The tumor microenvironment is a complex ecology of cells that evolves with and provides support to tumor cells during the transition to malignancy. Among the innate and adaptive immune cells ...recruited to the tumor site, macrophages are particularly abundant and are present at all stages of tumor progression. Clinical studies and experimental mouse models indicate that these macrophages generally play a protumoral role. In the primary tumor, macrophages can stimulate angiogenesis and enhance tumor cell invasion, motility, and intravasation. During monocytes and/or metastasis, macrophages prime the premetastatic site and promote tumor cell extravasation, survival, and persistent growth. Macrophages are also immunosuppressive, preventing tumor cell attack by natural killer and T cells during tumor progression and after recovery from chemo- or immunotherapy. Therapeutic success in targeting these protumoral roles in preclinical models and in early clinical trials suggests that macrophages are attractive targets as part of combination therapy in cancer treatment.
Macrophages promote tumor progression to malignancy. Noy and Pollard discuss how macrophages both stimulate tumor cell growth and suppress antitumor immune responses and propose ablation or redifferentiation of macrophages in tumors as a promising therapeutic approach to treat cancer.
Infiltration of macrophages in solid tumours is associated with poor prognosis and correlates with chemotherapy resistance in most cancers. In mouse models of cancer, macrophages promote cancer ...initiation and malignant progression by stimulating angiogenesis, increasing tumour cell migration, invasion and intravasation and suppressing antitumour immunity. At metastatic sites, macrophages promote tumour cell extravasation, survival and subsequent growth. Each of these pro-tumoural activities is promoted by a subpopulation of macrophages that express canonical markers but have unique transcriptional profiles, which makes tumour-associated macrophages (TAMs) good targets for anticancer therapy in humans through either their ablation or their re-differentiation away from pro-tumoural towards antitumoural states. In this Review, we evaluate the state of the art of TAM-targeting strategies, focusing on the limitations and potential side effects of the different therapies such as toxicity, rebound effects and compensatory mechanisms. We provide an extensive overview of the different types of therapy used in the clinic and their limitations in light of known macrophage biology and propose new strategies for targeting TAMs.
Specialized phagocytes are found in the most primitive multicellular organisms. Their roles in homeostasis and in distinguishing self from non-self have evolved with the complexity of organisms and ...their immune systems. Equally important, but often overlooked, are the roles of macrophages in tissue development. As discussed in this Review, these include functions in branching morphogenesis, neuronal patterning, angiogenesis, bone morphogenesis and the generation of adipose tissue. In each case, macrophage depletion impairs the formation of the tissue and compromises its function. I argue that in several diseases, the unrestrained acquisition of these developmental macrophage functions exacerbates pathology. For example, macrophages enhance tumour progression and metastasis by affecting tumour-cell migration and invasion, as well as angiogenesis.
Tumour progression is modulated by the local microenvironment. This environment is populated by many immune cells, of which macrophages are among the most abundant. Clinical correlative data and a ...plethora of preclinical studies in mouse models of cancers have shown that tumour-associated macrophages (TAMs) play a cancer-promoting role. Within the primary tumour, TAMs promote tumour cell invasion and intravasation and tumour stem cell viability and induce angiogenesis. At the metastatic site, metastasis-associated macrophages promote extravasation, tumour cell survival and persistent growth, as well as maintain tumour cell dormancy in some contexts. In both the primary and metastatic sites, TAMs are suppressive to the activities of cytotoxic T and natural killer cells that have the potential to eradicate tumours. Such activities suggest that TAMs will be a major target for therapeutic intervention. In this Perspective article, we chronologically explore the evolution of our understanding of TAM biology put into the context of major enabling advances in macrophage biology.
In many human cancers, the abundance of macrophages in the tumor microenvironment is correlated with poor prognosis. Experimental evidence for the causal relationship between macrophages and poor ...prognosis came from mouse models of breast cancer in which genetic ablation of macrophages resulted in attenuation of tumor progression and metastasis, and premature recruitment to hyperplastic lesions accelerated these processes. Malignancy is defined by the invasion of tumor cells into the stroma, a process that allows escape of these cells into the circulation and dissemination to distant sites. In this review, I argue that macrophages are recruited to the invasive front by expression of tumor‐derived chemotactic factors and in response to the disruption of the basement membrane. At this invasive site, macrophages enhance tumor cell migration and invasion through their secretion of chemotactic and chemokinetic factors including epidermal growth factor (EGF). They promote angiogenesis by the synthesis of angiogenic factors including vascular endothelial growth factor (VEGF), and they remodel the extracellular matrix and in particular, regulate collagen fibrillogenesis. A combination of these factors provides a triple‐whammy, as the more mobile and invasive tumor cells track along collagen fibers that are also anchored to blood vessels, which are fabricated at sites of invasion and through which macrophages potentiate tumor cell intravasation. All of these activities suggest that macrophage functions are significant targets for the generation of novel therapeutics that should improve the current cytotoxic armamentarium.
Tumor-associated macrophages are a major constituent of malignant tumors and are known to stimulate key steps in tumor progression. In our review in this journal in 2006, we postulated that ...functionally distinct subsets of these cells exist in different areas within solid tumors. Here, we review the many experimental and clinical studies conducted since then to investigate the function(s), regulation, and clinical significance of macrophages in these sites. The latter include three sites of cancer cell invasion, tumor nests, the tumor stroma, and areas close to, or distant from, the tumor vasculature. A more complete understanding of macrophage diversity in tumors could lead to the development of more selective therapies to restore the formidable, anticancer functions of these cells.
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Tumor cells metastasize to distant organs through a complex series of events that are driven by tumor intrinsic and extrinsic factors. In particular, non-malignant stromal cells, including immune ...cells, modify tumor metastatic behavior. Of these cells, tumor-associated innate immune cells, particularly macrophages and neutrophils, suppress the cytotoxic activity of innate and adaptive killer cells and interact with tumor cells to promote their growth and malignancy. These findings in mouse cancer models suggest that targeting these sub-populations of immune cells holds therapeutic promise in treating metastatic disease. In this review, we describe the origin and role of the macrophages, neutrophils, and their progenitors in the metastatic cascade and suggest strategies that might enhance cancer therapy.
Macrophages, the most plastic cells of the haematopoietic system, are found in all tissues and show great functional diversity. They have roles in development, homeostasis, tissue repair and ...immunity. Although tissue macrophages are anatomically distinct from one another, and have different transcriptional profiles and functional capabilities, they are all required for the maintenance of homeostasis. However, these reparative and homeostatic functions can be subverted by chronic insults, resulting in a causal association of macrophages with disease states. In this Review, we discuss how macrophages regulate normal physiology and development, and provide several examples of their pathophysiological roles in disease. We define the 'hallmarks' of macrophages according to the states that they adopt during the performance of their various roles, taking into account new insights into the diversity of their lineages, identities and regulation. It is essential to understand this diversity because macrophages have emerged as important therapeutic targets in many human diseases.
There is persuasive clinical and experimental evidence that macrophages promote cancer initiation and malignant progression. During tumor initiation, they create an inflammatory environment that is ...mutagenic and promotes growth. As tumors progress to malignancy, macrophages stimulate angiogenesis, enhance tumor cell migration and invasion, and suppress antitumor immunity. At metastatic sites, macrophages prepare the target tissue for arrival of tumor cells, and then a different subpopulation of macrophages promotes tumor cell extravasation, survival, and subsequent growth. Specialized subpopulations of macrophages may represent important new therapeutic targets.
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