Infectious bursal disease virus structural proteins expressed in a baculovirus recombinant confer protection in chickens

Evaluating the Breadth of Neutralizing Antibody Responses Elicited by Infectious Bursal Disease Virus Genogroup A1 Strains Using a Novel Chicken B-Cell Rescue System and Neutralization Assay

Eight infectious bursal disease virus (IBDV) genogroups have been identified based on the sequence of the capsid hypervariable region (HVR) (A1 to A8). Given reported vaccine failures, there is a need to evaluate the ability of vaccines to neutralize the different genogroups. To address this, we used a reverse genetics system and the chicken B-cell line DT40 to rescue a panel of chimeric IBDVs and perform neutralization assays. Chimeric viruses had the backbone of a lab-adapted strain (PBG98) and the HVRs from diverse field strains as follows: classical F52-70 (A1), U.S. variant Del-E (A2), Chinese variant SHG19 (A2), very virulent UK661 (A3), M04/09 distinct (A4), Italian ITA-04 (A6), and Australian variant Vic-01/94 (A8). Rescued viruses showed no substitutions at amino acid positions 253, 284, or 330, previously found to be associated with cell-culture adaptation. Sera from chickens inoculated with wild-type (wt) (F52-70) or vaccine (228E) A1 strains had the highest mean virus neutralization (VN) titers against the A1 virus (log₂ 15.4 and 12.7) and the lowest against A2 viruses (log₂ 7.4 to 7.9; P = 0.0001 to 0.0274), consistent with A1 viruses being most antigenically distant from A2 strains, which correlated with the extent of differences in the predicted HVR structure. VN titers against the other genogroups ranged from log₂ 9.3 to 13.3, and A1 strains were likely more closely antigenically related to genogroups A3 and A4 than A6 and A8. Our data are consistent with field observations and validate the new method, which can be used to screen future vaccine candidates for breadth of neutralizing antibodies and evaluate the antigenic relatedness of different genogroups. IMPORTANCE There is a need to evaluate the ability of vaccines to neutralize diverse IBDV genogroups and to better understand the relationship between HVR sequence, structure, and antigenicity. Here, we used a chicken B-cell line to rescue a panel of chimeric IBDVs with the HVR from seven diverse IBDV field strains and to conduct neutralization assays and protein modeling. We evaluated the ability of sera from vaccinated or infected birds to neutralize the different genogroups. Our novel chicken B-cell rescue system and neutralization assay can be used to screen IBDV vaccine candidates, platforms, and regimens for the breadth of neutralizing antibody responses elicited, evaluate the antigenic relatedness of diverse IBDV strains, and when coupled with structural modeling, elucidate immunodominant and conserved epitopes to strategically design novel IBDV vaccines in the future.

Soluble overexpression and purification of infectious bursal disease virus capsid protein VP2 in Escherichia coli and its nanometer structure observation

As part of the development of an infectious bursal disease virus (IBDV) subunit vaccine, this study was designed to improve the expression of highly soluble VP2-LS3 (Haemophilus parasuis lumazine synthase 3, LS3) protein by using different tagged vectors in E. coli. IBDV VP2-LS3 gene was designed and synthesized. Fusion tags, GST, NusA, MBP, Ppi, γ-crystallin, ArsC, and Grifin were joined to the N-terminus of VP2-LS3 protein. Seven expression plasmids were constructed, and each plasmid was transformed into E. coli BL21 (DE3) competent cells. After induction by IPTG, the solubility and expression levels of the various VP2-LS3 proteins were analyzed by SDS-PAGE and Western Blot analysis. The fusion tag that significantly promoted soluble expression of the VP2-LS3 protein was selected. Recombinant proteins were purified using Ni-NTA affinity chromatography, then cleaved by using TEV protease and detected by using transmission electron microscopy. Gel electrophoresis and sequencing analysis showed that all seven recombinant vectors were successfully constructed. GST, NusA, MBP, Ppi, γ-crystallin, ArsC, and Grifin enhanced the expression and solubility of VP2 protein; however, MBP was more effective for the high-purity production of VP2-LS3. Western Blot analysis confirmed successful generation of VP2-LS3 fusion protein in E. coli. The result of transmission electron microscopy showed that VP2-LS3 formed nano-sized particles with homogeneous shape and relatively uniform size. This study established a method to generate VP2-LS3 recombinant protein, which may lay a foundation for the development and subsequent study of IBDV subunit vaccines.

Infectious bursal disease virus in chickens: prevalence, impact, and management strategies

Infectious bursal disease (IBD), also known as Gumboro disease, is a highly contagious, immunosuppressive disease of young chickens. Although first observed about 60 years ago, to date, the disease is responsible for major economic losses in the poultry industry worldwide. IBD virus (IBDV), a double-stranded RNA virus, exists as two serotypes with only serotype 1 causing the disease in young chickens. The virus infects the bursa of Fabricius of particularly the actively dividing and differentiating lymphocytes of the B-cells lineage of immature chickens, resulting in morbidity, mortality, and immunosuppression. Immunosuppression enhances the susceptibility of chickens to other infections and interferes with vaccination against other diseases. Immunization is the most important measure to control IBD; however, rampant usage of live vaccines has resulted in the evolution of new strains. Although the immunosuppression caused by IBDV is more directed toward the B lymphocytes, the protective immunity in birds depends on inducement of both humoral and cell-mediated immune responses. The interference with the inactivated vaccine induced maternally derived antibodies in young chicks has become a hurdle in controlling the disease, thus necessitating the development of newer vaccines with improved efficacy. The present review illustrates the overall dynamics of the virus and the disease, and the recent developments in the field of virus diagnosis and vaccine research.

