A DNA vaccine for Ebola virus is safe and immunogenic in a phase I clinical trial

Abstract

Ebola viruses represent a class of filoviruses that causes severe hemorrhagic fever with high mortality. Recognized first in 1976 in the Democratic Republic of Congo, outbreaks continue to occur in equatorial Africa. A safe and effective Ebola virus vaccine is needed because of its continued emergence and its potential for use for biodefense. We report the safety and immunogenicity of an Ebola virus vaccine in its first phase I human study. A three-plasmid DNA vaccine encoding the envelope glycoproteins (GP) from the Zaire and Sudan/Gulu species as well as the nucleoprotein was evaluated in a randomized, placebo-controlled, double-blinded, dose escalation study. Healthy adults, ages 18 to 44 years, were randomized to receive three injections of vaccine at 2 mg (n = 5), 4 mg (n = 8), or 8 mg (n = 8) or placebo (n = 6). Immunogenicity was assessed by enzyme-linked immunosorbent assay (ELISA), immunoprecipitation-Western blotting, intracellular cytokine staining (ICS), and enzyme-linked immunospot assay. The vaccine was well-tolerated, with no significant adverse events or coagulation abnormalities. Specific antibody responses to at least one of the three antigens encoded by the vaccine as assessed by ELISA and CD4(+) T-cell GP-specific responses as assessed by ICS were detected in 20/20 vaccinees. CD8(+) T-cell GP-specific responses were detected by ICS assay in 6/20 vaccinees. This Ebola virus DNA vaccine was safe and immunogenic in humans. Further assessment of the DNA platform alone and in combination with replication-defective adenoviral vector vaccines, in concert with challenge and immune data from nonhuman primates, will facilitate evaluation and potential licensure of an Ebola virus vaccine under the Animal Rule.

Figures

FIG. 1.

Specific antibody responses to all vaccine components by IP-Western blot analysis. Sera from three representative subjects from each vaccine dose group are shown for each antigen (A). Sera were drawn at week 12, 4 weeks following the third vaccination. Antibody responses were specific and not cross-reactive to other vaccine antigens based on immunoblotting with monoclonal antibodies (B).

FIG. 2.

Kinetics and frequency of antibody responses. (A) Percentages of responders following the third vaccination by the IP-Western assay for all subjects are shown. The y axis represents the percentage of responders with a positive assay, and the x axis represents the vaccine dose group. White bars, GP (Z); black bars, GP (S/G); gray bars, NP (Z). (B) Kinetics of the antibody response for all subjects is shown over the 52 weeks of the study. The geometric mean titer of the log10 reciprocal dilution and standard deviation of the antibody response to GP (S/G) are plotted against the number of weeks after initial vaccination for each of the three dose levels. Vaccinations were given at 0, 4, and 8 weeks. The threshold for positivity in this assay was a reciprocal dilution of 30 and is shown as a dashed line. Of note, only six of eight subjects in the 8-mg dose group received all three vaccinations in the series, yet all vaccinees are included in the immunogenicity analysis.

FIG. 3.

Frequency of CD4+ and CD8+ T-cell responses by ICS and ELISPOT analysis. Frequency (percent responders) is represented on the left y axis. The week of analysis is shown on the lower x axis, the antigen assessed is shown on the upper x axis, and vaccine dose group is shown on the right y axis. The frequency of CD4+ ICS responses is shown by red bars, the frequency of CD8+ ICS responses is shown by green bars, and the frequency of positive ELISPOT responses is shown with blue bars. The schedule of the three DNA vaccinations is represented by arrows along the lower x axis.

FIG. 4.

Magnitude of antigen-specific T-cell responses to vaccine components. GP (S/G) (x axes) are shown in relation to each of the other two antigens, GP (Z) and NP (Z) (y axes). All antigen-specific CD4+ and CD8+ T-cell responses for all subjects (vaccine and placebo recipients) as assessed by ICS are shown. CD4+ and CD8+ T-cell responses are shown as a percentage of total CD4+ or CD8+ T cells on the x and y axes. CD4 responses are shown in the upper graphs, and CD8+ responses are shown in the lower graphs. The red dashed line represents the threshold of positivity (0.0445% for CD8+ T cells and 0.0241% for CD4+ T cells).

FIG. 5.

Kinetics of CD4+ and CD8+ T-cell responses to GP (S/G). Responses were assessed by ICS and are shown over the course of the study. Results are presented by dose group, with a blue line indicating the mean response over time for all subjects in a given group. The median of the response is represented in the box and whisker plots at each time point. The red dashed lines represent the positivity threshold for the ICS assay.