TLR7 and TLR8 Gene Variations and Susceptibility to Hepatitis C Virus Infection

Toll-like receptors (TLRs) play pivotal roles in the innate immune system and control inflammatory responses and adaptive immunity. We previously evaluated associations between TLR7 and TLR8 gene SNPs and susceptibility to hepatitis C virus (HCV) infection. Our results suggested that TLR7IVS2-151G and TLR8-129G alleles were present at higher frequency in males of an HCV-infected group as compared to a control group (24.1% vs. 14.4%, p = 0.028; 17.6% vs. 6.8%, p = 0.004, respectively). Based upon their recognition of single stranded viral RNA, this suggested that TLR7 and TLR8 played a significant role in anti-HCV immune responses. Here, we studied the functional effects of these polymorphisms by analyzing the mRNA expressions of TLR7 and TLR8 and cytokine production induced ex vivo by TLR7- and TLR8-specific agonists using whole blood of subjects with different genotypes. The percentage of CD14+ cells from those with an AG haplotype that expressed TLR7 and TLR8 was significantly lower, but higher in intensity compared to cells from those with GG and AC haplotypes. Cells from those with an AG haplotype produced more IFN-α and less amounts of pro-inflammatory cytokines upon stimulation. This suggests that variations in TLR7 and TLR8 genes might impair immune responses during HCV infection.


Introduction
Toll-like receptors (TLRs) are important pathogen recognition receptors (PRRs) at the interface between a host and the environment and are key molecules for both innate and adaptive immunity [1]. TLR7 and TLR8, which share a high degree of structural similarity, are located in the membranes of the endosomal compartment and recognize viral single-stranded RNA (ssRNA) [2,3,4]. TLR genetic variants and downstream signalling molecules can influence the ability of affected individuals to respond adequately to TLR ligands, which can result in their altered susceptibility to or the course of infectious disease [5,6,7]. The number of reported genotypic profiles of TLRs is rapidly expanding. However, the number of identified functional TLR polymorphisms remains limited, and parts of some studies have divergent results regarding the significance of polymorphisms and even have contradictory observations [8].
We previously identified a single nucleotide polymorphism (SNP) at IVS2-151 of TLR7 and a SNP resulting in a Met/Val change at a start codon TLR8 associated (G/C) SNP at position 2129 in the TLR8 promoter region [9]. The polymorphism, TLR7 IVS2-151 G.A, changes the 2151 nucleotide of the second intervening sequence (IVS-2 position 2151) from G to A and it's function is still unclear. In our Chinese population, TLR7 allele frequencies were A at 77%, and G at 22%, and TLR8 allele frequencies were C at 16%, and G at 83%. However, to date there have been few studies on the functional effects of TLR7 and TLR8 gene polymorphisms.
Hepatitis C virus (HCV), a single-stranded RNA virus, infects more than 170 million of the world's population [10]. The clinical outcome of HCV infection is highly variable, and genetic factors involving innate immunity are likely to affect disease susceptibility and disease progression after infection. A key aspect of the antiviral innate immune response is the synthesis and secretion of type I interferons (IFN), such as IFN-a and IFN-b [11]. Based on their recognition of single stranded viral RNA, TLR7 and TLR8, both of which genes are located on the X chromosome [12], have been suggested to play important roles in antiviral immune responses induced by IFN and inflammatory cytokines.
Recent reports have indicated that robust TLR7 and TLR8 agonists decrease the amounts of HCV RNA in HCV-infected patients [13,14]. The specific aims of this study were to evaluate possible associations between the susceptibility to HCV infection and TLR7 and TLR8 polymorphisms and to compare the expressions and functions of TLR7 and TLR8 variants.

