Inhibitory kappaBalpha control of nuclear factor-kappaB is dysregulated in endotoxin tolerant macrophages

Synergistic interactions between NOD receptors and TLRs: Mechanisms and clinical implications

Interactions between pattern recognition receptors (PRRs) shape innate immune responses to particular classes of pathogens. Here, we review interactions between TLRs and nucleotide-binding oligomerization domain 1 and 2 (NOD1 and NOD2) receptors, two major groups of PRRs involved in innate recognition of bacteria. Most of experimental data both in vitro and in vivo suggest that NODs and TLRs synergize with each other at inducing the production of cytokines and antimicrobial peptides. Molecular mechanisms of this synergy remain poorly understood, although several scenarios can be proposed: (i) direct interactions of signaling pathways downstream of NODs and TLRs; (ii) mutual transcriptional regulation of unique components of NOD-dependent and TLR-dependent signaling pathways; and (iii) interactions at the post-transcriptional level. Potential practical implications of NOD-TLR synergy are dual. In sepsis, where synergistic effects probably contribute to excessive proinflammatory cytokine production, blockade of NOD1, and/or NOD2 in addition to TLR4 blockade may be required to achieve therapeutic benefit. On the other hand, synergistic combinations of relatively small doses of NOD and TLR agonists administered before infection could be used to boost innate resistance against bacterial pathogens.

Review: Molecular mechanisms of endotoxin tolerance

The phenomenon of endotoxin tolerance has been widely investigated, but to date, the molecular mechanisms of endotoxin tolerance remain to be resolved clearly. The discovery of the Toll-like receptor (TLR) family as the major receptors for lipopolysaccharide (LPS) and other bacterial products has prompted a resurgence of interest in endotoxin tolerance mechanisms. Changes of cell surface molecules, signaling proteins, pro-inflammatory and anti -inflammatory cytokines and other mediators have been examined. During tolerance expression of LPS-binding protein (LBP), CD14, myeloid differentiation protein-2 (MD-2) and TLR2 are unchanged or up-regulated, whereas TLR4 is transiently suppressed or unchanged. Proximal post-receptor signaling proteins that are altered in tolerance include augmented degradation of interleukin-1 receptor-associated kinase (IRAK), and decreased TLR4-myeloid differentiation factor 88 (MyD88) and IRAK-MyD88 association. Tolerance has also been shown to be associated with decreased Gi protein content and activity, decreased protein kinase C (PKC) activity, reduction in mitogen-activated protein kinase (MAP kinase) activity, and reduced activator protein-1 (AP-1) and nuclear factor kappa B (NF-κB) induced gene transactivation. However, not all signaling proteins and pathways are suppressed in tolerance and induction of specific anti-inflammatory proteins and signaling pathways may serve important counter inflammatory functions. The latter include induction of IRAK-M and suppressor of cytokine-signaling-1 (SOCS-1), phosphoinositide-3-kinase (PI3K) signaling, and increased or maintained expression of inhibitor-κB (IκB) isoforms. Also at the nuclear level, increase in the NFκB subunit p50 homodimer expression and increased activation of peroxisome-proliferatoractivated receptors-γ (PPARγ) have been linked to tolerance phenotype. Although there are species and cellular variations in manifestation of the LPS tolerant phenotype, it is clear that the tolerance phenomena have evolved as a complex orchestrated counter regulatory response to inflammation.

Study of Protein Phosphatase 2A (PP2A) Activity in LPS-Induced Tolerance Using Fluorescence-Based and Immunoprecipitation-Aided Methodology

Protein phosphatase 2A (PP2A) is one of the most abundant intracellular serine/threonine (Ser/Thr) phosphatases accounting for 1% of the total cellular protein content. PP2A is comprised of a heterodimeric core enzyme and a substrate-specific regulatory subunit. Potentially, at least seventy different compositions of PP2A exist because of variable regulatory subunit binding that accounts for various activity modulating numerous cell functions. Due to the constitutive phosphatase activity present inside cells, a sensitive assay is required to detect the changes of PP2A activity under various experimental conditions. We optimized a fluorescence assay (DIFMU assay) by combining it with prior anti-PP2A immunoprecipitation to quantify PP2A-specific phosphatase activity. It is also known that prior exposure to lipopolysaccharides (LPS) induces “immune tolerance” of the cells to subsequent stimulation. Herein we report that PP2A activity is upregulated in tolerized peritoneal macrophages, corresponding to decreased TNF-α secretion upon second LPS stimulation. We further examined the role of PP2A in the tolerance effect by using PP2ACα^lox/lox;lyM-Cre conditional knockout macrophages. We found that PP2A phosphatase activity cannot be further increased by tolerance. TNF-α secretion from tolerized PP2ACα^lox/lox;lyM-Cre macrophages is higher than tolerized control macrophages. Furthermore, we showed that the increased TNF-α secretion may be due to an epigenetic transcriptionally active signature on the promoter of TNF-α gene rather than regulation of the NFκB/IκB signaling pathway. These results suggest a role for increased PP2A activity in the regulation of immune tolerance.

Lactobacillus plantarum MYL26 induces endotoxin tolerance phenotype in Caco-2 cells

BACKGROUND

Crohn's disease and ulcerative colitis are the major types of chronic inflammatory bowel disease occurring in the colon and small intestine. A growing body of research has proposed that probiotics are able to attenuate the inflammatory symptoms of these diseases in vitro and in vivo. However, the mechanism of probiotic actions remains unclear.