Advances in vaccine research against economically important viral diseases of food animals: Infectious bursal disease virus

Numerous reviews have been published on infectious bursal disease (IBD) and infectious bursal disease virus (IBDV). Many high quality vaccines are commercially available for the control of IBD that, when used correctly, provide solid protection against infection and disease caused by IBDV. Viruses are not static however; they continue to evolve and vaccines need to keep pace with them. The evolution of IBDV has resulted in very virulent strains and new antigenic types of the virus. This review will discuss some of the limitations associated with existing vaccines, potential solutions to these problems and advances in new vaccines for the control of IBD.

Tubular Bioreactor for Probing Baculovirus Infection and Protein Production

Probing the baculovirus infection process is essential in optimizing recombinant protein production. Typically, researchers monitor the infection process in stirred reactors that contain cells that have been infected at different times after virus inoculation, particularly if cells pass the primary infection and become infected by progeny virus. This chapter describes several alternative bioreactor systems for baculovirus infection. We provide an example alternative system that holds promise to avoid asynchronous distributions in infection time. Namely, we describe a two-stage reactor system consisting of an upstream continuous stirred tank reactor and a downstream tubular reactor with segmented plug flow for probing baculovirus infection and production.

Efficient self-assembly and protective efficacy of infectious bursal disease virus-like particles by a recombinant baculovirus co-expressing precursor polyprotein and VP4

Background

Virus-like particle (VLP) technology is considered one of the most promising approaches in animal vaccines, due to the intrinsic immunogenic properties as well as high safety profile of VLPs. In this study, we developed a VLP vaccine against infectious bursal disease virus (IBDV), which causes morbidity and mortality in chickens, by expressing a baculovirus in insect cells.

Methods

To improve the self-proteolytic processing of precursor polyprotein (PP), we constructed a recombinant baculovirus transfer vector that co-expresses PP and the VP4 protease gene of IBDV.

Results

Expression and VLP assembly of recombinant proteins and antigenicity of the VLP were examined by Western blotting, ELISA, and transmission electron microscopy. In animal experiments, vaccination with the recombinant VLP induced strong and uniform humoral immunity and provided complete protection against challenge with very virulent (vv) IBDV in SPF chickens (n = 12). As determined by the bursa of Fabricius (BF)/body weight (B/BW) ratio, the protection against post-challenge bursal atrophy was significantly higher (P < 0.001) in VLP-vaccinated birds than in non-vaccinated controls.

Conclusions

Since the protective efficacy of the VLP vaccine was comparable to that of a commercially available inactivated vaccine, the recombinant VLP merits further investigation as an alternative means of protection against vvIBD.

Development and evaluation of a Salmonella typhimurium flagellin based chimeric DNA vaccine against infectious bursal disease of poultry

Infectious bursal disease (IBD) is an acute immunosuppressive disease of young chicks, caused by a double-stranded RNA virus. VP2 being the major capsid protein of the virus is an ideal vaccine candidate possessing the neutralizing epitopes. The present study involves the use of flagellin (fliC) as a genetic adjuvant to improve the immune response of VP2 based DNA vaccine against IBD. Our findings revealed that birds immunized with plasmid pCIVP2fliC showed robust immune response than pCIVP2 immunized groups. Further, challenge study proved that genetic fusion of fliC and VP2 can provide a comparatively higher level of protection against vvIBDV challenge in chickens than VP2 alone. These results thus indicate that Salmonella flagellin could enhance the immune responses and protection efficacy of a DNA vaccine candidate against IBDV infection in chickens, highlighting the potential of flagellin as a genetic adjuvant in the prevention of vvIBDV infection.

Multivalent virus-like-particle vaccine protects against classic and variant infectious bursal disease viruses

Nucleotide sequences that encode the pVP2 proteins from a variant infectious bursal disease virus (IBDV) strain designated USA08MD34p and a classic IBDV strain designated Mo195 were produced with the use of reverse-transcriptase-polymerase chain reaction (RT-PCR) and cloned into a pGEM-T Easy vector. A nucleotide sequence that encodes the VP3 protein was also produced from the USA08MD34p viral genome with the use of RT-PCR and cloned into a pGEM-T Easy vector. The VP3 and pVP2 clones were inserted into the pVL1393 baculovirus transfer vector and sequenced to confirm their orientation to the promoter and to ensure they contained uninterrupted open reading frames. Recombinant baculoviruses were constructed by transfection in Sf9 cells. Three recombinant baculoviruses were produced and contained the USA08MD34p-VP3, USA08MD34p-pVP2, or Mo195-pVP2 genomic sequences. Virus-like particles (VLPs) were observed with the use of transmission electron microscopy when the USA08MD34p-VP3 baculovirus was co-inoculated into Sf9 cells with either of the pVP2 constructs. VLPs were also observed when the USA08MD34p-pVP2 and Mo195-pVP2 were coexpressed with USA08MD34p-VP3. These multivalent VLPs contained both classic and variant pVP2 molecules. Stability tests demonstrated the VLPs were stable at 4 and 24 C for 8 wk. The USA08MD34p, Mo195, and multivalent VLPs were used to vaccinate chickens. They induced an IBDV-specific antibody response that was detected by enzyme-linked immunosorbent assay (ELISA), and virus-neutralizing antibodies were detected in vitro. Chickens vaccinated with the multivalent VLPs were protected from a virulent variant IBDV strain (V1) and a virulent classic IBDV strain (STC). The results indicate the multivalent VLPs maintained the antigenic integrity of the variant and classic viruses and have the potential to serve as a multivalent vaccine for use in breeder-flock vaccination programs.

