Priming with an adenovirus 35-circumsporozoite protein (CS) vaccine followed by RTS,S/AS01B boosting significantly improves immunogenicity to Plasmodium falciparum CS compared to that with either malaria vaccine alone

Abstract

The RTS,S/AS02A protein-based vaccine consistently demonstrates significant protection against infection with Plasmodium falciparum malaria and also against clinical malaria and severe disease in children in areas of endemicity. Here we demonstrate with rhesus macaques that priming with a replication-defective human adenovirus serotype 35 (Ad35) vector encoding circumsporozoite protein (CS) (Ad35.CS), followed by boosting with RTS,S in an improved MPL- and QS21-based adjuvant formulation, AS01B, maintains antibody responses and dramatically increases levels of T cells producing gamma interferon and other Th1 cytokines in response to CS peptides. The increased T-cell responses induced by the combination of Ad35.CS and RTS,S/AS01B are sustained for at least 6 months postvaccination and may translate to improved and more durable protection against P. falciparum infection in humans.

FIG. 1.

ELISPOT data. Shown are antigen-specific IFN-γ ELISPOT results for CS C-terminal 15-mer peptides, expressed as the number of spots per 10⁶ PBMCs. (a) Two weeks after the last immunization. Geometric means (horizontal lines) were compared to those for the RTS,S-alone group. Statistical significance is indicated by asterisks: *, P < 0.05; **, P < 0.01. RTS,S, RTS,S/AS01B; Ad35, Ad35.CS. (b) Time course of geometric mean IFN-γ ELISPOT responses. Error bars, standard errors of the means, calculated on log-transformed data; vertical dashed lines, vaccination time points; horizontal dashed line, average background. Week 14 is 2 weeks after the final immunization; weeks 25 and 38 are 3 and 6 months later, respectively. R, RTS,S/AS01B.

FIG. 2.

Intracellular cytokine staining data. Results are numbers of cytokine-expressing CD4⁺ T cells per million CD4⁺ cells. (a) All cells expressing any two or all three cytokines evaluated. (b, c, and d) Cells expressing two or three cytokines including at least IL-2, IFN-γ, or TNF-α, respectively. Cells were harvested 2 weeks after the last vaccination. Geometric means were compared to those for the RTS,S/AS01B comparator group. *, P < 0.05; **, P < 0.01.

FIG. 3.

Antibody responses 2 weeks (a) and 3 months (b) after final immunization. RTS,S, RTS,S/AS01B; Ad35, Ad35.CS. **, P < 0.01 by Dunnett's comparison with RTS,S alone as the comparator and with the empty Ad35 group omitted. Horizontal lines represent geometric means (or the median in the case of the empty Ad35 group).