Toll-like receptor 3 and 7/8 function is impaired in hepatitis C rapid fibrosis progression post-liver transplantation

TLR8 agonist partially improves IFN-γ deficiency of NK cells in chronic hepatitis B through the synergy of monocytes

BACKGROUND

Natural killer (NK) cells exhibit a selective deficiency of IFN-γ production in chronic hepatitis B (CHB). Toll-like receptor 8 (TLR8) agonists could induce IFN-γ production in immune cells, although their effects on the deficiency in NK cells remain unclear.

AIMS

To investigate TLR8 expression in NK cells and the effect of TLR8 agonists in patients with CHB METHODS: We enrolled 32 patients with CHB and 19 healthy controls to assess TLR8 expression and IFN-γ production in NK cells. The sorted NK cells and monocytes were co-cultured to compare the extent of IFN-γ and IL-10 production after TLR8 agonist ssRNA40 stimulation. The synergic effect of NK cells and monocytes was assessed by blocking IL-12 and IL-18. We recruited another 22 patients with CHB undergoing nucleotide analogue (NA) therapy to explore the impact of antiviral treatment on the ssRNA40-mediated response of NK cells.

RESULTS

In patients with CHB, TLR8 expression in NK cells was up-regulated, accompanied by insufficient IFN-γ production. The enhanced IFN-γ secretion by ssRNA40 in NK cells depended on monocyte-derived IL-12 and IL-18. NK cells displayed an imbalanced response to ssRNA40 in patients with CHB with a weak increase in IFN-γ despite a higher IL-10 production. The response was improved in patients with CHB undergoing NA therapy.

CONCLUSIONS

In patients with CHB, targeting TLR8 partially rescues the IFN-γ insufficiency in NK cells. However, NK cells show an inhibitory response to TLR8 agonist stimulation. TLR8 agonist combined with NA may enhance the antiviral effect of NK cells.

Revealing immune infiltrate characteristics and potential immune-related genes in hepatic fibrosis: based on bioinformatics, transcriptomics and q-PCR experiments

Background

The occurrence and progression of hepatic fibrosis (HF) is accompanied by inflammatory damage. Immune genes play a pivotal role in fibrogenesis and inflammatory damage in HF by regulating immune cell infiltration. However, the immune mechanisms of HF are inadequately studied. Therefore, this research aims to identify the immune genes and biological pathway which involved in fibrosis formation and inflammatory damage in HF and explore immune target-based therapeutics for HF.

Methods

The expression dataset GSE84044 of HF was downloaded from the GEO database. The crucial module genes for HF were screened according to weighted gene co-expression network analysis (WGCNA). The crucial module genes were mapped to immune-related genes obtained from the ImmPort database to obtain the hepatic fibrosis immune genes (HFIGs). In addition, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) functional enrichment analyses were performed on HFIGs. Then, the protein-protein interaction (PPI) network was conducted on HFIGs and hub genes were identified from the PPI network. Moreover, immune infiltration analysis was performed to identified correlation between hub gene and immune cell infiltration. To verify the reliability of the GSE84044 expression profile data analysis, a rat model of CCl4-induced HF was established, followed by transcriptome sequencing and immunofluorescence analysis and quantitative reverse transcription (q-PCR) experiments were performed in HF rats and normal rat liver tissues. Finally, CMAP platform was used to explore immune target-based therapeutics for HF.

Results

In the bioinformatics analysis of GSE84044 data, 98 HFIGs were screened. These genes were mainly involved in inflammation-related biological pathways such as NOD-like receptor signaling pathway, NF-kappa B signaling pathway, Toll-like receptor signaling pathway and PI3K-Akt signaling pathway. From the PPI network, 10 hub genes were identified, including CXCL8, IL18, CXCL10, CD8A, IL7, PTPRC, CCL5, IL7R, CXCL9 and CCL2. Immune infiltration analysis showed that immune cells like neutrophils, natural killer (NK) cells, macrophages M1 and macrophages M2 were significantly correlated with the hepatic fibrosis process and hub gene expression was significantly correlated with these immune cells. Notably, most of the biological pathways HFIGs riched and all the hub gene expression except CXCL8 were validated in subsequent transcriptome and qRCR experiments. Finally, 15 small molecule compounds with the potential to reverse the high expression of hub genes were screen out as potential therapeutic agents for HF.