Purification of a recombinant protein produced in a baculovirus expression system by immobilized metal affinity chromatography

Over the past 10 years, the baculovirus-insect cell system has become a powerful and versatile tool for the expression of a variety of heterologous proteins. In order to simplify separation of a cloned protein from the baculovirus-insect expression system, we have cloned a gene encoding for the protein of interest, a structural protein (VP2) of a strain (E/DEL) of infectious bursal disease virus (IBDV), with a metal ion binding site (His)(5) at its C-terminus. This chimeric protein (VP2H) has been expressed and one-step affinity purified with immobilized metal ions (Ni(+2)). With antigen capture-enzyme-linked immunosorbent assay (AC-ELISA), we determined that the conformation of this chimeric protein was no different from the recombinant wild-type VP2 protein. However, the two proteins (VP2 and VP2H) can be distinguished and resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and detected immunologically following Western blotting. (c) 1994 John Wiley & Sons, Inc.

A tubular segmented-flow bioreactor for the infection of insect cells with recombinant baculovirus

A continuous process of insect cell (S f9) growth and baculovirus infection is tested with the sequential combination of a CSTR and a tubular reactor. A tubular infection reactor enables continuous introduction of baculovirus and therefore avoids the 'passage effect' observed in two-stage CSTR systems. Moreover, a tubular reactor can be used to test cell infection kinetics and the subsequent metabolism of infected insect cells. Unlike batch and CSTR culture, cells in a horizontally positioned tubular reactor settle due to poor mixing. We have overcome this problem by alternately introducing air bubbles and media and by maintaining a linear velocity sufficient to keep cells suspended. This article addresses the development of the tubular reactor and demonstrates its use as an infection system that complements the two-stage CSTR.

Current status of vaccines against infectious bursal disease

Infectious bursal disease virus (IBDV) is the aetiological agent of the acute and highly contagious infectious bursal disease (IBD) or "Gumboro disease". IBD is one of the economically most important diseases that affects commercially produced chickens worldwide. Along with strict hygiene management of poultry farms, vaccination programmes with inactivated and live attenuated viruses have been used to prevent IBD. Live vaccines show a different degree of attenuation; many of them may cause bursal atrophy and thus immunosuppression with poor immune response to vaccination against other pathogens and an increase in vulnerability to various types of infections as possible consequences. Depending on their intrinsic characteristics or on the vaccination procedures, some of the vaccines may not induce full protection against the very virulent IBDV strains and antigenic variants observed in the last three decades. As chickens are most susceptible to IBDV in their first weeks of life, active immunity to the virus has to be induced early after hatching. However, maternally derived IBDV-specific antibodies may interfere with early vaccination with live vaccines. Thus new technologies and second-generation vaccines including rationally designed and subunit vaccines have been developed. Recently, live viral vector vaccines have been licensed in several countries and are reaching the market. Here, the current status of IBD vaccines is discussed.

Virus-like particles of hepatitis B virus core protein containing five mimotopes of infectious bursal disease virus (IBDV) protect chickens against IBDV

Current infectious bursal disease virus (IBDV) vaccines suffer from maternal antibody interference and mimotope vaccines might be an alternative. Previously we demonstrated an IBDV VP2 five-mimotope polypeptide, 5EPIS, elicited protective immunity in chickens. In the current study, the 5epis gene was inserted into a plasmid carrying human hepatitis B virus core protein (HBc) gene at its major immunodominant region site. The recombinant gene was efficiently expressed in Escherichia coli to produce chimeric protein HBc-5EPIS which self-assembles to virus-like particles (VLP). Two-week old specific-pathogen-free chickens were immunized intramuscularly with HBc-5EPIS VLP or 5EPIS polypeptide without adjuvant (50 μg/injection) on day 0, 7, 14 and 21. Anti-5EPIS antibody was first detected on day 7 and day 21 in HBc-5EPIS and 5EPIS groups, respectively; on day 28, anti-5EPIS titers reached 12,800 or 1600 by ELISA, and 3200 or 800 by virus neutralization assay in HBc-5EPIS and 5EPIS groups, respectively. No anti-5EPIS antibody was detected in the buffer control group throughout the experiment. Challenge on day 28 with a virulent IBDV strain (GX8/99) resulted in 100%, 40.0% and 26.7% survival for chickens immunized with HBc-5EPIS, 5EPIS and buffer, respectively. These data suggest epitope presentation on chimeric VLP is a promising approach for improving mimotope vaccines for IBDV.

Alternative bioreactor strategy for probing infection and production

Probing the baculovirus infection process is essential in optimizing the recombinant protein production. Typically, researchers monitor the infection process in the stirred tank reactor, which, however, contains a population of cells infected at different times after virus inoculation. This chapter describes a two-stage reactor system consisting of an upstream continuous stirred tank reactor and a downstream tubular reactor with segmented plug flow for probing baculovirus infection and production.

