptor that not only regulates ADP-induced aggregation but can also dramatically potentiate secretion, when platelets are activated by other stimuli. Considering the importance of P2Y12 receptor in platelet function, a class of antiplatelet drugs, thienopyridines, have been designed and successfully used to prevent thrombosis. This review will focus on the role of activated platelets in inflammation and the effects that P2Y12 antagonism exerts on the inflammatory process. A change in platelet functions was noted in patients treated with thienopyridines during inflammatory conditions, suggesting that platelets may modulate the inflammatory response. Further experiments in a variety of animal models of diseases, such as sepsis, rheumatoid arthritis, myocardial infarction, pancreatitis and pulmonary inflammation have also demonstrated that activated platelets influence the inflammatory state. Platelets can secrete inflammatory modulators in a P2Y12-dependent manner, and, as a result, directly alter the inflammatory response. P2Y12 receptor may also be expressed in other cells of the immune system, indicating that thienopyridines could directly influence the immune system rather than only through platelets. Overall the results obtained to date strongly support the notion that activated platelets significantly contribute to the inflammatory process and that antagonizing P2Y12 receptor can influence the immune response.

Platelets in inflammation: regulation of leukocyte activities and vascular repair

There is now a large body of evidence that platelets are central actors of inflammatory reactions. Indeed, platelets play a significant role in a variety of inflammatory diseases. These diseases include conditions as varied as atherosclerosis, arthritis, dermatitis, glomerulonephritis, or acute lung injury. In this context, one can note that inflammation is a convenient but imprecise catch-all term that is used to cover a wide range of situations. Therefore, when discussing the role of platelets in inflammation, it is important to clearly define the pathophysiological context and the exact stage of the reaction. Inflammatory reactions are indeed multistep processes that can be either acute or chronic, and their sequence can vary greatly depending on the situation and organ concerned. Here, we focus on how platelets contribute to inflammatory reactions involving recruitment of neutrophils and/or macrophages. Specifically, we review past and recent data showing that platelets intervene at various stages of these reactions to regulate parameters such as endothelial permeability, the recruitment of neutrophils and macrophages and their effector functions, as well as inflammatory bleeding. The mechanisms underlying these various modulating effect of platelets are also discussed.

Thrombocytopenia impairs host defense in gram-negative pneumonia-derived sepsis in mice

Thrombocytopenia is a common finding in sepsis and associated with a worse outcome. We used a mouse model of pneumonia-derived sepsis caused by the human pathogen Klebsiella pneumoniae to study the role of platelets in host response to sepsis. Platelet counts (PCs) were reduced to less than a median of 5 × 10(9)/L or to 5 to 13 × 10(9)/L by administration of a depleting antibody in mice infected with Klebsiella via the airways. Thrombocytopenia was associated with strongly impaired survival during pneumonia-derived sepsis proportional to the extent of platelet depletion. Thrombocytopenic mice demonstrated PC-dependent enhanced bacterial growth in lungs, blood, and distant organs. Severe thrombocytopenia resulted in hemorrhage at the primary site of infection, but not in distant organs. PCs of 5 to 13 × 10(9)/L were sufficient to largely maintain hemostasis in infected lungs. Thrombocytopenia did not influence lung inflammation or neutrophil recruitment and did not attenuate local or systemic activation of coagulation or the vascular endothelium. PCs <5 × 10(9)/L even resulted in enhanced coagulation and endothelial cell activation, which coincided with increased proinflammatory cytokine levels. In accordance, low PCs in whole blood enhanced Klebsiella-induced cytokine release in vitro. These data suggest that platelets play an important role in host defense to Klebsiella pneumosepsis.

