Inhibition of human leukocyte mitosis by prednisolone in vitro.

The water-soluble glucocorticoid, prednisolone ~l-phosphatc, has been shown to inhibit mitotic activity of normal human leukocytes in short-term culture, with a roughly linear relationship between the log dose of steroid and the percentage decrease in mitosis. Over a concentration range of .002 ug/ml to 10 pg/ml, the mitotic index decreased to 25 per cent of control values. Further studies indicate that most of this inhibitory effect of the steroid operates during the first ~4 hours in tissue culture, although the effect is not demonstrable until the 2d and 3d day, when mitosis begins. The findings indicate that prednisolone does not significantly inhibit mitosis directly, in these cultures, but rather inhibits the conversion of partially differentiated circulating leukocytes to a state capable of mitosis. This action occurs at the same time as that of phytohemagglutinin, which initiates mitosis in these cultures by stimulating the conversion process.