Differential effects of CpG-DNA in Toll-like receptor-2/-4/-9 tolerance and cross-tolerance

Seasonal Variations in the Concentration of Particulate Matter in the Air of Cracow Affect the Magnitude of CD4+ T Cell Subsets Cytokine Production in Patients with Inflammatory and Autoimmune Disorders

Recently, the increased prevalence of chronic civilization diseases triggered by environmental pollution has been observed. In this context, the role of air pollution in the pathogenesis of autoimmune and/or inflammatory disorders is poorly elucidated. Here, we asked whether seasonal changes in the air quality of the city of Cracow affect the polarization of T cell subsets in healthy donors (HD) and patients with rheumatoid arthritis (RA), multiple sclerosis (MS), and atherosclerosis (AS). Peripheral blood mononuclear cells (PBMCs) from HD and patients were exposed in vitro to particulate matter isolated from the air of Cracow (PM CRC). Blood samples were collected in two seasons (winter and summer), with differences in air concentration of particulate matter of 10 μm (PM10) (below or above a daily limit of 50 µg/m3). The obtained data showed a significantly elevated frequency of CD4+ lymphocytes specific for IFN-γ and IL-17A after the exposure of PBMCs to PM CRC. This was observed for all patients’ groups and HD. In the case of patients, this effect was dependent on the seasonal concentration of PM in the air, paradoxically being less pronounced in the season with a higher concentration of air pollution. These observations may suggest the role of air pollution on the course of inflammatory and autoimmune disorders.

Myeloid Responses to Extracellular Vesicles in Health and Disease

Extracellular vesicles are mediators of cell-cell communication playing a key role in both steady-state and disease conditions. Extracellular vesicles carry diverse donor-derived cargos, including DNA, RNA, proteins, and lipids that induce a complex network of signals in recipient cells. Due to their ability to capture particulate matter and/or capacity to polarize and orchestrate tissue responses, myeloid immune cells (e.g., dendritic cells, macrophages, etc.) rapidly respond to extracellular vesicles, driving local and systemic effects. In cancer, myeloid-extracellular vesicle communication contributes to chronic inflammation, self-tolerance, and therapeutic resistance while in autoimmune disease, extracellular vesicles support inflammation and tissue destruction. Here, we review cellular mechanisms by which extracellular vesicles modulate myeloid immunity in cancer and autoimmune disease, highlighting some contradictory results and outstanding questions. We will also summarize how understanding of extracellular vesicle biology is being utilized for novel therapeutic and diagnostic applications.

Extracellular CIRP induces macrophage endotoxin tolerance through IL-6R-mediated STAT3 activation

Extracellular cold-inducible RNA-binding protein (eCIRP) is a damage-associated molecular pattern, whose effect on macrophages is not entirely elucidated. Here we identified that eCIRP promotes macrophage endotoxin tolerance. Septic mice had higher serum levels of eCIRP; this was associated with a reduced ex vivo immune response of their splenocytes to LPS. Pretreatment of macrophages with recombinant murine CIRP (rmCIRP) resulted in a tolerance to LPS stimulation as demonstrated by a reduction of TNF-α production. We found that eCIRP increased phosphorylated STAT3 (p-STAT3) in macrophages. A STAT3 inhibitor, Stattic, rescued macrophages from rmCIRP-induced tolerance by restoring the release of TNF-α in response to LPS stimulation. We discovered strong binding affinity between eCIRP and IL-6 receptor (IL-6R) as revealed by Biacore, fluorescence resonance energy transfer (FRET), and their colocalization in macrophages by immunostaining assays. Blockade of IL-6R with its neutralizing Ab inhibited eCIRP-induced p-STAT3 and restored LPS-stimulated TNF-α release in macrophages. Incubation of macrophages with rmCIRP skewed them toward an M2 phenotype, while treatment with anti-IL-6R Ab prevented rmCIRP-induced M2 polarization. Thus, we have demonstrated that eCIRP activates p-STAT3 via a novel receptor, IL-6R, to promote macrophage endotoxin tolerance. Targeting eCIRP appears to be a new therapeutic option to correct immune tolerance in sepsis.

Toll-like receptors in lupus nephritis

The pathogenesis of systemic autoimmune diseases such as systemic lupus erythematosus (SLE) is based on the loss of self-tolerance against ubiquitous autoantigens involving all mechanisms of adaptive immunity. However, data accumulating over the last decade imply an important role also for numerous elements of innate immunity, namely the Toll-like receptors in the pathogenesis of SLE. Here we discuss their role in the most common organ complication of SLE, i.e. lupus nephritis. We summarize experimental and clinical data on the expression and functional contribution of the Toll-like receptors in immune complex glomerulonephritis, and intrarenal inflammation. Based on these discoveries Toll-like receptors are evolving as therapeutic targets for the treatment of SLE and lupus nephritis.

The effects of repeated Toll-like receptors 2 and 4 stimulation in COPD alveolar macrophages

Background

COPD is a progressive inflammatory airway disease characterized by increased numbers of alveolar macrophages in the lungs. Bacterial colonization of the lungs is a common feature in COPD and can promote inflammation through continual and repeated Toll-like receptor (TLR) stimulation. We have studied the response of COPD alveolar macrophages to repetitive stimulation with TLR2 and TLR4 ligands. We investigated the effect of sequential stimulation with different ligands to determine whether this results in tolerance or amplification of the immune response.

Methods

We stimulated alveolar macrophages from COPD patients (n=9) and smokers (n=8) with the TLR4 agonist lipopolysaccharide (LPS) or the TLR2 agonist Pam3CSK4 for 24 hours before restimulating again for 24 hours. Cytokine protein release and gene expression were investigated.

Results

Repetitive stimulation of COPD and smokers macrophages with LPS for both 24-hour periods caused a reduction in tumor necrosis factor α, CCL5, and IL-10 production compared to cells that were not exposed initially to LPS. IL-6 and CXCL8 production were not significantly altered following repetitive LPS stimulation. The same pattern was observed for repeated stimulation with Pam3CSK4. Using COPD macrophages, LPS followed by Pam3CSK4 stimulation increased the levels of all cytokines compared to media followed by Pam3CSK4.

Conclusion

TLR tolerance in COPD alveolar macrophages occurs after repetitive stimulation with the same TLR ligand, but this only occurs for selected cytokines. CXCL8 production is not reduced after repetitive TLR stimulation with the same ligand; this may be an important mechanism for the increased CXCL8 levels that have been observed in COPD. We showed that TLR4 stimulation followed by TLR2 stimulation does not cause tolerance, but enhances cytokine production. This may be a relevant mechanism by which bacteria cause excessive inflammation in COPD patients.

Efficacy of a Virus-Like Nanoparticle As Treatment for a Chronic Viral Infection Is Hindered by IRAK1 Regulation and Antibody Interference

Although vaccination has been an effective way of preventing infections ever since the eighteenth century, the generation of therapeutic vaccines and immunotherapies is still a work in progress. A number of challenges impede the development of these therapeutic approaches such as safety issues related to the administration of whole pathogens whether attenuated or inactivated. One safe alternative to classical vaccination methods gaining recognition is the use of nanoparticles, whether synthetic or naturally derived. We have recently demonstrated that the papaya mosaic virus (PapMV)-like nanoparticle can be used as a prophylactic vaccine against various viral and bacterial infections through the induction of protective humoral and cellular immune responses. Moreover, PapMV is also very efficient when used as an immune adjuvant in an immunotherapeutic setting at slowing down the growth of aggressive mouse melanoma tumors in a type I interferon (IFN-I)-dependent manner. In the present study, we were interested in exploiting the capacity of PapMV of inducing robust IFN-I production as treatment for the chronic viral infection model lymphocytic choriomeningitis virus (LCMV) clone 13 (Cl13). Treatment of LCMV Cl13-infected mice with two systemic administrations of PapMV was ineffective, as shown by the lack of changes in viral titers and immune response to LCMV following treatment. Moreover, IFN-α production following PapMV administration was almost completely abolished in LCMV-infected mice. To better isolate the mechanisms at play, we determined the influence of a pretreatment with PapMV on secondary PapMV administration, therefore eliminating potential variables emanating from the infection. Pretreatment with PapMV led to the same outcome as an LCMV infection in that IFN-α production following secondary PapMV immunization was abrogated for up to 50 days while immune activation was also dramatically impaired. We showed that two distinct and overlapping mechanisms were responsible for this outcome. While short-term inhibition was partially the result of interleukin-1 receptor-associated kinase 1 degradation, a crucial component of the toll-like receptor 7 signaling pathway, long-term inhibition was mainly due to interference by PapMV-specific antibodies. Thus, we identified a possible pitfall in the use of virus-like particles for the systemic treatment of chronic viral infections and discuss mitigating alternatives to circumvent these potential problems.