Proinflammatory role of neutrophil extracellular traps in abdominal sepsis

Excessive neutrophil activation is a major component in septic lung injury. Neutrophil-derived DNA may form extracellular traps in response to bacterial invasions. The aim of the present study was to investigate the potential role of neutrophil extracellular traps (NETs) in septic lung injury. Male C57BL/6 mice were treated with recombinant human (rh)DNAse (5 mg/kg) after cecal ligation and puncture (CLP). Extracellular DNA was stained by Sytox green, and NET formation was quantified by confocal microscopy and cell-free DNA in plasma, peritoneal cavity, and lung. Blood, peritoneal fluid, and lung tissue were harvested for analysis of neutrophil infiltration, NET levels, tissue injury, as well as CXC chemokine and cytokine formation. We observed that CLP caused increased formation of NETs in plasma, peritoneal cavity, and lung. Administration of rhDNAse not only eliminated NET formation in plasma, peritoneal cavity, and bronchoalveolar space but also reduced lung edema and tissue damage 24 h after CLP induction. Moreover, treatment with rhDNAse decreased CLP-induced formation of CXC chemokines, IL-6, and high-mobility group box 1 (HMGB1) in plasma, as well as CXC chemokines and IL-6 in the lung. In vitro, we found that neutrophil-derived NETs had the capacity to stimulate secretion of CXCL2, TNF-α, and HMGB1 from alveolar macrophages. Taken together, our findings show that NETs regulate pulmonary infiltration of neutrophils and tissue injury via formation of proinflammatory compounds in abdominal sepsis. Thus we conclude that NETs exert a proinflammatory role in septic lung injury.

New strategy for sepsis: Targeting a key role of platelet-neutrophil interaction

Neutrophil and platelet are essential arms of the innate immune response. In sepsis, platelet abnormal activation as well as neutrophil paralysis are well recognized. For platelet, it is characterized by the contribution to disseminated intravascular coagulation (DIC) and the enhanced inflammation response. In terms of neutrophil, its dysfunction is manifested by the impaired recruitment and migration to the infectious foci, abnormal sequestration in the remote organs, and the delayed clearance. More recently, it has been apparent that together platelet-neutrophil interaction can induce a faster and harder response during sepsis. This article focuses on the activation of platelet, dysfunction of neutrophil, and the interaction between them during sepsis and profiles some of the molecular mechanisms and outcomes in these cellular dialogues, providing a novel strategy for treatment of sepsis.

The role of platelets in sepsis

Platelets are small circulating anucleate cells that are of crucial importance in haemostasis. Over the last decade, it has become increasingly clear that platelets play an important role in inflammation and can influence both innate and adaptive immunity. Sepsis is a potentially lethal condition caused by detrimental host response to an invading pathogen. Dysbalanced immune response and activation of the coagulation system during sepsis are fundamental events leading to sepsis complications and organ failure. Platelets, being major effector cells in both haemostasis and inflammation, are involved in sepsis pathogenesis and contribute to sepsis complications. Platelets catalyse the development of hyperinflammation, disseminated intravascular coagulation and microthrombosis, and subsequently contribute to multiple organ failure. Inappropriate accumulation and activity of platelets are key events in the development of sepsis-related complications such as acute lung injury and acute kidney injury. Platelet activation readouts could serve as biomarkers for early sepsis recognition; inhibition of platelets in septic patients seems like an important target for immune-modulating therapy and appears promising based on animal models and retrospective human studies.

Nuclear factor of activated T cells regulates neutrophil recruitment, systemic inflammation, and T-cell dysfunction in abdominal sepsis

