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PubMed2004

Anti-inflammatory cytokines induce lipopolysaccharide tolerance in human monocytes without modifying toll-like receptor 4 membrane expression.

Toll-like receptor 4 (TLR4) participates in innate immunity by detecting lipopolysaccharides (LPS) of Gram-negative bacterial cell walls. TLR4 macrophage expression in mice is modulated by LPS. This fact constitutes, at least partially, the molecular basis for LPS tolerance. Very recently, the effect of interferon-gamma (IFN-gamma), a pro-inflammatory cytokine, has been described on TLR4 membrane expression of human monocytes. IFN-gamma up-regulates TLR4 expression and antagonizes the LPS-induced TLR4 down-regulation. These data prompted us to study the expression of membrane TLR4 in human mono- cytes in which LPS tolerance was induced by LPS and by anti-inflammatory cytokines [interleukin-10 (IL-10) and transforming growth factor beta1 (TGFbeta1)]. Data concerning this latter model, and more specifically, the effect of anti-inflammatory cytokines over TLR4 expression, are not available at present. We show here that membrane TLR4 expression in human monocytes falls after LPS exposure. The effect was prolonged for 12 h, but then expression returned to normal levels. The incubation of human monocytes with IL-10, TGFbeta1 or a mixture of both induces no alterations in membrane TLR4 expression. However, these cytokines are able to substitute the tolerizing LPS exposure in order to induce LPS tolerance. Our data help to achieve a better understanding of the way cytokines control the cellular expression of TLR.

Adult↗
PubMed2005

Macrophage migration inhibitory factor contributes to the development of acute dextran sulphate sodium-induced colitis in Toll-like receptor 4 knockout mice.

Toll-like receptor 4 (TLR4), which recognizes lipopolysaccharides, plays an important role in the innate immune response. In this study, we investigated the role of TLR4 in the development of experimental colitis with regard to the biological actions of macrophage migration inhibitory factor (MIF) using TLR4 null ((-/-)) mice. TLR4(-/-) mice were given 2% dextran sulphate sodium (DSS) in drinking water to induce colitis, which was clinically and histologically as severe as that seen in wild-type (WT) mice. The level of tumour necrosis factor (TNF)-alpha in colon tissues was increased in WT mice but unchanged in TLR4(-/-) mice. The level of myeloperoxidase (MPO) activity in colon tissues was increased by DSS administration in both TLR4(-/-) and WT mice. The expression of MIF was up-regulated in the colons of TLR4(-/-) mice with acute DSS-induced colitis. An anti-MIF antibody significantly suppressed colitis and elevation of matrix metalloproteinase-13 in TLR4(-/-) mice. The current results obtained from TLR4(-/-) mice provide evidence that MIF plays a critical role in the development of acute DSS-induced colitis.

Acute Disease↗
PubMed2005

Bacterial endotoxin (lipopolysaccharide) stimulates interleukin-6 production and inhibits growth of pituitary tumour cells expressing the toll-like receptor 4.

Members of the Toll receptor (Tlr) family have a crucial role in the innate immune response following bacterial infection. The effects of Gram-negative bacteria-derived endotoxins (lipopolysaccharide, LPS) are predominantly mediated by Tlr4, and we have recently shown that pituitary folliculostellate cells express functional Tlr4. In the present study, we investigated whether Tlr4 is also present in normal and transformed endocrine epithelial pituitary cell types. By reverse transcriptase-polymerase chain reaction, Tlr4 mRNA expression was found in some pituitary epithelial tumour cell lines (AtT20, HP75), whereas others were negative (GH3, alphaT3-1). Tlr4 protein was detected by immunohistochemistry in a few epithelial cells in normal human anterior pituitaries and in 26 out of 67 human pituitary tumours analysed. LPS had no effect on adrenocorticotropic hormone secretion in Tlr4-positive AtT20 cells, but it suppressed the growth of these cells in a dose-dependent manner. As expected, neither hormone secretion, nor growth of Tlr4-negative GH3 cells was affected by LPS. In cell cultures of Tlr4-positive pituitary adenomas, LPS dose-dependently stimulated the production of interleukin (IL)-6, which is known to induce growth and hormone production in pituitary tumours. The LPS-induced IL-6 production was blocked by the specific p38alphaMAP kinase inhibitor, SB203580, and by the synthetic glucocorticoid, dexamethasone. The data suggest that, during Gram-negative bacteria-induced infections or inflammatory processes, LPS could affect pituitary tumour pathophysiology and progression in the subset of Tlr4-expressing pituitary adenomas.

