Abstract
The most common lymphoproliferative disease in chickens is Marek's disease (MD), which is caused by the oncogenic herpesvirus Marek's disease virus (MDV). The emergence of hypervirulent pathotypes of MDV has led to vaccine failures, which have become common and which have resulted in serious economic losses in some countries, and a revaccination strategy has been introduced in practice. The mechanism by which revaccination invokes superior immunity against MD is unknown. After field trials which showed that revaccination provided protection superior to that provided by a single vaccination were performed, experiments were conducted to explore the interaction between revaccinated chickens and MDV. The results showed that the chickens in the revaccination groups experienced two consecutive productive infections but that the chickens in the single-vaccination groups experienced one productive infection, demonstrating that revaccination of viruses caused the chickens to have productive and then latent infections. Revaccination of the virus induced in the chickens a higher and a longer temporary expansion of the CD8(+), CD4(+), and CD3(+) T-lymphocyte subpopulations, stronger peripheral blood lymphocyte proliferative activity; and higher levels of neutralizing antibody than single vaccination. These findings disagree with the postulate that MDV antigens persist, stimulate the immune system, and maintain a high level immunity after vaccination. The suppression of productive infection by maternal antibodies in chickens receiving the primary vaccination and a lower level of productive infection in the revaccination groups challenged with MDV were observed. The information obtained in this study suggests that the productive infection with revaccinated MDV in chickens plays a crucial role in the induction of superior immunity. This finding may be exploited for the development of a novel MD vaccine that results in the persistence of the antigen supply and that maintains a high level of immunity and may also have implications for other viral oncogenic diseases in humans and animals.