CpG-ODN exerts a neuroprotective effect via the TLR9/pAMPK signaling pathway by activation of autophagy in a neonatal HIE rat model

Hypoxic Ischemic Encephalopathy (HIE) is an injury caused to the brain due to prolonged lack of oxygen and blood supply which results in death or long-term disabilities. The main aim of this study was to investigate the role of Cytosine-phospho-guanine oligodeoxynucleotide (CpG-ODN) in autophagy after HIE. Ten-day old (P10) rat pups underwent right common carotid artery ligation followed by 2.5h of hypoxia as previously described by Rice-Vannucci. At 1h post HIE, rats were intranasally administered with recombinant CpG-ODN. Time-course expression levels of endogenous key proteins, TLR9, pAMPK/AMPK, LC3II/I, and LAMP1 involved in CpG-ODN's protective effects were measured using western blot. Short (48h) and long (4w) term neurobehavior studies were performed using righting reflex, negative geotaxis, water maze, foot fault and Rota rod tests. Brain samples were collected after long term for histological analysis. Furthermore, to elucidate the pathway via which CpG-ODN confers protection, TLR9 and AMPK inhibitors were used. Time course results showed that the expression of TLR9, pAMPK/AMPK, LC3II/I, LAMP1 increased after HIE. Neurobehavioral studies showed that HIE induced a significant delay in development and resulted in cognitive and motor function deficits. However, CpG-ODN ameliorated HIE-induced outcomes and improved long term neurological deficits. In addition, CpG-ODN increased expression of pAMPK/AMPK, p-ULK1/ULK1, P-AMBRA1/AMBRA1, LC3II/I and LAMP1 while inhibition of TLR9 and AMPK reversed those effects. In summary, CpG-ODN increased HIE-induced autophagy and improved short and long term neurobehavioral outcomes which may be mediated by the TLR9/pAMPK signaling pathway after HIE.

Toll-Like Receptor Ligand-Induced Liver Injury in D-Galactosamine-Sensitized Mice: Differences between TLR7/8 and TLR9 Ligands, Cytokine Patterns, and Cross-Tolerance Induction by TLR2 Ligand Pretreatment

Administration of Toll-like receptor ligands (TLRLs) is known to cause liver injury in D-galN-sensitized mice. In the present study, we aimed to complement preceding reports on the TLRL/D-galN system by analyzing comparisons among TLRLs, mouse strain dependence, effects on serum levels of cytokines, and effects of sequential administrations of different TLRLs. In a preliminary set of analyses, we first confirmed that liver failure can be induced by diverse TLRLs, including LTA and R848 in combination with D-galN. Analysis using TLR4-deficient mice excluded potential confounding effects of endogenous TLR4Ls that include those referred to as DAMPs in CpG DNA/D-galN hepatotoxicity. Subsequently, we showed that LTA pretreatment could prevent mortality in both CpG DNA/D-galN- and R848/D-galN-treated mice compared to without pretreatment. Incidentally, we observed that without the LTA pretreatment, CpG DNA/D-galN showed relatively higher liver-specific toxicity whereas R848/D-galN showed more symptoms of multiple organ failure. These findings suggest that, in D-galN-sensitized mice, different TLRLs not only show similarity in the ability to induce hepatic injury but also exhibit distinctive abilities in inducing systemic inflammation and multiple organ failure. These findings also suggest the potential usefulness of cross-tolerance induction using LTA in the prevention of organ failure in TLRL-mediated acute inflammation.

The ex vivo toll-like receptor 7 tolerance induction in donor lymphocytes prevents murine acute graft-versus-host disease

BACKGROUND AIMS

Acute graft-versus-host disease (aGVHD) remains a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation, mediated by alloreactive donor T cells. Toll-like receptors (TLRs), a family of conserved pattern-recognition receptors (PRRs), represent key players in donors' T-cell activation during aGVHD; however, a regulatory, tolerogenic role for certain TLRs has been recognized in a different context. We investigated whether the ex vivo-induced TLR-2,-4,-7 tolerance in donor cells could prevent alloreactivity in a mismatched transplantation model.

METHODS

TLR-2,-4,-7 tolerance was induced in mouse splenocytes, after stimulation with low doses of corresponding ligands. Cellular and molecular changes of the TLR-tolerant splenocytes and purified T cells were assessed by immunophenotypic and gene expression analyses. Incidence of aGVHD was evaluated by the clinical score and survival as well as histopathology of target tissues.

RESULTS

Only the R848-induced TLR7 tolerance prevented aGVHD. The TLR7 ligand-induced tolerance lasted for a critical post-transplant period and was associated with distinct cellular and molecular signatures characterized by induction of regulatory T cells, reduced alloreactivity and balanced regulation of inflammatory signaling and innate immune responses. The TLR7-tolerant T cells preserved the immunological memory and generated in vitro virus-specific T cells upon antigen stimulation. The anti-aGVHD tolerization effect was direct and specific to TLR7 and required the receptor-ligand interaction; TLR7^-/- T cells isolated from B6 TLR7^-/- mice presented a distinct gene expression profile but failed to prevent aGVHD.

DISCUSSION

We propose an effective and clinically applicable ex vivo approach for aGVHD prevention through a transient and reversible immune reprogramming exerted by TLR7-tolerant donor lymphocytes.

The Cytokine Response to Lipopolysaccharide Does Not Predict the Host Response to Infection

The magnitude of the LPS-elicited cytokine response is commonly used to assess immune function in critically ill patients. A suppressed response, known as endotoxin tolerance, is associated with worse outcomes, yet endotoxin tolerance-inducing TLR4 ligands are known to protect animals from infection. Thus, it remains unknown whether the magnitude of the LPS-elicited cytokine response provides an accurate assessment of antimicrobial immunity. To address this, the ability of diverse TLR ligands to modify the LPS-elicited cytokine response and resistance to infection were assessed. Priming of mice with LPS, monophosphoryl lipid A (MPLA), or poly(I:C) significantly reduced plasma LPS-elicited proinflammatory cytokines, reflecting endotoxin tolerance, whereas CpG-ODN-primed mice showed augmented cytokine production. In contrast, LPS, MPLA, and CpG-ODN, but not poly(I:C), improved the host response to a Pseudomonas aeruginosa infection. Mice primed with protective TLR ligands, including CpG-ODN, showed reduced plasma cytokines during P. aeruginosa infection. The protection imparted by TLR ligands persisted for up to 15 d yet was independent of the adaptive immune system. In bone marrow-derived macrophages, protective TLR ligands induced a persistent metabolic phenotype characterized by elevated glycolysis and oxidative metabolism as well as augmented size, granularity, phagocytosis, and respiratory burst. Sustained augmentation of glycolysis in TLR-primed cells was dependent, in part, on hypoxia-inducible factor 1-α and was essential for increased phagocytosis. In conclusion, the magnitude of LPS-elicited cytokine production is not indicative of antimicrobial immunity after exposure to TLR ligands. Additionally, protective TLR ligands induce sustained augmentation of phagocyte metabolism and antimicrobial function.

Therapeutic Potential for Targeting the Suppressor of Cytokine Signalling-1 Pathway for the Treatment of SLE

Although the specific events dictating systemic lupus erythematosus (SLE) pathology remain unclear, abundant evidence indicates a critical role for dysregulated cytokine signalling in disease progression. Notably, the suppressor of cytokine signalling (SOCS) family of intracellular proteins, in particular the kinase inhibitory region (KIR) bearing SOCS1 and SOCS3, plays a critical role in regulating cytokine signalling. To assess a relationship between SOCS1/SOCS3 expression and SLE, the goals of this study were to (1) evaluate the time kinetics of SOCS1/SOCS3 message and protein expression based on SLE-associated stimulations, (2) compare levels of SOCS1 and SOCS3 present in SLE patients and healthy controls by message and protein, (3) relate SOCS1/SOCS3 expression to inflammatory markers in SLE patients and (4) correlate SOCS1/SOCS3 levels to current treatments. We found that SOCS1 and SOCS3 were most abundant in murine splenic samples at 48 h subsequent to stimulation by anti-CD3, LPS or interferon-gamma. In addition, significant reductions in SOCS1 and SOCS3 were present within PMBCs of SLE patients compared to controls by both mRNA and protein expression. We also found that decreased levels of SOCS1 in SLE patients were correlated with enhanced levels of inflammatory markers and upregulated expression of MHC class II. Finally, we show that patients receiving steroid treatment possessed higher levels SOCS1 compared to SLE patient counterparts and that steroid administration to human PBMCs upregulated SOCS1 message in a dose- and time-dependent manner. Together, these results suggest that therapeutic strategies focused on SOCS1 signalling may have efficacy in the treatment of SLE.

