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

Oral delivery of Eimeria acervulina transfected sequentially with two copies of the VP2 gene induces immunity against infectious bursal disease virus in chickens

Chicken coccidiosis caused by Eimeria spp. can occur on almost all poultry farms, causing huge economic losses to the industry. Genetically manipulated Eimeria parasites as a vaccine vector to deliver viral antigens have been reported. In our preliminary study, transgenic E. acervulina expressing a VP2 gene (Ea-VP2) of the infectious bursal disease virus (IBDV) demonstrated partial protection against IBDV infection. To enhance immune responses, we aimed to increase the VP2 gene copy number in transgenic E. acervulina. In this study, we used a novel plasmid vector carrying a VP2 gene fused with three flag tags and a red fluorescent reporter gene (mCherry). The vector was introduced into Ea-VP2 sporozoites through nucleofection, leading to the generation of Ea-2VP2. Subsequent analysis revealed a notable escalation in the fluorescent rate, increasing from 0.11 to 95.1% following four consecutive passages facilitated by fluorescent-activated cell sorting. Verification via PCR, Western blot, and immunofluorescence confirmed the successful construction of the Ea-2VP2 population. Despite lower fecundity compared to wild-type E. acervulina, Ea-2VP2 maintained immunogenicity. Our research effectively created a transgenic E. acervulina strain transfected sequentially with two copies of the VP2 gene from IBDV. This modification resulted in an increased humoral immune response after primary immunization in chickens. Additionally, it demonstrated a degree of protection within the bursa against IBDV infection. Future studies will focus on further enhancing immune response levels.

Virus-like Particle Vaccines of Infectious Bursal Disease Virus Expressed in Escherichia coli Are Highly Immunogenic and Protect against Virulent Strain

OBJECTIVES

Infectious bursal disease virus (IBDV) is a highly contagious, acutely infectious agent that causes immunosuppression in chickens. We expressed IBDV VP2 proteins in Escherichia coli (E. coli) to develop an effective virus-like-particles (VLPs) vaccine and evaluated its immunogenicity.

METHODS

The VLPs produced in E. coli were used as an immunogen mixed with a water-in-mineral-oil adjuvant (MontanideTM ISA 71 VG, ISA 71 RVG) or a white oil (7#) adjuvant. VLPs without an adjuvant, commercial subunit vaccine, inactivated vaccine, and attenuated vaccine were used as controls. These test vaccines were intramuscularly injected into 19-day-old SPF chickens, which were challenged with the IBDV virulent strain at 30 days after vaccination.

RESULTS

The adjuvants boosted antibody production, and the adjuvant groups (except white oil) produced higher antibody levels than the non-adjuvanted controls and the commercial vaccine groups. In terms of cellular immunity, the VLPs plus adjuvant combinations produced higher levels of cytokines, IL-2, IL-4, and IFN-γ than the controls.

CONCLUSION

IBDV VLPs plus the ISA 71 RVG adjuvant can be used as an optimal vaccine combination for improving the immune efficacy of IBD subunit vaccines, which can protect against the virulent strain.

A comprehensive review and meta-analysis of recent advances in biotechnology for plant virus research and significant accomplishments in human health and the pharmaceutical industry

Secondary metabolites made by plants and used through their metabolic routes are today's most reliable and cost-effective way to make pharmaceuticals and improve health. The concept of genetic engineering is used for molecular pharming. As more people use plants as sources of nanotechnology systems, they are adding to this. These systems are made up of viruses-like particles (VLPs) and virus nanoparticles (VNPs). Due to their superior ability to be used as plant virus expression vectors, plant viruses are becoming more popular in pharmaceuticals. This has opened the door for them to be used in research, such as the production of medicinal peptides, antibodies, and other heterologous protein complexes. This is because biotechnological approaches have been linked with new bioinformatics tools. Because of the rise of high-throughput sequencing (HTS) and next-generation sequencing (NGS) techniques, it has become easier to use metagenomic studies to look for plant virus genomes that could be used in pharmaceutical research. A look at how bioinformatics can be used in pharmaceutical research is also covered in this article. It also talks about plant viruses and how new biotechnological tools and procedures have made progress in the field.

