Birnavirus precursor polyprotein is processed in Escherichia coli by its own virus-encoded polypeptide

gga-miR-27b-3p enhances type I interferon expression and suppresses infectious bursal disease virus replication via targeting cellular suppressors of cytokine signaling 3 and 6 (SOCS3 and 6)

MicroRNAs are small noncoding RNAs playing an important role in host response to pathogenic infection. Here we show that IBDV infection induced the demethylation of the pre-miR-27 promoter and upregulated gga-miR-27b-3p expression. We found that ectopic expression of miR-27b-3p in DF-1 cells enhanced the expression of chicken IFN-β, IRF3 and NF-κB, via directly targeting cellular suppressors of cytokine signaling 3 and 6 (SOCS3 and 6), inhibiting IBDV replication in host cells, while inhibition of endogenous miR-27b-3p by its inhibitors suppressed the expression of IFN-β, IRF3 and NF-κB, enhancing SOCS3 and 6 expressions and facilitating IBDV replication. Furthermore, transfection of DF-1 cells with miR-27b-3p markedly increased phosphorylation of STAT1 on Tyr701 in cells post chIFN-γ treatment. On the contrary, inhibition of endogenous miR-27b-3p reduced phosphorylation of STAT1 on Tyr701 in cells with chIFN-γ treatment. These findings indicate that gga-miR-27b-3p serves as an inducible antiviral mediator in host response to IBDV infection.

Epigenetic Upregulation of Chicken MicroRNA-16-5p Expression in DF-1 Cells following Infection with Infectious Bursal Disease Virus (IBDV) Enhances IBDV-Induced Apoptosis and Viral Replication

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression posttranscriptionally by silencing or degrading their targets and play important roles in the host response to pathogenic infection. Although infectious bursal disease virus (IBDV)-induced apoptosis in host cells has been established, the underlying molecular mechanism is not completely unraveled. Here, we show that infection of DF-1 cells by IBDV induced gga-miR-16-5p (chicken miR-16-5p) expression via demethylation of the pre-miR-16-2 (gga-miR-16-5p precursor) promoter. We found that ectopic expression of gga-miR-16-5p in DF-1 cells enhanced IBDV-induced apoptosis by directly targeting the cellular antiapoptotic protein B-cell lymphoma 2 (Bcl-2), facilitating IBDV replication in DF-1 cells. In contrast, inhibition of endogenous miR-16-5p markedly suppressed apoptosis associated with enhanced Bcl-2 expression, arresting viral replication in DF-1 cells. Furthermore, infection of DF-1 cells with IBDV reduced Bcl-2 expression, and this reduction could be abolished by inhibition of gga-miR-16-5p expression. Moreover, transfection of DF-1 cells with gga-miR-16-5p mimics enhanced IBDV-induced apoptosis associated with increased cytochrome c release and caspase-9 and -3 activation, and inhibition of caspase-3 decreased IBDV growth in DF-1 cells. Thus, epigenetic upregulation of gga-miR-16-5p expression by IBDV infection enhances IBDV-induced apoptosis by targeting the cellular antiapoptotic protein Bcl-2, facilitating IBDV replication in host cells.IMPORTANCE Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive disease in young chickens, causing severe economic losses to stakeholders across the globe. Although IBD virus (IBDV)-induced apoptosis in the host has been established, the underlying mechanism is not very clear. Here, we show that infection of DF-1 cells by IBDV upregulated gga-miR-16-5p expression via demethylation of the pre-miR-16-2 promoter. Overexpression of gga-miR-16-5p enhanced IBDV-induced apoptosis associated with increased cytochrome c release and caspase-9 and -3 activation. Importantly, we found that IBDV infection induced expression of gga-miR-16-5p that triggered apoptosis by targeting Bcl-2, favoring IBDV replication, while inhibition of gga-miR-16-5p in IBDV-infected cells restored Bcl-2 expression, slowing down viral growth, indicating that IBDV induces apoptosis by epigenetic upregulation of gga-miR-16-5p expression. These findings uncover a novel mechanism employed by IBDV for its own benefit, which may be used as a potential target for intervening IBDV infection.

gga-miR-155 Enhances Type I Interferon Expression and Suppresses Infectious Burse Disease Virus Replication via Targeting SOCS1 and TANK

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). MicroRNAs (miRNAs) are involved in host-pathogen interactions and innate immune response to viral infection. However, the role of miRNAs in host response to IBDV infection is not clear. We report here that gga-miR-155 acts as an anti-virus host factor inhibiting IBDV replication. We found that transfection of DF-1 cells with gga-miR-155 suppressed IBDV replication, while blockage of the endogenous gga-miR-155 by inhibitors enhanced IBDV replication. Furthermore, our data showed that gga-miR-155 enhanced the expression of type I interferon in DF-1 cells post IBDV infection. Importantly, we found that gga-miR-155 enhanced type I interferon expression via targeting SOCS1 and TANK, two negative regulators of type I IFN signaling. These results indicate that gga-miR-155 plays a critical role in cell response to IBDV infection.

The association of ribosomal protein L18 (RPL18) with infectious bursal disease virus viral protein VP3 enhances viral replication

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). IBDV VP3 is a multifunctional protein playing a key role in virus assembly and pathogenesis. To investigate the role of VP3 in pathogenesis, we transfected DF-1 cells with pRK5-FLAG-vp3 and found that VP3 enhanced type I interferon expression and suppressed IBDV replication. Furthermore we found that VP3 interacted with chicken Ribosomal Protein L18 (chRPL18) in host cells and knockdown of chRPL18 by RNAi significantly promoted Type I interferon expression and inhibited IBDV replication. Moreover, our data show that chicken double-stranded RNA-activated protein kinase (chPKR) interacted with both VP3 and chRPL18. Thus chRPL18 in association with VP3 and chPKR affects viral replication.

MicroRNA gga-miR-130b Suppresses Infectious Bursal Disease Virus Replication via Targeting of the Viral Genome and Cellular Suppressors of Cytokine Signaling 5

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression posttranscriptionally through silencing or degrading their targets, thus playing important roles in the immune response. However, the role of miRNAs in the host response against infectious bursal disease virus (IBDV) infection is not clear. In this study, we show that the expression of a series of miRNAs was significantly altered in DF-1 cells after IBDV infection. We found that the miRNA gga-miR-130b inhibited IBDV replication via targeting the specific sequence of IBDV segment A and enhanced the expression of beta interferon (IFN-β) by targeting suppressors of cytokine signaling 5 (SOCS5) in host cells. These findings indicate that gga-miR-130b-3p plays a crucial role in host defense against IBDV infection.IMPORTANCE This work shows that gga-miR-130b suppresses IBDV replication via directly targeting the viral genome and cellular SOCS5, the negative regulator for type I interferon expression, revealing the mechanism underlying gga-miR-130-induced inhibition of IBDV replication. This information will be helpful for the understanding of how host cells combat pathogenic infection by self-encoded small RNA and furthers our knowledge of the role of microRNAs in the cell response to viral infection.

