Gastroenterology
Volume 130, Issue 6 , Pages 1886-1900, May 2006

Toll-Like Receptor Signaling in the Liver

  • Robert F. Schwabe

      Affiliations

    • Corresponding Author InformationAddress requests for reprints to: Robert F. Schwabe, Department of Medicine, Columbia University, College of Physicians and Surgeons, P&S 9-460, 630 W. 168th Street, New York, New York 10032. fax: (212) 305-9822
    • ,
  • Ekihiro Seki
    • ,
  • David A. Brenner

published online 31 January 2006.

Article Outline

Abbreviations used in this paper:  ER, endoplasmic reticulum , LPS, lipopolysaccharide , LRR, leucine-rich repeat , PAMPs, pathogen-associated molecular patterns , PH, partial hepatectomy , SOCS1, suppressing of cytokine signaling 1 , TIR, Toll/interleukin-1 receptor , TIRAP, TIR domain containing adapter protein , TLRs, Toll-like receptors

 

Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns and are crucially involved in the regulation of innate immune responses. Despite chronic exposure to a high load of bacterial products, the normal liver shows no activation of TLR-signaling pathways. However, under pathologic conditions, Toll-like receptors promote proinflammatory signaling such as the nuclear factor-κB, c-Jun-N-terminal kinase (JNK), p38, and interferon pathways in the liver and regulate antiviral and antibacterial responses, hepatic injury, and wound healing. This review will summarize recent findings on TLR signaling; describe the functional expression of TLRs in resident and nonresident cell populations of the liver; and analyze the role of TLR-mediated signals in hepatic diseases such as hepatitis B and C virus infection, systemic endotoxemia, hepatic ischemia-reperfusion injury, liver regeneration, alcoholic liver injury, nonalcoholic steatohepatitis, primary biliary cirrhosis, and hepatic fibrosis.

Toll-like receptors (TLRs) are a group of highly conserved molecules that allow the immune system to sense molecules that are present in most classes of pathogens, but not the host, and to coordinate defense mechanisms against these pathogens. The recognition of pathogen-associated molecular patterns (PAMPs) by Toll-like receptors is a cornerstone of innate immunity and provides a quick and highly efficient response to pathogens in both vertebrate and invertebrate species.1, 2, 3, 4 In addition, there is accumulating evidence that TLRs contribute significantly to activation of adaptive immune responses such as dendritic cell maturation and T- and B-cell responses.5

Because of its anatomic links to the gut, the liver is constantly exposed to gut-derived bacterial products and functions as a major filter organ and a first line of defense. Eighty percent of intravenously injected endotoxin is detected in the liver within 20–30 minutes.6, 7 Kupffer cells, the resident macrophages of the liver, are able to take up efficiently endotoxin and phagocytose bacteria carried through portal vein blood and are considered to play a major role in the clearance of systemic bacterial infection.8, 9, 10, 11 Despite the constant exposure to low levels of gut-derived bacteria and bacterial products, there are no signs of ongoing inflammation in the normal liver. This lacking response is to some extent explained by very specific immunologic properties of the liver,12 which contribute to a high degree of tolerance as seen by graft survival across major histocompatibility antigen disparities, induction of systemic tolerance to food antigens, and persistence of some viral infections for decades.13 In addition, the healthy liver contains low messenger RNA (mRNA) levels of TLRs such as TLR1, TLR2, TLR4, TLR6, TLR7, TLR8, TLR9, and TLR10 and signaling molecules such as MD-2 and MyD88 in comparison with other organs.14, 15, 16 However, under pathologic conditions, TLRs activate inflammatory-signaling pathways in the liver and are actively involved in the pathophysiology in a large number of hepatic diseases. In this article, we will present an overview of recent advances in TLR signaling and review the role of TLRs in the pathophysiology of infectious, toxic, metabolic, and autoimmune liver disease.

Back to Article Outline

I. TLR Signaling 

TLRs and Coreceptors 

The human TLR family consists of currently 10 members, which are structurally characterized by the presence of a leucine-rich repeat (LRR) domain in their extracellular domain and a Toll/interleukin (IL)-1 receptor (TIR) domain in their intracellular domain. The existence of a large number of TLRs enables the innate immune system to discriminate between PAMPs that are characteristic of different microbial classes and launch specific defense mechanisms. A comparison of the amino acid sequences of the human TLRs reveals that members of the TLR family can be structurally divided into 5 subfamilies: the TLR3, TLR4, TLR5, TLR2, and TLR9 subfamilies. Whereas the TLR3, TLR4, and TLR5 subfamilies consist of only 1 member, the TLR2 subfamily is composed of TLR1, TLR2, TLR6, and TLR10 and the TLR9 subfamily of TLR7, TLR8, and TLR9. TLRs are able to detect a variety of PAMPs including lipopolysaccharide (LPS; TLR4), lipoproteins (TLR2/TLR1 and TLR2/TLR6 heterodimers), double-stranded RNA (TLR 3) and single-stranded RNA (TLR 7 and TLR8), flagellin (TLR5), and unmethylated CpG-containing DNA (TLR9) (Figure 1). TLR10 is an orphan receptor with currently unknown ligands. TLR1 and TLR2 form heterodimers with TLR6 and TLR10 as well as with each other, which may even broaden the ligand repertoire of these receptors. TLRs that mainly serve to detect bacterial lipopolysaccharides and lipoproteins are located on the cell surface. TLRs such as TLR3, TLR7, TLR8, and TLR9 that mainly recognize viral RNA and bacterial DNA are located in late endosome-lysosomes in which these materials are processed and host DNA is not present, thus avoiding aberrant self-recognition. Although each TLR detects specific PAMPs, many of the signaling molecules that mediate intracellular response are shared by the TLRs and form a complex signaling network that activates several pathways that initiate the transcription of a specific set of genes to induce proinflammatory, antiviral, and antibacterial responses. In addition, TLR ligands also repress the transcription of a large number of genes.17, 18 The TLR-signaling pathway shows remarkable similarity to the IL-1 receptor signaling pathway with which it shares many components including highly conserved cytoplasmic TIR domains and several intracellular adapter molecules.

  • View full-size image.
  • Figure 1. 

    Schematic overview of TLR signaling pathways. Viral PAMPS activate TLR3, TLR7, and TLR9, whereas bacterial PAMPs activate TLR1, TLR2, TLR4, TRL6, and TLR9. Each receptor interacts with 1 or several adapter molecules (MyD88, TRIF, TRAM, and TIRAP) to then induce activation of 1 or several downstream kinases and transcription factors, which up-regulate proinflammatory, antiviral, and antibacterial mediators. The main signaling pathways induced are the IRF7-IFN pathway (green), IRF3-IFN pathway (light blue), NF-κB pathway (red), AP-1 pathway (red), IRF5 pathway (dark blue), and p38 pathway (orange).

Two cell surface molecules, CD14 and MD-2, are involved in addition to TLR4 to transmit signals in response to LPS. Mice that are deficient in CD14 are resistant to the lethal effects of LPS but still able to respond to high concentrations of LPS.19 Recent data suggests that CD14 is not required for all TLR4-mediated signals and that MyD88-dependent signaling may occur in the absence of CD14.20 In contrast, mice deficient in MD-2 show an almost completely impaired response to LPS.21 MD-2 is required for TLR4 glycosylation, its release from gp96 in the endoplasmic reticulum (ER), and subsequent TLR4 cell surface expression.21, 22, 23 In addition, MD-2 has an important signaling function on the cell surface at which it tightly associates with TLR4 and is required for LPS responsiveness as demonstrated by the ability of MD-2 to block TLR4 signaling independently of effects on TLR4 cell surface expression.24, 25 Several other factors, among them the LPS-binding protein, enhance the response to LPS based on their ability to bind LPS and deliver it to CD14.26

Recent evidence suggests that CD36 acts as a co-receptor for a subset of TLR2 ligands. CD36-deficient mice show a severe defect in tumor necrosis factor (TNF)-α production after stimulation with the TLR2/6 ligands macrophage-activating lipopeptide 2 from Mycoplasma pneumoniae and lipoteichoic acid and are hypersusceptible to Staphylococcus aureus infection.27 In these mice, the responses to TLR 2/1 ligand PAM3CSK4 and the TLR2/6 ligand zymosan are preserved, indicating that only some TLR2 ligands are dependent on CD36. It is possible that other TLRs besides TLR4 and TLR2/6 also require co-receptors because Toll signaling in Drosophila has been shown to require several non-Toll receptors.28, 29

Back to Article Outline

Intracellular Adapter Molecules and Downstream Pathways 

Four adapter molecules interact with the TIR domains of TLRs to transduce proinflammatory and antiviral signals (Figure 1). Among these, MyD88 is the adapter molecule that is involved in the majority of pathways, which has led to classifying downstream signaling pathways as “MyD88-dependent” and “MyD88-independent.”

MyD88 interacts with TLR1, TLR2, TLR4, TLR5, TLR6, TLR7, and TLR9 as well as the IL-1 and IL-18 receptors. MyD88 has an N-terminal death domain and a C-terminal TIR domain through which it interacts with the TIR domains of TLRs or IL-1 and IL-18 receptors. MyD88-deficient mice are resistant to the effects of LPS and display an absent TNF-α secretion and a delayed nuclear factor (NF)-κB activation.30 Furthermore, NF-κB activation in response to lipopeptides, CpG-DNA, ssRNA, and flagellin is blunted in these mice, demonstrating that MyD88 is a crucial adapter in all TLR receptor signaling pathways except for TLR3.

