The Nuclear DNA Sensor IFI16 Acts as a Restriction Factor for Human Papillomavirus Replication through Epigenetic Modifications of the Viral Promoters. M. DE ANDREA CO-FIRST AUTHOR

The human interferon-inducible IFI16 protein, an innate immune sensor of intracellular DNA, was recently demonstrated to act as a restriction factor for human cytomegalovirus (HCMV) and herpes simplex virus type 1 (HSV-1) infection by inhibiting both viral DNA replication and transcription. Through the use of two distinct cellular models, this study provides strong evidence in support of the notion that IFI16 can also restrict HPV18 replication. In the first model, an immortalized keratinocyte cell line (NIKS) was used in which the IFI16 protein was knocked down through the use of siRNA technology, and overexpressed following transduction with the AdVIFI16 vector. The second model consisted of U2OS cells transfected by electroporation with HPV18 minicircles. In differentiated IFI16-silenced NIKS-HPV18 cells, viral load values were significantly increased compared with differentiated control cells. Consistent with this, IFI16 overexpression severely impaired HPV18 replication in both NIKS and U2OS cells, thus confirming its antiviral restriction activity. In addition to the inhibition of viral replication, IFI16 was also able to reduce viral transcription, as demonstrated by viral gene expression analysis in U2OS cells carrying episomal HPV18 minicircles and HeLa cells. We also provide evidence that IFI16 promotes the addition of heterochromatin marks and the reduction of euchromatin marks on viral chromatin at both early and late promoters, thus reducing both viral replication and transcription. Altogether, these results argue that IFI16 restricts chromatinised HPV DNA through epigenetic modifications and executes a broad surveillance role against viral DNA in the nucleus that is not restricted to Herpesviruses.