Adenoma↗
PubMed2006

Acute ethanol treatment modulates Toll-like receptor-4 association with lipid rafts.

BACKGROUND: Alcohol, a substance that is most frequently abused, suppresses innate immune responses to microbial pathogens. The host senses pathogens via Toll-like receptors (TLRs). Recent studies indicate that alcohol affects TLR signaling. METHODS: Here, we hypothesized that acute alcohol treatment may interfere with early steps of membrane-associated TLR2 and TLR4 signaling at the level of lipid rafts. Human monocytes and Chinese hamster ovary (CHO) cells, transfected with human TLR2, TLR4, or CD14, were stimulated with peptidoglycan (PGN, TLR2 ligand) or lipopolysaccharide (LPS, TLR4 ligand) with or without alcohol (50 mM) and analyzed for cytokine production (enzyme-linked immunosorbent assay), nuclear factor-kappaB (NF-kappaB) activation (electrophoretic mobility shift assay), membrane fluidity (fluorescent pyrene eximer formation), and partition of cellular membrane into cholesterol-rich, detergent-resistant domains (DRMs; Western blot). RESULTS: We determined that both TLR2 and TLR4 were located outside the rafts; flotillin, a DRM marker, was resident in the rafts, while CD14 was equally distributed in and outside the rafts in a steady-state condition. PGN forced TLR2 to migrate into DRMs. Engagement of TLR4 and CD14 with LPS induced their migration into the rafts. Alcohol prevented TLR4 partitioning; however, it did not affect TLR2 migration into the rafts. Furthermore, alcohol downregulated TLR4-induced, but not TLR2-induced, NF-kappaB activation and cytokine production in monocytes. We found that alcohol increased membrane fluidity and depleted cellular cholesterol in CHO cells without affecting cell viability. CONCLUSIONS: These data demonstrate for the first time that alcohol disturbs TLR4 and CD14 association with lipid rafts. We propose that alcohol-induced effects on lipid rafts may contribute to modulation of TLR4-CD14-triggered early cellular responses.

Adolescent↗
PubMed2003

Toll-like receptor 4-defective C3H/HeJ mice are not more susceptible than other C3H substrains to infection with Mycobacterium tuberculosis.

Mycobacterium tuberculosis produces a variety of molecules capable of activating Toll-like receptors, a family of pattern recognition receptors expressed by macrophages and a variety of other cells. To determine whether Toll-like receptor 4 (TLR4) was critical in resistance to M. tuberculosis infection, we compared the morbidity and mortality of TLR4-defective C3H/HeJ mice to those of TLR4-sufficient C3H mouse substrains. TLR4-defective C3H/HeJ mice and TLR4-sufficient C3H/HeSnJ, C3HeB/FeJ, and C3H/HeOuJ mice were infected by the aerosol route with M. tuberculosis. TLR4-defective C3H/HeJ mice had levels of cytokines in their bronchoalveolar lavage fluids and in vitro mycobacterial antigen-specific recall responses similar to those of other C3H mouse substrains. In addition, bacterial replication and long-term survival of mice following infection appeared to be independent of TLR4. Interestingly, C3HeB/FeJ mice were significantly more susceptible to M. tuberculosis infection, indicating that genetic heterogeneity among inbred C3H mouse substrains modifies resistance to infection. Therefore, cautious interpretation is required when the C3H/HeJ strain is used as a model of a TLR4-defective mouse strain, as there are significant allelic differences between C3H/HeJ and other C3H mouse substrains in response to M. tuberculosis infection. With this caveat, our data indicate that TLR4 may not be required for optimal immunity of mice to M. tuberculosis.

Animals↗
PubMed2004

Porphyromonas gingivalis lipopolysaccharide contains multiple lipid A species that functionally interact with both toll-like receptors 2 and 4.

