An Antibody Screen of a Plasmodium vivax Antigen Library Identifies Novel Merozoite Proteins Associated with Clinical Protection

Abstract

Background

Elimination of Plasmodium vivax malaria would be greatly facilitated by the development of an effective vaccine. A comprehensive and systematic characterization of antibodies to P. vivax antigens in exposed populations is useful in guiding rational vaccine design.

Methodology/Principal Findings

In this study, we investigated antibodies to a large library of P. vivax entire ectodomain merozoite proteins in 2 Asia-Pacific populations, analysing the relationship of antibody levels with markers of current and cumulative malaria exposure, and socioeconomic and clinical indicators. 29 antigenic targets of natural immunity were identified. Of these, 12 highly-immunogenic proteins were strongly associated with age and thus cumulative lifetime exposure in Solomon Islanders ( P<0.001–0.027). A subset of 6 proteins, selected on the basis of immunogenicity and expression levels, were used to examine antibody levels in plasma samples from a population of young Papua New Guinean children with well-characterized individual differences in exposure. This analysis identified a strong association between reduced risk of clinical disease and antibody levels to P12, P41, and a novel hypothetical protein that has not previously been studied, PVX_081550 (IRR 0.46–0.74; P<0.001–0.041).

Conclusion/Significance

These data emphasize the benefits of an unbiased screening approach in identifying novel vaccine candidate antigens. Functional studies are now required to establish whether PVX_081550 is a key component of the naturally-acquired protective immune response, a biomarker of immune status, or both.

Author Summary

Plasmodium vivax is now the predominant malaria parasite outside Africa. Because P. vivax can remain dormant in the liver for months, identifying and treating P. vivax in asymptomatic individuals is difficult. Additionally, current widely-used vector control measures are less efficient against mosquitoes that transmit P. vivax. An effective vaccine would therefore immensely facilitate P. vivax elimination. Unfortunately, little is known about P. vivax biology and only a few proteins have been investigated as targets for vaccine development. To address these knowledge gaps, we measured antibody levels to 34 entire ectodomain proteins predicted to be involved in P. vivax invasion of erythrocytes, in samples from individuals living in 2 malaria-endemic Asia-Pacific countries. We found that antibodies in malaria-exposed Solomon Islanders were reactive to the majority of proteins in our panel, and that antibodies to 12 of these proteins strongly reflected cumulative life-time exposure to P. vivax. In samples from Papua New Guinea children, we identified an association between antibodies to 3 proteins and protection against clinical malaria. Our results demonstrate that screening antibodies to a large number of P. vivax proteins is a useful approach in identifying novel targets of immunity. Functional studies are now required to establish whether these proteins are biomarkers of an individual’s immune status, potential vaccine candidates that warrant further development, or both.