Effect of IFN- pretreatment on IL-2–induced T-cell proliferation. T lymphocytes were activated with anti-CD3-antibodies and expanded in the presence of IL-2, after which IL-2–containing medium was removed. The cells were then left untreated or treated with 100 IU/mL of IFN- for 24 hours. The cells were collected, and an equal number of (□) untreated or (▪) IFN-–primed T cells was applied in microtiter plates. Different doses of IL-2 were added for 18 hours, followed by further incubation of 6 hours in the presence of 1 μCi/well of ³H-labeled thymidine. After harvesting the cells, the proliferation index was determined as described in Materials and Methods. The mean proliferation index (±SD) of six individual donors is shown.