The innate host response to lipopolysaccharide (LPS) obtained from Porphyromonas gingivalis is unusual in that different studies have reported that it can be an agonist for Toll-like receptor 2 (TLR2) as well as an antagonist or agonist for TLR4. In this report it is shown that P. gingivalis LPS is highly heterogeneous, containing more lipid A species than previously described. In addition, purification of LPS can preferentially fractionate these lipid A species. It is shown that an LPS preparation enriched for lipid A species at m/z 1,435 and 1,450 activates human and mouse TLR2, TLR2 plus TLR1, and TLR4 in transiently transfected HEK 293 cells coexpressing membrane-associated CD14. The HEK cell experiments further demonstrated that cofactor MD-2 was required for functional engagement of TLR4 but not of TLR2 nor TLR2 plus TLR1. In addition, serum-soluble CD14 effectively transferred P. gingivalis LPS to TLR2 plus TLR1, but poorly to TLR4. Importantly, bone marrow cells obtained from TLR2(-/-) and TLR4(-/-) mice also responded to P. gingivalis LPS in a manor consistent with the HEK results, demonstrating that P. gingivalis LPS can utilize both TLR2 and TLR4. No response was observed from bone marrow cells obtained from TLR2 and TLR4 double-knockout mice, demonstrating that P. gingivalis LPS activation occurred exclusively through either TLR2 or TLR4. Although the biological significance of the different lipid A species found in P. gingivalis LPS preparations is not currently understood, it is proposed that the presence of multiple lipid A species contributes to cell activation through both TLR2 and TLR4.

Animals↗
PubMed2005

Toll-like receptor 4 mediates tolerance in macrophages stimulated with Toxoplasma gondii-derived heat shock protein 70.

Peritoneal macrophages (PMs) from toll-like receptor 4 (TLR4)-deficient and wild-type (WT) mice were responsive to recombinant Toxoplasma gondii-derived heat shock protein 70 (rTgHSP70) and natural TgHSP70 (nTgHSP70) in NO release, but those from TLR2-, myeloid differentiation factor 88 (MyD88)-, and interleukin-1R-associated kinase 4 (IRAK4)-deficient mice were not. Polymyxin B did not inhibit PM activation by TgHSP70 and nTgHSP70 from WT and TLR4-deficient mice, while it inhibited PM activation by lipopolysaccharide. Pretreatment of PMs from WT but not from TLR4-deficient mice with rTgHSP70 resulted in suppression of NO release on restimulation with rTgHSP70. Similarly, pretreatment of PMs from WT but not TLR4-deficient mice with nTgHSP70 resulted in suppression of NO release on restimulation with nTgHSP70. Polymyxin B did not inhibit rTgHSP70- and nTgHSP70-induced tolerance of PMs from TLR4-deficient mice. Furthermore, PMs from WT mice increased suppressor of cytokine-signaling-1 (SOCS-1) expression after restimulation with rTgHSP70, while those from TLR4-deficient mice did not. Phosphorylation of JNK and I-kappaBalpha occurred in rTgHSP70-induced tolerance of PMs from TLR4-deficient mice, but not in that from WT mice. These data indicated that TgHSP70 signaling mechanisms were mediated by TLR2, MyD88, and IRAK4, but not by TLR4. On the other hand, signaling of TgHSP70-induced tolerance was mediated by TLR4, and the expression of SOCS-1 suppressed the TLR2 signaling pathway.

Adaptor Proteins, Signal Transducing↗
PubMed2005

Toll-like receptor-4 is required for intestinal response to epithelial injury and limiting bacterial translocation in a murine model of acute colitis.

