A distinguishing gene signature shared by tumor-infiltrating Tie2-expressing monocytes, blood “resident” monocytes, and embryonic macrophages suggests common functions and developmental relationships

Milestones in tumor vascularization and its therapeutic targeting

Research into the mechanisms and manifestations of solid tumor vascularization was launched more than 50 years ago with the proposition and experimental demonstrations that angiogenesis is instrumental for tumor growth and was, therefore, a promising therapeutic target. The biological knowledge and therapeutic insights forthcoming have been remarkable, punctuated by new concepts, many of which were not foreseen in the early decades. This article presents a perspective on tumor vascularization and its therapeutic targeting but does not portray a historical timeline. Rather, we highlight eight conceptual milestones, integrating initial discoveries and recent progress and posing open questions for the future.

Navigating tumor angiogenesis: therapeutic perspectives and myeloid cell regulation mechanism

Sustained angiogenesis stands as a hallmark of cancer. The intricate vascular tumor microenvironment fuels cancer progression and metastasis, fosters therapy resistance, and facilitates immune evasion. Therapeutic strategies targeting tumor vasculature have emerged as transformative for cancer treatment, encompassing anti-angiogenesis, vessel normalization, and endothelial reprogramming. Growing evidence suggests the dynamic regulation of tumor angiogenesis by infiltrating myeloid cells, such as macrophages, myeloid-derived suppressor cells (MDSCs), and neutrophils. Understanding these regulatory mechanisms is pivotal in paving the way for successful vasculature-targeted cancer treatments. Therapeutic interventions aimed to disrupt myeloid cell-mediated tumor angiogenesis may reshape tumor microenvironment and overcome tumor resistance to radio/chemotherapy and immunotherapy.

Improving cancer immunotherapy by preventing cancer stem cell and immune cell linking in the tumor microenvironment

Cancer stem cells are the key link between malignant tumor progression and drug resistance. This cell population has special properties that are different from those of conventional tumor cells, and the role of cancer stem cell-related exosomes in progression of tumor malignancy is becoming increasingly clear. Cancer stem cell-derived exosomes carry a variety of functional molecules involved in regulation of the microenvironment, especially with regard to immune cells, but how these exosomes exert their functions and the specific mechanisms need to be further clarified. Here, we summarize the role of cancer stem cell exosomes in regulating immune cells in detail, aiming to provide new insights for subsequent targeted drug development and clinical strategy formulation.

VEGF-C-expressing TAMs rewire the metastatic fate of breast cancer cells

The expression of pro-lymphangiogenic VEGF-C in primary tumors is associated with sentinel lymph node metastasis in most solid cancer types. However, the impact of VEGF-C on distant organ metastasis remains unclear. Perivascular tumor-associated macrophages (TAMs) play a crucial role in guiding hematogenous spread of cancer cells by establishing metastatic pathways within the tumor microenvironment. This process supports breast cancer cell intravasation and metastatic dissemination. We show here that VEGF-C-expressing TAMs reduce the dissemination of mammary cancer cells to the lungs while concurrently increasing lymph node metastasis. These TAMs express podoplanin and interact with normalized tumor blood vessels expressing VEGFR3. Moreover, clinical data suggest inverse association between VEGF-C-expressing TAMs and breast cancer malignancy. Thus, our study elucidates the paradoxical role of VEGF-C-expressing TAMs in redirecting cancer cells to preferentially disseminate to lymph nodes rather than to lungs, partially achieved by normalizing tumor blood vessels and promoting lymphangiogenesis.