The expression in plants of an engineered VP2 protein of Infectious Bursal Disease Virus induces formation of structurally heterogeneous particles that protect from a very virulent viral strain

Infectious Bursal Disease Virus (IBDV), the etiological agent of Gumboro disease, causes mortality and immunosuppression in chickens and major losses to poultry industry worldwide. The IBDV major capsid protein VP2 is considered the best candidate for the production of novel subunit vaccines. This structural protein contains the major conformational epitopes responsible for the induction of IBDV neutralizing antibodies in chickens and has been demonstrated able to form supramolecular structures in yeast and insect cells. The aim of this study was to express an engineered version of the VP2 protein (His-pVP2) to verify its ability to self-assemble into virus-like particles in plants. The recombinant VP2 was transiently expressed by agroinfiltration in Nicotiana benthamiana and transmission electron microscopy of sucrose density gradient fractions revealed the presence of a mixed population of differently shaped particles ranging from spherical capsids, with a diameter between ~25 and ~70 nm, to tubular structures, with variable length (from 100 to 400 nm). The recombinant VP2-based particles when used for the intramuscular immunization of specific-pathogen-free chicks resulted able to induce the production of anti-IBDV specific antibodies at titers comparable to those induced by a commercial vaccine. Moreover, all the immunized birds survived to the challenge with a Moroccan very virulent IBDV strain with no major histomorphological alterations of the Bursa of Fabricius, similarly to what obtained with the commercial inactivated vaccine.

Development of diagnostic tools for IBDV detection using plants as bioreactors

Infectious bursal disease virus (IBDV) is the etiological agent of an immunosuppressive and highly contagious disease that affects young birds, thus causing important economic losses in the poultry industry. Multimeric particles with different architectures based on the capsid protein VP2 have been widely produced for different purposes. We hereby show the production and easy recovery of IBDV subviral particles (SVP) from transiently transformed Nicotiana benthamiana. The SVP, which were observed by electronic microscopy, proved to be antigenically and immunogenically similar to the virion. Indeed, anti-IBDV antibodies from samples of infected birds recognized these SVP and, when injected intramuscularly, these subviral particles also evoked a humoral immune response in chickens. We developed an in-house ELISA using SVP as coating reagent that demonstrated to be highly accurate and in good agreement with a commercial ELISA. This study demonstrates that the recombinant antigen generated and the technology used to produce it are suitable for developing a diagnostic tool against Infectious bursal disease.

Current state-of-the-art in the use of plants for the production of recombinant vaccines against infectious bursal disease virus

Infectious bursal disease is a widely spread threatening contagious viral infection of chickens that induces major damages to the Bursa of Fabricius and leads to severe immunosuppression in young birds causing significant economic losses for poultry farming. The etiological agent is the infectious bursal disease virus (IBDV), a non-enveloped virus belonging the family of Birnaviridae. At present, the treatment against the spread of this virus is represented by vaccination schedules mainly based on inactivated or live-attenuated viruses. However, these conventional vaccines present several drawbacks such as insufficient protection against very virulent strains and the impossibility to differentiate vaccinated animals from infected ones. To overcome these limitations, in the last years, several studies have explored the potentiality of recombinant subunit vaccines to provide an effective protection against IBDV infection. In this review, we will give an overview of these novel types of vaccines with special emphasis on current state-of-the-art in the use of plants as "biofactories" (plant molecular farming). In fact, plants have been thoroughly and successfully characterized as heterologous expression systems for the production of recombinant proteins for different applications showing several advantages compared with traditional expression systems (Escherichia coli, yeasts and insect cells) such as absence of animal pathogens in the production process, improved product quality and safety, reduction of manufacturing costs, and simplified scale-up.

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.

