Initial upregulation of CD25 by IL-21 licenses IL-2 to maintain plasmablast generation in the absence of IL-21. Naive B cells from normal human spleens were initially cultured with CD40L together with IL-21. After 3 days, cells were harvested, washed, and recultured either in media alone or with IL-2. (A) After a further 3 days of culture, the proportion of plasmablasts (CD27^hiCD38^hi) was determined by flow cytometry. Each symbol corresponds to an individual experiment that used naïve B cells from a different normal donor spleen; the horizontal line represents the mean. (B) Secretion of IgM, IgG, and IgA was determined after 7 days of secondary culture in media alone or with IL-2 and Ig secretion was determined by enzyme-linked immunosorbent assay. Results are expressed as fold-change in Ig secretion relative to the “no cytokine” secondary culture (set to equal 1.0). The results for IgM represent mean ± SEM (n = 3); IgG secretion was detected in only 1 of 3 experiments; IgA was detected in 2 of 3 experiments.