Toll-like receptors in pathophysiology of liver diseases

Toll-like receptors (TLRs) are pattern recognition receptors that participate in host defense by recognizing pathogen-associated molecular patterns alongside inflammatory processes by recognizing damage associated molecular patterns. Given constant exposure to pathogens from gut, strict control of TLR-associated signaling pathways is essential in the liver, which otherwise may lead to inappropriate production of pro-inflammatory cytokines and interferons and may generate a predisposition to several autoimmune and chronic inflammatory diseases. The liver is considered to be a site of tolerance induction rather than immunity induction, with specificity in hepatic cell functions and distribution of TLR. Recent data emphasize significant contribution of TLR signaling in chronic liver diseases via complex immune responses mediating hepatocyte (i.e., hepatocellular injury and regeneration) or hepatic stellate cell (i.e., fibrosis and cirrhosis) inflammatory or immune pathologies. Herein, we review the available data on TLR signaling, hepatic expression of TLRs and associated ligands, as well as the contribution of TLRs to the pathophysiology of hepatic diseases.

TLR4, TLR7/8 agonist-induced miR-146a promotes macrophage tolerance to MyD88-dependent TLR agonists

TLRs facilitate the recognition of pathogens by immune cells and the initiation of the immune response, leading to the production of proinflammatory cytokines and chemokines. Production of proinflammatory mediators by innate immune cells, such as macrophages, is tightly regulated to facilitate pathogen clearance while limiting an adverse impact on host tissue. Exposure of innate immune cells to TLR ligands induces a state of temporary refractoriness to a subsequent exposure of a TLR ligand, a phenomenon referred to as "tolerance." This study sought to evaluate the mechanistic regulation of TLR4 and TLR7/8 ligand-induced tolerance to other TLRs by microRNA-146a. With the use of THP-1 macrophages, as well as human classic and alternative macrophages, we demonstrate that priming with a TLR4 agonist (LPS) or a TLR7/8 agonist (R848) induces homologous and heterologous tolerance to various TLR ligands in macrophages, leading to the impaired production of cytokines and chemokines. We also demonstrate that overexpression of microRNA-146a is sufficient to mimic LPS or R848-induced hyporesponsiveness. Conversely, inhibition of microRNA-146a activity leads to LPS- or R848-induced TLR hyper-responsiveness in TLR signaling tolerance. Furthermore, we demonstrate that microRNA-146a dampens cytokine production following a primary stimulus with MyD88-dependent but not MyD88-independent TLR pathways. Collectively, these data provide comprehensive evidence of the central role of microRNA-146a in TLR signaling tolerance to plasma membrane, as well as endosomal TLR ligands in human macrophages.

Macrophage polarization in cattle experimentally exposed to Mycobacterium avium subsp. paratuberculosis

Mycobacterium avium subspecies paratuberculosis (MAP), the causative agent of Johne's disease (JD) in cattle, has significant impacts on the livestock industry and has been implicated in the etiology of Crohn's disease. Macrophages play a key role in JD pathogenesis, which is driven by the manipulation of host immune mechanisms by MAP. A change in the macrophage microenvironment due to pathogenic or host-derived stimuli can lead to classical (M1) or alternative (M2) polarization of macrophages. In addition, prior exposure to antigenic stimuli has been reported to alter the response of macrophages to subsequent stimuli. However, macrophage polarization in response to MAP exposure and its possible implications have not been previously addressed. In this study, we have comprehensively examined monocyte/macrophage polarization and responsiveness to antigens from MAP-exposed and unexposed animals. At 3 years post-exposure, there was a heterogeneous macrophage activation pattern characterized by both classical and alternate phenotypes. Moreover, subsequent exposure of macrophages from MAP-exposed cattle to antigens from MAP and other mycobacterial species led to significant variation in the production of nitric oxide, interleukin-10 and tumour necrosis factor α. These results indicate the previously unreported possibility of changes in the activation state and responsiveness of circulating monocytes/macrophages from MAP-exposed cattle.

Flagellin suppresses experimental asthma by generating regulatory dendritic cells and T cells

BACKGROUND

Although the hygiene hypothesis suggests that microbial infections could subvert asthma and thus a microbial product might serve as a therapeutic adjuvant for asthma, the relationship between bacterial components and asthma is complex. Recently, low levels of flagellin, the Toll-like receptor (TLR) 5 ligand, have been reported to promote asthma.

OBJECTIVE

We show that a therapeutic dose of flagellin suppresses asthma and that the effect occurs through generating regulatory dendritic cells (rDCs) and regulatory T (Treg) cells.

METHODS

Ovalbumin (OVA)-induced wild-type and TLR5 knockout asthmatic mice were treated intranasally with a mixture of OVA and 10 μg of a flagellin B (FlaB; of Vibrio vulnificus). OVA/FlaB-treated rDCs were adoptively transferred to mice with OVA-induced asthma. Anti-CD25 mAb was used to deplete Treg cells. A mixture of house dust mite (HDM) and FlaB was used to treat mice with HDM-induced asthma. Blood CD14(+) monocyte-derived dendritic cells from HDM-sensitive asthmatic patients were treated with FlaB and incubated with autologous CD4(+) T cells.

RESULTS

An OVA/FlaB mixture ameliorated OVA-induced asthma by inhibiting TH1/TH2/TH17 responses in a TLR5-dependent manner through generating rDCs and Treg cells. The adoptive transfer of OVA/FlaB-treated dendritic cells inhibited OVA-induced asthma, whereas the depletion of CD25(+) cells eliminated the inhibitory effect. A similar effect of FlaB was observed in mice with HDM-induced asthma. In patients with HDM-sensitive asthma, FlaB-treated rDCs inhibited HDM-stimulated TH1/TH2 responses while enhancing Treg cells in an IL-10-dependent manner.

CONCLUSION

These findings collectively suggest that flagellin could be used as a tolerogenic adjuvant to treat allergic asthma.

Tolerance and Cross-Tolerance following Toll-Like Receptor (TLR)-4 and -9 Activation Are Mediated by IRAK-M and Modulated by IL-7 in Murine Splenocytes

Objective

Immune suppression during critical illness predisposes to serious infections. We sought to determine the mechanisms regulating tolerance and cross-tolerance to common pro-inflammatory danger signals in a model that recapitulates the intact in vivo immune response.

Materials and Methods

Flt3-expanded splenocytes obtained from wild-type or matching IRAK-M knockout (IRAK-M-/-), C57BL/6, male mice (8–10 weeks old) were treated repeatedly or alternately with either LPS or CpGA DNA, agonists of Toll-like receptor (TLR)-4 and -9, respectively, over successive 24-hour periods. Supernatants were collected following each 24-hour period with cytokine release (ELISA) and splenocyte IRAK-M expression (Western blot) determined. Tolerance and cross-tolerance were assessed in the absence or presence of programmed death receptor (PD)-1 blocking antibody or IL-7 pre-treatment.

Main Results

Splenocytes notably exhibited both tolerance and cross-tolerance to subsequent treatments with either LPS or CpGA DNA. The character of tolerance and cross-tolerance in this model was distinct following initial LPS or CpGA treatment in that TNFα and IFNγ release (not IL-10) were suppressed following LPS; whereas, initial CpGA treatment suppressed TNFα, IFNγ and IL-10 release in response to subsequent stimulation (LPS or CpGA). Tolerance and cross-tolerance were unrelated to IL-10 release or PD-1 but were attenuated in IRAK-M-/- splenocytes. IL-7 significantly suppressed IRAK-M expression and restored TNFα and IFNγ production without influencing IL-10 release.

Conclusions

In summary, acute immune tolerance and cross-tolerance in response to LPS or CpGA were distinct in that LPS selectively suppressed pro-inflammatory cytokine responses; whereas, CpGA suppressed both pro- and anti-inflammatory responses. The induction of tolerance and cross-tolerance in response to common danger signals was mechanistically unrelated to IL-10 or PD-1 but was directly influenced by IRAK-M expression. IL-7 reduced IRAK-M expression and attenuated immune tolerance induced by either LPS or CpGA, and thus may be useful for reversal of immune tolerance in the setting of critical illness.

