The missing link in EBV immune evasion: the BDLF3 gene induces ubiquitination and downregulation of MHC class I and MHC class II

The ability of Epstein-Barr virus (EBV) to spread and persist in human populations relies on a balance between host immune responses and EBV immune-evasion. CD8(+) cells specific for EBV late lytic cycle antigens show poor recognition of target cells compared to immediate early and early antigen-specific CD8(+) cells. This phenomenon is in part due to the early EBV protein, BILF1, whose immunosuppressive activity increases with lytic cycle progression. However, published data suggest the existence of a hitherto unidentified immune-evasion protein further enhancing protection against late EBV antigen-specific CD8(+) cells. We have now identified the late lytic gene, BDLF3, as the missing link accounting for the efficient evasion during late lytic cycle. Interestingly, BDLF3 also contributes to evasion of CD4(+) cell responses to EBV. We report that BDLF3 down-regulates expression of surface MHC class I and class II molecules in the absence of any effect upon other surface molecules screened, including CD54 (ICAM-1) and CD71 (Transferrin receptor). BDLF3 both enhanced internalization of surface MHC molecules and reduced the rate of their appearance at the cell surface. The reduced expression of surface MHC molecules correlated with functional protection against CD8(+) and CD4(+) T cell recognition. The molecular mechanism was identified as BDLF3-induced ubiquitination of MHC molecules and their subsequent downregulation in a proteasomal dependent manner.

IMPORTANCE: Immune-evasion is a necessary feature of viruses that establish life-long persistent infections in the face of strong immune-responses. EBV is an important human pathogen whose immune evasion mechanisms are only partly understood. Of the EBV immune-evasion mechanisms identified to date, none could explain why CD8(+) T cell responses to late lytic cycle genes are so infrequent and, when present, recognize lytically-infected target cells so poorly relative to CD8(+) T cells specific for early lytic cycle antigens. The present work identifies an additional immune-evasion protein, BDLF3 that is expressed late in lytic cycle and impairs CD8(+) T cell recognition by targeting cell surface MHC class I molecules for ubiquitination and proteasomal dependent downregulation. Interestingly, BDLF3 also targets MHC class II molecules, to impair CD4(+) T cell recognition. BDLF3 is therefore a rare example of a viral gene that impairs both the class I and class II MHC antigen presenting pathways.