Allele and haplotype frequencies in chronic hepatitis C patients
The genotypes involving TLR7 and TLR8 SNPs of 264 patients with chronic HCV infection and 243 control subjects were analyzed. The distributions of the allele and haplotype frequencies for the TLR7 IVS2-151 G.A (rs179009) and TLR8 -129G.C (rs3764879) polymorphisms are summarized in Table 1. The frequency of TLR7 IVS2-151G was significantly higher in male chronic HCV infection patients than control subjects (24.1% versus 14.4%; p = 0.028), with an odds ratios (OR) of 1.89 (95% CI = 1.06 to 3.33). TLR8 -129C also had a significantly higher frequency in male chronic HCV infection patients than controls (17.6% versus 6.8%; p = 0.004); OR = 2.91 (95% CI = 1.38 to 6.13). However, no associations were found between chronic HCV infection and TLR7 and TLR8 polymorphisms among females (Table 1). Due to X skewing might affect the interpretation of data in female, we disregard TLR7 SNP heterozygous females from analysis [15] and got a significant difference in homozygous TLR7 IVS2-151genotypes among controls and hepatitis C female patients (p = 0.021; OR = 8.55,95% CI = 0.99 to 73.4).
TLR7 and TLR8 expression studies Figure 1 shows the results of quantitative determinations for TLR7 and TLR8 mRNA expressions in healthy male volunteers with different TLR7 and TLR8 haplotypes. Individuals with the TLR7 IVS2-151A/TLR8 -129C (AC) haplotype had higher TLR7 and TLR8 mRNA expressions. TLR8 mRNA expression for those with the AC wild type was significantly higher than for those with the AG and GG haplotypes (p = 0.008; 0.001; respectively). Figure 2 shows the results of FASC analysis for the intracellular TLR7 and TLR8 protein expressions in monocytes from 37 healthy male subjects with different TLR7 and TLR8 genotypes: AG haplotype, N = 17; GG haplotype, N = 9; AC haplotype, N = 11. Those with the AG haplotype had a significantly lower percentage of CD14+ cells that expressed TLR7 (61.765.4%) compared to those with the GG haplotype (68.066.9%; p = 0.089) and the AC haplotype (68.068.3%; p = 0.066; Figure 2A). Also, the percentage of CD14+ cells from subjects with the AG haplotype that expressed TLR8 (61.564.9%) was significantly lower compared to cells from those with the GG haplotype (68.367.1%; p = 0.060) and the AC haplotype (68.168.3%; p = 0.048; Figure 2C). In contrast, the mean fluorescence intensities of TLR7 and TLR8 expressions were significantly higher in cells from subjects with the AG haplotype compared to cells from those with the GG and AC haplotypes ( Figure 2B & 2D).

Ex vivo stimulation assays
To examine whether the TLR7 and TLR8 SNPs affected cytokine induction, we stimulated whole blood samples from healthy donors using TLR7 and TLR8 specific agonists. Whole blood was obtained from healthy male volunteers with different haplotypes and exposed ex vivo to a TLR7-specific agonist (3M-001) or a TLR8-specific agonist (3M-002) for 12 hours, after which the supernatants collected for cytokines assays. In response to 3M-001, whole blood from those with the wild type (AG) haplotype had higher IFN-a production as compared to the GG and AC haplotypes (p = 0.003 and 0.004, respectively; Figure 3A). However, lower IL-1ß production from those with AG haplotyp compared to the GG haplotype  (p = 0.015; Figure 3B). Those with the AG haplotype exhibited decreased pro-inflammatory responses compared to the other haplotypes, as significant differences were observed for IL-6 and TNF-a after 3M-002 stimulation (p = 0.050 and 0.037, respectively; Figure 3G and 3I).

