Thymosin-induced suppression of proliferative response of human lymphocytes to mitogens

The proliferative response of human peripheral blood lymphocytes to phytohemagglutinin, concanavalin A, and pokeweed mitogen were suppressed by thymosin. Greatest decreases were observed when cells were preincubated with thymosin for 18 h before a 3-d culture with mitogen in the presence of thymosin. However, significant suppression also occurred when lymphocytes were preincubated for 2 h and cultured with thymosin or preincubated for either 2 or 18 h and washed free of thymosin before culture. These effects were related to the concentration of thymosin and time of exposure to thymosin but not merely to a delay in the response to mitogen or to toxicity. The suppression of mitogen-induced proliferation by thymosin appeared to result from effects of thymosin on a suppressor cell because lymphocytes incubated with thymosin did not acquire increased responsiveness to mitogens as did cells incubated for 18 h in its absence and because mixing thymosin-pretreated lymphocytes with cells not preincubated with thymosin resulted in decreased responsiveness to photohemagglutinin.