Self-assembly, antigenicity, and immunogenicity of the rabbit haemorrhagic disease virus (Czechoslovakian strain V-351) capsid protein expressed in baculovirus

Serological characterisation of Lagovirus virus-like particles originating from native and mutated VP60 of rabbit haemorrhagic disease virus 2 and European brown hare syndrome virus

Introduction

Since lagoviruses cannot be cultivated in vitro, using expression systems is an alternative and promising way of producing diagnostic viral antigens. It opens up their use as active immunogens for vaccine production.

Material and Methods

Virus-like particles (VLPs) were produced in a baculovirus expression system in Spodoptera frugiperda 9 (Sf9) insect cells based on wild-type and mutated variants of the virus capsid VP60 protein from a Polish strain of European brown hare syndrome virus (EBHSV) and wild-type and mutated versions of this protein from a Polish strain of rabbit haemorrhagic disease virus 2 (RHDV2). The mutations were the substitution of an arginylglycylaspartic acid (Arg-Gly-Asp/RGD) motif in the P2 subdomain and, in the S or P2 domain, the substitution of three lysines. The VLPs were purified with sucrose gradient ultracentrifugation.

Results

Protein production was confirmed by Western blot analysis using rabbit or hare sera and ELISA tests with different types of monoclonal antibody. The haemagglutination properties of some VLPs were also evaluated. Electron microscopy of wild-type EBHSV, wild-type RHDV2 and the four VP60 variants produced in this experiment revealed the formation of characteristic VLP structures.

Conclusion

For the first time, mutated VLPs of RHDV2 with an RGD motif in the VP60 sequence were obtained, which could potentially be used to deliver cargo to eukaryotic cells. Virus-like particles based on the VP60 proteins of EBHSV and RHDV with a three-lysine substitution in the S or P2 domains were also obtained. Potential exists for VLPs of EBHSV and RHDV2 as vaccine candidates.

Sulfated Lactosyl Archaeol Archaeosome-Adjuvanted Vaccine Formulations Targeting Rabbit Hemorrhagic Disease Virus Are Immunogenic and Efficacious

Vaccines play an important role in maintaining human and animal health worldwide. There is continued demand for effective and safe adjuvants capable of enhancing antigen-specific responses to a target pathogen. Rabbit hemorrhagic disease virus (RHDV) is a highly contagious calicivirus that often induces high mortality rates in rabbits. Herein, we evaluated the activity of an experimental sulfated lactosyl archaeol (SLA) archaeosome adjuvant when incorporated in subunit vaccine formulations targeting RHDV. The subunit antigens consisted of RHDV-CRM₁₉₇ peptide conjugates or recombinant RHDV2 VP60. SLA was able to enhance antigen-specific antibody titers and cellular responses in mice and rabbits. Three weeks following immunization, antigen-specific antibody levels in rabbits vaccinated with RHDV2 VP60 + SLA were significantly higher than those immunized with antigen alone, with geomean titers of 7393 vs. 117. In addition, the SLA-adjuvanted VP60-based formulations were highly efficacious in a rabbit RHDV2 challenge model with up to 87.5% animals surviving the viral challenge. These findings demonstrate the potential utility of SLA adjuvants in veterinary applications and highlight its activity in different types of mammalian species.

Rabbit hemorrhagic disease virus VP60 protein expressed in recombinant swinepox virus self-assembles into virus-like particles with strong immunogenicity in rabbits

Rabbit Hemorrhagic Disease (RHD) is an economically significant infectious disease of rabbits, and its infection causes severe losses in the meat and fur industry. RHD Virus (RHDV) is difficult to proliferate in cell lines in vitro, which has greatly impeded the progress of investigating its replication mechanism and production of inactivated virus vaccines. RHDV VP60 protein is a major antigen for developing RHD subunit vaccines. Herein, we constructed a TK-deactivated recombinant Swinepox virus (rSWPV) expressing VP60 protein and VP60 protein coupled with His-tag respectively, and the expression of foreign proteins was confirmed using immunofluorescence assay and western blotting. Transmission electron microscopy showed that the recombinant VP60, with or without His-tag, self-assembled into virus-like particles (VLPs). Its efficacy was evaluated by comparison with available commercial vaccines in rabbits. ELISA and HI titer assays showed that high levels of neutralizing antibodies were induced at the first week after immunization with the recombinant strain and were maintained during the ongoing monitoring for the following 13 weeks. Challenge experiments showed that a single immunization with 10⁶ PFU of the recombinant strain protected rabbits from lethal RHDV infection, and no histopathological changes or antigenic staining was found in the vaccine and rSWPV groups. These results suggest that rSWPV expressing RHDV VP60 could be an efficient candidate vaccine against RHDV in rabbits.

Chimeric VLPs Bearing VP60 from Two Serotypes of Rabbit Haemorrhagic Disease Virus Are Protective against Both Viruses

The VP60 capsid protein from rabbit haemorrhagic disease virus (RHDV), the causative agent of one of the most economically important disease in rabbits worldwide, forms virus-like particles (VLPs) when expressed using heterologous protein expression systems such as recombinant baculovirus, yeasts, plants or mammalian cell cultures. To prevent RHDV dissemination, it would be beneficial to develop a bivalent vaccine including both RHDV GI.1- and RHDV GI.2-derived VLPs to achieve robust immunisation against both serotypes. In the present work, we developed a strategy of production of a dual-serving RHDV vaccine co-expressing the VP60 proteins from the two RHDV predominant serotypes using CrisBio technology, which uses Tricholusia ni insect pupae as natural bioreactors, which are programmed by recombinant baculovirus vectors. Co-infecting the insect pupae with two baculovirus vectors expressing the RHDV GI.1- and RHDV GI.2-derived VP60 proteins, we obtained chimeric VLPs incorporating both proteins as determined by using serotype-specific monoclonal antibodies. The resulting VLPs showed the typical size and shape of this calicivirus as determined by electron microscopy. Rabbits immunised with the chimeric VLPs were fully protected against a lethal challenge infection with the two RHDV serotypes. This study demonstrates that it is possible to generate a dual cost-effective vaccine against this virus using a single production and purification process, greatly simplifying vaccine manufacturing.

An Adenovirus Vector Expressing FMDV RNA Polymerase Combined with a Chimeric VLP Harboring a Neutralizing Epitope as a Prime Boost Strategy to Induce FMDV-Specific Humoral and Cellular Responses

Foot and mouth disease is a highly contagious disease affecting cattle, sheep, and swine among other cloven-hoofed animals that imposes serious economic burden by its direct effects on farm productivity as well as on commerce of farmed produce. Vaccination using inactivated viral strains of the different serotypes is an effective protective measure, but has several drawbacks including a lack of cross protection and the perils associated with the large-scale growth of infectious virus. We have previously developed chimeric virus-like particles (VLPs) bearing an FMDV epitope which induced strong specific humoral responses in vaccinated pigs but conferred only partial protection against homologous challenge. While this and other FMD vaccines under development mostly rely on the induction of neutralizing responses, it is thought that induction of specific T-cell responses might improve both cross protective efficacy as well as duration of immunity. Therefore, we here describe the development of a recombinant adenovirus expressing the highly conserved nonstructural FMDV 3D protein as well as its capacity to induce specific T-cell responses in a murine model. We further describe the generation of an FMDV serotype C-specific chimeric VLP and analyze the immunogenicity of two different prime-boost strategies combining both elements in mice. This combination can effectively induce both humoral and cellular FMDV-specific responses eliciting high titers of ELISA and neutralizing antibodies anti-FMDV as well as a high frequency of IFNγ-secreting cells. These results provide the basis for further testing of this anti FMD vaccination strategy in cattle or pig, two of the most relevant natural host of this pathogen.