RESULTS

Our results suggested Lactobacillus plantarum MYL26 inhibited inflammation in Caco-2 cells through regulation of gene expressions of TOLLIP, SOCS1, SOCS3, and IκBα, rather than SHIP-1 and IRAK-3.

CONCLUSIONS

We proposed that live/ heat-killed Lactobacillus plantarum MYL26 and bacterial cell wall extract treatments impaired TLR4-NFκb signal transduction through Tollip, SOCS-1 and SOCS-3 activation, thus inducing LPS tolerance. Our findings suggest that either heat-killed probiotics or probiotic cell wall extracts are able to attenuate inflammation through pathways similar to that of live bacteria.

MicroRNA in TLR signaling and endotoxin tolerance

Toll-like receptors (TLRs) in innate immune cells are the prime cellular sensors for microbial components. TLR activation leads to the production of proinflammatory mediators and thus TLR signaling must be properly regulated by various mechanisms to maintain homeostasis. TLR4-ligand lipopolysaccharide (LPS)-induced tolerance or cross-tolerance is one such mechanism, and it plays an important role in innate immunity. Tolerance is established and sustained by the activity of the microRNA miR-146a, which is known to target key elements of the myeloid differentiation factor 88 (MyD88) signaling pathway, including IL-1 receptor-associated kinase (IRAK1), IRAK2 and tumor-necrosis factor (TNF) receptor-associated factor 6 (TRAF6). In this review, we comprehensively examine the TLR signaling involved in innate immunity, with special focus on LPS-induced tolerance. The function of TLR ligand-induced microRNAs, including miR-146a, miR-155 and miR-132, in regulating inflammatory mediators, and their impact on the immune system and human diseases, are discussed. Modulation of these microRNAs may affect TLR pathway activation and help to develop therapeutics against inflammatory diseases.

Immunoparalysis and adverse outcomes from critical illness

Proper immunologic balance between pro- and anti-inflammatory forces is necessary for recovery from critical illness. Persistence of a marked compensatory anti-inflammatory innate immune response after an insult is termed immunoparalysis. Critically ill patients demonstrating prolonged, severe reductions in monocyte HLA-DR expression or ex vivo tumor necrosis factor alpha production are at high risk for nosocomial infection and death. Reversal of immunoparalysis can be accomplished through the administration of immunostimulatory agents or tapering of exogenous immunosuppression. Evidence suggests that this may be associated with improved clinical outcomes. Immune-monitoring protocols are needed to identify patients who may benefit from immunomodulatory trials.

Heat shock factor 1 inhibits nuclear factor-kappaB nuclear binding activity during endotoxin tolerance and heat shock

RATIONALE

Sepsis, endotoxin tolerance, and heat shock (HS) all display down-regulation of innate immunity. We hypothesize that HS factor 1 (HSF-1) induces competitive inhibition of nuclear factor-kappaB (NF-kappaB)-induced signal transduction in both endotoxin tolerance and HS.

OBJECTIVES

We compared endotoxin tolerance and HS in RAW 264.7 cells. We transfected cells with an HS protein 70 (HSP70) plasmid to test whether HSP70 is the mediator of HS-induced NF-kappaB inhibition. We studied the effects of endotoxin stimulation and HS, both separately and together, on "wild-type" cells, cells transfected with the HSP70 plasmid, and cells transfected with vehicle.

FINDINGS

Heat shock protein 70 plasmid-transfected cells had increased HSP70 expression and demonstrated decreased nitric oxide (NO) release and inducible NO synthase messenger RNA expression in response to endotoxin compared with wild-type and empty plasmid-transfected cells. Heat shock completely abolished subsequent NO and inducible NO synthase messenger RNA expression in wild-type cells. Heat shock factor 1 reached maximum expression 60 to 90 minutes after HS. Heat shock protein 70-transfected cells still displayed endotoxin-induced NF-kappaB nuclear binding, whereas endotoxin tolerance, HS, and exposure to HSF-1, but not exposure to an unrelated promoter, inhibited NF-kappaB nuclear binding.

CONCLUSIONS

Endotoxin tolerance and HS appear to share a common immune suppressive effect, possibly through HSF-1-mediated competitive inhibition of NF-kappaB nuclear binding.

Inducible nitric oxide synthase and heme oxygenase-1 in the lung during lipopolysaccharide tolerance and cross tolerance

OBJECTIVE

Pretreatment with low-dose lipopolysaccharide protects cells/organs against a subsequent lethal Gram-negative (lipopolysaccharide tolerance) or Gram-positive (cross tolerance) stimulus. We determined whether this occurs in the rat lung. The involvement of inducible nitric oxide synthase and heme oxygenase-1 was evaluated.

DESIGN

Laboratory study.

SETTING

University hospital laboratory.

SUBJECTS

Anesthetized male Wistar rats.

INTERVENTIONS

To test the hypothesis, rats received saline or lipopolysaccharide (1 mg/kg). At 2, 4, 8, 16, or 24 hrs later, blood samples and lung tissue were taken to determine messenger RNA, protein concentration, and activity of inducible nitric oxide synthase and heme oxygenase-1. In additional experiments, rats were challenged with lipopolysaccharide (1 mg/kg) and subjected to Gram-negative (lipopolysaccharide) or Gram-positive (lipoteichoic acid and peptidoglycan) shock 24 hrs later. These studies were carried out in the presence and absence of inducible nitric oxide synthase or heme oxygenase-1 inhibitors (1400W or tin protoporphyrin IX). Following 6 hrs of shock, lung tissue was taken to determine lung damage and heme oxygenase-1 concentration and activity.

