A comparative analysis of cytokine production and tolerance induction by bacterial lipopeptides, lipopolysaccharides and Staphyloccous aureus in human monocytes

Staphylococcus aureus induces tolerance in human monocytes accompanied with expression changes of cell surface markers

Exposure of human monocytes to lipopolysaccharide (LPS) or other pathogen-associated molecular pattern (PAMPs) induces a temporary insensitivity to subsequent LPS challenges, a cellular state called endotoxin tolerance (ET), associated with the pathogenesis of sepsis. In this study, we aimed to characterize the cellular state of human monocytes from healthy donors stimulated with Staphylococcus aureus in comparison to TLR2-specific ligands. We analyzed S. aureus induced gene expression changes after 2 and 24 hours by amplicon sequencing (RNA-AmpliSeq) and compared the pro-inflammatory response after 2 hours with the response in re-stimulation experiments. In parallel, glycoprotein expression changes in human monocytes after 24 hours of S. aureus stimulation were analyzed by proteomics and compared to stimulation experiments with TLR2 ligands Malp-2 and Pam3Cys and TLR4 ligand LPS. Finally, we analyzed peripheral blood monocytes of patients with S. aureus bloodstream infection for their ex vivo inflammatory responses towards S. aureus stimulation and their glycoprotein expression profiles. Our results demonstrate that monocytes from healthy donors stimulated with S. aureus and TLR ligands of Gram-positive bacteria entered the tolerant cell state after activation similar to LPS treatment. In particular reduced gene expression of pro-inflammatory cytokines (TNF, IL1β) and chemokines (CCL20, CCL3, CCL4, CXCL2, CXCL3 and CXCL8) could be demonstrated. Glycoprotein expression changes in monocytes tolerized by the different TLR agonists were highly similar while S. aureus-stimulated monocytes shared some of the PAMP-induced changes but also exhibited a distinct expression profile. 11 glycoproteins (CD44, CD274, DSC2, ICAM1, LAMP3, LILRB1, PTGS2, SLC1A3, CR1, FGL2, and HP) were similarly up- or downregulated in all four comparisons in the tolerant cell state. Monocytes from patients with S. aureus bacteremia revealed preserved pro-inflammatory responsiveness to S. aureus stimulation ex vivo, expressed increased CD44 mRNA but no other glycoprotein of the tolerance signature was differentially expressed.

Staphylococcus aureus lipoproteins in infectious diseases

Infections with the Gram-positive bacterial pathogen Staphylococcus aureus remain a major challenge for the healthcare system and demand new treatment options. The increasing antibiotic resistance of S. aureus poses additional challenges, consequently inflicting a huge strain in the society due to enormous healthcare costs. S. aureus expresses multiple molecules, including bacterial lipoproteins (Lpps), which play a role not only in immune response but also in disease pathogenesis. S. aureus Lpps, the predominant ligands of TLR2, are important for bacterial survival as they maintain the metabolic activity of the bacteria. Moreover, Lpps possess many diverse properties that are of vital importance for the bacteria. They also contribute to host cell invasion but so far their role in different staphylococcal infections has not been fully defined. In this review, we summarize the current knowledge about S. aureus Lpps and their distinct roles in various infectious disease animal models, such as septic arthritis, sepsis, and skin and soft tissue infections. The molecular and cellular response of the host to S. aureus Lpp exposure is also a primary focus.

Anti-Inflammatory Effect of Muscle-Derived Interleukin-6 and Its Involvement in Lipid Metabolism

Interleukin (IL)-6 has been studied since its discovery for its role in health and diseases. It is one of the most important pro-inflammatory cytokines. IL-6 was reported as an exacerbating factor in coronavirus disease. In recent years, it has become clear that the function of muscle-derived IL-6 is different from what has been reported so far. Exercise is accompanied by skeletal muscle contraction, during which, several bioactive substances, collectively named myokines, are secreted from the muscles. Many reports have shown that IL-6 is the most abundant myokine. Interestingly, it was indicated that IL-6 plays opposing roles as a myokine and as a pro-inflammatory cytokine. In this review, we discuss why IL-6 has different functions, the signaling mode of hyper-IL-6 via soluble IL-6 receptor (sIL-6R), and the involvement of soluble glycoprotein 130 in the suppressive effect of hyper-IL-6. Furthermore, the involvement of a disintegrin and metalloprotease family molecules in the secretion of sIL-6R is described. One of the functions of muscle-derived IL-6 is lipid metabolism in the liver. However, the differences between the functions of IL-6 as a pro-inflammatory cytokine and the functions of muscle-derived IL-6 are unclear. Although the involvement of myokines in lipid metabolism in adipocytes was previously discussed, little is known about the direct relationship between nonalcoholic fatty liver disease and muscle-derived IL-6. This review is the first to discuss the relationship between the function of IL-6 in diseases and the function of muscle-derived IL-6, focusing on IL-6 signaling and lipid metabolism in the liver.

Specific features of human monocytes activation by monophosphoryl lipid A

We deciphered the mechanisms of production of pro- and anti-inflammatory cytokines by adherent human blood mononuclear cells (PBMC) activated by lipopolysaccharide (LPS) or monophosphoryl lipid A (MPLA). Both LPS and MPLA induced tumor necrosis factor (TNF) production proved to be dependent on the production of interleukin-1β (IL-1β). Of note, MPLA induced IL-1β release in human adherent PBMCs whereas MPLA was previously reported to not induce this cytokine in murine cells. Both LPS and MPLA stimulatory effects were inhibited by Toll-like receptor-4 (TLR4) antagonists. Only monocytes activation by LPS was dependent on CD14. Other differences were noticed between LPS and MPLA. Among the different donors, a strong correlation existed in terms of the levels of TNF induced by different LPSs. In contrast, there was no correlation between the TNF productions induced by LPS and those induced by MPLA. However, there was a strong correlation when IL-6 production was analyzed. Blocking actin polymerization and internalization of the agonists inhibited MPLA induced TNF production while the effect on LPS induced TNF production depended on the donors (i.e. high TNF producers versus low TNF producers). Finally, conventional LPS, tolerized adherent PBMCs to TLR2 agonists, while MPLA primed cells to further challenge with TLR2 agonists.

Glucose phosphorylated on carbon 6 suppresses lipopolysaccharide binding to lipopolysaccharide-binding protein and inhibits its bioactivities

Lipid A comprises the active region of lipopolysaccharide (LPS), and its phosphate group is required for LPS activities. Additionally, it is essential for effects of inhibitors of LPS-induced coagulation activity in limulus amebocyte lysate (LAL) tests. Lipid A has phosphorylated glucosamine residues, which are structurally similar to glucose 1-phosphate (G1P) and glucose 6-phosphate (G6P). This study focused on the antagonistic effects of glucose phosphates on the action of protein or non-protein inhibitors against LAL coagulation, LPS-LPS-binding protein (LBP) interaction, and LPS bioactivities. These effects of glucose phosphates were evaluated and compared with those of other charged sugars such as fructose 6-phosphate and glucuronic acid by LAL tests, ELISA-based LPS-LBP binding assay, cell-based assay, and using a mouse endotoxin shock model. G6P neutralized the interfering actions of drug substances and plasma proteins on LPS coagulation activity in LAL tests. Compared to other sugars, G6P more strongly inhibited LPS binding to LBP, leading to significant inhibition of LPS-induced cellular responses in human umbilical vein endothelial cells and in the THP-1 human leukemic line. Consistent herewith, G6P inhibited inflammatory cytokine release and decreased serum alanine aminotransferase and hepatic caspase-3/7 activities and mortality in LPS-stimulated d-galactosamine-sensitized mice. These data indicated that the structural properties of G6P, such as its glucose moiety and phosphorylation on carbon 6, are important for suppressing the interaction of proteins with LPS. Therefore, G6P is useful to improve sensitivity and accuracy of plasma and drug LPS assays, and such structural property is more suitable to antagonize LPS activities.

