Toll-like receptor 9 activation in neutrophils impairs chemotaxis and reduces sepsis outcome*:

Cytosolic nucleic acid sensing as driver of critical illness: mechanisms and advances in therapy

Nucleic acids from both self- and non-self-sources act as vital danger signals that trigger immune responses. Critical illnesses such as acute respiratory distress syndrome, sepsis, trauma and ischemia lead to the aberrant cytosolic accumulation and massive release of nucleic acids that are detected by antiviral innate immune receptors in the endosome or cytosol. Activation of receptors for deoxyribonucleic acids and ribonucleic acids triggers inflammation, a major contributor to morbidity and mortality in critically ill patients. In the past decade, there has been growing recognition of the therapeutic potential of targeting nucleic acid sensing in critical care. This review summarizes current knowledge of nucleic acid sensing in acute respiratory distress syndrome, sepsis, trauma and ischemia. Given the extensive research on nucleic acid sensing in common pathological conditions like cancer, autoimmune disorders, metabolic disorders and aging, we provide a comprehensive summary of nucleic acid sensing beyond critical illness to offer insights that may inform its role in critical conditions. Additionally, we discuss potential therapeutic strategies that specifically target nucleic acid sensing. By examining nucleic acid sources, sensor activation and function, as well as the impact of regulating these pathways across various acute diseases, we highlight the driving role of nucleic acid sensing in critical illness.

DANGER Signals Activate G -Protein Receptor Kinases Suppressing Neutrophil Function and Predisposing to Infection After Tissue Trauma

OBJECTIVE

Injured tissue predisposes the subject to local and systemic infection. We studied injury-induced immune dysfunction seeking novel means to reverse such predisposition.

BACKGROUND

Injury mobilizes primitive "DANGER signals" [danger-associated molecular patterns (DAMPs)] activating innate immunocyte (neutrophils, PMN) signaling and function. Mitochondrial formyl peptides activate G -protein coupled receptors (GPCR) like formyl peptide receptor-1. Mitochondrial DNA and heme activate toll-like receptors (TLR9 and TLR2/4). GPCR kinases (GRKs) can regulate GPCR activation.

METHODS

We studied human and mouse PMN signaling elicited by mitochondrial DAMPs (GPCR surface expression; protein phosphorylation, or acetylation; Ca 2+ flux) and antimicrobial functions [cytoskeletal reorganization, chemotaxis (CTX), phagocytosis, bacterial killing] in cellular systems and clinical injury samples. Predicted rescue therapies were assessed in cell systems and mouse injury-dependent pneumonia models.

RESULTS

Mitochondrial formyl peptides activate GRK2, internalizing GPCRs and suppressing CTX. Mitochondrial DNA suppresses CTX, phagocytosis, and killing through TLR9 through a novel noncanonical mechanism that lacks GPCR endocytosis. Heme also activates GRK2. GRK2 inhibitors like paroxetine restore functions. GRK2 activation through TLR9 prevented actin reorganization, implicating histone deacetylases (HDACs). Actin polymerization, CTX, bacterial phagocytosis, and killing were also rescued, therefore, by the HDAC inhibitor valproate. Trauma repository PMN showed GRK2 activation and cortactin deacetylation, which varied with severity and was most marked in patients developing infections. Either GRK2 or HDAC inhibition prevented loss of mouse lung bacterial clearance, but only the combination rescued clearance when given postinjury.

CONCLUSIONS

Tissue injury-derived DAMPs suppress antimicrobial immunity through canonical GRK2 activation and a novel TLR-activated GRK2-pathway impairing cytoskeletal organization. Simultaneous GRK2/HDAC inhibition rescues susceptibility to infection after tissue injury.

Short-term toll-like receptor 9 inhibition leads to left ventricular wall thinning after myocardial infarction

AIMS

Ischaemia-reperfusion injury (IRI) following myocardial infarction remains a challenging topic in acute cardiac care and consecutively arising heart failure represents a severe long-term consequence. The extent of neutrophil infiltration and neutrophil-mediated cellular damage are thought to be aggravating factors enhancing primary tissue injury. Toll-like receptor 9 was found to be involved in neutrophil activation as well as chemotaxis and may represent a target in modulating IRI, aspects we aimed to illuminate by pharmacological inhibition of the receptor.