MEASUREMENTS AND MAIN RESULTS

In the rat lung, lipopolysaccharide (1 mg/kg) induced a significant increase in inducible nitric oxide synthase protein at 8 hrs with a corresponding increase in plasma nitrate/nitrite at 8-16 hrs. Simultaneously, heme oxygenase-1 messenger RNA transcripts were observed at 8-16 hrs, and maximal expression of the protein followed (24 hrs). Pretreatment with low-dose lipopolysaccharide reduced myeloperoxidase activity (neutrophil infiltration) and wet-dry ratio (pulmonary edema) in the lungs of animals subjected to Gram-negative or Gram-positive shock, demonstrating tolerance. Pretreatment with low-dose lipopolysaccharide and the selective inducible nitric oxide synthase inhibitor 1400W reduced heme oxygenase-1 protein expression, and lung protection was abolished. Tin protoporphyrin IX did not affect heme oxygenase-1 expression, but heme oxygenase activity and lung protection were significantly reduced.

CONCLUSIONS

We propose that nitric oxide (most likely inducible nitric oxide synthase derived) regulates the induction of heme oxygenase-1 in the lung, which in turn plays an important part in pulmonary protection during lipopolysaccharide tolerance and cross tolerance.

Role of locus coeruleus heme oxygenase-carbon monoxide-cGMP pathway during hypothermic response to restraint

Central heme oxigenase-carbon monoxide (HO-CO) pathway has been shown to play a pyretic role in the thermoregulatory response to restraint. However, the specific site in the central nervous system where CO may act modulating this response remains unclear. LC is rich not only in sGC but also in heme oxygenase (HO; the enzyme that catalyses the metabolism of heme to CO, along with biliverdin and free iron). Therefore, the possible role of the HO-CO-cGMP pathway in the restraint-induced-hypothermia by LC neurons was investigated. Body temperature dropped about 0.7 degrees C during restraint. ZnDPBG (a HO inhibitor; 5 nmol, intra-LC) prevented the hypothermic response during restraint. Conversely, induction of the HO pathway in the LC with heme-lysinate (7.6 nmol, intra-LC) intensified the hypothermic response to restraint, and this effect was prevented by pretreatment with ODQ (a sGC inhibitor; given intracerebroventricularly, 1.3 nmol). Taken together, these data suggest that CO in the LC produced by the HO pathway and acting via cGMP is implicated in thermal responses to restraint.

Cellular reprogramming by gram-positive bacterial components: a review

LPS tolerance has been the focus of extensive scientific and clinical research over the last several decades in an attempt to elucidate the sequence of changes that occur at a molecular level in tolerized cells. Tolerance to components of gram-positive bacterial cell walls such as bacterial lipoprotein and lipoteichoic acid is a much lesser studied, although equally important, phenomenon. This review will focus on cellular reprogramming by gram-positive bacterial components and examines the alterations in cell surface receptor expression, changes in intracellular signaling, gene expression and cytokine production, and the phenomenon of cross-tolerance.

Characterization of calves exhibiting a novel inheritable TNF-α hyperresponsiveness to endotoxin: associations with increased pathophysiological complications

A subpopulation of calves, herein termed “hyperresponders” (HPR), was identified and defined by the patterns of plasma TNF-α concentrations that developed following two challenges with endotoxin (LPS, 0.8 μg Escherichia coli 055:B5 LPS/kg0.75live body wt) separated by 5 days. The principle characteristic of HPR calves was a failure to develop tolerance to repeated LPS challenge that was evident in the magnitude of the TNF-α concentrations and prolonged severity of pathological sequellae. Whereas calves failing to develop LPS tolerance were identified on the basis of their excessive in vivo plasma TNF-α concentration responses, in vitro TNF-α responses of peripheral blood mononuclear cells isolated from each calf and challenged with LPS or PMA did not correlate or predict the magnitude of in vivo plasma TNF response of the calf. Intentional breeding to obtain calves from bulls and/or cows documented as HPR resulted in offspring displaying the HPR character when similar progeny calves were tested with LPS in vivo, with extensive controls in place to account for sources of variability in the general TNF-α response to LPS that might compromise interpretation of the data. Feed intake, clinical serology and hematology profiles, and acute-phase protein responses of HPR calves following LPS were significantly different from those of calves displaying tolerance. These results suggest that the pattern of plasma TNF-α changes that evolve from a low-level double LPS challenge effectively reveal the presence of a genetic potential for animals to display excessive or prolonged pathological response to LPS-related stress and compromised prognosis for recovery.

Pathophysiology of polytrauma

Immediate and early trauma deaths are determined by primary brain injuries, or significant blood loss (haemorrhagic shock), while late mortality is caused by secondary brain injuries and host defence failure. First hits (hypoxia, hypotension, organ and soft tissue injuries, fractures), as well as second hits (e.g. ischaemia/reperfusion injuries, compartment syndromes, operative interventions, infections), induce a host defence response. This is characterized by local and systemic release of pro-inflammatory cytokines, arachidonic acid metabolites, proteins of the contact phase and coagulation systems, complement factors and acute phase proteins, as well as hormonal mediators: it is defined as systemic inflammatory response syndrome (SIRS), according to clinical parameters. However, in parallel, anti-inflammatory mediators are produced (compensatory anti-inflammatory response syndrome (CARS). An imbalance of these dual immune responses seems to be responsible for organ dysfunction and increased susceptibility to infections. Endothelial cell damage, accumulation of leukocytes, disseminated intravascular coagulation (DIC) and microcirculatory disturbances lead finally to apoptosis and necrosis of parenchymal cells, with the development of multiple organ dysfunction syndrome (MODS), or multiple organ failure (MOF). Whereas most clinical trials with anti-inflammatory, anti-coagulant, or antioxidant strategies failed, the implementation of pre- and in-hospital trauma protocols and the principle of damage control procedures have reduced post-traumatic complications. However, the development of immunomonitoring will help in the selection of patients at risk of post-traumatic complications and, thereby, the choice of the most appropriate treatment protocols for severely injured patients.

