Metastasis-associated Macrophages Research Articles

Abstract 6866: Investigating the role of Epha2 in breast cancer-mediated myeloid cell expansion and function

Abstract Bone metastases are present in over 70% of metastatic breast cancer patients, and current therapy reduces osteolysis in tumor-induced bone disease (TIBD) but does not improve overall survival. Frequently, TIBD is characterized by an increase in osteoclast proliferation and bone-resorbing activity, and patients often experience pain and pathologic fractures. Increased bone resorption releases growth factors that aid in the progression of tumor growth causing the ‘vicious cycle’. Additionally, bone metastatic patients are often resistant to first- and second-line therapies highlighting an importance for research in this area. The receptor tyrosine kinase, Ephrin-type-A 2 receptor (EphA2), participates in both forward and reverse signaling during receptor/ligand engagement and participates in many physiological and pathological processes, including bone homeostasis, immunity, and cancer. Interestingly, EphA2 is highly expressed in bone metastases, and previous evidence suggests that EphA2 can promote TIBD and reduce anti-tumor immunity. We therefore hypothesize that increased expression of EphA2 in breast cancer cells at the site of bone metastasis promotes expansion of myeloid cells, leading to TIBD. Previous data shows a reduction in osteoclast number and bone lesions following EphA2 knockout. However, the mechanism by which EphA2 mediates the expansion of osteoclasts is not well understood. In agreement with previous work, our data shows that EphA2 deletion in murine breast cancer (BC) cell line, 4T1, reduces bone lesion area and number and the presence of bone metastasis-associated macrophages. Additionally, we observed a significant reduction in Ly6Chi cells. To assess the effects of EphA2 deletion on the behavior of 4T1 cells, we conducted a wound healing assay and observed a decrease in wound healing capacity in 4T1 cells following EphA2 deletion. This suggests that EphA2 expression in 4T1 cells may increase their metastatic potential. We will repeat these studies and incorporate factors from the bone marrow. Our future studies will investigate the role of EphA2 signaling and reverse signaling in BC cell lines, myeloid progenitor cells, osteoclasts, and bone metastasis-associated macrophages. We will utilize murine and human BC cell lines to elucidate the effects of EphA2 deletion in BC on immune cell populations, bone metastasis, and tumor progression. Additionally, we will investigate how EphA2 reverse signaling impacts osteoclasts’ ability to modulate immune cells. We aim to reduce TIBD and tumor burden by reducing the expansion of osteoclasts and shifting the immune cells to an anti-tumorigenic phenotype. Our studies will provide additional insight into the underlying mechanisms of the vicious cycle which will offer new treatment strategies for patients with osteolytic breast cancer. Citation Format: Dominique Parker, Verra Ngwa, Jin Chen, Julie Rhoades. Investigating the role of Epha2 in breast cancer-mediated myeloid cell expansion and function [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6866.

Abstract 1324: EphA2 Expression in Breast Cancer Mediates Osteoclast Expansion and Promotes Osteolytic Bone Metastasis

Abstract Breast cancer is the most diagnosed cancer in the U.S., with a 5-year survival rate of less than 30% for patients with distant metastases. Bone metastases are present in over 70% of metastatic breast cancer patients, and current treatments treat the symptoms of osteolytic bone metastasis (OBM), but do not improve survival rates. OBM is characterized by an increase in osteoclast proliferation and bone-resorbing activity. Moreover, receptor tyrosine kinase, Ephrin-type-A 2 receptor (EphA2), is highly expressed in bone metastases, and previous evidence suggests its involvement in OBM and anti-tumor immunity. EphA2 participates in both forward and reverse signaling during receptor/ligand engagement and mediates various biological processes such as tissue organization and homeostasis and inflammation, as well as oncogenic processes like epithelial-mesenchymal transition. However, the mechanism by which EphA2 mediates the expansion of osteoclasts is not well understood. Analysis of transcriptomic data of patients with bone metastases in The Metastatic Breast Cancer Project (Provisional, December 2021) resulted in over 3500 genes that significantly correlated with EphA2 expression (Spearman's Correlation ≥ 0.35). Using a publicly available Gene Ontology reference list for myeloid differentiation, we identified 155 genes that have a significant positive correlation with EphA2 expression. To assess the effect of EphA2 expression on genes associated with osteoclasts, we compared the same samples to reference lists for osteoclast differentiation and proliferation. We identified 17 genes of interest that significantly correlate with EphA2 expression, further underscoring the possible involvement of EphA2 in OBM. We then compared the patients without bone metastases to the same osteoclast reference lists and found no significant correlations. We hypothesize that increased expression of EphA2 in breast cancer cells at the site of bone metastasis promotes osteoclast expansion, leading to OBM. We aim to investigate EphA2 reverse signaling in myeloid progenitor cells, osteoclasts, and bone metastasis-associated macrophages. We will utilize breast cancer mouse models to elucidate the effects of EphA2 inhibition on immune cell populations, bone metastasis, and tumor progression. We aim to reduce OBM and tumor burden by reducing the expansion of osteoclasts and shifting the immune cells to an anti-tumorigenic phenotype, which will offer new treatment strategies for osteolytic breast cancer. Citation Format: Dominique Parker, Verra Ngwa, Logan Northcutt, Natalie Bennett, Erik Beadle, Jade Miller, JIn Chen, Julie Rhoades. EphA2 Expression in Breast Cancer Mediates Osteoclast Expansion and Promotes Osteolytic Bone Metastasis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1324.

