Sequence of events in natural infection of Pekin duck embryos with duck hepatitis B virus

Hepatitis Virus Infections in Poultry

Viral hepatitis in poultry is a complex disease syndrome caused by several viruses belonging to different families including avian hepatitis E virus (HEV), duck hepatitis B virus (DHBV), duck hepatitis A virus (DHAV-1, -2, -3), duck hepatitis virus Types 2 and 3, fowl adenoviruses (FAdV), and turkey hepatitis virus (THV). While these hepatitis viruses share the same target organ, the liver, they each possess unique clinical and biological features. In this article, we aim to review the common and unique features of major poultry hepatitis viruses in an effort to identify the knowledge gaps and aid the prevention and control of poultry viral hepatitis. Avian HEV is an Orthohepevirus B in the family Hepeviridae that naturally infects chickens and consists of three distinct genotypes worldwide. Avian HEV is associated with hepatitis-splenomegaly syndrome or big liver and spleen disease in chickens, although the majority of the infected birds are subclinical. Avihepadnaviruses in the family of Hepadnaviridae have been isolated from ducks, snow geese, white storks, grey herons, cranes, and parrots. DHBV evolved with the host as a noncytopathic form without clinical signs and rarely progressed to chronicity. The outcome for DHBV infection varies by the host's ability to elicit an immune response and is dose and age dependent in ducks, thus mimicking the pathogenesis of human hepatitis B virus (HBV) infections and providing an excellent animal model for human HBV. DHAV is a picornavirus that causes a highly contagious virus infection in ducks with up to 100% flock mortality in ducklings under 6 wk of age, while older birds remain unaffected. The high morbidity and mortality has an economic impact on intensive duck production farming. Duck hepatitis virus Types 2 and 3 are astroviruses in the family of Astroviridae with similarity phylogenetically to turkey astroviruses, implicating the potential for cross-species infections between strains. Duck astrovirus (DAstV) causes acute, fatal infections in ducklings with a rapid decline within 1-2 hr and clinical and pathologic signs virtually indistinguishable from DHAV. DAstV-1 has only been recognized in the United Kingdom and recently in China, while DAstV-2 has been reported in ducks in the United States. FAdV, the causative agent of inclusion body hepatitis, is a Group I avian adenovirus in the genus Aviadenovirus. The affected birds have a swollen, friable, and discolored liver, sometimes with necrotic or hemorrhagic foci. Histologic lesions include multifocal necrosis of hepatocytes and acute hepatitis with intranuclear inclusion bodies in the nuclei of the hepatocytes. THV is a picornavirus that is likely the causative agent of turkey viral hepatitis. Currently there are more questions than answers about THV, and the pathogenesis and clinical impacts remain largely unknown. Future research in viral hepatic diseases of poultry is warranted to develop specific diagnostic assays, identify suitable cell culture systems for virus propagation, and develop effective vaccines.

Identification of natural recombination in duck hepatitis B virus

Due to its high similarity to human hepatitis B virus (HBV), duck HBV (DHBV) is often used as an important model for HBV research. While inter-genotypic recombination of HBV is common, it has not been reported with DHBV. In this study, 32 non-redundant DHBV complete genomes were analyzed using phylogenetic methods and classified into two clusters, corresponding to the 'Chinese' and 'Western country' branches previously reported based on geographical distribution. One 'Chinese' branch strain was isolated in Australia and three 'Western country' branch strains were isolated in China, suggesting cross-geographical distribution of both branches. Recombination analyses of the 32 DHBV genomes identified two possible inter-genotypic recombination events with high confidence value. These recombination events occurred between the lineages represented, respectively, by the Chinese isolate GD3 (AY536371, 'Chinese' branch) and the American isolate DHBV16 (K01834, 'Western country' branch), giving rise to two Chinese recombinant isolates CH4 (EU429324) and CH6 (EU429326). The identification of inter-genotypic recombination among circulating DHBV isolates suggests the usefulness of DHBV as a model for studying the mechanism of HBV recombination.

Avian hepatitis B viruses: Molecular and cellular biology, phylogenesis, and host tropism

The human hepatitis B virus (HBV) and the duck hepatitis B virus (DHBV) share several fundamental features. Both viruses have a partially double-stranded DNA genome that is replicated via a RNA intermediate and the coding open reading frames (ORFs) overlap extensively. In addition, the genomic and structural organization, as well as replication and biological characteristics, are very similar in both viruses. Most of the key features of hepadnaviral infection were first discovered in the DHBV model system and subsequently confirmed for HBV. There are, however, several differences between human HBV and DHBV. This review will focus on the molecular and cellular biology, evolution, and host adaptation of the avian hepatitis B viruses with particular emphasis on DHBV as a model system.

