Regulation of IL-21, but not CXCR5, expression by the IL-6/STAT3 axis. (A,B) T cells were stimulated with anti-CD3/anti-CD28 mAbs in the presence or absence of IL-6, rested 1 day, and restimulated for 6 hours in the presence of plastic-coated anti-CD3 mAbs. T cells were tested for FACS profile expression of CD40L, ICOS, and OX40 (A) and mRNA expression of selected genes by quantitative RT-PCR (B). Gray histograms in panel A represent isotype control-stained cells. (C) Control and T cell–specific STAT3^−/− mice were immunized in foot pads with KLH emulsified in CFA. Expression of CXCR5⁺ T_FH cells was analyzed on day 7. Numbers in dot-plot quadrants represent the percentages. (D) CXCR5⁺CD4⁺ T_FH cells were purified from control and T cell–specific STAT3^−/− mice and tested for T-cell proliferation in response to anti-CD3 mAbs. (E) Aliquots of cells in panel D were irradiated and tested for B-cell help function in the presence of WT B cells and anti-CD3 mAbs. (F) WT, IL-6–, STAT6-, and STAT3-deficient CD62L⁺CD4⁺ T cells were stimulated 48 hours in the presence or absence of recombinant IL-6. IL-21 detection in culture supernatants was performed by ELISA. Results represent mean plus or minus SD of triplicate culture. Similar results were obtained in 2 additional independent experiments.