Conclusion

The immune genes CXCL8, IL18, CXCL10, CD8A, IL7, PTPRC, CCL5, IL7R, CXCL9 and CCL2 may play an essential role in the fibrosis formation and inflammatory damage in HF. The outcomes of this research provide a basis for the study of the immune mechanisms of HF and contribute to the diagnosis and prevention and treatment of HF in clinical practice.

Toll-like Receptor Response to Hepatitis C Virus Infection: A Recent Overview

Hepatitis C virus (HCV) infection remains a major global health burden, causing chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Toll-like receptors (TLRs) are evolutionarily conserved pattern recognition receptors that detect pathogen-associated molecular patterns and activate downstream signaling to induce proinflammatory cytokine and chemokine production. An increasing number of studies have suggested the importance of TLR responses in the outcome of HCV infection. However, the exact role of innate immune responses, including TLR response, in controlling chronic HCV infection remains to be established. A proper understanding of the TLR response in HCV infection is essential for devising new therapeutic approaches against HCV infection. In this review, we discuss the progress made in our understanding of the host innate immune response to HCV infection, with a particular focus on the TLR response. In addition, we discuss the mechanisms adopted by HCV to avoid immune surveillance mediated by TLRs.

Toll-like receptors and hepatitis C virus infection

BACKGROUND

Hepatitis C virus (HCV) infection is a worldwide issue. However, the current treatment for hepatitis C has many shortcomings. Toll-like receptors (TLRs) are pattern recognition receptors involved in HCV infection, and an increasing number of studies are focusing on the role of TLRs in the progression of hepatitis C.

DATA SOURCES

We performed a PubMed search up to January 2021 with the following keywords: hepatitis C, toll-like receptors, interferons, inflammation, and immune evasion. We also used terms such as single-nucleotide polymorphisms (SNPs), susceptibility, fibrosis, cirrhosis, direct-acting antiviral agents, agonists, and antagonists to supplement the query results. We reviewed relevant publications analyzing the correlation between hepatitis C and TLRs and the role of TLRs in HCV infection.

RESULTS

TLRs 1-4 and 6-9 are involved in the process of HCV infection. When the host is exposed to the HCV, TLRs, as important participants in HCV immune evasion, trigger innate immunity to remove the virus and also promote inflammation and liver fibrosis. TLR gene SNPs affect hepatitis C susceptibility, treatment, and prognosis. The contribution of each TLR to HCV is different. Drugs targeting various TLRs are developed and validated, and TLRs can synergize with classic hepatitis C drugs, including interferon and direct-acting antiviral agents, constituting a new direction for the treatment of hepatitis C.

CONCLUSIONS

TLRs are important receptors in HCV infection. Different TLRs induce different mechanisms of virus clearance and inflammatory response. Although TLR-related antiviral therapy strategies exist, more studies are needed to explore the clinical application of TLR-related drugs.

Intrahepatic TLR3 and IFNL3 Expressions Are Associated with Stages of Fibrosis in Chronic Hepatitis C

