Final Report Summary - TIE2+MONOCYTES (Tie2-expressing monocytes: Role in tumor angiogenesis and therapeutic targeting)
Macrophages are inflammatory cells that play key roles in the defence of our bodies against pathogens, as well as the regulation of organ physiology and pathology. Macrophages are also found in tumors, where they appear to support tumor progression through several mechanisms. We previously identified a subpopulation of macrophages (termed TEMs, for TIE2-expressing macrophages) that promote tumor angiogenesis, that is, the formation of functional blood vessels required for nutrient and oxygen delivery to the growing tumor mass or its metastases. With the support of the European Research Council, we have investigated mechanisms whereby these pro-angiogenic macrophages regulate tumor angiogenesis, progression, and response to anticancer therapies. Moreover, we have developed strategies to either block their protumoral functions in tumors, or convert them into cell vehicles for the transport of gene therapy to tumors. In a first set of studies, we investigated whether TEMs’ pro-angiogenic activity limits the efficacy of various vascular-targeted treatments employed in the clinic, such as pure angiogenesis inhibitors that block the angiopoietin-2/TIE2 or vascular-endothelial growth factor-A (VEGFA)/VEGFA-receptor pathways, as well as cytotoxic agents that function as vascular-disrupting agents. We found that TEMs limit the efficacy of vascular-disrupting agents in experimental tumor models, suggesting that they may promote resistance to such treatments also in human cancer. Furthermore, we showed that some tumor types switch to an angiopoietin-2/TIE2–dependent mode of angiogenesis upon VEGFA blockade (a clinically approved anti-angiogenic treatment), providing a strong rationale for combining angiopoietin-2 and VEGFA inhibitors in selected patients with cancer. In a second set of studies, we have characterized the gene expression profiles of TEMs and other tumor-associated macrophages by RNA deep sequencing. These studies identified distinguishing molecular features of the pro-angiogenic macrophages – including a novel microRNA (miR-511-3p) that functions as a negative regulator of their pro-angiogenic and pro-tumoral genetic programs – that may provide new targets for selectively inhibiting these cells in human cancer. Moreover, RNA profiling of macrophages and vascular cells also revealed that these heterologous cell types may communicate with each other by releasing microvesicles enriched in specific microRNAs. In this regard, we discovered that the loading of specific microRNAs into macrophage-derived microvesicles is controlled by the activation state and transcriptomic profile of the cell, thus illustrating a general model for the sorting of microRNAs into cell-derived microvesicles. Overall, this work has provided novel insight into the transcriptional heterogeneity of tumor-associated macrophages, and how this can influence their properties and ability to communicate with other cells in the tumor microenvironment. In a third set of studies, we investigated whether the pro-angiogenic activity of TEMs could be exploited to promote the vascularization of tissues with insufficient (pathological) blood supply. By using experimental models of ischemia (interrupted blood flow), we showed that TEMs can be employed in the context of cell therapy to help re-vascularize ischemic tissues, hence suggesting new therapeutic avenues for the treatment of ischemic diseases in patients. Finally, the implementation of preclinical models based on human hematochimeric mice has shown the safety and feasibility of engineering TEMs for the targeted delivery of biotherapeutics to tumors. Taken together, the results of our studies have increased significantly our knowledge of the biological functions of pro-angiogenic macrophages in tumor and ischemic tissues, and provided proof-of-feasibility of cell and gene therapy approaches based on harnessing these cells for either anti- or pro-angiogenic therapy.