[Granule-accumulation in VERO cells used for determination of antitoxin titration by micro cell culture method: II. Nature of granules and influence on the potency testing of diphtheria toxoid].

Accumulation of granules occurs in VERO cells when mouse serum was added in high concentration. The nature of the granule and influence of the granule-accumulation on the potency testing of diphtheria antitoxin were studied. The concentration of triglyceride in VERO cells increased 15 folds after 24 hours of incubation by the addition of undiluted mouse serum. Kinetical study of the increase of triglyceride and electron microscope observation suggested that the accumulated granules might be oil droplets with an average diameter of 0.27 micron. The end point reaction of neutralization of diphtheria toxin by the standard antitoxin was analyzed by the addition of mouse serum to the micro cell culture using VERO cells. The mean values of the end point reactions in the cell cultures added with high concentrations of mouse serum were -2.324, -2.338 and -2.346, while that of the control group was -2.343. The variance of these mean values in the experiment and control groups were 0.0017, 0.0024, 0.0017 and 0.0021, respectively. A test of homogeneity of variance for these data showed no significant difference between them. Furthermore, antitoxin titer of the serum specimens of mouse immunized with diphtheria toxoids determined by the micro cell culture method were compared with that obtained by rabbit skin test; a high correlation between the antitoxin titers by the two techniques was recognized. The results were comparable with those of guinea-pig sera. From these results it is evident that little influence, if any, was observed in the potency testing of diphtheria antitoxin using VERO cells, in spite of the remarkable accumulation of granules.