Reprogramming tumor-associated macrophages to outcompete endovascular endothelial progenitor cells and suppress tumor neoangiogenesis

Tumors develop by invoking a supportive environment characterized by aberrant angiogenesis and infiltration of tumor-associated macrophages (TAMs). In a transgenic model of breast cancer, we found that TAMs localized to the tumor parenchyma and were smaller than mammary tissue macrophages. TAMs had low activity of the metabolic regulator mammalian/mechanistic target of rapamycin complex 1 (mTORC1), and depletion of negative regulator of mTORC1 signaling, tuberous sclerosis complex 1 (TSC1), in TAMs inhibited tumor growth in a manner independent of adaptive lymphocytes. Whereas wild-type TAMs exhibited inflammatory and angiogenic gene expression profiles, TSC1-deficient TAMs had a pro-resolving phenotype. TSC1-deficient TAMs relocated to a perivascular niche, depleted protein C receptor (PROCR)-expressing endovascular endothelial progenitor cells, and rectified the hyperpermeable blood vasculature, causing tumor tissue hypoxia and cancer cell death. TSC1-deficient TAMs were metabolically active and effectively eliminated PROCR-expressing endothelial cells in cell competition experiments. Thus, TAMs exhibit a TSC1-dependent mTORC1-low state, and increasing mTORC1 signaling promotes a pro-resolving state that suppresses tumor growth, defining an innate immune tumor suppression pathway that may be exploited for cancer immunotherapy.

AKR1B10 regulates M2 macrophage polarization to promote the malignant phenotype of gastric cancer

BACKGROUND

Immunotherapy has brought new hope to gastric cancer (GC) patients. Exploring the immune infiltration pattern in GC and the key molecules is critical for optimizing the efficacy of immunotherapy. Aldo-keto reductase family 1 member B10 (AKR1B10) is an inflammatory regulator and is closely related to the prognosis of patients with GC. However, the function of AKR1B10 in GC remains unclear.

METHODS

In the present study, the CIBERSORT algorithm was used to analyze the immune infiltration pattern in 373 samples in the Cancer Genome Atlas (TCGA) database. Differentially expressed genes (DEGs) were seared by combing the TCGA database and the Gene Expression Omnibus (GEO) database, and the key molecule AKR1B10 was identified by weighted gene coexpression network analysis (WGCNA). The biological functions of AKR1B10 in stomach adenocarcinoma (STAD) were investigated in vitro.

RESULTS

Macrophage polarization was the main immune infiltration pattern in GC, and the state of macrophage polarization was closely related to the pathological grading of GC and the clinical stage of patients. AKR1B10, MUC5AC, TFF2, GKN1, and PGC were significantly down-regulated in GC tissues. Low AKR1B10 expression induced M2 macrophage polarization and promoted the malignant phenotype of GC.

CONCLUSION

M2 macrophage polarization is the main immune infiltration pattern in GC. Low AKR1B10 expression induces M2 macrophage polarization and promotes the malignant transformation of GC.

Phenotypic and functional heterogeneity of monocytes in health and cancer in the era of high dimensional technologies

Monocytes have been traditionally classified in three discrete subsets, which can participate in the immune responses as effector cells or as precursors of myeloid-derived cells in circulation and tissues. However, recent advances in single-cell omics have revealed unprecedented phenotypic and functional heterogeneity that goes well beyond the three conventional monocytic subsets and propose a more fluid differentiation model. This novel concept does not only apply to the monocytes in circulation but also at the tissue site. Consequently, the binary model proposed for differentiating monocyte into M1 and M2 macrophages has been recently challenged by a spectrum model that more realistically mirrors the heterogeneous cues in inflammatory conditions. This review describes the latest results on the high dimensional characterization of monocytes and monocyte-derived myeloid cells in steady state and cancer. We discuss how environmental cues and monocyte-intrinsic properties may affect their differentiation toward specific functional and phenotypic subsets, the causes of monocyte expansion and reduction in cancer, their metabolic requirements, and the potential effect on tumor immunity.

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.

Estrogen Receptor Signaling in the Immune System

The immune system functions in a sexually dimorphic manner, with females exhibiting more robust immune responses than males. However, how female sex hormones affect immune function in normal homeostasis and in autoimmunity is poorly understood. In this review, we discuss how estrogens affect innate and adaptive immune cell activity and how dysregulation of estrogen signaling underlies the pathobiology of some autoimmune diseases and cancers. The potential roles of the major circulating estrogens, and each of the 3 estrogen receptors (ERα, ERβ, and G-protein coupled receptor) in the regulation of the activity of different immune cells are considered. This provides the framework for a discussion of the impact of ER modulators (aromatase inhibitors, selective estrogen receptor modulators, and selective estrogen receptor downregulators) on immunity. Synthesis of this information is timely given the considerable interest of late in defining the mechanistic basis of sex-biased responses/outcomes in patients with different cancers treated with immune checkpoint blockade. It will also be instructive with respect to the further development of ER modulators that modulate immunity in a therapeutically useful manner.

