Cutting edge: inhibition of TLR and FcR responses in macrophages by triggering receptor expressed on myeloid cells (TREM)-2 and DAP12

DAP12 is an ITAM-containing adapter that associates with receptors in myeloid and NK cells. DAP12-associated receptors can give activation signals leading to cytokine production; however, in some situations, DAP12 inhibits cytokine production stimulated through TLRs and FcRs. Here we show that Triggering Receptor Expressed on Myeloid cells (TREM)-2 is responsible for the DAP12-mediated inhibition in mouse macrophages. A chimeric receptor composed of the extracellular domain of TREM-2 and the cytoplasmic domain of DAP12 inhibited the TLR- and FcR-induced TNF production of DAP12-deficient macrophages, whereas a TREM-1 chimera did not. In wild-type macrophages, TREM-2 knockdown increased TLR-induced TNF production. A TREM-2 Fc fusion protein bound to macrophages, indicating that macrophages express a TREM-2 ligand. Thus, the interaction of TREM-2 and its ligand results in an inhibitory signal that can reduce the inflammatory response.

Expression and function of Toll-like receptors in T lymphocytes

Toll-like receptors (TLRs) are widely expressed in the innate immune system. They recognize conserved microbial ligands such as bacterial lipopolysaccharide, lipopeptides or viral and bacterial RNA and DNA. TLRs play an essential role in innate immune responses and in the initiation of adaptive immune responses. However, certain TLRs are also expressed in T lymphocytes, and the respective ligands can directly modulate T cell function. TLR2, TLR3, TLR5 and TLR9 act as co-stimulatory receptors to enhance proliferation and/or cytokine production of T-cell receptor-stimulated T lymphocytes. In addition, TLR2, TLR5 and TLR8 modulate the suppressive activity of naturally occurring CD25(+)CD4(+) regulatory T cells. The direct responsiveness of T lymphocytes to TLR ligands offers new perspectives for the immunotherapeutic manipulation of T cell responses.

Differential liver sensitization to toll-like receptor pathways in mice with alcoholic fatty liver

Gut-derived, endotoxin-mediated hepatocellular damage has been postulated to play a crucial role in the pathogenesis of alcohol-induced liver injury in rodents. Endotoxins induce production of tumor necrosis factor alpha (TNF-alpha) by Kupffer cells via Toll-like receptor (TLR) 4 and contribute to liver injury. This study addressed the contribution of other TLRs and ligands to alcoholic fatty liver. C57Bl6/J mice were fed a modified Lieber-DeCarli diet. Serum aminotransferase measurements, histological analysis, and quantification of liver TNF-alpha and TLR1-9 messenger RNA (mRNA) were performed. The effect of TLR ligands on liver injury was assessed in vivo. Neomycin and metronidazole or diphenyleneiodonium sulfate (DPI) were administered to evaluate the role of gut bacteria and NADPH oxidase activity, respectively, in hepatic TLR expression. Enteral ethanol (EtOH) exposure induced steatosis and increased liver weight, aminotransferase levels, and expression of TLR1, 2, 4, 6, 7, 8, and 9 liver mRNA. Injection of lipoteichoic acid, peptidoglycan (PGN), lipopolysaccharide (LPS), loxoribine, and oligonudeotide containing CpG (ISS-ODN) increased TNF-alpha mRNA expression more in the livers of EtOH-fed mice than in control mice. PGN, LPS, flagellin, and ISS-ODN induced liver inflammatory infiltrate in EtOH-fed mice but not control mice. Addition of antibiotics reduced the severity of alcoholic fatty liver without affecting TLR expression, whereas daily DPI injections reduced the EtOH-mediated upregulation of TLR2, 4, 6, and 9 mRNA. In conclusion, EtOH-fed mice exhibited an oxidative stress dependent on upregulation of multiple TLRs in the liver and are sensitive to liver inflammation induced by multiple bacterial products recognized by TLRs.

Adipose tissue as an immunological organ: Toll-like receptors, C1q/TNFs and CTRPs

Adipose tissue has long been regarded as a mostly resting tissue that is dedicated solely to energy storage and release. However, in recent years, this view has changed dramatically following new insights into the metabolic and immunological functions of preadipocytes and adipocytes. There are several lines of evidence for the involvement of adipose tissue in innate and acquired immune responses. First, adipocytes are potent producers of proinflammatory cytokines, such as interleukin-6 and tumor necrosis factor (TNF), and chemokines. Furthermore, adipocytes secrete high amounts of adipokines, such as leptin, adiponectin and resistin, that regulate monocyte/macrophage function, and also secrete molecules associated with the innate immune system, such as the C1qTNF-related protein superfamily. Finally, preadipocytes and adipocytes express a broad spectrum of functional Toll-like receptors and the former can convert into macrophage-like cells. Collectively, these data clearly establish the role of adipose tissue as a new member of the immune system.

