Characterization of the human immune response after vaccination with Bexsero through structural and functional studies of human Fabs from a longitudinal memory B-cell repertoire

The multi-component vaccine 4CMenB (Bexsero) is the first recombinant vaccine for the prevention of invasive meningococcal disease licensed in over 35 countries. The meningococcal factor H binding protein (fHbp), one of three main protein antigens in 4CMenB, specifically binds human factor H (hfH) and thereby down-regulates the human complement cascade. Over 1000 distinct amino acid sequences of fHbp have been identified and can be classified in three variants which are immunologically distinct. Within the fHbp variant groups the sequence identity is usually above 87%, while between variant groups the sequence identity can be as low as 62%. This high antigenic variability presumably underlies the apparent rarity of cross-reactive antibodies. We cloned and expressed a library of 110 anti-fHbp human monoclonal antibodies as Fabs (huFab), isolated from adults immunized with 4CMenB. We analyzed their antigen binding specificity and affinity, and their ability to inhibit binding of hfH to live meningococci. Interestingly, although 4CMenB vaccine contains only fHbp variant 1.1, thirteen of the 110 huFabs were found to be cross-reactive for fHbp variants 2 and 3. All 13 cross-reactive mAbs showed to be able to kill the bacterium when tested in a serum bactericidal assay using baby rabbit serum as complement source (rSBA); only those 3 mAbs competing with hfH binding showed bactericidial activity also with human complement (hSBA). The crystal structure of the fHbp/huFab 4B3 complex was determined and the identification of key antigen/antibody interactions was performed by structural analyses, obtaining atomic insights into the epitope recognized by this huFab. A detailed analysis of the interface revealed high conservation of key epitope residues some of which lie within the hFH binding site, which explains both the ability of huFab 4B3 to cross-react with different fHbp variants and its ability to inhibit hfH binding, thus explaining its unparalleled potency. In conclusion, we have obtained the first detailed characterization of cross-protective huMabs elicited by 4CMenB and in particular we gained structural data on huFab 4B3, which is able to compete for hFh binding to fHbp. These structural and functional data suggest that the hfH binding site on fHbp can be accessible to the human immune system upon immunization, enabling the generation of a highly potent antibody response