A double recombinant herpes virus of turkeys for the protection of chickens against Newcastle, infectious laryngotracheitis and Marek's diseases

A double recombinant strain of herpes virus of turkeys (HVT) was constructed that contains the fusion (F) gene from Newcastle disease virus (NDV) and the gD plus gI genes from infectious laryngotracheitis virus (ILTV) inserted into a non-essential region of the HVT genome. Expression of the F protein was controlled by a human cytomegalovirus promoter, whereas expression of gD plus gI was driven by an ILTV promoter. The double recombinant vaccine virus (HVT-NDV-ILT) was fully stable genetically and phenotypically following extended passage in cell culture and infection of chickens. Safety of the vaccine virus was confirmed by overdose and backpassage studies in specific-pathogen-free chickens. Chickens vaccinated with a single dose of HVT-NDV-ILT administered by the in ovo route were highly protected from challenge with the velogenic NDV (GB Texas), ILTV (LT 96-3) and Marek's disease virus (GA 5) strains (97%, 94% and 97%, respectively). Similarly, chickens vaccinated with a single dose by subcutaneous (SC) route at 1 day of age were highly protected from challenge with the same three viruses (100%, 100%, and 88%, respectively). The protection level of a single dose given by in ovo or SC route against challenge with a virulent Marek's disease virus strain demonstrates that insertion of multiple genes from two different pathogens within the HVT genome had no adverse effect on the capacity of HVT to protect against Marek's disease. These results demonstrate that HVT-NDV-ILT is a safe and efficacious vaccine for simultaneous control of NDV, ILTV and Marek's diseases.

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.

Generation of porcine reproductive and respiratory syndrome (PRRS) virus-like-particles (VLPs) with different protein composition

The causative agent of Porcine Reproductive and Respiratory Syndrome (PRRS) is an enveloped ssRNA (+) virus belonging to the Arteriviridae family. Gp5 and M proteins form disulfide-linked heterodimers that constitute the major components of PRRSV envelope. Gp2, Gp3, Gp4 and E are the minor structural proteins, being the first three incorporated as multimeric complexes in the virus surface. The disease has become one of the most important causes of economic losses in the swine industry. Despite efforts to design an effective vaccine, the available ones allow only partial protection. In the last years, VLPs have become good vaccine alternatives because of safety issues and their potential to activate both branches of the immunological response. The characteristics of recombinant baculoviruses as heterologous expression system have been exploited for the production of VLPs of a wide variety of viruses. In this work, two multiple baculovirus expression vectors (BEVs) with PRRS virus envelope proteins were engineered in order to generate PRRS VLPs: on the one hand, Gp5 and M cDNAs were cloned to generate the pBAC-Gp5M vector; on the other hand, Gp2, Gp3, Gp4 and E cDNAs have been cloned to generate the pBAC-Gp234E vector. The corresponding recombinant baculoviruses BAC-Gp5M and BAC-Gp234E were employed to produce two types of VLPs: basic Gp5M VLPs, by the simultaneous expression of Gp5 and M proteins; and complete VLPs, by the co-expression of the six PRRS proteins after co-infection. The characterization of VLPs by Western blot confirmed the presence of the recombinant proteins using the available specific antibodies (Abs). The analysis by Electron microscopy showed that the two types of VLPs were indistinguishable between them, being similar in shape and size to the native PRRS virus. This system represents a potential alternative for vaccine development and a useful tool to study the implication of specific PRRS proteins in the response against the virus.

An optimized, highly efficient, self-assembled, subvirus-like particle of infectious bursal disease virus (IBDV)

Infectious bursal disease virus (IBDV) causes immunosuppression in young chickens, leading to increased susceptibility to other diseases and a reduction in the immune response to other vaccines. Thus, IBDV results in great economic losses to the poultry industry. The most effective method of prevention is vaccination. However, medium-virulence vaccines can cause bursal pathological damage and immunosuppression. Here, we describe a safer, self-assembled, subvirus-like particle (sVP) vaccine without a complex purification process. The IBD-VP2 gene was cloned into Pichia pastoris, and the expressed protein self-assembled into T=1 sVPs (∼23nm). Immunization experiments showed that the sVP vaccine elicited high IBDV-neutralizing antibodies in each group, and all birds survived challenge with very virulent IBDV (vvIBDV). Additionally, IBDV RNA was not detected, and sterile immunity was achieved. In conclusion, the IBD-sVP is a suitable candidate for a recombinant subunit vaccine against IBDV.

High level soluble expression and one-step purification of IBDV VP2 protein in Escherichia coli

OBJECTIVES

To improve the expression of soluble IBDV VP2 protein by using different tagged vectors in Escherichia coli.

