Abstract
The ability of the host to distinguish between self and foreign nucleic acids is one of the critical factors contributing to the recognition of pathogens by Toll-like receptors (TLRs). Under certain circumstances, eukaryotic self-RNA may reach TLR-containing compartments allowing for self-recognition. Specific modifications were previously demonstrated to suppress immune activation when placed at several positions in an immune stimulatory RNA or silencing RNA (siRNA). However, we show that even a simple natural modification such as a single 2'-O-methylation at different nucleotide positions throughout a sequence derived from a self-RNA strongly interferes with TLR-mediated effects. Such a single modification can even have an inhibitory effect in vitro and in vivo when placed in a different than the immune stimulatory RNA strand acting as suppressive RNA. Several safeguard mechanisms appear to have evolved to avoid cellular TLR-mediated activation by self-RNAs that may under other circumstances result in inflammatory or autoimmune responses. This knowledge can be used to include as few as a single 2'-O-methyl modification at a specific position in a siRNA sense or anti-sense strand to avoid TLR immune effects.