IL-33 induces vessel sprouting ex vivo and angiogenesis in vivo. (A) Aortic segments were harvested from C57BL/6 mice. Aortic segments in Matrigel were treated with IL-33 (100 ng/mL) or VEGF (50 ng/mL) for 2 weeks (n = 5 per group). (B) Sprouting were classified from 0 (least positive) to 5 (most positive) as described in “Aortic ring assay.” (C-F) C57BL/6 mice were injected with 0.6 mL of Matrigel containing IL-33 (n = 6 per group). After 6 days, the mice were killed, and the Matrigel plugs were excised. (C) Representative Matrigel plugs were photographed. (D) Quantification of neovessel formation by measuring hemoglobin in the Matrigel. (E) Plugs were stained for infiltrating ECs by the use of anti-CD31 antibody. (F) Quantitative assessment of CD31⁺ ECs. Data are means ± SDs; *P < .05, **P < .01 vs untreated control.