Receptor-mediated monitoring of tissue well-being via detection of soluble heparan sulfate by Toll-like receptor 4

The nuclear factor HMGB1 mediates hepatic injury after murine liver ischemia-reperfusion

High-mobility group box 1 (HMGB1) is a nuclear factor that is released extracellularly as a late mediator of lethality in sepsis as well as after necrotic, but not apoptotic, death. Here we demonstrate that in contrast to the delayed role of HMGB1 in the systemic inflammation of sepsis, HMGB1 acts as an early mediator of inflammation and organ damage in hepatic ischemia reperfusion (I/R) injury. HMGB1 levels were increased during liver I/R as early as 1 h after reperfusion and then increased in a time-dependent manner up to 24 h. Inhibition of HMGB1 activity with neutralizing antibody significantly decreased liver damage after I/R, whereas administration of recombinant HMGB1 worsened I/R injury. Treatment with neutralizing antibody was associated with less phosphorylation of c-Jun NH₂-terminal kinase and higher nuclear factor–κB DNA binding in the liver after I/R. Toll-like receptor 4 (TLR4)-defective (C3H/Hej) mice exhibited less damage in the hepatic I/R model than did wild-type (C3H/HeOuj) mice. Anti-HMGB1 antibody failed to provide protection in C3H/Hej mice, but successfully reduced damage in C3H/Ouj mice. Together, these results demonstrate that HMGB1 is an early mediator of injury and inflammation in liver I/R and implicates TLR4 as one of the receptors that is involved in the process.

A Central Role for the Hsp90·Cdc37 Molecular Chaperone Module in Interleukin-1 Receptor-associated-kinase-dependent Signaling by Toll-like Receptors

Toll-like receptors (TLRs) serve crucial roles in innate immunity by mediating the activation of macrophages by microbial pathogens. The protein kinase interleukin-1 receptor associated kinase (IRAK-1) is a key component of TLR signaling pathways via its interaction with TRAF6, which subsequently leads to the activation of MAP kinases and various transcription factors. IRAK-1 is degraded following TLR activation, and this has been proposed to contribute to tolerance in macrophages by limiting further TLR-mediated signaling. Using a mass spectrometric-based approach, we have identified a cohort of chaperones and co-chaperones including Hsp90 and Cdc37, which bind to IRAK-1 but not IRAK-4 in 293T cells. Pharmacologic inhibition of Hsp90 led to a rapid decline in the expression level of IRAK-1, whereas overexpression of Cdc37 enhanced the activation and oligomerization of IRAK-1 in 293T cells. Significantly, the inhibition of Hsp90 in macrophages resulted in the destabilization and degradation of IRAK-1 but not IRAK-4. Concomitant with the loss of IRAK-1 expression was a reduction in the activation of p38 MAP kinase and Erk1/2 following stimulation with the bacterially derived TLR ligands, lipopolysaccharide and CpG DNA. Moreover, TLR ligand-induced expression of proinflammatory cytokines was also reduced. Thus we conclude that the level of on-going support provided to IRAK-1 by the Hsp90-Cdc37 chaperone module directly influences the magnitude of TLR-mediated macrophage activation. In addition, because further TLR signaling depends on the synthesis of new IRAK-1, the Hsp90-Cdc37 chaperone module could also contribute to tolerance in macrophages by controlling the rate at which nascent IRAK-1 is folded into a functional conformation.

Conditional signaling by Toll-like receptor 4

Signaling through Toll-like receptor 4 (TLR4) is thought to initiate innate and adaptive immune responses. Signaling of TLR4 is usually studied using isolated cells, which are activated by sub-nanomolar concentrations of lipopolysaccharide (LPS). However, in normal tissues, cells bearing TLR4 reside in microenvironments containing large amounts of endogenous substances that can stimulate the receptor. We developed an in vitro model system using the human cell line HEK 293 and an in vivo model using mice that have normal or that lack TLR4 receptors to study how TLR4 functions in such microenvironments. Here we report that signaling through TLR4 is strongly inhibited by intact extracellular matrix and that inhibition is abrogated and endogenous agonist(s) are liberated when the matrix is degraded. Thus, release from inhibition rather than direct stimulation by agonists such as LPS is the critical first event by which TLR4 initiates immune responses.

Minireview: functions of the renal tract epithelium in coordinating the innate immune response to infection

Infection of the urinary tract remains one of the most common infections affecting mankind. Renal epithelial cells, being one of the first cells to come into contact with invading organisms, are in a key position to coordinate host defense. The epithelium not only provides a physical barrier to infection, but can also augment the immune response via the production of a number of inflammatory mediators and antimicrobial proteins. Recent work has demonstrated that cells of the innate immune system, including epithelial cells, express toll-like receptors (TLRs), with the capacity to recognize bacterial components. Although the exact mechanisms remain unclear, engagement of TLRs can lead to epithelial cell activation and the production of inflammatory mediators. These include complement proteins, other bactericidal peptides, and chemotactic cytokines. The resulting inflammatory infiltrate serves to aid bacterial clearance, but can also lead to renal damage. In this review, we describe how renal epithelial cells contribute to the innate immune response to ascending urinary tract infection. We specifically relate previous work to more recent developments in this field. An improved understanding of the mechanisms involved may highlight potential therapeutic avenues to aid bacterial clearance and prevent the renal scarring associated with infection.

