Suboptimal Humoral Immune Response against Influenza A(H7N9) Virus Is Related to Its Internal Genes.

Influenza A(H7N9) virus pneumonia is associated with a high case fatality rate in humans. Multiple viral factors have been postulated to account for the high virulence of the virus. It has been reported that patients with influenza A(H7N9) virus infection have relatively low titers of neutralizing antibodies compared to those with seasonal influenza virus infections. In this study, we compared serum hemagglutination inhibition (HI) and microneutralization (MN) antibody titers of mice challenged with wild-type A(H7N9) viruses [H7N9(Anhui) and H7N9(Zhejiang)], an A(H1N1)pdm09 virus [pH1N1(2009)], and a recombinant A(H7N9) virus with PR8/H1N1 internal genes (rg-PR8-H7-N9). All mice infected by H7N9(Anhui) and H7N9(Zhejiang) developed serum HI antibodies at 14 days postinfection (dpi) but no detectable MN antibodies, even at 28 dpi. A low level of neutralizing activity was detected in H7N9(Anhui)- and H7N9(Zhejiang)-infected mice using fluorescent focus MN assay, but convalescent-phase serum samples obtained from H7N9(Anhui)-infected mice did not reduce the mortality of naive mice after homologous virus challenge. Reinfection with homologous A(H7N9) virus induced higher HI and MN titers than first infection. In contrast, pH1N1(2009) virus infection induced robust HI and MN antibody responses, even during the first infection. Moreover, rg-PR8-H7-N9 induced significantly higher HI and MN antibody titers than H7N9(Zhejiang). In conclusion, the internal genes of A(H7N9) virus can affect the humoral immune response against homologous viral surface proteins, which may also contribute to the virulence of A(H7N9) virus.