What goes around comes around: artificial circular RNAs bypass cellular antiviral responses

Natural circular RNAs have been found to sequester microRNAs and suppress their function. We have used this principle as a molecular tool, and produced artificial circular RNA sponges in a cell-free system by in vitro transcription and ligation. Formerly, we were able to inhibit hepatitis C virus propagation by applying a circular RNA decoy strategy against microRNA-122, which is essential for the viral life cycle. In another proof-of-principle study, we used circular RNAs to sequester microRNA-21, an oncogenic and pro-proliferative microRNA. This strategy slowed tumor growth in a 3D cell culture model system, as well as in xenograft mice upon systemic delivery. In the wake of the global use of an in vitro transcribed RNA in Covid-19 vaccines, the question arose if therapeutic circular RNAs trigger cellular antiviral defense mechanisms when delivered systemically. In this study, we present data on the cellular innate immune response as a consequence of liposome-based transfection of the circular RNA sponges we previously used to inhibit microRNA function. We find that circular RNAs produced by the presented methodology do not trigger the antiviral response and do not activate innate immune signaling pathways. A549 cells were transfected with 250 ng of artificial linear and circular RNA sponges ciRS-21-bu and ciRS-21-rnd (PMID: 34316687). 3 hours post transfection A549 cells were harvested, total RNA was isolated and RNA-sequencing was performed.