We have investigated DNA-mediated Type III interferon (IFN) induction using human cell lines and primary cells and reported that Ku70, a subunit of DNA repair protein, plays a pivotal role in the IFN induction. In the course of study, we previously reported that small interfering RNA (siRNA) enhances DNA transfection or DNA virus infection-mediated IFN-λ1 induction through a crosstalk signalling pathway between a RNA sensor, RIG-I and a DNA sensor, IFI16 in HeLa cells and primary dendritic cells. Here we provide new evidence for another role of siRNA. We found that siRNA containing a unique 5-nt motif sequence suppresses the siRNA/DNA-mediated induction of IFNs and inflammatory cytokines (CXCL10, TNFα and RANTES) up to 95%. Using a series of variants of the motif siRNA, we determined that the inhibition capability is dependent on the location of the motif. The motif siRNA is able to induce the suppression only if the motif is located at the 3’ or 5’ end. Further study using THP-1 Lucia ISG cells with various DNA stimulants indicated that the motif siRNA dose-dependently suppresses plasmid DNA, Poly(A:T), VACV-60 or HSV-70-mediated innate immune response, but has no impact on 2’3’-cGAMP-induced signaling. A DNA pull-down assay illustrated that the motif siRNA specifically competes with an interaction between DNA and IFI16, but not cGAS or Ku70. AlphaScreen proximity analysis further clarified that the motif siRNA has 2.5-fold higher affinity to IFI16 than siRNA without the motif. Collectively, these findings may provide a new strategy to regulate the over-response of the innate immune system for the host cells upon microbial infections, shedding light on a novel function of siRNA with the unique 5-nt motif as a quencher of innate immunity and a potential therapeutic agent for auto immune diseases.
(Funded by NCI Contract No. HHSN261200800001E.)