Clinical Significance of Tie-2-Expressing Monocytes/Macrophages and Angiopoietins in the Progression of Ovarian Cancer-State-of-the-Art

Tumour growth and metastasis are specific to advanced stages of epithelial ovarian cancer (EOC). Tumour angiogenesis is an essential part of these processes. It is responsible for providing tumours with nutrients, metabolites, and cytokines and facilitates tumour and immune cell relocation. Destabilised vasculature, a distinctive feature of tumours, is also responsible for compromising drug delivery into the bulk. Angiogenesis is a complex process that largely depends on how the tumour microenvironment (TME) is composed and how a specific organ is formed. There are contrary reports on whether Tie-2-expressing monocytes/macrophages (TEMs) reported as the proangiogenic population of monocytes have any impact on tumour development. The aim of this paper is to summarise knowledge about ovarian-cancer-specific angiogenesis and the unique role of Tie-2-expressing monocytes/macrophages in this process. The significance of this cell subpopulation for the pathophysiology of EOC remains to be investigated.

TIE-2 Signaling Activation by Angiopoietin 2 On Myeloid-Derived Suppressor Cells Promotes Melanoma-Specific T-cell Inhibition

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of immune suppressive cells detected in several human cancers. In this study, we investigated the features and immune suppressive function of a novel subset of monocytic MDSC overexpressing TIE-2 (TIE-2⁺ M-MDSC), the receptor for the pro-angiogenic factor angiopoietin 2 (ANGPT2). We showed that patients with melanoma exhibited a higher circulating rate of TIE-2⁺ M-MDSCs, especially in advanced stages, as compared to healthy donors. The distribution of the TIE-2⁺ M-MDSC rate toward the melanoma stage correlated with the serum level of ANGPT2. TIE-2⁺ M-MDSC from melanoma patients overexpressed immune suppressive molecules such as PD-L1, CD73, TGF-β, and IL-10, suggesting a highly immunosuppressive phenotype. The exposition of these cells to ANGPT2 increased the expression of most of these molecules, mainly Arginase 1. Hence, we observed a profound impairment of melanoma-specific T-cell responses in patients harboring high levels of TIE-2⁺ M-MDSC along with ANGPT2. This was confirmed by in vitro experiments indicating that the addition of ANGPT2 increased the ability of TIE-2⁺ M-MDSC to suppress antitumor T-cell function. Furthermore, by using TIE-2 kinase-specific inhibitors such as regorafenib or rebastinib, we demonstrated that an active TIE-2 signaling was required for optimal suppressive activity of these cells after ANGPT2 exposition. Collectively, these results support that TIE-2⁺ M-MDSC/ANGPT2 axis represents a potential immune escape mechanism in melanoma.

Isoforms of Neuropilin-2 Denote Unique Tumor-Associated Macrophages in Breast Cancer

