A DNA vaccine containing an infectious Marek's disease virus genome can confer protection against tumorigenic Marek's disease in chickens

DNA vaccine containing the infectious BAC20 clone of serotype 1 Marek's disease virus (MDV) was tested for its potential to protect against Marek's disease (MD). Chickens were immunized at 1 day old with BAC20 DNA suspended either in PBS, as calcium phosphate precipitates, incorporated into chitosan nanoparticles, in Escherichia coli DH10B cells, or bound to gold particles for gene-gun delivery. Challenge infection with MDV strain EU1 was performed at 12days old, and four out of seven birds immunized with BAC20 DNA in saline by the intramuscular route remained free of MD until day 77 after challenge infection. A delay in the development of the disease could be observed in some animals vaccinated with other BAC20 DNA formulations, but clinical MD and tumour formation were evident in all but one bird. Five out of seven animals immunized with the vaccine virus CV1988 were protected against MD, but none out of seven birds survived EU1 challenge infection after injection of negative-control plasmid DNA. In a second animal experiment, five out of 12 chickens immunized with BAC20 DNA and six out of eight birds immunized with virus reconstituted from BAC20 DNA remained free of MD after challenge infection. In contrast, none out of 12 chickens survived challenge infection after immunization with BAC20 DNA lacking the essential gE gene or with gE-negative BAC20 virus. The results suggested that an MDV BAC DNA vaccine has potential to protect chickens against MD, but that in vivo reconstitution of vaccine virus is a prerequisite for protection.