Effect of the selection pressure of vaccine antibodies on evolution of H9N2 avian influenza virus in chickens.

H9N2 avian influenza virus has spread worldwide, and vaccination with an inactivated virus is currently the major prevention method in China. To further understand the effect of the selection pressure from antibodies on the evolution of H9N2 avian influenza virus, F/98 (A/Chicken/Shanghai/F/98), which is the vaccine representative of H9N2 avian influenza virus in East China, was used for serial passaging for 20 generations in chickens with and without vaccination. After plaque purification from trachea and lung tissues, 390 quasispecies were obtained. The second-generation quasispecies under the selection pressure of vaccine antibodies had undergone 100% antigen variation, while after passaging to the fifth generation, only 30-40% of the quasispecies displayed antigen variation when there was no selection pressure of vaccine antibodies, implying that the selection pressure of vaccine antibodies promotes the antigen variation of F/98. We found for the first time that there were three mutation hotspots in the HA genes of the quasispecies under the selection pressure of vaccine antibodies, which were K131R, A168T, and N201D. Moreover, under the selection pressure of vaccine antibodies, 10 amino acids (67-76) of the NA protein of all quasispecies were deleted, and PB2 of the quasispecies had undergone a high-frequency R355K mutation. However, without selection pressure of vaccine antibodies, NP had undergone two high-frequency mutations, namely, V186I and L466I, and a high-frequency mutation of L77I appeared in the NS gene. This result shows that the vaccine antibody selection pressure could control and regulate gene variation of the F/98 virus. Compared to that of the parental virus F/98, the EID of the twentieth passaged virus under the selection pressure of vaccine antibodies did not change, while the EID of the twentieth passaged virus without selection pressure of vaccine antibodies was significantly enhanced by 794 times. Furthermore, the twentieth passaged virus with selection pressure from vaccine antibodies lost its lethal ability in embryonated chicken eggs, whereas the EID of the twentieth passaged virus without selection pressure of vaccine antibodies increased to 6.3 times that of the F/98 strain. All the above results show that the selection pressure of vaccine antibodies promotes the antigen variation of H9N2 avian influenza virus and plays a role in regulating and controlling gene mutation of H9N2 avian influenza virus.