Alterations in membrane cholesterol cause mobilization of lipid rafts from specific granules and prime human neutrophils for enhanced adherence-dependent oxidant production

Combustion-derived carbon nanoparticles cause delayed apoptosis in neutrophil-like HL-60 cells in vitro and in primed human neutrophilic granulocytes ex vivo

BACKGROUND

Inhalation of combustion-derived nanoparticles may contribute to the development or exacerbation of inflammatory lung diseases by direct interaction with neutrophilic granulocytes. Earlier studies have shown that exposure of human neutrophils to carbon nanoparticles ex vivo causes a prolongation of cellular life by the reduction of apoptosis rates. Accordingly, reduced neutrophil apoptosis rates were observed in neutrophils from bronchoalveolar lavages from carbon nanoparticle-exposed animals. The current study describes molecular and cellular modes of action responsible for this proinflammatory effect.

RESULTS

Experiments with human blood neutrophils or neutrophil-like differentiated HL-60 cells exposed to carbon nanoparticles revealed dose dependent reduction of apoptosis rates. In both experimental systems, intracellular reactive oxygen species proved to be causally linked to this endpoint. Among the human samples, only primed cells from donors with slightly elevated proinflammatory plasma factors responded by delayed apoptosis. These neutrophils are characterized by an immunophenotype (CD16bright CD62Ldim) which is also observed in inflammatory lung diseases. Upon exposure to carbon nanoparticles these cells are further activated in an oxidant dependent manner. This activation appears to be linked to reduced apoptosis as samples with unchanged apoptosis rates were also not responding at this level. As reactive oxygen species triggered by carbon nanoparticles are known to cause membrane rearrangements, lipid raft structures were investigated by ganglioside M1 staining. Exposure of neutrophils resulted in a reduction of raft structures which could be prevented by an antioxidant strategy. The destruction of lipid rafts by depleting cholesterol also caused an activated immunophenotype and delayed apoptosis, indicating that membrane rearrangements after carbon nanoparticle exposure in primed neutrophils are responsible for cell activation and delayed apoptosis.

CONCLUSIONS

The antiapoptotic reactions observed in two independent experimental systems, differentiated neutrophil-like HL-60 cells and primed neutrophils, may be considered as additional proinflammatory effect of inhaled combustion-derived nanoparticles. Particularly in chronic diseases, which are characterized by neutrophilic lung inflammation, this effect can be expected to contribute to the deterioration of the health status. The data describe a mode of action in which intracellular reactive oxygen species cause membrane rearrangements that are responsible for neutrophil activation and delayed apoptosis.

The influence of interleukin-27 on metabolic fitness in a murine neonatal model of bacterial sepsis

Human neonates are predisposed to an increased risk of mortality from infection due to fundamental differences in the framework of innate and adaptive immune responses relative to those in the adult population. As one key difference in neonates, an increase in the immunosuppressive cytokine, IL-27, is responsible for poor outcomes in a murine neonatal model of bacterial sepsis. In our model, the absence of IL-27 signaling during infection is associated with improved maintenance of body mass, increased bacterial clearance with reduced systemic inflammation, and decreased mortality rates that correlate to preservation of glucose homeostasis and insulin production. To further elucidate the mechanisms associated with IL-27 signaling and metabolic fitness, we analyzed global transcriptomes from spleen, liver, pancreas, and hindlimb muscle during Escherichia coli-induced sepsis in wild-type (WT) and IL-27Rα-deficient (KO) mice. Metabolically important tissues such as the liver, pancreas, and hindlimb muscle exhibit a shift in differential gene expression of pathways involved in oxidative phosphorylation, glycolysis, gluconeogenesis, lipid metabolism, and fatty acid β oxidation. The hindlimb muscle of KO pups demonstrated a significant reduction in all of these pathways during infection. The KO liver showed a significant down-regulation in gluconeogenesis and glycolytic pathways. Collectively, these findings suggest a negative influence of IL-27 on the metabolic profile during early-life infection. This is an important consideration for antagonization of IL-27 as a potential host-directed therapeutic opportunity as our findings point to an overall improvement in infectious disease parameters and metabolic fitness.NEW & NOTEWORTHY IL-27 has been linked with immune regulation during infection, but this is the first report of a combined influence of IL-27 on complete host response during systemic infection with metabolic fitness in a neonate. Novel findings demonstrate improved glucose homeostasis and insulin response supported by a reduced expression of genes involved in gluconeogenesis in the absence of IL-27 signaling. An increased expression of genes integral to cholesterol biosynthesis further supports a protective response during sepsis.

