IL-21 inhibits angiogenic sprouting of endothelial cells. (A) A mouse aortic ring treated with bFGF (10 ng/mL) and bFGF and IL-21 200 ng/mL. (B) Quantification of sprouting of primary mouse endothelial cells from isolated aortic rings in a three-dimensional collagen matrix. (C) Sprouting of bFGF (10 ng/mL) stimulated SVEC4-10 cells (mouse endothelial cell line) grown on beads embedded in a three-dimensional collagen matrix. (D) An example of sprouting of SVEC4-10 cells treated with bFGF (10 ng/mL, top panel) and bFGF and IL-21 (bottom panel). Data are relative mean values of at least 3 experiments (***P < .0005, *P < .05, compared with control).