Gliomas are highly vascularized tumors, and preclinical data have suggested that glioma growth critically depends on the generation of tumor associated blood vessels. Among multiple factors controlling the complex process of angiogenesis, VEGF (Vascular Endothelial Growth Factor) and its associated signaling cascade are considered of central importance. Glioma cells are a major source of VEGF, and high levels of VEGF generation have been reported to correlate with high grade malignancy and poor prognosis. Antiangiogenic therapy has been recently added to the battery of treatments to newly diagnosed malignant gliomas. However, preclinical data from our laboratory and others, as some clinical results, shown concomitantly elicit tumor adaptation and progression to stages of greater malignancy, with heightened invasiveness. These results warrant preclinical investigation. Here, we present evidence of accumulation of Tie2-expressing monocytes (TEMs) in the brain tumor/normal interphase regions in tumor-bearing animals treated with anti-VEGF agents. Immunohistochemistry using F4/80 or Iba1 antibodies revealed an increase number of microglia/macrophages in central and peripheral tumoral areas after antiangiogenesis treatment. Moreover, the recruitment of these tumor-associated microglia/macrophages (TAMs) was associated with an overrepresentation of TEMs. Almost 50% of Tie2-positive cells co-localized with microglia/macrophages after bevacizumab or prolonged VEGF-Trap/Aflibercept therapies. To determine the role of TEMs in the invasive phenotype of gliomas upon antiangiogenic therapies, we use monocytes isolated from buffy coats and sorted positively for Tie2 expression, and monocytic cultures polarized to M2 phenotype and forced to express Tie2. We found that TEMs express high level of MMP2, MMP9, CD44, and CXCR4, molecules related with extracellular matrix remodeling, using q-PCR, ELISA, FACS analysis and double immunofluorescence. Gelatin zymography, as well as collagenase assay confirmed the upregulation of MMP-2 and MMP-9 activity. Of interest, knocking down Tie2 by siRNA suppresses MMP9 activity emphasizes a role of Tie2 signaling in the regulation of MMP9. Additionally, in vitro invasion assay showed a pivotal role of TEMs in development of an invasive phenotype in glioma cells. Together, our results suggest a novel mechanism of escape of GBMs to anti-VEGF therapies and the necessity of targeting the TEM population in combination with anti-VEGF therapies to improve clinical outcome.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1020. doi:1538-7445.AM2012-1020