Endoplasmic Reticulum Protein 72 Regulates Integrin Mac-1 Activity to Influence Neutrophil Recruitment

BACKGROUND

Integrins mediate the adhesion, crawling, and migration of neutrophils during vascular inflammation. Thiol exchange is important in the regulation of integrin functions. ERp72 (endoplasmic reticulum-resident protein 72) is a member of the thiol isomerase family responsible for the catalysis of disulfide rearrangement. However, the role of ERp72 in the regulation of Mac-1 (integrin αMβ2) on neutrophils remains elusive.

METHODS

Intravital microscopy of the cremaster microcirculation was performed to determine in vivo neutrophil movement. Static adhesion, flow chamber, and flow cytometry were used to evaluate in vitro integrin functions. Confocal fluorescent microscopy and coimmunoprecipitation were utilized to characterize the interactions between ERp72 and Mac-1 on neutrophil surface. Cell-impermeable probes and mass spectrometry were used to label reactive thiols and identify target disulfide bonds during redox exchange. Biomembrane force probe was performed to quantitatively measure the binding affinity of Mac-1. A murine model of acute lung injury induced by lipopolysaccharide was utilized to evaluate neutrophil-associated vasculopathy.

RESULTS

ERp72-deficient neutrophils exhibited increased rolling but decreased adhesion/crawling on inflamed venules in vivo and defective static adhesion in vitro. The defect was due to defective activation of integrin Mac-1 but not LFA-1 (lymphocyte function-associated antigen-1) using blocking or epitope-specific antibodies. ERp72 interacted with Mac-1 in lipid rafts on neutrophil surface leading to the reduction of the C654-C711 disulfide bond in the αM subunit that is critical for Mac-1 activation. Recombinant ERp72, via its catalytic motifs, increased the binding affinity of Mac-1 with ICAM-1 (intercellular adhesion molecule-1) and rescued the defective adhesion of ERp72-deficient neutrophils both in vitro and in vivo. Deletion of ERp72 in the bone marrow inhibited neutrophil infiltration, ameliorated tissue damage, and increased survival during murine acute lung injury.

CONCLUSIONS

Extracellular ERp72 regulates integrin Mac-1 activity by catalyzing disulfide rearrangement on the αM subunit and may be a novel target for the treatment of neutrophil-associated vasculopathy.

Myeloid spatial and transcriptional molecular signature of ischemia-reperfusion injury in human liver transplantation

BACKGROUND

Ischemia-reperfusion injury (IRI) is a significant clinical concern in liver transplantation, with a key influence on short-term and long-term allograft and patient survival. Myeloid cells trigger and sustain tissue inflammation and damage associated with IRI, but the mechanisms regulating these activities are unknown. To address this, we investigated the molecular characteristics of intragraft myeloid cells present in biopsy-proven IRI- and IRI+ liver transplants.

METHODS

RNA-sequencing was performed on 80 pre-reperfusion and post-reperfusion biopsies from 40 human recipients of liver transplantation (23 IRI+, 17 IRI-). We used transcriptional profiling and computational approaches to identify specific gene coexpression network modules correlated with functional subsets of MPO+, lysozyme+, and CD68+ myeloid cells quantified by immunohistochemistry on sequential sections from the same patient biopsies.

