A recombinant envelope protein vaccine against West Nile virus.

West Nile (WN) virus is a flavivirus that first appeared in North America in 1999. Since then, more than 600 human deaths and 22,000 equine infections have been attributed to the virus in the United States. We expressed a truncated form of WN virus envelope (E) protein in Drosophila S2 cells. This soluble recombinant E protein was recognized by antibodies from naturally infected horses, indicating that it contains native epitopes. Mice and horses produced high-titer antibodies when immunized with recombinant E protein combined with aluminum hydroxide. Immunized mice were resistant to challenge with a lethal viral dose. Sera from immunized horses, administered to naive mice, conferred resistance against a lethal WN viral challenge. In addition, sera of immunized horses neutralized West Nile virus in vitro, as demonstrated by plaque reduction assays. This recombinant form of E protein, combined with aluminum hydroxide, is a candidate vaccine that may protect humans and horses against WN virus infections.