Discussion
We found that direct genetic fusion of M2e on noro-VLP raised few M2e antibodies in the mouse model, presumably because the short linker positions the peptide between the protruding domains of noro-VLP, limiting its accessibility. On the other hand, adding aluminum hydroxide adjuvant to the previously described SpyCatcher-M2e-decorated noro-VLP vaccine gave a strong response against M2e. Surprisingly, simple SpyCatcher-fused M2e without VLP display also functioned as a potent immunogen, which suggests that the commonly used protein linker SpyCatcher-SpyTag may serve a second role as an activator of the immune system in vaccine preparations. Based on the measured anti-M2e antibodies and cellular responses, both SpyCatcher-M2e as well as M2e presented on the noro-VLP via SpyTag/Catcher show potential for the development of universal influenza vaccines.
Methods
To compare SpyCatcher-mediated coupling and direct peptide fusion in experimental vaccination, we genetically fused the ectodomain of influenza matrix-2 protein (M2e) directly on the C-terminus of norovirus VP1 capsid protein. VLPs decorated with SpyCatcher-M2e and VLPs with direct M2 efusion were used to immunize mice.