The signaling mechanisms regulating neutrophil recruitment, systemic inflammation, and T-cell dysfunction in polymicrobial sepsis are not clear. This study explored the potential involvement of the calcium/calcineurin-dependent transcription factor, nuclear factor of activated T cells (NFAT), in abdominal sepsis. Cecal ligation and puncture (CLP) triggered NFAT-dependent transcriptional activity in the lung, spleen, liver, and aorta in NFAT-luciferase reporter mice. Treatment with the NFAT inhibitor A-285222 prior to CLP completely prevented sepsis-induced NFAT activation in all these organs. Inhibition of NFAT activity reduced sepsis-induced formation of CXCL1, CXCL2, and CXCL5 chemokines and edema as well as neutrophil infiltration in the lung. Notably, NFAT inhibition efficiently reduced the CLP-evoked increases in HMBG1, interleukin 6 (IL-6), and CXCL5 levels in plasma. Moreover, administration of A-285222 restored sepsis-induced T-cell dysfunction, as evidenced by markedly decreased apoptosis and restored proliferative capacity of CD4 T cells. Along these lines, treatment with A-285222 restored gamma interferon (IFN-γ) and IL-4 levels in the spleen, which were markedly reduced in septic mice. CLP-induced formation of regulatory T cells (CD4(+) CD25(+) Foxp3(+)) in the spleen was also abolished in A-285222-treated animals. All together, these novel findings suggest that NFAT is a powerful regulator of pathological inflammation and T-cell immune dysfunction in abdominal sepsis. Thus, our data suggest that NFAT signaling might be a useful target to protect against respiratory failure and immunosuppression in patients with sepsis.

Hydroxysafflor yellow A of Carthamus tinctorius attenuates lung injury of aged rats exposed to gasoline engine exhaust by down-regulating platelet activation

Long-term inhalation of gasoline engine exhaust (GEE) increases the risk of respiratory disease. Studies have suggested involvement of platelets in the development of some lung diseases. Hydroxysafflor yellow A (HSYA), a flavonoid compound, prevents hemostasis. Therefore, we investigated its effects on GEE-induced lung injury, and role of platelets in injury. Sixty-week-old male Sprague-Dawley rats were exposed to GEE for 4h/day for 6 weeks, and then grouped as follows: control, GEE, GEE+HSYA, GEE+HSYA+GW9662, and GEE+GW9662. Arterial oxygen tension (PaO2), carbon dioxide tension (PaCO2), pH, and the PaO2/fraction of inspired oxygen ratio (PaO2/FiO2) in the blood were detected using a blood gas analyzer. Wet/dry lung weight ratio, total protein in bronchoalveolar lavage fluid (BALF), and cytokine concentrations in serum and BALF were determined. Furthermore, cyclic adenosine monophosphate (cAMP) level and expression levels of target proteins were analyzed. Platelets were counted and their state was evaluated. HSYA attenuated GEE-mediated decreases in PaO2, PaO2/FiO2, platelet cAMP level, protein kinase A (PKA) activity, and peroxisome proliferator-activated receptor γ (PPARγ) expression. HSYA also attenuated GEE-mediated increases in lung permeability, cytokine levels in serum and BALF, plasma platelet count, and ADP-mediated platelet aggregation. Moreover, it suppressed GEE-induced increases in the expression of adhesion molecules and proinflammatory cytokines in platelets and lung tissue. Therefore, HSYA is therapeutically effective for GEE-mediated lung injury and acts by enhancing PKA activity and inhibiting platelet activation.

Targeting platelet-derived soluble CD40 ligand: a new treatment strategy for HIV-associated neuroinflammation?

Human immunodeficiency virus type 1 (HIV) continues to be one of the most prevalent global health afflictions to date. The advent and introduction of combined antiretroviral therapy (cART) has made a significant impact on the course of infection. However, as patients are living longer, many HIV-associated illnesses are becoming prevalent among the infected population, especially those associated with chronic inflammation. Consistently, HIV-associated neuroinflammation is believed to be a major catalyst in the development of HIV-associated neurocognitive disorders (HAND), which are estimated to persist in approximately 50% of infected individuals regardless of cART. This dramatically underscores the need to develop effective adjunctive therapies capable of controlling this aspect of the disease, which are currently lacking.