Multiple TLRs are expressed in human cholangiocytes and mediate host epithelial defense responses to Cryptosporidium parvum via activation of NF-kappaB

Infection of epithelial cells by Cryptosporidium parvum triggers a variety of host-cell innate and adaptive immune responses including release of cytokines/chemokines and up-regulation of antimicrobial peptides. The mechanisms that trigger these host-cell responses are unclear. Thus, we evaluated the role of TLRs in host-cell responses during C. parvum infection of cultured human biliary epithelia (i.e., cholangiocytes). We found that normal human cholangiocytes express all known TLRs. C. parvum infection of cultured cholangiocytes induces the selective recruitment of TLR2 and TLR4 to the infection sites. Activation of several downstream effectors of TLRs including IL-1R-associated kinase, p-38, and NF-kappaB was detected in infected cells. Transfection of cholangiocytes with dominant-negative mutants of TLR2 and TLR4, as well as the adaptor molecule myeloid differentiation protein 88 (MyD88), inhibited C. parvum-induced activation of IL-1R-associated kinase, p-38, and NF-kappaB. Short-interfering RNA to TLR2, TLR4, and MyD88 also blocked C. parvum-induced NF-kappaB activation. Moreover, C. parvum selectively up-regulated human beta-defensin-2 in directly infected cells, and inhibition of TLR2 and TLR4 signals or NF-kappaB activation were each associated with a reduction of C. parvum-induced human beta-defensin-2 expression. A significantly higher number of parasites were detected in cells transfected with a MyD88 dominant-negative mutant than in the control cells at 48-96 h after initial exposure to parasites, suggesting MyD88-deficient cells were more susceptible to infection. These findings demonstrate that cholangiocytes express a variety of TLRs, and suggest that TLR2 and TLR4 mediate cholangiocyte defense responses to C. parvum via activation of NF-kappaB.

Expression and regulation of toll-like receptors in cerebral endothelial cells

Cerebral endothelial cells - the principal components of the blood-brain barrier (BBB) - fulfill several important functions in the central nervous system (CNS). They form an active interface between blood and neuronal tissue and play a key role in the maintenance of the homeostasis of the CNS. Infections caused by different pathogens are often associated with systemic symptoms and may compromise the functional integrity of the BBB as well. In the mediation of the systemic effect of pathogens Toll-like receptors (TLRs) play a significant role. TLRs are a type of pattern recognition receptor and recognize molecules that are broadly shared by pathogens but distinguishable from host molecules. TLRs are broadly distributed on cells of the immune system and function as primary sensors of invading pathogens. There is also growing experimental evidence indicating that Toll-like receptors are expressed on different non-immune cell types as well, like epithelial or endothelial cells. Here we demonstrate the expression of TLR2, TLR3, TLR4 and TLR6 on rat and human cerebral endothelial cells. Oxidative stress significantly upregulated the expression of these receptors whereas TNF-alpha upregulated the expression of TLR2 and TLR3. Furthermore we have shown, that activation of TLR2/6 leads to an increased permeability which is accompanied by a downregulation of occludin and claudin-5 expression and disappearance of these tight junction proteins from the cell membrane. Changes in occludin expression and localization could be inhibited by the ERK1/2 inhibitor U0126. Our results suggest a significant role of the cerebral endothelium in mediation of the neural effects of different inflammatory processes.

Toll-like receptor and antimicrobial peptide expression in the bovine endometrium

BACKGROUND

The endometrium is commonly infected with bacteria leading to severe disease of the uterus in cattle and humans. The endometrial epithelium is the first line of defence for this mucosal surface against bacteria and Toll-like receptors (TLRs) are a critical component of the innate immune system for detection of pathogen associated molecular patterns (PAMPs). Antimicrobial peptides, acute phase proteins and Mucin-1 (MUC-1) also provide non-specific defences against microbes on mucosal surfaces. The present study examined the expression of innate immune defences in the bovine endometrium and tested the hypothesis that endometrial epithelial cells express functional receptors of the TLR family and the non-specific effector molecules for defence against bacteria.

METHODS

Bovine endometrial tissue and purified populations of primary epithelial and stromal cells were examined using RT-PCR for gene expression of TLRs, antimicrobial peptides and MUC-1. Functional responses were tested by evaluating the secretion of prostaglandin E(2) and acute phase proteins when cells were treated with bacterial PAMPs such as bacterial lipopolysaccharide (LPS) and lipoproteins.

