Species-Specific Inhibition of RIG-I Ubiquitination and IFN Induction by the Influenza A Virus NS1 Protein

Influenza A viruses can adapt to new host species, leading to the emergence of novel pathogenic strains. There is evidence that highly pathogenic viruses encode for non-structural 1 (NS1) proteins that are more efficient in suppressing the host immune response. The NS1 protein inhibits type-I interferon (IFN) production partly by blocking the TRIM25 ubiquitin E3 ligase-mediated Lys63-linked ubiquitination of the viral RNA sensor RIG-I, required for its optimal downstream signaling. In order to understand possible mechanisms of viral adaptation and host tropism, we examined the ability of NS1 encoded by human (Cal04), avian (HK156), swine (SwTx98) and mouse-adapted (PR8) influenza viruses to interact with TRIM25 orthologues from mammalian and avian species. Using co-immunoprecipitation assays we show that human TRIM25 binds to all tested NS1 proteins, whereas the chicken TRIM25 ortholog binds preferentially to the NS1 from the avian virus. Strikingly, none of the NS1 proteins were able to bind mouse TRIM25. Since NS1 can inhibit IFN production in mouse, we tested the impact of TRIM25 and NS1 on RIG-I ubiquitination in mouse cells. While NS1 efficiently suppressed human TRIM25-dependent ubiquitination of RIG-I 2CARD, NS1 inhibited the ubiquitination of full-length mouse RIG-I in a mouse TRIM25-independent manner. Therefore, we tested if the ubiquitin E3 ligase Riplet, which has also been shown to ubiquitinate RIG-I, interacts with NS1. We found that NS1 binds mouse Riplet and inhibits its activity to induce IFN-β in murine cells. Furthermore, NS1 proteins of human but not swine or avian viruses were able to interact with human Riplet, thereby suppressing RIG-I ubiquitination. In conclusion, our results indicate that influenza NS1 protein targets TRIM25 and Riplet ubiquitin E3 ligases in a species-specific manner for the inhibition of RIG-I ubiquitination and antiviral IFN production

Viral unmasking of cellular 5S rRNA pseudogene transcripts induces RIG-I mediated immunity

The sensor retinoic acid-inducible gene-I (RIG-I) detects double-stranded RNA derived from RNA viruses. Although RIG-I is also known to play a role in the antiviral response to DNA viruses, physiological RNA species recognized by RIG-I during DNA virus infection are largely unknown. Using next-generation RNA sequencing (RNAseq), we found that host-derived RNAs, most prominently 5S ribosomal RNA pseudogene 141 (RNA5SP141), bind to RIG-I during herpes simplex virus 1 (HSV-1) infection. HSV-1 infection induced relocalization of RNA5SP141 from the nucleus to the cytoplasm, and virus-induced shutoff of host protein synthesis downregulated RNA5SP141-interacting proteins, thereby allowing RNA5SP141 to bind RIG-I and induce type I interferon. Silencing of RNA5SP141 strongly dampened the antiviral response to HSV-1 and the related 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 viral depletion of their respective binding proteins

Profit enhancing competitive pressure in vertically related industries

Coevolution of viruses and their hosts represents a dynamic molecular battle between the immune system and viral factors that mediate immune evasion. After the abandonment of smallpox vaccination, cowpox virus infections are an emerging zoonotic health threat, especially for immunocompromised patients. Here we delineate the mechanistic basis of how cowpox viral CPXV012 interferes with MHC class I antigen processing. This type II membrane protein inhibits the coreTAP complex at the step after peptide binding and peptide-induced conformational change, in blocking ATP binding and hydrolysis. Distinct from other immune evasion mechanisms, TAP inhibition is mediated by a short ER-lumenal fragment of CPXV012, which results from a frameshift in the cowpox virus genome. Tethered to the ER membrane, this fragment mimics a high ER-lumenal peptide concentration, thus provoking a trans-inhibition of antigen translocation as supply for MHC I loading. These findings illuminate the evolution of viral immune modulators and the basis of a fine-balanced regulation of antigen processing

What viruses can teach us about the human immune system

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A phosphomimetic-based mechanism of dengue virus to antagonize innate immunity

14-3-3 proteins regulate biological processes by binding to phospho-Ser or phospho-Thr motifs of cellular proteins. Among them, 14-3-3ε is crucial for antiviral immunity by mediating the cytosol-to-mitochondrial-membrane translocation of the pathogen sensor RIG-I. Here we show that the NS3 protein of dengue virus (DV) binds to 14-3-3ε and prevents RIG-I translocation to the adaptor MAVS, thereby blocking antiviral signaling. Intriguingly, a highly conserved phosphomimetic RxEP motif in NS3 is essential for 14-3-3ε binding. A recombinant mutant DV deficient in 14-3-3ε binding is impaired in RIG-I antagonism and elicits a markedly augmented innate immune response and enhanced T cell activation. Our work reveals a novel phosphomimetic-based mechanism for viral antagonism of 14-3-3-mediated immunity, which may guide the rational design of therapeutics

RIG-I Works Double Duty

The pathogen sensor RIG-I recognizes viral RNA and signals to induce an antiviral response. In this issue of Cell Host & Microbe, Weber et al. (2015), along with recent work by Sato et al. (2015), demonstrate that RIG-I directly inhibits viral replication independent of antiviral signaling

TRIM proteins: New players in virus-induced autophagy

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