Tumor-associated macrophages (TAMs) exert profound influence over breast cancer progression, promoting immunosuppression, angiogenesis, and metastasis. Neuropilin-2 (NRP2), consisting of the NRP2a and NRP2b isoforms, is a co-receptor for heparin-binding growth factors including VEGF-C and Class 3 Semaphorins. Selective upregulation in response to environmental stimuli and independent signaling pathways endow the NRP2 isoforms with unique functionality, with NRP2b promoting increased Akt signaling via receptor tyrosine kinases including VEGFRs, MET, and PDGFR. Although NRP2 has been shown to regulate macrophage/TAM biology, the role of the individual NRP2a/NRP2b isoforms in TAMs has yet to be evaluated. Using transcriptional profiling and spectral flow cytometry, we show that NRP2 isoform expression was significantly higher in TAMs from murine mammary tumors. NRP2a/NRP2b levels in human breast cancer metastasis were dependent upon the anatomic location of the tumor and significantly correlated with TAM infiltration in both primary and metastatic breast cancers. We define distinct phenotypes of NRP2 isoform-expressing TAMs in mouse models of breast cancer and within malignant pleural effusions from breast cancer patients which were exclusive of neuropilin-1 expression. Genetic depletion of either NRP2 isoform in macrophages resulted in a dramatic reduction of LPS-induced IL-10 production, defects in phagosomal processing of apoptotic breast cancer cells, and increase in cancer cell migration following co-culture. By contrast, depletion of NRP2b, but not NRP2a, inhibited production of IL-6. These results suggest that NRP2 isoforms regulate both shared and unique functionality in macrophages and are associated with distinct TAM subsets in breast cancer.

Histidine-rich glycoprotein (HRGP): Pleiotropic and paradoxical effects on macrophage, tumor microenvironment, angiogenesis, and other physiological and pathological processes

Histidine-rich glycoprotein (HRGP) is a relatively less known glycoprotein, but it is abundant in plasma with a multidomain structure, which allows it to interact with many ligands and regulate various biological processes. HRGP ligands includes heme, Zn²⁺, thrombospondin, plasmin/plasminogen, heparin/heparan sulfate, fibrinogen, tropomyosin, IgG, FcγR, C1q. In many conditions, the histidine-rich region of HRGP strengthens ligand binding following interaction with Zn²⁺ or exposure to low pH, such as sites of tissue injury or tumor growth. The multidomain structure and diverse ligand binding attributes of HRGP indicates that it can act as an extracellular adaptor protein, connecting with different ligands, especially on cell surfaces. Also, HRGP can selectively target IgG, which blocks the production of soluble immune complexes. The most common cell surface ligand of HRGP is heparan sulfate proteoglycan, and the interaction is also potentiated by elevated Zn²⁺ concentration and low pH. Recent reports have shown that HRGP can modulate macrophage polarization and possibly regulate other physiological processes such as angiogenesis, anti-tumor immune response, fibrinolysis and coagulation, soluble immune complex clearance and phagocytosis of apoptotic/necrosis cells. In addition, it has also been reported that HRGP has antibacterial and anti-HIV infection effects and may be used as a novel clinical biomarker accordingly. This review outlines the molecular, structural and biological properties of HRGP as well as presenting an update on the function of HRGP in various physiological processes.

Heterogeneity and function of macrophages in the breast during homeostasis and cancer

Macrophages are diverse immune cells populating all tissues and adopting a unique tissue-specific identity. Breast macrophages play an essential role in the development and function of the mammary gland over one's lifetime. In the recent years, with the development of fate-mapping, imaging and scRNA-seq technologies we grew a better understanding of the origin, heterogeneity and function of mammary macrophages in homeostasis but also during breast cancer development. Here, we aim to provide a comprehensive review of the latest improvements in studying the macrophage heterogeneity in healthy mammary tissues and breast cancer.

Decrease of Pro-Angiogenic Monocytes Predicts Clinical Response to Anti-Angiogenic Treatment in Patients with Metastatic Renal Cell Carcinoma

The modulation of subpopulations of pro-angiogenic monocytes (VEGFR-1+CD14 and Tie2+CD14) was analyzed in an ancillary study from the prospective PazopanIb versus Sunitinib patient preferenCE Study (PISCES) (NCT01064310), where metastatic renal cell carcinoma (mRCC) patients were treated with two anti-angiogenic drugs, either sunitinib or pazopanib. Blood samples from 86 patients were collected prospectively at baseline (T1), and at 10 weeks (T2) and 20 weeks (T3) after starting anti-angiogenic therapy. Various subpopulations of myeloid cells (monocytes, VEGFR-1+CD14 and Tie2+CD14 cells) decreased during treatment. When patients were divided into two subgroups with a decrease (defined as a >20% reduction from baseline value) (group 1) or not (group 2) at T3 for VEGFR-1+CD14 cells, group 1 patients presented a median PFS and OS of 24 months and 37 months, respectively, compared with a median PFS of 9 months (p = 0.032) and a median OS of 16 months (p = 0.033) in group 2 patients. The reduction in Tie2+CD14 at T3 predicted a benefit in OS at 18 months after therapy (p = 0.04). In conclusion, in this prospective clinical trial, a significant decrease in subpopulations of pro-angiogenic monocytes was associated with clinical response to anti-angiogenic drugs in patients with mRCC.