Comparison of cell culture systems for duck hepatitis B virus using SyBr green quantitative PCR

The Hepadnaviridae family contains DNA viruses such as human hepatitis B virus (HBV), woodchuck hepatitis B virus (WHV), and duck hepatitis B virus (DHBV). DHBV is distributed in both wild and domestic ducks. HBV is a worldwide health problem with carriers at risk of developing cirrhosis and liver cancer. All medical staff and scientists working with HBV must be vaccinated, because of its highly contagious nature. DHBV is a safe surrogate for HBV because of their similarities. Several cell culture systems have been developed to study anti-DHBV drugs and disinfectants. However, differences in their capabilities to support DHBV propagation have not been reported. Therefore, a sensitive and reproducible quantitative PCR based on SyBr green dye was developed. This system does not need electrophoresis for analysis of PCR products, thus reducing processing time and potential for cross-contamination. It allowed precise quantification of DHBV over 8-logarithm dynamic range with a good correlation (R(2) = 0.9689) and showed minimal run-to-run deviation. Sensitivity was 820 copies of DHBV genome and specificity was confirmed by melting curve analysis. It demonstrated good repeatability in quantification of DHBV loads from serum of infected ducks. This assay compared DHBV yields from different cultured cells. All cells had similar kinetic curves for DHBV replication and replication peaks appeared 4 days post-infection. Duck embryonic hepatocytes showed the highest (P > 0.05) replication peak for DHBV. Therefore, duck embryonic hepatocytes and quantitative PCR based on SyBr green dye are a good choice for anti-DHBV drug and disinfectant testing.

Hepatitis B virus biology

Hepadnaviruses (hepatitis B viruses) cause transient and chronic infections of the liver. Transient infections run a course of several months, and chronic infections are often lifelong. Chronic infections can lead to liver failure with cirrhosis and hepatocellular carcinoma. The replication strategy of these viruses has been described in great detail, but virus-host interactions leading to acute and chronic disease are still poorly understood. Studies on how the virus evades the immune response to cause prolonged transient infections with high-titer viremia and lifelong infections with an ongoing inflammation of the liver are still at an early stage, and the role of the virus in liver cancer is still elusive. The state of knowledge in this very active field is therefore reviewed with an emphasis on past accomplishments as well as goals for the future.

Characterization of the antiviral effects of 2' carbodeoxyguanosine in ducks chronically infected with duck hepatitis B virus

This study was carried out to evaluate benefits and limitations of long-term therapy of hepatitis B virus infections with a nucleoside analog inhibitor of virus replication. The model we used was the domestic duck chronically infected with duck hepatitis B virus by in ovo infection. 2' Carbodeoxyguanosine was used as an inhibitor of viral DNA synthesis. In all animals examined there was a reduction in virus production during therapy. A dose of 2' carbodeoxyguanosine of 10 micrograms/kg every other day reduced the number of infected hepatocytes from greater than 95% to 25% to 50% in less than 3 mo, whereas a 10-fold higher dose produced a decline to less than 10%. Histological evaluation revealed mild to moderate liver injury in ducks receiving the higher dose of 2' carbodeoxyguanosine, suggesting that disappearance of infected hepatocytes may have been accelerated by a toxic effect of the drug. Drug treatment did not completely eliminate duck hepatitis B virus from any duck, and replication was restored in all hepatocytes within a few weeks to several months after antiviral therapy was terminated. Our results suggest that elimination of a chronic infection with a single inhibitor of replication may be difficult in a host that lacks an antiviral immune response capable of eliminating at least a portion of the infected hepatocytes and of ultimately producing antibodies capable of neutralizing residual virus.

