Are the opsonophagocytic activities of antibodies in infant sera measured by different pneumococcal phagocytosis assays comparable?

Host protection against Streptococcus pneumoniae is mainly mediated by opso nin-dependent phagocytosis. Several techniques for measuring opsonophagocyt ic activity (OPA) of antibodies to S. pneumoniae have been standardized and used. These include the viable cell-assay, flow-cytometric assays, and an assay utilizing radiolabeled bacteria. Using these different methods, we me asured the OPA of antibodies to S, pneumoniae types 6B and 19F from the ser a of infants immunized with a pneumococcal conjugate vaccine, PncCRM. Gener ally, the results obtained by the various techniques correlated well, altho ugh serotype-specific differences were found (6B, r = 0.78 to 0.95, P < 0.0 01; 19F, r = 0.50 to 0.84, P < 0.001). The same serotype-specific differenc es were observed for the relationship between the concentrations of specifi c immunoglobulin G antibodies measured by enzyme immunoassay and the OPA, S ince the sensitivities of the OPA assays differed, the most prominent discr epancies between the techniques were found at low antibody concentrations.