Emerging roles of circular RNAs in innate immunity

•CircRNAs are a new class of RNA with diverse roles in innate immunity.•Endogenous circRNAs regulate the activity of innate immune proteins and cells.•Viruses encode circRNAs, though their roles require further investigation.•Foreign circRNAs that lack the RNA modification N6-methyladenosine (m6A), a ‘self’ marker, trigger an innate immune response. The proper function of the innate immune system depends on an intricate network of regulation that promotes effective responses to pathogens while avoiding autoimmunity. Circular RNAs (circRNAs), a class of RNAs that lack 5′ and 3′ ends, have emerged as key actors in these networks. Recent studies have demonstrated that endogenous circRNAs in eukaryotes regulate the activation of innate immune proteins and cells through diverse modes of action. Some DNA viruses also encode circRNAs, and foreign circRNAs have been found to stimulate an innate immune response. This review summarizes recent investigations that reveal the critical roles that circRNAs play in innate immunity and points to future areas of study in this emerging field.