Evaluation of tick-borne encephalitis DNA vaccines in monkeys

Tick-borne encephalitis is usually caused by infection with one of two flaviviruses: Russian spring summer encephalitis virus (RSSEV) or Central European encephalitis virus (CEEV). We previously demonstrated that gene gun inoculation of mice with naked DNA vaccines expressing the prM and E genes of these viruses resulted in long-lived homologous and heterologous protective immunity (Schmaljohn et al., 1997). To further evaluate these vaccines, we inoculated rhesus macaques by gene gun with the RSSEV or CEEV vaccines or with both DNA vaccines and compared resulting antibody titers with those obtained by vaccination with a commercial, formalin-inactivated vaccine administered at the human dose. Vaccinations were given at days 0, 30, and 70. All of the vaccines elicited antibodies detected by ELISA and by plaque-reduction neutralization tests. The neutralizing antibody responses persisted for at least 15 weeks after the final vaccination. Because monkeys are not uniformly susceptible to tick-borne encephalitis, the protective properties of the vaccines were assessed by passive transfer of monkey sera to mice and subsequent challenge of the mice with RSSEV or CEEV. One hour after transfer, mice that received 50 microl of sera from monkeys vaccinated with both DNA vaccines had circulating neutralizing antibody levels <20-80. All of these mice were protected from challenge with RSSEV or CEEV. Mice that received 10 microl of sera from monkeys vaccinated with the individual DNA vaccines, both DNA vaccines, or a commercial vaccine were partially to completely protected from RSSEV or CEEV challenge. These data suggest that DNA vaccines may offer protective immunity to primates similar to that obtained with a commercial inactivated-virus vaccine.