CONTRIBUTION OF MKP-1 REGULATION OF p38 TO ENDOTOXIN TOLERANCE

Endotoxin tolerance has been characterized as diminished TNF-alpha expression after a second LPS stimulus and is dependent on new protein synthesis. LPS-induced expression of TNF-alpha is partly regulated by the p38 mitogen-activated protein (MAP) kinase, which post-transcriptionally stabilizes TNF-alpha mRNA. The dual-specific phosphatase, MKP-1, has been shown to negatively regulate p38 via dephosphorylation. We hypothesized that MKP-1 expression induced during tolerance regulates TNF-alpha expression by inhibiting p38 activity. To test this hypothesis, tolerance was induced in THP-1 cells, and naive or tolerized cells were rechallenged 18 h later with LPS (1 microg/mL) and TNF-alpha production was measured. Under similar conditions, nuclear proteins were isolated after LPS stimulation and were analyzed for phospho-p38 and MKP-1 by Western blot. Transient overexpression of MKP-1 was achieved using an adenoviral expression strategy and infected cells subsequently treated with LPS for TNF-alpha production and p38 activation. Results showed that LPS tolerance was induced as reflected by decreased TNF-alpha. Induction of LPS hyporesponsiveness could be mimicked by overexpression of MKP-1 but not beta-gal. MKP-1 expression was noted only in LPS-tolerized or Ad-MKP-1 infected cells. In the canonical and Ad-MKP-1-mediated tolerance models, decreased phospho-p38 activity was observed. MKP-1s role in mediating endotoxin tolerance was further confirmed by demonstrating the inability to fully tolerize peritoneal macrophages isolated from MKP-1 null mutant (vs. wild type) mice (24% vs. 72% reductions, respectively). These data demonstrate that the dual specific phosphatase MKP-1 is an important mediator of endotoxin tolerance via p38 regulation.

Glycosylphosphatidylinositols in malaria pathogenesis and immunity: potential for therapeutic inhibition and vaccination

Glycosylphosphatidylinositols (GPIs) are found in the outer cell membranes of all eukaryotes. GPIs anchor a diverse range of proteins to the surface of Plasmodium falciparum, but may also exist free of protein attachment. In vitro and in vivo studies have established GPIs as likely candidate toxins in malaria, consistent with the prevailing paradigm that attributes induction of inflammatory cytokines, fever and other pathology to parasite toxins released when schizonts rupture. Although evolutionarily conserved, sufficient structural differences appear to exist that impart upon plasmodial GPIs the ability to activate second messengers in mammalian cells and elicit immune responses. In populations exposed to P. falciparum, the antibody response to purified GPIs is characterised by a predominance of immunoglobulin (Ig)G over IgM and an increase in the prevalence, level and persistence of responses with increasing age. It remains unclear, however, if these antibodies or other cellular responses to GPIs mediate anti-toxic immunity in humans; anti-toxic immunity may comprise either reduction in the severity of disease or maintenance of the malaria-tolerant state (i.e. persistent asymptomatic parasitaemia). P. falciparum GPIs are potentially amenable to specific therapeutic inhibition and vaccination; more needs to be known about their dual roles in malaria pathogenesis and protection for these strategies to succeed.

Toxoplasma gondii interferes with lipopolysaccharide-induced mitogen-activated protein kinase activation by mechanisms distinct from endotoxin tolerance

We show in this study that Toxoplasma gondii infection induces rapid activation of p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase 1/2, and stress-activated protein kinase/c-Jun N-terminal kinase MAPK, followed promptly by their deactivation in mouse macrophages. Nevertheless, when infected cells were subsequently subjected to LPS triggering, MAPK activation was severely defective, in particular in the case of p38 MAPK, which is required for LPS-triggered TNF-alpha and IL-12 production. Similar effects occurred during endotoxin tolerance, but the phenomena were distinct. LPS pretriggering failed to activate the major p38 MAPK kinase, MAPK kinase 3/6. Toxoplasma infection, in contrast, resulted in sustained activation of this kinase. Furthermore, endotoxin pre-exposure blocked IkappaBalpha degradation upon subsequent LPS triggering, but this was not the case for Toxoplasma preinfection. Endotoxin-mediated down-regulation of the LPS receptor, Toll-like receptor 4, has been suggested as one possible mechanism contributing to tolerance, and we found in this study that LPS down-modulated Toll-like receptor 4 expression. In contrast, Toxoplasma infection induced up-regulation of this pattern recognition receptor. Our results show that T. gondii blocks LPS-triggered cytokine production in part through MAPK inactivation, and that this occurs through pathways distinct from endotoxin-induced tolerance.

