Cell-surface RNA forms ternary complex with RNA-binding proteins and heparan sulfate to recruit immune receptors

Recent discoveries have shown the presence of ribonucleic acid (RNA) on the cell surface, defying the view that RNA only functions intracellularly. However, how RNA is presented on the cell surface and what its biological relevance is are poorly understood. We established Toll-like receptor 7 (TLR7) as a cell-surface RNA (csRNA) probe. Employing it in a genome-wide knockout screening, we identified heparan sulfate (HS) as a crucial factor for csRNA presentation. Cell-surface proximity labeling revealed that HS-associated csRNAs (hepRNAs) are in the vicinity of RNA-binding proteins (RBPs). These observations led us to a model wherein cell-surface HS, RNA, and RBP form ternary complexes, validated by our spatio-selective RNA-protein crosslinking technology in a TLR7-orthogonal manner. We further revealed the identities of hepRNA and found that they can recruit the immune receptor killer cell immunoglobulin-like receptor 2DL5 (KIR2DL5), potentially enhancing receptor-ligand interactions. Employing human cell lines, our findings lay the groundwork for investigating how cell-surface ribonucleoproteins contribute to immune modulation.