A recombinant turkey herpesvirus expressing chicken interleukin-2 increases the protection provided by in ovo vaccination with infectious bursal disease and infectious bronchitis virus

In ovo vaccination remains an attractive option for the mass application of vaccines to poultry, ensuring a uniform application of vaccine in a cost-effective manner. However, the number of vaccines that can be delivered safely by this method is limited. Several infectious bursal disease virus (IBDV) vaccines can be given in ovo though most are delivered post-hatch and there are no currently licensed embryo-safe infectious bronchitis virus (IBV) vaccines. Reduction in the dose of vaccines given in ovo is one possibility to ensure embryo safety though efficacy can be reduced when low doses are used. We have investigated the use of embryo-safe IBDV and IBV vaccines and the effects of co-delivery of a turkey herpesvirus recombinant expressing bioactive chicken IL-2 (IL-2/HVT). Co-delivery of the IL-2/HVT with low doses of the IBDV or IBV vaccines significantly increased the antibody response against these viruses. In addition the protection against challenge with virulent IBDV or IBV was increased significantly. This suggests that the co-delivery of IL-2/HVT with low doses of other vaccines in ovo may be one method to increase the number of vaccines that can be given safely and efficaciously via in ovo vaccination.

Development of a multi-mimotope peptide as a vaccine immunogen for infectious bursal disease virus

To explore the mimotope vaccine approach against infectious bursal disease virus (IBDV), five IBDV-specific monoclonal antibodies (mAbs) were prepared and their binding peptides were screened against a phage-displayed 12-mer peptide library. After three rounds of biopanning, 12 phages were selected for each mAbs and their specificity to IBDV was verified by sandwich and competitive inhibition ELISAs. Seven phages per mAb were sequenced and their amino acid sequences were deduced. The five representative sequences of mimotopes corresponding mAbs were determined. An artificial gene, designated 5epis (5 epitopes) and consisting of the five mimotopes arranged in tandem (F1-F7-B34-2B1-2G8) with four GGGS spacers, was chemically synthesized and cloned into a prokaryotic expression plasmid pET28b. The protein, designated r5EPIS, was efficiently expressed in Escherichia coli and showed a size of 10kDa in SDS-PAGE. The r5EPIS protein reacted with anti-IBDV mAbs and polyclonal antibodies in Western blot immunoassays. Immunization of SPF chickens with r5EPIS protein (with Freund adjuvant, 50mug per injection on day 0 and 14) evoked high levels of antibody (12,800 by ELISA/1600 by virus neutralizing assay at day 21) and protected 100% of the chickens against a challenge of 200 ELD(50) of IBDV GX8/99 strain, which sharply contrasted with the, respectively, 13.3% and 6.6% survival rate in the adjuvant group and the untreated group. The multi-mimotope protein r5EPIS promises to be a novel subunit vaccine candidate for IBDV.

VP1 protein of infectious bursal disease virus modulates the virulence in vivo

Infectious bursal disease viruses (IBDVs), belonging to the Birnaviridae family, cause severe immunodeficiency in young chickens by destroying the precursors of antibody-producing B cells in the bursa of Fabricius (BF). Different pathotypes of IBDVs, including cell culture-adapted viruses, differ markedly in virulence, which is characterized by mortality and bursal damage. To study the molecular determinants of virulence in IBDV, the genomic segments A and B of GLS bursa-derived (GLSBD) and tissue culture-adapted (GLSTC) viruses were cloned and sequenced. Comparison of the deduced amino acid sequences of segments A and B revealed only two amino acid substitutions at positions 87 (Q --> R) and 261 (P --> L) in segment B, and at positions 253 (Q --> H) and 284 (A --> T) in segment A; the latter of which has been shown to be involved in tissue culture adaptation and attenuation of the virus. To study the function of VP1 protein encoded by segment B, reassortant viruses between tissue culture-adapted strains, GLSTC and D78, and GLSBD were recovered using the reverse genetics system. The recombinant virus rGLSBDB containing segment B of GLSBD was able to replicate in Vero and chicken embryo fibroblast (CEF) cells but exhibited delayed replication kinetics. To evaluate the characteristics of these viruses in vivo, 3-week-old chickens were given equal doses of parental viruses or reassortant viruses by ocular inoculation. The pathological lesions and viral antigen distribution in BF were analyzed at 1, 2, or 3 days postinfection. Parental GLSBD and the recovered rGLSBDB viruses propagate most efficiently in the BF and cause severe bursal lesions, whereas the tissue culture-adapted GLSTC virus replicates less efficiently and induces mild bursal lesions at 3 days postinfection. Taken together, our results demonstrate that the VP1 protein of IBDV is involved in the efficiency of viral replication and modulates the virulence in vivo.

A recombinant Newcastle disease virus (NDV) expressing VP2 protein of infectious bursal disease virus (IBDV) protects against NDV and IBDV

Infectious bursal disease virus (IBDV) causes a highly immunosuppressive disease in chickens. Currently available, live IBDV vaccines can lead to generation of variant viruses. We have developed an alternative vaccine that will not create variant IBDV. By using the reverse genetics approach, we devised a recombinant Newcastle disease virus (NDV) vector from a commonly used vaccine strain LaSota to express the host-protective immunogen VP2 of a variant IBDV strain GLS-5. The gene encoding the VP2 protein of the IBDV was inserted into the most 3'-proximal locus of a full-length NDV cDNA for high-level expression. We successfully recovered the recombinant virus, rLaSota/VP2. The rLaSota/VP2 was genetically stable, at least up to 12 serial passages in chicken embryos, and was shown to express the VP2 protein. The VP2 protein was not incorporated into the virions of recombinant virus. Recombinant rLaSota/VP2 replicated to a titer similar to that of parental NDV strain LaSota in chicken embryos and cell cultures. To assess protective efficacy of the rLaSota/VP2, 2-day-old specific-pathogen-free chickens were vaccinated with the recombinant virus and challenged with a highly virulent NDV strain Texas GB or IBDV variant strain GLS-5 at 3 weeks postvaccination. Vaccination with rLaSota/VP2 generated antibody responses against both NDV and IBDV and provided 90% protection against NDV and IBDV. Booster immunization induced higher levels of antibody responses against both NDV and IBDV and conferred complete protection against both viruses. These results indicate that the recombinant NDV can be used as a vaccine vector for other avian pathogens.