A timeline of tumour-associated macrophage biology.

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.

CD36-mediated metabolic crosstalk between tumor cells and macrophages affects liver metastasis

Liver metastasis is highly aggressive and treatment-refractory, partly due to macrophage-mediated immune suppression. Understanding the mechanisms leading to functional reprogramming of macrophages in the tumor microenvironment (TME) will benefit cancer immunotherapy. Herein, we find that the scavenger receptor CD36 is upregulated in metastasis-associated macrophages (MAMs) and deletion of CD36 in MAMs attenuates liver metastasis in mice. MAMs contain more lipid droplets and have the unique capability in engulfing tumor cell-derived long-chain fatty acids, which are carried by extracellular vesicles. The lipid-enriched vesicles are preferentially partitioned into macrophages via CD36, that fuel macrophages and trigger their tumor-promoting activities. In patients with liver metastases, high expression of CD36 correlates with protumoral M2-type MAMs infiltration, creating a highly immunosuppressive TME. Collectively, our findings uncover a mechanism by which tumor cells metabolically interact with macrophages in TME, and suggest a therapeutic potential of targeting CD36 as immunotherapy for liver metastasis.

Interleukin 4 Controls the Pro-Tumoral Role of Macrophages in Mammary Cancer Pulmonary Metastasis in Mice.

Simple SummaryMetastasis is the main cause of death from breast cancer. In mouse models of breast cancer lung metastasis, macrophages enhance metastasis by promoting tumor cell seeding and persistent growth. Here, we show that interleukin-4 (IL4) is required for this process as IL4 receptor (IL4rα)-null mice develop fewer and smaller lung metastases. This deficiency is partially rescued by adoptive transfer of wild-type monocytes. IL4 signaling in macrophages upregulates the expression of the chemokine receptor CXCR2, necessary for IL4-mediated tumor cell extravasation in vitro. In addition, expression of several other genes already causally associated with lung metastasis including Ccl2, Csf1, Ccr1, Hgf and Flt1 are upregulated in macrophages. High-resolution intravital imaging at the time of metastatic seeding showed reduced physical interaction between tumor cells and IL4rα-deficient macrophages, showing the dependence on IL4. We conclude that IL4 signaling in monocytes and macrophages is important during seeding and growth of breast metastasis in the lung.Metastasis is the systemic manifestation of cancer and the main cause of death from breast cancer. In mouse models of lung metastases, recruitment of classical monocytes from blood to the lung and their differentiation to metastasis-associated macrophages (MAMs) facilitate cancer cell extravasation, survival and growth. Ablation of MAMs or their monocytic progenitors inhibits metastasis. We hypothesized that factors controlling macrophage polarization modulate tumor cell extravasation in the lung. We evaluated whether signaling by Th1 or Th2 cytokines in macrophages affected transendothelial migration of tumor cells in vitro. Interferon gamma and LPS inhibited macrophage-dependent tumor cell extravasation while the Th2 cytokine interleukin-4 (IL4) enhanced this process. We demonstrated that IL4 receptor (IL4rα)-null mice developed fewer and smaller lung metastasis in E0771-LG mammary cancer models of this disease. Adoptive transfer of wild-type monocytes to IL4rα-deficient mice partially rescued this phenotype. IL4 signaling in macrophages controlled the expression of the chemokine receptor CXCR2, necessary for IL4-mediated tumor cell extravasation in vitro. Furthermore, IL4 signaling in macrophages regulated the transcript abundance of several other genes already causally associated with mammary cancer lung metastasis including Ccl2, Csf1, Ccr1, Hgf and Flt1. The central role of IL4 signaling in MAMs was confirmed by high-resolution intravital imaging of the lung in mice at the time of metastatic seeding, which showed reduced physical interaction between tumor cells and IL4rα-deficient macrophages. This interaction with wild-type MAMs enhanced tumor cell survival and seeding, which was lost in the IL4rα mice. These data indicate that IL4 signaling in monocytes and macrophages is key during seeding and growth of breast metastasis in the lung, as it regulates pro-tumoral paracrine signaling between cancer cells and macrophages.

