Preliminary results on in vitro murine norovirus susceptibility to Mx-mediated innate immunity

Objectives
Murine norovirus (MuNoV) belongs to the family Caliciviridae, genus Norovirus and is currently used as study model for human noroviruses, major aetiologic agents of gastroenteritis worldwide. Myxovirus (Mx) protein is an interferon-induced protein that host cell can oppose virus infection and was detected in several species including human being. In mice, two genes encoding Mx1 and 2 proteins are present but it was evidenced that these genes were inactivated by deletions in several laboratory mouse strains. Mx antiviral activity was detected on several negative stranded RNA viruses but information are still lacking for most of positive stranded RNA viruses. In this study, the susceptibility of the MuNoV to specific and interspecific Mx proteins was investigated.
Methods
RAW264.7 cells (murine macrophages) were first tested for constitutive expression of Mx1 proteins. Plasmids containing the murine Mx1, bovine Mx1 and human MxA genes under the CMV immediate early promotor were then used to transfect RAW264.7 cells for transient Mx expression. Negative control consisted in a plasmid expressing eGFP. Four hours after transfection, cells were infected at low MOI with the CW1 MuNoV strain. Cells and supernatants were harvested 24h post infection. RNA was extracted and viral genomic copies were measured by real time RT-PCR.
Results
An effect was confirmed on CW1 replication for both specific and interspecific Mx proteins. The highest effect was obtained with the bovine Mx1 protein.
Conclusion
In conclusion, we showed in these preliminary in vitro studies the MuNoV susceptibility to specific and interspecific Mx proteins. Bovine Mx1 protein was already demonstrated to have important antiviral activity on negative stranded RNA viruses (influenza- and paramyxoviruses) and co-evolution with the host could explain a higher susceptibility to interspecific Mx proteins. Important implications of this adaptation could be expected on zoonotic concerns associated to NoV. Moreover, even if several control studies are still be conducted to validate these preliminary results, they could drive several pertaining questions on the MuNoV model used with laboratory mice. Perspectives of this work consist to validate the susceptibility in vivo and also to test the murine Mx2 antiviral activity on MuNoV.