Discussion
Although an antiviral immune response is staged in almost all patients infected with HCV, only some of these patients achieve a spontaneous resolution and the factors that determine the quality  of these immune responses remain mostly unknown [16]. In addition to viral factors, host factors, including age, gender and genetic factors, influence the spontaneous outcome after HCVinfection, the progression to chronic disease and the response to therapy [17]. Thus, a role for TLR7 and TLR8 in the immune response against HCV, a single-stranded RNA virus, is conceivable, as an antiviral effect of TLR7 stimulation has been demonstrated in early clinical studies [13].
Our results showed that the TLR7IVS2-151G and TLR8-129G polymorphisms were statistically significantly more frequent in male subjects with chronic HCV infection as compared to controls. In addition, our results suggest that the TLR7 (IVS2-151A.G) and TLR8 (-129G.C) polymorphisms also altered these gene expressions when quantified by mRNA. However, ex vivo stimulation assays demonstrated that these TLR7 and TLR8 polymorphisms resulted in significantly different production of IFN-a, IL-1ß and IL-6 after stimulation with a TLR7 agonist, whereas only IL-1ß and TNF-a production were altered after stimulation with a TLR8 agonist ( Figure 3). Taken together, the immune cells from those with the wild type AG haplotype produced more IFN-a and less amounts of pro-inflammatory cytokines upon stimulation. We propose the variations in the TLR7 and TLR8 genes may impair the immune responses to HCV due to less IFN-a production during stimulation.
The distributions of TLR7 IVS2-151G.A (rs179009) and TLR8-129G.C (rs3764879) allelic frequencies in other populations are shown in Table 3. In our Taiwanese study population, the genotype distributions for the TLR7 IVS2-151G.A and TLR8-129G.C polymorphisms were: 413 had the A allele (15.0%), 73 had the G allele (85.0%), and 46 had the C allele (9.5%), and 440 had the G allele (90.5%), respectively. Compared with other ethnic populations from the website of the international HapMap project, statistical analysis did not reveal a difference in allele frequencies for the TLR7 IVS2-151G.A polymorphism when comparing our population with Caucasians and Japanese, but there was a significant difference between our results and those from a Nigerian population (p = 0.009).
In our previous study, SNPs resulting in a Met/Val change at the start codon for TLR8 and a (G/C) SNP at position 2129 in the TLR8 promoter region were shown to be in linkage [9]. As compared to other ethnic populations, Taiwanese exhibited an allele frequency for TLR8 Met1Val and TLR8 -129 G/C opposite to those of Caucasians and Nigerians ( Table 3). The TLR8 -129 G/C linkage with the TLR8 A1G polymorphism alters the start ATG of TLR8 into a GTG triplet. A methionine located at position 4 could be used as an alternate start codon, resulting in a truncated TLR8 (1038 aa vs. 1041 aa) with a shorter signal peptide. The functional effect of this mutation may result in a more rapid decay of TLR8 mRNA or may affect protein function [18].
In overexpression assays, it has been demonstrated that a TLR8 A1G polymorphism resulted in impaired NF-kB activation in vitro. The mutated receptor variant was associated with modulating the cytokine secretion profiles and lipid mediator synthesis patterns in monocytes and neutrophils [18]. The functional TLR8 variant may have implications for assessing the risks for individual patients infected with HIV and other RNA viruses [18]. In this study we also demonstrated that those with the TLR8-129G associated TLR8 variant had a higher susceptibility to HCV infection. This is consistent with previous studies [19,20].
Undesired self-destructive immune responses result in a loss of a robust immune response against HCV replication and perturbations in virus eradication [21,22]. An increasing body of evidence suggests a role for innate immunity in the control of HCV infection, although the precise mechanisms have not been defined. The potency of an innate immune response is genetically predetermined and can result in HCV clearance [22]. TLR7 and TLR8 are PRRs of interest in the setting of HCV-infection as they can bind ssRNAs and lead to the production of large amounts of the antiviral cytokine interferon-a by dendritic cells [23]. Therefore, we hypothesized that genetic variants of TLR7 and TLR8 might influence the immune response against HCV.
We selected TLR7-and TLR8-specific agonists for whole blood ex vivo stimulation. Whole blood samples from subjects with the wild type AG haplotype had higher IFN-a production as compared to the GG and AC haplotypes upon stimulation with 5 mM of 3M-001, but there was no induction of IFN-a after stimulation with the TLR8 agonist 3M-002 regardless of haplotype. The reason may be that plasmacytoid dendritic cells in peripheral blood, which express TLR7 but not TLR8, are the primary producers of IFN-a [23].
In contrast, upon stimulation with 5 mM of 3M-002, whole blood samples from those with the AG haplotype exhibited decreased pro-inflammatory responses reflected by IL-1b and TNF-a induction. High levels of these cytokines have been shown to be correlated with increased liver damage in HCV patients [16,17,24,25]. Thus, male volunteers with the TLR7 IVS2-151A/ TLR8 -129G haplotype had higher induced interferon production and lower amounts of inflammatory cytokines than those with the GG and AC haplotypes after ex vivo whole blood stimulation with TLR7 and TLR8 agonists.
The allele frequencies of both variations were gender-dependent. Because the TLR7 and TLR8 genes are located on the Xchromosome [4], gender-specific effects come as no surprise. The  skewing of X-chromosomal activation in women is an age-and tissue-dependent process that may affect the analysis of the effects of X-chromosomal variations [15,26]. Therefore, associations between chronic HCV chronic infection and TLR7 and TLR8 polymorphisms were not found among females in our study. Due to the accumulating data on genetic polymorphisms in HCVinfection [22,23,24], we suggest that patient's sex needs to be taken into account when future individual risk profiles for HCV infection are generated.
Our results suggest that TLR7 and TLR8 variants have functional relevance in the setting of HCV-infection by conferring susceptibility to infection. The TLR7 and TLR8 variants result in reduced IFN-a release. Accordingly, this could be responsible for a lower level of immune activation and explain the higher susceptibility to chronic HCV chronic infection among males with these mutations. Thus, our results support the concept that HCV infection depends on TLR7/8-mediated immune activation. It will be of interest to observe the results from further trials on the use of TLR agonists, such as isatoribine or CpG-DNA [13,27], for the treatment of chronic HCV infection and how these are correlated with the genetic markers of the associated genes.
A different approach for investigating the roles of TLR7 and TLR8 SNPs in the resolution of HCV-infection would require a comparison of patients who are chronically infected with HCV with individuals that have spontaneously cleared infections. Unfortunately, patients with proven spontaneous recovery from HCV-infection were not included in this study. Therefore, we investigated the effects of these mutations in healthy volunteers. In humans, TLR7 is expressed by plasmacytoid DCs and B cells [28,29], whereas TLR8 is abundant on monocytes and monocytederived DCs [30,31]. Our results showed that subjects with the TLR7 IVS2-151A/TLR8 -129G haplotype had a lower percentage of monocytes that expressed TLR7 or TLR8, but these cells had higher intensity expressions compared to cells from those with other genotypes.
In conclusion, the results presented here demonstrate a protective effect of the TLR7 IVS2-151A/TLR8 -129G haplotype against HCV chronic infection. We showed that the presence of this mutation leads to lower IFN-a and higher pro-inflammatory cytokine production. The modulation of immune responses is not only compatible with the restriction of HCV infection, but leads us to propose that future studies should address the relevance of these polymorphisms during the clinical course of HCV infection. The results of this study may have implications for assessing individual patient's risk profiles, as well as for the use of TLR agonists in the prevention of or in therapy for HCV infection.

