Aberrant activation of innate immune receptors can cause a spectrum of immune disorders, such as Aicardi-Goutières syndrome (AGS). One such receptor is MDA5, a viral dsRNA sensor that induces antiviral immune response. Using a newly developed RNase-protection/RNA-seq approach, we demonstrate here that constitutive activation of MDA5 in AGS results from the loss of tolerance to cellular dsRNAs formed by Alu retroelements. While wild-type MDA5 cannot efficiently recognize Alu-dsRNAs because of its limited filament formation on imperfect duplexes, AGS variants of MDA5 display reduced sensitivity to duplex structural irregularities, assembling signaling-competent filaments on Alu-dsRNAs. Moreover, we identified an unexpected role of an RNA-rich cellular environment in suppressing aberrant MDA5 oligomerization, highlighting context dependence of self versus non-self discrimination. Overall, our work demonstrates that the increased efficiency of MDA5 in recognizing dsRNA comes at a cost of self-recognition and implicates a unique role of Alu-dsRNAs as virus-like elements that shape the primate immune system.

Highlights d Unmodified circRNA adjuvant induces antigen-specific T and B cell responses d m 6 A RNA modification marks self circRNAs and abrogates circRNA immunity d Unmodified circRNA and K63-polyubiquitin activate RIG-I and innate immune signaling d YTHDF2 binding of m 6 A-circRNA additionally suppresses circRNA immunity

Highlights d RIPLET, not TRIM25, is the obligatory ubiquitin E3 ligase for RIG-I d RIPLET recognizes pre-assembled RIG-I oligomers on dsRNA and ubiquitinates RIG-I d RIPLET can cross-bridge RIG-I filaments formed on longer dsRNA d The two binding modes synergize for length dependent dsRNA discrimination by RIG-I