Endotoxin-induced cross-tolerance to Gram-positive sepsis

The manifestations of Gram-positive sepsis and Gram-negative sepsis share some common clinical features suggesting common pathways of activation. The goal of this study was to assess whether lipopolysaccharide (LPS) can produce cross-tolerance to Gram-positive sepsis induced by group B streptococcus (GBS). Thromboxane (TxB2), tumor necrosis factor (TNFα), and nitric oxide (NO) production by in vitro LPS- and heat killed GBS-stimulated rat peritoneal macrophages were measured. Since our previous studies have demonstrated altered macrophage activation of extracellular signal-regulated kinases 1 and 2 (ERK 1/2) in tolerance, we also examined the effect of LPS and killed GBS on ERK 1/2 activation in normal and LPS tolerant macrophages. Tolerance was induced in rats by intraperitoneal injection of Salmonella enteritidis LPS or vehicle for two consecutive days at doses of 0.1 and 0.5 mg/kg body weight. Three days after the second LPS dose, rats were injected intravenously with viable GBS (5 x l09 cfu/kg) and D-galactosamine (1 g/kg). LPS tolerance significantly prolonged ( P <0.05) mean survival time to severe GBS sepsis in D-galactosamine sensitized rats from 12.9 ± 1.7 h in control rats to 44.0 ± 8.9 h in tolerant rats. Peritoneal macrophages from LPS tolerant rats exhibited suppressed LPS induced in vitro TxB2 and TNFα production ( P <0.05). Tolerance also decreased in vitro heat killed GBS-induced TNFα production, but did not significantly affect TxB2 production. NO production stimulated by LPS (10 µg/ml was not impaired in LPS tolerance; however at lower doses (0.02—1.25 µg/ml), NO production was significantly decreased ( P <0.05). NO production was augmented ( P <0.05) in response to stimulation with GBS (10 µg/ml) and unaltered at lower doses (0.02—1.25 µg/ml) in tolerant cells. LPS activated ERK 1/2 in control macrophages, but activation of ERK 1/2 was suppressed in LPS tolerance. GBS did not significantly affect ERK 1/2 activity in control or tolerant macrophages. Nevertheless, the selective mitogen-activated kinase (MAPK)/ERK kinase (MEK) inhibitor, PD 98059 blocked ( P <0.05) both GBS- and LPS-induced TNFα and TxB2 production, but not NO production. Thus, some level of ERK 1/2 activity appears essential for GBS- and LPS-induced macrophage activation. In conclusion, LPS tolerance induces partial cross-tolerance to Gram-positive sepsis induced lethality, and alters LPS- and GBS-induced in vitro peritoneal macrophage mediator production. This suggests common pathways of cellular activation for GBS and LPS that are altered by LPS tolerance.

Toll-like receptor-2 transduces signals for NF-κB activation, apoptosis and reactive oxygen species production

The innate immune system coordinates the inflammatory response to pathogens. To do so, cells of the innate immune system must rapidly discriminate between self and non-self. All bacteria express membrane-associated lipoproteins. These molecules activate cells of the innate immune system to initiate host defense mechanisms. However, it is currently unknown how the innate immune system recognizes bacterial lipoproteins. Here, we describe that in response to bacterial lipoprotein, human Toll-like receptor-2 activates three different cellular responses: nuclear factor-κB dependent transcription, programmed cell death and reactive oxygen species production. We propose that Toll-like receptor-2 fulfils multiple roles in the genesis of the immune response to bacterial pathogens.

High glucose-induced oxidative stress increases IL-8 production in human gingival epithelial cells

OBJECTIVE

Diabetes is often associated with increased prevalence and severity of periodontal disease. We hypothesized that gingival epithelial cells modify periodontal disease progression and predicted that hyperglycemia would activate an inflammatory response in human gingival epithelial cells (HGECs).

MATERIALS AND METHODS

We tested our hypothesis in immortalized HGECs (epi 4 cells) isolated from periodontal tissue and transfected with the simian virus 40 T antigen. The epi 4 cells were cultured in high (25 mM, HG) and normal (6 mM, NG) glucose conditions.

RESULTS

The epi 4 cells showed increased interleukin-8 (IL-8) protein secretion and mRNA expression when cultured in HG, compared with in NG. These effects were not associated with increased cell proliferation and were not observed in a hyperosmolar control group (normal glucose with 19 mM mannitol). Increased IL-8 secretion in HG was inhibited by pretreatment with an antioxidant, N-acetylcysteine, or a protein kinase C inhibitor, Ro31-8220. Hyperglycemia did not affect IL-8 secretion by gingival fibroblasts or periodontal ligament cells. In epi 4 cells, hyperglycemia also induced expression of toll-like receptor 2 (TLR2) but not TLR4.

CONCLUSION

These findings suggest a potential participation of epithelial cells in periodontal disease during diabetes by evoking an excessive host inflammatory response.

Immunostimulation by phospholipopeptide biosurfactant from Staphylococcus hominis in Oreochromis mossambicus

The immunostimulatory effect of phospholipopeptide biosurfactant from Staphylococcus hominis (GenBank Accession No: KJ564272) was assessed with Oreochromis mossambicus. The non-specific (serum lysozyme activity, serum antiprotease activity, serum peroxidase activity and serum bactericidal activity), specific (bacterial agglutination assay) immune responses and disease resistance activity against Aeromonas hydrophila were examined. Fish were intraperitonially injected with water soluble secondary metabolite (biosurfactant) of S. hominis at a dose of 2 mg, 20 mg and 200 mg kg(-1) body weight. Commercial surfactant surfactin (sigma) at 20 mg kg(-1) was used as standard and saline as negative control. All the doses of water soluble biosurfactant tested, significantly enhanced the specific, nonspecific immunity and disease resistance from the day of post administration of phospholipopeptide biosurfactant till the tail of the experimental period. These results clearly indicated that the secondary metabolite isolated from S. hominis stimulates the immunity of finfish thereby could enhance aquaculture production.

From cytokine to myokine: the emerging role of interleukin-6 in metabolic regulation

The lack of physical activity and overnutrition in our modern lifestyle culminates in what we now experience as the current obesity and diabetes pandemic. Medical research performed over the past 20 years identified chronic low-grade inflammation as a mediator of these metabolic disorders. Hence, finding therapeutic strategies against this underlying inflammation and identifying molecules implicated in this process is of significant importance. Following the observation of an increased plasma concentration of interleukin-6 (IL-6) in obese patients, this protein, known predominantly as a pro-inflammatory cytokine, came into focus. In an attempt to clarify its importance, several studies implicated IL-6 as a co-inducer of the development of obesity-associated insulin resistance, which precedes the development of type 2 diabetes. However, the identification of IL-6 as a myokine, a protein produced and secreted by skeletal muscle to fulfil paracrine or endocrine roles in the insulin-sensitizing effects following exercise, provides a contrasting and hence paradoxical identity of this protein in the context of metabolism. We review here the literature considering the complex, pleiotropic role of IL-6 in the context of metabolism in health and disease.

