Cell cycle activation by virion-shed and virion-associated IL2-SU chimeras. Go/G1-arrested J3-13 cells were incubated for 24 hours in media conditioned with either rIL-2 (recombinant IL-2; bottom right panel) or with viral supernatants containing retroviruses coincorporating IL2-SU and wild-type amphotropic envelope glycoproteins (top left panel). Viral particles were removed by ultracentrifugation before the addition of supernatants to cultures of Go/G1 arrested J3-13 cells (top right panel). Cells were stained with propidium iodide to assess the DNA content of the treated cells. Before stimulation, 80% of J3-13 cells were in the Go/G1 phase of the cell cycle (bottom left panel).