Serum bactericidal antibody assays - The role of complement in infection and immunity

Complement is an essential component of the immune system and human pathogenic organisms have developed various mechanisms for evading complement mediated serum killing. The “gold standard” for measuring the ability of vaccine-induced antibody to kill Neisseria meningitidis is the serum bactericidal antibody (SBA) assay which measures complement mediated killing via antibody. This assay requires active complement, either intrinsic from the serum being tested or the addition of exogenous complement, either from a human or from another species such as rabbit. For serogroup C, an SBA titre of ≥4 was established as the correlate of protection when using human complement and ≥8 as the threshold when using rabbit complement, based on comparative assay results. Licensure of meningococcal vaccines, including polysaccharide protein conjugate vaccines and serogroup B vaccines has been based on the immune responses measured with the SBA assay, thus on a surrogate of vaccine efficacy. This review examines the use of complement and the SBA assay to assess immunity to meningococcal infection, and provides examples of vaccine trials in different age groups where various assays have been used.