Incorporation of monocytic EPCs into vascular lumen in vivo in a tumor neovascularization model. (a) Representative tumor sections stained for mouse CD31 (red) and human CD31 (green), from mice with transplants of CT-26 cells alone or of CT-26 cells and monocytic EPCs from an SSc patient or a healthy control. Nuclei were counterstained with TO-PRO3 (blue). Arrows indicate blood vessels. Scale bars = 200 μm. (b) Representative tumor sections stained for mouse CD31 (red) and human CD31 or HLA class I (green) from mice with transplants of CT-26 cells and monocytic EPCs from a healthy control. Negative controls were sections incubated with isotype-matched mouse or rat mAb to an irrelevant antigen, instead of the primary antibody. Nuclei were counterstained with TO-PRO3 (blue). Asterisks indicate blood vessel lumen, while arrows indicate transplanted human monocytic EPCs located at the vascular wall. Scale bars = 50 μm. (c) The vasculogenic potency of monocytic EPCs was assessed by determining the proportion of vascular lumens carrying human CD31+ endothelial cells in tumors arising from CT-26 cells co-transplanted with monocytic EPCs (104 or 105) from SSc patients or healthy controls.