Research on infectious bursal disease--the past, the present and the future

Infectious bursal disease (IBD) virus (IBDV) is the etiological agent of "Gumboro disease". Although first observed about 40 years ago, this disease continues to pose an important threat to the commercial poultry industry. The emergence of antigenic variant as well as very virulent strains in vaccinated flocks considerably stimulated research efforts on both, IBD and IBDV. In this review, some of the recent advances in the understanding of the structure, morphogenesis and molecular biology of the virus as well as in development of new diagnostic approaches and new strategies for vaccination against IBD are briefly summarized.

Vaccination with E. coli recombinant empty viral particles of infectious bursal disease virus (IBDV) confer protection

The A genome segment of the highly virulent Infectious bursal disease virus (IBDV) was amplified using long and accurate-RT-PCR (LA-RT-PCR). The entire sequence region encoding VP2, VP4, and VP3 in that order was cloned and sequenced. Following subcloning into the Escherichia coli expression vector pET21a under the T7 promoter, viral proteins were expressed and processed as demonstrated by Western blot analysis. Virus-like particles could be visualized by immuno-electron microscopy in IPTG-induced cells suggesting that viral assembly can take place in E. coli. Induction of anti-IBDV antibodies was detected in chickens immunized with purified recombinant IBDV by intra muscular (i.m.) injection. Furthermore, the vaccinated chickens were protected when challenged with the Gep 5 isolate of IBDV.

Expression of VP2 gene protein of infectious bursal disease virus detected in Korea

The VP2 gene DNA (1.4 kb in approximate) of a very virulent infectious bursal disease virus (vvIBDV) Chinju strain detected in Chinju, Korea was cloned into the bacmid, a baculovirus shuttle vector, through transposition of the gene from initially cloned pFastBacHTa plasmid, a baculovirus expression vector, and was subsequently expressed in Spodoptera frugiperda (Sf) cells. Biological properties of the expressed VP2 subunit protein were characterized to aid in the development of genetically engineered diagnostic reagents and vaccines against the vvIVDV. When the VP2 DNA-recombinant bacmid was transfected and propagated in the Sf cells, the cells showed no occlusion formation, which is a positive evidence for the insertion of the VP2 DNA into the polyhedrin gene of the bacmid, whereas the occlusions were observed in the cells infected by the Autographa californica nuclear polyhedrosis virus, a wild baculovirus. The expression of VP2 DNA was identified by strong positive reaction in fluorescent antibody test using chicken anti-IBDV serum. The VP2 protein was determined as a polypeptide band with Mr of 48 kDa by the sodium dodecyl-polyacrylamide gel electrophoresis for the lysate of the Sf cells infected with the recombinant bacmid. The VP2 protein was successfully purified from the cell lysate by Ni-NTA affinity chromatography. The expressed VP2 subunit protein reacted specifically with chicken anti-IBDV serum in Western blotting.

Structure-dependent efficacy of infectious bursal disease virus (IBDV) recombinant vaccines

The immunogenicity and protective capability of several baculovirus-expressed infectious bursal disease virus (IBDV)-derived assemblies as VP2 capsids, VPX tubules and polyprotein (PP)-derived mixed structures, were tested. Four-week-old chickens were immunised subcutaneously with one dose of each particulate antigen. VP2 icosahedral capsids induced the highest neutralising response, followed by PP-derived structures and then VPX tubules. All vaccinated animals were protected when challenged with a very virulent IBDV (vvIBDV) isolate, however the degree of protection is directly correlated with the levels of neutralising antibodies. VP2 capsids elicited stronger protective immunity than tubular structures and 3 micrograms of them were sufficient to confer a total protection comparable to that induced by an inactivated vaccine. Therefore, VP2 capsids represent a suitable candidate recombinant vaccine instead of virus-like particles (VLPs) for IBDV infections. Our results also provide clear evidence that the recombinant IBDV-derived antigens are structure-dependent in order to be efficient as vaccine components.

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.

Cloning, expression, and characterization of avian reovirus guanylyltransferase

We have cloned and sequenced the L3 genome segment of avian reovirus strain 1733, which specifies the viral guanylyltransferase protein, lambdaC. The L3 gene is 3907 nucleotides long and encodes, in a single large open-reading frame, a polypeptide of 1285 amino acid residues, with a calculated M(r) of 142.2 kDa. Expression of this gene in a baculovirus/insect cell system produced a recombinant protein that comigrated with reovirion lambdaC and reacted with anti-reovirus polyclonal serum in a Western blot assay. Incubation of recombinant lambdaC with GTP led to the formation GMP-lambdaC complex via a phosphoamide linkage. Interestingly, a 42-kDa amino-terminal proteolytic fragment of recombinant lambdaC protein also exhibited autoguanylylation activity, demonstrating both that this fragment is necessary and sufficient for autoguanylylation activity and that the 100-kDa complementary fragment is expendable for that activity. Comparison of the deduced amino acid sequence of protein lambdaC with those of the mammalian and grass carp reovirus guanylyltransferases revealed that only two of eight lysine residues within the amino-terminal 42-kDa region are conserved. Interestingly, these two lysines match with the lysine residues in the mammalian reovirus capping enzyme proposed to be essential for autoguanylylation activity. Our alignment analysis also showed that the S-adenosyl-l-methionine-binding pocket previously detected in the mammalian reovirus capping enzyme is fully conserved in its avian and grass carp reovirus counterparts, suggesting that all three enzymes have methylase activity.

