siRNA and Innate Immunity

Abstract

Canonical small interfering RNA (siRNA) duplexes are potent activators of the mammalian innate immune system. The induction of innate immunity by siRNA is dependent on siRNA structure and sequence, method of delivery, and cell type. Synthetic siRNA in delivery vehicles that facilitate cellular uptake can induce high levels of inflammatory cytokines and interferons after systemic administration in mammals and in primary human blood cell cultures. This activation is predominantly mediated by immune cells, normally via a Toll-like receptor (TLR) pathway. The siRNA sequence dependency of these pathways varies with the type and location of the TLR involved. Alternatively nonimmune cell activation may also occur, typically resulting from siRNA interaction with cytoplasmic RNA sensors such as RIG1. As immune activation by siRNA-based drugs represents an undesirable side effect due to the considerable toxicities associated with excessive cytokine release in humans, understanding and abrogating this activity will be a critical component in the development of safe and effective therapeutics. This review describes the intracellular mechanisms of innate immune activation by siRNA, the design of appropriate sequences and chemical modification approaches, and suitable experimental methods for studying their effects, with a view toward reducing siRNA-mediated off-target effects.

Akt2 Deficiency Protects from Acute Lung Injury via Alternative Macrophage Activation and miR-146a Induction in Mice

“…The siRNA used for in vivo targeting of Akt2 contain 29-O-methyl modifications, increasing their in vivo stability and reducing immune response activation (44,45). Its effectiveness in suppressing Akt2 was verified in vitro before in vivo application.…”

The Business of RNAi Therapeutics

“…7,8 It almost came to be assumed that an in vivo RNAi efficacy claim was in fact an innate immunostimulatory artefact. Importantly, this suspicion extended to the preclinical data that formed the rationale for the industry's lead clinical candidates in wet age-related macular edema (Acuity/Opko's Cand5, Merck/Allergan's Sirna-027/AGN-745, Quark/Pfizer's PF-4523655) 9,10 and respiratory viral infection (Alnylam's ALN-RSV01), 11 approaches which incidentally did not involve specific delivery chemistries.…”

“…8 Indeed, RNAi-related "flu-like symptoms" with inflammatory cytokine elevations have been observed in a number of clinical trials (e.g., TKM-ApoB, NucB1000, CALAA-01), 12 and activation of the alternative complement pathway was seen in the Atu027 trial. Although the clinical importance of such observations can vary considerably, it is necessary to improve on that record.…”

HIF2α-Targeted RNAi Therapeutic Inhibits Clear Cell Renal Cell Carcinoma

“…Questions such as maximum tolerated dose, biodegradability, and toxicity upon repeat dosing, need to be assessed before this technology advances to the clinic. Despite significant efforts to reduce the immunomodulatory potential of the HIF2a-RGD-DPC therapeutic, evaluation of possible cytokine induction, complement activation and other immune reactions warrants attention (46,50). A successful first-in-class RNAi therapeutic for ccRCC will provide a muchneeded alternative treatment choice, not only as a potential monotherapy, but more attractively in combination with current treatment options.…”

“…Sequences containing known SNPs directed against target sites were excluded from the screen set. The candidate sequences were synthesized with 2 0 -O-methyl/2 0 -fluoro-modified nucleotides to provide nuclease resistance, and to abrogate potential immune activation (46). The gene inhibition potency of the RNAi trigger screen set was evaluated by in vitro transfection.…”