Defective lipopolysaccharide-dependent ERK 1/2 activation in endotoxin tolerant murine macrophages is reversed by direct protein kinase C stimulation

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.

Lutzomyia longipalpis saliva triggers lipid body formation and prostaglandin E₂ production in murine macrophages

Background

Sand fly saliva contains molecules that modify the host's hemostasis and immune responses. Nevertheless, the role played by this saliva in the induction of key elements of inflammatory responses, such as lipid bodies (LB, also known as lipid droplets) and eicosanoids, has been poorly investigated. LBs are cytoplasmic organelles involved in arachidonic acid metabolism that form eicosanoids in response to inflammatory stimuli. In this study, we assessed the role of salivary gland sonicate (SGS) from Lutzomyia (L.) longipalpis, a Leishmania infantum chagasi vector, in the induction of LBs and eicosanoid production by macrophages in vitro and ex vivo.

Methodology/Principal Findings

Different doses of L. longipalpis SGS were injected into peritoneal cavities of C57BL/6 mice. SGS induced increased macrophage and neutrophil recruitment into the peritoneal cavity at different time points. Sand fly saliva enhanced PGE₂ and LTB₄ production by harvested peritoneal leukocytes after ex vivo stimulation with a calcium ionophore. At three and six hours post-injection, L. longipalpis SGS induced more intense LB staining in macrophages, but not in neutrophils, compared with mice injected with saline. Moreover, macrophages harvested by peritoneal lavage and stimulated with SGS in vitro presented a dose- and time-dependent increase in LB numbers, which was correlated with increased PGE₂ production. Furthermore, COX-2 and PGE-synthase co-localized within the LBs induced by L. longipalpis saliva. PGE₂ production by macrophages induced by SGS was abrogated by treatment with NS-398, a COX-2 inhibitor. Strikingly, SGS triggered ERK-1/2 and PKC-α phosphorylation, and blockage of the ERK-1/2 and PKC-α pathways inhibited the SGS effect on PGE₂ production by macrophages.

Conclusion

In sum, our results show that L. longipalpis saliva induces lipid body formation and PGE₂ production by macrophages ex vivo and in vitro via the ERK-1/2 and PKC-α signaling pathways. This study provides new insights regarding the pharmacological mechanisms whereby L. longipalpis saliva influences the early steps of the host's inflammatory response.

After the injection of saliva into the host's skin by sand flies, a transient erythematous reaction is observed, which is related to an influx of inflammatory cells and the release of various molecules that actively facilitate the blood meal. It is important to understand the specific mechanisms by which sand fly saliva manipulates the host's inflammatory responses. Herein, we report that saliva from Lutzomyia (L.) longipalpis, a widespread Leishmania vector, induces early production of eicosanoids. Intense formation of intracellular organelles called lipid bodies (LBs) was noted within those cells that migrated to the site of saliva injection. In vitro and ex vivo, sand fly saliva was able to induce LB formation and PGE₂ release by macrophages. Interestingly, PGE₂ production induced by L. longipalpis saliva was dependent on intracellular mechanisms involving phosphorylation of signaling proteins such as PKC-α and ERK-1/2 and subsequent activation of cyclooxygenase-2. Thus, this study provides new insights into the pharmacological properties of sand fly saliva and opens new opportunities for intervening with the induction of the host's inflammatory pathways by L. longipalpis bites.

Substance P inhibits natural killer cell cytotoxicity through the neurokinin-1 receptor

SP is a potent neuroimmunomodulator that functions through ligating members of the neurokinin receptor family, one of which, NK1R, is widely expressed in immune cells. As in humans, circulating SP levels are increased in pathologic states associated with impairment of NK cell functions, such as depression and HIV infection, we hypothesized that SP has a direct, inhibitory effect upon NK cells. We have studied a clonal human NK cell line (YTS) as well as ex vivo human NK cells and have determined that truncated and full-length NK1R isoforms are expressed in and SP bound by ex vivo NK cells and the YTS NK cell line. Incubation of YTS cells with 10⁻⁶ M SP and ex vivo NK cells with 10⁻⁵ M SP inhibited cytotoxic ability by ∼20% and reduced degranulation. This inhibitory effect upon cytotoxicity was partially prevented by the NK1R antagonist CP96,345. The treatment of YTS or ex vivo NK cells with SP neither down-modulated NCR expression nor affected triggering receptor-induced NF-κB activation. Preincubation of YTS cells with SP, however, did abbreviate the typically prolonged intracellular calcium increase induced by target cell engagement and reduced triggering receptor-induced pERK. Thus, SP has the potential to regulate NK cell functions and acts downstream from neurokinin receptors to modulate NK cell activation signaling. This mechanism may contribute to impairment of NK cell function in certain disease states associated with increased circulating SP. Antagonism of this system may present an opportunity to augment NK cell function therapeutically in selected human diseases.

