Toll-like receptors (TLRs) can recognize highly conserved pathogen-associated molecular patterns (PAMPs), which function as agonists or TLR agonists/ligands (TLRLs). Specifically, TLRs on the cell surface are mostly responsible for identifying different components of microbial membranes, such as proteins, lipids, and lipoproteins (11). For instance, TLR5 recognizes extracellular flagellin and activates the production of pro-inflammatory cytokines. On the other hand, the Nod-like receptor (NLR) Ipaf recognizes flagellin in the cytoplasm of macrophages and activates caspase-1 (12). Several studies have shown that certain elements derived from the host, such as fibrinogen, heat shock proteins, RNA, and DNA, can also act as TLR ligands (13, 14). TLRs are expressed on cells implicated in the innate immune reaction (myeloid and NK cells) and several cells of the adaptive immune response (regulatory and triggered T cells) and intercede innate immune reactions versus microbial pathogens and stimulate adaptive immune reactions (13, 14). In the cell membrane, TLRs 1, 2, 4, 5, 6, and 10 are expressed. Lysosomes and endosomes include TLRs 3, 7, 8, and 9. TLRs 11, 12, and 13 are more active in other mammals. For instance, in mice, TLR11 plays a role in recognizing certain bacterial components ( Figure 1 ). TLRs can recognize PAMPs of bacteria, viruses, and parasites based on their various kinds (17). Dendritic cells (DCs) and macrophages produce TLRs that specifically detect nucleic acids inside endosomes. TLR3, TLR7/8, and TLR9 are among the TLRs in this subgroup that are in charge of detecting, respectively, double-stranded (ds)RNA, single-stranded (ss)RNA, and ssDNA (18). Pattern recognition receptors (PRRs) belong to several families that monitor the cellular microenvironment for signs of viral infection. Viruses contain structural PAMPs, which are perfect targets for PRRs like TLRs. These receptors set off an intracellular signaling cascade that results in the production of cytokines and type-1 interferon (IFN), which drive the innate antiviral immune response. This defensive mechanism acts quickly while controlling the adaptive immune system (19, 20). The inflammatory cytokines induced by TLRs are associated with several hepatic disorders, such as inflammation of the liver, hepatic fibrosis, cirrhosis, alcohol-related liver disease (ARLD), non-alcoholic liver disease (NAFLD), reoxygenation injury, and HCC (21, 22). TLRs diagnose preserved PAMPs, which serve as TLRLs. TLRs can form homodimers or heterodimers, activated by certain ligands. Following ligand binding to TLRs, TLRs dimerize and change their conformation to attract downstream adaptor proteins, including myeloid differentiation primary response protein 88 (MyD88), TIRAP/MyD88-adaptor-like (Mal), TIR domain-containing adaptor inducing IFN-β (TRIF)/TIR domain-containing adaptor molecule-1 (TICAM-1), SARM (sterile α and armadillo-motif-containing protein), and TRF-related adaptor molecule (TRAM) ( Figure 2 ) (24–30). The type of TLRs involved in dimerization depends on the type of PAMPs (17). TLRs tag along with one another following ligand binding. They are dimerized based on their TLR type. Most TLRs undergo homodimerization, including 3, 4, 5, 7, 8, and 9. However, the remainder is heterodimerized, such as TLR1/2, TLR2/6, and occasionally TLR7 and TLR8 (15, 31, 32). TLR4 is a well-known example of homodimerization; it is triggered by lipopolysaccharide (LPS) that is extracted from the outer membrane of Gram-negative bacteria, such as Escherichia coli. On the other hand, TLR2 can form heterodimers with TLR1 or TLR6, depending on the ligand (33). After the dimerization of TLRs, the signal might go along two major routes. The first, known as MyD88-dependent, employs the MyD88 mediator and leads to the yield of inflammatory cytokines such as tumor necrosis factor (TNF), IL-6, and IL-1, and chemokines such as C-C motif ligand 4, CCL4 (25, 26). The other pathway, TRIF-mediated, uses the TRIF mediator and eventuates the expression of type I IFNs like IFNα/β (34). TLR3 is the lonely TLR that triggers the TRIF-dependent path (35). Moreover, TLRs are associated with cell reproduction, survival, apoptosis, angiogenesis, tissue remodeling, repair, and the evolvement of innate and adaptive immune responses. These proteins become active through hetero or homodimerization (11, 36, 37).

