Relationship of cloning inhibition factor, "lymphotoxin" factor, and proliferation inhibition factor release in vitro by mitogen-activated human lymphoctes.

Human lymphocytes were stimulated in vitro with mitogens, phytohemagglutinin and concanavalin A to secrete proliferation inhibitory factor (PIF), cloning inhibitory factor (CIF), and lymphotoxin (LT). These three activities were demonstrable in the same supernatant, moreover, the particular effect observed was shown to depend on the concentration of the medium and the type of target cell employed. In general, the medium effects on target cells were: a) cytotoxic at high concentrations, b) growth inhibitory at intermediate concentrations, and c) only temporarily growth inhibitory at low concentrations. The absolute concentration producing a certain effect varied, depending on the target cell type employed. In addition, the sensitivity of the target cells to LT parallels the sensitivity of the cell to each of the other activities, PIF and CIF, and no species specificity was observed.