Assessment Of Avidity Of Igg Antibodies Against Meningococcal Serogroup A Polysaccharide And Its Correlation With Pre-existing Data On Bactericidal Activity In Meningitis Patients And Controls From Ethiopia

Studies in the “meningitis belt” countries such as Burkina Faso, Gambia, Ethiopia and Sudanrnshowed the presence of high antibody levels against serogroup A capsular polysaccharide (APS)rnof Neisseria meningitidis. The antibody concentrations usually exceeded the suggested protectivernantibody level (≥ 2μg/ml of total Ig against APS). In spite of this, the populations in this area arernaffected by recurring epidemics of meningococcal meningitis. Therefore, to determine the level ofrnanti-APS IgG that correlates with protection in the “meningitis belt” area, it may be appropriate tornconsider the quality of the antibodies along with quantity. Though quality of antibodies might bernaffected by several factors, this study considered the avidity of antibodies against APS, which isrnbelieved to better correlate with serum bactericidal activity (SBA). Two modified ELISArnmethods using the chaotropic agent ammonium thiocyanate (NH4SCN) that disrupts weakrnantibody binding were employed to measure avidity indices (AI) of anti-APS IgG. In one of thernmethods a constant concentration of SCN- and varying serum dilutions, and in the other, a singlernserum dilution with varying SCN- concentrations were used. 114 acute and convalescent serarnfrom Ethiopian meningitis patients and controls were analysed. High correlation was observedrnbetween the avidity indices calculated from the two methods (r=0.8). When assaying patient sera,rnanti-APS IgG concentrations correlated with SBA using baby rabbit sera as a complement sourcern(rSBA) both before and after elution with 120mM SCN- (r=0.64 and r=0.65, p

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