The envelope (E) gene of Japanese encephalitis virus (JEV) plays a major protective role against JEV
infection. In order to locate the part of E gene that is responsible for this protection, an N-terminal fragment EA (
nucleotide number 933-1877 bp of JEV genome) and a C-terminal fragment EB (
nucleotide number 1851-2330 bp of JEV genome) from E gene were prepared. Both of these fragments were used in the form of
recombinant proteins (rEA and rEB) and plasmid
DNA (pEA, pM15EA and pEB) for immunizations. Recombinant EA
protein (rEA) was previously found to be non-protective because it was expressed in an insoluble form. Plasmid EA (pEA) was also found to be non-protective unless it is preceded by a 15 mer
signal peptide derived from the very C-terminal of the membrane gene (M) of JEV to form pM15EA plasmid indicating the importance of the
signal peptide in the expression of EA immunogenicity. Although pM15EA and pEB are both immunogenic and protective against JEV lethal
infection, the protection by both fragments however is not optimal. Even when pM15EA and pEB were used together for immunization, maximum protection as those induced by control
vaccine was not achieved. However, if individual fragments (EA or EB) were used in
a DNA priming-
protein boosting or
protein priming-
DNA boosting strategy, high levels of protection were achieved by both fragments. This was especially true for EA fragment where the level of protection against JEV lethal
infection was equal to that induced by commercially available
vaccine alone. The protection correlated very well with the
neutralizing antibody titers and the T helper cell involved in this process in mainly the Th1 type.