Safety evaluation of a recombinant myxoma-RHDV virus inducing horizontal transmissible protection against myxomatosis and rabbit haemorrhagic disease

Screening optimal DC-targeting peptide to enhance the immune efficacy of recombinant Lactobacillus expressing RHDV VP60

Dendritic cells (DCs) present an ideal target for delivering immunogenic cargo due to their potent antigen-presenting capabilities. This targeting approach holds promise in vaccine development by enhancing the efficiency of antigen recognition and capture by DCs. To identify a high-affinity targeting peptide binding to rabbit DCs, rabbit monocyte-derived DCs (raMoDCs) were isolated and cultured, and a novel peptide, HS (HSLRHDYGYPGH), was identified using a phage-displayed peptide library. Alongside HS, two other DC-targeting peptides, KC1 and MY, previously validated in our laboratory, were employed to construct recombinant Lactgobacillus reuteri fusion-expressed rabbit hemorrhagic disease virus (RHDV) capsid protein VP60. These recombinant Lactobacillus strains were named HS-VP60/L. reuteri, KC1-VP60/L. reuteri, and MY-VP60/L. reuteri. The ability of these recombinant Lactobacillus to bind rabbit DCs was evaluated both in vivo and in vitro. Results demonstrated that the DC-targeting peptide KC1 significantly enhanced the capture efficiency of recombinant Lactobacillus by raMoDCs, promoted DC maturation, and increased cytokine secretion. Furthermore, oral administration of KC1-VP60/L. reuteri effectively induced SIgA and IgG production in rabbits, prolonged rabbit survival post-challenge, and reduced RHDV copies in organs. In summary, the DC-targeting peptide KC1 exhibited robust binding to raMoDCs, and recombinant Lactobacillus expressing KC1-VP60 protein antigens efficiently induced systemic and mucosal immune responses in rabbits, conferring protective efficacy against RHDV. This study offers valuable insights for the development of novel RHDV vaccines.

Immunological Cross-Protection between Different Rabbit Hemorrhagic Disease Viruses—Implications for Rabbit Biocontrol and Vaccine Development

The use of rabbit hemorrhagic disease virus (RHDV) as a biocontrol agent to control feral rabbit populations in Australia, in combination with circulating endemic strains, provides a unique environment to observe the interactions between different lagoviruses competing for the same host. Following the arrival of RHDV2 (GI.2) in Australia, it became necessary to investigate the potential for immunological cross-protection between different variants, and the implications of this for biocontrol programs and vaccine development. Laboratory rabbits of various immune status—(1) rabbits with no detectable immunity against RHDV; (2) rabbits with experimentally acquired immunity after laboratory challenge; (3) rabbits immunised with a GI.2-specific or a multivalent RHDV inactivated virus prototype vaccine; or (4) rabbits with naturally acquired immunity—were challenged with one of three different RHDV variants (GI.1c, GI.1a or GI.2). The degree of cross-protection observed in immune rabbits was associated with the variant used for challenge, infectious dose of the virus and age, or time since acquisition of the immunity, at challenge. The immune status of feral rabbit populations should be determined prior to intentional RHDV release because of the high survival proportions in rabbits with pre-existing immunity. In addition, to protect domestic rabbits in Australia, a multivalent RHDV vaccine should be considered because of the limited cross-protection observed in rabbits given monovalent vaccines.

Progression of rabbit haemorrhagic disease virus 2 upon vaccination in an industrial rabbitry: a laboratorial approach

<p>Rabbit haemorrhagic disease virus 2 (RHDV2) emerged recently in several European countries, leading to extensive economic losses in the industry. In response to this new infection, specific inactivated vaccines were developed in Europe and full and rapid setup of protective immunity induced by vaccination was reported. However, data on the efficacy of these vaccines in an ongoing-infection scenario is unavailable. In this study we investigated an infected RHDV2 indoor industrial meat rabbitry, where fatalities continued to occur after the implementation of the RHDV2 vaccination, introduced to control the disease. The aim of this study was to understand if these mortalities were RHDV2-related, to discover if the dead animals showed any common features such as age or time distance from vaccination, and to identify the source of the outbreak. Anatomo-pathological analysis of vaccinated animals with the virus showed lesions compatible with systemic haemorrhagic disease and RHDV2-RNA was detected in 85.7% of the animals tested. Sequencing of the <em>vp60</em> gene amplified from liver samples led to the recognition of RHDV2 field strains demonstrating that after the implementation of vaccination, RHDV2 continued to circulate in the premises and to cause sporadic deaths. A nearby, semi-intensive, RHDV2 infected farm belonging to the same owner was identified as the most probable source of the virus. The main risk factors for virus introduction in these two industries were identified. Despite the virus being able to infect a few of the vaccinated rabbits, the significant decrease in mortality rate observed in vaccinated adult rabbits clearly reflects the efficacy of the vaccination. Nonetheless, the time taken to control the infection also highlights the importance of RHDV2 vaccination prior to the first contact with the virus, highly recommendable in endemic areas, to mitigate the infection’s impact on the industry.</p>

