Endothelial Progenitor Cell Marker CD133 Identifies Tumor Endothelium

CD133, a pentaspan surface antigen, is considered a marker of tissue-specific stem cells, including hematopoietic stem cells and endothelial progenitors. We and others have previously shown that CD133 expression is high on endothelial progenitor cells; that it is downregulated during their differentiation and that it is not found on mature endothelium. In addition, CD133⁺ cells in brain tumors were shown to exhibit features of cancer stem cells. Recent report showed that CD133⁺ tumor cells are located in perivascular niches in astrocytomas and glioblastomas. It was also demonstrated that in glioblastomas, CD133⁺ cancer stem cells are tightly associated with tumor vasculature and that signals from endothelial cells are the key factors in self-renewal and proliferation of cancer stem cells. However, the contribution of CD133⁺ endothelial progenitor cells to tumor angiogenesis remains unknown. We sought to investigate the role of CD133⁺ cells in human brain tumors and their relation to endothelial progenitor cells and to the mature endothelium. To this end we examined tumor samples resected from patients with glioblastomas. Histological and immunohistochemical analyses revealed that CD133⁺ cells in these tumors are predominantly endothelial cells as demonstrated by co-staining with CD31 and CD133. We further analyzed a population of endothelial cells using multicolor flow cytometry analysis and we demonstrated that on average 60% of endothelial cells, as defined by the expression of CD31 and VE-cadherin, also express CD133. However, we also found significant heterogeneity between glioblastomas obtained from different patients. Additionally, CD133 was not expressed on CD45⁻CD31⁺ subpopulation in meningiomas. Taken together, we demonstrated that high percentage of endothelial cells in glioblastomas expresses CD133, while CD133 could also be found on other cell types within the tumor. Furthermore, our data suggest that tumor vasculature might be enriched with bone marrow derived hemangiogenic progenitors.