Endotoxin tolerance in sepsis: concentration-dependent augmentation or inhibition of LPS-stimulated macrophage TNF secretion by LPS pretreatment

BACKGROUND

We previously showed that macrophages (MPhi) pretreated with bacterial endotoxin (lipopolysaccharide [LPS]) develop an altered state of LPS-responsiveness--"LPS tolerance": LPS tolerance was associated with inhibition of tumor necrosis factor (TNF) release and decreased extracellular signal-regulated kinase and p38 kinase activation when MPhi were restimulated with LPS. However, the concentration of LPS used for pretreatment (most frequently 10 ng/mL) may be much higher than LPS concentrations observed in patients. Therefore, in the current study we examined the effect of lower and higher pretreatment LPS concentrations on subsequent LPS-stimulated MPhi responses.

METHODS

RAW 264.7 MPhi-like cells were pretreated in vitro (PreRx) for 24 hours in medium or a range of LPS concentrations (0 ng/mL, 1 ng/mL, 10 ng/mL, or 100 ng/mL of E. coli 0111B4 LPS). Culture medium was discarded after 24 hours and MPhi were restimulated with LPS (0 ng/mL, 1 ng/mL, 10 ng/mL or 100 ng/mL). Three different lots of LPS (Sigma) were used. Supernatant TNF secretion at 3 hour was measured using enzyme-linked immunosorbent assay (pg/mL +/- SEM). Statistics by Chi-square and student's t test.

RESULTS

Pretreatment with 100 ng/mL of LPS profoundly inhibited TNF release at all LPS restimulation concentrations (p < 0.05 vs. Medium PreRx). In contrast, very low dose LPS pretreatment (1 ng/mL) significantly augmented TNF release versus medium (p < 0.05). There was no further augmentation observed when even lower doses of LPS (0.1 ng/mL) were used for pretreatment. Similar results were obtained with three different lots E. coli 0111B4 LPS or using LPS from E. coli 0127B8.

CONCLUSION

Prior exposure of MPhi to bacterial ligands alters MPhi cytokine production in response to subsequent LPS-stimulated activation. This modulated MPhi response is critically dependent on the concentration of LPS pretreatment.

Both ERK and JNK are required for enhancement of MD-2 gene expression during differentiation of HL-60 cells

BACKGROUND INFORMATION

MD-2 is associated with the extracellular domain of TLR4 (Toll-like receptor 4) and augments TLR4-dependent LPS (lipopolysaccharide) responses in vitro. Our previous investigation found that PMA-induced HL-60 cell differentiation to macrophages is associated largely with TLR2 and CD14 and, to a much lesser extent, with TLR4.

RESULTS

We studied the MD-2 expression during differentiation of HL-60 cells induced by PMA. The results showed that PMA, but not VitD(3) (1alpha,25-dihydroxy-vitamin D(3)), strongly induces MD-2 gene expression by HL-60 cells in a time- and dose-dependent manner. Treatment with an MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase] inhibitor (PD98059) and a JNK (c-Jun N-terminal kinase) inhibitor (SP600125) suppresses PMA-induced MD-2 gene expression, whereas impairment of p38 function by treatment with the inhibitor SB203580 has no effect on MD-2 mRNA. In order to reveal the possible molecular mechanism for such a regulation of MD-2 gene expression, we cloned and analysed the putative MD-2 gene promoter. Transient transfection of different deletion mutants demonstrated that the region -185/-171 (5'-TCCTTTACAGGAAGT-3') of the MD-2 gene promoter is closely related to gene transcription in response to PMA. Additionally, the transcription factor Elk-1 has been found to bind this specific motif.

CONCLUSIONS

These results suggest that ERK and JNK pathways are involved in PMA-mediated MD-2 gene expression during HL-60 cell differentiation, and the activation of the MEK/possible ERK/Elk signal pathway is the mechanism responsible for PMA-induced MD-2 gene expression in differentiated HL-60 cells.

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.

