The effect of IL-21 on IL-21 receptor (R) of B cells and B cell activation in patients with rheumatoid arthritis (RA). (a) IL-21R expression on B cells was substantially upregulated in RA patients (n = 45) compared to healthy controls (HC) (n = 38). (b) Peripheral blood mononuclear cells (PBMC) from either RA patients (n = 14) or HC (n = 11) were cultured with or without rIL-21 (100 ng/ml), and IL-21R expression on B cells was analyzed after 48 h. (c) Activation marker expressions on B cells were significantly upregulated in RA patients (n = 20) compared to HC (n = 15). (d) PBMC isolated from RA patients (n = 10) and HC (n = 10) were stimulated with rIL-21 (100 ng/ml) or rIL-21 in combination with anti-IL-21R antibody (10 ug/ml), and activation marker expressions on B cells were examined after 72 h. (e) Microarray analysis of cytokines in serum from RA patients (n = 6) and HC (n = 4). **P < 0.01; *P < 0.05; n.s., not significant. PBS, phosphate-buffered saline.