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 that 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, while cells entering the lytic cycle become sensitized to NK cell killing, we observed that cells in the 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.
IMPORTANCEThis 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 the lytic cycle. Induction of the lytic cycle is associated with acquired sensitization to NK cell killing, while progress through the late lytic cycle is associated with acquired resistance to killing. We provide mechanistic explanations for this novel observation, indicating important roles for the BZLF1 immediate early transactivator, the BHRF1 vBcl-2 homologue, and a novel ligand for the DNAM-1 NK cell receptor. E pstein-Barr Virus (EBV), one of eight human herpesviruses, is carried by over 90% of the world's adult population. Primary EBV infection occurs in the oropharynx, leading to infection of B lymphocytes (1, 2). These infected B cells can support the lytic cycle, in which more than 80 viral genes are expressed to generate new infectious virus, but they more frequently host nonproductive infections through expression of a limited number of socalled latent EBV genes (latency III genes) that drive lymphoproliferation as an alternative mechanism of expanding the infected cell pool. In vitro, this growth transformation is demonstrated by the ready establishment of lymphoblastoid cell lines (LCLs) following infection of resting B cells. Following initial infection in vivo, EBV downregulates the expression of all viral proteins and enters a true latent phase (latency 0) in the memory B cell population, where it establishes a lifelong infection (1). Periodically the virus reactivates and undergoes fu...