Deleterious effects of hepatitis delta virus replication on host cell proliferation

Hepatitis Delta Virus and Hepatocellular Carcinoma

The hepatitis D virus (HDV) is a compact, enveloped, circular RNA virus that relies on hepatitis B virus (HBV) envelope proteins to initiate a primary infection in hepatocytes, assemble, and secrete new virions. Globally, HDV infection affects an estimated 12 million to 72 million people, carrying a significantly elevated risk of developing cirrhosis, liver failure, and hepatocellular carcinoma (HCC) compared to an HBV mono-infection. Furthermore, HDV-associated HCC often manifests at a younger age and exhibits more aggressive characteristics. The intricate mechanisms driving the synergistic carcinogenicity of the HDV and HBV are not fully elucidated but are believed to involve chronic inflammation, immune dysregulation, and the direct oncogenic effects of the HDV. Indeed, recent data highlight that the molecular profile of HCC associated with HDV is unique and distinct from that of HBV-induced HCC. However, the question of whether the HDV is an oncogenic virus remains unanswered. In this review, we comprehensively examined several crucial aspects of the HDV, encompassing its epidemiology, molecular biology, immunology, and the associated risks of liver disease progression and HCC development.

HDV Pathogenesis: Unravelling Ariadne's Thread

Hepatitis Delta virus (HDV) lies in between satellite viruses and viroids, as its unique molecular characteristics and life cycle cannot categorize it according to the standard taxonomy norms for viruses. Being a satellite virus of hepatitis B virus (HBV), HDV requires HBV envelope glycoproteins for its infection cycle and its transmission. HDV pathogenesis varies and depends on the mode of HDV and HBV infection; a simultaneous HDV and HBV infection will lead to an acute hepatitis that will resolve spontaneously in the majority of patients, whereas an HDV super-infection of a chronic HBV carrier will mainly result in the establishment of a chronic HDV infection that may progress towards cirrhosis, liver decompensation, and hepatocellular carcinoma (HCC). With this review, we aim to unravel Ariadne’s thread into the labyrinth of acute and chronic HDV infection pathogenesis and will provide insights into the complexity of this exciting topic by detailing the different players and mechanisms that shape the clinical outcome.

Human hepatitis D virus-specific T cell epitopes

HDV is a small, defective RNA virus that requires the HBsAg of HBV for its assembly, release, and transmission. Chronic HBV/HDV infection often has a severe clinical outcome and is difficult to treat. The important role of a robust virus-specific T cell response for natural viral control has been established for many other chronic viral infections, but the exact role of the T cell response in the control and progression of chronic HDV infection is far less clear. Several recent studies have characterised HDV-specific CD4+ and CD8+ T cell responses on a peptide level. This review comprehensively summarises all HDV-specific T cell epitopes described to date and describes our current knowledge of the role of T cells in HDV infection. While we now have better tools to study the adaptive anti-HDV-specific T cell response, further efforts are needed to define the HLA restriction of additional HDV-specific T cell epitopes, establish additional HDV-specific MHC tetramers, understand the degree of cross HDV genotype reactivity of individual epitopes and understand the correlation of the HBV- and HDV-specific T cell response, as well as the breadth and specificity of the intrahepatic HDV-specific T cell response.

The role of telomerase and viruses interaction in cancer development, and telomerase-dependent therapeutic approaches

Human telomerase reverse transcriptase (hTERT) is an enzyme that is critically involved in elongating and maintaining telomeres length to control cell life span and replicative potential. Telomerase activity is continuously expressed in human germ-line cells and most cancer cells, whereas it is suppressed in most somatic cells. In normal cells, by reducing telomerase activity and progressively shortening the telomeres, the cells progress to the senescence or apoptosis process. However, in cancer cells, telomere lengths remain constant due to telomerase's reactivation, and cells continue to proliferate and inhibit apoptosis, and ultimately lead to cancer development and human death due to metastasis. Studies demonstrated that several DNA and RNA oncoviruses could interact with telomerase by integrating their genome sequence within the host cell telomeres specifically. Through the activation of the hTERT promoter and lengthening the telomere, these cells contributes to cancer development. Since oncoviruses can activate telomerase and increase hTERT expression, there are several therapeutic strategies based on targeting the telomerase of cancer cells like telomerase-targeted peptide vaccines, hTERT-targeting dendritic cells (DCs), hTERT-targeting gene therapy, and hTERT-targeting CRISPR/Cas9 system that can overcome tumor-mediated toleration mechanisms and specifically apoptosis in cancer cells. This study reviews available data on the molecular structure of telomerase and the role of oncoviruses and telomerase interaction in cancer development and telomerase-dependent therapeutic approaches to conquest the cancer cells.