Reactive myelopoiesis and FX-expressing monocyte-derived macrophages triggered by chemotherapy promotes cancer lung metastasis

Abstract Lung metastasis-associated macrophages (MAMs) play a very important role in tumor metastasis. In mouse models of lung metastasis, interstitial macrophages (IMs) markedly accumulate and differentiate into MAMs. Several pre-clinical studies have demonstrated that certain cytotoxic drugs enhance metastasis. In this study, we hypothesize that host responses to chemotherapy treatment enhance breast cancer metastasis by modulation of lung macrophage toward a pro-metastatic phenotype. Here, we show that multi-dose chemo treatment promotes breast cancer lung metastasis in the spontaneous breast cancer animal model. Pretreatment with Gemcitabine and Paclitaxel induces a significantly higher number of pulmonary metastases in the experimental metastasis model. Mechanistically, multi-dose of chemotherapy induces reactive myelopoiesis characterized by increase of BM LSK cells and multipotent progenitors in tumor-free mice, which is associated with the increase of mitochondrial ROS. Further, chemokine CCL2 expression is significantly increased in the plasma and lung tissues of chemo-treated tumor-free mice. Consequently, the percentage of BM Ly6C+ monocytes are significantly increased because of increased progenitor monocyte differentiation potential and monocyte proliferation, which results in an increase of lung monocytes and IMs with enhanced expression of coagulation factor X (FX). Inhibition of activated FX (FXa) can inhibit chemotherapy-induced lung metastasis. Together, these studies suggest a novel mechanism for chemotherapy induced metastasis via the modulation of BM-derived monocytes/macrophages and extravascular clotting in lung tissues. Supported by grants from NIH (P20GM135004)

Activatable Fluorophores for Imaging Immune Cell Function.

ConspectusOptical imaging has become an essential tool to study biomolecular processes in live systems with unprecedented spatial resolution. New fluorescent technologies and advances in optical microscopy have revolutionized the ways in which we can study immune cells in real time. For example, activatable fluorophores that emit signals after target recognition have enabled direct imaging of immune cell function with enhanced readouts and minimal background. In this Account, we summarize recent advances in the chemical synthesis and implementation of activatable fluorescent probes to monitor the activity and the role of immune cells in different pathological processes, from infection to inflammatory diseases or cancer. In addition to the contributions that our group has made to this field, we review the most relevant literature disclosed over the past decade, providing examples of different activatable architectures and their application in diagnostics and drug discovery. This Account covers the imaging of the three major cell types in the immune system, that is, neutrophils, macrophages, and lymphocytes. Attracted by the tunability and target specificity of peptides, many groups have designed strategies based on fluorogenic peptides whose fluorescence emission is regulated by the reaction with enzymes (e.g., MMPs, cathepsins, granzymes), or through Förster resonance energy transfer (FRET) mechanisms. Selective imaging of immune cells has been also achieved by targeting different intracellular metabolic routes, such as lipid biogenesis. Other approaches involve the implementation of diversity-oriented fluorescence libraries or the use of environmentally sensitive fluorescent scaffolds (e.g., molecular rotors). Our group has made important progress by constructing probes to image metastasis-associated macrophages in tumors, apoptotic neutrophils, or cytotoxic natural killer (NK) cells against cancer cells, among other examples. The chemical probes covered in this Account have been successfully validated in vitro in cell culture systems, and in vivo in relevant models of inflammation and cancer. Overall, the range of chemical structures and activation mechanisms reported to sense immune cell function is remarkable. However, the emergence of new strategies based on new molecular targets or activatable mechanisms that are yet to be discovered will open the door to track unexplored roles of immune cells in different biological systems. We anticipate that upcoming generations of activatable probes will find applications in the clinic to help assessing immunotherapies and advance precision medicine. We hope that this Account will evoke new ideas and innovative work in the design of fluorescent probes for imaging cell function.

