Abstract
Microbial pathogens bind host complement regulatory proteins to evade the immune system. The bacterial pathogen Neisseria meningitidis, or meningococcus, binds several complement regulators, including human Factor H (FH). FH binding protein (FHbp) is a component of two licensed meningococcal vaccines and in mice FHbp elicits antibodies that inhibit binding of FH to FHbp, which defeat the bacterial evasion mechanism. However, humans vaccinated with FHbp develop antibodies that enhance binding of FH to the bacteria, which could limit the effectiveness of the vaccines. In the present study, we show that two vaccine-elicited antibody fragments (Fabs) isolated from different human subjects increase binding of complement FH to meningococcal FHbp by ELISA. The two Fabs have different effects on the kinetics of FH binding to immobilized FHbp as measured by surface plasmon resonance. The 1.7- and 2.0-Å resolution X-ray crystal structures of the Fabs in complexes with FHbp illustrate that the two Fabs bind to similar epitopes on the amino-terminal domain of FHbp, adjacent to the FH binding site. Superposition models of ternary complexes of each Fab with FHbp and FH show that there is likely minimal contact between the Fabs and FH. Collectively, the structures reveal that the Fabs enhance binding of FH to FHbp by altering the conformations and mobilities of two loops adjacent to the FH binding site of FHbp. In addition, the 1.5 Å-resolution structure of one of the isolated Fabs defines the structural rearrangements associated with binding to FHbp. The FH-enhancing human Fabs, which are mirrored in the human polyclonal antibody responses, have important implications for tuning the effectiveness of FHbp-based vaccines.
Highlights
Like many microbial pathogens, the bacterium Neisseria meningitidis, which is known as meningococcus, uses surface proteins to recruit soluble complement inhibitors
Factor H (FH) binding to the opsonin C3b promotes decay of the alternative pathway (AP) C3 convertase, C3bBb, by competing for the Bb binding site, a process known as decay acceleration activity [3,4]
We tested commercially produced lots of two human anti-FH binding protein (FHbp) Fabs and two control mousehuman chimeric Fabs for concentration-dependent binding to purified FHbp by enzymelinked immunosorbent assay (ELISA)
Summary
The bacterium Neisseria meningitidis, which is known as meningococcus, uses surface proteins to recruit soluble complement inhibitors. One of these inhibitors is Factor H (FH), which down-regulates complement activation on the bacterial surface [1,2]. FH serves as a cofactor for Factor I to cleave active C3b into inactive iC3b, which prevents the formation of the AP C3 convertase [5,6] Together, these FH functions effectively block the amplification loop of the AP and limit the generation of C3b, which binds and activates the classical pathway C3 convertase, C4bC2b [7]. FHbp was developed as a key antigen in two licensed meningococcal serogroup B vaccines [17,18]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