We previously demonstrated that the inflammatory mediator soluble CD40 ligand (sCD40L) is elevated in both the plasma and cerebrospinal fluid of cognitively impaired infected individuals compared to their non-impaired infected counterparts. Our group, and others have recently demonstrated that there is an increasing role for this inflammatory mediator in the pathogenesis of HIV-associated neuroinflammation, thereby identifying this molecule as a potential therapeutic target for the management of HAND. Platelets are the major source of circulating sCD40L, and these small cells are increasingly implicated in a multitude of inflammatory disorders, including those common during HIV infection. Thus, antiplatelet therapies that minimize the release of platelet-derived inflammatory mediators such as sCD40L are an innovative, non-traditional approach for the treatment of HIV-associated neuroinflammation, with the potential to benefit other HIV-associated illnesses.

Ticagrelor reduces neutrophil recruitment and lung damage in abdominal sepsis

Abstract Platelets play an important role in abdominal sepsis and P2Y12 receptor antagonists have been reported to exert anti-inflammatory effects. Herein, we assessed the impact of platelet inhibition with the P2Y12 receptor antagonist ticagrelor on pulmonary neutrophil recruitment and tissue damage in a model of abdominal sepsis. Wild-type C57BL/6 mice were subjected to cecal ligation and puncture (CLP). Animals were treated with ticagrelor (100 mg/kg) or vehicle prior to CLP induction. Edema formation and bronchoalveolar neutrophils as well as lung damage were quantified. Flow cytometry was used to determine expression of platelet-neutrophil aggregates, neutrophil activation and CD40L expression on platelets. CLP-induced pulmonary infiltration of neutrophils at 24 hours was reduced by 50% in ticagrelor-treated animals. Moreover, ticagrelor abolished CLP-provoked lung edema and decreased lung damage score by 41%. Notably, ticagrelor completely inhibited formation of platelet-neutrophil aggregates and markedly reduced thrombocytopenia in CLP animals. In addition, ticagrelor reduced platelet shedding of CD40L in septic mice. Our data indicate that ticagrelor can reduce CLP-induced pulmonary neutrophil recruitment and lung damage suggesting a potential role for platelet antagonists, such as ticagrelor, in the management of patients with abdominal sepsis.

Platelet shedding of CD40L is regulated by matrix metalloproteinase-9 in abdominal sepsis

BACKGROUND AND OBJECTIVES

Platelet-derived CD40L is known to regulate neutrophil recruitment and lung damage in sepsis. However, the mechanism regulating shedding of CD40L from activated platelets is not known. We hypothesized that matrix metalloproteinase (MMP)-9 might cleave surface-expressed CD40L and regulate pulmonary accumulation of neutrophils in sepsis.

METHODS

Abdominal sepsis was induced by cecal ligation and puncture (CLP) in wild-type and MMP-9-deficient mice. Edema formation, CXC chemokine levels, myeloperoxidase levels, neutrophils in the lung and plasma levels of CD40L and MMP-9 were quantified.

RESULTS

CLP increased plasma levels of MMP-9 but not MMP-2. The CLP-induced decrease in platelet surface CD40L and increase in soluble CD40L levels were significantly attenuated in MMP-9 gene-deficient mice. Moreover, pulmonary myeloperoxidase (MPO) activity and neutrophil infiltration in the alveolar space, as well as edema formation and lung injury, were markedly decreased in septic mice lacking MMP-9. In vitro studies revealed that inhibition of MMP-9 decreased platelet shedding of CD40L. Moreover, recombinant MMP-9 was capable of cleaving surface-expressed CD40L on activated platelets. In human studies, plasma levels of MMP-9 were significantly increased in patients with septic shock as compared with healthy controls, although MMP-9 levels did not correlate with organ injury score.

CONCLUSIONS

Our novel data propose a role of MMP-9 in regulating platelet-dependent infiltration of neutrophils and tissue damage in septic lung injury by controlling CD40L shedding from platelets. We conclude that targeting MMP-9 may be a useful strategy to limit acute lung injury in abdominal sepsis.

