Using a mouse model, we previously demonstrated that subcutaneous infection with the JaTH160 strain of Japanese encephalitis virus (JEV) causes significantly higher virulence and stronger virus propagation in the brain compared with that of the JaOArS982 strain. We also showed that the JaTH160 strain, but not JaOArS982, expresses the NS1’ protein and that NS1’ enhances JEV production in avian cells and embryonated chicken eggs. In this study, we examined whether NS1’ expression affects virulence in mice infected with the JaOArS982 and JaTH160 strains using the corresponding recombinant viruses S982-IC and JaTH-IC. Expression of the NS1’ protein in S982-IC diminished the mortality in mice, whereas S982-IC viruses without NS1’ caused 40–60% mortality. However, the viral loads in the brains of these mice were not significantly different despite the dvariation in NS1’ expression. JaTH-IC viruses depleted of the NS1’ protein exhibited high mortality levels, similar to those of the virus expressing NS1’. Previous studies showed that the NS1’ protein plays a role in the enhanced virulence of the JEV SA14 strain in mice. However, our current data suggest that NS1’ protein expression in S982-IC reduces, rather than enhances, the mortality in mice. Thus, the effect of NS1’ on pathogenicity in vivo may vary among virus strains. Our data also suggest that the reduced mortality resulting from NS1’ expression in S982-IC is not simply due to viral replication in the brains. Further investigation is needed to uncover the mechanism by which NS1’ affects pathogenicity in JEV-infected animals.

Using a mouse model,we previously demonstrated that subcutaneous infection with the JaTH160 strainof Japaneseencephalitis virus (JEV) causes significantly higher virulence and strongervirus propagation in the brain compared with that of the JaOArS982strain. We also showed that the JaTH160 strain,but not JaOArS982, expresses the NS1’ protein and that NS1’ enhances JEVproduction in avian cells and embryonated chicken eggs. In this study, weexamined whether NS1’ expression affects virulence in mice infected with theJaOArS982 and JaTH160 strains using the corresponding recombinant viruses S982-ICand JaTH-IC.Expression of the NS1’ protein in S982-IC diminishedthe mortality in mice, whereas S982-IC viruses without NS1’ caused 40% mortality.However, the viral loads in the brains of these mice were not significantlydifferent despite the difference in NS1’ expression. JaTH-IC viruses depletedof the NS1’ protein exhibited high mortality levels, similar to those of thevirus expressing NS1’.Previousstudies showed that the NS1’ protein plays a role in the enhanced virulenceof the JEV SA14 strain in mice. However, ourcurrent data suggest that NS1’ protein expression in S982-IC reduces,rather than enhances, the mortality in mice. Thus, the effect of NS1’ on pathogenicity invivo may vary among virus strains. Our data also suggest that the reducedmortality resulting from NS1’ expression in S982-IC is not simply due to viralreplication in the brains. Furtherinvestigation is needed to uncover the mechanism by which NS1’ affectspathogenicity in JEV-infected animals.