RESULTS

A global molecular map showed gene signatures related to myeloid activation in all patients regardless of IRI status; however, myeloid cell subsets differed dramatically in their spatial morphology and associated gene signatures. IRI- recipients were found to have a natural corticosteroid production and response profile from pre-reperfusion to post-reperfusion, particularly among monocytes/macrophages. The pre-reperfusion signature of IRI+ recipients included acute inflammatory responses in neutrophils and increased translation of adaptive immune-related genes in monocytes/macrophages coupled with decreased glucocorticoid responses. Subsequent lymphocyte activation at post-reperfusion identified transcriptional programs associated with the transition to adaptive immunity found only among IRI+ recipients.

CONCLUSIONS

Myeloid subset-specific genes and related signaling pathways provide targets for the development of therapeutic strategies aimed at limiting IRI in the clinical setting of liver transplantation.

Alpha-1 Antitrypsin Therapy Modifies Neutrophil Adhesion in Patients with Obstructive Lung Disease

Alpha-1 antitrypsin (AAT) deficiency (AATD) is characterized by neutrophil-dominated inflammation resulting in emphysema. The cholesterol-rich neutrophil outer plasma membrane plays a central role in adhesion and subsequent transmigration to underlying tissues. This study aimed to investigate mechanisms of increased neutrophil adhesion in AATD, and whether AAT augmentation therapy abrogates this effect. Plasma and blood neutrophils were donated by healthy controls (n=20), AATD (n=30) and AATD patients post AAT augmentation therapy (n=6). Neutrophil membrane protein expression was investigated using liquid chromatography-tandem mass spectrometry. The effect of once weekly intravenous AAT augmentation therapy was assessed by ELISAs, and calcium fluorometric, μ-calpain and cell adhesion assays. Decreased neutrophil plasma membrane cholesterol content (P=0.03), yet increased abundance of integrin alpha-M (fold change 1.91), integrin alpha-L (fold change 3.76) and cytoskeletal adaptor proteins including talin-1 (fold change 4.04), were detected on AATD neutrophil plasma membrane fractions. The described inflammatory induced structural changes were a result of >2 fold increased cytosolic calcium levels (P=0.02), leading to significant calcium dependent μ-calpain activity (3.5 fold change, P=0.005), resulting in proteolysis of the membrane cholesterol trafficking protein caveolin-1. Treatment of AAT-deficient individuals with AAT augmentation therapy resulted in increased caveolin-1 and membrane cholesterol content (111.8 ± 15.5 vs 64.18 ± 7.8 µg/ 2x10⁷ cells pre- and post-treatment respectively, P=0.02), with concurrent decreased neutrophil integrin expression and adhesion. Results demonstrate an auxiliary benefit of AAT augmentation therapy, evident by a decrease in circulating inflammation and controlled neutrophil adhesion.

The Many Roles of Cholesterol in Sepsis: A Review

RATIONALE

The biological functions of cholesterol are diverse, ranging from cell membrane integrity and signalling, immunity, to the synthesis of steroid and sex hormones, Vitamin D, bile acids and oxysterols. Multiple studies have demonstrated hypocholesterolemia in sepsis, the degree of which is an excellent prognosticator of poor outcomes. However, the clinical significance of hypocholesterolemia has been largely unrecognized.

OBJECTIVES/METHODS

We undertook a detailed review of the biological roles of cholesterol, the impact of sepsis, its reliability as a prognosticator in sepsis, and the potential utility of cholesterol as a treatment.

MEASUREMENTS AND MAIN RESULTS

Sepsis affects cholesterol synthesis, transport and metabolism. This likely impacts upon its biological functions including immunity, hormone and vitamin production, and cell membrane receptor sensitivity. Early preclinical studies show promise for cholesterol as a pleiotropic therapeutic agent.

CONCLUSIONS

Hypocholesterolemia is a frequent condition in sepsis and an important early prognosticator. Low plasma levels are associated with wider changes in cholesterol metabolism and its functional roles, and these appear to play a significant role in sepsis pathophysiology. The therapeutic impact of cholesterol elevation warrants further investigation.