RESULTS

The endometrium expressed TLRs 1 to 10, whilst purified populations of epithelial cells expressed TLRs 1 to 7 and 9, and stromal cells expressed TLRs 1 to 4, 6, 7, 9 and 10. The TLRs appear to be functional as epithelial cells secreted prostaglandin E(2) in response to bacterial PAMPs. In addition, the epithelial cells expressed antimicrobial peptides, such as Tracheal and Lingual Antimicrobial Peptides (TAP and LAP) and MUC-1, which were upregulated when the cells were treated with LPS. However, the epithelial cells did not express appreciable amounts of the acute phase proteins haptoglobin or serum amyloid A.

CONCLUSION

Epithelial cells have an essential role in the orchestration of innate immune defence of the bovine endometrium and are likely to be the key to prevention of endometrial infection with bacteria.

Induction of a novel chicken Toll-like receptor following Salmonella enterica serovar Typhimurium infection

Toll-like receptors (TLRs) are a group of highly conserved molecules that initiate the innate immune response to pathogens by recognizing structural motifs expressed by microbes. We have identified a novel TLR, TLR15, by bioinformatic analysis of the chicken genome, which is distinct from any known vertebrate TLR and thus appears to be avian specific. The gene for TLR15 was sequenced and is found on chromosome 3, and it has archetypal TIR and transmembrane domains and a distinctive arrangement of extracellular leucine-rich regions. mRNA for TLR15 was detected in the spleen, bursa, and bone marrow of healthy chickens, suggesting a role for this novel receptor in constitutive host defense. Following in vivo Salmonella enterica serovar Typhimurium infection, quantitative real-time PCR demonstrated significant upregulation of TLR15 in the cecum of infected chickens. Interestingly, similar induction of TLR2 expression following infection was also observed. In vitro studies revealed TLR15 upregulation in chicken embryonic fibroblasts stimulated with heat-killed S. enterica serovar Typhimurium. Collectively, these results suggest a role for the TLR in avian defense against bacterial infection. We hypothesize that TLR15 may represent an avian-specific TLR that has been either retained in chicken and lost in other taxa or gained in the chicken.

Pulmonary surfactant protein A regulates TLR expression and activity in human macrophages

The pulmonary innate immune system responds to various airborne microbes. Although its specificity is broad and based on the recognition of pathogen-associated molecular patterns, it is uniquely regulated to limit inflammation and thereby prevent damage to the gas-exchanging alveoli. Macrophages, critical cell determinants of this system, recognize microbes through pattern recognition receptors such as TLRs, which typically mediate proinflammatory responses. The lung collectin, surfactant protein A (SP-A), has emerged as an important innate immune determinant that regulates microbe-macrophage interactions in this environment. In this study, we report the basal and SP-A-induced transcriptional and posttranslational regulation of TLR2 and TLR4 expression during the differentiation of primary human monocytes into macrophages. Despite SP-A's ability to up-regulate TLR2 expression on human macrophages, it dampens TLR2 and TLR4 signaling in these cells. SP-A decreases the phosphorylation of IkappaBalpha, a key regulator of NF-kappaB activity, and nuclear translocation of p65 which result in diminished TNF-alpha secretion in response to TLR ligands. SP-A also reduces the phosphorylation of TLR signaling proteins upstream of NF-kappaB, including members of the MAPK family. Finally, we report for the first time that SP-A decreases the phosphorylation of Akt, a major cell regulator of NF-kappaB and potentially MAPKs. These data identify a critical role for SP-A in modulating the lung inflammatory response by regulating macrophage TLR activity.

Expression of functional Toll-like receptors by salivary gland epithelial cells: increased mRNA expression in cells derived from patients with primary Sjögren's syndrome

Toll-like receptors (TLR) play an essential role in the activation of both innate and adaptive immune responses. Salivary gland epithelial cells (SGEC) may participate in the development of glandular inflammatory reactions that characterize primary Sjögren's syndrome (pSS). In this study we sought to assess the expression and function of several TLR molecules in cultured non-neoplastic SGEC obtained from pSS patients and disease controls. Long-term cultured non-neoplastic SGEC derived from pSS patients (SS-SGEC) and disease controls (control-SGEC), as well as the monocytic cell line THP-1 (positive control cell line), were examined by reverse transcription-polymerase chain reaction (RT-PCR) analysis and quantitative real-time PCR for mRNA expression of TLR1, -2, -3 and -4 molecules. TLR function was assessed by the induction of the expression (flow cytometry) of the immunoregulatory molecules CD54/intercellular adhesion molecule-1 (ICAM-1), CD40, CD86/B7 x 2, major histocompatibility complex (MHC) class I and MHC class II following treatment with the TLR ligands: Staphylococcus aureus peptidoglycan (TLR2), the synthetic dsRNA analogue polyinosinic:cytidylic acid (TLR3) and Escherichia coli lipopolysaccharide (TLR4). SGEC were found to express functional TLR2, -3 and -4 molecules, as attested by dose-dependent up-regulation of surface ICAM-1, CD40 and MHC-I expression (as well as of reciprocal TLR mRNA) following treatment with the respective TLR-ligands. SS-SGEC lines displayed significantly higher constitutive expression of TLR1 (P=0 x 0027), TLR2 (P=0 x 01) and TLR4 (P=0 x 03) mRNA compared to control-SGEC. This study demonstrates that cultured SGEC express functional TLR molecules; the high constitutive TLR expression by SS-SGEC is probably suggestive of the intrinsic activation of epithelial cells in pSS and further supports the role of this type of tissue in pathogenesis of the disorder.

