Source and biological role of IL-3 in the interaction of T cells with PDC. (a) Mature PDC were cocultured with CD4+ CD45RO+ T cells for 24 hr in the presence of SEB, brefeldin A was added for the last 7 hr, and IL-3 was detected by intracellular flow cytometry. Inset numbers indicate the percentage of positive cells. (b) Naive PDC were cocultured with CD4+ CD45RO+ T cells and SEB in the absence or presence of blocking mAb directed to IL-3, the IL-3 receptor, or CD40. PDC survival was determined after 3 days, the medium control was set to 100%. (c,d) Naive PDC were cultured in medium alone or in the presence of IL-3. After 48 hr, the survival rate of PDC (c) and the expression of CD80, CD86, ICOS-L and MHC class-II on PDC cultured in the presence of IL-3 (d) were determined. One representative experiment out of three is shown.