Plasmids including NP158HHDpBudCE4.1, NP-189HHDpBudCE4.1, NP373HHDpBudCE4.1, and NP458HHDpBudCE4.1 were prepared as follows. In brief, the single-chain trimer gene (HHD) was constructed by connecting the leader sequence, the human b 2 -microglobulin, the human HLA-A*0201 a-1, a-2 domains, and the mouse H-2D b a-3 domain.