TLR response to HBV infection in many animal models has been well investigated (46, 47). In animal models, TLR2 activation has been shown to expedite HBV clearance and improve HBV-specific T-cell responses (48). Isogawa et al. (49) conducted research whereby they observed that a single intravenous injection of TLR3, 4, 5, 7, and 9 agonists reduced HBV replication, hence inducing IFN-α/β in the liver of HBV-transgenic mice and indicating the anti-HBV effect of these TLRs. TLR-agonist-mediated regulation of HBV replication has also been shown in mouse non-parenchymal liver cells (50). The woodchuck model of CHB’s transcriptome study also showed a limited intrahepatic type I IFN response, like in individuals with CHB (51). A suppressive tendency of TLR expression was seen in the hepatocytes of the woodchuck model of chronic hepatitis compared to that of healthy animals. This finding suggests that the innate immune response is compromised in chronic infection (52). Yet, a recent study using a woodchuck model of hepatitis revealed the function of TLR2 in the resolution of HBV infection (48). Other recent research has shown, using an HBV mouse model, how TLR signaling enhances HBV-specific CD8+ T-cell responses, which regulate the infection (47). In an HBV hydrodynamic mouse model, intraperitoneal inoculation of 5,6-dimethylxanthenone-4-acetic acid (DMXAA), a STING agonist, induced type I IFNs that decreased HBV DNA replication intermediates in the mouse livers, demonstrating the role of the innate immune response in suppressing HBV replication (53). Tree shrews (Tupaia belangeri) be a useful animal model for HBV infection as HBV induces human-like symptoms in them, such as hepatitis and chronic infection. Tree shrews’ whole genomes have shown that they are more closely linked to humans than to rodents (54). Numerous studies have indicated that tree shrews are vulnerable to HBV infection (39). In research, Kayesh et al. (39) examined the infectivity of HBV-F infection in tree shrews, both wild-type and mutant (HBV-F Wt/Mt), and they also described the innate immune response against HBV infection in this model. found that HBV-F Wt/Mt may cause an acute infection in adult tree shrews, as evidenced by elevated alanine aminotransferase (ALT) levels in the sera and the presence of HBV DNA in the liver tissues of both 7 and 14 dpi-infected tree shrews. Only HBV-F-Wt-infected tree shrew liver tissues showed induction of IFN-β and upstream TLR1, 3, 7, 8, and 9, indicating a more stealthy form of HBV-F-Wt infection. TLR response characterization revealed several distinct responses between Mt and HBV-F-Wt, including downregulating TLR8 in Tupaia liver tissue infected with HBV-F-Wt. These variations may thus be related to the increased pathogenicity and replication activity of HBV-F-Mt compared to Wt; however, more research employing the tree shrew model is needed (39). Overall, these results point to a significant role that innate immunity plays in inhibiting HBV replication; further research is needed to determine the precise mechanisms (47).

