Coupling of the M2e peptide to the PapMV vaccine platforms and assessment of the humoral response directed to M2e in mice. (A) The M2e antigen was coupled to the PapMV-N and PapMV-C vaccine platforms using the sortase A (SrtA) enzyme. Coupled bands for both platforms can be visualized on SDS-PAGE slightly over the 25 kDa marker. The arrow pointing to the right indicates the signal of the PapMV-N coupled to the M2e peptide (lane 1), and the arrow pointing to the left indicates the signal of the PapMV-C coupled to the M2e peptide (lane 2). Signals corresponding to PapMV-N CP (lane 3) and PapMV-C CP (lane 5) are also observed. The signal corresponding to SrtA is shown in lane 4. Remaining SrtA after the coupling reaction is seen in lanes 1 and 2. Proteins from the SDS-PAGE were transferred to a membrane to perform Western blotting using anti-M2e (middle panel) or anti-PapMV (lower panel) antibodies. (B) Balb/C mice, five per group, were immunized once, intramuscularly (i.m.), with formulation buffer (Buffer), PapMV-C (10 µg) with 1 µg of free peptide M2e, 10 µg PapMV-C/M2e 19%, PapMV-N (10 µg) with 1 µg of free peptide M2e, and 10 µg PapMV-C/M2e 14%. ELISA was performed with serum harvested at day 20 to assess immunoglobulin G (IgG) 2a titers directed to the M2e peptide. **** p > 0.0001.