Human Factor H and anti-Neisserial surface protein A (NspA) antibodies compete for overlapping binding sites on meningococcal NspA.

Neisserial surface protein A (NspA) is a small, conserved outer membrane protein that has been investigated as a vaccine antigen against meningococcal disease. After NspA had been tested in humans, this antigen was discovered to recruit the human complement regulator Factor H (FH). Previous studies in transgenic mice showed that human FH decreased the protective antibody responses to NspA. The purpose of the present study was to map the binding sites for human FH and anti-NspA antibodies. We found that an anti-NspA monoclonal antibody (mAb), AL-12, inhibits binding of FH to NspA by enzyme-linked immunosorbent assay (ELISA). Based on this result, we tested the roles of the 10 charged residues on the external loops of NspA in binding these two molecules by site-specific mutagenesis and binding experiments. Through ELISA and surface plasmon resonance experiments, we show that three aspartate (D) residues, D77 on loop 2 and D113 and D118 on loop 3, are important for binding human FH. Further, residues D113 and D118, as well as lysine 79 and arginine 109, are involved in binding mAb AL-12, which binds to a conformational epitope. The results have implications for strategies to increase NspA immunogenicity by decreasing binding to human FH, as has been done with other antigens that recruit this complement regulator.