Tumor-Associated Macrophages: From Mechanisms to Therapy

“…the ways in which the microenvironmental support to tumors is the acquisition of a vasculature that provides oxygenation, as well as the nutrition and waste disposal required for growth above a certain size in a process often referred to as the ‘‘angiogenic switch’’ (Hanahan and Weinberg, 2011).”

“Furthermore, inflammatory conditions such as Crohn’s disease dramatically increase the risk of colorectal cancer (Balkwill et al., 2005; Balkwill and Mantovani, 2012; Coussens and Werb, 2002; Grivennikov et al., 2010).”

“Furthermore, inflammatory conditions such as Crohn’s disease dramatically increase the risk of colorectal cancer (Balkwill et al., 2005; Balkwill and Mantovani, 2012; Coussens and Werb, 2002; Grivennikov et al., 2010).”

“…SPARC) that increases tumor cell-ECM interaction and thus migration (Sangaletti et al., 2008), Cathepsin proteases that remodel the matrix and release sequestered growth factors (Laoui et al., 2011; Quail and Joyce, 2013) and TGF-b that promotes EMT of the invading tumor cells (Bonde et al., 2012).”

“TheseM-MDSCs have greater immunosuppressive potency than the granulocytic ones and are further defined as F4/80+ a marker also found on inflammatory monocytes (Gabrilovich and Nagaraj, 2009; Gabrilovich et al., 2012).”

“References to these polarizing agents can be found in (De Palma and Lewis, 2013; Sica and Mantovani, 2012). above strongly support targeting the destruction or redifferentiation of macrophages as an important part of combinatorial therapies in human cancer patients.”

“…the activated macrophages responding to IFN-g, TNF-a, and Toll-like receptor 4 (TLR4) activation capable of killing pathogens through mechanisms such as iNOS, and (2) alternatively activated macrophages responding to IL-4 and IL-13 involved in antiparasitic immunity and in asthma (Gordon, 2003).”

“Furthermore, in most large-scale transcriptome analysis, macrophages have a mixed phenotype expressing both M1 and M2 markers (Qian and Pollard, 2010).”

“In addition, TAMs isolated from human renal cell carcinoma induce the expression of CTLA4 and Foxp3 in CD4 T cells (Daurkin et al., 2011).”

“In contrast, macrophages 50 Immunity 41, July 17, 2014 ª2014 Elsevier Inc. involved in pathogen responses appear to come from circulating BM monocytes (Wynn et al., 2013).”

“However, even early on, there is also engagement of cells of the acquired system—particularly T cells (Gajewski et al., 2013).”

“that were shown to be differentially secreted bymacrophages as a function of their activation state (proinflammatory and anti-inflammatory, respectively) (Biswas and Mantovani, 2010).”

“Myeloid cells also promote mesenchymal-to-epithelial transition (MET) and tumor growth by inhibiting TGF-b signaling in these epithelial metastatic cells (Gao et al., 2012).”

“Monocytes and/or macrophages are essential metastasis promoters acting both to prepare sites and also to promote the extravasation, survival, and persistent growth of metastatic cells (Joyce and Pollard, 2009; Qian et al., 2009).”

“TheseM-MDSCs have greater immunosuppressive potency than the granulocytic ones and are further defined as F4/80 a marker also found on inflammatory monocytes (Gabrilovich and Nagaraj, 2009; Gabrilovich et al., 2012).”

“TAMs have been found to express IL-10 and TGF-b in different pathological scenarios including human and mouse cancers (Pollard, 2004).”

“Nevertheless, several recent studies suggest that most TAM subpopulations arise from the Ly6C+ population of circulating mouse monocytes in grafted tumors (Movahedi et al., 2010), primary mouse mammary tumors (Franklin et al., 2014), and in lung metastases (Qian et al., 2011).”

“Because this process is dependent on a different cell type to classical macrophages it will not be reviewed further here, but readers are referred to recent reviews that discuss the process and therapeutic opportunities (Camacho and Pienta, 2014; Esposito and Kang, 2014; Mundy, 2002).”

“This causes tumor cells andmacrophages to rapidly stream along collagen fibers in lock-step, ending up in tumor cells clustering around blood vessels (Condeelis and Pollard, 2006; Wyckoff et al., 2007) (Figure 1).”

