Preclinical development of TLR ligands as drugs for the treatment of chronic viral infections

Regulation of immune responses to infection through interaction between stem cell-derived exosomes and toll-like receptors mediated by microRNA cargoes

Infectious diseases are among the factors that account for a significant proportion of disease-related deaths worldwide. The primary treatment approach to combat microbial infections is the use of antibiotics. However, the widespread use of these drugs over the past two decades has led to the emergence of resistant microbial species, making the control of microbial infections a serious challenge. One of the most important solutions in the field of combating infectious diseases is the regulation of the host's defense system. Toll-like receptors (TLRs) play a crucial role in the first primary defense against pathogens by identifying harmful endogenous molecules released from dying cells and damaged tissues as well as invading microbial agents. Therefore, they play an important role in communicating and regulating innate and adaptive immunity. Of course, excessive activation of TLRs can lead to disruption of immune homeostasis and increase the risk of inflammatory reactions. Targeting TLR signaling pathways has emerged as a new therapeutic approach for infectious diseases based on host-directed therapy (HDT). In recent years, stem cell-derived exosomes have received significant attention as factors regulating the immune system. The regulation effects of exosomes on the immune system are based on the HDT strategy, which is due to their cargoes. In general, the mechanism of action of stem cell-derived exosomes in HDT is by regulating and modulating immunity, promoting tissue regeneration, and reducing host toxicity. One of their most important cargoes is microRNAs, which have been shown to play a significant role in regulating immunity through TLRs. This review investigates the therapeutic properties of stem cell-derived exosomes in combating infections through the interaction between exosomal microRNAs and Toll-like receptors.

Innate Immunity, Inflammation, and Intervention in HBV Infection

Hepatitis B virus (HBV) infection is still one of the most dangerous viral illnesses. HBV infects around 257 million individuals worldwide. Hepatitis B in many individuals ultimately develops hepatocellular carcinoma (HCC), which is the sixth most common cancer and the third leading cause of cancer-related deaths worldwide. The innate immunity acts as the first line of defense against HBV infection through activating antiviral genes. Along with the immune responses, pro-inflammatory cytokines are triggered to enhance the antiviral responses, but this may result in acute or chronic liver inflammation, especially when the clearance of virus is unsuccessful. To a degree, the host innate immune and inflammatory responses dominate the HBV infection and liver pathogenesis. Thus, it is crucial to figure out the signaling pathways involved in the activation of antiviral factors and inflammatory cytokines. Here, we review the interplay between HBV and the signal pathways that mediates innate immune responses and inflammation. In addition, we summarize current therapeutic strategies for HBV infection via modulating innate immunity or inflammation. Characterizing the mechanisms that underlie these HBV-host interplays might provide new approaches for the cure of chronic HBV infection.

Novel Molecular Therapeutics Targeting Signaling Pathway to Control Hepatitis B Viral Infection

Numerous canonical cellular signaling pathways modulate hepatitis B virus (HBV) replication. HBV genome products are known to play a significant role in regulating these cellular pathways for the liver’s viral-related pathology and physiology and have been identified as the main factor in hepatocarcinogenesis. Signaling changes during viral replication ultimately affect cellular persistence, multiplication, migration, genome instability, and genome damage, leading to proliferation, evasion of apoptosis, block of differentiation, and immortality. Recent studies have documented that numerous signaling pathway agonists or inhibitors play an important role in reducing HBV replication in vitro and in vivo, and some have been used in phase I or phase II clinical trials. These optional agents as molecular therapeutics target cellular pathways that could limit the replication and transcription of HBV or inhibit the secretion of the small surface antigen of HBV in a signaling-independent manner. As principle-based available information, a combined strategy including antiviral therapy and immunomodulation will be needed to control HBV infection effectively. In this review, we summarize recent findings on interventions of molecular regulators in viral replication and the interactions of HBV proteins with the components of the various targeting cellular pathways, which may assist in designing novel agents to modulate signaling pathways to prevent HBV replication or carcinogenesis.

