Toll-Like Receptor Response To Hepatitis B Virus Infection And Potential Of TLR Agonists As Immunomodulators For Treating Chronic Hepatitis B: An Overview Part 2
Jun 21, 2023
3. Inhibition of Innate Immune Response by HBV Infection
The exact mechanisms of immune evasion or inhibition by HBV remain unclear. Moreover, whether HBV evades or inhibits innate recognition, or stimulates innate immunity has not yet been verified [75]. However, HBV protein levels, including HBsAg and HBeAg, are associated with HBV persistence, which is suggestive of the role of viral proteins in the suppression of host immune response [76–78].
HBV immune evasion refers to the ability of the hepatitis B virus to mutate or selectively suppress the host's immune response so that the immune system cannot effectively clear the virus infection. This evasion mechanism is one of the important reasons for the long-term persistence, replication, and reproduction of HBV.
Immunity refers to the ability of the human immune system to resist pathogens. For HBV-infected people, people with strong immunity can produce a large number of antibodies in their bodies, which can promote immune response and help clear the virus; people with weak immunity will experience multiple hepatitis B virus infections and the possibility of turning into chronic hepatitis B.
Therefore, HBV immune evasion is closely related to immunity. Strong immunity can overcome the escape mechanism of the virus, while weak immunity may be exploited by the virus to further damage the body's health. Therefore, improving immunity is one of the important means for hepatitis B patients to enhance their resistance to the virus. From this point of view, we need to improve immunity. Cistanche can significantly improve immunity. The polysaccharides in Cistanche can regulate the immune response of the human immune system, improve the stress ability of immune cells, and enhance the immunity of immune cells. Bactericidal effect.
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Several studies have enhanced our understanding of the innate immune response modulation by HBV [38,79]. HBV has been reported to suppress TLR-mediated antiviral response in hepatic cells by interfering with the activation of IRF-3, nuclear factor kappa B (NF-κB), and extracellular signal-regulated kinase (ERK) 1/2 [80]. HBV has been shown to inhibit TLR9 response by inhibiting the MyD88-IRAK4 axis in pDCs obtained from patients [54]. In another study, TLR9 expression and TLR9-mediated B cell functions were suppressed in all peripheral B cell subsets exposed to HBV [81].
Impaired expression of TLR4, 8, and 9 in peripheral DC subsets from patients with chronic HBV infection has also been reported [82], which induced a reduced innate immune function. HBV polymerase was reported to block IRF activation by disrupting the interaction between IκB kinase-ε and DEAD-box RNA helicase and inhibiting IFN production [83]. Another study showed the inhibition of STING-mediated IRF-3 activation and IFN-β production by HBV polymerase [84]. HBsAg has been reported to suppress the activation of NF-κB, IRF-3, and MAPKs in murine hepatocytes [85]. Moreover, T-cell activation induced by TLR3-stimulated murine KCs or LSECs was also suppressed by HBsAg [85], suggesting the potential of HBsAg to attenuate or inhibit TLR-mediated immune response. Using RPMI 8226, a human myeloma B cell line, it was shown that HBsAg induced TLR9 dysfunction by suppressing HBsAg-induced phosphorylation, which activated the transcription factor, CREB, thereby preventing TLR9 promoter activity [81].
A recent study also reported HBsAg-mediated interference of the NF-κB pathway by interaction with TAK1 and TAB2, leading to a suppressed immune response [86]. Moreover, HBc-mediated downregulation of interferon-induced transmembrane protein 1 (IFITM1) expression [87] and HBeAg-mediated suppression of the NF-κB signaling pathway, inhibiting the innate immune response [88,89], has also been reported. The HBV X protein (HBx) plays a critical role in inhibiting the innate immune response, and several studies have indicated the association of HBx in the suppression of type I IFN production by downregulating MAVS protein [90–93]. HBx-mediated downregulation of TIR-domain-containing adaptor-inducing interferon-beta (TRIF), a key component of innate immune signaling, has also been shown [43]. HBx can suppress the transcription of TRIM22 through a single CpG methylation in its 5’-UTR, reducing the binding affinity of IRF-1 and inhibiting IFN-mediated anti-HBV response [94]. Recently, a key mechanism of innate immune evasion by HBx in hepatocytes has been reported, where HBx-induced adenosine deaminases acting on RNA 1 (ADAR1) deaminates adenosine to generate inosine by interacting with HBV RNAs and avoiding host immune recognition [19].
