Abstract
Neisseria meningitidis serogroup B (MenB) is the leading cause of meningococcal meningitis and sepsis in industrialized countries, with the highest incidence in infants and adolescents. Two recombinant protein vaccines that protect against MenB are now available (i.e. 4CMenB and MenB-fHbp). Both vaccines contain the Factor H Binding Protein (fHbp) antigen, which can bind the Human Factor H (fH), the main negative regulator of the alternative complement pathway, thus enabling bacterial survival in the blood. fHbp is present in meningococcal strains as three main variants which are immunologically distinct. Here we sought to obtain detailed information about the epitopes targeted by anti-fHbp antibodies induced by immunization with the 4CMenB multicomponent vaccine. Thirteen anti-fHbp human monoclonal antibodies (mAbs) were identified in a library of over 100 antibody fragments (Fabs) obtained from three healthy adult volunteers immunized with 4CMenB. Herein, the key cross-reactive mAbs were further characterized for antigen binding affinity, complement-mediated serum bactericidal activity (SBA) and the ability to inhibit binding of fH to live bacteria. For the first time, we identified a subset of anti-fHbp mAbs able to elicit human SBA against strains with all three variants and able to compete with human fH for fHbp binding. We present the crystal structure of fHbp v1.1 complexed with human antibody 4B3. The structure, combined with mutagenesis and binding studies, revealed the critical cross-reactive epitope. The structure also provided the molecular basis of competition for fH binding. These data suggest that the fH binding site on fHbp v1.1 can be accessible to the human immune system upon immunization, enabling elicitation of human mAbs broadly protective against MenB. The novel structural, biochemical and functional data are of great significance because the human vaccine-elicited mAbs are the first reported to inhibit the binding of fH to fHbp, and are bactericidal with human complement. Our studies provide molecular insights into the human immune response to the 4CMenB meningococcal vaccine and fuel the rationale for combined structural, immunological and functional studies when seeking deeper understanding of the mechanisms of action of human vaccines.