An inefficient immune response against the hepatitis C virus (HCV), combined with viral evasion mechanisms, is responsible for the chronicity of infection. The need to evaluate the innate mechanisms of the immune response, such as TLR3 and IFN-λ3, and their relationship with the virus-host interaction is important for understanding the pathogenesis of chronic hepatitis C. The present study aimed to investigate the gene expressions of TRL3 and IFNL3 in liver tissue, seeking to evaluate whether these could be potential biomarkers of HCV infection. A total of 23 liver biopsy samples were collected from patients with chronic HCV, and 8 biopsies were collected from healthy control patients. RNA extraction, reverse transcription and qPCR were performed to quantify the relative gene expressions of TLR3 and IFNL3. Data on the viral load; AST, ALT, GGT and AFP levels; and the viral genotype were collected from the patients' medical records. The intrahepatic expression of TLR3 (p = 0.0326) was higher in chronic HCV carriers than in the control group, and the expression of IFNL3 (p = 0.0037) was lower in chronic HCV carriers than in the healthy control group. The expression levels of TLR3 (p = 0.0030) and IFNL3 (p = 0.0036) were higher in the early stages of fibrosis and of necroinflammatory activity in the liver; in contrast, TLR3 and IFNL3 expressions were lower in the more advanced stages of fibrosis and inflammation. There was no correlation between the gene expression and the serum viral load. Regarding the initial METAVIR scale scores, liver transaminase levels were lower in patients with advanced fibrosis when correlated with TLR3 and IFNL3 gene expressions. The results suggest that in the early stages of the development of hepatic fibrosis, TLR3 and IFN-λ3 play important roles in the antiviral response and in the modulation of the tolerogenic liver environment because there is a decrease in the intrahepatic expressions of TLR3 and IFNL3 in the advanced stages of fibrosis, probably due to viral evasion mechanisms.

Interrelationship between Toll-like receptors and infection after orthotopic liver transplantation

Early microbial recognition by the innate immune system is accomplished by Toll-like receptors (TLRs), with resultant initiation of a pro-inflammatory response against infecting organisms. In spite of presence of an abundance of Toll-like receptors on the surface of the liver, gut bacteria does not elicit an inflammatory reaction in healthy individuals due to tolerance to these TLRs, suggesting that the inflammatory responses seen in the liver are the result of breakdown of this tolerance. While orthotopic liver transplantation is often life saving in many instances, death following this procedure is most commonly due to infection that occurs in up to 80% of transplant recipients, most commonly due to microbial causes in up to 70% of cases and viral infections in 20%, while fungal infections affect only 8% of cases. The probability of acquiring infection following hepatic transplantation is heightened due to affection of the innate immune defense mechanisms of the host following this procedure. Single nucleotide polymorphisms of TLRs have been associated with increased likelihood of either development of post-transplant infection or eradication of infecting organism. However, conflicting reports from other studies reveal that prevalence of this single nucleotide polymorphism is not increased in infected patients.

Peripheral blood and hepatic Toll-like receptor 7 expression and interferon lambda 1 levels in chronic hepatitis C: Relation to virus replication and liver injury

BACKGROUND AND AIM

Toll-like receptor 7 (TLR7) can recognize single-stranded RNA viruses like hepatitis C virus (HCV) with subsequent induction of different interferon (IFN) types including IFN lambda (IFNL), which activate an immediate anti-viral response. However, the role of TLR7 in inflammation and fibrosis, characteristics of HCV-induced liver injury, is still controversial. The present work was designed to investigate the potential role of TLR7 and IFNL1 in chronic hepatitis C (CHC) in relation to viral replication and liver injury.

METHODS

Forty two treatment-naïve patients with CHC and 20 healthy subjects were enrolled in the study. TLR7 expression on peripheral blood CD14+ monocytes was studied by color flow cytometry and the frequency of TLR7+CD14+ cells was expressed as percentage of total monocyte count. Quantification of IFNL1 levels in serum was determined using enzyme-linked immunosorbant assay. Liver biopsies were examined for assessment of histological activity grade (A0-A3) and fibrosis stage (F0-F4) according to METAVIR scoring system as well as steatosis grade. Immunohistochemical staining was performed using human antibodies against TLR7 and IFNL1 and was scored semi-quantitatively (score 0-3). Hepatic expression of TLR7 and IFNL1 was further classified using a two-grade scale as low expression (score 0 or 1) and high expression (score 2 or 3).

