Immunity in Pekin ducks experimentally and naturally infected 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.

The early host innate immune response to duck hepatitis B virus infection

The early phase after hepatitis B virus infection could play a crucial role in clearance and/or persistence of the virus, particularly in neonates. This work compared the early phase of duck hepatitis B virus infection in 1-day-old (D1) and 28-day-old (D28) ducks to determine whether differences in viral or host innate immune response can be related to the difference in outcome. In the first phase, almost immediately after inoculation, virus was taken up by components of the reticulo-endothelial systems, particularly liver-specific macrophages, Kupffer cells. Very early after infection, the induction of alpha interferon by infected hepatocytes occurred and was rapidly reinforced by recruitment of effector lymphocytes, which directly or indirectly caused apoptosis, eliminating infected hepatocytes, as was seen in mature birds. In addition, a lack of lymphocytic infiltration of the liver was found in D1 ducks, which supports the suggestion that the innate immune network is less effective in D1 ducks. Taken together, these results suggest that failure of the co-ordinated innate immune response rather than a defect in induced antiviral cell-mediated immunity may be the key factor which makes baby ducks vulnerable to persistence of hepadnavirus infection.

Duck immunoglobulins: structure, functions and molecular genetics

Four types of immunoglobulin (Ig) have been identified in ducks: IgM, a secretory Ig resembling IgM, a 7.8S IgG, and a 5.7S IgG. Structurally and antigenically the 5.7S IgG resembles an F(ab')2 fragment of the 7.8S IgG. When ducks mount serum antibody responses, the sequence of Ig involvement is IgM --> 7.8S IgG --> 5.7S IgG. Although serum Ig levels increase, and antigen-binding Igs can be demonstrated, sera from repeatedly immunized ducks commonly lack secondary antibody activities such as agglutination, precipitation, complement fixation and tissue sensitization. These deficiencies are most likely attributable to the absence of functionally important components of the predominant Ig (5.7S IgG) and/or a possibly unusual steric structure of duck Igs. A related issue concerns production of the two antigenically related IgGs: what are the cellular and molecular events involved, and how are they controlled? Evidence from current molecular genetic studies has confirmed the similarity of the VH, CH1 and CH2 domains of the 7.8S and 5.7S IgGs and shown, by virtue of the existence of separate mature messages for the heavy (H) chains of these molecules, that they are biosynthesized independently. Models for the possibilities that the two H chains are products of one gene or of two genes are presented. Cloning and sequencing the duck H chain gene locus, which is in progress, is providing data supporting the one gene hypothesis. The results obtained from cDNA sequencing also confirm that the duck IgGs are unusual in terms of the anatomy of the hinge region and of the number and location of intra- and inter-chain disulphide bonds, observations which will be of importance for understanding structure/function relationships of these unusual and interesting molecules.

Identification of T-cell epitopes associated with immunity within the surface protein of duck hepatitis B virus

Duck hepatitis is a convenient model of hepatitis B virus (HBV) infection, but the lack of immunological reagents hampers investigation of pathogenesis and vaccine development. The aim of this study was to define T-cell epitopes in the surface peptide recognized by vaccinated immune birds. Blastogenesis assays were used to test the proliferative response of spleen mononuclear cells to synthetic peptides spanning the pre-S/S region in 22 naïve and 13 immunized and challenged immune ducks. Roughly > or = 50% of the immune ducks responded to five immunodominant peptides eliciting a statistically greater proliferative response than in naïve birds. Fewer ducks responded to an additional six peptides. No statistically significant difference could be shown for the response to 11 peptides between the immune ducks and the naïve ducks. There was no clustering of the immunodominant peptides which were located throughout the surface antigen at sites of major swings in hydrophobicity. A number of peptides which induce lymphoblastogenesis in vaccinated immune ducks have been identified. Their role in spontaneous recovery from duck hepatitis B infection merits investigation.