Innate immune recognition and modulation in hepatitis D virus infection

Hepatitis D virus (HDV) is a global health threat with more than 15 million humans affected. Current treatment options are largely unsatisfactory leaving chronically infected humans at high risk to develop liver cirrhosis and hepatocellular carcinoma. HDV is the only human satellite virus known. It encodes only two proteins, and requires Hepatitis B virus (HBV) envelope protein expression for productive virion release and spread of the infection. How HDV could evolve and why HBV was selected as a helper virus remains unknown. Since the discovery of Na⁺-taurocholate co-transporting polypeptide as the essential uptake receptor for HBV and HDV, we are beginning to understand the interactions of HDV and the immune system. While HBV is mostly regarded a stealth virus, that escapes innate immune recognition, HBV-HDV coinfection is characterized by a strong innate immune response. Cytoplasmic RNA sensor melanoma differentiation antigen 5 has been reported to recognize HDV RNA replication and activate innate immunity. Innate immunity, however, seems not to impair HDV replication while it inhibits HBV. In this review, we describe what is known up-to-date about the interplay between HBV as a helper and HDV’s immune evasion strategy and identify where additional research is required.

Amino Acid Substitutions within HLA-B*27-Restricted T Cell Epitopes Prevent Recognition by Hepatitis Delta Virus-Specific CD8+ T Cells

Virus-specific CD8 T cell response seems to play a significant role in the outcome of hepatitis delta virus (HDV) infection. However, the HDV-specific T cell epitope repertoire and mechanisms of CD8 T cell failure in HDV infection have been poorly characterized. We therefore aimed to characterize HDV-specific CD8 T cell epitopes and the impacts of viral mutations on immune escape. In this study, we predicted peptide epitopes binding the most frequent human leukocyte antigen (HLA) types and assessed their HLA binding capacities. These epitopes were characterized in HDV-infected patients by intracellular gamma interferon (IFN-γ) staining. Sequence analysis of large hepatitis delta antigen (L-HDAg) and HLA typing were performed in 104 patients. The impacts of substitutions within epitopes on the CD8 T cell response were evaluated experimentally and by in silico studies. We identified two HLA-B*27-restricted CD8 T cell epitopes within L-HDAg. These novel epitopes are located in a relatively conserved region of L-HDAg. However, we detected molecular footprints within the epitopes in HLA-B*27-positive patients with chronic HDV infections. The variant peptides were not cross-recognized in HLA-B*27-positive patients with resolved HDV infections, indicating that the substitutions represent viral escape mutations. Molecular modeling of HLA-B*27 complexes with the L-HDAg epitope and its potential viral escape mutations indicated that the structural and electrostatic properties of the bound peptides differ considerably at the T cell receptor interface, which provides a possible molecular explanation for the escape mechanism. This viral escape from the HLA-B*27-restricted CD8 T cell response correlates with a chronic outcome of hepatitis D infection. T cell failure resulting from immune escape may contribute to the high chronicity rate in HDV infection.IMPORTANCE Hepatitis delta virus (HDV) causes severe chronic hepatitis, which affects 20 million people worldwide. Only a small number of patients are able to clear the virus, possibly mediated by a virus-specific T cell response. Here, we performed a systematic screen to define CD8 epitopes and investigated the role of CD8 T cells in the outcome of hepatitis delta and how they fail to eliminate HDV. Overall the number of epitopes identified was very low compared to other hepatotropic viruses. We identified, two HLA-B*27-restricted epitopes in patients with resolved infections. In HLA-B*27-positive patients with chronic HDV infections, however, we detected escape mutations within these identified epitopes that could lead to viral evasion of immune responses. These findings support evidence showing that HLA-B*27 is important for virus-specific CD8 T cell responses, similar to other viral infections. These results have implications for the clinical prognosis of HDV infection and for vaccine development.