The adapter molecule TIRAP (also called Mal) associates with MyD88 and is required for MyD88-dependent TLR2 and TLR4 signaling.31, 32 TIRAP possesses a C-terminal TIR domain but lacks an N-terminal death domain. TIRAP-deficient mice have defects in cytokine production, NF-κB activation, and mitogen-activated protein kinases in response to the TLR4 ligand LPS.32 In addition, TIRAP-deficient mice are impaired in their responses to ligands of the heterodimeric TLR2-TLR1 and TLR2-TLR6 complexes.31

The adapter molecule TRIF exclusively associates with TLR3 and TLR4 and is crucially involved in mediating the antiviral interferon response as demonstrated by the absent up-regulation of type I interferons and an enhanced susceptibility to cytomegalovirus in the TRIF-mutated Lps2 mice and TRIF-deficient mice.33, 34 TRIF also mediates NF-κB activation in response to TLR3 ligands and is involved in the MyD88-independent prolonged NF-κB activation in response to TLR4 ligands.34 Whereas the interaction between TLR3 and TRIF appears to be direct, the interaction between TRIF and TLR4 is mediated by yet another adapter protein called TRAM. TRAM acts as a bridge between TLR4 and TRIF, allowing for activation of IRF3 in the MyD88-independent pathway. TRAM-deficient mice demonstrate defects in cytokine production in response to LPS but show normal responses to the ligands for TLR2, TLR3, TLR7, and TLR9 and also IL-1.35

MyD88-NF-κB/AP-1/IRF5/p38-Proinflammatory Pathways 

MyD88 induces the transcription of proinflammatory and antibacterial mediators by activating the NF-κB, AP-1, IRF5, and p38 pathways (Figure 1). MyD88 activates NF-κB in response to all TLRs, except for TLR3. After stimulation with LPS, MyD88 mediates the early phase strong NF-κB activation, whereas late phase NF-κB activation depends on TRIF.34 Activation of the NF-κB pathway by MyD88 requires its interaction with IRAK4 and IRAK1,36, 37 which in turn recruit TRAF6 to activate TAK1 and the IκB kinase complex resulting in IκB phosphorylation and NF-κB liberation.38 After its translocation to the nucleus, NF-κB activates several hundreds of proinflammatory genes containing κB-binding sites. Many inflammatory genes that contain NF-κB sites also contain AP-1 sites in their promoter, and the transcription of these genes additionally depends on a TLR4-IRAK-TRAF6-TAK1-dependent activation of JNK and subsequent phosphorylation of several members of the AP-1 group of transcription factors.38 MyD88 binds to and activates the transcription factor IRF5 in concert with TRAF6 to induce the transcription of proinflammatory genes such as IL-6, IL-12, and TNF-α but not interferon (IFN) α.39 Accordingly, IRF5-deficient mice are resistant to lethal shock induced by injection of unmethylated CpG-DNA or LPS.39 The activation of p38, but not NF-κB or JNK, by TLR4 ligands depends on the formation of reactive oxygen species and activation of ASK1.40 The MAP kinase p38 is believed to up-regulate inflammatory reaction by stabilizing the mRNA of proinflammatory mediators. Accordingly, ASK1-deficient mice are resistant to lipopolysaccharide-induced septic shock.40 TNF-α is an important transcriptional target of the TLR4-MyD88-dependent pathway and is a potent inducer of NF-κB and AP-1 itself. This mechanism may serve as an amplification loop to induce prolonged NF-κB signaling after LPS stimulation41, 42 and spread inflammatory and antibacterial signals to cells that express TNF receptors but not TLRs.

MyD88-IRF7-IFN Pathway 

TLR7 and TLR9 confer antiviral responses by up-regulating IFN-α and IFN-β. This pathway is mediated by a signaling cascade that involves MyD88, IRAK-4, IRAK-1, and IRF7.43, 44 Mice that are deficient in either MyD88, IRAK-4, or IRF7 show a severe impairment in the up-regulation of IFN-α and IFN-β and are more vulnerable to viral infection.36, 45

TRIF-IRF3-IFN Pathway 

TRIF mediates antiviral responses after TLR3 and TLR4 stimulation through the transcription factor IRF3, which activates a large set of antiviral genes including IFN-α and IFN-β (Figure 1). The noncanonical IκB kinase (IKK) homologs IKK-ϵ and TANK-binding kinase-1 (TBK1) provide the link between TRIF and IRF3 and mediate IRF3 phosphorylation and the transcription of IRF3-dependent genes.46, 47 Whereas the expression of IKK-ϵ is restricted to cells of the immune system and requires induction by LPS or proinflammatory cytokines such as IL-1, IL-6, or TNF-α,48 TBK1 is widely expressed and may thus play a more ubiquitous role in mediating antiviral responses.49

TRIF-NF-κB Pathway 

TRIF mediates NF-κB activation in response to TLR3 and TLR4 agonists. Although TRIF is the only adapter to mediate NF-κB after TLR3 stimulation, NF-κB activation in response to TLR4 depends on TRIF and MyD88 with MyD88 mediating an early phase strong NF-κB activation and TRIF mediating a late phase and weaker NF-κB activation. TRIF was shown to associate with RIP1 through its C-terminal portion50 and with TRAF6 through its N-terminus.51, 52 Embryonic fibroblast cells from RIP1-deficient mice show an impaired NF-κB activation in response to TLR3 ligands, whereas the role of TRAF6 is still controversial.53 It is likely that the slower onset and prolonged activation of TRIF lead to the up-regulation of NF-κB-dependent genes that serve different purposes than those up-regulated by the rapid MyD88-dependent NF-κB activation.

Negative Regulation of TLR Signaling 

Although a strong proinflammatory response after TLR stimulation may be beneficial in the short term to eradicate pathogens, an excess of these mediators may be deleterious. Therefore, several mechanisms have evolved that negatively regulate TLR-induced cellular responses.54 These mechanisms act at the receptor level (ST2 and SIGGIR), at the level of adapter molecules (mainly MyD88), at the level of receptor proximal kinases (mainly IRAK), and/or at the level of receptor distal kinases by suppressor of cytokine signaling 1 (SOCS1) (Figure 2). LPS stimulation results in a reduced surface expression of the LPS receptor complex composed of TLR4 and MD-2.55, 56 TLR4 and TLR9 signaling is negatively regulated by proteolytic degradation of TLR4 and TLR 9 after being marked for degradation by the E3 ubiquitin ligase Triad3A.57 ST2 and SIGIRR are cell-surface receptors with TIR domains that inhibit TLR4 by sequestering signaling proteins from the pathway.58, 59 In SIGIRR- and ST2-deficient mice, the LPS-induced inflammatory response is enhanced.58, 59 MyD88s, an alternatively spliced variant of MyD88 that lacks the intermediary domain of MyD88, is induced in monocytes upon LPS stimulation and blocks LPS-mediated NF-κB activation by its inability to recruit IRAK-4.43, 60 IRAK-M, a member of the IRAK family of serine/threonine kinases that lacks kinase activity and negatively regulates TLR signaling, is induced by TLR stimulation in macrophages.61 IRAK-M-deficient mice show increased production of inflammatory cytokines in response to TLR ligands and defective induction of LPS tolerance.61 Two splice variants of IRAK2 (termed IRAK2c and IRAK2d) also block TLR4 signaling.62 SOCS-1 is induced by LPS and CpG DNA and blocks LPS-induced NF-κB activation.63, 64 SOCS-1-deficient mice are hypersensitive to LPS-induced endotoxin shock and show a defective induction of LPS tolerance.63 Accordingly, macrophages from SOCS1-deficient cells display an increased phosphorylation of IκB, JNK, and p38.65

  • View full-size image.
  • Figure 2. 

    Negative regulation of TLR signaling. TLR signaling is negatively regulated at several levels. At the receptor level, the TIR domain containing receptors ST2 and SIGGIR sequester signaling molecules and inhibit TLR4 signaling. The ubiquitin ligase TRIAD3A induces the degradation of TLR4 and TLR9. At the level of adapater molecules, MyD88s blocks the recruitment of IRAK4. At the level of receptor proximal kinases, the IRAK family member IRAK-M and the splice variants of IRAK2, IRAK2c, and 2d, inhibit IRAK activity. SOCS1 inhibits NF-κB, JNK, and p38 activation, which are at least in part mediated by effects of IRAK.

Back to Article Outline

II. TLR Expression and Signaling in Hepatic Cell Populations 

Kupffer Cells 

Kupffer cells are resident macrophages of the liver and perform multiple functions, including phagocytosis and antigen processing and presentation, and secrete proinflammatory mediators, including cytokines, prostanoids, nitric oxide, and reactive oxygen intermediates. Because of their anatomical localization, Kupffer cells are among the first cells in the liver to be hit by gut-derived toxins and orchestrate the inflammatory response within the liver. Kupffer cells express TLR4 and are responsive to LPS.66 Kupffer cells have been shown to be involved in the uptake and possibly hepatic excretion of LPS.9, 67 After LPS stimulation, Kupffer cells produce TNF-α, IL-1β, IL-6, IL-12, IL-18, and several chemokines.68, 69 Kupffer cell-derived IL-12 and IL-18 in turn activate hepatic NK cells to produce IFN-γ, a key cytokine involved in microbial eradication and hepatic wound healing.70 Kupffer cells also appear to express functional TLR2 because TLR2-deficient Kupffer cells show a greatly diminished response toward Listeria monocytogenes.71 Because of the continuous exposure to low amounts of LPS, Kupffer cells have evolved mechanisms to evade some of the proinflammatory actions of LPS. In comparison with peripheral blood monocytes, Kupffer cells express low levels of CD14.15 Freshly isolated human Kupffer cells secrete the anti-inflammatory cytokine IL-10 in response to stimulation with LPS, which contributes to the down-regulation of proinflammatory cytokines such as IL-6 and TNF-α after LPS stimulation.72 LPS induces hyporesponsiveness to LPS in Kupffer cells, whereas the TLR9 ligand CpG DNA may even enhance the response to LPS.73 However, the expression of negative regulators of TLR signaling such as IRAK-M, MyD88s, IRAK2c, and IRAK2d has not been determined in Kupffer cells.

