Envelope binding assays to IL-2 receptor-expressing target cells. Kit225 cells were used as an IL-2 receptor-expressing target cell. The background fluorescence was determined by incubating cells with supernatant from nontransfected TELCeB6 packaging cells (white area). Binding assays were performed with viruses pseudotyped with the wild-type amphotropic envelope, A, the IL2-SU, or the IL2-SUX chimeric envelopes. Target cells were either treated (broken line) with recombinant IL-2 (100 ng for 30 minutes at 37°C) or not treated (black area) before binding assays with the various virions.