Chen et al. showed that serum anti-HBV response induced by vaccination was shown to be strongly related to four SNPs (rs2243248, rs1805015, rs1295686, and rs3804100) in the IL-4, IL-4RA, IL-13, and TLR2 genes (94). In addition, the length and strength of the protective humoral immune response to the hepatitis B vaccination may also be impacted by sequence changes in genes related to pathogen identification, antigen processing and presentation, and cell differentiation and maturation. In this study, the frequencies of 53 known SNPs within 21 putative genes were compared between 24 non-responders and 46 responders. There was a significant (P < 0.05) correlation found between four SNPs in the IL-4, IL-4RA, IL-13, and TLR2 genes and the vaccination-induced serum anti-HBV response in the vaccinated individuals. When examined in conjunction with risk variables, including age and gender, two SNPs (rs1295686 within the 5-hydroxytryptamine receptor 2A (HTR2A) gene and rs1805015 within the solute carrier family 22 member 4 (SLC22A4) gene) also showed a significant connection with the vaccine-induced immune response (P < 0.05) by multivariable logistic regression analysis. Moreover, haplotype analysis showed that non-responders had a higher frequency of the AG haplotype, which is generated by SNPs rs1143633 (IL-1B; intron) and rs1143627 (IL-1B; intron), than did responders. Consequently, the status of the hepatitis B vaccine-induced protective humoral immune response was associated with SNPs in the cytokine/cytokine receptor genes and TLR2. Also, TLR2 rs3804100 polymorphisms may be protective factors for HBV-related HCC (94, 97). Genotyping assists in identifying HBV response opponents. The rs3804099 in TLR2 gene CT and TT genotypes have been demonstrated to influence hepatitis B progression (95). Xie et al. also conducted a haplotype study on the loci rs3804100 and rs3804099. They observed that the TT haplotype was importantly linked with lower HCC invasion. Mutually, the CC haplotype enhances the risk of HCC in patients over time (101). Using TLR2/MyD88/NF-κB signaling, STAT1-Ser727 phosphorylation, and IL-10-associate, partially autocrine mechanism, HBV increases monocyte production of inflammatory cytokines and inhibits IFN-α-stimulated stat1, stat2, and ch25h expression. In patients with CHB, coupling mRNA rates of IL-1 and IL-10 were usually more remarkable than in healthy people. To disrupt the antiviral innate immune response, HBV enhances the inflammatory response of monocytes while suppressing their IFN-α/β responsiveness. These efficacies have interceded through differential phosphorylation of Tyr701 and Ser727 of STAT1 (102). Researchers found that in a different study, rs4696480 and rs3804099 in TLR2 mutant carriers improved liver action parameters and reduced the rate of HBsAg, suggesting that TLR gene mutations may be able to lessen the inflammatory damage that results in long-term HBV diseases. Researchers showed that the mutations in rs3804099 and rs4696480 were related to an inhibited rate of TNF-α and IL-6 between patients with CHB compared to wild-type carriers. Additionally, the TLR2 mutant carriers had lower levels of pro-inflammatory cytokines and less inflammation due to the mutations in rs4696480 and rs3804099 (95).

TLR3 is an innate immune system component that provides an early immunological response to foreign antigens by recognizing double-stranded RNA (dsRNA). Genetic variations, such as SNPs in TLR3, may contribute to an individual’s vulnerability to viral infections, such as HBV (103). Seven SNPs in the TLR3 gene are related to HBV, one of which is the rs1879026 polymorphism. The T allele of the rs1879026 polymorphism was shown to be substantially correlated with Saudis’ risk of contracting HBV infection in research conducted on a Saudi population (103). Nonetheless, a Chinese cohort revealed no correlation between the mutation and HBV-related liver illness. This shows that HBV susceptibility or ethnic variances may be influenced by the rs1879026 polymorphism, but not HBV-associated liver disorders (104). The haplotype GCGA of rs1879026, rs5743313, rs5743314, and rs5743315 were related to HBV in Saudi Arabians, as determined by haplotype analysis. The haplotype GT transporters of rs1879026-rs3775290 had a detracted risk of developing CHB, liver cirrhosis (LC), and HCC (41). However, a study conducted by Xu et al. did not find any significant link between the haplotype GT and HBV infection in a meta-analysis of diverse ethnic groups (41). There is disagreement over the relationship between the rs3775290 polymorphism and HBV (103). According to Goktas et