Hepatocytes 

Hepatocytes fulfill metabolic and detoxifying functions in the liver, and are important mediators of the acute phase response. Hepatocytes express TLR2 and TLR4 receptors and are responsive to LPS, but this response is fairly weak with only 2-fold elevated levels of serum amyloid A after LPS stimulator.74 Similarly, stimulation with the TLR2 ligand bacterial lipoprotein induces NF-κB activation and a weak induction of serum amyloid A.74 The expression of TLR2 in hepatocytes is up-regulated by LPS, TNF-α, bacterial lipoprotein, and IL-1β in an NF-κB-dependent manner, indicating that hepatocytes become more responsive to TLR2 ligands under inflammatory conditions.74 On the other hand, TLR4 expression in hepatocytes is not up-regulated by proinflammatory mediators.75 A recent report shows that the transformed hepatocyte cell line PH5CH8 expresses TLR3 and up-regulates IFN-β-promoter activity and interferon-responsive genes in response to poly-I:C,76 but it is not known whether primary hepatocytes express TLR3 to activate antiviral-signaling pathways. Hepatocytes are also believed to play a role in the uptake of endotoxin and its removal from the systemic circulation through secretion into the bile.67, 77 Based on the available data, it seems plausible that hepatocytes, most likely in concert with Kupffer cells, serve to remove LPS from systemic circulation and, for this reason, express low levels of TLR4 and only weakly respond to LPS. It is likely that the uptake of LPS in hepatocytes is not mediated by CD14 or TLRs but by other molecules, eg, scavenger receptors.78, 79

Hepatic Stellate Cells 

After liver injury, hepatic stellate cells undergo an activation process and become the predominant extracellular matrix-producing cell type in the liver.80 Activated human hepatic stellate cells express TLR4 and CD14 and respond to LPS with the activation of IKK/NF-κB and JNK as well as the secretion of proinflammatory cytokines.81 Activated mouse hepatic stellate cells express TLR4 and CD14 as well as low levels of TLR2 and respond to LPS, lipoteichoic acid, and N-acetyl muramyl peptide with an up-regulation of Erk phosphorylation and IL-6, TGF-β1, and MCP-1 secretion.82 In quiescent rat hepatic stellate cells, LPS stimulates the synthesis of TNF-α, IL-6, and IL-1 but not of transforming growth factor (TGF)-β.83 Thus, LPS and other TLR ligands may enhance fibrogenic responses in the liver through direct effects on hepatic stellate cells.

Biliary Epithelial Cells 

Biliary epithelial cells form the biliary tree that connects the liver with the intestinal lumen to deliver bile to the intestine. Mouse biliary epithelial cells express CD14; MD-2; and TLR2, TLR3, TLR4, and TLR584 and display NF-κB activation and TNF-α after LPS stimulator.84 Human biliary epithelial cells express TLR1–10.85 In an in vitro model of biliary cryptosporidiosis, C parvum recruits TLR2 and TLR4 to the host-cell-parasite interface and induces NF-κB activation, IL-8, and human β-defensin 2 expression.85

Sinusoidal Endothelial Cells 

Sinusoidal endothelial cells constitutively express TLR4 and CD14 and show an increase in NF-κB activation after LPS stimulation.86 After repetitive LPS challenges, sinusoidal endothelial cells show reduced NF-κB activation, decreased CD54 expression, and reduced ability to promote leukocyte adhesion.86 In sinusoidal endothelial cells, LPS tolerance is not regulated at the level of TLR4 surface expression but appears to be linked to prostanoid expression.86 Although some authors propose sinusoidal endothelial cells to be involved in the hepatic uptake of LPS, several studies have not found such a role.67, 77

Hepatic Dendritic Cells 

Hepatic dendritic cells are the professional antigen-presenting cells of the liver. During inflammation, dendritic cells are recruited into the liver sinusoids from which they can migrate to periportal and pericentral areas. Hepatic CD11c+/CD11b+ dendritic cells express the TLR4/MD-2 complex and respond to LPS, peptidoglycan, poly-I:C, and CpG-DNA to produce inflammatory cytokines, such as IL-12 and TNF-α, and to express co-stimulatory molecules, such as CD40, CD80, and CD86 on their cell surface (Seki E et al, unpublished data). Liver CD11c+, CD8α− (myeloid), and CD11c+, CD8-α+ (“lymphoid-related”) dendritic cells express lower TLR4 mRNA compared with their splenic counterparts.14 Lower TLR4 expression correlates with the reduced capacity of LPS-stimulated, but not anti-CD40-stimulated, hepatic dendritic cells to induce naive allogeneic (C3H/HeJ) T-cell proliferation.14 In contrast to LPS-stimulated splenic dendritic cells, these LPS-activated hepatic dendritic cells induce alloantigen-specific, T-cell hyporesponsiveness in vitro and are inferior allogeneic T-cell stimulators compared with splenic dendritic cells.14 These data suggest that the low expression of TLR4 by hepatic dendritic cells may contribute to the reduced or altered activation of hepatic adaptive immune responses.

Back to Article Outline

III. TLR Signaling in Hepatic Disease 

Alcohol-Induced Liver Injury 

Orally ingested alcohol disrupts the intestinal epithelial barrier causing enhanced permeability87 and subsequent elevations of endotoxin levels in the portal vein.88, 89 LPS-mediated activation of Kupffer cells plays a crucial role in ethanol-induced liver injury (Figure 3). Liver injury is strongly reduced when gram-negative microflora is eliminated from the gut by lactobacillus or antibiotics or when Kupffer cells are depleted with gadolinium chloride.90, 91, 92 Conversely, long-term ethanol exposure sensitizes rats to the effects of LPS and strongly increases TNF-α levels and liver injury.93 The activation of Kupffer cells in alcoholic liver disease largely depends on TLR4 because TLR4-mutated C3H/HeJ mice display strongly reduced levels of proinflammatory mediators in the liver and blunted liver injury despite elevated endotoxin levels.94 NADP(H) oxidase is a crucial downstream mediator of TLR4 in Kupffer cells during alcohol-induced liver injury.95 Mice deficient in p47phox, the main cytosolic component of NADP(H) oxidase, show an absence of free radical production, NF-κB activation, TNF-α mRNA induction, and liver pathology after ethanol exposure.95

  • View full-size image.
  • Figure 3. 

    TLR signaling in alcoholic liver injury. Orally ingested alcohol destroys the intestinal barrier and increases the levels of LPS in the portal vein. LPS binds to TLR4, which is expressed on Kupffer cells. TLR4 activated NF-κB, JNK/AP-1, and NADPH oxidase. These factors are involved in the release of TNF-α, IL-1β, IL-12, and IL-18, which then promote hepatocyte injury through the recruitment of neutrophils and platelets and through direct effects on hepatocytes.

Nonalcocoholic Steatohepatitis 

Genetically obese Fa/Fa rats and ob/ob mice exhibit increased hepatic sensitivity to endotoxin and develop steatohepatitis after exposure to low doses of LPS.96 Intestinal bacteria are thought to contribute to steatohepatitis in mice with fatty liver.97 Accordingly, probiotics reduce hepatic injury in ob/ob mice.98 Mice with nonalcoholic fatty liver display increased liver injury and inflammatory cytokine induction after challenge with the TLR4 ligand LPS but not the TLR2 ligand peptidoglycan.99 TLR2-deficient mice are not protected against the development of steatohepatitis after a methionine-choline-deficient diet.99 There is preliminary evidence that TLR4-deficient mice have less severe steatohepatitis in the model of a methionine-choline-deficient diet.100

Liver Fibrosis 

Hepatic fibrosis is the reaction to chronic hepatic injury induced by a variety of stimuli, including viral hepatitis, alcohol, and autoimmune and metabolic disease.80 The changes in intestinal motility and subsequent alteration of microflora content, decreased mucosal integrity, and suppressed immunity in hepatic fibrosis contribute to failure of the intestinal mucosal barrier with subsequent increases in bacterial translocation and LPS levels in hepatic fibrosis and cirrhosis.101, 102, 103, 104, 105, 106 Although there is abundant data demonstrating that LPS is elevated in experimental models of hepatic fibrosis107, 108 and in patients with cirrhosis,109, 110, 111 there are only a few studies addressing the role of LPS signaling in hepatic fibrogenesis. Studies from the middle of the last century have shown that antibiotics prevent hepatic injury and fibrosis induced by CCl4 or choline-deficient diet and that endotoxin enhances hepatic fibrosis by choline-deficient diet.112, 113 TLR4 is expressed on 2 key mediators of hepatic fibrogenesis: Kupffer cells and hepatic stellate cells. Kupffer cells initiate fibrogenesis by a secreting proinflammatory and profibrogenic cytokines, whereas hepatic stellate cells are the predominant source of extracellular matrix production in the fibrotic liver.80 We have shown that hepatic fibrogenesis and inflammation are strongly reduced in the TLR4-mutated C3H/HeJ strain following bile duct ligation, indicating that the LPS-TLR4 pathway plays an important role in hepatic fibrogenesis.114 It is currently unclear whether TLR4 ligands such as LPS target Kupffer cells in hepatic fibrogenesis or whether hepatic stellate cells are a direct target (Figure 4). Activated HSCs are highly responsive to LPS through a TLR4-dependent pathway.80, 81 In hepatic stellate cells, LPS induces IL-8 and MCP-1 production and activates transcription factor NF-κB and c-Jun through TLR4, indicating that LPS exerts direct effects on hepatic stellate cells during fibrogenesis.81 In addition, hepatic stellate cells express TLR2, implying a potential role for TLR2 ligands such as HCV core and NS3 in the proinflammatory or profibrogenic signaling in hepatic stellate cells.115, 116 Thus, TLR2 and TLR4 ligands may be involved in hepatic stellate cell activation and perpetuation.

  • View full-size image.
  • Figure 4. 

    TLR signaling in hepatic fibrogenesis. During liver injury, LPS levels are increased because of increased permeability of the intestinal mucosal barrier. Toll-like signaling may contribute to fibrogenesis through 2 pathways: (1) LPS stimulate TLR4 on Kupffer cells to (a) enhance hepatocyte damage, (b) increase leukocyte infiltration, and (c) secrete profibrogenic cytokines such as TGF-β and PDGF. These factors are believed to then act in concert to induce activation of hepatic stellate cells and fibrogenesis. (2) LPS may directly stimulate TLR4 on hepatic stellate cells to activate NF-κB and JNK and perpetuate hepatic stellate cell activation. In patients with hepatitis C, HCV-NS3, and HCV-core, may act as TLR2 ligands and activate proinflammatory and profibrogenic pathways in Kupffer cells and HSCs.

Hepatitis B Virus Infection 

Hepatitis B virus (HBV) is a DNA virus that causes an acute hepatitis, which is self-limited in 80%–90% of adults but becomes chronic in the remaining 10%–20% of adult patients. Chronic hepatitis B is the leading cause of cirrhosis and hepatocellular carcinoma worldwide. In contrast to hepatitis C virus (HCV) infection, acute HBV infection does not induce a strong up-regulation of interferon-dependent genes during the lag phase of infection or the log phase of viral spread.117 It is possible that HBV evades the innate immune response either because of an inability of TLRs to sense HBV or because HBV has evolved strategies to prevent TLR signaling in a similar manner as HCV (see below). Despite the lack of strong innate immune responses during acute HBV infection, it has been shown that activation of TLRs blocks HBV replication. In HBV transgenic mice, activation of TLR3 by poly-I:C leads to an IFN-dependent inhibition of HBV replication.118 These findings have been extended in a recent study that investigated the influence of TLR2, TLR4, TLR5, TLR7, and TLR9 ligands on HBV replication.119 All TLR ligands except for TLR2 inhibited HBV replication in the liver within 24 hours of injection in an interferon-dependent manner.119 The TLR9 agonist CPG 7909 enhanced immunogenicity of hepatitis B vaccination.120 Thus, TLR 3, 4, 5, 7, and 9 ligands may provide novel tools for the treatment of acute or chronic HBV infection and vaccination against HBV.

