Antibody and T-cell responses to a virosomal adjuvanted H9N2 avian influenza vaccine: impact of distinct additional adjuvants.

A highly efficacious vaccine is required to counteract a threat of an avian influenza pandemic. Increasing the potency of vaccines by adjuvation is essential not only to overcome generally low immunogenicity of pandemic strains, but also to allow dose sparing and as such to make it feasible to satisfy huge global production demands. In this study we evaluated the ability of four distinct adjuvants to further increase immune responses to a virosomal adjuvanted avian H9N2 influenza vaccine in mice. Currently registered adjuvants aluminium phosphate, aluminium hydroxide and MF59, as well as a novel promising adjuvant MATRIX-M were included in the study. Our results demonstrate that all adjuvants significantly increased the H9N2 haemagglutinin (HA) inhibition and ELISA antibody titers induced with the virosomal adjuvanted vaccine. The adjuvants exhibited different effect on the isotype of virus specific antibodies, with MATRIX-M inducing the most pronounced skewing to IgG2a, i.e. towards Th1 type of response. While the virosomal adjuvanted pandemic influenza vaccine efficiently induced CD4(+) T-cell response, with no further increase upon adjuvation, the CD8(+) T-cell responses induced with virosomal adjuvanted vaccine could be significantly improved upon additional adjuvation with MATRIX-M or MF59. All adjuvants demonstrated a dose sparing effect, i.e. in combination with the virosomal adjuvanted pandemic influenza vaccine they increased immune responses to comparable level independent of the tested vaccine dose. In conclusion, our results demonstrate that immune responses to a virosomal adjuvanted pandemic influenza vaccine can be further enhanced by add-on adjuvants, with MATRIX-M being overall the most potent adjuvant in combination with virosomes, followed by MF59 and finally aluminium-based adjuvants.