al., patients with the TT genotype had higher HBV DNA levels than those with the CC and CT genotypes. This implies that a continued CHB condition may be associated with the TT genotype. A study conducted in Tunisian showed that the T allele became significantly related to chronic HBV infection and elevated the risk of HBV-associated HCC (105, 106). The TT genotype can inhibit CHB, HBV-associated LC, and HCC in Chinese populations but can decrease the likelihood of HBV transmission. The rs3775290 polymorphism on chromosome 4’s TLR3 gene can inhibit TLR3 activation, hindering pathogen recognition and weakening the immune response, thereby increasing the disease’s infectiousness (104). A meta-analysis validated the association between the rs3775291 polymorphism and HBV susceptibility, with the T allele and TT genotype enhancing the chances of HBV infection (107). Rong et al. found that individuals with the CT and TT genotypes had a 1.4-overlay and 2.3-fold greater probability of developing CHB, respectively, compared to those with the CC genotype. Similarly, they increased the CHB-associated high risk of acute chronic liver failure (ACLF) (108). A meta-analysis indicated that rs3775291 is a substantial risk factor for HBV-associated liver infection (107, 109). Li and Zheng discovered that it is associated with prolonged susceptibility to HBV-associated HCC (110). Chen et al. demonstrated, however, that polymorphism is likely an inhibitor factor for HBV-associated HCC. It was determined that the rs3775291 GG genotype was related to independence from HBsAg and HBeAg restriction. On the other hand, the A allele was linked to an increased risk of CHB (97, 111). The rs3775291 polymorphism was unrelated to HBV impotence in a Brazilian individual, possibly due to ethnic differences. This mutation causes the substitution of leucine with phenylalanine at position 412 of the TLR3 protein, and in vitro analysis has shown that it impairs the ability of TLR3 (112). TLR3 expression in HCC tissues may influence apoptosis and decrease HCC cell proliferation and angiogenesis (89).

TLR4 initiates downstream signaling by using adaptor proteins TRIF, TRAM, and MyD88, which results in the activation of protein kinases and the production of IFN-β. TLR-mediated activation of the MyD88-dependent pathway regulates the production of genes necessary for adaptive immunity and inflammation, including pro-inflammatory IL-6 (113). rs1800769, rs2069845, and rs1880242 are SNPs in the TLR4 gene, which encodes protein. These SNPs have been linked to changes in the TLR4 gene, which produces the TLR4 protein. The innate immune system relies on this protein to recognize infections and trigger immunological responses (113). Polymorphism rs4986790 refers to a missense variant in the coding region that substitutes aspartic acid (69) for glycine (Gly) at the 299th position of the TLR4 protein (Gly). This molecule is found in TLR4’s extracellular domain near the myeloid differentiation protein-2 (MD-2) connection point (114, 115). In an investigation, there was a higher frequency of TLR3 rs3775290 CC genotype and TLR4 rs4986790 AA genotype among patients who had a stable response to the viral treatment, as compared to those who did not respond to the treatment. The study suggested that the TLR3 and TLR4 gene variants played a significant role in the treatment response, with TLR3 being more effective than TLR4 in reducing the viral load (106). In addition, rs1800796, rs2069845, and rs1880242 were associated with sustained viral response. Furthermore, researchers demonstrated that the common favorable IL-28B variant is essential for TLR-triggered antiviral preservation (113). It has been found that the AA wild-type genotype may have an impact on the severity of HBV-associated liver fibrosis in men, which can affect the early diagnosis of CHB (116). The TLR4 mRNA contains a polymorphism known as rs11536889 in its 3’-untranslated region (3`-UTR), which hinders LP-induced transmembrane signal transduction via post-transcriptional control of 3`-UTR. Moreover, this polymorphism is also responsible for preventing the recurrence of HBV following liver transplantation (117, 118). Based on a distinct study, it was observed that patients with CHB having CG and GG genotypes were independently associated with HCC. Further stratified studies revealed that the G allele of rs11536889 and the A allele of rs2149356 were identified as risk factors for cirrhosis in patients with CHB who do not consume alcohol (119, 120). A study by Abdelwahed et al. found that rs2069845 was associated with a reduced risk of CHB in a Saudi Arabian cohort (121).