Cellular Nuclear Export Factors TAP and Aly Are Required for HDAg-L-mediated Assembly of Hepatitis Delta Virus

Hepatitis delta virus (HDV) is a satellite virus of hepatitis B virus (HBV). HDV genome encodes two forms of hepatitis delta antigen (HDAg), small HDAg (HDAg-S), which is required for viral replication, and large HDAg (HDAg-L), which is essential for viral assembly. HDAg-L is identical to HDAg-S except that it bears a 19-amino acid extension at the C terminus. Both HDAgs contain a nuclear localization signal (NLS), but only HDAg-L contains a CRM1-independent nuclear export signal at its C terminus. The nuclear export activity of HDAg-L is important for HDV particle formation. However, the mechanisms of HDAg-L-mediated nuclear export of HDV ribonucleoprotein are not clear. In this study, the host cellular RNA export complex TAP-Aly was found to form a complex with HDAg-L, but not with an export-defective HDAg-L mutant, in which Pro²⁰⁵ was replaced by Ala. HDAg-L was found to colocalize with TAP and Aly in the nucleus. The C-terminal domain of HDAg-L was shown to directly interact with the N terminus of TAP, whereas an HDAg-L mutant lacking the NLS failed to interact with full-length TAP. In addition, small hairpin RNA-mediated down-regulation of TAP or Aly reduced nuclear export of HDAg-L and assembly of HDV virions. Furthermore, a peptide, TAT-HDAg-L(198-210), containing the 10-amino acid TAT peptide and HDAg-L(198-210), inhibited the interaction between HDAg-L and TAP and blocked HDV virion assembly and secretion. These data demonstrate that formation and release of HDV particles are mediated by TAP and Aly.

The hepatitis delta virus: Replication and pathogenesis

Hepatitis delta virus (HDV) is a defective virus and a satellite of the hepatitis B virus (HBV). Its RNA genome is unique among animal viruses, but it shares common features with some plant viroids, including a replication mechanism that uses a host RNA polymerase. In infected cells, HDV genome replication and formation of a nucleocapsid-like ribonucleoprotein (RNP) are independent of HBV. But the RNP cannot exit, and therefore propagate, in the absence of HBV, as the latter supplies the propagation mechanism, from coating the HDV RNP with the HBV envelope proteins for cell egress to delivery of the HDV virions to the human hepatocyte target. HDV is therefore an obligate satellite of HBV; it infects humans either concomitantly with HBV or after HBV infection. HDV affects an estimated 15 to 20 million individuals worldwide, and the clinical significance of HDV infection is more severe forms of viral hepatitis--acute or chronic--, and a higher risk of developing cirrhosis and hepatocellular carcinoma in comparison to HBV monoinfection. This review covers molecular aspects of HDV replication cycle, including its interaction with the helper HBV and the pathogenesis of infection in humans.

Animal models of chronic hepatitis delta virus infection host-virus immunologic interactions

Hepatitis delta virus (HDV) is a defective RNA virus that has an absolute requirement for a virus belonging to the hepadnaviridae family like hepatitis B virus (HBV) for its replication and formation of new virions. HDV infection is usually associated with a worsening of HBV-induced liver pathogenesis, which leads to more frequent cirrhosis, increased risk of hepatocellular carcinoma (HCC), and fulminant hepatitis. Importantly, no selective therapies are available for HDV infection. The mainstay of treatment for HDV infection is pegylated interferon alpha; however, response rates to this therapy are poor. A better knowledge of HDV–host cell interaction will help with the identification of novel therapeutic targets, which are urgently needed. Animal models like hepadnavirus-infected chimpanzees or the eastern woodchuck have been of great value for the characterization of HDV chronic infection. Recently, more practical animal models in which to perform a deeper study of host virus interactions and to evaluate new therapeutic strategies have been developed. Therefore, the main focus of this review is to discuss the current knowledge about HDV host interactions obtained from cell culture and animal models.

Joint infectious causation of human cancers

Joint infectious causation of cancer has been accepted in a few well-studied instances, including Burkitt's lymphoma and liver cancer. In general, evidence for the involvement of parasitic agents in oncogenesis has expanded, and recent advances in the application of molecular techniques have revealed specific mechanisms by which host cells are transformed. Many parasites evolve to circumvent immune-mediated detection and destruction and to control critical aspects of host cell reproduction and survival: cell proliferation, apoptosis, adhesion, and immortalization. The host has evolved tight regulation of these cellular processes-the control of each represents a barrier to cancer. These barriers need to be compromised for oncogenesis to occur. The abrogation of a barrier is therefore referred to as an essential cause of cancer. Alternatively, some aspects of cellular regulation restrain but do not block oncogenesis. Relaxation of a restraint is therefore referred to as an exacerbating cause of cancer. In this chapter, we explore past and current evidence for joint infectious causation of cancer in the context of essential and exacerbating causes. We stress that discovery of joint infectious causation may provide great improvements in controlling cancer, particularly through the identification of many additional nonhuman targets for synergistic interventions for prevention and treatment.