Function and mechanism of toll-like receptors in cerebral ischemic tolerance: from preconditioning to treatment

Increasing evidence suggests that toll-like receptors (TLRs) play an important role in cerebral ischemia-reperfusion injury. The endogenous ligands released from ischemic neurons activate the TLR signaling pathway, resulting in the production of a large number of inflammatory cytokines, thereby causing secondary inflammation damage following cerebral ischemia. However, the preconditioning for minor cerebral ischemia or the preconditioning with TLR ligands can reduce cerebral ischemic injury by regulating the TLR signaling pathway following ischemia in brain tissue (mainly, the inhibition of the TLR4/NF-κB signaling pathway and the enhancement of the interferon regulatory factor-dependent signaling), resulting in TLR ischemic tolerance. Additionally, recent studies found that postconditioning with TLR ligands after cerebral ischemia can also reduce ischemic damage through the regulation of the TLR signaling pathway, showing a significant therapeutic effect against cerebral ischemia. These studies suggest that the ischemic tolerance mediated by TLRs can serve as an important target for the prevention and treatment of cerebral ischemia. On the basis of describing the function and mechanism of TLRs in mediating cerebral ischemic damage, this review focuses on the mechanisms of cerebral ischemic tolerance induced by the preconditioning and postconditioning of TLRs and discusses the clinical application of TLRs for ischemic tolerance.

Inhibiting TNF-α signaling does not attenuate induction of endotoxin tolerance

Tumor necrosis factor-alpha (TNF-α) is a central mediator of inflammatory responses elicited by Toll-like receptor agonists, such as the Gram-negative bacterial outer membrane antigen lipopolysaccharide (LPS). TNF-α is responsible for altering vascular permeability and activating infiltrating inflammatory cells, such as monocytes and neutrophils. Interestingly, TNF-α has also demonstrated the ability to induce tolerance to subsequent challenges with TNF-α or LPS in monocyte and macrophage cell populations. Tolerance is characterized by the inability to mount a typical inflammatory response during subsequent challenges following the initial exposure to an inflammatory mediator such as LPS. The ability of TNF-α to induce a tolerant-like state with regard to LPS is most likely a regulatory mechanism to prevent excessive inflammation. We hypothesized that the induction of tolerance or the degree of tolerance is dependent upon the production of TNF-α during the primary response to LPS. To investigate TNF-α-dependent tolerance, human monocytic THP-1 cells were treated with TNF-α-neutralizing antibodies or antagonistic TNF-α receptor antibodies before primary LPS stimulation and then monitored for the production of TNF-α during the primary and challenge stimulation. During the primary stimulation, anti-TNF-α treatment effectively attenuated the production of TNF-α and interleukin-1β; however, this reduced production did not impact the induction of endotoxin tolerance. These results demonstrate that interfering with TNF-α signaling attenuates production of inflammatory cytokines without affecting the induction of tolerance.

Over-activation of TLR5 signaling by high-dose flagellin induces liver injury in mice

Flagellin is a potent activator of a broad range of cell types that are involved in innate and adaptive immunity. Therefore, it is a good adjuvant candidate for vaccines, and it might function as a biological protectant against both major acute radiation syndrome during cancer radiotherapy and a mitigator of radiation emergencies. However, accumulating evidence has implicated flagellin in the occurrence of some inflammatory diseases, such as acute lung inflammation, cardiovascular collapse and inflammatory bowel disease. The aim of this study was to elucidate whether only flagellin-TLR5 signaling activation plays a role in the pathophysiology of liver or whether some other flagellin activity also contributes to liver injury either via bacterial infections or during clinical applications. Recombinant flagellin proteins with or without TLR5-stimulating activity were used to evaluate the role of flagellin-TLR5 signaling in liver injury in wild-type and TLR5 KO mice. Gross lesions and large areas of hepatocellular necrosis were observed in liver tissue 12 h after the intraperitoneal administration of 100 or 200 µg flagellin (FliC) in a dose- and time-dependent manner in wild-type mice, but not in TLR5 KO mice. Deletion of the N-terminal or TLR5 binding domain of flagellin inhibited flagellin-induced inflammatory responses and the subsequent acute liver function abnormality and damage. These data confirmed that flagellin is an essential determinant of liver injury and demonstrated that the over-activation of TLR5 signaling by high-dose flagellin caused acute inflammatory responses, neutrophil accumulation and oxidative stress in the liver, which contributes to the progression and severity of flagellin-induced liver injury.

The impact of single and pairwise Toll-like receptor activation on neuroinflammation and neurodegeneration

Background

Toll-like receptors (TLRs) enable innate immune cells to respond to pathogen- and host-derived molecules. The central nervous system (CNS) exhibits most of the TLRs identified with predominant expression in microglia, the major immune cells of the brain. Although individual TLRs have been shown to contribute to CNS disorders, the consequences of multiple activated TLRs on the brain are unclear. We therefore systematically investigated and compared the impact of sole and pairwise TLR activation on CNS inflammation and injury.

Methods

Selected TLRs expressed in microglia and neurons were stimulated with their specific TLR ligands in varying combinations. Cell cultures were then analyzed by immunocytochemistry, FlowCytomix, and ELISA. To determine neuronal injury and neuroinflammation in vivo, C57BL/6J mice were injected intrathecally with TLR agonists. Subsequently, brain sections were analyzed by quantitative real-time PCR and immunohistochemistry.

Results

Simultaneous stimulation of TLR4 plus TLR2, TLR4 plus TLR9, and TLR2 plus TLR9 in microglia by their respective specific ligands results in an increased inflammatory response compared to activation of the respective single TLR in vitro. In contrast, additional activation of TLR7 suppresses the inflammatory response mediated by the respective ligands for TLR2, TLR4, or TLR9 up to 24 h, indicating that specific combinations of activated TLRs individually modulate the inflammatory response. Accordingly, the composition of the inflammatory response pattern generated by microglia varies depending on the identity and combination of the activated TLRs engaged. Likewise, neuronal injury occurs in response to activation of only selected TLRs and TLR combinations in vitro. Activation of TLR2, TLR4, TLR7, and TLR9 in the brain by intrathecal injection of the respective TLR ligand into C57BL/6J mice leads to specific expression patterns of distinct TLR mRNAs in the brain and causes influx of leukocytes and inflammatory mediators into the cerebrospinal fluid to a variable extent. Also, the intensity of the inflammatory response and neurodegenerative effects differs according to the respective activated TLR and TLR combinations used in vivo.

Conclusions

Sole and pairwise activation of TLRs modifies the pattern and extent of inflammation and neurodegeneration in the CNS, thereby enabling innate immunity to take account of the CNS diseases’ diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0166-7) contains supplementary material, which is available to authorized users.

Bruton tyrosine kinase mediates TLR9-dependent human dendritic cell activation

BACKGROUND

Bruton tyrosine kinase (BTK) plays an essential role in various biologic functions of different cell types. Mutations in BTK lead to X-linked agammaglobulinemia (XLA) in humans. BTK was recently linked to the innate immune system, in particular, the Toll-like receptor (TLR) pathway; however, the TLR9 pathway has never been studied in dendritic cells (DCs) of patients with XLA.

OBJECTIVE

The aim of this study was to investigate the role of BTK in human DC activation upon TLR9 stimulation.

METHODS

DCs of patients with XLA and healthy donors were stimulated via TLR4/9 and evaluated for cell activation and cytokine production.

RESULTS

We showed that BTK plays an essential role in DC responses to unmethylated CpG oligodeoxynucleotide: although responses to lipopolysaccaride/TLR4 induce normal DC activation in terms of upregulation of specific markers (CD86, CD83, CD80, HLA-DR), the CpG/TLR9 pathway is completely impaired in patients with XLA. Furthermore, cytokine production upon TLR9 activation in patients with XLA is radically impaired in terms of IL-6, IL-12, TNF-α, and IL-10 production. Interestingly, BTK mediated STAT1/3 upregulation in a TLR9-dependent manner. The important role of BTK in human DC activation was confirmed after incubation of healthy DCs with ibrutinib, the specific BTK inhibitor, which resulted in impairment of TLR9 responses as seen in patients with XLA.

CONCLUSION

Analysis of these data suggests that BTK regulates human DC responses upon TLR9 engagement in terms of activation, cytokine production, and STAT1/3 upregulation. These findings may be of important significance for better understanding and managing different clinical conditions, such as agammaglobulinemia and lymphoid malignancies.

