Target-agnostic identification of human antibodies toPlasmodium falciparumsexual forms reveals cross stage recognition of glutamate-rich repeats

Circulating sexual stages ofPlasmodium 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 sexual stages ofPfin the form of gamete and gametocyte extract. We isolated mAbs reactive against a range ofPfproteins including well-established targets Pfs48/45 and Pfs230. One mAb, 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 ofPfproteins, 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 thePlasmodiumparasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response againstPf.