Hepatocytic expression of human sodium-taurocholate cotransporting polypeptide enables hepatitis B virus infection of macaques

Major open questions in the hepatitis B and D field - Proceedings of the inaugural International emerging hepatitis B and hepatitis D researchers workshop

The early and mid-career researchers (EMCRs) of scientific communities represent the forefront of research and the future direction in which a field takes. The opinions of this key demographic are not commonly aggregated to audit fields and precisely demonstrate where challenges lie for the future. To address this, we initiated the inaugural International Emerging Researchers Workshop for the global Hepatitis B and Hepatitis D scientific community (75 individuals). The cohort was split into small discussion groups and the significant problems, challenges, and future directions were assessed. Here, we summarise the outcome of these discussions and outline the future directions suggested by the EMCR community. We show an effective approach to gauging and accumulating the ideas of EMCRs and provide a succinct summary of the significant gaps remaining in the Hepatitis B and Hepatitis D field.

Progress of Infection and Replication Systems of Hepatitis B Virus

Despite the long-standing availability of effective prophylaxis, chronic hepatitis B virus (HBV) infection remains a formidable public health threat. Antiviral treatments can limit viral propagation, but prolonged therapy is necessary to control HBV replication. Robust in vitro models of HBV infection are indispensable prerequisites for elucidating viral pathogenesis, delineating virus-host interplay and developing novel therapeutic, preventative countermeasures. Buoyed by advances in molecular techniques and tissue culture systems, investigators have engineered numerous in vitro models of the HBV life cycle. However, all current platforms harbor limitations in the recapitulation of natural infection. In this article, we comprehensively review the HBV life cycle, provide an overview of existing in vitro HBV infection and replication systems, and succinctly present the benefits and caveats in each model with the primary objective of constructing refined experimental models that closely mimic native viral infection and offering robust support for the ambitious "elimination of hepatitis by 2030" initiative.

Hepatitis B: Model Systems and Therapeutic Approaches

Hepatitis B virus (HBV) infection is a major global health issue and ranks among the top causes of liver cirrhosis and hepatocellular carcinoma. Although current antiviral medications, including nucleot(s)ide analogs and interferons, could inhibit the replication of HBV and alleviate the disease, HBV cannot be fully eradicated. The development of cellular and animal models for HBV infection plays an important role in exploring effective anti-HBV medicine. During the past decades, advancements in several cell culture systems, such as HepG2.2.15, HepAD38, HepaRG, hepatocyte-like cells, and primary human hepatocytes, have propelled the research in inhibiting HBV replication and expression and thus enriched our comprehension of the viral life cycle and enhancing antiviral drug evaluation efficacy. Mouse models, in particular, have emerged as the most extensively studied HBV animal models. Additionally, the present landscape of HBV therapeutics research now encompasses a comprehensive assessment of the virus's life cycle, targeting numerous facets and employing a variety of immunomodulatory approaches, including entry inhibitors, strategies aimed at cccDNA, RNA interference technologies, toll-like receptor agonists, and, notably, traditional Chinese medicine (TCM). This review describes the attributes and limitations of existing HBV model systems and surveys novel advancements in HBV treatment modalities, which will offer deeper insights toward discovering potentially efficacious pharmaceutical interventions.

High-Throughput Screening for the Prevalence of Neutralizing Antibodies against Human Adenovirus Serotype 5

Adenoviral vectors based on the human adenovirus species C serotype 5 (HAdV-C5) are commonly used for vector-based gene therapies and vaccines. In the preclinical stages of development, their safety and efficacy are often validated in suitable animal models. However, pre-existing neutralizing antibodies may severely influence study outcomes. Here, we generated a new HAdV-C5-based reporter vector and established a high-throughput screening assay for the multivalent detection of HAdV-C5-neutralizing antibodies in serum. We screened the sera of rhesus macaques at different primate centers, and of rabbits, horses, cats, and dogs, showing that HAdV-C5-neutralizing antibodies can be found in all species, albeit at different frequencies. Our results emphasize the need to prescreen model animals in HAdV-C5-based studies.

Aborted infection of human sodium taurocholate cotransporting polypeptide (hNTCP) expressing woodchuck hepatocytes with hepatitis B virus (HBV)

Due to the limited host range of HBV, research progress has been hindered by the absence of a suitable animal model. The natural history of woodchuck hepatitis virus (WHV) infection in woodchuck closely mirrors that of HBV infection in human, making this species a promising candidate for establishing both in vivo and in vitro HBV infection models. Therefore, this animal may be a valuable species to evaluate HBV vaccines and anti-HBV drugs. A significant milestone in HBV and hepatitis D virus (HDV) infection is the discovery of sodium taurocholate cotransporting polypeptide (NTCP) as the functional receptor. In an effort to enhance susceptibility to HBV infection, we introduced hNTCP into the woodchuck hepatocytes by multiple approaches including transduction of vLentivirus-hNTCP in woodchuck hepatocytes, transfection of p-lentivirus-hNTCP-eGFP plasmids into these cells, as well as transduction of vAdenovirus-hNTCP-eGFP. Encouragingly, our findings demonstrated the successful introduction of hNTCP into woodchuck hepatocytes. However, it was observed that these hNTCP-expressing hepatocytes were only susceptible to HDV infection but not HBV. This suggests the presence of additional crucial factors mediating early-stage HBV infection that are subject to stringent species-specific restrictions.

Interspecies comparison of the early transcriptomic changes associated with hepatitis B virus exposure in human and macaque immune cell populations

Background and aims

Hepatitis B virus (HBV) infection affects 300 million individuals worldwide, representing a major factor for the development of hepatic complications. Although existing antivirals are effective in suppressing replication, eradication of HBV is not achieved. Therefore, a multi-faceted approach involving antivirals and immunomodulatory agents is required. Non-human primates are widely used in pre-clinical studies due to their close evolutionary relationship to humans. Nonetheless, it is fundamental to identify the differences in immune response between humans and these models. Thus, we performed a transcriptomic characterization and interspecies comparison of the early immune responses to HBV in human and cynomolgus macaques.

Methods

We characterized early transcriptomic changes in human and cynomolgus B cells, T cells, myeloid and plasmacytoid dendritic cells (pDC) exposed to HBV ex vivo for 2 hours. Differentially-expressed genes were further compared to the profiles of HBV-infected patients using publicly-available single-cell data.

Results

HBV induced a wide variety of transcriptional changes in all cell types, with common genes between species representing only a small proportion. In particular, interferon gamma signaling was repressed in human pDCs. At the gene level, interferon gamma inducible protein 16 (IFI16) was upregulated in macaque pDCs, while downregulated in humans. Moreover, IFI16 expression in pDCs from chronic HBV-infected patients anti-paralleled serum HBsAg levels.

Conclusion

Our characterization of early transcriptomic changes induced by HBV in humans and cynomolgus macaques represents a useful resource for the identification of shared and divergent host responses, as well as potential immune targets against HBV.

Targeted viral adaptation generates a simian-tropic hepatitis B virus that infects marmoset cells

Hepatitis B virus (HBV) only infects humans and chimpanzees, posing major challenges for modeling HBV infection and chronic viral hepatitis. The major barrier in establishing HBV infection in non-human primates lies at incompatibilities between HBV and simian orthologues of the HBV receptor, sodium taurocholate co-transporting polypeptide (NTCP). Through mutagenesis analysis and screening among NTCP orthologues from Old World monkeys, New World monkeys and prosimians, we determined key residues responsible for viral binding and internalization, respectively and identified marmosets as a suitable candidate for HBV infection. Primary marmoset hepatocytes and induced pluripotent stem cell-derived hepatocyte-like cells support HBV and more efficient woolly monkey HBV (WMHBV) infection. Adapted chimeric HBV genome harboring residues 1-48 of WMHBV preS1 generated here led to a more efficient infection than wild-type HBV in primary and stem cell derived marmoset hepatocytes. Collectively, our data demonstrate that minimal targeted simianization of HBV can break the species barrier in small NHPs, paving the path for an HBV primate model.