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.

Exploring evidence-based innovative therapy for the treatment of chronic HBV infection: experimental and clinical

With the advent of various vaccines and antimicrobial agents during the 20th century, the control and containment of infectious diseases appeared to be a matter of time. However, studies unveiled the diverse natures of microbes, their lifestyle, and pathogenetic potentials. Since the ground-breaking discovery of the hepatitis B virus (HBV) by Baruch Blumberg and the subsequent development of a vaccine in the early 1980s, the main task of the scientific community has been to develop a proper management strategy for HBV-induced chronic liver diseases. In the early 1980’s, standard interferon (IFN) induced a reduction of HBV DNA levels, followed by the normalization of serum transaminases (alanine aminotransferase, ALT), in some chronic hepatitis B (CHB) patients. However, in the course of time, the limitations of standard IFN became evident, and the search for an alternative began. In the late 1980’s, nucleoside analogs entered the arena of CHB treatment as oral drugs with potent antiviral capacities. At the beginning of the 21st century, insights were developed into the scope and limitations of standard IFN, pegylated-IFN as well as nucleoside analogs for treating CHB. Considering the non-cytopathic nature of the HBV, the presence of covalently closed circular DNA (cccDNA) in the nucleus of the infected hepatocytes and HBV-induced immune-mediated liver damages, a new field of CHB management was initiated by modulating the hosts’ immune system through immune therapy. This review will discuss the nature and design of innovative immune therapy for CHB.

Recent Advances in Understanding, Diagnosing, and Treating Hepatitis B Virus Infection

Chronic hepatitis B virus (HBV) infection affects 292 million people worldwide and is associated with a broad range of clinical manifestations including cirrhosis, liver failure, and hepatocellular carcinoma (HCC). Despite the availability of an effective vaccine HBV still causes nearly 900,000 deaths every year. Current treatment options keep HBV under control, but they do not offer a cure as they cannot completely clear HBV from infected hepatocytes. The recent development of reliable cell culture systems allowed for a better understanding of the host and viral mechanisms affecting HBV replication and persistence. Recent advances into the understanding of HBV biology, new potential diagnostic markers of hepatitis B infection, as well as novel antivirals targeting different steps in the HBV replication cycle are summarized in this review article.

Advances in Targeting the Innate and Adaptive Immune Systems to Cure Chronic Hepatitis B Virus Infection

“Functional cure” is being pursued as the ultimate endpoint of antiviral treatment in chronic hepatitis B (CHB), which is characterized by loss of HBsAg whether or not anti-HBs antibodies are present. “Functional cure” can be achieved in <10% of CHB patients with currently available therapeutic agents. The dysfunction of specific immune responses to hepatitis B virus (HBV) is considered the major cause of persistent HBV infection. Thus, modulating the host immune system to strengthen specific cellular immune reactions might help eliminate HBV. Strategies are needed to restore/enhance innate immunity and induce HBV-specific adaptive immune responses in a coordinated way. Immune and resident cells express pattern recognition receptors like TLRs and RIG I/MDA5, which play important roles in the induction of innate immunity through sensing of pathogen-associated molecular patterns (PAMPs) and bridging to adaptive immunity for pathogen-specific immune control. TLR/RIG I agonists activate innate immune responses and suppress HBV replication in vitro and in vivo, and are being investigated in clinical trials. On the other hand, HBV-specific immune responses could be induced by therapeutic vaccines, including protein (HBsAg/preS and HBcAg), DNA, and viral vector-based vaccines. More than 50 clinical trials have been performed to assess therapeutic vaccines in CHB treatment, some of which display potential effects. Most recently, using genetic editing technology to generate CAR-T or TCR-T, HBV-specific T cells have been produced to efficiently clear HBV. This review summarizes the progress in basic and clinical research investigating immunomodulatory strategies for curing chronic HBV infection, and critically discusses the rather disappointing results of current clinical trials and future strategies.

