Development and Use of a Serum Bactericidal Assay Using Pooled Human Complement To Assess Responses to a Meningococcal Group A Conjugate Vaccine in African Toddlers

Ameningococcal group A polysaccharide (PS) conjugate vaccine (PsA-TT) has been developed for African countries affected by epidemic meningitis caused byNeisseria meningitidis. Complement-mediated serum bactericidal antibody (SBA) assays are used to assess protective immune responses to meningococcal vaccination. Human complement (hC=) was used in early studies demonstrating antibody-mediated protection against disease, but it is difficult to obtain and standardize. We developed and evaluated a method for sourcing hC = and then used the SBA assay with hC = (hSBA) to measure bactericidal responses to PsA-TT vaccination in 12- to 23-month-old African children. Sera with active complement from 100 unvaccinated blood donors were tested for intrinsic bactericidal activity, SBA titer using rabbit complement (rSBA), and anti-group A PS antibody concentration. Performance criteria and pooling strategies were examined and then verified by comparisons of three independently prepared hC = lots in two laboratories. hSBA titers of clinical trial sera were then determined using this complement sourcing method. Two different functional antibody tests were necessary for screening hC=. hSBA titers determined using three independent lots of pooled hC = were within expected assay variation among lots and between laboratories. In African toddlers, PsA-TT elicited higher hSBA titers thanmeningococcal polysaccharide or Hib vaccines. PsA-TT immunization or PS challenge of PsA-TT-primed subjects resulted in vigorous hSBAmemory responses, and titers persisted in boosted groups for over a year. Quantify