Proliferative signals for suppressor T cells. Helper cells stimulated with pokeweed mitogen in vitro produce a suppressor cell growth factor

To define molecular signals elaborated by inducer populations supporting growth or differentiation of T8⁺ cells, we collected supernatants of pokeweed mitogen (PWM)-stimulated cultures depleted of T8⁺ cells. When added to purified T8⁺ cells, these supernatants caused significant proliferation. PWM plus interleukin 2 (IL-2) in amounts equivalent to those in the supernatant could not reconstitute the response caused by the supernatant. T8⁺ cells activated by supernatants obtained from PWM-pulsed T4⁺ cells suppressed fresh PWM cultures. Although exhibiting little proliferation, T8⁺ cells cultured for 6 d in PWM plus IL-2 still suppressed a fresh PWM response. The supernatants therefore contain an additional T suppressor cell growth factor (TsGF). Elaboration of TsGF required radiosensitive T4⁺Leu8⁺ cells. Molecular weight determination by high performance liquid chromatography gave a single peak of TsGF activity at ~8,000. Finally, whereas TsGF in the absence of IL-2 could not support the proliferation of T suppressor cells, it did cause T8⁺ cells to become strongly IL-2 receptor-positive.