REGULATION OF ADOLESCENT RAT INTESTINAL EPITHELIAL INDUCIBLE NITRIC OXIDE SYNTHASE EXPRESSION IN ENDOTOXIN TOLERANCE: MODULATION OF SIGNAL TRANSDUCTION

Endotoxin/lipopolysaccharide (LPS) tolerance is a state of induced hyporesponsiveness to endotoxin or LPS characterized by alterations in the release of inflammatory mediators. As the gut is both a source of infection and target of injury, we tested the hypothesis that alterations in intestinal epithelial signal transduction would account for the acquisition of endotoxin tolerance as defined by decreased induction of a key mediator of gut injury, inducible nitric oxide synthase (iNOS). Rats (15 days of age) were injected with saline or LPS (1 microg/g i.p.). Tissue was harvested after 1, 4, or 6 h for assessment of signaling and iNOS expression. Other animals received a second dose of LPS 1 to 7 days after the initial dose. Selected animals received the p38 inhibitor, SB203580 (10 microg/g), which was co-administered with the first dose of LPS. Induction of iNOS mRNA and protein was significantly attenuated after repeated LPS administration. Epithelial cells from LPS-tolerant rats showed a minimal level of iNOS expression by immunohistochemistry. The down-regulation of intestinal iNOS was not gender dependent. p38 inhibition enhanced tolerance rather than blocking it. LPS-mediated activation of NF-kappaB was attenuated in a manner consistent with a primary role in the induction of tolerance. Endotoxin tolerance can be demonstrated in intestinal epithelial cells using an in vivo model. Modulation of NF-kappaB signaling may be key in the down-regulation of LPS effect seen in tolerance.

Endotoxin tlerance inhibits lipopolysaccharide-initiated acute pulmonary inflammation and lung injury in rats by the mechanism of nuclear factor-kappaB

In this study, the effect of endotoxin tolerance on lipopolysaccharide (LPS)-initiated pulmonary inflammation, the local production of tumour necrosis factor-alpha (TNF-alpha) and the cytokine-induced neutrophil attractant (CINC), as well as the activation of nuclear factor-kappaB (NF-kappaB) and its subunit composition, were examined in vivo. Endotoxin tolerance was reproduced by four consecutive daily intraperitoneal injections of 0.6 mg/kg of Escherichia coli 055:B5 LPS. Compared with control rats, endotoxin-tolerant rats failed to increase the permeability of pulmonary microvascular or recruit neutrophil to lung tissue upon restimulation with 6 mg/kg of LPSs. Pretreatment with LPSs inhibited the protein level of TNF-alpha in bronchoalveolar lavage fluid (BALF) and mRNA expression of CINC in lung tissue in response to subsequent LPS stimulation. These changes were accompanied by the suppression of activation of NF-kappaB, including the low level of total amount of DNA-binding activity and high percentage of non-transactive p50 homodimers. These data demonstrate that endotoxin tolerance can alleviate the LPS-induced acute neutrophilic pulmonary inflammation in rats and can inhibit the proinflammatory cytokines in lung and suggest that endotoxin tolerance might result from the unresponsiveness of NF-kappaB and persistent high percentage of p50 homodimers. Therefore, the phenomenon of endotoxin tolerance might be used as a strategy for the prevention or treatment of LPS-associated acute respiratory distress syndrome in which excessive or dysregulated inflammation leads to acute lung injury.

Lower levels of whole blood LPS-stimulated cytokine release are associated with poorer clinical outcomes in surgical ICU patients

BACKGROUND

In vitro pretreatment of human monocytes with lipopolysaccharide (LPS) induces "endotoxin tolerance" with blunted TNF and IL-6 release to rechallenge with LPS. The pro-inflammatory cytokines TNF and IL-6 are important mediators in sepsis. A high IL-6 concentration has been used as a marker of infection severity, but IL-6 may also have beneficial effects as an acute-phase protein. We sought to address two questions: (a) What is the relationship between TNF and IL-6 release? (b) Is the clinical outcome different for intensive care unit (ICU) patients with ex vivo characteristics of endotoxin tolerance (low levels of ex vivo LPS-stimulated cytokine release)?

MATERIALS AND METHODS

Heparinized whole blood was obtained from 62 surgical ICU patients and 15 control subjects and incubated for 3 h at 37 degrees C in the presence or absence of 10 ng/mL LPS. Concentrations of TNF and IL-6 were measured in plasma samples using an enzyme-linked immunosorbent assay (pg/mL). Clinical data on ICU length of stay (LOS), ventilator days, white blood cell count (WBC), and documented clinical infection were obtained by chart review. Outcome parameters for patients with low ex vivo LPS-stimulated cytokine release (low = IL-6 < 3000 pg/mL and TNF <2100 pg/mL) were compared to patients with Normal/High concentrations of cytokines.

RESULTS

Cytokines were essentially undetectable in ICU patients or controls without LPS stimulation, however a range of values was measured for LPS-stimulated release in both ICU patients (IL-6, 7847 +/- 857 pg/mL; TNF, 4390 +/- 457 pg/mL) and controls (IL-6, 7704 +/- 793 pg/mL; TNF, 6706 +/- 715 pg/mL). There were no differences in age between High/Normal concentrations of cytokines compared to the Low cytokine group, however there were significant differences in WBC, cytokine concentrations, ICU LOS, incidence of clinical infection, and mortality. The Low group also required an average of 6.9 more days of mechanical ventilation (p < 0.05). LPS-stimulated TNF release seemed to correlate better with the observed mortality than did IL-6 release.

CONCLUSION

The data suggest that ICU patients with characteristics of endotoxin tolerance (low LPS-stimulated cytokine release capacity) have significantly poorer clinical outcomes. Ex vivo LPS-stimulated whole blood cytokine production may be useful to identify ICU patients with severe sepsis.

