Polyinosinic acid is a ligand for toll-like receptor 3.
Marshall-Clarke S., Downes JE., Haga IR., Bowie AG., Borrow P., Pennock JL., Grencis RK., Rothwell P.
Innate immune responses are critical in controlling viral infections. Viral proteins and nucleic acids have been shown to be recognized by pattern recognition receptors of the Toll-like receptor (TLR) family, triggering downstream signaling cascades that lead to cellular activation and cytokine production. Viral DNA is sensed by TLR9, and TLRs 3, 7, and 8 have been implicated in innate responses to RNA viruses by virtue of their ability to sense double-stranded (ds) RNA (TLR3) or single-stranded RNA (murine TLR7 and human TLR8). Viral and synthetic dsRNAs have also been shown to be a potent adjuvant, promoting enhanced adaptive immune responses, and this property is also dependent on their recognition by TLR3. It has recently been shown that mRNA that is largely single-stranded is a ligand for TLR3. Here we have investigated the ability of single-stranded homopolymeric nucleic acids to induce innate responses by murine immune cells. We show for the first time that polyinosinic acid (poly(I)) activates B lymphocytes, dendritic cells, and macrophages and that these responses are dependent on the expression of both TLR3 and the adaptor molecule, Toll/IL-1 receptor domain-containing adaptor inducing IFN-beta (TRIF). We therefore conclude that TLR3 is able to sense both single-stranded RNA and dsRNA.