Liver organoids as a primary human model to study HBV-mediated Hepatocellular carcinoma. A review

Hepatitis B Virus (HBV) is the prevailing cause of chronic liver disease, which progresses to Hepatocellular carcinoma (HCC) in 75% of cases. It represents a serious health concern being the fourth leading cause of cancer-related mortality worldwide. Treatments available to date fail to provide a complete cure with high chances of recurrence and related side effects. The lack of reliable, reproducible, and scalable in vitro modeling systems that could recapitulate the viral life cycle and represent virus-host interactions has hindered the development of effective treatments so far. The present review provides insights into the current in-vivo and in-vitro models used for studying HBV and their major limitations. We highlight the use of three-dimensional liver organoids as a novel and suitable platform for modeling HBV infection and HBV-mediated HCC. HBV organoids can be expanded, genetically altered, patient-derived, tested for drug discovery, and biobanked. This review also provides the general guidelines for culturing HBV organoids and highlights their several prospects for HBV drug discovery and screening.

Forthcoming Developments in Models to Study the Hepatitis B Virus Replication Cycle, Pathogenesis, and Pharmacological Advancements

Hepatitis, liver cirrhosis, and hepatocellular carcinoma are all manifestations of chronic hepatitis B. Its pathogenesis and molecular mechanism remain mysterious. As medical science progresses, different models are being used to study the disease from the physiological and molecular levels. Animal models have played an unprecedented role in achieving in-depth knowledge of the disease while posing no risk of harming humans throughout the study. The scarcity of acceptable animal models has slowed progress in hepatitis B virus (HBV) research and preclinical testing of antiviral medicines since HBV has a narrow species tropism and exclusively infects humans and higher primates. The development of human chimeric mice was supported by a better understanding of the obstacles to interspecies transmission, which has substantially opened the way for HBV research in vivo and the evaluation of possible chronic hepatitis B therapeutics. Animal models are cumbersome to handle, not accessible, and expensive. Hence, it is herculean to investigate the HBV replication cycle in animal models. Therefore, it becomes essential to build a splendid in vitro cell culture system to demonstrate the mechanisms attained by the HBV for its multiplication and sustenance. We also addressed the advantages and caveats associated with different models in examining HBV.

Activation of distinct antiviral T-cell immunity: A comparison of bi- and trispecific T-cell engager antibodies with a chimeric antigen receptor targeting HBV envelope proteins

Despite the availability of an effective prophylactic vaccine, 820,000 people die annually of hepatitis B virus (HBV)-related liver disease according to WHO. Since current antiviral therapies do not provide a curative treatment for the 296 million HBV carriers around the globe, novel strategies to cure HBV are urgently needed. A promising approach is the redirection of T cells towards HBV-infected hepatocytes employing chimeric antigen receptors or T-cell engager antibodies. We recently described the effective redirection of T cells employing a second-generation chimeric antigen receptor directed against the envelope protein of hepatitis B virus on the surface of infected cells (S-CAR) as well as bispecific antibodies that engage CD3 or CD28 on T cells employing the identical HBV envelope protein (HBVenv) binder. In this study, we added a trispecific antibody comprising all three moieties to the tool-box. Cytotoxic and non-cytolytic antiviral activities of these bi- and trispecific T-cell engager antibodies were assessed in co-cultures of human PBMC with HBV-positive hepatoma cells, and compared to that of S-CAR-grafted T cells. Activation of T cells via the S-CAR or by either a combination of the CD3- and CD28-targeting bispecific antibodies or the trispecific antibody allowed for specific elimination of HBV-positive target cells. While S-CAR-grafted effector T cells displayed faster killing kinetics, combinatory treatment with the bispecific antibodies or single treatment with the trispecific antibody was associated with a more pronounced cytokine release. Clearance of viral antigens and elimination of the HBV persistence form, the covalently closed circular (ccc) DNA, through cytolytic as well as cytokine-mediated activity was observed in all three settings with the combination of bispecific antibodies showing the strongest non-cytolytic, cytokine-mediated antiviral effect. Taken together, we demonstrate that bi- and trispecific T-cell engager antibodies can serve as a potent, off-the-shelf alternative to S-CAR-grafted T cells to cure HBV.

Recent Drug Development in the Woodchuck Model of Chronic Hepatitis B

Infection with hepatitis B virus (HBV) is responsible for the increasing global hepatitis burden, with an estimated 296 million people being carriers and living with the risk of developing chronic liver disease and cancer. While the current treatment options for chronic hepatitis B (CHB), including oral nucleos(t)ide analogs and systemic interferon-alpha, are deemed suboptimal, the path to finding an ultimate cure for this viral disease is rather challenging. The lack of suitable laboratory animal models that support HBV infection and associated liver disease progression is one of the major hurdles in antiviral drug development. For more than four decades, experimental infection of the Eastern woodchuck with woodchuck hepatitis virus has been applied for studying the immunopathogenesis of HBV and developing new antiviral therapeutics against CHB. There are several advantages to this animal model that are beneficial for performing both basic and translational HBV research. Previous review articles have focused on the value of this animal model in regard to HBV replication, pathogenesis, and immune response. In this article, we review studies of drug development and preclinical evaluation of direct-acting antivirals, immunomodulators, therapeutic vaccines, and inhibitors of viral entry, gene expression, and antigen release in the woodchuck model of CHB since 2014 until today and discuss their significance for clinical trials in patients.

Host cell-dependent late entry step as determinant of hepatitis B virus infection

Hepatitis B virus (HBV) has a highly restricted host range and cell tropism. Other than the human sodium taurocholate cotransporting polypeptide (huNTCP), the HBV entry receptor, host determinants of HBV susceptibility are poorly understood. Woodchucks are naturally infected with woodchuck hepatitis virus (WHV), closely related to HBV, but not with HBV. Here, we investigated the capabilities of woodchuck hepatic and human non-hepatic cell lines to support HBV infection. DNA transfection assays indicated that all cells tested supported both HBV and WHV replication steps post entry, including the viral covalently closed circular DNA (cccDNA) formation, which is essential for establishing and sustaining infection. Ectopic expression of huNTCP rendered one, but not the other, woodchuck hepatic cell line and the non-hepatic human cell line competent to support productive HBV entry, defined here by cccDNA formation during de novo infection. All huNTCP-expressing cell lines tested became susceptible to infection with hepatitis D virus (HDV) that shares the same entry receptor and initial steps of entry with HBV, suggesting that a late entry/trafficking step(s) of HBV infection was defective in one of the two woodchuck cell lines. In addition, the non-susceptible woodchuck hepatic cell line became susceptible to HBV after fusion with human hepatic cells, suggesting the lack of a host cell-dependent factor(s) in these cells. Comparative transcriptomic analysis of the two woodchuck cell lines revealed widespread differences in gene expression in multiple biological processes that may contribute to HBV infection. In conclusion, other than huNTCP, neither human- nor hepatocyte-specific factors are essential for productive HBV entry. Furthermore, a late trafficking step(s) during HBV infection, following the shared entry steps with HDV and before cccDNA formation, is subject to host cell regulation and thus, a host determinant of HBV infection.