The problem of antiviral therapy for chronic hepadnavirus infections

Attempts at antiviral therapy of patients with active liver disease as a consequence of chronic hepatitis B virus infection have been moderately successful. The molecular and cellular basis for a successful outcome in these patients is not understood and the same therapies do not appear to benefit carriers that still have fairly normal livers and only a moderate hepatitis as a result of the immune response to the infection. Most carriers fall into this latter classification, at least during the early years of infection, and a therapy that could be successfully applied before extensive liver damage had occurred would presumably reduce the risk of subsequent liver damage and the progression to primary hepatocellular carcinoma. Traditionally, it has been assumed that the primary reason that individuals become chronically infected is that the cytotoxic T-cell response and/or antibody-dependent killing of infected hepatocytes is insufficient to clear the infection. Less attention has been focused on the role of the antibody response in the generation of virus-neutralizing antibodies as the possible major deficiency predisposing some individuals to become carriers. However, carriers normally are antigenemic for HBsAg and virus, and carriers with only antibodies to these structures in their circulation are virtually unknown. In addition, it is usually assumed that the hepatocyte, the major target of infection, does not spontaneously turn over and that, in the absence of an immune response to the infected cell, hepatocellular viability is unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid resolution of duck hepatitis B virus infections occurs after massive hepatocellular involvement

A study was carried out to determine some of the factors that might distinguish transient from chronic hepadnavirus infection. First, to better characterize chronic infection, Pekin ducks, congenitally infected with the duck hepatitis B virus (DHBV), were used to assess age-dependent variations in viremia, percentage of DHBV-infected hepatocytes, and average levels of DNA replication intermediates in the cytoplasm and of covalently closed circular DNA in the nuclei of infected hepatocytes. Levels of viremia and viral DNA were found to peak at about the time of hatching but persisted at relatively constant levels in chronically infected birds up to 2 years of age. The percentage of infected hepatocytes was also constant, with DHBV replication in virtually 100% of hepatocytes in all birds. Next, we found that adolescent ducks inoculated intravenously with a large dose of DHBV also developed massive infection of hepatocytes with an early but low-level viremia, followed by rapid development of a neutralizing antibody response. No obvious quantitative or qualitative differences between transiently and chronically infected liver tissue were detected in the intracellular markers of viral replication examined. However, in the adolescent duck experiment, DHBV infection was rapidly cleared from the liver even when up to 80% of hepatocytes were initially infected. In all of these ducks, clearance of infection was accompanied by only a mild hepatitis, with no evidence that massive cell death contributed to the clearance. This finding suggested that mechanisms in addition to immune-mediated destruction of hepatocytes might make major contributions to clearance of infections, including physiological turnover of hepatocytes in the presence of a neutralizing antibody response and/or spontaneous loss of the capacity of hepatocytes to support virus replication.

Validation of virus inactivation by heat treatment in the manufacture of diaspirin crosslinked hemoglobin

Diaspirin crosslinked hemoglobin (DCLHb), a hemoglobin based oxygen carrying solution prepared from outdated human blood, is subjected to a heat treatment step to inactivate viruses in our manufacturing process. To validate the efficacy of this inactivation, we have simulated the heat treatment procedure at a reduced scale using hemoglobin solution spiked with representative viruses. Human Immuno-deficiency Virus (HIV), Cytomegalovirus (CMV), and Duck Hepatitis B Virus (DHBV) were used in this validation. Inoculation with concentrated virus was performed just prior to the heat treatment to determine the effect of that specific process step. Samples were taken before, during, and after heat treatment and assayed for virus titer in an attempt to assess the rate as well as the extent of virus inactivation. CMV was analyzed in a plaque assay using MRC-5 indicator cells. The titer was reduced from 3.3 x 10(6) plaque forming units (PFU) per mL to less than 5 x 10(1) PFU/mL (detection limit) within 30 minutes. DHBV was analyzed by inoculation of serially diluted samples into Pekin ducklings, followed at intervals by screening sera for DHBV DNA by dot blot hybridization. The titer was reduced from 5.0 x 10(6) duck infectious units (DIU) per mL to less than 5 x 10(0) DIU/mL (detection limit) within 1 hour. HIV titers were determined through an ELISA assay for p24 antigen present in peripheral blood lymphocyte cocultivation supernatants. The titer was reduced from 2.0 x 10(4) infectious units (IU) per mL to less than 2 x 10(0) IU/mL (detection limit) within 1 hour. These data indicate that high titers of these blood borne viruses are rapidly inactivated by this heat treatment process.

