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* Department of Medical Microbiology and Hygiene, University of Heidelberg, Heidelberg; and
Institute for Organic Chemistry, University of Karlsruhe, Karlsruhe, Germany
Synthetic small interfering RNA (siRNA) can suppress the expression of endogenous mRNA through RNA interference. It has been reported that siRNA can induce type I IFN production from plasmacytoid dendritic cells, leading to off-target effects. To separate immunostimulation from the desired gene-specific inhibitory activity, we designed RNA strands with chemical modifications at strategic positions of the ribose or nucleobase residues. Substitution of uridine residues by 2'-deoxyuridine or thymidine residues was found to decrease type I IFN production upon in vitro stimulation of human PBMC. Thymidine residues in both strands of a siRNA duplex further decreased immunostimulation. Fortunately, the thymidine residues did not affect gene-silencing activity. In contrast, 2'-O-methyl groups at adenosine and uridine residues reduced both IFN-
secretion and gene-silencing activity. Oligoribonucleotides with 2'-O-methyladenosine residues actively inhibited IFN- secretion induced by other immunostimulatory RNAs, an effect not observed for strands with 2'-deoxynucleosides. Furthermore, neither 5-methylcytidine nor 7-deazaguanosine residues in the stimulatory strands affected IFN- secretion, suggesting that recognition does not involve sites in the major groove of duplex regions. The activity data, together with structure prediction and exploratory UV-melting analyses, suggest that immunostimulatory sequences adopt folded structures. The results show that immunostimulation can be suppressed by suitable chemical modifications without losing siRNA potency by introducing seemingly minor structural changes.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 This work was supported by a grant of the Landesstiftung Baden-Württemberg to K.H., C.R., and A.D. (P-LS-RNS/25).
2 Address correspondence and reprint requests to Dr. Alexander H. Dalpke, Department of Medical Microbiology and Hygiene, Hygiene-Institute, University of Heidelberg, Im Neuenheimer Feld 324, 69120 Heidelberg, Germany. E-mail address: alexander.dalpke{at}med.uni-heidelberg.de
3 Abbreviations used in this paper: RNAi, RNA interference; DOTAP, N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methylsulfate; eGFP, enhanced green fluorescent protein; MFI, mean fluorescence intensity; RISC, RNA-induced silencing complex; siRNA, small interfering RNA; TBDMS, tert-butyldimethylsilyl.
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