IL-21R is expressed on DLBCL cell lines and mediates IL-21–induced activation of STAT1, STAT3, and STAT5. (A) DLBCL cell lines were stained for IL-21R cell-surface expression as described in “Cell-surface receptor staining.” Solid histograms represent staining with biotinylated anti–IL-21R antibody and dashed histograms represent isotype control. (B) RC-K8, OCI-LY-10, MC116, and OCI-LY-3 DLBCL cells were treated with IL-21 (100 ng/mL) for the indicated time periods and STAT activation was measured by immunoblotting with phosphorylation-specific antibodies. Immunoblotting for nonphosphorylated STATs served as loading controls. Results in panels A and B are representative of 3 independent experiments. (C) Densitometric analysis of STAT activation from 3 independent experiments. The values in specimens at time point 0 were arbitrarily defined as 1. Error bars represent SE. *A statistically significant difference (P < .05) between experimental conditions, marked by arrowheads.