Cutting edge: TLR4 activation mediates liver ischemia/reperfusion inflammatory response via IFN regulatory factor 3-dependent MyD88-independent pathway

The triggering molecular mechanism of ischemia-reperfusion injury (IRI), which in clinical settings results in excessive and detrimental inflammatory responses, remains unclear. This study analyzes the role of the TLR system in an established murine model of liver warm ischemia followed by reperfusion. By contrasting in parallel TLR knockout mice with their wild-type counterparts, we found that TLR4, but not TLR2, was specifically required in initiating the IRI cascade, as manifested by liver function (serum alanine aminotransferase levels), pathology, and local induction of proinflammatory cytokines/chemokines (TNF-alpha, IL-6, IFN-inducible protein 10). We then investigated the downstream signaling pathway of TLR4 activation. Our results show that IFN regulatory factor 3, but not MyD88, mediated IRI-induced TLR4 activation leading to liver inflammation and hepatocellular damage. This study documents the selective usage of TLR in a clinically relevant noninfectious disease model, and identifies a triggering molecular mechanism in the pathophysiology of liver IRI.

Basic mechanisms of humoral rejection

Humoral rejection is among the most vexing problems afflicting organ transplants. Triggered by antibodies predominantly against donor human leukocyte (HLA), humoral rejection can now be understood through consideration of basic mechanisms of immunity to foreign antigens and impact of humoral immunity on blood vessels. Basic considerations may also shed light on mechanisms by which various treatments have recently brought about vastly improved outcomes.

Thematic review series: The immune system and atherogenesis. Paying the price for pathogen protection: toll receptors in atherogenesis

Atherosclerosis is a chronic inflammatory response characterized by the accumulation of cells of innate and acquired immune systems within the intima of the arterial wall. Macrophages are the predominant participant in innate immune responses in atherosclerosis. Protein receptors expressed by macrophages and endothelial cells recognize components and products of microorganisms and play a vital role in innate immunity. In particular, the members of the toll-like receptor (TLR) family play a critical role in the inflammatory components of atherosclerosis. Both exogenous ligands involved in microbial recognition as well as endogenous ligands involved in sterile inflammation pathways are implicated in the pathology of atherosclerosis. In this review, we discuss our current understanding of the role of TLRs and their coactivators in atherosclerosis, with particular emphasis on studies in atherosclerosis-prone hypercholesterolemic mice.

Expression of toll-like receptors 2 and 4 in rheumatoid synovial tissue and regulation by proinflammatory cytokines interleukin-12 and interleukin-18 via interferon-gamma

OBJECTIVE

To study the expression of Toll-like receptor 2 (TLR-2) and TLR-4 and its association with proinflammatory cytokines in synovial tissue from patients with rheumatoid arthritis (RA), osteoarthritis (OA), and healthy individuals.

METHODS

Synovial tissue specimens from 29 RA patients were stained for TLR-2, TLR-4, and proinflammatory cytokines (interleukin-1beta [IL-1beta], IL-12, IL-17, IL-18, and tumor necrosis factor alpha [TNFalpha]). The expression of TLR-2, TLR-4, and cytokines as well as the degree of inflammation in synovial tissue were compared between patients with RA, patients with OA (n = 5), and healthy individuals (n = 3). Peripheral blood mononuclear cells (PBMCs) were incubated with IL-12 and IL-18, and TLR expression was assessed using fluorescence-activated cell sorter analysis. Production of TNFalpha and IL-6 was measured using Luminex bead array technology.

RESULTS

In RA synovial tissue, the expression of TLR-2 was slightly higher than that of TLR-4. Interestingly, both TLR-2 and TLR-4 were expressed at higher levels in moderately inflamed synovium, as compared with synovial tissue with no or severe inflammation. TLR expression in both the lining and the sublining was associated with the presence of IL-12 and IL-18, but no other cytokines, in the lining. The expression of both TLRs was low in synovial tissue from OA patients and healthy donors. Stimulation of PBMCs with IL-12 and IL-18 resulted in increased expression of both TLR-2 and TLR-4; this could be blocked with anti-interferon-gamma (anti-IFNgamma) antibodies, suggesting a role for IFNgamma. Lipopolysaccharide- or lipoteichoic acid-mediated triggering of PBMCs incubated with IL-12/IL-18 or IFNgamma led to an increased production of both TNFalpha and IL-6, indicating the functionality of TLR-2 and TLR-4.

CONCLUSION

TLR-2 and TLR-4 are expressed in synovial tissue of patients with clinically active disease and are associated with the levels of both IL-12 and IL-18. The synergistic effect of IL-12 and IL-18 on T cell IFNgamma production seems to regulate expression of TLR-2 and TLR-4 in the synovial tissue of RA patients.

Tumor necrosis factor α blockade treatment down-modulates the increased systemic and local expression of toll-like receptor 2 and toll-like receptor 4 in spondylarthropathy

OBJECTIVE

Abnormal host defense against pathogens has been implicated in the pathogenesis of spondylarthropathy (SpA), a disease characterized by abundant synovial infiltration with innate immune cells. Given the role of Toll-like receptors (TLRs) in activation of innate inflammation and the occurrence of TLR-dependent infections after tumor necrosis factor alpha (TNFalpha) blockade treatment, the present study was undertaken to analyze TLRs and their modulation by TNFalpha blockade in SpA.

METHODS

Peripheral blood mononuclear cells (PBMCs) were obtained from SpA and rheumatoid arthritis (RA) patients during infliximab therapy, and from healthy controls. TLR-2 and TLR-4 expression and TNFalpha production upon lipopolysaccharide (LPS) stimulation were analyzed by flow cytometry on different monocyte subsets. Synovial biopsy specimens from 23 SpA patients before and after infliximab or etanercept treatment, from 15 RA patients, and from 18 osteoarthritis (OA) patients were analyzed by immunohistochemistry.

RESULTS

Expression of TLR-4, but not TLR-2, was increased on PBMCs from patients with SpA, whereas both TLRs were increased in RA patients. TLR expression was particularly increased on the CD163+ macrophage subset. Infliximab reduced TLR-2 and TLR-4 expression on monocytes of SpA and RA patients, leading to lower levels than in controls and to impaired TNFalpha production upon LPS stimulation. In inflamed synovium, the expression of both TLRs and of CD163 was significantly higher in patients with SpA than in those with RA or OA. Paralleling the systemic effect, TLRs in synovium were down-regulated following treatment with infliximab as well as etanercept, indicating a class effect of TNFalpha blockers.

