Recombinant baculovirus production and characteristic analysis. (a) Structural schematics of SIV HA proteins, where the amino acid sequences of the TM domain are shown. H3-WT denotes H3 wild-type (WT) HA protein; H1-WT denotes H1 wild-type HA protein; and H1-TM denotes H1 wild-type HA protein with the replaced H3 TM. Amino acid sequences of H3-WT, H1-WT, and H1-TM were aligned and designated. (b) Western blots of H1-WT, H1-TM, and H3-WT protein expression. The molecular weight of HA was approximately 70 kD. (c) Hemagglutination assays of H1-WT, H1-TM, and H3-WT protein. PBS with 0.5% chicken erythrocytes was included as a negative control (NC). All experiments were performed 3 times. (d) Viral titers of H1-WT, H1-TM, and H3-WT recombinant viruses of different generations of viral stocks (**** p < 0.0001).