Molecular characterization of the interaction between sialylated Neisseria gonorrhoeae and factor H.

Human factor H (HufH), a key inhibitor of the alternative pathway of complement, binds to Neisseria gonorrhoeae and constitutes an important mechanism of human-specific complement evasion. The C-terminal domain 20 of HufH contains the binding site for sialylated gonococci. We exploited differences in amino acid sequences between human and non-binding chimpanzee fH domain 20 to create cross-species mutations to define amino acids important for binding to sialylated gonococci. We used fH/Fc fusion constructs that contained contiguous fH domains 18-20 fused to Fc fragments of murine IgG2a. The Fc region was used both as a tag for detection of each fusion molecule on the bacterial surface and as an indicator for complement-dependent killing. Arg-1203 was critical for binding to both porin (Por) B.1A and PorB.1B strains. Modeling of the R1203N human-to-chimpanzee mutation using the crystal structure of HufH19-20 as a template showed a loss of positive charge that protrudes at the C terminus of domain 20. We tested the functional importance of Arg-1203 by incubating sialylated gonococci with normal human serum, in the presence of wild-type HufH18-20/Fc or its R1203A mutant. Gonococci bound and were killed by wild-type HufH18-20/Fc but not by the R1203A mutant. A recombinant fH/Fc molecule that contained chimpanzee domain 20, humanized only at amino acid 1203 (N1203R) also bound to sialylated gonococci and restored killing. These findings provide further insights into the species specificity of gonococcal infections and proof-of-concept of a novel therapeutic approach against gonorrhea, a disease rapidly becoming resistant to conventional antibiotics.