RESULTS

Percentages of circulating TLR7+CD14+ monocytes and serum IFNL1 levels were significantly higher in patients with CHC than in healthy controls (P = 0.025 and P < 0.001 respectively) and were positively correlated with corresponding hepatic TLR7 and IFNL1 expression (P < 0.001 and P = 0.010 respectively). Significantly lower peripheral blood and hepatic TLR7 expression and IFNL1 levels were found in patients with viral loads between 200,000-600,000 IU/ml and >600,000 IU/ml than in those with viral load <200,000 IU/ml (P < 0.05), in patients with severe necroinflammation than in those with mild-to-moderate necroinflammation (P < 0.05) and in patients with advanced fibrosis than in those with early fibrosis (P < 0.01). Also, changes in TLR7 expression and IFNL1 production in peripheral blood and the liver were inversely correlated with serum levels of aspartate and alanine aminotransferases (P < 0.05) and HCV RNA (P < 0.01), histological activity grade (P < 0.01) and fibrosis stage (P < 0.01). By plotting receiver operating characteristics (ROC) curve, serum IFNL1 showed higher sensitivity and specificity than percentages of circulating TLR7+CD14+ monocytes in discriminating patients with CHC according to the severity of hepatic necroinflammation (area under the curve (AUC) = 0.901 vs. 0.816 respectively) and fibrosis (AUC = 0.971 vs. 0.825 respectively) at a cut-off value of 44.75 pg/ml and 10.25% respectively.

CONCLUSIONS

TLR7 activation and IFNL1 production in CHC may play an important role in controlling viral replication and limiting hepatic inflammation and fibrosis and their downregulation may result in viral persistence and disease progression. The immunoregulatory role of TLR7-IFNL1 pathway in the pathogenesis of chronic HCV infection should be further studied. Clinical trials with a large number of patients are needed to assess the usefulness of serum IFNL1 as a potential biomarker for severity of liver injury in chronic HCV infection and other liver diseases.

Modulation of the Immune System in Chronic Hepatitis C and During Antiviral Interferon-Free Therapy

The treatment of patients with chronic hepatitis C virus (HCV) infection has changed tremendously over the past 2 years, with an increasing variety of all-oral direct-acting antiviral (DAA) treatment regimens available for different HCV genotypes and distinct clinical settings. These treatments have significantly improved safety in patients with advanced liver disease compared with interferon (IFN)-based regimens. HCV modifies the human immune system to escape immunosurveillance via several mechanisms. One of the basic mechanisms of HCV is the ability to “switch” the immune response by reducing the activity of cells responsible for the elimination of virus-infected cells. IFN-free DAA treatment regimens provide a unique opportunity to assess the effect of HCV elimination on the immune system. Abrupt changes in the immune system can in some cases be responsible for two alarming processes: viral reactivation in patients with chronic hepatitis B and recurrence of hepatocellular carcinoma in patients with previous successful cancer treatment.

Secreted and intracellular cytokines are complementary measures for human monocytes treated with Toll-like receptor agonists