HCV Infection 

HCV is a single-stranded hepatotrophic RNA virus that causes a chronic infection of the liver in 70%–80% of patients that may lead to the development of cirrhosis and hepatocellular carcinoma. HCV has developed several strategies to evade the immune system resulting in failure to eradicate the virus in most infected individuals.121 Recent reports provide evidence that HCV may escape attack from the innate immune system by interfering with the TRIF-TBK1-IRF3 pathway at several levels (Figure 5). NS3 induces the degradation of the TLR adapter molecule TRIF.122 NS3/4a impedes both IRF3 and NF-κB activation by reducing functional TRIF abundance and by generating cleavage products with dominant-negative activity.122 Additionally, NS3/4a protein interacts directly with TBK1, resulting in decreased TBK1-IRF3 interaction and inhibition of IRF3 activation.123 Thus, HCV has not only developed extensive strategies to avoid attack from the adaptive immune system but also employs several mechanisms to prevent activation of the innate immune system.121 On the other hand, HCV core and NS3 proteins activate TLR2 in monocytes and macrophages to induce TNF-α, IL-6, and IL-8 production through NF-κB, JNK/AP-1, p38, and ERK pathways115 (Figure 5). In patients with HCV infection, the expression of TLR4 in B cells is elevated 3- to 7-fold.124 The induction of TLR4 is mediated by NS5A and leads to higher secretion of IFN-α and IL-6.124 It has been demonstrated that the immortalized human hepatocyte cell line PH5CH8 expresses TLR3 and up-regulates IFN-β-promoter activity and interferon-responsive genes after poly-I:C exposure, which may represent a potential defense mechanism of hepatocytes against HCV infection.76 Although these TLR-dependent pathways are part of innate immune responses to fight HCV infection, it is possible that up-regulation of TLR4 and activation of NF-κB, JNK, and p38 contribute to the proinflammatory environment of later stages of chronic HCV infection, which is associated with hepatocyte damage, fibrosis, and hepatocellular carcinoma development. Recent studies have suggested that the TLR system can be exploited for the treatment of chronic hepatitis C infection. One-week treatment with the TLR7 agonist isatoribine caused a significant reduction of plasma HCV RNA, an increase in the levels of OAS, a marker of antiviral immunity, and an increase in the levels of the chemokine IP-10 and neopterin, a marker of macrophage activation.125

  • View full-size image.
  • Figure 5. 

    Effects of HCV on TLR signaling. Double-stranded RNA from HCV binds to TLR3. However, TLR3 signaling is inhibited by NS3/4A through degradation of TRIF and by NS3 by binding to TBK1 and blocking the association between TBK1 and IRF3. HCV mediates activation of NF-κB, JNK/AP-1, and p38 through a TLR2-MyD88-IRAK pathway.

Ischemia-Reperfusion Liver Injury 

Ischemia-reperfusion injury of the liver occurs in procedures such as partial hepatectomy and liver transplantation. Kupffer cells play a prominent role in ischemia/reperfusion injury of liver (Figure 6). After activation, Kupffer cells produce proinflammatory cytokines that damage endothelial cells, hepatocytes, and neutrophils, leading to the recruitment of T lymphocytes and hepatic inflammation. Ischemia-reperfusion-induced hepatic inflammation and hepatocellular damage are almost completely prevented in TLR4-deficient mice and IRF3-deficient mice, whereas ischemia-reperfusion liver injury remains unchanged in MyD88-deficient mice.126, 127 It remains to be clarified whether other TLR4-mediated MyD88-independent pathways, eg, TRIF-mediated NF-κB and AP-1 activation, also contribute to hepatic ischemia-reperfusion injury. Experiments in chimeric mice suggest that nonparenchymal cells mediate the majority of toxic TLR4 effects after hepatic ischemia-reperfusion. High mobility group box 1 (HMGB1) is strongly induced after hepatic ischemia-reperfusion and mediates hepatic injury. Because HMGB1 acts as a TLR4 agonist, and inhibition of HMGB1 protected TLR4-intact C3H/OuJ mice but not TLR4-mutated C3H/HeJ, it is likely that HMGB1 promotes hepatic injury by functioning as an endogenous TLR ligand.126, 127

  • View full-size image.
  • Figure 6. 

    Dual role of Toll-like receptor signaling in liver injury. After ischemia or partial hepatectomy, TLR ligands increase to activate Kupffer cells. In the case of liver regeneration, a modest amount of TLR stimulation induces NF-κB and AP-1 activation in the Kupffer cells, which leads to IL-6 and TNF-α secretion and a regenerative hepatocyte response. Strong TLR stimulation, eg, after ischemia-reperfusion injury, leads to the activation of cytotoxic mediators, which directly and indirectly cause hepatocyte injury.

Liver Regeneration After Partial Hepatectomy 

As the main detoxifying organ in the body, the liver has a high likelihood of toxic injury and possesses astonishing regenerative properties that will restore the liver to full size within 7–10 days and ensure survival.128, 129 During liver regeneration, a complex network of cytokines (TNF-α and IL-6), growth factors (HGF, EGF, TGF-α), kinases (Erk, JNK), and transcription factors (AP-1, NF-κB, Stat3) drives hepatocytes out of the G0 phase to enter 1–2 rounds of replication.114 After partial hepatectomy (PH), LPS is elevated in portal vein blood and is believed to contribute to the initiation of liver regeneration.130 LPS triggers the secretion of TNF-α and IL-6 in Kupffer cells, which then initiate liver regeneration (Figure 6). However, the currently available data on the role of TLRs and TLR ligands are not sufficient to draw a clear picture. Whereas some studies have reported that liver regeneration after CCl4 is suppressed in TLR4-mutated mice,130, 131 we do not find a role for TLR2, TLR4, or TLR9 in liver regeneration after PH.132 On the other hand, MyD88-deficient mice display decreased TNF-α and IL-6 levels, impaired NF-κB nuclear translocation in Kupffer cells, and decreased levels of immediately early genes,132 suggesting that liver regeneration is likely to be driven by other TLR receptors, which may possibly bind endogenous TLR ligands to signal through MyD88. In addition, strong activation of TLRs may also impair liver regeneration. Injection of the TLR4 ligand LPS or the TLR3 ligands murine CMV and poly-I:C suppress liver regeneration after hepatectomy.133 Taken together, moderate TLR-dependent activation of MyD88 signaling appears to be required for beginning of liver regeneration, but strong activation of this pathway may induce opposite signals and suppress liver regeneration.

Endotoxin-Induced Liver Injury 

Endotoxin-induced fulminant hepatitis is a common clinical complication during sepsis and accounts for a high percentage of sepsis-associated mortality. In rodents, LPS does not induce significant liver pathology, even at lethal doses.134 Pretreatment with D-galactosamine, an inhibitor of transcription in hepatocytes, sensitizes mice to LPS-induced liver injury.135 Several TLR agonists have been demonstrated to modulate LPS-mediated injury. Pretreatment with the TLR3 agonist reduces hepatic injury induced by LPS plus D-galactosamine by down-regulating TLR4 expression.136 On the other hand, pretreatment with heat-killed Propionibacterium acnes, which acts as agonist of several TLRs, renders mice highly susceptible to LPS challenge.134, 137, 138 P acnes leads to a recruitment of macrophages and dendritic cells into the liver and sensitizes the liver to LPS in an IFN-γ-, IL-12-, and IL-18-dependent manner.134, 137, 138 Hepatic granulomas, consisting of macrophages, dendritic cells, and lymphocytes, are not found in MyD88-deficient mice and in TLR9-deficient mice after P acnes administration, suggesting that MyD88 and TLR9 are crucial in mediating the sensitizing effects of P acnes.139, 140 Although P acnes also acts as a TLR2 ligand, P acnes induces hepatic granuloma formation in TLR2-deficient mice.139 P acnes sensitizes the liver to LPS by up-regulating the hepatic levels of TLR4 and its co-receptor MD-2 but does not sensitize the liver to TLR2 ligands.141

TLRs and Microbial Infection in the Liver 

The liver is a target of a wide range of microbes including Listeria, Salmonella, and Plasmodium species. L monocytogenes is a gram-positive facultative intracellular bacterium that infects hepatocytes and Kupffer cells, leading to destruction of the host cells and bacterial replication. L monocytogenes-infected Kupffer cells produce proinflammatory cytokines, such as TNF-α and IL-12 via TLR2 through a MyD88-dependent pathway.71, 142 MyD88-deficient mice display an almost absent clearance of L monocytogenes, leading to high mortality.71, 142 Although the levels of proinflammatory cytokines are decreased in TLR2-deficient mice, L monocytogenes clearance is almost identical to wild-type mice. This discrepancy in L monocytogenes eradication between TLR2-deficient and Myd88-deficient mice suggests that either an activation of multiple TLRs or of additional non-TLR receptors is required to achieve L monocytogenes eradication.

Typhoid fever is an acute infectious disease caused by Salmonella typhimurium and is often accompanied by abnormal liver tests and, in a minority of patients, by salmonella hepatitis. In mice, TLR4 seems to required for defense against S typhimurium because TLR4-mutated C3H/HeJ mice display an enhanced susceptibility to S typhimurium.143, 144 Functional TLR4 is required to up-regulate TLR2 mRNA, down-regulate TLR4 mRNA, and initiate granuloma formation in the liver.145 Furthermore, C3H/HeJ mice show an impairment in their NO-dependent antimicrobial activity and display high levels of S typhimurium in Kupffer cells.144 Salmonella choleraesuis infection induces liver injury through up-regulation of Fas-ligand on NKT cells that depends on TLR2 but not TLR4.146 However, eradication of S choleraesuis does not depend on TLR2 and TLR4, suggesting that Salmonella infection may activate other TLRs in addition to TLR2 and TLR4.