Inhibition of a p38/stress-activated protein kinase-2-dependent phosphatase restores function of IL-1 receptor-associate kinase-1 and reverses Toll-like receptor 2- and 4-dependent tolerance of macrophages

Pretreatment of macrophages with Toll-like receptor (TLR)2 or TLR4 agonists leads to a stage of cell hyporesponsiveness to a second stimulation with TLR agonists. This tolerance state is accompanied by the repression of IL-1 receptor-associated kinase-1, mitogen-activated protein kinases, and IkappaB phosphorylation and expression of genes encoding proinflammatory cytokines, like IL-1beta and TNF-alpha. In this report, we demonstrated that mucin-like glycoprotein (tGPI-mucin) of Trypanosoma cruzi trypomastigotes (TLR2 agonist) and LPS (TLR4 agonist) induce cross-tolerance in macrophages and we addressed the role of phosphatase activity in this process. Analysis of the kinetic of phosphatase activity induced by tGPI-mucin or LPS revealed maximum levels between 12 and 24 h, which correlate with the macrophage hyporesponsiveness stage. The addition of okadaic acid, an inhibitor of phosphatase activity, reversed macrophage hyporesponsiveness after exposure to either LPS or tGPI-mucin, allowing phosphorylation of IL-1R-associated kinase-1, mitogen-activated protein kinases, and IkappaB and leading to TNF-alpha gene transcription and cytokine production. Furthermore, pretreatment with either the specific p38/stress-activated protein kinase-2 inhibitor (SB203580) or the NF-kappaB translocation inhibitor (SN50) prevented the induction of phosphatase activity and hyporesponsiveness in macrophage, permitting cytokine production after restimulation with LPS. These results indicate a critical role of p38/stress-activated protein kinase-2 and NF-kappaB-dependent phosphatase in macrophage hyporesponsiveness induced by microbial products that activate TLR2 and TLR4.

Carbon monoxide modulates endotoxin-induced production of granulocyte macrophage colony-stimulating factor in macrophages

The stress-inducible gene heme oxygenase-1 (HO-1) provides protection against oxidative stress. Although the mechanisms by which HO-1 exerts its cytoprotection are not clearly understood, it has been speculated that carbon monoxide (CO), a catalytic byproduct following heme catabolism by HO-1, may mediate cellular cytoprotection via its anti-inflammatory properties. Granulocyte macrophage colony-stimulating factor (GM-CSF) is a potent cytokine generated in response to bacterial endotoxin (lipopolysaccharide [LPS]) to stimulate proliferation, maturation, and effector functions of leukocytes, contributing to the proinflammatory responses to LPS. We hypothesized that HO-1 and/or CO could regulate the expression and production of GM-CSF. HO-1 overexpression, as well as exposure to a low concentration of CO, inhibited LPS-induced GM-CSF production in macrophages. Furthermore, CO inhibited LPS-induced GM-CSF induction via inhibition in the activation of the transcription factor NF-kappaB. CO inhibited LPS-induced activation of NF-kappaB, which has been shown to regulate GM-CSF transcription, by preventing the phosphorylation and degradation of the regulatory subunit IkappaB-alpha. These data raise the intriguing possibility that CO at low concentrations may play an important role in inflammatory disease states and thus has potential therapeutic implications.

Lipopolysaccharide preconditioning and cross-tolerance: the induction of protective mechanisms for rat intestinal ileus

BACKGROUND & AIMS

Endotoxin elicits an inflammatory response within the intestinal muscularis and causes intestinal muscle dysfunction. Our aims were to investigate intestinal muscle recovery after a single or repeated lipopolysaccharide (LPS) injections. We also investigated the ability of LPS to induce cross-tolerance to postoperative ileus.

METHODS

Motility was measured in vivo and in vitro by transit and organ-bath techniques. Nuclear factor kappa-B, nuclear factor interleukin 6, and signal transducer and activator of transcription were quantified by using electrophoretic mobility shift assay, and tumor necrosis factor alpha, interleukin 6, inducible nitric oxide synthase, and cyclooxygenase 2 were measured with reverse-transcription polymerase chain reaction. Myeloperoxidase histochemistry for neutrophils was performed in jejunal muscularis whole mounts.

RESULTS

Endotoxin-induced suppression of in vitro muscle contractility temporally recovered over 7 days with a similar profile whether after a single dose or during the continuous daily injection of LPS. Functional adaptation to continuous LPS was reflected in a significant blunting of transcription factor activation and cytokine messenger RNA up-regulation compared with the naive LPS-stimulated muscularis. Preconditioning of the muscularis showed significant cross-tolerance to the functional, molecular, and leukocytic sequelae of intestinal manipulation.

CONCLUSIONS

The muscularis externa recovered and developed tolerance to endotoxin over 7 days, which conferred cross-tolerance to intestinal manipulation. Thus, preconditioning induces protective mechanisms to a subsequent insult within the muscularis externa.

Toll receptors, CD14, and macrophage activation and deactivation by LPS

This review will focus on the molecular mechanisms of macrophage activation and desensitization by bacterial lipopolysaccharide (LPS). The most recent advances in the understanding of the function of the LPS receptor complex and its role in the development of the septic shock syndrome and endotoxin tolerance will be discussed.

