This article reviews the evidence for macrophages playing an important role in the regulation of tumor angiogenesis. Findings in mouse models show that macrophages promote angiogenesis in tumors both by producing excessive amounts of proangiogenic factors and by physically assisting sprouting blood vessels to augment the complexity of the intra-tumoral vascular network. Recent studies however suggest that macrophages may be dispensable for the initiation of angiogenesis in tumors. Rather, these cells express proangiogenic programs that enhance the complexity of the tumor-associated vasculature, leading to aberrant, plethoric and dysfunctional angiogenesis. Gene expression and cell depletion studies further indicate that tumor-associated macrophages (TAMs) comprise phenotypically and functionally distinct subsets. This may reflect "education" of the macrophage phenotype by signals in some areas of the tumor microenvironment and/or TAM subsets derived from distinct macrophage precursors. Among the better characterized TAM subsets are the proangiogenic (TIE2(+)) and the angiostatic/inflammatory (CD11c(+)) macrophages, which coexist in tumors. Such antagonizing TAM subsets occupy distinct niches in the tumor microenvironment and are present at ratios that vary according to the tumor type and grade. Specifically targeting TAMs or reprogramming them from a proangiogenic to an angiostatic function may "normalize" the tumor vasculature and improve the efficacy of various anticancer therapies, including radiotherapy, chemotherapy and vascular-disrupting agents.
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