Of the most well characterized types of tumor-infiltrating inflammatory cells are bone marrow-derived myeloid (BMM) cells, which include a variety of described cell types including tumor-associated macrophages (TAMs), TIE2-expressing monocytes (TEMs), myeloid-derived suppressor cells (MDSCs), and neutrophils. Thus, it is apparent that tumor associated myeloid cells represent a diverse array of cell types with distinct, yet often overlapping functions. As the number of studies on BMM cells grows, it has become clear that characteristics bestowed upon these cells likely describe a heterogeneous population of cells whose common origin is still controversial. In addition, many of these cell types are plastic and may change in differentiation status over the course of tumor progression. The exact relationship between these myeloid cell types and how they change phenotypically and functionally in various settings over the course of tumor progression is still a topic of debate. BMM cells infiltrate tumors as an initial innate immune response to inflammatory agents released by the primary tumor and other stromal cells. However, signals produced within the tumor microenvironment stimulate many pro-tumorigenic, pro-angiogenic, and invasive functions of these cells. Since the late 1970's, the infiltration of myeloid cells has been well documented, and for the most part, associated with poor prognosis. Macrophages associated with solid tumor tissue have been reported to constitute up to 50% of the tumor mass and have strong implications in tumor progression and metastasis.
The role of BMM cells at the primary tumor is multi-faceted, and in many cases, provides a supportive environment for pre-existing malignant cells. However, there is increasing evidence supporting the role of BMM cells in stimulating tumor growth and inducing oncogenic mutations in surrounding epithelial cells associated with the earliest stages of carcinogenesis. Indeed, BMM cells are likely recruited to tumor sites as a part of an innate immune response, and their continuous presence parallels chronic inflammation, which can be a contributing event for many types of cancer. For example, TAMs can produce high amounts reactive oxygen and nitrogen species that can interact with DNA, inducing mutations in surrounding epithelium. This property may explain mutations seen in local tumor endothelium and stromal cells. Alternatively, TAMs can produce cytokines that are capable of inducing genetic abnormalities. For instance, generation of migration inhibitory factor (MIF) suppresses p53 transcription in tumor cells, resulting in defective DNA damage repair and the accumulation of genetic mutations. TAMs can also produce several growth factors capable of directly promoting the growth of tumor cells. These include fibroblast growth factor (FGF), hepatocyte growth factor (HGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and transforming growth factor - β (TGF-β). EGF appears to be especially important in many types of cancers, including breast cancer, in which it has been shown to enhance tumor-cell migration via direct regulation of integrin-binding focal adhesion proteins, tensin-3 and cten. Together, these studies provide a substantial link between inflammation associated with macrophage infiltration and tumorigenesis.
Likely precursors of TAMs are monocytes that are actively recruited to the tumor from the blood. Monocytes were originally shown to be recruited to these sites in response to a tumor-derived chemokine CC chemokine ligand 2 (CCL2/MCP-1) where they then differentiated into TAMs. Subsequent studies have identified additional factors involved in attracting monocytes to the tumor. Chemokines such as CCL2, CCL5, CXCL8/IL-8 and SDF-1 expressed by tumor cells, fibroblasts, endothelial cells and TAMs, act as monocyte chemoattractants. Cytokines and growth factors, including CSF-1, VEGF-A, and P1GF have also been implicated in initiating monocyte infiltration. In addition to promoting cell migration of peripheral monocytes to the tumor microenvironment, certain chemoattractants may also indirectly enhance monocyte infiltration through indirect means. For instance, both CCL2 and CCL5 have been shown to stimulate monocytes to secrete MMP-9, MMP-19, and urokinase-type plasminogen activator (uPA), which act to degrade the basement membrane and ECM components to further promote monocyte infiltration. The accumulation of TAMs into hypoxic regions of tumors is well documented and is likely regulated by a hypoxia-mediated chemoattractive gradient involving hypoxia-inducible factor 1 (HIF-1) induced growth factors such as VEGF. Not surprisingly, high ex
BMM cells have been shown to be important mediators of the angiogenic switch. Though anti-angiogenic functions of macrophages have been reported, BMM cells generally play a pro-angiogenic role. In the polyoma middle T oncoprotein model of mammary tumors, a significant infiltration of macrophages occurs at stages directly preceding those changes associated with angiogenesis. When given tumors, mice deficient in macrophages, through genetic deletion of the CSF-1 gene, manifest a delayed angiogenic switch. TAMs are an important producer of VEGF-A within the tumor, which may be regulated by hypoxia as well as CSF-1 activation. Transgenic mice made to express GFP under the control of the human VEGF-A promoter illustrate that both stromal cells as well as bone marrow-derived macrophages are a large source of this growth factor. The release of MMPs and uPA that breakdown the ECM also serves as a mechanism to release bound VEGF-A functions during vascular remodeling. TAMs tend to infiltrate regions of poor vascularization, which induces transcriptional activation of HIF-1 and HIF-2-regulated promoters, resulting in the upregulation of proteins such as VEGF-A, MMPs, interleukins, and chemokines. Thus, it appears that TAMs not only function to promote vascular sprouting through the direct or indirect release of angiogenic factors, but also provide enzymes capable of vascular remodeling after vessel formation.
As mentioned, BMM cells are an important source of VEGF in the primary tumor microenvironment, which was long thought to promote angiogenesis and tumor progression. However, studies specifically assessing the role of BMM cell-derived VEGF on tumor vessel formation report interesting, yet paradoxical results. Deletion of VEGF-A in myeloid cells inhibits high-density tumor vessel formation, resulting in vascular normalization. Vessels in tumors lacking BMM cell-derived VEGF were less torturous and had increased pericyte coverage. Notably, the growth of various tumor models in these mice was accelerated. These tumor possessed less areas of necrosis and hypoxia. Thus, myeloid cells, via ex
A population of peripheral, tumor-infiltrating monocytes, considered precursors to TAMs, has been shown to play an essential role in angiogenesis as well. Monocytes expressing the angiopoietin receptor, Tie2, referred as Tie2 expressing monocytes (TEMs) associate closely with tumor blood vessels. Elimination of TEMs by means of a suicide gene significantly impairs tumor growth and vascularity in mouse glioma models. Though the molecular mechanisms behind the pro-angiogenic role of TEMs needs to be elucidated, the human counterpart of TEMs has recently been identified in the peripheral blood of cancer patients, with angiogenic activity in xenotransplanted human tumors. Thus, TEMs represent a distinct myeloid sub-population of monocyte/macrophages that may prove to be an attractive anti-angiogenic target.