IL-21–induced CD25 expression enables naïve B cells to respond to IL-2 with enhanced plasmablast generation and Ig secretion. Naïve B cells were sorted from normal spleens and then cultured with CD40L alone in the presence or absence of IL-2 and/or IL-21. The proportion of plasmablasts (CD27^hiCD38^hi) (A,C) and expression of CD25 (B) were determined by flow cytometry after 5 days. The data depicted are representative of 4 (A) or 3 (B) separate experiments using naïve B cells isolated from different donor spleens. C represents the mean ± SEM from 4 separate experiments; *P < .05, comparing cultures with and without IL-2. Secretion of IgM and IgG (D) or IgG subclasses (E) was determined after 10 to 12 days by enzyme-linked immunosorbent assay. Statistical analysis using 2-way ANOVA with Bonferroni post-test analysis confirmed a statistically significant difference between cultures with or without IL-2 (P < .005 for both IgM and IgG) and between different concentrations of IL-21 (P < .005 for IgM; P < .001 for IgG). Data in D show mean ± SEM from 3 independent experiments on 3 different donor spleens, each performed in triplicate. Data in E show mean ± SEM from a single experiment performed in triplicate but is representative of 2 independent experiments performed on different normal donor spleens. (F) Naïve and memory B cells were isolated from a normal donor and a CD25-deficient patient and then cultured with CD40L/IL-21 alone or together with IL-2. Secretion of IgM and IgG was determined after 10 days. Values represent the mean ± SEM of triplicate cultures for normal B cells and the mean of single cultures for CD25-deficient B cells. (G) Carboxyfluorescein diacetate succinimidyl ester profiles of splenic naïve B cells from normal donors cultured for 5 days with CD40L alone or with varying concentrations of IL-21 in the absence or presence of IL-2. Results are representative of 4 independent experiments using different normal donor spleens.