Fundamental studies on, and development of therapies against, chronic hepatitis B virus (HBV) infection, which inflicts hundreds of millions worldwide, are impeded by deficiencies in HBV-susceptible animal models. HBV displays a strict species and cell tropism that are not clearly understood. Here, by studying replication of HBV, and the related woodchuck hepatitis virus, in human and woodchuck hepatic or non-hepatic cells, we found that non-hepatic human cells and some woodchuck hepatic cells could support productive HBV entry after expression of the human cell receptor for HBV. Moreover, by studying the infection of hepatitis D virus, which shares the same entry receptor and initial steps of entry with HBV, we could narrow down a host determinant of HBV infection operating at a late entry/trafficking step(s). Our study thus provides new insights into determinants of HBV host tropism and facilitates the development of HBV-susceptible animal models.

Long-term hepatitis B virus infection of rhesus macaques requires suppression of host immunity

Hepatitis B virus has infected a third of the world's population, and 296 million people are living with chronic infection. Chronic infection leads to progressive liver disease, including hepatocellular carcinoma and liver failure, and there remains no reliable curative therapy. These gaps in our understanding are due, in large part, to a paucity of animal models of HBV infection. Here, we show that rhesus macaques regularly clear acute HBV infection, similar to adult humans, but can develop long-term infection if immunosuppressed. Similar to patients, we longitudinally detected HBV DNA, HBV surface antigen, and HBV e antigen in the serum of experimentally infected animals. In addition, we discovered hallmarks of HBV infection in the liver, including RNA transcription, HBV core and HBV surface antigen translation, and covalently closed circular DNA biogenesis. This pre-clinical animal model will serve to accelerate emerging HBV curative therapies into the clinic.

Advances in designing Adeno-associated viral vectors for development of anti-HBV gene therapeutics

Despite the five decades having passed since discovery of the hepatitis B virus (HBV), together with development of an effective anti-HBV vaccine, infection with the virus remains a serious public health problem and results in nearly 900,000 annual deaths worldwide. Current therapies do not eliminate the virus and viral replication typically reactivates after treatment withdrawal. Hence, current endeavours are aimed at developing novel therapies to achieve a functional cure. Nucleic acid-based therapeutic approaches are promising, with several candidates showing excellent potencies in preclinical and early stages of clinical development. However, this class of therapeutics is yet to become part of standard anti-HBV treatment regimens. Obstacles delaying development of gene-based therapies include lack of clinically relevant delivery methods and a paucity of good animal models for preclinical characterisation. Recent studies have demonstrated safety and efficiency of Adeno-associated viral vectors (AAVs) in gene therapy. However, AAVs do have flaws and this has prompted research aimed at improving design of novel and artificially synthesised AAVs. Main goals are to improve liver transduction efficiencies and avoiding immune clearance. Application of AAVs to model HBV replication in vivo is also useful for characterising anti-HBV gene therapeutics. This review summarises recent advances in AAV engineering and their contributions to progress with anti-HBV gene therapy development.

Hepatitis-D Virus Infection Is Not Impaired by Innate Immunity but Increases Cytotoxic T-Cell Activity

Approximately 70 million humans worldwide are affected by chronic hepatitis D, which rapidly leads to liver cirrhosis and hepatocellular carcinoma due to chronic inflammation. The triggers and consequences of this chronic inflammation, induced by co-infection with the hepatitis D virus (HDV) and the hepatitis B virus (HBV), are poorly understood. Using CRISPR technology, we characterized the recognition of HDV mono- and co-infection by intracellular innate immunity and determined its influence on the viral life cycle and effector T-cell responses using different HBV and HDV permissive hepatoma cell lines. We showed that HDV infection is detected by MDA5 and -after a lag phase -induces a profound type I interferon response in the infected cells. The type I interferon response, however, was not able to suppress HDV replication or spread, thus providing a persistent trigger. Using engineered T-cells directed against the envelope proteins commonly used by HBV and HDV, we found that HDV immune recognition enhanced T-cell cytotoxicity. Interestingly, the T-cell effector function was enhanced independently of antigen presentation. These findings help to explain immune mediated tissue damage in chronic hepatitis D patients and indicate that combining innate triggers with T-cell activating therapies might allow for a curative approach.

CRISPR/Cas and Hepatitis B Therapy: Technological Advances and Practical Barriers

After almost a decade of using CRISPR/Cas9 systems to edit target genes, CRISPR/Cas9 and related technologies are rapidly moving to clinical trials. Hepatitis B virus (HBV), which causes severe liver disease, cannot be cleared by modern antivirals, but represents an ideal target for CRISPR/Cas9 systems. Early studies demonstrated very high antiviral potency of CRISPR/Cas9 and supported its use for developing a cure against chronic HBV infection. This review discusses the key issues that must be solved to make CRISPR/Cas9 an anti-HBV therapy.

In Vivo Modelling of Hepatitis B Virus Subgenotype A1 Replication Using Adeno-Associated Viral Vectors

The paucity of animal models that simulate the replication of the hepatitis B virus (HBV) is an impediment to advancing new anti-viral treatments. The work reported here employed recombinant adeno-associated viruses (AAVs) to model HBV subgenotype A1 and subgenotype D3 replication in vitro and in vivo. Infection with subgenotype A1 is endemic to parts of sub-Saharan Africa, and it is associated with a high risk of hepatocellular carcinoma. Recombinant AAV serotype 2 (AAV2) and 8 (AAV8) vectors bearing greater-than-genome-length sequences of HBV DNA from subgenotype A1 and D3, were produced. Transduced liver-derived cultured cells produced HBV surface antigen and core antigen. Administration of AAV8 carrying HBV subgenotype A1 genome (AAV8-A1) to mice resulted in the sustained production of HBV replication markers over a six-month period, without elevated inflammatory cytokines, expression of interferon response genes or alanine transaminase activity. Markers of replication were generally higher in animals treated with subgenotype D3 genome-bearing AAVs than in those receiving the subgenotype A1-genome-bearing vectors. To validate the use of the AAV8-A1 murine model for anti-HBV drug development, the efficacy of anti-HBV artificial primary-microRNAs was assessed. Significant silencing of HBV markers was observed over a 6-month period after administering AAVs. These data indicate that AAVs conveniently and safely recapitulate the replication of different HBV subgenotypes, and the vectors may be used to assess antivirals’ potency.

Na+-Taurocholate Co-Transporting Polypeptide (NTCP) in Livers, Function, Expression Regulation, and Potential in Hepatitis B Treatment

Chronic hepatitis B virus (HBV) infection has become one of the leading causes of liver cirrhosis and hepatocellular carcinoma globally. The discovery of sodium taurocholate co-transporting polypeptide (NTCP), a solute carrier, as a key receptor for HBV and hepatitis D virus (HDV) has opened new avenues for HBV treatment. Additionally, it has led researchers to generate hepatoma cell lines (including HepG2-NTCP and Huh-7-NTCP) susceptible to HBV infection in vitro, hence, paving the way to develop and efficiently screen new and novel anti-HBV drugs. This review summarizes the history, function and critical findings regarding NTCP as a viral receptor for HBV/HDV, and it also discusses recently developed drugs targeting NTCP.