Identification and expression of Toll-like receptors 1-10 in selected bovine and ovine tissues

Members of the Toll-like receptor (TLR) family are vital to immune function through the sensing of pathogenic agents and initiation of an appropriate immune response. More specifically, tissue and cell specific TLR expression patterns have been correlated with the ability to respond to various pathogenic challenges. Bovine sequence exists for 4 of the 10 human TLR Reference Sequences and no ovine TLR sequence has been reported. The main goal of this study was to determine if homologues of human TLRs 1-10 exist within the cattle and sheep. Subsequent to this, quantitative real time PCR assays were to be developed to produce transcript expression profiles in cattle skin and sheep gut-associated lymphoid tissue, as these epithelial tissues are the primary sites of host/pathogen interactions for numerous pathogens. Our findings show that homologues of human TLRs 1-10 do indeed exist within both cattle and sheep, with respective bovine and ovine homologues sharing at least 95% nucleotide sequence identity and 83-90% identity to the corresponding human Reference Sequences. Conservation of the amino acid sequence between homologous ruminant and human TLRs ranged between 84 and 97%. Quantitative real time PCR (qPCR) assays confirmed expression of all 10 TLRs within ovine jejunum, Peyer's patch and mesenteric lymph nodes. While in bovine skin all TLRs apart from TLR6 were detected. The most abundant TLR transcripts within the ovine jejunum were TLRs 3, 5 and 6, while TLRs 6, 7 and 10 were abundant in both ovine Peyer's patch and mesenteric lymph node. In bovine skin TLRs 2 and 7 were most abundant. In all tissues tested TLR4 expression was at the lower limit of detection.

Glucocorticoids severely impair differentiation and antigen presenting function of dendritic cells despite upregulation of Toll-like receptors

Glucocorticoids (GCs) are widely used as anti-inflammatory and immunosuppressive agents. Effects of GC have mainly been attributed to the suppression of T cells. Recently, several studies have indicated the role of dendritic cells (DC) in GC-mediated immunosuppression. We investigated the effect of GC on characteristics of DC. Given the crucial role of Toll-like receptor (TLR) triggering for the initiation of DC maturation program, we analyzed the expression of TLR2, 3, 4 by GC-treated DC. To extend our in vitro findings, we analyzed the distribution of DC subsets in the blood of patients treated with high-dose corticosteroids. DC differentiation in presence of GC was skewed to a qualitatively distinct population incapable of inducing an efficient immune response, whereas GC presence during the process of maturation significantly reduced DC IL-12 p70 and TNF production and T cell stimulatory function. Despite the fact that GC increased expression of TLR2, 3 and 4 on DC, their stimulation with TLR-derived signals did not induce maturation. Administration of high-dose GC to the patients with systemic autoimmunity induced a decrease of circulating myeloid DC and abrogated plasmacytoid DC. These findings provide further insights into the mechanisms of GC immunosuppressive functions and reveal additional mechanisms of their therapeutic efficiency.

Toll-like receptor 2, 3, and 4 expression and function in human airway smooth muscle

BACKGROUND

Host defense against microbial pathogens is elicited through the innate immune system by means of Toll-like receptors (TLRs). Airway smooth muscle cells (ASMCs) display proinflammatory and immunomodulatory functions. ASMCs might participate in airway inflammatory responses associated with innate immune activation.

OBJECTIVES

We determined the effects of cytokines, TLR ligands, and corticosteroids on TLR expression and function in human ASMCs.

METHODS

Real-time PCR and flow cytometry were used to assess TLR mRNA and protein expression, respectively. ASMCs were stimulated with TLR ligands, and chemokine release was measured by means of ELISA.

RESULTS

ASMCs expressed TLR1 to TLR10 mRNA, and TLR2 and TLR3 protein expression was demonstrated. TNF-alpha and double-stranded RNA (dsRNA; TLR3 ligand) were potent inducers of TLR2 and TLR3 mRNA expression, and both stimuli had additive or synergistic effects with IFN-gamma on TLR2 and TLR4, but not TLR3, mRNA expression. Peptidoglycan (TLR2 ligand) and LPS (TLR4 ligand) weakly enhanced TLR2 mRNA expression. Peptidoglycan, dsRNA, and LPS induced IL-8 and eotaxin release, with dsRNA being most potent. dsRNA also modulated cytokine-induced chemokine release in a differential manner. Dexamethasone inhibited cytokine- and ligand-induced TLR2, TLR3, and TLR4 expression and chemokine release. However, dexamethasone potentiated TLR2 expression induced by combined IFN-gamma and TNF-alpha stimulation.