RESULTS

Fusion tags, Grifin, MBP, SUMO, thioredoxin, γ-crystallin, ArsC and PpiB, enhanced the expression and solubility of VP2 protein. The fusion proteins were purified by Ni-NTA chromatography, MBP-VP2 showed the highest purity about 90 %. After removing the MBP tag, VP2 self-assembled into virus-like particles, ~25 nm diam. Results from AGP suggested the recombinant IBDV VP2 protein identified by reference serum like IBDV.

CONCLUSION

All the seven tags enhanced the expression and solubility of IBDV VP2 protein. The recombinant protein self-assembly into virus like particles and possess antigenicity as reference IBDV.

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.

Enhanced immune responses of chickens to oral vaccination against infectious bursal disease by ginseng stem-leaf saponins

Infectious bursal disease (IBD), caused by infectious bursal disease virus (IBDV), is an immunosuppressive infectious disease of global economic importance in poultry. This study was designed to evaluate the effect of oral administration of ginseng stem-leaf saponins (GSLS) on humoral and gut mucosal immunity in chickens vaccinated with live IBDV vaccine, and furthermore, to test its protective efficacy against virulent IBDV challenge following vaccination. In experiment 1, chickens were orally administered with GSLS at 5 mg/kg of BW for 7 d, and then immunized with live IBDV vaccine via the oral route. Serum was sampled on 0, 1, 2, 3, 4, and 5 wk postvaccination for detecting antibody titers by ELISA, and intestinal tissues were collected on 0, 1, 3, and 5 wk postvaccination for measurement of IgA-positive cells and intestinal intraepithelial lymphocytes by immunohistochemical and hematoxylin-eosin staining, respectively. Result showed that antibody titers, IgA-positive cells and intestinal intraepithelial lymphocytes were significantly higher in chickens drinking GSLS than the control, suggesting an enhanced effect of GSLS on humoral and gut mucosal immune responses. In experiment 2, chickens were delivered with GSLS and then vaccinated in the same way as in experiment 1. The birds were challenged with virulent IBDV at wk 3 postvaccination. Then the birds were weighed, bled, and necropsied at d 3 postchallenge and the bursae were sampled for gross and histopathological examination. Results demonstrated that GSLS provided a better protection against virulent IBDV challenge following vaccination than the control. In conclusion, oral administration of GSLS enhances both humoral and gut mucosal immune responses to IBDV and offers a better protection against virulent IBDV challenge. Considering its immunomodulatory properties to IBDV vaccine, GSLS might be a promising oral adjuvant for vaccination against infectious diseases in poultry.

Overexpression of recombinant infectious bursal disease virus (IBDV) capsid protein VP2 in the middle silk gland of transgenic silkworm

Infectious bursal disease virus (IBDV) is the causative agent of a highly contagious disease affecting young chickens and causes serious economic losses to the poultry industry worldwide. Development of subunit vaccine using its major caspid protein, VP2, is one of the promising strategies to protect against IBDV. This study aim to test the feasibility of using silkworm to produce recombinant VP2 protein (rVP2) derived from a very virulent strain of IBDV (vvIBDV). A total of 16 transgenic silkworm lines harboring a codon-optimized VP2 gene driven by the sericin1 promoter were generated and analyzed. The results showed that the rVP2 was synthesized in the middle silk gland of all lines and secreted into their cocoons. The content of rVP2 in the cocoon of each line was ranged from 0.07 to 16.10 % of the total soluble proteins. The rVP2 was purified from 30 g cocoon powders with a yield of 3.33 mg and a purity >90 %. Further analysis indicated that the rVP2 was able to tolerate high temperatures up to 80 °C, and exhibited specific immunogenic activity in mice. To our knowledge, this is the first report of overexpressing rVP2 in the middle silk gland of transgenic silkworm, which demonstrates the capability of silkworm as an efficient tool to produce recombinant immunogens for use in new vaccines against animal diseases.