A Review on the Methods Used for the Detection and Diagnosis of Rabbit Hemorrhagic Disease Virus (RHDV)

Since the early 1980s, the European rabbit (Oryctolagus cuniculus) has been threatened by the rabbit hemorrhagic disease (RHD). The disease is caused by a lagovirus of the family Caliciviridae, the rabbit hemorrhagic disease virus (RHDV). The need for detection, identification and further characterization of RHDV led to the development of several diagnostic tests. Owing to the lack of an appropriate cell culture system for in vitro propagation of the virus, much of the methods involved in these tests contributed to our current knowledge on RHD and RHDV and to the development of vaccines to contain the disease. Here, we provide a comprehensive review of the RHDV diagnostic tests used since the first RHD outbreak and that include molecular, histological and serological techniques, ranging from simpler tests initially used, such as the hemagglutination test, to the more recent and sophisticated high-throughput sequencing, along with an overview of their potential and their limitations.

Immunity against Lagovirus europaeus and the Impact of the Immunological Studies on Vaccination

In the early 1980s, a highly contagious viral hemorrhagic fever in rabbits (Oryctolagus cuniculus) emerged, causing a very high rate of mortality in these animals. Since the initial occurrence of the rabbit hemorrhagic disease virus (RHDV), several hundred million rabbits have died after infection. The emergence of genetically-different virus variants (RHDV GI.1 and GI.2) indicated the very high variability of RHDV. Moreover, with these variants, the host range broadened to hare species (Lepus). The circulation of RHDV genotypes displays different virulences and a limited induction of cross-protective immunity. Interestingly, juvenile rabbits (<9 weeks of age) with an immature immune system display a general resistance to RHDV GI.1, and a limited resistance to RHDV GI.2 strains, whereas less than 3% of adult rabbits survive an infection by either RHDV GI.1. or GI.2. Several not-yet fully understood phenomena characterize the RHD. A very low infection dose followed by an extremely rapid viral replication could be simplified to the induction of a disseminated intravascular coagulopathy (DIC), a severe loss of lymphocytes—especially T-cells—and death within 36 to 72 h post infection. On the other hand, in animals surviving the infection or after vaccination, very high titers of RHDV-neutralizing antibodies were induced. Several studies have been conducted in order to deepen the knowledge about the virus’ genetics, epidemiology, RHDV-induced pathology, and the anti-RHDV immune responses of rabbits in order to understand the phenomenon of the juvenile resistance to this virus. Moreover, several approaches have been used to produce efficient vaccines in order to prevent an infection with RHDV. In this review, we discuss the current knowledge about anti-RHDV resistance and immunity, RHDV vaccination, and the further need to establish rationally-based RHDV vaccines.

Construction and immunogenicity of novel bivalent virus-like particles bearing VP60 genes of classic RHDV(GI.1) and RHDV2(GI.2)

Rabbit hemorrhagic disease (RHD) is an acute, inflammatory, septic, and devastating infectious disease caused by Rabbit hemorrhagic disease virus (RHDV), which poses a serious threat to the rabbit industry. RHDV2 (GI.2/RHDVb), a recently reported new variant could cause RHD in wild populations, but also RHDV-vaccinated rabbits. For now, both RHDV and RHDV2 are the main causes of RHD. To develop a new subunit vaccine that could protect rabbits against both classic RHDV and RHDV2 infections, we constructed a recombinant baculovirus (Bac-classic RHDV VP60-RHDV2 VP60) containing the VP60 genes of classic RHDV and RHDV2. Both VP60 genes were well expressed simultaneously in Spodoptera frugiperda cells (Sf9) after infection with the recombinant baculovirus. Transmission electron microscopy showed that the recombinant VP60 self-assembled into virus-like particles (VLPs). The antigenicity and immunogenicity of the bivalent VLPs vaccine were examined with animal experiments. Our results demonstrated that both the humoral and cellular immune responses were efficiently induced in rabbits by a subunit vaccine based on the recombinant baculovirus. In addition, all rabbits immunized with the bivalent VLPs vaccine survived after challenged with classic RHDV, and showed no clinical signs of RHD, whereas all the rabbits in the negative control group died from classic RHDV infection and showed typical clinical signs of RHD. In summary, our results indicated that the recombinant baculovirus carrying two VP60 genes is a candidate construct from which to develop a bivalent VLPs vaccine against both classic RHDV and RHDV2 infections.

Immunogenicity in Rabbits of Virus-Like Particles from a Contemporary Rabbit Haemorrhagic Disease Virus Type 2 (GI.2/RHDV2/b) Isolated in The Netherlands

Rabbit haemorrhagic disease virus (RHDV) type 2 (GI.2/RHDV2/b) is an emerging pathogen in wild rabbits and in domestic rabbits vaccinated against RHDV (GI.1). Here we report the genome sequence of a contemporary RHDV2 isolate from the Netherlands and investigate the immunogenicity of virus-like particles (VLPs) produced in insect cells. RHDV2 RNA was isolated from the liver of a naturally infected wild rabbit and the complete viral genome sequence was assembled from sequenced RT-PCR products. Phylogenetic analysis based on the VP60 capsid gene demonstrated that the RHDV2 NL2016 isolate clustered with other contemporary RHDV2 strains. The VP60 gene was cloned in a baculovirus expression vector to produce VLPs in Sf9 insect cells. Density-gradient purified RHDV2 VLPs were visualized by transmission electron microscopy as spherical particles of around 30 nm in diameter with a morphology resembling authentic RHDV. Immunization of rabbits with RHDV2 VLPs resulted in high production of serum antibodies against VP60, and the production of cytokines (IFN-γ and IL-4) was significantly elevated in the immunized rabbits compared to the control group. The results demonstrate that the recombinant RHDV2 VLPs are highly immunogenic and may find applications in serological detection assays and might be further developed as a vaccine candidate to protect domestic rabbits against RHDV2 infection.

Characterization of protective humoral and cellular immune responses against RHDV2 induced by a new vaccine based on recombinant baculovirus

Rabbit hemorrhagic disease (RHD) is a lethal disease in rabbits caused by RHD virus (RHDV). Protection is only possible through vaccination. A new virus variant (RHDV2) which emerged in 2010 in France differed from the classical RHDV1 variant in certain aspects and vaccines against RHDV1 induced limited cross protection only. In a previous study, we designed a recombinant baculovirus based RHDV2-VP1 vaccine, which provided a protective immunity in rabbits against RHDV2. In the present study this newly created vaccine is characterized with regard to onset and duration of protection, and possible cross protection against classical RHDV1. Furthermore, humoral and cellular immune mechanisms in vaccinated and infected rabbits were analyzed. In all experiments, the recombinant vaccine was compared to a conventional liver-based RHDV2 vaccine. The RHDV2-VP1 vaccine induced a protective immune response already seven days after single vaccination and fully protected for at least 14 months. A booster vaccination 21 days after the first had a negative influence on long-term protection. The cross protection provided by the RHDV2-VP1 vaccine against classical RHDV1 was limited since only 50% of vaccinated rabbits survived the infection. Conclusively, the new, baculovirus-based RHDV2-VP1 vaccine has the potential to protect rabbits against the infection with RHDV2, blocks completely the disease progression and prevents the spread of RHDV2 at the population level.