CONCLUSION

Inflammation in SpA is characterized by increased TLR-2 and TLR-4 expression, which is sharply reduced by TNFalpha blockade. These findings suggest a potential role of innate immunity-mediated inflammation in SpA and provide an additional clue regarding the mechanism of action as well as the potential side effects of TNFalpha blockade.

Up-regulation of cell surface Toll-like receptor 4-MD2 expression on dendritic epidermal T cells after the emigration from epidermis during cutaneous inflammation

BACKGROUND

Mouse epidermis contains a population of gammadelta T cells, termed dendritic epidermal T cells (DETCs), which uniformly express the invariant Vgamma3 T cell receptor. Certain DETC lines were reported to respond to Gram-negative bacteria in the presence of immobilized anti-CD3 monoclonal antibody or to lipopolysaccharide (LPS) in the presence of B cell lines.

OBJECTIVE

To determine whether DETCs express the primary signaling receptor for LPS, Toll-like receptor (TLR) 4-MD2.

METHODS

We analyzed expression of TLR4-MD2 in three independent DETC lines as well as in freshly isolated DETCs.

RESULTS

All DETC lines expressed TLR4 and MD2 transcripts and TLR4-MD2 protein complex intracellularly, but none expressed TLR4-MD2 on the cell surface. By immunoblotting, only the immature form of TLR4 protein was detected in the DETC lines. The DETC lines did not respond to LPS even in the presence of immobilized anti-CD3 monoclonal antibody. Freshly isolated DETCs and their fetal thymic precursors also lacked cell surface expression of TLR4-MD2, but a small subpopulation of dermal Vgamma3 T cells isolated from croton oil-painted skin expressed TLR4-MD2 on the cell surface. Similarly, Vgamma3 T cells emigrated from organ-cultured epidermis expressed cell surface TLR4-MD2.

CONCLUSIONS

These results demonstrate that DETCs do not constitutively express cell surface TLR4-MD2, but TLR4-MD2 expression may be up-regulated when DETCs emigrate from epidermis during cutaneous inflammation.

Evidence for a role of toll-like receptor 4 in development of chronic allograft rejection after cardiac transplantation

BACKGROUND

Long-term success of cardiac transplantation is limited by various forms of graft rejection. The specific mechanisms initiating and controlling these highly specialized immune processes remain unclear so far.

METHODS

To investigate the role of innate immunity in the development of allograft rejection, we assessed toll-like receptor (TLR)4 expression and typical downstream effects of TLR signaling (B7-1, interleukin [IL]-12, tumor necrosis factor [TNF]-alpha) in circulating CD14+ monocytes in 38 transplant recipients 1 to 3 years and in 10 transplant recipients 6 to 10 years after transplantation and compared them with 20 healthy controls using reverse transcription polymerase chain reaction, flow cytometry, and enzyme-linked immunoadsorbent assay. The results were matched with endothelial function testing as an early clinical indicator of transplant vasculopathy early after transplantation.

RESULTS

Allograft endothelial dysfunction (ED) was defined as a compromised coronary flow reserve (CFVR) to acetylcholine (CFVR<2 in 13 of 38 transplant recipients). In these patients, mRNA transcript levels for TLR4 (P<0.05) and surface expression of TLR4 (P<0.005) and B7-1 (P<0.05) on circulating monocytes as well as secretion of IL-12 (P<0.02) and TNF-alpha (P<0.05) were significantly higher than in the remaining 25 patients without ED. Compared with the controls, recipients late after transplantation did not show significantly elevated levels of TLR4 or dependent mediators. These results were compared with mRNA levels in a mice model of acute and chronic rejection. Rejecting mice exhibited elevated mRNA levels for mTLR4 and mB7-1.

CONCLUSIONS

Our results suggest activation of innate immunity in heart-transplant recipients through TLR4 contributes to the development of chronic rejection after cardiac transplantation.

Inhibition of toll-like receptor and cytokine signaling--a unifying theme in ischemic tolerance

Cerebral ischemia triggers acute inflammation, which exacerbates primary brain damage. Activation of the innate immune system is an important component of this inflammatory response. Inflammation occurs through the action of proinflammatory cytokines, such as TNF, IL-1 beta and IL-6, that alter blood flow and increase vascular permeability, thus leading to secondary ischemia and accumulation of immune cells in the brain. Production of these cytokines is initiated by signaling through Toll-like receptors (TLRs) that recognize host-derived molecules released from injured tissues and cells. Recently, great strides have been made in understanding the regulation of the innate immune system, particularly the signaling mechanisms of TLRs. Negative feedback inhibitors of TLRs and inflammatory cytokines have now been identified and characterized. It is also evident that lipid rafts exist in membranes and play a role in receptor-mediated inflammatory signaling events. In the present review, using this newly available large body of knowledge, we take a fresh look at studies of ischemic tolerance. Based on this analysis, we recognize a striking similarity between ischemic tolerance and endotoxin tolerance, an immune suppressive state characterized by hyporesponsiveness to lipopolysaccharide (LPS). In view of this analogy, and considering recent discoveries related to molecular mechanisms of endotoxin tolerance, we postulate that inhibition of TLR and proinflammatory cytokine signaling contributes critically to ischemic tolerance in the brain and other organs. Ischemic tolerance is a protective mechanism induced by a variety of preconditioning stimuli. Tolerance can be established with two temporal profiles: (i) a rapid form in which the trigger induces tolerance to ischemia within minutes and (ii) a delayed form in which development of protection takes several hours or days and requires de-novo protein synthesis. The rapid form of tolerance is achieved by direct interference with membrane fluidity, causing disruption of lipid rafts leading to inhibition of TLR/cytokine signaling pathways. In the delayed form of tolerance, the preconditioning stimulus first triggers the TLR/cytokine inflammatory pathways, leading not only to inflammation but also to simultaneous upregulation of feedback inhibitors of inflammation. These inhibitors, which include signaling inhibitors, decoy receptors, and anti-inflammatory cytokines, reduce the inflammatory response to a subsequent episode of ischemia. This novel interpretation of the molecular mechanism of ischemic tolerance highlights new avenues for future investigation into the prevention and treatment of stroke and related diseases.

