The Epstein-Barr Virus G-Protein-Coupled Receptor Contributes to Immune Evasion by Targeting MHC Class I Molecules for Degradation

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Abstract
Epstein-Barr virus (EBV) is a human herpesvirus that persists as a largely subclinical infection in the vast majority of adults worldwide. Recent evidence indicates that an important component of the persistence strategy involves active interference with the MHC class I antigen processing pathway during the lytic replication cycle. We have now identified a novel role for the lytic cycle gene, BILF1, which encodes a glycoprotein with the properties of a constitutive signaling G-protein-coupled receptor (GPCR). BILF1 reduced the levels of MHC class I at the cell surface and inhibited CD8+ T cell recognition of endogenous target antigens. The underlying mechanism involves physical association of BILF1 with MHC class I molecules, an increased turnover from the cell surface, and enhanced degradation via lysosomal proteases. The BILF1 protein of the closely related CeHV15 γ1-herpesvirus of the Rhesus Old World primate (80% amino acid sequence identity) downregulated surface MHC class I similarly to EBV BILF1. Amongst the human herpesviruses, the GPCR encoded by the ORF74 of the KSHV γ2-herpesvirus is most closely related to EBV BILF1 (15% amino acid sequence identity) but did not affect levels of surface MHC class I. An engineered mutant of BILF1 that was unable to activate G protein signaling pathways retained the ability to downregulate MHC class I, indicating that the immune-modulating and GPCR-signaling properties are two distinct functions of BILF1. These findings extend our understanding of the normal biology of an important human pathogen. The discovery of a third EBV lytic cycle gene that cooperates to interfere with MHC class I antigen processing underscores the importance of the need for EBV to be able to evade CD8+ T cell responses during the lytic replication cycle, at a time when such a large number of potential viral targets are expressed. Epstein-Barr virus (EBV) is a herpesvirus and an important human pathogen that can cause diseases ranging from non-malignant proliferative disease to fully malignant cancers of lymphocytes and epithelial cells. Nevertheless, the vast majority of people in all populations worldwide are infected with EBV. After primary infection the virus persists for the life of the infected individual, and usually without clinical symptoms. To understand how EBV causes disease, we also need to know how this virus is able to persist in healthy individuals with potent immune-responses. A picture is emerging which suggests that EBV, similar to non-tumorigenic herpesvirus, has multiple co-operating mechanisms to modulate immune responses by interfering with antigen processing pathway in cells undergoing lytic virus replication. We have now identified an EBV-encoded protein, BILF1, which targets MHC class I molecules for lysosomal degradation, leading to impaired recognition by immune T cells. BILF1 is a constitutively active G-protein-coupled receptor, but its effect on MHC class I degradation is independent of its signaling functions, and the molecular mechanisms are distinct from those identified in other viruses that induce degradation of MHC class I. This work aids our understanding of EBV biology and emphasizes the complexity of mechanisms evolved by viruses to enable persistent infection.