The maturation process of pVP2 requires assembly of infectious bursal disease virus capsids

Infectious bursal disease virus (IBDV) is a nonenveloped avian virus with a two-segment double-stranded RNA genome. Its T=13 icosahedral capsid is most probably assembled with 780 subunits of VP2 and 600 copies of VP3 and has a diameter of about 60 nm. VP1, the RNA-dependent RNA polymerase, resides inside the viral particle. Using a baculovirus expression system, we first observed that expression of the pVP2-VP4-VP3 polyprotein encoded by the genomic segment IBDA results mainly in the formation of tubules with a diameter of about 50 nm and composed of pVP2, the precursor of VP2. Very few virus-like particles (VLPs) and VP4 tubules with a diameter of about 25 nm were also identified. The inefficiency of VLP assembly was further investigated by expression of additional IBDA-derived constructs. Expression of pVP2 without any other polyprotein components results in the formation of isometric particles with a diameter of about 30 nm. VLPs were observed mainly when a large exogeneous polypeptide sequence (the green fluorescent protein sequence) was fused to the VP3 C-terminal domain. Large numbers of VLPs were visualized by electron microscopy, and single particles were shown to be fluorescent by standard and confocal microscopy analysis. Moreover, the final maturation process converting pVP2 into the VP2 mature form was observed on generated VLPs. We therefore conclude that the correct scaffolding of the VP3 can be artificially induced to promote the formation of VLPs and that the final processing of pVP2 to VP2 is controlled by this particular assembly. To our knowledge, this is the first report of the engineering of a morphogenesis switch to control a particular type of capsid protein assembly.

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.

Effect of MOI ratio on the composition and yield of chimeric infectious bursal disease virus-like particles by baculovirus co-infection: deterministic predictions and experimental results

Virus-like particles (VLPs) are empty particles consisting of virus capsid proteins that closely resemble native virus but are devoid of the native viral nucleic acids and therefore have attracted significant attention as noninfectious vaccines. A recombinant baculovirus, vIBD-7, which encodes the structural proteins (VP2, VP3, and VP4) of infectious bursal disease virus (IBDV), produces native IBD VLPs in infected Spodoptera frugiperda insect cells. Another baculovirus, vEDLH-22, encodes VP2 that is fused with a histidine affinity-tag (VP2H) at the C-terminus. By co-infection with these two baculoviruses, hybrid VLPs with histidine tags were formed and purified by immobilized metal affinity chromatography (Hu et al., 1999). Also, we demonstrated that varying the MOI ratio of these infecting viruses altered the extent of VP2H incorporated into the particles. A dynamic mathematical model that described baculovirus infection and VLP synthesis (Hu and Bentley, 2000) was adapted here for co-infection and validated by immunofluorescence labeling. It was shown to predict the VLP composition as a dynamic function of MOI. A constraint in the VP2H content incorporated into the particles was predicted and shown by experiments. Also, the MOI ratio of both infecting viruses was shown to be the major factor influencing the composition of the hybrid particles and an important factor in determining the overall yield. ELISA results confirmed that VP2H was exhibited to a varied extent on the outer surface of the particles. This model provides insight on the use of virus co-infection in virus-mediated recombinant protein expression systems and aids in the optimization of chimeric VLP synthesis.

Recombinant Semliki Forest virus vector exhibits potential for avian virus vaccine development

The Semliki Forest virus (SFV) expression system was evaluated as a basis for avian vaccine development. Initial studies indicated that 1-day-old specific pathogen-free (SPF) chicks were susceptible to infection with an infectious strain of SFV, producing SFV-specific antibodies but no clinical disease. One-day-old SPF chicks immunised intramuscularly with recombinant replication-defective SFV (rSFV) particles expressing the Escherichia coli (E. coli) lacZ reporter gene developed high titres of beta-gal- specific antibodies at 4 weeks p.i. after two inoculations. In contrast, significantly lower antibody levels were elicited in chicks immunised with a recombinant SFV-based DNA construct or a conventional CMV promoter-based DNA plasmid. rSFV particles encoding the protective VP2 protein or the VP2/VP4/VP3 polyprotein of infectious bursal disease virus (IBDV) were produced and the expressed antigens were characterised in cell culture. Proteins of the correct size were generated and found to react against a range of IBDV-specific monoclonal antibodies. Immunisation of 1-day-old SPF chicks with rSFV particles encoding the IBDV proteins resulted in specific antibodies being elicited in all birds, neutralising antibodies being induced in some but not all birds.

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.

Chimeric infectious bursal disease virus-like particles expressed in insect cells and purified by immobilized metal affinity chromatography

Chimeric virus-like particles (VLPs) of infectious bursal disease virus (IBDV) were produced by coinfecting Spodoptera frugiperda (Sf-9) insect cells with two recombinant baculoviruses, vIBD-7 and vEDLH-22. vIBD-7 encodes VP2, VP3, and VP4 of the IBDV structural proteins. vEDLH-22 encodes VP2 with five histidine residues at the carboxy-terminus (VP2H). Coinfection produced hybrid VLPs composed of VP2, VP2H, and VP3. The additional histidine residues on VP2H enabled the efficient purification of VLPs based on immobilized metal affinity chromatography (IMAC). These results demonstrated that the VLPs formed are comprised of chimeric subunits with attached affinity ligands, and further, that sufficient His5 ligand was available for binding to the IMAC metal-chelating resin. Additionally, these novel particles were fully characterized for antigenicity by a series of monoclonal antibodies, and appeared identical to the two wild-type IBDV strains contributing subunits to the chimeric VLP. IMAC purification provides a promising low-cost and simple scheme to purify VLPs as vaccines.