Pattern recognition receptors and their involvement in the pathogenesis of arthritis

PURPOSE OF REVIEW

Pattern recognition receptors are germ-line encoded receptors that recognize specific pathogen-associated molecules, thereby allowing the innate immune system to distinguish self from nonself structures. Pattern recognition receptors mediate activation of different signaling pathways, resulting in the production of proinflammatory cytokines and the expression of antimicrobial genes. Additionally, pattern recognition receptors play a central role in the activation and direction of the adaptive immune response. This review summarizes recent advances in research trying to elucidate the link between different pattern recognition receptors and inflammatory autoimmune disorders.

RECENT FINDINGS

The best known pattern recognition receptors, the toll-like receptors, are involved in the regulation of inflammation during infectious diseases. They affect apoptotic pathways and dendritic cell maturation, and interact with B-cell receptors in priming T-cell responses to host-derived DNA. This brought toll-like receptors and other pattern recognition receptors into focus as potential players in the induction of autoimmune diseases. Indeed, several inflammatory autoimmune diseases have been linked during the past few years to defects or polymorphisms of genes encoding pattern recognition receptors.

SUMMARY

The discovery of toll-like receptors and other groups of pattern recognition receptors, such as the caspase recruitment domains or the triggering receptors expressed by myeloid cells, allowed one to draw an increasingly complex picture of immune responses to pathogens. The growing evidence for an involvement of pattern recognition receptors in the pathogenesis of autoimmune disorders warrants further investigation of the expression and function of pattern recognition receptors to develop novel therapeutics for diseases such as rheumatoid arthritis.

Toll-like receptors and other links between innate and acquired alloimmunity

Innate immunity represents the first line of defense against invading pathogens and noxious stimuli. The Toll-like receptors (TLRs) are essential innate immune receptors that alert the immune system to the presence of invading microbes. Emerging evidence shows that TLR signaling is important in allograft rejection. In a murine model, the rejection of minor mismatched allografts cannot occur in the absence of MyD88, an important TLR signal adaptor protein, owing to a defect in dendritic cell maturation, which leads to diminished T-helper cell type 1 immune responses. A recent clinical study also suggests that recipients with a mutant TLR4 genotype manifest reduced lung allograft rejection. Thus, innate immune signaling via TLRs is important for alloimmunity.

The immunobiology of cancer immunosurveillance and immunoediting

The last fifteen years have seen a reemergence of interest in cancer immunosurveillance and a broadening of this concept into one termed cancer immunoediting. The latter, supported by strong experimental data derived from murine tumor models and provocative correlative data obtained by studying human cancer, holds that the immune system not only protects the host against development of primary nonviral cancers but also sculpts tumor immunogenicity. Cancer immunoediting is a process consisting of three phases: elimination (i.e., cancer immunosurveillance), equilibrium, and escape. Herein, we summarize the data supporting the existence of each of the three cancer immunoediting phases. The full understanding of the immunobiology of cancer immunosurveillance and immunoediting will hopefully stimulate development of more effective immunotherapeutic approaches to control and/or eliminate human cancers.

Innate immunity in the retina: Toll-like receptor (TLR) signaling in human retinal pigment epithelial cells

Toll-like receptors (TLRs) are crucial components of innate immunity that participate in host defense against microbial pathogens. We evaluated the expression and function of TLRs in human retinal pigment epithelial (RPE) cells. Real time PCR analysis revealed gene expression for TLRs 1–7, 9, and 10 in RPE cells. TLRs 1 and 3 were the most highly expressed TLRs. Protein expression for TLRs 2, 3, and 4 was observed on RPE cells and this expression was augmented by treatment with poly I:C or interferon-γ (IFN-γ). TLR 3 is the receptor for dsRNA, an intermediate of virus replication. Because RPE cells express TLR 3 and are frequently the site of virus replication within the retina, we evaluated TLR 3 signaling. RPE cells treated with poly I:C produced IFN-β but not IFN-α, and this was inhibited by the treatment of RPE cells with anti-TLR 3 antibody. Human recombinant IFN-β was shown to be biologically active on RPE cells by inhibiting viral replication. Poly I:C treatment of RPE resulted in an increase in the production of IL-6, IL-8, MCP-1, and sICAM-1. The presence of TLRs on RPE cells and the resultant TLR signaling in RPE cells suggest that these molecules may play an important role in innate and adaptive immune responses within the retina.

TH1 immune responses to fully MHC mismatched allografts are diminished in the absence of MyD88, a toll-like receptor signal adaptor protein

Toll-like receptors (TLRs) are innate immune receptors that are critical for recognizing conserved microbial motifs by inducing TH1 immunity. The majority of TLRs utilize the adaptor protein MyD88 for signal transduction, although other adaptors have been recently described. As the role of innate immunity in transplantation is unclear, we examined the importance of the MyD88 pathway in acute rejection of fully MHC-mismatched murine allografts and specifically investigated whether MyD88 signaling is important for DC (dendritic cell) function and TH1 alloimmune responses. Our results demonstrate that acute rejection of both fully allogeneic skin and cardiac allografts occurs in the absence of MyD88. However, priming of naïve recipient T cells by allogeneic DCs and TH1 immune responses were diminished in the absence of MyD88, although TH2 immunity remained intact. Thus, these results demonstrate that MyD88 signaling is important for DC function and TH1 responses during fully MHC-mismatched solid-organ transplantation, although graft rejection occurs independently of MyD88.

