Structural basis of antigenic escape of a malaria vaccine candidate.

Department of Epitope Mapping, Division of Malaria Vaccine Development, Walter Reed Army Institute of Research, Silver Spring, MD 20910, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 08/2007; 104(30):12488-93. DOI: 10.1073/pnas.0701464104
Source: PubMed

ABSTRACT Antibodies against the malaria vaccine candidate apical membrane antigen-1 (AMA-1) can inhibit invasion of merozoites into RBC, but antigenic diversity can compromise vaccine efficacy. We hypothesize that polymorphic sites located within inhibitory epitopes function as antigenic escape residues (AER). By using an in vitro model of antigenic escape, the inhibitory contribution of 24 polymorphic sites of the 3D7 AMA-1 vaccine was determined. An AER cluster of 13 polymorphisms, located within domain 1, had the highest inhibitory contribution. Within this AER cluster, antibodies primarily targeted five polymorphic residues situated on an alpha-helical loop. A second important AER cluster was localized to domain 2. Domain 3 polymorphisms enhanced the inhibitory contribution of the domain 2 AER cluster. Importantly, the AER clusters could be split, such that chimeras containing domain 1 of FVO and domain 2 + 3 of 3D7 generated antisera that showed similarly high level inhibition of the two vaccine strains. Antibodies to this chimeric protein also inhibited unrelated strains of the parasite. Interstrain AER chimeras can be a way to incorporate inhibitory epitopes of two AMA-1 strains into a single protein. The AER clusters map in close proximity to conserved structural elements: the hydrophobic trough and the C-terminal proteolytic processing site. This finding led us to hypothesize that a conserved structural basis of antigenic escape from anti-AMA-1 exists. Genotyping high-impact AER may be useful for classifying AMA-1 strains into inhibition groups and to detect allelic effects of an AMA-1 vaccine in the field.

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    ABSTRACT: Background Polymorphism in antigens is a common mechanism for immune evasion used by many important pathogens, and presents major challenges in vaccine development. In malaria, many key immune targets and vaccine candidates show substantial polymorphism. However, knowledge on antigenic diversity of key antigens, the impact of polymorphism on potential vaccine escape, and how sequence polymorphism relates to antigenic differences is very limited, yet crucial for vaccine development. Plasmodium falciparum apical membrane antigen 1 (AMA1) is an important target of naturally-acquired antibodies in malaria immunity and a leading vaccine candidate. However, AMA1 has extensive allelic diversity with more than 60 polymorphic amino acid residues and more than 200 haplotypes in a single population. Therefore, AMA1 serves as an excellent model to assess antigenic diversity in malaria vaccine antigens and the feasibility of multi-allele vaccine approaches. While most previous research has focused on sequence diversity and antibody responses in laboratory animals, little has been done on the cross-reactivity of human antibodies.Methods We aimed to determine the extent of antigenic diversity of AMA1, defined by reactivity with human antibodies, and to aid the identification of specific alleles for potential inclusion in a multi-allele vaccine. We developed an approach using a multiple-antigen-competition enzyme-linked immunosorbent assay (ELISA) to examine cross-reactivity of naturally-acquired antibodies in Papua New Guinea and Kenya, and related this to differences in AMA1 sequence.ResultsWe found that adults had greater cross-reactivity of antibodies than children, although the patterns of cross-reactivity to alleles were the same. Patterns of antibody cross-reactivity were very similar between populations (Papua New Guinea and Kenya), and over time. Further, our results show that antigenic diversity of AMA1 alleles is surprisingly restricted, despite extensive sequence polymorphism. Our findings suggest that a combination of three different alleles, if selected appropriately, may be sufficient to cover the majority of antigenic diversity in polymorphic AMA1 antigens. Antigenic properties were not strongly related to existing haplotype groupings based on sequence analysis.Conclusions Antigenic diversity of AMA1 is limited and a vaccine including a small number of alleles might be sufficient for coverage against naturally-circulating strains, supporting a multi-allele approach for developing polymorphic antigens as malaria vaccines.
    BMC medicine. 10/2014; 12(1):183.
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    ABSTRACT: After more than 50 years of intensive research and development, only one malaria vaccine candidate, "RTS,S," has progressed to Phase 3 clinical trials. Despite only partial efficacy, this candidate is now forecast to become the first licensed malaria vaccine. Hence, more efficacious second-generation malaria vaccines that can significantly reduce transmission are urgently needed. This review will focus on a major obstacle hindering development of effective malaria vaccines: parasite antigenic diversity. Despite extensive genetic diversity in leading candidate antigens, vaccines have been and continue to be formulated using recombinant antigens representing only one or two strains. These vaccine strains represent only a small fraction of the diversity circulating in natural parasite populations, leading to escape of non-vaccine strains and challenging investigators' abilities to measure strain-specific efficacy in vaccine trials. Novel strategies are needed to overcome antigenic diversity in order for vaccine development to succeed. Many studies have now cataloged the global diversity of leading Plasmodium falciparum and Plasmodium vivax vaccine antigens. In this review, we describe how population genetic approaches can be applied to this rich data source to predict the alleles that best represent antigenic diversity, polymorphisms that contribute to it, and to identify key polymorphisms associated with antigenic escape. We also suggest an approach to summarize the known global diversity of a given antigen to predict antigenic diversity, how to select variants that best represent the strains circulating in natural parasite populations and how to investigate the strain-specific efficacy of vaccine trials. Use of these strategies in the design and monitoring of vaccine trials will not only shed light on the contribution of genetic diversity to the antigenic diversity of malaria, but will also maximize the potential of future malaria vaccine candidates.
    Frontiers in Immunology 01/2014; 5:359.
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    ABSTRACT: Apical membrane antigen 1 (AMA1) is a leading malarial vaccine candidate however its polymorphic nature may limit its success in the field. This study aimed to circumvent AMA1 diversity by dampening the antibody response to the highly polymorphic loop Id, previously identified as a major target of strain-specific, invasion-inhibitory antibodies. To achieve this, five polymorphic residues within this loop were mutated to alanine, glycine or serine on a 3D7 and FVO AMA1 backbone. Initially the corresponding antigens were displayed on the surface of bacteriophage where the alanine and serine but not glycine mutants folded correctly. The alanine and serine AMA1 mutants were expressed in E. coli, refolded in vitro and used to immunize rabbits. Serological analyses indicated that immunization with a single mutated form of 3D7 AMA1 was sufficient to increase the cross-reactive antibody response. Targeting the corresponding residues in an FVO backbone did not achieve this outcome. The inclusion of at least one engineered form of AMA1 in a bi-allelic formulation resulted in an antibody response with broader reactivity against different AMA1 alleles than combining the wild type forms of 3D7 and FVO AMA1 alleles. For one combination, this extended to enhanced relative growth inhibition of a heterologous parasite line, although this was at the cost of reduced overall inhibitory activity. These results suggest that targeted mutagenesis of AMA1 is a promising strategy for overcoming antigenic diversity in AMA1 and reducing the number of variants required to induce an antibody response that protects against a broad range of P. falciparum AMA1 genotypes However, optimization of the immunization regime and mutation strategy will be required for this potential to be realized.
    Infection and immunity. 08/2014;


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