Toll-like receptors, long non-coding RNA NEAT1, and RIG-I expression are associated with HBeAg-positive chronic hepatitis B patients in the active phase

BACKGROUND

Innate immunity plays a crucial role in host-virus interactions and greatly influences viral replication including HBV infection. However, few studies have investigated the possible antiviral immune roles played by TLRs, RIG-I, and long no-coding RNA NEAT1 in chronic HBV infection (CHB) patients in clinical samples and their relationships among immune responses. In this study, we sought to investigate the mRNA expression levels of TLR1-10, RIG-I, and NEAT1 expression in HBeAg-positive CHB treatment-naïve patients with the active phase.

METHODS

The expression levels of TLR1-10, RIG-I, and NEAT1 of CHB patients with the active phase and healthy controls were measured by qPCR. Serum HBV DNA and routine liver biochemistry including ALT, etc were also measured to evaluate the impaired physiological function of the liver affected by CHB.

RESULTS

The expression levels of TLR1 and TLR6 in CHB with active phase were remarkably lower than that in healthy controls. The levels of TLR3 in CHB patients with active phase were remarkably higher than that in healthy controls. The total NEAT1 expression was abnormally decreased in CHB patients as compared with healthy controls. The levels of RIG-I were significantly decreased in CHB patients in the active phase when compared to healthy controls. The expression of TLR6 and RIG-I was closely correlated with NEAT1 expression. TLR6 level was positively correlated with RIG-I level.

CONCLUSION

Chronic HBV infection can alter the innate immune response by downregulating functional expression of TLR1, TLR6, NEAT1.

Toward Curative Immunomodulation Strategies for Chronic Hepatitis B Virus Infection

Chronic hepatitis B virus (HBV) infection remains a major cause of morbidity and mortality worldwide. HBV surface antigen loss is considered a functional cure and is an ideal goal for antiviral therapy. However, current treatment regimens, including nucleos(t)ide analogues or interferons monotherapy and combination therapy, rarely achieve this goal in chronic hepatitis B patients. Nucleos(t)ide analogues (NAs), as well as many direct antiviral drugs in ongoing development, are able to inhibit HBV replication and gene expression, but it is hard to achieve immune control and prevent recurrence after therapy cessation. Host immunity, especially HBV-specific T cell response, is proven to play a critical role in control or clearance of HBV infection. Considering HBV chronically infected patients display varying degrees of dysfunction regarding their immune system, novel approaches to enhancing antiviral immune responses are necessary in order to combine with current antiviral agents. In this Review, we focus on the role of innate and adaptive immune responses in HBV immunopathogenesis and discuss attractive strategies or drugs that aim to activate or rebuild antiviral immunity to achieve the goal of an HBV functional cure.

Pre-Activation of Toll-Like Receptor 2 Enhances CD8+ T-Cell Responses and Accelerates Hepatitis B Virus Clearance in the Mouse Models

Toll-like receptors (TLRs) play a crucial role in activation of innate immunity, which is essential for inducing effective adaptive immune responses. Our previous studies have shown that toll-like receptor 2 (TLR2) is required to induce effective virus-specific T-cell responses against hepatitis B virus (HBV) in vivo. However, the contribution of TLR2 activation to adaptive immunity and HBV clearance remains to be clarified. In this study, we explored the hydrodynamic injection (HI) mouse models for HBV infection and examined how the TLR2 agonist Pam3CSK (P3C) influences HBV control and modulates HBV-specific T-cell response if applied in vivo. We found that TLR2 activation by P3C injection leads to the rapid but transient production of serum proinflammatory factors interleukin-6 and tumor necrosis factor-α and activation of CD8⁺ T cells in vivo. Then, the anti-HBV effect and HBV-specific T-cell immunity were investigated by TLR2 activation in the mouse models for persistent or acute HBV infections using HBV plasmids pAAV-HBV1.2 and pSM2, respectively. Both P3C application at early stage and pre-activation promoted HBV clearance, while only TLR2 pre-activation enhanced HBV-specific T-cell response in the liver. In the mouse model for acute HBV infection, P3C application had no significant effect on HBV clearance though P3C significantly enhanced the HBV-specific T-cell response. Collectively, TLR2 pre-activation enhances HBV-specific T-cell responses and accelerates HBV clearance in HI mouse models. Thus, the modulation of host immune status by TLR2 agonists may be explored for immunotherapeutic strategies to control HBV infection.