Myeloid cell-based delivery of IFN-γ reprograms the leukemia microenvironment and induces anti-tumoral immune responses

The immunosuppressive microenvironment surrounding tumor cells represents a key cause of treatment failure. Therefore, immunotherapies aimed at reprogramming the immune system have largely spread in the past years. We employed gene transfer into hematopoietic stem and progenitor cells to selectively express anti-tumoral cytokines in tumor-infiltrating monocytes/macrophages. We show that interferon-γ (IFN-γ) reduced tumor progression in mouse models of B-cell acute lymphoblastic leukemia (B-ALL) and colorectal carcinoma (MC38). Its activity depended on the immune system's capacity to respond to IFN-γ and drove the counter-selection of leukemia cells expressing surrogate antigens. Gene-based IFN-γ delivery induced antigen presentation in the myeloid compartment and on leukemia cells, leading to a wave of T cell recruitment and activation, with enhanced clonal expansion of cytotoxic CD8+ T lymphocytes. The activity of IFN-γ was further enhanced by either co-delivery of tumor necrosis factor-α (TNF-α) or by drugs blocking immunosuppressive escape pathways, with the potential to obtain durable responses.

Tumor-Associated Macrophages and Their Functional Transformation in the Hypoxic Tumor Microenvironment

Tumor-associated macrophages (TAMs) are some of the most abundant immune cells within tumors and perform a broad repertoire of functions via diverse phenotypes. On the basis of their functional differences in tumor growth, TAMs are usually categorized into two subsets of M1 and M2. It is well established that the tumor microenvironment (TME) is characterized by hypoxia along with tumor progression. TAMs adopt an M1-like pro-inflammatory phenotype at the early phases of oncogenesis and mediate immune response that inhibits tumor growth. As tumors progress, anabatic hypoxia of the TME gradually induces the M2-like functional transformation of TAMs by means of direct effects, metabolic influence, lactic acidosis, angiogenesis, remodeled stroma, and then urges them to participate in immunosuppression, angiogenesis and other tumor-supporting procedure. Therefore, thorough comprehension of internal mechanism of this TAM functional transformation in the hypoxic TME is of the essence, and might provide some novel insights in hypoxic tumor immunotherapeutic strategies.

Multi-target weapons: diaryl-pyrazoline thiazolidinediones simultaneously targeting VEGFR-2 and HDAC cancer hallmarks

In anticancer drug discovery, multi-targeting compounds have been beneficial due to their advantages over single-targeting compounds. For instance, VEGFR-2 has a crucial role in angiogenesis and cancer management, whereas HDACs are well-known regulators of epigenetics and have been known to contribute significantly to angiogenesis and carcinogenesis. Herein, we have reported nineteen novel VEGFR-2 and HDAC dual-targeting analogs containing diaryl-pyrazoline thiazolidinediones and their in vitro and in vivo biological evaluation. In particular, the most promising compound 14c has emerged as a dual inhibitor of VEGFR-2 and HDAC. It demonstrated anti-angiogenic activity by inhibiting in vitro HUVEC proliferation, migration, and tube formation. Moreover, an in vivo CAM assay showed that 14c repressed new capillary formation in CAMs. In particular, 14c exhibited cytotoxicity potential on different cancer cell lines such as MCF-7, K562, A549, and HT-29. Additionally, 14c demonstrated significant potency and selectivity against HDAC4 in the sub-micromolar range. To materialize the hypothesis, we also performed molecular docking on the crystal structures of both VEGFR-2 (PDB ID: 1YWN) and HDAC4 (PDB-ID: 4CBY), which corroborated the designing and biological activity. The results indicated that compound 14c could be a potential lead to develop more optimized multi-target analogs with enhanced potency and selectivity.

