Bystander Killing during Avian Leukosis Virus Subgroup B Infection Requires TVB S3 Signaling

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Abstract
Cell killing by avian leukosis virus subgroup B (ALV-B) in cultures has been extensively studied, but the molecular basis of this process has not been established. Here we show that superinfection, which has been linked to cell killing by ALV-B, plays no crucial role in cell death induction. Instead, we show that signaling by the ALV-B receptor, TVB S3 , a member of the tumor necrosis factor receptor family, is essential for ALV-B-mediated cell death. TVB S3 activated caspase-dependent apoptosis during ALV-B infection. Strikingly, apoptosis induction occurred predominantly in uninfected cells, while ALV-B-infected cells were protected against cell death. This bystander killing phenomenon was reproduced in a virus-free system by cocultivating ALV-B Env-expressing cells with TVB S3 -expressing cells. Taken together, our results indicated that ALV-B-mediated apoptosis is triggered by ALV-B Env-TVB S3 interactions.