SNP rs5744174 is one of the TLR5 gene’s genetic mutations. It has been demonstrated that the SNP rs5744174 mutation affects how the body reacts to infections. Research has examined the relationship between this SNP and the severity or susceptibility to certain infectious illnesses (122). According to a study by Katrinli and colleagues, the rs5744174 polymorphism of the TLR5 gene in Turkish society is linked to CHB. The rs5744174 mutation converts Phe to Leu at position 616 of TLR5 (25). Due to differences in protein sequence and structure, this variation significantly impacts HBV sensitivity more than TLR5 expression levels (123). The TT genotype may protect against hepatitis B. Additionally, the T allele was observed to be associated with spontaneous conversion of blood HBeAg to TT, as well as higher IFN secretion levels (124, 125). Recent research reveals that genetic variations may lead to HBV-related severe liver damage (125). In addition, a study by Wu et al. found that the T allele at TLR5 rs5744174 was associated with earlier spontaneous HBeAg seroconversion in a Taiwanese cohort. This SNP may affect the IFN-γ production in response to HBV infection (126). In contrast, a study by Zhang et al. found no significant association between the rs5744174 SNP and CHB infection in a Chinese population. This SNP may not have a consistent effect on HBV infection across different ethnic groups (41).

Through its ability to stimulate robust type I IFN production in pDCs, as well as to encourage B-cell activation and germinal center (94) responses, TLR7 is crucial for the induction of antiviral immunity. TLR7 is an X-linked gene-encoded single-stranded RNA receptor subject to strong purifying selection, indicating that it has a non-redundant biological role in host survival. TLR7 has been shown to play a crucial component in the GC responses to virus-like particles and ssRNA viruses (VLP). B-cell internal To counteract exogenous retrovirus, the GC responses depend on Myd88/TLR7 signaling (127). It has been demonstrated that TLRs, such as TLR7, promote B cell responses to immunogenic nucleic acids and immunological complexes (128) containing nucleic acids. Under normal circumstances, B cells can use nucleic acid recognition to react to early microbial assaults right away. Autoantibodies and autoimmunity, however, are encouraged when severe cellular or tissue damage takes place and B cell responses to an endogenous cellular nucleic acid are not inhibited. Many data points to the coordinated activation of TLR7 and the B cell receptor (BCR) in aberrantly activated B cells. By learning more about the possible molecular synergy between the TLR7 and BCR pathways in B cells, treatments that may be able to stop autoimmune states in patients can be developed (129). Zhu et al. conducted a study to determine the susceptibility and progression of CHB in Chinese individuals with altered TLR7 polymorphisms (rs179010, rs2074109, and rs179009). Another study revealed that Chinese men with CHB had a high frequency of the C allele of TLR7 rs179010, indicating an increased risk of CHB infection (23, 96). A study conducted by Al-Qahtani et al. found that the TLR7 rs179010 polymorphism is linked with a lower risk of CHB infection in Saudi Arabians (41). This polymorphism may impact the expression and function of TLR7, a receptor that detects viral RNA and triggers the immune response. The study suggests that the TLR7 rs179010 polymorphism may have an impact on the B-cell responses to HBV infection by altering the production of interferon-α (IFN-α) and immunoglobulin G (IgG) (47).

