Implications from predicted B-cell and T-cell epitopes of Plasmodium falciparum merozoite proteins EBA175-RII and Rh5.

Kevin Kariuki Wamae,Lynette Isabella Ochola-Oyier

doi:10.6026/97320630012082

Abstract

The leading circumsporozoite protein (CSP) based malaria vaccine, RTS,S, though promising, has shown limited efficacy in field studies. There is therefore, still a need to identify other malaria vaccine targets. Merozoite antigens are potential vaccine candidates, since naturally acquired antibodies generated against them inhibit erythrocyte invasion and in some cases result in the clinical protection from disease. We thus used in silico tools (BCPreds, NetMHCcons and NetMHCIIpan 3.0) to predict B-cell epitopes (BCEs) and T-cell epitopes (TCEs) in two merozoite invasion proteins, EBA175-RII and Rh5. Initially, we validated these tools using CSP to determine whether the algorithms could predict the epitopes in the RTS,S vaccine. In EBA175-RII, we prioritised three BCEs 15REKRKGMKWDCKKKNDRSNY34, 420SNRKLVGKINTNSNYVHRNKQ440 and 528WISKKKEEYNKQAKQYQEYQ547, a CD8+ epitope 553KMYSEFKSI561 and a CD4+ epitope 440QNDKLFRDEWWK VIKKD456. Three Rh5 epitopes were prioritised, a BCE 344SCYNNNFCNTNGIRYHYDEY363, a CD8+ epitope 198STYGKCIAV206 and a Rh5 CD4+ epitope 180TFLDYYKHLSYNSIYHKSSTY200. All these epitopes are in the region involved in the proteins’ interaction with their erythrocyte receptors, thus enabling erythrocyte invasion. Therefore, upon validation of their immunogenicity, by ELISA using serum from a malaria endemic population, antibodies to these epitopes may inhibit erythrocyte invasion. All the epitopes we predicted in EBA175-RII and Rh5 are novel. We also identified polymorphic epitopes that may escape host immunity, as some variants were not predicted as epitopes, suggesting that they may not be immunogenic regions. We present a set of epitopes that following in vitro validation provide a set of molecules to screen as potential vaccine candidates.

Highlights

Malaria is caused by the unicellular protozoan parasite, Plasmodium falciparum, that remains an important public health concern due to the high rates of mortality and morbidity in children under 5 years of age [1]
Prediction of B-cell epitopes (BCEs) and T-cell epitopes (TCEs) in circumsporozoite protein (CSP) After clustering the 22 predicted BCEs, we remained with 18 unique epitopes (Table 2) of which 7 contained the CSP BCE, NANP3
Since all the predicted CSP BCEs had antigenic scores of 1, we used this value in our selection of erythrocyte binding antigen-175 (EBA175)-region II (RII) and Rh5 BCEs

Summary

Introduction

Malaria is caused by the unicellular protozoan parasite, Plasmodium falciparum, that remains an important public health concern due to the high rates of mortality and morbidity in children under 5 years of age [1]. The malaria vaccine candidate (MosquirixTM), RTS,S, based on the circumsporozoite protein (CSP), was approved for use by European regulators in July 2015, it has shown limited success and waning efficacy over time [9]. Previous studies have shown that in silico tools can identify Bcell epitopes (BCE) and T-cell epitopes (TCE) [10, 11], making this approach a quicker way to prioritise potential immunogenic targets for in vitro validation. A prime target for the design of a malaria vaccine is the invasive blood-stage form of the parasite, the merozoite, which invades red blood cells (RBCs) initiating the blood stage infection and the clinical symptoms of disease [12]

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