In Vivo Models of HDV Infection: Is Humanizing NTCP Enough?

Mice Engrafted with Human Liver Cells

Rodents are commonly employed to model human liver conditions, although species differences can restrict their translational relevance. To overcome some of these limitations, researchers have long pursued human hepatocyte transplantation into rodents. More than 20 years ago, the first primary human hepatocyte transplantations into immunodeficient mice with liver injury were able to support hepatitis B and C virus infections, as these viruses cannot replicate in murine hepatocytes. Since then, hepatocyte chimeric mouse models have transitioned into mainstream preclinical research and are now employed in a diverse array of liver conditions beyond viral hepatitis, including malaria, drug metabolism, liver-targeting gene therapy, metabolic dysfunction-associated steatotic liver disease, lipoprotein and bile acid biology, and others. Concurrently, endeavors to cotransplant other cell types and humanize immune and other nonparenchymal compartments have seen growing success. Looking ahead, several challenges remain. These include enhancing immune functionality in mice doubly humanized with hepatocytes and immune systems, efficiently creating mice with genetically altered grafts and reliably humanizing chimeric mice with renewable cell sources such as patient-specific induced pluripotent stem cells. In conclusion, hepatocyte chimeric mice have evolved into vital preclinical models that address many limitations of traditional rodent models. Continued improvements may further expand their applications.

Bulevirtide and emerging drugs for the treatment of hepatitis D

INTRODUCTION

Hepatitis delta virus (HDV) causes acute and chronic liver disease that requires the co-infection of the Hepatitis B virus and can lead to significant morbidity and mortality. Bulevirtide is a recently introduced entry inhibitor drug that acts on the sodium taurocholate cotransporting peptide, thereby preventing viral entry to target cells in chronic HDV infection. The mainstay of chronic HDV therapy prior to bulevirtide was interferon alpha, which has an undesirable side effect profile.

AREAS COVERED

We review bulevirtide data from recent clinical trials in Europe and the United States. Challenges to development and implementation of bulevirtide are discussed. Additionally, we review ongoing trials of emerging drugs for HDV, such as pegylated interferon lambda and lonafarnib.

EXPERT OPINION

Bulevirtide represents a major shift in treatment for chronic HDV, for which there is significant unmet need. Trials that compared bulevirtide in combination with interferon alpha vs interferon alpha monotherapy demonstrated significant increase in virologic response. Overall, treatment with different doses of bulevirtide were comparable. Bulevirtide was generally well tolerated, and no serious adverse events occurred. Understanding the true prevalence of HDV, as well as continued studies of emerging drugs will prove valuable to the larger goal of eradication of Hepatitis D.

In vitro cell culture models to study hepatitis B and D virus infection

Chronic infection with the hepatitis B virus (HBV) and hepatitis D virus (HDV) can cause a major global health burden. Current medication regimens can repress viral replication and help to control disease progression, but a complete cure is hardly achieved due to the difficulties to eradicate viral templates (cccDNA and integrates). To develop novel curative antiviral therapies for HBV/HDV infection, it is vital to precisely understand the details of the molecular biology of both viruses and the virus-host interactions. One important prerequisite for gaining this aim is the availability of suitable in vitro models that support HBV/HDV infection, replicate both viruses via their authentic template and allow to adequately study host cell responses. The discovery of sodium taurocholate cotransporting polypeptide (NTCP) receptor as the most crucial host factor promoted HBV/HDV research to a new era. Recently, the structure of human NTCP was solved, gaining a deeper understanding of HBV recognition as the bona fide receptor. After decades of continuous efforts, new progress has been achieved in the development of cell culture models supporting HBV/HDV study. This review summarizes the cell culture models currently available, discusses the advantages and disadvantages of each model, and highlights their future applications in HBV and HDV research.

Studying T Cell Responses to Hepatotropic Viruses in the Liver Microenvironment

T cells play an important role in the clearance of hepatotropic viruses but may also cause liver injury and contribute to disease progression in chronic hepatitis B and C virus infections which affect millions of people worldwide. The liver provides a unique microenvironment of immunological tolerance and hepatic immune regulation can modulate the functional properties of T cell subsets and influence the outcome of a virus infection. Extensive research over the last years has advanced our understanding of hepatic conventional CD4+ and CD8+ T cells and unconventional T cell subsets and their functions in the liver environment during acute and chronic viral infections. The recent development of new small animal models and technological advances should further increase our knowledge of hepatic immunological mechanisms. Here we provide an overview of the existing models to study hepatic T cells and review the current knowledge about the distinct roles of heterogeneous T cell populations during acute and chronic viral hepatitis.

