Suppressor cells induced by purified protein derivative of tuberculin (PPD): the suppression is mediated by cells that proliferate in response to stimulation with PPD.

Lymphocytes stimulated with purified protein derivative of tuberculin (PPD) were found to inhibit the PPD stimulation of fresh, autologous lymphocytes. This suppressor effect was exerted after preincubation with both high and low concentrations of PPD. Optimal suppression occurred after preincubation with PPD in concentrations of 5 micrograms/ml and higher, the same concentrations that gave optimal stimulation of DNA synthetsis in primary cultures. The suppressor effect was abolished completely by 'hot pulse' treatment and partly by treatment with colchicine during PPD preincubation, showing that the PPD-induced suppressr cells are generated by cell division. When fresh lymphocytes were incubated together with PPD-pretreated cells in cultures that were not stimulated with PPD, the PPD-stimulated lymphocytes exerted a stimulatory effect on the fresh lymphocytes. This effect was maximal for cells preincubated for 1 h with PPD, decreasing with increasing duration of preincubation with PPD. Possible explanations of this observation are discussed.