Neutrophil and platelet complexes and their relevance to neutrophil recruitment and activation

The manifestation of platelet 'satallitism' around neutrophils in whole blood is a long acknowledged phenomenon [1]. Circulating platelet-neutrophil complexes (PNC) occur in a diverse range of inflammatory disorders and infections that affect numerous organs of the body. Animal models have revealed that the formation of PNC is required for the recruitment of neutrophils to inflamed tissue, since platelets 'prime' neutrophils for efficient adhesion to vascular endothelium via the up-regulation of integrins and enhanced responsiveness to chemokines (Fig. 1). Perhaps surprisingly, the surface contact between platelets and neutrophils additionally enhances other neutrophil functions, such as chemotaxis that is required for migration into tissues, trans-cellular production of eicosanoids, phagocytosis and trapping of pathogens, increased respiratory burst leading to the production of reactive oxygen species (ROS), and modulation of neutrophil apoptosis (Fig. 1). Platelet P-selectin appears to have a particular role in enhancing the majority of these activities, and the influence of platelet P-selectin is not therefore confined to the initial rolling events in the process of neutrophil extravasation.

Platelets promote bacterial dissemination in a mouse model of streptococcal sepsis

Platelets have been reported to contribute to inflammation and inflammatory disorders. In the present study, we demonstrate that platelets contribute to the acute response to bacterial infection in a mouse model of invasive Streptococcus pyogenes infection. Thrombocytopenia occurred rapidly in infected animals and this was associated with platelet activation, formation of platelet-neutrophil complexes and neutrophil activation. In order to assess the role of platelets during infection, platelets were depleted prior to infection. Platelet-depleted animals had significantly decreased platelet-neutrophil complex formation and neutrophil activation in response to infection. Importantly, significantly fewer bacteria disseminated to the blood, lungs, and spleen of platelet-depleted animals. Platelet-depleted animals did not decrease as significantly in weight as the infected control animals. The results demonstrate a previously unappreciated role for platelets during the pathophysiological response to infection, whereby S. pyogenes bacteria bind to platelets and platelets facilitate bacterial dissemination.

Antiplatelet therapy is associated with decreased transfusion-associated risk of lung dysfunction, multiple organ failure, and mortality in trauma patients

OBJECTIVE

To determine whether prehospital antiplatelet therapy was associated with reduced incidence of acute lung dysfunction, multiple organ failure, and mortality in blunt trauma patients.

DESIGN

Secondary analysis of a cohort enrolled in the National Institute of General Medical Sciences Trauma Glue Grant database.

SETTING

Multicenter study including nine U.S. level-1 trauma centers.

PATIENTS

A total of 839 severely injured blunt trauma patients at risk for multiple organ failure (age > 45 yr, base deficit > 6 mEq/L or systolic blood pressure < 90 mm Hg, who received a blood transfusion). Severe/isolated head injuries were excluded.

MEASUREMENTS AND MAIN RESULTS

Primary outcomes were lung dysfunction (defined as grades 2-3 by the Denver multiple organ failure score), multiple organ failure (Denver multiple organ failure score >3), and mortality. Patients were documented as on antiplatelet therapy if taking acetylsalicylic acid, clopidogrel, and/or ticlopidine. Fifteen percent were taking antiplatelet therapy prior to injury. Median injury severity score was 30 (interquartile range 22-51), mean age 61 + 0.4 yr and median RBCs volume transfused was 1700 mL (interquartile range 800-3150 mL). Overall, 63% developed lung dysfunction, 19% had multiple organ failure, and 21% died. After adjustment for age, gender, comorbidities, blood products, crystalloid/12 hrs, presence of any head injury, injury severity score, and 12 hrs base deficit > 8 mEq/L, 12 hrs RBC transfusion was associated with a significantly smaller risk of lung dysfunction and multiple organ failure among the group receiving antiplatelet therapy compared with those not receiving it (lung dysfunction p = 0.0116, multiple organ failure p = 0.0291). In addition, antiplatelet therapy had a smaller risk (albeit not significant, p = 0.06) of death for patients receiving RBC compared to those not on antiplatelet therapy after adjustment for confounders,

CONCLUSIONS

Pre-injury antiplatelet therapy is associated with a decreased risk of lung dysfunction, multiple organ failure, and possibly mortality in high-risk blunt trauma patients who received blood transfusions. These findings suggest platelets have a role in organ dysfunction development and have potential therapeutic implications.

