Mechanisms responsible for differential bactericidal activities of human and rabbit complement for Neisseria meningitidis

Abstract

Polysaccharide conjugate vaccines are available to prevent disease caused by Neisseria meningitidis serogroups A, C, W and Y. Meningococcal vaccine efficacy is assessed in clinical trials using serum bactericidal assays (SBAs). Baby rabbit serum is usually used as the source of complement for SBAs as it lacks endogenous bactericidal activity compared to human serum. Previous studies have shown that SBA activities determined with rabbit (rSBA) and human (hSBA) complement correlate poorly, possibly due to different interactions between antibody subclasses and the complement source, and species-specific interaction of Neisseria meningitidis with complement regulators. The aim of this project was to investigate the mechanisms responsible for differential bactericidal activities of human and rabbit complement against Neisseria meningitidis. The serum concentration of polysaccharide-specific antibody subclasses was measured following vaccination with quadrivalent meningococcal polysaccharide vaccines; data showed that the concentration of polysaccharide-specific IgG1 antibody correlated most significantly with hSBA titres whereas the concentration of polysaccharide-specific IgM antibody correlated most significantly with rSBA titres. The interaction of human IgM and IgG subclasses with human and rabbit complement was compared at the level of C1q and C3 using both binding and functional assays. These data define important differences in the ability of human antibody subclasses to fix human and rabbit complement. Specifically, polysaccharide-specific IgM contributes significantly more to bactericidal titres in rSBAs compared to hSBAs. As a consequence, rSBAs produce misleadingly high titres in individuals with large IgM responses to vaccination. Using a series of pathway-specific inhibitors, it was shown that the alternative pathway contributes significantly more to the bactericidal activity of rabbit complement towards Neisseria meningitidis than human complement. This project provides significant insight into the difficulties and challenges associated with the interpretation of rSBA data, enhances the understanding of antibody responses to meningococcal vaccines and will support improvements in the development and testing of meningococcal vaccines.