Mutations in STAT3 or STAT1 partially reduce IL-10– and IL-21–mediated proliferation and survival of CD40L-activated B cells. (A and B) Naive B cells were sort purified from healthy controls, STAT3_MUT patients, or a STAT1_MUT patient; labeled with CFSE; and cultured for 5 d with CD40L alone or together with IL-4, IL-10, or IL-21. (A) CFSE profiles of control and STAT3_MUT naive B cells stimulated under the indicated conditions. (B) Frequency of B cells in each division; the values for the control and STAT3_MUT B cells represent means ± SEM from five independent experiments. Data for STAT1_MUT B cells are from a single experiment. (C and D) Naive B cells from healthy controls or STAT3_MUT patients were cultured for 3 d with CD40L alone or together with IL-10 or IL-21. (C) The percentage of apoptotic cells was determined by annexin V staining. The values represent the mean percentage of annexin V⁺ cells. (D) The number of viable cells recovered from each culture was also determined. (E and F) Expression of IL-10R (red; E) and IL-21R (red; F) on naive B cells was determined by flow cytometry. The overlays (blue) represent isotype control mAb. The values are means ± SEM (n = 4) of the fluorescence intensity of cells labeled with the isotype control or anti–IL-10R or –IL-21R mAb.