Crude extracts of recombinant baculovirus expressing rabbit hemorrhagic disease virus 2 VLPs from both insect and rabbit cells protect rabbits from rabbit hemorrhagic disease caused by RHDV2

Vaccines against viral pathogens are often composed of recombinant proteins expressed in different systems. Such proteins expressed by recombinant baculoviruses have been proven to be effective for vaccination. Especially, after codon usage optimization high amounts of recombinant viral proteins can be obtained which can assemble to virus like particles (VLPs) spontaneously. In this study we compared two different codon usages of RHDV2-VP1 to improve the expression of recombinant VP1 of RHDV2 by recombinant baculoviruses after infection of insect SF9 cells or transduction of mammalian RK13 cells in order to gain high protein yields. Also the influence on the auto-assembly of RHDV2-VP1 to VLPs was investigated. Finally, the immunogenic potential of such recombinant vaccines against RHDV2 to induce a protective immune response in rabbits against RHDV2 should be characterized. There was no influence of different codon usages on RHDV2-VP1 gene expression in the respective cell lines detected. However, in insect cell line SF9 higher rates of recombinant VP1 were measured in comparison to the transduction of mammalian cells RK13. Auto-assembly of RHDV2-VP1 to VLPs was observed in both cell systems by electron microscopy. Finally, both RHDV-VP1 VLPs derived from mammalian and insect cells were able to induce a protective humoral immune response in rabbits against RHDV2.

Clinical chemistry, haematology, immune response and histological evaluation of rabbits after immunisation and challenge with rabbit haemorrhagic disease (RHD) virus

<p>Following their immunisation and infection with a VSHI-CN-6 viral strain, a group of 15 rabbits were examined in a study of Rabbit Haemorrhagic Disease (RHD). Serum samples were collected from the external ear vein at 0, 15, 30 and 60 days post-immunisation. The recorded platelet numbers were closer to the lower physiological limit, indicating a mild thrombocytopenia, with values ranging from 26.6 to 30.43×10⁴/mm³. The phagocytic index revealed significant differences (P&lt;0.001) between the mean values obtained before vaccination (day 0) and the 3 post-vaccination measurements, confirming the increase in phagocytic capacity after immunisation. Additionally, the serum lysozyme average value equalled 9.14 mg/mL post-vaccination. The analysis of variance revealed significant statistical differences (P&lt;0.05) between the average values obtained before vaccination (0) and the post-vaccination values, measured on day 14 and 30, respectively. The morphology of the samples collected from the main organs involved in immune protection, spleen and gastric and portal lymph nodes highlighted changes corresponding to the post-vaccination immunological response. The white pulp of the spleen appeared as a diffuse lymphoid tissue, presenting with primary and secondary lymphoid follicles. The ratio of white/red pulp was in favour of the white pulp and multiple lymphoid follicles were present, indicating their reactivation. In the medullary area of gastric and portal lymph nodes, narrow lymphoid cords, circumscribed by relatively large lymphatic sinuses, and well defined lymphocytolysis were observed. Moreover, the exudate and lymphoid follicles during activation were noted in the cortical area. Furthermore, the inflammatory processes were identified, morphologically manifested by the thickening of connective tissue in the lymph node capsule, dilacerations of the connective fibres and the presence of light acidophilic serous exudate with rare inflammatory cells (serous lymphoreticulitis).</p>

Development of a VLP-based vaccine in silkworm pupae against rabbit hemorrhagic disease virus

Rabbit hemorrhagic disease virus (RHDV) is the etiological agent behind rabbit hemorrhagic disease (RHD), which is lethal and contagious in rabbits. The virus does not replicate in cell culture and the only commercial inactivated vaccine available is derived from infected rabbit livers. RHDV capsid protein, VP60, is the main antigen comprising the virion. We used a baculovirus-silkworm pupae system to express VP60, which self-assembled into virus-like particles (VLPs) with a similar size and morphology to RHDV. Hemagglutination assays (HAs) showed that VP60 expression levels of VP60 reached as high as 10⁷HA units (HAU) per pupa. A single intramuscular injection with 10⁴HAU of VLPs completely protected rabbits for at least 180days against RHDV challenge, and for at least 360days when the VLPs were emulsified with Freund's complete adjuvant. These data suggest that silkworm pupae can be used to develop VLP-based vaccines which confer durable protection against RHD.

Self-assembly of virus-like particles of rabbit hemorrhagic disease virus capsid protein expressed in Escherichia coli and their immunogenicity in rabbits

In this study, virus-like particles (VLPs) derived from rabbit hemorrhagic disease virus (RHDV) were evaluated for the development of a vaccine against RHDV infection. The VP60 gene was cloned and inserted into a pSMK expression vector containing a small ubiquitin-like modifier (SUMO) tag that can promote the soluble expression of heterologous proteins in Escherichia coli cells. After expression and purification of His-SUMO-VP60 and cleavage of the SUMO tag, we found that the RHDV VP60 protein had self-assembled into VLPs with a similar shape and smaller size compared with authentic RHDV capsid. Next, the antigenicity and immunogenicity of the VLPs were examined. The results showed that RHDV-specific responses were clearly induced in rabbits and that all rabbits in the VLP group survived while those in the negative control group died within 72 h post-infection. These results suggest that VLP-based RHDV could be a promising RHDV vaccine candidate.

Improved Production Efficiency of Virus-Like Particles by the Baculovirus Expression Vector System

Vaccines based on virus-like particles (VLPs) have proven effective in humans and animals. In this regard, the baculovirus expression vector system (BEVS) is one of the technologies of choice to generate such highly immunogenic vaccines. The extended use of these vaccines for human and animal populations is constrained because of high production costs, therefore a significant improvement in productivity is crucial to ensure their commercial viability. Here we describe the use of the previously described baculovirus expression cassette, called TB, to model the production of two VLP-forming vaccine antigens in insect cells. Capsid proteins from porcine circovirus type 2 (PCV2 Cap) and from the calicivirus that causes rabbit hemorrhagic disease (RHDV VP60) were expressed in insect cells using baculoviruses genetically engineered with the TB expression cassette. Productivity was compared to that obtained using standard counterpart vectors expressing the same proteins under the control of the polyhedrin promoter. Our results demonstrate that the use of the TB expression cassette increased the production yields of these vaccine antigens by around 300% with respect to the standard vectors. The recombinant proteins produced by TB-modified vectors were fully functional, forming VLPs identical in size and shape to those generated by the standard baculoviruses, as determined by electron microscopy analysis. The use of the TB expression cassette implies a simple modification of the baculovirus vectors that significantly improves the cost efficiency of VLP-based vaccine production, thereby facilitating the commercial viability and broad application of these vaccines for human and animal health.

The application of virus-like particles as vaccines and biological vehicles

Virus-like particles (VLPs) can be spontaneously self-assembled by viral structural proteins under appropriate conditions in vitro while excluding the genetic material and potential replication probability. In addition, VLPs possess several features including can be rapidly produced in large quantities through existing expression systems, highly resembling native viruses in terms of conformation and appearance, and displaying repeated cluster of epitopes. Their capsids can be modified via genetic insertion or chemical conjugation which facilitating the multivalent display of a homologous or heterogeneous epitope antigen. Therefore, VLPs are considered as a safe and effective candidate of prophylactic and therapeutic vaccines. VLPs, with a diameter of approximately 20 to 150 nm, also have the characteristics of nanometer materials, such as large surface area, surface-accessible amino acids with reactive moieties (e.g., lysine and glutamic acid residues), inerratic spatial structure, and good biocompatibility. Therefore, assembled VLPs have great potential as a delivery system for specifically carrying a variety of materials. This review summarized recent researches on VLP development as vaccines and biological vehicles, which demonstrated the advantages and potential of VLPs in disease control and prevention and diagnosis. Then, the prospect of VLP biology application in the future is discussed as well.