Recent advances in the discovery and development of TLR ligands as novel therapeutics for chronic HBV and HIV infections

INTRODUCTION

Toll-like receptor (TLR) ligands remain as promising antiviral drug candidates for the treatment of chronic viral infections. Basic research on the mechanisms of antiviral activity of TLR ligands in preclinical animal models and clinical testing of drug candidates have been carried out in recent years. Areas covered: This review provides an overview of the preclinical and clinical testing of TLR ligands in two major viral infections: hepatitis B virus (HBV) and human immunodeficiency virus (HIV). Recent results have further demonstrated the potent antiviral activity of various TLR ligands . A TLR7 agonist is in clinical trials for the treatment of chronic HBV infection while a HBV vaccine using a TLR9 ligand as an adjuvant has proven to be superior to conventional HBV vaccines and has been approved for clinical use. Generally, TLR activation may achieve viral control mainly by promoting adaptive immunity to viral proteins. Expert opinion: Recent research in this field indicates that TLR ligands could be developed as clinically effective drugs if the obstacles concerning toxicity and application routes are overcome. TLR-mediated promotion of adaptive immunity is a major issue for future studies and will determine the future development of TLR ligands as drugs for immunomodulation.

Anti-hepatitis B virus (HBV) response of imiquimod based toll like receptor 7 ligand in hbv-positive human hepatocelluar carcinoma cell line

Background

Toll like receptors (TLRs) play an important role in innate immunity and various studies suggest that TLRs play a crucial role in pathogenesis of hepatitis B virus (HBV) infection. The present study aims in looking into the status of crucial host and viral gene expression on inciting TLR7.

Methods

The transcription of TLR7 pathway signaling molecules and HBV DNA viral load were quantified by Real Time-PCR after stimulation of TLR7 with its imiquimod based ligand, R837. Cell cycle analysis was performed using flow-cytometry. Expression of TLR7 and chief cell cycle regulator governing G1/S transition, p53 was also seen in liver biopsysss samples of CHB patients. HBV induced alteration in histone modifications in HepG2 cells and its restoration on TLR7 activation was determined using western blot.

Results

The TLR7 expression remains downregulated in HepG2.2.15 cells and in liver biopsy samples from CHB patients. Interestingly HBV DNA viral load showed an inverse relationship with the TLR7 expression in the biopsy samples. We also evaluated the anti-viral activity of R837, an agonist of TLR7. It was observed that there was a suppression of HBV replication and viral protein production upon TLR7 stimulation. R837 triggers the anti-viral action probably through the Jun N-terminal Kinase (JNK) pathway. We also observed a downregulation of histone H3K9Me3 repression mark upon R837 treatment in HBV replicating HepG2.2.15 cells, mimicking that of un-infected HepG2 cells. Additionally, the G1/S cell cycle arrest introduced by HBV in HepG2.2.15 cells was released upon ligand treatment.

Conclusion

The study thus holds a close insight into the changes in hepatocyte micro-environment on TLR7 stimulation in HBV infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-017-2189-z) contains supplementary material, which is available to authorized users.

