Marek's disease virus spread: In and out of chickens

Marek's disease (MD) is a highly contagious virus-induced lymphoma in poultry caused by Marek's disease virus (MDV), an alphaherpesvirus heavily affecting chicken industry. In spite of the widespread use of live attenuated vaccines during the last 40 years, MD remains a major threat for poultry production with estimated annual economic losses in the range of 1 billion dollars worldwide. In Europe as well as in other countries, MDV has evolved towards a substantially increased virulence observed in more recent strains, thereby threatening the sustainability of the current vaccination strategy. The endemic nature of the disease is clearly linked to the mode of virus transmission and to incomplete protection provided by current vaccines. It is well known that while vaccines protect against the disease, infection and virus shedding are not halted. We hypothesize that the persistence and bird-to-bird transmission of pathogenic strains in vaccinated birds provide the basis for the emergence of very virulent strains. Successful transmission of MDV entails efficient viral shedding from infected birds and the entry of the virus via the respiratory route, in both vaccinated and naïve birds. Improved knowledge about these two essential steps in virus replication is critical to ultimately reduce MDV contamination of the environment and infection of the animals. To date, entry and shedding of MDV in vivo have been poorly studied compared to virus-induced tumourigenesis, host resistance and virulence. Our project aims to study the key elements on the cellular and molecular levels that are involved in virus entry and shedding. The novelty of our project is to focus on differentiated cells, which represent the major target cells infected in vivo and are derived either from the haematopoeitic lineage (macrophages and lymphocytes) or from the skin epithelium.

Marek's disease is a major disease of poultry that induces major economic and welfare implications for poultry health. Despite the widespread vaccination for the last 40 years, the threat from the disease has not disappeared, as the viral strains continue to increase in virulence. The proposed project aims at generating underpinning knowledge on the keystone of MDV entry and shedding and at defining its interaction with and replication in differentiated cells from the hematopoeitic (macrophages and lymphocytes) and from the skin epithelium lineages. the feather follicle epithelium is the only site in the infected chickens where a fully productive infection of the virus resulting in the continuous shedding of the virus occurs. At the same time the lymphocytes from the infected birds carry a latent infection with very limited expression of viral antigens. Our preliminary studies have also suggested a direct correlation between virus virulence and the levels of replication in the feather follicle, with highly virulent strains replicating at higher levels resulting in increased shedding. The project will aim to exploit the current knowledge on avian cell differentiation (with an emphasis on skin and lymphoid lineages), type-specific cellular determinants controlling MDV spread and shedding and on the vaccines that control shedding and entry of pathogenic MDV strains to develop viable and cost-effective alternative to primary avian cells for vaccine production and virus studies, and improve vaccine efficacy in the control of spread and shedding of pathogenic viruses. By studying virus entry and shedding, we aim at promoting sustainable methods to reduce MDV endemics and to improve biosecurity by generating vaccines with improved efficiency regarding the reduction of environmental contamination by MDV.

By targeting viral spread, which is essential for viral persistence in vaccinated flocks, our project aims at developing novel control strategies, which will reduce MD in poultry and create conditions in which the emergence of MDV with increased virulence can be stopped. A better knowledge of the conditions in which a virus contaminates naïve or vaccinated sentinel birds is a prerequisite to the development of sustainable methods to reduce the impact of MD that is endemic worldwide. From our research into cellular elements that are essential for virus production in skin, 'favourable' genotypes of chickens could be selected that would show reduced shedding of virus. A vaccine that would induce a 'sterile immunity' would also prevent contamination of the environment by virulent viruses. Infections with Marek's disease virus (MDV) have a high economic impact worldwide and the prevention strategies employed universally rely on vaccination of chickens either in ovo or at 1-day-old. The chicken industry in all of Europe is a tightly interwoven net with respect to the use of only a small number of different chicken breeds, farm structures, and management regimens applied. Therefore, national approaches to improve control of MDV and perform state-of-the art research on the continually evolving pathogen would seem unreasonable. The rationale for the joining of forces of the premier scientists and national research centers with one of the most prominent companies providing broiler as well as layer breeds in Europe and beyond was the driving force for this consortium that has worked together over more than a decade now. Who will benefit from this research? The proposed research will benefit the poultry industry as an improved understanding of the molecular mechanisms of MDV spread could lead to new strategies of designing new vaccine. Judging from the interest in the lay press, there is a considerable interest among the public for the scientific understanding of how some diseases produce damage to animal health. This proposal could unravel some of the intricate pathways of cancer in animals, and hence will have significant impact on animal welfare through all poultry production systems. What will be done to ensure that they have the opportunity to benefit from this research? A special attention will be paid to the exploitation of results as this project will identify genes, molecular and cellular mechanisms which may be at the origin of industrial developments. A consortium agreement will be engaged and negotiated through the institutional representative of each partner. This consortium agreement will follow the frame described in annex 5, with a particular focus on i) confidentiality as the developments in WP1 and 2 will engage the exchange of confidential information and ii) exploitation of the results, as commercial exploitation may concern either new cell-culture systems for the production of vaccines or the design of more efficient vaccines. Identification of genes/pathways/molecular mechanisms (WP1-2-3) may lead to the application for joined patent(s) in conditions that will have to be defined in the IPR section of the consortium agreement. Material rights may concern cell-lines or vaccines which may arise as breakthrough developments from WP1 - 2 and 4. The project aims mainly to target the poultry vaccine market with a promising new offer in MDV replication process that can be monitored in vitro and therefore that can lead to a better production and control of the poultry vaccines.