Viral dsRNA is known to induce type I IFNs through three main systems: an endosomal pathway involving Toll-like receptor 3 (TLR3) and TIR domain-containing adaptor protein inducing IFNβ (TRIF); a mitochondrial pathway through RIG-I or MDA5 and mitochondrial antiviral signalling protein (MAVS; also known as IPS1, VISA and CARDIF); and a cytosolic RNA-activated protein kinase (PKR) pathway. However, TRIF-deficient cells have a more severely attenuated IFN response to polyinosinic–polycytidylic acid (polyI:C; a synthetic mimic of viral dsRNA) than TLR3-deficient cells, which indicates the possibility of a TLR3-independent, TRIF-dependent sensor.
The authors identified three members of the DExD/H helicase family (of which RIG-I is a member) — DDX1, DDX21 and DHX36 — that could be purified with polyI:C in cultured D2SC cells (immature dendritic cells). To investigate the potential role of these helicases in recognizing dsRNA, they established stable D2SC cell lines in which the expression of DDX1, DDX21, DHX36, RIG-I, TRIF, MAVS, TLR3 or MDA5 was knocked down. The results showed that DDX1, DDX21, DHX36 and TRIF have a role in sensing both short and long polyI:C in D2SC cells; RIG-I–MAVS preferentially senses short polyI:C, whereas MDA5–MAVS preferentially senses long polyI:C; and TLR3 senses extracellular but not intracellular polyI:C (which is consistent with its endosomal localization). The roles of the helicases and TRIF in sensing viral dsRNA were confirmed in response to influenza A virus and reovirus.
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