Cross- immunity of a H9N2 live attenuated influenza vaccine against H5N2 highly pathogenic avian influenza virus in chickens.

The most commonly utilized inactivated influenza vaccines (IIVs) are usually deficient in cross immunity against divergent viruses. On the other hand, live attenuated influenza vaccines (LAIVs) are proved to be more effective in cross-protective immunity. We previously developed a H9N2 LAIV and verified its effective protection against a broad spectrum of H9N2 strains. In the present study, we evaluated its cross-immunity against H5N2 virus, a representative subtype of currently predominant H5 highly pathogenic avian influenza viruses. All chickens vaccinated with this LAIV survived from challenge of H5N2 virus in a lethal dose, and viral proliferation was effectively inhibited, as well as pathological lesions. Vaccination of this LAIV significantly activated H5N2-reactive CD4+ and CD8+ T cells in lungs. These LAIV-activated cross-reactive T cells expanded robustly following H5N2 exposure, and the increasing tendency was temporally correlated with viral clearance. Besides cellular immunity, factors of humoral immunity also play a contributing role in cross-immunity. Passively transferring H9N2 LAIV anti-serum resulted in 100% survival rate to chickens against H5N2 virus. Within components of the anti-serum, cross-binding IgGs against nucleoprotein (NP) of H5N2 virus were found of a contributing role in the cross immunity. These results indicate that this H9N2 LAIV represents a promising strategy for controlling highly pathogenic H5N2 virus in chickens. The cross immunity was partly attributed to LAIV activated H5N2-cross-reactive T cells and partly attributed to cross-binding IgGs against NP.