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ABSTRACTSynthetic small interfering RNAs (siRNAs) can trigger a strong innate immune response in mammalian cells. This nonspecific side effect may hinder the application of siRNAs as tools in gene silencing. Chemically synthesized siRNAs, including traditional 19-mers with 2-nt 3′ overhangs, longer duplexes with blunt or 3′ overhangs, and asymmetric duplexes with a blunt end and a 2-nt 3′ overhang, can evoke strong dose-dependent interferon-α(IFN-α) and tumor necrosis factor-α(TNF-α) release in human peripheral blood mononuclear cells (PBMCs). This response is independent of retinoic acid-inducible gene I but may involve endosomal toll-like receptors (TLRs). The immunostimulatory effect of the siRNAs is directly related to either or both of the strands of the duplex in a sequence-dependent manner. However, although some single-stranded RNAs and siRNAs potently evoked both IFN-αand TNF-αinduction, these responses were not always coupled. In accordance with this, specific chemical modifications differentially altered cytokine production, suggesting recruitment of different TLRs in a sequence-dependent manner. [ABSTRACT FROM AUTHOR]

RNA interference (RNAi) is one of the most promising tools for deciphering the human genome and has great therapeutic potential. However, its high target specificity limits its efficiency for therapeutic protection from viruses with high rates of genetic mutation. This limitation may be overcome by the expression of long hairpin RNAs (lhRNAs). Indeed, lhRNAs have been shown recently to have increased efficacy over short interfering RNAs (siRNAs) as protective antiviral agents. Here, we investigate the expression of lhRNAs and demonstrate unintended effects. We show that overexpressed lhRNAs are exported to the cytoplasm. As a consequence, we detect activation of innate immune signaling pathways by lhRNAs. With growing concerns about the complexity of cytoplasmic detection of dsRNAs by the innate immune machinery, this work highlights the need for closer scrutiny when using lhRNAs as potential antiviral agents. [ABSTRACT FROM AUTHOR]