Innate immune response of alveolar macrophage to house dust mite allergen is mediated through TLR2/-4 co-activation

House dust mite, Dermatophagoides pteronyssinus (Der p), is one of the major allergens responsible for allergic asthma. However, the putative receptors involved in the signalization of Der p to the innate immune cells are still poorly defined as well as the impact of their activation on the outcome of the allergen-induced cell response. We previously reported that the HDM activation of mouse alveolar macrophages (AM) involves the TLR4/CD14 cell surface receptor complex. Here using a TLR ligand screening essay, we demonstrate that HDM protein extract engages the TLR2, in addition to the TLR4, in engineered TLR-transfected HEK cells but also in the MH-S mouse alveolar macrophage cell line model. Moreover we found that the concomitant recruitment of the MH-S cell’s TLR2 and TLR4 receptors by the HDM extract activates the MyD88-dependent signaling pathway and leads to the secretion of the NF-κB regulated pro-inflammatory factors NO and TNF-α. However unlike with the canonical TLR4 ligand (i.e. the bacterial LPS) mobilization of TLR4 by the HDM extract induces a reduced production of the IL-12 pro-inflammatory cytokine and fails to trigger the expression of the T-bet transcription factor. Finally we demonstrated that HDM extract down-regulates LPS induced IL-12 and T-bet expression through a TLR2 dependent mechanism. Therefore, we propose that the simultaneous engagement of the TLR2 and TLR4 receptors by the HDM extract results in a cross regulated original activation pattern of the AM which may contribute to the Th2 polarization of the allergen-induced immune response. The deciphering of these cross-regulation networks is of prime importance to open the way for original therapeutic strategies taking advantage of these receptors and their associated signaling pathways to treat allergic asthma.

Gram-positive bacteria as an antigen topically applied into gingival sulcus of immunized rat accelerates periodontal destruction

BACKGROUND AND OBJECTIVE

Periodontitis is generally accepted to relate to gram-negative bacteria, and the host defense system influences its onset and progression. However, little is known about the relation between gram-positive bacteria and periodontitis. In this study, we topically applied gram-positive and gram-negative bacterial suspensions to the gingival sulcus in rats after immunization, and then histopathologically examined their influence on periodontal destruction.

MATERIALS AND METHODS

Rats previously immunized with heat-treated and sonicated Staphylococcus aureus or Aggregatibacter actinomycetemcomitans were used as immunized groups. The non-immunized group received only sterile phosphate-buffered saline. In each animal, S. aureus or A. actinomycetemcomitans suspension was applied topically to the palatal gingival sulcus of first molars every 24 h for 10 d. Blood samples were collected and the serum level of anti-S. aureus or anti-A. actinomycetemcomitans immunoglobulin G (IgG) antibodies was determined by enzyme-linked immunosorbent assay. The first molar regions were resected and observed histopathologically. Osteoclasts were stained with tartrate-resistant acid phosphatase (TRAP). The formation of immune complexes was confirmed by immunohistological staining of C1qB.

RESULTS

Serum levels of anti-S. aureus and anti-A. actinomycetemcomitans IgG antibodies in the immunized groups were significantly higher than those in the non-immunized groups were. The loss of attachment, increase in apical migration of the junctional epithelium, and decreases in alveolar bone level and number of TRAP-positive multinuclear cells in each immunized group were significantly greater than in each non-immunized group. The presence of C1qB was observed in the junctional epithelium and adjacent connective tissue in the immunized groups.

CONCLUSIONS

Heat-treated and sonicated S. aureus and A. actinomycetemcomitans induced attachment loss in rats immunized with their suspensions. Our results suggest that not only gram-negative but also gram-positive bacteria are able to induce periodontal destruction.

Innate immune regulation by toll-like receptors in the brain

The innate immune system plays an important role in cerebral health and disease. In recent years the role of innate immune regulation by toll-like receptors in the brain has been highlighted. In this paper the expression of toll-like receptors and endogenous toll-like receptor ligands in the brain and their role in cerebral ischemia will be discussed. Further, the ability of systemic toll-like receptor ligands to induce cerebral inflammation will be reviewed. Finally, the capacity of toll-like receptors to both increase (sensitization) and decrease (preconditioning/tolerance) the vulnerability of the brain to damage will be disclosed. Studies investigating the role of toll-like receptors in the developing brain will be emphasized.

Escherichia coli K1-induced cytopathogenicity of human brain microvascular endothelial cells

Pathophysiology of Escherichia coli sepsis is complex involving circulating bacterial products, cytokine release, and sustained bacteremia resulting in the damage of vascular endothelium. Here, it is shown that E. coli K1 produced cytopathogenicity of human brain microvascular endothelial cells (HBMEC), that constitute the blood-brain barrier. Whole bacteria or their conditioned medium produced severe HBMEC damage suggesting E. coli K1-cytopathogenicity is a contact-independent process. Using lipopolysaccharide (LPS) inhibitor, polymyxin B, purified LPS extracted from E. coli K1 as well as LPS mutant derived from E. coli K1, we showed that LPS is not the sole determinant of E. coli K1-mediated HBMEC death. Bacterial product(s) for HBMEC cytopathogenicity was heat-labile suggesting LPS-associated proteins. Several isogenic gene-deletion mutants (ΔompA, ΔibeA, ΔibeB, Δcnf1) exhibited HBMEC cytopathogenicity similar to that produced by wild type E. coli K1. E. coli K1-mediated HBMEC death was independent of phosphatidylinositol 3-kinase (PI3K) but dependent partially on focal adhesion kinase (FAK) using HBMEC expressing dominant negative FAK and PI3K.

Contraction-induced interleukin-6 gene transcription in skeletal muscle is regulated by c-Jun terminal kinase/activator protein-1

Exercise increases the expression of the prototypical myokine IL-6, but the precise mechanism by which this occurs has yet to be identified. To mimic exercise conditions, C2C12 myotubes were mechanically stimulated via electrical pulse stimulation (EPS). We compared the responses of EPS with the pharmacological Ca(2+) carrier calcimycin (A23187) because contraction induces marked increases in cytosolic Ca(2+) levels or the classical IκB kinase/NFκB inflammatory response elicited by H(2)O(2). We demonstrate that, unlike H(2)O(2)-stimulated increases in IL-6 mRNA, neither calcimycin- nor EPS-induced IL-6 mRNA expression is under the transcriptional control of NFκB. Rather, we show that EPS increased the phosphorylation of JNK and the reporter activity of the downstream transcription factor AP-1. Furthermore, JNK inhibition abolished the EPS-induced increase in IL-6 mRNA and protein expression. Finally, we observed an exercise-induced increase in both JNK phosphorylation and IL-6 mRNA expression in the skeletal muscles of mice after 30 min of treadmill running. Importantly, exercise did not increase IL-6 mRNA expression in skeletal muscle-specific JNK-deficient mice. These data identify a novel contraction-mediated transcriptional regulatory pathway for IL-6 in skeletal muscle.

