Thus, we evaluated whether unadjuvanted single immunisations with low doses of our VLP-vaccine containing baculovirus were effective in eliciting protective immune responses in an in vivo mouse experiment using a stringent 100 mLD50 Regorafenib challenge dose. We assessed protection conferred by three different concentrations of SH1-VLPs (3 μg, 0.3 μg and 0.03 μg in terms of HA content, administered intramuscularly). We also compared groups that received
a single vaccine dose with a group that received two immunisations on days 0 and 14 (0.3 μg in terms of HA content). To explore whether a prime-only strategy could protect against a heterologous strain as well, we included a VLP formulation that contained HA of AH1, a divergent H7N9 isolate. [4]. Mice that received two immunisations with 0.3 μg SH1, expectedly showed a 100% survival rate and little weight loss ( Fig. TSA HDAC 1A and C). Similarly,
no weight loss was observed for the SH1-3 μg prime-only group. Mice in the prime-only vaccination groups that received lower vaccine doses (0.3 μg and 0.03 μg) showed more weight loss (7% and 10%, respectively) than mice in the high dose or prime-boost groups (both 3%), but the mice were completely protected from mortality and regained weight after day 5 post challenge ( Fig. 1A and C). Mice vaccinated with AH1-VLPs lost slightly more weight than mice that received the same dose of SH1-VLPs (0.3 μg of HA) but were fully protected
from mortality ( Fig. 1A and C). Animals that received an M1-only preparation containing similar amounts of baculovirus as the SH1- and AH1-VLP preparation showed no enhanced protection as compared to naïve mice ( Fig. 1A and C). This proves that neither M1 or the baculovirus or a combination of both was able to induce significant protective immune responses in our challenge model. Since previous studies highlight the isothipendyl critical role of CD8+ T-cells in protective immunity to influenza infection [26] and [27], we assessed whether a single low vaccine dose could also induce full protection in CD8+ T-cell-depleted mice. Minimal weight loss for CD8+-depleted, SH1-0.3 μg-vaccinated mice after challenge and a 100% survival rate ( Fig. 1B and D) suggested that the humoral response was sufficient to robustly protect these animals. As previous studies reported a remarkable cross-reactivity of H7 antibodies [13] and [28], we tested sero-reactivity to a panel of divergent recombinant H7 proteins and a representative HA from each influenza subtype (H3, H4, H10, H14 and H15 – in addition to H7) that cluster into phylogenetic group 2 (Fig. 2A). An H1 HA (group 1) was added as a negative control antigen. Strong sero-reactivity was detected against the HA of the vaccine strains SH1 and AH1.