The down-regulation of casein kinase 1 alpha as a host defense response against infectious bursal disease virus infection

Infectious bursal disease virus (IBDV) is an important immunosuppressive virus of chickens. Although the gene functions of IBDV have been well characterized, the host responses during IBDV infection remain much poor. In the present study, casein kinase 1 alpha (CK1α), a novel VP2-associated protein, was down-regulated during IBDV replication in DF1 cells. Further experiments showed that siRNA-mediated knockdown of CK1α inhibited IBDV replication, while overexpression of CK1α promoted IBDV growth. Finally, we revealed that the effects of CK1α expression level on IBDV replication were involved in the negative regulation of CK1α on type I interferon receptor (IFNAR1), because ubiquitination assay analyses demonstrated that CK1α could promote the ubiquitination of IFNAR1, thereby affecting the stability of this receptor. In conclusion, down-regulation of CK1α during IBDV infection as a host defense response increased abundance of IFNAR1, which in turn enhanced an inhibitory effect on IBDV replication.

Infectious bursal disease virus as a replication-incompetent viral vector expressing green fluorescent protein

Infectious bursal disease virus (IBDV) has been established as a replication-competent viral vector capable of carrying an epitope at multiple loci in the genome. To enhance the safety and increase the insertion capacity of IBDV as a vector, a replication-incompetent IBDV vector was developed in the present study. The feasibility of replacing one of the viral gene loci, including pvp2, vp3, vp1, or the polyprotein vp243, with the sequence of green fluorescent protein (GFP) was explored. A method combining TCID₅₀ and immunoperoxidase monolayer assay (IPMA) determined the most feasible locus for gene replacement to be pvp2. The genomic segment containing gfp at the pvp2 locus was able to be encapsidated into IBDV particles. Furthermore, the expression of GFP in GFP-IBDV infected cells was confirmed by Western blotting and GFP-IBDV particles showed similar morphology and size to that of wildtype IBDV by electron microscopy. By providing the deleted protein in trans in a packaging cell line (pVP2-DF1), replication-incompetent GFP-IBDV particles were successfully plaque-quantified. The gfp sequence from the plaque-forming GFP-IBDV in pVP2-DF1 was confirmed by RT-PCR and sequencing. To our knowledge, GFP-IBDV developed in the present study is the first replication-incompetent IBDV vector which expresses a foreign protein in infected cells without the capability to produce viral progeny. Additionally, such replication-incompetent IBDV vectors could serve as bivalent vaccine vectors for conferring protection against infections with IBDV and other economically important, or zoonotic, avian pathogens.

Evidence of genetic drift and reassortment in infectious bursal disease virus and emergence of outbreaks in poultry farms in India

Recurrent outbreaks of infectious bursal disease (IBD) have become a burning problem to the poultry industry worldwide. Here, we performed genetic analysis of IBD virus (IBDV) field isolates from recent outbreaks in various poultry farms in India. The sequence analysis of IBDV VP2 hypervariable region revealed amino acid pattern similar to that of very virulent (222A, 242I, 253Q, 256I, 272I, 279D, 284A, 294I, 299S and 330S) and intermediate plus virulent (222A, 242I, 253Q, 256I, 272T, 279N, 284A, 294I, 299S and 330S) type whereas analysis of VP1 revealed presence of sequence similar to that of very virulent (61I, 145T) and unique (61I, 141I, 143D, 145S) type in field isolates. Among the eight field isolates, two isolates contained very virulent type VP2 and unique type VP1, three contained intermediate plus virulent type VP2 and unique type VP1 whereas five contained both VP2 and VP1 of very virulent type. The phylogenetic analysis based on VP2 nucleotide sequence showed clustering of all eight isolates close to known very virulent strains whereas based on VP1, five isolates formed unique cluster and three isolates were placed close to very virulent strains. The isolates forming unique VP1 cluster showed highest similarity with classical virulent IBDVs suggesting their possible evolution from segment B of non-very virulent IBDVs. Interestingly, these five isolates were responsible for outbreaks in four different farms located at three different geographic locations in India. These observations indicates genetic reassortment between segment A and segment B from co-infecting IBDV strains leading to emergence of very virulent strains and their widespread prevalence in Indian poultry farms. The presence of 272I and 279D in VP2 protein of five field isolates may explain possible cause of Gumboro intermediate plus vaccine failure in prevention of the outbreaks. However, mortality caused by other three strains which are antigenically similar to VP1 of intermediate plus vaccine strains could not be explained and the possible role of their unique VP1 in enhancing the pathogenesis needs to be investigated further.

Ribosomal protein L4 interacts with viral protein VP3 and regulates the replication of infectious bursal disease virus

VP3 protein is a structural protein which plays important roles in the virus assembly and the inhibition of antiviral innate immunity of infectious bursal disease virus (IBDV). To explore the potential roles of VP3 in the interplay of IBDV with the host cell, an immunoprecipitation (IP)-coupled mass spectra (MS) screening was performed and the host cellular ribosomal protein L4 (RPL4) was identified as a putative interacting partner of VP3 protein. The interaction of RPL4 with VP3 was further confirmed by co-immunoprecipitation (co-IP) and their colocalization in DF1 cells were observed by confocal microscopy. In addition, knockdown of RPL4 in DF1 cells resulted in reductions of the viral protein pVP2 expression and the virus titers, which reveals a significant role of RPL4 in IBDV replication. Taken together, we indicated for the first time that ribosomal protein L4 (RPL4) was an interacting partner of VP3 and involved in the modulation of IBDV replication. The present study contributes to further understanding the pathogenic mechanism of IBDV.

Chondroitin sulfate N-acetylgalactosaminyltransferase-2 contributes to the replication of infectious bursal disease virus via interaction with the capsid protein VP2

Infectious bursal disease virus (IBDV) is a birnavirus that causes a highly contagious immunosuppressive disease in young chickens. The capsid protein VP2 of IBDV plays multiple roles in its life cycle. To more comprehensively understand the functions of VP2 involved in the communication between virus and host, we used yeast two-hybrid screening to identify the cellular factors that interact with this protein. We found that chondroitin sulfate N-acetylgalactosaminyltransferase-2 (CSGalNAcT2), a typical type II transmembrane protein located in Golgi apparatus, could interact with VP2, and we confirmed this interaction by co-immunoprecipitation and confocal laser scanning microscopy assays. Additionally, up-regulation of CSGalNAcT2 during IBDV infection was observed. Overexpression and siRNA-mediated knockdown of CSGalNAcT2 assays suggested that CSGalNAcT2 promoted IBDV replication. Moreover, this enhancing effect of CSGalNAcT2 could be inhibited by Brefeldin A, which is a Golgi-disturbing agent. This indicated that the integrity of Golgi apparatus structure was involved in the function of CSGalNAcT2. Taken together, we concluded that CSGalNAcT2, located in the Golgi apparatus, contributed to the replication of IBDV via interaction with VP2.