Bacterial superantigens enhance the in vitro proinflammatory response and in vivo lethality of the TLR2 agonist bacterial lipoprotein

Bacterial superantigens are Gram-positive exotoxins that induce proinflammatory cytokine release in vitro, cause lethal shock in vivo, and can be detected in the bloodstream of critically ill patients. They also have a powerful priming effect on the TLR4 agonist LPS. The aim of this study was to investigate the relationship between superantigens and the TLR2 agonist bacterial lipoprotein (BLP). Priming of human monocytes or PBMCs with superantigens significantly enhanced proinflammatory cytokine TNF-α and IL-6 release in response to BLP stimulation. The priming effect of superantigens could be blocked by inhibiting p38 MAPK during the priming phase as opposed to NF-κB or ERK inhibition. This was consistent with higher expression of the phosphorylated p38 after superantigen priming and BLP or LPS stimulation. C57BL/6 mice with superantigen priming (10 μg/mouse) when challenged with BLP (600 μg/mouse) exhibited substantially higher mortality (100%) compared with mice without superantigen priming (zero). Mice given superantigen alone did not demonstrate any signs of illness. Mice challenged with both superantigen and BLP had significantly higher levels of serum TNF-α and IL-6 compared with those of mice challenged with either agent alone. Depletion of the monocyte/macrophage subpopulation significantly reduced the mortality rate from 100 to 20% in superantigen-primed, BLP-challenged C57BL/6 mice, with a 5- to 10-fold decrease in serum TNF-α and IL-6. Our results demonstrate that bacterial superantigens enhance the in vitro proinflammatory cytokine release and in vivo lethality of BLP. This novel finding may help to explain the massive proinflammatory cytokine release seen in superantigen-mediated septic shock.

Human monocytes tolerant to LPS retain the ability to phagocytose bacteria and generate reactive oxygen species

Tolerance to lipopolysaccharide (LPS) occurs when animals or cells exposed to LPS become hyporesponsive to a subsequent challenge with LPS. This mechanism is believed to be involved in the down-regulation of cellular responses observed in septic patients. The aim of this investigation was to evaluate LPS-induced monocyte tolerance of healthy volunteers using whole blood. The detection of intracellular IL-6, bacterial phagocytosis and reactive oxygen species (ROS) was determined by flow cytometry, using anti-IL-6-PE, heat-killed Staphylococcus aureus stained with propidium iodide and 2',7'-dichlorofluorescein diacetate, respectively. Monocytes were gated in whole blood by combining FSC and SSC parameters and CD14-positive staining. The exposure to increasing LPS concentrations resulted in lower intracellular concentration of IL-6 in monocytes after challenge. A similar effect was observed with challenge with MALP-2 (a Toll-like receptor (TLR)2/6 agonist) and killed Pseudomonas aeruginosa and S. aureus, but not with flagellin (a TLR5 agonist). LPS conditioning with 15 ng/mL resulted in a 40% reduction of IL-6 in monocytes. In contrast, phagocytosis of P. aeruginosa and S. aureus and induced ROS generation were preserved or increased in tolerant cells. The phenomenon of tolerance involves a complex regulation in which the production of IL-6 was diminished, whereas the bacterial phagocytosis and production of ROS was preserved. Decreased production of proinflammatory cytokines and preserved or increased production of ROS may be an adaptation to control the deleterious effects of inflammation while preserving antimicrobial activity.

Checks and balances: the ocular response to infection

Bacterial corneal infections threaten vision. With the widespread use of contact lenses and the increasing number of vision-correction (refractive) surgeries, the number of bacterial corneal infection (keratitis) cases has dramatically increased over the past decade. These infections are often blinding, as bacteria multiply in the corneal epithelium and stroma, provoking inflammatory cell migration into the cornea, and ultimately damage or destruction of corneal tissue.

Response of corneal epithelial cells to Staphylococcus aureus

Staphylococcus aureus is a leading cause of invasive infection. It also infects wet mucosal tissues including the cornea and conjunctiva. Conflicting evidence exists on the expression of Toll-like receptors by human corneal epithelial cells. It was therefore of interest to determine how epithelial cells from this immune privileged tissue respond to S. aureus. Further, it was of interest to determine whether cytolytic toxins, with the potential to cause ion flux or potentially permit effector molecule movement across the target cell membrane, alter the response. Microarrays were used to globally assess the response of human corneal epithelial cells to S. aureus. A large increase in abundance of transcripts encoding the antimicrobial dendritic cell chemokine, CCL20, was observed. CCL20 release into the medium was detected, and this response was found to be largely TLR2 and NOD2 independent. Corneal epithelial cells also respond to S. aureus by increasing the intracellular abundance of mRNA for inflammatory mediators, transcription factors, and genes related to MAP kinase pathways, in ways similar to other cell types. The corneal epithelial cell response was surprisingly unaffected by toxin exposure. Toxin exposure did, however, induce a stress response. Although model toxigenic and non-toxigenic strains of S. aureus were employed in the present study, the results obtained were strikingly similar to those reported for stimulation of vaginal epithelial cells by clinical toxic shock toxin expressing isolates, demonstrating that the initial epithelial cellular responses to S. aureus are largely independent of strain as well as epithelial cell tissue source.

Muscle as an endocrine organ: focus on muscle-derived interleukin-6

Skeletal muscle has recently been identified as an endocrine organ. It has, therefore, been suggested that cytokines and other peptides that are produced, expressed, and released by muscle fibers and exert paracrine, autocrine, or endocrine effects should be classified as "myokines." Recent research demonstrates that skeletal muscles can produce and express cytokines belonging to distinctly different families. However, the first identified and most studied myokine is the gp130 receptor cytokine interleukin-6 (IL-6). IL-6 was discovered as a myokine because of the observation that it increases up to 100-fold in the circulation during physical exercise. Identification of IL-6 production by skeletal muscle during physical activity generated renewed interest in the metabolic role of IL-6 because it created a paradox. On one hand, IL-6 is markedly produced and released in the postexercise period when insulin action is enhanced but, on the other hand, IL-6 has been associated with obesity and reduced insulin action. This review focuses on the myokine IL-6, its regulation by exercise, its signaling pathways in skeletal muscle, and its role in metabolism in both health and disease.

MyD88-dependent and MyD88-independent pathways in synergy, priming, and tolerance between TLR agonists

TLRs sense components of microorganisms and are critical host mediators of inflammation during infection. Different TLR agonists can profoundly alter inflammatory effects of one another, and studies suggest that the sequence of exposure to TLR agonists may importantly impact on responses during infection. We tested the hypothesis that synergy, priming, and tolerance between TLR agonists follow a pattern that can be predicted based on differential engagement of the MyD88-dependent (D) and the MyD88-independent (I) intracellular signaling pathways. Inflammatory effects of combinations of D and I pathway agonists were quantified in vivo and in vitro. Experiments used several D-specific agonists, an I-specific agonist (poly(I:C)), and LPS, which acts through both the D and I pathways. D-specific agonists included: peptidoglycan-associated lipoprotein, Pam3Cys, flagellin, and CpG DNA, which act through TLR2 (peptidoglycan-associated lipoprotein and Pam3Cys), TLR5, and TLR9, respectively. D and I agonists were markedly synergistic in inducing cytokine production in vivo in mice. All of the D-specific agonists were synergistic with poly(I:C) in vitro in inducing TNF and IL-6 production by mouse bone marrow-derived macrophages. Pretreatment of bone marrow-derived macrophages with poly(I:C) led to a primed response to subsequent D-specific agonists and vice versa, as indicated by increased cytokine production, and increased NF-kappaB translocation. Pretreatment with a D-specific agonist augmented LPS-induced IFN-beta production. All D-specific agonists induced tolerance to one another. Thus, under the conditions studied here, simultaneous and sequential activation of both the D and I pathways causes synergy and priming, respectively, and tolerance is induced by agonists that act through the same pathway.