OTME-6. Deep sequencing reveals heterogeneity of brain metastasis-associated macrophages and microglia and uncovers their cell type-specific functions within the tumor microenvironment

Macrophages represent a highly plastic cell type,indispensable for tissue and organ homeostasis, as well as innate immunity. Basic and translational research attributed tumor-promoting functions to macrophages, and their presence is often associated to poor patient prognosis and therapy resistance. While brain-resident macrophages, the so-called microglia (MG), represent the major immune cell type in the parenchyma under normal conditions, primary and metastatic brain tumors induce the recruitment of different immune cell types from the periphery, including monocyte-derived macrophages (MDM). Controversy remained about the redundancy of disease-associated molecular signatures and functions. The identification of markers that reliably distinguish brain-resident from blood-borne tumor-associated macrophages (TAMs) allowed the interrogation of molecular traits of different TAM populations in mouse and human brain tumors.Using RNA-Seq, we demonstrated that TAMs rapidly acquire disease-associated transcriptional programs upon initial tumor infiltration, while gene expression remained stable during different stages of BrM progression. Across different BrM models, disease-associated transcriptional changes revealed lineage-specific, non-redundant functions of TAM populations, which was further reflected by cell type-specific occupation of different niches within the BrM microenvironment. Furthermore, we observed dose- and cell type-specific immune modulatory effects of whole brain radiotherapy on myeloid cells in BrM leading to a transient loss of disease-associated transcriptional programs predominately in blood-borne myeloid populations. This effect can at least in part be attributed to a replenishment of the recruited macrophage pool. This observation was further supported by scRNA-Seq analyses revealing higher heterogeneity of TAM-MDM compared to TAM-MG under treatment-naïve conditions and in response to radiotherapy.Together, our results point towards the phenotypic plasticity of TAMs, especially MDMs, and the contribution of each compartment in instigating cancer-associated inflammation or the establishment of an immuno-suppressive TME. While TAM-MG exert functions related to pro-inflammatory responses, TAM-MDM are rather involved in tissue repair and regulation of adaptive immune cell functions.

Abstract CT144: Development of GMI-1359, a novel agent targeting tumor-microenvironment cross-talk in bone metastatic cancer

Abstract Pre-clinical and clinical data suggest that the bone marrow (BM) environment provides breast cancer (BC) cells a protective haven against chemotherapeutic insult, endocrine therapies, and immune recognition. Novel agents that intercept cross-talk between BC cells and the host could therefore improve the depth of response to therapies and potentially increase overall survival in metastatic BC. We have previously shown that bone metastatic BC cells reside in perivascular niches expressing high levels of SDF-1α (CXCL12) and E-selectin, two molecules for which emerging data have demonstrated a significant role in metastatic BC progression. Our prior pre-clinical studies have shown that E-selectin inhibition specifically prevents circulating BC cells from homing to the BM, whereas CXCR4 blockade mobilizes micrometastases from the marrow into circulation, where they may be sensitized to chemotherapeutic cell kill. Given the normal role of these molecules in host immune responses, E-selectin/CXCR4 blockade has additional potential to impact the tumor immune microenvironment. GMI-1359 is a small molecule, glycomimetic compound with dual inhibitory activity against E-selectin and CXCR4. We hypothesize that GMI-1359 can block E-selectin and CXCR4 binding to E-selectin ligands and SDF-1, disrupting the protective effects of the bone microenvironment on tumor cells. A single-center, phase 1b, open-label, single and multiple ascending dose study (NCT04197999) is therefore evaluating GMI-1359 in patients with HR+ metastatic breast cancer with bony metastases who are stable or minimally progressive on endocrine therapy, with or without a CDK4/6 inhibitor. To date, 2 patients have been enrolled and received single monthly doses of 3.5, 5.0 and 7.0 mg/kg followed by 3 daily doses of 7 mg/kg. Both patients have completed treatment with no dose limiting toxicities observed. Coincident with GMI-1359 administration at all dose levels, we observed mobilization of CD34+ cells coupled with a reduction in elevated serum sE-selectin levels, demonstrating the dual functionality of the compound. Initial peripheral blood immunophenotype data on one patient revealed a redistribution of myeloid derived suppressor cells and a shift in macrophage polarization from M2 to M1. Notably, we observed similar immune alterations in ongoing preclinical studies in the E0771 syngeneic BC mouse model. We found that orthotopically-engrafted mice treated with single agent 1359 had a significant increase in the ratio of CD8/Tregulatory cells isolated from the primary tumor and BM. We also observed a decrease in primary tumor M-MDSCs and lung metastasis-associated macrophages. In summary, our preliminary clinical data suggest that GMI-1359 is well-tolerated and elicits on-target effects expected from disruption of the host microenvironment. Ongoing clinical and preclinical work will define the efficacy of this strategy to enhance responses to chemo and immune therapies. Citation Format: P. Kelly Marcom, Trevor T. Price, Andrew S. Murray, William E. Fogler, John L. Magnani, Helen Thackray, Eric Feldman, Dorothy Sipkins. Development of GMI-1359, a novel agent targeting tumor-microenvironment cross-talk in bone metastatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT144.

