The Supporting Information files were omitted from the article. The legends and links to files are available below. Figure S1. The expression of CD3 and IL-21 in consecutive serial sections of spleen. Immunohistochemical staining of consecutive serial sections showing CD3⁺ and IL-21⁺ cells in spleens of MRL/lpr mice (A) and B6 mice (B). Further magnification (under 400 x magnification) of the black-bordered box (under 200 x magnification) shows the typical of CD3⁺ and IL-21⁺lymphocytes.

Figure S2. Neutralization of IL-21 reduces autoantibody production and Tfh cells in vivo. MRL/lpr mice were treated with an anti-IL-21-neutralizing antibody (100 µg/mouse/treatment) once per week for 4 weeks via intraperitoneal injection. (A) The renal score of MRL/lpr mice was analyzed (n=6 for each group). (B) Serum level of ANA in MRL/lpr mice was analyzed by ELISA (n=6 for each group). (C) Serum level of ds-DNA in MRL/lpr mice was analyzed by ELISA (n=6 for each group). (D) The percentages of CXCR5⁺PD-1⁺ cells among CD4⁺ T cells was analyzed by flow cytometry in MRL/lpr (n=6 for each group). Figure S3. IL-21 intracellular expression in sorted Tfh cells. Sorted CD4⁺CXCR5⁺PD-1⁺ Tfh cells from MRL/lpr and B6 mice were cultured in the presence of anti-CD3 and anti-CD28 for 2 days, and stimulated with PMA+ionomycin+brefeldin A (PIB) for the last 5 hours, IL-21 intracellular expression in CD4⁺ cells was detected by flow cytometry. Results shown are representative of at least three independent experiments. Figure S4. The production of IL-21 by Tfh cells from different weeks of age of MRL/lpr mice. Sorted CD4⁺CXCR5⁺PD-1⁺ Tfh cells from 5 and 20 weeks of age of MRL/lpr mice were cultured in the presence of anti-CD3 and anti-CD28 for 2 days, detection of IL-21 in the supernatants by ELISA. Results shown are representative of at least three independent experiments. Figure S5. The expression of CD19 and IL-10 in consecutive serial sections of spleen. Immunohistochemical staining of consecutive serial sections showing CD19⁺ and IL-10⁺ cells in spleens of MRL/lpr mice (A) and B6 mice (B). Further magnification (under 400 x magnification) of the black-bordered box (under 200 x magnification) shows the typical of CD19⁺ and IL-10⁺lymphocytes. Figure S6. IL-21 promotes the differentiation of B10 cells. (A) Naïve B cells sorted from B6 mice were cultured in the presence of LPS with or without 20 ng/ml IL-21 for the indicated times and stimulated with PIB for the final 5 hours. IL-10 mRNA expression was detected by real-time RT-PCR. Results shown are representative of at least three independent experiments. (B) Naïve B cells sorted from B6 mice were cultured in the presence of LPS (L) and the indicated concentrations of IL-21 for 48 hours followed by stimulation with PI for the final 5 hours. IL-10 in supernatants was detected by ELISA. Results shown are representative of at least three independent experiments. (C) Naïve B cells sorted from B6 mice were cultured in the presence of LPS with or without 20 ng/ml IL-21 for 24 hours. Cells were stimulated with PIB for the last 5 hours. IL-10⁺ cells were analyzed in a CD19 gate by flow cytometry. Results shown are representative of at least three independent experiments. Figure S7. IL-21 induces the activation of STAT3. Naïve B cells sorted from B6 mice were cultured with or without 20 ng/ml IL-21 or AG490 for the indicated times, and the relative levels of p-STAT3 (A) and STAT3 (B) proteins were determined by Western blot. The relative p-STAT3 and STAT3 protein expression were showed as ration to β-actin and to calculate fold-induction relative to vehicle control. The data showed the quantitation of the blot bands with averages of 3 experiments.