The evolution of poxvirus vaccines

After Edward Jenner established human vaccination over 200 years ago, attenuated poxviruses became key players to contain the deadliest virus of its own family: Variola virus (VARV), the causative agent of smallpox. Cowpox virus (CPXV) and horsepox virus (HSPV) were extensively used to this end, passaged in cattle and humans until the appearance of vaccinia virus (VACV), which was used in the final campaigns aimed to eradicate the disease, an endeavor that was accomplished by the World Health Organization (WHO) in 1980. Ever since, naturally evolved strains used for vaccination were introduced into research laboratories where VACV and other poxviruses with improved safety profiles were generated. Recombinant DNA technology along with the DNA genome features of this virus family allowed the generation of vaccines against heterologous diseases, and the specific insertion and deletion of poxvirus genes generated an even broader spectrum of modified viruses with new properties that increase their immunogenicity and safety profile as vaccine vectors. In this review, we highlight the evolution of poxvirus vaccines, from first generation to the current status, pointing out how different vaccines have emerged and approaches that are being followed up in the development of more rational vaccines against a wide range of diseases.

Cutaneous lesions in pet rabbits following subcutaneous administration of a novel bivalent vaccine against myxomatosis and rabbit haemorrhagic disease

BACKGROUND

A novel bivalent vaccine to protect against myxomatosis and rabbit haemorrhagic disease is commercially available for pet rabbits.

HYPOTHESIS/OBJECTIVES

To describe the appearance of cutaneous lesions arising in pet rabbits positive for myxoma virus (MV) by RT-PCR evaluation shortly after vaccination.

ANIMALS

Four pet rabbits presenting with papular, crusting skin lesions ~10 days after vaccination.

METHODS

Histological evaluation of formalin-fixed skin biopsies obtained from lesional skin (case 1). Real-time polymerase chain reaction (RT-PCR) evaluation of paraffin-embedded tissue from skin biopsies (case 1) and crusts obtained from the lesion surface (cases 2-4) for myxoma virus are reported as cycle threshold (Ct ) values.

RESULTS

Lesions affecting the ear pinna, dorsal aspect of the nose, vulva and/or conjunctiva are reported. Histopathological findings included severe ulcerative, necrotizing dermatitis and intralesional cytoplasmic inclusion bodies in myxoma cells. DNA was amplified from all the paraffin-embedded skin biopsies (Ct  = 34-35) and crusts (Ct  = 20-24).

CONCLUSIONS AND CLINICAL IMPORTANCE

Although a wild virus challenge cannot be definitively excluded, veterinarians and pet-owners should be aware that cutaneous lesions have been observed after vaccination with this novel vaccine in low numbers of rabbits.

Myxomatosis in Australia and Europe: a model for emerging infectious diseases

Myxoma virus is a poxvirus naturally found in two American leporid (rabbit) species (Sylvilagus brasiliensis and Sylvilagus bachmani) in which it causes an innocuous localised cutaneous fibroma. However, in European rabbits (Oryctolagus cuniculus) the same virus causes the lethal disseminated disease myxomatosis. The introduction of myxoma virus into the European rabbit population in Australia in 1950 initiated the best known example of what happens when a novel pathogen jumps into a completely naïve new mammalian host species. The short generation time of the rabbit and their vast numbers in Australia meant evolution could be studied in real time. The carefully documented emergence of attenuated strains of virus that were more effectively transmitted by the mosquito vector and the subsequent selection of rabbits with genetic resistance to myxomatosis is the paradigm for pathogen virulence and host-pathogen coevolution. This natural experiment was repeated with the release of a separate strain of myxoma virus in France in 1952. The subsequent spread of the virus throughout Europe and its coevolution with the rabbit essentially paralleled what occurred in Australia. Detailed molecular studies on myxoma virus have dissected the role of virulence genes in the pathogenesis of myxomatosis and when combined with genomic data and reverse genetics should in future enable the understanding of the molecular evolution of the virus as it adapted to its new host. This review describes the natural history and evolution of myxoma virus together with the molecular biology and experimental pathogenesis studies that are informing our understanding of evolution of emerging diseases.