Plasmodium berghei, a lethal strain of mouse malaria, induces stage-specific pathologic changes in the host, including asymptomatic changes during the liver stage and symptomatic changes during the erythrocyte stage, characterized by the presence of apoptotic and necrotic hepatocytes and dense infiltration of lymphocytes. Liver injury in this model is mediated by infiltrating lymphocytes in response to increased IL-12 production. In mice deficient in MyD88, IL-12 secretion and liver injury are completely blunted, suggesting that the MyD88 pathway is an essential mediator of hepatic manifestations of malaria infection.147

Primary Biliary Cirrhosis and Hepatic Autoimmune Disease 

Recent evidence suggest that TLRs make a significant contribution to adaptive immune responses and are involved in the enhanced immune response seen in autoimmune processes. TLRs have been shown to trigger autoimmune processes through their ability to enhance interferon γ production in diseases such as diabetes, autoimmune encephalitis, and systemic lupus erythematodes.148, 149, 150, 151 In the liver, there is increasing evidence that TLRs are involved in the pathogenesis of primary biliary cirrhosis (PBC), an autoimmune disorder that primarily affects the biliary tree. TLR3, TLR4, and TLR9 expression are significantly elevated in PBC.152, 153 Monocytes from PBC patients produce higher levels of proinflammatory cytokines in response to TLR2, TLR3, TLR4, TLR5, and TLR9 stimulation, suggesting that an increased responsiveness to TLR ligands contributes to the failing self-tolerance in biliary cirrhosis.154 In vitro incubation of peripheral blood mononuclear cells from PBC patients with CpG-B, but not CpG-A, leads to a high frequency of intracellular IgM-positive B cells, associated with high levels of synthesized IgM.155 CD27+ memory B cells are the main type of B cells to increase IgM after CpG-B, and this memory B-cell subset also expresses higher densities of TLR9 as compared with naive B cells. It remains to be investigated whether the enhanced response to CpG-B plays a causative role in the pathogenesis of PBC or whether it merely represents a correlate of an enhanced immune response in PBC.155 It has also been reported that the 2848 AA TLR9 genotype is associated with enhanced gene expression and higher frequency of intracellular IgM(+) B cells following CpG stimulation, but there are no differences in the distribution of TLR9 genotypes between patients and controls, suggesting that this polymorphism is not causatively involved in the pathogenesis of PBC.156 The role of TLRs in other hepatic autoimmune diseases such as autoimmune hepatitis, primary sclerosing cholangitis, and overlap syndromes has not been studied.

Back to Article Outline

Conclusion and Future Directions 

TLR-mediated signals play an important role in the pathophysiology of a number of hepatic diseases. TLR4 and its ligand LPS are involved in alcoholic liver disease and hepatic fibrogenesis. TLR4 and its downstream mediator IRF3 are crucial mediators of injury after hepatic ischemia-reperfusion. The TRIF-TBK1-IRF3 pathway is targeted by HCV to down-regulate immune responses and establish persistent infection. However, there is still a big gap in our knowledge about the role of TLRs in many hepatic diseases. Future studies need to characterize further the role of TLR pathways in animal models and patients with hepatic diseases. Research efforts should be focused on (1) the role of TLRs in hepatic autoimmune disease, (2) the involvement of TLR4 in nonalcoholic fatty liver disease, (3) the HCV-induced down-regulation of TLR pathways, (4) the role of TLRs in HBV infection, (5) the role of TLRs in hepatic wound healing and fibrogenesis, (6) the negative regulation of TLR signaling in the liver, and (7) the potential contribution of TLR polymorphisms to hepatic diseases. Furthermore, an attempt should be made to characterize further the “good” TLR-induced signals, ie, as enhanced immune response, protection from apoptosis and proliferation vs the “bad” signals such as hepatotoxicity and chronic inflammation. Using mouse models with cell-type-specific gene inactivation and conditional knock-out or knock-in animal models will allow us to pinpoint further the cell populations that are involved in these processes and define the benefits of long-term vs short-term inhibition of TLR pathways. Better understanding of the TLR-signaling pathways in the liver will help to shape new concepts with TLRs and their downstream signaling mediators as pharmacologic targets in liver disease.

