Heterodimerization of the cytoplasmic domain of β_IL3 and _IL3 induces mitogenesis in CTLL-2 cells. (A) Flow cytometric analysis confirming expression of the hCD8 chimeric receptors. Thin lines represent results with cells treated with FITC-coupled secondary antibody alone; thick lines represent results with cells treated with CD8 and FITC-coupled secondary antibody. (B) Immunoprecipitation of surface biotinylated proteins confirming expression of both CD8/ and CD8/β in doubly transfected cells as described in the legend to Fig 4B. Lane 1, parental CTLL-2; lane 2, CTLL-2 transfected with CD8/β_IL-3; lane 3, CTLL-2 transfected with CD8/_IL-3; lane 4, CTLL-2 transfected with CD8/_IL-3 and CD8/β_IL-3. (C) [³H]-thymidine incorporation of transfectants in the absence of IL-2. Cells were cultured with or without IL-2 for 40 hours and then incubated for an additional 8 hours in the presence of [³H]-thymidine. Data are plotted as the percent of maximal incorporation observed in IL-2. Error bars represent the SEM of triplicate samples.