Geranylgeranyl transferase regulates CXC chemokine formation in alveolar macrophages and neutrophil recruitment in septic lung injury

Overwhelming accumulation of neutrophils is a significant component in septic lung damage, although the signaling mechanisms behind neutrophil infiltration in the lung remain elusive. In the present study, we hypothesized that geranylgeranylation might regulate the inflammatory response in abdominal sepsis. Male C57BL/6 mice received the geranylgeranyl transferase inhibitor, GGTI-2133, before cecal ligation and puncture (CLP). Bronchoalveolar lavage fluid and lung tissue were harvested for analysis of neutrophil infiltration, as well as edema and CXC chemokine formation. Blood was collected for analysis of Mac-1 on neutrophils and CD40L on platelets. Gene expression of CXC chemokines, tumor necrosis factor-α (TNF-α), and CCL2 chemokine was determined by quantitative RT-PCR in isolated alveolar macrophages. Administration of GGTI-2133 markedly decreased CLP-induced infiltration of neutrophils, edema, and tissue injury in the lung. CLP triggered clear-cut upregulation of Mac-1 on neutrophils. Inhibition of geranylgeranyl transferase reduced CLP-evoked upregulation of Mac-1 on neutrophils in vivo but had no effect on chemokine-induced expression of Mac-1 on isolated neutrophils in vitro. Notably, GGTI-2133 abolished CLP-induced formation of CXC chemokines, TNF-α, and CCL2 in alveolar macrophages in the lung. Geranylgeranyl transferase inhibition had no effect on sepsis-induced platelet shedding of CD40L. In addition, inhibition of geranylgeranyl transferase markedly decreased CXC chemokine-triggered neutrophil chemotaxis in vitro. Taken together, our findings suggest that geranylgeranyl transferase is an important regulator of CXC chemokine production and neutrophil recruitment in the lung. We conclude that inhibition of geranylgeranyl transferase might be a potent way to attenuate acute lung injury in abdominal sepsis.

Sepsis-induced leukocyte adhesion in the pulmonary microvasculature in vivo is mediated by CD11a and CD11b

Leukocyte accumulation is a rate-limiting step in inflammatory lung injury. The aim of this study was to define the role of CD11a/CD18 and CD11b/CD18 in sepsis-induced leukocyte rolling and adhesion in lung arterioles, capillaries and venules in male C57BL/6 mice using intravital fluorescence microscopy. Cecal ligation and puncture (CLP) markedly increased leukocyte rolling in arterioles and venules but not in capillaries in the lung. Immunoneutralization of CD11a, but not CD11b, decreased CLP-provoked leukocyte rolling in lung arterioles. Inhibition of CD11a or CD11b abolished CLP-induced arteriolar and venular leukocyte adhesion. Immunoneutralization of CD11a and CD11b reduced sepsis-induced leukocyte sequestration in pulmonary capillaries. Moreover, blocking CD11a or CD11b function improved microvascular blood flow in the lung of CLP animals. Considered together, our novel findings show that CD11a and CD11b mediate leukocyte adhesion in both arterioles and venules as well as trapping in capillaries in the lung. In addition, our data demonstrate that CD11a but not CD11b supports leukocyte rolling in pulmonary arterioles. Thus, these findings elucidate the molecular mechanisms behind leukocyte-endothelium interactions in the lung during systemic inflammation.

