Blood vessels expressing CD90 in human and rat brain tumors

Blood vessels in brain tumors, particularly glioblastomas, have been shown to express CD90. CD90+ cells in and around blood vessels in cancers including brain tumors have been identified as endothelial cells, cancer stem cells, fibroblasts or pericytes. In this study, we aimed to determine the nature or type(s) of cells that express CD90 in human brain tumors as well as an experimental rat glioma model by double immunofluorescence staining. The majority of CD90+ cells in human glioblastoma tissue expressed CD31, CD34 and von Willebrand factor, suggesting that they were endothelial cells. Vasculatures in a metastatic brain tumor and meningioma also expressed CD90. CD90+ cells often formed glomeruloid structures, typical of angiogenesis in malignant tumors, not only in glioblastoma but also in metastatic tumors. Some cells in the middle and outer layers of the vasculatures expressed CD90. Similar results were obtained in the rat glioma model. There were cells expressing both α‐smooth muscle actin and CD90 in the middle layer of blood vessels, indicating that smooth muscle cells and/or pericytes may express CD90. CD90+ vasculatures were surrounded by tumor‐associated macrophages (TAMs). Thus, in addition to endothelial cells, some other types of cells, such as smooth muscle cells, pericytes and fibroblasts constituting the vasculature walls in brain tumors expressed CD90. Because CD90 has been shown to interact with integrins expressed by circulating monocytes, CD90 might be involved in angiogenesis through recruitment and functional regulation of TAMs in tumors. CD90+ vasculatures may also interact with tumor cells through interactions with integrins. Because CD90 was not expressed by vasculatures in normal brain tissue, it might be a possible therapeutic target to suppress angiogenesis and tumor growth.