Fluid shear stress-mediated mechanotransduction in circulating leukocytes and its defect in microvascular dysfunction

Leukocytes (neutrophils, monocytes) in the active circulation exhibit multiple phenotypic indicators for a low level of cellular activity, like lack of pseudopods and minimal amounts of activated, cell-adhesive integrins on their surfaces. In contrast, before these cells enter the circulation in the bone marrow or when they recross the endothelium into extravascular tissues of peripheral organs they are fully activated. We review here a multifaceted mechanism mediated by fluid shear stress that can serve to deactivate leukocytes in the circulation. The fluid shear stress controls pseudopod formation via the FPR receptor, the same receptor responsible for pseudopod projection by localized actin polymerization. The bioactivity of macromolecular factors in the blood plasma that interfere with receptor stimulation by fluid flow, such as proteolytic cleavage in the extracellular domain of the receptor or the membrane actions of cholesterol, leads to a defective ability to respond to fluid shear stress by actin depolymerization. The cell reaction to fluid shear involves CD18 integrins, nitric oxide, cGMP and Rho GTPases, is attenuated in the presence of inflammatory mediators and modified by glucocorticoids. The mechanism is abolished in disease models (genetic hypertension and hypercholesterolemia) leading to an increased number of activated leukocytes in the circulation with enhanced microvascular resistance and cell entrapment. In addition to their role in binding to biochemical agonists/antagonists, membrane receptors appear to play a second role: to monitor local fluid shear stress levels. The fluid shear stress control of many circulating cell types such as lymphocytes, stem cells, tumor cells remains to be elucidated.

How Long Does a Neutrophil Live?-The Effect of 24 h Whole Blood Storage on Neutrophil Functions in Pigs

Neutrophils are important effector cells of the innate immune system, traditionally regarded to have a short life span. The goal of this study was to evaluate the effect of the whole blood storage on neutrophil functions, e.g., viability, antimicrobial effect, neutrophil extracellular trap (NET) formation and phagocytosis. Therefore, fresh porcine whole blood was compared to whole blood stored for 24 h in the dark at room temperature. Different cell parameters in whole blood and in isolated neutrophils were analyzed. The following parameters were analyzed: cell count, band and segmented neutrophil count, viability, cholesterol content, release of free DNA as a marker for cell death, phagocytic activity in whole blood and in isolated neutrophils, the transmigration rate of neutrophils to IL8 stimulus, the production of reactive oxygen species (ROS), and the formation of NETs. It was observed that the number of isolated neutrophils decreased over time, indicating cell death occurs during 24 h of blood storage. However, the surviving neutrophils isolated from stored blood reacted comparably or even showed enhanced antimicrobial activity in the case of phagocytosis of Streptococcus (S.) suis, ROS production, and transmigration. The slightly altered cholesterol level of the harvested neutrophils in stored blood when compared to fresh blood partially explains some of the detected differences.

High-Density Lipoprotein from Chronic Kidney Disease Patients Modulates Polymorphonuclear Leukocytes

The anti-inflammatory properties of high-density lipoproteins (HDL) are lost in uremia. These HDL may show pro-inflammatory features partially as a result of changed protein composition. Alterations of polymorphonuclear leukocytes (PMNLs) in chronic kidney disease (CKD) may contribute to chronic inflammation and high vascular risk. We investigated if HDL from uremic patients is related to systemic inflammation by interfering with PMNL function. PMNL apoptosis was investigated by assessing morphological features and DNA content. CD11b surface expression was quantified by flow cytometry. Oxidative burst was measured via cytochrome c reduction assay. Chemotaxis was assessed by using an under-agarose migration assay. We found that HDL from CKD and hemodialysis (HD) patients significantly attenuated PMNL apoptosis, whereas HDL isolated from healthy subjects had no effect on PMNL apoptosis. The use of signal transduction inhibitors indicated that uremic HDL exerts anti-apoptotic effects by activating pathways involving phosphoinositide 3-kinase and extracellular-signal regulated kinase. Healthy HDL attenuated the surface expression of CD11b, whereas HDL from CKD and HD patients had no effect. All tested isolates increased the stimulation of oxidative burst, but did not affect PMNL chemotactic movement. In conclusion, HDL may contribute to the systemic inflammation in uremic patients by modulating PMNL functions.