Advances in potential M-protein peptide-based vaccines for preventing rheumatic fever and rheumatic heart disease

Rheumatic fever (RF) and rheumatic heart disease (RHD) are postinfectious complications of an infection (or repeated infection) with the Gram-positive bacterium, Streptococcus pyogenes (also known as group A streptococcus, GAS). RF and RHD are global problems and affect many indigenous populations of developed countries and many developing countries. However, RF and RHD are only part of a larger spectrum of diseases caused by this organism. The development of a vaccine against GAS has primarily targeted the abundant cell-surface protein called the M-protein. This review focuses on different M-protein-based-subunit vaccine approaches and the different delivery technologies used to administer these vaccine candidates in preclinical studies.

High Expression Levels of Receptor Activator of Nuclear Factor-Kappa B Ligand Associated With Human Chronic Periodontitis Are Mainly Secreted by CD4+T Lymphocytes

BACKGROUND

Chronic periodontitis is an infectious disease characterized by alveolar bone destruction and teeth loss. Receptor activator of nuclear factor-kappa B ligand (RANKL) is an osteoclastogenic cytokine, a central regulatory factor in the osteoclast's lifespan, and a participant in physiological and pathological bone resorption. Gingival T cells synthesize RANKL, contributing to molecular local imbalance that entails the alveolar bone resorption seen in periodontitis. Our study was aimed at associating the levels of RANKL with the CD4(+) T-cell activity present in gingival tissues of chronic periodontitis patients.

METHODS

Gingival biopsies were obtained from 33 chronic periodontitis patients and 20 healthy controls. Specimens were either formalin fixed and paraffin embedded for real-time reverse transcription-polymerase chain reaction (RT-PCR) and histologic analysis or tissue digestion processed for cell culture and flow-cytometry analysis. RANKL mRNA and protein levels were determined by quantitative RT-PCR and enzyme-linked immunosorbent assay (ELISA) in gingival-cell culture supernatants. Gingival leukocytes were quantified by flow cytometry. RANKL and CD4 immunoreactivity were analyzed by flow cytometry and confocal microscopy.

RESULTS

RANKL mRNA levels were higher in patients with periodontitis than in healthy subjects, and spontaneous and lipopolysaccharide (LPS)- and phytohemagglutinin (PHA)-stimulated RANKL synthesis were higher also in patients than controls. CD4(+) T lymphocytes were the predominant infiltrate cell subset present in gingival tissues of periodontitis patients. Furthermore, an association between RANKL and CD4(+) T cells was determined by double-staining flow cytometry and confocal microscopy.

CONCLUSION

Taken together, these data demonstrate that gingival CD4(+) T cells are the main cells responsible for higher levels of RANKL observed in human chronic periodontitis patients.

The phosphatidylinositol 3 kinase pathway regulates tolerance to lipopolysaccharide and priming responses to Staphylococcus aureus and lipopolysaccharide

Our previous studies have demonstrated that although LPS and Staphylococcus aureus induce homologous tolerance, they induce priming to each other instead of cross-tolerance. The phosphatidylinositol 3 (PI3) kinase pathway has been implicated in microbial signaling and inflammatory gene expression regulation. We hypothesized that LPS or S. aureus induced tolerance and priming responses to each other are PI3 kinase pathway-dependent. CD1 mice received intraperitoneal injections of 1% Biogel and were treated intraperitoneally with vehicle, LPS, or S. aureus (5 mg/kg) 3 days later. Peritoneal macrophages (MØ) were harvested 24 h later and exposed to vehicle or the PI3 kinase inhibitors wortmannin (10 nmol/L) or LY294002 (10 nmol/L) 1 h before in vitro stimulation with LPS or S. aureus (10 microg/mL). Both LPS and S. aureus significantly induced tumor necrosis factor alpha and thromboxane B2 synthesis (P < 0.05, n = 3) in naive cells. LPS and S. aureus induced homologous tolerance were associated with suppressed tumor necrosis factor alpha and thromboxane B2 levels but augmented interleukin 10 production. However, LPS and S. aureus induced priming to each other, as shown by augmented mediator production. Wortmannin and LY294002 reversed LPS tolerance yet had no effect on S. aureus tolerance. PI3 kinase blockade attenuated the priming responses to both LPS and S. aureus. Mice pretreated with LPS and challenged with LPS were protected. In contrast, mice pretreated with LPS and wortmannin demonstrated LPS tolerance reversal. These data suggest that PI3 kinase is essential for LPS induced homologous tolerance and reciprocal LPS and S. aureus induced priming responses.

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.

Immunostimulatory CpG-oligodeoxynucleotides (CpG-ODN) induce early hepatic injury, but provide a late window for protection against endotoxin-mediated liver damage

BACKGROUND/AIMS

An impaired immunologic response to infection has been recognized as a major defect in the pathogenesis of sepsis and multi-organ failure. Sepsis-associated liver dysfunction and damage are main determinants for the course of the disease. CpG-motif-containing DNA-sequences (CpG-ODN) were previously shown to confer protection in models of infection by stimulating both innate and specific immune responses. Herein, we studied the effect of CpG-ODN in lipopolysaccharide (LPS)-associated hepatotoxicity.

METHODS

Sprague Dawley rats pre-treated at day 6 with either CpG-ODN or inert DNA were challenged with E. coli LPS and subsequently studied for liver injury at 6 and 16 h using in vivo fluorescence microscopy and immunohistochemistry. Western blot protein analysis served for assessment of expression of TLR4, TNF receptor-associated factor 6 (TRAF6), NFkappaB and caspase-3. To evaluate CpG-ODN effects during non-septic conditions, additional animals were solely exposed to CpG-ODN and studied after 1 and 6 days.

RESULTS

CpG-ODN application induced marked hepatic microcirculatory deterioration and liver dysfunction at day 1, however, with almost complete recovery to normal at day 6. Interestingly, CpG-ODN pre-treatment decreased LPS-induced leukocyte-endothelial cell interaction, sinusoidal perfusion failure and caspase-3-dependent apoptotic cell death. Although Kupffer cell phagocytic activity was not affected, CpG-ODN pre-treatment in LPS-challenged animals attenuated hepatic protein expression of TRAF6 and NFkappaB and increased TLR4 by almost 100%.

CONCLUSIONS

CpG-containing DNA-sequences induce early hepatic injury, but mediate long-term protection against LPS hepatotoxicity. The mechanism of protection is based on the induction of cross-tolerance, probably via inhibition of the downstream TRAF6-NFkappaB signaling pathway and upregulation of the TLR4 surface receptor.

Reprogramming of circulatory cells in sepsis and SIRS

Immune status is altered in patients with sepsis or non-infectious systemic inflammatory response syndrome (SIRS). Reduced ex-vivo TNF production by endotoxin-activated monocytes has been regularly reported. This observation is reminiscent of the phenomenon of endotoxin tolerance, and the term 'leukocyte reprogramming' well defines this phenomenon. This review will outline that the hyporesponsiveness of circulating leukocytes is not a generalized phenomenon in sepsis and SIRS. Indeed, the nature of the insult (i.e. infectious versus non-infectious SIRS; under anesthesia [surgery] or not [trauma, burn]), the nature of the activator used to trigger leukocytes (i.e. different Toll-like receptor ligands or whole bacteria), the nature of the cell culture (i.e. isolated monocytes versus peripheral blood mononuclear cells versus whole blood assays), and the nature of the analyzed cytokines (e.g. IL-1beta versus IL-1ra; TNF versus IL-10) have a profound influence on the outcome of the response.

