The B16 tumor vasculature in CL1-R2–treated mice is mature and functional. All data presented were obtained from mice 16 d after tumor implantation (A, Left) Representative B16 tumor vessels stained with isolectin B4 (brown staining; counterstaining, hematoxylin) from mice treated with CL1-R2 or control Ig in combination with cyclophosphamide. Bar, 50 µm. (A, Right) Histogram showing the number of vessels with an open lumen. Number of fields: four. n = 5 mice per group. *, P = 0.0341 (Student’s t test). (B, Left) CD34-positive endothelial cells (green) are covered by SMA (red) pericyte cells in B16 tumors from CL1-R2–treated mice (bottom) compared with IgG1-treated Ctrl mice (top). Nuclei are labeled blue. Images were taken with confocal fluorescence microscopy. Bar, 20 µm. (B, Right) Quantification of the number of endothelial cells covered by pericytes. The ratio of total area red staining (SMA) to green staining (CD34) was calculated (10 fields per tumor, 5 tumors). Values are means ± SEM. ***, P < 0.0001. (C, Left) Cryosections of B16 tumors from control and CL1-R2–treated mice stained with isolectin B4 (red). Hoechst staining (blue) indicates perfused vessels. Insets show images at higher magnification. Bars, 50 µm. (C, Right) Quantification. n = 5. Number of fields: 4. **, P = 0.0047 (unpaired Student’s t test with Welch’s correction). Control = 24.59 ± 7.086 (n = 5); CL1-R2 = 65.52 ± 1.158 (n = 5). (D, Left) Histopathology of B16 tumors stained with hematoxylin and eosin. Necrosis is indicated by arrowheads. Bar, 200 µm. (D, Right) Quantification of necrotic areas in B16 tumors from CL1-R2–treated and control mice. n = 5 mice/group. Number of fields: 4. **, P = 0.0092 (Student’s t test with Welch’s correction). Images and quantitative data are representative of three independent experiments. The data in A, C, and D are means ± SEM of four different microscope fields for each tumor and a total of five different tumors in each treatment group.