P. falciparum and P. vivax Epitope-Focused VLPs Elicit Sterile Immunity to Blood Stage Infections.

David C Whitacre,Diego A Espinosa,Fidel P Zavala,Cory J Peters,Darrell L Peterson,Joyce E Jones,David R Milich,Amy E Tucker,Denise L Doolan

doi:10.1371/journal.pone.0124856

Abstract

In order to design P. falciparum preerythrocytic vaccine candidates, a library of circumsporozoite (CS) T and B cell epitopes displayed on the woodchuck hepatitis virus core antigen (WHcAg) VLP platform was produced. To test the protective efficacy of the WHcAg-CS VLPs, hybrid CS P. berghei/P. falciparum (Pb/Pf) sporozoites were used to challenge immunized mice. VLPs carrying 1 or 2 different CS repeat B cell epitopes and 3 VLPs carrying different CS non-repeat B cell epitopes elicited high levels of anti-insert antibodies (Abs). Whereas, VLPs carrying CS repeat B cell epitopes conferred 98% protection of the liver against a 10,000 Pb/Pf sporozoite challenge, VLPs carrying the CS non-repeat B cell eptiopes were minimally-to-non-protective. One-to-three CS-specific CD4/CD8 T cell sites were also fused to VLPs, which primed CS-specific as well as WHcAg-specific T cells. However, a VLP carrying only the 3 T cell domains failed to protect against a sporozoite challenge, indicating a requirement for anti-CS repeat Abs. A VLP carrying 2 CS repeat B cell epitopes and 3 CS T cell sites in alum adjuvant elicited high titer anti-CS Abs (endpoint dilution titer >1x106) and provided 80–100% protection against blood stage malaria. Using a similar strategy, VLPs were constructed carrying P. vivax CS repeat B cell epitopes (WHc-Pv-78), which elicited high levels of anti-CS Abs and conferred 99% protection of the liver against a 10,000 Pb/Pv sporozoite challenge and elicited sterile immunity to blood stage infection. These results indicate that immunization with epitope-focused VLPs carrying selected B and T cell epitopes from the P. falciparum and P. vivax CS proteins can elicit sterile immunity against blood stage malaria. Hybrid WHcAg-CS VLPs could provide the basis for a bivalent P. falciparum/P. vivax malaria vaccine.

Highlights

Malaria is an important tropical parasitic disease that kills more people than any other communicable disease with the exception of tuberculosis
Using a similar strategy we developed a hybrid VLP carrying P. vivax CS repeat B cell epitopes and immunized mice challenged with P. berghei/P. vivax (Pb/Pv) hybrid sporozoites [17] demonstrated full protection from blood stage malaria
The P. berghei/P. falciparum (Pb/P. falciparum (Pf)) sporozoite technology allows evaluation of efficacy of these candidate vaccines by inserting the Pf B and T cell candidate epitopes in the CS protein of Pb sporozoites

Summary

Introduction

Malaria is an important tropical parasitic disease that kills more people than any other communicable disease with the exception of tuberculosis. P. vivax is the most prevalent species outside of sub-Saharan Africa and responsible for approximately 50% of all malaria cases worldwide [1]. Worldwide prevalence of the disease is estimated to be on the order of 135–287 million clinical cases each year. The P. falciparum malaria parasite has 14 chromosomes, an estimated 5,300 genes (many of which vary extensively between strains) and a complex four-stage life cycle as it passes from a mosquito vector to humans and back again. A vaccine must out perform the immune response to the natural infection. This complexity and the lack of suitable animal models has impeded vaccine development against both P. falciparum and P. vivax

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Conclusion