Deletion of the viral thymidine kinase in a meq-deleted recombinant Marek's disease virus reduces lymphoid atrophy but is less protective

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 but is less protective. Microorganisms. 10(1):7. 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 disease of domesticated chickens and its etiologic agent is the Gallid alphaherpesvirus 2 (GaHV-2), also known as Marek’s disease virus (MDV). MD is currently controlled by vaccination using live attenuated strains of MDV (e.g., CVI988/Rispens), non-pathogenic serotypes of MDV (GaHV-3), or non-pathogenic strains of the related Melagrid alphaherpesvirus 1 (MeHV-1). One attractive strategy for the production of new vaccine strains is a recombinant MDV attenuated by the deletion of the major viral oncogene meq. However, meq-deleted variants of MDV cause atrophy of the bursa and thymus in maternal antibody-negative chickens, and the resulting immunosuppression makes them unsuitable. 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. We demonstrate that ablation of the viral tk from the meq-deleted virus rMd5B40/'meq resulted in a virus attenuated for replication in vitro and which spared chickens from atrophy of the lymphoid organs in vivo. When the rMd5B40/'meq/'tk/GFP was used as a vaccine it was protective against challenge with the vv+MDV strain 686, but the protection was less than that provided by the CVI988/Rispens vaccine.