Eliciting specific humoral immunity from a plasmid DNA encoding infectious bursal disease virus polyprotein gene fused with avian influenza virus hemagglutinin gene

DNA vaccine coding for infectious bursal disease virus (IBDV) polyprotein gene and that for avian influenza virus (AIV) hemagglutinin (HA) gene have been shown to induce immunity and provide protection against the respective disease. The present study was carried out to determine whether an IBDV polyprotein gene-based DNA fused with AIV HA gene could trigger immune response to both IBDV and AIV. After transfection, VP2 and HA were detected in the cytoplasm and at cell membrane, respectively, by immunofluorescent antibody double staining method, suggesting the fusion strategy did not affect the location of protein expression. VP4 cleavage between VP2 and HA was confirmed by Western blot, indicating the fusion strategy did not affect VP4 function in transfected cells. After vaccination in chickens, the DNA construct VP24-HA/pcDNA induced ELISA and virus neutralizing antibodies against VP2 and hemagglutination inhibition antibody against the HA subtype. The results indicated that a single plasmid construct carrying IBDV VP243 gene-based DNA fused with AIV HA gene can elicit specific antibody responses to both IBDV and AIV by DNA vaccination.

Mutations of residues 249 and 256 in VP2 are involved in the replication and virulence of infectious Bursal disease virus

Infectious bursal disease virus (IBDV) is a pathogen of worldwide significance to the poultry industry. Although the PDE and PFG domains of the capsid protein VP2 contribute significantly to virulence and fitness, the detailed molecular basis for the pathogenicity of IBDV is still not fully understood. Because residues 253 and 284 of VP2 are not the sole determinants of virulence, we hypothesized that other residues involved in virulence and fitness might exist in the PDE and PFG domains of VP2. To test this, five amino acid changes selected by sequence comparison of the PDE and PFG domains of VP2 were introduced individually using a reverse genetics system into the virulent strain (rGx-F9VP2). Then reverse mutations of the selected residues 249 and 256 were introduced individually into the attenuated strain (rGt). Seven modified viruses were generated and evaluated in vitro (CEF cells) and in vivo (SPF chicken). For residue 249, Q249R could elevate in vitro and reduce in vivo the replication of rGx-F9VP2 while R249Q could reduce in vitro and elevate in vivo the replication of rGt; meanwhile Q249R reduced the virulence of rGx-F9VP2 while R249Q increased the virulence of rGt, which indicated that residue 249 significantly contributed to the replication and virulence of IBDV. For residue 256, I256V could elevate in vitro and reduce in vivo the replication of rGx-F9VP2 while V256I could reduce in vitro but didn't change in vivo the replication of rGt; although V256I didn't increase the virulence of rGt, I256V obviously reduced the virulence of virulent IBDV. The present results demonstrate for the first time, to different extent, residues 249 and 256 of VP2 are involved in the replication efficiency and virulence of IBDV; this is not only beneficial to further understanding of pathogenic mechanism but also to the design of newly tailored vaccines against IBDV.

Critical roles of glucocorticoid-induced leucine zipper in infectious bursal disease virus (IBDV)-induced suppression of type I Interferon expression and enhancement of IBDV growth in host cells via interaction with VP4

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). Although IBDV-induced immunosuppression has been well established, the underlying exact molecular mechanism for such induction is not very clear. We report here the identification of IBDV VP4 as an interferon suppressor by interaction with the glucocorticoid-induced leucine zipper (GILZ) in host cells. We found that VP4 suppressed the expression of type I interferon in HEK293T cells after tumor necrosis factor alpha (TNF-α) treatment or Sendai virus (SeV) infection and in DF-1 cells after poly(I·C) stimulation. In addition, the VP4-induced suppression of type I interferon could be completely abolished by knockdown of GILZ by small interfering RNA (siRNA). Furthermore, knockdown of GILZ significantly inhibited IBDV growth in host cells, and this inhibition could be markedly mitigated by anti-alpha/beta interferon antibodies in the cell cultures (P < 0.001). Thus, VP4-induced suppression of type I interferon is mediated by interaction with GILZ, a protein that appears to inhibit cell responses to viral infection.

Critical role for voltage-dependent anion channel 2 in infectious bursal disease virus-induced apoptosis in host cells via interaction with VP5

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). Although IBDV-induced host cell apoptosis has been established, the underlying molecular mechanism is still unclear. We report here that IBDV viral protein 5 (VP5) is a major apoptosis inducer in DF-1 cells by interacting with the voltage-dependent anion channel 2 (VDAC2) in the mitochondrion. We found that in DF-1 cells, VP5-induced apoptosis can be completely abolished by 4,4'-diisothiocyanatostibene-2,2'-disulfonic acid (DIDS), an inhibitor of VDAC. Furthermore, knockdown of VDAC2 by small interfering RNA markedly inhibits IBDV-induced apoptosis associated with decreased caspase-9 and -3 activation and cytochrome c release, leading to increased IBDV growth in host cells. Thus, VP5-induced apoptosis during IBDV infection is mediated by interacting with VDAC2, a protein that appears to restrict viral replication via induction of cell death.

Baculovirus virions displaying infectious bursal disease virus VP2 protein protect chickens against infectious bursal disease virus infection

Infectious bursal disease (IBD) is an acute and contagious viral infection of young chickens caused by IBD virus (IBDV). The VP2 protein of IBDV is the only antigen for inducing neutralizing antibodies and protective immunity in the natural host. In the current study, we have succeeded in construction of one recombinant baculovirus BacSC-VP2 expressing His6-tagged VP2 with the baculovirus envelope protein gp64 transmembrane domain (TM) and cytoplasmic domain (CTD). The His6-tagged recombinant VP2 was expressed and anchored on the plasma membrane of Sf-9 cells, as examined by western blot and confocal microscopy. Immunogold electron microscopy demonstrated that the VP2 protein of IBDV was successfully displayed on the viral surface. Vaccination of chickens with the VP2-pseudotyped baculovirus vaccine (BacSC-VP2) elicited significantly higher levels of VP2-specific enzyme-linked immunosorbent assay antibodies and neutralizing antibodies than the control groups. IBDV-specific proliferation of lymphocytes was observed in chickens immunized with the recombinant BacSC-VP2. An in vivo challenge study of the recombinant baculovirus BacSC-VP2 showed effective protection against a very virulent (vv) IBDV infection in chickens. In addition, mortality and gross and histopathological findings in the bursa demonstrated the efficacy of the vaccine in reducing virulence of the disease. These results indicate that the recombinant baculovirus BacSC-VP2 can be a potential vaccine against IBDV infections.

Infectious bursal disease virus activates the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway by interaction of VP5 protein with the p85α subunit of PI3K

Phosphatidylinositol 3-kinase (PI3K)/Akt signaling is commonly activated upon virus infection and has been implicated in the regulation of diverse cellular functions such as proliferation and apoptosis. The present study demonstrated for the first time that infectious bursal disease virus (IBDV), the causative agent of a highly contagious disease in chickens, can induce Akt phosphorylation in cultured cells, by a mechanism that is dependent on PI3K. Inhibition of PI3K activation greatly enhanced virus-induced cytopathic effect and apoptotic cell death as evidenced by cleavage of poly-ADP ribose polymerase and activation of caspase-3. Investigations into the mechanism of PI3K/Akt activation revealed that IBDV activates PI3K/Akt signaling through binding of the non-structural protein VP5 to regulatory subunit p85α of PI3K resulting in the suppression of premature apoptosis and improved virus growth after infection. The results presented here provide a basis for understanding molecular mechanism of IBDV infection.