Modulation of toll-like receptor 4 expression on human monocytes by tumor necrosis factor and interleukin-6: tumor necrosis factor evokes lipopolysaccharide hyporesponsiveness, whereas interleukin-6 enhances lipopolysaccharide activity

Toll-like receptors (TLR) play a pivotal role in the innate immune response, and the expression levels of these receptors may reflect the sensitivity of immune cells to infections. The binding of lipopolysaccharide (LPS) to TLR-4 triggers human monocytes to produce cytokines, which play a dominant role in the inflammatory response, as can be observed during sepsis and after polytrauma. Here, we evaluated TLR-4 expression of isolated monocytes in the presence of tumor necrosis factor (TNF)-alpha, interleukin (IL) 6, IL-8, and IL-10, and we investigated cellular activation of this treatment. TNF-alpha significantly down-regulated TLR-4 mRNA expression after 6 h (100% vs. 38.5% +/- 4%; P < 0.05). This down-regulation was followed by a dose- and time-dependent diminished expression of TLR-4 surface protein (100% vs. 8.0% +/- 5%; P < 0.01). Forty-eight hours after TNF-alpha treatment, a reduced nuclear factor (NF)-kappaB translocation and a diminished IL-6 secretion after LPS stimulation were found (100% vs. 42.0% +/- 23%; P < 0.05). In contrast, IL-6 incubation upregulated TLR-4 cell surface protein (100% vs. 165.8% +/- 24%; P < 0.05) and increased the ability to activate NF-kappaB and AP-1 after LPS stimulation. Stimulation with IL-8 or IL-10 had no significant effects. We conclude that not only LPS but also TNF-alpha and IL-6 have the potency to regulate the immune response via TLR-4. Down-regulation of TLR-4 by TNF-alpha is associated with LPS hyporeactivity for NF-kappaB formation, whereas upregulation of TLR-4 via IL-6 can increase the responsiveness of mononuclear phagocytes.

Relative hyperoxia augments lipopolysaccharide-stimulated cytokine secretion by murine macrophages

BACKGROUND

Increased systemic levels of inflammatory mediators are seen after open abdominal operations. Macrophages that are exposed to lipopolysaccharide secrete cytokines. Peritoneal macrophages normally reside in a pO(2) of 40 mm Hg. We hypothesize that exposure of lipopolysaccharide-stimulated macrophages to "non-physiologic" pO(2) augments cytokine secretion.

METHOD

Murine macrophages were preconditioned to a pO(2) of 40 mm Hg for 24 hours. The medium then was discarded and exchanged for a medium containing a pO(2) of 40, 150, or 440 mm Hg. Macrophages were incubated in the desired pO(2) for 6 and 24 hours while stimulated with lipopolysaccharide (0 to 100 ng/mL). The effect of pO(2) was compared. Supernatant tumor necrosis factor (TNF) and interleukin-6 were measured with enzyme-linked immunosorbent assay. Statistics were performed with analysis of variance.

RESULTS

We found dose-dependent lipopolysaccharide-stimulated TNF and interleukin-6 production with macrophages incubated at physiologic pO(2). Higher pO(2) did not stimulate TNF and interleukin-6 in the absence of lipopolysaccharide. However, a pO(2) of 150 and 440 mm Hg significantly (P <.05) increased lipopolysaccharide-stimulated TNF and interleukin-6 production versus 45 mm Hg.

CONCLUSION

Our data suggest synergy between increased pO(2) and lipopolysaccharide for macrophage TNF and interleukin-6 production. Similar pO(2) elevations may occur with an open peritoneum or high supplemental O(2). Cytokines from peritoneal macrophages may contribute to the increased systemic inflammation after open operations.

Evidence for a CD14- and serum-independent pathway in the induction of endotoxin-tolerance in human monocytes and THP-1 monocytic cells

Lipopolysaccharide (LPS) stimulation of macrophages or monocytes is believed to occur via a serum- and CD14-dependent signaling pathway via toll-like receptor 4 (TLR4). We sought to determine whether serum and/or CD14 are required for LPS to induce the endotoxin-tolerant state in human monocytes. LPS treatments were performed in the presence or absence of an anti-CD14 monoclonal antibody and with or without fetal bovine serum. Endotoxin tolerance was assessed after an 18-h exposure (pretreatment) to 10 ng/mL of LPS. Medium was discarded and cells were challenged with activating (1-1000 ng/mL) doses of LPS. LPS-stimulated tumor necrosis factor (TNF) secretion into culture supernatants was determined after 5 h by ELISA and p44/p42 ERK kinase activation was measured after 30 min by Western blot. Statistical analysis was by ANOVA. LPS induced endotoxin-tolerance with a significant inhibition of LPS-stimulated TNF secretion and less p44/p42 ERK kinase activation. When LPS-stimulation of naïve (nontolerant) monocytes was performed in medium with anti-CD14 antibody or without serum, there was marked blunting of TNF release. However, LPS pretreatment in medium without serum or in medium containing anti-CD14 antibody resulted in changes in monocyte activation and function characteristic of endotoxin tolerance. LPS-stimulated p44/p42 ERK kinase activation and TNF release were diminished whether or not anti-CD14 antibody was present during LPS pretreatment. LPS-stimulated TNF secretion and p44/p42 ERK kinase activation require the presence of serum and are inhibited by anti-CD14 antibody. Our findings suggest that LPS induces endotoxin tolerance in human monocytic cells via a pathway that does not require serum or cell surface CD14.

