Gene Expression Signature and Pro-Angiogenic Function of Tie2-Expressing Macrophages

Abstract

Chronic inflammation and activation of angiogenesis sustain tumor progression. Recent data indicate that tumor-infiltrating myeloid cells support tumor angiogenesis by regulating multiple pro-angiogenic pathways. However, the functional heterogeneity of myeloid cells - macrophages in particular - may be greater than currently appreciated, and the relative contribution of distinct myeloid cell types to tumor angiogenesis is currently poorly known. Our laboratory identified a subset of tumor-associated macrophages (TAMs) that are required for tumor angiogenesis - the Tie2-expressing macrophages (TEMs). TEMs appear pre-committed to a pro-angiogenic function already when they circulate as monocytes and their elimination from tumor-bearing mice blunts tumor angiogenesis without affecting the recruitment of other myeloid subsets to the tumors. Yet, both the molecular bases of TEMs pro-angiogenic activity and the functional role of the TIE2 receptor in TEM biology are currently unknown. To investigate the molecular identity of TEMs, I developed protocols to purify TEMs and TIE2-TAMs from tumors and compared the gene expression profile of these 2 tumor macrophage subsets. I found that the TEM gene signature is consistent with enhanced pro-angiogenic and tissue-remodelling activity. From such analyses, novel surface markers were established that easily and unambiguously identify TEMs in tumors. Furthermore, I found that circulating Ly6C-monocytes and tumor-infiltrating TEMs express similar gene signatures, suggesting possible developmental relationships between the two myeloid subsets. TEMs, but not other tumor-infiltrating myeloid cells, express the angiopoietin receptor TIE2. To study the functional role of TIE2 in TEMs, I developed a novel lentiviral vector-based, conditional gene knock down platform. Remarkably, Tie2 knock-down in TEMs significantly inhibited angiogenesis and microvascular perfusion in a spontaneous tumor model, recapitulating several features of TEM elimination. In conclusion, these studies provided a molecular characterization of TEMs and related myeloid cells and identified TIE2 as a critical regulator of their proangiogenic function in tumors.