Antigenic characterisation of infectious bursal disease virus to improve vaccination strategies and vaccine design

The poultry industry is essential to securing enough food for the expanding human population. However, infectious bursal disease virus (IBDV), responsible for Gumboro Disease in chickens, is recognised as a worldwide concern for the industry. The disease can be severe and lead to high death rates and suppressed immune responses that can exacerbate other infections. So great is the concern that every year billions of chickens are vaccinated against IBDV and there has been considerable investment into new types of vaccine. Vaccines typically work by stimulating the production of neutralising antibodies that bind the virus and prevent it from entering the host cells. However, despite these developments, numerous vaccine failures have been reported, with some flocks experiencing high death rates, despite being vaccinated, for reasons that are poorly understood.

When a vaccine failure occurs, the sequence of the virus circulating in the flock (field strain) is determined and if there are mutations found, it is often presumed that a mis-match between the vaccine strain and the field strain is responsible for the failure. However, while some changes in the virus sequence will cause a vaccine to be ineffective, this is not the case for every mutation, and there are many other potential reasons why a vaccine may fail, including poor timing of vaccination or inadequate dosing. It is therefore important to determine which sequence changes result in vaccine failures and which do not, in order to determine whether a more matched vaccine is needed in a particular region. One way to discern this is to determine the ability of the vaccine to induce antibodies that neutralise the field strain, in so-called 'neutralisation assays'. However, neutralisation assays are not routinely conducted, as field strains of IBDV do not infect the cells that are typically used.

Field strains of IBDV infect cells in an organ known as the bursa of Fabricius. Dr Andrew Broadbent at The Pirbright Institute has shown that when cells are removed from the bursa and kept alive in the laboratory, field strains of IBDV will infect and replicate in them. He has used these cells to conduct neutralisation assays with field strains of IBDV in order to quantify neutralising antibodies. In addition, his group has developed IBDV strains that are tagged with a small molecule (GFP11). When the tag interacts with another, larger molecule (GFP1-10), it fluoresces green. Professor Helen Sang's group at The Roslin Institute has developed chickens that contain the GFP1-10 molecule in every cell. A collaboration between the two groups has already shown that cells from these chickens fluoresce green when infected with the tagged IBDV, meaning they can be used in the neutralisation assay to make it higher throughput.

We aim to generate a panel of tagged IBDVs from diverse strains from different geographical areas, and use bursal cells containing GFP1-10 to determine whether IBDV vaccines cross-neutralise different IBDV strains. This will help determine the extent to which vaccine failure is due to changes in the virus sequence. Moreover, we will screen the panel of viruses to determine whether certain strains induce antibodies that cross-neutralise others, and we will identify common regions that the antibodies bind to that are shared between the different strains. If we identify common antibody binding sites between strains that cross-neutralise, then it may be possible to develop a more universal IBDV vaccine that can protect against multiple strains in the future.

Infectious bursal disease virus (IBDV), which causes disease and immunosuppression in chickens, is recognised as a threat to food security. Vaccination is key to disease control, however, vaccine failures have been reported for reasons that are complex. It is important to determine the contribution of antigenic drift to vaccine failure in order to evaluate the need for a more antigenically matched vaccine in a given region. Additionally, the IBDV population has recently been divided into 7 genogroups based on the sequence of the capsid hypervariable region, however, how the genogroups are related to one another antigenically, & the extent to which antibodies elicited by one genogroup cross-neutralise the others, is poorly understood. A cell-based method for quantifying neutralising antibody titres against IBDV field strains is lacking, as IBDV field strains do not readily infect the cells typically used in neutralisation assays. Andrew Broadbent (AB) has shown that primary bursal cells cultured ex vivo can support the replication of IBDV field strains and be used in neutralisation assays. AB has also developed an IBDV reverse genetics system and engineered a split GFP IBDV containing a GFP11 tag. Helen Sang (HS) has engineered transgenic chickens that express GFP1-10. AB and HS have shown that cells from these chickens fluoresce green when infected with split GFP IBDV. We plan to combine these tools to generate a higher throughput neutralisation assay to determine the breadth of neutralising antibody responses generated by commercial IBDV vaccines against a panel of GFP11-tagged IBDVs from diverse strains. This will help determine the extent to which vaccine failure is due to antigenic drift. Moreover, we will evaluate the ability of the strains to induce antibodies that cross-neutralise one another, and we will identify common epitopes between these strains to potentially design antigens that elicit more broadly neutralising antibodies in the future.

This proposal combines novel tools generated by scientists at both The Pirbright Institute (TPI) and The Roslin Institute (TRI) in order to create new knowledge regarding infectious bursal disease virus (IBDV), an economically important pathogen of poultry that threatens global food security. Data from this project can be readily applied to improve vaccination strategies and develop future vaccines, thus driving innovation. Those who will benefit from the project include:

Vaccine companies. The drive for antigenically matched vaccines against locally circulating strains creates smaller markets which are a disincentive for larger companies to invest in IBDV vaccines. The results from this proposal will help determine the need for a matched vaccine in a region and aid the discovery of antigens and epitopes that elicit more broadly cross-neutralising antibodies. This knowledge could be applied to the development of a more universal IBDV vaccine in the future. Such a vaccine would have a worldwide market, being more profitable for the companies, while leading to enhanced disease control. To this end we have received letters of support from MSD Animal Health.

Farmers and poultry producers. If it became policy to screen how well field isolates in a particular region were neutralised by different commercial vaccines, it would enable a farmer or poultry producer to choose the most appropriate vaccine to use in the flock. Moreover, if a more universal vaccine was made available, IBDV will be better controlled in the future, meaning farmers will be able to make more money due to better flock performance, leading to improved economic resilience. It should be noted that, as IBDV also causes immunosuppression that exacerbates secondary infections and reduces the effectiveness of vaccination programmes, there will be an added economic benefit in excess of controlling morbidity and mortality caused by the initial infection.

People concerned with animal health and welfare. As the knowledge generated from this proposal could be applied to improve IBDV control there will be less morbidity and mortality in infected flocks, with an overall benefit to animal welfare. As IBDV is endemic worldwide, improved control would have a significant impact on animal welfare across the globe.

The public. Should IBDV be better controlled, the poultry industries will be more productive, boosting the economy in general. Moreover, if the productivity of the poultry industry improves, there will be more food to feed the growing human population and improve the overall nutritional status of communities.

The UK poultry industry. The data generated in this project will be useful for preventing the incursion of exotic strains of IBDV into the UK as it will be possible to assess which exotic strains are neutralised by current vaccines that are in use in the UK and which are not, aiding surveillance efforts by highlighting strains to watch out for.

UK veterinary vaccinology. The postdoctoral research assistant (PDRA) will be trained in valuable techniques broadly applicable to veterinary vaccinology, for example gene cloning, reverse genetics, in vivo vaccine immunogenicity studies, and neutralisation assays. Should the PDRA show promise, they could go on to become a future leader in veterinary vaccinology. The BBSRC acknowledges there is a lack of career path for veterinary vaccinologists and there is a need for support across the skills pipeline. The current project would therefore help contribute to fulfilling this unmet need.

UK attractiveness for research and innovation. The proposed work drives innovation through collaboration between scientists at TPI and TRI. This collaborative research programme therefore capitalises on the investment made into two major research capabilities, strengthening the UK research infrastructure and enhancing the UK attractiveness for research and innovation investment.