Subviral particle as vaccine and vaccine platform

Recombinant subvirual particles retain similar antigenic features of their authentic viral capsids and thus have been applied as nonreplicating subunit vaccines against viral infection and illness. Additionally, the self-assembled, polyvalent subviral particles are excellent platforms to display foreign antigens for immune enhancement for vaccine development. These subviral particle-based vaccines are noninfectious and thus safer than the conventional live attenuated and inactivated vaccines. While several VLP vaccines are available in the markets, numerous others, including dual vaccines against more than one pathogen, are under clinical or preclinical development. This article provides an update of these efforts.

Protective oral vaccination against infectious bursal disease virus using the major viral antigenic protein VP2 produced in Pichia pastoris

Infectious bursal disease virus (IBDV) causes economically important immunosuppressive disease in young chickens. The self-assembling capsid protein (VP2) from IBDV strain IR01 was expressed in Pichia pastoris resulting in the formation of homomeric, 23-nm infectious bursal disease subviral particles (IBD-SVPs) with a yield of 76 mg/l before and 38 mg/l after purification. Anti-IBDV antibodies were detected in chickens injected with purified IBD-SVPs or fed with either purified IBD-SVPs or inactivated P. pastoris cells containing IBD-VP2 (cell-encapsulated). Challenge studies using the heterologous classical IBDV strain (MB3) showed that intramuscular vaccination with 20 µg purified IBD-SVPs conferred full protection, achieved complete virus clearance and prevented bursal damage and atrophy, compared with only 40% protection, 0-10% virus clearance accompanied by severe atrophy and substantial bursal damage in mock-vaccinated and challenge controls. The commercial IBDV vaccine also conferred full protection and achieved complete virus clearance, albeit with partial bursal atrophy. Oral administration of 500 µg purified IBD-SVPs with and without adjuvant conferred 100% protection but achieved only 60% virus clearance with adjuvant and none without it. Moderate bursal damage was observed in both cases but the inclusion of adjuvant resulted in bursal atrophy similar to that observed with live-attenuated vaccine and parenteral administration of 20 µg purified IBD-SVPs. The oral administration of 250 mg P. pastoris cells containing IBD-VP2 resulted in 100% protection with adjuvant and 60% without, accompanied by moderate bursal damage and atrophy in both groups, whereas 25 mg P. pastoris cells containing IBD-VP2 resulted in 90-100% protection with moderate bursal lesions and severe atrophy. Finally, the oral delivery of 50 µg purified IBD-SVPs achieved 40-60% protection with severe bursal lesions and atrophy. Both oral and parenteral administration of yeast-derived IBD-VP2 can therefore induce a specific and protective immune response against IBDV without affecting the growth rate of chickens.

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.

Virus-like particles as a highly efficient vaccine platform: diversity of targets and production systems and advances in clinical development

Virus-like particles (VLPs) are a class of subunit vaccines that differentiate themselves from soluble recombinant antigens by stronger protective immunogenicity associated with the VLP structure. Like parental viruses, VLPs can be either non-enveloped or enveloped, and they can form following expression of one or several viral structural proteins in a recombinant heterologous system. Depending on the complexity of the VLP, it can be produced in either a prokaryotic or eukaryotic expression system using target-encoding recombinant vectors, or in some cases can be assembled in cell-free conditions. To date, a wide variety of VLP-based candidate vaccines targeting various viral, bacterial, parasitic and fungal pathogens, as well as non-infectious diseases, have been produced in different expression systems. Some VLPs have entered clinical development and a few have been licensed and commercialized. This article reviews VLP-based vaccines produced in different systems, their immunogenicity in animal models and their status in clinical development.

Virus-like particle-based vaccines for animal viral infections

Vaccination is considered one of the most effective ways to control pathogens and prevent diseases in humans as well as in the veterinary field. Traditional vaccines against animal viral diseases are based on inactivated or attenuated viruses, but new subunit vaccines are gaining attention from researchers in animal vaccinology. Among these, virus-like particles (VLPs) represent one of the most appealing approaches opening up interesting frontiers in animal vaccines. VLPs are robust protein scaffolds exhibiting well-defined geometry and uniformity that mimic the overall structure of the native virions but lack the viral genome. They are often antigenically indistinguishable from the virus from which they were derived and present important advantages in terms of safety. VLPs can stimulate strong humoral and cellular immune responses and have been shown to exhibit self-adjuvanting abilities. In addition to their suitability as a vaccine for the homologous virus from which they are derived, VLPs can also be used as vectors for the multimeric presentation of foreign antigens. VLPs have therefore shown dramatic effectiveness as candidate vaccines; nevertheless, only one veterinary VLP-base vaccine is licensed. Here, we review and examine in detail the current status of VLPs as a vaccine strategy in the veterinary field, and discuss the potential advantages and challenges of this technology.

