Abstract Recognition of RNA fragments by Toll-like receptor 7 (TLR7) and TLR8 helps to initiate the innate immune response against pathogens. An outstanding question is why RNA fragments generated during clearance of apoptotic cells fail to activate TLR7 and TLR8 signaling. Here we show that select 2′- O -methyl (2′-OMe) guanosine RNA fragments, including those derived from host RNAs, function as potent TLR7 and TLR8 antagonists and reduce TLR7 sensing in vivo. Mechanistically, these fragments bind to an antagonistic site on these proteins via their 5′-end 2′-OMe guanosine. These findings indicate that host RNAs evade detection because abundant ribosomal 2′-OMe-modified fragments naturally antagonize TLR7 and TLR8. Crucially, rare TLR7 and TLR8 mutations at this antagonist binding site decrease inhibition by 2′-OMe guanosine RNA fragments, leading to autoimmunity in patients. Collectively, this work redefines TLR7 and TLR8 sensing by introducing 2′-OMe guanosine as a natural immune checkpoint for their activation.
Alharbi et al. (Tue,) studied this question.