Bulevirtide in the Treatment of Hepatitis Delta: Drug Discovery, Clinical Development and Place in Therapy

It has been ten years since the identification of NTCP as the cell surface receptor for HBV and HDV entry into hepatocytes. The search for molecules interfering with the binding of NTCP and HBV/HDV led to design bulevirtide (BLV). This large polypeptide mimics a region of the pre-S1 HBsAg and blocks viral entry by inhibitory competition. BLV was initially tested in cell cultures, animal models and more recently in Phase I-III human trials (called 'MYRS'). As monotherapy or in combination with peginterferon, BLV is well tolerated and exhibits potent antiviral activity. Plasma viremia significantly declines and/or becomes undetectable in more than 75% of patients treated for >24 weeks. However, serum HBsAg concentrations remain unchanged. No selection of BLV resistance in HBV/HDV has been reported in vivo to date. BLV is administered subcutaneously once daily at doses between 2 and 10 mg. BLV received conditional approval in Europe in 2020 to treat chronic hepatitis delta. The advent of peginterferon lambda or new specific anti-HDV antivirals (lonafarnib, etc.) will open the door for combination therapies with BLV. Since there is no stable reservoir for HDV-RNA within infected hepatocytes, viral clearance might be achieved using antivirals for a minimum timeframe. This is what happens in hepatitis C combining several antivirals, curing nearly all patients treated for 3 months. Clearance of HDV-RNA genomes may occur despite HBV persistence as cccDNA or chromosome integrated HBV-DNA within hepatocytes. This is supported by cases of HDV elimination using BLV despite persistence of serum HBsAg. Another path for HDV cure will derive from achieving HBsAg clearance, the goal of new promising anti-HBV gene therapies (bepirovirsen, etc.). In summary, the advent of BLV has triggered a renovated interest for antiviral therapy in hepatitis delta. We envision combination therapies that will lead to HDV cure in the near future.

Hepatitis B and Hepatitis D Viruses: A Comprehensive Update with an Immunological Focus

Hepatitis B virus (HBV) and hepatitis delta virus (HDV) are highly prevalent viruses estimated to infect approximately 300 million people and 12-72 million people worldwide, respectively. HDV requires the HBV envelope to establish a successful infection. Concurrent infection with HBV and HDV can result in more severe disease outcomes than infection with HBV alone. These viruses can cause significant hepatic disease, including cirrhosis, fulminant hepatitis, and hepatocellular carcinoma, and represent a significant cause of global mortality. Therefore, a thorough understanding of these viruses and the immune response they generate is essential to enhance disease management. This review includes an overview of the HBV and HDV viruses, including life cycle, structure, natural course of infection, and histopathology. A discussion of the interplay between HDV RNA and HBV DNA during chronic infection is also included. It then discusses characteristics of the immune response with a focus on reactions to the antigenic hepatitis B surface antigen, including small, middle, and large surface antigens. This paper also reviews characteristics of the immune response to the hepatitis D antigen (including small and large antigens), the only protein expressed by hepatitis D. Lastly, we conclude with a discussion of recent therapeutic advances pertaining to these viruses.

Virome of Three Termite Species from Southern Vietnam

Modern metagenomic approaches enable the effective discovery of novel viruses in previously unexplored organisms. Termites are significant ecosystem converters and influencers. As with the majority of tropical forest insects, termites are studied insufficiently, and termite virome remains especially understudied. Here, we studied the virome of lichenophagous and mycophagous termites (Hospitalitermes bicolor, Macrotermes carbonarius and Odontotermes wallonensis) collected in the Cat Tien National Park (Vietnam). We assembled four full genomes of novel viruses related to Solemoviridae, Lispiviridae, Polycipiviridae and Kolmioviridae. We also found several contigs with relation to Chuviridae and Deltaflexiviridae that did not correspond to complete virus genomes. All the novel viruses clustered phylogenetically with previously identified viruses of the termites. Deltaflexi-like contigs were identified in the fungi-cultivating M. carbonarius and showed homology with viruses recently discovered in the edible basidiomycete mushrooms.