RNAi as New Class of Nanomedicines - Harnessing Biomaterials in Nanomedicine: Preparation, Toxicity, and Applications - page 180

Innate immune responses function as a mechanism to recognize
and eliminate pathogens, while aiding in the establishment of
adaptive immunity. This recognition requires a diverse family of
pattern recognition receptors to detect a variety of bacterial and
viral-associated molecules, such as, lipopolysaccharides, bacterial
DNA, and viral RNAs [77, 78]. Originally, siRNAs were believed to be
unable to engage the RNA-dependent protein kinase (PKR) pathway
due to their small size (<30bp) [38]. However, more recent studies
have shown that, in some cases, synthetic siRNAs could stimulate
aspects of the innate immune response through the engagement of
Toll-like receptors (TLRs), in particular, TLR7 and TLR8 [68, 79, 132].
It has been suggested that the sequence-independent engagement
of TLR3 on dendritic cells and endothelial cells could be a source
of additional toxicity [80, 133]. However, TLR3 signaling does not
appear to be a major determinant of innate immune activation by
siRNAs. On the other hand, the sequence-specific engagement of
TLR7 and TLR8 seems to be the primary mechanism for the induction
of innate immune activation by synthetic siRNAs [28, 46, 64]. Both
TLR7 and TLR8 are expressed in the endosomes and their signaling
requires endosomal acidification. Although similar in function and
intracellular localization, they are expressed in diff erent subsets
of cells, with TLR7 predominating in plasmacytoid dendritic cells
and B cells, while TLR8 expression is restricted to dendritic cells,
monocytes and macrophages. Although the immunostimulatory
eff ects of siRNAs have not been fully characterized, it appears
that these receptors recognize GU-rich RNA sequences [28, 64,
68, 79]. Selecting siRNAs that avoid these GU-rich motifs can help
to reduce the immunostimulatory nature of certain siRNAs [79].
However, GU-rich sequences alone can't account for the full range
of siRNA immunogenicity. While GU-rich sequences have been
shown to engage TLR7 and TLR8, AU-rich sequences were found to
preferentially activate TLR8 [46]. Additional factors, including the
position of immunostimulatory motifs within the siRNA, sequence
context, and the length of the siRNA molecule, have been suggested
to influence the immunostimulatory nature of siRNAs [54, 68].
The incorporation of chemical modifications into the siRNA
has been used to improve several aspects of the pharmacokinetic
properties of siRNAs including serum nuclease stability and
reduction of off -target gene silencing [5]. Several diff erent
modifications have been incorporated into siRNAs including the