In vitro bioassay of endotoxin using fluorescein as a pH indicator in a macrophage cell culture system

Based on the biological activity of endotoxin, we propose a possible new method for detecting endotoxin using a pH-indication system of macrophage culture media. After RAW 264.7 macrophage cells were treated with lipopolysaccharide (LPS), the addition of fluorescein to the LPS-treated media reproductively reduced its absorption and emission spectra (it was a dose-dependent reduction). The advantages of this LPS-detection method were compared with the Limulus Amebocyte Lysate (LAL) test by using purified bacterial LPS (Salmonella minnessota, Escherichia coli, and Pseudomonas aeruginosa). Additionally, the absorption and fluorescence intensity of fluorescein, following treatment of RAW 264.7 cells with a high concentration of Staphylococcus aureus (Gram-positive, lysed bacteria), could not generally be detected by the LAL test, but they were found to be reduced, in a dose-response relationship, with this new system. The macrophage culture system-method might be a good supplement to the LAL assay for detection of LPS, Gram-negative and Gram-positive bacteria.

Binding of lipopeptide to CD14 induces physical proximity of CD14, TLR2 and TLR1

Lipoproteins or lipopeptides (LP) are bacterial cell wall components detected by the innate immune system. For LP, it has been shown that TLR2 is the essential receptor in cellular activation. However, molecular mechanisms of LP recognition are not yet clear. We used a FLAG-labeled derivative of the synthetic lipopeptide N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2R,S)-propyl]-(R)-cysteinyl-seryl-(lysyl)(3)-lysine (Pam(3)CSK(4)) to study the roles of CD14, TLR2 and TLR1 in binding and signaling of LP and their molecular interactions in human cells. The activity of Pam(3)CSK(4)-FLAG was TLR2 dependent, whereas the binding was enabled by CD14, as evaluated by flow cytometry and confocal microscopy. Using FRET and FRAP imaging techniques to study molecular associations, we could show that after Pam(3)CSK(4)-FLAG binding, CD14 and Pam(3)CSK(4)-FLAG associate with TLR2 and TLR1, and TLR2 is targeted to a low-mobility complex. Thus, LP binding to CD14 is the first step in the LP recognition, inducing physical proximity of CD14 and LP with TLR2/TLR1 and formation of the TLR2 signaling complex.

Triacyl-lipopentapeptide adjuvants: TLR2-dependent activation of macrophages and modulation of receptor-mediated cell activation by altering acyl-moieties

Synthetic lipopeptides derived from bacterial lipoprotein are efficient immunoadjuvants. In vitro they activate antigen presenting cells (APCs) to induce the translocation of nuclear factor kappa B (NF-kappaB) and the activation of further transcription factors. This results in the expression of genes encoding cytokines such as IL-1, IL-6, TNF-alpha and in the release of reactive oxygen/nitrogen intermediates. The molecular structure of microbial products determines TLR specificity and thus their activatory potential and immunoadjuvanticity. In the present study, we investigated the lipopeptide-induced activation of leukocytes at different cellular levels by applying derivatives of a synthetic lipopentapeptide-fatty acid library. Our results show that TLR2 plays a key role for the activation of bone marrow-derived macrophages (BMDMs) by lipopentapeptide derivatives and that the fatty acid composition of the lipopeptides determines their activation potential and TLR specificity.

Synthetic lipopeptide adjuvants and Toll-like receptor 2--structure-activity relationships

Bacterial lipoproteins and their synthetic analogues (sLP) are strong immune modulators of the early host responses after infection. Synthetic lipopeptides are strong adjuvants for the adaptive immune system. Lipoproteins and lipopeptides induce signalling in immune cells through Toll-like receptor-TLR2/TLR1 heterodimers. By screening a combinatorial lipohexapeptide amide collection in an in vitro IL-8 induction assay, we systematically evaluated the potential of 19 proteinogenic amino acids in the peptide moiety of Pam3Cys-lipopeptides to interact with TLR2. New Pam3Cys-lipopeptides with high activity were obtained. Different fatty acids were introduced to investigate the influence of the acyl moiety. Lipopeptides with modifications in the core structure of the unusual amino acid S-glycerylcysteine were synthesized and tested for IL-8 induction via TLR2.

Staphylococcus aureus and Lipopolysaccharide Induce Homologous Tolerance but Heterologous Priming: Role of Interferon-??

Lipopolysaccharide (LPS), the gram-negative bacterial cell wall component, induces tolerance to a secondary challenge of LPS in macrophages (Mphi) as evidenced by reduced inflammatory mediator production. However, it is uncertain if heat-killed (HK) gram-positive bacteria Staphylococcus aureus (Sa) can induce a similar tolerance and alter responses to LPS. We hypothesized that HKSa induces homologous tolerance and cross tolerance to LPS stimulation in human promonocytic THP-1 cells. We measured TNF-alpha, TxB2, and IFN-gamma production and the phosphorylation of p38, JNK, and ERK-1/2 in human promonocytic THP-1 cells. HKSa (10 microg/mL) significantly stimulated naive (nonpretreated) cell TNF-alpha (P<0.05) and TxB2 production (P<0.05). However, HKSa-pretreated cells challenged secondarily with HKSa (10 microg/mL) exhibited a decrease in the production of TNF-alpha (89 +/- 5%, P<0.05) and TxB2 (85 +/- 3%, P<0.05) compared with HKSa-stimulated naive cells. By contrast, secondary LPS challenge of HKSa-pretreated cells augmented TNF-alpha (41 +/- 3%, P<0.05) and TxB2 (42 +/- 6%, P<0.05) compared with LPS-stimulated naive cells. In naive cells, HKSa and LPS stimulation also significantly phosphorylated the mitogen-activated kinases (MAPKs) p38, JNK, and ERK-1/2 (P<0.005) compared with basal levels. HKSa and LPS induced homologous tolerance as evidenced by the down-regulation of the three MAPK (P<0.05), thus paralleling data on mediator production. HKSa-pretreated cells' priming responses to LPS correlated with augmented phosphorylation of JNK and p38 (P<0.05), whereas ERK-1/2 phosphorylation remained down-regulated. In contrast to TNF-alpha and TxB2 production, HKSa-induced IFN-gamma was up-regulated (26 +/- 5%) in HKSa-pretreated cells compared with HKSa-stimulated naive cells. IFN-gamma antibody exhibited reversed priming in HKSa-pretreated cells as evidenced by a reduction in TNF-alpha. Exogenous human IFN-gamma- (1 microg/mL) and HKSa-pretreated cells secondarily stimulated with HKSa did not prevent the induction of tolerance. In contrast, exogenous IFN-gamma pretreatment prevented the induction of LPS homologous tolerance resulting in an increase in TNF-alpha production. The data demonstrate that HKSa induces homologous tolerance but causes priming to LPS.

