Abstract
Affinity maturation selects B cells expressing somatically mutated antibody variants with improved antigen-binding properties to protect from invading pathogens.We determined the molecular mechanism underlying the clonal selection and affinity maturation of human B cells expressing protective antibodies against the circumsporozoite protein of the malaria parasite Plasmodium falciparum (PfCSP).We show in molecular detail that the repetitive nature of PfCSP facilitates direct homotypic interactions between two PfCSP repeat-bound monoclonal antibodies, thereby improving antigen affinity and B cell activation. These data provide a mechanistic explanation for the strong selection of somatic mutations that mediate homotypic antibody interactions after repeated parasite exposure in humans. Our findings demonstrate a different mode of antigen-mediated affinity maturation to improve antibody responses to PfCSP and presumably other repetitive antigens.
Highlights
S porozoites of the human malaria parasite Plasmodium falciparum (Pf) express a surface protein, circumsporozoite protein (PfCSP), with an immunodominant central NANP (Asn-Ala-Asn-Pro) repeat region [1,2,3]
Antigen binding was abrogated when the original Ig Vk1-5 light chain was replaced by Vk2-28 or when the native Ig heavy chains were paired with a Vk1-5 light chain with 9–amino acid–long KCDR3 (Fig. 1B), demonstrating the importance of these specific Ig features in antigen recognition
heavy-chain CDR2 (HCDR2) W52→S (H.W52_S) and H.W52_R mutants of the selected antibodies, as well as an H.W52_A mutant of antibody 2140 and a double mutant (H.V50_F_W52_R) to mimic the IGHV330*02 and IGHV3-30-5*02 alleles, all showed reduced PfCSP repeat reactivity associated with reduced in vitro parasite inhibitory activity (Fig. 1, C and D; single-letter amino acid abbreviations are defined in the legend to Fig. 1)
Summary
1210_YY IgG, with its restricted ability to engage in homotypic antibody interactions, showed a lower binding avidity to fulllength PfCSP than 1210 Affinity maturation selects for mutations that improve homotypic antibody interactions, thereby indirectly increasing PfCSP NANP binding. BCR signaling was delayed in cells expressing 1210_GL compared with that in cells expressing 1210. This effect was even more pronounced in 1210_YY mutant cells. 1210_H.V50_Imut (1210 with HCDR2 V50→I), with high repeat affinity, mediated stronger signals than 1210, especially with low antigen concentrations, whereas 1210_NS showed no significant differences (Fig. 3D). Despite a 2-log difference in NANP3 affinities (Fig. 1, G and H) and the varied potential of these antibodies to engage in homotypic interactions, all showed similar capacities to inhibit Pf sporozoites in vitro
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