The primate lentivirus auxiliary protein Vpx counteracts an unknown restriction factor that renders human dendritic and myeloid cells largely refractory to HIV-1 infection. Here we identify SAMHD1 as this restriction factor. SAMHD1 is a protein involved in Aicardi-Goutières syndrome, a genetic encephalopathy with symptoms mimicking congenital viral infection, that has been proposed to act as a negative regulator of the interferon response. We show that Vpx induces proteasomal degradation of SAMHD1. Silencing of SAMHD1 in non-permissive cell lines alleviates HIV-1 restriction and is associated with a significant accumulation of viral DNA in infected cells. Concurrently, overexpression of SAMHD1 in sensitive cells inhibits HIV-1 infection. The putative phosphohydrolase activity of SAMHD1 is probably required for HIV-1 restriction. Vpx-mediated relief of restriction is abolished in SAMHD1-negative cells. Finally, silencing of SAMHD1 markedly increases the susceptibility of monocytic-derived dendritic cells to infection. Our results demonstrate that SAMHD1 is an antiretroviral protein expressed in cells of the myeloid lineage that inhibits an early step of the viral life cycle.

After integration in the host cell genome, the HIV‐1 provirus is packaged into chromatin. A specific chromatin disruption occurs in the HIV‐1 promoter during transcriptional activation in response to TNF‐alpha, suggesting that chromatin plays a repressive role in HIV‐1 transcription and that chromatin modification(s) might result in transcriptional activation. We have treated several cell lines latently infected with HIV‐1 with two new specific inhibitors of histone deacetylase, trapoxin (TPX) and trichostatin A (TSA), to cause a global hyperacetylation of cellular histones. Treatment with both drugs results in the transcriptional activation of the HIV‐1 promoter and in a marked increase in virus production. Dose‐response curves and kinetic analysis show a close correlation between the level of histone acetylation and HIV‐1 gene expression. In contrast, both TPX and TSA have little or no effect on HIV‐1 promoter activity following transient transfection of an HIV‐1 promoter‐reporter plasmid. Activation of HIV‐1 transcription by TSA and TPX treatment occurs in the absence of NF‐kappa B induction. Chromatin analysis of the HIV‐1 genome shows that a single nucleosome (nuc‐1) located at the transcription start and known to be disrupted following TNF‐alpha treatment, is also disrupted following TPX or TSA treatment. This disruption is independent of transcription as it is resistant to alpha‐amanitin. These observations further support the crucial role played by nuc‐1 in the suppression of HIV‐1 transcription during latency and demonstrate that transcriptional activation of HIV‐1 can proceed through a chromatin modification.