The current status and future directions of myxoma virus, a master in immune evasion

Myxoma virus (MYXV) gained importance throughout the twentieth century because of the use of the highly virulent Standard Laboratory Strain (SLS) by the Australian government in the attempt to control the feral Australian population of Oryctolagus cuniculus (European rabbit) and the subsequent illegal release of MYXV in Europe. In the European rabbit, MYXV causes a disease with an exceedingly high mortality rate, named myxomatosis, which is passively transmitted by biting arthropod vectors. MYXV still has a great impact on European rabbit populations around the world. In contrast, only a single cutaneous lesion, restricted to the point of inoculation, is seen in its natural long-term host, the South-American Sylvilagus brasiliensis and the North-American S. Bachmani. Apart from being detrimental for European rabbits, however, MYXV has also become of interest in human medicine in the last two decades for two reasons. Firstly, due to the strong immune suppressing effects of certain MYXV proteins, several secreted virus-encoded immunomodulators (e.g. Serp-1) are being developed to treat systemic inflammatory syndromes such as cardiovascular disease in humans. Secondly, due to the inherent ability of MYXV to infect a broad spectrum of human cancer cells, the live virus is also being developed as an oncolytic virotherapeutic to treat human cancer. In this review, an update will be given on the current status of MYXV in rabbits as well as its potential in human medicine in the twenty-first century.

Table of contents

Abstract

1. The virus

2. History

3. Pathogenesis and disease symptoms

4. Immunomodulatory proteins of MYXV

4.1. MYXV proteins with anti-apoptotic functions

4.1.1. Inhibition of pro-apoptotic molecules

4.1.2. Inhibition by protein-protein interactions by ankyrin repeat viral proteins

4.1.3. Inhibition of apoptosis by enhancing the degradation of cellular proteins

4.1.4. Inhibition of apoptosis by blocking host Protein Kinase R (PKR)

4.2. MYXV proteins interfering with leukocyte chemotaxis

4.3. MYXV serpins that inhibit cellular pro-inflammatory or pro-apoptotic proteases

4.4. MYXV proteins that interfere with leukocyte activation

4.5. MYXV proteins with sequence similarity to HIV proteins

4.6. MYXV proteins with unknown immune function

5. Vaccination strategies against myxomatosis

5.1. Current MYXV vaccines

5.2. Vaccination campaigns to protect European rabbits in the wild

6. Applications of myxoma virus for human medicine

6.1. MYXV proteins as therapeutics for allograft vasculopathy and atherosclerosis

6.2. Applications for MYXV as a live oncolytic virus to treat cancer

7. Discussion and Conclusions

8. List of Abbreviations

References

Author Details

Authors' contributions

Competing interests

Figure Legends

Acknowledgements

Antigen delivery systems for veterinary vaccine development. Viral-vector based delivery systems

The recent advances in molecular genetics, pathogenesis and immunology have provided an optimal framework for developing novel approaches in the rational design of vaccines effective against viral epizootic diseases. This paper reviews most of the viral-vector based antigen delivery systems (ADSs) recently developed for vaccine testing in veterinary species, including attenuated virus and DNA and RNA viral vectors. Besides their usefulness in vaccinology, these ADSs constitute invaluable tools to researchers for understanding the nature of protective responses in different species, opening the possibility of modulating or potentiating relevant immune mechanisms involved in protection.

Genome comparison of a nonpathogenic myxoma virus field strain with its ancestor, the virulent Lausanne strain

One of the best-studied examples of host-virus coevolution is the release of myxoma virus (MV) for biological control of European rabbits in Australia and Europe. To investigate the genetic basis of MV adaptation to its new host, we sequenced the genome of 6918, an attenuated Spanish field strain, and compared it with that of Lausanne, the strain originally released in Europe in 1952. Although isolated 43 years apart, the genomes were highly conserved (99.95% identical). Only 32 of the 159 MV predicted proteins revealed amino acid changes. Four genes (M009L, M036L, M135R, and M148R) in 6918 were disrupted by frameshift mutations.