CONCLUSION

Expression of TLR2, TLR3, and TLR4 is regulated by cytokines and TLR ligands, and their activation mediates chemokine release in ASMCs.

CLINICAL IMPLICATIONS

Proinflammatory responses mediated by activation of pathogen-recognition receptors in ASMCs might contribute to infectious exacerbations of airway inflammatory conditions, such as asthma and chronic obstructive pulmonary disease.

Toll-like receptors expressed in tumor cells: targets for therapy

Toll-like receptors (TLRs), mainly expressing in human immune related cells and epithelial cells, play an essential role in the host defense against microbes by recognizing conserved bacterial molecules. Recently, the expression or up-regulation of TLRs has been detected in many tumor cell lines or tumors, especially epithelial derived cancers. Although the TLR profile varies on different tumor cells, the current evidences indicate that the expression of TLRs is functionally associated with tumor progression. TLR expression may promote malignant transformation of epithelial cells. Engagement of TLRs increases tumor growth and tumor immune escape, and induces apoptosis resistance and chemoresistance in some tumor cells. These findings demonstrate that TLR is a promising target for the development of anticancer drugs and make TLR agonists or antagonists the potential agents for tumor therapy.

Molecular basis of age-associated cytokine dysregulation in LPS-stimulated macrophages

Aged humans and rodents are susceptible to infection with Streptococcus pneumoniae bacteria as a result of an inability to make antibodies to capsular polysaccharides. This is partly a result of decreased production of proinflammatory cytokines and increased production of interleukin (IL)-10 by macrophages (Mphi) from aged mice. To understand the molecular basis of cytokine dysregulation in aged mouse Mphi, a microarray analysis was performed on RNA from resting and lipopolysaccharide (LPS)-stimulated Mphi from aged and control mice using the Affymetrix Mouse Genome 430 2.0 gene chip. Two-way ANOVA analysis demonstrated that at an overall P < 0.01 level, 853 genes were regulated by LPS (169 in only the young, 184 in only the aged, and 500 in both). Expression analysis of systematic explorer revealed that immune response (proinflammatory chemokines, cytokines, and their receptors) and signal transduction genes were specifically reduced in aged mouse Mphi. Accordingly, expression of Il1 and Il6 was reduced, and Il10 was increased, confirming our previous results. There was also decreased expression of interferon-gamma. Genes in the Toll-like receptor-signaling pathway leading to nuclear factor-kappaB activation were also down-regulated but IL-1 receptor-associated kinase 3, a negative regulator of this pathway, was increased in aged mice. An increase in expression of the gene for p38 mitogen-activated protein kinase (MAPK) was observed with a corresponding increase in protein expression and enzyme activity confirmed by Western blotting. Low doses of a p38 MAPK inhibitor (SB203580) enhanced proinflammatory cytokine production by Mphi and reduced IL-10 levels, indicating that increased p38 MAPK activity has a role in cytokine dysregulation in the aged mouse Mphi.

Functional TLRs and NODs in human gingival fibroblasts

Since human gingival fibroblasts are the major cells in periodontal tissues, we hypothesized that gingival fibroblasts are endowed with receptors for bacterial components, which induce innate immune responses against invading bacteria. We found clear mRNA expression of Toll-like receptors (TLR)1, TLR2, TLR3, TLR4, TLR5, TLR6, TLR7, TLR8, TLR9, MD-2, MyD88, NOD1, and NOD2 in gingival fibroblasts. Gingival fibroblasts constitutively expressed these molecules. Upon stimulation with chemically synthesized ligands mimicking microbial products for these receptors, the production of pro-inflammatory cytokines, such as interleukin (IL)-6, IL-8, and monocyte chemoattractant protein-1, was markedly up-regulated. Furthermore, the production of pro-inflammatory cytokines induced by TLR and NOD ligands was significantly inhibited by an RNA interference assay targeted to NF-kappaB. These findings indicate that these innate immunity-related molecules in gingival fibroblasts are functional receptors involved in inflammatory reactions in periodontal tissues, which might be responsible for periodontal pathogenesis.