Induction of interferon signaling and allograft inflammatory factor 1 in macrophages in a mouse model of breast cancer metastases.

Background: Metastatic breast cancer cells recruit macrophages (metastasis-associated macrophages, or MAMs) to facilitate their seeding, survival and outgrowth. However, a comprehensive understanding of the gene expression program in MAMs and how this program contributes to metastasis remain elusive. Methods: We compared the transcriptomes of MAMs recruited to lung metastases and resident alveolar macrophages (RAMs) and identified a large variety of differentially expressed genes and their associated signaling pathways. Some of the changes were validated using qRT-PCR and immunofluorescence. To probe the functional relevance to metastatic growth, a gene-targeting mouse model of female mice in the C57BL6/J background was used to study allograft inflammatory factor 1 (AIF1, also known as ionized calcium-binding adapter molecule 1 or IBA1). Results: Interferon signaling is one of the most activated pathways in MAMs, with strong upregulation of multiple components of the pathway and a significant enrichment for the gene signatures of interferon-alpha-treated human macrophages. Aif1, an interferon-responsive gene that regulates multiple macrophage activities, was robustly induced in MAMs. Aif1 deficiency in MAMs, however, did not affect development of lung metastases, suggesting that AIF1 indicates MAM activation but is dispensable for regulating metastasis. Conclusions: The drastically different gene expression profile of MAMs as compared to RAMs suggests an important role in promoting metastatic growth. Dissection of the underlying mechanisms and functional validation of potential targets in the profile may provide novel therapeutic strategies for the treatment of metastatic diseases.

Induction of interferon signaling and allograft inflammatory factor 1 in macrophages in a mouse model of breast cancer metastases

Background: Metastatic breast cancer cells recruit macrophages (metastasis-associated macrophages, or MAMs) to facilitate their seeding, survival and outgrowth. However, a comprehensive understanding of the gene expression program in MAMs and how this program contributes to metastasis remain elusive. Methods: We compared the transcriptomes of MAMs recruited to lung metastases and resident alveolar macrophages (RAMs) and identified a large variety of differentially expressed genes and their associated signaling pathways. Some of the changes were validated using qRT-PCR and immunofluorescence. To probe the functional relevance to metastatic growth, a gene-targeting mouse model of female mice in the C57BL6/J background was used to study allograft inflammatory factor 1 (AIF1, also known as ionized calcium-binding adapter molecule 1 or IBA1). Results: Interferon signaling is one of the most activated pathways in MAMs, with strong upregulation of multiple components of the pathway and a significant enrichment for the gene signatures of interferon-alpha-treated human macrophages. Aif1, an interferon-responsive gene that regulates multiple macrophage activities, was robustly induced in MAMs. Aif1 deficiency in MAMs, however, did not affect development of lung metastases, suggesting that AIF1 indicates MAM activation but is dispensable for regulating metastasis. Conclusions: The drastically different gene expression profile of MAMs as compared to RAMs suggests an important role in promoting metastatic growth. Dissection of the underlying mechanisms and functional validation of potential targets in the profile may provide novel therapeutic strategies for the treatment of metastatic diseases.

Tissue-Resident and Recruited Macrophages in Primary Tumor and Metastatic Microenvironments: Potential Targets in Cancer Therapy.

Macrophages within solid tumors and metastatic sites are heterogenous populations with different developmental origins and substantially contribute to tumor progression. A number of tumor-promoting phenotypes associated with both tumor- and metastasis-associated macrophages are similar to innate programs of embryonic-derived tissue-resident macrophages. In contrast to recruited macrophages originating from marrow precursors, tissue-resident macrophages are seeded before birth and function to coordinate tissue remodeling and maintain tissue integrity and homeostasis. Both recruited and tissue-resident macrophage populations contribute to tumor growth and metastasis and are important mediators of resistance to chemotherapy, radiation therapy, and immune checkpoint blockade. Thus, targeting various macrophage populations and their tumor-promoting phenotypes holds therapeutic promise. Here, we discuss various macrophage populations as regulators of tumor progression, immunity, and immunotherapy. We provide an overview of macrophage targeting strategies, including therapeutics designed to induce macrophage depletion, impair recruitment, and induce repolarization. We also provide a perspective on the therapeutic potential for macrophage-specific acquisition of trained immunity as an anti-cancer agent and discuss the therapeutic potential of exploiting macrophages and their traits to reduce tumor burden.