Back to Article Outline

References 

  1. Aderem A , Ulevitch RJ . Toll-like receptors in the induction of the innate immune response . Nature . 2000;406:782–787
  2. Janeway CA , Medzhitov R . Innate immune recognition . Annu Rev Immunol . 2002;20:197–216
  3. Lemaitre B , Nicolas E , Michaut L , Reichhart JM , Hoffmann JA . The dorsoventral regulatory gene cassette spatzle/Toll/cactus controls the potent antifungal response in Drosophila adults . Cell . 1996;86:973–983
  4. Roach JC, Glusman G, Rowen L, Kaur A, Purcell MK, Smith KD, et al. The evolution of vertebrate Toll-like receptors . Proc Natl Acad Sci U S A . 2005;102:9577–9582
  5. Schnare M , Barton GM , Holt AC , Takeda K , Akira S , Medzhitov R . Toll-like receptors control activation of adaptive immune responses . Nat Immunol . 2001;2:947–950
  6. Mathison JC , Ulevitch RJ . The clearance, tissue distribution, and cellular localization of intravenously injected lipopolysaccharide in rabbits . J Immunol . 1979;123:2133–2143
  7. Ruiter DJ, van der Meulen J, Brouwer A, Hummel MJ, Mauw BJ, van der Ploeg JC, et al. Uptake by liver cells of endotoxin following its intravenous injection . Lab Invest . 1981;45:38–45
  8. Benacerraf B , Sebestyen MM , Schlossman S . A quantitative study of the kinetics of blood clearance of P32-labelled Escherichia coli and Staphylococci by the reticuloendothelial system . J Exp Med . 1959;110:27–48
  9. Fox ES , Thomas P , Broitman SA . Clearance of gut-derived endotoxins by the liver. Release and modification of 3H, 14C-lipopolysaccharide by isolated rat Kupffer cells . Gastroenterology . 1989;96:456–461
  10. Gregory SH , Wing EJ . Neutrophil-Kupffer-cell interaction in host defenses to systemic infections . Immunol Today . 1998;19:507–510
  11. van Egmond M, van Garderen E, van Spriel AB, Damen CA, van Amersfoort ES, van Zandbergen G, et al. FcαRI-positive liver Kupffer cells (reappraisal of the function of immunoglobulin A in immunity) . Nat Med . 2000;6:680–685
  12. Mehal WZ , Azzaroli F , Crispe IN . Immunology of the healthy liver (old questions and new insights) . Gastroenterology . 2001;120:250–260
  13. Crispe IN . Hepatic T cells and liver tolerance . Nat Rev Immunol . 2003;3:51–62
  14. De Creus A , Abe M , Lau AH , Hackstein H , Raimondi G , Thomson AW . Low TLR4 expression by liver dendritic cells correlates with reduced capacity to activate allogeneic T cells in response to endotoxin . J Immunol . 2005;174:2037–2045
  15. Lichtman SN , Wang J , Lemasters JJ . LPS receptor CD14 participates in release of TNF-α in RAW 264.7 and peritoneal cells but not in Kupffer cells . Am J Physiol . 1998;275:G39–G46
  16. Zarember KA , Godowski PJ . Tissue expression of human Toll-like receptors and differential regulation of Toll-like receptor mRNAs in leukocytes in response to microbes, their products, and cytokines . J Immunol . 2002;168:554–561
  17. Gao JJ, Diesl V, Wittmann T, Morrison DC, Ryan JL, Vogel SN, et al. Regulation of gene expression in mouse macrophages stimulated with bacterial CpG-DNA and lipopolysaccharide . J Leukoc Biol . 2002;72:1234–1245
  18. Wells CA, Ravasi T, Faulkner GJ, Carninci P, Okazaki Y, Hayashizaki Y, et al. Genetic control of the innate immune response . BMC Immunol . 2003;4:5
  19. Haziot A, Ferrero E, Kontgen F, Hijiya N, Yamamoto S, Silver J, et al. Resistance to endotoxin shock and reduced dissemination of gram-negative bacteria in CD14-deficient mice . Immunity . 1996;4:407–414
  20. Jiang Z, Georgel P, Du X, Shamel L, Sovath S, Mudd S, et al. CD14 is required for MyD88-independent LPS signaling . Nat Immunol . 2005;6:565–570
  21. Nagai Y, Akashi S, Nagafuku M, Ogata M, Iwakura Y, Akira S, et al. Essential role of MD-2 in LPS responsiveness and TLR4 distribution . Nat Immunol . 2002;3:667–672
  22. Visintin A , Mazzoni A , Spitzer JA , Segal DM . Secreted MD-2 is a large polymeric protein that efficiently confers lipopolysaccharide sensitivity to Toll-like receptor 4 . Proc Natl Acad Sci U S A . 2001;98:12156–12161
  23. Ohnishi T , Muroi M , Tanamoto K . MD-2 is necessary for the toll-like receptor 4 protein to undergo glycosylation essential for its translocation to the cell surface . Clin Diagn Lab Immunol . 2003;10:405–410
  24. Schromm AB, Lien E, Henneke P, Chow JC, Yoshimura A, Heine H, et al. Molecular genetic analysis of an endotoxin nonresponder mutant cell line (a point mutation in a conserved region of MD-2 abolishes endotoxin-induced signaling) . J Exp Med . 2001;194:79–88
  25. da Silva Correia J , Ulevitch RJ . MD-2 and TLR4 N-linked glycosylations are important for a functional lipopolysaccharide receptor . J Biol Chem . 2002;277:1845–1854
  26. Schumann RR, Leong SR, Flaggs GW, Gray PW, Wright SD, Mathison JC, et al. Structure and function of lipopolysaccharide binding protein . Science . 1990;249:1429–1431
  27. Hoebe K, Georgel P, Rutschmann S, Du X, Mudd S, Crozat K, et al. CD36 is a sensor of diacylglycerides . Nature . 2005;433:523–527
  28. Michel T , Reichhart JM , Hoffmann JA , Royet J . Drosophila Toll is activated by gram-positive bacteria through a circulating peptidoglycan recognition protein . Nature . 2001;414:756–759
  29. Tanji T , Ip YT . Regulators of the Toll and Imd pathways in the Drosophila innate immune response . Trends Immunol . 2005;26:193–198
  30. Kawai T , Adachi O , Ogawa T , Takeda K , Akira S . Unresponsiveness of MyD88-deficient mice to endotoxin . Immunity . 1999;11:115–122
  31. Horng T , Barton GM , Flavell RA , Medzhitov R . The adaptor molecule TIRAP provides signalling specificity for Toll-like receptors . Nature . 2002;420:329–333
  32. Yamamoto M, Sato S, Hemmi H, Sanjo H, Uematsu S, Kaisho T, et al. Essential role for TIRAP in activation of the signalling cascade shared by TLR2 and TLR4 . Nature . 2002;420:324–329
  33. Hoebe K, Du X, Georgel P, Janssen E, Tabeta K, Kim SO, et al. Identification of Lps2 as a key transducer of MyD88-independent TIR signalling . Nature . 2003;424:743–748
  34. Yamamoto M, Sato S, Hemmi H, Hoshino K, Kaisho T, Sanjo H, et al. Role of adaptor TRIF in the MyD88-independent toll-like receptor signaling pathway . Science . 2003;301:640–643
  35. Yamamoto M, Sato S, Hemmi H, Uematsu S, Hoshino K, Kaisho T, et al. TRAM is specifically involved in the Toll-like receptor 4-mediated MyD88-independent signaling pathway . Nat Immunol . 2003;4:1144–1150
  36. Suzuki N, Suzuki S, Duncan GS, Millar DG, Wada T, Mirtsos C, et al. Severe impairment of interleukin-1 and Toll-like receptor signalling in mice lacking IRAK-4 . Nature . 2002;416:750–756
  37. Swantek JL , Tsen MF , Cobb MH , Thomas JA . IL-1 receptor-associated kinase modulates host responsiveness to endotoxin . J Immunol . 2000;164:4301–4306
  38. Sato S, Sanjo H, Takeda K, Ninomiya-Tsuji J, Yamamoto M, Kawai T, et al. Essential function for the kinase TAK1 in innate and adaptive immune responses . Nat Immunol . 2005;6:1087–1095
  39. Takaoka A, Yanai H, Kondo S, Duncan G, Negishi H, Mizutani T, et al. Integral role of IRF-5 in the gene induction programme activated by Toll-like receptors . Nature . 2005;434:243–249
  40. Matsuzawa A, Saegusa K, Noguchi T, Sadamitsu C, Nishitoh H, Nagai S, et al. ROS-dependent activation of the TRAF6-ASK1-p38 pathway is selectively required for TLR4-mediated innate immunity . Nat Immunol . 2005;6:587–592
  41. Covert MW , Leung TH , Gaston JE , Baltimore D . Achieving stability of lipopolysaccharide-induced NF-κB activation . Science . 2005;309:1854–1857
  42. Werner SL , Barken D , Hoffmann A . Stimulus specificity of gene expression programs determined by temporal control of IKK activity . Science . 2005;309:1857–1861
  43. Burns K , Janssens S , Brissoni B , Olivos N , Beyaert R , Tschopp J . Inhibition of interleukin 1 receptor/Toll-like receptor signaling through the alternatively spliced, short form of MyD88 is due to its failure to recruit IRAK-4 . J Exp Med . 2003;197:263–268
  44. Uematsu S, Sato S, Yamamoto M, Hirotani T, Kato H, Takeshita F, et al. Interleukin-1 receptor-associated kinase-1 plays an essential role for Toll-like receptor (TLR)7- and TLR9-mediated interferon (α) induction . J Exp Med . 2005;201:915–923
  45. Honda K, Yanai H, Negishi H, Asagiri M, Sato M, Mizutani T, et al. IRF-7 is the master regulator of type-I interferon-dependent immune responses . Nature . 2005;434:772–777
  46. Fitzgerald KA, McWhirter SM, Faia KL, Rowe DC, Latz E, Golenbock DT, et al. IKKϵ and TBK1 are essential components of the IRF3 signaling pathway . Nat Immunol . 2003;4:491–496
  47. McWhirter SM , Fitzgerald KA , Rosains J , Rowe DC , Golenbock DT , Maniatis T . IFN-regulatory factor 3-dependent gene expression is defective in Tbk1-deficient mouse embryonic fibroblasts . Proc Natl Acad Sci U S A . 2004;101:233–238
  48. Shimada T, Kawai T, Takeda K, Matsumoto M, Inoue J, Tatsumi Y, et al. IKK-i, a novel lipopolysaccharide-inducible kinase that is related to IκB kinases . Int Immunol . 1999;11:1357–1362
  49. Hemmi H, Takeuchi O, Sato S, Yamamoto M, Kaisho T, Sanjo H, et al. The roles of two IκB kinase-related kinases in lipopolysaccharide and double stranded RNA signaling and viral infection . J Exp Med . 2004;199:1641–1650
  50. Meylan E, Burns K, Hofmann K, Blancheteau V, Martinon F, Kelliher M, et al. RIP1 is an essential mediator of Toll-like receptor 3-induced NF-κB activation . Nat Immunol . 2004;5:503–507
  51. Sato S, Sugiyama M, Yamamoto M, Watanabe Y, Kawai T, Takeda K, et al. Toll/IL-1 receptor domain-containing adaptor inducing IFN-β (TRIF) associates with TNF receptor-associated factor 6 and TANK-binding kinase 1, and activates two distinct transcription factors, NF-κB and IFN-regulatory factor-3, in the Toll-like receptor signaling . J Immunol . 2003;171:4304–4310
  52. Jiang Z , Zamanian-Daryoush M , Nie H , Silva AM , Williams BR , Li X . Poly(I-C)-induced Toll-like receptor 3 (TLR3)-mediated activation of NFκB and MAP kinase is through an interleukin-1 receptor-associated kinase (IRAK)-independent pathway employing the signaling components TLR3-TRAF6-TAK1-TAB2-PKR . J Biol Chem . 2003;278:16713–16719
  53. Gohda J , Matsumura T , Inoue J . Cutting edge (TNFR-associated factor (TRAF) 6 is essential for MyD88-dependent pathway but not toll/IL-1 receptor domain-containing adaptor-inducing IFN-β (TRIF)-dependent pathway in TLR signaling) . J Immunol . 2004;173:2913–2917
  54. Liew FY , Xu D , Brint EK , O’Neill LA . Negative regulation of toll-like receptor-mediated immune responses . Nat Rev Immunol . 2005;5:446–458