Immune evasion by attenuating RIG-I signaling via N6 methyladenosine (m6A) modification of HBV RNAs has been reported previously [95]. Recently, HBV-induced N6 m6A modification of phosphatase and tensin homolog (PTEN) affecting innate immunity with decreased IRF-3 dephosphorylation and IFN synthesis has also been reported [96]. Zhou et al. reported that HBV inhibited RIG-I-induced IFN production by forming a ternary complex including hexokinase and sequestering MAVS from RIG-I [97]. Impairment of NK cell function by HBV has also been reported [98]. HBV can evade cGAS sensing in HepG2-NTCP cells with a fully functional cGAS-STING pathway [99]; however, the mechanism remains unknown. Moreover, the demonstration of the sensing of naked HBV rcDNA but not the infectious HBV virions by cGAS in PHH indicates the impairment of cGAS sensing during HBV infection [99,100], although the exact mechanism remains unclear.
From the above findings, it is reasonable to consider that establishing HBV infection and its protein components can inhibit/suppress the host innate immune response (Figure 2) by different known and unknown mechanisms, and therefore, a detailed understanding of the immune inhibition/evasion is crucial for devising new therapeutic and preventive strategies to control HBV infection.
4. Potential of TLR Agonists as Immunomodulators
As the immune response is suppressed in chronic HBV infection, restoration of the immune response is crucial for improving the chronic infection outcome, which can be achieved using immunomodulators, such as TLR agonists, checkpoint inhibitors, and therapeutic vaccines [4,101,102]. In this study, we mainly focused on the use of TLR agonists as immunomodulators to enhance the immune response in chronic infection. TLR agonists play a significant role in modulating the immunotherapeutic effects [103,104]. Recently, TLR agonists have attracted interest for use as vaccine adjuvants or immune modulators because of their ability to induce the production of IFN, proinflammatory cytokines, and chemokines, which may exert anti-HBV effects [105,106]. TLR1/2 and TLR3 agonists can inhibit HBV replication in PHH [105].
Another study showed that GS-9620 (vesatolimod), an oral TLR7 agonist, along with nucleos(t)ide analogs (NAs) positively enhanced the immunomodulatory effects by increasing the T cell and NK cell responses and reduced the ability of NK cells to suppress T cells in chronically infected patients [107]. However, no significant change in HBsAg level was found after combination therapy with NAs and GS-9620 [107]. A recent study demonstrated that dual-acting TLR7/8 (R848) and TLR2/7 (CL413) agonists are more potent in inhibiting HBV replication than single-acting TLR7 (CL264) or TLR9 (CpG-B) agonists [108], which highlights the higher potential of dual-acting TLR agonists in inducing broad cytokine repertoires. Several studies have demonstrated the ability of TLR agonists to suppress HBV in animal models.
In a previous study, GS-9620 was shown to induce an immune response by increasing the production of IFN-α, other cytokines, and chemokines in chronically infected chimpanzees. It reduced viral loads over 2 logs and serum HBsAg and HBeAg levels by 50% [109], highlighting the effectiveness of TLR7 agonists in the treatment of chronic HBV infection and supporting the further investigation of this candidate in human CHB. In another recent study, the mechanism of GS-9620 action was investigated in CHB chimpanzees, and it was reported that GS-9620 exerts an anti-HBV response associated with aggregation of immune cells in the liver that can either kill HBV-infected cells or can prevent HBV from infecting new cells by producing antibodies [110].
In the woodchuck model of CHB, the administration of the GS-9620 also induced a significant reduction in serum viral DNA and intrahepatic woodchuck hepatitis virus (WHV) DNA replicative intermediates, WHV cccDNA and WHV RNA [111], indicating that the TLR7 agonist is a potent immunomodulator with anti-HBV effects in the woodchuck model. Moreover, the incidence of HCC was remarkably reduced in GS-9620-treated woodchucks [111]. In another study, it was shown that oral administration of another TLR7 agonist, APR002, in combination with entecavir (ETV) enhanced ISG expression and reduced WHV cccDNA in chronically infected woodchucks with WHV [112]. In a phase 1b clinical trial, oral administration of GS-9620 was found to be safe and well-tolerated, which enhanced peripheral ISG15 production without significant systemic IFN-α levels [113]. AL-034, an oral TLR7 agonist, showed efficacy against HBV in a mouse model [114].