“Recently, Noman et al. showed that MDSCs and TAMs in hypoxic tumor regions upregulate the expression of PD-L1 as a consequence of HIF-1a signaling (Noman et al., 2014).”

“…evidence from studies on the DC-T cell immunological synapse suggests that interaction of CTLA-4 with B7 ligands not only signals for the Immunity 41, July 17, 2014 ª2014 Elsevier Inc. 53 inhibition of T cells but also induces a DCs inhibitory phenotype (Butte et al., 2007; Mellor et al., 2004).”

“Both PD-L1 and L2 are regulated in TAMs and myeloid-derived suppressor cells (See below—MDSC) (Belai et al., 2014; Duraiswamy et al., 2013).”

“…example, CCL2 acting via its receptor CCR2 is a direct mediator of monocyte recruitment to the primary tumor and to metastases in the PYMT model (Cortez-Retamozo et al., 2012; Franklin et al., 2014; Qian et al., 2011) even though this recruitment requires CSF1 (Lin et al., 2001; Qian et al., 2011).”

“Nevertheless, several recent studies suggest that most TAM subpopulations arise from the Ly6C+ population of circulating mouse monocytes in grafted tumors (Movahedi et al., 2010), primary mouse mammary tumors (Franklin et al., 2014), and in lung metastases (Qian et al., 2011).”

“This Th2 environment is characterized by transforming growth factor-b1 (TGF-b1) and Arginase 1, as well as increased numbers of CD4 T cells (DeNardo et al., 2009).”

“L-arginine is necessary for T cells function, and its depletion inhibits the re-expression of the CD3 z chain after internalization caused by antigen stimulation and TCR signaling (Rodriguez et al., 2003, 2004).”

“Even before tumor cells arrive, the frequency and site specificity of metastatic growth can be influenced by primary tumors through the formation of sites that enhance homing of circulating tumor cells known as premetastatic niches (Psaila and Lyden, 2009).”

“L-arginine is necessary for T cells function, and its depletion inhibits the re-expression of the CD3 z chain after internalization caused by antigen stimulation and TCR signaling (Rodriguez et al., 2003, 2004).”

“Nevertheless, several recent studies suggest that most TAM subpopulations arise from the Ly6C+ population of circulating mouse monocytes in grafted tumors (Movahedi et al., 2010), primary mouse mammary tumors (Franklin et al., 2014), and in lung metastases (Qian et al., 2011).”

“These descriptions were captured to suggest that TAMs could be either tumor killing (M1) or tumor promoting (M2) (Sica et al., 2008).”

“The original in vitro characterizations were extended to in vivo models by Mills and coworkers who called these states M1 (activated) and M2 (alternatively activated) (Mills, 2012).”

“Macrophages also promote neoangiogenesis in glioblastoma models (Du et al., 2008) Characterization of angiogenic TAMs show that they express TIE2.”

“Several recent studies suggest that such an approach is feasible and therapeutic (Coussens et al., 2013; De Palma and Lewis, 2013).”

“Consequently, ablation of TAMs for example by genetic depletion of their major growth factor, CSF-I, diminishes the number of circulating tumor cells and reduces metastasis (Wyckoff et al., 2007).”

“Several recent studies suggest that such an approach is feasible and therapeutic (Coussens et al., 2013; De Palma and Lewis, 2013).”

“TAMs have been found to express IL-10 and TGF-b in different pathological scenarios including human and mouse cancers (Pollard, 2004).”

“Similarly, monocytes from patients with hepatocellular carcinoma express PD-L1 that contributes to human tumor xenograft growth in vivo, while the blocking of PD-L1 reverses this effect (Kuang et al., 2009).”

“Studies of lung metastasis show that upon their arrival at the target site, tumor cells together with associated platelets recruited via their expression of tissue factor form microclots and arrest in the target tissue vessels (Gil-Bernabé et al., 2012).”

“TAM expression of B7-1 and B7-2 was shown to be dependent on their activation phenotype; both molecules are expressed by proinflammatory macrophages and are downregulated by antiinflammatory macrophages (Ding et al., 1993; Flores Villanueva et al., 1994; Kennedy et al., 2013).”

“Some factors such as GM-CSF act in protumoral or antitumoral fashion dependent on context (Pyonteck et al., 2013; Su et al., 2014).”

