The Influence Of Neuraminidase On The Receptor Binding Specificity Of Influenza A Viruses And The Biological Characteristics Of Duck H7N9 Viruses

The influenza virus receptor binding specificity is a key determinant of revealing the viral pathogenesis and transmission.The convertion of influenza viruses’ receptor binding specificity from SA α-2,3Gal(avian receptor) to SA α-2,6Gal(human receptor) is a key factor for the viruses’ adaptation to human hosts which might cause an influenza pandemic. Two H7 subtype influenza viruses, A / chicken / Jilin / SD020 / 2014(H7N2)(JL/020) and A / Anhui / 1/2013(H7N9)(AH/1), were selected to study the mechanism of receptor-binding specificity in this study. To investigate which factor influences the receptor-binding property, a series of reassortant and mutant viruses were constructed by using plasmid-based reverse genetics. Solid-phase ELISA assay showed that the receptor-binding specificity of AH/1 were not affected by the R57 K and R312 K of HA, while N2 NA increased the binding affinity against SA α-2,3Gal. Homologous modeling further showed that the 57 th and 312 th amino acids were not located in the receptor-binding domains. The hemagglutination inhibition(HI) antibody titers of the H7 HA single factor and H7N9 whole virus sera against JL/020 and AH/1 virus were both 4 and 7, respectively, while the HI titers of the N2 NA sera against the two viruses were different. All these results indicated that NA protein influences the receptor-binding properties. This study demonstrated that, in addition to HA, NA has also an important role on receptor-binding specificity of influenza A virus, which provides a theoretical basis for further study of receptor-binding properties.In December 2014, we isolated four H7N9 viruses from ducks in the live poultry markets in Zhejiang, Fujian, and Guangxi in China during our avian influenza routine suveillance. To understand the biological characteristics of the four duck H7N9 influenza virus, we analyzed the four duck H7N9 viruses genetically and antigenetically. Receptor binding specificity and the BALB/c mice pathogenicity of the four duck influenza viruses were also analyzed. Sequences analysis revealed that these duck viruses are complicated reassortants and clearly distinct from the 2013/H7N9 viruses. The four duck H7N9 viruses were divided into 3 genotypes according to the nucleotide similarity and phylogenetic tree of the eight gene segments,with DK/ZJ/S4488/2014 and DK/ZJ/S4489/2014 belonging to the same genotype. The four H7N9 duck viruses bound to both α-2,3-linked Sias and α-2,6-linked Sias, although their affinity for the α-2,3-linked Sias was higher than that for the α-2,6-linked Sias.They are antigenically different from the 2013/H7N9 viruses and replicate more efficiently inmice than the 2013/H7N9 avian influenza viruses. This study showed that duck H7N9 influenza viruses have a strong pathogenicity, and its threats to public health cannot be ignored.