Sertoli cells initiate testicular innate immune responses through TLR activation

TLRs play a crucial role in early host defense against invading pathogens. In the seminiferous epithelium, Sertoli cells are the somatic nurse cells that mechanically segregate germ cell autoantigens by means of the blood-tubular barrier and create a microenvironment that protects germ cells from both interstitial and ascending invading pathogens. The objective of this study was to examine TLR expression and their functional responses to specific agonists in mouse Sertoli cells. We measured the expression of TLR2, TLR4, TLR5, and TLR6 mRNAs and confirmed by FACS analysis the presence of proteins TLR2 and TLR5 on which we focused our study. Stimulation of Sertoli cells with macrophage-activating lipopeptide-2, agonist of TLR2/TLR6, and with flagellin, agonist of TLR5, induces augmented secretion of the chemokine MCP-1. To assess the functional significance of MCP-1 production following TLR stimulation, conditioned medium from either macrophage-activating lipopeptide-2 or flagellin-treated Sertoli cells was tested for in vitro chemotaxis assay, and a significant increase of macrophage migration was observed in comparison with unstimulated conditioned medium. Moreover, we studied the role of NF-kappaB and of MAPKs in regulating TLR-mediated MCP-1 secretion by using inhibitors specific for each transduction pathway and we demonstrated a pivotal role of the IkappaB/NF-kappaB and JNK systems. In addition, TLR2/TLR6 and TLR5 stimulation induces increased ICAM-1 expression in Sertoli cells. Collectively, this study demonstrates the novel ability of Sertoli cells to potentially respond to a wide variety of bacteria through TLR stimulation.

Expression and regulation of Toll-like receptors in lupus-like immune complex glomerulonephritis of MRL-Fas(lpr) mice

BACKGROUND

How microbial infections exacerbate immune complex glomerulonephritis remains speculative. Toll-like receptors (TLRs) may be involved in this phenomenon, because TLRs have potent immunostimulatory functions when exposed to selected pathogen-associated molecules.

METHODS

We addressed this issue by characterizing the expression of TLR1-9 in MRLlpr/lpr mice that spontaneously develop immune complex glomerulonephritis as part of a systemic lupus-like autoimmune syndrome.

RESULTS

Five-week-old healthy MRLlpr/lpr mice expressed TLR3 mRNA in kidneys at comparable levels as in the spleen, while all other TLRs were expressed at low levels in the kidney. In 20-week-old nephritic MRLlpr/lpr mice, renal mRNA levels had increased for TLR1-9. Renal TLR mRNA originated at least in part from glomeruli as evidenced by real-time RT-PCR from laser capture microdissected glomeruli. Immunostaining for TLR3, TLR7 and TLR9 revealed their expression by F4/80-positive infiltrating macrophages in 20-week-old nephritic MRLlpr/lpr mice. In addition, TLR3 localized to glomerular mesangial cells. Cultured mesangial cells expressed TLR1-4 and TLR6, while murine macrophages expressed TLR1-9. TNF-alpha and IFN-gamma induced TLR2, TLR3 and TLR6 mRNA in mesangial cells, while they down-regulated TLR1-9 mRNA in macrophages. Stimulation of both cell types with ligands for TLR1-4, TLR5, TLR7 and TLR9 induced IL-6 production consistent with their respective TLR expression patterns. TNF-alpha and IFN-gamma enhanced ligand-induced IL-6 production in both cell types irrespective of their modulatory effect on respective TLR mRNA levels.

CONCLUSION

Thus, cell-type-specific expression and regulation of TLRs may be involved in infection-associated exacerbation of immune complex glomerulonephritis of MRLlpr/lpr mice.

Identification, mRNA expression and genomic structure of TLR22 and its association with GCRV susceptibility/resistance in grass carp (Ctenopharyngodon idella)

Toll-like receptor 22 (TLR22) plays a crucial role in response to virus infection by recognizing double stranded RNA (dsRNA) in aquatic animals. In the present study, a TLR22 homologue gene was identified and characterized from grass carp (Ctenopharyngodon idella) (CiTLR22). CiTLR22 genomic sequence comprises 4754 base pairs (bp), containing one intron. The cDNA sequence consists of 3831bp, encoding a protein of 954 amino acid residues. CiTLR22 was constitutively expressed in all 15 investigated tissues, highly in gill and lowly in liver and spleen. The expression profile of CiTLR22 in spleen was rapidly and significantly up-regulated at 6h (456.13-fold, P<0.05), then rapidly recovered to normal level at 12h (P>0.05) post-injection of grass carp reovirus (GCRV). The expression levels of CiTLR22 were rapidly elevated post-poly(I:C) stimulation in dose- and time-dependent manners in CIK (C. idella kidney) cell line. After GCRV infection, CiTLR22 transcripts were inhibited at the early stage, then were up-regulated and reached a peak at 24h post-infection, latterly down-regulated in CIK cell culture. In the whole genomic sequence, six single nucleotide polymorphisms (SNPs) were detected. Five of them were sited in the coding region and all synonymous, and another located in the 5' untranslated region (UTR). The following SNP analysis revealed that 2406 C/T was just a mutation. Only 417 G/T was significantly associated with the resistance of grass carp to GCRV both in genotype (P=0.013) and allele (P=0.015). -8 A/T and 2574 C/T, 863 C/T and 1923 G/T, 863 C/T and 2574 C/T were pairwise linkage disequilibrium. None of the haplotype was associated with the resistance of grass carp to GCRV. The results indicate that CiTLR22 plays an important role in the responses to dsRNA and GCRV, and is partially inhibited by GCRV in vitro. The potential molecular marker lays foundation for the selective breeding of the GCRV-resistant grass carp.