METHODS AND RESULTS

Forty-nine male adult Sprague-Dawley rats were used. IRI was induced by occlusion of the left coronary artery and subsequent snare removal after 30 min. Oligonucleotide (ODN) 2088, a toll-like receptor 9 (TLR9) antagonist, control-ODN, or DNase, were administered at the time of reperfusion and over 24 h via a mini-osmotic pump. The hearts were harvested 24 h or 4 weeks after left coronary artery occlusion and immunohistochemical staining was performed. Echocardiography was done after 1 and 4 weeks to determine ventricular function. Inhibition of TLR9 by ODN 2088 led to left ventricular wall thinning (P = 0.003) in association with drastically enhanced neutrophil infiltration (P = 0.005) and increased markers of tissue damage. Additionally, an up-regulation of the chemotactic receptor CXCR2 (P = 0.046) was found after TLR9 inhibition. No such effects were observed in control-ODN or DNase-treated animals. We did not observe changes in monocyte content or subset distribution, hinting towards neutrophils as the primary mediators of the exerted tissue injury.

CONCLUSIONS

Our data indicate a TLR9-dependent, negative regulation of neutrophil infiltration. Blockage of TLR9 appears to prevent the down-regulation of CXCR2, followed by an uncontrolled migration of neutrophils towards the area of infarction and the exertion of disproportional tissue injury resulting in potential aneurysm formation. In comparison with previous studies conducted in TLR^-/- mice, we deliberately chose a transient pharmacological inhibition of TLR9 to highlight effects occurring in the first 24 h following IRI.

The Regulation of Neutrophil Migration in Patients with Sepsis: The Complexity of the Molecular Mechanisms and Their Modulation in Sepsis and the Heterogeneity of Sepsis Patients

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Common causes include gram-negative and gram-positive bacteria as well as fungi. Neutrophils are among the first cells to arrive at an infection site where they function as important effector cells of the innate immune system and as regulators of the host immune response. The regulation of neutrophil migration is therefore important both for the infection-directed host response and for the development of organ dysfunctions in sepsis. Downregulation of CXCR4/CXCL12 stimulates neutrophil migration from the bone marrow. This is followed by transmigration/extravasation across the endothelial cell barrier at the infection site; this process is directed by adhesion molecules and various chemotactic gradients created by chemotactic cytokines, lipid mediators, bacterial peptides, and peptides from damaged cells. These mechanisms of neutrophil migration are modulated by sepsis, leading to reduced neutrophil migration and even reversed migration that contributes to distant organ failure. The sepsis-induced modulation seems to differ between neutrophil subsets. Furthermore, sepsis patients should be regarded as heterogeneous because neutrophil migration will possibly be further modulated by the infecting microorganisms, antimicrobial treatment, patient age/frailty/sex, other diseases (e.g., hematological malignancies and stem cell transplantation), and the metabolic status. The present review describes molecular mechanisms involved in the regulation of neutrophil migration; how these mechanisms are altered during sepsis; and how bacteria/fungi, antimicrobial treatment, and aging/frailty/comorbidity influence the regulation of neutrophil migration.

How are granulocytes for transfusion best used? The past, the present and the future

Granulocyte transfusions continue to be used in clinical practice, predominantly for treatment of refractory infection in the setting of severe neutropenia. There is biological plausibility for effectiveness in these patients with deficiencies of neutrophils, either as a consequence of disease or treatment. However, there is a chequered history of conducting and completing interventional trials to define optimal use, and many uncertainties remain regarding schedule and dose. Practice and clinical studies are severely limited by the short shelf life and viability of current products, which often restricts the timely access to granulocyte transfusions. In the future, methods are needed to optimise donor-derived granulocyte products. Options include use of manufactured neutrophils, expanded and engineered from stem cells. Further possibilities include manipulation of neutrophils to enhance their function and/or longevity. Granulocyte transfusions contain a heterogeneous mix of cells, and there is additional interest in how these transfusions may have immunomodulatory effects, including for potential uses as adjuncts for anti-cancer effects.

