The full-length cDNA fragments corresponding to H2-H16 HA subtypes of IAV were inserted into the pCAGGS vector (purchased from Addgene) to express entire HA proteins (unpublished data). For H1 proteins, HA gene representing human IAV strains from different years (1918, 1934, 1977, 2007 and 2009 ) and a swine influenza virus strain were expressed by inserting the corresponding cDNA fragments into the pCAGGS vector in a similar manner. For the details of these influenza virus strains, please refer to Fig. 4 . Recombinant plasmids were transfected into 293T cells (ATCC Number CRL-11268) using Lipofectamine 2000 (Invitrogen, Carlsbad, CA) according to the manufacturer's instructions. The cells were harvested and lyzed 72 hours after transfection. The expression of HA proteins was verified by Western blot analysis using murine antibodies against corresponding HA1 proteins (unpublished data).