Cytokine production by human peripheral blood mononuclear cells including monocytes, is frequently assessed by measuring secreted cytokines using enzyme linked immunosorbent assay (ELISA), whereby the total concentration of one cytokine of interest is obtained without information regarding the cell type responsible for making the cytokine. Cytokines can be retained inside the cell using protein transport inhibitors. Subsequent analysis by flow cytometry not only identifies the cell type producing the cytokine but can semi-quantitate the amount of cytokine produced by measuring the geometric mean fluorescence intensity (gMFI) and is amenable to analyzing more than one protein associated with the same cell (multiplexing). We hypothesized that a more comprehensive and biologically meaningful cytokine profile could be acquired by measuring both secreted and the retained intracellular cytokines in parallel cultures of magnetic-sorted CD14+ monocytes. Peripheral monocytes were isolated from 18 healthy donors and treated with standardized molecules that stimulate cytokine production; Toll-like receptor (TLR)4 agonist (lipopolysaccharide, LPS) or TLR7/8 agonist (R848). Pro-inflammatory cytokines (interleukin (IL)-6, IL-8 and tumour necrosis factor (TNF)) secreted into the culture medium were measured by ELISA. Parallel cultures were treated with LPS and R848 in the presence of brefeldin A (protein transport inhibitor) and the accumulated intracellular cytokines measured by flow cytometry. Each cytokine (IL-6/IL-8/TNF) gave a unique general pattern when secreted versus intracellular cytokine measurements (frequency and gMFI) were plotted to determine correlation. For monocytes treated with the TLR4 agonist, secreted IL-8 correlated with the frequency of IL-8 positive cells (R = 0.559, p = .016) and not with the amount (gMFI) of IL-8 per cell. In contrast, monocytes treated with the TLR7/8 agonist showed no correlation of secreted IL-8 with the frequency of IL-8 positive cells, but with this treatment secreted IL-6 was correlated with an increase in the frequency of IL-6 positive cells (R = 0.501, p = .034). TNF secretion from monocytes treated with either the TLR4 or TLR7/8 agonist did not correlate with the frequency or gMFI of TNF positive cells. However, there were significant correlations between the TLR4 and TLR7/8 induced TNF response (secreted and gMFI). We conclude that there are fundamental differences in secreted and intracellular IL-6/IL-8/TNF production after monocytes are treated with TLR agonists. Furthermore, secreted and intracellular cytokine analyses are complementary measures that should be used in parallel to explore inflammatory response and cytokine biology.

Rapidity of fibrosis progression in liver transplant recipients with recurrent hepatitis C is influenced by toll-like receptor 3 polymorphism

Liver transplantation activates the innate immune system through toll-like receptors (TLRs), potentially leading to allograft rejection and graft failure. We evaluated the association of single-nucleotide polymorphisms in TLR genes with the severity of hepatitis C virus recurrence after liver transplantation (LT). This is a two-center study of 176 adult patients who received a first LT from deceased donors for hepatitis C virus (HCV) cirrhosis. Eleven polymorphisms were evaluated by real-time polymerase chain reaction and melting curves analyses: TLR1 (Asp248Ser and Ser602Ile), TLR2 (Arg753Gln), TLR3 (Leu412Phe), TLR4 (Asp299Gly), TLR5 (Arg392Stop), TLR6 (Ser249Pro), TLR7 (Gln11Leu), TLR8 (Met1Val), and TLR9 (-1237T/C and -1486C/T). The CC genotype of TLR3 Leu412Phe in liver recipients was associated with severe recurrence (odds ratio (OR) = 2.01, 95% confidence interval (95% CI) = 1.02-3.93, p = 0.04). We also analyzed this polymorphism in 72 of their donors but no association was found with severity of HCV recurrence (p = 0.89). Multivariate analysis showed donor age older than 40 yr (OR=2.93; 95% CI = 1.49-5.8, p = 0.002) and the TLR3 Leu412Phe CC genotype (OR=2.02, 95%CI=1.01-4.05, p = 0.046) were independently associated with severe HCV recurrence. Our results show that the TLR3 Leu412Phe CC genotype is independently associated with severity of hepatitis C recurrence after LT.

Toll-like receptor 3 gene polymorphisms are not associated with the risk of hepatitis B and hepatitis C virus infection

INTRODUCTION

The present study investigated the prevalence of two single-nucleotide polymorphisms (SNPs) in the Toll-like receptor 3 (TLR3) gene in patients infected with hepatitis B virus (HBV) and hepatitis C virus (HCV).

METHODS

Samples collected from HCV (n = 74) and HBV (n = 35) carriers were subjected to quantitative real-time PCR (qPCR) to detect the presence of the SNPs rs5743305 and rs3775291 in TLR3 and to measure the following biomarkers: alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transpeptidase (GGT), and prothrombin time (PT). A healthy control group was investigated and consisted of 299 HCV- and HBV-seronegative individuals.