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.

An overview of infectious bursal disease

Infectious bursal disease (IBD) is a viral immunosuppressive disease of chickens attacking mainly an important lymphoid organ in birds [the bursa of Fabricius (BF)]. The emergence of new variant strains of the causative agent [infectious bursal disease virus (IBDV)] has made it more urgent to develop new vaccination strategies against IBD. One of these strategies is the use of recombinant vaccines (DNA and viral-vectored vaccines). Several studies have investigated the host immune response towards IBDV. This review will present a detailed background on the disease and its causative agent, accompanied by a summary of the most recent findings regarding the host immune response to IBDV infection and the use of recombinant vaccines against IBD.

Virus-like particles: the new frontier of vaccines for animal viral infections

Vaccination continues to be the main approach to protect animals from infectious diseases. Until recently, all licensed vaccines were developed using conventional technologies. Subunit vaccines are, however, gaining attention from researchers in the field of veterinary vaccinology, and among these, virus-like particles (VLPs) represent one of the most appealing approaches. VLPs are robust protein cages in the nanometer range that mimic the overall structure of the native virions but lack the viral genome. They are often antigenically indistinguishable from the virus from which they were derived and present important advantages in terms of safety. VLPs can stimulate strong humoral and cellular immune responses and have been shown to exhibit self-adjuvanting abilities. In addition to their suitability as a vaccine for the homologous virus from which they are derived, VLPs can also be used as vectors for the multimeric presentation of foreign antigens. VLPs have therefore shown dramatic effectiveness as candidate vaccines. Here, we review the current status of VLPs as a vaccine technology in the veterinary field, and discuss the potential advantages and challenges of this technology.

Induction of protective immunity in chickens immunized with plant-made chimeric Bamboo mosaic virus particles expressing very virulent Infectious bursal disease virus antigen

Very virulent Infectious bursal disease virus (vvIBDV) causes a highly contagious disease in young chickens and leads to significant economic loss in the poultry industry. Effective new vaccines are urgently needed. Autonomously replicating plant virus-based vector provides attractive means for producing chimeric virus particles (CVPs) in plants that can be developed into vaccines. In this study, we demonstrate the potential for vaccine development of Bamboo mosaic virus (BaMV) epitope-presentation system, where the antigen from vvIBDV VP2 was fused to the N-terminus of BaMV coat protein. Accordingly, an infections plasmid, pBIBD2, was constructed. Inoculation of the recombinant BaMV clone pBIBD2 enabled the generation of chimeric virus, BIBD2, and stable expression of IBDV VP2 antigen on its coat protein. After intramuscular immunization with BIBD2 CVPs, chickens produced antibodies against IBDV and were protected from vvIBDV (V263/TW strain) challenges. These results corroborate the feasibility of BaMV-based CVP platform in plants for the development and production of vaccines against IBDV.

Chimeric calicivirus-like particles elicit specific immune responses in pigs

Virus-like particles (VLPs) have received considerable attention due to their potential application in veterinary vaccines and, in particular, VLPs from rabbit haemorrhagic disease virus (RHDV) have successfully shown to be good platforms for inducing immune responses against an inserted foreign epitope in mice. The aim of this study was to assess the immunogenicity of chimeric RHDV-VLPs as vaccine vectors in pigs. For this purpose, we have generated chimeric VLPs containing a well-known T epitope of 3A protein of foot-and-mouth disease virus (FMDV). Firstly, RHDV-VLPs were able to activate immature porcine bone marrow-derived dendritic cells (poBMDCs) in vitro. Secondly, pigs were inoculated twice in a two-week interval with chimeric RHDV-VLPs at different doses intranasally or intramuscularly. One intramuscularly treated group was also inoculated with adjuvant Montanide™ ISA 206 at the same time. Specific IgG and IgA antibodies against RHDV-VLPs were induced and such levels were higher in the adjuvanted group compared with other groups. Interestingly, anti-RHDV-VLP IgA responses were higher in groups inoculated intramuscularly than those that received the VLPs intranasally. Two weeks after the last immunisation, specific IFN-γ-secreting cells against 3A epitope and against RHDV-VLPs were detected in PBMCs by ELISPOT. The adjuvanted group exhibited the highest IFN-γ-secreting cell numbers and lymphoproliferative specific T cell responses against 3A epitope and RHDV-VLP. This is the first immunological report on the potential use of chimeric RHDV-VLPs as antigen carriers in pigs.