Lipoteichoic acid and peptidoglycan from Staphylococcus aureus synergistically induce neutrophil influx into the lungs of mice

Staphylococcus aureus is an important pathogen in nosocomial pneumonia. Lipoteichoic acid (LTA) and peptidoglycan (PepG) are part of the staphylococcal cell wall. Here we show that LTA and PepG act in synergy to cause polymorphonuclear cell recruitment in the pulmonary compartment during S. aureus pneumonia.

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.

Toll-like receptor 2- and 6-mediated stimulation by macrophage-activating lipopeptide 2 induces lipopolysaccharide (LPS) cross tolerance in mice, which results in protection from tumor necrosis factor alpha but in only partial protection from lethal LPS doses

Patients or experimental animals previously exposed to lipopolysaccharide (LPS) become tolerant to further LPS challenge. We investigated the potential of the macrophage-activating lipopeptide 2 (MALP-2) to induce in vivo cross tolerance to tumor necrosis factor alpha (TNF-alpha) and LPS. MALP-2-induced tolerance could be of practical interest, as MALP-2 proved much less pyrogenic in rabbits than LPS. Whereas LPS signals via Toll-like receptor 4 (TLR4), MALP-2 uses TLR2 and TLR6. LPS-mediated cytokine release was studied in mice pretreated with intraperitoneal injections of MALP-2. No biologically active TNF-alpha could be detected in the serum of MALP-2-treated animals when challenged with LPS 24 or 72 h later, whereas suppression of LPS-dependent interleukin (IL)-6 lasted for only 24 h. Protection from lethal TNF-alpha shock was studied in galactosamine-treated mice. Dose dependently, MALP-2 prevented death from lethal TNF-alpha doses in TLR4(-/-) but not in TLR2(-/-) mice, with protection lasting from 5 to 24 h. To assay protection from LPS, mice were pretreated with MALP-2 doses of up to 10 micro g. Five and 24 h later, the animals were simultaneously sensitized and challenged by intravenous coinjection of galactosamine and a lethal dose of 50 ng of LPS. There was only limited protection (four of seven mice survived) when mice were challenged 5 h after MALP-2 pretreatment, and no protection when mice were challenged at later times. The high effectiveness of MALP-2 in suppressing TNF-alpha, the known ways of biological inactivation, and low pyrogenicity make MALP-2 a potential candidate for clinical use.

Lipopeptides: adjuvanticity in conventional and genetic immunization

Synthetic lipopeptides derived from the bacterial cell wall component lipoprotein activate B-lymphocytes and macrophages/monocytes in vitro. In vivo they constitute potent immunoadjuvants for a broad range of different antigens and species comparable or superior to Freund's adjuvant. Here, we demonstrate that P(3)CSK(4), representing a highly active lipopentapeptide derivative in vitro, significantly enhances and accelerates the humoral immune response to tetanus toxoid. P(3)CSK(4) could substitute for up to 90% of the antigen without any decrease in the specific IgG level, and the presence of the lipopeptide resulted in a prolonged production of specific IgG in time. Investigations using P(3)CSK(4) as an adjuvant in genetic immunization confirmed earlier data demonstrating that lipopeptides constitute adjuvants for low-immunogenic DNA constructs and/or for application routes resulting in weak immune responses. We monitored a lipopeptide-dependent shift from a Th1-type to Th2-type response, when DNA immunization was followed by i.p. administration of the lipopeptide adjuvant.

Borrelia burgdorferi-induced tolerance as a model of persistence via immunosuppression

If left untreated, infection with Borrelia burgdorferi sensu lato may lead to chronic Lyme borreliosis. It is still unknown how this pathogen manages to persist in the host in the presence of competent immune cells. It was recently reported that Borrelia suppresses the host's immune response, thus perhaps preventing the elimination of the pathogen (I. Diterich, L. Härter, D. Hassler, A. Wendel, and T. Hartung, Infect. Immun. 69:687-694, 2001). Here, we further characterize Borrelia-induced immunomodulation in order to develop a model of this anergy. We observed that the different Borrelia preparations that we tested, i.e., live, heat-inactivated, and sonicated Borrelia, could desensitize human blood monocytes, as shown by attenuated cytokine release upon restimulation with any of the different preparations. Next, we investigated whether these Borrelia-specific stimuli render monocytes tolerant, i.e. hyporesponsive, towards another Toll-like receptor 2 (TLR2) agonist, such as lipoteichoic acid from gram-positive bacteria, or towards the TLR4 agonist lipopolysaccharide. Cross-tolerance towards all tested stimuli was induced. Furthermore, using primary bone marrow cells from TLR2-deficient mice and from mice with a nonfunctional TLR4 (strain C3H/HeJ), we demonstrated that the TLR2 was required for tolerance induction by Borrelia, and using neutralizing antibodies, we identified interleukin-10 as the key mediator involved. Although peripheral blood mononuclear cells tolerized by Borrelia exhibited reduced TLR2 and TLR4 mRNA levels, the expression of the respective proteins on monocytes was not decreased, ruling out the possibility that tolerance to Borrelia is attributed to a reduced TLR2 expression. In summary, we characterized tolerance induced by B. burgdorferi, describing a model of desensitization which might mirror the immunosuppression recently attributed to the persistence of Borrelia in immunocompetent hosts.

Toll-like receptor 2 (TLR2) and TLR9 signaling results in HIV-long terminal repeat trans-activation and HIV replication in HIV-1 transgenic mouse spleen cells: implications of simultaneous activation of TLRs on HIV replication

Opportunistic infections are common in HIV-infected patients; they activate HIV replication and contribute to disease progression. In the present study we examined the role of Toll-like receptor 2 (TLR2) and TLR9 in HIV-long terminal repeat (HIV-LTR) trans-activation and assessed whether TLR4 synergized with TLR2 or TLR9 to induce HIV replication. Soluble Mycobacterium tuberculosis factor (STF) and phenol-soluble modulin from Staphylococcus epidermidis induced HIV-LTR trans-activation in human microvessel endothelial cells cotransfected with TLR2 cDNA. Stimulation of ex vivo spleen cells from HIV-1 transgenic mice with TLR4, TLR2, and TLR9 ligands (LPS, STF, and CpG DNA, respectively) induced p24 Ag production in a dose-dependent manner. Costimulation of HIV-1 transgenic mice spleen cells with LPS and STF or CpG DNA induced TNF-alpha and IFN-gamma production in a synergistic manner and p24 production in an additive fashion. In the THP-1 human monocytic cell line stably expressing the HIV-LTR-luciferase construct, LPS and STF also induced HIV-LTR trans-activation in an additive manner. This is the first time that TLR2 and TLR9 and costimulation of TLRs have been shown to induce HIV replication. Together these results underscore the importance of TLRs in bacterial Ag- and CpG DNA-induced HIV-LTR trans-activation and HIV replication. These observations may be important in understanding the role of the innate immune system and the molecular mechanisms involved in the increased HIV replication and HIV disease progression associated with multiple opportunistic infections.