Platelets can enhance vascular permeability

Platelets survey blood vessels, searching for endothelial damage and preventing loss of vascular integrity. However, there are circumstances where vascular permeability increases, suggesting that platelets sometimes fail to fulfill their expected function. Human inflammatory arthritis is associated with tissue edema attributed to enhanced permeability of the synovial microvasculature. Murine studies have suggested that such vascular leak facilitates entry of autoantibodies and may thereby promote joint inflammation. Whereas platelets typically help to promote microvascular integrity, we examined the role of platelets in synovial vascular permeability in murine experimental arthritis. Using an in vivo model of autoimmune arthritis, we confirmed the presence of endothelial gaps in inflamed synovium. Surprisingly, permeability in the inflamed joints was abrogated if the platelets were absent. This effect was mediated by platelet serotonin accumulated via the serotonin transporter and could be antagonized using serotonin-specific reuptake inhibitor antidepressants. As opposed to the conventional role of platelets to microvascular leakage, this demonstration that platelets are capable of amplifying and maintaining permeability adds to the rapidly growing list of unexpected functions for platelets.

Isoflurane prevents acute lung injury through ADP-mediated platelet inhibition

BACKGROUND

Growing evidence suggests platelets are essential in posttraumatic, acute lung injury (ALI). Halogenated ethers interfere with the formation of platelet-granulocyte aggregates. The potential benefit of halogenated ethers has not been investigated in models of trauma/hemorrhagic shock (T/HS). Therefore, we hypothesized that isoflurane decreases T/HS-mediated ALI through platelet inhibition.

METHODS

Sprague-Dawley rats (n = 47) were anesthetized by either pentobarbital or inhaled isoflurane and placed into (1) control, (2) trauma (laparotomy) sham shock, (3) T/HS (mean arterial pressure, 30 mmHg × 45 min), (4) pretreatment with an ADP receptor antagonist, or (5) T/HS with isoflurane initiated during resuscitation groups. ALI was determined by protein and pulmonary immunofluorescence bronchoalveolar lavage (BAL) fluid. Platelet Mapping specifically evaluated thrombin-independent inhibition of the ADP and AA pathways of platelet activation.

RESULTS

Pretreatment with isoflurane abrogated ALI as measured by both BAL fluid protein and pulmonary immunofluorescence (P < .001). Platelet Mapping revealed specific inhibition of the platelet ADP-pathway with isoflurane (P < .001). Pretreatment with an ADP receptor antagonist decreased ALI to sham levels, confirming that specific platelet ADP inhibition decreases ALI. Isoflurane initiated during resuscitation also decreased ALI (P < .001).

CONCLUSION

Isoflurane attenuates ALI through an antiplatelet mechanism, in part, through inhibition of the platelet ADP pathway. Isoflurane given postinjury also protects against ALI, and highlights the potential applications of this therapy in various clinical scenarios of ischemia/reperfusion.

Do aspirin and statins prevent severe sepsis?

PURPOSE OF REVIEW

Sepsis is an inflammatory condition associated with significant morbidity and mortality. Given the lack of specific therapies for the condition, prevention has garnered significant interest and increased importance. The article reviews the current literature regarding the use of aspirin and statins for the prevention of sepsis.

RECENT FINDINGS

Aspirin and statins have been integral in the prevention of atherosclerotic disease. Additionally, statins have proven beneficial in the prevention of nonatherosclerotic conditions secondary to their pleiotropic effects. In animal models, this pleiotropism modulates many inflammatory pathways of sepsis. The platelet also plays an integral role in this inflammatory cascade of sepsis. Scientific data indicates that antiplatelet therapy, including aspirin, may attenuate these undesirable effects of platelets. Finally, observational studies have shown that patients taking statins have a decreased incidence of sepsis and septic shock, and aspirin may potentiate these benefits.