Molecular and clinical aspects of hepatitis D virus infections

Hepatitis D virus (HDV) is a defective virus with circular, single-stranded genomic RNA which needs hepatitis B virus (HBV) as a helper virus for virion assembly and infectivity. HDV virions are composed of a circular shape HDV RNA and two types of viral proteins, small and large HDAgs, surrounded by HBV surface antigen (HBsAg). The RNA polymerase II from infected hepatocytes is responsible for synthesizing RNAs with positive and negative polarities for HDV, as the virus does not code any enzyme to replicate its genome. HDV occurs as co-infection or super-infection in up to 5% of HBsAg carriers. A recent multi-center study highlighted that pegylated interferon α-2a (PEG-IFN) is currently the only treatment option for delta hepatitis. Nucleotide/nucleoside analogues, which are effective against HBV, have no relevant effects on HDV. However, additional clinical trials combining PEG-IFN and tenofovir are currently ongoing. The molecular interactions between HDV and HBV are incompletely understood. Despite fluctuating patterns of HBV viral load in the presence of HDV in patients, several observations indicate that HDV has suppressive effects on HBV replication, and even in triple infections with HDV, HBV and HCV, replication of both concomitant viruses can be reduced. Additional molecular virology studies are warranted to clarify how HDV interacts with the helper virus and which key cellular pathways are used by both viruses. Further clinical trials are underway to optimize treatment strategies for delta hepatitis.

Hepatitis D virus isolates with low replication and epithelial-mesenchymal transition-inducing activity are associated with disease remission

Clearance of hepatitis D virus (HDV) viremia leads to disease remission. Large hepatitis delta antigen (L-HDAg) has been reported to activate transforming growth factor β, which may induce epithelial-mesenchymal transition (EMT) and fibrogenesis. This study analyzed serum HDV RNA "quasispecies" in HDV-infected patients at two stages of infection: before and after alanine aminotransferase (ALT) elevations. Included in the study were four patients who went into remission after ALT elevation and three patients who did not go into remission and progressed to cirrhosis or hepatocellular carcinoma. Full-length HDV cDNA clones were obtained from the most abundant HDV RNA species at the pre- and post-ALT elevation stages. Using an in vitro model consisting of Huh-7 cells transfected with cloned HDV cDNAs, the pre- or post-ALT elevation dominant HDV RNA species were characterized for (i) their replication capacity by measuring HDV RNA and HDAg levels in transfected cells and (ii) their capacity to induce EMT by measuring the levels of the mesenchymal-cell-specific protein vimentin, the EMT regulators twist and snail, and the epithelial-cell-specific protein E-cadherin. Results show that in patients in remission, the post-ALT elevation dominant HDV RNA species had a lower replication capacity in vitro and lower EMT activity than their pre-ALT elevation counterparts. This was not true of patients who did not go into remission. The expression of L-HDAg, but not small HDAg, increased the expression of the EMT-related proteins. It is concluded that in chronically infected patients, HDV quasispecies with a low replication capacity and low EMT activity are associated with disease remission.

Hepatitis D virus: an update

Hepatitis D virus (HDV) infection involves a distinct subgroup of individuals simultaneously infected with the hepatitis B virus (HBV) and characterized by an often severe chronic liver disease. HDV is a defective RNA agent needing the presence of HBV for its life cycle. HDV is present worldwide, but the distribution pattern is not uniform. Different strains are classified into eight genotypes represented in specific regions and associated with peculiar disease outcome. Two major specific patterns of infection can occur, i.e. co-infection with HDV and HBV or HDV superinfection of a chronic HBV carrier. Co-infection often leads to eradication of both agents, whereas superinfection mostly evolves to HDV chronicity. HDV-associated chronic liver disease (chronic hepatitis D) is characterized by necro-inflammation and relentless deposition of fibrosis, which may, over decades, result in the development of cirrhosis. HDV has a single-stranded, circular RNA genome. The virion is composed of an envelope, provided by the helper HBV and surrounding the RNA genome and the HDV antigen (HDAg). Replication occurs in the hepatocyte nucleus using cellular polymerases and via a rolling circle process, during which the RNA genome is copied into a full-length, complementary RNA. HDV infection can be diagnosed by the presence of antibodies directed against HDAg (anti-HD) and HDV RNA in serum. Treatment involves the administration of pegylated interferon-α and is effective in only about 20% of patients. Liver transplantation is indicated in case of liver failure.