RESULTS

No significant differences in allele, genotype and haplotype frequencies were observed between the investigated groups, and no association was observed between the polymorphisms and histopathological results. Nevertheless, genotypes TA/AA (rs5743305) and GG (rs3775291) appear to be associated with higher levels of ALT (p<0.01), AST (p<0.05) and PT (p<0.05). In addition, genotypes TT (rs5743305; p<0.05) and GG (rs3775291; p<0.05) were associated with higher GGT levels.

CONCLUSIONS

This genetic analysis revealed the absence of an association between the polymorphisms investigated and susceptibility to HBV and HCV infection; however, these polymorphisms might be associated with a greater degree of biliary damage during the course of HCV infection.

Toll-like receptor 8 (TLR8) polymorphisms are associated with non-progression of chronic hepatitis C in HIV/HCV coinfected patients

Toll-like receptor 8 (TLR8) polymorphisms have been related to hepatitis C virus (HCV) infection. The aim was to estimate the association of TLR8 polymorphisms with HCV-related outcomes in HIV/HCV coinfected patients. We performed a cross-sectional study of 220 patients who underwent a liver biopsy. TLR8 polymorphisms were genotyped using GoldenGate® assay. The outcome variables were non-fibrosis (F0), mild-inflammation (A0/A1), and non-steatosis [fatty hepatocytes (FH) <10%]. Logistic regression analysis was used to compare the outcome variables according to TLR8 polymorphisms. Four polymorphisms were analyzed (rs1013151, rs5744069, rs17256081 and rs3764880rs1013151). Female patients had higher frequency of TLR8 major alleles at rs17256081 and rs101315, and minor alleles at rs3764880 and rs5744069. Male patients had higher frequency of TLR8 minor alleles except for rs3764880, where major alleles were higher (p<0.01). Two TLR8 polymorphisms (rs1013151 and rs5744069) were significantly associated with non-fibrosis (F0) [adjusted odds ratio (aOR)=4.42 (95% of confidence interval (95%CI)=1.54; 12.68) (p=0.006) and aOR=4.76 (95%CI=1.69; 13.37) (p=0.003); respectively]. When data were stratified by gender, rs1013151 and rs5744069 polymorphisms remained significant for male patients [adjusted odds ratio (aOR)=4.49 (95%CI=1.08; 18.62) (p=0.039) and aOR=6.17 (95%CI=1.45; 26.20) (p=0.014); respectively]. When data were stratified by major HCV genotypes, patients infected with HCV genotype 1 (GT1) had significant values for both rs1013151 and rs5744069 polymorphisms [aOR=5.79 (95%CI=1.44; 23.32) (p=0.013) and aOR=8.01 (95%CI=2.16; 35.65) (p=0.005); respectively]. Finally, none of the TLR8 polymorphisms were significantly associated with mild-inflammation or non-steatosis. In conclusion, TLR8 polymorphisms seem to be related to non-progression of liver fibrosis in HIV/HCV coinfected patients, particularly in males and those patients infected with GT1.

Toll-like receptor 3 expression and function in childhood idiopathic nephrotic syndrome

The efficacy of steroids and immunosuppressive treatments in idiopathic nephrotic syndrome (INS) hints at the implication of immune cells in the pathophysiology of the disease. Toll-like receptor (TLR) dysfunctions are involved in many kidney diseases of immune origin, but remain little described in INS. We investigated the expression and function of TLRs in peripheral blood mononuclear cells (PBMC) of INS children, including 28 in relapse, 23 in remission and 40 controls. No child had any sign of infection, but a higher Epstein-Barr virus viral load was measured in the PBMC of relapsing patients. TLR-3 expression was increased in B cells only during INS remission. There was a negative correlation between proteinuria and TLR-3 expression in total and the main subsets of PBMC from INS patients. The expression of TLR-8 was also increased in both CD4(+) T cells and B cells in INS remission. There was a negative correlation between proteinuria and TLR-8 expression in total PBMC, CD4(+) T cells and B cells of INS patients. Nevertheless, TLR-3 and TLR-8 expression was normalized in all PBMC subsets in an additional group of 15 INS patients in remission with B cell repletion after rituximab therapy. Paradoxically, interferon (IFN) regulatory factor 3 transactivation was increased in PBMC of all INS patients. In-vitro secretion of IFN-α and interleukin 6 were increased spontaneously in PBMC of INS remission patients, whereas PBMC from all INS patients displayed an impaired IFN-α secretion after TLR-3 stimulation. Thus, TLR-3 pathway dysfunctions may be closely involved in INS pathogenesis.

