Germline-encoded pattern recognition receptors (PRRs) mediate an early innate immune response against viral pathogens. Retinoic acid-inducible gene I protein (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) are cytoplasmic sensors that detect viral RNA species and signal through the mitochondrial antiviral-signalling protein (MAVS) to induce the production of type I interferons (IFNs) and other cytokines. Intracellular DNA sensors, such as cyclic GMP–AMP synthase (cGAS) and IFNγ-inducible protein 16 (IFI16), detect viral DNA in the cytoplasm and/or nucleus and signal through the stimulator of IFN genes (STING) to trigger an immune response. Viruses antagonize PRRs through the sequestration or modification of their viral genomes and through the manipulation of post-translational modifications of PRRs or their adaptor proteins. Some viruses cleave or degrade PRRs or their adaptors, or sequester and relocalize PRRs to escape immunity. A better understanding of the cellular immune-surveillance machinery and viral immune evasion strategies may guide the development of antiviral therapeutics and vaccines.

Infection with SARS-CoV-2 can lead to excessive production of pro-inflammatory cytokines, but the production of type I interferons, which are key antiviral mediators, is reportedly blunted. Here, we discuss how imbalanced interferon responses may contribute to the pathology of COVID-19. In this Comment, Michaela Gack and colleagues discuss how the dysregulation of type I interferon responses may contribute to COVID-19 pathology.

Tripartite motif (TRIM) proteins are a versatile family of ubiquitin E3 ligases involved in a multitude of cellular processes. Studies in recent years have demonstrated that many TRIM proteins play central roles in the host defense against viral infection. While some TRIM proteins directly antagonize distinct steps in the viral life cycle, others regulate signal transduction pathways induced by innate immune sensors, thereby modulating antiviral cytokine responses. Furthermore, TRIM proteins have been implicated in virus-induced autophagy and autophagy-mediated viral clearance. Given the important role of TRIM proteins in antiviral restriction, it is not surprising that several viruses have evolved effective maneuvers to neutralize the antiviral action of specific TRIM proteins. Here, we describe the major antiviral mechanisms of TRIM proteins as well as viral strategies to escape TRIM-mediated host immunity.

The sensor RIG-I detects double-stranded RNA derived from RNA viruses. Although RIG-I is also known to have a role in the antiviral response to DNA viruses, physiological RNA species recognized by RIG-I during infection with a DNA virus are largely unknown. Using next-generation RNA sequencing (RNAseq), we found that host-derived RNAs, most prominently 5S ribosomal RNA pseudogene 141 (RNA5SP141), bound to RIG-I during infection with herpes simplex virus 1 (HSV-1). Infection with HSV-1 induced relocalization of RNA5SP141 from the nucleus to the cytoplasm, and virus-induced shutoff of host protein synthesis downregulated the abundance of RNA5SP141-interacting proteins, which allowed RNA5SP141 to bind RIG-I and induce the expression of type I interferons. Silencing of RNA5SP141 strongly dampened the antiviral response to HSV-1 and the related virus Epstein-Barr virus (EBV), as well as influenza A virus (IAV). Our findings reveal that antiviral immunity can be triggered by host RNAs that are unshielded following depletion of their respective binding proteins by the virus.