Abstract
The InterFeron Induced TransMembrane (IFITM) proteins are interferon stimulated genes that restrict many viruses, including HIV-1. SAMHD1 is another restriction factor blocking replication of HIV-1 and other viruses. Some lentiviruses evolved Vpx/Vpr proteins to degrade SAMHD1. However, this viral antagonism can be perturbed by host mechanisms: a recent study showed that in interferon (IFN) treated THP1 cells, Vpx is unable to degrade SAMHD1. In the present work, we designed an Interferon Stimulated Genes (ISGs)-targeted CRISPR knockout screen in order to identify ISGs regulating this phenotype. We found that IFITM proteins contribute to the IFNα-mediated protection of SAMHD1 by blocking VSV-G-mediated entry of the lentiviral particles delivering Vpx. Consistent with this, IFNα treatment and IFITM expression had no effect when the A-MLV envelope was used for pseudotyping. Using an assay measuring viral entry, we show that IFNα and IFITMs directly block the delivery of Vpx into cells by inhibiting VSV-G viral fusion. Strikingly, the VSV-G envelope was significantly more sensitive to this IFNα entry block and to IFITMs than HIV-1’s natural envelope. This highlights important differences between VSV-G pseudotyped and wild-type HIV-1, in particular relative to the pathways they use for viral entry, suggesting that HIV-1 may have evolved to escape restriction factors blocking entry.
Highlights
Type I IFNs play a central role in activation of innate immunity by turning on a vast transcriptional program that results in enhanced expression of hundreds of Interferon Stimulated Genes (ISGs)
We show that IFNα protects SAMHD1 from degradation when Vpx-containing Virus-Like Particles (VLPs) are pseudotyped with Vesicular Stomatitis Virus glycoprotein (VSV-G) for viral entry, but not when they are pseudotyped with the envelope from amphotropic murine leukemia virus (A-MLV), suggesting that the IFNα block occurs at the level of viral entry
SAMHD1 degradation in THP1 cells was achieved by overnight treatment with Virus-Like Particles (VLPs) that consist of the SIVMAC Gag/Pol structural proteins and naturally package both Vpr and Vpx [26], with only Vpx being able to degrade SAMHD1 [14]
Summary
Type I IFNs play a central role in activation of innate immunity by turning on a vast transcriptional program that results in enhanced expression of hundreds of Interferon Stimulated Genes (ISGs). Many of these ISGs encode proteins that have antiviral activity, inhibiting viruses at various steps of their life cycle including at the earliest stages of entry [1]. Retroviruses release genomes and associated proteins into the cytoplasm during the uncoating process, resulting in the synthesis of reverse transcription products, formation of the preintegration complex, and nuclear import [7]. Vpx/Vpr proteins directly bind to Roesch et al Retrovirology (2018) 15:26
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