Nonhuman primate to human immunobridging to infer the protective effect of an Ebola virus vaccine candidate

Abstract

It has been proven challenging to conduct traditional efficacy trials for Ebola virus (EBOV) vaccines. In the absence of efficacy data, immunobridging is an approach to infer the likelihood of a vaccine protective effect, by translating vaccine immunogenicity in humans to a protective effect, using the relationship between vaccine immunogenicity and the desired outcome in a suitable animal model. We here propose to infer the protective effect of the Ad26.ZEBOV, MVA-BN-Filo vaccine regimen with an 8-week interval in humans by immunobridging. Immunogenicity and protective efficacy data were obtained for Ad26.ZEBOV and MVA-BN-Filo vaccine regimens using a fully lethal EBOV Kikwit challenge model in cynomolgus monkeys (nonhuman primates [NHP]). The association between EBOV neutralizing antibodies, glycoprotein (GP)-binding antibodies, and GP-reactive T cells and survival in NHP was assessed by logistic regression analysis. Binding antibodies against the EBOV surface GP were identified as the immune parameter with the strongest correlation to survival post EBOV challenge, and used to infer the predicted protective effect of the vaccine in humans using published data from phase I studies. The human vaccine-elicited EBOV GP-binding antibody levels are in a range associated with significant protection against mortality in NHP. Based on this immunobridging analysis, the EBOV GP-specific-binding antibody levels elicited by the Ad26.ZEBOV, MVA-BN-Filo vaccine regimen in humans will likely provide protection against EBOV disease.

Fig. 1. Experiment schematic and comparison of Ebola virus disease course in NHP and humans.

a The disease course and outcome of EVD was compared between untreated NHP and humans*. As far as the authors are aware, IM infection in humans via needle reusage is only identified in a single report. There is rapid disease progression in NHP relative to humans, irrespective of route of infection. Duration indicated for NHP represents the average of 13 untreated controls from seven independent NHP studies (C25#1, 12, C29#1, C29#2, C29#8, TO14#1, TO14#2). In humans, time to symptoms, time to outcome, and time to hospital are based on cited papers^–, and shown as the range of the mean values reported in the cited studies. Symptom onset in NHP was defined based on recorded clinical score. b Experimental schematic of immunogenicity and efficacy studies in NHP. Vaccine regimens containing Ad26.ZEBOV and MVA-BN-Filo were tested with variations in dose, dose interval, dose order, and valency of the adenovector vaccine. Four weeks after the final vaccination, NHP were challenged with EBOV Kikwit (IM) at a target dose of 100 pfu (actual range 50–1615 pfu), and followed up for signs and symptoms of EVD for up to 4 weeks. EBOV Ebola virus, EVD Ebola virus disease, IM intramuscular, NHP nonhuman primates.

Fig. 2. Immunogenicity and protective efficacy of Ad26.ZEBOV, MVA-BN-Filo regimens in NHP.

Nonsurviving NHP are depicted by open circles. a EBOV GP-binding antibody concentrations (ELISA units [EU]/mL, log₁₀ transformed) by regimen as determined by Filovirus Animal Non-Clinical Group (FANG) EBOV GP ELISA (BBRC); group means are indicated; samples below the lower limit of detection (LOD) are plotted at a common value (dotted line). b EBOV neutralizing antibody titers (IC₅₀, log₁₀ transformed) as determined by pseudovirus neutralization assay (psVNA; Monogram); group means are indicated; dotted line is LOD. Three samples could not be analyzed due to sample volume limitations. c EBOV GP-reactive IFN-γ T cells (spot-forming units [SFU]/10⁶ cells, log₁₀ transformed) as enumerated by IFN-γ ELISpot (TBRI); group means are indicated; samples below LOD are plotted at a common value (dotted line). d Table shows vaccine dose, dose order, and dose interval, as well as the associated survival after challenge. Additional regimens tested, as well as an overview by regimen can be found in Supplementary Fig. 2 and Supplementary Table 1. Dose of Ad26.ZEBOV (green) and MVA-BN-Filo (blue) as well as dose interval (shades of red and pink). Yellow shading identifies regimens with 100% protection. ELISA enzyme-linked immunosorbent assay, IFN-γ ELISpot interferon-gamma enzyme-linked immunospot, EU ELISA units, IC₅₀ half-maximal inhibitory concentration, InfU infectious units, LOD limit of detection, psVNA pseudovirus neutralization assay, SFU spot-forming units, PBMC peripheral blood mononuclear cells, vp viral particles.