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.

Current strategies for subunit and genetic viral veterinary vaccine development

Developing vaccines for livestock provides researchers with the opportunity to perform efficacy testing in the natural hosts. This enables the evaluation of different strategies, including definition of effective antigens or antigen combinations, and improvement in delivery systems for target antigens so that protective immune responses can be modulated or potentiated. An impressive amount of knowledge has been generated in recent years on vaccine strategies and consequently a wide variety of antigen delivery systems is now available for vaccine research. This paper reviews several antigen production and delivery strategies other than those based on the use of live viral vectors. Genetic and protein subunit vaccines as well as alternative production systems are considered in this review.

Virus-like particles as a vaccine delivery system: myths and facts

Vaccines against viral disease have traditionally relied on attenuated virus strains or inactivation of infectious virus. Subunit vaccines based on viral proteins expressed in heterologous systems have been effective for some pathogens, but have often suffered from poor immunogenicity due to incorrect protein folding or modification. In this chapter we focus on a specific class of viral subunit vaccine that mimics the overall structure of virus particles and thus preserves the native antigenic conformation of the immunogenic proteins. These virus-like particles (VLPs) have been produced for a wide range of taxonomically and structurally distinct viruses, and have unique advantages in terms of safety and immunogenicity over previous approaches. With new VLP vaccines for papillomavirus beginning to reach the market place we argue that this technology has now ‘come-of-age’ and must be considered a viable vaccine strategy.

Oral immunization with transgenic rice seeds expressing VP2 protein of infectious bursal disease virus induces protective immune responses in chickens

The expression of infectious bursal disease virus (IBDV) host-protective immunogen VP2 protein in rice seeds, its immunogenicity and protective capability in chickens were investigated. The VP2 cDNA of IBDV strain ZJ2000 was cloned downstream of the Gt1 promoter of the rice glutelin GluA-2 gene in the binary expression vector, pCambia1301-Gt1. Agrobacterium tumefaciens containing the recombinant vector was used to transform rice embryogenic calli, and 121 transgenic lines were obtained and grown to maturity in a greenhouse. The expression level of VP2 protein in transgenic rice seeds varied from 0.678% to 4.521% microg/mg of the total soluble seed protein. Specific pathogen-free chickens orally vaccinated with transgenic rice seeds expressing VP2 protein produced neutralizing antibodies against IBDV and were protected when challenged with a highly virulent IBDV strain, BC6/85. These results demonstrate that transgenic rice seeds expressing IBDV VP2 can be used as an effective, safe and inexpensive vaccine against IBDV.

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.

A novel recombinant virus-like particle vaccine for prevention of porcine parvovirus-induced reproductive failure

A novel vaccine against porcine parvovirus (PPV), composed of recombinant virus-like particles (PPV-VLPs) produced with the baculovirus expression vector system (BEVS) at industrial scale, was tested for its immunogenicity and protective potency. A formulation of submicrogram amounts of PPV-VLPs in a water-in-mineral oil adjuvant evoked high serum antibody titres in both guinea pigs, used as reference model, and target species, pigs. A single immunisation with 0.7microg of this antigen yielded complete foetal protection against PPV infection after challenge with a virulent strain of this virus. Furthermore, also in the presence of mild adjuvants the protective action of these PPV-VLPs is excellent. This recombinant subunit vaccine overcomes some of the drawbacks of classical PPV vaccines.

