Induction of broader protection against influenza A virus by sequential-boosting of inactivated influenza vaccines in Balb/c mouse

Abstract

Background: The continuous threat of unpredictable influenza virus outbreaks in human necessitates the investigation of new types of vaccines that target a comprehensive range of influenza viruses. However, the current vaccine formulations should always be updated annually for the upcoming influenza season and this shortcoming is mainly because such vaccines only enable a very limited level of homologous and heterologous protection. Therefore, novel vaccines are needed that target the conserved viral proteins which provide a broader immunity against influenza. These vaccine methodologies can lead to the development of universal protection against influenza. Aim: To compare the magnitude of the homologous and heterologous protection developed by sequential-boosting influenza vaccine strategy. Method: BALB/c mice were immunized intramuscularly with 2 or 4 doses (3 weeks interval) of inactivated whole influenza A virus antigen combined with adjuvant (Group A: seasonal H1N1/Brisbane/07-pandemic H1N1/California/09-PBS-PBS, Group B: seasonal H1N1/Brisbane/07-pandemic H1N1/California/09-H5N1/Vietnam/04-H5N1/Vietnam/04, (-) Control: PBS-PBS- PBS-PBS). 3 weeks post-vaccination, a lethal dose of pandemic H1N1/PR/8 or H3N2/HK/68 (10LD50) was given intranasally, and weight loss (21 dpi), morbidity were monitored. The lung viral titers (in both Day 3 and Day 7 post-infection) were measured by TCID50 assay. Specific Ab measurement using different hemagglutinin (HA) proteins was conducted using an ELISA assay format. Results: Significantly different kinetics against both homologous and heterologous infection between the two groups was seen (P<0.05), which were demonstrated by less weight loss and morbidity in Group B (H1N1/PR/8 challenge: survival ratio is in 55.0% in A, 100% in B; H3N2/HK/68 challenge: survival ratio is 33.3% in A, 91.7% in B). Furthermore in Group B, the lung viral load during the challenge represented a faster clearance of influenza virus (for both H1N1/PR/8 and H3N2/HK/68 challenge, P<0.05). Meanwhile, reduced clinical morbidity in Group B was partially associated with higher HA antibody generation (for H1N1/PR/8, A Vs B, P<0.05). Conclusion: The presence of an enhanced homologous and heterologous protection after sequential-boosting influenza A vaccine in mouse model may assist in the development of a potential strategy of future influenza prophylactics.