Complement-Mediated Bactericidal Activity of Human Antibodies to Poly  2->8 N-Acetylneuraminic Acid, the Capsular Polysaccharide of Neisseria meningitidis Serogroup B

Serum antibodies to Neisseria meningitidis group B (MenB) polysaccharide are reported not to elicit bacteriolysis in the presence of human complement. To reexamine this question, we evaluated the ability of two human IgM anti-MenB polysaccharide monoclonal antibodies (MAbs) and seven human MenB polysaccharide-reactive human IgM paraproteins to elicit bacteriolysis. In the presence of human complement, both MAbs and five of the seven paraproteins were bactericidal at antibody concentrations of 0.25-9.6 micrograms/mL (50% killing). Activity of the respective antibodies was enhanced 200- to > 10,000-fold when rabbit complement was used instead of human complement. With rabbit complement, the bactericidal activity of human IgM polyclonal antibody or MAb to Haemophilus influenzae type b (Hib) polysaccharide but not human IgG polyclonal antibody or MAb to Hib polysaccharide was similarly augmented. Thus, for both MenB and Hib, IgM antipolysaccharide antibodies elicit complement-mediated bactericidal activity in the presence of human complement, and the use of rabbit complement yields spuriously high activity.