Deletion of the viral thymidine kinase in a meq-deleted recombinant Marek's disease virus reduces lymphoid atrophy and is an effective strategy for viral attenuation

Authors: Conrad, Steven; OLUWAYINKA, ENIOPE - Federal University Of Agriculture, Abeokuta; Mays, Jody; Heidari, Mohammad; Dunn, John

Submitted to: Microorganisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2021
Publication Date: 12/21/2021
Citation: Conrad, S.J., Oluwayinka, E.B., Mays, J.K., Heidari, M., Dunn, J.R. 2021. Deletion of the viral thymidine kinase in a meq-deleted recombinant Marek's disease virus reduces lymphoid atrophy and is an effective strategy for viral attenuation. Microorganisms. https://doi.org/10.3390/microorganisms10010007.
DOI: https://doi.org/10.3390/microorganisms10010007

Interpretive Summary: Marek’s disease (MD), caused by Marek's disease virus (MDV), is a highly contagious disease of domesticated chickens and the economic losses caused by MD are a concern for the poultry industry both in the United States and globally. Disease caused by MDV infection is currently controlled by universal vaccination using either attenuated strains of MDV, non-pathogenic serotypes of MDV, or non-pathogenic strains of the highly-related virus isolated from turkeys. New vaccines are needed to control the historical trend of MDV evolution and vaccine breaks. One promising vaccine candidate is a strain of MDV in which the major oncogene has been knocked out. While highly protective, this vaccine candidate has some residual virulence. In this study, we attempted to eliminate this residual virulence by also knocking out a gene called thymidine kinase, which is an important component of DNA replication. We were successful in reducing replication of the virus and eliminating the residual virulence, however, the protection against virulent challenge was reduced and resulted in slightly less protection compared to the current gold standard CVI988/Rispens vaccine.

Technical Abstract: Marek’s disease (MD) is a ubiquitous and highly contagious disease of domesticated chickens and the economic losses caused by MD are a concern for the poultry industry both in the United States and globally. The etiologic agent of MD is the Gallid alphaherpesvirus 2 (GaHV-2), also known as Marek’s disease virus (MDV). Disease caused by MDV infection is currently controlled by universal vaccination using either attenuated strains of MDV (e.g., CVI988/Rispens), non-pathogenic serotypes of MDV (GaHV-3), or non-pathogenic strains of the highly-related (heterologous) Melagrid alphaherpesvirus type 1 (MeHV-1). While adequately protective at present, the history of MDV is one of continuing viral evolution and higher virulence in field isolates, underscoring the need for newer and more protective live attenuated vaccine strains. One promising strategy recently utilized by several groups is a recombinant MDV in which the major viral oncogene meq has been deleted. A consistent problem with vaccine strains based upon these meq-deleted variants of MDV is that, while extremely protective, they cause atrophy of the lymphoid organs (the thymus and bursa) in maternal antibody-negative chickens and concomitant immunosuppression which makes them unsuitable for widespread use as vaccines. Herein we detail our attempt to mitigate the lymphoid atrophy caused by meq-deleted MDV by further attenuation of the virus through ablation of the viral thymidine kinase (tk) gene, a time-tested strategy of herpesvirus attenuation. In this study we demonstrate that deletion of the viral tk from the meq-deleted virus rMd5B40delmeq resulted in a virus, rMd5B40/delmeq/deltk/GFP, which was attenuated for replication in vitro and which spared chickens from atrophy of the lymphoid organs in vivo. When the rMd5B40/delmeq/deltk/GFP was used as a vaccine it was protective against challenge with the vv+MDV strain 686, but the protection afforded was slightly but significantly less than that provided by the current standard for MD protection, the CVI988/Rispens vaccine.