The evaluation of the HBV therapeutic vaccine that consisted of a novel TLR7 agonist (named T7-EA), an alum adjuvant, and a recombinant HBsAg protein, indicated that T7-EA induced Th1-type immune responses, as well as increased T-cell response and HBsAg-specific IgG2a titer in a mouse model [115]. An earlier study reported that TLR8 agonist ssRNA40 could selectively activate liver-resident innate immune cells, and mucosal-associated invariant T cells and NK cells were identified as IFN-γ-producing cells after TLR8 activation [116], which highlighted the possibility of using TLR8 agonist in treating CHB. GS-9688 (selgantolimod), an oral TLR8 agonist, was found as a potent anti-HBV molecule in HBV-infected primary human hepatocytes [117].
In another study, GS-9688 was reported to exert an anti-HBV effect in the woodchuck model of CHB [118], where it reduced viral loads over 5 logs and suppressed woodchuck hepatitis surface antigen (WHsAg) in 50% of the treated chronically infected woodchucks [118]. Another recent study also showed the therapeutic potential of GS-9688 in CHB, where GS-9688 induced cytokines in human PBMCs that could activate antiviral effector function by different immune mediators, including NK cells, HBV-specific CD8+ T cells, CD4+ follicular helper T cells, and mucosal-associated invariant T cells [119]. The anti-HBV activity of TLR9-ligand has been reported previously [120,121]. The TLR9 agonist, CpG oligodeoxynucleotides (ODNs), was also evaluated in the woodchuck model, and it was observed that the WHsAg serum level was suppressed by the combined administration of CpG and ETV, but no effect was observed with either agent alone [122], suggesting synergistic effects. CpG oligonucleotides increased HBV-specific IL2 and IFN-γ responses in whole blood stimulated with HBsAg or HBcAg [123]. An earlier study reported the ability of AIC649, a TLR9 agonist, in reducing WHV DNA, and WHsAg in a unique biphasic response pattern [124].
Recently, it was shown that AIC649, in combination with ETV, could effectively suppress WHV DNA and WHsAg in the woodchuck model of CHB [125]. A previous study showed the use of the TLR3 ligand, poly (I: C), as an effective adjuvant for HBV therapeutic vaccine (named pHBV-vaccine), which effectively suppressed HBV replication [126]. A recent study also reported the improvement of liver infection status when poly (I: C) was delivered by calcium phosphate nanoparticles conjugated with an F4/80 antibody. It also enhanced T-cell responses and the production of intrahepatic cytokines and chemokines, which significantly reduced HBsAg, HBeAg, and HBV DNA levels in mice [127]. A recent study showed that HBV interacted with SIGLEC-3 (CD33) and served as an immune checkpoint receptor for HBV infection and impaired the host immunity [128].
In PBMCs from CHB patients, it has also been shown that anti–SIGLEC-3 mAb could reverse the effect and induce a cytokine response to the TLR-7 agonist, GS-9620 [128]. TLR agonists as vaccine adjuvants are currently under investigation for different human vaccines, including HBV vaccines, and appear promising in vaccine studies [129,130]. Vaccines containing particular TLR agonist(s) may activate specific TLR(s) and enhance vaccine efficacy without direct participation in protective immunity [131,132].
In an investigation of a therapeutic synthetic long peptide (SLP)-based vaccine to treat chronic HBV, it has been shown that TLR2-ligand conjugation of the prototype HBV-core SLP triggered functional patient T cell responses ex vivo [133], suggesting that TLR agonists may also act as potential adjuvants in HBV vaccines. A recent study has also demonstrated the ability of PRR ligands to induce innate immunity toward HBV control [134]. Therefore, the use of TLR agonists in HBV therapeutics/vaccines seems promising and could be an effective tool in the control of HBV chronic infection, which requires further investigation. The TLR agonists as immunomodulators in development are shown in Table 1.
5. Conclusions
Despite the availability of an effective HBV vaccine, chronic HBV infection remains a global health problem. Recent studies have highlighted HBV as a cunning virus, which also raises questions about the stealth properties of HBV and shows the ability of the virus to interfere with the innate immune response and establish an infection. From the available data, it is assumed that chronic HBV infection induces the dysregulation of host innate immunity, including the suppression of TLR response and downstream cytokines.