Induction of interferon signaling and allograft inflammatory factor 1 in macrophages in a mouse model of breast cancer metastases.

Background: Metastatic breast cancer cells recruit macrophages (metastasis-associated macrophages, or MAMs) to facilitate their seeding, survival and outgrowth. However, a comprehensive understanding of the gene expression program in MAMs and how this program contributes to metastasis remain elusive. Methods: We compared the transcriptomes of MAMs recruited to lung metastases and resident alveolar macrophages (RAMs) and identified a large variety of differentially expressed genes and their associated signaling pathways. Some of the changes were validated using qRT-PCR and immunofluorescence. To probe the functional relevance to metastatic growth, a gene-targeting mouse model of female mice in the C57BL6/J background was used to study allograft inflammatory factor 1 (AIF1, also known as ionized calcium-binding adapter molecule 1 or IBA1). Results: Interferon signaling is one of the most activated pathways in MAMs, with strong upregulation of multiple components of the pathway and a significant enrichment for the gene signatures of interferon-alpha-treated human macrophages. Aif1, an interferon-responsive gene that regulates multiple macrophage activities, was robustly induced in MAMs. Aif1 deficiency in MAMs, however, did not affect development of lung metastases, suggesting that AIF1 indicates MAM activation but is dispensable for regulating metastasis. Conclusions: The drastically different gene expression profile of MAMs as compared to RAMs suggests an important role in promoting metastatic growth. Dissection of the underlying mechanisms and functional validation of potential targets in the profile may provide novel therapeutic strategies for the treatment of metastatic diseases.

Abstract IA001: Deconvoluting the tumor microenvironment to enhance immunotherapy

Abstract Cancer evolution to metastasis (the metastatic cascade) involves not only the acquisition of mutations or epigenetic modifications in oncogenes and tumor suppressor gene but also an interplay with the surrounding tumor microenvironment (TME). The TME promotes every step of the metastatic cascade. In this TME immune cells of both the acquired and innate system are highly represented and generally these act in a pro-tumoral fashion or are inert. Much work has been performed on primary tumors and studies of metastasis have lagged behind despite the fact that ~90% of deaths from solid tumors are caused by this process. We have addressed this issue using mouse models of breast cancer lung and bone metastasis. Our studies have particularly focussed upon metastasis associated macrophages (MAMs) that are distinct from the tumor associated macrophages in primary tumors. We have shown that MAMs and their progenitors promote metastatic seeding and persistent growing in both organs. In the lung particularly we have described complex signalling pathways including the regulation of recruitment of classical monocytes to the metastatic site and stages of MAM differentiation form these monocytes. Our data show not only direct pro-tumoral effects of these cells on tumor cells such as conferring survival signals but also indirect ones via suppression of tumor cell cytotoxic T and NK cell activity (Kitamura et al., 2017; Kitamura et al., 2019; Ma et al., 2020). In parallel we have profiled TAMs in primary human breast and endometrial cancers and shown that they are different from circulating monocytes but surprisingly, are also different from each other. Further bioinformatic analysis and multiplex immunohistochemistry using breast cancer data sets shows that TAM signatures can be derived that predict poor disease specific overall survival and disease sub-classes. These TAMs have many different subtypes within a tumor and thus may play independent roles including anti-tumoral roles particularly in the face of immunotherapy. Exploiting these possibilities for therapy will be discussed (Lopez-Yrigoyen et al., 2020). Kitamura, T., Doughty-Shenton, D., Cassetta, L., Fragkogianni, S., Brownlie, D., Kato, Y., Carragher, N., and Pollard, J.W. (2017). Monocytes Differentiate to Immune Suppressive Precursors of Metastasis-Associated Macrophages in Mouse Models of Metastatic Breast Cancer. Frontiers in immunology 8, 2004. Kitamura, T., Kato, Y., Brownlie, D., Soong, D.Y.H., Sugano, G., Kippen, N., Li, J., Doughty-Shenton, D., Carragher, N., and Pollard, J.W. (2019). Mammary Tumor Cells with High Metastatic Potential Are Hypersensitive to Macrophage-Derived HGF. Cancer immunology research 7, 2052-2064. Lopez-Yrigoyen, M., Cassetta, L., and Pollard, J.W. (2020). Macrophage targeting in cancer. Ann. N. Y. Acad. Sci. Ma, R.Y., Zhang, H., Li, X.F., Zhang, C.B., Selli, C., Tagliavini, G., Lam, A.D., Prost, S., Sims, A.H., Hu, H.Y., et al. (2020). Monocyte-derived macrophages promote breast cancer bone metastasis outgrowth. J. Exp. Med. 217. Citation Format: Jeffrey W. Pollard. Deconvoluting the tumor microenvironment to enhance immunotherapy [abstract]. In: Proceedings of the AACR Virtual Special Conference on the Evolving Tumor Microenvironment in Cancer Progression: Mechanisms and Emerging Therapeutic Opportunities; in association with the Tumor Microenvironment (TME) Working Group; 2021 Jan 11-12. Philadelphia (PA): AACR; Cancer Res 2021;81(5 Suppl):Abstract nr IA001.

