Epitope specificity of antibody-mediated protection induced in mice by the malaria vaccine RTS,S/AS01

Abstract

Antibodies induced by the malaria vaccine RTS,S/AS01 neutralize infectivity of transgenic sporozoites expressing Plasmodium falciparum CSP (PfCSP). These antibodies recognize the junctional, minor repeats, central repeats, and C-term regions of this antigen. The epitope specificity of antibodies mediating protection in mice was characterized in vivo using transgenic sporozoites expressing restricted antigenic portions of PfCSP. In this model, we found protection is mediated mostly by antibodies specific for the central repeats.

Fig. 1. Epitope mapping of anti-RTS,S/AS01 antibodies.

Binding of anti-RTS,S/AS01 antibodies to specific epitopes of the PfCSP protein by ELISA. Immune sera obtained after 3 immunizations with 5 μg of RTS,S adjuvanted with 10-fold diluted original AS01_E adjuvant dose, representing 2.5 µg of the TLR4 ligand 3-O-desacyl-4’-monophosphoryl lipid A (MPL), and 2.5 µg of the QS-21 saponin was assayed. Maxisorp 96 well plates were coated with different peptides at 1 μg/mL, (A) Junctional region, (B) Minor repeat, (C) central repeats and D C-term, (S Table 2). Serum dilutions were run in duplicates, starting at 1:100 followed by 3-fold-dilution.

Fig. 2. Liver burden reduction in mice passively immunized with polyclonal anti-RTS,S/AS01 sera and challenged with knock-in (KI) transgenic sporozoites expressing different epitopes or the full PfCSP.

C57Bl/6 mice were passively immunized with anti-RTS,S/AS01 sera, naïve sera or the related positive control monoclonal antibodies, CIS43, L9, 317, 369 or 1512 and 6 h later challenged with 2000 transgenic KI parasites expressing the A full PfCSP, B PfJR, C PfMR, (D) PfNPNA11 or (E) PfC-term. Results expressed as bioluminescence (photons/sec) and percent reduction, which was calculated using the naïve group as 100% infection. For statistical analysis Mann-Whitney t-test was used. ns: not significant, p > 0.05; *p = 0.02. These figures show results of a representative experiment. Two independent experiments performed for each transgenic parasite.