Inducible nitric oxide synthase is not required in the development of endotoxin tolerance in mice

We investigated the role of inducible nitric oxide synthase (iNOS) in endotoxin tolerance, which was induced in mice genetically deficient of iNOS (iNOS-/-) and in wild-type littermates. In non-tolerant wild-type mice, endotoxin induced high mortality, elevation of plasma levels of nitrite and nitrate, tumor necrosis factor a (TNFalpha), and interleukin 10 (IL-10). These events were preceded by degradation of inhibitors kappaBalpha (IkappaBalpha) and kappaBI (IkappaBbeta), and activation of nuclear factor-kappaB (NF-kappaB) in the lung. Pretreatment of wild-type mice with a sublethal dose of endotoxin prior to lethal endotoxin administration ameliorated lethality and blunted TNFalpha production, whereas IL-10, nitrite, and nitrate production was maintained. These events were associated with reduction of IKBa degradation and NF-kappaB activation in the lung. The kinetics of degradation of IkappaBbeta were also altered. In parallel experiments, nontolerant iNOS-/- mice experienced similar mortality after endotoxin as nontolerant wild-type mice. Plasma levels of nitrite and nitrate were not elevated after lethal endotoxin administration. IL-10 levels were significantly reduced in comparison to nontolerant wild-type mice, whereas TNFalpha levels were similarly increased. These events were preceded by lesser degradation of IkappaBalpha and reduced NF-kappaB activation in the lung. Pretreatment of iNOS-/- mice with a sublethal endotoxin ameliorated lethality. TNFalpha production was significantly reduced, whereas IL-10 production was significantly increased when compared to nontolerant iNOS-/- mice. Degradation of IkappaBalpha and activation of NF-kappaB in the lung were not altered by endotoxin tolerance, whereas kinetics of IkappaBbeta degradation was only delayed. Our data suggests that iNOS is not required for the development of endotoxin tolerance, and that other signal transduction pathways, rather than NF-kappaB, may regulate induction of endotoxin tolerance in the absence of iNOS.

Endotoxin tolerance: A review

Endotoxin tolerance was initially described when it was observed that animals survived a lethal dose of bacterial endotoxin if they had been previously treated with a sublethal injection. In animal models, two phases of endotoxin tolerance are described, an early phase associated with altered cellular activation and a late phase associated with the development of specific antibodies against the polysaccharide side chain of Gram-negative organisms. Recently, there has been a tremendous resurgence of interest in the mechanisms responsible for altered responsiveness to bacterial endotoxin. Host immune cells, particularly macrophages and monocytes, that are exposed to endotoxin for 3 to 24 hrs are rendered "tolerant" and manifest a profoundly altered response when rechallenged with bacterial endotoxin or lipopolysaccharide. The "lipopolysaccharide-tolerant" phenotype is characterized by inhibition of lipopolysaccharide-stimulated tumor necrosis factor production, altered interleukin-1 and interleukin-6 release, enhanced cyclooxygenase-2 activation, inhibition of mitogen-activated protein kinase activation, and impaired nuclear factor-kappaB translocation. Human monocytes and macrophages can be induced to become tolerant, and there is increasing evidence that monocytic cells from patients with systemic inflammatory response syndrome and sepsis have many characteristics of endotoxin tolerance.

Role of host phosphotyrosine phosphatase SHP-1 in the development of murine leishmaniasis

Activation of host phosphotyrosine phosphatase SHP-1 by Leishmania and its subsequent impact on tyrosine phosphorylation-based signaling cascades were shown to represent an important mechanism whereby this pathogen may alter host cell functions. Herein, we report that Leishmania-induced macrophage SHP-1 activity is necessary for its survival within phagocytes through the attenuation of nitric oxide-dependent and -independent microbicidal mechanisms. In vivo, Leishmania major infection, which footpad inflammation is mostly undetectable in SHP-1-deficient viable motheaten mice, was accompanied by increased inducible nitric oxide synthase and activation of neutrophils. These enhanced cellular activities were paralleled by a marked activation of signaling events usually negatively regulated by SHP-1. Overall, this study firmly establishes that modulation of the signaling terminator SHP-1 by Leishmania is essential for its installment and propagation.

Differential regulation of myocardial NF kappa B following acute or chronic TNF-alpha exposure

Tumor necrosis factor alpha (TNF-alpha) is a critical mediator of myocardial dysfunction during acute inflammatory states. TNF-alpha is also present in the serum of patients with chronic cardiac diseases. In monocytes, TNF-alpha stimulates cells by activating distinct signaling pathways that involve nuclear translocation of NF kappa B. Since NF kappa B may also regulate the expression of genes that could contribute to myocardial dysfunction, the cardiomyocyte NF kappa B activation following acute or chronic TNF-alpha challenges was investigated. To accomplish this, the authors either acutely administered TNF-alpha to healthy mice, or used transgenic mice which chronically overexpress TNF-alpha exclusively in cardiac myocytes. Following acute administration of TNF-alpha, cardiac NF kappa B translocation was detected from 15 min to 2 h post-challenge. The time course of I kappa B alpha degradation was consistent with the kinetics of NF kappa B translocation. I kappa B beta degradation was slower and less dramatic. In transgenic mice chronically overexpressing TNF-alpha, myocardial NF kappa B activation was detected at all ages tested (21, 40, and 75 days). In contrast to acutely challenged animals, two distinct NF kappa B proteins were activated in chronically challenged animals, p50--65 heterodimers as well as p50 homodimers. Activation of both could be transiently blocked by administration of a recombinant chimeric TNF-alpha receptor antagonist (rhTNFR:Fc). I kappa B alpha, but not I kappa B beta, levels were elevated in transgenics when compared to wild-type animals. These data indicate that following acute TNF-alpha administration, which simulates bacterial sepsis, myocardial p50-p65 translocates within minutes. Chronic TNF-alpha exposure, which is thought to occur in long-standing congestive heart failure, results in translocation of transcriptionally inactive p50 homodimers in addition to transcriptionally active p50--65 heterodimers. It is speculated that activation of p50 homodimers constitutes an adaptive response to minimize the inflammatory consequences of chronic cardiac TNF-alpha exposure.