Inflammatory bowel disease (IBD) arises from a dysregulated mucosal immune response to luminal bacteria. Toll-like receptor (TLR)4 recognizes LPS and transduces a proinflammatory signal through the adapter molecule myeloid differentiation marker 88 (MyD88). We hypothesized that TLR4 participates in the innate immune response to luminal bacteria and the development of colitis. TLR4-/- and MyD88-/- mice and littermate controls were given 2.5% dextran sodium sulfate (DSS) for 5 or 7 days followed by a 7-day recovery. Colitis was assessed by weight loss, rectal bleeding, and histopathology. Immunostaining was performed for macrophage markers, chemokine expression, and cell proliferation markers. DSS treatment of TLR4-/- mice was associated with striking reduction in acute inflammatory cells compared with wild-type mice despite similar degrees of epithelial injury. TLR4-/- mice experienced earlier and more severe bleeding than control mice. Similar results were seen with MyD88-/- mice, suggesting that this is the dominant downstream pathway. Mesenteric lymph nodes from TLR4-/- and MyD88-/- mice more frequently grew gram-negative bacteria. Altered neutrophil recruitment was due to diminished macrophage inflammatory protein-2 expression by lamina propria macrophages in TLR4-/- and MyD88-/- mice. The similarity in crypt epithelial damage between TLR4-/- or MyD88-/- and wild-type mice was seen despite decreased epithelial proliferation in knockout mice. TLR4 through the adapter molecule MyD88 is important in intestinal response to injury and in limiting bacterial translocation. Despite the diversity of luminal bacteria, other TLRs do not substitute for the role of TLR4 in this acute colitis model. A defective innate immune response may result in diminished bacterial clearance and ultimately dysregulated response to normal flora.

Adaptor Proteins, Signal Transducing↗
PubMed2004

LPS activation of Toll-like receptor 4 signals CD11b/CD18 expression in neutrophils.

We identify herein a novel signaling function of the Toll-like receptor-4 (TLR4), the lipopolysaccharide (LPS) receptor mediating the innate immune response, in inducing the expression of CD11b/CD18 integrin in polymorphonuclear leukocytes (PMNs). Studies were made in PMNs isolated from TLR4-deficient (TLR4(-/-)) and C57BL/6 [wild-type (WT)] mice. We observed increased CD11b expression in WT PMNs within 3 h after LPS challenge, whereas CD11b was not expressed in TLR4(-/-) PMNs above basal levels. TLR4-activated CD11b expression was cycloheximide sensitive and involved the activation of transcription factors, NF-kappaB and c-Jun/PU.1. TLR4(-/-) PMNs challenged with LPS were functionally defective as the result of the impaired CD11b expression in that they failed to adhere and did not migrate across endothelial cells in response to N-formylmethionyl-leucyl-phenylalanine. TLR4 also promoted increased binding of LPS to PMNs on the basis of expression of CD11b. Thus TLR4 signaling activates synthesis and upregulation of CD11b and is essential for PMN adhesion and transmigration. Our data suggest an important role of TLR4-activated CD11b expression in the mechanism of the PMN host-defense response to LPS.

Animals↗
PubMed2004

Expression of toll-like receptor 4 on dendritic cells is significant for anticancer effect of dendritic cell-based immunotherapy in combination with an active component of OK-432, a streptococcal preparation.

A lipoteichoic acid-related molecule OK-PSA is an active component of OK-432, a Streptococcus-derived anticancer immunotherapeutic agent. In the present study, we first examined the effect of OK-PSA on the maturation of dendritic cells (DCs) in vitro by using the DCs derived from 5 healthy donors and 10 patients with head and neck cancer with or without expression of toll-like receptor 4 (TLR4) or MD-2 mRNA. OK-PSA treatment effectively increased the surface expression of MHC class II, CD80, CD83, and CD86. OK-PSA-stimulated DCs secreted the cytokines that can induce helper T-cell 1 (Th1)-type T-cell response, and stimulated allogeneic T cells to produce IFN-gamma and to elicit an allogeneic antigen-specific cytotoxicity. These activities almost depended on expression of TLR4 and MD-2 genes. We next investigated the in vivo anticancer effect of intratumoral administration of syngeneic DCs followed by OK-PSA against established tumors in mice. C57BL/6 mice, which express wild-type TLR4, and C57BL/6-derived TLR4-knockout (TLR4(-/-)) mice were used. Although OK-PSA accelerated the antitumor effect of intratumoral DC administration in wild-type mice bearing syngeneic tumors, the antitumor effect of OK-PSA as well as of the combination therapy with DCs and OK-PSA was not significant in TLR4(-/-) mice. Interestingly, an administration of wild-type-mouse-derived DCs followed by OK-PSA exhibited a marked antitumor effect even in the TLR4(-/-) mice. These findings suggest that OK-PSA may be a potent adjuvant for local DC therapy, and that DC therapy followed by OK-PSA is able to elicit anticancer activity even in a TLR4-deficient host when TLR4 is expressed only in DCs injected intratumorally.

