Ad35.CS.01-RTS,S/AS01 Heterologous Prime Boost Vaccine Efficacy against Sporozoite Challenge in Healthy Malaria-Naïve Adults

Abstract

Methods: In an observer blind, phase 2 trial, 55 adults were randomized to receive one dose of Ad35.CS.01 vaccine followed by two doses of RTS,S/AS01 (ARR-group) or three doses of RTS,S/AS01 (RRR-group) at months 0, 1, 2 followed by controlled human malaria infection.

Results: ARR and RRR vaccine regimens were well tolerated. Efficacy of ARR and RRR groups after controlled human malaria infection was 44% (95% confidence interval 21%-60%) and 52% (25%-70%), respectively. The RRR-group had greater anti-CS specific IgG titers than did the ARR-group. There were higher numbers of CS-specific CD4 T-cells expressing > 2 cytokine/activation markers and more ex vivo IFN-γ enzyme-linked immunospots in the ARR-group than the RRR-group. Protected subjects had higher CS-specific IgG titers than non-protected subjects (geometric mean titer, 120.8 vs 51.8 EU/ml, respectively; P = .001).

Conclusions: An increase in vaccine efficacy of ARR-group over RRR-group was not achieved. Future strategies to improve upon RTS,S-induced protection may need to utilize alternative highly immunogenic prime-boost regimens and/or additional target antigens.

Trial registration: ClinicalTrials.gov NCT01366534.

Conflict of interest statement

Competing Interests: JH, IV, MP, JS are employees or own stock of Crucell Holland, BV. the manufacturer of Ad35.CS.01 vaccine. YV, ML, DH, PM, DM, EJ, J. Cohen, GV, WB, JV are employees and/or own stock of GSK Vaccines, the manufacturer of RTS,S/AS01 vaccine described in this study. WB is listed as an inventor on patents or has patent applications covering various malaria vaccine candidates. J. Cohen is listed as an inventor on patents or patent applications related to RTS,S, TRAP and other malaria vaccine candidates, all assigned to GSK. All authors have no other competing interests either personal or professional. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.

Figures

Fig 1. Flow diagram.

The number of subjects completing each immunization and controlled human malaria infection. ARR = first dose with Ad35.CS, second and third doses with RTS,S/AS01B. RRR = three doses of RTS,S/AS01B

Fig 2. Kaplan-Meier curves for time to parasitemia by blood smear (A) and PCR (B) for ARR (green), RRR (red), infectivity control (black) subjects (ATP efficacy).

Log-rank analysis showed statistically significant differences between ARR and control subjects (P

Fig 3. Cellular immune responses by vaccine group and days post immunization.

A. Geometric mean CS-specific CD4+ T cells producing IFN-γ by ELISpot. Comparisons between ARR and RRR on days 42, 77, 105,140, and 236 were significant (P < .05) by Student’s t-test on log-transformed data. Error bars represent standard deviation of the mean. B. Geometric mean CS-specific CD4+ T cell responses expressing ≥2 cytokine/activation markers among IL2, IFN-γ, TNF-α, and CD40L, per million PBMC. Comparisons between ARR and RRR on days 42, 77, 105, 140 were significant (P < .05) by Student’s t-test on log-transformed data. Error bars represent standard deviation of the mean. C. Pie charts showing the pattern of CS-specific polyfunctional CD4+ CD40L+ T cells expressing ≥2 cytokine markers at Day 77 (day of challenge). Each slice within the pie chart represents a specific combination of two or three cytokines. The size of the pie chart is proportional to the magnitude of the response.

Fig 4. Reverse cumulative distribution curves by post challenge protection status for CS anti-repeat antibodies on day 77-Day of challenge for subjects in ARR protected (solid red) and non-protected (dashed red) compared to RRR protected (solid black) and non-protected (dashed black) subjects.

Y-axis represents proportion of subjects at each antibody level.