Transforming growth factor β-activated kinase 1 transcriptionally suppresses hepatitis B virus replication

Hepatitis B Virus (HBV) replication in hepatocytes is restricted by the host innate immune system and related intracellular signaling pathways. Transforming growth factor β-activated kinase 1 (TAK1) is a key mediator of toll-like receptors and pro-inflammatory cytokine signaling pathways. Here, we report that silencing or inhibition of endogenous TAK1 in hepatoma cell lines leads to an upregulation of HBV replication, transcription, and antigen expression. In contrast, overexpression of TAK1 significantly suppresses HBV replication, while an enzymatically inactive form of TAK1 exerts no effect. By screening TAK1-associated signaling pathways with inhibitors and siRNAs, we found that the MAPK-JNK pathway was involved in TAK1-mediated HBV suppression. Moreover, TAK1 knockdown or JNK pathway inhibition induced the expression of farnesoid X receptor α, a transcription factor that upregulates HBV transcription. Finally, ectopic expression of TAK1 in a HBV hydrodynamic injection mouse model resulted in lower levels of HBV DNA and antigens in both liver and serum. In conclusion, our data suggest that TAK1 inhibits HBV primarily at viral transcription level through activation of MAPK-JNK pathway, thus TAK1 represents an intrinsic host restriction factor for HBV replication in hepatocytes.

Potential strategies for "cure" of hepatitis B

Hepatitis B is a worldwide health problem and the main cause of liver cirrhosis, liver failure, and liver cancer. The steady state of hepatitis B virus (HBV) covalently closed circular DNA (cccDNA) in HBV infected hepatocytes and virus specific immune tolerance contribute to the chronic persistent infection and hard-to-cure of hepatitis B. The presently available therapeutics for hepatitis B can control viral replication, but rarely eliminate HBV surface antigen (HBsAg) or HBV cccDNA. The "cure" of hepatitis B, which is characterized by the HBsAg loss or HBsAg seroconversion, and cccDNA clearance, has been the goal of researchers for years. In recent years, with the robust progress in understanding the HBV pathogenesis and the rapid development of gene editing technology, the "cure" of hepatitis B becomes prospective. This paper aims to summarize the potential strategies for the "cure" of hepatitis B.

Treatment of hepatitis B virus: an update

Chronic hepatitis B virus infection is a global health concern as it affects over 240 million people worldwide and an estimated 686,000 people die annually as a result of complications of the disease. With the development of newer antiviral drugs, viral suppression of HBV is achievable, however elimination of HBV from infected individuals (functional cure) remains an issue. Due to persistence of HBV DNA (cccDNA) in infected cells, chronically infected patients who discontinue therapy prior to HBsAg loss or seroconversion are likely to relapse. Several novel therapeutic strategies are being researched and studied in clinical trials. Here we review these novel strategies to achieve sustained cure or elimination of HBV. These strategies include the targeting of the host or viral factors required for viral persistence as well as therapeutic vaccines.

Combination therapy including CpG oligodeoxynucleotides and entecavir induces early viral response and enhanced inhibition of viral replication in a woodchuck model of chronic hepadnaviral infection

CpG oligodeoxynucleotides (ODNs) stimulate immune cells via TLR9 and are potentially useful immunomodulators for the treatment of chronic viral infections. In the present study, different classes of CpGs were tested for their capacities for innate immune activation and antiviral activities in the woodchuck model. A class P CpG ODN was found to stimulate interferon (IFN) production in woodchuck peripheral blood mononuclear cells (PBMCs) in vitro, and following subcutaneous administration in vivo, it was observed to induce IFN and MxA expression in woodchuck PBMCs. Combination treatment with CpG ODN and entecavir (ETV) led to effective suppression of the woodchuck hepatitis virus (WHV) load in the woodchucks, with early viral responses and inhibition of replication. The woodchuck hepatitis surface antigen (WHsAg) serum concentrations were strongly decreased by CpG and ETV together but not by either agent alone, indicating synergistic effects. However, viral control post-treatment was still transient, similar to that observed with ETV alone. Significantly elevated levels of serum aspartate aminotransferase (AST) but not of alanine aminotransferase (ALT) in some of the woodchucks receiving CpG ODN were noted, but these increases were resolved before the completion of treatment and were not associated with an elevated serum bilirubin level or coagulation disorders, suggesting the absence of a significant safety concern.

