Immunogenicity of West Nile virus infectious DNA and its noninfectious derivatives.

The exceptionally high virulence of the West Nile NY99 strain makes its suitability in the development of a live WN vaccine uncertain. The aim of this study is to investigate the immunogenicity of noninfectious virus derivatives carrying pseudolethal mutations, which preclude virion formation without affecting preceding steps of the viral infectious cycle. When administered using DNA immunization, such constructs initiate an infectious cycle but cannot lead to a viremia. While the magnitude of the immune response to a noninfectious replication-competent construct was lower than that of virus or infectious DNA, its overall quality and the protective effect were similar. In contrast, a nonreplicating construct of similar length induced only a marginally detectable immune response in the dose range used. Thus, replication-competent noninfectious constructs derived from infectious DNA may offer an advantageous combination of the safety of noninfectious formulations with the quality of the immune response characteristic of infectious vaccines.

The suitability of yellow fever and Japanese encephalitis vaccines for immunization against West Nile virus.

Seven volunteers involved in flavivirus studies have been immunized with commercial Japanese encephalitis and yellow fever vaccines JE-VAX and YF-VAX. Strong homologous and cross-reactive with West Nile virus (WNV) antibody responses with titers 1:1600 to 1:51200 were found in all donors. All donors developed high levels of yellow fever virus (YFV) and Japanese encephalitis virus (JEV) neutralizing antibodies with titers 1:50 to 1:1600 and 1:20 to 1:640, respectively, and WNV neutralizing antibodies with titers 1:10 to 1:80. In contrast, predominantly YF-specific cell-mediated immunity was detected in all immunized donors. Responses to YFV were long lasting, but the anti-JEV humoral immunity was found to decrease with time. Cross-reactive anti-WNV responses were following the same trend dropping below detectable level at 4 years post-immunization and sharply coming back after booster immunization with the JE vaccine. Thus, immunization with the commercial flavivirus JE vaccine may be beneficial for individuals at high risk of exposure to WNV, such as personnel involved in WN research.

An attenuated West Nile prototype virus is highly immunogenic and protects against the deadly NY99 strain: a candidate for live WN vaccine development.

In a short time, West Nile virus has developed into a nationwide health and veterinary problem. The high virulence of the circulating virus and related lineage 1 WN strains hinders development of an attenuated live vaccine. We describe an attenuated WN isolate, WN1415, which is a molecularly cloned descendant of the WN prototype B956 strain. The parent virus belongs to lineage 2, members of which have not been associated with epidemic or epizootic outbreaks. A set of non-conservative mutations, mostly in non-structural protein genes, distinguishes the WN1415 isolate from the parent B956 prototype strain. Immunization with WN1415 (55-550,000 pfu) established a potent immunity, which protected the majority of mice against lethal challenge with WN NY99. The attenuated nature of the isolate and its excellent growth characteristics combined with the availability of a highly stable infectious clone make the isolate an attractive candidate for live WN vaccine development.