M2e5x VLP-supplemented vaccination modulates virus-specific IgG antibody isotypes. BALB/c mice were immunized with split vaccine alone (Split, N = 18), split vaccine supplemented with M2e5x VLPs (Split+M2e5x, N = 18), or split vaccine supplemented with M1 VLPs (split+M1, N = 8). Sera were collected 3 weeks after boost (B) vaccination. The isotypes of IgG, IgG1, and IgG2a, were detected using M2e peptide or influenza A viruses, A/California/04/09 (A/Cali, H1N1) as an ELISA coating antigen for antibody detection. (a) IgG isotype antibody responses to M2e peptide in M2e5x VLP supplemented split vaccine group. (b) IgG isotype antibody responses to virus in split vaccine group. (c) IgG isotype antibody responses to virus in M2e5x VLP-supplemented split vaccine group. (d) IgG isotype antibody responses to virus in M1 VLP-supplemented split vaccine group. (e) Ratios of IgG2a/IgG1 isotype antibodies. Error bars indicate mean ± SEM. Asterisk (P < 0.05) indicates significant difference between IgG1 and IgG2a isotypes. ELISA, enzyme-linked immunosorbent assay; M2e, M2 ectodomain; VLP, virus-like particles.