Expression of a Functional Mx1 Protein Is Essential for the Ability of RIG-I Agonist Prophylaxis to Provide Potent and Long-Lasting Protection in a Mouse Model of Influenza A Virus Infection

Abstract

RIG-I is an innate sensor of RNA virus infection and its activation induces interferon-stimulated genes (ISGs). In vitro studies using human cells have demonstrated the ability of synthetic RIG-I agonists (3pRNA) to inhibit IAV replication. However, in mouse models of IAV the effectiveness of 3pRNA reported to date differs markedly between studies. Myxoma resistance (Mx)1 is an ISG protein which mediates potent anti-IAV activity, however most inbred mouse strains do not express a functional Mx1. Herein, we utilised C57BL/6 mice that do (B6.A2G-Mx1) and do not (B6-WT) express functional Mx1 to assess the ability of prophylactic 3pRNA treatment to induce ISGs and to protect against subsequent IAV infection. In vitro, 3pRNA treatment of primary lung cells from B6-WT and B6.A2G-Mx1 mice resulted in ISG induction however inhibition of IAV infection was more potent in cells from B6.A2G-Mx1 mice. In vivo, a single intravenous injection of 3pRNA resulted in ISG induction in lungs of both B6-WT and B6.A2G-Mx1 mice, however potent and long-lasting protection against subsequent IAV challenge was only observed in B6.A2G-Mx1 mice. Thus, despite broad ISG induction, expression of a functional Mx1 is critical for potent and long-lasting RIG-I agonist-mediated protection in the mouse model of IAV infection.

Induction and antiviral activity of ferret myxovirus resistance (Mx) protein 1 against influenza A viruses

“…Recently, Horman et al reported induction of ferret IFITM1, 2 and 3 in vitro in response to treatment of cells with ferret IFN-α or infection with IAV 44 , although these studies did not investigate induction in vivo, cellular localisation or the antiviral activity of the different ferret IFITMs. Further studies characterising similarities and differences between restriction factors expressed by humans and ferrets are of particular importance given the widespread use of ferrets as animal models to study IAV and other human respiratory virus infections, and of new prophylactic and therapeutic approaches that are based on activation of innate immune receptors such as RIG-I 1 – 4 .…”

“…As first line of defence, highly conserved cell -associated innate immune sensing receptors such as the cytoplasmic helicase RIG-I can detect viruses, specifically viral nucleic acids, triggering a signalling cascade that results in the rapid induction of diverse antiviral effectors. In our previous work we have demonstrated that the activation of those pathways with synthetic RNA ligands of RIG-I provides potent protection from viral infection with influenza A virus (IAV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and respiratory syncytial virus (RSV) in mice and ferrets 1 – 4 . Cell-intrinsic innate antiviral defences include a range of intracellular proteins with antiviral activity that are regulated independently of interferons (IFNs), as well as IFN stimulating genes (ISGs).…”