The human Cytomegalovirus tegument protein pp65 (pUL83) dampens interferon-type I production by inactivating the DNA sensor cGAS

The innate immune response against Human Cytomegalovirus (HCMV) plays a pivotal role during primary infection. Indeed, HCMV infection of primary fibroblasts rapidly triggers a strong induction of interferons-type I (IFN-type I) accompanied by proinflammatory cytokine release. Here, we show that primary human foreskin fibroblasts (HFFs) produce IFN-type I when infected with the wild type HCMV strain TB40/E (v65Rev). However, significantly higher IFN­type I levels were observed when HFFs were infected with HCMV unable to express UL83­encoded pp65 (v65Stop), suggesting that the tegument pp65 protein may downregulate IFN­type I production. To clarify the mechanisms pp65 relies on to inhibit IFN­type I production, we analyzed the activation of the cGMP­AMP synthase (cGAS)/STING/IRF3 axis in HFFs infected with v65Rev or with a mutant unable to express pp65 (v65Stop). The results obtained reveal that pp65 selectively binds to cGAS and prevents its interaction with STING, thus inactivating the signaling pathway through the cGAS/STING/IRF3 axis. Notably, during the first 6 hours of HCMV infection, STING undergoes proteasome degradation regardless of the presence of pp65. Additionally, down-regulation of cGAS activity by pp65 blocked IRF3 dimerization and DNA binding activity, leading to the inhibition of IFN-type I production. Collectively, our data provide mechanistic insight into the interplay between HCMV pp65 and cGAS leading to subsequent immune evasion by this prominent DNA virus.