Endothelial Nox2 Limits Systemic Inflammation and Hypotension in Endotoxemia by Controlling Expression of Toll-Like Receptor 4

ABSTRACT

Leukocyte Nox2 is recognized to have a fundamental microbicidal function in sepsis but the specific role of Nox2 in endothelial cells (EC) remains poorly elucidated. Here, we tested the hypothesis that endothelial Nox2 participates in the pathogenesis of systemic inflammation and hypotension induced by LPS. LPS was injected intravenously in mice with Tie2-targeted deficiency or transgenic overexpression of Nox2. Mice with Tie2-targeted Nox2 deficiency had increased circulating levels of TNF-α, enhanced numbers of neutrophils trapped in lungs, and aggravated hypotension after LPS injection, as compared to control LPS-injected animals. In contrast, Tie2-driven Nox2 overexpression attenuated inflammation and prevented the hypotension induced by LPS. Because Tie2-Cre targets both EC and myeloid cells we generated bone marrow chimeric mice with Nox2 deletion restricted to leukocytes or ECs. Mice deficient in Nox2 either in leukocytes or ECs had reduced LPS-induced neutrophil trapping in the lungs and lower plasma TNF-α levels as compared to control LPS-injected mice. However, the pronounced hypotensive response to LPS was present only in mice with EC-specific Nox2 deletion. Experiments in vitro with human vein or aortic endothelial cells (HUVEC and HAEC, respectively) treated with LPS revealed that EC Nox2 controls NF-κB activation and the transcription of toll-like receptor 4 (TLR4), which is the recognition receptor for LPS. In conclusion, these results suggest that endothelial Nox2 limits NF-κB activation and TLR4 expression, which in turn attenuates the severity of hypotension and systemic inflammation induced by LPS.

Liver X Receptor Activation Impairs Neutrophil Functions and Aggravates Sepsis

BACKGROUND

Liver X receptors (LXRs) are nuclear receptors activated by oxidized lipids and were previously implicated in several metabolic development and inflammatory disorders. Although neutrophils express both LXR-α and LXR-β, the consequences of their activation, particularly during sepsis, remain unknown.

METHODS

We used the model of cecal ligation and puncture (CLP) to investigate the role of LXR activation during sepsis.

RESULTS

In this study, we verified that LXR activation reduces neutrophil chemotactic and killing abilities in vitro. Mice treated with LXR agonists showed higher sepsis-induced mortality, which could be associated with reduced neutrophil infiltration at the infectious foci, increased bacteremia, systemic inflammatory response, and multiorgan failure. In contrast, septic mice treated with LXR antagonist showed increased number of neutrophils in the peritoneal cavity, reduced bacterial load, and multiorgan dysfunction. More important, neutrophils from septic patients showed increased ABCA1 messenger ribonucleic acid levels (a marker of LXR activation) and impaired chemotactic response toward CXCL8 compared with cells from healthy individuals.

CONCLUSIONS

Therefore, our findings suggest that LXR activation impairs neutrophil functions, which might contribute to poor sepsis outcome.

The G Protein-Coupled Receptor Kinases (GRKs) in Chemokine Receptor-Mediated Immune Cell Migration: From Molecular Cues to Physiopathology

Although G protein-coupled receptor kinases (GRKs) have long been known to regulate G protein-coupled receptor (GPCR) desensitization, their more recently characterized functions as scaffolds and signalling adapters underscore that this small family of proteins governs a larger array of physiological functions than originally suspected. This review explores how GRKs contribute to the complex signalling networks involved in the migration of immune cells along chemokine gradients sensed by cell surface GPCRs. We outline emerging evidence indicating that the coordinated docking of several GRKs on an active chemokine receptor determines a specific receptor phosphorylation barcode that will translate into distinct signalling and migration outcomes. The guidance cues for neutrophil migration are emphasized based on several alterations affecting GRKs or GPCRs reported to be involved in pathological conditions.

Targeting nitric oxide as a key modulator of sepsis, arthritis and pain

Nitric oxide (NO) is produced by enzymatic activity of neuronal (nNOS), endothelial (eNOS), and inducible nitric oxide synthase (iNOS) and modulates a broad spectrum of physiological and pathophysiological conditions. The iNOS isoform is positively regulated at transcriptional level and produces high levels of NO in response to inflammatory mediators and/or to pattern recognition receptor signaling, such as Toll-like receptors. In this review, we compiled the main contributions of our group for understanding of the role of NO in sepsis and arthritis outcome and the peripheral contributions of NO to inflammatory pain development. Although neutrophil iNOS-derived NO is necessary for bacterial killing, systemic production of high levels of NO impairs neutrophil migration to infections through inhibiting neutrophil adhesion on microcirculation and their locomotion. Moreover, neutrophil-derived NO contributes to multiple organ dysfunction in sepsis. In arthritis, NO is chief for bacterial clearance in staphylococcal-induced arthritis; however, it contributes to articular damage and bone mass degradation. NO produced in inflammatory sites also downmodulates pain. The mechanism involved in analgesic effect and inhibition of neutrophil migration is dependent on the activation of the classical sGC/cGMP/PKG pathway. Despite the increasing number of studies performed after the identification of NO as an endothelium-derived relaxing factor, the underlying mechanisms of NO in inflammatory diseases remain unclear.