The hepatitis delta virus RNA genome interacts with eEF1A1, p54(nrb), hnRNP-L, GAPDH and ASF/SF2

Because of its extremely limited coding capacity, the hepatitis delta virus (HDV) takes over cellular machineries for its replication and propagation. Despite the functional importance of host factors in both HDV biology and pathogenicity, little is known about proteins that associate with its RNA genome. Here, we report the identification of several host proteins interacting with an RNA corresponding to the right terminal stem-loop domain of HDV genomic RNA, using mass spectrometry on a UV crosslinked ribonucleoprotein complex, RNA affinity chromatography, and screening of a library of purified RNA-binding proteins. Co-immunoprecipitation was used to confirm the interactions of eEF1A1, p54(nrb), hnRNP-L, GAPDH and ASF/SF2 with the right terminal stem-loop domain of HDV genomic RNA in vitro, and with both polarities of HDV RNA within HeLa cells. Our discovery that HDV RNA associates with RNA-processing pathways and translation machinery during its replication provides new insights into HDV biology and its pathogenicity.

Kinetics of WHV-HDV replication in acute fatal course of woodchuck hepatitis

The objective of this study was to evaluate, by developing one-step real-time PCR, the outcome of superinfection with hepatitis D virus (HDV) genotype I in woodchucks that were chronic carriers of woodchuck hepatitis virus (WHV) and did not show relevant signs of liver damage. Three woodchucks (Marmota monax) chronically infected with WHV were superinfected with a woodchuck HDV inoculum. The evolution of the WHV and HDV infections was monitored by quantifying HDV-RNA, WHV-DNA, and HDV-WHV antigens and antibodies. WHV and HDV sequencing was also performed and liver markers were evaluated. Liver damage was assessed using the Ishak method. All woodchucks showed a high HDV viral load, antigenemia and short survival after superinfection. Histopathological examination of autoptic liver samples showed massive liver necrosis compatible with an acute fatal course of hepatitis. The WHV sequencing showed that the virus population was not substituted by the WHV inoculum. The HDV sequencing performed during superinfection and at autopsy indicated amino acid changes in immune dominant regions of the HDV antigen. The strong correlation between acute infection with HDV genotype I and rapid and fatal liver failure indicates that HDV can be an important factor in the prognosis of HDV-WHV-superinfected woodchucks.

Large hepatitis delta antigen is a novel clathrin adaptor-like protein

Clathrin-mediated endocytosis is a common pathway for viral entry, but little is known about the direct association of viral protein with clathrin in the cytoplasm. In this study, a putative clathrin box known to be conserved in clathrin adaptors was identified at the C terminus of the large hepatitis delta antigen (HDAg-L). Similar to clathrin adaptors, HDAg-L directly interacted with the N terminus of the clathrin heavy chain through the clathrin box. HDAg-L is a nucleocytoplasmic shuttle protein important for the assembly of hepatitis delta virus (HDV). Here, we demonstrated that brefeldin A and wortmannin, inhibitors of clathrin-mediated exocytosis and endosomal trafficking, respectively, specifically blocked HDV assembly but had no effect on the assembly of the small surface antigen of hepatitis B virus. In addition, cytoplasm-localized HDAg-L inhibited the clathrin-mediated endocytosis of transferrin and the degradation of epidermal growth factor receptor. These results indicate that HDAg-L is a new clathrin adaptor-like protein, and it may be involved in the maturation and pathogenesis of HDV coinfection or superinfection with hepatitis B virus through interaction with clathrin.

