Neutrophils express distinct RNA receptors in a non-canonical way

RNAs are capable of modulating immune responses by binding to specific receptors. Neutrophils represent the major fraction of circulating immune cells, but receptors and mechanisms by which neutrophils sense RNA are poorly defined. Here, we analyzed the mRNA and protein expression patterns and the subcellular localization of the RNA receptors RIG-I, MDA-5, TLR3, TLR7, and TLR8 in primary neutrophils and immortalized neutrophil-like differentiated HL-60 cells. Our results demonstrate that both neutrophils and differentiated HL-60 cells express RIG-I, MDA-5, and TLR8 at the mRNA and protein levels, whereas TLR3 and TLR7 are not expressed at the protein level. Subcellular fractionation, flow cytometry, confocal laser scanning microscopy, and immuno-transmission electron microscopy provided evidence that, besides the cytoplasm, RIG-I and MDA-5 are stored in secretory vesicles of neutrophils and showed that RIG-I and its ligand, 3p-RNA, co-localize at the cell surface without triggering neutrophil activation. In summary, this study demonstrates that neutrophils express a distinct pattern of RNA recognition receptors in a non-canonical way, which could have essential implications for future RNA-based therapeutics.