Prototype foamy virus downregulates RelB expression to facilitate viral replication.

Foamy viruses (FVs) are classified in the subfamily Spumaretrovirinae and bridge the gap between Orthoretrovirinae and Hepadnaviridae. FVs have strong cytopathic effects against cells cultured in vitro. However, they establish lifelong latent infections without evident pathology in the host. The roles of cellular factors in FV replication are poorly understood. To better understand this area, we determined the transcriptomes of HT1080 cells infected with prototype foamy virus (PFV) to measure the effect of PFV infection on the expression of cellular genes. We found that the level of RelB mRNA, a member of the nuclear factor-κB (NF-κB) protein family, was significantly decreased as a result of PFV infection, and this was further confirmed with real-time PCR. Interestingly, overexpression of RelB reduced PFV replication, whereas its depletion using small interfering RNA increased PFV replication. This inhibitory effect of RelB results from diminished transactivation of the viral long terminal repeat (LTR) promoter and an internal promoter (IP) by viral Tas protein. Together, these data demonstrate that PFV infection downregulates the viral inhibitory host factor RelB, which otherwise restricts viral gene expression.