Induction of phosphorylation of STAT proteins in stimulated CB B cells. (A) CB B cells were cultured in the absence or presence of CD40L with or without IL-4, IL-21, or both cytokines. After 16 hours, the cells were harvested, fixed, and permeabilized, and expression of phospho-STAT1, 3, 5, and 6 determined by immunofluorescence and flow cytometry. Solid histograms represent expression of phosphorylated STAT1, 3, 5, or 6 in the presence of the indicated stimulus; the dashed outline histograms represents expression of phosphorylated STATs in cells cultured in the absence of any exogenous stimulus. (B) Coexpression of STAT3 and STAT6 was determined for CB B cells stimulated with CD40L alone or together with IL-4, IL-21, or IL-4 and IL-21. The values represent the percentage of cells in each quadrant of the contour plot.