Interaction of surfactant protein A with lipopolysaccharide and regulation of inflammatory cytokines in the THP-1 monocytic cell line

Pulmonary surfactant protein A (SP-A) is involved in innate immunity in the lung. In this study we investigated the interaction of SP-A with different serotypes of lipopolysaccharide (LPS) on the regulation of inflammatory cytokines in vitro. In the human monocytic cell line, THP-1, combining SP-A with lipid A or rough LPS further enhanced lipid A- or rough LPS-stimulated tumor necrosis factor alpha (TNF-alpha) mRNA levels, while SP-A-elicited increases in TNF-alpha mRNA levels were partially neutralized. In contrast, the combination of smooth LPS and SP-A resulted in additive effects on TNF-alpha mRNA levels. We also demonstrated that there was cross-tolerance between SP-A and LPS in THP-1 cells. Pretreatment of THP-1 cells with LPS modestly inhibited the response of these cells to subsequent challenge with SP-A, with regard to the production of TNF-alpha, whereas there was no or little effect on the production of interleukin-1beta (IL-1beta) and IL-8. Conversely, pretreatment of THP-1 cells with SP-A markedly increased the response to subsequent challenge with LPS with regard to the production of IL-1beta and IL-8, although the production of TNF-alpha was modestly decreased. However, a synergistic stimulatory effect was observed when the two agents were added simultaneously to the cells. NF-kappaB formation was downregulated in SP-A- but not in LPS-induced tolerant cells. These results suggested that SP-A exhibits different interactions with distinct serotypes of LPS. In addition, SP-A is different from LPS with regard to the induction of cross-tolerance, and these actions may be mediated, at least in part, through different mechanisms.

NF-kappaB expression in mononuclear cells of patients with sepsis resembles that observed in lipopolysaccharide tolerance

The expression of NF-kappaB was studied in freshly isolated peripheral blood mononuclear cells (PBMC) of patients with severe sepsis and major trauma. The expression of p65p50 heterodimer, the active form of NF-kappaB, was significantly reduced for all patients as compared with control subjects. The p50p50 homodimer, an inhibitory form of NF-kappaB, was reduced in the survivors of sepsis and in patients with trauma. Subsequent in vitro stimulation of PBMC with lipopolysaccharide (LPS) did not induce further NF-kappaB nuclear translocation: the survivors of sepsis and trauma patients showed low expression of both p65p50 and p50p50, whereas nonsurvivors of sepsis showed a predominance of the inactive homodimer and a low p65p50/p50p50 ratio when compared with control subjects. In the later group of patients there was a reverse correlation between plasma IL-10 levels and the p65p50/p50p50 ratio after in vitro LPS stimulation (r = -0.8, p = 0.04). The reduced expression of nuclear NF-kappaB was not due to its inhibition by IkappaBalpha, as very low expression of IkappaBalpha, as well as low levels of p65 and p50 were found in the cytoplasm of PBMC from patients with sepsis and trauma when compared with control subjects. These results demonstrate that upon LPS activation, PBMC of patients with systemic inflammatory response syndrome show patterns of NF-kappaB expression that resemble those reported during LPS tolerance: global down-regulation of NF-kappaB in survivors of sepsis and trauma patients and the presence of large amounts of the inactive homodimer in the nonsurvivors of sepsis.

Suppression of hemopoiesis during CCl(4)-induced hepatic fibrosis: role of systemic endotoxemia

CCl(4)-induced hepatic fibrosis in mice was accompanied by insufficiency of bone marrow myelo- and erythropoiesis. Polymyxin B completely abolished these changes. This phenomenon can be explained by the development of macrophage endotoxin tolerance during hepatic fibrosis.

Inhibition of lipopolysaccharide-induced signal transduction in endotoxin-tolerized mouse macrophages: dysregulation of cytokine, chemokine, and toll-like receptor 2 and 4 gene expression

In this study, the effect of in vitro endotoxin tolerance on LPS-induced mitogen-activated protein kinase activation, transcription factor induction, and cytokine, chemokine, and Toll-like receptor (TLR) 2 and 4 gene expression, as well as the involvement of TNF and IL-1 signaling pathways in tolerance, were examined. Pretreatment of mouse macrophages with LPS inhibited phosphorylation of the extracellular signal-regulated kinases, c-Jun NH2-terminal kinases, and p38 kinase; degradation of I-kappaBalpha (inhibitory protein that dissociates from NF-kappaB) and I-kappaBbeta; and activation of the transcription factors NF-kappaB and AP-1 in response to subsequent LPS stimulation. These changes were accompanied by suppression of LPS-induced expression of mRNA for GM-CSF, IFN-gamma-inducible protein-10, KC, JE/monocyte chemoattractant protein-1, macrophage-inflammatory protein-1beta, and macrophage-inflammatory protein-2, with concurrent inhibition of chemokine secretion. In contrast to control cells, endotoxin-tolerant macrophages exhibited an increased basal level of TLR2 mRNA, and failed to increase levels of TLR2 mRNA or to down-regulate TLR4 gene expression upon restimulation with LPS. As judged by transcription factor activation, LPS and IL-1 were found to induce a state of cross-tolerance against each other, while no such reciprocal effect was seen for LPS and TNF-alpha. In addition, macrophages from TNFR I/II double knockout mice were LPS tolerizable, and blocking of endogenous TNF-alpha with TNFR-Fc fusion protein did not affect the capacity of LPS to tolerize macrophages. These data extend our understanding of LPS-signaling mechanisms that are inhibited in endotoxin-tolerized macrophages and suggest that endotoxin tolerance might result from impaired expression and/or functions of common signaling intermediates involved in LPS and IL-1 signaling.