Neutrophil Membrane Cholesterol Content is a Key Factor in Cystic Fibrosis Lung Disease

Background

Identification of mechanisms promoting neutrophil trafficking to the lungs of patients with cystic fibrosis (CF) is a challenge for next generation therapeutics. Cholesterol, a structural component of neutrophil plasma membranes influences cell adhesion, a key step in transmigration. The effect of chronic inflammation on neutrophil membrane cholesterol content in patients with CF (PWCF) remains unclear. To address this we examined neutrophils of PWCF to evaluate the cause and consequence of altered membrane cholesterol and identified the effects of lung transplantation and ion channel potentiator therapy on the cellular mechanisms responsible for perturbed membrane cholesterol and increased cell adhesion.

Methodology

PWCF homozygous for the ΔF508 mutation or heterozygous for the G551D mutation were recruited (n = 48). Membrane protein expression was investigated by mass spectrometry. The effect of lung transplantation or ivacaftor therapy was assessed by ELISAs, and calcium fluorometric and μ-calpain assays.

Findings

Membranes of CF neutrophils contain less cholesterol, yet increased integrin CD11b expression, and respond to inflammatory induced endoplasmic reticulum (ER) stress by activating μ-calpain. In vivo and in vitro, increased μ-calpain activity resulted in proteolysis of the membrane cholesterol trafficking protein caveolin-1. The critical role of caveolin-1 for adequate membrane cholesterol content was confirmed in caveolin-1 knock-out mice. Lung transplant therapy or treatment of PWCF with ivacaftor, reduced levels of circulating inflammatory mediators and actuated increased caveolin-1 and membrane cholesterol, with concurrent normalized neutrophil adhesion.

Interpretation

Results demonstrate an auxiliary benefit of lung transplant and potentiator therapy, evident by a reduction in circulating inflammation and controlled neutrophil adhesion.

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This study explored neutrophil adhesion in cystic fibrosis.

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Altered membrane cholesterol lead to increased adhesion.

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Circulating inflammatory mediators caused increased calpain activity and reduced membrane cholesterol content.

In patients with cystic fibrosis (CF), chronic inflammation in the circulation, in part originating from the pulmonary compartment, leads to decreased membrane cholesterol in circulating neutrophils, resulting in increased cell adhesion. The mechanism of action involves proteolytic down-regulation of the cholesterol trafficking protein caveolin-1. The overall effect of lung transplant therapy, or CFTR potentiator treatment, was to significantly diminish the circulating inflammatory burden thereby permitting caveolin-1 expression, with concomitant decreased CF cell adhesion and significant clinical improvement.

NOX2 is critical for heterotypic neutrophil-platelet interactions during vascular inflammation

Platelet-leukocyte interactions on activated endothelial cells play an important role during microvascular occlusion under oxidative stress conditions. However, it remains poorly understood how neutrophil-platelet interactions are regulated during vascular inflammation. By using intravital microscopy with mice lacking nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) and their bone marrow chimera, we demonstrated that NOX2 from both hematopoietic and endothelial cells is crucial for neutrophil-platelet interactions during tumor necrosis factor alpha-induced venular inflammation. Platelet NOX2-produced reactive oxygen species (ROS) regulated P-selectin exposure upon agonist stimulation and the ligand-binding function of glycoprotein Ibα. Furthermore, neutrophil NOX2-generated ROS enhanced the activation and ligand-binding activity of αMβ2 integrin following N-formyl-methionyl-leucyl phenylalanine stimulation. Studies with isolated cells and a mouse model of hepatic ischemia/reperfusion injury revealed that NOX2 from both platelets and neutrophils is required for cell-cell interactions, which contribute to the pathology of hepatic ischemia/reperfusion injury. Platelet NOX2 modulated intracellular Ca(2+) release but not store-operated Ca(2+) entry (SOCE), whereas neutrophil NOX2 was crucial for SOCE but not intracellular Ca(2+) release. Different regulation of Ca(2+) signaling by platelet and neutrophil NOX2 correlated with differences in the phosphorylation of AKT, ERK, and p38MAPK. Our results indicate that platelet and neutrophil NOX2-produced ROS are critical for the function of surface receptors essential for neutrophil-platelet interactions during vascular inflammation.

