Inhibition of tumor cell line alkaline phosphatase by human peripheral blood monocytes. Correlation with growth inhibitory activity

Alkaline phosphatase (AP) activity in intact Ag8 cells was measured by a colorimetric assay in which spectrophotometer readings were obtained directly through wells of microtiter plates using a Multiskan ELISA reader. The wide difference between enzymatic activity observed in cells of this cell line and human peripheral blood monocytes permitted an assessment of inhibition of cell line growth in the presence of plastic-adherent peripheral blood cells. Comparison at 24 h of incubation of enzymatic measurements with those obtained using 3H-thymidine uptake showed comparable results, although values obtained with the enzyme were generally lower. Similarly, direct comparison of results obtained by the enzyme-based method showed good correlation with the number of viable tumor cells remaining in culture. Inhibition of AP activity was observed earlier than inhibition of 3H-thymidine incorporation, reaching its peak after 10 h of incubation. This technique has potential application for other enzyme systems and provides a new tool for analyzing mechanisms whereby effector cells are able to control proliferation of transformed cells.