  55. Nomura F, Akashi S, Sakao Y, Sato S, Kawai T, Matsumoto M, et al. Cutting edge (endotoxin tolerance in mouse peritoneal macrophages correlates with down-regulation of surface toll-like receptor 4 expression) . J Immunol . 2000;164:3476–3479
  56. Akashi S, Shimazu R, Ogata H, Nagai Y, Takeda K, Kimoto M, et al. Cutting edge (cell surface expression and lipopolysaccharide signaling via the toll-like receptor 4-MD-2 complex on mouse peritoneal macrophages) . J Immunol . 2000;164:3471–3475
  57. Chuang TH , Ulevitch RJ . Triad3A, an E3 ubiquitin-protein ligase regulating Toll-like receptors . Nat Immunol . 2004;5:495–502
  58. Brint EK, Xu D, Liu H, Dunne A, McKenzie AN, O’Neill LA, et al. ST2 is an inhibitor of interleukin 1 receptor and Toll-like receptor 4 signaling and maintains endotoxin tolerance . Nat Immunol . 2004;5:373–379
  59. Wald D, Qin J, Zhao Z, Qian Y, Naramura M, Tian L, et al. SIGIRR, a negative regulator of Toll-like receptor-interleukin 1 receptor signaling . Nat Immunol . 2003;4:920–927
  60. Janssens S , Burns K , Vercammen E , Tschopp J , Beyaert R . MyD88S, a splice variant of MyD88, differentially modulates NF-κB- and AP-1-dependent gene expression . FEBS Lett . 2003;548:103–107
  61. Kobayashi K , Hernandez LD , Galan JE , Janeway CA , Medzhitov R , Flavell RA . IRAK-M is a negative regulator of Toll-like receptor signaling . Cell . 2002;110:191–202
  62. Hardy MP , O’Neill LA . The murine IRAK2 gene encodes four alternatively spliced isoforms, two of which are inhibitory . J Biol Chem . 2004;279:27699–27708
  63. Nakagawa R, Naka T, Tsutsui H, Fujimoto M, Kimura A, Abe T, et al. SOCS-1 participates in negative regulation of LPS responses . Immunity . 2002;17:677–687
  64. Dalpke AH , Opper S , Zimmermann S , Heeg K . Suppressors of cytokine signaling (SOCS)-1 and SOCS-3 are induced by CpG-DNA and modulate cytokine responses in APCs . J Immunol . 2001;166:7082–7089
  65. Kinjyo I, Hanada T, Inagaki-Ohara K, Mori H, Aki D, Ohishi M, et al. SOCS1/JAB is a negative regulator of LPS-induced macrophage activation . Immunity . 2002;17:583–591
  66. Su GL, Klein RD, Aminlari A, Zhang HY, Steinstraesser L, Alarcon WH, et al. Kupffer cell activation by lipopolysaccharide in rats (role for lipopolysaccharide binding protein and toll-like receptor 4) . Hepatology . 2000;31:932–936
  67. Van Bossuyt H , De Zanger RB , Wisse E . Cellular and subcellular distribution of injected lipopolysaccharide in rat liver and its inactivation by bile salts . J Hepatol . 1988;7:325–337
  68. Seki E, Tsutsui H, Nakano H, Tsuji N, Hoshino K, Adachi O, et al. Lipopolysaccharide-induced IL-18 secretion from murine Kupffer cells independently of myeloid differentiation factor 88 that is critically involved in induction of production of IL-12 and IL-1β . J Immunol . 2001;166:2651–2657
  69. Kopydlowski KM, Salkowski CA, Cody MJ, van Rooijen N, Major J, Hamilton TA, et al. Regulation of macrophage chemokine expression by lipopolysaccharide in vitro and in vivo . J Immunol . 1999;163:1537–1544
  70. Tsutsui H , Matsui K , Okamura H , Nakanishi K . Pathophysiological roles of interleukin-18 in inflammatory liver diseases . Immunol Rev . 2000;174:192–209
  71. Seki E, Tsutsui H, Tsuji NM, Hayashi N, Adachi K, Nakano H, et al. Critical roles of myeloid differentiation factor 88-dependent proinflammatory cytokine release in early phase clearance of Listeria monocytogenes in mice . J Immunol . 2002;169:3863–3868
  72. Knolle P , Schlaak J , Uhrig A , Kempf P , Meyer zum Buschenfelde KH , Gerken G . Human Kupffer cells secrete IL-10 in response to lipopolysaccharide (LPS) challenge . J Hepatol . 1995;22:226–229
  73. Schuchmann M , Hermann F , Herkel J , van der Zee R , Galle PR , Lohse AW . HSP60 and CpG-DNA-oligonucleotides differentially regulate LPS-tolerance of hepatic Kupffer cells . Immunol Lett . 2004;93:199–204
  74. Matsumura T, Degawa T, Takii T, Hayashi H, Okamoto T, Inoue J, et al. TRAF6-NF-κB pathway is essential for interleukin-1-induced TLR2 expression and its functional response to TLR2 ligand in murine hepatocytes . Immunology . 2003;109:127–136
  75. Matsumura T , Ito A , Takii T , Hayashi H , Onozaki K . Endotoxin and cytokine regulation of toll-like receptor (TLR) 2 and TLR4 gene expression in murine liver and hepatocytes . J Interferon Cytokine Res . 2000;20:915–921
  76. Li K , Chen Z , Kato N , Gale M , Lemon SM . Distinct poly-I (C and virus-activated signaling pathways leading to interferon-β production in hepatocytes) . J Biol Chem . 2005;280:16739–16747
  77. Mimura Y , Sakisaka S , Harada M , Sata M , Tanikawa K . Role of hepatocytes in direct clearance of lipopolysaccharide in rats . Gastroenterology . 1995;109:1969–1976
  78. Hampton RY , Golenbock DT , Penman M , Krieger M , Raetz CR . Recognition and plasma clearance of endotoxin by scavenger receptors . Nature . 1991;352:342–344
  79. Vishnyakova TG, Bocharov AV, Baranova IN, Chen Z, Remaley AT, Csako G, et al. Binding and internalization of lipopolysaccharide by Cla-1, a human orthologue of rodent scavenger receptor B1 . J Biol Chem . 2003;278:22771–22780
  80. Bataller R , Brenner DA . Liver fibrosis . J Clin Invest . 2005;115:209–218
  81. Paik YH , Schwabe RF , Bataller R , Russo MP , Jobin C , Brenner DA . Toll-like receptor 4 mediates inflammatory signaling by bacterial lipopolysaccharide in human hepatic stellate cells . Hepatology . 2003;37:1043–1055
  82. Brun P , Castagliuolo I , Pinzani M , Palu G , Martines D . Exposure to bacterial cell wall products triggers an inflammatory phenotype in hepatic stellate cells . Am J Physiol Gastrointest Liver Physiol . 2005;289:G571–G578
  83. Thirunavukkarasu C , Uemura T , Wang LF , Watkins SC , Gandhi CR . Normal rat hepatic stellate cells respond to endotoxin in LBP-independent manner to produce inhibitor(s) of DNA synthesis in hepatocytes . J Cell Physiol . 2005;204:654–665
  84. Harada K, Ohira S, Isse K, Ozaki S, Zen Y, Sato Y, et al. Lipopolysaccharide activates nuclear factor-κB through toll-like receptors and related molecules in cultured biliary epithelial cells . Lab Invest . 2003;83:1657–1667
  85. Chen XM, O’Hara SP, Nelson JB, Splinter PL, Small AJ, Tietz PS, et al. Multiple TLRs are expressed in human cholangiocytes and mediate host epithelial defense responses to Cryptosporidium parvum via activation of NF-κB . J Immunol . 2005;175:7447–7456
  86. Uhrig A, Banafsche R, Kremer M, Hegenbarth S, Hamann A, Neurath M, et al. Development and functional consequences of LPS tolerance in sinusoidal endothelial cells of the liver . J Leukoc Biol . 2005;77:626–633
  87. Bjarnason I , Peters TJ , Wise RJ . The leaky gut of alcoholism (possible route of entry for toxic compounds) . Lancet . 1984;1:179–182
  88. Draper LR , Gyure LA , Hall JG , Robertson D . Effect of alcohol on the integrity of the intestinal epithelium . Gut . 1983;24:399–404
  89. Parlesak A , Schafer C , Schutz T , Bode JC , Bode C . Increased intestinal permeability to macromolecules and endotoxemia in patients with chronic alcohol abuse in different stages of alcohol-induced liver disease . J Hepatol . 2000;32:742–747
  90. Nanji AA , Khettry U , Sadrzadeh SM . Lactobacillus feeding reduces endotoxemia and severity of experimental alcoholic liver (disease) . Proc Soc Exp Biol Med . 1994;205:243–247
  91. Adachi Y , Moore LE , Bradford BU , Gao W , Thurman RG . Antibiotics prevent liver injury in rats following long-term exposure to ethanol . Gastroenterology . 1995;108:218–224
  92. Adachi Y , Bradford BU , Gao W , Bojes HK , Thurman RG . Inactivation of Kupffer cells prevents early alcohol-induced liver injury . Hepatology . 1994;20:453–460
  93. Hansen J , Cherwitz DL , Allen JI . The role of tumor necrosis factor-α in acute endotoxin-induced hepatotoxicity in ethanol-fed rats . Hepatology . 1994;20:461–474
  94. Uesugi T , Froh M , Arteel GE , Bradford BU , Thurman RG . Toll-like receptor 4 is involved in the mechanism of early alcohol-induced liver injury in mice . Hepatology . 2001;34:101–108
  95. Kono H, Rusyn I, Yin M, Gabele E, Yamashina S, Dikalova A, et al. NADPH oxidase-derived free radicals are key oxidants in alcohol-induced liver disease . J Clin Invest . 2000;106:867–872
  96. Yang SQ , Lin HZ , Lane MD , Clemens M , Diehl AM . Obesity increases sensitivity to endotoxin liver injury (implications for the pathogenesis of steatohepatitis) . Proc Natl Acad Sci U S A . 1997;94:2557–2562
  97. Solga SF , Diehl AM . Non-alcoholic fatty liver disease (lumen-liver interactions and possible role for probiotics) . J Hepatol . 2003;38:681–687
  98. Li Z, Yang S, Lin H, Huang J, Watkins PA, Moser AB, et al. Probiotics and antibodies to TNF inhibit inflammatory activity and improve nonalcoholic fatty liver disease . Hepatology . 2003;37:343–350
  99. Szabo G , Velayudham A , Romics L , Mandrekar P . Modulation of non-alcoholic steatohepatitis by pattern recognition receptors in mice (the role of toll-like receptors 2 and 4) . Alcohol Clin Exp Res . 2005;29:S140–S145
  100. Igolnikov AC , Spatz K , Green RM . C3H/HeJ mice with mutations of the toll-like receptor 4 (TLR-4) are resistant to methionine-choline deficient (MCD) diet induced non-alcoholic steatohepatitis (NASH) . Hepatology . 2002;36:A965
  101. Wiest R , Garcia-Tsao G . Bacterial translocation (BT) in cirrhosis . Hepatology . 2005;41:422–433
  102. Chesta J , Defilippi C , Defilippi C . Abnormalities in proximal small bowel motility in patients with cirrhosis . Hepatology . 1993;17:828–832
  103. Guarner C , Runyon BA , Young S , Heck M , Sheikh MY . Intestinal bacterial overgrowth and bacterial translocation in cirrhotic rats with ascites . J Hepatol . 1997;26:1372–1378
  104. Ramachandran A , Prabhu R , Thomas S , Reddy JB , Pulimood A , Balasubramanian KA . Intestinal mucosal alterations in experimental cirrhosis in the rat (role of oxygen free radicals) . Hepatology . 2002;35:622–629
  105. Rajkovic IA , Williams R . Abnormalities of neutrophil phagocytosis, intracellular killing and metabolic activity in alcoholic cirrhosis and hepatitis . Hepatology . 1986;6:252–262
  106. Garcia-Tsao G , Lee FY , Barden GE , Cartun R , West AB . Bacterial translocation to mesenteric lymph nodes is increased in cirrhotic rats with ascites . Gastroenterology . 1995;108:1835–1841
