Sequential Proliferation Induced in Human Peripheral Blood Lymphocytes by Mitogens

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Abstract
The addition of PHA-stimulated human lymphocytes inhibits the continued proliferation of autologous lymphocytes growing in a feeder-layer system in response to mitogen. Velocity sedimentation and 3HTdR autoradiographic studies of the suppressive population indicated that the effect is mediated by proliferating, large cells (“blasts”). Other populations of actively dividing cells, including normal lymphocytes stimulated with pokeweed mitogen or the calcium ionophore A23187, the Raji B cell line, and human and mouse fibroblasts, suppressed 3HTdR uptake as well, in a similar dose range. The suppressive effect of these cells could be roughly correlated with the proportion of blasts in the cell population. Inhibitory activity in this system was not demonstrated in supernatants from PHA blasts compared to saline controls, but intact cells suppressed across a nucleopore membrane but not a dialysis membrane. Supplementing the assay culture with low m.w. constituents of the medium did not decrease the suppressive effect. However, when fresh serum was added in increasing concentrations with the inhibitory cells, their suppressive effect was partially abrogated. These results may be interpreted in at least two ways that are not mutually exclusive: i) PHA blasts may elaborate a labile inhibitory substance that may be bound, inactivated, or degraded to some degree by serum and/or ii) actively proliferating cells may exhaust an essential high m.w. nutrient in the assay culture. It is likely that the inhibitory effects observed in many culture systems involving mitogen-induced suppressor lymphocytes are not restricted to immunologic phenomena but may include a more general function of actively dividing cells, which is neither tissue nor species specific.

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