Diabetes Mellitus and Sepsis: A Challenging Association

Sepsis is a life-threatening organ dysfunction caused by a deregulated host response to infection. This inappropriate response to micro-organism invasion is characterized by an overwhelmed systemic inflammatory response and cardiovascular collapse that culminate in high mortality and morbidity in critical care units. The occurrence of sepsis in diabetes mellitus (DM) patients has become more frequent, as the prevalence of DM has increased dramatically worldwide. These two important diseases represent a global public health concern and highlight the importance of increasing our knowledge of the key elements of the immune response related to both conditions. In this context, it is well established that the cells taking part in the innate and adaptive immune responses in diabetic patients have compromised function. These altered responses favor micro-organism growth, a process that contributes to sepsis progression. The present review provides an update on the characteristics of the immune system in diabetic and septic subjects. We also explore the beneficial effects of insulin on the immune response in a glycemic control-dependent and independent manner.

Wip1 Deficiency Promotes Neutrophil Recruitment to the Infection Site and Improves Sepsis Outcome

Sepsis is defined as an uncontrolled host response to infection, and no specific therapy or drugs have been used in clinical trials currently. Discovering new therapeutic targets for sepsis treatment has always been a central problem in the field of sepsis research. Neutrophils stand at the first line in controlling infection and have been identified to be dysregulated with impaired migration and antimicrobial function during sepsis. Based on our previous results on demonstrating wild-type p53-induced phosphatase 1 in controlling neutrophil development, we explored the possible relationship among Wip1, neutrophils, and sepsis in the present study. Wip1-deficient mice exhibited improved outcomes in cecal ligation and puncture (CLP)-induced sepsis model with enhanced bacterial clearance and less multi-organ damage. The protection seen in Wip1 KO mice was mainly due to an increased accumulation of neutrophils in the primary infectious locus mediated by the decreased internalization of CXCR2, as well as by an increased antimicrobial function. Additionally, we also identified a negative correlation between CXCR2 and Wip1 in human neutrophils during sepsis. Pharmacological inhibition of Wip1 with its inhibitor can also prevent the internalization of CXCR2 on human neutrophils treated with lipopolysaccharides in vitro and significantly improve the outcome in CLP-induced sepsis model. Taken together, our results demonstrate that Wip1 can negatively regulate neutrophil migration and antimicrobial immunity during sepsis and inhibition of Wip1 can be a potential therapeutic target for sepsis treatment.

SOCS1 is a negative regulator of metabolic reprogramming during sepsis

Sepsis can induce an overwhelming systemic inflammatory response, resulting in organ damage and death. Suppressor of cytokine signaling 1 (SOCS1) negatively regulates signaling by cytokine receptors and Toll-like receptors (TLRs). However, the cellular targets and molecular mechanisms for SOCS1 activity during polymicrobial sepsis are unknown. To address this, we utilized a cecal ligation and puncture (CLP) model for sepsis; C57BL/6 mice subjected to CLP were then treated with a peptide (iKIR) that binds the SOCS1 kinase inhibitory region (KIR) and blocks its activity. Treatment with iKIR increased CLP-induced mortality, bacterial burden, and inflammatory cytokine production. Myeloid cell-specific SOCS1 deletion (Socs1Δmyel) mice were also more susceptible to sepsis, demonstrating increased mortality, higher bacterial loads, and elevated inflammatory cytokines, compared with Socs1fl littermate controls. These effects were accompanied by macrophage metabolic reprograming, as evidenced by increased lactic acid production and elevated expression of the glycolytic enzymes hexokinase, lactate dehydrogenase A, and glucose transporter 1 in septic Socs1Δmyel mice. Upregulation was dependent on the STAT3/HIF-1α/glycolysis axis, and blocking glycolysis ameliorated increased susceptibility to sepsis in iKIR-treated CLP mice. These results reveal a role of SOCS1 as a regulator of metabolic reprograming that prevents overwhelming inflammatory response and organ damage during sepsis.

