Single-stranded (ss) and double-stranded (ds) small interfering RNAs (siRNAs) containing immunostimulatory RNA motifs can activate innate immunity through Toll-like receptor 7/8 (TLR7/8), leading to the production of proinflammatory cytokines and type I interferon. More recently, we have noted that 2'-uridine modified ss or ds siRNAs not only evade immune activation, but can suppress TLR signaling triggered by their unmodified counterparts. Here we compared the inhibitory effects of several T-modifications. In contrast to 2'-deoxy uridine modified ss siRNAs, 2'-O-methyl uridine modified ss siRNAs inhibited at nanomolar concentrations the production of TNF-alpha induced by a variety of immunostimulatory RNA sequences. Using oligonucleotide microarrays, we highlight the strong suppressive effect of RNA-containing 2'-O-methyl uridines. Indeed, nearly all of the 270 genes induced by an immunostimulatory ss siRNA were completely inhibited or downregulated by cotreatment with its 2'-O-methyl modified version. Also, 2'-O-methyl modified RNAs inhibited E coli total RNA or mitochondrial RNA to induce TNF-alpha production in human monocytes. Collectively, these data indicate that 2'-modified RNAs, in particular those containing 2'-O-methyl modification, are recognized with high affinity by TLR7/8, but do not induce downstream signaling. Therefore, this new generation of TLR antagonists can be used as immunosuppressive agents to interfere with TLR signaling. (c) 2007 Elsevier Inc. All rights reserved.