Comparative analysis of rabbit hemorrhagic disease virus (RHDV) and new RHDV2 virus antigenicity, using specific virus-like particles

In 2010 a new Lagovirus related to rabbit haemorrhagic disease virus (RHDV) emerged in France and has since rapidly spread throughout domestic and wild rabbit populations of several European countries. The new virus, termed RHDV2, exhibits distinctive genetic, antigenic and pathogenic features. Notably, RHDV2 kills rabbits previously vaccinated with RHDV vaccines. Here we report for the first time the generation and characterization of RHDV2-specific virus-like particles (VLPs). Our results further confirmed the differential antigenic properties exhibited by RHDV and RHDV2, highlighting the need of using RHDV2-specific diagnostic assays to monitor the spread of this new virus.

Immunogenicity of different recombinant rabbit hemorrhagic disease virus-like particles carrying CD8+ T cell epitope from chicken ovalbumin (OVA)

To explore the capacity and immunogenicity of virus-like particles (VLPs) of rabbit hemorrhagic disease virus (RHDV) accommodating foreign amino acid sequences, integrations were performed at the following four locations of the structural protein VP60 of RHDV using the OVA257-264 CD8+ T cell epitope (SIINFEKL): (1) inserting at the N-terminus of the VP60 protein (N1); (2) replacing amino acid positions 2-14 of the VP60 protein (N2); (3) replacing amino acid positions 196-207 of the VP60 protein (I1); and (4) replacing amino acid positions 217-228 of the VP60 protein (I2). The recombinant proteins were expressed by baculovirus expression system. The ability to form RHDV-like particles was confirmed by electron microscopy. The immunogenicity of the four recombinant proteins (N1, N2, I1 and I2) was evaluated in mice without any adjuvants. The results indicated that the four recombinant proteins (N1, N2, I1 and I2) could assemble into VLPs. All of the recombinant proteins could induce a specific immune response. Recombinant proteins I1 and I2 were able to elicit both high levels of IFN-γ secretion and anti-VP60 specific immune responses in the murine model. The levels of the VP60-specific IgG antibody in groups I1 and I2 displayed higher optical density (OD) values than those of groups N1 and N2 (P<0.001, P<0.001). The number of IFN-γ-producing splenocytes in mice that were immunized with recombinant proteins I1 and I2 was also significantly greater compared with mice that were immunized with recombinant proteins N1 and N2 (P<0.01). All of these above mentioned results might be beneficial to the establishment of the RHDV-VLPs display system.

An enhanced heterologous virus-like particle for human papillomavirus type 16 tumour immunotherapy

Cervical cancer is caused by high-risk, cancer-causing human papillomaviruses (HPV) and is the second highest cause of cancer deaths in women globally. The majority of cervical cancers express well-characterized HPV oncogenes, which are potential targets for immunotherapeutic vaccination. Here we develop a rabbit haemorrhagic disease virus (RHDV) virus-like particle (VLP)-based vaccine designed for immunotherapy against HPV16 positive tumours. An RHDV-VLP, modified to contain the universal helper T cell epitope PADRE and decorated with an MHC I-restricted peptide (aa 48–57) from the HPV16 E6, was tested for its immunotherapeutic efficacy against the TC-1 HPV16 E6 and E7-expressing tumour in mice. The E6-RHDV-VLP-PADRE was administered therapeutically for the treatment of a pre-existing TC-1 tumour and was delivered with antibodies either to deplete regulatory T cells (anti-CD25) or to block T cell suppression mediated through CTLA-4. As a result, the tumour burden was reduced by around 50% and the median survival time of mice to the humane endpoint was almost doubled the compared to controls. The incorporation of PADRE into the RHDV-VLP was necessary for an E6-specific enhancement of the anti-tumour response and the co-administration of the immune modifying antibodies contributed to the overall efficacy of the immunotherapy. The E6-RHDV-VLP-PADRE shows immunotherapeutic efficacy, prolonging survival for HPV tumour-bearing mice. This was enhanced by the systemic administration of immune-modifying antibodies that are commercially available for use in humans. There is potential to further modify these particles for even greater efficacy in the path to development of an immunotherapeutic treatment for HPV precancerous and cancer stages.

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.

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.

Virus-like particles and α-galactosylceramide form a self-adjuvanting composite particle that elicits anti-tumor responses

Virus-like particles (VLP) are effective vehicles for delivery of heterologous antigen to antigen-presenting cells. However VLP alone are insufficiently stimulatory to generate the signals required to facilitate effective priming of naïve T cells. We show that the VLP derived from rabbit hemorrhagic disease virus can bind the galactose-containing adjuvant α-galactosylceramide to form a composite particle for co-delivery of antigen and adjuvant to the same antigen-presenting cell. Vaccination with VLP and α-galactosylceramide activated splenic iNKT cells to produce IFN-γ and IL-4, led to the generation of antigen-specific T cells that protected prophylactically against subcutaneous tumor challenge, and was more effective at generating anti-tumor immune responses than either component individually. These data demonstrate a novel method for immunopotentiating VLP to increase their efficacy in the generation of anti-tumor responses via the innate ligand recognition properties of calicivirus-derived nanoparticles.

Rabbit haemorrhagic disease (RHD) and rabbit haemorrhagic disease virus (RHDV): a review

Rabbit haemorrhagic disease virus (RHDV) is a calicivirus of the genus Lagovirus that causes rabbit haemorrhagic disease (RHD) in adult European rabbits (Oryctolagus cuniculus). First described in China in 1984, the virus rapidly spread worldwide and is nowadays considered as endemic in several countries. In Australia and New Zealand where rabbits are pests, RHDV was purposely introduced for rabbit biocontrol. Factors that may have precipitated RHD emergence remain unclear, but non-pathogenic strains seem to pre-date the appearance of the pathogenic strains suggesting a key role for the comprehension of the virus origins. All pathogenic strains are classified within one single serotype, but two subtypes are recognised, RHDV and RHDVa. RHD causes high mortality in both domestic and wild adult animals, with individuals succumbing between 48-72 h post-infection. No other species has been reported to be fatally susceptible to RHD. The disease is characterised by acute necrotising hepatitis, but haemorrhages may also be found in other organs, in particular the lungs, heart, and kidneys due to disseminated intravascular coagulation. Resistance to the disease might be explained in part by genetically determined absence or weak expression of attachment factors, but humoral immunity is also important. Disease control in rabbitries relies mainly on vaccination and biosecurity measures. Such measures are difficult to be implemented in wild populations. More recent research has indicated that RHDV might be used as a molecular tool for therapeutic applications. Although the study of RHDV and RHD has been hampered by the lack of an appropriate cell culture system for the virus, several aspects of the replication, epizootology, epidemiology and evolution have been disclosed. This review provides a broad coverage and description of the current knowledge on the disease and the virus.