Trained immunity or tolerance: opposing functional programs induced in human monocytes after engagement of various pattern recognition receptors

Upon priming with Candida albicans or with the fungal cell wall component β-glucan, monocytes respond with an increased cytokine production upon restimulation, a phenomenon termed "trained immunity." In contrast, the prestimulation of monocytes with lipopolysaccharide has long been known to induce tolerance. Because the vast majority of commensal microorganisms belong to bacterial or viral phyla, we sought to systematically investigate the functional reprogramming of monocytes induced by the stimulation of pattern recognition receptors (PRRs) with various bacterial or viral ligands. Monocytes were functionally programmed for either enhanced (training) or decreased (tolerance) cytokine production, depending on the type and concentration of ligand they encountered. The functional reprogramming of monocytes was also associated with cell shape, granulocity, and cell surface marker modifications. The training effect required p38- and Jun N-terminal protein kinase (JNK)-mediated mitogen-activated protein kinase (MAPK) signaling, with specific signaling patterns directing the functional fate of the cell. The long-term effects on the function of monocytes were mediated by epigenetic events, with both histone methylation and acetylation inhibitors blocking the training effects. In conclusion, our experiments identify the ability of monocytes to acquire adaptive characteristics after prior activation with a wide variety of ligands. Trained immunity and tolerance are two distinct and opposing functional programs induced by the specific microbial ligands engaging the monocytes.

Fractal-like kinetics of intracellular enzymatic reactions: a chemical framework of endotoxin tolerance and a possible non-specific contribution of macromolecular crowding to cross-tolerance

BACKGROUND

The response to endotoxin (LPS), and subsequent signal transduction lead to the production of cytokines such as tumor necrosis factor-α (TNF-α) by innate immune cells. Cells or organisms pretreated with endotoxin enter into a transient state of hyporesponsiveness, referred to as endotoxin tolerance (ET) which represents a particular case of negative preconditioning. Despite recent progress in understanding the molecular basis of ET, there is no consensus yet on the primary mechanism responsible for ET and for the more complex cases of cross tolerance. In this study, we examined the consequences of the macromolecular crowding (MMC) and of fractal-like kinetics (FLK) of intracellular enzymatic reactions on the LPS signaling machinery. We hypothesized that this particular type of enzyme kinetics may explain the development of ET phenomenon.

METHOD

Our aim in the present study was to characterize the chemical kinetics framework in ET and determine whether fractal-like kinetics explains, at least in part, ET. We developed an ordinary differential equations (ODE) mathematical model that took into account the links between the MMC and the LPS signaling machinery leading to ET. We proposed that the intracellular fractal environment (MMC) contributes to ET and developed two mathematical models of enzyme kinetics: one based on Kopelman's fractal-like kinetics framework and the other based on Savageau's power law model.

RESULTS

Kopelman's model provides a good image of the potential influence of a fractal intracellular environment (MMC) on ET. The Savageau power law model also partially explains ET. The computer simulations supported the hypothesis that MMC and FLK may play a role in ET.

CONCLUSION

The model highlights the links between the organization of the intracellular environment, MMC and the LPS signaling machinery leading to ET. Our FLK-based model does not minimize the role of the numerous negative regulatory factors. It simply draws attention to the fact that macromolecular crowding can contribute significantly to the induction of ET by imposing geometric constrains and a particular chemical kinetic for the intracellular reactions.

Glycogen synthase kinase 3 inhibitor attenuates endotoxin-induced liver injury

BACKGROUND/AIMS

Endotoxin (lipopolysaccharide, LPS)-induced acute liver injury was attenuated by endotoxin tolerance (ET), which is characterized by phosphatidylinositol 3-kinase pathway/Akt signaling. Glycogen synthase kinase 3 (GSK-3) acts downstream of phosphatidylinositol 3-kinase pathway/Akt and GSK-3 inhibitor protects against organic injury. This study evaluates the hypothesis that ET attenuated LPS-induced liver injury through inhibiting GSK-3 functional activity and downstream signaling.

METHODS

Sprague-Dawley rats with or without low-dose LPS pretreatment were challenged with or without large dose of LPS and subsequently received studies. Serum tumor necrosis factor-alpha, interleukin-10, alanine aminotransferase, lactate dehydrogenase, and total bilirubin levels were analyzed, morphology of liver tissue was performed, glycogen content, myeloperoxidase content, phagocytosis activity of Kupffer cells, and the expression and inhibitory phosphorylation as well as kinase activity of GSK-3 were examined. Survival after LPS administration was also determined.

RESULTS

LPS induced significant increases of serum TNF-α, alanine aminotransferase, lactate dehydrogenase, and total bilirubin (P < 0.05), which were companied by obvious alterations in liver: the injury of liver tissue, the decrease of glycogen, the infiltration of neutrophils, and the enhancement of phagocytosis of Kupffer cells (P < 0.05). LPS pretreatment significantly attenuated these alterations, promoted the inhibitory phosphorylation of GSK-3 and inhibited its kinase activity, and improved the survival rate (P < 0.05).

CONCLUSIONS

ET attenuated LPS-induced acute liver injury through inhibiting GSK-3 functional activity and its downstream signaling.

β-1,4-mannobiose stimulates innate immune responses and induces TLR4-dependent activation of mouse macrophages but reduces severity of inflammation during endotoxemia in mice

β-1,4-Mannobiose (MNB) has been shown to exert prebiotic activity and modulate mucosal gene expression. In this study, the immune-modulating effect of MNB in healthy and endotoxemic mice and its role in Toll-like receptor (TLR) 2/4-mediated macrophage activation were investigated. Mice were supplemented daily with MNB (0, 5, 10, or 25 mg/kg) for 14 d. To examine the effect of MNB during endotoxemia, mice were supplemented with or without MNB (25 mg/kg) for 14 d, followed by challenge with intraperitoneal LPS or saline. MNB induced expression of both T helper (Th) 1- and Th2-type cytokines in the ileum (P < 0.05) and increased fecal IgA production and splenic NK cell activity (P < 0.05) in healthy mice. In endotoxemic mice, MNB reduced the expression of Tnfa, Il-6, iNos (P < 0.05), and Il-10 (P < 0.05), and reduced LPS-induced weight loss but increased Ifng, Il-12p40, Il-5, and Ifna expression (P < 0.05) and NK cell activity relative to positive control (LPS) mice. Treatment of RAW 264.7 macrophages with MNB induced TNF-α and IL-6 secretion (P < 0.05), and this effect was abrogated by inhibiting TLR4, but not TLR2, signaling. Pretreatment of RAW 264.7 cells with MNB induced tolerance to TLR2 and TLR4 agonists, reducing TNF-α production (P < 0.05) upon secondary stimulation with LPS or lipoteichoic acid. These results indicate that MNB can modulate intestinal and systemic immune responses in healthy and endotoxemic mice and prevent LPS-induced immune suppression, as well as directly stimulating innate immune mechanisms in vitro as a TLR4 agonist.

A critical role for MAPK signalling pathways in the transcriptional regulation of toll like receptors

Toll-like Receptors (TLR) are phylogenetically conserved transmembrane proteins responsible for detection of pathogens and activation of immune responses in diverse animal species. The stimulation of TLR by pathogen-derived molecules leads to the production of pro-inflammatory mediators including cytokines and nitric oxide. Although TLR-induced events are critical for immune induction, uncontrolled inflammation can be life threatening and regulation is a critical feature of TLR biology. We used an avian macrophage cell line (HD11) to determine the relationship between TLR agonist-induced activation of inflammatory responses and the transcriptional regulation of TLR. Exposure of macrophages to specific TLR agonists induced upregulation of cytokine and nitric oxide pathways that were inhibited by blocking various components of the TLR signalling pathways. TLR activation also led to changes in the levels of mRNA encoding the TLR responsible for recognising the inducing agonist (cognate regulation) and cross-regulation of other TLR (non-cognate regulation). Interestingly, in most cases, regulation of TLR mRNA was independent of NFκB activity but dependent on one or more of the MAPK pathway components. Moreover, the relative importance of ERK, JNK and p38 was dependent upon both the stimulating agonist and the target TLR. These results provide a framework for understanding the complex pathways involved in transcriptional regulation of TLR, immune induction and inflammation. Manipulation of these pathways during vaccination or management of acute inflammatory disease may lead to improved clinical outcome or enhanced vaccine efficacy.

MicroRNA-146a in autoimmunity and innate immune responses

MicroRNA (miRNA) are approximately 22 nucleotide single-stranded RNA that regulate the stability of target messenger RNA by selective binding to specific sites at the 3'-untranslated regions (UTR). This triggers repression in translation and mRNA degradation. It has been estimated that approximately 60% of all mRNA are under the control of miRNA. Among the known hundreds of miRNA, some are considered master regulators controlling either a single or multiple cellular pathways. Some miRNA are known to affect development and cell differentiation, while others are implicated in immunity and autoimmune diseases. A very interesting example is miR-146a, which has been reported to be downregulated in systemic lupus erythematosus and upregulated in rheumatoid arthritis (RA). Several groups have recently focused their attention on miRNA in the pathogenesis of RA. Interestingly, the expression of miR-146a is upregulated in different cell types and tissues in RA patients. miRNA in RA could also be considered as possible future targets for new therapeutic approaches. This discussion will focus on the current understanding in the function of miR-146a in endotoxin tolerance and cross-tolerance, and how it may contribute to modulate the overproduction of known pathogenic cytokines, such as tumour necrosis factor α.