Adjuvants, Immunologic↗
PubMed2005

Respiratory epithelial cells require Toll-like receptor 4 for induction of human beta-defensin 2 by lipopolysaccharide.

BACKGROUND: The respiratory epithelium is a major portal of entry for pathogens and employs innate defense mechanisms to prevent colonization and infection. Induced expression of human beta-defensin 2 (HBD2) represents a direct response by the epithelium to potential infection. Here we provide evidence for the critical role of Toll-like receptor 4 (TLR4) in lipopolysaccharide (LPS)-induced HBD2 expression by human A549 epithelial cells. METHODS: Using RTPCR, fluorescence microscopy, ELISA and luciferase reporter gene assays we quantified interleukin-8, TLR4 and HBD2 expression in unstimulated or agonist-treated A549 and/or HEK293 cells. We also assessed the effect of over expressing wild type and/or mutant TLR4, MyD88 and/or Mal transgenes on LPS-induced HBD2 expression in these cells. RESULTS: We demonstrate that A549 cells express TLR4 on their surface and respond directly to Pseudomonas LPS with increased HBD2 gene and protein expression. These effects are blocked by a TLR4 neutralizing antibody or functionally inactive TLR4, MyD88 and/or Mal transgenes. We further implicate TLR4 in LPS-induced HBD2 production by demonstrating HBD2 expression in LPS non-responsive HEK293 cells transfected with a TLR4 expression plasmid. CONCLUSION: This data defines an additional role for TLR4 in the host defense in the lung.

Epithelial Cells↗
PubMed2003

CD40-mediated up-regulation of Toll-like receptor 4-MD2 complex on the surface of murine dendritic cells.

Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns, which are non-self macromolecular components of pathogens that allow the innate-immune system to recognize infection. TLRs are expressed on macrophages and dendritic cells (DC). TLR stimulation or CD40 agonists can induce inflammatory cytokine secretion from macrophages and DC, and promote DC maturation. The regulation of TLR expression by inflammation has begun to be explored. Our studies have focused on the regulation of TLR4 surface expression on DC. TLR4, along with the adaptor molecule MD2, is involved in the recognition of lipopolysaccharide (LPS). CD40 stimulation via cross-linked anti-CD40 monoclonal antibody (mAb) up-regulates TLR4-MD2 surface expression on a DC cell line (DC2.4) and on ex vivo-cultured splenic DC. LPS treatment down-regulated surface TLR4-MD2 on DC2.4 cells, but if combined with anti-CD40 mAb, increased TLR4-MD2 expression was observed. The increased TLR4-MD2 surface expression by any treatment did not correlate with TLR4 mRNA levels. The functional consequence of increased TLR4-MD2 expression following LPS and anti-CD40 treatment was examined. Although CD40 prestimulation did slightly enhance interleukin-12p70 secretion after LPS restimulation, simultaneous anti-CD40 mAb and LPS treatment, which up-regulates TLR4-MD2 complex, does not restore DC responsiveness to subsequent LPS.

Adjuvants, Immunologic↗
PubMed2006

Phosphatidylcholine-specific phospholipase C (PC-PLC) is required for LPS-mediated macrophage activation through CD14.

Lipid rafts, composed of sphingolipids, are critical to Toll-like receptor 4 (TLR4) assembly during lipopolysaccharide (LPS) exposure, as a result of protein kinase C (PKC)-zeta activation. However, the mechanism responsible for this remains unknown. The purpose of this study is to determine if LPS-induced TLR4 assembly and activation are dependent on the sphingolipid metabolite ceramide produced by phosphatidylcholine-specific phospholipase C (PC-PLC) or CD14. To study this, THP-1 cells were stimulated with LPS. Selected cells were pretreated with the PC-PLC inhibitor D609, exogenous C2 ceramide, CD14 neutralizing antibody, or TLR4 neutralizing antibody. LPS led to production of ceramide, phosphorylation of PKC-zeta, and assembly of the TLR4 within lipid rafts. This was followed by activation of the mitogen-activated protein kinase family and the liberation of cytokines. Pretreatment with D609 or CD14 blockade was associated with attenuated LPS-induced ceramide production, TLR4 assembly on lipid rafts, and cytokine production. Pretreatment with TLR4 blockade did not affect LPS-induced ceramide production but was associated with significant attenuation in cytokine production. Treatment with C2 ceramide prior to LPS reversed the inhibitory effects induced by D609 but not of CD14 or TLR4 blockade. C2 ceramide alone induced the activation of PKC-zeta and the assembly of TLR4 but was not associated with cytokine liberation. This study demonstrates that TLR4 assembly and activation following LPS exposure require the production of ceramide by PC-PLC, which appears to be CD14-dependent.