Plasmodium falciparum infection causes proinflammatory priming of human TLR responses

TLRs are a major group of pattern recognition receptors that are crucial in initiating innate immune responses and are capable of recognizing Plasmodium ligands. We have investigated TLR responses during acute experimental P. falciparum (P.f.) infection in 15 malaria-naive volunteers. TLR-4 responses in whole blood ex vivo stimulations were characterized by significantly (p < 0.01) up-regulated proinflammatory cytokine production during infection compared with baseline, whereas TLR-2/TLR-1 responses demonstrated increases in both proinflammatory and anti-inflammatory cytokine production. Responses through other TLRs were less obviously modified by malaria infection. The degree to which proinflammatory TLR responses were boosted early in infection was partially prognostic of clinical inflammatory parameters during the subsequent clinical course. Although simultaneous costimulation of human PBMC with P.f. lysate and specific TLR stimuli in vitro did not induce synergistic effects on cytokine synthesis, PBMC started to respond to subsequent TLR-4 and TLR-2 stimulation with significantly (p < 0.05) increased TNF-alpha and reduced IL-10 production following increasing periods of preincubation with P.f. Ag. In contrast, preincubation with preparations derived from other parasitic, bacterial, and fungal pathogens strongly suppressed subsequent TLR responses. Taken together, P.f. primes human TLR responses toward a more proinflammatory cytokine profile both in vitro and in vivo, a characteristic exceptional among microorganisms.

Saccharomyces boulardii decreases inflammation and intestinal colonization by Candida albicans in a mouse model of chemically-induced colitis

The present study was designed to investigate the effects of Saccharomyces boulardii on inflammation and intestinal colonization by Candida albicans in a BALB/c mouse model of colitis that had been induced by dextran-sulfate-sodium (DSS). Colonization with C. albicans was established by oral gavage with a 200 microL suspension of 10(7) yeast cells. A 1.5% solution of DSS was administered in drinking water 1 h after C. albicans oral challenge, while 10(7) cells of S. boulardii was inoculated daily by oral gavage for 1 week. Faeces were collected daily for 2 weeks. Seven groups of mice consisting of those that were administered either C. albicans or S. boulardii or both were sacrificed after 14 days and samples of the colon were taken for histological scoring and real-time PCR (RT-PCR) analysis of inflammatory cytokines and toll-like receptors (TLRs). Compared to control animals that did not receive DSS, the number of C. albicans colonies recovered from faeces was significantly greater in mice receiving DSS. In contrast, the colony forming units (CFUs) of C. albicans were greatly reduced in mice receiving S. boulardii. The administration of this yeast decreased the severity of DSS-induced clinical scores and histological inflammation. At the mRNA expression level, an increase in TLR2 and TLR4 resulting from the presence of S. boulardii was associated with a reduction in the inflammatory cytokines TNFalpha and INFgamma. In mice receiving DSS and C. albicans, TLR4 was over-expressed by stimulation with both yeasts, but TLR2 and TNFalpha, which were increased by the administration of C. albicans alone, were decreased in the presence of S. boulardii. These results indicate that S. boulardii decreased inflammation and C. albicans colonization in this BALB/c mouse model of colitis.

Th2 Cytokines Down-Regulate TLR Expression and Function in Human Intestinal Epithelial Cells

TLRs serve important immune and nonimmune functions in human intestinal epithelial cells (IECs). Proinflammatory Th1 cytokines have been shown to promote TLR expression and function in IECs, but the effect of key Th2 cytokines (IL-4, IL-5, IL-13) on TLR signaling in IECs has not been elucidated so far. We stimulated human model IECs with Th2 cytokines and examined TLR mRNA and protein expression by Northern blotting, RT-PCR, real-time RT-PCR, Western blotting, and flow cytometry. TLR function was determined by I-kappaBalpha phosphorylation assays, ELISA for IL-8 secretion after stimulation with TLR ligands and flow cytometry for LPS uptake. IL-4 and IL-13 significantly decreased TLR3 and TLR4 mRNA and protein expression including the requisite TLR4 coreceptor MD-2. TLR4/MD-2-mediated LPS uptake and TLR ligand-induced I-kappaBalpha phosphorylation and IL-8 secretion were significantly diminished in Th2 cytokine-primed IECs. The down-regulatory effect of Th2 cytokines on TLR expression and function in IECs also counteracted enhanced TLR signaling induced by stimulation with the hallmark Th1 cytokine IFN-gamma. In summary, Th2 cytokines appear to dampen TLR expression and function in resting and Th1 cytokine-primed human IECs. Diminished TLR function in IECs under the influence of Th2 cytokines may protect the host from excessive TLR signaling, but likely also impairs the host intestinal innate immune defense and increases IEC susceptibility to chronic inflammation in response to the intestinal microenvironment. Taken together, our data underscore the important role of Th2 cytokines in balancing TLR signaling in human IECs.

