“Play it again, SAMHD1”: the hard life of an antiviral restriction factor during the intrinsic immune response against human cytomegalovirus infection

Sterile α-motif and histidine-aspartate domain-containing protein 1 (SAMHD1) is a cellular deoxynucleotide triphosphates (dNTPs) triphosphohydrolase that hydrolyses dNTPs into deoxynucleosides and inorganic triphosphates. This activity is associated with its antiviral restriction function and it inhibits replication of many RNA and DNA viruses. The antiviral activity is negatively regulated by Thr-592 phosphorylation, that destabilizes the active tetrameric form of the protein. Human cytomegalovirus (HCMV) is an opportunistic pathogen in immunocompromised hosts, while it is asymptomatic in the general population. Infection is life-long, and latency is a pivotal feature of the virus, which interferes with host anti-viral responses, including intracellular restriction factors. During my PhD project, we investigated the role of SAMHD1 in HCMV replication and potential viral evasion mechanisms, which could contribute to the unsuccessful clearance of the virus. After infection of different cell types with different HCMV strains, we observed an increase in SAMHD1 mRNA and protein levels. This was associated with the induction of phosphorylation at the regulatory residue Thr-592, likely involving the cellular kinase Cdk1, but not the viral kinase pUL97. Both SAMHD1 knock-down and, on the other hand, overexpression of wild-type and Thr-592 mutants showed negligible influence on HCMV viral production, suggesting that SAMHD1 activity could be overcome by HCMV during lytic infection, independently from its phosphorylation status. We also observed, by various experimental approaches, phospho-SAMHD1 localization in the cytoplasm of infected fibroblasts and its association with viral particles, suggesting the idea of a mechanism of de-localization and evasion from its nuclear-associated antiviral activity. Despite recent observations of SAMHD1 restriction during early steps of HCMV replication, our work suggests that SAMHD1 is unable to limit viral lytic infection, probably due to re-localization in the cytoplasm, setting a cellular environment permissive for a productive HCMV replication.