The induction of host innate immune response by using TLR agonists may aid in the better understanding of the host innate immune responses, particularly the interaction between TLRs and viral components. Thus, although TLR agonists have shown promising results in improving the innate immune response during HBV infection, further studies are required to investigate the mechanisms behind modulating the host immune response and restoring immunity.
Author Contributions:
Conceptualization, M.E.H.K., M.K. and K.T.-K.; writing-original draft preparation, M.E.H.K.; writing-review and editing, M.E.H.K., M.K. and K.T.-K.; supervision, K.T.-K. All authors have read and agreed to the published version of the manuscript.
Funding:
This study was supported by a grant (innovative drug development network) from the Japan Agency for Medical Research and Development (AMED).
Institutional Review Board Statement:
Not applicable.
Informed Consent Statement:
Not applicable.
Acknowledgments:
We would like to thank Takahiro Sanada and Sayeh Ezzikouri for their kind collaboration.
Conflicts of Interest:
The authors declare no conflict of interest.
References
1. Liang, T.J. Hepatitis B: The virus and disease. Hepatology 2009, 49, S13–S21. [CrossRef]
2. Mastrodomenico, M.; Muselli, M.; Provvidenti, L.; Scatigna, M.; Bianchi, S.; Fabiani, L. Long-term immune protection against HBV: Associated factors and determinants. Hum. Vaccines Immunother. 2021, 17, 2268–2272. [CrossRef]
3. World Health Organization. Hepatitis B. Updated on 27 July. Available online: https://www.who.int/news-room/fact-sheets/ detail/hepatitis-b (accessed on 28 July 2021).
4. Ezzikouri, S.; Kayesh, M.E.H.; Benjelloun, S.; Kohara, M.; Tsukiyama-Kohara, K. Targeting Host Innate and Adaptive Immunity to Achieve the Functional Cure of Chronic Hepatitis B. Vaccines 2020, 8, 216. [CrossRef]
5. Lee, W.M. Hepatitis B virus infection. N. Engl. J. Med. 1997, 337, 1733–1745. [CrossRef]
6. Huang, H.L.; Jeng, K.S.; Hu, C.P.; Tsai, C.H.; Lo, S.J.; Chang, C. Identification and characterization of a structural protein of hepatitis B virus: A polymerase and surface fusion protein encoded by a spliced RNA. Virology 2000, 275, 398–410. [CrossRef]
7. Hu, J.; Seeger, C. Hepadnavirus Genome Replication and Persistence. Cold Spring Harb. Perspect. Med. 2015, 5, a021386. [CrossRef]
8. Glebe, D.; Bremer, C.M. The molecular virology of hepatitis B virus. Semin. Liver Dis. 2013, 33, 103–112. [CrossRef]
9. Yan, H.; Zhong, G.; Xu, G.; He, W.; Jing, Z.; Gao, Z.; Huang, Y.; Qi, Y.; Peng, B.; Wang, H.; et al. Sodium taurocholate transporting polypeptide is a functional receptor for the human hepatitis B and D viruses. eLife 2012, 1, e00049. [CrossRef] [PubMed]
10. Takeuchi, J.S.; Fukano, K.; Iwamoto, M.; Tsukuda, S.; Suzuki, R.; Aizaki, H.; Muramatsu, M.; Wakita, T.; Sureau, C.; Watashi, K. A Single Adaptive Mutation in Sodium Taurocholate Cotransporting Polypeptide Induced by Hepadnaviruses Determines Virus Species Specificity. J. Virol. 2019, 93, e01432-18. [CrossRef] [PubMed]
11. Rouse, B.T.; Sehrawat, S. Immunity and immunopathology to viruses: What decides the outcome? Nat. Rev. Immunol. 2010, 10, 514–526. [CrossRef] [PubMed]
12. Zeisel, M.B.; Lucifora, J.; Mason, W.S.; Sureau, C.; Beck, J.; Levrero, M.; Kann, M.; Knolle, P.A.; Benkirane, M.; Durantel, D.; et al. Towards an HBV cure: State-of-the-art and unresolved questions–report of the ANRS workshop on HBV cure. Gut 2015, 64, 1314–1326. [CrossRef]
