Recombinant interleukin 2 inhibits pokeweed mitogen-induced proliferation of human adult peripheral blood and cord blood mononuclear cells.

We studied the effects of recombinant interleukin 2 (rIL-2) on pokeweed mitogen (PWM)-induced proliferation of unfractionated human mononuclear cells from the peripheral blood (PBMCs) and cord blood (CBMCs). rIL-2 was found to inhibit PWM-induced proliferation of both PBMCs and CBMCs in a dose-dependent manner. The response was statistically significant at concentration 10 U/ml. The inhibition of adult PBMC proliferation was relatively mild, but in newborns up to 75% inhibition was found. The inhibitory effect of rIL-2 was best detectable at day 4, at the same time as PWM-induced proliferation peaked. However, also at days 8 and 10, the response induced by PWM plus rIL-2 was much lower than that induced by rIL-2 alone. These finding suggest that the inhibition was due to rIL-2-mediated stimulation of the function of PWM-activated suppressor cells. CD8+ T cells were, however, not alone responsible for the rIL-2-induced inhibition of PWM-induced proliferation, since the responses were essentially similar independent of whether CD8+ T cells were present or absent. In addition, the release of interferon-gamma (IFN-gamma) in response to rIL-2 was not a major cause of the inhibitory effects, because rIL-2 inhibited PWM-induced proliferation of CBMCs, which are deficient in producing IFN-gamma. Our results provide novel data to the ongoing discussion of IL-2 as a down-regulator of immune functions.