Chronic hepatitis B: Are we close to a cure?

Approximately 300 million people worldwide are persistently infected with the hepatitis B virus and are at risk of developing hepatocellular carcinoma and liver cirrhosis, which can progress to end-stage liver disease. Despite the effectiveness of the current vaccination policy, the prevalence of the disease remains high, and the burden for health services is considerable. The currently available antiviral strategies are either poorly effective or only effective for non-curative suppression of viral replication. Recent efforts have been focused on improving the cure rate for chronic hepatitis B and developing strategies to eliminate infected cells. Several approaches are under evaluation, and these include targeting the virus at different stages of its life cycle and boosting the antiviral immune response. This article reviews these latest approaches and comments on their feasibility and potential translation into clinical applications.

Innate immune recognition of hepatitis B virus

Hepatitis B virus (HBV) is a hepatotropic DNA virus and its infection results in acute or chronic hepatitis. It is reported that the host innate immune system contributes to viral control and liver pathology, while whether and how HBV can trigger the components of innate immunity remains controversial. In recent years, the data accumulated from HBV-infected patients, cellular and animal models have challenged the concept of a stealth virus for HBV infection. This editorial focuses on the current findings about the innate immune recognition to HBV. Such evaluation could help us to understand HBV immunopathogenesis and develop novel immune therapeutic strategies to combat HBV infection.

Toll-like receptor (TLR)-mediated innate immune responses in the control of hepatitis B virus (HBV) infection

The role of adaptive immune responses in the control of hepatitis B virus (HBV) infection is well accepted. The contribution of innate immune responses to the viral control is recognized but yet not fully understood. Toll-like receptors (TLRs) sense pathogen-associated molecule patterns and activate antiviral mechanisms including the intracellular antiviral pathways and the production of antiviral effectors like interferons (IFNs) and pro-inflammatory cytokines. Activation of the TLR3 pathway and the production of IFN-β represent one of the major mechanisms leading to the suppression of HBV replication in the liver, as shown in different in vitro and in vivo models. TLR4 signaling and TLR2 signaling result in the activation of intracellular pathways including MAPK and PI-3 K/Akt in hepatocytes and reduce HBV replication in an IFN-independent manner. HBV is able to counteract the actions of TLR3 and TLR2/4 through downregulation of TLR expression and attenuation of the cellular signaling pathways. Thus, TLR ligands are promising candidates as immunomodulators and therapeutics for the treatment of chronic HBV infection. Specific antiviral treatment against HBV could recover the TLR functions in chronic HBV infection and increase the effectiveness of therapeutic approaches based on TLR activation.

Control of hepatitis B virus replication by interferons and Toll-like receptor signaling pathways

Hepatitis B virus (HBV) infection is one of the major causes of liver diseases, affecting more than 350 million people worldwide. The interferon (IFN)-mediated innate immune responses could restrict HBV replication at the different steps of viral life cycle. Indeed, IFN-α has been successfully used for treatment of patients with chronic hepatitis B. However, the role of the innate immune response in HBV replication and the mechanism of the anti-HBV effect of IFN-α are not completely explored. In this review, we summarized the currently available knowledge about the IFN-mediated anti-HBV effect in the HBV life cycle and the possible effectors downstream the IFN signaling pathway. The antiviral effect of Toll-like receptors (TLRs) in HBV replication is briefly discussed. The strategies exploited by HBV to evade the IFN- and TLR-mediated antiviral actions are summarized.