Improving rigor and reproducibility in nonhuman primate research

Nonhuman primates (NHPs) are a critical component of translational/preclinical biomedical research due to the strong similarities between NHP and human physiology and disease pathology. In some cases, NHPs represent the most appropriate, or even the only, animal model for complex metabolic, neurological, and infectious diseases. The increased demand for and limited availability of these valuable research subjects requires that rigor and reproducibility be a prime consideration to ensure the maximal utility of this scarce resource. Here, we discuss a number of approaches that collectively can contribute to enhanced rigor and reproducibility in NHP research.

Nonhuman primates (NHP) are a crucial component of biomedical research due to their similarities with human physiology and pathobiology.

Increased rigor and reproducibility in NHP research are critical to maximizing the information obtained from this scarce and valuable resource.

More widespread adoption of approaches such as normative protocols, preregistration, and data sharing, as well as more extensive training in biostatistics, can enhance rigor and reproducibility in NHP studies.

In-vitro and in-vivo models for hepatitis B cure research

PURPOSE OF REVIEW

Antiviral therapy for chronic hepatitis B infection is rarely curative, thus research in HBV cure strategies is a priority. Drug development and testing has been hampered by the lack of robust cell culture systems and small animal models. This review summarizes existing models for HBV cure research and focuses on recent developments since 2017 until today.

RECENT FINDINGS

The field has progressed in the development of cell culture and animal models to study HBV. Although early cell culture systems relied on transfection of HBV genomes in hepatoma cell lines, novel models expressing the entry receptor for HBV are susceptible to infection. Improved culture conditions for primary human hepatocytes, the primary target of HBV, have enabled the screening and validation of novel antivirals. Mouse models grafted with partially humanized livers are suitable for testing viral entry inhibitors or direct acting antivirals, and can be reconstituted with human immune cells to analyze immunotherapies. Other immunocompetent models include mice transduced with HBV genomes or woodchucks infected with their native hepatitis virus.

SUMMARY

Model systems for HBV research have helped lay the groundwork for the development and optimization of antiviral and immune-based therapeutic approaches that are now moving to clinical trials.

Rapid and Robust Continuous Purification of High-Titer Hepatitis B Virus for In Vitro and In Vivo Applications

Available treatments for hepatitis B can control the virus but are rarely curative. This led to a global initiative to design new curative therapies for the 257 million patients affected. Discovery and development of these new therapies is contingent upon functional in vitro and in vivo hepatitis B virus (HBV) infection models. However, low titer and impurity of conventional HBV stocks reduce significance of in vitro infections and moreover limit challenge doses in current in vivo models. Therefore, there is a critical need for a robust, simple and reproducible protocol to generate high-purity and high-titer infectious HBV stocks. Here, we outline a three-step protocol for continuous production of high-quality HBV stocks from supernatants of HBV-replicating cell lines. This purification process takes less than 6 h, yields to high-titer stocks (up to 1 × 10¹¹ enveloped, DNA-containing HBV particles/mL each week), and is with minimal equipment easily adaptable to most laboratory settings.

Animal Models for the Study of Hepatitis B Virus Pathobiology and Immunity: Past, Present, and Future

Hepatitis B virus (HBV) infection is a global public health problem that plagues approximately 240 million people. Chronic hepatitis B (CHB) often leads to liver inflammation and aberrant repair which results in diseases ranging from liver fibrosis, cirrhosis, to hepatocellular carcinoma. Despite its narrow species tropism, researchers have established various in vivo models for HBV or its related viruses which have provided a wealth of knowledge on viral lifecycle, pathogenesis, and immunity. Here we briefly revisit over five decades of endeavor in animal model development for HBV and summarize their advantages and limitations. We also suggest directions for further improvements that are crucial for elucidation of the viral immune-evasion strategies and for development of novel therapeutics for a functional cure.

Advances in treatment and prevention of hepatitis B

Chronic hepatitis B (CHB) continues to contribute to worldwide morbidity and mortality significantly. Scientists, clinicians, pharmaceutical companies, and health organizations have dedicated substantial Intellectual and monetary resources to finding a cure, increasing immunization rates, and reducing the global burden of CHB. National and international health-related organizations including the center for disease control, the national institute of health, the American Association for the study of liver disease (AASLD), The European association for the study of the Liver (EASL), The Asia Pacific association for the study of the Liver (APASL) and the world health organization release periodic recommendations for disease prevention and treatment. Our review of the most recent guidelines by EASL, AASLD, APASL, and Taiwan Association for the Study of the Liver revealed that an overwhelming majority of cited studies were published before 2018. We reviewed Hepatitis B-related literature published 2018 onwards to identify recent developments and current barriers that will likely direct future efforts towards eradicating hepatitis B. The breakthrough in our understanding of the hepatitis B virus life cycle and resulting drug development is encouraging with significant room for further progress. Data from high-risk populations, most vulnerable to the devastating effects of hepatitis B infection and reactivation remain sparse. Utilization of systems approach, optimization of experimental models, identification and validation of next-generation biomarkers, and precise modulation of the human immune response will be critical for future innovation. Within the foreseeable future, new treatments will likely complement conventional therapies rather than replace them. Most Importantly, pragmatic management of CHB related population health challenges must be prioritized to produce real-world results.

Molecular regulation of the hepatic bile acid uptake transporter and HBV entry receptor NTCP

Transporters expressed by hepatocytes and enterocytes play a critical role in maintaining the enterohepatic circulation of bile acids. The sodium taurocholate cotransporting polypeptide (NTCP), exclusively expressed at the basolateral side of hepatocytes, mediates the uptake of conjugated bile acids. In conditions where bile flow is impaired (cholestasis), pharmacological inhibition of NTCP-mediated bile acid influx is suggested to reduce hepatocellular damage due to bile acid overload. Furthermore, NTCP has been shown to play an important role in hepatitis B virus (HBV) and hepatitis Delta virus (HDV) infection by functioning as receptor for viral entry into hepatocytes. This review provides a summary of current molecular insight into the regulation of NTCP expression at the plasma membrane, hepatic bile acid transport, and NTCP-mediated viral infection.

Animal Models of Hepatitis B Virus Infection-Success, Challenges, and Future Directions

Chronic hepatitis B virus (HBV) infection affects more than 250 million people worldwide, which greatly increases the risk for terminal liver diseases, such as liver cirrhosis and hepatocellular carcinoma (HCC). Even though current approved antiviral therapies, including pegylated type I interferon (IFN) and nucleos(t)ide analogs, can effectively suppress viremia, HBV infection is rarely cured. Since HBV exhibits a narrow species tropism and robustly infects only humans and higher primates, progress in HBV research and preclinical testing of antiviral drugs has been hampered by the scarcity of suitable animal models. Fortunately, a series of surrogate animal models have been developed for the study of HBV. An increased understanding of the barriers towards interspecies transmission has aided in the development of human chimeric mice and has greatly paved the way for HBV research in vivo, and for evaluating potential therapies of chronic hepatitis B. In this review, we summarize the currently available animal models for research of HBV and HBV-related hepadnaviruses, and we discuss challenges and future directions for improvement.

