225: Inosine-mediated modulation of RNA sensing by innate immune sensors

Abstract

RNA-specific adenosine deaminase (ADAR)-mediated adenosine to inosine (A-to-I) editing is a critical arm of the antiviral response. However, mechanistic insights into how A-to-I RNA editing affects viral infection are lacking. We posited that inosine incorporation into RNA facilitates sensing of non-self RNA by innate immune sensors, and accordingly investigated the impact of inosine-modified RNA on Toll Like Receptor (TLR) 7/8 sensing. Inosine incorporation into synthetic ssRNA potentiated TNF-α or IFN-α production in human PBMCs, in a sequence-dependent manner, indicative of TLR7/8 recruitment. The effect of inosine incorporation on TLR7/8 sensing was restricted to immunostimulatory ssRNAs, and was not seen with inosine-containing short dsRNAs, nor with a deoxy-inosine-modified ssRNA. Inosine-mediated increase of self-secondary structure of an ssRNA resulted in potentiated mouse Tlr7 activation, as established through the use of Tlr7-deficient cells. There was a correlation between hyper-editing of influenza A viral ssRNA and its ability to stimulate TNF-α, independent of 5′-triphosphate residues. Furthermore, A-to-I editing of viral ssRNA directly enhanced mouse Tlr7 sensing, when present in proportions reproducing biologically relevant levels of RNA editing. Thus we demonstrate for the first time that inosine incorporation into immunostimulatory ssRNA can potentiate TLR7/8 activation. Our results suggest a novel function of A-to-I RNA editing, which is to facilitate sensing of phagocytosed viral RNA by innate immune sensors TLR7/8.