Recognition of human oncogenic viruses by host pattern-recognition receptors

Human oncogenic viruses include Epstein-Barr virus, hepatitis B virus, hepatitis C virus, human papilloma virus, human T-cell lymphotropic virus, Kaposi's associated sarcoma virus, and Merkel cell polyomavirus. It would be expected that during virus-host interaction, the immune system would recognize these pathogens and eliminate them. However, through evolution, these viruses have developed a number of strategies to avoid such an outcome and successfully establish chronic infections. The persistent nature of the infection caused by these viruses is associated with their oncogenic potential. In this article, we will review the latest information on the interaction between oncogenic viruses and the innate immune system of the host. In particular, we will summarize the available knowledge on the recognition by host pattern-recognition receptors of pathogen-associated molecular patterns present in the incoming viral particle or generated during the virus' life cycle. We will also review the data on the recognition of cell-derived danger associated molecular patterns generated during the virus infection that may impact the outcome of the host-pathogen interaction and the development cancer.

The natural course of chronic hepatitis B virus infection and its management

Chronic infection with the hepatitis B virus (HBV) runs a long natural course during which underlying changes in liver histology can progress to cirrhosis and hepatic decompensation, as well as to hepatocellular carcinoma. Therapeutic intervention is currently aiming at suppression of HBV replication by applying a number of pharmacological agents. For an optimum use of available therapies, good knowledge of the natural course of chronic infection, as well as of the role played by several viral, host, and environmental factors, is mandatory. The larger part of this chapter deals with how to treat the various subsets of patients with chronic hepatitis B (CHB), using mainly three first-line drugs: pegylated interferon-α2a, entecavir, and tenofovir, administered either in finite courses or indefinitely. The frequency of virological, serological, biochemical, and histological responses in the various subsets of patients, both during and after stopping treatment, is reviewed. It is stressed that the application of the highly potent antivirals entecavir and tenofovir, with acceptable safety records and with a high barrier to HBV resistance, represents major progress in the treatment of CHB. Despite the hitherto important developments in the treatment of viral hepatitis B, clinical cure of chronic HBV infection with HBsAg loss is achievable only in a few treated patients while eradication of HBV infection appears unrealistic. Development of new pharmacological agents acting at multiple targets of the replicative cycle of HBV may achieve higher efficacy and even cure of CHB.

Tickling the TLR7 to cure viral hepatitis

Chronic hepatitis B and C are the leading causes of liver disease and liver transplantation worldwide. Ability to mount an effective immune response against both HBV and HCV is associated with spontaneous clearance of both infections, while an inability to do so leads to chronicity of both infections. To mount an effective immune response, both innate and adaptive immune responses must work in tandem. Hence, developing protective immunity to hepatitis viruses is an important goal in order to reduce the global burden of these two infections and prevent development of long-term complications. In this regard, the initial interactions between the pathogen and immune system are pivotal in determining the effectiveness of immune response and subsequent elimination of pathogens. Toll-like receptors (TLRs) are important regulators of innate and adaptive immune responses to various pathogens and are often involved in initiating and augmenting effective antiviral immunity. Immune-based therapeutic strategies that specifically induce type I interferon responses are associated with functional cure for both chronic HBV and HCV infections. Precisely, TLR7 stimulation mediates an endogenous type I interferon response, which is critical in development of a broad, effective and protective immunity against hepatitis viruses. This review focuses on anti-viral strategies that involve targeting TLR7 that may lead to development of protective immunity and eradication of hepatitis B.

Understanding the interaction of hepatitis C virus with host DEAD-box RNA helicases

The current therapeutic regimen to combat chronic hepatitis C is not optimal due to substantial side effects and the failure of a significant proportion of patients to achieve a sustained virological response. Recently developed direct-acting antivirals targeting hepatitis C virus (HCV) enzymes reportedly increase the virologic response to therapy but may lead to a selection of drug-resistant variants. Besides direct-acting antivirals, another promising class of HCV drugs in development include host targeting agents that are responsible for interfering with the host factors crucial for the viral life cycle. A family of host proteins known as DEAD-box RNA helicases, characterized by nine conserved motifs, is known to play an important role in RNA metabolism. Several members of this family such as DDX3, DDX5 and DDX6 have been shown to play a role in HCV replication and this review will summarize our current knowledge on their interaction with HCV. As chronic hepatitis C is one of the leading causes of hepatocellular carcinoma, the involvement of DEAD-box RNA helicases in the development of HCC will also be highlighted. Continuing research on the interaction of host DEAD-box proteins with HCV and the contribution to viral replication and pathogenesis could be the panacea for the development of novel therapeutics against HCV.

