Heterodimerization of the cytoplasmic domain of β_IL-3 and _IL-3 is required to induce mitogenesis in Ba/F3 cells. (A) Flow cytometric analyses confirming expression of the CD8/IL-3R chimeric receptors in Ba/F3 cells. Thin lines represent controls treated with FITC-coupled secondary antibody alone; thick lines represent results with cells stained with CD8 and FITC-coupled secondary antibody. The bottom righthand panel represents the CD8 signal in cells expressing both CD8/_IL-3 and CD8/β_IL-3. (B) Immunoprecipitation of biotinylated cell-surface molecules to confirm the expression of CD8/_IL-3 and CD8/β_IL-3 in doubly transfected cells. Lane 1, control Ba/F3; lane 2, Ba/F3 transfected with CD8/β_IL-3; lane 3, Ba/F3 transfected with CD8/_IL-3; lane 4, Ba/F3 transfected with CD8/_IL-3 and CD8/β_IL-3. (C) [³H]-thymidine incorporation by transfectants incubated in the absence of IL-3. Cells were cultured with or without IL-3 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-3. Error bars represent the SEM of triplicate samples.