Novel Image-Based Titration Method for Potency Testing of Diphtheria Toxoid in a Vero Cell Assay

The potency of diphtheria toxoid is determined by titrating neutralizing antibody levels in immunized mice using the Vero cell assay. The WHO manual recommends titration by determining the cytotoxic endpoint through measuring cell metabolism and observing cell morphology under a microscope, which has been performed in our laboratory; however, the former is time-consuming and the latter subjective. To address these limitations, we have developed a novel endpoint determination method based on cell image analysis. We assessed the feasibility of this image analysis and compared it with microscopic observation and cell metabolism measurement. Consequently, image analysis proved effective for endpoint determination, yielding potency values comparable to existing methods in measuring the potencies of commercial vaccines. These findings suggest that image analysis, being objective and convenient, can serve as a reliable alternative method for potency testing of diphtheria vaccines.