Toll-like receptors and the genetics of innate immunity

PURPOSE OF REVIEW

The discovery that mammalian Toll-like receptors recognize microbial products and initiate innate immune responses to them has spawned a new field of biology, namely the study of molecular interactions linking microbial recognition to innate and adaptive immune responses. This field has grown very rapidly in recent years, due largely to recent advances in genetic technology. This review summarizes recent work in which genetic approaches have been used to identify novel and important facets of Toll-like receptor function.

RECENT FINDINGS

Recent genetic studies have uncovered a wealth of information relating to ligand-receptor interactions, Toll-like receptor gene regulation, signal transduction, dendritic cell activation and allele-phenotype associations.

SUMMARY

Information emerging from genetic studies of Toll-like receptors has improved our understanding of innate and acquired immunity. This improved understanding promises to facilitate the future development of novel therapies for many different inflammatory diseases including asthma, sepsis and atherosclerosis.

A genetic basis for the “Adonis” phenotype of low adiposity and strong bones

Toll receptors in Drosophila contribute to host defense and establish the body plan. Mammalian homologues of Toll, the Toll-like receptors (TLRs), are thought to function only in host defense. Here, we report that mice harboring mutations in TLR4 or in CD14, a co-receptor for TLR4, have an "ideal" body plan consisting of increased bone mineral content, density, and size as well as decreased body fat. These mutant mice live long lives, have normal activity and fertility, and show no evidence of infection. Unlike many strains of caged wild-type mice, they do not become obese. Although all mice continue to gain body fat, bone content, and overall weight, the difference in bone content and body fat between mutant and wild-type mice increases with age. Thus, defects in TLR4/CD14 complex generate an "Adonis" phenotype, characterized by this ideal body type, and this function could potentially be exploited for the treatment of osteoporosis and obesity.

Endogenous ligands of Toll-like receptors

Extensive work has suggested that a number of endogenous molecules such as heat shock proteins (hsp) may be potent activators of the innate immune system capable of inducing proinflammatory cytokine production by the monocyte-macrophage system and the activation and maturation of dendritic cells. The cytokine-like effects of these endogenous molecules are mediated via the Toll-like receptor (TLR) signal-transduction pathways in a manner similar to lipopolysaccharide (LPS; via TLR4) and bacterial lipoproteins (via TLR2). However, recent evidence suggests that the reported cytokine effects of hsp may be a result of the contaminating LPS and LPS-associated molecules. The reasons for previous failure to recognize the contaminant(s) being responsible for the putative TLR ligands of hsp include failure to use highly purified hsp free of LPS contamination; failure to recognize the heat sensitivity of LPS; and failure to consider contaminant(s) other than LPS. Whether other reported putative endogenous ligands of TLR2 and TLR4 are a result of contamination of pathogen-associated molecular patterns is not clear. It is essential that efforts should be directed to conclusively determine whether the reported putative endogenous ligands of TLRs are a result of the endogenous molecules or of contaminant(s), before exploring further the implication and therapeutic potential of these putative TLR ligands.

Multimeric tyrosine sulfate acts as an endothelial cell protectant and prevents complement activation in xenotransplantation models

BACKGROUND

Activation of endothelial cells (EC) in xenotransplantation is mostly induced through binding of antibodies (Ab) and activation of the complement system. Activated EC lose their heparan sulfate proteoglycan (HSPG) layer and exhibit a procoagulant and pro-inflammatory cell surface. We have recently shown that the semi-synthetic proteoglycan analog dextran sulfate (DXS, MW 5000) blocks activation of the complement cascade and acts as an EC-protectant both in vitro and in vivo. However, DXS is a strong anticoagulant and systemic use of this substance in a clinical setting might therefore be compromised. It was the aim of this study to investigate a novel, fully synthetic EC-protectant with reduced inhibition of the coagulation system.

METHOD

By screening with standard complement (CH50) and coagulation assays (activated partial thromboplastin time, aPTT), a conjugate of tyrosine sulfate to a polymer-backbone (sTyr-PAA) was identified as a candidate EC-protectant. The pathway-specificity of complement inhibition by sTyr-PAA was tested in hemolytic assays. To further characterize the substance, the effects of sTyr-PAA and DXS on complement deposition on pig cells were compared by flow cytometry and cytotoxicity assays. Using fluorescein-labeled sTyr-PAA (sTyr-PAA-Fluo), the binding of sTyr-PAA to cell surfaces was also investigated.

RESULTS

Of all tested compounds, sTyr-PAA was the most effective substance in inhibiting all three pathways of complement activation. Its capacity to inhibit the coagulation cascade was significantly reduced as compared with DXS. sTyr-PAA also dose-dependently inhibited deposition of human complement on pig cells and this inhibition correlated with the binding of sTyr-PAA to the cells. Moreover, we were able to demonstrate that sTyr-PAA binds preferentially and dose-dependently to damaged EC.

CONCLUSIONS

We could show that sTyr-PAA acts as an EC-protectant by binding to the cells and protecting them from complement-mediated damage. It has less effect on the coagulation system than DXS and may therefore have potential for in vivo application.

Cutting edge: an endogenous pathway to systemic inflammatory response syndrome (SIRS)-like reactions through Toll-like receptor 4

Systemic inflammatory response syndrome (SIRS) is typically associated with trauma, surgery, or acute pancreatitis. SIRS resembles sepsis, triggered by exogenous macromolecules such as LPS acting on Toll-like receptors. What triggers SIRS in the absence of infection, however, is unknown. In this study, we report that a SIRS-like response can be induced in mice by administration of soluble heparan sulfate, a glycosaminoglycan associated with nucleated cells and extracellular matrices, and by elastase, which cleaves and releases heparan sulfate proteoglycans. The ability of heparan sulfate and elastase to induce SIRS depends on functional Toll-like receptor 4, because mutant mice lacking that receptor or its function do not respond. These results provide a molecular explanation for the initiation of SIRS.