The isolation and culture of DHBV-infected embryo and duckling hepatocytes and the effect of aflatoxin B1 or irradiation on these cells

The preparation of primary cultures of control and DHBV-infected duck hepatocytes from embryos and young ducklings is described. Cultures of both embryo and duckling hepatocytes secreted duck serum proteins. Cultures of hepatocytes established from ducklings maintained initial morphology for up to 3 weeks in culture and also exhibited high levels of metabolism of aflatoxin B1. Embryonic cell cultures rapidly lost ability to metabolise AFB1 and became overgrown by spindle-shaped cells. Both embryo and duckling cell cultures secreted infective DHBV, and had intracellular replicative forms of the virus. No integration of the virus into the duck genome was observed, and attempts to induce viral integration in the duckling hepatocytes using irradiation and aflatoxin B1 toxicity were unsuccessful. The results of the study lend further support to the suggestion that the rarity of liver cancer in DHBV-infected experimental ducks is related to an innate resistance of the hepatocytes to develop DHBV-DNA integration. Another possibility may be related to the lower oncogenic potential of the DHBV strain used for the study. However DHBV infected duckling hepatocytes would appear to offer a suitable material for studying viral replication and mechanisms of aflatoxin B1 toxicity during prolonged cell culture.

Duck hepatitis B virus infection of non-hepatocytes

One hundred and seventeen ducklings, 42 inoculated with duck hepatitis B virus (DHBV) 2 days after hatching and 55 connatally infected, were studied over a 6-month period in parallel with 20 ducklings without DHBV infection. Using immunohistochemical, in situ and blot hybridization analyses, the natural course of hepatic and extrahepatic infection was examined. DHBV infection started in the liver 2-4 days post-inoculation. There, DHBV was found not only in hepatocytes, but also in bile duct epithelial cells. Further, DHBV infection occurred in exocrine and endocrine pancreas (beginning 6-10 days and 20 days post-inoculation, respectively) and in germinal centers of the spleen (beginning 8 weeks post-inoculation). Occasionally viral DNA was also found in kidney glomeruli. Using strand-specific RNA probes, viral DNA in pancreas and spleen was clearly demonstrated to be replicating intermediates. Hepatic and extrahepatic infection with DHBV was not associated with histologic inflammation or pathologic changes in these tissues or the liver. These data indicate that DHBV can infect cells other than hepatocytes. The biological significance of non-hepatocyte infection for the life-cycle of the virus and its potential significance for viral persistence remain to be determined.

Natural infection of duck embryos with duck hepatitis B virus: time and tissue sequence of infection

There have been no studies addressing the detailed sequence of embryonic infection with duck hepatitis B virus (DHBV). Therefore, duck embryos from flocks infected with DHBV were examined to study the sequence of infection by DHBV in various embryonic tissues. Embryos from flocks infected with DHBV were harvested in duplicates from 7 to 25 days of incubation. Whole embryos (to 12 days) or dissected embryonic tissues were fixed, paraffin embedded, and stained for DHBV surface antigen (DHBsAg) using a peroxidase-antiperoxidase technique. Isolated hepatic cells were infected in 7-day-old embryos, and these increased in number until 11 days, when most cells were positive for DHBsAg. Endocrine pancreatic cells were positive from day 10, but only an occasional exocrine pancreatic cell was infected after day 20. Renal tubule cells were positive for DHBV by day 11, increasing in number until about day 18, after which a decline in numbers of infected cells occurred. Renal glomeruli became positive for DHBsAg from day 24. When present in the developing embryo, thymus, bursa of Fabricius, spleen, bone marrow, lung, and duodenum remained negative for DHBsAg. It was concluded that the timing of infection of specific tissues was not necessarily related to cellular maturity but may reflect a need for specific metabolic functions that permit viral replication.

Immunity in Pekin ducks experimentally and naturally infected with duck hepatitis B virus

The immune response to duck hepatitis B virus (DHBV) had not been elucidated. An assay was therefore established to detect the presence of antibody to DHB surface antigen (anti-DHBs) in serum of experimentally inoculated and naturally infected ducks. Anti-DHBs in serum was detected by indirect RIA from the percentage inhibition of binding of rabbit anti-DHBs to purified DHBsAg. Specificity was confirmed by positive and negative controls, infected and noninfected sera, and a mouse monoclonal antibody to DHB core antigen (anti-DHBc). Serum and liver samples were tested for DHBV DNA by dot-blot hybridization assay. Adult ducks repeatedly inoculated with DHBV remained non-viraemic but developed anti-DHBs. This antibody activity neutralized the infectivity of DHBV, which was experimentally inoculated into 1-day-old ducklings. In naturally infected flocks anti-DHBs was detected in a proportion of noninfected adult ducks as well as 1-day-old hatchlings. Anti-DHBs activity in hatchlings neutralized the infectivity of experimentally inoculated DHBV. Pekin ducks can therefore mount a neutralizing antibody response to DHBV, and immunity may be transferred in ovo from dam to off-spring.