Comparison of the kinetics of the specific cellular immune response to duck hepatitis B virus in infected and immune ducks

The kinetics of the cell mediated immune response by ducks acutely and chronically infected with, or immune to infection by duck hepatitis B virus (DHBV) was determined. This was measured by an antigen specific blastogenesis assay to duck hepatitis B surface antigen (DHBsAg) and duck hepatitis B core antigen (DHBcAg) using peripheral blood mononuclear cells (PBMC). The three outcomes of acute infection by DHBV were either clearance from both serum and liver, clearance from serum but not liver, or the development of persistent viraemia. Acutely infected ducks that failed to clear the infection also failed to develop a significant cellular immune response to both antigens. Ducks with chronic infection acquired as neonates or as the result of the failure to clear acute infection had an increasing cellular immune response over time. Two groups of immune ducks were examined. These were either ducks that had become immune following infection or that had been vaccinated. Both groups of ducks demonstrated significant cellular responses following challenge with DHBV irrespective of the level of their responses before challenge. However, there was a reduction in the response of their PBMC over a 4-week-period postchallenge. The range of cellular immune responses to DHBV antigens observed in this study has a number of counterparts in hepatitis B infection of humans. Coupled with the defined clinical outcomes that can be established in the duck/DHBV model, further study of the cellular immune response to DHBV is warranted.

Antigen-specific blastogenesis assays for duck hepatitis B virus using duck peripheral blood and splenic mononuclear cells

An antigen-specific lymphoblastogenesis assay for duck hepatitis B surface antigen (DHBsAg) and duck hepatitis B core antigen (DHBcAg) was developed using mononuclear cells from the peripheral blood (PBMC) or spleens (SMC) of immune ducks. Optimal culture conditions for the assay were determined by testing a number of variables, including antigen concentration, cell numbers/well, and the day of harvest. The specificity of the assay was assessed. The assay used 10% pooled duck serum supplement, and 8 x 10(5) cells/well for PBMC or 5 x 10(5) cells/well for SMC. The optimum antigen concentration ranged from 0.01 to 0.1 microgram/ml for both DHBsAg and DHBcAg. Maximum antigen-specific blastogenesis occurred between 4 to 7 days after establishment of the culture. The use of PHA (10 micrograms/ml) mitogenesis could predict the optimal cell numbers/well for antigen-specific blastogenesis. The assay demonstrated specific responses by immune ducks compared with those of unexposed ducklings and adult ducks (for DHBsAg P < 0.001; DHBcAg P < 0.05). For immune ducks, PBMC from all 8 ducks responded to DHBsAg, however, cells from only 4 of 7 immune ducks, responded to DHBcAg. Splenic mononuclear cells from all immune ducks responded to either DHBsAg or DHBcAg or both antigens.

DHBV manipulation and prediction of the outcome of infection

BACKGROUND/AIMS

The immunological response of ducks to acute infection with duck hepatitis B virus (DHBV) has not been fully characterised. In this study the relationship between viral dose and the outcome of infection in immune competent 26-day-old ducks was examined.

METHODS

Indirect ELISA assays were developed to detect the presence of antibody to DHB surface antigen and DHB core antigen. A DHBV serum pool was titrated in 1-day-old and 26-day-old ducklings.

RESULTS

The ID50 dose of the ducks injected at 26 days of age was found to be 1000 times that of the ducks injected on day of hatch. The antibody responses and serum DHBV DNA were followed in eight ducks inoculated with DHBV positive serum when 26 days of gene and in three ducks infected with DHBV on day of hatch. The three ducks infected on day of hatch were viraemic by day 7 and remained highly viraemic throughout the experimental period. In the older ducks, inoculation with 1000ID50 resulted in the development of chronic carriage, while inoculation with either 100 or 10ID50 doses resulted in acute infection with or without viraemia. These ducks were able to clear the infection from their circulation, but only 50% cleared DHBV from the liver within the experimental period. All infected ducks developed anti-core activity. Only non-viraemic ducks developed anti-surface activity.

CONCLUSION

DHBV infection can be established in immune competent adolescent ducks, with variable disease outcomes comparable to HBV infection in humans.