Pea-derived vaccines demonstrate high immunogenicity and protection in rabbits against rabbit haemorrhagic disease virus

Vaccines against rabbit haemorrhagic disease virus (RHDV) are commercially produced in experimentally infected rabbits. A genetically engineered and manufactured version of the major structural protein of RHDV (VP60) is considered to be an alternative approach for vaccine production. Plants have the potential to become an excellent recombinant production system, but the low expression level and insufficient immunogenic potency of plant-derived VP60 still hamper its practical use. In this study, we analysed the expression of a novel multimeric VP60-based antigen in four different plant species, including Nicotiana tabacum L., Solanum tuberosum L., Brassica napus L. and Pisum sativum L. Significant differences were detected in the expression patterns of the novel fusion antigen cholera toxin B subunit (CTB)::VP60 (ctbvp60(SEKDEL)) at the mRNA and protein levels. Pentameric CTB::VP60 molecules were only detected in N. tabacum and P. sativum, and displayed equal levels of CTB, at approximately 0.01% of total soluble protein (TSP), and traces of detectable VP60. However, strong enhancement of the CTB protein content via self-fertilization was only observed in P. sativum, where it reached up to 0.7% of TSP. In rabbits, a strong decrease in the protective vaccine dose required from 48-400 microg potato-derived VP60 [Castanon, S., Marin, M.S., Martin-Alonso, J.M., Boga, J.A., Casais, R., Humara, J.M., Ordas, R.J. and Parra, F. (1999) Immunization with potato plants expressing VP60 protein protects against rabbit hemorrhagic disease virus. J. Virol. 73, 4452-4455; Castanon, S., Martin-Alonso, J.M., Marin, M.S., Boga, J.A., Alonso, P., Parra, F. and Ordas, R.J. (2002) The effect of the promoter on expression of VP60 gene from rabbit hemorrhagic disease virus in potato plants. Plant Sci. 162, 87-95] to 0.56-0.28 microg antigenic VP60 (measured with VP60 enzyme-linked immunosorbent assay) of crude CTB::VP60 pea extracts was demonstrated. Rabbits immunized with pea-derived CTB::VP60 showed anti-VP60-specific antibodies, similar to RikaVacc((R))-immunized rabbits, and survived RHDV challenge.

Versatile RHDV virus-like particles: incorporation of antigens by genetic modification and chemical conjugation

Virus-like particles have proved to be excellent molecular scaffolds, yet the individual characteristics and immune responses generated against each VLP requires the development of a wide range of capsids for use as vaccines, molecular delivery vessels, and nanoscale templates. Here we describe the development of Rabbit haemorrhagic disease virus (RHDV)-like particles as a rapidly versatile molecular workbench, overcoming limitations imposed by established genetic antigen incorporation procedures with chimeric VLP. Production of the RHDV capsid protein in a baculovirus system led to the self-assembly of VLP which were recovered at over 99% purity and manipulated both genetically and chemically. Fusion of small peptide sequences to RHDV VLP was well tolerated, forming chimeric capsids that enhanced the presentation of foreign peptide to hybridoma T helper cells 700-fold. Rapid and simple conjugation techniques employing the hetero-bifunctional chemical linker sulfo-SMCC enabled both small peptides and whole proteins to be conjugated to the surface of RHDV VLP, overcoming limitations imposed on VLP formation and yield experienced with chimeric VLP. Administration of VLP/ovalbumin conjugate provoked high titre ovalbumin-specific antibody in mice, demonstrating the immune stimulatory properties of the capsid were conferred to conjugated foreign antigen. VLP facilitated delivery of conjugated antigen to dendritic cells, eliciting proliferative responses in naïve TCR transgenic T helper cells that were at least 10-fold greater than ovalbumin antigen delivered alone.

Development of a low-cost, insect larvae-derived recombinant subunit vaccine against RHDV

Vaccine antigens against rabbit hemorrhagic disease virus (RHDV) are currently derived from inactivated RHDV obtained from livers of experimentally infected rabbits. Several RHDV-derived recombinant immunogens have been reported. However, their application in vaccines has been restricted due to their high production costs. In this paper, we describe the development of an inexpensive, safe, stable vaccine antigen for RHDV. A baculovirus expressing a recombinant RHDV capsid protein (VP60r) was used to infect Trichoplusia ni insect larvae. It reached an expression efficiency of 12.5% of total soluble protein, i.e. approximately 2 mg of VP60r per larva. Preservation of the antigenicity and immunogenicity of the VP60r was confirmed by immunological and immunization experiments. Lyophilized crude larvae extracts, containing VP60r, were stable, at room temperature, for at least 800 days. In all cases, rabbits immunized with a single dose of VP60r by the intramuscular route were protected against RHDV challenge. Doses used were as low as 2 microg of VP60r in the presence of adjuvant or 100 microg without one. Orally administered VP60r in the absence of an adjuvant gave no protection. The potential costs of an RHDV vaccine made using this technology would be reduced considerably compared with producing the same protein in insect cells maintained by fermentation. In conclusion, the larva expression system may provide a broad-based strategy for production of recombinant subunit antigens (insectigens) for human or animal medicines, especially when production costs restrain their use.

Towards a unique and transmissible vaccine against myxomatosis and rabbit haemorrhagic disease for rabbit populations

Currently available vaccines against myxomatosis and rabbit hemorrhagic disease virus (RHDV) are not suited to immunise wild rabbit populations, as vaccines need to be delivered individually by conventional veterinary practices. As an alternative approach, research in Spain has focused on the development of a transmissible vaccine. A recombinant virus has been constructed based on a naturally attenuated myxoma virus (MV) field strain, expressing the RHDV capsid protein (VP60). Following inoculation of rabbits, the recombinant virus (MV-VP60) induced specific antibody responses against MV and RHDV, conferring protection against lethal challenges with both viruses. Furthermore, the recombinant MV-VP60 virus showed a limited horizontal transmission capacity, either by direct contact or in a flea-mediated process, promoting immunisation of contact uninoculated animals. Efficacy and safety of the vaccine have been extensively evaluated under laboratory conditions and in a limited field trial. The development of the transmissible vaccine strategy and the steps being taken to obtain the marketing authorisation for the vaccine in the European Union are presented in this review.

Small RNA Viruses of Insects: Expression in Plants and RNA Silencing

Interest in insect small RNA viruses (SRVs) has grown slowly but steadily. A number of new viruses have been analyzed at the sequence level, adding to our knowledge of their diversity at the level of both individual virus species and families. In particular, a number of possible new virus families have emerged. This research has largely been driven by interest in their potential for pest control, as well as in their importance as the causal agents of disease in beneficial arthropods. At the same time, research into known viruses has made valuable contributions to our understanding of an emerging new field of central importance to molecular biology-the existence of RNA-based gene silencing, developmental control, and adaptive immune systems in eukaryotes. Subject to RNA-based adaptive immune responses in their hosts, viruses have evolved a variety of genes encoding proteins capable of suppressing the immune response. Such genes were first identified in plant viruses, but the first examples known from animal viruses were identified in insect RNA viruses. This chapter will address the diversity of insect SRVs, and attempts to harness their simplicity in the engineering of transgenic plants expressing viruses for resistance to insect pests. We also describe RNA interference and antiviral pathways identified in plants and animals, how they have led viruses to evolve genes capable of suppressing such adaptive immunity, and the problems presented by these pathways for the strategy of expressing viruses in transgenic plants. Approaches for countering these problems are also discussed.