“Moreover, the inhibition of B7-H4 restores the stimulating function of TAMs and contributes to tumor regression (Kryczek et al., 2006).”

“…can inhibit the activation of natural killer (NK) cells and a subsets of activated T cells upon their ligation to killer cell immunoglobulin like receptor CD94 (also known asNKG2) (Borrego et al., 1998) or the inhibitory leukocyte immunoglobulin-like receptors LIT-2 (HLA-E and HLA-G respectively).”

“probably because the immune system has access to the products of mutated genes and/or recognizes tissue damage caused by invasion (Coussens and Pollard, 2011; Gajewski et al., 2013).”

“Furthermore, inflammatory conditions such as Crohn’s disease dramatically increase the risk of colorectal cancer (Balkwill et al., 2005; Balkwill and Mantovani, 2012; Coussens and Werb, 2002; Grivennikov et al., 2010).”

“in vivo both in the primary and metastatic sites (Biswas et al., 2013).”

“…can cause tumor initiation by creating a mutagenic microenvironment either directly through free radical generation or indirectly via alterations in the microbiome and barrier functions that allow access of genotoxic bacteria to the epithelial cells (Dedon and Tannenbaum, 2004; Jobin, 2013).”

“Exosomes derived from different melanoma strains can also redirect metastatic cell target tropisms from one tissue to another (Peinado et al., 2011).”

“Monocytes and/or macrophages are essential metastasis promoters acting both to prepare sites and also to promote the extravasation, survival, and persistent growth of metastatic cells (Joyce and Pollard, 2009; Qian et al., 2009).”

“In other models, M-MDSCs modulate the efficacy of antivascular therapies (Shojaei et al., 2007).”

“Hypoxia acting via hypoxia inducible factor 1- a (HIF-1a) also induces T cell suppression by TAMS although the mechanism is unknown (Doedens et al., 2010).”

“For example, it has been shown that INF-g secretion by activated NK cells that have migrated to the lymph node is an important mediator of CD8+ T cell activation and that HLA-G and HLA-E inhibits this NK cells migration and INF-g secretion (Kelly et al., 2002).”

“Substantial evidence indicates that macrophages, rather than being tumoricidal as suggested after their activation in vitro (Fidler, 1988), adopt a protumoral phenotype in vivo both in the primary and metastatic sites (Biswas et al., 2013).”

“Exosomes derived from different melanoma strains can also redirect metastatic cell target tropisms from one tissue to another (Peinado et al., 2011).”

“L-arginine is necessary for T cells function, and its depletion inhibits the re-expression of the CD3 z chain after internalization caused by antigen stimulation and TCR signaling (Rodriguez et al., 2003, 2004).”

“In this context and importantly, the combination of macrophages and a high ratio of CD4+ regulatory versus CD8+ T cells in human breast cancer is predictive of poorer survival (Ruffell et al., 2011).”

“Genetic gain-of-function of VEGFA over a loss-of function of CSF1 in the PyMT mouse model also resulted in a dramatic recruitment of macrophages and a rescue of angiogenesis that resulted in an acceleration of tumor progression to malignancy (Lin et al., 2007).”

“TAMs in this mouse model expressed some of the CCR5 ligands (Schlecker et al., 2012).”

“Similar results can be seen in cervical and breast cancermodels (Strachan et al., 2013).”

“These effects seem not to be restricted to chemotherapy because TIE+ positive TAMs limit the efficacy of antivascular reagents, and their ablation strongly increases the therapeutic efficacy of these agents (Priceman et al., 2010; Welford et al., 2011).”

“TGF-b inhibits cytotoxic T lymphocyte, Th1, and Th2 CD4+ T cells (Oh and Li, 2013), whereas IL-10 inhibits Th1 and Th2 CD4+ T cell helper functions (Ng et al., 2013).”

“Studies of lung metastasis show that upon their arrival at the target site, tumor cells together with associated platelets recruited via their expression of tissue factor form microclots and arrest in the target tissue vessels (Gil-Bernabé et al., 2012).”

“There are numerous additional reports of TAMs affecting angiogenesis in a wide range of models, mostly xenograft ones, and for further information the reader is referred to recent reviews on this topic (Coffelt et al., 2009; Nucera et al., 2011).”