Vaccines for viral and parasitic diseases produced with baculovirus vectors

The baculovirus-insect cell expression system is an approved system for the production of viral antigens with vaccine potential for humans and animals and has been used for production of subunit vaccines against parasitic diseases as well. Many candidate subunit vaccines have been expressed in this system and immunization commonly led to protective immunity against pathogen challenge. The first vaccines produced in insect cells for animal use are now on the market. This chapter deals with the tailoring of the baculovirus-insect cell expression system for vaccine production in terms of expression levels, integrity and immunogenicity of recombinant proteins, and baculovirus genome stability. Various expression strategies are discussed including chimeric, virus-like particles, baculovirus display of foreign antigens on budded virions or in occlusion bodies, and specialized baculovirus vectors with mammalian promoters that express the antigen in the immunized individual. A historical overview shows the wide variety of viral (glyco)proteins that have successfully been expressed in this system for vaccine purposes. The potential of this expression system for antiparasite vaccines is illustrated. The combination of subunit vaccines and marker tests, both based on antigens expressed in insect cells, provides a powerful tool to combat disease and to monitor infectious agents.

Vaccination against very virulent infectious bursal disease virus using recombinant T4 bacteriophage displaying viral protein VP2

In order to develop a desirable inexpensive, effective and safe vaccine against the very virulent infectious bursal disease virus (vvIBDV), we tried to take advantage of the emerging T4 bacteriophage surface protein display system. The major immunogen protein VP2 from the vvIBDV strain HK46 was fused to the nonessential T4 phage surface capsid protein, a small outer capsid (SOC) protein, resulting in the 49 kDa SOC-VP2 fusion protein, which was verified by sodium dodecylsulfate polyacrylamide gel electrophoresis and Western blot. Immunoelectromicroscopy showed that the recombinant VP2 protein was successfully displayed on the surface of the T4 phage. The recombinant VP2 protein is antigenic and showed reactivities to various monoclonal antibodies (mAbs) against IBDV, whereas the wild-type phage T4 could not react to any mAb. In addition, the recombinant VP2 protein is immunogenic and elicited specific antibodies in immunized specific pathogen free (SPF) chickens. More significantly, immunization of SPF chickens with the recombinant T4-VP2 phage protected them from infection by the vvIBDV strain HK46. When challenged with the vvIBDV strain HK46 at a dose of 100 of 50% lethal dose (LD50) per chicken 4 weeks after the booster was given, the group vaccinated with the T4-VP2 recombinant phage showed no clinical signs of disease or death, whereas the unvaccinated group and the group vaccinated with the wild-type T4 phage exhibited 100% clinical signs of disease and bursal damages, and 30%-40% mortality. Collectively, the data herein showed that the T4-displayed VP2 protein might be an inexpensive, effective and safe vaccine candidate against vvIBDV.

DNA immunization against very virulent infectious bursal disease virus with VP2-4-3 gene and chicken IL-6 gene

The present study was to investigate the feasibility and efficiency of the DNA vaccine to protect chickens against very virulent infectious bursal disease virus (vvIBDV) infection. A plasmid DNA carrying VP2-4-3 genes of vvIBDV SH95 and a plasmid DNA carrying chicken interleukin-6 (ChIL-6) genes were constructed and designated as pALTER-MAX-VP2-4-3 and pALTER-MAX-ChIL-6 respectively. Several DNA vaccination experiments were performed: 1-week-old chickens were intramuscularly injected with only plasmid pcDNA3-VP2, pALTER-MAX-VP2-4-3 or mixture with pALTER-MAX-ChIL-6. The chickens at 4 weeks old were orally inoculated with vvIBDV SH95. The results showed that immunization with the mixture of pALTER-MAX-VP2-4-3 and pALTER-MAX-ChIL-6 three times conferred protection for 90% of chickens. Enzyme-linked immunosorbent assay (ELISA) antibody titres in chickens immunized together with pALTER-MAX-ChIL-6 were higher than those immunized simply with plasmid pcDNA3-VP2 or pALTER-MAX-VP2-4-3. IBDV was not detected in the bursa of the protected chickens at 8 days after challenge by RT-PCR. The results indicate that protection against vvIBDV can be achieved by using the VP2-4-3 gene of vvIBDV as a DNA vaccine. Furthermore, the simultaneous injection of ChIL-6 plasmid significantly increased the protection after challenge with the very virulent strain.

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.

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.

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.