A hepatitis B virus causes chronic infections in equids worldwide

Preclinical testing of novel therapeutics for chronic hepatitis B (CHB) requires suitable animal models. Equids host homologs of hepatitis C virus (HCV). Because coinfections of hepatitis B virus (HBV) and HCV occur in humans, we screened 2,917 specimens from equids from five continents for HBV. We discovered a distinct HBV species (Equid HBV, EqHBV) in 3.2% of donkeys and zebras by PCR and antibodies against EqHBV in 5.4% of donkeys and zebras. Molecular, histopathological, and biochemical analyses revealed that infection patterns of EqHBV resembled those of HBV in humans, including hepatotropism, moderate liver damage, evolutionary stasis, and potential horizontal virus transmission. Naturally infected donkeys showed chronic infections resembling CHB with high viral loads of up to 2.6 × 10⁹ mean copies per milliliter serum for >6 mo and weak antibody responses. Antibodies against Equid HCV were codetected in 26.5% of donkeys seropositive for EqHBV, corroborating susceptibility to both hepatitis viruses. Deltavirus pseudotypes carrying EqHBV surface proteins were unable to infect human cells via the HBV receptor NTCP (Na⁺/taurocholate cotransporting polypeptide), suggesting alternative viral entry mechanisms. Both HBV and EqHBV deltavirus pseudotypes infected primary horse hepatocytes in vitro, supporting a broad host range for EqHBV among equids and suggesting that horses might be suitable for EqHBV and HBV infections in vivo. Evolutionary analyses suggested that EqHBV originated in Africa several thousand years ago, commensurate with the domestication of donkeys. In sum, EqHBV naturally infects diverse equids and mimics HBV infection patterns. Equids provide a unique opportunity for preclinical testing of novel therapeutics for CHB and to investigate HBV/HCV interplay upon coinfection.

In vitro transcribed mRNA for expression of designer nucleases: Advantages as a novel therapeutic for the management of chronic HBV infection

Chronic infection with the hepatitis B virus (HBV) remains a significant worldwide medical problem. While diseases caused by HIV infection, tuberculosis and malaria are on the decline, new cases of chronic hepatitis B are on the rise. Because often fatal complications of cirrhosis and hepatocellular carcinoma are associated with chronic hepatitis B, the need for a cure is as urgent as ever. Currently licensed therapeutics fail to eradicate the virus and this is attributable to persistence of the viral replication intermediate comprising covalently closed circular DNA (cccDNA). Elimination or inactivation of the viral cccDNA is thus a goal of research aimed at hepatitis B cure. The ability to engineer nucleases that are capable of specific cleavage of a DNA sequence now provides the means to disable cccDNA permanently. The scientific literature is replete with many examples of using designer zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and RNA-guided endonucleases (RGENs) to inactivate HBV. However, important concerns about safety, dose control and efficient delivery need to be addressed before the technology is employed in a clinical setting. Use of in vitro transcribed mRNA to express therapeutic gene editors goes some way to overcoming these concerns. The labile nature of RNA limits off-target effects and enables dose control. Compatibility with hepatotropic non-viral vectors is convenient for the large scale preparation that will be required for advancing gene editing as a mode of curing chronic hepatitis B.

Efficient and reproducible depletion of hepatitis B virus from plasma derived extracellular vesicles

Extracellular vesicles (EVs) are emerging fundamental players in viral infections by shuttling viral components, mediating immune responses and likely the spread of the virus. However, the obstacles involved in purifying EVs and removing contaminating viral particles in a reliable and effective manner bottlenecks the full potential for the development of clinical and diagnostic treatment options targeting EV. Because of the similarities in size, density, membrane composition and mode of biogenesis of EVs and virions there are no standardized approaches for virus-removal from EV preparations yet. Functional EV studies also require EV samples that are devoid of antibody contaminants. Consequently, the study of EVs in virology needs reliable and effective protocols to purify EVs and remove contaminating antibodies and viral particles. Here, we established a protocol for EV purification from hepatitis B virus (HBV)-containing plasma by a combination of size-exclusion chromatography and affinity-based purification. After purification, EV samples were free of virus-sized particles, HBV surface antigen, HBV core antigen, antibodies or infectious material. Viral genomic contamination was also decreased following purification. By using appropriate antibodies and size parameters, this protocol could potentially be applied to purification of EVs from other viral samples. In summary, we established a fast, reproducible and robust approach for the removal of HBV from EV preparations. Looking forward to the point of purifying EVs from clinical samples, this method should enable studies shedding light on the underlying mechanisms of EVs in viral infections and their diagnostic and prognostic potential.

Synchronised infection identifies early rate-limiting steps in the hepatitis B virus life cycle

Hepatitis B virus (HBV) is an enveloped DNA virus that contains a partially double-stranded relaxed circular (rc) DNA. Upon infection, rcDNA is delivered to the nucleus where it is repaired to covalently closed circular (ccc) DNA that serves as the transcription template for all viral RNAs. Our understanding of HBV particle entry dynamics and host pathways regulating intracellular virus trafficking and cccDNA formation is limited. The discovery of sodium taurocholate co-transporting peptide (NTCP) as the primary receptor allows studies on these early steps in viral life cycle. We employed a synchronised infection protocol to quantify HBV entry kinetics. HBV attachment to cells at 4°C is independent of NTCP, however, subsequent particle uptake is NTCP-dependent and reaches saturation at 12 h post-infection. HBV uptake is clathrin- and dynamin dependent with actin and tubulin playing a role in the first 6 h of infection. Cellular fractionation studies demonstrate HBV DNA in the nucleus within 6 h of infection and cccDNA was first detected at 24 h post-infection. Our studies show the majority (83%) of cell bound particles enter HepG2-NTCP cells, however, only a minority (<1%) of intracellular rcDNA was converted to cccDNA, highlighting this as a rate-limiting in establishing infection in vitro. This knowledge highlights the deficiencies in our in vitro cell culture systems and will inform the design and evaluation of physiologically relevant models that support efficient HBV replication.

Advances with RNAi-Based Therapy for Hepatitis B Virus Infection

Infection with hepatitis B virus (HBV) remains a global health challenge. Approximately 292 million people worldwide are chronically infected with HBV and the annual mortality from the infection is approaching 900,000. Despite the availability of an effective prophylactic vaccine, millions of individuals are at risk of potentially fatal complicating cirrhosis and hepatocellular carcinoma. Current drug treatments can suppress viral replication, slow the progression of liver fibrosis, and reduce infectivity, but can rarely clear the viral covalently closed circular DNA (cccDNA) that is responsible for HBV persistence. Alternative therapeutic strategies, including those based on viral gene silencing by harnessing the RNA interference (RNAi) pathway, effectively suppress HBV replication and thus hold promise. RNAi-based silencing of certain viral genes may even lead to disabling of cccDNA during chronic infection. This review summarizes different RNAi activators that have been tested against HBV, the advances with vectors used to deliver artificial potentially therapeutic RNAi sequences to the liver, and the current status of preclinical and clinical investigation.

Animal models for the study of human hepatitis B and D virus infection: New insights and progress

Hepatitis B virus (HBV) is a member of the Hepadnaviridae family and infects hepatocytes, leading to liver pathology in acutely and chronically infected individuals. Co-infection with Hepatitis D virus (HDV), which requires the surface proteins of HBV to replicate, can exacerbate this disease progression. Thus, the >250 million people living with chronic HBV infection, including 13 million co-infected with HDV, would significantly benefit from an effective and affordable curative treatment. Animal models are crucial to the development of innovative disease therapies, a paradigm repeated again and again throughout the fields of immunology, neurology, reproduction, and development. Unfortunately, HBV has a highly-restricted species tropism, infecting limited species including humans, chimpanzees, and treeshrews. The first experimentally controlled studies of HBV infection were following inoculation of human volunteers in 1942, which identified the transmissibility of hepatitis through serum transfer and led to the hypothesis that the etiological agent was viral. Subsequent research in chimpanzees (Desmyter et al., 1971; Lichter, 1969) and later in other species, such as the treeshrews (Walter et al., 1996; Yan et al., 1996), further confirmed the viral origin of hepatitis B. Shortly thereafter, HBV-like viral infections were identified in woodchucks (Summers et al., 1978; Werner et al., 1979) and ducks, and much of our understanding of HBV replication can be attributed to these important models. However, with the exodus of chimpanzees from research and the limited reagents and historical data for treeshrews and other understudied species, there remains an urgent need to identify physiologically relevant models of chronic HBV infection. While large strides have been made in generating such models, particularly over the past two decades, there is still no available model that faithfully recapitulates the immunity and pathogenesis of HBV infection. Here, we discuss recent advancements in the generation of murine and non-human primate (NHP) models of HBV/HDV infection.