Study participants
We recruited a population of 264 chronic hepatitis C patients (including 187 men and 77 female adults) according to suggestions by Lu et al [32] ( mean age 52.9615.7 years) and 243 control subjects (including 146 men and 97 female adults) who were anti-HCV negative ( mean age 48.3619.8 years) for determinations of TLR7 and TLR8 gene variations. Patients with HBV or HIV infection were excluded. Chronic HCV infection was proven by detection of anti-HCV or HCV RNA in the patient's sera over a period of at least 6 months. Serum activity of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were recorded. The detail demographic characteristics of study subjects are summarized in Table S1. Informed consent was obtained from all individuals. This study was approved by the E-DA Hospital ethics committee.
Determinations of TLR7 and TLR8 mRNA expressions by quantitative reverse transcription polymerase chain reaction (RT-PCR) Healthy male volunteers (laboratory staff members and college students) were randomly selected from among healthy male subjects after TLR7 IVS2-151 and TLR8-129 genotyping. Total RNA was extracted from peripheral blood samples using a Qiagen RNA Blood isolation kit (Qiagen, Hilden, Germany) according to the manufacturer's instructions. Complementary DNA (cDNA) was synthesized by reverse transcription with oligo-dT primers. The mRNA expressions of TLR7 and TLR8 were analyzed by quantitative RT-PCR. Fluorescence real-time PCR analysis used an ABI PRISM 7700 Sequence Detection System (Applied Biosystems, Warrington, WA). The PCR amplification reaction used the conditions: 50uC for 2 min, 95uC for 15 min, and 40 cycles at 95uC for 15 sec followed by 58uC for 1 min. We used the comparative CT (DDCT) method, in which CT is the threshold cycle number that is the minimum required for sample detection. The arithmetic formula for the DCT method is the difference in CT for the TLR7 or TLR8 gene and a housekeeping gene, GAPDH (DCT = CT TLR7 or CT TLR8 -CT GADPH ). Then, the relative amounts of mRNA were normalized to the lowest TLR7 or TLR8 mRNA level given by 2 2DDCT and given as the logarithm of 2 2DDCT .