Construction and testing of a novel host-range defective myxoma virus vaccine with the M063 gene inactivated that is non-permissive for replication in rabbit cells

Deletion of the M063 gene from myxoma virus produces a virus that is unable to replicate in rabbit cells in vitro or in live rabbits but can be propagated in non-rabbit cell lines. A targeted M063 deletion mutant was constructed in the attenuated Uriarra strain of myxoma virus and the ability of this virus to act as a safe, non-transmissible vaccine against myxomatosis was tested in outbred laboratory rabbits. Immunization with the M063 deletion vaccine provided good short-term protection against lethal challenge with virulent myxoma virus. Long-term protection was similar to reported results with heterologous live virus, with some rabbits protected but others succumbing to challenge. Replication-deficient poxvirus vaccines, like the Modified Vaccinia Virus Ankara (MVA) in man and the myxoma virus vaccine described here in rabbits, are very attractive from a safety perspective. Seasonal boosting would be predicted to provide long-term protection. Targeted host-range gene deletions could have potential for rapid development of poxvirus vaccines in general.

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.

Pest or prized possession? Genetically modified biocontrol from an international perspective

This article provides an overview of current research, regulations and international issues concerning genetically modified (GM) organisms for use as biological controls of vertebrates. There is increasing interest in using biotechnology to solve vertebrate pest problems around the world. A major issue lies in the fact that individual countries focusing on internal problems of pest management may overlook the potential of transborder entry. Animals considered a pest in one country may well be prized possessions in another, and research and management strategies should consider the adverse effects of biocontrol agents entering the ‘wrong’ country. There is a wealth of guidance in the form of national and international regulations and ethics guidelines. However, current legislation and agreements may not be adequate to ensure that all risks of GM biocontrols, particularly disseminating agents, have been considered from an international perspective. Major issues include concerns of transboundary movement, non-target effects and the need for an international body to consult with and regulate the use of GM biocontrols. We live in a finite and interconnected world: it is vital that impacts of potential control strategies are assessed at a local and international level, and from social, environmental and economic perspectives.

A review of mutual transmission of important infectious diseases between livestock and wildlife in Europe

Oral vaccination of red foxes against rabies has been practiced in Europe since 1978 and has succeeded in greatly reducing the occurrence of this disease in foxes: this is an example of coordinated activity against a disease that affects both wild and domestic animals as well as humans. Some examples of diseases that affect both domestic and wild animals in Europe are: classical swine fever (hog cholera) in wild boars and domestic swine; myxomatosis and rabbit hemorrhagic disease in domestic and wild rabbits; bovine viral diarrhea (BVD) in cattle and roe deer; contagious ecthyma in domestic sheep and goats and also in, e.g., chamois, muskox, and reindeer; Mycobacterium bovis in cattle, wild boars, badgers, and deer; and brucellosis in a broad range of livestock and wildlife in all European countries. In addition, serological surveys performed in different free-ranging ungulate species revealed the presence of alphaherpesviruses related to bovine herpesvirus-1 in 7 European countries; and a study of malignant catarrhal fever in deer in Germany might indicate that in this case sheep are the main reservoir species. Although many data on infectious diseases are available in various European countries, there is more need for systematic surveillance and coordinated research.

First field trial of a transmissible recombinant vaccine against myxomatosis and rabbit hemorrhagic disease

As a novel approach for immunisation of wild rabbits, we have recently developed a transmissible vaccine against myxomatosis and rabbit hemorrhagic disease (RHD) based on a recombinant myxoma virus (MV) expressing the RHDV capsid protein [J. Virol. 74 (2000) 1114]. The efficacy and safety of the vaccine have been extensively evaluated under laboratory conditions. In this study, we report the first limited field trial of the candidate vaccine that was undertaken in an island of 34 Has containing a population of around 300 rabbits. Following administration by the subcutaneous route to 76 rabbits, the vaccine induced specific antibody responses against both myxomatosis and RHDV in all the inoculated rabbits. Furthermore, the recombinant virus exhibited a limited horizontal transmission capacity, promoting seroconversion of around 50% of the uninoculated rabbit population. No evidence of undesirable effects due to the recombinant virus field release was detected.