Role of lipid raft components and actin cytoskeleton in fibronectin-binding, surface expression, and de novo synthesis of integrin subunits in PGE2- or 8-Br-cAMP-stimulated mastocytoma P-815 cells

Integrins are heterodimeric adhesion receptors essential for adhesion of non-adherent cells to extracellular ligands such as extracellular matrix components. The affinity of integrins for ligands is regulated through a process termed integrin activation and de novo synthesis. Integrin activation is regulated by lipid raft components and the actin structure. However, there is little information on the relationship between integrin activation and its de novo synthesis. Cancerous mouse mast cells, mastocytoma P-815 cells (P-815 cells) are known to bind to fibronectin through de novo synthesis of integrin subtypes by prostaglandin (PG) E2 stimulation. The purpose of this study was to clarify the relationship between lipid raft components and the actin cytoskeleton, and PGE2-induced P-815 cells adhesion to fibronectin and the increase in surface expression and mRNA and protein levels of αvβ3 and αIIbβ3 integrins. Cholesterol inhibitor 6-O-α-maltosyl-β cyclodextrin, glycosylphosphatidylinositol-anchored proteins inhibitor phosphatidylinositol-specific phospholipase C and actin inhibitor cytochalasin D inhibited PGE2-induced cell adhesion to fibronectin, but did not regulate the surface expression and mRNA and protein levels of αv and αIIb, and β3 integrin subunits. In addition, inhibitor of integrin modulate protein CD47 had no effect on PGE2- and 8-Br-cAMP-induced cell adhesion. These results suggest that lipid raft components and the actin cytoskeleton are directly involved in increasing of adhesion activity of integrin αIIb, αv and β3 subunits to fibronectin but not in stimulating of de novo synthesis of them in PGE2-stimulated P-815 cells. The modulation of lipid rafts and the actin structure is essential for P-815 cells adhesion to fibronectin.

Biophysical description of multiple events contributing blood leukocyte arrest on endothelium

Blood leukocytes have a remarkable capacity to bind to and stop on specific blood vessel areas. Many studies have disclosed a key role of integrin structural changes following the interaction of rolling leukocytes with surface-bound chemoattractants. However, the functional significance of structural data and mechanisms of cell arrest are incompletely understood. Recent experiments revealed the unexpected complexity of several key steps of cell-surface interaction: (i) ligand-receptor binding requires a minimum amount of time to proceed and this is influenced by forces. (ii) Also, molecular interactions at interfaces are not fully accounted for by the interaction properties of soluble molecules. (iii) Cell arrest depends on nanoscale topography and mechanical properties of the cell membrane, and these properties are highly dynamic. Here, we summarize these results and we discuss their relevance to recent functional studies of integrin-receptor association in cells from a patient with type III leukocyte adhesion deficiency. It is concluded that an accurate understanding of all physical events listed in this review is needed to unravel the precise role of the multiple molecules and biochemical pathway involved in arrest triggering.

Extracellular protein disulfide isomerase regulates ligand-binding activity of αMβ2 integrin and neutrophil recruitment during vascular inflammation

β2 integrins play a crucial role during neutrophil recruitment into the site of vascular inflammation. However, it remains unknown how ligand-binding activity of the integrin is regulated. Using fluorescence intravital microscopy in mice generated by crossing protein disulfide isomerase (PDI) floxed mice with lysozyme-Cre transgenic mice, we demonstrate that neutrophil PDI is required for neutrophil adhesion and crawling during tumor necrosis factor-α-induced vascular inflammation in vivo. Rescue experiments show that the isomerase activity of extracellular PDI is critical for its regulatory effect on neutrophil recruitment. Studies with blocking anti-PDI antibodies and αLβ2 or αMβ2 null mice suggest that extracellular PDI regulates αMβ2 integrin-mediated adhesive function of neutrophils during vascular inflammation. Consistently, we show that neutrophil surface PDI is important for αMβ2 integrin-mediated adhesion of human neutrophils under shear and static conditions and for binding of soluble fibrinogen to activated αMβ2 integrin. Confocal microscopy and biochemical studies reveal that neutrophil surface PDI interacts with αMβ2 integrin in lipid rafts of stimulated neutrophils and regulates αMβ2 integrin clustering, presumably by changing the redox state of the integrin. Thus, our results provide the first evidence that extracellular PDI could be a novel therapeutic target for preventing and treating inappropriate neutrophil sequestration.