Neutrophil dysregulation during sepsis: an overview and update

Sepsis remains a leading cause of death worldwide, despite advances in critical care, and understanding of the pathophysiology and treatment strategies. No specific therapy or drugs are available for sepsis. Neutrophils play a critical role in controlling infection under normal conditions, and it is suggested that their migration and antimicrobial activity are impaired during sepsis which contribute to the dysregulation of immune responses. Recent studies further demonstrated that interruption or reversal of the impaired migration and antimicrobial function of neutrophils improves the outcome of sepsis in animal models. In this review, we provide an overview of the associated mediators and signal pathways involved which govern the survival, migration and antimicrobial function of neutrophils in sepsis, and discuss the potential of neutrophils as a target to specifically diagnose and/or predict the outcome of sepsis.

Neutrophil Dysfunction in Sepsis

Objective:

Sepsis is defined as life-threatening organ dysfunction due to a dysregulated host response to infection. In this article, we reviewed the correlation between neutrophil dysfunction and sepsis.

Data Sources:

Articles published up to May 31, 2016, were selected from the PubMed databases, with the keywords of “neutrophil function”, “neutrophil dysfunction”, and “sepsis”.

Study Selection:

Articles were obtained and reviewed to analyze the neutrophil function in infection and neutrophil dysfunction in sepsis.

Results:

We emphasized the diagnosis of sepsis and its limitations. Pathophysiological mechanisms involve a generalized circulatory, immune, coagulopathic, and/or neuroendocrine response to infection. Many studies focused on neutrophil burst or cytokines. Complement activation, impairment of neutrophil migration, and endothelial lesions are involved in this progress. Alterations of cytokines, chemokines, and other mediators contribute to neutrophil dysfunction in sepsis.

Conclusions:

Sepsis represents a severe derangement of the immune response to infection, resulting in neutrophil dysfunction. Neutrophil dysfunction promotes sepsis and even leads to organ failure. Mechanism studies, clinical practice, and strategies to interrupt dysregulated neutrophil function in sepsis are desperately needed.

Mincle activation enhances neutrophil migration and resistance to polymicrobial septic peritonitis

Sepsis is a systemic inflammatory response to bacterial infection. The therapeutic options for treating sepsis are limited. Impaired neutrophil recruitment into the infection site is directly associated with severe sepsis, but the precise mechanism is unclear. Here, we show that Mincle plays a key role in neutrophil migration and resistance during polymicrobial sepsis. Mincle-deficient mice exhibited lower survival rates in experimental sepsis from cecal ligation and puncture and Escherichia coli–induced peritonitis. Mincle deficiency led to higher serum inflammatory cytokine levels and reduced bacterial clearance and neutrophil recruitment. Transcriptome analyses revealed that trehalose dimycolate, a Mincle ligand, reduced the expression of G protein–coupled receptor kinase 2 (GRK2) in neutrophils. Indeed, GRK2 expression was upregulated, but surface expression of the chemokine receptor CXCR2 was downregulated in blood neutrophils from Mincle-deficient mice with septic injury. Moreover, CXCL2-mediated adhesion, chemotactic responses, and F-actin polymerization were reduced in Mincle-deficient neutrophils. Finally, we found that fewer Mincle-deficient neutrophils infiltrated from the blood circulation into the peritoneal fluid in bacterial septic peritonitis compared with wild-type cells. Thus, our results indicate that Mincle plays an important role in neutrophil infiltration and suggest that Mincle signaling may provide a therapeutic target for treating sepsis.

Therapeutic potential and limitations of cholinergic anti-inflammatory pathway in sepsis

Sepsis is one of the main causes of mortality in hospitalized patients. Despite the recent technical advances and the development of novel generation of antibiotics, severe sepsis remains a major clinical and scientific challenge in modern medicine. Unsuccessful efforts have been dedicated to the search of therapeutic options to treat the deleterious inflammatory components of sepsis. Recent findings on neuronal networks controlling immunity raised expectations for novel therapeutic strategies to promote the regulation of sterile inflammation, such as autoimmune diseases. Interesting studies have dissected the anatomical constituents of the so-called "cholinergic anti-inflammatory pathway", suggesting that electrical vagus nerve stimulation and pharmacological activation of beta-2 adrenergic and alpha-7 nicotinic receptors could be alternative strategies for improving inflammatory conditions. However, the literature on infectious diseases, such as sepsis, is still controversial and, therefore, the real therapeutic potential of this neuroimmune pathway is not well defined. In this review, we will discuss the beneficial and detrimental effects of neural manipulation in sepsis, which depend on the multiple variables of the immune system and the nature of the infection. These observations suggest future critical studies to validate the clinical implications of vagal parasympathetic signaling in sepsis treatment.

