Terheggen Ulrich, Drew Damien R, Hodder Anthony N, Cross Nadia J, Mugyenyi Cleopatra K, Barry Alyssa E, Anders Robin F, Dutta Sheetij, Osier Faith H A, Elliott Salenna R, Senn Nicolas, Stanisic Danielle I, Marsh Kevin, Siba Peter M, Mueller Ivo, Richards Jack S, Beeson James G
The Burnet Institute of Medical Research and Public Health, 85 Commercial Road, Melbourne, Victoria, 3004, Australia.
Department of Medicine, University of Melbourne, Melbourne, Victoria, Australia.
BMC Med. 2014 Oct 16;12:183. doi: 10.1186/s12916-014-0183-5.
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.
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.
We 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.
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.
抗原多态性是许多重要病原体用于免疫逃逸的常见机制，也是疫苗研发面临的主要挑战。在疟疾中，许多关键免疫靶点和候选疫苗都表现出显著的多态性。然而，关于关键抗原的抗原多样性、多态性对潜在疫苗逃逸的影响以及序列多态性与抗原差异之间的关系，我们所知非常有限，但这些对于疫苗研发至关重要。恶性疟原虫顶膜抗原1（AMA1）是疟疾免疫中自然获得性抗体的重要靶点，也是主要的候选疫苗。然而，AMA1具有广泛的等位基因多样性，在单一种群中存在超过60个多态性氨基酸残基和200多种单倍型。因此，AMA1是评估疟疾疫苗抗原中抗原多样性以及多等位基因疫苗方法可行性的理想模型。尽管此前大多数研究都集中在实验室动物的序列多样性和抗体反应上，但关于人类抗体的交叉反应性研究较少。
我们旨在确定AMA1的抗原多样性程度（通过与人类抗体的反应性来定义），并协助鉴定可能纳入多等位基因疫苗的特定等位基因。我们开发了一种使用多抗原竞争酶联免疫吸附测定（ELISA）的方法，以检测巴布亚新几内亚和肯尼亚自然获得性抗体的交叉反应性，并将其与AMA1序列差异相关联。
我们发现，尽管抗体与等位基因的交叉反应模式相同，但成年人的抗体交叉反应性比儿童更强。不同人群（巴布亚新几内亚和肯尼亚）之间以及不同时间的抗体交叉反应模式非常相似。此外，我们的结果表明，尽管AMA1等位基因存在广泛的序列多态性，但其抗原多样性出人意料地受到限制。我们的研究结果表明，如果选择得当，三种不同等位基因的组合可能足以覆盖多态性AMA1抗原中的大部分抗原多样性。基于序列分析，抗原特性与现有的单倍型分组没有很强的相关性。
AMA1的抗原多样性有限，包含少数等位基因的疫苗可能足以覆盖自然传播的菌株，这支持了采用多等位基因方法来开发作为疟疾疫苗的多态性抗原。
