Fig. 4: Adjuvanted M2e-H3 stalk vaccination induces heterologous cross-protection against group 2 influenza A viruses.

A M2e-specific serum IgG antibody levels in the different vaccine groups. B Serum IgG antibodies specific for M2e-H3 stalk antigen in the M2e-H3 stalk groups with (M2e-H3 stalk) and without adjuvant (M2e-H3 stalk/No adj). C–H The groups of mice (n = 5, 6–8 weeks old) vaccinated (prime or prime-boost) with M2e-H3 stalk protein (20 μg +/- adjuvant) or M2e-RBD (20 μg plus adjuvant) were intranasally challenged with group 2 influenza A viruses (H3N2, H7N9). Body weight changes and survival rates were daily monitored. C, D A/Phil/1982 H3N2 (3xLD₅₀, 2.3 × 10² EID₅₀), E A/Nanchang/1995 H3N2 (2xLD₅₀, 3 × 10⁶ EID₅₀), F A/Sha/2013 H7N9 (3xLD₅₀, 1.1 × 10⁴ EID₅₀), G A/HK/1968 H3N2 (3xLD₅₀, 4 × 10¹ EID₅₀). H Efficacy of thermostable M2e-H3 stalk protein. Mice vaccinated with M2e-H3 stalk protein pre-incubated at 50 °C for 11 days prior to prime-boost vaccination were challenged with A/Nanchang/1995 H3N2 (2xLD₅₀, 3 × 10⁶ EID₅₀). I Efficacy of single dose M2e-H3 stalk vaccination. Mice with M2e-H3 stalk prime dose only were challenged with A/Nanchang/1995 H3N2 (2xLD₅₀, 3 × 10⁶ EID₅₀) at 4 weeks after vaccination. Mock inf: mock group (adjuvant only) with virus infection, No Adj: M2e-H3 stalk vaccinated group without adjuvant. Statistical significance was determined using the two-way ANOVA followed by Bonferroni post-test. Error bars indicate means ± SEM; *P < 0.05; **P < 0.01; ***P < 0.001.