A Statistical Interaction between Circumsporozoite Protein-Specific T Cell and Antibody Responses and Risk of Clinical Malaria Episodes following Vaccination with RTS,S/AS01E

Francis M Ndungu,Johan Vekemans,Philip Bejon,Domtila Kimani,Ally Olotu,Oscar Kai,Jedidah Mwacharo,Erik Jongert,Philippe Moris,Georges Snounou

doi:10.1371/journal.pone.0052870

Abstract

The candidate malaria vaccine RTS,S/AS01E provides significant but partial protection from clinical malaria. On in vitro circumsporozoite protein (CSP) peptide stimulation and intra-cellular cytokine staining of whole blood taken from 407 5–17 month-old children in a phase IIb trial of RTS,S/AS01E, we identified significantly increased frequencies of two CSP-specific CD4+ T cells phenotypes among RTS,S/AS01E vaccinees (IFNγ-IL2+TNF− and IFNγ-IL2+TNF+ CD4+ T cells), and increased frequency of IFNγ-IL2-TNF+ CD4+ T cells after natural exposure. All these T cells phenotypes were individually associated with reductions in the risk of clinical malaria, but IFNγ-IL2-TNF+ CD4+ T cells independently predicted reduced risk of clinical malaria on multi-variable analysis (HR = 0.29, 95% confidence intervals 0.15–0.54, p<0.0005). Furthermore, there was a strongly significant synergistic interaction between CSP-specific IFNγ-IL2-TNF+ CD4+ T cells and anti-CSP antibodies in determining protection against clinical malaria (p = 0.002). Vaccination strategies that combine potent cellular and antibody responses may enhance protection against malaria.

Highlights

The current lead candidate malaria vaccine is RTS,S/AS01E [1]
Anti-circumsporozoite protein (CSP) antibodies correlate with protection against infection in malaria-naıve-adult challenge studies [4] and field studies in young children [6], against clinical malaria in trials with young children in Kenya/Tanzania [7] and in Gabon/Ghana/Tanzania [8], but anti-CSP antibodies did not correlate with protection against clinical malaria in a trial with older children in Mozambique [9]
6 (0.5% of 1200) samples failed quality control because of high background (.5% IFNc+ CD4+ or CD8+ T cells on media-only control conditions), and 38 samples failed because the positive control (i.e. staphylococcal enterotoxin B (SEB) stimulation) did not result in a four-fold increase in IFNc+ CD4+ T cells over media-only control

Summary

Introduction

The current lead candidate malaria vaccine is RTS,S/AS01E [1]. The RTS,S antigen consists of the C-terminal region of the P falciparum CSP including 19 copies of the central tandem repeats, fused to the hepatitis B surface antigen (HBsAg), co-expressed with unfused HBsAg in Saccharomyces cerevisiae cells. RTS,S, formulated with AS01 and at a paediatric dose, is referred to as RTS,S/AS01E. Anti-CSP antibodies could protect by a variety of mechanisms including complement activation, antibody dependent cellular cytotoxicity, sporozoite neutralization, and/or FccR mediated phagocytosis [10]. CD4+ T cells might mediate protection indirectly by providing help to B cells for the production of highly effective anti-CSP Abs, or directly by secreting effector/cytotoxic cytokines (e.g. TNF or IFNc) [11,12]

Methods

Results

Conclusion