Impact of Extended Dosing Intervals and Ipsilateral Versus Contralateral Boosting on mRNA Vaccine Immunogenicity in Mice

BACKGROUND: Although mRNA vaccines have the potential to be developed and deployed rapidly to combat infectious diseases, the ideal method of administration and boosting schedule strategy for generating optimal immunogenicity is an area of active research. We compared the immune responses resulting from different schedules for prime-boost and boosting either ipsilaterally or contralaterally in relation to the initial vaccine dose. METHODS: Influenza hemagglutinin (HA) was used as a model antigen for different vaccination regimens in mice using both mRNA lipid nanoparticles (mRNA-LNP) and AF03-adjuvanted recombinant protein (rHA-AF03) vaccines. RESULTS: Increasing the prime-boost interval resulted in higher levels of serum anti-HA IgG and functional antibody hemagglutination inhibition (HAI) responses in mRNA-LNP-vaccinated animals, which correlated with an induction of germinal center (GC) B cells and follicular helper T (Tfh) cells in lymph nodes. In addition, longer prime-boost intervals resulted in higher levels of IL-2 and TNF-α producing CD4+ T cells two weeks after boosting. The number of Ig-secreting long-lived plasma cells increased with the length of prime-boost intervals. Contralateral boosting resulted in an increase in HAI titers and GC B cells compared to an ipsilateral boost. However, significantly higher numbers of GC B cells were induced in the draining lymph nodes following ipsilateral boosting than in the non-draining lymph nodes. CONCLUSIONS: Overall, our data provides insights into the immune mechanisms of action of mRNA-LNP to develop the optimal vaccine regimen for mRNA vaccine platforms.