Kaposi's Sarcoma-Associated Herpesvirus ORF54/dUTPase Downregulates a Ligand for the NK Activating Receptor NKp44

Abstract

Kaposi's sarcoma-associated herpesvirus (KSHV) establishes long-term latent infection in humans and can cause cancers in endothelial and B cells. A functioning immune system is vital for restricting viral proliferation and preventing KSHV-dependent neoplasms. While natural killer (NK) lymphocytes are known to target virus-infected cells for destruction, their importance in the anti-KSHV immune response is not currently understood. Activating receptors on NK cells recognize ligands on target cells, including the uncharacterized ligand(s) for NKp44, termed NKp44L. Here we demonstrate that several NK ligands are affected when KSHV-infected cells are induced to enter the lytic program. We performed a screen of most of the known KSHV genes and found that the product of the ORF54 gene could downregulate NKp44L. The ORF54-encoded protein is a dUTPase; however, dUTPase activity is neither necessary nor sufficient for the downregulation of NKp44L. In addition, we find that ORF54 can also target proteins of the cytokine receptor family and the mechanism of downregulation involves perturbation of membrane protein trafficking. The ORF54-related proteins of other human herpesviruses do not possess this activity, suggesting that the KSHV homolog has evolved a novel immunoregulatory function and that the NKp44-NKp44L signaling pathway contributes to antiviral immunity.

Identification of a cellular ligand for the natural cytotoxicity receptor NKp44

“…39 Saliently, MLL5 has been described as a nuclear protein, 28 whereas NKp44L is not found in the nucleus, consistently with data of Madrid and Ganem. 40 They also showed that during lytic Kaposi's sarcoma-associated herpes virus infection, NKp44L is instead mislocalized and concentrated in the nucleus, whereas its expression on the cell surface decreases, as previously shown in chronic HIV infection. 21,38 Intriguingly, no obvious transmembrane sequence or glycosylphosphatidylinositol anchor motif was deduced for the NKp44L sequence.…”

Modulation of CD112 by the alphaherpesvirus gD protein suppresses DNAM-1–dependent NK cell-mediated lysis of infected cells

“…The significance of the NK cell response against herpesviruses is also reflected by the various mechanisms that these pathogens have evolved to evade or delay it (4). Indeed, for beta-and gammaherpesviruses, a variety of molecular mechanisms avoiding the NK-mediated antiviral activity have been defined (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). Remarkably and paradoxically, such mechanisms have remained largely elusive for the alphaherpesviruses (17).…”

NCR2 (natural cytotoxicity triggering receptor 2)

“…Interestingly, during lytic infection, only surface levels of the NKp44 ligand are reduced as overall cellular levels are unchanged, indicating a defect in cellular trafficking (Madrid and Ganem, 2012). Also, while the NKp44 ligand is typically located outside of the nucleus, during lytic infection the ligand is found localizing to the nucleus (Madrid and Ganem, 2012). This localization is concurrent with a burst of lytic gene expression, mainly consisting of immune related genes (Madrid and Ganem, 2012).…”

“…In the case of Kaposi's sarcoma-associated herpes virus, the extracellular ligand expression is reduced during de novo infection. Interestingly, during lytic infection, only surface levels of the NKp44 ligand are reduced as overall cellular levels are unchanged, indicating a defect in cellular trafficking (Madrid and Ganem, 2012). Also, while the NKp44 ligand is typically located outside of the nucleus, during lytic infection the ligand is found localizing to the nucleus (Madrid and Ganem, 2012).…”

“…Also, while the NKp44 ligand is typically located outside of the nucleus, during lytic infection the ligand is found localizing to the nucleus (Madrid and Ganem, 2012). This localization is concurrent with a burst of lytic gene expression, mainly consisting of immune related genes (Madrid and Ganem, 2012).…”