“For example, it has been shown that INF-g secretion by activated NK cells that have migrated to the lymph node is an important mediator of CD8 T cell activation and that HLA-G and HLA-E inhibits this NK cells migration and INF-g secretion (Kelly et al., 2002).”

“TGF-b inhibits cytotoxic T lymphocyte, Th1, and Th2 CD4+ T cells (Oh and Li, 2013), whereas IL-10 inhibits Th1 and Th2 CD4+ T cell helper functions (Ng et al., 2013).”

“In addition, HLA-G-transfected APCs can inhibit CD4+ T cell activation and induce immunosuppressive differentiation in vitro (LeMaoult et al., 2004).”

“inhibition of T cells but also induces a DCs inhibitory phenotype (Butte et al., 2007; Mellor et al., 2004).”

“…SPARC) that increases tumor cell-ECM interaction and thus migration (Sangaletti et al., 2008), Cathepsin proteases that remodel the matrix and release sequestered growth factors (Laoui et al., 2011; Quail and Joyce, 2013) and TGF-b that promotes EMT of the invading tumor cells (Bonde et al., 2012).”

“This Th2 environment is characterized by transforming growth factor-b1 (TGF-b1) and Arginase 1, as well as increased numbers of CD4+ T cells (DeNardo et al., 2009).”

“…in the polarization ofmacrophages by a number of factors including IL-4 synthesized by CD4+ T cells and/or tumor cells (Coussens et al., 2013; Gocheva et al., 2010) and growth factors produced by tumor cells such as colony stimulating factor-1 CSF1 (Lin et al., 2002) and GM-CSF (Su et al., 2014).”

“Furthermore CCL9 acting through its receptor, CCR1, recruits immature myeloid cells in colon cancer models (Kitamura et al., 2010; Kitamura et al., 2007).”

“Both PD-L1 and L2 are regulated in TAMs and myeloid-derived suppressor cells (See below—MDSC) (Belai et al., 2014; Duraiswamy et al., 2013).”

“CSF1-regulated macrophages regulate this switch in the PyMT model in part through production of VEGF (Lin and Pollard, 2007).”

“In colorectal cancer, CCL20 secreted by TAMs recruit CCR6+ nTreg cells (Liu et al., 2011).”

“Chemokine receptors CCR4, CCR5, CCR6, and CCR10 expressed by nTreg cells are involved in their migration into the tumor microenvironment (Adeegbe and Nishikawa, 2013).”

“Several recent studies suggest that such an approach is feasible and therapeutic (Coussens et al., 2013; De Palma and Lewis, 2013).”

“Savage and coworkers investigated the ability of human macrophages to induce regulatory T cells and showed that IL-10 expressing anti-inflammatory macrophages but not proinflammatory macrophages are responsible for induction of iTreg cells (Savage et al., 2008).”

“This correlates with enhanced engagement of the acquired immune system indicating antigen recognition probably because the immune system has access to the products of mutated genes and/or recognizes tissue damage caused by invasion (Coussens and Pollard, 2011; Gajewski et al., 2013).”

“There are numerous additional reports of TAMs affecting angiogenesis in a wide range of models, mostly xenograft ones, and for further information the reader is referred to recent reviews on this topic (Coffelt et al., 2009; Nucera et al., 2011).”

“Similarly, amphotericin B enhances macrophage-mediated inhibition of glioma tumor-initiating cells (Sarkar et al., 2014).”

“be further discussed here, and there are excellent reviews defining their functions and classification elsewhere (Gabrilovich andNagaraj, 2009; Gabrilovich et al., 2012;Montero et al., 2012).”

“Importantly, an anatomical structure consisting of macrophages, endothelial, and tumor cells named the tumor microenvironment for metastasis (TMEM) is recognizable in histological sections and is predictive of metastatic potential in primary human breast cancers (Rohan et al., 2014).”

“Indeed, the culture of bonemarrow-derived DCs with anti PD-L1 antibody inhibits their activation, induces IL-10 expression, and suppresses cocultured CD4 T cell activation (Kuipers et al., 2006).”

“In this model, macrophage-synthesized WNT7b targets vascular endothelial cells, stimulating their production of VEGF, resulting in the angiogenic switch (Yeo et al., 2014).”

