West Nile virus/dengue type 4 virus chimeras that are reduced in neurovirulence and peripheral virulence without loss of immunogenicity or protective efficacy.

A candidate live attenuated vaccine strain was constructed for West Nile virus (WN), a neurotropic flavivirus that has recently emerged in the U.S. Considerable attenuation for mice was achieved by chimerization with dengue virus type 4 (DEN4). The genes for the structural premembrane and envelope proteins of DEN4 present in an infectious cDNA clone were replaced by the corresponding genes of WN strain NY99. Two of 18 cDNA clones of a WN/DEN4 chimera yielded full-length RNA transcripts that were infectious when transfected into susceptible cells. The two infectious clones shared a motif in the transmembrane signal domain located immediately downstream of the NS2B-NS3 protease cleavage site that separates the DEN4 capsid protein and the WN premembrane protein of the chimera. This motif, Asp and Thr at a position 3 and 6 amino acids downstream of the cleavage site, respectively, was not present in the 16 noninfectious cDNA clones. The WN/DEN4 chimera was highly attenuated in mice compared with its WN parent; the chimera was at least 28,500 times less neurovirulent in suckling mice inoculated intracerebrally and at least 10,000 times less virulent in adult mice inoculated intraperitoneally. Nonetheless, the WN/DEN4 chimera and a deletion mutant derived from it were immunogenic and provided complete protection against lethal WN challenge. These observations provide the basis for pursuing the development of a live attenuated WN vaccine.