Protection of chickens against very virulent infectious bursal disease virus (IBDV) and Marek's disease virus (MDV) with a recombinant MDV expressing IBDV VP2

To develop a herpes virus vaccine that can induce immunity for an extended period, a recombinant Marek's disease (MD) virus (MDV) CVI-988 strain expressing infectious bursal disease virus (IBDV) host-protective antigen VP2 at the US2 site (rMDV) was developed under the control of an SV40 early promoter. Chickens vaccinated with the rMDV showed no clinical signs and no mortality and 55% of the chickens were considered protected histopathologically after challenge with very virulent IBDV (vvIBDV), whereas all of the chickens vaccinated with the conventional IBDV vaccine showed no clinical signs and were protected. Chickens vaccinated with the CVI-988 or chickens in the challenge control showed severe clinical signs and high mortality (70-75%) and none of them were protected. Also, the rMDV conferred full protection to chickens against vvMDV just as the CVI-988 strain did, whereas 90% of the challenge control chickens died of MD. Antibody levels against IBDV and MDV following the vaccination increased continuously for at least 10 weeks. No histopathological lesions in the rMDV-vaccinated chickens and no contact transmission of the rMDV to their penmates were confirmed. These results demonstrate that an effective and safe recombinant herpesvirus-based IBD vaccine could be constructed by expressing the VP2 antigen at the US2 site of the CVI-988 vaccine strain.

Introduction to poultry vaccines and immunity

The poultry industry constitutes a significant sector of world agriculture. In the United States, more than 8 billion birds are produced yearly with a value exceeding $20 billion. Broiler chickens are the largest segment of the industry. Birds raised under commercial conditions are vulnerable to environmental exposure to a number of pathogens. Therefore, disease prevention by vaccination is an integral part of flock health management protocols. Active immunization using live vaccines is the current industry standard. Routinely used vaccines in chickens include MDV, NDV, IBV, and IBDV, and in turkeys NDV and HEV. Newer vaccines, including molecular recombinants in which genes of immunogenic proteins from infectious agents are inserted into a live viral vector, are also being examined for commercial use. Efforts are under way to enhance vaccine efficacy by the use of adjuvants, particularly cytokines. The vaccine delivery systems include in ovo injection, aerosol, spray, drinking water, eye drop, and wing web injection. The in ovo vaccination procedure is relatively new and at the present time it is used primarily to vaccinate broiler chickens against MDV. Birds respond to vaccines by developing humoral and cellular immune responses. Bursa of Fabricius and the thymus serve as the primary lymphoid organs of the immune system. B cells use surface immunoglobulins as antigen receptors and differentiate into plasma cells to secrete antibodies. Three classes of antibodies are produced: IgM, IgG (also called IgY), and IgA. Successful vaccinal response in a flock is often monitored by demonstrating a rise in antibody titer within a few days of vaccination. ELISA is used most commonly for serologic monitoring. T cells are the principal effector cells of specific cellular immunity. T cells differentiate into alpha beta and gamma delta cells. In adult birds, gamma delta cells may constitute up to 50% of the circulating T cells. Functionally, CD4+ cells serve as helper cells and CD8+ cells as cytotoxic/suppressor cells.

Generation of a mutant infectious bursal disease virus that does not cause bursal lesions

A reverse genetics system for birnavirus, based on synthetic transcripts of the infectious bursal disease virus (IBDV) genome, was recently developed (E. Mundt and V. N. Vakharia, Proc. Natl. Acad. Sci. USA 93:11131-11136, 1996). To study the function of the 17-kDa nonstructural (NS) protein in viral growth and pathogenesis, we constructed a cDNA clone of IBDV segment A in which the first and only initiation codon (ATG) of NS protein was mutated to a stop codon (TAG). Transfection of Vero cells with combined transcripts of either modified or unmodified segment A, and with segment B, generated viable IBDV progeny. When chicken embryo fibroblast cells infected with transfectant viruses were analyzed by immunofluorescence assays using NS-specific antiserum, the mutant virus did not yield a fluorescence signal, indicating a lack of NS protein expression. Furthermore, replication kinetics and cytotoxic effects of the mutant virus were compared with those of the parental attenuated vaccine strain of IBDV (D78) in vitro. The mutant virus grew to slightly lower titers than D78 virus and exhibited decreased cytotoxic and apoptotic effects in cell culture. To evaluate the characteristics of the recovered viruses in vivo, we inoculated 3-week-old chickens with D78 or mutant virus and analyzed their bursa for histopathological lesions. The recovered D78 virus caused microscopic lesions and atrophy of the bursa, while the mutant virus failed to induce any pathological lesions or clinical signs of disease. In both instances, the virus was recovered from the bursa, and the presence or absence of mutation in these viruses was confirmed by nucleotide sequence analysis of NS gene. Although the mutant virus exhibited a delay in replication in vivo, it induced levels of IBDV neutralizing antibodies that were similar to those of D78 virus. In addition, no reversion of mutation was detected in the mutant virus recovered from inoculated chickens. These results demonstrate that NS protein is dispensable for viral replication in vitro and in vivo and that it plays an important role in viral pathogenesis. Thus, generation of such NS protein-deficient virus will facilitate the study of immunosuppression and aid in the development of live-attenuated vaccines for IBDV.

