Yellow fever vector live-virus vaccines: West Nile virus vaccine development

Abstract

By combining molecular-biological techniques with our increased understanding of the effect of gene sequence modification on viral function, yellow fever 17D, a positive-strand RNA virus vaccine, has been manipulated to induce a protective immune response against viruses of the same family (e.g. Japanese encephalitis and dengue viruses). Triggered by the emergence of West Nile virus infections in the New World afflicting humans, horses and birds, the success of this recombinant technology has prompted the rapid development of a live-virus attenuated candidate vaccine against West Nile virus.

Fig. 1

Two-plasmid system encoding the YF–WN chimeric vaccine. Plasmid YF5′3′IV WN preMembrane and envelope protein genes (prME) encodes the 5′ UTR, yellow fever capsid (YFC), West Nile virus prM (gray) and 5′ end of E (blue), and the 3′ end of yellow fever NS5 and UTR. Plasmid YFM5.2 WN encodes the second half of E (blue) and the non-structural genes of yellow fever NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5. West Nile prME gene fragments were amplified by RT-PCR and subcloned into the two-plasmid system by overlap-extension PCR. Silent Eag I and Bsp EI sites were introduced for in vitro ligation steps necessary to create a full-length cDNA before in vitro transcription. Naked RNA initiates productive infections after transfection of a Vero cell line.

Fig. 2

Sequence alignment of the E protein of WN NY-99 strain and the JE SA14-14-2 vaccine strain. Identical and conserved residues are shown in blue and green, respectively. A 77% identity was predicted by CLUSTAL W alignment. Red arrows map the location of ten amino acid residues that distinguish virulent JE Nakayama strain E protein (seeTable 1).