Restimulation of PPD-stimulated lymphocytes: decreased responsiveness of prestimulated cells to a second challenge with PPD and PHA.

Lymphocytes stimulated with purified protein derivative (PPD) were inhibited in their response to a second stimulation with PPD or phytohaemagglutinin (PHA). The degree of inhibition was related to the PPD concentration during prestimulation, the dose-response curve for inhibition resembling very much that of stimulation. The decreased reactivity was neither due to a toxic effect of PPD nor to altered proliferation kinetics of the prestimulated cells. Lymphocytes preincubated for 6 h or 16 days with PPD were equally refractory, and the non-reactivity persisted even if the cells were incubated without stimulant for 1 week or more. The prestimulated cells were able to suppress the PHA stimulation of fresh lymphocytes. These results indicate that the decreased reactivity of the prestimulated lymphocytes is due to the action of suppressor lymphocytes, generated during the primary stimulation.