Visualizing murine breast and melanoma tumor microenvironment using intravital multiphoton microscopy

Intravital multiphoton imaging of the tumor milieu allows for the dissection of intricate and dynamic biological processes in situ. Herein, we present a step-by-step protocol for setting up an experimental cancer imaging model that has been optimized for solid tumors such as breast cancer and melanoma implanted in the flanks of mice. This protocol can be utilized for dissecting tumor-immune cell dynamics in vivo or other tumor-specific biological questions.

For complete details on the use of this protocol for intravital imaging of breast cancer, please refer to Tikoo et al. (2021a), and for intravital imaging of melanoma, please refer to Tikoo et al. (2021b).

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Detailed protocol for setting up high-resolution intravital imaging of murine tumors

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3D printing of custom stage inserts for tumor stabilization

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Procedures for cannulation of blood vessels

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Surgical preparation and tissue stabilization for imaging tumor milieu in vivo

Macrophage regulation of angiogenesis in health and disease

Macrophages are primarily known as phagocytic innate immune cells, but are, in fact, highly dynamic multi-taskers that interact with many different tissue types and have regulatory roles in development, homeostasis, tissue repair, and disease. In all of these scenarios angiogenesis is pivotal and macrophages appear to play a key role in guiding both blood vessel sprouting and remodelling wherever that occurs. Recent studies have explored these processes in a diverse range of models utilising the complementary strengths of rodent, fish and tissue culture studies to unravel the mechanisms underlying these interactions and regulatory functions. Here we discuss how macrophages regulate angiogenesis and its resolution as embryonic tissues grow, as well as their parallel and different functions in repairing wounds and in pathologies, with a focus on chronic wounds and cancer.

Macrophage depletion induces edema through release of matrix-degrading proteases and proteoglycan deposition

Colony-stimulating factor 1 receptor (CSF1R) blockade abates tumor-associated macrophage (TAM) infiltrates and provides marked clinical benefits in diffuse-type tenosynovial giant cell tumors. However, facial edema is a common adverse event associated with TAM elimination in patients. In this study, we examined molecular and cellular events associated with edema formation in mice and human patients with cancer treated with a CSF1R blocking antibody. Extended antibody treatment of mice caused marked body weight gain, an indicator of enhanced body fluid retention. This was associated with an increase of extracellular matrix-remodeling metalloproteinases (MMPs), namely MMP2 and MMP3, and enhanced deposition of hyaluronan (HA) and proteoglycans, leading to skin thickening. Discontinuation of anti-CSF1R treatment or blockade of MMP activity restored unaltered body weight and normal skin morphology in the mice. In patients, edema developed at doses well below the established optimal biological dose for emactuzumab, a CSF1R dimerization inhibitor. Patients who developed edema in response to emactuzumab had elevated HA in peripheral blood. Our findings indicate that an early increase of peripheral HA can serve as a pharmacodynamic marker for edema development and suggest potential interventions based on MMP inhibition for relieving periorbital edema in patients treated with CSF1R inhibitors.

Immune Cell Modulation of the Extracellular Matrix Contributes to the Pathogenesis of Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal malignancy with a five-year survival rate of only 9%. PDAC is characterized by a dense, fibrotic stroma composed of extracellular matrix (ECM) proteins. This desmoplastic stroma is a hallmark of PDAC, representing a significant physical barrier that is immunosuppressive and obstructs penetration of cytotoxic chemotherapy agents into the tumor microenvironment (TME). Additionally, dense ECM promotes hypoxia, making tumor cells refractive to radiation therapy and alters their metabolism, thereby supporting proliferation and survival. In this review, we outline the significant contribution of fibrosis to the pathogenesis of pancreatic cancer, with a focus on the cross talk between immune cells and pancreatic stellate cells that contribute to ECM deposition. We emphasize the cellular mechanisms by which neutrophils and macrophages, specifically, modulate the ECM in favor of PDAC-progression. Furthermore, we investigate how activated stellate cells and ECM influence immune cells and promote immunosuppression in PDAC. Finally, we summarize therapeutic strategies that target the stroma and hinder immune cell promotion of fibrogenesis, which have unfortunately led to mixed results. An enhanced understanding of the complex interactions between the pancreatic tumor ECM and immune cells may uncover novel treatment strategies that are desperately needed for this devastating disease.

