The Final (Oral Ebola) Vaccine Trial on Captive Chimpanzees?

Abstract

Could new oral vaccine technologies protect endangered wildlife against a rising tide of infectious disease? We used captive chimpanzees to test oral delivery of a rabies virus (RABV) vectored vaccine against Ebola virus (EBOV), a major threat to wild chimpanzees and gorillas. EBOV GP and RABV GP-specific antibody titers increased exponentially during the trial, with rates of increase for six orally vaccinated chimpanzees very similar to four intramuscularly vaccinated controls. Chimpanzee sera also showed robust neutralizing activity against RABV and pseudo-typed EBOV. Vaccination did not induce serious health complications. Blood chemistry, hematologic, and body mass correlates of psychological stress suggested that, although sedation induced acute stress, experimental housing conditions did not induce traumatic levels of chronic stress. Acute behavioral and physiological responses to sedation were strongly correlated with immune responses to vaccination. These results suggest that oral vaccination holds great promise as a tool for the conservation of apes and other endangered tropical wildlife. They also imply that vaccine and drug trials on other captive species need to better account for the effects of stress on immune response.

Figure 1. Relative health impact of EBOV infection versus sedation for EBOV vaccination.

(A) Carcass of a chimpanzee killed by EBOV in Odzala-Kokoua National Park, Republic of Congo. (B) Sedated chimpanzee being orally vaccinated against EBOV at the New Iberia Research Center, USA.

Figure 2. Filorab1 vaccine is strongly immunogenic in chimpanzees.

Top Panels: Highest serum dilution factor at which serum antibodies isolated from chimpanzees achieved 50% neutralization of (A) EBOV pseudotyped VSV and (B) RABV. Red bars IM vaccinated chimpanzees, blue bars orally vaccinated chimpanzees. Serum antibodies from all subjects except one IM vaccinated chimpanzee achieved 50% RABV neutralization at dilutions much higher than the lowest dilution factor (dashed line in (B) considered by the World Health Organization to be robustly protective against RABV challenge. No comparable standard is accepted for EBOV. Bottom Panels: ELISA optical densities (OD) for chimpanzee serum titers of IgG against (C) EBOV GP and D) RABV. Day 0, 14, and 28 OD’s for 1/150 dilutions of chimpanzee IgG are plotted as a proportion of the OD for the positive control: post-challenge IgG from macaques vaccinated with filorab1 in a previous study. Circles are averages for the six orally vaccinated (in blue) and four IM vaccinated (in red) chimpanzees. Least squares regression lines through the orally vaccinated chimpanzee data show very close to exponential growth of IgG against EBOV GP IgG (R² = 0.98) and RABV IgG (R² = 0.99). Error bars are 95% confidence intervals (1.96 standard errors). Lack of confidence interval overlap between successive sampling days indicates highly significant rises in IgG on Days 14 and 28. RABV IgG titers for orally vaccinated chimpanzees grew more slowly than for IM vaccinated chimpanzees or macaques in the previous study (black X’s). EBOV GP IgG titers of orally vaccinated chimpanzees grew at a rate similar to that of macaques.

Figure 3. Correlates of acute stress.

(A) Mean change in body mass between sedations (1 kg equals 1.4% of initial mean body mass). (B) Serum glucose and (D) WBC for ten chimpanzees in this study (red circles), six chimpanzees from previous Ebola VLP vaccine study (solid black circles), and 26 newly captive baboons (open black circles). Glucose in this study and the baboon study peaked on about the same day and exhibited similar baseline levels. Higher peak glucose in baboons is consistent with more severe stress. Chimpanzee WBC in this study peaked on Day 0 then plateaued. Chimpanzee WBC in the Ebola VLP study returned to baseline by Day 56. WBC not available for baboon study. The three orally vaccinated chimpanzees that presented voluntarily for sedation (blue circles) exhibited lower values of (C) glucose and (E) WBC than the three that did not always present voluntarily (red circles). Voluntary presenters showed significantly lower serum glucose (t test Day 7 p = 0.04, Day 14 p = 0.017) and WBC (Day 7 p = 0.039) than non-voluntary presenters.(F) Alkaline phosphatase (ALP) in this chimpanzee study and the baboon study peaked on the first sampling day then decayed at similar rates to baseline concentrations that were, in both cases, 71% of the peak value, suggesting that similar mechanisms down-regulate stress responses in the two species. Baboon values have been normalized to the peak chimpanzee concentration to illustrate this similarity. Error bars in all panels are 95% confidence intervals (i.e. 1.96 standard errors).

Figure 4. Immune responses enhanced by stress.

ELISA optical densities (OD) of EBOV GP-specific IgG (left column) and RABV-specific IgG (right column) were correlated with three well-known correlates of acute stress (A,B) serum glucose, (C,D) WBC, and (E,F) Alkaline phosphatase (ALP). R² and p value in each plot are for univariate least squares regression of each stress correlate versus Day 28 IgG OD at 1:150 dilution. Note that IM vaccinated chimpanzees (blue circles) tended to exhibit higher values for all stress correlates. Because of correlation between stress variables, multivariate stepwise regression was used to test for independent effects of WBC, glucose, and ALP on IgG OD (see main text for results).