The coat protein of Rabbit hemorrhagic disease virus contains a molecular switch at the N-terminal region facing the inner surface of the capsid

To function adequately, many if not all proteins involved in macromolecular assemblies show conformational polymorphism as an intrinsic feature. This general strategy has been described for many essential cellular processes. Here we describe this structural polymorphism in a viral protein, the coat protein of Rabbit hemorrhagic disease virus (RHDV), which is required during virus capsid assembly. By combining genetic, structure modeling, and cryo-electron microscopy and image processing analysis, we have established the mechanism that allows RHDV coat protein to switch among quasi-equivalent conformational states to achieve the appropriate curvature for the formation of a closed shell. The RHDV capsid structure is based on a T = 3 lattice, containing 180 copies of identical subunits, similar to those of other caliciviruses. The quasi-equivalent interactions between the coat proteins are achieved by the N-terminal region of a subset of subunits, which faces the inner surface of the capsid shell. Mutant coat protein lacking this N-terminal sequence assembles into T = 1 capsids. Our results suggest that the polymorphism of the RHDV T = 3 capsid might bear resemblance to that of plant virus T = 3 capsids.

Rabbit Haemorrhagic Disease Virus: serological evidence of a non-virulent RHDV-like virus in south-western Australia

Although several different cELISAs have been used to assess the exposure of European rabbits to rabbit haemorrhagic disease (RHD), the interpretation of the results of such assays is not always straight-forward. Here we report on such difficulties, and on the likely presence of a non-virulent rabbit haemorrhagic disease virus–like virus (nvRHDV-LV) in south-western Australia. Analysis of sera collected from European rabbits at Kojaneerup (near Albany) in Western Australia provided the first serological evidence of the likely presence of a nvRHDV-LV in wild rabbit populations outside the east coast of Australia and New Zealand, before the deliberate introduction of RHDV as biological control agent in both countries. Six out of 30 rabbits (20%) sampled 1–2 months before the known arrival of RHDV at Kojaneerup were seropositive to RHD on the basis of their IgG isoELISAs. However, none of these positive samples were positive for the RHDV antibody cELISA (1 : 10), indicating likely exposure to nvRHDV-LV. Subsequent serological analysis of 986 rabbits sampled between September 1996 and August 1999 at Kojaneerup indicated that nvRHDV-LV persisted in these rabbits following the natural arrival of RHDV in September 1996. At least 10–34% of rabbits appeared to have been exposed to nvRHDV-LV during the 3-year study. The presence of nvRHDV-LV seemed to offer only limited protection to rabbits from RHDV during the initial epizootic; however, persistence of nvRHDV-LV may have mitigated further RHDV activity after this epizootic. Fewer than 1% of rabbits (9 of 986) showed evidence of RHDV-challenge during the 30 months following the initial RHDV epizootic. Furthermore, except for the epizootic in September 1996, no clinical signs of the disease were apparent in the population until RHDV was deliberately reintroduced in April 1999. Mortality of rabbits exposed to RHDV at this time appeared to be correlated with their IgG isoELISA titre.

Rabbit hemorrhagic disease virus: identification of a cleavage site in the viral polyprotein that is not processed by the known calicivirus protease

The positive-strand RNA genome of the calicivirus rabbit hemorrhagic disease virus (RHDV) contains one long open reading frame (ORF) covering almost 95% of the genomic RNA. Translation of this ORF leads to a polyprotein that is proteolytically processed at eight sites. Site nos. 4 and 5 from the amino-terminus are located within the protein p41 and obviously belong to alternative cleavage pathways leading to p23/2 and p18 or p29 and p13. Seven of the eight cleavage sites were identified before and the flanking sequences fulfill the requirements of the known RHDV protease, so that it is very likely that all theses sites are cleaved by this enzyme. The last unknown cleavage site was no. 4, one of the two alternative sites within p41 that separates the nonstructural proteins p23/2 and the VPg precursor p18. Mutagenesis studies identified aspartic acid at position 936 and arginine at position 937 as the residues that flank the cleavage site. The sequence at the processing site is unusual for the RHDV 3C-like protease since other sites display glutamic acid or glutamine at the P1 site and glycine, aspartic acid, or threonine at the P1' site. Expression of a polyprotein fragment lacking the viral protease revealed that the newly identified site is not cleaved by the RHDV protease but by an unknown proteolytic activity.

Structural requirements for the assembly of Norwalk virus-like particles

Norwalk virus (NV) is the prototype strain of a group of human caliciviruses responsible for epidemic outbreaks of acute gastroenteritis. While these viruses do not grow in tissue culture cells or animal models, expression of the capsid protein in insect cells results in the self-assembly of recombinant NV virus-like particles (rNV VLPs) that are morphologically and antigenically similar to native NV. The X-ray structure of the rNV VLPs has revealed that the capsid protein folds into two principal domains: a shell (S) domain and a protruding (P) domain (B. V. V. Prasad, M. E. Hardy, T. Dokland, J. Bella, M. G. Rossmann, and M. K. Estes, Science 286:287-290, 1999). To investigate the structural requirements for the assembly of rNV VLPs, we performed mutational analyses of the capsid protein. We examined the ability of 10 deletion mutants of the capsid protein to assemble into VLPs in insect cell cultures. Deletion of the N-terminal 20 residues, suggested by the X-ray structure to be involved in a switching mechanism during assembly, did not affect the ability of the mutant capsid protein to self-assemble into 38-nm VLPs with a T=3 icosahedral symmetry. Further deletions in the N-terminal region affected particle assembly. Deletions in the C-terminal regions of the P domain, involved in the interactions between the P and S domains, did not block the assembly process, but they affected the size and stability of the particles. Mutants carrying three internal deletion mutations in the P domain, involved in maintaining dimeric interactions, produced significantly larger 45-nm particles, albeit in low yields. The complete removal of the protruding domain resulted in the formation of smooth particles with a diameter that is slightly smaller than the 30-nm diameter expected from the rNV structure. These studies indicate that the shell domain of the NV capsid protein contains everything required to initiate the assembly of the capsid, whereas the entire protruding domain contributes to the increased stability of the capsid by adding intermolecular contacts between the dimeric subunits and may control the size of the capsid.

DNA vaccination against feline calicivirus infection using a plasmid encoding the mature capsid protein

Feline calicivirus (FCV), a member of the diverse family Caliciviridae, is a respiratory and oral pathogen of cats. Although conventional FCV vaccines are available, there are some safety and efficacy problems associated with their use. The potential of DNA vaccination against FCV infection was therefore explored. Four cats were inoculated intramuscularly with three 100 microg doses, 2 weeks apart, with a plasmid (pF9VAC) containing the mature capsid protein gene of FCV strain F9. Four control cats received the same plasmid lacking the FCV gene insert. All eight cats showed clinical signs following heterologous challenge with FCV strain LS027. However, rectal temperatures and general clinical sign scores were significantly lower in vaccinates compared to controls, and there was a marked difference in ulcer distribution between the two groups. Although no serological responses were detected in either group prior to challenge, post-challenge titres in the vaccinated group were generally higher. The results indicate that partial protection against a calicivirus is possible by DNA vaccination but that other approaches to enhance efficacy such as the use of cytokine genes or prime-boost protocols may also be required.

