Induction of lytic cycle sensitizes Epstein-Barr virus infected B cells to NK cell killing that is counteracted by virus-mediated NK cell evasion mechanisms in late lytic cycle

Epstein-Barr Virus (EBV) persists for the lifetime of the infected host despite eliciting strong immune responses. This persistence requires a fine balance between the host immune system and EBV immune evasion. Accumulating evidence suggests an important role for natural killer (NK) cells in this balance. NK cells can kill EBV infected cells undergoing lytic replication in-vitro and studies in both humans, and mice with reconstituted human immune systems have shown NK cells can limit EBV replication and prevent infectious mononucleosis. We now show that NK cells, via NKG2D and DNAM-1 interactions, recognize and kill EBV infected cells undergoing lytic replication, and that expression of a single EBV lytic gene, BZLF1, is sufficient to trigger sensitization to NK cell killing. We also present evidence suggesting the possibility of the existence of an as yet unidentified DNAM-1 ligand which may be particularly important for killing lytically infected normal B cells. Furthermore, whilst cells entering lytic cycle become sensitized to NK cell killing, we observed that cells in late lytic cycle are highly resistant. We identified expression of the vBcl-2 protein, BHRF1, as one effective mechanism by which EBV mediates this protection. Thus, contrary to the view expressed in some reports, EBV has evolved the ability to evade NK cell responses.

IMPORTANCE: This report extends our understanding of the interaction between EBV and host innate responses. It provides the first evidence that the susceptibility to NK cell lysis of EBV infected B cells undergoing lytic replication is dependent upon the phase of lytic cycle. Induction of lytic cycle is associated with acquired sensitization to NK cell killing, while progress through late lytic cycle is associated with acquired resistance to killing. We provide mechanistic explanations for this novel observation, implicating important roles for the BZLF1 immediate-early transactivator, the BHRF1 vBcl-2 homologue, and a novel ligand for the DNAM-1 NK cell receptor.