A Monovalent Chimpanzee Adenovirus Ebola Vaccine Boosted with MVA

Abstract

Background: The West African outbreak of Ebola virus disease that peaked in 2014 has caused more than 11,000 deaths. The development of an effective Ebola vaccine is a priority for control of a future outbreak.

Methods: In this phase 1 study, we administered a single dose of the chimpanzee adenovirus 3 (ChAd3) vaccine encoding the surface glycoprotein of Zaire ebolavirus (ZEBOV) to 60 healthy adult volunteers in Oxford, United Kingdom. The vaccine was administered in three dose levels--1×10(10) viral particles, 2.5×10(10) viral particles, and 5×10(10) viral particles--with 20 participants in each group. We then assessed the effect of adding a booster dose of a modified vaccinia Ankara (MVA) strain, encoding the same Ebola virus glycoprotein, in 30 of the 60 participants and evaluated a reduced prime-boost interval in another 16 participants. We also compared antibody responses to inactivated whole Ebola virus virions and neutralizing antibody activity with those observed in phase 1 studies of a recombinant vesicular stomatitis virus-based vaccine expressing a ZEBOV glycoprotein (rVSV-ZEBOV) to determine relative potency and assess durability.

Results: No safety concerns were identified at any of the dose levels studied. Four weeks after immunization with the ChAd3 vaccine, ZEBOV-specific antibody responses were similar to those induced by rVSV-ZEBOV vaccination, with a geometric mean titer of 752 and 921, respectively. ZEBOV neutralization activity was also similar with the two vaccines (geometric mean titer, 14.9 and 22.2, respectively). Boosting with the MVA vector increased virus-specific antibodies by a factor of 12 (geometric mean titer, 9007) and increased glycoprotein-specific CD8+ T cells by a factor of 5. Significant increases in neutralizing antibodies were seen after boosting in all 30 participants (geometric mean titer, 139; P<0.001). Virus-specific antibody responses in participants primed with ChAd3 remained positive 6 months after vaccination (geometric mean titer, 758) but were significantly higher in those who had received the MVA booster (geometric mean titer, 1750; P<0.001).

Conclusions: The ChAd3 vaccine boosted with MVA elicited B-cell and T-cell immune responses to ZEBOV that were superior to those induced by the ChAd3 vaccine alone. (Funded by the Wellcome Trust and others; ClinicalTrials.gov number, NCT02240875.).

Figure 1. Antibody Responses to the Zaire ebolavirus (ZEBOV) Glycoprotein.

Panel A shows the geometric mean titer of antibody responses to increasing doses of the chimpanzee adenovirus 3 (ChAd3) vaccine encoding the surface glycoprotein of ZEBOV, followed by a booster dose of a modified vaccinia Ankara (MVA) strain. Antibody responses are shown according to measurements on a standardized enzyme-linked immunosorbent assay (ELISA) for doses of 1×10¹⁰ viral particles (in 19 participants), 2.5×10¹⁰ viral particles (in 20 participants), and 5×10¹⁰ viral particles (in 20 participants). Solid symbols indicate that participants received only the ChAd3 vaccine, and open symbols, that participants received the ChAd3 vaccine followed by booster MVA. Antibody responses increased significantly by 7 days after the MVA dose and peaked at day 14 after boosting and then decreased slightly by day 28. There were no significant differences in responses among the dose groups in the cohort that received the MVA booster at any time point after vaccination. The days of the analysis are indicated by a plus sign after administration of the ChAd3 vaccine (A) and the MVA vaccine (M). Panel B shows the responses of participants after administration of the prime and booster vaccines, according to results on anti-ZEBOV glycoprotein (GP) IgG ELISA. The solid horizontal lines represent the geometric mean titer. Percentages of vaccinees with positive antibody responses at each time point are indicated below the graph. The horizontal dashed line represents the threshold for a positive result (arbitrary ELISA units, +0.561), calculated as the mean plus 3 standard deviations of the response on day 0 for all participants. Panel C shows antibody titers to inactivated whole ZEBOV virions (Makona strain) as measured on ELISA. The data show that immunogenicity at 4 weeks after priming with ChAd3 was similar to that measured after immunization with a recombinant vesicular stomatitis virus–based vaccine expressing ZEBOV glycoprotein (rVSV-ZEBOV) in 10 vaccinees in Hamburg, Germany. Panel D shows titers of neutralizing antibodies against live ZEBOV (Mayinga strain) from all participants who received the MVA booster, as measured at 28 days after the ChAd3 dose and 14 days after the MVA dose. Low levels of neutralizing antibodies that were detected in participants at 28 days were similar to levels reported after the administration of the rVSV-ZEBOV vaccine. By 14 days after MVA vaccination, the levels had increased by a factor of 9. In Panels C and D, the columns represent the geometric mean titer, the I bars represent 95% confidence intervals, and the horizontal dashed lines represent the positive threshold. In Panels B and D, the asterisk denotes P<0.001 by the two-tailed Mann–Whitney test.

Figure 2. T-Cell Responses and Induction of Cytokines after Boosting with MVA.

Panel A shows the median T-cell responses to ChAd3 vaccination and MVA boosting on enzyme-linked immunosorbent spot (ELISPOT) assay at all time points, as measured in spot-forming cells (SFCs) per million peripheral-blood mononuclear cells (PBMCs). The dose of MVA is indicated in plaque-forming units (PFU). Panel B shows the relationship between the prime–boost interval and the peak ELISPOT response 7 days after MVA vaccination, as calculated by means of a two-tailed Spearman’s test. Panel C shows the total cytokine response on flow cytometry with intracellular cytokine staining at 28 days after priming (post-prime) or 7 days after boosting, according to the MVA dose. The secretion of interferon-γ, interleukin-2, and tumor necrosis factor α (TNF-α) by CD4+ and CD8+ T cells was quantified for each booster-dose group and expressed as the frequency of cells expressing any one of the three cytokines. Panel D shows the expression of the degranulation marker CD107a 28 days after priming or 7 days after boosting. In Panels C and D, the solid horizontal lines indicate median values, and the dashed horizontal lines indicate the positive threshold. Panel E shows the proportions of CD4+ and CD8+ T cells that secreted any combination of interferon-γ, interleukin-2, and TNF-α after stimulation with two different MVA doses.

Figure 3. Effect of Reduced Prime–Boost Intervals on Cellular Immunogenicity.

Panel A shows the results of reducing the interval between prime vaccination with ChAd3 and booster vaccination with MVA to either 1 week or 2 weeks (as compared with 3 to 10 weeks) in two groups of eight participants each. The results are shown as median T-cell responses on ELISPOT assay, as measured in SFCs per million PBMCs. D0 indicates the beginning of the prime–boost interval for each group. Responses in all three groups peaked at 7 days after boosting, regardless of the prime–boost interval. Panel B shows the relationship between the prime–boost interval and the peak ELISPOT response 7 days after boosting, with a modest negative correlation between the prime–boost interval and peak T-cell immunogenicity. Also shown are individual ELISPOT responses to summed glycoprotein peptide pools at 7 days (Panel C) and 28 days (Panel D) after boosting; no significant differences between the groups were seen at either 7 days or 28 days. The black horizontal lines indicate median values. In these analyses, all the participants received 2.5×10¹⁰ viral particles of the ChAd3 vaccine and a booster dose of 1.5×10⁸ PFU of MVA.