Early to Phase II drugs currently under investigation for the treatment of liver fibrosis

INTRODUCTION

Chronic liver diseases represent a high unmet medical need and are characterized by persistent inflammation, parenchymal damage and fibrotic remodeling, leading eventually to cirrhosis and hepatic failure. Besides the persisting high prevalence of chronic viral hepatitis B and C, the dramatic increase in nonalcoholic steatohepatitis is now considered to be a major pathophysiologic driver for fibrosis development and subsequently cirrhosis. Increasing evidence suggests that also liver cirrhosis can regress when treated adequately.

AREAS COVERED

Herein, the authors review the underlying pathophysiologic mechanisms leading to fibrotic remodeling in the liver. They also highlight the options for novel treatment strategies by using molecular targeted agents.

EXPERT OPINION

New in vitro and preclinical animal models, and the careful selection of patients with high disease dynamics for clinical studies, provide a sound basis for the clinical development of antifibrotic agents in humans. Surrogate parameters of liver function, inflammation, tissue remodeling and damage, as well as noninvasive imaging techniques, can be applied in clinical trials to provide fast readouts and novel and reliable endpoints for trial design, and provide an attractive regulatory environment for this emerging disease area.

Genetic variants of innate immune receptors and infections after liver transplantation

Infection is the leading cause of complication after liver transplantation, causing morbidity and mortality in the first months after surgery. Allograft rejection is mediated through adaptive immunological responses, and thus immunosuppressive therapy is necessary after transplantation. In this setting, the presence of genetic variants of innate immunity receptors may increase the risk of post-transplant infection, in comparison with patients carrying wild-type alleles. Numerous studies have investigated the role of genetic variants of innate immune receptors and the risk of complication after liver transplantation, but their results are discordant. Toll-like receptors and mannose-binding lectin are arguably the most important studied molecules; however, many other receptors could increase the risk of infection after transplantation. In this article, we review the published studies analyzing the impact of genetic variants in the innate immune system on the development of infectious complications after liver transplantation.

Role of toll-like receptors in immune activation and tolerance in the liver

Liver has a unique vascular system receiving the majority of the blood supply from the gastrointestinal tract through the portal vein and faces continuous exposure to foreign pathogens and commensal bacterial products. These gut-derived antigens stimulate liver cells and result in a distinctive immune response via a family of pattern recognition receptors, the Toll-like receptors (TLRs). TLRs are expressed on Kupffer cells, dendritic cells, hepatic stellate cells, endothelial cells, and hepatocytes in the liver. The crosstalk between gut-derived antigens and TLRs on immune cells trigger a distinctive set of mechanisms to induce immunity, contributing to various acute and chronic liver diseases including liver cirrhosis and hepatocellular carcinoma. Accumulating evidence has shown that TLRs stimulation by foreign antigens induces the production of immunoactivating and immunoregulatory cytokines. Furthermore, the immunoregulatory arm of TLR stimulation can also control excessive tissue damage. With this knowledge at hand, it is important to clarify the dual role of disease-specific TLRs as activators and regulators, especially in the liver. We will review the current understanding of TLR signaling and subsequent immune activation and tolerance by the innate immune system in the liver.

