The therapeutic application of small interfering RNAs (siRNAs) is limited by the induction of severe off-target effects, especially in the liver. Therefore, we assessed the potential of differently modified siRNAs to induce the hepatic innate immune system in vitro and in vivo.

Primary isolated liver cells were transfected with siRNAs against apolipoprotein B1 (APOB1), luciferase (LUC) or galactosidase (GAL). For in vivo use, siRNAs were formulated in lipid nanoparticles (LNPs) and administered intravenously to C57BL/6 mice. Liver tissue was collected 6–48h after injection and knock-down efficiency or immune responses were determined by quantitative reverse-transcription-linked PCR.

Unmodified GAL siRNA transiently induced the expression of TNF-α, IL-6, IL-10, IFN-β and IFN-sensitive gene 15 in vivo, whereas a formulation of 2′-O-methylated-LUC siRNA had no such effects. Formulation of unmodified APOB1-specific siRNA suppressed APOB1 mRNA levels by ~80% in the liver 48h after application. The results were paralleled in vitro, where transfection of liver cells with unmodified siRNAs, but not with chemically modified siRNAs, led to cell-type-specific induction of immune genes. These immune responses were not observed in MYD88-deficient mice or in chloroquine-treated cells in vitro.

Our data indicate that siRNAs activate endosomal Toll-like receptors in different liver-derived cell types to various degrees, in vitro. LNP-formulated siRNA selectively leads to hepatic knock-down of target genes in vivo. Here, off-target immune responses are restricted to non-parenchymal liver cells. However, 2′-O-methyl modifications of siRNA largely avoid immune-stimulatory effects, which is a crucial prerequisite for the development of safe and efficient RNA-interference-based therapeutics.