Metastasis-associated macrophages constrain antitumor capability of natural killer cells in the metastatic site at least partially by membrane bound transforming growth factor β

BackgroundMetastatic breast cancer is a leading cause of cancer-related death in women worldwide. Infusion of natural killer (NK) cells is an emerging immunotherapy for such malignant tumors, although elimination of...

Glufosinate constrains synchronous and metachronous metastasis by promoting anti-tumor macrophages.

Glutamine synthetase (GS) generates glutamine from glutamate and controls the release of inflammatory mediators. In macrophages, GS activity, driven by IL10, associates to the acquisition of M2‐like functions. Conditional deletion of GS in macrophages inhibits metastasis by boosting the formation of anti‐tumor, M1‐like, tumor‐associated macrophages (TAMs). From this basis, we evaluated the pharmacological potential of GS inhibitors in targeting metastasis, identifying glufosinate as a specific human GS inhibitor. Glufosinate was tested in both cultured macrophages and on mice bearing metastatic lung, skin and breast cancer. We found that glufosinate rewires macrophages toward an M1‐like phenotype both at the primary tumor and metastatic site, countering immunosuppression and promoting vessel sprouting. This was also accompanied to a reduction in cancer cell intravasation and extravasation, leading to synchronous and metachronous metastasis growth inhibition, but no effects on primary tumor growth. Glufosinate treatment was well‐tolerated, without liver and brain toxicity, nor hematopoietic defects. These results identify GS as a druggable enzyme to rewire macrophage functions and highlight the potential of targeting metabolic checkpoints in macrophages to treat cancer metastasis.

Targeting Macrophages: Friends or Foes in Disease?

Macrophages occupy a prominent position during immune responses. They are considered the final effectors of any given immune response since they can be activated by a wide range of surface ligands and cytokines to acquire a continuum of functional states. Macrophages are involved in tissue homeostasis and in the promotion or resolution of inflammatory responses, causing tissue damage or helping in tissue repair. Knowledge in macrophage polarization has significantly increased in the last decade. Biomarkers, functions, and metabolic states associated with macrophage polarization status have been defined both in murine and human models. Moreover, a large body of evidence demonstrated that macrophage status is a dynamic process that can be modified. Macrophages orchestrate virtually all major diseases—sepsis, infection, chronic inflammatory diseases (rheumatoid arthritis), neurodegenerative disease, and cancer—and thus they represent attractive therapeutic targets. In fact, the possibility to “reprogram” macrophage status is considered as a promising strategy for designing novel therapies. Here, we will review the role of different tissue macrophage populations in the instauration and progression of inflammatory and non-inflammatory pathologies, as exemplified by rheumatoid arthritis, osteoporosis, glioblastoma, and tumor metastasis. We will analyze: 1) the potential as therapeutic targets of recently described macrophage populations, such as osteomacs, reported to play an important role in bone formation and homeostasis or metastasis-associated macrophages (MAMs), key players in the generation of premetastatic niche; 2) the current and potential future approaches to target monocytes/macrophages and their inflammation-causing products in rheumatoid arthritis; and 3) the development of novel intervention strategies using oncolytic viruses, immunomodulatory agents, and checkpoint inhibitors aiming to boost M1-associated anti-tumor immunity. In this review, we will focus on the potential of macrophages as therapeutic targets and discuss their involvement in state-of-the-art strategies to modulate prevalent pathologies of aging societies.

Targeting Macrophage-Recruiting Chemokines as a Novel Therapeutic Strategy to Prevent the Progression of Solid Tumors.

