PATHOGENESIS OF MAREKS-DISEASE (MD) AND POSSIBLE MECHANISMS OF IMMUNITY INDUCED BY MD VACCINE

Marek's disease (MD) is a lymphoproliferative disease of chicken, whic h is characterized by malignant T cell-lymphoma formation. This diseas e can be effectively prevented by vaccination with attenuated MD virus (MDV), apathogenic MDV or herpesvirus of turkey. MD vaccines are ones of a few vaccines which can prevent virus-induced tumor among mammali an and avian species. To determine the roles of T cell subsets in the protection mechanism, chickens vaccinated with an attenuated MDV (CV19 88) were depleted of either CD4(+) or CD8(+)T cells by neonatal thymec tomy and injections of monoclonal antibodies against chicken CD4 or CD 8 molecules and then challenged with an oncogenic MDV. These birds wer e effectively protected from MDV-induced tumors. However, virus titers in CD4(+)T cells, which are the main target cells for MDV-latent infe ction and subsequent transformation, were much higher in CD8-deficient vaccinated chickens than in untreated vaccinated chickens at the earl y stage of the latent phase. These results suggested that CD8(+)T cell responses induced by the MD vaccine are essential for anti-virus but not anti-tumor effects. Here, we will discuss how the attenuated vacci ne prevents chickens from lymphoma-formation by an oncogenic MDV.