Author contributions: T.P.M., J.L., N.K.-T., J.A., F.G., S.G., and P.B. designed research; J.L., G.A.M., R.N., A.D., K.M., C.J., J.A., J.S.K., and S.G. performed research; J.L. contributed new reagents/analytic tools; T.P.M., J.L., N.K.-T., R.N., T.E., S.S., F.G., J.S.K., F.A.E., S.G., and P.B. analyzed data; and T.P.M., J.L., and S.G. wrote the paper.

Received 2006 Jan 11

Freely available online through the PNAS open access option.

Abstract

West Nile (WN) virus is an important cause of febrile exanthem and encephalitis. Since it invaded the U.S. in 1999, >19,000 human cases have been reported. The threat of continued epidemics has spurred efforts to develop vaccines. ChimeriVax-WN02 is a live, attenuated recombinant vaccine constructed from an infectious clone of yellow fever (YF) 17D virus in which the premembrane and envelope genes of 17D have been replaced by the corresponding genes of WN virus. Preclinical tests in monkeys defined sites of vaccine virus replication in vivo. ChimeriVax-WN02 and YF 17D had similar biodistribution but different multiplication kinetics. Prominent sites of replication were skin and lymphoid tissues, generally sparing vital organs. Viruses were cleared from blood by day 7 and from tissues around day 14. In a clinical study, healthy adults were inoculated with 5.0 log₁₀ plaque-forming units (PFU) (n = 30) or 3.0 log₁₀ PFU (n = 15) of ChimeriVax-WN02, commercial YF vaccine (YF-VAX, n = 5), or placebo (n = 30). The incidence of adverse events in subjects receiving the vaccine was similar to that in the placebo group. Transient viremia was detected in 42 of 45 (93%) of ChimeriVax-WN02 subjects, and four of five (80%) of YF-VAX subjects. All subjects developed neutralizing antibodies to WN or YF, respectively, and the majority developed specific T cell responses. ChimeriVax-WN02 rapidly elicits strong immune responses after a single dose, and is a promising candidate warranting further evaluation for prevention of WN disease.

Keywords: clinical trial, non-human primate, yellow fever

Abstract

West Nile (WN) virus (family Flaviviridae) first appeared in the Western Hemisphere in 1999, causing an outbreak in New York City. In successive years, WN expanded its geographic range. Over 19,000 human cases have been reported in the United States (1). The spectrum of disease extends from a mild febrile exanthem to fatal encephalitis. Mosquito control has failed to stop progression of the disease. However, the introduction of vaccines for horses (2, 3) has controlled the veterinary disease. In late 1999, we initiated development of a vaccine for humans.

ChimeriVax-WN02 is a live, attenuated chimeric vaccine derived from an infectious clone of yellow fever (YF) 17D virus in which the premembrane and envelope proteins of YF 17D virus have been replaced by the corresponding genes of WN (4). The vector, YF 17D vaccine, is approved by regulatory authorities worldwide, and has a 70-year history of use in >400 million persons (5). Three mutations were introduced into the WN E gene at sites predicted to reduce neurovirulence (4, 6), resulting in a highly attenuated phenotype. The vaccine candidate was incapable of being transmitted by mosquitoes (7). ChimeriVax-WN02 vaccine protected hamsters against challenge with WT WN virus (8). In rhesus macaques, the vaccine caused a transient viremia, induced neutralizing (N) Ab, and protected against intracerebral challenge with WT WN virus (4).

Here, we report tests in monkeys that elucidate sites of virus replication in vivo, and the results of a clinical trial of a vaccine for prevention of human infection with WN virus.

CV-WN, ChimeriVax-WN02; YF, yellow fever 17D; Pos, number monkeys positive by PCR; Eq/ml, plaque-forming equivalents per ml; inoc, inoculation site; contra, contralateral arm; mandib, mandibular; mesen, mesenteric; skel, skeletal.

Acknowledgments

We thank Danell Mathis, Nathan Arthur, and Peter Joo (all from PRA International, Lenexa, KS); Danny Vellom, John Hamberger, Robert Schrader, Chris Murphy, David Hymes, Seth Shapiro, and Judy Newberne (all from Acambis, Cambridge, MA); Robert Beck, Thad Pullano, and Linna Wei (all from Acambis, Canton, MA); and John Cruz, Alan Rothman, and George Reed (all from University of Massachusetts Medical School, Worcester) for their assistance. This work was supported by Acambis and National Institutes of Health Grants K08 AI01729 (to S.G.), N01 AI25490, and U19 AI057319.

Acknowledgments

Abbreviations

WN	West Nile
YF	yellow fever
N	neutralizing
PFU	plaque-forming unit
AE	adverse event
GMT	geometric mean titer
PBMC	peripheral blood mononuclear cell.

Abbreviations

Footnotes

Conflict of interest statement: Acambis is developing ChimeriVax-WN vaccine with the intent to sell and distribute in the United States and other countries.

Data deposition: The sequence reported in this paper has been deposited in the GenBank database (accession no. {"type":"entrez-nucleotide","attrs":{"text":"AF196835","term_id":"11597239"}}AF196835).

Footnotes

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