Induction of DNA synthesis in cells incubated with IL-2–displaying retroviruses. 3H-thymidine incorporation was measured in four different IL-2–dependent cell types: F7, Bclp75, HT-2, and W4E9 cells. Cells, which were arrested in Go/G1 by overnight deprivation of IL-2, were either incubated in 96-well plates for 24 hours in media conditioned with 50 U of recombinant IL-2 (rIL-2), 70 μL of viral supernatant containing nonenveloped retroviruses (−), 70 μL of viral supernatant containing retroviruses coated with amphotropic envelopes (A), or 70 μL of viral supernatant containing retroviruses coated with IL-2 chimeric envelopes (IL2-SU). The levels of 3H-thymidine incorporation are expressed as percentages relative to 3H-thymidine incorporation in rIL-2–stimulated cells. Experiments were performed in triplicate, and the means ± standard deviations are shown.