“However, it is challenging to determine the specific impact of TAM PD-1 ligand expression on effector cells inhibition in vivo since numerous cells in the tumor microenvironment express PD-L1 (Greaves and Gribben, 2013).”

“a loss-of function of CSF1 in the PyMT mouse model also resulted in a dramatic recruitment of macrophages and a rescue of angiogenesis that resulted in an acceleration of tumor progression to malignancy (Lin et al., 2007).”

“…elegant experiments using photoconvertible fluorescent lineage tracing of spleen and BM monocytes suggest that the splenic contribution is minor and that BM is the primary source of monocytes that generate TAMs at least in the Lewis Lung carcinoma syngeneic transplant model (Shand et al., 2014).”

“Furthermore CCL9 acting through its receptor, CCR1, recruits immature myeloid cells in colon cancer models (Kitamura et al., 2010; Kitamura et al., 2007).”

“This differential affinity suggests direct competition for the ligand binding as a mechanism to induce suppression (Greenwald et al., 2002).”

“TAM expression of B7-1 and B7-2 was shown to be dependent on their activation phenotype; both molecules are expressed by proinflammatory macrophages and are downregulated by antiinflammatory macrophages (Ding et al., 1993; Flores Villanueva et al., 1994; Kennedy et al., 2013).”

“Consequently, they will not be further discussed here, and there are excellent reviews defining their functions and classification elsewhere (Gabrilovich andNagaraj, 2009; Gabrilovich et al., 2012;Montero et al., 2012).”

“However, the specific inhibitory effect mediated by TAMs in vivo is still unknown and as with PD1 ligands, CTLA-4 ligands are expressed on some human tumors and other immune cells (Greaves and Gribben, 2013; Tamura et al., 2005; Tirapu et al., 2006).”

“Because this process is dependent on a different cell type to classical macrophages it will not be reviewed further here, but readers are referred to recent reviews that discuss the process and therapeutic opportunities (Camacho and Pienta, 2014; Esposito and Kang, 2014; Mundy, 2002).”

“However, the specific inhibitory effect mediated by TAMs in vivo is still unknown and as with PD1 ligands, CTLA-4 ligands are expressed on some human tumors and other immune cells (Greaves and Gribben, 2013; Tamura et al., 2005; Tirapu et al., 2006).”

“These HLA-G negative tumors might rely on myeloid cell 52 Immunity 41, July 17, 2014 ª2014 Elsevier Inc. HLA-G expression as an effector of inhibitory mechanisms.”

“chemotherapy, suggesting an immunosuppressive mechanism (Boissonnas et al., 2013; Engelhardt et al., 2012).”

“In addition, clotting upregulates VECM1 on endothelial cells that promote myeloid cell attachment and thus their recruitment (Ferjan ci c et al., 2013).”

“TAM expression of B7-1 and B7-2 was shown to be dependent on their activation phenotype; both molecules are expressed by proinflammatory macrophages and are downregulated by antiinflammatory macrophages (Ding et al., 1993; Flores Villanueva et al., 1994; Kennedy et al., 2013).”

“Monocytes and macrophages can themselves express members of the LILRB inhibitory receptors family (LIT-2 and LIT-4) that upon binding HLA-G causes them to acquire immunosuppressive phenotype through the secretion of IL-10 and TGF-b1 (Brown et al., 2004;McIntire et al., 2004).”

“affected tumor malignancy in both xenograft and GEM models of cancer (Abraham et al., 2010; Lin et al., 2001; Qian and Pollard, 2010; Quail and Joyce, 2013).”

“Both PD-L1 and L2 are regulated in TAMs and myeloid-derived suppressor cells (See below—MDSC) (Belai et al., 2014; Duraiswamy et al., 2013).”

“In addition, the expression of B7-H4 on TAMs was found to correlate with clinical stage of lung carcinoma and gastric cancer (Chen et al., 2012; Matsunaga et al., 2011).”

“Because this process is dependent on a different cell type to classical macrophages it will not be reviewed further here, but readers are referred to recent reviews that discuss the process and therapeutic opportunities (Camacho and Pienta, 2014; Esposito and Kang, 2014; Mundy, 2002).”

“Substantial evidence indicates that macrophages, rather than being tumoricidal as suggested after their activation in vitro (Fidler, 1988), adopt a protumoral phenotype in vivo both in the primary and metastatic sites (Biswas et al., 2013).”