Department of Molecular Microbiology, Washington University School of Medicine, Saint Louis, Missouri, United States of America.
PLoS Pathog. 2013;9(5):e1003390. doi: 10.1371/journal.ppat.1003390. Epub 2013 May 23.
Disrupting erythrocyte invasion by Plasmodium falciparum is an attractive approach to combat malaria. P. falciparum EBA-175 (PfEBA-175) engages the host receptor Glycophorin A (GpA) during invasion and is a leading vaccine candidate. Antibodies that recognize PfEBA-175 can prevent parasite growth, although not all antibodies are inhibitory. Here, using x-ray crystallography, small-angle x-ray scattering and functional studies, we report the structural basis and mechanism for inhibition by two PfEBA-175 antibodies. Structures of each antibody in complex with the PfEBA-175 receptor binding domain reveal that the most potent inhibitory antibody, R217, engages critical GpA binding residues and the proposed dimer interface of PfEBA-175. A second weakly inhibitory antibody, R218, binds to an asparagine-rich surface loop. We show that the epitopes identified by structural studies are critical for antibody binding. Together, the structural and mapping studies reveal distinct mechanisms of action, with R217 directly preventing receptor binding while R218 allows for receptor binding. Using a direct receptor binding assay we show R217 directly blocks GpA engagement while R218 does not. Our studies elaborate on the complex interaction between PfEBA-175 and GpA and highlight new approaches to targeting the molecular mechanism of P. falciparum invasion of erythrocytes. The results suggest studies aiming to improve the efficacy of blood-stage vaccines, either by selecting single or combining multiple parasite antigens, should assess the antibody response to defined inhibitory epitopes as well as the response to the whole protein antigen. Finally, this work demonstrates the importance of identifying inhibitory-epitopes and avoiding decoy-epitopes in antibody-based therapies, vaccines and diagnostics.
破坏恶性疟原虫（Plasmodium falciparum）对红细胞的入侵是一种有吸引力的抗疟疾方法。恶性疟原虫 EBA-175（PfEBA-175）在入侵过程中与宿主受体糖蛋白 A（GpA）结合，是一种领先的疫苗候选物。识别 PfEBA-175 的抗体可以阻止寄生虫的生长，但并非所有抗体都是抑制性的。在这里，我们使用 X 射线晶体学、小角度 X 射线散射和功能研究，报告了两种 PfEBA-175 抗体抑制的结构基础和机制。两种抗体与 PfEBA-175 受体结合域复合物的结构揭示了最有效的抑制性抗体 R217 与关键的 GpA 结合残基和 PfEBA-175 的拟议二聚体界面结合。第二种弱抑制性抗体 R218 结合到富含天冬酰胺的表面环。我们表明，结构研究中确定的表位对于抗体结合至关重要。总的来说，结构和映射研究揭示了不同的作用机制，R217 直接阻止受体结合，而 R218 允许受体结合。我们使用直接受体结合测定法表明，R217 直接阻断 GpA 结合，而 R218 则不会。我们的研究详细阐述了 PfEBA-175 与 GpA 之间的复杂相互作用，并强调了针对恶性疟原虫入侵红细胞的分子机制的新方法。研究结果表明，旨在通过选择单个或组合多个寄生虫抗原来提高血液阶段疫苗的功效的研究，应该评估针对定义的抑制性表位的抗体反应以及针对整个蛋白质抗原的反应。最后，这项工作证明了在抗体为基础的治疗、疫苗和诊断学中，识别抑制性表位和避免诱饵表位的重要性。