Structural characterisation and expression analysis of toll-like receptor 2 gene from catfish

Toll-like receptors (TLRs) are important components of innate immunity. They were found to recognise specific structures on pathogens termed pathogen-associated molecular patterns (PAMPs) and utilise conserved signaling pathways to activate pro-inflammatory cytokines and type-1 interferons. In spite of much understanding gained from the mammalian systems, many fish TLRs are unknown. Recent studies in Japanese flounder as well as in zebrafish suggested that the ligand binding and activation of inflammatory responses in fish may be different from and more complex than those found in mammals. In channel catfish, the major aquaculture species in the United States, only partial sequences of TLR3 and TLR5 were reported. As a part of efforts to characterise the innate immune components in channel catfish, here we cloned and sequenced both the cDNA and the gene for TLR2, a receptor believed mostly responsible for recognition of lipopeptides on the surface of most Gram-positive bacteria. However, expression analysis after infection with a Gram-negative bacterium, Edwardsiella ictaluri indicated that TLR2 was modestly down-regulated in the head kidney tissue of blue catfish, and with a similar pattern in the head kidney of channel catfish though the down-regulation in channel catfish was not statistically significant. In the spleen, an insignificant down-regulation was initially observed early after infection, with an increase of TLR expression later after infection. These results suggest the involvement of TLR2 in the responses after the bacterial infection. As LPS is believed to be the major PAMP for Gram-negative bacteria, additional research is warranted to determine the functions and mechanisms of TLR2 in infections of Gram-negative bacteria.

Is the trophoblast an immune regulator? The role of Toll-like receptors during pregnancy

In the early 1950s, Medawar recognized for the first time the unique immunology of the maternal/ fetal interface and its potential relevance for transplantation. In his original work, he described the fetal allograft analogy whereby the fetus may be viewed as a semi-allogeneic conceptus that has evaded rejection by the maternal immune system. Although numerous hypotheses have been proposed to prove this observation, none have demonstrated that the maternal immune system is antagonist to the invading trophoblast. In the present article, we have reviewed recent studies demonstrating the expression and function of Toll-like receptors (TLRs) in trophoblast cells, and, based on these data, we propose an alternative view for maternal/fetal immune interactions.

Toll-like receptor expression in chronic hepatitis C: correlation with pro-inflammatory cytokine levels and liver injury

BACKGROUND/AIMS

Toll-like receptors (TLR's) are critical receptors that promote innate immune responses to pathogen-associated molecular patterns. Activation of TLR's leads to production of pro-inflammatory cytokines such as tumour necrosis factor (TNF)-alpha. This study investigates whether peripheral blood monocyte expression of TLR's is disturbed in patients with chronic hepatitis C and whether levels of expression of these molecules are significantly correlated with hepatitis C virus (HCV) genotype, viral load, hepatic necroinflammatory activity, histological stage and circulating TNF-alpha concentrations.

METHODS

In 18 non-cirrhotic patients with biopsy-proven, virologically-confirmed chronic hepatitis C and 32 controls, we measured expression of TLR2 and TLR4 on peripheral blood monocytes. HCV genotype, viral load, serum alanine aminotransferase (ALT) levels, histological stage of disease and circulating TNF-alpha and endotoxin levels were also determined.

RESULTS

Peripheral blood monocyte expression of TLR2 and TLR4 were significantly increased in patients with chronic hepatitis C compared to controls, irrespective of HCV genotype or histological stage of disease. Circulating levels of TNF-alpha were also significantly increased in patients with chronic hepatitis C. In both the overall study cohort and patients with chronic hepatitis C, monocyte expression of TLR2, but not of TLR4, correlated significantly with serum TNF-alpha levels. In patients with chronic hepatitis C, monocyte expression of TLR2, but not of TLR4, also correlated significantly with serum ALT levels. Expression of TLR's was not significantly correlated with viral load.

CONCLUSIONS

Up-regulation of peripheral blood monocyte expression of TLR2 and TLR4 occurs in patients with chronic hepatitis C. Increased monocyte expression of TLR2, but not of TLR4, correlates significantly with both increased circulating TNF-alpha levels and hepatic necroinflammatory activity in this disorder.