Marek's disease: a model for protection against herpesvirus-induced tumours.

Marek's disease (MD) is a lymphoproliferative disease of chickens caused by a herpesvirus (MDV). Several effective vaccines have been developed and MD is therefore often considered as a model for studying antitumour vaccines. Key factors for the understanding of vaccine-induced immunity are discussed. Three serotypes have been characterized: serotype 1 or oncogenic MDV, serotype 2 or non-oncogenic MDV and serotype 3 or herpesvirus of turkeys. The three serotypes have clearly different genomes and proteins. The pathogenesis of infection with serotype 1 MDV can be divided into an early cytolytic phase, a latent phase and a second cytolytic infection combined with the development of tumours and permanent immunosuppression. The activation of T cells during the early cytolytic phase is important for establishing infection in the target cells for latency and transformation. Immune responses by B cells, and especially by T cells, are directed against viral infection. Evidence is presented that Marek's disease tumour-associated surface antigen is not involved in the antitumour responses. Moreover, the importance of antitumour immune responses in MD is questioned. Vaccinal immunity is dependent more on T cells than B cells and is directed against virus antigens. It is proposed that the evidence claimed for an antitumour response induced by vaccination actually relates to an antiviral response. The relevance of new, highly oncogenic strains causing MD in vaccinated chickens is discussed.