Intracellular lipid flux and membrane microdomains as organizing principles in inflammatory cell signaling

Lipid rafts and caveolae play a pivotal role in organization of signaling by TLR4 and several other immune receptors. Beyond the simple cataloguing of signaling events compartmentalized by these membrane microdomains, recent studies have revealed the surprisingly central importance of dynamic remodeling of membrane lipid domains to immune signaling. Simple interventions upon membrane lipid, such as changes in cholesterol loading or crosslinking of raft lipids, are sufficient to induce micrometer-scale reordering of membranes and their protein cargo with consequent signal transduction. In this review, using TLR signaling in the macrophage as a central focus, we discuss emerging evidence that environmental and genetic perturbations of membrane lipid regulate protein signaling, illustrate how homeostatic flow of cholesterol and other lipids through rafts regulates the innate immune response, and highlight recent attempts to harness these insights toward therapeutic development.

T-cell activation differentially mediates the host response to sepsis

Survival during sepsis requires both swift control of infectious organisms and tight regulation of the associated inflammatory response. As the role of T cells in sepsis is somewhat controversial, we examined the impact of increasing antigen-dependent activation of CD4 T cells in a murine model of cecal ligation and puncture using T-cell receptor transgenic II (OT-II) mice that are specific for chicken ovalbumin (OVA) in the context of major histocompatibility complex II. Here, we injected OT-II mice with 0, 1, or 100 μg of OVA and demonstrate that increased antigen treatment resulted in increased numbers of activated splenic CD4 T cells. Vehicle-treated, septic OT-II mice had decreased survival, increased bacterial load, and increased levels of IL-6. Interestingly, this decrease in survival was abrogated when OT-II mice were injected with 1 μg OVA, which was correlated with normalized bacterial load and levels of IL-6. However, when OT-II mice were injected with 100 μg OVA, decreased survival was restored but, in contrast to vehicle-treated OT-II mice, had decreased bacterial load and enhanced IL-6 levels. We also observed that neutrophil oxidative burst and phagocytosis were dependent on CD4 T-cell activation. Further, at extreme levels of T-cell activation, intestinal permeability was significantly increased. Altogether, we conclude that too little CD4 T-cell activation produces dysfunctional neutrophils leading to decreased bacteria clearance and survival, whereas too much CD4 T-cell activation produces a neutrophil phenotype that leads to efficient bacterial clearance but with increased tissue damage and mortality.

Interleukin-7 (IL-7) treatment accelerates neutrophil recruitment through gamma delta T-cell IL-17 production in a murine model of sepsis

The sepsis syndrome represents an improper immune response to infection and is associated with unacceptably high rates of mortality and morbidity. The interactions between T cells and the innate immune system while combating sepsis are poorly understood. In this report, we observed that treatment with the potent, antiapoptotic cytokine interleukin-7 (IL-7) accelerated neutrophil recruitment and improved bacterial clearance. We first determined that T cells were necessary for the previously observed IL-7-mediated enhanced survival. Next, IL-7 increased Bcl-2 expression in T cells isolated from septic mice as early as 3 h following treatment. This treatment resulted in increased gamma interferon (IFN-γ) and IP-10 production within the septic peritoneum together with local and systemic increases of IL-17 in IL-7-treated mice. We further demonstrate that the increase in IL-17 was largely due to increased recruitment and production by γδ T cells, which express CXCR3. Consistent with increased IL-17 production, IL-7 treatment increased CXCL1/KC production, neutrophil recruitment, and bacterial clearance. Significantly, end-organ tissue injury was not significantly different between vehicle- and IL-7-treated mice. Collectively, these data illustrate that IL-7 can mediate the cross talk between Th1 and Th17 lymphocytes during sepsis such that neutrophil recruitment and bacterial clearance is improved while early tissue injury is not increased. All together, these observations may underlay novel potential therapeutic targets to improve the host immune response to sepsis.