Fig. 3. Immunological correlates of protection in NHP.

Graphs represent logistic regression models with immune parameters as predictors of survival after EBOV Kikwit challenge (100 pfu, IM) in cynomolgus monkeys for: a EBOV GP-binding antibody levels (ELISA, red), and b EBOV neutralizing antibodies (psVNA, brown). c EBOV GP-reactive T cells (IFN-γ ELISpot, purple). Individual immune response levels are identified with open circles and the associated survival as a binary variable with survival as 1 (top) and nonsurvival as 0 (bottom). Assay LODs are indicated by dotted lines. d ROC curves for sensitivity and specificity of GP-binding antibody levels (red), EBOV neutralizing antibodies (brown) and EBOV GP-reactive T cells (purple) in predicting NHP survival after challenge. ROC AUC are indicated in the panel. AUC area under the curve, ELISA enzyme-linked immunosorbent assay, IFN-γ ELISpot interferon-gamma enzyme-linked immunospot, EU ELISA units, IC₅₀ half-maximal inhibitory concentration, LOD limit of detection, psVNA pseudovirus neutralization assay, SFU spot-forming units.

Fig. 4. Vaccine dose-down studies refine logistic model based on GP-binding antibody levels for immunobridging.

a EBOV GP-binding antibody concentrations (EU/mL, log₁₀ transformed) as determined by FANG ELISA (BBRC); group means are indicated; dotted line is LOD. b Table identifies dose of Ad26.ZEBOV and MVA-BN-Filo with highest dose represented in green and lowest dose in red. Vaccine dose interval is 8 weeks. An overview by regimen can be found in Supplementary Table 4. c Logistic regression models of GP-binding antibodies as predictor of survival after EBOV Kikwit challenge (100 pfu, IM) in NHP for all vaccine regimens combined from five NHP studies (red), all regimens combined from seven NHP studies (blue), Ad26.ZEBOV, MVA-BN-Filo regimen with an 8-week interval only (green). Individual immune response levels are identified with open circles and the associated survival as a binary variable with survival as 1 (top) and nonsurvival as 0 (bottom). Assay LOD is indicated by dotted line. d ROC curves for sensitivity and specificity of GP-binding antibody levels in predicting NHP survival after challenge for all vaccine regimens combined from five NHP studies (red), all regimens combined from seven NHP studies (blue), Ad26.ZEBOV, MVA-BN-Filo regimen with an 8-week interval only (green). ROC AUC are indicated in the panel. BBRC Battelle Biomedical Research Institute, ELISA enzyme-linked immunosorbent assay, EU ELISA units, FANG Filovirus Animal Non-Clinical Group, InfU infectious units, vp viral particles.

Fig. 5. Immunobridging of human EBOV GP-binding antibodies to the NHP-challenge model.

a Visual representation of bootstrap method to derive 95% confidence interval. Logistic model is shown in blue. Gray lines represent 10,000 bootstraps of NHP dataset, to derive the 95% confidence bands (blue dashed lines) as the 250th and 9750th values of the ranked bootstraps above each ELISA value. The human EBOV GP-binding antibody data are bootstrapped 10,000 times on each of the NHP iterations to derive a 95% CI for survival probability. b Immunobridging of human GP-binding antibody levels from two phase I studies^, using the logistic model based on GP-binding antibody levels and survival in NHP (a). Green line is logistic regression model based on Ad26.ZEBOV, MVA-BN-Filo regimen with an 8-week interval. Individual human GP-binding antibody levels and the associated survival probability based on the logistic model in NHP are indicated by blue lines. The point estimate for mean predicted survival probability (magenta solid line) is the average of the individual survival probabilities. The human EBOV GP-binding antibody data (magenta open circles, x-axis) are bootstrapped 10,000 times on each of the NHP iterations (b) to derive a 95% confidence interval (CI) for mean predicted survival probability (magenta dashed lines). Survival probabilities for the main logistic model (green closed circles, y-axis) are shown. EU ELISA units.