Insights into Hepatitis B Virus DNA Integration-55 Years after Virus Discovery

Hepatitis B virus (HBV), which was discovered in 1965, is a threat to global public health. HBV infects human hepatocytes and leads to acute and chronic liver diseases, and there is no cure. In cells infected by HBV, viral DNA can be integrated into the cellular genome. HBV DNA integration is a complicated process during the HBV life cycle. Although HBV integration normally results in replication-incompetent transcripts, it can still act as a template for viral protein expression. Of note, it is a primary driver of hepatocellular carcinoma (HCC). Recently, with the development of detection methods and research models, the molecular biology and the pathogenicity of HBV DNA integration have been better revealed. Here, we review the advances in the research of HBV DNA integration, including molecular mechanisms, detection methods, research models, the effects on host and viral gene expression, the role of HBV integrations in the pathogenesis of HCC, and potential treatment strategies. Finally, we discuss possible future research prospects of HBV DNA integration.

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HBV DNA integration is associated with hepatocarcinogenesis via multiple mechanisms

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HBV double-stranded linear DNA (dslDNA) is the dominant substrate for integration into the host genome

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The insertion sites of HBV DNA integration occur throughout the whole host genome using the NHEJ or MMEJ DNA repair pathway

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HBV DNA integration should be used as a clinical indicator for disease monitoring and treatment of patients with HBV infection

Innovative HBV Animal Models Based on the Entry Receptor NTCP

Hepatitis B is a major global health problem, with an estimated 257 million chronically infected patients and almost 1 million deaths per year. The causative agent is hepatitis B virus (HBV), a small, enveloped, partially double-stranded DNA virus. HBV has a strict species specificity, naturally infecting only humans and chimpanzees. Sodium taurocholate co-transporting polypeptide (NTCP), a bile acid transporter expressed on hepatocytes, has been shown to be one of the key factors in HBV infection, playing a crucial role in the HBV entry process in vitro and in vivo. Variations in the amino acid sequence of NTCP can inhibit HBV infection and, therefore, contributes, in part, to the species barrier. This discovery has revolutionized the search for novel animal models of HBV. Indeed, it was recently shown that variations in the amino acid sequence of NTCP represent the sole species barrier for HBV infection in macaques. Here, we review what is known about HBV entry through the NTCP receptor and highlight how this knowledge has been harnessed to build new animal models for the study of HBV pathogenesis and curative therapies.

MHC-E-Restricted CD8+ T Cells Target Hepatitis B Virus-Infected Human Hepatocytes

Currently 247 million people are living with chronic hepatitis B virus infection (CHB), and the development of novel curative treatments is urgently needed. Immunotherapy is an attractive approach to treat CHB, yet therapeutic approaches to augment the endogenous hepatitis B virus (HBV)-specific T cell response in CHB patients have demonstrated little success. In this study, we show that strain 68-1 rhesus macaque (RM) CMV vaccine vectors expressing HBV Ags engender HBV-specific CD8+ T cells unconventionally restricted by MHC class II and the nonclassical MHC-E molecule in RM. Surface staining of human donor and RM primary hepatocytes (PH) ex vivo revealed the majority of PH expressed MHC-E but not MHC class II. HBV-specific, MHC-E-restricted CD8+ T cells from RM vaccinated with RM CMV vaccine vectors expressing HBV Ags recognized HBV-infected PH from both human donor and RM. These results provide proof-of-concept that MHC-E-restricted CD8+ T cells could be harnessed for the treatment of CHB, either through therapeutic vaccination or adoptive immunotherapy.

In Vitro Systems for Studying Different Genotypes/Sub-Genotypes of Hepatitis B Virus: Strengths and Limitations

Hepatitis B virus (HBV) infects the liver resulting in end stage liver disease, cirrhosis, and hepatocellular carcinoma. Despite an effective vaccine, HBV poses a serious health problem globally, accounting for 257 million chronic carriers. Unique features of HBV, including its narrow virus-host range and its hepatocyte tropism, have led to major challenges in the development of suitable in vivo and in vitro model systems to recapitulate the HBV replication cycle and to test various antiviral strategies. Moreover, HBV is classified into at least nine genotypes and 35 sub-genotypes with distinct geographical distributions and prevalence, which have different natural histories of infection, clinical manifestation, and response to current antiviral agents. Here, we review various in vitro systems used to study the molecular biology of the different (sub)genotypes of HBV and their response to antiviral agents, and we discuss their strengths and limitations. Despite the advances made, no system is ideal for pan-genotypic HBV research or drug development and therefore further improvement is required. It is necessary to establish a centralized repository of HBV-related generated materials, which are readily accessible to HBV researchers, with international collaboration toward advancement and development of in vitro model systems for testing new HBV antivirals to ensure their pan-genotypic and/or customized activity.

The E3 Ubiquitin Ligase TRIM21 Promotes HBV DNA Polymerase Degradation

The tripartite motif (TRIM) protein family is an E3 ubiquitin ligase family. Recent reports have indicated that some TRIM proteins have antiviral functions, especially against retroviruses. However, most studies mainly focus on the relationship between TRIM21 and interferon or other antiviral effectors. The effect of TRIM21 on virus-encoded proteins remains unclear. In this study, we screened candidate interacting proteins of HBV DNA polymerase (Pol) by FLAG affinity purification and mass spectrometry assay and identified TRIM21 as its regulator. We used a coimmunoprecipitation (co-IP) assay to demonstrate that TRIM21 interacted with the TP domain of HBV DNA Pol. In addition, TRIM21 promoted the ubiquitination and degradation of HBV DNA Pol using its RING domain, which has E3 ubiquitin ligase activity. Lys260 and Lys283 of HBV DNA Pol were identified as targets for ubiquitination mediated by TRIM21. Finally, we uncovered that TRIM21 degrades HBV DNA Pol to restrict HBV DNA replication, and its SPRY domain is critical for this activity. Taken together, our results indicate that TRIM21 suppresses HBV DNA replication mainly by promoting the ubiquitination of HBV DNA Pol, which may provide a new potential target for the treatment of HBV.