Bridged-Ring Compounds↗
PubMed2004

Toll-like receptor 4 in normal and inflamed lungs and other organs of pig, dog and cattle.

Bacterial diseases, especially those of the lung caused by Gram-negative bacteria, inflict significant economic loss associated with mortality and morbidity in domestic animals. Toll-like receptor 4 (TLR4) has recently been recognized as a major receptor for cellular interactions with lipopolysaccharides derived from Gram-negative bacteria. However, there are no data on the expression of TLR4 in various organs of domestic animals. We performed immunohistochemistry and immuno-gold electron microscopy to localize TLR4 in lung and seven other organs from normal pig, dog and calf (n=2 each) and in inflamed lungs from calves (n=4) challenged with Mannheimia hemolytica. The data show TLR4 in macrophages in lung, small intestine, liver and spleen in all the species and pulmonary intravascular macrophages in calves and pigs. Epithelium in lung, small intestine, cornea and convoluted and straight renal tubules was stained for TLR4. Vascular endothelium of large blood vessels only in lungs and skin was positive, and skeletal muscles were negative for TLR4. In inflamed lungs, airway epithelium showed reduced staining for TLR4 while staining in macrophages remained unaltered. These are the first immunocytochemical data on TLR4 expression in domestic animal species and show similarity in TLR4 staining in macrophages, epithelium and vascular endothelium among dog, pig and cattle.

Animals↗
PubMed2005

Toll-like receptor 4 on nonhematopoietic cells sustains CNS inflammation during endotoxemia, independent of systemic cytokines.

Inflammatory agonists such as lipopolysaccharide (LPS) induce robust systemic as well as CNS responses after peripheral administration. Responses in the innate immune system require triggering of toll-like receptor 4 (TLR4), but the origin of CNS sequelas has been controversial. We demonstrate expression of TLR4 transcripts in mouse brain in the meninges, ventricular ependyma, circumventricular organs, along the vasculature, and in parenchymal microglia. The contribution of TLR4 expressed in CNS resident versus hematopoietic cells to the development of CNS inflammation was examined using chimeric mice. Reciprocal bone marrow chimeras between wild-type and TLR4 mutant mice show that TLR4 on CNS resident cells is critically required for sustained inflammation in the brain after systemic LPS administration. Hematopoietic TLR4 alone supported the systemic release of acute phase cytokines, but transcription of proinflammatory genes in the CNS was reduced in duration. In contrast, TLR4 function in radiation-resistant cells was sufficient for inflammatory progression in the brains of chimeric mice, despite the striking absence of cytokine elevations in serum. Surprisingly, a temporal rise in serum corticosterone was also dependent on TLR4 signaling in nonhematopoietic cells. Our findings demonstrate a requirement for TLR4 function in CNS-resident cells, independent of systemic cytokine effects, for sustained CNS-specific inflammation and corticosterone rise during endotoxemia.

Animals↗
PubMed2001

MD-2 enables Toll-like receptor 2 (TLR2)-mediated responses to lipopolysaccharide and enhances TLR2-mediated responses to Gram-positive and Gram-negative bacteria and their cell wall components.