  107. Nolan JP , Leibowitz AI . Endotoxin and the liver. III. Modification of acute carbon tetrachloride injury by polymyxin b—an antiendotoxin . Gastroenterology . 1978;75:445–449
  108. Grinko I , Geerts A , Wisse E . Experimental biliary fibrosis correlates with increased numbers of fat-storing and Kupffer cells, and portal endotoxemia . J Hepatol . 1995;23:449–458
  109. Chan CC, Hwang SJ, Lee FY, Wang SS, Chang FY, Li CP, et al. Prognostic value of plasma endotoxin levels in patients with cirrhosis . Scand J Gastroenterol . 1997;32:942–946
  110. Fukui H , Brauner B , Bode JC , Bode C . Plasma endotoxin concentrations in patients with alcoholic and non-alcoholic liver disease (reevaluation with an improved chromogenic assay) . J Hepatol . 1991;12:162–169
  111. Lin RS, Lee FY, Lee SD, Tsai YT, Lin HC, Lu RH, et al. Endotoxemia in patients with chronic liver diseases (relationship to severity of liver diseases, presence of esophageal varices, and hyperdynamic circulation) . J Hepatol . 1995;22:165–172
  112. Luckey TD , Reyniers JA , Gyorgy P , Forbes M . Germfree animals and liver necrosis . Ann N Y Acad Sci . 1954;57:932–935
  113. Rutenburg AM , Sonnenblick E , Koven I , Aprahamian HA , Reiner L , Fine J . The role of intestinal bacteria in the development of dietary cirrhosis in rats . J Exp Med . 1957;106:1–14
  114. Seki E , Osawa Y , Uchinami H , Brenner DA , Schwabe RF . TLR4 mediates inflammation and fibrogenesis after bile duct ligation . Hepatology . 2005;42:A265
  115. Dolganiuc A, Oak S, Kodys K, Golenbock DT, Finberg RW, Kurt-Jones E, et al. Hepatitis C core and nonstructural 3 proteins trigger toll-like receptor 2-mediated pathways and inflammatory activation . Gastroenterology . 2004;127:1513–1524
  116. Bataller R , Paik YH , Lindquist JN , Lemasters JJ , Brenner DA . Hepatitis C virus core and nonstructural proteins induce fibrogenic effects in hepatic stellate cells . Gastroenterology . 2004;126:529–540
  117. Wieland S , Thimme R , Purcell RH , Chisari FV . Genomic analysis of the host response to hepatitis B virus infection . Proc Natl Acad Sci U S A . 2004;101:6669–6674
  118. McClary H , Koch R , Chisari FV , Guidotti LG . Relative sensitivity of hepatitis B virus and other hepatotropic viruses to the antiviral effects of cytokines . J Virol . 2000;74:2255–2264
  119. Isogawa M , Robek MD , Furuichi Y , Chisari FV . Toll-like receptor signaling inhibits hepatitis B virus replication in vivo . J Virol . 2005;79:7269–7272
  120. Cooper CL, Davis HL, Morris ML, Efler SM, Adhami MA, Krieg AM, et al. CPG 7909, an immunostimulatory TLR9 agonist oligodeoxynucleotide, as adjuvant to Engerix-B HBV vaccine in healthy adults (a double-blind phase I/II study) . J Clin Immunol . 2004;24:693–701
  121. Rehermann B , Nascimbeni M . Immunology of hepatitis B virus and hepatitis C virus infection . Nat Rev Immunol . 2005;5:215–229
  122. Li K, Foy E, Ferreon JC, Nakamura M, Ferreon AC, Ikeda M, et al. Immune evasion by hepatitis C virus NS3/4A protease-mediated cleavage of the Toll-like receptor 3 adaptor protein TRIF . Proc Natl Acad Sci U S A . 2005;102:2992–2997
  123. Otsuka M, Kato N, Moriyama M, Taniguchi H, Wang Y, Dharel N, et al. Interaction between the HCV NS3 protein and the host TBK1 protein leads to inhibition of cellular antiviral responses . Hepatology . 2005;41:1004–1012
  124. Machida K , Cheng KT , Sung VM , Levine AM , Foung S , Lai MM . Hepatitis C virus induces toll-like receptor 4 expression, leading to enhanced production of Beta interferon and interleukin-6 . J Virol . 2006;80:866–874
  125. Horsmans Y, Berg T, Desager JP, Mueller T, Schott E, Fletcher SP, et al. Isatoribine, an agonist of TLR7, reduces plasma virus concentration in chronic hepatitis C infection . Hepatology . 2005;42:724–731
  126. Zhai Y, Shen XD, O’Connell R, Gao F, Lassman C, Busuttil RW, et al. Cutting edge (TLR4 activation mediates liver ischemia/reperfusion inflammatory response via IFN regulatory factor 3-dependent MyD88-independent pathway) . J Immunol . 2004;173:7115–7119
  127. Tsung A, Sahai R, Tanaka H, Nakao A, Fink MP, Lotze MT, et al. The nuclear factor HMGB1 mediates hepatic injury after murine liver ischemia-reperfusion . J Exp Med . 2005;201:1135–1143
  128. Taub R . Liver regeneration (from myth to mechanism) . Nat Rev Mol Cell Biol . 2004;5:836–847
  129. Higgins GM , Anderson RM . Experimental pathology of the liver. I. Restoration of the liver of the white rat following partial surgical removal . Arch Pathol . 1931;12:186–202
  130. Cornell RP , Liljequist BL , Bartizal KF . Depressed liver regeneration after partial hepatectomy of germ-free, athymic and lipopolysaccharide-resistant mice . Hepatology . 1990;11:916–922
  131. Su GL , Wang SC , Aminlari A , Tipoe GL , Steinstraesser L , Nanji A . Impaired hepatocyte regeneration in toll-like receptor 4 mutant mice . Dig Dis Sci . 2004;49:843–849
  132. Seki E, Tsutsui H, Iimuro Y, Naka T, Son G, Akira S, et al. Contribution of Toll-like receptor/myeloid differentiation factor 88 signaling to murine liver regeneration . Hepatology . 2005;41:443–450
  133. Sun R , Gao B . Negative regulation of liver regeneration by innate immunity (natural killer cells/interferon-gamma) . Gastroenterology . 2004;127:1525–1539
  134. Shimizu Y , Margenthaler JA , Landeros K , Otomo N , Doherty G , Flye MW . The resistance of P. acnes—primed interferon γ-deficient mice to low-dose lipopolysaccharide-induced acute liver injury . Hepatology . 2002;35:805–814
  135. Galanos C , Freudenberg MA , Reutter W . Galactosamine-induced sensitization to the lethal effects of endotoxin . Proc Natl Acad Sci U S A . 1979;76:5939–5943
  136. Jiang W , Sun R , Wei H , Tian Z . Toll-like receptor 3 ligand attenuates LPS-induced liver injury by down-regulation of toll-like receptor 4 expression on macrophages . Proc Natl Acad Sci U S A . 2005;102:17077–17082
  137. Tsuji H, Mukaida N, Harada A, Kaneko S, Matsushita E, Nakanuma Y, et al. Alleviation of lipopolysaccharide-induced acute liver injury in Propionibacterium acnes-primed IFN-γ-deficient mice by a concomitant reduction of TNF-α, IL-12, and IL-18 production . J Immunol . 1999;162:1049–1055
  138. Tsutsui H, Matsui K, Kawada N, Hyodo Y, Hayashi N, Okamura H, et al. IL-18 accounts for both TNF-α- and Fas ligand-mediated hepatotoxic pathways in endotoxin-induced liver injury in mice . J Immunol . 1997;159:3961–3967
  139. Kalis C, Gumenscheimer M, Freudenberg N, Tchaptchet S, Fejer G, Heit A, et al. Requirement for TLR9 in the immunomodulatory activity of Propionibacterium acnes . J Immunol . 2005;174:4295–4300
  140. Romics L, Dolganiuc A, Velayudham A, Kodys K, Mandrekar P, Golenbock D, et al. Toll-like receptor 2 mediates inflammatory cytokine induction but not sensitization for liver injury by Propionibacterium acnes . J Leukoc Biol . 2005;78:1255–1264
  141. Romics L, Dolganiuc A, Kodys K, Drechsler Y, Oak S, Velayudham A, et al. Selective priming to Toll-like receptor 4 (TLR4), not TLR2, ligands by P. acnes involves up-regulation of MD-2 in mice . Hepatology . 2004;40:555–564
  142. Edelson BT , Unanue ER . MyD88-dependent but Toll-like receptor 2-independent innate immunity to Listeria (no role for either in macrophage listericidal activity) . J Immunol . 2002;169:3869–3875
  143. O’Brien AD , Rosenstreich DL , Scher I , Campbell GH , MacDermott RP , Formal SB . Genetic control of susceptibility to Salmonella typhimurium in mice (role of the LPS gene) . J Immunol . 1980;124:20–24
  144. Vazquez-Torres A, Vallance BA, Bergman MA, Finlay BB, Cookson BT, Jones-Carson J, et al. Toll-like receptor 4 dependence of innate and adaptive immunity to Salmonella (importance of the Kupffer cell network) . J Immunol . 2004;172:6202–6208
  145. Totemeyer S , Foster N , Kaiser P , Maskell DJ , Bryant CE . Toll-like receptor expression in C3H/HeN and C3H/HeJ mice during Salmonella enterica serovar Typhimurium infection . Infect Immun . 2003;71:6653–6657
  146. Shimizu H, Matsuguchi T, Fukuda Y, Nakano I, Hayakawa T, Takeuchi O, et al. Toll-like receptor 2 contributes to liver injury by Salmonella infection through Fas ligand expression on NKT cells in mice . Gastroenterology . 2002;123:1265–1277
  147. Adachi K, Tsutsui H, Kashiwamura S, Seki E, Nakano H, Takeuchi O, et al. Plasmodium berghei infection in mice induces liver injury by an IL-12- and toll-like receptor/myeloid differentiation factor 88-dependent mechanism . J Immunol . 2001;167:5928–5934
  148. Bach JF . A Toll-like trigger for autoimmune disease . Nat Med . 2005;11:120–121
  149. Kerfoot SM, Long EM, Hickey MJ, Andonegui G, Lapointe BM, Zanardo RC, et al. TLR4 contributes to disease-inducing mechanisms resulting in central nervous system autoimmune disease . J Immunol . 2004;173:7070–7077
  150. Lang KS, Recher M, Junt T, Navarini AA, Harris NL, Freigang S, et al. Toll-like receptor engagement converts T-cell autoreactivity into overt autoimmune disease . Nat Med . 2005;11:138–145
  151. Patole PS, Grone HJ, Segerer S, Ciubar R, Belemezova E, Henger A, et al. Viral double-stranded RNA aggravates Lupus Nephritis through Toll-like receptor 3 on glomerular mesangial cells and antigen-presenting cells . J Am Soc Nephrol . 2005;16:1326–1338
  152. Takii Y, Nakamura M, Ito M, Yokoyama T, Komori A, Shimizu-Yoshida Y, et al. Enhanced expression of type I interferon and toll-like receptor-3 in primary biliary cirrhosis . Lab Invest . 2005;85:908–920
  153. Wang AP, Migita K, Ito M, Takii Y, Daikoku M, Yokoyama T, et al. Hepatic expression of toll-like receptor 4 in primary biliary cirrhosis . J Autoimmun . 2005;25:85–91
  154. Mao TK, Lian ZX, Selmi C, Ichiki Y, Ashwood P, Ansari AA, et al. Altered monocyte responses to defined TLR ligands in patients with primary biliary cirrhosis . Hepatology . 2005;42:802–808
  155. Kikuchi K, Lian ZX, Yang GX, Ansari AA, Ikehara S, Kaplan M, et al. Bacterial CpG induces hyper-IgM production in CD27(+) memory B cells in primary biliary cirrhosis . Gastroenterology . 2005;128:304–312
  156. Kikuchi K, Lian ZX, Kimura Y, Selmi C, Yang GX, Gordon SC, et al. Genetic polymorphisms of toll-like receptor 9 influence the immune response to CpG and contribute to hyper-IgM in primary biliary cirrhosis . J Autoimmun . 2005;24:347–352

 Supported by a Research Scholar Award from the American Gastroenterological Association and Procter & Gamble (to R.F.S.).

PII: S0016-5085(06)00065-5

doi:10.1053/j.gastro.2006.01.038

Gastroenterology
Volume 130, Issue 6 , Pages 1886-1900, May 2006