Molecular epizootiology of infectious bursal disease (IBD) in Korea

We conducted a molecular epizootiological study of infectious bursal disease (IBD) in Korea by analyzing 85 IBD viruses (IBDVs) obtained from vaccinated or unvaccinated flocks between 1980 and 2007. Phylogenetic analysis of the partial nucleotide sequence of the hypervariable region of the VP2 gene (nucleotides 661-1020) and pathogenicity tests revealed more genetic and phenotypic diversity of IBDV in Korea than has been reported previously. We showed that very virulent IBDVs (vvIBDVs) were already present in Korea in 1986. Moreover, vvIBDVs were repeatedly detected in Korean poultry that had been vaccinated, which casts doubt on the IBD vaccine programs. We also identified novel putative antigenic variant (AV)-like IBDV isolates on the basis of their antigenic indices and the presence of amino acid changes (P222S or P222T-A321D) that are known to affect the antigenicity of VP2. These observations suggest that future studies examining the efficacy of conventional vaccines against atrophy of the bursa of Fabricius and vvIBDV shedding may be useful. Moreover, it will be of interest to determine the prevalence of putative Korean antigenic variants and whether these strains exert immunosuppressive effects in vaccinated birds.

DNA vaccination with VP2 gene of very virulent infectious bursal disease virus (vvIBDV) delivered by transgenic E. coli DH5alpha given orally confers protective immune responses in chickens

The efficacy of different doses of oral DNA vaccines carrying VP2 gene of vvIBDV delivered by E. coli DH5alpha was studied and compared with purified VP2 recombinant expression plasmid DNA vaccine injected intradermally and whole virus vaccine either from homologous virus or from commercial source. The recombinant plasmid pRc-VP2 was transformed in a non-pathogenic strain of E. coli, the DH5alpha and designated as EC/pRC-VP2. Oral immunization of maternal antibody free broiler chickens at 7 and 14-day-old with different dosages of EC/pRc-VP2 elicited specific humoral immune response as measured by ELISA. Protection in different groups was calculated through clinical signs, gross and histopathological lesions, bursa of Fabricius to body weight ratio, humoral and cellular immune responses and mortality in the chickens. Vaccination with EC/pRc-VP2 at the dose rate of 10(9)CFU per chicken conferred 95.4% protection of the chickens against the challenge with homologous virulent field strain of vvIBDV. Protection afforded by attenuated vero cell adapted UAF-06 strain of vvIBDV was comparable (94%) to that by EC/pRc-VP2 and pRc-VP2 vaccines, which was significantly higher (P<0.05) than the protection provided by a commercial attenuated IBDV stain D-78 vaccine (D-78 vaccine was used as positive control due to its frequent use in the field for vaccination of poultry chickens) and other control groups in the study. The results revealed that DNA vaccines against IBDV may be successfully done by adopting bacterial-vectored oral delivery system and vaccination with homologous vvIBDV (UAF-06) conferred significantly higher protection as compared with imported non-homologous commercial IBDV vaccine.

Diagnostic potential of recombinant protein of hexahistidine tag and infectious bursal disease virus VPX expressed in Escherichia coli

The current method to detect antibody titre against infectious bursal disease virus (IBDV) in chickens is based on enzyme-linked immunosorbent assay (ELISA) using whole virus as coating antigen. Coating the ELISA plates requires a purified or at least semi-purified preparation of virus as antigen, which needs special skills and techniques. In this study, instead of using whole virus, recombinant protein of hexahistidine tag (His 6 tag) and VPX protein of IBDV expressed in E. coli was used as an alternative antigen to coat the ELISA plates. There was a good correlation coefficient (R2 = 0.972) between the results of the ELISA using plates coated with monoclonal antibody against His 6 tag and those of the commercial IBDV ELISA kit. Hence, His 6 tag and VPX recombinant protein expressed in E. coli has the potential for the development of ELISA for the measurement of IBDV-specific antibody.

Reduction of infectious bursal disease virus replication in cultured cells by proteasome inhibitors

Infectious bursal disease virus (IBDV) is the etiological agent of a highly contagious disease in chickens. In a recent report, proteasome inhibitor MG132 has been shown to completely inhibit IBDV-induced apoptosis. This raises the possibility that the ubiquitin–proteasome pathway may be used by the virus to promote viral replication. In this study, we examined the interplay between IBDV replication and the ubiquitin–proteasome pathway in cultured cells. Treatment of DF-1 cells with the proteasome inhibitors MG132 or lactacystin significantly decreased virus release in the supernatant and prevented virus-induced cytopathic effect. Inhibition of the ubiquitin–proteasome pathway did reduce markedly viral RNA transcription and protein translation but not affect virus internalization. We also demonstrated that IBDV activates caspase pathway via triggering the efflux of cytochrome c in mitochondria into cytosol of infected cells. This activity was dose-dependently reduced by proteasome inhibitor treatment. Taken together, our data suggest that proteasome inhibitor reduces IBDV replication through inhibition of viral RNA transcription and protein synthesis, and thus preventing IBDV-induced apoptosis.

Nucleotide Sequence Analysis of VP2 Hypervariable Domain of Infectious Bursal Disease Virus Detected in Japan from 1993 to 2004

Bursae of Fabricius were collected from 20 chickens diagnosed with infectious bursal disease virus (IBDV) infection from 15 prefectures in 1993 to 2004. Here we report the nucleotide sequence analysis of VP2 hypervariable domain of IBDV genome detected by reverse transcription-polymerase chain reaction from these samples. Ten sequences derived from 10 prefectures in 1996 to 2003 were of the classical type and other 10 sequences derived from 6 prefectures in 1993 to 2004 were of the highly virulent type. Of the classical type sequences, 9 sequences were closely related to the sequence of classical attenuated vaccines used in Japan. Furthermore, two were identical to the sequence of B-Chi5 which represents Vaccine B passaged 5 times in chickens and was reported to be reverted the virulence during the passages. The 10 highly virulent type sequences were classified into four sequences, none of which had been previously detected in Japan. However, the deduced amino acid sequences were identical to each other and to the sequences of highly virulent IBDVs previously detected in Japan. The most common nucleotide sequences, which accounted for 6 of the sequences, were identical to 34 highly virulent type sequences detected in various countries in BLAST search. This is the first report of detection of the sequence in Japan which is identical to highly virulent strains detected in other countries. These findings show the prevalence of classical IBDVs closely related to the attenuated vaccines and highly virulent IBDVs derived from other countries throughout Japan since 1993.

