Vaccines in development against West Nile virus

Abstract

West Nile encephalitis emerged in 1999 in the United States, then rapidly spread through the North American continent causing severe disease in human and horses. Since then, outbreaks appeared in Europe, and in 2012, the United States experienced a new severe outbreak reporting a total of 5,387 cases of West Nile virus (WNV) disease in humans, including 243 deaths. So far, no human vaccine is available to control new WNV outbreaks and to avoid worldwide spreading. In this review, we discuss the state-of-the-art of West Nile vaccine development and the potential of a novel safe and effective approach based on recombinant live attenuated measles virus (MV) vaccine. MV vaccine is a live attenuated negative-stranded RNA virus proven as one of the safest, most stable and effective human vaccines. We previously described a vector derived from the Schwarz MV vaccine strain that stably expresses antigens from emerging arboviruses, such as dengue, West Nile or chikungunya viruses, and is strongly immunogenic in animal models, even in the presence of MV pre-existing immunity. A single administration of a recombinant MV vaccine expressing the secreted form of WNV envelope glycoprotein elicited protective immunity in mice and non-human primates as early as two weeks after immunization, indicating its potential as a human vaccine.

Figure 1

Schwarz/Moraten vaccine strain (MVSchw)-sE_WNV vector. The MV genes are indicated: N (nucleoprotein), PVC (phosphoprotein and V/C proteins), M (matrix), F (fusion), H (hemagglutinin), L (polymerase). T7 = T7 RNA polymerase promoter; hh = hammerhead ribozyme; T7t = T7 RNA polymerase terminator; ∂ = hepatitis delta virus (HDV) ribozyme; in red, the additional transcription units (ATU1) with the WNV sequence inserted. Total nucleotides in measles virus (MV) genome: 15,894.

Figure 2

West Nile virus (WNV) replication in Saimiri sciureus squirrel monkeys. Six animals were inoculated intravenously with WNV (IV1 to IV6), and serum was collected to assess infection. (A) The presence of viral RNA was determined by quantitative real-time polymerase chain reaction. (B) Infectious virus was determined by the plaque formation assay. (C) WNV antibody levels were measured by enzyme-linked immunosorbent assay. Blood samples were collected on days 0, 2 and 5 for animals IV1 and IV2; on days 0, 3, 6 and 11 for animals IV3 and IV4; and on days 0, 4, 9 and 12 for animals IV5 and IV6. Monkeys were euthanized on day 7 for IV1 and IV2; on day 14 for IV3 and IV4; and on day 15 for IV5 and IV6. The black line represents the mean value for each time point. The dashed line in panel C represents the baseline determined from naive animal sera. Abbreviations: OD, optical density; pfu, plaque-forming units.

Figure 3

West Nile virus (WNV) challenge of squirrel monkeys immunized with MVSchw-sE_WNV. Twelve monkeys immunized with MVSchw-sE_WNV and four monkeys mock immunized (with empty MV vaccine) or phosphate-buffered saline (PBS)) were challenged with WNV IS-98-ST1, either 15 or 30 days after immunization. (A) and (B): WNV-specific antibody levels were determined by enzyme-linked immunosorbent assay on the day of immunization, 15 days after immunization, the day of challenge, seven days after challenge and on the day of euthanasia. (C) The presence of WNV genomic RNA was determined by quantitative real-time polymerase chain reaction on day 2 after challenge in the serum of challenged animals. Animals 3, 4, 5 and 6 received one vaccination on day 30 before challenge; animals 9, 10, 11 and 12 received one vaccination on day 15 before challenge; and animals 1 and 2 (PBS) and 7 and 8 (MV) were mock infected. (D) The mean WNV titer in the serum of vaccinated animals, determined by the plaque formation assay at day 2 after challenge. Abbreviations: OD, optical density; pfu, plaque-forming units.