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.

In mammals, RNA 2'-O-methylation is a molecular signature for the discrimination of endogenous from exogenous mRNA by the cellular innate immune system 1-3 . Here we purified TRBP and its interacting partners to identify a DICERindependent TRBP complex containing FTSJ3, a putative 2'-O-methyltransferase (2'O-MTase). In vitro and ex-vivo experiments show that FTSJ3 is a 2'O-MTase that is recruited to HIV RNA through TRBP. Using RiboMethSeq analysis 4 , we identified 2'-Omethylated residues on viral genome, and found that FTSJ3 modifies HIV RNA at specific sites. HIV-1 viruses produced in FTSJ3 knock-down cells show reduced 2'-Omethylation and trigger type 1-interferons (IFN) expression in human dendritic cells through the RNA sensor MDA5. This IFN-α and IFN-β induction leads to a reduced HIV expression. Altogether, our study revealed an unexpected mechanism used by HIV to evade innate immune recognition involving the recruitment of TRBP/FTSJ3 complex to viral RNA and its 2'-O-methylation.Among post-transcriptional modifications of RNA, called epitranscriptome, 2'-Omethylation is present at the 5'-extremity of all mRNAs in mammals and internally at specific residues of tRNA, rRNA, snRNA and cellular mRNA 5 , 6 . Known for decades for its role in RNA stabilization, its key function in innate immunity has been uncovered recently 1,2 .Cellular mRNAs are capped with a 7-methyl-guanosine ( m7 G), a 5'-modification crucial for mRNA splicing, export to the cytoplasm, and efficient translation into proteins 7 . In addition, mRNAs in higher Eucaryotes acquire a second modification: a 2'-O-ribose methylation on the first (cap1) and the second transcribed nucleotide (cap2). This modification provides a molecular signature for discrimination of self from non-self mRNA. In the cell, cytoplasmic sensors, such as MDA5 and RIG-I, recognize exogenous unmodified RNAs and activate type-1 interferon production to establish an antiviral state 8,9 . To evade innate immune response, some viruses, including Flaviviruses, encode for their own viral 2'O-MTase that can add a cap1/2 on viral transcripts 10 and in the case of West Nile, Dengue and Ebola viruses can also methylate internal adenosine residues of viral genome in vitro 11,12 . In addition, in other pathogens, such as E.coli strains, tRNA 2'-O-methylation also plays an important role in immunostimulation of human dendritic cells since the loss of unique Gm18 residue in mutant TrmH E.coli strain abolishes immunosuppression 13,14 . This is due to the G18 residue of tRNA Tyr that has to be 2'-O-methylated to avoid innate sensing in human cells. In contrast to Flaviviruses, HIV does not encode a 2'O-MTase. However, FTSJ3, a putative host 2'O-MTase, was purified as a partner of TAR RNA binding protein (TRBP), a dsRNA binding