Memory B cells exhibit greater sensitivity to the differentiation-inducing effects of STAT3-activating cytokines IL-21 and IL-10. (A and B) Naive and memory B cells were sort-purified from normal donor spleens and then cultured with CD40L alone or with increasing concentrations of IL-21 (A) or IL-10 (B). After 4 d, expression of PRDM1 and XBP1 was determined by qPCR. The values are presented as the percentage of the maximum response, defined as the levels of expression induced in memory B cells by the highest dose of IL-21 tested (50 ng/ml). The data represent the mean ± SEM of experiments using naive and memory B cells from three different normal donor spleens.