Structure and replication of hepatitis delta virus RNA

While this volume covers many different aspects of hepatitis delta virus (HDV) replication, the focus in this chapter is on studies of the structure and replication of the HDV RNA genome. An evaluation of such studies is not only an integral part of our understanding of HDV infections but it also sheds new light on some important aspects of cell biology, such as the fidelity of RNA transcription by a host RNA polymerase and on various forms of post-transcriptional RNA processing. Representations of the replication of the RNA genome are frequently simplified to a form of rolling-circle model, analogous to what have been described for plant viroids. One theme of this review is that such models, even after some revision, deceptively simplify the complexity of HDV replication and can fail to make clear major questions yet to be solved.

Immunology of HDV infection

Hepatitis delta virus (HDV) infection may occur as coinfection with hepatitis B virus (HBV) or as superinfection of a chronically HBV-infected patient. A strong antibody response is mounted, which persists for many years; however, it is not able to modulate the course of infection. In most cases the superinfection takes a chronic course. In patients with inactive disease (HDV PCR negative) an oligospecific T-helper cell immune response and a cytotoxic T-cell response were found, which were absent in patients with persistent viremia. The role of the cellular immune response in liver injury during acute infection has not been investigated. Vaccination strategies tested in the woodchuck model induced specific B- and T-cell responses but failed to protect from HDV infection.

Development of a novel system to study hepatitis delta virus genome replication

Hepatitis delta virus (HDV) genome replication requires the virus-encoded small delta protein (deltaAg). During replication, nucleotide sequence changes accumulate on the HDV RNA, leading to the translation of deltaAg species that are nonfunctional or even inhibitory. A replication system was devised where all deltaAg was conditionally provided from a separate and unchanging source. A line of human embryonic kidney cells was stably transfected with a single copy of cDNA encoding small deltaAg, with expression under tetracycline (TET) control. Next, HDV genome replication was initiated in these cells by transfection with a mutated RNA unable to express deltaAg. Thus, replication of this RNA was under control of the TET-inducible deltaAg. In the absence of TET, there was sufficient deltaAg to allow a low level of HDV replication that could be maintained for at least 1 year. When TET was added, both deltaAg and genomic RNA increased dramatically within 2 days. With clones of such cells, designated 293-HDV, the burst of HDV RNA replication interfered with cell cycling. Within 2 days, there was a fivefold enhancement of G1/G0 cells relative to both S and G2/M cells, and by 6 days, there was extensive cell detachment and death. These findings and those of other studies that are under way demonstrate the potential applications of this experimental system.

Immunization of woodchucks (Marmota monax) with hepatitis delta virus DNA vaccine

We investigated the DNA immunization approach in order to induce a protective immune response against hepatitis delta virus (HDV) superinfection of chronically woodchuck hepatitis virus (WHV) infected woodchucks. The animals were immunized with an expression vector encoding HDAg by gene gun. T cell and humoral immune responses induced by this protocol were determined and compared with those induced by HDAg immunization using a CpG oligonucleotide as an adjuvant. After immunization the woodchucks were challenged with 10(6) genome equivalents of HDV. The protein immunization with HDAg induced good humoral and T helper cell responses in the woodchucks, but did not protect them from HDV superinfection. The DNA immunized woodchucks were also not protected from HDV superinfection, however, the course of infection was modified: HDV viremia occurred later, the typical fluctuation of the HDV RNA titer with several peaks was absent, and antibodies to HDV were not detectable.

Efficient hepatitis delta virus RNA replication in avian cells requires a permissive factor(s) from mammalian cells

Hepatitis delta virus (HDV) is a highly pathogenic human RNA virus whose genome is structurally related to those of plant viroids. Although its spread from cell to cell requires helper functions supplied by hepatitis B virus (HBV), intracellular HDV RNA replication can proceed in the absence of HBV proteins. As HDV encodes no RNA-dependent RNA polymerase, the identity of the (presumably cellular) enzyme responsible for this reaction remains unknown. Here we show that, in contrast to mammalian cells, avian cells do not support efficient HDV RNA replication and that this defect cannot be rescued by provision of HDV gene products in trans. Contrary to earlier assertions, this defect is not due to enhanced apoptosis triggered in avian cells by HDV. Fusion of avian cells to mammalian cells rescues HDV replication in avian nuclei, indicating that the nonpermissive phenotype of avian cells is not due to the presence of dominantly acting inhibitors of replication. Rather, avian cells lack one or more essential permissive factors present in mammalian cells. These results set the stage for the identification of such factors and also explain the failure of earlier efforts to transmit HDV infection to avian hosts harboring indigenous hepadnaviruses.