Protection by humoral elements independent of virus neutralization activity against an influenza virus challenge.

To investigate the protective efficacy of a seasonal trivalent inactivated whole virion influenza vaccine (TIV) adjuvanted with aluminum phosphate (Fluval AB, referred to here as TIV+Al), we immunized mice with the TIV+Al, and as controls, with TIV, TIV+Al combined with Freund adjuvant (TIV+Al+F), inactivated A/PR/8/34(H1N1) (PR8) strain or PBS, and challenged them with a lethal dose of a mouse-adapted PR8 virus. Serum pools from immunized mice were passively transferred to recipient mice that were then challenged similarly. All actively immunized mice survived the challenge. Of recipient mice receiving serum from mice actively immunized with TIV, TIV+Al or TIV+Al+F, 20%, 80%, and 100% survived, respectively. Rates of mortality and morbidity of recipient mice were inversely proportional to the hemagglutination inhibition (HI) antibody level to the vaccine virus in the absence of detectable PR8-specific HI, neuraminidase inhibition (NI) and virus neutralization (VN) antibodies. No cross-reactivity was observed between vaccine and PR8 strains in in vitro HI, NI or VN assays. In splenocytes from TIV+Al-immunized mice production of IFN-γ or granzyme-B protein and mRNA expression increased (p<0.05). Thus, antibodies play a major role in the protection against a mismatched challenge infection independent of HI, NI and VN activity, but cellular immune responses may contribute to full protection in actively immunized mice.