Solid tumors are initiated by genetic mutations in non-hematopoietic cells and progress into invasive malignant tumors. This tumor progression often culminates in metastatic disease that is largely refractory to current therapeutic modalities and thus dramatically reduces survival of tumor patients. As solid tumors account for more than 80% of cancer-related deaths, it is necessary to develop novel therapeutic strategies to treat the diseases. An attractive strategy is to target macrophages in both primary tumors [known as tumor-associated macrophages (TAMs)] and metastatic tumors [called metastasis-associated macrophages (MAMs)]. TAMs and MAMs are abundant in most solid tumors and can promote tumor metastasis. Several studies in various models of solid tumors suggest that the accumulation of TAMs, MAMs, and their progenitor cells is regulated by chemokine ligands released by tumor and stromal cells. Consequently, these macrophage-recruiting chemokines could be potential therapeutic targets to prevent malignant tumor development through disruption of the accumulation of pro-metastatic macrophages. This review will discuss the role of chemokine ligands and their receptors in TAM and MAM accumulation in primary and secondary tumor sites, and finally discuss the therapeutic potential of inhibitors against these macrophage-recruiting chemokines.

Monocytes Differentiate to Immune Suppressive Precursors of Metastasis-Associated Macrophages in Mouse Models of Metastatic Breast Cancer.

Metastasis-associated macrophages (MAMs) play pivotal roles in breast cancer metastasis by promoting extravasation and survival of metastasizing cancer cells. In a metastatic breast cancer mouse model, we previously reported that circulating classical monocytes (C-MOs) preferentially migrated into the tumor-challenged lung where they differentiated into MAMs. However, the fate and characteristics of C-MOs in the metastatic site has not been defined. In this study, we identified that adoptively transferred C-MOs (F4/80lowCD11b+Ly6C+) differentiated into a distinct myeloid cell population that is characterized as F4/80highCD11bhighLy6Chigh and gives rise to MAMs (F4/80lowCD11bhighLy6Clow) within 18 h after migration into the metastatic lung. In mouse models of breast cancer, the CD11bhighLy6Chigh MAM precursor cells (MAMPCs) were commonly found in the metastatic lung, and their accumulation was increased during metastatic tumor growth. The morphology and gene expression profile of MAMPCs were distinct from C-MOs and had greater similarity to MAMs. For example MAMPCs expressed mature macrophage markers such as CD14, CD36, CD64, and CD206 at comparable levels with MAMs, suggesting that MAMPCs have committed to a macrophage lineage in the tumor microenvironment. MAMPCs also expressed higher levels of Arg1, Hmox1, and Stab1 than C-MOs to a comparable level to MAMs. Expression of these MAM-associated genes in MAMPCs was reduced by genetic deletion of colony-stimulating factor 1 receptor (CSF1R). On the other hand, transient CSF1R blockade did not reduce the number of MAMPCs in the metastatic site, suggesting that CSF1 signaling is active in MAMPCs but is not required for their accumulation. Functionally MAMPCs suppressed the cytotoxicity of activated CD8+ T cells in vitro in part through superoxide production. Overall, our results indicate that immediately following migration into the metastatic tumors C-MOs differentiate into immunosuppressive cells that have characteristics of monocytic myeloid-derived suppressor cell phenotype and might be targeted to enhance efficacy of immunotherapy for metastatic breast cancer.

Imaging macrophage distribution and density in mammary tumors and lung metastases using fluorine-19 MRI cell tracking.

PurposeThe presence of tumor‐associated macrophages (TAMs) correlates with breast cancer progression and metastatic spread. Metastasis‐associated macrophages (MAMs) are also recruited to distant sites, where they support metastatic growth. In this study, we demonstrate that in vivo fluorine‐19 (19F)‐based MRI cell tracking can evaluate the density and distribution of macrophages within murine breast cancer tumors and associated metastases.MethodsThree murine breast cancer cell lines with different metastatic potentials (4T1, 168FARN, and 67NR) were implanted into the mammary fat pad in mice. In vivo whole body 19F MRI was performed on tumor‐bearing mice 24 hours post‐intravenous injection of a perfluorocarbon (PFC) agent, which was taken up by macrophages in situ.ResultsTAMs were detected mainly in the periphery of primary tumors, and higher numbers of TAMs were detected in the more aggressive 4T1 tumors. Tumors had significantly greater 19F spins/mm3 when they were smaller, suggesting more TAM infiltration in early‐stage tumors. 19F signal was observed within lung metastases in mice with 4T1 tumors, and fluorescence microscopy confirmed the presence of PFC‐positive macrophages.ConclusionThis study shows for the first time proof of the ability to use MRI cell tracking to visualize MAMs in the lungs. The ability to detect and monitor the number of TAMs in individual tumors with 19F MRI would allow for identification of breast tumors with heavy infiltration of TAMs and could be used as a biomarker for decisions about how to best treat these patients as well as for monitoring responses to therapy. Magn Reson Med 80:1138–1147, 2018. © 2018 International Society for Magnetic Resonance in Medicine.