Varicella-Zoster Virus Inhibition of the NF-κB Pathway during Infection of Human Dendritic Cells: Role for Open Reading Frame 61 as a Modulator of NF-κB Activity

Dendritic cells (DC) are antigen-presenting cells essential for initiating primary immune responses and therefore an ideal target for viral immune evasion. Varicella-zoster virus (VZV) can productively infect immature human DCs and impair their function as immune effectors by inhibiting their maturation, as evidenced by the expression modulation of functionally important cell surface immune molecules CD80, CD86, CD83, and major histocompatibility complex I. The NF-kappa B pathway largely regulates the expression of these immune molecules, and therefore we sought to determine whether VZV infection of DCs modulates the NF-kappa B pathway. Nuclear localization of NF-kappa B p50 and p65 indicates pathway activation; however, immunofluorescence studies revealed cytoplasmic retention of these NF-kappa B subunits in VZV-infected DCs. Western blotting revealed phosphorylation of the inhibitor of kappa B alpha (I kappa B alpha) in VZV-infected DCs, indicating that the pathway is active at this point. We conclude that VZV infection of DC inhibits the NF-kappa B pathway following protein phosphorylation but before the translocation of NF-kappa B subunits into the nucleus. An NF-kappa B reporter assay identified VZV open reading frame 61 (ORF61) as an inhibitor of tumor necrosis factor alpha-induced NF-kappa B reporter activity. Mutational analysis of ORF61 identified the E3 ubiquitin ligase domain as a region required for NF-kappa B pathway inhibition. In summary, we provide evidence that VZV inhibits the NF-kappa B signaling pathway in human DCs and that the E3 ubiquitin ligase domain of ORF61 is required to modulate this pathway. Thus, this work identifies a mechanism by which VZV modulates host immune function.