The vascular phenotype of α2-null mice is dependent on α2-null endothelial cells within the wild-type tumor microenvironment. (A) Tumor volume stimulated from B16F10 melanoma cells mixed at a ratio of 5:1 with wild-type or α2-null primary pulmonary endothelial cells in wild-type mice as a function of time is shown. The data are presented as the mean plus or minus SEM of 5 animals per endothelial cell genotype repeated 3 times (P = .009, statistical analysis by ANOVA). Therefore, tumors intermixed with α2-null endothelial cells grew faster at all time points than those intermixed with wild-type endothelial cells. (B) Immunofluorescence analysis of CD31⁺ tumor vessels (green) within the tumors of wild-type mice receiving α2-null endothelial cells (KO-ECs) or wild-type endothelial cells (WT-ECs). Vessels formed by α2-null endothelial cells in wild-type mice were larger than vessels formed by wild-type endothelial cells in wild-type mice (20×/0.75 NA objective). (C) Immunofluorescence analysis of tumor vessels in wild-type mice receiving α2-null endothelial cells. Tumor vessels were highlighted with anti-CD31 (green) and those cells that expressed the α2β1 integrin were stained with the anti-α2β1 integrin antibody (red). The tumor vessels were a mixture of wild-type (CD31⁺ and α2β1 integrin–positive [yellow]) and α2-null endothelial cells (CD31⁺ and α2β1 integrin–negative [green]; 20×/0.75 NA objective).