Interleukin-1 receptor-associated kinase 2- and protein kinase D1-dependent regulation of IRAK-monocyte expression by CpG DNA

As a part of the negative feedback mechanism, CpG DNA induces IRAK-M expression in monocytic cells. In the present study we investigated a biochemical signaling pathway and the transcription factors responsible for CpG DNA-mediated Irak-m gene expression. CpG DNA-induced Irak-m expression did not require new protein synthesis and was regulated at the transcriptional level through an endosomal pH-sensitive TLR9/MyD88 signaling pathway. Over-expression of the dominant negative (DN) form of or gene-specific knockdown of signaling modulators in the TLR9 pathway demonstrated that IRAK4, IRAK1, IRAK2, and PKD1 are required for Irak-m transcription induced by CpG DNA. Over-expression of DN-IRAK1 only partially, but significantly, inhibited CpG DNA-induced Irak-m promoter activity. While IRAK1 was critical for the initial phase, IRAK2 was required for the late phase of TLR9 signaling by sustaining activation of PKD1 that leads to activation of NF-κB and MAPKs. Irak-m promoter-luciferase reporters with alterations in the predicted cis-acting transcriptional regulatory elements revealed that the NF-κB consensus site in the Irak-m promoter region is absolutely required for Irak-m gene expression. AP-1 and CREB binding sites also contributed to the optimal Irak-m expression by CpG DNA. Collectively, our results demonstrate that IRAK2 plays a key role in the TLR9-mediated transcriptional regulation of Irak-m expression by sustaining activation of PKD1 and NF-κB.

Toll-like receptor 9 in systemic lupus erythematosus, impact on glucocorticoid treatment

AIM

To assess TLR9 expression in systemic lupus erythematosus (SLE) patients, its correlation with disease activity, and impact of TLR9 expression on the response to oral glucocorticoids.

METHODS

Twenty-five active SLE, 15 inactive, and 15 control subjects were included. Anti-DNA, ANA, C3, C4, and TLR9 mRNA expressions were assessed. Active SLE patients only received oral steroid for 6 weeks. Post therapy, they were classified into steroid sensitive and steroid resistant. Data were reassessed after treatment.

RESULTS

SLEDAI, anti-DNA, ANA, and TLR9 expressions were significantly higher in active SLE patients. Based on retrograde analysis, TLR9 expression was significantly higher in steroid-resistant versus steroid-sensitive group before treatment, with no significant difference between them after treatment. There was a significant positive correlation between TLR9 expression and SLEDAI score and anti-DNA and negative correlation with C3 and C4 in all patients.

CONCLUSION

TLR9 may play a role in the pathogenesis of SLE and correlates with the disease activity. Corticosteroids have no effect on TLR9 expression, explaining lack of corticosteroid response in some SLE patients. TLR 9 expression can be used in predicting glucocorticoid response in active SLE patients. New treatment modalities targeting TLR9 expression may be of value in steroid-resistant patients.

The role of innate immunity in HBV infection

Hepatitis B virus (HBV) infection is one of the main causes of chronic liver diseases. Whether HBV infection is cleared or persists is determined by both viral factors and host immune responses. It becomes clear that innate immunity is of importance in protecting the host from HBV infection and persistence. However, HBV develops strategies to suppress the antiviral immune responses. A combined therapeutic strategy with both viral suppression and enhancement of antiviral immune responses is needed for effective long-term clearance and cure for chronic HBV infection. We and others confirmed that bifunctional siRNAs with both gene silencing and innate immune activation properties are beneficial for inhibition of HBV and represent a potential approach for treatment of viral infection. Understanding the nature of liver innate immunity and their roles in chronic HBV progression and HBV clearance may aid in the design of novel therapeutic strategies for chronic HBV infection.

The Toll-like receptor 9 ligand CPG-C attenuates acute inflammatory cardiac dysfunction

Stimulation of toll-like receptor 9 (TLR9) by CpG-C containing oligonucleotides attenuates ischemic injury in the brain and liver. In this study, we investigate whether any of the three classes of CpG (A, B, or C) mitigate ischemia-induced cardiac dysfunction. We measured left ventricular ejection fraction (LVEF) in C57BL/6 mice using transthoracic echocardiography. Using LPS as an inflammatory stimulus, CpG-C was uniquely able to prevent cardiac dysfunction; its activity was confirmed through nuclear factor κB transcriptional activity assay in HL-1 cardiomyocytes. We went on to investigate CpG-C's efficacy and mechanism in the treatment of ischemia-reperfusion. Compared with baseline, no class of CpG significantly altered LVEF at 6 or 24 h; 40 mg/kg LPS induced a rapid, profound suppression of LVEF compared with baseline (26% ± 1.4% vs. 65% ± 1.4%), whereas pretreatment with CpG demonstrated that of the three classes, only CpG-C prevented the LPS -induced decrease in LVEF (51% ± 5.8%). In separate mice, 1-h ischemia followed by reperfusion of the left anterior descending artery resulted in a 7-day suppression of the LVEF (66% ± 5.2% at baseline; 46% ± 4.7% at day 1, and 46% ± 4.0% at day 7), whereas mice either pretreated with or begun on an infusion of CpG-C during the ischemia had no significant decline in LVEF. Gene expression microarray of CpG-C-stimulated cells revealed upregulation of the nuclear factor κB pathway inhibitors TNFAIP3, NFKBIA, TRIM30, and TNIP1. These may play a role in attenuation of cardiac inflammation. The TLR9 ligand CpG-C attenuates the acute inflammatory cardiac dysfunction induced by both LPS and ischemia-reperfusion of the left anterior descending artery.

Bacterial immunotherapy of gastrointestinal tumors

Background

Cancer immunotherapy using bacteria dates back over 150 years. The deeper understanding on how the immune system interferes with the tumor microenvironment has led to the re-emergence of bacteria or their related products in immunotherapeutic concepts. In this review, we discuss recent approaches on experimental bacteriolytic therapy, emphasizing the specific interplay between bacteria, immune cells and tumor cells to break the tumor-induced tolerance.

Results

Experimental research during the last decades demonstrated beneficial but also adverse influence of bacteria on tumor growth. There is a strong correlation between chronic infections and tumor incidence. However, acute bacterial infections have favourable effects on tumor growth often contributing to complete remission. Tumor regression is usually attributable to both direct tumor cell killing (via apoptosis and/or necrosis, depending on the applied bacteria) and indirect immune stimulation. This includes (I) elimination of immunosuppressive immune cells (i.e. tumor-associated macrophages, myeloid-derived suppressor, and regulatory T cells), (II) suppression of Th2-directed cytokine secretion (TGFα, IL10), (III) providing a pro-inflammatory micro-milieu (tumor infiltrating neutrophils) and (IV) supporting the influx of cytotoxic T cells into tumors. This finally forces the development of an immunological memory and may provide long-term protection against cancer.

Conclusion

Immunotherapy using bacteria is still a double-edged sword. Experiences from the last years have substantially contributed to when bacteria and defined components thereof might be integrated into immunotherapeutic concepts. Attempts in transferring this approach into the clinics are on their way.

Toll-like receptors in cerebral ischemic inflammatory injury

Cerebral ischemia triggers acute inflammation, which has been associated with an increase in brain damage. The mechanisms that regulate the inflammatory response after cerebral ischemia are multifaceted. An important component of this response is the activation of the innate immune system. However, details of the role of the innate immune system within the complex array of mechanisms in cerebral ischemia remain unclear. There have been recent great strides in our understanding of the innate immune system, particularly in regard to the signaling mechanisms of Toll-like receptors (TLRs), whose primary role is the initial activation of immune cell responses. So far, few studies have examined the role of TLRs in cerebral ischemia. However, work with experimental models of ischemia suggests that TLRs are involved in the enhancement of cell damage following ischemia, and their absence is associated with lower infarct volumes. It may be possible that therapeutic targets could be designed to modulate activities of the innate immune system that would attenuate cerebral brain damage. Ischemic tolerance is a protective mechanism induced by a variety of preconditioning stimuli. Interpreting the molecular mechanism of ischemic tolerance will open investigative avenues into the treatment of cerebral ischemia. In this review, we discuss the critical role of TLRs in mediating cerebral ischemic injury. We also summarize evidence demonstrating that cerebral preconditioning downregulates pro-inflammatory TLR signaling, thus reducing the inflammation that exacerbates ischemic brain injury.