Efficacy of VP2 protein expressed in E. coli for protection against highly virulent infectious bursal disease virus

The ability of a heat-inactivated whole virus from a highly virulent infectious bursal disease virus (hvIBDV) and VP2 protein from hvIBDV expressed in E. coli provided protection against a hvIBDV challenge in specificpathogen-free (SPF) chickens. Six out of seven chickens that were injected three times with crude VP2 protein developed significant antibody titer against IBDV. However, only four out of the seven chickens survived the hvIBDV challenge. Despite showing low antibody titer profiles, all chickens immunized with the heat-inactivated whole virus also survived the challenged with hvIBDV. However, all of these chickens had bursal atrophy and mild to moderate depletion of lymphocytes. Thus, antibodies raised against IBDV VP2 protein expressed in E. coli and denatured IBDV proteins induced some degree of protection against mortality but not against bursal damage following challenge with hvIBDV.

Nonstructural protein of infectious bursal disease virus inhibits apoptosis at the early stage of virus infection

Infectious bursal disease virus (IBDV), the causative agent of a highly contagious disease in chickens, carries a small nonstructural protein (NS). This protein has been implicated to play a role in the induction of apoptosis. In this study, we investigate the kinetics of viral replication during a single round of viral replication and examine the mechanism of IBDV-induced apoptosis. Our results show that it is caspase dependent and activates caspases 3 and 9. Nuclear factor kappa B (NF-kappaB) is also activated and is required for IBDV-induced apoptosis. The NF-kappaB inhibitor MG132 completely inhibited IBDV-induced DNA fragmentation, caspase 3 activation, and NF-kappaB activation. To study the function of the NS protein in this context, we generated the recombinant rGLS virus and an NS knockout mutant, rGLSNSdelta virus, using reverse genetics. Comparisons of the replication kinetics and markers for virally induced apoptosis indicated that the NS knockout mutant virus induces earlier and increased DNA fragmentation, caspase activity, and NF-kappaB activation. These results suggest that the NS protein has an antiapoptotic function at the early stage of virus infection.

Development of recombinant VP2 vaccine for the prevention of infectious bursal disease of chickens

The objective of the present study was to investigate the feasibility of a subunit vaccine and a live bacteria vaccine to protect chickens against infectious bursal disease virus (IBDV) infection. The gene for VP2 of a new wild-type very virulent IBDV (vvIBDV) strain was cloned into an Escherichia coli expression system. Following expression, the recombinant VP2 and the induced expression bacteria were used to vaccinate chickens against virulent IBDV (vIBDV). Three weeks after the vaccination, chickens were inoculated with IBDV strain BC 6/85 by intranasal route or eyedrop route, prior to challenge anti-IBDV serum antibody was detected by AGP. All chickens vaccinated with recombinant VP2 could be detected anti-IBDV antibody. The subunit vaccine of recombinant VP2 conferred protection for 90--100% chickens, live bacteria vaccine of recombinant VP2 conferred protection for 85.7% chickens. The results indicate that E. coli BL 21/pET 28 a-VP2 could be used to develop recombinant VP2 vaccine against infectious bursal disease in chickens.

Characterization of particles formed by the precursor protein VPX of infectious bursal disease virus in insect Hi-5 cells: implication on its proteolytic processing

The precursor (VPX) of host immunogen VP2 protein for infectious bursal disease virus (IBDV) was expressed in insect Sf9 and Hi-5 cells, and the types of particles generated as well as the immunogenicity induced by these particles were examined. Recombinant VPXH (rVPXH) protein, expressed in Hi-5 cells at an expression level 4x higher than in Sf9 cells, was efficiently processed by proteases to yield VP2-like proteins with corresponding molecular weight, a phenomenon not observed previously. At least three structures of particles were observed for VPXH and VP2-like proteins purified by immobilized metal-ion affinity chromatography (MAC). In addition to the two previously identified twisted tubular and isometric particle structures, there was a new one: icosahedral particles of approximately 25 nm in diameter. The purified particles were further separated by gel-filtration chromatography (GFC) linking with HPLC, which was able to resolve the isometric from icosahedral particles better than ultracentrifugation. Chromatographic results indicate that rVPXH protein mainly involved in the formation of the isometric particle structure and occasionally twisted tubular structure, and the icosahedral particles were formed by the degraded products of rVPXH (VP2-like proteins). Thus, by combining IMAC and GFC, it was shown that VPX was processed efficiently to yield VP2-like protein that could form small virus-like particles in Hi-5 cells. Finally, we demonstrated that virus-neutralizing antibodies were induced when susceptible chickens were vaccinated with the IMAC-purified rVPXH protein (40 microg per bird). This indicates that these particles are highly immunogenic and might serve as an alternative vaccine candidate for the development of IBDV subunit vaccine.

Expression of immunogenic VP2 protein of infectious bursal disease virus in Arabidopsis thaliana

VP2 protein is the major host-protective immunogen of infectious bursal disease virus (IBDV) of chickens. Transgenic lines of Arabidopsisthaliana expressing recombinant VP2 were developed. The VP2 gene of an IBDV antigenic variant E strain was isolated, amplified by RT-PCR and introduced into a plant expression vector, pE1857, having a strong promoter for plant expression. A resulting construct with a Bar gene cassette for bialaphos selection in plant (rpE-VP2) was introduced into Agrobacterium tumefaciensby electroporation. Agrobacterium containing the rpE-VP2 construct was used to transform Ar. thaliana and transgenic plants were selected using bialaphos. The presence of VP2 transgene in plants was confirmed by PCR and Southern blot analysis and its expression was confirmed by RT-PCR. Western blot analysis and antigen-capture ELISA assay using monoclonal anti-VP2 were used to determine the expression of VP2 protein in transgenic plants. The level of VP2 protein in the leaf extracts of selected transgenic plants varied from 0.5% to 4.8% of the total soluble protein. Recombinant VP2 protein produced in plants induced antibody response against IBDV in orally-fed chickens.

Enhancement of the immunogenicity of DNA vaccine against infectious bursal disease virus by co-delivery with plasmid encoding chicken interleukin 2

The immunoregulatory activity of a nonmammalian interleukin 2 (IL-2), chicken IL-2 (chIL-2), was investigated using a DNA vaccine against infectious bursal disease virus (IBDV) as a model. Coadministration of a plasmid encoding the VP2 gene of IBDV (pCI-VP2) and a plasmid encoding chicken IL-2 gene (pCI-chIL-2) enhances bursal protection against both the homologous IBDV strain ZJ2000 and the heterologous strain BC6/85 compared to administration of pCI-VP2 alone. Vaccination with pCI-VP2 alone induces low bursal protection against ZJ2000 and only protects chickens from clinical outbreaks and mortality, but not from bursal damage caused by BC6/85. Co-administration of the plasmid encoding the polyprotein gene of IBDV (pCI-VP2/4/3) and pCI-chIL-2 provides complete protection (15/15) against ZJ2000 and satisfactory protection (13/15) against BC6/85. In contrast, only 10 out of 15 chickens and 6 out of 15 chickens were protected against ZJ2000 and BC6/85, respectively, using the pCI-VP2/4/3 vaccination alone. A significant increase in the IBDV-specific neutralizing antibody response was also observed in chickens that received pCI-VP2/4/3 plus pCI-chIL-2 as compared with those that received the pCI-VP2/4/3 vaccination alone. By administrating different amounts of plasmid DNA, we confirmed that the pCI-chIL-2, but not the backbone plasmid pCI, contributes to increased immunoprotection of DNA vaccine against IBDV. These results strongly indicate that the efficacy of avian DNA vaccine can be modulated by co-administration of a plasmid encoding chIL-2.

