Inactivation of a Daxx-mediated cellular intrinsic immune defense against human cytomegalovirus contributes to viral gene expression.

by Ryan Thomas Saffert

Human cytomegalovirus (HCMV) is a medically relevant human pathogen ubiquitously present in the population. Infection of neonates and immunosuppressed individuals can result in severe morbidity and mortality. Disease manifests during primary lytic infection or when latent infections reactivate to lytic ones. Presently, no effective vaccine exists. Understanding regulation of immediate early events during the viral lifecycle and the molecular switch between lytic and latent infection may provide insight into ways to control or inhibit viral replication through the development of more effective antivirals.;HCMV immediate early (IE) gene expression is initiated from the major IE promoter (MIEP) and commits the virus to a lytic replication cycle. Augmenting IE gene expression are viral proteins packaged within the virion tegument that are delivered to the cell upon infection. A subset of these proteins, including pp71, positively contributes to MIEP activation. The association of pp71 with Daxx, a cellular transcriptional regulator, affected IE gene expression through an unknown mechanism. We demonstrated that Daxx repressed the MIEP through recruitment of histone deacetylases, mediating an intrinsic immune defense against HCMV by blocking IE gene expression. HCMV inactivated Daxx-mediated repression of the MIEP by nuclear delivery of pp71, which associated with Daxx and induced its proteasomal degradation.;HCMV establishes a life-long association with the host through a latent state. Latency is characterized by a lack of lytic gene expression, exemplified by a block to IE gene transcription. Despite its importance to the viral lifecycle, viral or cellular proteins which facilitated HCMV latency remained largely uncharacterized. We demonstrated that HCMV failed to induce the degradation of Daxx at the start of latency because pp71, which normally localized to the nucleus, was sequestered in the cytoplasm. Consequently, Daxx silenced the MIEP. HCMV thus usurped the cell's own defense mechanism to block IE gene expression and promote latency. Our work was the first to identify a host protein directly required to facilitate HCMV latency. In total, we mechanistically elucidated the contribution of Daxx and pp71 in regulating one of the earliest events during HCMV infection, the expression of viral IE genes.

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