Role of toll-like receptors in liver transplantation

Toll-like receptors (TLRs) are pathogen recognition receptors that orchestrate the innate immune response and the subsequent adaptive immune response. TLRs can be triggered by exogenous ligands expressed by invading pathogens or by the release of endogenous ligands, such as that occurring through cellular injury during the transplantation process. They are now recognized to play an important role in many facets of transplantation biology, including rejection and tolerance, ischemia/reperfusion injury (IRI), and infections after transplantation. The role of TLRs in liver transplantation is unique with respect to other organ transplants because the portal circulation is a continuous source of TLR2 and TLR4 ligands, and this influences TLR signaling pathways, which have a central role in transplantation immunity. This review provides a critical update on recent data outlining the important role of TLRs in liver transplantation, and there is a particular focus on emerging advances in our understanding of rejection and tolerance, IRI, and infections after transplantation and on the ways in which these events may influence the recurrence of diseases such as hepatitis C infection after liver transplantation.

Roles of Toll-like receptors in transplantation

PURPOSE OF REVIEW

Beyond their crucial role in host defence against invading pathogens, Toll-like receptors (TLRs) are now known to be key mediators of the 'sterile' innate immune response triggered by tissue injury. Here, we will review recent evidence examining the role of TLRs in determining the fate of the transplanted organ.

RECENT FINDINGS

Experimental studies have delineated a crucial role for TLRs in the pathogenesis of ischaemia-reperfusion injury (IRI). Following transplantation, experimental models and observational human studies have confirmed TLRs as mediators of IRI but also suggest that TLRs modify the subsequent adaptive alloimmune response, which determines whether an organ is rejected or accepted.

SUMMARY

As a key determinant of organ damage following ischaemia-reperfusion and as a modulator of the adaptive alloimmune response, we propose that TLR-mediated innate immune responses represent a potential therapeutic target to improve organ outcomes following transplantation.

Cyclosporine and tacrolimus have inhibitory effects on toll-like receptor signaling after liver transplantation

Toll-like receptors (TLRs) play a key role in transplantation biology. The effect of immunosuppression on TLR function after liver transplantation is unknown. Peripheral blood mononuclear cells (PBMCs) from 113 post-liver transplant patients and 13 healthy controls were stimulated with TLR-specific ligands [lipopolysaccharide (TLR4), pan-3-cys (P3C) (TLR2), Poly (I:C) (PIC) (TLR3), R848 (TLR7/8), and CpG (TLR9)] for 24 hours. PBMCs from 5 healthy controls were also cultured with therapeutic concentrations of cyclosporine A (CYA) and tacrolimus (TAC). Cytokine production was measured with enzyme-linked immunosorbent assays and flow cytometry. PBMCs from patients on calcineurin inhibitors after liver transplantation produced less interleukin-6 (IL-6) and tumor necrosis factor α (TNFα) in response to TLR2 stimulation (IL-6: P=0.02; TNFα: P=0.01), TLR4 stimulation (IL-6: P=0.02; TNFα: P=0.01), and TLR7/8 stimulation (IL-6: P=0.02; TNFα: P=0.02), compared with healthy controls. Both CD56(bright) and CD56(dim) natural killer (NK) cells from patients on calcineurin inhibitors also produced less interferon-γ (IFNγ) with TLR7/8 stimulation compared with healthy controls (CD56(bright) : P=0.002; CD56(dim) : P=0.004). Similar findings were demonstrated in healthy PBMCs cultured with CYA (PBMCs: TLR2, IL-6: P=0.005; TLR4, IL-6: P=0.03, TNFα: P=0.03; TLR7/8, IL-6: P=0.02, TNFα: P=0.01; CD56(dim) NK cells: TLR7/8, IFNγ: P=0.03). TAC impaired TLR4-mediated IL-6 and TNFα production by PBMCs (IL-6; P = 0.02; TNFα P = 0.009). In conclusion, patients on calcineurin inhibitors had impaired inflammatory cytokine production in response to TLR2, TLR4, and TLR7/8 stimulation compared comparison with healthy controls. Importantly, TAC and CYA appear to have different effects on TLR signaling. Impaired TLR function has important repercussions for risk of infection, graft rejection, and disease recurrence after transplantation, and the different immunosuppressive profiles of CYA and TAC may guide the choice of therapy to improve disease outcomes.