Immunization of Chickens with VP2 Protein of Infectious Bursal Disease Virus Expressed in Arabidopsis thaliana

Transgenic plants represent a safe, effective, and inexpensive way to produce vaccines. The immunogenicity of VP2 protein of an infectious bursal disease (IBD) virus variant E isolate expressed in transgenic Arabidopsis thaliana was compared with a commercial vaccine in specific-pathogen-free broiler chickens. The VP2 coding sequence was isolated and integrated into A. thaliana genome by Agrobacterium tumefaciens-mediated transformation. Soluble VP2 expressed in transgenic plants was used to immunize chickens. Chickens receiving oral immunization with plant-derived VP2 at 1 and 3 wk of age had an antibody response using enzyme-linked immunosorbent assay and 80% protection against challenge infection at 4 wk. Chickens primed with a commercial vaccine at 1 wk followed by an oral booster with VP2 expressed in plants at 3 wk of age showed 90% protection. Chickens immunized with a commercial vaccine at 1 and 3 wk showed 78% protection. Results supported the efficacy of plant-produced VP2 as a vaccine against IBD.

Ability of Lactococcus lactis to export viral capsid antigens: a crucial step for development of live vaccines

The food grade bacterium Lactococcus lactis is a potential vehicle for protein delivery in the gastrointestinal tract. As a model, we constructed lactococcal strains producing antigens of infectious bursal disease virus (IBDV). IBDV infects chickens and causes depletion of B-lymphoid cells in the bursa of Fabricius and subsequent immunosuppression, morbidity, or acute mortality. The two major IBDV antigens, i.e., VP2 and VP3, that form the viral capsid were expressed and targeted to the cytoplasm, the cell wall, or the extracellular compartment of L. lactis. Whereas VP3 was successfully targeted to the three compartments by the use of relevant expression and export vectors, VP2 was recalcitrant to export, thus confirming the difficulty of translocating naturally nonsecreted proteins across the bacterial membrane. This defect could be partly overcome by fusing VP2 to a naturally secreted protein (the staphylococcal nuclease Nuc) that carried VP2 through the membrane. Lactococcal strains producing Nuc-VP2 and VP3 in various bacterial compartments were administered orally to chickens. The chickens did not develop any detectable immune response against VP2 and VP3 but did exhibit an immune response against Nuc when Nuc-VP2 was anchored to the cell wall of lactococci.

Plasmid DNA encoding antigens of infectious bursal disease viruses induce protective immune responses in chickens: factors influencing efficacy

The complete polyprotein (VP2/4/3) and VP2 genes of two infectious bursal disease viruses (IBDVs) (one attenuated strain JD1 and one virulent strain ZJ2000) were amplified by long and accurate polymerase chain reaction (LA-PCR), cloned, sequenced and inserted into plasmids pCI and pcDNA3 under the control of human cytomegalovirus (hCMV) immediate early enhancer and promoter. A series of DNA vaccine preparations were made using liposome as the adjuvant to examine their immunogenicity. Although VP2 is the main protective immunogen of IBDV, DNA encoding VP2 initiated a very low level of neutralizing antibody and only protected chickens from clinical outbreak and morality, but not bursal damage. In contrast, DNA encoding VP2/4/3 induced neutralizing antibody and satisfactory protection against virulent IBDV. Recombinant plasmids encoding the polyprotein gene of strain ZJ2000 were more efficient at inducing an immune response than that of strain JD1. Polyprotein expressed by the pCI vector induced better immune response than that expressed by the pcDNA3. Delivery of DNA through intramuscular and/or intradermal routes elicited much higher protective responses than that of oral and eyedrop routes. Most of the chickens vaccinated with high doses of DNA were protected from challenge. Additionally, the immune response to the DNA vaccine was significantly enhanced by a liposome adjuvant. These results indicate that the source of the target genes (from different IBDV strains), the eukaryotic expression vector, the adjuvant, the delivery route and the dosage might play a role of varying degree in influencing the efficacy of the DNA vaccine against IBDV.

DNA vaccination with plasmids containing various fragments of large segment genome of infectious bursal disease virus

The present study was undertaken to determine the effectiveness in including VP2 gene of the large segment genome of infectious bursal disease virus (IBDV) in the DNA vaccine for protection of chickens against infectious bursal disease (IBD). Different fragments of the large segment gene of IBDV standard challenge (STC) strain were successfully amplified by reverse transcription-polymerase chain reaction (RT-PCR) followed by cloning into a eukaryotic expression plasmid vector (pCR3.1) as DNA vaccines. Transient expression of the encoded genes from various constructed plasmids was characterized in COS-7 cells by positive immunofluorescent staining with polyclonal or monoclonal antibody to IBDV. Chickens (1-day old) were intramuscularly injected with the individual plasmid three times at weekly intervals, challenged with IBDV strain STC at 21-day old, and observed for 10 days. Chickens receiving the plasmids containing VP2 genes, including VP243, VP24, or VP2 fragment, did not have clinical signs, mortality, and bursal atrophy and were effectively protected against IBDV infection. On the contrary, chickens receiving plasmids without containing VP2 genes, including VP4, VP3, or VP43 fragment, had marked bursal atrophy and were not protected against IBD. Antigen detection was correlated with protection; chickens protected from IBDV infection had undetectable IBDV antigen in bursae. Enzyme-linked immunosorbent assay (ELISA) antibody titer to IBDV was low or undetectable prior to or after challenge with IBDV in protected chickens receiving the plasmids containing the VP2 gene. The results indicate that inclusion of VP2 gene in the plasmid DNA is essential in achieving effective protection mediated by DNA vaccination against IBDV infection in chickens.

