A West Nile virus DNA vaccine induces neutralizing antibody in healthy adults during a phase 1 clinical trial

Abstract

Background: West Nile virus (WNV) is a mosquito-borne flavivirus that can cause severe meningitis and encephalitis in infected individuals. We report the safety and immunogenicity of a WNV DNA vaccine in its first phase 1 human study.

Methods: A single-plasmid DNA vaccine encoding the premembrane and the envelope glycoproteins of the NY99 strain of WNV was evaluated in an open-label study in 15 healthy adults. Twelve subjects completed the 3-dose vaccination schedule, and all subjects completed 32 weeks of evaluation for safety and immunogenicity. The development of a vaccine-induced immune response was assessed by enzyme-linked immunosorbant assay, neutralization assays, intracelluar cytokine staining, and enzyme-linked immunospot assay.

Results: The vaccine was safe and well tolerated, with no significant adverse events. Vaccine-induced T cell and antibody responses were detected in the majority of subjects. Neutralizing antibody to WNV was detected in all subjects who completed the 3-dose vaccination schedule, at levels shown to be protective in studies of horses, an incidental natural host for WNV.

Conclusions: Further assessment of this DNA platform for human immunization against WNV is warranted.

Trial registration: ClinicalTrials.gov identifier: NCT00106769 .

Figure 1

Serum samples from vaccinees at week 8 (before the third vaccination), week 12 (4 weeks after the third vaccination), and week 32, assessed for the presence of neutralizing antibody by a West Nile virus reporter-virus particle (RVP) neutralization assay. The X-axis shows individual vaccine clinical trial subjects, and the Y-axis shows the log₁₀ reciprocal EC₅₀ neutralizing antibody titer. Subject B was not assessed at the week 12 time point because of visit noncompliance. Subjects G and H received 2 doses of vaccine. Subject C received 1 dose of vaccine. ND, not done.

Figure 2

Serum samples from vaccinees at week 12 (4 weeks after the third vaccination) and serum samples from horses 3 weeks after receipt of a 1-mg dose of pCBWN West Nile virus (WNV) DNA vaccine, assessed by WNV reporter-virus particles (RVP) neutralization assay and by plaque reduction neutralization (PRNT). The X-axis shows individual vaccine clinical trial subjects or horse samples, and the Y-axis shows the log₁₀ reciprocal antibody titer. Subject B was not assessed at the week 12 time point because of visit noncompliance. Subjects G and H received 2 doses of vaccine. Subject C received 1 dose of vaccine. VRC, Vaccine Research Center.

Figure 3

T cell responses specific for West Nile virus (WNV) DNA vaccine insert–specific peptide pools (WNV-E [left] and WNV-M [right]), assessed by enzyme-linked immunospot (ELISpot) assay and intracelluar cytokine staining (ICS). Data are shown for the entire study (weeks 0−32). The magnitude of ELISpot and ICS responses are shown in the top graphs, and the frequency of responses among all subjects is shown in the bottom graphs. The threshold for positivity by ELISpot assay is a mean of 59 sfc per million peripheral blood mononuclear cells. For ICS responses, a positive response required that the P value from a Fisher's exact test of the difference between the antigen-specific response and the negative control response be <.01 and that the antigen-specific response be at least 0.0241% (for CD3⁺CD4⁺ responses) or at least 0.0445% (for CD3⁺CD8⁺ responses) higher than the negative control response.