RNA interference (RNAi) is a mechanism of post-transcriptional gene silencing that has been described both in animal and in plant cells. Short double-stranded RNA (dsRNA) duplexes lead to specific deletion of RNAs containing the same sequence. In plants, RNAi is an important defence against dsRNA-containing viruses and transposons, but whether this process can be used as a tool in human antiviral responses is unclear. A recent paper in Nature Medicine describes how RNAi can be targeted to inhibit HIV-infection in human cells, so possibly forming the basis of new antiviral therapies.

To assess the effects of RNAi on HIV-1 infection, Novina and colleagues targeted both cellular and viral RNAs. The HeLa-derived cell line Magi-CCR5 (which expresses human CD4, and the chemokine receptors CCR5 and CXCR4) was transfected with short interfering RNA (siRNA) specific for the gene of interest and then infected with HIV-1. Cells transfected with siRNA specific for CD4 (the principle receptor for HIV-1) expressed CD4 mRNA at a level eight times lower than control cells, which led to a fourfold reduction in HIV-1 entry. Therefore, siRNA-directed silencing of CD4 specifically inhibited HIV entry and hence replication.

Next, the viral structural protein Gag was targeted by transfecting cells with siRNA specific for the p24 component of this polyprotein. P24-siRNA-transfected cells showed a fourfold decrease in viral protein compared with controls, implying that viral amplification was inhibited by this approach.

The authors also carried out transfection assays on human T cells, to assess the effect of RNAi on viral infectivity in a more physiological context. H9 cells were transfected with siRNA against green fluorescent protein (GFP) and were infected with an HIV-1 strain in which the nef gene had been replaced with GFP. Again, silencing of viral gene expression occurred, resulting in reduced GFP and HIV-1 protein expression.

But can siRNA-directed silencing reduce viral production in an established infection? Novina et al. tested the effect of p24-siRNA on previously infected Hela-CD4 cells and on a latently infected T-cell clone (ACH2) and again saw silencing of p24 expression. So, HIV-1 gene expression can be silenced by this approach even after viral integration has occurred in an established infection.

This study extends work by Lee and colleagues, published in Nature Biotechnology, who used a vector-based RNAi strategy to silence an HIV-1 gene, and establishes that siRNA technology can be used to suppress multiple steps of the HIV-1 life cycle.