Modification of H1N1 Influenza Luciferase Reporter Viruses Using StopGo Translation and/or Mouse-Adapted Mutations

Reporter viruses are valuable tools for studying infections at the cellular level and in living animals. They also enable rapid, high-throughput antiviral drug screening and serological studies. We previously developed a bioluminescence-based reporter virus, rTN09-PA-Nluc, derived from influenza A/Tennessee/1-560/2009 (TN09, pH1N1) in which a NanoLuc (Nluc) reporter protein was fused to the PA protein. Reduced growth of rTN09-PA-Nluc in MDCK cells and mice was restored by mutations arising from mouse adaptation. Here, to test the hypothesis that the growth defect resulted from the PA-Nluc protein fusion, we generated the luciferase reporter virus rTN09-PA-Nluc/SG, which undergoes StopGo translation to yield separate PA and NLuc proteins along with a proportion of the PA-Nluc fusion. The rTN09-PA-Nluc/SG virus had greater protein expression and increased replication in MDCK cells compared to rTN09-PA-Nluc. The reporter virus encoding StopGo translation was superior to the virus without it in bioluminescence-based virus neutralization assays in vitro, providing results in 24 h as opposed to 3 days using unmodified influenza virus and standard neutralization assay protocols. However, the reporter virus encoding StopGo translation remained attenuated in mice. Mouse-adaptive mutations were needed for full virulence and efficient non-invasive imaging in mice. Overall, these findings demonstrate the benefit of incorporating StopGo translation into influenza reporter viruses for in vitro assays, yet mouse-adapted mutations appeared superior in mice.