The use of differential staining of sister chromatid to estimate the in vitro effect of human alpha interferon on cell division in normal and tumour cells.

Concentration of 10, 100 and 1000 I.U./ml of human leukocyte alpha interferon (IFN) were added into peripheral human blood (PBL) cultures and in KB cell cultures in the presence of 5-bromdeoxyuridine (BrdU) at 10 micrograms/ml. After 72 hours the differential staining of sister chromatid (harlequin) technique was applied in order to differentiate among the metaphases of successive cell generations occurring in the presence of IFN. The frequency of the first (M1), second (M2) and third (M3) metaphases was recorded and the replication index (RI) as well as the average generation time (AGT) was calculated for untreated controls and for each of the IFN concentrations used, both in the blood cultures and in the KB cells. In the PBL cultures a clear dose-related inhibitory effect of IFN on cell division was observed, the RI values being lessened whereas the AGT concomitantly increased by increasing the IFN concentrations. An increase in M1 metaphase frequency was observed concomitantly with a diminished number of M3 cells. In KB cells the division kinetics was not influenced by IFN as indicated by similar RI and AGT values observed in controls and in IFN treated cells. However, the frequencies of both M1 and M3 cells were slightly diminished concomitantly with a discrete augmentation of M2 cell number. The differential staining of sister chromatid thus proved a highly useful technique to investigate the different sensitivity of the normal and malignant cells to the growth inhibitory effect induced by alpha IFN in vitro.