Monocyte-induced cell cycle synchronization of a leukemic cell line in vitro

Human monocytes cultured in vitro exert cytostatic activity against neoplastic cell lines. The cytostatic effect was examined on the nonadherent human leukemic cell line K-562. After 24 hours of co-culture between monocytes and K-562 cells (in a ratio of 10: 1), the K-562 cells were removed and their DNA-synthesis and DNA-content were examined by methyl-3H-thymidine incorporation and flow cytofluorometry. Cell proliferation curves were obtained. A partial and reversible cell cycle block in the G0/G1-phase was observed. After removal of the target cells from the monocyte monolayers, the target cells regained their normal proliferation rate during the following 1 to 3 days, with a maximal number of cells in S-phase at 7 to 9 hours after separation of target cells from the monocytes. The most marked effect was induced by monocytes cultured in vitro for 12 days, and with monocytes stimulated with lymphokine, i.e. supernatants from BCG-exposed lymphocyte cultures.