Strategies to eliminate HBV infection

Chronic HBV infection is a major public health concern affecting over 240 million people worldwide. Although suppression of HBV replication is achieved in the majority of patients with currently available newer antivirals, discontinuation of therapy prior to hepatitis B surface antigen loss or seroconversion is associated with relapse of HBV in the majority of cases. Thus, new therapeutic modalities are needed to achieve eradication of the virus from chronically infected patients in the absence of therapy. The basis of HBV persistence includes viral and host factors. Here, we review novel strategies to achieve sustained cure or elimination of HBV. The novel approaches include targeting the viral and or host factors required for viral persistence, and novel immune-based therapies, including therapeutic vaccines.

Basis of HBV persistence and new treatment options

The majority of the morbidity and mortality associated with hepatitis B virus infection is due to viral persistence and its consequences. The heterogeneity of outcomes from HBV infection suggests that both viral and host factors influence the development of chronic infection. Study of host genetic susceptibility has revealed a number of genes including MHC class II loci and cytokine receptors, which decrease the risk of persistence. On the viral side, the replication system is adapted to generate high levels of virions without stimulating the innate immune system. Secreted viral proteins (HBsAg and HBeAg) suppress innate responses through inhibition of TLR signaling, which leads to a weak adaptive immune response with an exhausted phenotype that is incapable of inducing viral elimination. However, even when the adaptive immune system begins to take effect after HBe seroconversion, the ability of the virus to mutate and evade T and B cell-mediated responses helps to sustain persistent infection. Understanding the mechanisms of persistence is important for the design of therapeutic strategies. Although there are currently no specific drugs that target the viral minichromosome (cccDNA), it is expected that in the future we will be able to use existing drugs more effectively to eliminate the infection.

Adjuvant immunotherapies as a novel approach to bacterial infections

The rapid emergence of multidrug-resistant pathogens, especially Gram-negative bacteria and mycobacteria, represents one of the major medical challenges of the 21st century. The gradual loss of effective classical antibiotics for many bacterial pathogens, combined with an increasing population density and mobility, urgently calls for the development of novel treatments. Here, we discuss the potential of adjuvant immunotherapies to selectively stimulate protective immune responses as a treatment option for bacterial infections. In order to elicit appropriate immune responses and to avoid unwanted inflammatory tissue damage, it is essential to identify ligands and receptor pathways that specifically control protective responses at the site of infection. We summarize existing data and discuss suitable candidate targets for future immunotherapies of infectious diseases.

Host-targeting agents for prevention and treatment of chronic hepatitis C - perspectives and challenges

Hepatitis C virus (HCV) infection is a major cause of chronic liver disease and hepatocellular carcinoma worldwide. Furthermore, HCV-induced liver disease is a major indication of liver transplantation. In the past years, direct-acting antivirals (DAAs) targeting HCV enzymes have been developed. DAAs increase the virologic response to anti-HCV therapy but may lead to selection of drug-resistant variants and treatment failure. To date, strategies to prevent HCV infection are still lacking and antiviral therapy in immunocompromised patients, patients with advanced liver disease and HIV/HCV-co-infection remains limited. Alternative or complementary approaches addressing the limitations of current antiviral therapies are to boost the host's innate immunity or interfere with host factors required for pathogenesis. Host-targeting agents (HTAs) provide an interesting perspective for novel antiviral strategies against viral hepatitis since they have (i) a high genetic barrier to resistance, (ii) a pan-genotypic antiviral activity, and (iii) complementary mechanisms of action to DAAs and might therefore act in a synergistic manner with current standard of care or DAAs in clinical development. This review highlights HTAs against HCV infection that have potential as novel antivirals and are in preclinical or clinical development.