A poorly neutralizing IgG2a/c response elicited by a DNA vaccine protects mice against Japanese encephalitis virus.

We demonstrated previously that immunization with a DNA vaccine expressing the Japanese encephalitis virus (JEV) envelope (E) protein conferred a high level of protection through a poorly neutralizing antibody response. Here, we further investigated the role of the IgG subclass in this antibody-dependent protection using cytokine co-immunization and cytokine-deficient mice. A significant difference in IgG2a/c but not IgG1 was observed between mice that survived or died following a lethal challenge. Correspondingly, the IgG2a/c response and protection increased in IL-4-deficient mice but decreased in IFN-γ-deficient mice, highlighting the importance of IgG2a/c. In addition, the restoration of protection and E-specific IgG2a/c production in IFN-γ-deficient mice by a T helper (Th) type 1-biased intramuscular immunization suggested that IgG2a/c but not IFN-γ was the major component for protection. The failure of protection against a direct intracranial challenge indicated that IgG2a/c-mediated protection was restricted to outside the central nervous system. Consistent with this conclusion, passive transfer of E-specific antisera conferred protection only pre-exposure to JEV. Therefore, our data provided evidence that the IgG subclass plays an important role in protection against JEV, particular in poorly neutralizing E-specific antibodies, and Th1-biased IgG2a/c confers better protection than Th2-biased IgG1 against JEV.