13. McMahon, B.J. Epidemiology and natural history of hepatitis B. Semin. Liver Dis. 2005, 25 (Suppl. S1), 3–8. [CrossRef]
14. Wieland, S.; Thimme, R.; Purcell, R.H.; Chisari, F.V. Genomic analysis of the host response to hepatitis B virus infection. Proc. Natl. Acad. Sci. USA 2004, 101, 6669–6674. [CrossRef] [PubMed]
15. Fletcher, S.P.; Chin, D.J.; Cheng, D.T.; Ravindran, P.; Bitter, H.; Gruenbaum, L.; Cote, P.J.; Ma, H.; Klumpp, K.; Menne, S. Identification of an intrahepatic transcriptional signature associated with self-limiting infection in the woodchuck model of hepatitis B. Hepatology 2013, 57, 13–22. [CrossRef] [PubMed]
16. Dunn, C.; Peppa, D.; Khanna, P.; Nebbia, G.; Jones, M.; Brendish, N.; Lascar, R.M.; Brown, D.; Gilson, R.J.; Tedder, R.J.; et al. Temporal analysis of early immune responses in patients with acute hepatitis B virus infection. Gastroenterology 2009, 137, 1289–1300. [CrossRef] [PubMed]
17. Suslov, A.; Boldanova, T.; Wang, X.; Wieland, S.; Heim, M.H. Hepatitis B Virus Does Not Interfere With Innate Immune Responses in the Human Liver. Gastroenterology 2018, 154, 1778–1790. [CrossRef]
18. Ma, Z.; Zhang, E.; Yang, D.; Lu, M. Contribution of Toll-like receptors to the control of hepatitis B virus infection by initiating antiviral innate responses and promoting specific adaptive immune responses. Cell Mol. Immunol. 2015, 12, 273–282. [CrossRef] [PubMed]
19. Wang, L.; Sun, Y.; Song, X.; Wang, Z.; Zhang, Y.; Zhao, Y.; Peng, X.; Zhang, X.; Li, C.; Gao, C.; et al. Hepatitis B virus evades immune recognition via RNA adenosine deaminase ADAR1-mediated viral RNA editing in hepatocytes. Cell Mol. Immunol. 2021, 18, 1871–1882. [CrossRef]
20. Tsuge, M.; Takahashi, S.; Hiraga, N.; Fujimoto, Y.; Zhang, Y.; Mitsui, F.; Abe, H.; Kawaoka, T.; Imamura, M.; Ochi, H.; et al. Effects of hepatitis B virus infection on the interferon response in immunodeficient human hepatocyte chimeric mice. J. Infect. Dis. 2011, 204, 224–228. [CrossRef]
21. Mutz, P.; Metz, P.; Lempp, F.A.; Bender, S.; Qu, B.; Schoneweis, K.; Seitz, S.; Tu, T.; Restuccia, A.; Frankish, J.; et al. HBV Bypasses the Innate Immune Response and Does Not Protect HCV From the Antiviral Activity of Interferon. Gastroenterology 2018, 154, 1791–1804.e1722. [CrossRef]
22. Luangsay, S.; Gruffaz, M.; Isorce, N.; Testoni, B.; Michelet, M.; Faure-Dupuy, S.; Maadadi, S.; Ait-Goughoulte, M.; Parent, R.; Rivoire, M.; et al. Early inhibition of hepatocyte innate responses by hepatitis B virus. J. Hepatol. 2015, 63, 1314–1322. [CrossRef]
23. Sanada, T.; Tsukiyama-Kohara, K.; Shin, I.T.; Yamamoto, N.; Kayesh, M.E.H.; Yamane, D.; Takano, J.I.; Shiogama, Y.; Yasutomi, Y.; Ikeo, K.; et al. Construction of complete Tupaia belangeri transcriptome database by whole-genome and comprehensive RNA sequencing. Sci. Rep. 2019, 9, 12372. [CrossRef] [PubMed]
24. Liu, H.Y.; Zhang, X.Y. Innate immune recognition of hepatitis B virus. World J. Hepatol. 2015, 7, 2319–2322. [CrossRef] [PubMed]
25. Zhang, Z.; Trippler, M.; Real, C.I.; Werner, M.; Luo, X.; Schefczyk, S.; Kemper, T.; Anastasiou, O.E.; Ladiges, Y.; Treckmann, J.; et al. Hepatitis B Virus Particles Activate Toll-Like Receptor 2 Signaling Initially Upon Infection of Primary Human Hepatocytes. Hepatology 2020, 72, 829–844. [CrossRef]
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