Abstract
Circulating sexual stages of Plasmodium falciparum (Pf) can be transmitted from humans to mosquitoes, thereby furthering the spread of malaria in the population. It is well established that antibodies (Abs) can efficiently block parasite transmission. In search for naturally acquired Ab targets on sexual stages, we established an efficient method for target-agnostic single B cell activation followed by high-throughput selection of human monoclonal antibodies (mAbs) reactive to natural sexual stages of Pf in the form of gamete and gametocyte extract. We isolated mAbs reactive against a range of Pf proteins including well-established targets Pfs48/45 and Pfs230. One of these mAbs, B1E11K, was cross-reactive to various proteins containing glutamate-rich repetitive elements expressed at different stages of the parasite life cycle. A crystal structure of two B1E11K Fab domains in complex with its main antigen, RESA, expressed on asexual blood stages, showed binding of B1E11K to a repeating epitope motif in a head-to-head conformation engaging in affinity-matured homotypic interactions. Thus, this mode of recognition of Pf proteins, previously described only for PfCSP, extends to other repeats expressed across various stages. The findings augment our understanding of immune-pathogen interactions to repeating elements of the Plasmodium parasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response against Pf .
Summary
Proteins with amino acid repeats enriched in glutamate residues are prominently expressed in the asexual and sexual stages of Plasmodium falciparum ( Pf ), the main causative agent of malaria. Here, we present a target-agnostic approach for the isolation of sexual stage antibodies (Abs), leading to the identification of B1E11K, an Ab cross-reactive to glutamate-rich repeats in proteins present in various life cycle stages of Pf . The Ab binds in a head-to-head conformation, leveraging affinity-matured homotypic interactions – an uncommon characteristic of Ab somatic hypermutation that results in the optimization of Ab-Ab interactions in the context of binding their repetitive epitope. Our data provides further credence that repetitive elements of Pf can significantly modulate the humoral response, and that antibody recognition through homotypic interactions is occurring throughout the Pf life cycle.
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