Screening And Immunogenicity Of Swine Influenza Virus Vaccine Strains Of H3N2 Subtype In China From 2013 To 2018

Swine influenza virus(SIV)is a member of type A influenza virus of Orthomyxoviridae,which can cause acute,febrile and highly transmitted respiratory diseases in pigs.Although the mortality rate of swine influenza virus is lower,it plays an important role in the genetic evolution and ecological distribution of influenza virus.On the one hand,pigs,act as the common susceptible host of human influenza virus and avian influenza virus,so that different sources of influenza viruses have rearranged in pigs;on the other hand,swine influenza viruses can also bind to the surface receptors of human or poultry cells,causing infection to humans or birds,which plays an important role in the adaptive evolution of influenza viruses.Therefore,the development of a vaccine to control swine influenza is not only the need of the breeding industry,but also has far-reaching public health significance.Nine H3N2 subtype SIVs were isolated from asymptomatic pigs in China from 2013 to2018.Based on the genetic evolution analysis of their surface genes,it was found that the HA and NA of the 9 viruses had high homology,and they all belonged to recent human lineage swine influenza virus.The antigenicity difference analysis of 9 viruses showed that all viruses could cross-react,but there were some differences in antigenicity among different H3 subtypes of viruses,among which A/swine/Guangxi/1659/2017(H3N2)had broad-spectrum antigenicity.According to the results of genetic evolution analysis and antigenicity analysis,using reverse genetic technique,the surface gene of A/swine/Guangxi/1659/2017(H3N2)was selected as the donor of the surface gene of H3N2 subtype swine influenza vaccine strain,and the internal gene of PR8 virus was used as the skeleton to successfully rescue the H3subtype recombinant influenza virus GX1659/PR8.The surface gene of GX1659/PR8 was sequenced,and the results showed that its gene sequence was identical to that of its donor strain GX1659.Chicken embryos were infected with 100EID₅₀dose of GX1659 and GX1659/PR8.The hemagglutination titer of chicken embryo allantoic fluid was determined at different times and the growth curve was drawn by EID₅₀.The results showed that the hemagglutination titer of GX1659 and GX1659/PR8 reached the peak at 60 h and 48 h after inoculation,respectively,and the titer of GX1659/PR8 was significantly higher than that of the donor strain GX1659.Compared with GX1659,the replication ability of recombinant virus GX1659/PR8 in chicken embryos has been greatly improved.Considering the superiority of the reassortant virus GX1659/PR8,this study further used GX1659/PR8 as a candidate strain to prepare inactivated vaccine and evaluate its immunogenicity and protective efficacy in mice and pigs.The results showed that GX1659/PR8 vaccine strain could induce animals to produce high titers of HI antibody,NT antibody and specific IgG antibody.After the mice were immunized,the virus content in the organs of mice was titrated by chicken embryo titration.The results showed that the vaccine could significantly inhibit the replication of homologous viruses in animals.The vaccine could not provide adequate protection when challenged with the obviously different antigenicity strain isolated in earlier years.In this study,active immunity and duration of immunity of pigs were also carried out.The results of hemagglutination inhibition showed that the vaccine could provide lasting protection for pigs for more than 180 days.