IL-21, but not IL-10, induces CD25 expression on all activated naïve B cells. (A) Naïve B cells from healthy donors were cultured for 5 days with CD40L alone (blue histogram) or in the presence of IL-10 (orange histogram) or IL-21 (red histogram). Expression of CD25 was determined after 5 days. (B-D) Normal naïve B cells were cultured for 5 days with CD40L alone or together with IL-21 for 5 days. (B) Differential expression of CD27 and CD38 delineates IL-21–induced plasmablasts (CD38^hiCD27^hi; red gate) and nonplasmablasts (CD38^loCD27^lo; blue gate). Expression of CD25 (C) or DNAM-1 (D) was determined on all B cells in cultures stimulated with CD40L alone or on nonplasmablasts and plasmablasts present in cultures of CD40L/IL-21–stimulated B cells. Gray histograms represent isotype controls. Numbers represent percentage of live cells within each gate.