IL-21 receptor α-chain is expressed by endothelial cells. (A) FACS analysis of human umbilical vein-derived endothelial cells (HUVECs) stained for IL-21-R α-chain (white), conjugate control is represented by gray histogram. (B) SVEC4–10 stained for IL-21-Rα. (C) mRNA expression of IL-21-Rα mouse endothelial cell lines SVEC and TME in primary (pr) human (h) and mouse (m) endothelial cells, and freshly isolated human tumor (TEC) and normal endothelial cells (NECs).³⁹  (D) mRNA expression of IL-2-Rα in primary human, primary mouse, and freshly isolated human tumor and normal endothelial cells. Positive control is spleen tissue; negative control is no template in the PCR reaction (H₂O).