Nonprotective and Temperature-Sensitive Variants of Marek’s Disease Vaccine Viruses2

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Abstract
In an attempt to elucidate mechanisms of vaccinal immunity in Marek’s disease (MD), two nonprotective variants of MD vaccine viruses were compared with their counterpart protective viruses. The variant viruses were 200D, an MD virus stock passaged over 200 times in duck embryo fibroblast cultures, and HVT/hub, a stock of turkey herpesvirus passed over 70 times in chicken embryo fibroblast cultures. At doses up to 1.4×105 plaqueforming units, the viruses were partly (HVT/hub) or totally (200D) deficient in their abilities to protect chickens against MD. Similarly, they were partly (HVT/hub) or totally (200D) deficient in their abilities to replicate in vivo as measured by virus reisolation and antibody assays; however, both viruses replicated well in vitro. Attempts were unsuccessful to implicate host range mutation, temperature sensitivity, or autointerference as the basis for the defective in vivo replication. The variant viruses were antigenically similar to their counterpart protective vaccine viruses except that 200D lacked the A-antigen. The 200D (but not the HVT/hub) variant virus was identified as a temperature-sensitive (ts) mutant characterized by 380°/41° C replication, plating efficiencies of greater than 103, and a high reversion frequency. These findings not only caution against the danger of over-attenuation of MD vaccine viruses with loss of immunizing potential but also identify a unique ts mutant of MD virus potentially useful as a tool for further studies.