Duck hepatitis B virus (DHBV) as a model for understanding hepadnavirus neutralization

The role of the immune response to the human hepatitis B virus (HBV) envelope proteins in neutralization of viral infectivity has been well documented. The similarity between HBV, prototype member of the hepadnavirus family, and the closely related duck hepatitis B virus (DHBV) has allowed, use of the latter as a convenient model for the study of molecular mechanisms of HBV replication and neutralization. In this brief review, we will examine the HBV and DHBV envelope proteins and their role as targets for virus neutralization.

Effects of Phyllanthus plant extracts on duck hepatitis B virus in vitro and in vivo

The effects of extracts of five Australian Phyllanthus species (P. hirtellus, P. gunnii, P. gasstroemii, P. similis and P. tenellus), other plant extracts and the antiviral drug foscarnet on duck hepatitis B virus (DHBV) endogenous DNA polymerase (DNAp) activity were compared. All 5 Phyllanthus species caused 50% inhibition at concentrations of dry weight between 350-800 micrograms/ml, which is comparable with the effect described for P. amarus on the DNAp of human and woodchuck hepatitis B viruses. Incubation of P. hirtellus with 100 ID50 DHBV neutralized infection. However, neither P. gasstroemi extract, given by intraperitoneal injection (i.p.) at a dose of 20 mg/kg 3 times per week to ducklings early in the incubation period, or P. hirtellus extract, given to established DHBV carrier ducklings, prevented or eliminated infection.

Mechanism, kinetics, and role of duck hepatitis B virus e-antigen expression in vivo

No duck hepatitis B virus (DHBV) pre-C transcript has been identified so far, and neither the interrelationship of e-antigen (DHBeAg) with the expression of other viral antigens or virus replication nor its function is known. In this study we identified in infected livers a minor transcript from which the precursor protein of DHBeAg could be synthesized. Mutation of the first AUG on this transcript abolished expression of DHBeAg. DHBV genomes containing this mutation were infectious in Pekin ducks, the kinetics of pre-S envelope protein expression and virus secretion were not significantly different from wild-type, and the mutant genomes did not revert to wild-type to a detectable level after several passages. In contrast to pre-S protein, the level of DHBeAg in the serum was independent of the level of viremia, accumulated gradually to a high and constant level after a lag phase, and was also easily detectable in a mixed infection containing less than 0.1% of wild-type in a pre-C mutant virus containing inoculum. These data indicate that precore protein is synthesized from a minor pre-C mRNA with translation initiation at the pre-C AUG codon, and leads to high levels of DHBeAg rather late in infection. High levels of DHBeAg can even be produced efficiently by a very small subpopulation of wild-type virus in a mixed infection with predominantly pre-C mutant virus. Lack of DHBeAg appears to have no effect on DHBV viability and kinetics of virus secretion into the bloodstream when ducklings are infected with the pre-C AUG mutant virus a few days after birth.

Epitope-specific antibody response to the surface antigen of duck hepatitis B virus in infected ducks

In order to investigate the immune response to duck hepatitis B virus (DHBV) infection, newly hatched DHBV DNA negative ducklings were injected with infectious serum of sufficiently low DHBV-DNA titer to allow clearance of viremia. Of 20 injected ducklings, 13 (65%) became viremic. Of these, 6 (46%) cleared virus from the serum 3 to 22 weeks postinjection. The convalescent sera of these 6 animals were tested for an epitope-specific antibody response in a highly specific competitive inhibition assay using a panel of monoclonal antibodies against duck hepatitis B surface antigen (DHBsAg) that had been well-characterized. All 6 animals recovering from DHBV infection developed antibodies to epitopes on the preS and S proteins of DHBV. Antibody responses were highly variable with marked differences between animals in the extent and specificity of the antibody response. The humoral response to DHBsAg was prolonged in some animals but transient in others. No antibody to preS or S was detected in either preimmune sera or sera of control animals from an uninfected flock. Infected animals that did not clear viremia also remained antibody negative. The humoral responses to neutralizing preS epitopes III and V were weak but antibodies to two immunodominant epitopes on the preS region (II and B) were present in all 6 animals. The humoral response to the two epitopes in the S region was transient and of lower titer when compared to the two immunodominant preS epitopes. The two immunodominant preS epitopes may play an important role in clearance of DHBV infection in ducks.