Role of Thymus-Dependent Immune System in HVT Protection against Marek's Disease

Abstract

SUMMARY An experiment was conducted to study the role thymusdependent and bursa-dependent immune systems in protecting chickens against Marek's disease (MD) using herpesvirus of turkeys (HVT). Chickens were rendered T-cell-deficient (TX) by surgical thymectomy at one day of age and by antithymus serum treatment. The chicks were unable to mount graft-versus-host reaction in embryonating eggs. A separate group of chicks was made B-celldeficient (BX) by cyclophosphamide and antibursal serum treatment, and the chicks produced no antibodies against HVT or MD virus (MDV). A group of chicks with intact immune systems was also included. The chicks were vaccinated at 11 days of age with HVT vaccine and then challenged with MDV three weeks postvaccination. Vaccinated uninfected and unvaccinated MDV-infected birds of BX, TX, and normal immune status were also included. Vaccinated control birds (BX, TX, and normal) did not develop MD lesions. Normal and TX birds of the unvaccinated MDVinfected group developed severe MD lesions, but the BX chicks did not develop gross lesons. Vaccinated and challenged normal and BX birds had no MD lesions, but TX birds had a high incidence of lesions. Thus, in the absence of thymus, the HVT vaccine fails to protect birds against MD infection. This indicates that cellmediated immunity probably plays a major role in HVT vaccinal protection against MD.