Role of myeloid cells in vascular endothelial growth factor-independent tumor angiogenesis

Purpose of review Targeting the vascular endothelial growth factor (VEGF) pathway has had a significant impact in the therapy of cancer and intraocular neovascular disorders. Similar to other therapies, inherent/acquired resistance to anti-VEGF drugs may occur in cancer patients, leading to disease recurrence. This review describes recent findings on the role of myeloid cells in refractoriness or/and acquired resistance to such therapies. Recent findings Various myeloid cell types, including tumor-associated macrophages, Tie2-expressing monocytes and neutrophils, have been implicated in tumor angiogenesis. Several cytokines involved in the mobilization and/or proangiogenic effects of these cells represent therapeutic targets. CD11b(+)Gr1(+) myeloid cells have been shown to render tumors refractory to angiogenic blockade by VEGF antibodies. This effect was mediated by the secreted protein Bv8, which is upregulated by granulocyte colony-stimulating factor. Summary Progress in unraveling proangiogenic mechanisms dependent on various myeloid cell types has expanded our understanding of tumor angiogenesis and has generated a number of potential therapeutic targets.