Woolly Monkey-HBV Infection in Squirrel Monkeys as a Surrogate Nonhuman Primate Model of HBV Infection

Development of curative therapies for chronic hepatitis B virus (HBV) infection will likely require new animal models. Here, we evaluate HBV infection in squirrel monkeys based on the high-sequence homology of the HBV receptor, Na+/taurocholate co-transporting peptide (NTCP), between humans and squirrel monkeys. HBV PreS1 peptide was examined for binding human and squirrel monkey NTCP. Immunodeficient Fah -/- , NOD, Rag1 -/- , Il2Rg null (FNRG) mice engrafted with human or squirrel monkey hepatocytes were challenged with HBV or Woolly Monkey HBV (WMHBV). In addition, adult squirrel monkeys were inoculated with HBV, WMHBV, adeno-associated virus containing an infectious genome of HBV (AAV-HBV), and AAV-WMHBV. Finally, neonate squirrel monkeys were assessed for the potential of chronic infection with WMHBV. PreS1 peptide efficiently bound to human and squirrel monkey NTCP but not to mouse or capuchin NTCP. FNRG mice engrafted with squirrel monkey hepatocytes were susceptible to infection by WMHBV but not human HBV. Similarly, adult squirrel monkeys could be infected with WMHBV but not human HBV, whereas chimeric mice engrafted with human hepatocytes were susceptible to HBV but not WMHBV. Infection of squirrel monkeys with AAV-WMHBV yielded maximum viremia of 108 genomes/mL with detectable virus for up to 8 months. Notably, covalently closed circular DNA was detected in the liver of these animals. Infection of neonates with WMHBV led to detectable viremia for up to 6 months. Conclusions: Adult and neonate squirrel monkeys exhibited prolonged WMHBV viremia lasting 6-8 months. This is greater than twice the duration of viremia achieved in other nonhuman primates and suggests that squirrel monkeys may be a suitable model for testing HBV therapeutics.

Small Animal Models for Human Immunodeficiency Virus (HIV), Hepatitis B, and Tuberculosis: Proceedings of an NIAID Workshop

The main advantage of animal models of infectious diseases over in vitro studies is the gain in the understanding of the complex dynamics between the immune system and the pathogen. While small animal models have practical advantages over large animal models, it is crucial to be aware of their limitations. Although the small animal model at least needs to be susceptible to the pathogen under study to obtain meaningful data, key elements of pathogenesis should also be reflected when compared to humans. Well-designed small animal models for HIV, hepatitis viruses and tuberculosis require, additionally, a thorough understanding of the similarities and differences in the immune responses between humans and small animals and should incorporate that knowledge into the goals of the study. To discuss these considerations, the NIAID hosted a workshop on 'Small Animal Models for HIV, Hepatitis B, and Tuberculosis' on May 30, 2019. Highlights of the workshop are outlined below.

A New Role for Capsid Assembly Modulators To Target Mature Hepatitis B Virus Capsids and Prevent Virus Infection

Hepatitis B virus (HBV) is a major human pathogen, killing an estimated 887,000 people per year. Therefore, potentially curative therapies are of high importance. Following infection, HBV deposits a covalently closed circular DNA (cccDNA) in the nucleus of infected cells that serves as a transcription template and is not affected by current therapies. HBV core protein allosteric modulators (CpAMs) prevent correct capsid assembly but may also affect early stages of HBV infection. In this study, we aimed to determine the antiviral efficacy of a novel, structurally distinct heteroaryldihydropyrimidine (HAP)-type CpAM, HAP_R01, and investigated whether and how HAP_R01 prevents the establishment of HBV infection. HAP_R01 shows a significant inhibition of cccDNA formation when applied during the first 48 h of HBV infection. Inhibiting cccDNA formation, however, requires >1-log₁₀-higher concentrations than inhibition of the assembly of newly forming capsids (half-maximal effective concentration [EC₅₀], 345 to 918 nM versus 26.8 to 43.5 nM, respectively). Biophysical studies using a new method to detect the incoming capsid in de novo infection revealed that HAP_R01 can physically change mature capsids of incoming virus particles and affect particle integrity. Treating purified HBV virions with HAP_R01 reduced their infectivity, highlighting the unique antiviral activity of CpAMs to target the capsid within mature HBV particles. Accordingly, HAP_R01 shows an additive antiviral effect in limiting de novo infection when combined with viral entry inhibitors. In summary, HAP_R01 perturbs capsid integrity of incoming virus particles and reduces their infectivity and thus inhibits cccDNA formation in addition to preventing HBV capsid assembly.

Vaccine-Mediated Inhibition of the Transporter Associated with Antigen Processing Is Insufficient To Induce Major Histocompatibility Complex E-Restricted CD8+ T Cells in Nonhuman Primates

Major histocompatibility complex E (MHC-E) is a highly conserved nonclassical MHC-Ib molecule that tightly binds peptides derived from leader sequences of classical MHC-Ia molecules for presentation to natural killer cells. However, MHC-E also binds diverse foreign and neoplastic self-peptide antigens for presentation to CD8+ T cells. Although the determinants of MHC-E-restricted T cell priming remain unknown, these cells are induced in humans infected with pathogens containing genes that inhibit the transporter associated with antigen processing (TAP). Indeed, mice vaccinated with TAP-inhibited autologous dendritic cells develop T cells restricted by the murine MHC-E homologue, Qa-1b. Here, we tested whether rhesus macaques (RM) vaccinated with viral constructs expressing a TAP inhibitor would develop insert-specific MHC-E-restricted CD8+ T cells. We generated viral constructs coexpressing SIVmac239 Gag in addition to one of three TAP inhibitors: herpes simplex virus 2 ICP47, bovine herpes virus 1 UL49.5, or rhesus cytomegalovirus Rh185. Each TAP inhibitor reduced surface expression of MHC-Ia molecules but did not reduce surface MHC-E expression. In agreement with modulation of surface MHC-Ia levels, TAP inhibition diminished presentation of MHC-Ia-restricted CD8+ T cell epitopes without impacting presentation of peptide antigen bound by MHC-E. Vaccination of macaques with vectors dually expressing SIVmac239 Gag with ICP47, UL49.5, or Rh185 generated Gag-specific CD8+ T cells classically restricted by MHC-Ia but not MHC-E. These data demonstrate that, in contrast to results in mice, TAP inhibition alone is insufficient for priming of MHC-E-restricted T cell responses in primates and suggest that additional unknown mechanisms govern the induction of CD8+ T cells recognizing MHC-E-bound antigen.IMPORTANCE Due to the near monomorphic nature of MHC-E in the human population and inability of many pathogens to inhibit MHC-E-mediated peptide presentation, MHC-E-restricted T cells have become an attractive vaccine target. However, little is known concerning how these cells are induced. Understanding the underlying mechanisms that induce these T cells would provide a powerful new vaccine strategy to an array of neoplasms and viral and bacterial pathogens. Recent studies have indicated a link between TAP inhibition and induction of MHC-E-restricted T cells. The significance of our research is in demonstrating that TAP inhibition alone does not prime MHC-E-restricted T cell generation and suggests that other, currently unknown mechanisms regulate their induction.

Efficient long-term amplification of hepatitis B virus isolates after infection of slow proliferating HepG2-NTCP cells

BACKGROUND & AIMS

As hepatitis B virus (HBV) spreads through the infected liver it is simultaneously secreted into the blood. HBV-susceptible in vitro infection models do not efficiently amplify viral progeny or support cell-to-cell spread. We sought to establish a cell culture system for the amplification of infectious HBV from clinical specimens.

METHODS

An HBV-susceptible sodium-taurocholate cotransporting polypeptide-overexpressing HepG2 cell clone (HepG2-NTCPsec+) producing high titers of infectious progeny was selected. Secreted HBV progeny were characterized by native gel electrophoresis and electron microscopy. Comparative RNA-seq transcriptomics was performed to quantify the expression of host proviral and restriction factors. Viral spread routes were evaluated using HBV entry- or replication inhibitors, visualization of viral cell-to-cell spread in reporter cells, and nearest neighbor infection determination. Amplification kinetics of HBV genotypes B-D were analyzed.