Microbial recognition by Toll-like receptors

Toll-like receptors (TLRs) sense invasion of microorganisms by detecting microbial components that are conserved among pathogens. Recognition of microbial components by TLRs triggers activation of the innate immune system. Signaling pathways via TLRs originate from conserved cytoplasmic Toll/IL-1 receptor (TIR) domains. Recent accumulating evidence demonstrates that TIR domain-containing adaptors, such as MyD88, TIRAP/Mal, TRIF, and TRAM, regulate TLR-mediated signaling pathways. MyD88 is common to all TLR-mediated pathways, which lead to the production of inflammatory cytokines, whereas TRIF mediates induction of IFN-beta in TLR3 and TLR4 signaling pathways. TIRAP/Mal is implicated in the TLR2- and TLR4-mediated MyD88-dependent signaling pathway. TRAM is specifically involved in the TLR4-mediated TRIF-dependent pathway. Thus, TIR domain-containing adaptors play a pivotal role in TLR signaling pathways, which culminate in pathogen-specific immune responses.

Why are dendritic cells important in allergic diseases of the respiratory tract?

Increasing evidence points to the role of antigen-presenting dendritic cells (DC) in regulating adaptive immune responses. DC are especially sensitive to signals derived from microbes, allergens, and the airway tissue microenvironment, can polarize naïve T-cells into either Th1 or Th2 effector cells, and are increasingly recognized as having a central role in the establishment of T-cell memory and tolerance to inhaled antigens. DC form a closely meshed network within the respiratory mucosa and are rapidly recruited from the circulation in response to a variety of proinflammatory stimuli. Studies using animal models have highlighted the role of DC in both initiation and maintenance of allergic airway inflammation. Increased numbers of airway mucosal DC are found in both allergic rhinitis and asthma, and an increasing number of investigators have highlighted important functional differences between DC from atopic and normal individuals. This article reviews recent information on the involvement of DC in the pathogenesis of allergic airway disease and the means by which DC could be exploited as targets for therapy in asthma and allergic rhinitis.

Hyaluronan fragments stimulate endothelial recognition of injury through TLR4

Tissues must quickly recognize injury to respond to the rapid pace of microbial growth. In skin, dermal microvascular endothelial cells must also react to danger signals from the surrounding tissue and immediately participate by initiating the wound repair process. Components of the extracellular matrix such as hyaluronan are rapidly broken down into smaller molecular weight oligosaccharides in a wound, and these can activate a variety of biological processes. This study set out to determine if hyaluronan fragments released following injury can stimulate endothelial cells and what mechanism is responsible for this response. Using genechip microarray analysis, a response to hyaluronan fragments was detected in endothelial cells with the most significant increase observed for the chemokine IL-8. This observation was verified with qualitative reverse transcriptase-PCR and ELISA in human endothelial cell culture, and in a mouse model by observing serum levels of MIP-2 and KC following hyaluronan fragment administration in vivo. Activation was TLR4-dependent, as shown by use of TLR4 blocking antibody and TLR4-deficient mice, but not due to the presence of undetected contaminants as shown by inactivation following digestion with the hyaluronan-degrading enzyme chondroitinase ABC or incubation with the hyaluronan-specific blocking peptide Pep-1. Inactivation of LPS activity failed to diminish the action of hyaluronan fragments. These observations suggest that endogenous components of the extracellular matrix can stimulate endothelia to trigger recognition of injury in the initial stages of the wound defense and repair response.

Toll-like receptor-4 and allograft responses

Signaling through toll-like receptors (TLRs) is believed to be the critical first step in the activation of antigen presenting cells and the initiation of adaptive immune responses. Of these receptors, TLR-4 particularly recognizes endogenous agonists and may be important for allograft responses. We tested this concept using mice with defective function and structure of TLR-4 as recipients of grafts across major and minor histocompatibility barriers. The kinetics of rejection was the same in mutant mice and wild-type controls. Our results highlight an important difference between alloimmune and conventional immune responses.

mRNA is an endogenous ligand for Toll-like receptor 3

Toll-like receptors (TLRs) are the basic signaling receptors of the innate immune system. They are activated by molecules associated with pathogens or injured host cells and tissue. TLR3 has been shown to respond to double stranded (ds) RNA, a replication intermediary for many viruses. Here we present evidence that heterologous RNA released from or associated with necrotic cells or generated by in vitro transcription also stimulates TLR3 and induces immune activation. To assess RNA-mediated TLR3 activation, human embryonic kidney 293 cells stably expressing TLR3 and containing a nuclear factor-kappaB-dependent luciferase reporter were generated. Exposing these cells to in vitro transcribed RNA resulted in a TLR3-dependent induction of luciferase activity and interleukin-8 secretion. Treatment with in vitro transcribed mRNA activated nuclear factor-kappaB via TLR3 through a process that was dose-dependent and involved tyrosine phosphorylation. Furthermore, in vitro transcribed natural or 2'-fluoro-substituted mRNA induced the expression of TLR3, interferon regulatory factor-1, tumor necrosis factor-alpha, and interleukin-1 receptor-associated kinase-M mRNA in human dendritic cells (DCs). DCs responded to mRNA treatment by expressing activation markers, and this maturation was inhibited by antagonistic TLR3-specific antibody. Endogenous RNA released from or associated with necrotic cells also stimulated DCs, leading to interferon-alpha secretion, which could be abolished by pretreatment of necrotic cells with RNase. These results demonstrate that RNA, likely through secondary structure, is a potent host-derived activator of TLR3. This finding has potential physiologic relevance because RNA escaping from damaged tissue or contained within endocytosed cells could serve as an endogenous ligand for TLR3 that induces or otherwise modulates immune responses.

