Mobile genetic elements have significantly shaped our genomic landscape. LINE-1 retroelements are the only autonomously active elements left in the human genome. Since new insertions can have detrimental consequences, cells need to efficiently control LINE-1 retrotransposition. Here, we demonstrate that the intrinsic immune factor TRIM5α senses and restricts LINE-1 retroelements. Previously, rhesus TRIM5α has been shown to efficiently block HIV-1 replication, while human TRIM5α was found to be less active. Surprisingly, we found that both human and rhesus TRIM5α efficiently repress human LINE-1 retrotransposition. TRIM5α interacts with LINE-1 ribonucleoprotein complexes in the cytoplasm, which is essential for restriction. In line with its postulated role as pattern recognition receptor, we show that TRIM5α also induces innate immune signaling upon interaction with LINE-1 ribonucleoprotein complexes. The signaling events activate the transcription factors AP-1 and NF-κB, leading to the down-regulation of LINE-1 promoter activity. Together, our findings identify LINE-1 as important target of human TRIM5α, which restricts and senses LINE-1 via two distinct mechanisms. Our results corroborate TRIM5α as pattern recognition receptor and shed light on its previously undescribed activity against mobile genetic elements, such as LINE-1, to protect the integrity of our genome.

Methylation of the mRNA 5 0 cap by cellular methyltransferases enables efficient translation and avoids recognition by innate immune factors. Coronaviruses encode viral 2 0 -O-methyltransferases to shield their RNA from host factors. Here, we generate recombinant SARS-CoV-2 harboring a catalytically inactive 2 0 -O-methyltransferase Nsp16, Nsp16mut, and analyze viral replication in human lung epithelial cells. Although replication is only slightly attenuated, we find SARS-CoV-2 Nsp16mut to be highly immunogenic, resulting in a strongly enhanced release of type I interferon upon infection. The elevated immunogenicity of Nsp16mut is absent in cells lacking the RNA sensor MDA5. In addition, we report that Nsp16mut is highly sensitive to type I IFN treatment and demonstrate that this strong antiviral effect of type I IFN is mediated by the restriction factor IFIT1. Together, we describe a dual role for the 2 0 -O-methyltransferase Nsp16 during SARS-CoV-2 replication in avoiding efficient recognition by MDA5 and in shielding its RNA from interferon-induced antiviral responses, thereby identifying Nsp16 as a promising target for generating attenuated and highly immunogenic SARS-CoV-2 strains and as a potential candidate for therapeutic intervention.

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We thank Richard O.S. Karoo and members of the Spire Cardiff Hospital (Wales, UK) for 91 providing skin samples. We are grateful to J. Paul Mitchell, Sion A. Coulman (Cardiff, Wales, 92 UK) and all members of the Viral Trafficking, Restriction and Innate Signaling team for 93 excellent technical help and/or critical reading of the manuscript. We are also grateful to 94 Caroline Goujon and Olivier Moncorgé (IRIM, Montpellier, France) for the kind gift of 95 reagents. Immunofluorescence and part of the flow cytometry acquisitions were performed at 96 the Montpellier RIO Imaging facility (Montpellier, France).