Angiopoietin 2 stimulates TIE2-expressing monocytes to suppress T cell activation and to promote regulatory T cell expansion.

Angiopoietin 2 (ANGPT2) is a proangiogenic cytokine whose expression is often upregulated by endothelial cells in tumors. Expression of its receptor, TIE2, defines a highly proangiogenic subpopulation of myeloid cells in circulation and tumors called TIE2-expressing monocytes/macrophages (TEMs). Genetic depletion of TEMs markedly reduces tumor angiogenesis in various tumor models, emphasizing their essential role in driving tumor progression. Previously, we demonstrated that ANGPT2 augments the expression of various proangiogenic genes, the potent immunosuppressive cytokine, IL-10, and a chemokine for regulatory T cells (Tregs), CCL17 by TEMs in vitro. We now show that TEMs also express higher levels of IL-10 than TIE2(-) macrophages in tumors and that ANGPT2-stimulated release of IL-10 by TEMs suppresses T cell proliferation, increases the ratio of CD4(+) T cells to CD8(+) T cells, and promotes the expansion of CD4(+)CD25(high)FOXP3(+) Tregs. Furthermore, syngeneic murine tumors expressing high levels of ANGPT2 contained not only high numbers of TEMs but also increased numbers of Tregs, whereas genetic depletion of tumor TEMs resulted in a marked reduction in the frequency of Tregs in tumors. Taken together, our data suggest that ANGPT2-stimulated TEMs represent a novel, potent immunosuppressive force in tumors.

Switching of vascular phenotypes within a murine breast cancer model induced by angiopoietin-2.

Sustained growth of solid tumours can rely on both the formation of new and the co-option of existing blood vessels. Current models suggest that binding of angiopoietin-2 (Ang-2) to its endothelial Tie2 receptor prevents receptor phosphorylation, destabilizes blood vessels, and promotes vascular permeability. In contrast, binding of angiopoietin-1 (Ang-1) induces Tie2 receptor activation and supports the formation of mature blood vessels covered by pericytes. Despite the intense research to decipher the role of angiopoietins during physiological neovascularization and tumour angiogenesis, a mechanistic understanding of angiopoietin function on vascular integrity and remodelling is still incomplete. We therefore assessed the vascular morphology of two mouse mammary carcinoma xenotransplants (M6378 and M6363) which differ in their natural angiopoietin expression. M6378 displayed Ang-1 in tumour cells but no Ang-2 in tumour endothelial cells in vivo. In contrast, M6363 tumours expressed Ang-2 in the tumour vasculature, whereas no Ang-1 expression was present in tumour cells. We stably transfected M6378 mouse mammary carcinoma cells with human Ang-1 or Ang-2 and investigated the consequences on the host vasculature, including ultrastructural morphology. Interestingly, M6378/Ang-2 and M6363 tumours displayed a similar vascular morphology, with intratumoural haemorrhage and non-functional and abnormal blood vessels. Pericyte loss was prominent in these tumours and was accompanied by increased endothelial cell apoptosis. Thus, overexpression of Ang-2 converted the vascular phenotype of M6378 tumours into a phenotype similar to M6363 tumours. Our results support the hypothesis that Ang-1/Tie2 signalling is essential for vessel stabilization and endothelial cell/pericyte interaction, and suggest that Ang-2 is able to induce a switch of vascular phenotypes within tumours.

Colon capsule endoscopy: current status and future directions.

Colon capsule endoscopy (CCE; PillCam Colon; Given Imaging; Yoqneam, Israel) is a minimally invasive wireless technique for the visualization of the colon. With the recent introduction of the second generation colon capsule the diagnostic accuracy of CCE for polyp detection has significantly improved and preliminary data suggest it may be useful to monitor mucosal inflammation in patients with inflammatory bowel disease. Limitations include the inability to take biopsies and the procedural costs. However, given the potentially higher acceptance within an average risk colorectal cancer (CRC) screening population, its usefulness as a screening tool with regard to CRC prevention should be further evaluated.

Angiopoietin-2 regulates gene expression in TIE2-expressing monocytes and augments their inherent proangiogenic functions.

TIE2-expressing monocytes/macrophages (TEM) are a highly proangiogenic subset of myeloid cells in tumors. Here, we show that circulating human TEMs are already preprogrammed in the circulation to be more angiogenic and express higher levels of such proangiogenic genes as matrix metalloproteinase-9 (MMP-9), VEGFA, COX-2, and WNT5A than TIE2(-) monocytes. Additionally, angiopoietin-2 (ANG-2) markedly enhanced the proangiogenic activity of TEMs and increased their expression of two proangiogenic enzymes: thymidine phosphorylase (TP) and cathepsin B (CTSB). Three "alternatively activated" (or M2-like) macrophage markers were also upregulated by ANG-2 in TEMs: interleukin-10, mannose receptor (MRC1), and CCL17. To investigate the effects of ANG-2 on the phenotype and function of TEMs in tumors, we used a double-transgenic (DT) mouse model in which ANG-2 was specifically overexpressed by endothelial cells. Syngeneic tumors grown in these ANG-2 DT mice were more vascularized and contained greater numbers of TEMs than those in wild-type (WT) mice. In both tumor types, expression of MMP-9 and MRC1 was mainly restricted to tumor TEMs rather than TIE2(-) macrophages. Furthermore, tumor TEMs expressed higher levels of MRC1, TP, and CTSB in ANG-2 DT tumors than WT tumors. Taken together, our data show that although circulating TEMs are innately proangiogenic, exposure to tumor-derived ANG-2 stimulates these cells to exhibit a broader, tumor-promoting phenotype. As such, the ANG-2-TEM axis may represent a new target for antiangiogenic cancer therapies.