Vitamin E (alpha-tocopherol) attenuates cyclo-oxygenase 2 transcription and synthesis in immortalized murine BV-2 microglia

One of the immediate early microglial genes that are up-regulated in response to proinflammatory stimuli is cyclo-oxygenase 2 (COX-2). In the present study, we have investigated the effects of alpha-tocopherol (alpha TocH), an essential constituent of the nervous system, on the activation of COX-2 in lipopolysaccharide (LPS)-stimulated mouse BV-2 microglia. In unstimulated BV-2 cells, COX-2 mRNA and protein were almost undetectable but were strongly up-regulated in response to LPS. Activation of COX-2 protein synthesis in LPS-stimulated BV-2 cells involved activation of the extracellular-signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK) pathway and was sensitive to the protein kinase C (PKC) inhibitors staurosporine and chelerythrine, and the MAP kinase/ERK kinase 1/2 inhibitors PD98059 and U0126. Supplementation of BV-2 cells with alpha TocH before LPS stimulation resulted in pronounced up-regulation of protein phosphatase 2A (PP2A) activity, down-regulation of PKC activity, ERK1/2 phosphorylation and nuclear factor kappa B (NF kappa B) activation. As a result, COX-2 protein levels and prostaglandin E(2) production were significantly lower in alpha TocH-supplemented cells. The effects of alpha TocH on PKC activity could be reverted by calyculin A and okadaic acid, two PP inhibitors. In summary, our results suggest that alpha TocH activates microglial PP2A activity and thereby silences an LPS-activated PKC/ERK/NF kappa B signalling cascade resulting in significantly attenuated COX-2 protein synthesis. These in vitro results imply that alpha TocH could induce quiescence to pathways that are associated with acute or chronic inflammatory conditions in the central nervous system.

Intracellular protein tyrosine phosphorylation of adherent human macrophages on adsorbed fibronectin

Fibronectin (FN) was pre-adsorbed onto physicochemically distinct substrates: polyethyleneglycol-based networks or tissue culture polystyrene (TCPS). The role of these substrates in modulating FN-mediated intracellular protein tyrosine phosphorylation and cell adhesion was analyzed with human primary blood derived macrophages. Although macrophage adhesion on both FN-pre-adsorbed TCPS and networks was similarly dependent on protein tyrosine kinase (PTK) and protein serine/threonine kinase (PSK), the compensation between PTK and PSK, and the involvement of signaling molecules (such as protein kinase C (PKC) isoforms) were distinct between the substrates. The pattern and the extent of tyrosine phosphorylation of several proteins (i.e. approximately 70, approximately 44, approximately 30kDa) were differentially regulated by PKCs. FN-derived peptides were employed to probe this material-dependency in macrophage adhesion and tyrosine phosphorylation. The PHSRN domain in the peptide sequence was predominant in mediating this substrate-dependent FN signaling event. We conclude that the tyrosine phosphorylation and the cross talk between PTK and PSK are modulated by FN and the substrate onto which the protein is adsorbed.

Human macrophage adhesion on fibronectin: the role of substratum and intracellular signalling kinases

Fibronectin and Arg-Gly-Asp (RGD)- and/or Pro-His-Ser-Arg-Asn (PHSRN)-containing oligopeptides were immobilized onto physicochemically distinct substrata: polyethyleneglycol-based networks or tissue culture polystyrene (TCPS). The role of selected signalling kinases in the adhesion of human primary blood-derived macrophages on these modified substrata was investigated. We demonstrated that the protein tyrosine kinase (PTK) or protein serine/threonine kinase (PSK) dependency and the PTK-PSK cross-talk compensation for macrophage adhesion varied dynamically with the substratum modification and the culture time. The inhibition of MAPK kinase (MAPKK) decreased macrophage adhesion on TCPS, whereas the inhibition of phosphoinositide-3 kinase (PI3 kinase) decreased macrophage adhesion on networks at 24 h. The PI3 kinase-protein kinase C (PKC)-MAPK cascade was involved in macrophage adhesion on fibronectin-preadsorbed TCPS or networks but not on fibronectin-grafted networks. This fibronectin-mediated adhesion signalling involved both RGD and PHSRN sequences in a form of G(3)RGDG(6)PHSRNG on TCPS but not on networks. Furthermore, G(3)RGDG(6)PHSRNG grafted onto networks evoked unique signalling in macrophage adhesion from that preadsorbed onto networks. Thus, macrophage adhesion and the role of selected signalling kinases were modulated by the substratum and the ligand conjugation method.