Astragaloside IV alleviates E. coli-caused peritonitis via upregulation of neutrophil influx to the site of infection

Astragaloside IV (AS-IV), an active saponin purified from Astragali Radix, has been identified with broad biological and pharmacological activities. In the present study, we continue to explore the potential effect of AS-IV on antibacterial response using an acute E. coli peritoneal infection model. Our findings implied that administration of AS-IV decreases mortality in mice challenged by lethal E. coli infection. The protection of AS-IV was related to promotion of neutrophil extravasation into the peritoneum and bacterial clearance. Toll-like receptor (TLR) activation in neutrophils has been reported to reduce CXCR2 expression and subsequent neutrophil migration. Our data indicated that AS-IV prevented the reduction of CXCR2 expression and neutrophil migration induced by LPS, the activator for TLR4. Moreover, we found that AS-IV blocks LPS-induced suppression of CXCR2 on neutrophils by inhibiting the expression of G protein-coupled receptor kinase-2 (GRK2), an agonist that regulates desensitization and internalization of chemokine receptors. Taken together, these data propose that AS-IV, through modulating GRK2-CXCR2 signal in neutrophils, offers an essential efficacy on host antibacterial immunity.

Role of Mitochondrial DNA in Septic AKI via Toll-Like Receptor 9

Toll-like receptor 9 (TLR9) contributes to the development of polymicrobial septic AKI. However, the mechanisms that activate the TLR9 pathway and cause kidney injury during sepsis remain unknown. To determine the role of mitochondrial DNA (mtDNA) in TLR9-associated septic AKI, we established a cecal ligation and puncture (CLP) model of sepsis in wild-type (WT) and Tlr9-knockout (Tlr9KO) mice. We evaluated systemic circulation and peritoneal cavity dynamics and immune response and tubular mitochondrial dysfunction to determine upstream and downstream effects on the TLR9 pathway, respectively. CLP increased mtDNA levels in the plasma and peritoneal cavity of WT and Tlr9KO mice in the early phase, but the increase in the peritoneal cavity was significantly higher in Tlr9KO mice than in WT mice. Concomitantly, leukocyte migration to the peritoneal cavity increased, and plasma cytokine production and splenic apoptosis decreased in Tlr9KO mice compared with WT mice. Furthermore, CLP-generated renal mitochondrial oxidative stress and mitochondrial vacuolization in the proximal tubules in the early phase were reversed in Tlr9KO mice. To elucidate the effects of mtDNA on immune response and kidney injury, we intravenously injected mice with mitochondrial debris (MTD), including substantial amounts of mtDNA. MTD caused an immune response similar to that induced by CLP, including upregulated levels of plasma IL-12, splenic apoptosis, and mitochondrial injury, but this effect was attenuated by Tlr9KO. Moreover, MTD-induced renal mitochondrial injury was abolished by DNase pretreatment. These findings suggest that mtDNA activates TLR9 and contributes to cytokine production, splenic apoptosis, and kidney injury during polymicrobial sepsis.

Paradoxical Roles of the Neutrophil in Sepsis: Protective and Deleterious

Sepsis, an overwhelming inflammatory response syndrome secondary to infection, is one of the costliest and deadliest medical conditions worldwide. Neutrophils are classically considered to be essential players in the host defense against invading pathogens. However, several investigations have shown that impairment of neutrophil migration to the site of infection, also referred to as neutrophil paralysis, occurs during severe sepsis, resulting in an inability of the host to contain and eliminate the infection. On the other hand, the neutrophil antibacterial arsenal contributes to tissue damage and the development of organ dysfunction during sepsis. In this review, we provide an overview of the main events in which neutrophils play a beneficial or deleterious role in the outcome of sepsis.

