The human herpesvirus-7 (HHV-7) U21 immunoevasin subverts NK-mediated cytoxicity through modulation of MICA and MICB

Herpesviruses have evolved numerous immune evasion strategies to facilitate establishment of lifelong persistent infections. Many herpesviruses encode gene products devoted to preventing viral antigen presentation as a means of escaping detection by cytotoxic T lymphocytes. The human herpesvirus-7 (HHV-7) U21 gene product, for example, is an immunoevasin that binds to class I major histocompatibility complex molecules and redirects them to the lysosomal compartment. Virus infection can also induce the upregulation of surface ligands that activate NK cells. Accordingly, the herpesviruses have evolved a diverse array of mechanisms to prevent NK cell engagement of NK-activating ligands on virus-infected cells. Here we demonstrate that the HHV-7 U21 gene product interferes with NK recognition. U21 can bind to the NK activating ligand ULBP1 and reroute it to the lysosomal compartment. In addition, U21 downregulates the surface expression of the NK activating ligands MICA and MICB, resulting in a reduction in NK-mediated cytotoxicity. These results suggest that this single viral protein may interfere both with CTL-mediated recognition through the downregulation of class I MHC molecules as well as NK-mediated recognition through downregulation of NK activating ligands.

The long coevolution of herpesviruses with their hosts has resulted in the development of a diverse array of viral immune evasion strategies and host counter-strategies. The identification of viral proteins that impair the function of cellular immune-recognition receptors has proven fertile ground for the discovery of fundamental concepts in immunology and cell biology. While the cytomegaloviruses have demonstrated an extraordinary array of immunoevasive tactics, little is known about the immunoevasive strategies of the closely-related human herpesvirus-7 (HHV-7). We have previously demonstrated that the U21 gene product from HHV-7 likely interferes with viral antigen presentation to cytotoxic T cells by rerouting class I major histocompatibility molecules to lysosomes for degradation. In addition to the host's cytotoxic T cell response, virus infection also induces the expression of Natural-Killer (NK) activating ligands, alerting cytotoxic NK cells to identify and kill virus-infected cells. Here we describe a novel function for the same viral protein - U21 - in interfering with NK cell recognition. Our findings provide the first indication that HHV-7, too, may have found it necessary to strategize mechanisms of NK escape.