Immune Sensing of Synthetic, Bacterial, and Protozoan RNA by Toll-like Receptor 8 Requires Coordinated Processing by RNase T2 and RNase 2

Human toll-like receptor 8 (TLR8) activation induces a potent T helper-1 (Th1) cell response critical for defense against intracellular pathogens, including protozoa. The receptor harbors two distinct binding sites, uridine and di- and/or trinucleotides, but the RNases upstream of TLR8 remain poorly characterized. We identified two endolysosomal endoribonucleases, RNase T2 and RNase 2, that act synergistically to release uridine from oligoribonucleotides. RNase T2 cleaves preferentially before, and RNase 2 after, uridines. Live bacteria, P. falciparum-infected red blood cells, purified pathogen RNA, and synthetic oligoribonucleotides all required RNase 2 and T2 processing to activate TLR8. Uridine supplementation restored RNA recognition in RNASE2−/− or RNASET2−/− but not RNASE2−/−RNASET2−/− cells. Primary immune cells from RNase T2-hypomorphic patients lacked a response to bacterial RNA but responded robustly to small-molecule TLR8 ligands. Our data identify an essential function of RNase T2 and RNase 2 upstream of TLR8 and provide insight into TLR8 activation. •Endolysosomal RNase 2 and RNase T2 cooperatively process RNA into TLR8 ligands•RNase 2 and RNase T2 synergistically release uridine from RNA ligands•RNase T2-hypomorphic patients have an impaired to TLR8 response to pathogen RNA•Sensing of live pathogens by TLR8 requires both RNase 2 and RNase T2 Toll-like receptor 8 (TLR8) is an endosomal RNA sensor that induces a robust T-helper 1 response downstream of pathogen recognition. Ostendorf and colleagues identify two RNases, RNase 2 and RNase T2, which synergistically process pathogen RNA into TLR8 ligands and are critically required for the sensing of live bacteria and plasmodia.