SUMMARY

Sepsis is a deadly and costly condition with no available, specific treatment options. The statins and aspirin are well tolerated and widely used for prevention of cardiovascular disease. Because of their effects on the immune system and inflammatory pathways, they may present viable medical options for the prevention of sepsis.

Inflection points in sepsis biology: from local defense to systemic organ injury

Sepsis and septic shock lead to considerable morbidity and mortality in developed and developing countries. Despite advances in understanding the innate immune events that lead to septic shock, molecular therapies based on these advances have failed to improve sepsis mortality. The clinical failure of laboratory-derived therapies may be, in part, due to the pleiotropic consequences of the acute inflammatory response, which is the focus of this review. A brisk response to infecting organism is essential for pathogen containment and eradication. However, systemic spread of inflammation beyond a single focus leads to organ injury and higher mortality. The primary goal of this article is to discuss recent animal- and human-based scientific advances in understanding the host response to infection and to highlight how these defense mechanisms can be locally beneficial but systemically detrimental. There are other factors that determine the severity of sepsis that are beyond the scope of this review, including the virulence of the pathogen and regulation by Toll-like receptors. Specifically, this review focuses on how the effector mechanisms of platelets, mast cells, neutrophil extracellular traps (NETs), and the endothelium participate in combating local infections yet can induce organ injury during systemic infection.

Involvement of thrombopoietin in acinar cell necrosis in L-arginine-induced acute pancreatitis in mice

Thrombopoietin (TPO) plays an important role in injuries of different tissues. However, the role of TPO in acute pancreatitis (AP) is not yet known. The aim of the study was to determine the involvement of TPO in AP. Serum TPO was assayed in necrotizing pancreatitis induced by L-arginine in mice. Recombinant TPO and anti-TPO antibody were given to mice with necrotizing pancreatitis. Amylase, lipase, lactate dehydrogenase, myeloperoxidase activity and pancreatic water content were assayed in serum and tissue samples. Pancreas and lung tissue samples were also collected for histological evaluation. Immunohistochemistry of amylase α and PCNA were applied for the study of acinar regeneration and TUNEL assay for the detection of apoptosis in the pancreas. Increased levels of serum TPO were found in necrotizing pancreatitis. After TPO administration, more severe acinar necrosis was found and blockade of TPO reduced the acinar necrosis in this AP model. Acinar regeneration and apoptosis in the pancreas were affected by TPO and antibody treatment in necrotizing pancreatitis. The severity of pancreatitis-associated lung injury was worsened after TPO treatment, but attenuated after Anti-TPO antibody treatment. In conclusion, serum TPO is up-regulated in the necrotizing pancreatitis induced by L-arginine in mice and may be a risk factor for the pancreatic acinar necrosis in AP. As a pro-necrotic factor, blockade of TPO can attenuate the acinar necrosis in AP and may be a possible therapeutic intervention for AP.

Platelet-mediated vascular dysfunction during acute lung injury

CONTEXT

Platelets have significant roles in initiating and mediating reduced alveolar blood flow, microvascular leak, and ventilation/perfusion mismatch caused by metabolic changes and altered signal transduction caused by ischemia-reperfusion.

OBJECTIVE

This review focuses on platelet mechanisms of vascular dysfunction in the lung and presents a hypothesis for interplay between platelet activation, endothelial damage and fibrinogen. The purpose is to discuss current knowledge regarding mechanisms of platelet-mediated endothelial injury and implications for new strategies to treat vascular dysfunction associated with acute lung injury (ALI).

METHODS

Literature from a number of fields was searched using Medline and Google Scholar.

RESULTS

Activated platelets contribute to redox imbalance through reactive oxygen species production, pro-leak molecules such as PAF and serotonin, and recruitment of inflammatory cytokines and leukocytes to the damaged endothelium.

CONCLUSION

Platelets are a critical component of pulmonary ALI, acting in conjunction with fibrinogen to mediate endothelial damage through multiple signal transduction pathways.