Antibodies elicited by Newcastle disease virus-vectored H7N9 avian influenza vaccine are functional in activating the complement system

H7N9 subtype avian influenza virus poses a great challenge for poultry industry. Newcastle disease virus (NDV)vectored H7N9 avian influenza vaccines (NDVvecH7N9) are effective in disease control because they are protective and allow mass administration. Of note, these vaccines elicit undetectable H7N9-specific hemagglutinationinhibition (HI) but high IgG antibodies in chickens. However, the molecular basis and protective mechanism underlying this particular antibody immunity remain unclear. Herein, immunization with an NDVvecH7N9 induced low anti-H7N9 HI and virus neutralization titers but high levels of hemagglutinin (HA)-binding IgG antibodies in chickens. Three residues (S150, G151 and S152) in HA of H7N9 virus were identified as the dominant epitopes recognized by the NDVvecH7N9 immune serum. Passively transferred NDVvecH7N9 immune serum conferred complete protection against H7N9 virus infection in chickens. The NDVvecH7N9 immune serum can mediate a potent lysis of HA-expressing and H7N9 virus-infected cells and significantly suppress H7N9 virus infectivity. These activities of the serum were significantly impaired after heat-inactivation or treatment with complement inhibitor, suggesting the engagement of the complement system. Moreover, mutations in the 150-SGS-152 sites in HA resulted in significant reductions in cell lysis and virus neutralization mediated by the NDVvecH7N9 immune serum, indicating the requirement of antibody-antigen binding for complement activity. Therefore, antibodies induced by the NDVvecH7N9 can activate antibody-dependent complement-mediated lysis of H7N9 virus-infected cells and complement-mediated neutralization of H7N9 virus. Our findings unveiled a novel role of the complement in protection conferred by the NDVvecH7N9, highlighting a potential benefit of engaging the complement system in H7N9 vaccine design.