Toll-like receptors and lupus nephritis

A variety of immune mechanisms, both humoral and cellular, are involved in the onset and amplification of the inflammatory response in lupus nephritis (LN). Accumulating evidence substantiates the view that innate immunity pathways may also amplify inflammatory reactions within the kidneys. Toll-like receptors (TLRs) are essential modulators of the innate immune response thanks to their ability to rocognize conserved molecular patterns that are microbe specific and other danger signals. Their recognition of endogenous molecules released from injured cells may also contribute to renal inflammation. Studies conducted in vivo and in vitro provide experimental evidence for the functional role of TLRs in LN. Intriguingly, these data suggest that pharmacological TLR signal suppression could be a useful approach to the treatment of systemic lupus erythematosus.

MicroRNA in TLR signaling and endotoxin tolerance

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

Toll-like receptors in acute liver injury and regeneration

Liver is the lymphoid organ with an overwhelming innate immune system, which functions as a filter organ at the first line between the digestive tract and the rest of the body, with receiving 80% of the blood supply through portal vein. TLRs are widely expressed on parenchymal and non-parenchymal cells in the liver, which play critical roles for the liver health. Recent studies indicate that TLR-medicated signals have been involved in almost all liver diseases such as acute and chronic hepatitis, liver fibrosis and cirrhosis, alcoholic and non-alcoholic liver disease, ischemia/reperfusion liver injury, liver regeneration and hepatocellular carcinoma. In this review, the expressions of TLRs in hepatic cell populations including hepatocytes, LSECs, Kupffer cells, lymphocytes, DCs, biliary epithelial cells and HSCs, and TLR ligands and signaling in the liver are summarized. Further, recent advances in the roles of TLRs in acute liver injury and regeneration as mediator and regulator, and their potential therapeutic targets are discussed.

The synthetic triterpenoid, CDDO-Me, modulates the proinflammatory response to in vivo lipopolysaccharide challenge

The synthetic triterpenoid, CDDO-Me, has potent antiproliferative and antioxidant properties. However, its immunomodulatory effects in the context of LPS challenge are incompletely defined. Pretreatment with oral CDDO-Me significantly improved survival following lethal-dose LPS challenge in mice. To define this protection further, we measured effects of CDDO-Me pretreatment on splenocyte populations and cytokine production following LPS challenge, using low-level LPS pretreatment as an in vivo control for reducing cytokine production. Despite similar decreases in levels of LPS-inducible, circulating proinflammatory cytokines (IL-12p70, IFN-gamma, IL-6, IL-17, and IL-23) and increases in heme oxygenase 1 (HO-1) protein expression, low-dose LPS and CDDO-Me pretreatments markedly differed in their overall response profiles. Splenocytes from LPS-pretreated mice contained reduced numbers of dendritic cells, increased percentages of Th17 and T-regulatory cells, lower levels of TLR-inducible IL-6, and higher levels of TLR-inducible IL-10. In contrast, CDDO-Me protection against LPS challenge had no impact on absolute numbers or distribution of splenocyte subsets, despite attenuating in vivo induction of proinflammatory cytokines in an IL-10-independent manner. Together, these results suggest that CDDO-Me pretreatment uniquely confers protection against LPS challenge by modulating the in vivo immune response to LPS. Thus, CDDO-Me potentially represents a novel oral agent for use in LPS-mediated inflammatory diseases.

Mechanistic role of microRNA-146a in endotoxin-induced differential cross-regulation of TLR signaling

Human TLRs are critical sensors for microbial components leading to the production of proinflammatory cytokines that are controlled by various mechanisms. Monocytes pretreated with LPS exhibit a state of hyporesponsiveness, referred to as cross-tolerance, to both homologous and heterologous ligands, which play a broader role in innate immunity. To date, LPS-induced cross-tolerance has not been examined regarding microRNA expression kinetics. In this study, THP-1 monocytes treated with various inflammatory ligands showed a continuous amplification of microRNA (miR)-146a over 24 h that is inversely correlated to TNF-α production. In contrast, inhibition of miR-146a showed a reciprocal effect. Thus, the characteristic upregulation of miR-146a in LPS-exposed THP-1 monocytes was studied for cross-tolerance. Strikingly, in LPS-tolerized THP-1 monocytes, only miR-146a showed a continuous overexpression, suggesting its crucial role in cross-tolerance. Similarly, peptidoglycan-primed THP-1 cells showed homologous tolerance associated with miR-146a upregulation. Subsequently, interchangeable differential cross-regulation was observed among non-LPS ligands. TLR2 and TLR5 ligands showed both homologous and heterologous tolerance correlated to miR-146a overexpression. More importantly, inflammatory responses to TLR4, TLR2, and TLR5 ligands were reduced due to knockdown of miR-146a targets IL-1R-associated kinase 1 or TNFR-associated factor 6, suggesting the regulatory effect of miR-146a on these TLRs signaling. Transfection of miR-146a into THP-1 cells caused reduction of TNF-α production, mimicking LPS-induced cross-tolerance. Aside from individual ligands, a whole bacterial challenge in LPS-primed THP-1 monocytes was accompanied by less TNF-α production, which is conversely correlated to miR-146a expression. Our studies have thus demonstrated that miR-146a plays a crucial role for in vitro monocytic cell-based endotoxin-induced cross-tolerance.

Lactobacillus plantarum lipoteichoic acid down-regulated Shigella flexneri peptidoglycan-induced inflammation

Bacterial peptidoglycans (PGNs) are recognized by the host's innate immune system. This process is mediated by the NOD/CARD family of proteins, which induces inflammation by activating nuclear factor (NF)-κB. Excessive activation of monocytes by Shigella flexneri PGN (flexPGN) leads to serious inflammatory diseases such as intestinal bowel diseases (IBD) and Crohn's disease. In this study, we examined whether Lactobacillus plantarum lipoteichoic acid (pLTA) could attenuate the pro-inflammatory signaling induced by flexPGN in human monocytic THP-1 cells. Compared to control THP-1 cells, pLTA-tolerant cells showed a significant reduction in TNF-α and IL-1β production in response to flexPGN. We also examined the inhibition of NF-κB and the activation of mitogen-activated protein kinase (MAPK) in pLTA-tolerant cells. We found that the expression of NOD2 in pLTA-tolerant cells was down-regulated at the mRNA and protein levels, suggesting that pLTA is a potent modulator of the pro-inflammatory NOD2-related signaling pathways induced by flexPGN. Together, these data indicate that pLTA induces cross-tolerance against flexPGN. Notably, these effects are related not only to IL-1 signaling, which is known to play a role in LPS tolerance, but also to NOD-Rick signaling. This study provides insight into how commensal microflora may contribute to homeostasis of the host intestinal tract.

Macroparasites, innate immunity and immunoregulation: developing natural models

Innate immune receptors carry out surveillance for infection threats and are a proximal controller of the threshold and intensity at which inflammatory responses occur. As such, they are a natural focus for understanding how inflammatory immune reactivity is regulated. This review highlights how little data there are relating to the effect of macroparasites on systemic innate receptor responses. The idea is developed that studies on innate immune function in wild animals exposed to a natural profile of infections, including macroparasites, might be a valuable model in which to test hypotheses about the ultimate cause of aberrant inflammation in modern human populations.

Toll-like receptor 9 promotes steatohepatitis by induction of interleukin-1beta in mice

BACKGROUND & AIMS

Development of nonalcoholic steatohepatitis (NASH) involves the innate immune system and is mediated by Kupffer cells and hepatic stellate cells (HSCs). Toll-like receptor 9 (TLR9) is a pattern recognition receptor that recognizes bacteria-derived cytosine phosphate guanine (CpG)-containing DNA and activates innate immunity. We investigated the role of TLR9 signaling and the inflammatory cytokine interleukin-1beta (IL-1beta) in steatohepatitis, fibrosis, and insulin resistance.

METHODS

Wild-type (WT), TLR9(-/-), IL-1 receptor (IL-1R)(-/-), and MyD88(-/-) mice were fed a choline-deficient amino acid-defined (CDAA) diet for 22 weeks and then assessed for steatohepatitis, fibrosis, and insulin resistance. Lipid accumulation and cell death were assessed in isolated hepatocytes. Kupffer cells and HSCs were isolated to assess inflammatory and fibrogenic responses, respectively.

