Immunogenicity and protective efficacy of prime-boost immunization in mice vaccinated with live and inactivated influenza A (H5N1) vaccines

Avian influenza A (H1N1) in humans is characterized by severe clinical manifestation and high mortality. The main drawback of current human H5N1 vaccines is related to low immunogenicity. Prime-boost vaccination is considered as an effective approach to enhance vaccine immunogenicity. The aim of this study was to compare immune response and protective efficacy of diverse prime-boost immunization protocols: 1) prime and boost with live influenza vaccine (LAIV) А/17/Turkey/Turkey/05/133 (H5N2); 2) prime with LAIV А/17/Turkey/Turkey/05/133 (H5N2) followed by boost with inactivated influenza vaccine (IIV) “Orniflu” (H5N1). Both vaccination protocols were found to increase serum antibody level against homologous and heterologous influenza A virus strains. In particular, serum HAI antibodies were significantly elevated solely after LAIV/LAIV vaccination. A more sensitive sandwich ELISA assay revealed that serum virus-specific IgG antibody levels were significantly increased after both vaccination protocols as well as after a single LAIV or IIV vaccination. Both LAIV and IIV boost increased titers of serum IgG specific against unrelated influenza A (H5N1) strains: homologous A/NIBRG-23 (clade 2.2), A/Indonesia (clade 2.1) and, to a lesser extent, against clade 1 virus A/ Vietnam and even against heterologous А/New York (H1N1). Single LAIV vaccination was also able to induce antibody responses against all strains examined, though to a lesser degree as compared with either prime-boost protocols. However, amount of splenic CD8+ Тcells specific to homologous influenza A virus strain was solely observed after LAIV/IIV vaccination. Moreover, both LAIV and IIV boosting effect demonstrated high protection level against lethal challenge with А (H1N1) WT virus and significantly decreased lung viral titer compared to control group. Furthermore, both regimens resulted in lung virus clearance after non-lethal challenge with clade 1, 2.1 or 2.2 influenza А (H5N1). In conclusion, we demonstrated that both LAIV/LAIV and LAIV/IIV regimens were able to induce cross-clade A (H5N1) response and that prime-boost immunization was a promising approach to improve immunogenicity of influenza A (H5N1) virus vaccine