The HHV-6A Proteins U20 and U21 Target NKG2D Ligands to Escape Immune Recognition

Abstract

The coevolution of the human immune system and herpesviruses led to the emergence and diversification of both cellular danger molecules recognized by immune cells on the one hand and viral countermeasures that prevent the expression of these proteins on infected cells on the other. There are eight ligands for the activating receptor NKG2D in humans – MICA, MICB, ULBP1-6. Several of them are induced and surface-expressed on herpesvirus-infected cells to serve as danger signals to activate the immune system. Therefore, these ligands are frequently targeted for suppression by viral immune evasion mechanisms. Mechanisms to downregulate NKG2D ligands and thereby escape immune recognition have been identified in all other human herpesviruses (HHV), except for HHV-6A. In this study, we identify two HHV-6A encoded immunoevasins, U20 and U21, which suppress the expression of the NKG2D ligands ULBP1 and ULBP3, respectively, during infection. Additionally, MICB is targeted by a so far unexplored viral protein. Due to the diminished NKG2D ligand surface expression on infected cells, recognition of HHV-6A infected cells by innate immune cells is impaired. Importantly, our study indicates that immune escape mechanisms between the related herpesviruses HHV-6A and HHV-6B are evolutionary conserved as the same NKG2D ligands are targeted. Our data contribute an additional piece of evidence for the importance of the NKG2D receptor – NKG2D ligand axis during human herpesvirus infections and sheds light on immune evasion mechanisms of HHV-6A.

Structural Models for Roseolovirus U20 And U21: Non-Classical MHC-I Like Proteins From HHV-6A, HHV-6B, and HHV-7

“…In HHV-7, U21 was shown to bind and downregulate the nonclassical MHCs HLA-E and HLA-G, as well as the NK activating ligands MICA and MICB ( 19 , 20 ). Also, U21 expressed by HHV-6A is able to downregulate ULBP3 ( 18 ). In HHV-6B, the precise role of U21 has yet to be defined, but it is clear that the NK ligands ULBP1, ULBP3, and MICB are downregulated during infection ( 17 ).…”

“…In HHV-6B, the precise role of U21 has yet to be defined, but it is clear that the NK ligands ULBP1, ULBP3, and MICB are downregulated during infection ( 17 ). Very recently, HHV-6A U20 was shown to downregulate ULBP1 from the cell surface, decreasing NK activation in degranulation assays ( 18 ). In addition, U20 from HHV-6B has been shown to affect TNFR signaling, inhibiting PARP cleavage, caspase 3 and 8 activation, and IκBα and NF-κB transcriptional activity ( 48 ).…”

“…U21 redirects and sequesters its targets in the ER where they are eventually redirected to the lysosome ( 21 ). However, U20 accomplishes ULBP1 downregulation by an alternative, lysosome-independent mechanism that remains to be elucidated ( 18 ). To better understand how these roseolovirus immunoevasins fit into the picture of T cell and NK cell evasion presented above, we utilized advanced structure prediction tools to generate structural models and draw parallels to the better-characterized viral immunoevasins just discussed.…”

“…We examined the predicted structures for compatibility with previously identified NK immunoevasin mechanisms involving MHC-Ia and MHC-Ib proteins for which structural information is available. U20 from HHV-6A recently has been reported to down-regulate the stress-induced MHC-platform-only MHC-Ib protein ULBP-1 ( 18 ). We investigated whether the predicted U20 structures and structural mechanisms from previous work would be consistent with this activity.…”

“…Although these vMHCs have a diverse range of functions they all share the characteristic class I MHC fold [reviewed in ( 12 – 14 )]. Roseoloviruses HHV-6A, HHV-6B, and HHV-7 all have been shown to downregulate host-derived cellular MHC-Ia (cMHC-Ia) ( 15 , 16 ) and cellular MHC-Ib (cMHC-Ib) molecules ( 17 – 20 ) with the roseolovirus U21 glycoprotein responsible for this activity by intercepting cMHC-Ia in the ER and directing it to lysosomes for degradation ( 21 , 22 ), and the U20 protein recently implicated in MHC-Ib downregulation ( 18 ). However, many questions remain surrounding the structures and molecular mechanisms of these two proteins.…”