RESULTS

Infected HepG2-NTCPsec+ secreted high levels of large HBV surface protein-enveloped infectious HBV progeny with typical appearance under electron microscopy. RNA-seq transcriptomics revealed that HBV does not induce significant gene expression changes in HepG2-NTCPsec+, however, transcription factors favoring HBV amplification were more strongly expressed than in less permissive HepG2-NTCPsec-. Upon inoculation with HBV-containing patient sera, rates of infected cells increased from 10% initially to 70% by viral spread to adjacent cells, and viral progeny and antigens were efficiently secreted. HepG2-NTCPsec+ supported up to 1,300-fold net amplification of HBV genomes depending on the source of virus. Viral spread and amplification were abolished by entry and replication inhibitors; viral rebound was observed after inhibitor discontinuation.

CONCLUSIONS

The novel HepG2-NTCPsec+ cells efficiently support the complete HBV life cycle, long-term viral spread and amplification of HBV derived from patients or cell culture, resembling relevant features of HBV-infected patients.

LAY SUMMARY

Currently available laboratory systems are unable to reproduce the dynamics of hepatitis B virus (HBV) spread through the infected liver and release into the blood. We developed a slowly dividing liver-derived cell line which multiplies infectious viral particles upon inoculation with patient- or cell culture-derived HBV. This new infection model can improve therapy by measuring, in advance, the sensitivity of a patient's HBV strain to specific antiviral drugs.

In Vivo Model Systems for Hepatitis B Virus Research

Hepatitis B virus (HBV) affects more than 257 million people globally, resulting in progressively worsening liver disease, manifesting as fibrosis, cirrhosis, and hepatocellular carcinoma. The exceptionally narrow species tropism of HBV restricts its natural hosts to humans and non-human primates, including chimpanzees, gorillas, gibbons, and orangutans. The unavailability of completely immunocompetent small-animal models has contributed to the lack of curative therapeutic interventions. Even though surrogates allow the study of closely related viruses, their host genetic backgrounds, immune responses, and molecular virology differ from those of HBV. Various different models, based on either pure murine or xenotransplantation systems, have been introduced over the past years, often making the choice of the optimal model for any given question challenging. Here, we offer a concise review of in vivo model systems employed to study HBV infection and steps in the HBV life cycle or pathogenesis.

A global scientific strategy to cure hepatitis B

Chronic hepatitis B virus (HBV) infection is a global public health challenge on the same scale as tuberculosis, HIV, and malaria. The International Coalition to Eliminate HBV (ICE-HBV) is a coalition of experts dedicated to accelerating the discovery of a cure for chronic hepatitis B. Following extensive consultation with more than 50 scientists from across the globe, as well as key stakeholders including people affected by HBV, we have identified gaps in our current knowledge and new strategies and tools that are required to achieve HBV cure. We believe that research must focus on the discovery of interventional strategies that will permanently reduce the number of productively infected cells or permanently silence the covalently closed circular DNA in those cells, and that will stimulate HBV-specific host immune responses which mimic spontaneous resolution of HBV infection. There is also a pressing need for the establishment of repositories of standardised HBV reagents and protocols that can be accessed by all HBV researchers throughout the world. The HBV cure research agenda outlined in this position paper will contribute markedly to the goal of eliminating HBV infection worldwide.

Design, synthesis, and biological evaluation of novel 2'-deoxy-2'-fluoro-2'-C-methyl 8-azanebularine derivatives as potent anti-HBV agents

Hepatitis B virus (HBV) is a global health problem requiring more efficient and better tolerated anti-HBV agent. In this paper, a series of novel 2'-deoxy-2'-fluoro-2'-C-methyl-β-d-arabinofuranosyl 8-azanebularine analogues (1 and 2a) and N⁴-substituted 8-azaadenosine derivatives (2b-g) were designed, synthesized and screened for in vitro anti-HBV activity. Two concise and practical synthetic routes were developed toward the structural motif construction of 2'-deoxy-2'-fluoro-2'-C-methyl-β-d-arabinofuranosyl 8-azainosine from the ribonolactone 3 under mild conditions. The in vitro anti-HBV screening results showed that these 8-azanebularine analogues had a significant inhibitory effect on the expression of HBV antigens and HBV DNA at a concentration of 20 μM. Among them, halogen-substituted 8-azaadenosine derivative 2g displayed activities comparable to that of 3TC. In particular, 2g retained excellent activity against lamivudine-resistant HBV mutants.

A Single Adaptive Mutation in Sodium Taurocholate Cotransporting Polypeptide Induced by Hepadnaviruses Determines Virus Species Specificity

Hepatitis B virus (HBV) and its hepadnavirus relatives infect a wide range of vertebrates, from fish to human. Hepadnaviruses and their hosts have a long history of acquiring adaptive mutations. However, there are no reports providing direct molecular evidence for such a coevolutionary "arms race" between hepadnaviruses and their hosts. Here, we present evidence suggesting that the adaptive evolution of the sodium taurocholate cotransporting polypeptide (NTCP), an HBV receptor, has been influenced by virus infection. Evolutionary analysis of the NTCP-encoding genes from 20 mammals showed that most NTCP residues are highly conserved among species, exhibiting evolution under negative selection (dN/dS ratio [ratio of nonsynonymous to synonymous evolutionary changes] of <1); this observation implies that the evolution of NTCP is restricted by maintaining its original protein function. However, 0.7% of NTCP amino acid residues exhibit rapid evolution under positive selection (dN/dS ratio of >1). Notably, a substitution at amino acid (aa) 158, a positively selected residue, converting the human NTCP to a monkey-type sequence abrogated the capacity to support HBV infection; conversely, a substitution at this residue converting the monkey Ntcp to the human sequence was sufficient to confer HBV susceptibility. Together, these observations suggested a close association of the aa 158 positive selection with the pressure by virus infection. Moreover, the aa 158 sequence determined attachment of the HBV envelope protein to the host cell, demonstrating the mechanism whereby HBV infection would create positive selection at this NTCP residue. In summary, we provide the first evidence in agreement with the function of hepadnavirus as a driver for inducing adaptive mutation in host receptor.IMPORTANCE HBV and its hepadnavirus relatives infect a wide range of vertebrates, with a long infectious history (hundreds of millions of years). Such a long history generally allows adaptive mutations in hosts to escape from infection while simultaneously allowing adaptive mutations in viruses to overcome host barriers. However, there is no published molecular evidence for such a coevolutionary arms race between hepadnaviruses and hosts. In the present study, we performed coevolutionary phylogenetic analysis between hepadnaviruses and the sodium taurocholate cotransporting polypeptide (NTCP), an HBV receptor, combined with virological experimental assays for investigating the biological significance of NTCP sequence variation. Our data provide the first molecular evidence supporting that HBV-related hepadnaviruses drive adaptive evolution in the NTCP sequence, including a mechanistic explanation of how NTCP mutations determine host viral susceptibility. Our novel insights enhance our understanding of how hepadnaviruses evolved with their hosts, permitting the acquisition of strong species specificity.

Cell and Animal Models for Studying Hepatitis B Virus Infection and Drug Development

Many cell culture and animal models have been used to study hepatitis B virus (HBV) replication and its effects in the liver; these have facilitated development of strategies to control and clear chronic HBV infection. We discuss the advantages and limitations of systems for studying HBV and developing antiviral agents, along with recent advances. New and improved model systems are needed. Cell culture systems should be convenient, support efficient HBV infection, and reproduce responses of hepatocytes in the human body. We also need animals that are fully permissive to HBV infection, convenient for study, and recapitulate human immune responses to HBV and effects in the liver. High-throughput screening technologies could facilitate drug development based on findings from cell and animal models.