Direct Observations on the Effect of “Proliferation Inhibitory Factor” on the Clonal Growth of Target Cells

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Abstract
A colony assay system is described in which the division of target cells can be microscopically observed for several days in the presence of proliferation inhibitory factor (PIF). At dilutions of 1:20 and higher PIF suppresses cell division in HeLa target cell cultures without loss of viability. Dilutions of 1:10 and lower have some cytotoxic activity. Both unstimulated and PHA-stimulated leucocyte culture supernatants have PIF activity; however, unstimulated supernatants are not as potent, and dilutions of 1:10 suppress cell division by 50% in the absence of cytotoxicity. The cytotoxic effect can be abrogated by dilution or by adding the PIF to colonies of more than one cell. Loss of target cells from the glass surface measured by visual observation does not account for the inhibition of DNA synthesis. The PIF effect can be reversed by removal of the PIF-containing medium from the target cells and its replacement with fresh or conditioned medium. Although highly effective on HeLa cells, PIF has no effect on cell replication of mouse L cells.

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