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.

Impaired mitogen-activated protein kinase activation and altered cytokine secretion in endotoxin-tolerant human monocytes

BACKGROUND

Dysregulation of monocyte/macrophage cytokine production after exposure to multiple inflammatory stimuli may contribute to multiple organ failure and sepsis. Endotoxin (lipopolysaccharide [LPS]) activation of murine macrophage results in the phosphorylation of kinases in the mitogen-activated protein kinase cascade. Pretreatment of murine macrophages with LPS induces LPS-tolerance, with inhibition of LPS-stimulated activation of kinases (ERK1/2 and p38) and diminished release of tumor necrosis factor (TNF). We sought to determine whether similar alterations in LPS-dependent signal transduction are present in LPS-tolerant human peripheral blood monocytes.

METHODS

Human peripheral blood monocytes from healthy volunteer donors (n = 12) were incubated in RPMI 1640 culture medium +/- 10 ng/mL of LPS for 18 hours, then stimulated with 0 to 1,000 ng/mL of LPS. Supernatant TNF and interleukin-1 (IL-1) levels were measured after 5 hours by enzyme-linked immunosorbent assay. Activation of the p42/p44 kinases (ERK1/2) was measured 15 minutes after LPS with monoclonal antibodies to diphosphorylated (active) ERK1/2 using novel flow cytometric methods.

RESULTS

LPS-tolerant (10 ng/mL LPS pretreatment) human monocytes had significant inhibition of LPS-stimulated TNF secretion but augmented IL-1 release (p < 0.05). Nontolerant human monocytes had a dramatic increase in the percentage of ERK1/2-positive cells in response to an initial stimulation with LPS. This did not occur in the LPS-tolerant cells. Phorbol-12-myristate-13 acetate restored ERK1/2 activation in LPS-tolerant human monocytes.

CONCLUSION

LPS-tolerance in human monocytes is associated with inhibition of LPS-stimulated TNF secretion, augmented release of IL-1, and defective activation of mitogen-activated protein kinase cascade (ERK1/2). These results suggest a method of identifying LPS-tolerance and monocyte dysfunction in patients with sepsis.

Endotoxin tolerance from lipopolysaccharide pretreatment induces nuclear factor-kappaB alterations not present in C3H/HeJ mice

BACKGROUND

Lipopolysaccharide (LPS) activation of macrophage (MO) cytokine secretion requires activation and translocation of nuclear factor-kappaB (NF-kappaB). Endotoxin tolerance induced in LPS-responsive C3H/HeN MOs by LPS pretreatment results in decreased tumor necrosis factor (TNF) secretion and altered NF-kappaB activation. C3H/HeJ MOs have a genetic defect that renders them tolerant to LPS activation. We hypothesized that the alterations of NF-kappaB activation seen with LPS tolerance in HeN MOs would be present in HeJ mice.

METHODS

MOs from C3H/HeJ and C3H/HeN mice were cultured with +/- 10 ng/mL LPS pretreatment for 24 hours and then stimulated with 1 to 1,000 ng/mL LPS. Activation of NF-kappaB was assayed by gel shift using a 32P-labeled specific oligonucleotide 30 minutes after LPS activation. TNF secretion 6 hours after LPS stimulation was measured by bioassay.

RESULTS

LPS stimulation activated NF-kappaB in both HeN and HeJ MOs. We observed decreased NF-kappaB activation and a characteristic mobility shift in endotoxin-tolerant MOs from HeN mice that were not present in HeJ MOs. In contrast with the results in HeN mice, LPS pretreatment did not induce any alterations in NF-kappaB activation in HeJ MOs. LPS-stimulated TNF secretion was decreased in HeN MOs after LPS pretreatment. There was no change in TNF secretion in HeJ MOs, but, overall, TNF secretion by these cells was much less than that seen in HeN cells.

CONCLUSION

MOs from C3H/HeN mice rendered LPS-tolerant by low-dose LPS pretreatment have alterations in activation of NF-kappaB not present in LPS-hyporesponsive C3H/HeJ mice.