Interaction of serum and colony‐stimulating factor for survival of a factor‐dependent hemopoietic progenitor cell line

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Abstract
The growth, in vitro of the murine myeloid cell line FDC‐P1 depends on the presenc of serum and a murine hemopoietic growth factor (either granulocyte/ macrophage colony‐stimulating factor (GM‐CSF) or multipotential colony‐stimu‐lating factor (multi‐CSF, IL3)). To determine the differential roles of serum and colony‐stimulating factor (CSF) during the growth of FDC‐P1 cultures, we investigated the kinetics of proliferation and death after withdrawal of serum or CSF, using flow cytometry to quantitate the numbers of vital and dead cells. After withdrawal of CSF, the cells died without entering a quiescent state. The life span of cultures lacking CSF increased with increasing concentrations of serum (> 50 h at 30% serum), and the cells kept dividing until they died. During the period of population death caused by the absence of CSF, the re‐addition of CSF immediately prevented further cells from dying. After the withdrawal of serum in the presence of CSF, the cells continued to live and proliferate for weeks, but required high cell densities (>> 105/ml), which suggests that the cells produced an active substance that can substitute for serum. Serum as well as serum‐free conditioned medium from dense cultures made the survival and growth of FDC‐P1 cultures independent of cell density. Without sufficient quantities of this activity, all cells of the population died within an interval that was much shorter than one cell cycle, which indicates that the factor acts throughout most of the cell cycle. The results suggest that both the CSF and the serum factor act together to permit cell survival, rather than to drive proliferation.