A sequential histologic and immunohistochemical study of duck hepatitis B virus infection in Pekin ducks

Twenty-nine Pekin ducks were inoculated with duck hepatitis B virus (DHBV), DHBV-free serum, or saline at 1 day of age. Congenitally DHBV-infected ducks were also studied. Ducks were killed periodically during a 92-week study and examined histologically and immunohistochemically to assess liver and extrahepatic inflammation and to detect and characterize DHBV core antigen tissue distribution. DHBV infection produced an asymptomatic but persistent DHBV viremia in all ducks associated with a mild to moderate transient hepatic inflammation apparent at 3 to 6 weeks post-inoculation and waning afterwards. DHBV core antigen was detected in hepatocyte cytoplasm at 1 week post-inoculation, and by 3 weeks post-inoculation scattered pancreatic acinar and islet cells also contained viral antigen. Small numbers of mononuclear cells in the splenic white pulp also contained viral antigen. Viral antigen persisted in all of these tissues throughout the duration of the experiment. No inflammation or tissue injury was detected in any of the extrahepatic tissues during the course of DHBV infection. One DHBV-injected duck developed a hepatocellular carcinoma at 88 weeks of age. Isolated patches of neoplastic hepatocytes contained cytoplasmic DHBV core antigen. The results of this study indicate that DHBV, like mammalian hepadnavirus, is capable of producing a persistent infection of the liver and several extrahepatic tissues and suggest that persistent infection may be associated with the development of hepatocellular carcinoma.

Vertical transmission of woodchuck hepatitis virus

One newborn and 24 fetal woodchuck litters from a woodchuck hepatitis virus (WHV) endemic population were examined for serological or hepatic evidence of WHV. In 18 of 24 fetal litters, there was detectable WHV DNA in the livers, either at explant culture or tissue extract. Most of those WHV DNA-positive liver extracts, which were examined by Southern blot, showed integration of WHV. However, WHV DNA replicative forms without integration were demonstrated in livers of two litters from late gestation. Woodchuck hepatitis surface antigen was detected in the sera of two other fetal litters from the late gestation period. WHV DNA was demonstrated in sera of three litters at different stages of ontogeny.

Isolation and characterization of a hepatitis B virus endemic in herons

A new hepadnavirus (designated heron hepatitis B virus [HHBV]) has been isolated; this virus is endemic in grey herons (Ardea cinerea) in Germany and closely related to duck hepatitis B virus (DHBV) by morphology of viral particles and size of the genome and of the major viral envelope and core proteins. Despite its striking similarities to DHBV, HHBV cannot be transmitted to ducks by infection or by transfection with cloned viral DNA. After the viral genome was cloned and sequenced, a comparative sequence analysis revealed an identical genome organization of HHBV and DHBV (pre-C/C-, pre-S/S-, and pol-ORFs). An open reading frame, designated X in mammalian hepadnaviruses, is not present in DHBV. DHBV and HHBV differ by 21.6% base exchanges, and thus they are less closely related than the two known rodent hepatitis B viruses (16.4%). The nucleocapsid protein and the 17-kilodalton envelope protein sequences of DHBV and HHBV are well conserved. In contrast, the pre-S part of the 34-kilodalton envelope protein which is believed to mediate virus attachment to the cell is highly divergent (less than 50% homology). The availability of two closely related avian hepadnaviruses will now allow us to test recombinant viruses in vivo and in vitro for host specificity-determining sequences.

Presence of antibodies to the polymerase gene product(s) of hepatitis B and woodchuck hepatitis virus in natural and experimental infections

Antibodies against synthetic peptides derived from the polymerase gene of the hepatitis B virus (HBV) were present in 80% of renal dialysis patients infected with HBV and in woodchucks infected with woodchuck hepatitis virus (WHV). Polymerase antibody (anti-pol) appeared as the earliest marker of both HBV and WHV infections in approximately half of the individuals tested, suggesting that these antibodies were generated following early viral replication in the liver during the incubation period and prior to the appearance of virus in the blood. Many HBV- or WHV-infected individuals negative for surface antigen throughout infection also had anti-pol, but anti-pol appeared only after anti-surface, anti-core and/or anti-e. The presence of anti-pol did not correlate with other serologic markers of HBV or WHV infection, nor did it correlate with histologically confirmed hepatitis in woodchucks. However, there was a significant correlation between the presence of anti-pol and elevated liver enzyme levels in the sera of renal dialysis patients. In several cases, anti-pol was the sole marker of infection, suggesting that underlying infection and low levels of virus replication were present. Most individuals with anti-pol had antibodies to one of the three synthetic peptides, suggesting it may be immunodominant in natural infections. In human populations, groups with a high frequency of HBV infection have a high frequency of polymerase antibodies, and groups with a low frequency of HBV infection have a low frequency of polymerase antibodies. A standard assay for the detection of polymerase antibodies is described, and possible clinical applications are discussed.