While NK cells are the primary source of IFN-γ production, TLR8 agonists can selectively engage the immune system in the liver to increase the output of IFN-γ. It has been demonstrated that the TLR8 agonist GS-9688 may cause a long-lasting antiviral response in woodchucks infected with the hepatitis virus. GS-9688 is regarded as a possible medication to treat CHB since it may also cause the generation of cytokines in human immune cells to activate antiviral effector function. Many studies have demonstrated that HBV may block the TLR signaling pathway to evade the immune response, even if activating the TLR8 signaling pathway might cause NK cells to release IFN-γ (128). TLR8, a critical pyogenic bacteria sensor, is diminished by TLR signaling on the cell surface. Viral and bacterial infections are recognized by TLR8, an endosomal sensor of RNA breakdown products in human phagocytes (130). Uridine and small oligomers, both byproducts of RNA degradation, bind cooperatively at two separate locations in the N-terminal domain. The already-formed TLR8-dimer undergoes a conformational shift, allowing for the recruitment and signaling of MyD88 (131). Monocytes and myeloid dendritic cells are the primary cell types that express TLR8 (132–134). The researchers presented data obtained outside the living organism (ex vivo) that supports the theory that RNA from borrelia bacteria, delivered through endosomal vacuoles, activates TLR8. These investigations also showed that TLR8 is self-amplifying and solely responsible for inducing IFN-β in highly pure human monocytes through IRF-7, a similar signaling pathway linked to RNA viruses (135). The TLR8 SNP rs3764880 (Met1Val) modulates the translation of the two main TLR8 isoforms and plays an essential role in the immune reaction (136). In investigations, researchers screened about 250 Han Chinese individuals in Taiwan. They showed that a TLR8 Met/Val replacement at the start codon and a G>C alteration (rs3764880) at the TLR8−129 situation in the promoter zone (rs3764879) are in complete relation disequilibrium, with TLR8-129G/+1G allele frequencies of 84·0%. An SNP at the beginning codon (ATG>GTG, Met 1 Val) of TLR8 causes a frame-shift mutation, resulting in the organization of a truncated shape of TLR8 (1038 versus 1041 amino acids) comprising a three-amino-acid elimination at the N-terminus, which is as well as forecasted to be a part of the signal peptide (137, 138). However, the lack of animal models that accurately replicate the function of the TLR8 receptor is hindering research into its precise role in CHB. Furthermore, a thorough investigation of TLR8 expression patterns in CHB patients’ PBMCs is required, as is knowledge of how TLR8 is regulated in response to antiviral medication (42, 139, 140).

Chihab et al. discovered three SNPs on the TLR9 gene related to HBV (93). The rs18708 G allele inhibits the development of HBV illnesses, whereas the AA genotype may significantly contribute to the progress of HBV infection relevant to progressive liver disease (AdLD). Moreover, it was substantially linked with HBV DNA load in genotype AA individuals with more DNA than the AG genotype. The rs573836 polymorphism was similarly linked to HBV DNA load, with the AA genotype having a lower DNA burden than the GG genotype (93). Additionally, Wu et al. demonstrated that the rs573836 CT genotype is linked with the initial spontaneous conversion of HBeAg (123). The rs5743836 polymorphism influences the transcription of TLR9 mRNA in the promoter region of the TLR9 gene. It has been discovered that its C allele increases promoter action, resulting in elevated TLR9 expression and thymosin 1, which signal and limit HBV growth via downstream cytokines (141). He et al. demonstrated that the rs352140 polymorphism was related to chronic HBV infection vulnerability and risk ratio for CT genotype carriers. It was insignificant to CC and TT genotypes (89). In an investigation, researchers examined the possibility of a relationship between an individual’s sensitivity to HBV intrauterine transmission and polymorphisms in TLR3 (rs3775290) and TLR9 (rs352140). The synonymous +2848 G > A polymorphism exists in exon two of the TLR9 gene and presumably influences expression at the mRNA rate. Following regulating risk agents, including maternal HBeAg, maternal HBV DNA, and mode of transfer, the minor allele ‘A’ of TLR9 was remarkably related to HBV intrauterine transmission capability, and the GA genotype preserved neonates from HBV intrauterine transmission. Carrier analysis showed that TLR9 GA+AA was mainly associated with intrauterine transmission capability (53, 142) ( Figure 3 ).