RESULTS

The CDAA diet induced NASH in WT mice, characterized by steatosis, inflammation, fibrosis, and insulin resistance. TLR9(-/-) mice showed less steatohepatitis and liver fibrosis than WT mice. Among inflammatory cytokines, IL-1beta production was suppressed in TLR9(-/-) mice. Kupffer cells produced IL-1beta in response to CpG oligodeoxynucleotide. IL-1beta but not CpG-oligodeoxynucleotides, increased lipid accumulation in hepatocytes. Lipid accumulation in hepatocytes led to nuclear factor-kappaB inactivation, resulting in cell death in response to IL-1beta. IL-1beta induced fibrogenic responses in HSCs, including secretion of tissue inhibitor of metalloproteinase-1. IL-1R(-/-) mice had reduced steatohepatitis and fibrosis, compared with WT mice. Mice deficient in MyD88, an adaptor molecule for TLR9 and IL-1R signaling, also had reduced steatohepatitis and fibrosis. TLR9(-/-), IL-1R(-/-), and MyD88(-/-) mice had less insulin resistance than WT mice on the CDAA diet.

CONCLUSIONS

In a mouse model of NASH, TLR9 signaling induces production of IL-1beta by Kupffer cells, leading to steatosis, inflammation, and fibrosis.

Inadequate clearance of translocated bacterial products in HIV-infected humanized mice

Bacterial translocation from the gut and subsequent immune activation are hallmarks of HIV infection and are thought to determine disease progression. Intestinal barrier integrity is impaired early in acute retroviral infection, but levels of plasma lipopolysaccharide (LPS), a marker of bacterial translocation, increase only later. We examined humanized mice infected with HIV to determine if disruption of the intestinal barrier alone is responsible for elevated levels of LPS and if bacterial translocation increases immune activation. Treating uninfected mice with dextran sodium sulfate (DSS) induced bacterial translocation, but did not result in elevated plasma LPS levels. DSS-induced translocation provoked LPS elevation only when phagocytic cells were depleted with clodronate liposomes (clodrolip). Macrophages of DSS-treated, HIV-negative mice phagocytosed more LPS ex vivo than those of control mice. In HIV-infected mice, however, LPS phagocytosis was insufficient to clear the translocated LPS. These conditions allowed higher levels of plasma LPS and CD8+ cell activation, which were associated with lower CD4+/CD8+ cell ratios and higher viral loads. LPS levels reflect both intestinal barrier and LPS clearance. Macrophages are essential in controlling systemic bacterial translocation, and this function might be hindered in chronic HIV infection.

Neonatal bacterial infection alters fever to live and simulated infections in adulthood

Fever is a critical component of the host immune response to infection. An emerging literature demonstrates that experience with infectious organisms early in life, during the perinatal period, may permanently program immune responses later in life, including fever. We explored the influence of neonatal infection with Escherichia coli on fever responses to lipopolysaccharide (LPS) and E. coli in adulthood. Fever to a low dose of LPS in adulthood did not significantly differ as a consequence of early-life infection. Eight days after the LPS injection, the same group of rats received a high dose of live E. coli. This time, neonatally infected rats exhibited a markedly longer fever than controls. In a subsequent experiment, fever to a single high dose of E. coli without prior LPS in adulthood did not differ by group, suggesting that the previous difference was a lack of tolerance to the dual challenges in early-infected rats. Finally, both groups exhibited decreased tumor necrosis factor (TNF)-alpha and toll-like-receptor (TLR) 4 production to dual LPS challenges in isolated splenocytes, whereas only rats infected as neonates exhibited increased cyclooxygenase-2 within the hypothalamus in response to adult infection, suggesting that early infection-induced changes in fever regulation may involve a change in central mechanisms. Taken together, these data indicate that early-life infection is associated with marked changes in host temperature regulation in adulthood.

Signaling crosstalk during sequential TLR4 and TLR9 activation amplifies the inflammatory response of mouse macrophages

The TLR family of pattern recognition receptors is largely responsible for meditating the activation of macrophages by pathogens. Because macrophages may encounter multiple TLR ligands during an infection, signaling crosstalk between TLR pathways is likely to be important for the tailoring of inflammatory reactions to pathogens. Here, we show that rather than inducing tolerance, LPS pretreatment primed the inflammatory response (e.g., TNF production) of mouse bone marrow-derived macrophages (BMM) to the TLR9 ligand, CpG DNA. The priming effects of LPS, which correlated with enhanced Erk1/2, JNK, and p38 MAPK activation, appeared to be mediated via both c-Fms-dependent and -independent mechanisms. LPS pretreatment and inhibition of the M-CSF receptor, c-Fms, with GW2580 had comparable effects on CpG DNA-induced Erk1/2 and p38 MAPK activation. However, c-Fms inhibition did not enhance CpG DNA-induced JNK activation; also, the levels of TNF produced were significantly lower than those from LPS-primed BMM. Thus, the priming effects of LPS on TLR9 responses appear to be largely mediated via the c-Fms-independent potentiation of JNK activity. Indeed, inhibition of JNK abrogated the enhanced production of TNF by LPS-pretreated BMM. The c-Fms-dependent priming effects of LPS are unlikely to be a consequence of the inhibitory constraints of M-CSF signaling on TLR9 expression being relieved by LPS; instead, LPS may exert its priming effects via signaling molecules downstream of TLR9. In summary, our findings highlight the importance of signaling crosstalk between TLRs, as well as between TLRs and c-Fms, in regulating the inflammatory reaction to pathogens.

A20 is an early responding negative regulator of Toll-like receptor 5 signalling in intestinal epithelial cells during inflammation

Several negative regulatory mechanisms control Toll-like receptor (TLR)-mediated inflammatory responses and restore immune system balance, including the zinc-finger protein A20, a negative regulator of TLR signalling that inhibits nuclear factor kappa B (NF-kappaB) activity. In the present study, we investigated TLR-5-mediated A20 expression and its role in intestinal epithelial cells (IECs) during inflammation. HCT-15 and HT-29 cells were stimulated with flagellin, then the expressions of A20, interleukin-1 receptor-associated kinase (IRAK-M) and Tollip were evaluated using RNase protection assay. Furthermore, experimental colitis was induced in tlr4-deficient CH3/HeJ mice by administration of dextran sodium sulphate (DSS), then flagellin was injected anally, and the colonic expression of A20 was examined by real-time polymerase chain reaction (PCR) and immunohistochemistry. To confirm flagellin-induced expression of A20, we employed an organ culture system. The role of A20 in flagellin-induced tolerance induction was evaluated in vitro, using a gene knock-down method targeting A20. A20 expression increased rapidly and peaked at 1 h after flagellin stimulation in cultured IECs, then declined gradually to the basal level. In vivo, anal injection of flagellin induced epithelial expression of A20 in injured colonic tissue, whereas flagellin did not cause a significant increase in A20 expression in non-injured normal tissue, which was also confirmed in vitro using the organ culture system. Gene knock-down using A20 siRNA did not influence tolerance induced by restimulation with flagellin. A20 is an early response negative regulator of TLR-5 signalling in IECs that functions during intestinal inflammation. Our results provide new insights into the negative feedback regulation of TLR-5 signalling that maintains the innate immune system in the gut.

Effects of repeated injections of fibroblast-stimulating lipopeptide-1 on fever, formation of cytokines, and on the responsiveness to endotoxin in guinea-pigs

AIMS

We investigated, whether the Toll-like receptors (TLRs)-2/6-agonist fibroblast-stimulating lipopeptide-1 (FSL-1), like the TLR-4 agonist lipopolysaccharide (LPS), induces a state of tolerance. We further tested the influence of repeated pre-treatment with FSL-1 on the animals' responsiveness to LPS.

METHODS

Abdominal temperature was recorded in unrestrained guinea-pigs with intra-abdominally implanted radiotransmitters. Circulating concentrations of tumour necrosis factor (TNF) and interleukin-6 (IL-6) were measured with specific bioassays. We tested the effects of intra-arterial (i.a.) or intraperitoneal (i.p.) injections of 100 microg kg(-1) FSL-1, repeated five times at intervals of 3 days. The animals' responses to i.a. or i.p. injections of 10 microg kg(-1) LPS were determined another 3 days later and compared to those of naïve guinea-pigs.

RESULTS

The FSL-1-induced TNF peak was significantly attenuated starting with the third i.a. administration, while fever was unimpaired and the IL-6-peak just tended to decrease. Fever and IL-6 in response to i.a. injections of LPS were identical in both groups, while circulating TNF was higher in naïve compared to FSL-1 pre-treated animals. The effects of repeated i.p. injections of FSL-1 were more pronounced resulting in attenuation of fever as well as circulating TNF and IL-6, the strongest reduction observed after the third stimulation with FSL-1. Repeated i.p. pre-treatment with FSL-1 induced hyporesponsiveness to i.p. administration of LPS compared to naïve animals with regard to fever and especially with regard to LPS-induced formation of cytokines.

CONCLUSIONS

There is a development of tolerance to FSL-1 and cross-tolerance between FSL-1 and LPS depending on the route of administration of the respective TLR-2/6 and TLR-4 agonists.