Epitope and Paratope Mapping by HDX-MS Combined with SPR Elucidates the Difference in Bactericidal Activity of Two Anti-NadA Monoclonal Antibodies

Characterization
of antigen–antibody interactions is crucial
for understanding antibody-mediated protection against pathogens,
biopharmaceutical development, as well as evaluation of the immune
response post vaccination. Bexsero is a multicomponent vaccine against Neisseria meningitidis serogroup B in which one of the key
vaccine antigens is Neisserial adhesin A (NadA), a trimeric coiled-coil
protein. Two NadA-specific monoclonal antibodies (mAbs) isolated from
Bexsero-vaccinated individuals have been shown to have similar binding
affinity and appear to recognize a similar antigen region, yet only
one of the mAbs is bactericidal. In this study, we use hydrogen/deuterium
exchange mass spectrometry (HDX-MS) to perform an in-depth study of
the interaction of the two mAbs with NadA antigen using a combined
epitope and paratope mapping strategy. In addition, we use surface
plasmon resonance (SPR) to investigate the stoichiometry of the binding
of the two mAbs to NadA. While epitope mapping only identifies a clear
binding impact of one of the mAbs on NadA, the paratope mapping analyses
shows that both mAbs are binding to NadA through several complementarity
determining regions spanning both heavy and light chains. Our results
highlight the advantage of combined epitope and paratope mapping HDX-MS
experiments and supporting biochemical experiments to characterize
antigen–antibody interactions. Through this combined approach,
we provide a rationale for how the binding stoichiometry of the two
mAbs to the trimeric NadA antigen can explain the difference in bactericidal
activity of the two mAbs.