[NP gene of pandemic H1N1 virus attenuates virulence of mouse-adapted human influenza virus].

: The authors studied a possible role of the caspase cleavage motif located in the nucleoprotein (NP) of pandemic influenza virus H1N1 in the regulation of viral virulence properties. A reverse genetics method was used to obtain chimeric seasonal-like mouse-adapted influenza virus hvA/PE/8/34 (H1N10) carrying either the NP gene of wild type pandemic virus with incomplete caspase motif ETGC or mutated pandemic NP with natural caspase cleavage site of human type ETDG. The wild-type NP gene of the pandemic virus was found to poorly fit to the gene pattern of closely related seasonal-like hvA/PR/8/34 virus (H1N1) and did not rescue mature virus production whereas a mutated NP with human-type caspase cleavage site maintained gene fitness, giving rise to a chimeric virus. The generated chimeric virus hvA/PR/8/34 carrying the mutated pandemic NP successfully replicated in the murine lung, but was attenuated and did not reach the virulence level of seasonal-like mouse-adapted virus hvA/PR/8/34. The findings indicate that the NP caspase cleavage site plays a role in viral adaptation and viral virulence in mammals.