Biology of Toll receptors: lessons from insects and mammals

Toll receptors are type I transmembrane proteins that play important roles in development and immunity in animals. Comparison of the genomes of mouse and human on one side and of the fruitfly Drosophila and the mosquito Anopheles (two dipteran insects) on the other, revealed that the four species possess a similar number of Toll receptors (approximately 10). However, phylogenetic analyses indicate that the families of Toll receptors expanded independently in insects and mammals. We review recent results on these receptors, which point to differences in the activation and signaling between Tolls in insects and Toll-like receptors (TLRs) in mammals. Whereas mammalian TLRs appear to be solely dedicated to host-defense, insect Tolls may be predominantly linked to other functions, probably developmental.

TLR2 mRNA upregulation in ischemic lobes in mouse partial hepatic ischemia/reperfusion injury model

To investigate TLR2 (Toll-like receptor 2) mRNA expression in ischemic hepatic lobes under the condition of partial hepatic ischemia/reperfusion injury in BALB/c mice and its relationship with liver function impairment. A partial ischemia/reperfusion injury model was established. The portal vein and hepatic artery supply to the median and left lobes of the liver were obstructed by an atraumatic artery micro-clip, with the obstruction lasting for about 60 min. Then reperfusion was fulfilled by removal of the clip. The liver samples were collected at the 4th h after the restoration of blood inflow. Total RNA was extracted from the liver samples and analyzed quantitatively by method of real-time PCR. At the same time, portal vein serum and plasma were taken respectively for further detection of the level of endotoxin, tumor necrosis factor alpha (TNF-alpha) and plasmic alanine aminotransferase (pALT). The results indicated that TLR2 mRNA in ischemic lobe was up-regulated markedly in mice partial liver ischemia/reperfusion injury model compared to that in sham operation group (deltaCt: 1.05 +/- 1.02 vs 5.08 +/- 1.36, P<0.001). The level of portal vein pALT and TNF-alpha increased significantly (112.32 +/- 17.56 pg/ml vs 6.07 +/- 5.33 pg/ml, P<0.01; 890 +/- 127 microm/L vs 30 +/- 5 microm/L, P<0.001) . However, the level of portal vein endotoxin remained below the normal line, suggesting a state of non-endotoxemia. TLR2 mRNA expression in ischemic lobe, as well as portal vein pALT and TNF-alpha, was up-regulated in the model of mice partial ischemia/reperfusion injury, suggesting the involvement of TLR2 in ischemia/reperfusion pathological process.

Skin immune responses to peptide and protein antigen are TLR4 independent

Little is known about the innate immune mechanisms regulating adaptive immune responses elicited through the skin. Tissue injury is postulated to liberate Toll like receptor 4 (TLR4) ligands. In this study, we determined whether TLR4 signaling modulates the response to epidermal injury induced by tape stripping (TS) and whether it alters humoral and cellular immune responses generated through epicutaneous immunization with peptide+cholera toxin (CT). The combined use of cholera toxin and TS with antigen promoted optimal antigen-specific CD4(+) and CD8(+) T cell proliferation in Balb/c and C57BL/6 mice, respectively. TLR4 mutant mice had similar T cell responses to wild type mice. Further, OVA-protein specific IgG, IgG(1), IgG(2a), and IgE titers were similar in wild type and TLR4 mutant mice. Thus, TLR4 signaling was not required for the generation of epicutaneous T cell or antibody mediated immune responses and did not alter the quality of the immune responses elicited.

Toll-like receptors

The innate immune system in drosophila and mammals senses the invasion of microorganisms using the family of Toll receptors, stimulation of which initiates a range of host defense mechanisms. In drosophila antimicrobial responses rely on two signaling pathways: the Toll pathway and the IMD pathway. In mammals there are at least 10 members of the Toll-like receptor (TLR) family that recognize specific components conserved among microorganisms. Activation of the TLRs leads not only to the induction of inflammatory responses but also to the development of antigen-specific adaptive immunity. The TLR-induced inflammatory response is dependent on a common signaling pathway that is mediated by the adaptor molecule MyD88. However, there is evidence for additional pathways that mediate TLR ligand-specific biological responses.

The role of dendritic cells in immune regulation and allergic airway inflammation

Dendritic cells (DC) are potent antigen presenting cells that display an extraordinary capacity to present antigen to naïve T-cells and initiate primary immune responses. In the context of the lung and upper airway it is clear that DC play a key role in the regulation of adaptive immune responses to inhaled antigen. DC are particularly sensitive to signals derived from microbes, allergens and the airway tissue microenvironment. By the nature of the signals they provide at the time of antigen presentation, DC can polarize naïve T-cells into either T-helper type 1 (Th1) or Th2 effector cells, and are increasingly recognized as having a central role in the establishment of T-cell memory and peripheral immune tolerance. DC form a network within the upper airway and lung, and are rapidly recruited from the circulation in response to a variety of proinflammatory stimuli. Studies using animal models have highlighted the role of DC in both the initiation and maintenance of allergic airway inflammation. In early childhood, human DC are functionally immature, and this is thought to contribute to the development of allergic sensitization in those children who are genetically at risk for the development of atopy. Increased numbers of airway mucosal DC are found in both allergic rhinitis and asthma, while studies of blood-derived DC have emphasized important differences between the function of DC from atopic and normal individuals. This article reviews recent information on the involvement of DC in allergic airway disease, and the mechanisms by which DC could be exploited as targets for therapy in asthma and allergic rhinitis.

Critical role of the Toll-like receptor signal adaptor protein MyD88 in acute allograft rejection

The Toll-like receptors (TLRs) are recently discovered germline-encoded receptors on APCs that are critically important in innate immune recognition of microbial pathogens. However, their role in solid-organ transplantation is unknown. To explore this role, we employed a skin allograft model using mice with targeted deletion of the universal TLR signal adaptor protein, MyD88. We report that minor antigen-mismatched (HY-mismatched) allograft rejection cannot occur in the absence of MyD88 signaling. Furthermore, we show that the inability to reject these allografts results from a reduced number of mature DCs in draining lymph nodes, leading to impaired generation of anti-graft-reactive T cells and impaired Th1 immunity. Hence, this work demonstrates that TLRs can be activated in a transplant setting and not solely by infections. These results link innate immunity to the initiation of the adaptive alloimmune response.

