The paracrine role of Tie-2-expressing monocytes in tumor angiogenesis.

Literature data have demonstrated that tumor neovascularization is regulated in part by myelomonocytic cells. Luigi Naldini's group has reported the identification in human peripheral blood of a novel subset of Tie-2-expressing monocytes (TEMs) that promote angiogenesis in paracrine manner. Although recruited to tumors in lower numbers than tumor-associated macrophages (TAMs), TEMs are a more potent source of proangiogenic signals, suggesting that they significantly contribute to tumor angiogenesis. Moreover, TEMs, while stimulating angiogenesis, do not actively incorporate into blood vessels and this subpopulation of Tie-2+ cells, rather than bone marrow-derived endothelial progenitor cells (EPCs), which are incorporated in new-forming blood vessels, promote tumor neovascularization through the release of proangiogenic factors.