Protection of rabbits against rabbit hemorrhagic disease virus by immunization with the VP60 protein expressed in plants with a potyvirus-based vector

A new plum pox potyvirus (PPV)-based vector has been constructed for the expression of full-length individual foreign proteins. The foreign sequences are cloned between the NIb replicase and capsid protein (CP) cistrons. The heterologous protein is split from the rest of the potyviral polyprotein by cleavage at the site that originally separated the NIb and CP proteins and at an additional NIa protease recognition site engineered at its amino-terminal end. This vector (PPV-NK) has been used to clone different genes, engendering stable chimeras with practical applications. We have constructed a chimera expressing high levels of jellyfish green fluorescent protein, which can be very useful for the study of PPV molecular biology. The VP60 structural protein of rabbit hemorrhagic disease virus (RHDV) was also successfully expressed by making use of the PPV-NK vector. Inoculation of extracts from VP60-expressing plants induced a remarkable immune response against RHDV in rabbits, its natural host. Moreover, these animals were protected against a lethal challenge with RHDV.

Gene transfer using recombinant rabbit hemorrhagic disease virus capsids with genetically modified DNA encapsidation capacity by addition of packaging sequences from the L1 or L2 protein of human papillomavirus type 16

The aim of this study was to produce gene transfer vectors consisting of plasmid DNA packaged into virus-like particles (VLPs) with different cell tropisms. For this purpose, we have fused the N-terminally truncated VP60 capsid protein of the rabbit hemorrhagic disease virus (RHDV) with sequences which are expected to be sufficient to confer DNA packaging and gene transfer properties to the chimeric VLPs. Each of the two putative DNA-binding sequences of major L1 and minor L2 capsid proteins of human papillomavirus type 16 (HPV-16) were fused at the N terminus of the truncated VP60 protein. The two recombinant chimeric proteins expressed in insect cells self-assembled into VLPs similar in size and appearance to authentic RHDV virions. The chimeric proteins had acquired the ability to bind DNA. The two chimeric VLPs were therefore able to package plasmid DNA. However, only the chimeric VLPs containing the DNA packaging signal of the L1 protein were able efficiently to transfer genes into Cos-7 cells at a rate similar to that observed with papillomavirus L1 VLPs. It was possible to transfect only a very limited number of RK13 rabbit cells with the chimeric RHDV capsids containing the L2-binding sequence. The chimeric RHDV capsids containing the L1-binding sequence transfer genes into rabbit and hare cells at a higher rate than do HPV-16 L1 VLPs. However, no gene transfer was observed in human cell lines. The findings of this study demonstrate that the insertion of a DNA packaging sequence into a VLP which is not able to encapsidate DNA transforms this capsid into an artificial virus that could be used as a gene transfer vector. This possibility opens the way to designing new vectors with different cell tropisms by inserting such DNA packaging sequences into the major capsid proteins of other viruses.

Rabbit hemorrhagic disease virus: genome organization and polyprotein processing of a calicivirus studied after transient expression of cDNA constructs

Rabbit hemorrhagic disease virus (RHDV) belongs to the family Caliciviridae. Studies on this virus are hampered by the lack of a convenient cell culture system. To study viral protein expression a cDNA construct containing the entire protein-coding region of the virus was established and used for transient expression studies. After metabolic labeling of transfected cells and immunoprecipitation with a set of RHDV-specific antisera a variety of polypeptides were identified and assigned to defined regions of the viral genome. The consensus sequences of already identified or putative proteolytic cleavage sites in the viral polyprotein were changed by the introduction of mutations into the expression construct. Expression of these mutated constructs and analysis of the protein patterns allowed us to identify novel cleavage sites in the polyprotein and revealed the first details regarding the order of polyprotein processing.

Horizontal transmissible protection against myxomatosis and rabbit hemorrhagic disease by using a recombinant myxoma virus

We have developed a new strategy for immunization of wild rabbit populations against myxomatosis and rabbit hemorrhagic disease (RHD) that uses recombinant viruses based on a naturally attenuated field strain of myxoma virus (MV). The recombinant viruses expressed the RHDV major capsid protein (VP60) including a linear epitope tag from the transmissible gastroenteritis virus (TGEV) nucleoprotein. Following inoculation, the recombinant viruses induced specific antibody responses against MV, RHDV, and the TGEV tag. Immunization of wild rabbits by the subcutaneous and oral routes conferred protection against virulent RHDV and MV challenges. The recombinant viruses showed a limited horizontal transmission capacity, either by direct contact or in a flea-mediated process, promoting immunization of contact uninoculated animals.

Core particles of hepatitis B virus as carrier for foreign epitopes

To be effective as vaccines, most monomeric proteins and peptides either require chemical coupling to high molecular weight carriers or application together with adjuvants. More recently, recombinant DNA techniques have been used to insert foreign epitopes into proteins with inherent multimerization capacity, such as particle-forming viral capsid or envelope proteins. The core protein of hepatitis B virus (HBcAg), because of its unique structural and immunological properties, has gained widespread interest as a potential antigen carrier. Foreign sequences of up to approximately 40 amino acid residues at the N terminus, 50 or 100 amino acids in the central immunodominant c/e 1 epitope region of HBcAg, and up to 100 or even more residues at the C terminus, did not interfere with particle formation. The humoral immunogenicity of inserted epitopes is determined by the immunogenicity of the peptide itself and its surface exposure, and is influenced by the route of application. The probably flexible and surface-exposed c/e1 region emerged as the most promising insertion site. When applied together with adjuvants approved for human and veterinary use, or even without adjuvants, such chimeric particles induced B and T cell immune responses against the inserted epitopes. In some cases neutralizing antibodies, cytotoxic T cells and protection against challenge with the intact pathogen were demonstrated. Major factors for the potentiated immune response against the foreign epitopes are the multimeric structure of chimeric HBcAg that results in a high epitope density per particle, and the provision of T cell help by the carrier moiety. Beyond its use as subunit vaccine, chimeric HBcAg produced in attenuated Salmonella strains may be applicable as live vaccine.

Viral zoonoses and food of animal origin: caliciviruses and human disease

Caliciviruses are important veterinary and human pathogens. The viruses gain their name from characteristic cup-shaped structures seen on the virion surface by negative stain electron microscopy. In humans caliciviruses are a major cause of diarrhoeal disease. There are two fundamentally different genome structures amongst human caliciviruses. The Norwalk-like or small round structured viruses (SRSVs) are viruses that have an amorphous structure when viewed by EM, they have a genome composed of 3 major open reading frames (ORFs). These viruses cause epidemic gastroenteritis amongst all age groups. In contrast, the 'classic' human caliciviruses (HuCVs) display the typical calicivirus surface structure and have their capsid ORF fused to and contiguous with the non structural proteins forming one giant polyprotein. HuCVs are predominantly associated with paediatric infections and are only a minor cause of disease in humans. Spread of disease for both SRSVs and HuCVs is usually by faecal oral transmission. SRSVs are a major cause of foodborne gastroenteritis especially linked to the consumption of sewage-contaminated shellfish. However, there is no evidence that these viruses replicate in shellfish or that they originate from an animal source.

