Neuraminidase (NA) 370-Loop Mutations of the 2009 Pandemic H1N1 Viruses Affect NA Enzyme Activity, Hemagglutination Titer, Mouse Virulence, and Inactivated-Virus Immunogenicity

Hemagglutinin (HA) and neuraminidase (NA) are the two major envelope proteins of influenza viruses. The spatial organization of HA and NA on the virus surface needs to be optimized to promote viral fitness, host specificity, transmissibility, infectivity, and virulence. We previously demonstrated that the recombinant NA protein of the 2009 pandemic H1N1 (pH1N1) with the I365T/S366N mutation in the NA 370-loop elicited higher NA-inhibition antibody titers against the homologous pH1N1 virus and three heterologous H5N1, H3N2, and H7N9 viruses in mice. In this study, we used PR8-based reverse genetics (RG) by replacing the HA and NA genes of A/Texas/05/2009 pH1N1 virus to obtain the wild-type pH1N1 and three NA 370-loop mutant viruses of pH1N1 (I365T/S366N), RG pH1N1 (I365E/S366D), and RG pH1N1 (I365T/S366A). Our results revealed that the viral NA enzyme activity increased for the RG pH1N1(I365T/S366N) and RG pH1N1 (I365E/S366D) viruses but reduced for the RG pH1N1 (I365T/S366A) virus. The increased or decreased NA enzyme activity was found to correlate with the increase or decrease in HA titers of these NA 370-loop mutant viruses. All of these three NA 370-loop mutant RG pH1N1 viruses were less virulent than the wild-type RG pH1N1 virus in mice. Immunizations with the inactivated viruses carrying the three NA 370-loop mutations and the wild-type RG pH1N1 virus were found to elicit approximately the same titers of NA-inhibition antibodies against H1N1 and H5N1 viruses. These results may provide information for developing NA-based influenza virus vaccines.