Figure 1: The diagram of epitope-based DNA vaccine design. To construct pSV-Env , pSV-triEnv , and pSV-sextEnv , a Kozak sequence, an ER signal sequence, and a universal Th2 epitope (Pan DR epitope, PADRE) were introduced into the 5′ end of each epitope encoding gene and a 6 × His-tag was added at the 3′ end. A linker sequence was also inserted between adjacent epitopes. pSV-Env2 was constructed without adding ER signal sequence nor PADRE.