Non-lethal viral challenge of influenza haemagglutinin and nucleoprotein DNA vaccinated mice results in reduced viral replication

Influenza DNA vaccines have been widely studied in experimental animal models and protection documented after lethal viral challenge. In this study, we have investigated the humoral response after a non-lethal viral challenge of mice vaccinated with plasmids encoding the influenza haemagglutinin (HA) or nucleoprotein (NP) genes. BALB/c mice were immunized intramuscularly with three doses (100 mug) of HA, NP or backbone plasmid at 3-week intervals, or alternatively infected intranasally, before being challenged with homologous virus 13 weeks later. Mice were then sacrificed at weekly intervals and the antibody-secreting cell response was examined systemically (spleen and bone marrow) and in the respiratory tract (nasal associated lymphoid tissue (NALT) and lungs). Sera were collected after each dose of vaccine and at sacrifice and analyzed by ELISA, haemagglutination inhibition and virus neutralization assays. We found that previous viral infection apparently elicits sterilizing immunity. Vaccination with HA or NP DNA significantly reduced viral replication in the nasal cavity after viral challenge, however, increases in serum antibody titres were observed after challenge. Prior to challenge, specific antibody-secreting cells were observed in the systemic compartment after HA or NP DNA vaccination but were also found in the NALT after viral challenge. In conclusion, intramuscular DNA vaccination resulted in immunological memory in the systemic compartment, which was rapidly reactivated upon viral challenge.