Macrophage Biology and Mechanisms of Immune Suppression in Breast Cancer

Macrophages are crucial innate immune cells that maintain tissue homeostasis and defend against pathogens; however, their infiltration into tumors has been associated with adverse outcomes. Tumor-associated macrophages (TAMs) represent a significant component of the inflammatory infiltrate in breast tumors, and extensive infiltration of TAMs has been linked to poor prognosis in breast cancer. Here, we detail how TAMs impede a productive tumor immunity cycle by limiting antigen presentation and reducing activation of cytotoxic T lymphocytes (CTLs) while simultaneously supporting tumor cell survival, angiogenesis, and metastasis. There is an urgent need to overcome TAM-mediated immune suppression for durable anti-tumor immunity in breast cancer. To date, failure to fully characterize TAM biology and classify multiple subsets has hindered advancement in therapeutic targeting. In this regard, the complexity of TAMs has recently taken center stage owing to their subset diversity and tightly regulated molecular and metabolic phenotypes. In this review, we reveal major gaps in our knowledge of the functional and phenotypic characterization of TAM subsets associated with breast cancer, before and after treatment. Future work to characterize TAM subsets, location, and crosstalk with neighboring cells will be critical to counteract TAM pro-tumor functions and to identify novel TAM-modulating strategies and combinations that are likely to enhance current therapies and overcome chemo- and immuno-therapy resistance.

A vasculature-centric approach to developing novel treatment options for glioblastoma

Introduction: Glioblastoma is invariably deadly and is characterized by extensive vascularization and macrophage-dominant immunosuppression; nevertheless, anti-angiogenesis has so far failed to prolong overall survival of patients. Regardless of the problems in clinical development, the rationale for the application of anti-angiogenics in glioblastoma remains.Areas covered: Resistance to anti-angiogenics is discussed, including vessel co-option and amplification of hypoxic signaling in response to vessel destruction. The modulation of GSC and tumor-associated macrophages by dysfunctional tumor vessels and by hypoxia are outlined. Pharmacologic approaches to sensitizing glioblastomas to anti-angiogenics and evidence for the cooperation of anti-angiogenics with immunotherapies are summarized. Database search: https://pubmed.ncbi.nlm.nih.gov prior to December 12, 2020.Expert opinion: Despite drawbacks in the clinical development of vascular endothelial growth factor A (VEGF)-targeted agents, there is still rationale for the use of anti-angiogenics. The better understanding of vascular co-option and adverse effects of blood vessel destruction guides to improve strategies for vascular targeting. The pivotal role of the vasculature and of angiogenic factors such as VEGF for the induction and maintenance of immunosuppression in glioblastoma supports the use of anti-angiogenics in combination with immunotherapy. Proinflammatory repolarization of perivascular and perinecrotic tumor-associated macrophages is probably paramount for overcoming treatment resistance to virtually any treatment.

Contributions of Embryonic HSC-Independent Hematopoiesis to Organogenesis and the Adult Hematopoietic System

During ontogeny, the establishment of the hematopoietic system takes place in several phases, separated both in time and location. The process is initiated extra-embryonically in the yolk sac (YS) and concludes in the main arteries of the embryo with the formation of hematopoietic stem cells (HSC). Initially, it was thought that HSC-independent hematopoietic YS cells were transient, and only required to bridge the gap to HSC activity. However, in recent years it has become clear that these cells also contribute to embryonic organogenesis, including the emergence of HSCs. Furthermore, some of these early HSC-independent YS cells persist into adulthood as distinct hematopoietic populations. These previously unrecognized abilities of embryonic HSC-independent hematopoietic cells constitute a new field of interest. Here, we aim to provide a succinct overview of the current knowledge regarding the contribution of YS-derived hematopoietic cells to the development of the embryo and the adult hematopoietic system.

