Links between cancer and inflammation were first made in the nineteenth century, on the basis of observations that tumours often arose at sites of chronic inflammation and that inflammatory cells were present in biopsied samples from tumours 1 . The idea that these processes are connected was out of favour for more than a century, but there has been a recent resurgence in interest. Several lines of evidence 1-4 (Box 1) -based on a range of findings, from epidemiological studies of patients to molecular studies of genetically modified mice -have led to a general acceptance that inflammation and cancer are linked.Epidemiological studies have shown that chronic inflammation predisposes individuals to various types of cancer. It is estimated that underlying infections and inflammatory responses are linked to 15-20% of all deaths from cancer worldwide 1 . There are many triggers of chronic inflammation that increase the risk of developing cancer. Such triggers include microbial infections (for example, infection with Helicobacter pylori is associated with gastric cancer and gastric mucosal lymphoma), autoimmune diseases (for example, inflammatory bowel disease is associated with colon cancer) and inflammatory conditions of unknown origin (for example, prostatitis is associated with prostate cancer). Accordingly, treatment with non-steroidal anti-inflammatory agents decreases the incidence of, and the mortality that results from, several tumour types [5][6][7] .The hallmarks of cancer-related inflammation include the presence of inflammatory cells and inflammatory mediators (for example, chemokines, cytokines and prostaglandins) in tumour tissues, tissue remodelling and angiogenesis similar to that seen in chronic inflammatory responses, and tissue repair. These signs of 'smouldering' inflammation 2 are also present in tumours for which a firm causal relationship to inflammation has not been established (for example, breast tumours). Indeed, inflammatory cells and mediators are present in the microenvironment of most, if not all, tumours, irrespective of the trigger for development.Studies of genetically modified mice, adoptive-transfer experiments in mice, and analyses of human tumours have allowed researchers to begin to unravel the molecular pathways that link inflammation and cancer. Here we review current knowledge of the molecular and cellular pathways that link inflammation and cancer, and we describe how these pathways suppress effective antitumour immunity during tumour progression. We also discuss how cancer-related inflammation affects many aspects of malignancy, including the proliferation and survival of malignant cells, angiogenesis (which is required for the survival of cells within tumours of a certain size), tumour metastasis, and tumour response to chemotherapeutic drugs and hormones.Advances in understanding the genetic pathways involved in cancer have led to the development of a range of therapies that target malignant cells. Understanding the pathways involved in cancer-related inflammation could ...

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(Retnla, Fizz1), and chitinase 3-like 3 (Chi3l3, Ym1) (14). IL-10 activates STAT3-mediated expression of genes (Il10, Tgfb1, Mrc1) associated with an M2-like phenotype (4,15,16). STAT-mediated activation of macrophages is regulated by members of the SOCS family. IL-4 and IFN-γ, the latter in concert with TLR stimulation, upregulate SOCS1 and SOCS3, which in turn inhibit the action of STAT1 and STAT3, respectively (17,18).Downstream of, or in parallel with, the IRF/STAT/SOCS pathway, a panel of transcription factors orchestrates polarized macrophage activation. The nuclear receptors PPARγ (19) and PPARδ (20,21) control distinct subsets of genes associated with M2 macrophage activation and oxidative metabolism (Figure 1). Interestingly, STAT6 coordinates and synergizes with both PPARγ (22) and Krüppel-like factor 4 (KLF4), a member of a family of proteins that contribute to macrophage function (23, 24). KLF4 cooperates with Stat6 to induce M2 genes (Arg-1, Mrc1, Fizz1, PPARγ) and inhibit M1 genes (TNFa, Cox-2, CCL5, iNOS) via sequestration of coactivators required for NF-κB activation. KLF2 regulates macrophage activation by inhibiting NF-κB/ HIF-1α activities (25). IL-4 also induces c-Myc activity in human macrophages (26), which controls genes of M2 activation (Scarb1, Alox15, and Mrc1) as well as STAT6 and PPARγ activation (26).TLR engagement leads to NF-κB activation and production of inflammatory mediators (27) associated with M1 macrophages. However, NF-κB activation also activates a genetic program essential for resolution of inflammation (28) and for M2 polarization of tumor-associated macrophages (TAMs) (29). Moreover, induction of p50 NF-κB homodimers is essential for M2 polarization in vitro and in vivo (30). The hypoxia-inducible factors HIF-1α and HIF-2α are expressed differentially in M1- and M2-polarized macrophages (31) and regulate inducible NOS2 (M1) and arginase 1 (M2), respectively. Figure 1Mechanisms of macrophage polarization. The major pathways of macrophage polarization are outlined. The crosstalk between the M1-M2 macrophage-polarizing pathways is also indicated. The balance between activation of STAT1 and STAT3/STAT6 finely regulates macrophage polarization and activity. A predominance of NF-κB and STAT1 activation promotes M1 macrophage polarization, resulting in cytotoxic and inflammatory functions. In contrast, a predominance of STAT3 and STAT6 activation results in M2 macrophage polarization, associated with immune suppression and tumor progression. PPARγ and PPARδ control distinct aspects of M2 macrophage activation and oxidative metabolism. KLF4 and KLF2 participate in the promotion of M2 macrophage functions by cooperating with STAT6 and suppressing the NF-κB/HIF-1α-dependent transcription, respectively. IL-4-induced c-Myc activity controls a subset of M2-associated genes. IL-4 also induces the M2-polarizing Jmjd3-IRF4 axis to inhibit IRF5-mediated M1 polarization. IL-10 promotes M2 polarization through the induction of p50 NF-κB homodimer, c-Maf, and STAT3 activities. Epigenetic ...

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Summary Description of macrophage activation is currently contentious and confusing. Like the biblical Tower of Babel, macrophage activation encompasses a panoply of descriptors used in different ways. The lack of consensus on how to define macrophage activation in experiments in vitro and in vivo impedes progress in multiple ways, including the fact that many researchers still consider there to be only the two types of activated macrophages often termed M1 and M2. Here we describe a set of standards for the field encompassing three principles: the source of macrophages, definition of the activators, and a consensus collection of markers to describe macrophage activation, with the goal of unifying experimental standards for diverse experimental scenarios. Collectively, we propose a common framework for macrophage activation nomenclature.