Toll-like receptor 9 signaling in dendritic cells regulates neutrophil recruitment to inflammatory foci following Leishmania infantum infection

Leishmania infantum is a protozoan parasite that causes visceral leishmaniasis (VL). This infection triggers dendritic cell (DC) activation through the recognition of microbial products by Toll-like receptors (TLRs). Among the TLRs, TLR9 is required for DC activation by different Leishmania species. We demonstrated that TLR9 is upregulated in vitro and in vivo during infection. We show that C57BL/6 mice deficient in TLR9 expression (TLR9(-/-) mice) are more susceptible to infection and display higher parasite numbers in the spleen and liver. The increased susceptibility of TLR9(-/-) mice was due to the impaired recruitment of neutrophils to the infection foci associated with reduced levels of neutrophil chemoattractants released by DCs in the target organs. Moreover, both Th1 and Th17 cells were also committed in TLR9(-/-) mice. TLR9-dependent neutrophil recruitment is mediated via the MyD88 signaling pathway but is TIR domain-containing adapter-inducing interferon beta (TRIF) independent. Furthermore, L. infantum failed to activate both plasmacytoid and myeloid DCs from TLR9(-/-) mice, which presented reduced surface costimulatory molecule expression and chemokine release. Interestingly, neutrophil chemotaxis was affected both in vitro and in vivo when DCs were derived from TLR9(-/-) mice. Our results suggest that TLR9 plays a critical role in neutrophil recruitment during the protective response against L. infantum infection that could be associated with DC activation.

MyD88-, but not Nod1- and/or Nod2-deficient mice, show increased susceptibility to polymicrobial sepsis due to impaired local inflammatory response

Pathogen recognition and triggering of the inflammatory response following infection in mammals depend mainly on Toll-like and Nod-like receptors. Here, we evaluated the role of Nod1, Nod2 and MyD88-dependent signaling in the chemokine production and neutrophil recruitment to the infectious site during sepsis induced by cecal ligation and puncture (CLP) in C57Bl/6 mice. We demonstrate that Nod1 and Nod2 are not involved in the release of chemokines and recruitment of neutrophils to the infectious site during CLP-induced septic peritonitis because these events were similar in wild-type, Nod1-, Nod2-, Nod1/Nod2- and Rip2-deficient mice. Consequently, the local and systemic bacterial loads were not altered. Accordingly, neither Nod1 nor Nod2 was involved in the production of the circulating cytokines and in the accumulation of leukocytes in the lungs. By contrast, we showed that MyD88-dependent signaling is crucial for the establishment of the local inflammatory response during CLP-induced sepsis. MyD88-deficient mice were susceptible to sepsis because of an impaired local production of chemokines and defective neutrophil recruitment to the infection site. Altogether, these data show that Nod1, Nod2 and Rip2 are not required for local chemokine production and neutrophil recruitment during CLP-induced sepsis, and they reinforce the importance of MyD88-dependent signaling for initiation of a protective host response.

Mitochondrial damage-associated molecular patterns released by abdominal trauma suppress pulmonary immune responses

BACKGROUND

Historically, fever, pneumonia, and sepsis after trauma are ascribed to pain and poor pulmonary toilet. No evidence supports that assertion however, and no known biologic mechanisms link injury to infection. Our studies show that injured tissues release mitochondria (MT). Mitochondrial damage-associated molecular patterns (mtDAMPs) however can mimic bacterial pathogen-associated danger molecules and attract neutrophils (PMN). We hypothesized that mtDAMPs from traumatized tissue divert neutrophils from the lung, causing susceptibility to infection.

METHODS

Anesthetized rats (6-10 per group) underwent pulmonary contusion (PC) by chest percussion. When modeling traumatic MT release, some rats had MT isolated from the liver (equal to 5% liver necrosis) injected intraperitoneally (IPMT). Negative controls had PC plus buffer intraperitoneally. Positive controls underwent PC plus cecal ligation and puncture. At 16 hours, bronchoalveolar and peritoneal lavages were performed. Bronchoalveolar lavage fluid (BALF) and peritoneal lavage fluid were assayed for PMN count, albumin, interleukin β, (IL-β), and CINC-1. Assays were normalized to blood urea nitrogen to calculate absolute concentrations.

RESULTS

PC caused alveolar IL-1β and CINC production and a 34-fold increase in BALF neutrophils. As expected, IPMT increased peritoneal IL-1β and CINC and attracted PMN to the abdomen. However, remarkably, IPMT after PC attenuated BALF cytokine accumulation and decreased BALF PMN. Cecal ligation and puncture had no direct effect on BALF PMNs but, like IPMT, blunted BALF leukocytosis after PC.