MD-2 is associated with Toll-like receptor 4 (TLR4) on the cell surface and enables TLR4 to respond to LPS. We tested whether MD-2 enhances or enables the responses of both TLR2 and TLR4 to Gram-negative and Gram-positive bacteria and their components. TLR2 without MD-2 did not efficiently respond to highly purified LPS and LPS partial structures. MD-2 enabled TLR2 to respond to nonactivating protein-free LPS, LPS mutants, or lipid A and enhanced TLR2-mediated responses to both Gram-negative and Gram-positive bacteria and their LPS, peptidoglycan, and lipoteichoic acid components. MD-2 enabled TLR4 to respond to a wide variety of LPS partial structures, Gram-negative bacteria, and Gram-positive lipoteichoic acid, but not to Gram-positive bacteria, peptidoglycan, and lipopeptide. MD-2 physically associated with TLR2, but this association was weaker than with TLR4. MD-2 enhanced expression of both TLR2 and TLR4, and TLR2 and TLR4 enhanced expression of MD-2. Thus, MD-2 enables both TLR4 and TLR2 to respond with high sensitivity to a broad range of LPS structures and to lipoteichoic acid, and, moreover, MD-2 enhances the responses of TLR2 to Gram-positive bacteria and peptidoglycan, to which the TLR4-MD-2 complex is unresponsive.

Adjuvants, Immunologic↗
PubMed2003

Overexpression of Toll-like receptor 4 amplifies the host response to lipopolysaccharide and provides a survival advantage in transgenic mice.

Toll-like receptors are transmembrane proteins that are involved in the innate immune recognition of microbial constituents. Among them, Toll-like receptor 4 (Tlr4) is a crucial signal transducer for LPS, the major component of Gram-negative bacteria outer cell membrane. The contribution of Tlr4 to the host response to LPS and to infection with virulent Salmonella typhimurium was studied in four transgenic (Tg) strains including three overexpressing Tlr4. There was a good correlation between the level of Tlr4 mRNA expression and the sensitivity to LPS both in vitro and in vivo: Tg mice possessing the highest number of Tlr4 copies respond the most to LPS. Overexpression of Tlr4 by itself appears to have a survival advantage in Tg mice early during infection: animals possessing more than two copies of the gene survived longer and in a greater percentage to Salmonella infection. The beneficial effect of Tlr4 overexpression is greatly enhanced when the mice present a wild-type allele at natural resistance-associated macrophage protein 1, another critical innate immune gene involved in resistance to infection with Salmonella. Tlr4 and natural resistance-associated macrophage protein 1 exhibit functional epistatic interaction to improve the capacity of the host to control bacterial replication. However, this early improvement in disease resistance is not conducted later during infection, because mice overexpressing Tlr4 developed an excessive inflammatory response detrimental to the host.

Adjuvants, Immunologic↗
PubMed2003

Toll-like receptor 3 mediates a more potent antiviral response than Toll-like receptor 4.

We have recently described an IFN regulatory factor 3-mediated antiviral gene program that is induced by both Toll-like receptor (TLR)3 and TLR4 ligands. In our current study, we show that activation of IFN/viral response gene expression in primary macrophage cells is stronger and prolonged with TLR3 stimulation compared with that of TLR4. Our data also reveal that the cytoplasmic tails of both TLR3 and TLR4 can directly interact with myeloid differentiation factor 88 (MyD88). However, although Toll/IL-1 receptor homology domain-containing adaptor protein/MyD88 adaptor-like is able to associate with TLR4, we were unable to detect any interaction between Toll/IL-1 receptor homology domain-containing adaptor protein/MyD88 adaptor-like and TLR3. By using quantitative real-time PCR assays, we found that TLR3 expression is inducible by both TLR3 and TLR4 ligands, while TLR4 expression is not inducible by these same stimuli. Furthermore, using cells derived from mice deficient in the IFN-alphabetaR, we show that both TLR3 and TLR4 require IFN-beta autocrine/paracrine feedback to induce TLR3 expression and activate/enhance genes required for antiviral activity. More specifically, a subset of antiviral genes is initially induced independent of IFN-beta, yet the cytokine further enhances expression at later time points. This was in contrast to a second set of genes (including TLR3) that is induced only after IFN-beta production. Taken together, our data argue that, despite both TLR3 and TLR4 being able to use IFN-beta to activate/enhance antiviral gene expression, TLR3 uses multiple mechanisms to enhance and sustain the antiviral response more strongly than TLR4.

Adaptor Proteins, Signal Transducing↗