Protection against Marek's disease-derived tumor transplants by the nononcogenic SB-1 strain of Marek's disease virus.

A series of experiments was conducted to study the in vivo protection against Marek's disease-derived tumor transplants by the nononcogenic SB-1 strain of Marek's disease virus. Intact, embryonally bursectomized (Bx), thymectomized (Tx), or cyclophosphamide (Cy)-treated chickens of four genetic lines were vaccinated with live or inactivated SB-1. JMV, a non-virus-producing transplant, and GA/Tr-1 and MDT-198, two virus-producing transplants were used for challenge. Optimal protection against JMV was present 7 days postvaccination, but there was significant protection even when SB-1 and JMV were administered together. Protection was abolished by an increase in the number of tumor cells used for challenge or by combined Tx and Cy treatment. Inactivated SB-1-infected cells were unable to induce protection against JMV challenge. Protection was also present against challenge with GA/Tr-1, but not against MDT-198, except in vaccinated, Bx chickens. It was concluded that protection against JMV was T-cell dependent and required the induction of neo-antigens not present in an inactivated SB-1 cellular preparation. The absence of protection in intact chickens against MDT-198 could not be explained.