HSV infection activates a robust innate response through engagement of multiple pattern recognition receptors (PRR) including both TLR (TLR2 and TLR9) as well as non-TLR. Signaling events downstream of these receptors activate NF-κB and IRF3 responsive genes and initiate an innate inflammatory response aimed at controlling viral replication and spread. In this work, we studied immune suppressive activity of a replication-defective HSV virus and identified the immediate early protein ICP0 as a negative regulator of both NF-κB and IRF3 signaling. ICP0 possesses an ubiquitin E3 ligase function through its NH2 RING domain, as well as de-ubiquitinating activity through its association with the cellular de-ubiquitinating enzyme USP7 (HAUSP). We show that these two domains of ICP0 function independently to suppress IRF3 and NF-κB signaling, respectively and, in the process, effectively shut down host innate immunity to HSV infection. Although ICP0 inhibition of IRF3 has been reported, inhibition of TLR-mediated NF-κB response has not been previously described. We show that ICP0 globally inhibits NF-κB response to all TLR receptors as well as IL-1R. ICP0 exerts this activity by associating with USP7 and altering its cellular localization from a nuclear to cytoplasmic protein. In the cytosol, USP7 associates with and de-ubiquitinates TRAF6 and IKK-γ (NEMO), two signaling components of the TLR-mediated NF-κB pathway that are poly-ubiquitinated upon TLR activation. ICP-0 expression vectors harboring point mutations/deletions that target the RING domain E3 ligase function or compromise ICP-0 ability to bind USP7 would selectively inhibit its ability to interfere with either IRF3 or NF-kB signaling, respectively. In support of this, knockdown of endogenous USP7 by RNAi severely impaired ICP0-mediated inhibition of NF-κB response while leaving its capacity to inhibit IRF3 intact. In contrast, over-expression of USP-7 was sufficient to inhibit TLR-mediated NF-κB response. Ability of ICP-0 to inhibit both IRF3 and NF-κB signaling pathways, the former through its E3 ligase function and the latter through its association with USP-7, affords HSV comprehensive protection from host immunity during repeated cycles of lytic infection and reactivation from latency. The work also identifies a rare example of how two seemingly contradictory biologic functions resident within ICP0, namely ubiquitin E3 ligase activity at the NH2 terminus RING domain and de-ubiquitinating activity through association with USP-7, could cooperate to inhibit multiple signaling pathways necessary for efficient silencing of innate immunity. Finally, the work also identifies a previously unknown function for USP7 in regulating innate signaling beyond its known function as a regulator of p53.
Disclosure: No relevant conflicts of interest to declare.