Molecular determinants of virulence, cell tropism, and pathogenic phenotype of infectious bursal disease virus

Infectious bursal disease viruses (IBDVs), belonging to the family Birnaviridae, exhibit a wide range of immunosuppressive potential, pathogenicity, and virulence for chickens. The genomic segment A encodes all the structural (VP2, VP4, and VP3) and nonstructural proteins, whereas segment B encodes the viral RNA-dependent RNA polymerase (VP1). To identify the molecular determinants for the virulence, pathogenic phenotype, and cell tropism of IBDV, we prepared full-length cDNA clones of a virulent strain, Irwin Moulthrop (IM), and constructed several chimeric cDNA clones of segments A and B between the attenuated vaccine strain (D78) and the virulent IM or GLS variant strain. Using the cRNA-based reverse-genetics system developed for IBDV, we generated five chimeric viruses after transfection by electroporation procedures in Vero or chicken embryo fibroblast (CEF) cells, one of which was recovered after propagation in embryonated eggs. To evaluate the characteristics of the recovered viruses in vivo, we inoculated 3-week-old chickens with D78, IM, GLS, or chimeric viruses and analyzed their bursae for pathological lesions 3 days postinfection. Viruses in which VP4, VP4-VP3, and VP1 coding sequences of the virulent strain IM were substituted for the corresponding region in the vaccine strain failed to induce hemorrhagic lesions in the bursa. In contrast, viruses in which the VP2 coding region of the vaccine strain was replaced with the variant GLS or virulent IM strain caused rapid bursal atrophy or hemorrhagic lesions in the bursa, as seen with the variant or classical virulent strain, respectively. These results show that the virulence and pathogenic-phenotype markers of IBDV reside in VP2. Moreover, one of the chimeric viruses containing VP2 sequences of the virulent strain could not be recovered in Vero or CEF cells but was recovered in embryonated eggs, suggesting that VP2 contains the determinants for cell tropism. Similarly, one of the chimeric viruses containing the VP1 segment of the virulent strain could not be recovered in Vero cells but was recovered in CEF cells, suggesting that VP1 contains the determinants for cell-specific replication in Vero cells. By comparing the deduced amino acid sequences of the D78 and IM strains and their reactivities with monoclonal antibody 21, which binds specifically to virulent IBDV, the putative amino acids involved in virulence and cell tropism were identified. Our results indicate that residues Gln at position 253 (Gln253), Asp279, and Ala284 of VP2 are involved in the virulence, cell tropism, and pathogenic phenotype of virulent IBDV.

DNA-mediated vaccination against infectious bursal disease in chickens

The objective of the present study was to investigate the feasibility of a DNA vaccine to protect chickens against infectious bursal disease virus (IBDV) infection. A plasmid DNA carrying VP2, VP4, and VP3 genes of the standard challenge (STC) strain of IBDV was constructed and designated as pCR3.1-VP243-STC. One-day-old chickens were intramuscularly injected with the plasmid pCR3.1-VP243-STC once (group D1), twice (group D2), or three times (group D3) at weekly intervals. Chickens at 3 weeks old were orally inoculated with IBDV strain STC and observed for 10 days after challenge. Immunization twice (group D2) or three times (group D3) with the plasmid pCR3.1-VP243-STC conferred protection for 50-100 or 80-100% of chickens, respectively, as evidenced by the absence of clinical signs, mortality, and bursal atrophy. Although chickens vaccinated once (group D1) with the plasmid pCR3.1-VP243-STC did not have clinical signs, they exhibited varying degree of bursal atrophy after challenge. Enzyme-linked immunosorbent assay (ELISA) antibody titers in chickens protected by the plasmid pCR3.1-VP243-STC were significantly lower (P<0.05) than those not protected 10 days after challenge. IBDV antigen was not detected in the bursae of chickens that were protected by receiving the plasmid pCR3.1-VP243-STC twice or three times. The results indicate that the constructed plasmid pCR3.1-VP243-STC as a DNA vaccine provided efficacious protection for chickens against IBDV infection.

Induction of apoptosis in vitro by the 17-kDa nonstructural protein of infectious bursal disease virus: possible role in viral pathogenesis

Infectious bursal disease virus (IBDV) causes severe immunodeficiency in young chickens by destroying the precursors of antibody-producing B cells in the bursa of Fabricius. It has been shown that IBDV infection induces apoptosis in chicken embryo and tissue culture cells. We previously reported that an IBDV mutant lacking the expression of 17-kDa nonstructural (NS) protein exhibited decreased apoptotic effects in cell culture as compared to the parental IBDV, suggesting that the NS protein may be involved in induction of apoptosis. Here, we report that the NS protein of IBDV alone is capable of inducing apoptosis in cell culture. Transfection of chicken B-lymphocyte cell line (RP9) and chicken embryo fibroblast cells with a plasmid DNA, containing the NS protein gene under the control of the immediate-early promoter-enhancer region of human cytomegalovirus, induced programmed cell death in both cell lines. Apoptosis changes, such as chromatin condensation, DNA fragmentation, and the appearance of apoptotic nuclear bodies, were observed in cell cultures 48-h posttransfection. As reported earlier, the mutant IBDV grew to lower titers with slower replication kinetics and lower cytopathogenicity when compared to that of the parental virus. Here, we demonstrate that the mutant virus is closely associated with cells and its yield from the supernatant was approximately 30-fold lower than the wild-type due to increased cell association, indicating a deficiency in lysis of virus-infected cells. Taken together, our results indicate that the NS protein of IBDV is highly cytotoxic, which brings about the release of the viral progeny from cells, and thus play an important role in viral pathogenesis.

Separation of pure and immunoreactive virus-like particles using gel filtration chromatography following immobilized metal ion affinity chromatography

A purification process was developed to obtain highly pure rVP2H particles, formed by a structural protein (VP2) of the infectious bursal disease virus (IBDV) with six additional histidine residues at its C-terminus. The ultimate goal was the development of an efficient subunit vaccine against IBDV infection. The particles within the infected High-Five (Hi-5) cell lysates were partially purified by employing immobilized metal ion (Ni(2+)) affinity chromatography (IMAC). The initial step could recover approximately 85% of immunoreactive rVP2H proteins but failed to separate the rVP2H particles from the free rVP2H proteins or its degraded products. To separate the particulate form from the free form of rVP2H, an additional step was added, which used either gel filtration chromatography or CsCl density gradient ultracentrifugation. Both were able to produce extremely pure rVP2H particles with a buoyant density close to 1.27 g/cm(3). However, the former method can process a larger sample volume than does the latter. By integrating IMAC and gel filtration chromatography, 1 mg of extremely pure rVP2H particles was routinely obtained from a 500 mL Hi-5 cell culture broth. The separation of the particulate form from the free form of rVP2H proteins exposes their respective immunogenicity to induce the virus-neutralizing antibodies and the ability to protect chickens from IBDV infection. Additionally, the abundant quantities of pure rVP2H particles coupled with their uniform dimensions facilitates an understanding of higher order structure of the immunogenic particles and can therefore result in improved vaccines against the virus.

Different architectures in the assembly of infectious bursal disease virus capsid proteins expressed in insect cells

Infectious bursal disease virus (IBDV) capsid is formed by the processing of a large polyprotein and subsequent assembly of VPX/VP2 and VP3. To learn more about the processing of the polyprotein and factors affecting the correct assembly of the viral capsid in vitro, different constructs were made using two baculovirus transfer vectors, pFastBac and pAcYM1. Surprisingly, the expression of the capsid proteins gave rise to different types of particles in each system, as observed by electron microscopy and immunofluorescence. FastBac expression led to the production of only rigid tubular structures, similar to those described as type I in viral infection. Western blot analysis revealed that these rigid tubules are formed exclusively by VPX. These tubules revealed a hexagonal arrangement of units that are trimer clustered, similar to those observed in IBDV virions. In contrast, pAcYM1 expression led to the assembly of virus-like particles (VLPs), flexible tubules, and intermediate assembly products formed by icosahedral caps elongated in tubes, suggesting an aberrant morphogenesis. Processing of VPX to VP2 seems to be a crucial requirement for the proper morphogenesis and assembly of IBDV particles. After immunoelectron microscopy, VPX/VP2 was detected on the surface of tubules and VLPs. We also demonstrated that VP3 is found only on the inner surfaces of VLPs and caps of the tubular structures. In summary, assembly of VLPs requires the internal scaffolding of VP3, which seems to induce the closing of the tubular architecture into VLPs and, thereafter, the subsequent processing of VPX to VP2.

