Although it is clear that high-risk human papillomaviruses (HPVs) can selectively infect keratinocytes and persist in the host, it still remains to be unequivocally determined whether they can escape
antiviral innate immunity by interfering with
pattern recognition receptor (PRR) signaling. In this study, we have assessed the innate immune response in monolayer and organotypic raft cultures of NIKS cells harboring multiple copies of episomal HPV18 (NIKSmcHPV18), which fully recapitulates the persistent state of
infection. We show for the first time, to our knowledge, that NIKSmcHPV18, as well as HeLa cells (a cervical
carcinoma-derived cell line harboring integrated HPV18
DNA), display marked downregulation of several
PRRs, as well as other PRR downstream effectors, such as the adaptor
protein stimulator of IFN genes and the
transcription factors IRF1 and 7. Importantly, we provide evidence that downregulation of stimulator of IFN genes,
cyclic GMP-AMP synthase, and
retinoic acid-inducible gene I
mRNA levels occurs at the transcriptional level through a novel epigenetic silencing mechanism, as documented by the accumulation of repressive
heterochromatin markers seen at the promoter region of these genes. Furthermore, stimulation of NIKSmcHPV18 cells with salmon sperm
DNA or poly(deoxyadenylic-deoxythymidylic)
acid, two potent inducers of PRR signaling, only partially restored PRR
protein expression. Accordingly, the production of IFN-β and IFN-λ1 was significantly reduced in comparison with the parental NIKS cells, indicating that HPV18 exerts its immunosuppressive activity through downregulation of PRR signaling. Altogether, our findings indicate that high-risk human papillomaviruses have evolved broad-spectrum mechanisms that allow simultaneous depletion of multiple effectors of the innate immunity network, thereby creating an unreactive cellular milieu suitable for viral persistence.