Enzyme-linked immunosorbent assay-based detection of antibodies to antigenic subtypes of infectious bursal disease viruses of chickens

An enzyme-linked immunosorbent assay (ELISA) using a fragment of the infectious bursal disease virus (IBDV) VP2 gene expressed in baculovirus was developed. A 944-bp portion of the VP2 gene from the Del-A strain of IBDV was ligated into the pAc360 transfer vector and transfected into baculovirus, Recombinant baculoviruses were identified by dot blot hybridization. The recombinant baculovirus 9A5 expressed a 57-kDa VP2 fusion protein, which was immunoprecipitated. This baculovirus-expressed VP2 was used as an antigen in an ELISA (Ohio State University [OSU]-ELISA). Titers of sera from specific-pathogen-free chickens infected with different strains of IBDV in our laboratory were determined by the OSU-ELISA, commercial ELISAs, and a virus neutralization assay. The results indicate that all sera from the specific-pathogen-free chickens infected with IBDV strains contained high titers of neutralizing antibodies. Each of these antisera also tested positive with the commercial ELISA kits. The OSU-ELISA did not detect antibodies to all strains of IBDV tested. This ELISA detected antibodies to the antigenically similar Delaware variant strains Del-A, Del-E, and GLS but did not detect or detected very poorly antibodies to antigenically heterologous classic IBDV strains STC, D78, and BVM. Although the antiserum to the IN strain of IBDV contained a virus-neutralizing antibody titer, the OSU-ELISA did not detect antibodies in this serum, suggesting IN is antigenically heterologous to Del-A. Titers to the IN strain were detected with the commercial ELISA kits. The OSU-ELISA detected antibodies to the SAL strain, suggesting this virus is antigenically similar to Del-A, which is supported by previously reported vaccination and challenge studies. In conclusion, the OSU-ELISA could be used to detect antibodies to a subgroup of IBDV strains, while commercially available ELISA kits detected antibodies to all the antigenic subtypes of IBDV tested.

Recombinant rabbit hemorrhagic disease virus capsid protein expressed in baculovirus self-assembles into viruslike particles and induces protection

VP60, the unique component of rabbit hemorrhagic disease virus capsid, was expressed in the baculovirus system. The recombinant VP60, released in the supernatant of infected insect cells, assembled without the need of any other viral component to form viruslike particles (VLPs), structurally and immunologically indistinguishable from the rabbit hemorrhagic disease virion. Intramuscular vaccination of rabbits with the VLPs conferred complete protection in 15 days; this protection was found to be effective from the fifth day after VLP injection and was accompanied by a strong humoral response.

Poultry vaccines of the future

Current poultry vaccines are based either on live attenuated organisms or on killed organisms. Future vaccines also may be based on deletion mutants, live viral or bacterial vectors that express foreign genes, and naked DNA. Vaccines have different purposes, depending on the disease, which govern their intrinsic characteristics. Improvement of vaccine efficacy can be addressed by modifications of the vaccine and its administration, modifications in the capacity of the host to mount an immune response, and modifications of environmental factors. The concept of "designer vaccines" for matching vaccines that deliver specific antigenic peptides to chickens with the MHC haplotype that best presents those peptides to T cells is discussed.

Active and passive protection against variant and classic infectious bursal disease virus strains induced by baculovirus-expressed structural proteins

Infectious bursal disease virus (IBDV) is responsible for a highly immunosuppressive disease in young chickens which causes significant economic losses to the poultry industry worldwide. The structural protein genes (VP2, VP3 and VP4) of a variant IBDV strain (GLS) were expressed in insect cells using a baculovirus expression system. Susceptible chickens vaccinated with a single dose of the recombinant IBDV antigens were completely protected against challenge with two variant strains of IBDV (GLS and Delaware), and partially protected against the standard challenge strain (STC). A booster dose of the recombinant antigens induced higher levels of neutralizing antibodies and afforded complete protection against both variant and standard virus challenges. Specific-pathogen-free hens vaccinated with a single dose of the same subunit vaccine produced virus-neutralizing antibodies that were capable of passively protecting the progeny from infection with variant IBDV.

Molecular basis of antigenic variation in infectious bursal disease virus

Four antigenically different strains of infectious bursal disease virus (IBDV), characterized by their reactivities with a panel of neutralizing monoclonal antibodies (MAbs), were selected to determine the molecular basis of antigenic variation. The large genome segment A, encoding the structural proteins of the U.S. variants GLS, DS326, E/Del and the vaccine strain D78, was cloned and sequenced. Comparison of the deduced amino acid sequences of the U.S. variants with other IBDV strains showed that most of the amino acid substitutions occur in the central region between residues 212 to 332, especially in the two hydrophilic regions between residues 212 to 223 and residues 314 to 324 of VP2 protein. By comparing the amino acid sequences of these variant viruses and their reactivities with IBDV specific MAbs, the putative amino acids involved in the formation of virus-neutralizing epitopes were identified. Comparison of the D78 versus PBG98 sequence showed that Gln at position 249 (Gln249) appears to be critical in binding with MAb B69. Similarly, comparison of the U.S. variant sequences with other serotype 1 sequences showed unique substitution(s) at residue Glu321 in GLS, residues Ile286, Asp318, Glu323 in E/Del, and residues Glu311 and Gln320 in DS326, which could be potential residue(s) involved in the recognition of MAb57, MAb67, and MAb179 epitopes, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)