Early infection during burn-induced inflammatory response results in increased mortality and p38-mediated neutrophil dysfunction

Following burn injury, the host is susceptible to bacterial infections normally cleared by healthy patients. We hypothesized that during the systemic immune response that follows scald injury, the host's altered immune status increases infection susceptibility. Using a murine model of scald injury under inhaled anesthesia followed by intraperitoneal infection, we observed increased neutrophil numbers and function at postburn day (PBD) 1 compared with sham-burned and PBD4 mice. Further, increased mortality, bacteremia, and serum IL-6 were observed in PBD1 mice after Pseudomonas aeruginosa (PA) infection compared with sham-burned and PBD4 mice infected with PA. To examine these disparate responses, we investigated neutrophils isolated at 5 and 24 h following PA infection from PBD1 and sham-burned mice. Five hours after infection, there was no significant difference in number of recruited neutrophils; however, neutrophils from injured mice had decreased activation, active-p38, and oxidative burst compared with sham-burned mice. In direct contrast, 24 h after infection, we observed increased numbers, active-p38, and oxidative burst of neutrophils from PBD1 mice. Finally, we demonstrated that in neutrophils isolated from PBD1 mice, the observed increase in oxidative burst was p38 dependent. Altogether, neutrophil activation and function from thermally injured mice are initially delayed and later exacerbated by a p38-dependent mechanism. This mechanism is likely key to the observed increase in bacterial load and mortality of PBD1 mice infected with PA.

Divergent adaptive and innate immunological responses are observed in humans following blunt trauma

BACKGROUND

The immune response to trauma has traditionally been modeled to consist of the systemic inflammatory response syndrome (SIRS) followed by the compensatory anti-inflammatory response syndrome (CARS). We investigated these responses in a homogenous cohort of male, severe blunt trauma patients admitted to a University Hospital surgical intensive care unit (SICU). After obtaining consent, peripheral blood was drawn up to 96 hours following injury. The enumeration and functionality of both myeloid and lymphocyte cell populations were determined.

RESULTS

Neutrophil numbers were observed to be elevated in trauma patients as compared to healthy controls. Further, neutrophils isolated from trauma patients had increased raft formation and phospho-Akt. Consistent with this, the neutrophils had increased oxidative burst compared to healthy controls. In direct contrast, blood from trauma patients contained decreased naïve T cell numbers. Upon activation with a T cell specific mitogen, trauma patient T cells produced less IFN-gamma as compared to those from healthy controls. Consistent with these results, upon activation, trauma patient T cells were observed to have decreased T cell receptor mediated signaling.

CONCLUSIONS

These results suggest that following trauma, there are concurrent and divergent immunological responses. These consist of a hyper-inflammatory response by the innate arm of the immune system concurrent with a hypo-inflammatory response by the adaptive arm.

The ins and outs of leukocyte integrin signaling

Integrins are the principal cell adhesion receptors that mediate leukocyte migration and activation in the immune system. These receptors signal bidirectionally through the plasma membrane in pathways referred to as inside-out and outside-in signaling. Each of these pathways is mediated by conformational changes in the integrin structure. Such changes allow high-affinity binding of the receptor with counter-adhesion molecules on the vascular endothelium or extracellular matrix and lead to association of the cytoplasmic tails of the integrins with intracellular signaling molecules. Leukocyte functional responses resulting from outside-in signaling include migration, proliferation, cytokine secretion, and degranulation. Here, we review the key signaling events that occur in the inside-out versus outside-in pathways, highlighting recent advances in our understanding of how integrins are activated by a variety of stimuli and how they mediate a diverse array of cellular responses.