ELISA. The M2e epitopes, including the nanoparticle platforms with M2e epitopes (Tetra-M2e and Mono-M2e) and M2e linked to GCN4, (M2eN-GCN4), were used for coating of the ELISA plates. Briefly, individual wells of the flat-bottom 96-well Immulon 1B plates (NUNC/Thermo Fisher Scientific, Rochester, NY) were coated with 5 μg/mL of tetrameric M2e (M2eN-GCN4) or the nanoparticle of interest. Antigen adhesion was allowed to proceed at 4 • C overnight. Plates were rinsed with 2% Tween 20 in phosphate-buffered saline (PBS) (PBS/Tween 20 Ther-moFisher) and blocked with a 3% BSA in PBS solution.