A monoclonal antibody-based immunoassay to measure the antibody response against the repeat region of the circumsporozoite protein of Plasmodium falciparum.

Kristina Radin,Geert Leroux-Roels,Franck Lemiale,Yann G J Sterckx,Jason Regules,Frederic Clement,Erik Jongert,Christian Ockenhouse

doi:10.1186/s12936-016-1596-8

Abstract

BackgroundThe malaria vaccine candidate RTS,S/AS01 (GSK Vaccines) induces high IgG concentration against the circumsporozoite protein (CSP) of Plasmodium falciparum. In human vaccine recipients circulating anti-CSP antibody concentrations are associated with protection against infection but appear not to be the correlate of protection. However, in a humanized mouse model of malaria infection prophylactic administration of a human monoclonal antibody (MAL1C), derived from a RTS,S/AS01-immunized volunteer, directed against the CSP repeat region, conveyed full protection in a dose-dependent manner suggesting that antibodies alone are able to prevent P. falciparum infection when present in sufficiently high concentrations. A competition ELISA was developed to measure the presence of MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients and study their possible contribution to protection against infection.ResultsMAL1C-like antibodies present in polyclonal vaccine-induced sera were evaluated for their ability to compete with biotinylated monoclonal antibody MAL1C for binding sites on the capture antigen consisting of the recombinant protein encompassing 32 NANP repeats of CSP (R32LR). Serum samples were taken at different time points from participants in two RTS,S/AS01 vaccine studies (NCT01366534 and NCT01857869). Vaccine-induced protection status of the study participants was determined based on the outcome of experimental challenge with infected mosquito bites after vaccination. Optimal conditions were established to reliably detect MAL1C-like antibodies in polyclonal sera. Polyclonal anti-CSP antibodies and MAL1C-like antibody content were measured in 276 serum samples from RTS,S/AS01 vaccine recipients using the standard ELISA and MAL-1C competition ELISA, respectively. A strong correlation was observed between the results from these assays. However, no correlation was found between the results of either assay and protection against infection.ConclusionsThe competition ELISA to measure MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients was robust and reliable but did not reveal the elusive correlate of protection.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-016-1596-8) contains supplementary material, which is available to authorized users.

Highlights

The malaria vaccine candidate RTS,S/AS01 (GSK Vaccines) induces high IgG concentration against the circumsporozoite protein (CSP) of Plasmodium falciparum
To examine whether the biotinylation procedure had altered structural and/or antigen-binding properties of MAL1C, the IgG content and the antigen-binding capacity of the Biotinylated MAL1C (B-MAL1C) were measured with the IgG ELISA Quantitation Set and the standard R32LR ELISA, respectively
Development of the MAL1C‐competition ELISA Plates were coated with the capturing antigen HisR32LR as described in “Methods” section

Summary

Introduction

The malaria vaccine candidate RTS,S/AS01 (GSK Vaccines) induces high IgG concentration against the circumsporozoite protein (CSP) of Plasmodium falciparum. How‐ ever, in a humanized mouse model of malaria infection prophylactic administration of a human monoclonal antibody (MAL1C), derived from a RTS,S/AS01-immunized volunteer, directed against the CSP repeat region, conveyed full protection in a dose-dependent manner suggesting that antibodies alone are able to prevent P. falciparum infection when present in sufficiently high concentrations. The most advanced malaria vaccine candidate directed against P. falciparum is RTS,S/AS01 (GSK Vaccines). This vaccine targets the pre-erythrocytic stage of the parasite and focuses on the circumsporozoite protein (CSP). Efficacy trials have shown that over the first 18 months following three doses of RTS,S/AS01, malaria cases were reduced by almost half in children aged 5–17 months at the time of first vaccination and by 27% in infants aged 6–12 weeks. In July 2015, the Committee for Medicinal Products for Human Use (CHMP) of the European Medicines Agency (EMA) has adopted a positive scientific opinion for the RTS,S/AS01 vaccine in children aged 6 weeks to 17 months

Methods

Results

Conclusion