Intracellular TLR7 and TLR8 protein expression assay
The intracellular protein expressions of TLR7 and TLR8 in monocytes from healthy male volunteers with different haplotypes were analyzed by flow cytometric analysis. First, 250 mL of freshly collected EDTA-whole blood was stained with 5 mL of PEconjugated anti-CD14 antibody (BD Biosciences, San Jose, CA) for 30 min at room temperature. Red blood cells were lysed by adding 2 mL of working 16 BD FACSTM Lysing Solution into each reaction tube for 10 min at room temperature, and then excess unbound antibody was washed away with phosphate buffered saline (PBS). For intracellular staining, the remaining leukocytes were fixed and permeabilised by adding 250 mL of Fixation and Permeabilization Solution (BD Biosciences, San Jose, CA) for 20 min at 4uC, after which cells were incubated with 2 mL of FITC-conjugated anti-TLR7 (IMGENEX, San Diego, CA) or anti-TLR8 (IMGENEX, San Diego, CA) diluted with 18 mL of 10% foetal bovine serum in PBS containing 0.2% saponin for 1 hr at 4uC. Cells were finally resuspended in PBS and analyzed on a FACSCalibur flow cytometer with WinMDI98 software (BD Biosciences, San Jose, CA).
Cytokine amounts were quantified using Luminex technology, which utilizes microspheres as the solid support for conventional sandwich immunoassay to simultaneously measure multiple cytokines. The combinations of different types of beads allows for the simultaneous measurements of IFN-a, IL-1b, IL-6, TNF-a, and MIP-1a (Biosource International, Camarillo, CA). The cytokine kit assay used the manufacturer's protocol. Briefly, to each designated pre-wetted well with a filter bottom in a 96-well microplate format (Millipore, Billerica, MA), a bead suspension was added, and the beads were washed and blocked for 10 min with PBS/bovine serum albumin. To generate a standard curve, appropriate standards were prepared in diluents. Standards and samples (50 ml) were diluted 1:1 with assay diluents and pipetted into duplicate wells. The plates were incubated for 1 h at room temperature on a microtitre shaker. After washing with PBS, a phycoerythrin (PE)-conjugated detection antibody cocktail was added to each well and the plates were incubated for an additional 45 min in the dark. Wells were washed twice, an assay buffer was added to each well, and samples were analyzed using the Luminex system (Bio-Rad Laboratories, Hercules, CA). The experimental data were analyzed using a five-parameter standard curve that was fit to the results for the provided standards.

Statistical analysis
Genotype frequencies of TLR7 and TLR8 polymorphisms were compared between chronic HCV-infected patients and control subjects using chi-square tests. Quantitative results from three groups are given as means 6 SD and assessed with nonparametric Kruskal-Wallis test. A p-value,0.05 was considered statistically significant. SPSS (version 14.01) was used for data management and statistical analyses and Graph Pad Prism 5 software was used to generate the figures.

Supporting Information
Table S1 Demographic characteristics of study subjects. (DOCX)