Toll receptors and pathogen resistance

Toll receptors in insects, mammals and plants are key players that sense the invasion of pathogens. Toll-like receptors (TLRs) in mammals have been established to detect specific components of bacterial and fungal pathogens. Furthermore, recent evidence indicates that TLRs are involved in the recognition of viral invasion. Signalling pathways via TLRs originate from the conserved Toll/IL-1 receptor (TIR) domain. The TIR domain-containing MyD88 acts as a common adaptor that induces inflammatory cytokines; however, there exists a MyD88-independent pathway that induces type I IFNs in TLR4 and TLR3 signalling. Another TIR domain-containing adaptor, TIRAP/Mal has recently been shown to mediate the MyD88-dependent activation in the TLR4 and TLR2 signalling pathway. Thus, individual TLRs may have their own signalling systems that characterize their specific activities.

Regulatory T cells selectively express toll-like receptors and are activated by lipopolysaccharide

Regulatory CD4 T cells (Treg) control inflammatory reactions to commensal bacteria and opportunist pathogens. Activation of Treg functions during these processes might be mediated by host-derived proinflammatory molecules or directly by bacterial products. We tested the hypothesis that engagement of germline-encoded receptors expressed by Treg participate in the triggering of their function. We report that the subset of CD4 cells known to exert regulatory functions in vivo (CD45RB(low) CD25(+)) selectively express Toll-like receptors (TLR)-4, -5, -7, and -8. Exposure of CD4(+) CD25(+) cells to the TLR-4 ligand lipopolysaccharide (LPS) induces up-regulation of several activation markers and enhances their survival/proliferation. This proliferative response does not require antigen-presenting cells and is augmented by T cell receptor triggering and interleukin 2 stimulation. Most importantly, LPS treatment increases CD4(+) CD25(+) cell suppressor efficiency by 10-fold and reveals suppressive activity in the CD4(+) CD45RB(low) CD25(-) subset that when tested ex-vivo, scores negative. Moreover, LPS-activated Treg efficiently control naive CD4 T cell-dependent wasting disease. These findings provide the first evidence that Treg respond directly to proinflammatory bacterial products, a mechanism that likely contributes to the control of inflammatory responses.

New frontiers: the 2002 FASEB Summer Research Conference in Transplant Immunology

The first Federation of American Societies for Experimental Biology (FASEB) summer research conference on transplantation immunology was organized by Angus Thomson (University of Pittsburgh), Robert Lechler (Imperial College London), Laurence Turka (University of Pennsylvania) and Megan Sykes (Massachusetts General Hospital). Over the past four decades, patient and graft survival rates for solid organ transplant recipients have improved dramatically; however, chronic rejection and the untoward effects of potent immunosuppressive drugs continue to loom. This symposium is a testament to the importance of bringing investigators from diverse biological backgrounds together in a single forum to discuss the fundamental issues of immune biology and advance the goal of transplant-specific tolerance.

Synthesis and surface expression of hyaluronan by dendritic cells and its potential role in antigen presentation

Hyaluronan (HA) is a large glycosaminoglycan consisting of repeating disaccharide units of glucuronic acid and N-acetylglucosamine. HA is known to act as a filling material of extracellular matrices and as an adhesive substrate for cellular migration. Here we report that dendritic cells (DC) express mRNAs for HA synthases and hyaluronidases, actively synthesize HA, and display HA on their surfaces. Interestingly, HA expression levels on DC were not significantly altered by their maturation states. With respect to physiological function, three specific HA inhibitors, i.e., bovine proteoglycan, a 12-mer HA-binding peptide (GAHWQFNALTVR) termed Pep-1, and an oligomeric Pep-1 formulation, all interfered with DC-induced activation of CD4(+) T cells isolated from DO11.10 TCR transgenic mice. For example, Pep-1 oligomer efficiently inhibited DC-dependent cluster formation, IL-2 and IFN-gamma production, and proliferation by DO11.10 T cells in vitro without affecting the viabilities of DC or T cells, DC function to uptake exogenous proteins, or DC-T cell conjugate formation at earlier time points. These observations suggest a paracrine mechanism by which DC-associated HA facilitates some of the late changes in T cell activation. Although T cells constitutively expressed mRNAs for HA synthases and hyaluronidases, their surface HA expression became detectable only after activation. Oligomeric Pep-1 and bovine proteoglycan both inhibited mitogen-triggered T cell activation in the absence of DC, suggesting an autocrine mechanism by which HA expressed by T cells assists their own activation processes. Finally, adoptively transferred DO11.10 T cells showed progressive mitosis when stimulated with Ag-pulsed DC in living animals, and this clonal expansion was inhibited significantly by administration of Pep-1 oligomer. Our findings may introduce a new concept that relatively simple carbohydrate moieties expressed on DC and perhaps T cells play an important immunomodulatory role during Ag presentation.

Immature, semi-mature and fully mature dendritic cells: which signals induce tolerance or immunity?

Dendritic cells (DCs) are currently divided into tolerogenic immature and immunogenic mature differentiation stages. However, recent findings challenge this model by reporting mature DCs as inducers of regulatory CD4+ T cells in vivo. This implies that decisive tolerogenic and immunogenic maturation signals for DCs might exist. Closer inspection reveals that tolerance is observed when partial- or semi-maturation of DCs occurs, whereas only full DC maturation is immunogenic. The decisive immunogenic signal seems to be the release of proinflammatory cytokines from the DCs. Moreover, the semi-mature DC phenotype is comparable to steady-state migratory veiled DCs within the lymphatics, which seem to continuously tolerize lymph node T cells against tissue-derived self-antigens or apoptotic cells.