The anamnestic neutralizing antibody response is critical for protection of mice from challenge following vaccination with a plasmid encoding the Japanese encephalitis virus premembrane and envelope genes

For Japanese encephalitis (JE), we previously reported that recombinant vac cine-induced protection from disease does not prevent challenge virus repli cation in mice. Moreover, DNA vaccines for JE: can provide protection from high challenge doses in the absence of detectable prechallenge neutralizing antibodies. In the present study, we evaluated the role of postchallenge i mmune responses in determining the outcome of JE virus infection, using mic e immunized with a plasmid, pcDNA3JEME, encoding the JE virus premembrane ( prM) and envelope (E) coding regions. In the first experiment, 10 mice were vaccinated once (five animals) or twice (remainder) with 100 mu g of pcDNA 3JEME. All of these mice showed low (6 of 10) or undetectable (4 of 10) lev els of neutralizing antibodies. Interestingly, eight of these animals showe d a rapid rise in neutralizing antibody following challenge with 10,000 50% lethal doses of JE virus and survived for 21 days, whereas only one of the two remaining animals survived. No unimmunized animals exhibited a rise of neutralizing antibody or survived challenge. Levels of JE virus-specific i mmunoglobulin M class antibodies were elevated following challenge in half of the unimmunized mice and in the single pcDNA3JEME-immunized mouse that d ied. In the second experiment, JE virus-specific primary cytotoxic T-lympho cyte (CTL) activity was detected in BALB/c mice immunized once with 100 mu g of pcDNA3JEME 4 days after challenge, indicating a strong postchallenge r ecall of CTLs. In the third experiment, evaluation of induction of CTLs and antibody activity by plasmids containing portions of the prM/E cassette de monstrated that induction of CTL responses alone were not sufficient to pre vent death. Finally, we showed that antibody obtained from pcDNA3JEME-immun ized mice 4 days following challenge could partially protect recipient mice from lethal challenge. Taken together, these results indicate that neutral izing antibody produced following challenge provides the critical protectiv e component in pcDNA3JEME-vaccinated mice.