Role of Ser-652 and Lys-692 in the protease activity of infectious bursal disease virus VP4 and identification of its substrate cleavage sites

The polyprotein of infectious bursal disease virus (IBDV), an avian birnavirus, is processed by the viral protease, VP4. Previous data obtained on the VP4 of infectious pancreatic necrosis virus (IPNV), a fish birnavirus, and comparative sequence analysis between IBDV and IPNV suggest that VP4 is an unusual eukaryotic serine protease that shares properties with prokaryotic leader peptidases and other bacterial peptidases. IBDV VP4 is predicted to utilize a serine-lysine catalytic dyad. Replacement of the members of the predicted catalytic dyad (Ser-652 and Lys-692) confirmed their indispensability. The two cleavage sites at the pVP2-VP4 and VP4-VP3 junctions were identified by N-terminal sequencing and probed by site-directed mutagenesis. Several additional candidate cleavage sites were identified in the C-terminal domain of pVP2 and tested by cumulative site-directed mutagenesis and expression of the mutant polyproteins. The results suggest that VP4 cleaves multiple (Thr/Ala)-X-Ala downward arrowAla motifs. A trans activity of the VP4 protease of IBDV, and also IPNV VP4 protease, was demonstrated by co-expression of VP4 and a polypeptide substrate in Escherichia coli. For both proteases, cleavage specificity was identical in the cis- and trans-activity assays. An attempt was made to determine whether VP4 proteases of IBDV and IPNV were able to cleave heterologous substrates. In each case, no cleavage was observed with heterologous combinations. These results on the IBDV VP4 confirm and extend our previous characterization of the IPNV VP4, delineating the birnavirus protease as a new type of viral serine protease.

Induction of protective immunity in chickens immunised with plasmid DNA encoding infectious bursal disease virus antigens

Direct DNA inoculations were used to determine the efficacy of gene immunisation of chickens to elicit protective immune responses against infectious bursal disease virus (IBDV). The vp2 gene of IBDV strains GP40 and D78, and the vp2-vp4-vp3 encoding segment of strain D78 were cloned in an expression vector which consisted of human cytomegalovirus (HCMV) immediate early enhancer and promoter, adenovirus tripartite leader sequences and SV40 polyadenylation signal. For purification of vaccine-quality plasmid DNA from E. coli, an effective method was developed. Chickens were vaccinated by inoculation of DNA by two routes (intramuscular and intraperitoneal). Two weeks later, chickens were boosted with DNA, and at 2 weeks post-boost, they were challenged with virulent IBDV strain. Low to undetectable levels of IBDV-specific antibodies and no protection were observed with DNA encoding VP2. However, plasmids encoding VP2-VP4-VP3 induced IBDV-specific antibodies and protection in the chickens. DNA immunisation opens a new approach to the development of gene vaccines for chickens against infectious diseases.

Interactions in vivo between the proteins of infectious bursal disease virus: capsid protein VP3 interacts with the RNA-dependent RNA polymerase, VP1

Little is known about the intermolecular interactions between the viral proteins of infectious bursal disease virus (IBDV). By using the yeast two-hybrid system, which allows the detection of protein-protein interactions in vivo, all possible interactions were tested by fusing the viral proteins to the LexA DNA-binding domain and the B42 transactivation domain. A heterologous interaction between VP1 and VP3, and homologous interactions of pVP2, VP3, VP5 and possibly VP1, were found by co-expression of the fusion proteins in Saccharomyces cerevisiae. The presence of the VP1-VP3 complex in IBDV-infected cells was confirmed by co-immunoprecipitation studies. Kinetic analyses showed that the complex of VP1 and VP3 is formed in the cytoplasm and eventually is released into the cell-culture medium, indicating that VP1-VP3 complexes are present in mature virions. In IBDV-infected cells, VP1 was present in two forms of 90 and 95 kDa. Whereas VP3 initially interacted with both the 90 and 95 kDa proteins, later it interacted exclusively with the 95 kDa protein both in infected cells and in the culture supernatant. These results suggest that the VP1-VP3 complex is involved in replication and packaging of the IBDV genome.

Sequence comparison of the VP2 variable region of infectious bursal disease virus isolates from Vietnam

The variable region in the VP2 gene of twenty-three infectious bursal disease virus (IBDV) isolates, collected in Vietnam in 1997 and 1998, was amplified as cDNA by using the reverse transcription-polymerase chain reaction and sequenced. Analysis of amino acid substitutions and phylogenetic relationships of the deduced amino acid sequences (residues 206-350) showed that the nineteen Vietnamese vv IBDVs clustered with the European vv IBDVs, Japanese vv IBDVs and Chinese vv strains, and that the four vietnamese virulent strains were closely related to European virulent strain 52/70. These results suggest that Vietnamese vv IBDVs, European vv IBDVs, Japanese vv IBDVs and Chinese vv strains have the same origin.

Formation of virus-like particles when the polyprotein gene (segment A) of infectious bursal disease virus is expressed in insect cells

The baculovirus expression vector system was used to examine the expression of the full-length infectious bursal disease virus (IBDV) segment A cDNA, which encodes the structural proteins in a polyprotein precursor that is autocatalytically cleaved to VPX, VP3, and VP4. No VP2 was observed in lysates of recombinant baculovirus infected cells indicating the lack of processing of VPX to VP2 in this system. Virus-like particles (VLP) were purified from the infected insect cells, and on negative staining electron microscopy, looked very similar to authentic IBDV particles in shape and size, suggesting that processing of VPX to VP2 is not necessary for capsid assembly.

Involvement of serine proteinase in infectious pancreatic necrosis virus capsid protein maturation and NS proteinase cleavage in CHSE-214 cells

An investigation of virus-specific protein maturation in infectious pancreatic necrosis virus (IPNV) infected Chinook salmon embryo cells (CHSE-214) was undertaken. The precursor protein (pVP2-1) of the major mature capsid protein (VP2) was processed sequentially from pVP2-1 to pVP2-2 and VP2. Experiments using serine proteinase inhibitors showed that the maturation of the VP2 was blocked in the pVP2-1 post-translational cleavage steps. A protinin, a potent proteinase inhibitor, at 800 μg ml(-1) blocked pVP2-2 to VP2 and the cleavage of VP4 (28 kDa) to VP4-1 (25 kDa). Therefore, our data showed that the maturation of the capsid protein (VP2) and cleavage of VP4 (NS proteinase) can be blocked by serine proteinase inhibitors.