Nucleoside-modified circRNA: Reduced immunogenicity and expansive applications beyond vaccines
mRNA drives the production of functional proteins to achieve therapeutic intervention, rendering it an attractive molecular platform for biomedical applications. In the 1990s, researchers established in vitro transcription (IVT) systems to produce linear mRNA encoding target proteins. Neverthe-less, unmodified mRNA exhibits potent immunogenicity and elicits robust innate immune responses in vivo , which brought early translational research of mRNA to a prolonged standstill. In 2005, the Karikó and Weissman team demonstrated that re-placing native uridine (U) with chemically modified pseudouridine ( ψ ) during IVT could drastically reduce mRNA immunogenicity and evade innate immune sensing. 1,2 This breakthrough resolved the safety bottleneck of mRNA and fueled the rapid advancement of mRNA vaccines. During the COVID-19 pandemic, mRNA vaccines received regulatory approval and were widely administered owing to their short development timelines, high production efficiency, and potent protective immunity. Linear mRNA suffers from poor stability and a short in vivo half-life. In 2022, a research team from Peking University pio-neered a circular RNA (circRNA) vaccine platform worldwide. The covalently closed circular conformation of circRNA markedly improves its in vivo stability and substantially extends its half-life. 3 Since then, circRNA-based cancer vaccines and in vivo chimeric antigen receptor T (CAR-T) cell immunotherapies have been documented successively. 4 However, the issue of circRNA immunogenicity remains unresolved. Analogous to linear mRNA, IVT produced circRNA triggers vigorous innate immune reactions in mammalian cells. Unlike mRNA, circRNA cannot attenuate immunogenicity