Bivalent binding of a neutralising antibody to a calicivirus involves the torsional flexibility of the antibody hinge

The structure of a complex between rabbit haemorrhagic disease virus (RHDV) virus-like particles (VLPs) and a neutralising monoclonal antibody mAb-E3 has been determined at low resolution by cryo-electron microscopy and three-dimensional (3-D) reconstruction techniques. The atomic co-ordinates of an Fab were fitted to the cryo-electron microscope density map to produce a binding model. The VLP has a T = 3 icosahedral lattice consisting of a hollow spherical shell with 90 protruding arches. Each dimeric arch presents two mAb binding sites; however, steric hindrance between the variable domains of the Fabs prevents the occupation of both sites simultaneously. Thus the maximum mAb occupation is 50%. Once a mAb is bound to one site it may bind to either of two neighbouring sites related by a local 3-fold axis. The mAbs are bound bivalently on epitopes not related by a 2-fold symmetry axis. This binding geometry implies a torsional flexibility of the mAb hinge region, involving a 60 degrees rotation of one Fab arm with respect to the other. Owing to extreme flexibility of the hinge region, the Fc domains occupy random orientations and are not visible in the reconstruction. The bivalent attachment of mAb-E3 to RHDV suggests that the neutralisation mechanism(s) involves inhibition of viral decapsidation and/or the inhibition of binding to the receptor.

Particulate vaccine candidate for Japanese encephalitis induces long-lasting virus-specific memory T lymphocytes in mice

We previously reported that extracellular particles (EPs) composed of premembrane (prM) and envelope (E) proteins were released from cells infected with recombinant vaccinia viruses encoding Japanese encephalitis (JE) virus prM and E genes. In the present study, EPs were evaluated for induction of JE virus-specific antibody and specific T lymphocytes in mice. Six- to 8-week-old male Balb/c mice were inoculated intraperitoneally once or twice (at a 3-week interval) with purified EPs containing 1 microgram of E without adjuvant. Neutralizing antibody was detected and spleen cells proliferated against JE viral antigen 3 weeks after the second immunization with EPs. Neutralizing antibody and JE virus-specific T lymphocytes were also detected 10 months after immunization with EPs containing 2 micrograms of E. Spleen cells obtained from EP-immunized mice and stimulated in vitro with live JE virus, expressed JE virus-specific cytotoxic activity. The cytotoxic activity was reduced by treatment with anti-CD3 antibody and complement. These results indicate that immunization with EPs induces long-lasting specific antibody and memory T cells in mice.

The complete nucleotide sequence of rabbit haemorrhagic disease virus (Czech strain V351): use of the polymerase chain reaction to detect replication in Australian vertebrates and analysis of viral population sequence variation

The complete nucleotide sequence of the Czech strain of rabbit haemorrhagic disease virus (RHDV) was determined to be 7437 nucleotides in length with a 5-terminal non-coding region of 9 nucleotides and a 3'-terminal non-coding region of 59 nucleotides. Two open reading frames (ORFs) were found within this sequence coding for polypeptides of 2344 (nucleotides 10-7044) and 117 amino acids (nucleotides 7025-7378). The sequence of this isolate was approximately 1% different from that reported by Meyers et al., having 78 nucleotide changes which resulted in 30 amino acid differences, the majority of these clustering in the N-terminus of the large ORF and the middle of the viral coat protein. Only a single conservative amino acid change was seen in the smaller 3'-terminal ORF. Since the virus cannot at present be propagated in tissue culture, but isolated only after replication in rabbits, the reported sequence must be considered as a consensus sequence from the viral population. To gain some understanding of the possible sequence diversity within this virus population, 97 clones were sequenced from a polymerase chain reaction (PCR) fragment to determine the sequence diversity of the virus population. Four major classes of variant were described with mutations generally in the third base position of codons. A nested reverse transcriptase (RT) PCR (using sequence derived for the coat protein of RHDV) was used to determine the presence or absence of RHDV inoculated into non-host animal species. No replication of the virus was detected in 28 different vertebrate species other than rabbits. PCR tests on both mosquitoes and fleas feeding on RHDV infected rabbits were positive. The RT-PCR test was more sensitive when compared with an antigen capture ELISA to detect the presence of genomic RNA/or virus in infected rabbits.

Genetic map of the calicivirus rabbit hemorrhagic disease virus as deduced from in vitro translation studies

The 7.5-kb plus-stranded genomic RNA of rabbit hemorrhagic disease virus contains two open reading frames of 7 kb (ORF1) and 351 nucleotides (ORF2) that cover nearly 99% of the genome. The aim of the present study was to identify the proteins encoded in these open reading frames. To this end, a panel of region-specific antisera was generated by immunization of rabbits with bacterially expressed fusion proteins that encompass in total 95% of the ORF1 polyprotein and almost the complete ORF2 polypeptide. The antisera were used to analyze the in vitro translation products of purified virion RNA of rabbit hemorrhagic disease virus. Our studies show that the N-terminal half of the ORF1 polyprotein is proteolytically cleaved to yield three nonstructural proteins of 16, 23, and 37 kDa (p16, p23, and p37, respectively). In addition, a cleavage product of 41 kDa which is composed of VPg and a putative nonstructural protein of approximately 30 kDa was identified. Together with the results of previous studies which identified a trypsin-like cysteine protease (TCP) of 15 kDa, a putative RNA polymerase (pol) of 58 kDa, and the major capsid protein VP60, our data establish the following gene order in ORF1: NH2-p16-p23-p37 (helicase)-p30-VPg-TCP-pol-VP60-COOH. Immunoblot analyses showed that a minor structural protein of 10 kDa is encoded in ORF2. The data provide the first complete genetic map of a calicivirus. The map reveals a remarkable similarity between caliciviruses and picornaviruses with regard to the number and order of the genes that encode the nonstructural proteins.

Expression of the major inner capsid protein of the epizootic haemorrhagic disease virus in baculovirus and potential diagnostic use

The RNA 7 encoding the major capsid protein (VP7) of epizootic haemorrhagic disease virus (EHDV), Australian serotype 2 (strain CS439), was cloned and the complete nucleotide sequence was determined. The coding region contained 1047 nucleotides (nt) capable of encoding a predicted 349 amino acid (aa) polypeptide with a calculated molecular size of 38.087 kDa. When the VP7 gene was expressed in bacterial or yeast expression systems, the expression product showed weak or no reactivity with polyclonal antibodies to EHDV. Therefore, the expression of the VP7 gene in baculovirus was pursued. The expressed EHDV VP7 was similar in antigenicity to that of the native virus as revealed by its reactivity in ELISA with monoclonal antibody (MAb) specific to EHDV. Preliminary ELISA results indicated that the recombinant protein binds to EHDV antibodies in serum and that these antibodies block the binding of EHDV-specific MAb. The availability of a reliable EHDV recombinant VP7 could enhance our existing assay for detection of EHDV-specific antibodies.

Presence of rabbit haemorrhagic disease virus antigen in rabbit tissues as revealed by a monoclonal antibody dependent capture ELISA

The production of monoclonal antibodies (mAbs) to a common antigenic region on rabbit haemorrhagic disease virus (RHDV) has enabled the development of a capture ELISA for virus detection. The assay was shown to detect reliably the presence of viral antigen in crude homogenates of a range of rabbit tissues and has provided the first evidence for the presence of RHDV in blood, serum and heart tissue. A limited time course study of the progression of virus infection revealed viral antigen is first detected in the liver at between 18 and 24 h post-infection (p.i.) and in the spleen between 24 and 30 h p.i. The assay displayed a high level of specificity, clearly differentiating between groups of infected (lowest observed optical density (O.D.) 0.45) and uninfected (highest O.D. 0.06) rabbits. No viral antigen was detected in the urine or faeces collected from infected rabbits at post-mortem. Experiments employing the 'spiking' of pools of urine and faecal homogenates collected from uninfected rabbits, with a known amount of virus, indicated that RHDV may not be present in the faeces of infected animals and if present in urine, it appears to undergo substantial degradation.