Comparative evaluation of the antiproliferative effect of cyclosporin A and gamma-interferon on normal and HPV-transformed keratinocytes by cell counting, MTT assay and tritiated thymidine incorporation.

We compared three techniques, the MTT tetrazolium assay, cell counting, and tritiated thymidine ([3H]TdR) incorporation assay to measure the antiproliferative effect of cyclosporin A (CsA) and interferon-gamma (IFN-gamma) on normal human skin keratinocyte cultures (NHK) used at the second passage and human papilomavirus type 16- and 18-transformed cell lines (EK16 and EK18) exposed continuously to the drugs for 3 days. The three techniques showed that under CsA (0.5 and 8 micrograms/ml) and IFN-gamma (5 and 160 U/ml) treatments the cells remained viable and that the growth of keratinocytes was inhibited. For IFN-gamma, the MTT colorimetric assay consistently underestimated its growth inhibitory activity as compared to cell counting or [3H]TdR incorporation, whatever the cells used. For high doses of CsA, MTT and cell counting gave similar percentages, of inhibitory activity whatever the cells; MTT underestimated this activity as compared to [3H]TdR incorporation only in NHK and EK18 cells, whereas similar results were obtained with EK16 cells. In conclusion, this investigations shows that MTT sensitivity differed with the drug and also according to the keratinocyte cultures. The MTT test is clearly not appropriate for study of IFN-gamma treatment whatever the keratinocytes used. Such discrepancies indicate that the MTT test should be done with care on cultures to measure the effects of drugs on cell growth; the growth inhibition should be carefully considered and it would be best if two different methods were used.