Measurement of ex vivo ELISpot interferon-gamma recall responses to Plasmodium falciparum AMA1 and CSP in Ghanaian adults with natural exposure to malaria.

Dorothy Anum,Michael R. Hollingdale,Daniel Dodoo,Kwadwo A. Koram,Bjoern Peters,Michael F. Ofori,Jo Glenna Banania,Ben A. Gyan,Harini Ganeshan,Martha Sedegah,Maria Belmonte,Kwadwo A. Kusi,Eileen Villasante,Jun Huang,John K. A. Tetteh,Yohan Kim

doi:10.1186/s12936-016-1098-8

Abstract

BackgroundMalaria eradication requires a concerted approach involving all available control tools, and an effective vaccine would complement these efforts. An effective malaria vaccine should be able to induce protective immune responses in a genetically diverse population. Identification of immunodominant T cell epitopes will assist in determining if candidate vaccines will be immunogenic in malaria-endemic areas. This study therefore investigated whether class I-restricted T cell epitopes of two leading malaria vaccine antigens, Plasmodium falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1), could recall T cell interferon-γ responses from naturally exposed subjects using ex vivo ELISpot assays.MethodsThirty-five subjects aged between 24 and 43 years were recruited from a malaria-endemic urban community of Ghana in 2011, and their peripheral blood mononuclear cells (PBMCs) were tested in ELISpot IFN-γ assays against overlapping 15mer peptide pools spanning the entire CSP and AMA1 antigens, and 9–10mer peptide epitope mixtures that included previously identified and/or predicted human leukocyte antigen (HLA) class 1-restricted epitopes from same two antigens.ResultsFor CSP, 26 % of subjects responded to at least one of the nine 15mer peptide pools whilst 17 % responded to at least one of the five 9–10mer HLA-restricted epitope mixtures. For AMA1, 63 % of subjects responded to at least one of the 12 AMA1 15mer peptide pools and 51 % responded to at least one of the six 9–10mer HLA-restricted epitope mixtures. Following analysis of data from the two sets of peptide pools, along with bioinformatics predictions of class I-restricted epitopes and the HLA supertypes expressed by a subset of study subjects, peptide pools that may contain epitopes recognized by multiple HLA supertypes were identified. Collectively, these results suggest that natural transmission elicits ELISpot IFN-γ activities to class 1-restricted epitopes that are largely HLA-promiscuous.ConclusionsThese results generally demonstrate that CSP and AMA1 peptides recalled ELISpot IFN-γ responses from naturally exposed individuals and that both CSP and AMA1 contain diverse class 1-restricted epitopes that are HLA-promiscuous and are widely recognized in this population.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-016-1098-8) contains supplementary material, which is available to authorized users.

Highlights

Malaria eradication requires a concerted approach involving all available control tools, and an effective vaccine would complement these efforts
No prediction of the human leukocyte antigen (HLA)-restriction was possible with the remaining seven subjects that were positive with apical membrane antigen-1 (AMA1) 15mer pools but not the predicted AMA1 HLA-binding 9–10mer peptide pool (A-HLA) pools. These outcomes collectively suggest that it is possible to identify the HLArestriction of responses to AMA1 epitopes in the absence of HLA-typing in 10 of 17 (59 %) non-HLA-typed subjects. These results suggest that positive activities of most subjects are detected by both peptide pools spanning regions of circumsporozoite protein (CSP) or AMA1 or by HLA-specific peptide pools, but epitope promiscuity makes a significant contribution to determining agreement between positive CSP or AMA1 15mer peptide pools and HLA peptide pools
The aim of this study was to assess 15mer peptides that cover the entire sequences of CSP and AMA1, as well as predicted/ known HLA class I-restricted epitopes from the same antigens, for their ability to induce IFN-γ recall responses in peripheral blood mononuclear cells (PBMCs) from naturally exposed individuals in Ghana, and to better determine optimal reagents for further site characterization

Summary

Introduction

Malaria eradication requires a concerted approach involving all available control tools, and an effective vaccine would complement these efforts. This study investigated whether class I-restricted T cell epitopes of two leading malaria vaccine antigens, Plasmodium falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1), could recall T cell interferon-γ responses from naturally exposed subjects using ex vivo ELISpot assays. The role of antibodies to malaria antigens, especially the immunodominant circumsporozoite protein (CSP) and other liver and blood stage antigens, has been widely investigated [6]. Sub-unit vaccines are being widely developed and the leading candidate, RTS, S which is based on the CSP, elicits protection in up to 50 % of subjects, but begins to wane after the first year [9, 10]

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