FIG 4.

Neutralization of ACT cytotoxicity by serial dilutions of convalescent human serum. (A) The concentration of ACT to be used for the toxin neutralization assay was chosen based on cytotoxicity of ACT toward J774 cells. ACT was added to 30,000 J774 cells grown overnight on a 96-well plate. After 2 h, cytotoxicity (percent viable cells) was measured by a CCK-8 assay. The arrow indicates the concentration of ACT (80 ng/ml) chosen for use in the toxin neutralization assay. Data represent the mean and SD of the results from 3 experiments performed in triplicate. (B) ACT was mixed with serum at the indicated dilution and added to J774 cells as described in Materials and Methods. The final dilution was 1:x, where x is the value on the x axis. Percent toxin neutralization was calculated as described in Materials and Methods. The reference standard for human convalescent anti-pertussis serum was provided by NIBSC (89/530). Data represent the mean and SD of the results from 2 experiments performed in triplicate.