Relationship of CIF, LT, and PIF released in vitro by activated human lymphocytes. II. A further functional comparison of LT and PIF activities on HeLa and L-929 target cells.

Lymphotoxin (LT), and proliferation inhibition factor (PIF) activities found in 5-day supernatants of mitogen-activated human lymphocytes (SAL) were further compared. In agreement with previous results, the activities could not be distinguished functionally. Quantitative differences in the amount of activity detected in the SAL could be accounted for on the basis of target cell differences, concentration of the lymphocyte effector molecules in the supernatant, and the parameter employed to assess cell function. Growth inhibitory activity detected at high supernatant dilutions was completely reversible, whereas the cytotoxic activity detected at low supernatant dilutions was irreversible. When the active medium was fractionated on DEAE, two peaks of inhibitory activity were detected. Depending upon the amount of activity and target cell, both peaks of activity were growth inhibitory or cytotoxic. Since both peaks of material affected HeLa and L-929 cells, the materials were not species specific. Thus, it appears that cloning inhibitor factor, LT, and PIF activities may actually be measures of the same stable materials found in 5-day activated lymphocyte supernatants.