Duck hepatitis B virus replicates in the yolk sac of developing embryos

Duck hepatitis B virus (DHBV) is the only member of the hepadnavirus family in which nearly 100% vertical transmission from carrier mother to embryo has been reported. Large quantities of maternally transmitted virus particles are present in the yolk prior to incubation of the eggs, and replicative forms of DHBV DNA are detectable in the liver at 6 days of incubation. Since the yolk sac is similar to the liver in its production of serum proteins, we examined the yolk sacs of developing embryos for signs of viral replication. We detected the supercoiled form of DHBV DNA, DHBV RNA transcripts similar to those in the virus-replicating liver, and DNA polymerase activity and viral DNA in corelike particles in extracts of yolk sac tissue of naturally infected eggs. DHBV core antigen was strongly stained in only the endodermal layer of the yolk sac by immunofluorescence. DHBV RNA was detectable in the yolk sac from 4 days of incubation until hatching, and a larger quantity of DHBV RNA was present in the yolk sac than in the liver during all the stages of embryogenesis. Our data indicate that DHBV replicates actively in the yolk sac from an earlier stage than that previously reported in studies of embryonic liver and that replication is limited to the endodermal cell layer, which is ontogenetically and functionally related to the liver. The yolk sac may support the vertical transmission of DHBV.

Humoral immune responsiveness in duck hepatitis B virus-infected ducks

Immunofluorescence assays with fixed tissue sections were used to characterize antibody reactivity in sera obtained from duck hepatitis B virus-infected ducks. Under conditions of experimental infection, antibody to core antigen but not to surface antigen was detectable. A majority of the ducks infected at 8 days after hatching and a minority of those infected at 1 day after hatching showed a transient anti-core antigen humoral response; this response was stronger in the antibody-positive ducks infected on day 8 than in those infected on day 1. Antiviral antibody was not detected in the sera of ducks congenitally infected with duck hepatitis B virus. Several of the infected ducks, but none of the uninfected ducks, exhibited autoantibody reactivity for alpha-islet-cell-associated antigen.

Production of hepatitis B virus by a differentiated human hepatoma cell line after transfection with cloned circular HBV DNA

Closed-circular HBV DNA was introduced into cells of the established human hepatoma culture HepG2. The culture medium of one of 40 single-cell clones contained HBV surface antigen (HBsAg), core-related antigens (HBc/eAg), and HBV DNA sequences. HBV DNA and DNA polymerase activity were detected in particles resembling both nucleocapsids and complete virions (Dane particles). Intracellular integrated and extrachromosomal HBV DNA sequences were detected. Relaxed-circular and single-stranded forms of viral DNA were identified as likely replicative intermediates of the HBV genome. In conclusion, in vitro production of Dane-like particles by transformed human hepatocytes has been achieved. This model should be valuable as a cell culture system for studying virus replication and virus-host cell interactions.

Viral antigen expression in the pancreas of DHBV-infected embryos and young ducks

The time course of appearance of viral antigen-positive pancreatic cells was examined in both congenitally duck hepatitis B virus (DHBV)-infected duck embryos and experimentally DHBV-infected posthatch ducks. In the embryos, the earliest detectable viral antigen-positive pancreatic cells were localized to islets and identifiable as endocrine on the basis of hormone expression. Non-islet-associated, viral antigen-positive cells appeared at a late stage of embryogenesis, following the onset of chymotrypsinogen production by exocrine tissue; a number of these viral antigen-positive cells were directly identifiable as exocrine on the basis of chymotrypsinogen expression. By contrast, in the pancreas of experimentally infected posthatch ducks, the appearance of viral antigen-positive exocrine cells (chymotrypsinogen-positive) predated the appearance of antigen-positive islet cells. These results are consistent with the possibility that viral antigen expression in exocrine tissue is dependent on the state of cell maturation.