CONCLUSION

Rather than acting as a "second hit" to enhance PMN-mediated lung injury, mtDAMPs from trauma and pathogen-associated danger molecules from peritoneal infection diminish PMN accumulation in a contused lung. This may make the lung susceptible to pneumonia. This paradigm provides a novel mechanistic model of the relationship among blunt tissue trauma, systemic inflammation, and pneumonia that can be studied to improve trauma outcomes.

Evaluation of Toll-like, chemokine, and integrin receptors on monocytes and neutrophils from peripheral blood of septic patients and their correlation with clinical outcomes

Recognition of pathogens is performed by specific receptors in cells of the innate immune system, which may undergo modulation during the continuum of clinical manifestations of sepsis. Monocytes and neutrophils play a key role in host defense by sensing and destroying microorganisms. This study aimed to evaluate the expression of CD14 receptors on monocytes; CD66b and CXCR2 receptors on neutrophils; and TLR2, TLR4, TLR5, TLR9, and CD11b receptors on both cell types of septic patients. Seventy-seven septic patients (SP) and 40 healthy volunteers (HV) were included in the study, and blood samples were collected on day zero (D0) and after 7 days of therapy (D7). Evaluation of the cellular receptors was carried out by flow cytometry. Expression of CD14 on monocytes and of CD11b and CXCR2 on neutrophils from SP was lower than that from HV. Conversely, expression of TLR5 on monocytes and neutrophils was higher in SP compared with HV. Expression of TLR2 on the surface of neutrophils and that of TLR5 on monocytes and neutrophils of SP was lower at D7 than at D0. In addition, SP who survived showed reduced expression of TLR2 and TLR4 on the surface of neutrophils at D7 compared to D0. Expression of CXCR2 for surviving patients was higher at follow-up compared to baseline. We conclude that expression of recognition and cell signaling receptors is differentially regulated between SP and HV depending on the receptor being evaluated.

PPAR-γ/IL-10 axis inhibits MyD88 expression and ameliorates murine polymicrobial sepsis

Polymicrobial sepsis induces organ failure and is accompanied by overwhelming inflammatory response and impairment of microbial killing. Peroxisome proliferator-activated receptor (PPAR)-γ is a nuclear receptor with pleiotropic effects on lipid metabolism, inflammation, and cell proliferation. The insulin-sensitizing drugs thiazolidinediones (TZDs) are specific PPAR-γ agonists. TZDs exert anti-inflammatory actions in different disease models, including polymicrobial sepsis. The TZD pioglitazone, which has been approved by the U.S. Food and Drug Administration, improves sepsis outcome; however, the molecular programs that mediate its effect have not been determined. In a murine model of sepsis, we now show that pioglitazone treatment improves microbial clearance and enhances neutrophil recruitment to the site of infection. We also observed reduced proinflammatory cytokine production and high IL-10 levels in pioglitazone-treated mice. These effects were associated with a decrease in STAT-1-dependent expression of MyD88 in vivo and in vitro. IL-10R blockage abolished PPAR-γ-mediated inhibition of MyD88 expression. These data demonstrate that the primary mechanism by which pioglitazone protects against polymicrobial sepsis is through the impairment of MyD88 responses. This appears to represent a novel regulatory program. In this regard, pioglitazone provides advantages as a therapeutic tool, because it improves different aspects of host defense during sepsis, ultimately enhancing survival.

Neutrophils in innate and adaptive immunity

Neutrophils have long been viewed as short-lived cells crucial for the elimination of extracellular pathogens, possessing a limited role in the orchestration of the immune response. This dogma has been challenged by recent lines of evidence demonstrating the expression of an increasing number of cytokines and effector molecules by neutrophils. Moreover, in analogy with their "big brother" macrophages, neutrophils integrate the environmental signals and can be polarized towards an antitumoural or protumoural phenotype. Neutrophils are a major source of humoral fluid phase pattern recognition molecules and thus contribute to the humoral arm of innate immunity. Neutrophils cross talk and shape the maturation and effector functions of other leukocytes in a direct or indirect manner, through cell-cell contact or cytokine production, respectively. Therefore, neutrophils are integrated in the activation and regulation of the innate and adaptive immune system and play an important role in the resolution or exacerbation of diverse pathologies, including infections, chronic inflammation, autoimmunity and cancer.