Expression of functional Toll-like receptor 2 on human epidermal keratinocytes

Epidermal keratinocytes secrete cytokines, chemokines, and anti-microbial peptides in response to various microbial pathogens and their components including lipopolysaccharide (LPS). To identify the receptor(s) involved in the anti-microbial responses of epidermal keratinocytes, we analyzed expression of CD14, Toll-like receptor 2 (TLR2), and TLR4 on cultured normal human epidermal keratinocytes (NHEK). Although CD14 and TLR2 mRNA were expressed in cultured NHEK, only TLR2 was detected on the cell surface. Cultured NHEK did not express TLR4 mRNA or protein. Commercial LPS preparations could stimulate epidermal keratinocytes to produce beta-defensin-2 and IL-8, and the LPS response was inhibited with mAb specific for TLR2, but not for CD14 or TLR4. Repurified LPS and lipid A did not stimulate epidermal keratinocytes, whereas peptidoglycan (PGN) from Gram-positive bacteria and yeast cell wall particle induced beta-defensin-2 and IL-8 production. Thus, cultured NHEK express functional TLR2, but not CD14 or TLR4, and the "LPS" response of epidermal keratinocytes shown in the previous studies might be mediated by TLR2-dependent recognition of non-LPS bacterial components contaminating in commercial LPS preparations. In the normal human skin, however, epidermal keratinocytes expressed both TLR2 and TLR4. Because TLR4 was induced in epidermal keratinocytes by in vitro stimulation with PGN from Gram-positive bacteria, constitutive expression of TLR4 on epidermal keratinocytes in vivo might also be induced by continuous recognition of the resident skin flora containing Gram-positive bacteria through TLR2.

SOCS-1 participates in negative regulation of LPS responses

SOCS-1 is a negative regulatory molecule of the JAK-STAT signal cascade. Here, we demonstrate that SOCS-1 is a critical downregulating factor for LPS signal pathways. SOCS-1 expression was promptly induced in macrophages upon LPS stimulation. SOCS-1-deficient mice were highly sensitive to LPS-induced shock and produced increased levels of inflammatory cytokines. Introduction of SOCS-1 inhibited LPS-induced NF-kappaB and STAT1 activation in macrophages. Furthermore, LPS tolerance, a refractory state to second LPS stimulation, was not observed in SOCS-1-deficient mice. These results suggest SOCS-1 as an essential, negative regulator in LPS responses that protects the host from harmful overresponses to LPS and may provide new insight into the endotoxin-induced fatal syndrome that occasionally occurs following infection.

Induction of bacterial lipoprotein tolerance is associated with suppression of toll-like receptor 2 expression

Tolerance to bacterial cell wall components including lipopolysaccharide (LPS) may represent an essential regulatory mechanism during bacterial infection. Two members of the Toll-like receptor (TLR) family, TLR2 and TLR4, recognize the specific pattern of bacterial cell wall components. TLR4 has been found to be responsible for LPS tolerance. However, the role of TLR2 in bacterial lipoprotein (BLP) tolerance and LPS tolerance is unclear. Pretreatment of human THP-1 monocytic cells with a synthetic bacterial lipopeptide induced tolerance to a second BLP challenge with diminished tumor necrosis factor-alpha and interleukin-6 production, termed BLP tolerance. Furthermore, BLP-tolerized THP-1 cells no longer responded to LPS stimulation, indicating a cross-tolerance to LPS. Induction of BLP tolerance was CD14-independent, as THP-1 cells that lack membrane-bound CD14 developed tolerance both in serum-free conditions and in the presence of a specific CD14 blocking monoclonal antibody (MEM-18). Pre-exposure of THP-1 cells to BLP suppressed mitogen-activated protein kinase phosphorylation and nuclear factor-kappaB activation in response to subsequent BLP and LPS stimulation, which is comparable with that found in LPS-tolerized cells, indicating that BLP tolerance and LPS tolerance may share similar intracellular pathways. However, BLP strongly enhanced TLR2 expression in non-tolerized THP-1 cells, whereas LPS stimulation had no effect. Furthermore, a specific TLR2 blocking monoclonal antibody (2392) attenuated BLP-induced, but not LPS-induced, tumor necrosis factor-alpha and interleukin-6 production, indicating BLP rather than LPS as a ligand for TLR2 engagement and activation. More importantly, pretreatment of THP-1 cells with BLP strongly inhibited TLR2 activation in response to subsequent BLP stimulation. In contrast, LPS tolerance did not prevent BLP-induced TLR2 overexpression. These results demonstrate that BLP tolerance develops through down-regulation of TLR2 expression.

Lipopeptide adjuvants: monitoring and comparison of P3CSK4- and LPS-induced gene transcription

Bacteria-derived synthetic lipoproteins constitute potent macrophage activators in vivo and are effective stimuli, enhancing the immune response especially with respect to low or non-immunogenic compounds. N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2R,S)-propyl]-(R)-cysteinyl-seryl-(lysyl)3-lysine (P3CSK4), exhibiting one of the most effective lipopeptide derivatives, represents a highly efficient immunoadjuvant in parenteral, oral, nasal and genetic immunization either in combination with or after covalent linkage to antigen. In order to further elucidate its molecular mode of action with respect to the transcriptional level, we focused our investigations on the P3CSK4-induced modulation of gene transcription. We could show that P3CSK4 activates/represses an array of at least 140 genes partly involved in signal transduction and regulation of the immune response. P3CSK4 activates the expression of tumor suppressor protein p53 (p53), c-rel, inhibitor of nuclear factor kappa B (NFkappaB) alpha (IkappaB alpha), type 2 (inducible) nitric oxide (NO) synthase (iNOS), CD40-LR, intercellular adhesion molecule-1 (ICAM-1) and interleukin 1/6/15 (IL-1/6/15). We detected no activation of heat shock protein (HSP) 27, 60, 84 and 86, osmotic stress protein 94 (Osp 94), IL-12, extracellular signal-regulated protein kinase 1 (ERK1), p38 mitogen activated protein (MAP)-kinase (p38), c-Jun NH2-terminal kinase (JNK), signal transducer and activator of transcription 1 (STAT1), CD14 and caspase genes. Furthermore, we monitored inhibition of STAT6, Janus kinase 3 (Jak3) and cyclin D1/D3 gene transcription after stimulating bone marrow-derived macrophages (BMDM) with lipopeptide. In addition, we monitored significant differences after lipopeptide and lipopolysaccharide (LPS) stimulation of bone marrow-derived murine macrophages. Our findings are of importance for further optimizing both conventional and genetic immunization, and for the development of novel synthetic vaccines.

Monocyte deactivation in severe human sepsis or following cardiopulmonary bypass

We investigated the specificity for gram-negative stimuli as well as the contribution of signal transduction pathways for leukocyte hyporesponsiveness in sepsis or following cardiopulmonary bypass (CPB). Whole blood of nine patients undergoing CPB and 25 patients with severe sepsis was stimulated ex vivo with LPS (E. coli O111:B4) or with Staphylococcus aureus Cowan strain I (SAC-I) lysate in the absence or presence of inhibitors of protein kinase C (PKC), protein-tyrosine kinase (PTK), or protein-tyrosine phosphatase (PTP). Both toxins stimulated a TNF-alpha response through PTK signaling. Although suppression of the cytokine response was similar for LPS and SAC-I after CPB, it was significantly more pronounced for SAC-I in sepsis. Inhibition of PTP failed to increase TNF-alpha upon LPS, whereas a moderate increase was observed with SAC-I. Impaired TNF-alpha responses occur in sepsis and after CPB. Although this has primarily been reported for gram-negative stimuli, our data suggest that this is even more pronounced for gram-positive stimuli in severe sepsis. Although PTK was the predominant signaling pathway, inhibition of PTP only partially restored the TNF-alpha response to SAC-I. Our results suggest that cellular mechanisms underlying monocyte deactivation are different in sepsis or following CPB and are discriminate for gram-positive and gram-negative toxins.