Significant non-S-phase DNA synthesis visualized by flow cytometry in activated and in malignant human lymphoid cells

The development of a monoclonal antibody to the deoxynucleoside bromodeoxyuridine (BrdU), combined with two parameter flow cytometry, has allowed us to examine large numbers of cells for non-S-phase DNA synthesis. Three human lymphoid cell populations were studied to determine the level of deoxynucleoside (dN) incorporation as a function of DNA content. In each population, non-S-phase DNA synthesis was observed. In a rapidly growing human T-lymphoblastoid cell line (CCRF-CEM), 53% of dN incorporation occurred in G0/G1 plus G2 + M. In chronic lymphocytic leukemia (CLL) cells stimulated with tetradecanoylphorbol acetate (TPA), 45% of the observed burst in thymidine incorporation was found to be localized to G0/G1 cells. Non-S-phase incorporation was not, however, limited to neoplastic cells. Normal human peripheral blood B cells treated with the Cowan strain of Staphylococcus aureus (CSA) undergo a transient burst in thymidine incorporation, but do not go on to divide in the absence of other stimuli. Flow-cytometric analysis showed that 80% of this CSA-stimulated dN incorporation was into G0/G1 cells. These data are consistent with a more dynamic state of DNA synthesis than usually envisioned. Furthermore, the data show that although thymidine incorporation levels are related to incorporation of dN into DNA, they can be unrelated to cell proliferation.