HDAC inhibition potentiates anti-tumor activity of macrophages and enhances anti-PD-L1-mediated tumor suppression

Despite the widespread use of the blockade of immune checkpoints, for a significant number of cancer patients, these therapies have proven ineffective, presumably due to the immunosuppressive nature of the tumor microenvironment (TME). Critical drivers of immune escape in the TME include tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs), which not only mediate immune suppression, but also facilitate metastatic dissemination and impart resistance to immunotherapies. Thus, strategies that convert them into tumor fighters may offer great therapeutic potential. In this study, we evaluated whether pharmacologic modulation of macrophage phenotype by HDAC inhibitors (HDACi) could produce an anti-tumor effect. We demonstrated that low-dose HDACi trichostatin-A (TSA) markedly reshaped the tumor immune microenvironment by modulating the suppressive activity of infiltrating macrophages and inhibiting the recruitment of MDSCs in various tumors. These actions, in turn, augmented anti-tumor immune responses and further enhanced anti-tumor effects of immunotherapies. HDAC inhibition, however, also upregulated PD-L1, thereby limiting the beneficial therapeutic effects. Indeed, combining low-dose TSA with anti-PD-L1 in this model significantly enhanced the durability of tumor reduction and prolonged survival of tumor-bearing mice, compared with the effect of either treatment alone. These data introduce HDAC inhibition as a potential means to harness the anti-tumor potential of macrophages in cancer therapy.

Tie2-expressing monocytes as a novel angiogenesis-related cellular biomarker for non-small cell lung cancer

To investigate the clinical value of Tie2-expressing monocytes (TEMs) in the early diagnosis of lung cancer and assess its correlation with angiogenesis, a total of 184 patients with non-small cell lung cancer (NSCLC), 101 patients with benign pulmonary disease (BPD), and 77 healthy controls were enrolled in our study. The distribution of TEMs in lung tissue was determined by immunofluorescence staining. Lung microvascular density was assessed by immunohistochemical staining. Receiver-operating characteristic (ROC) curve analysis was performed to assess the diagnostic value of TEM frequency. Patients with NSCLC were followed up for 26 months. We found that the TEM frequency in peripheral blood monocytes of patients with NSCLC was significantly greater than that in patients with BPD and healthy controls. TEM frequency showed a correlation with NSCLC recurrence. The majority of TEMs in tumor tissues were localized around blood vessels; tumoral TEM frequency showed a positive correlation with microvascular density. High percentage of TEMs in the peripheral blood was associated with poor overall survival. ROC curve analysis revealed the potential diagnostic value of circulating TEM frequency in NSCLC. Thus, we believe that TEM frequency is related to angiogenesis in tumor tissues and may serve as a diagnostic marker for NSCLC.

Hypoxia-Inducible Factor α Subunits Regulate Tie2-Expressing Macrophages That Influence Tumor Oxygen and Perfusion in Murine Breast Cancer

Tie2-expressing monocytes/macrophages (TEMs) are a distinct subset of proangiogenic monocytes selectively recruited to tumors in breast cancer. Because of the hypoxic nature of solid tumors, we investigated if oxygen, via hypoxia-inducible transcription factors HIF-1α and HIF-2α, regulates TEM function in the hypoxic tumor microenvironment. We orthotopically implanted PyMT breast tumor cells into the mammary fat pads of syngeneic LysMcre, HIF-1α fl/fl /LysMcre, or HIF-2α fl/fl /LysMcre mice and evaluated the tumor TEM population. There was no difference in the percentage of tumor macrophages among the mouse groups. In contrast, HIF-1α fl/fl /LysMcre mice had a significantly smaller percentage of tumor TEMs compared with control and HIF-2α fl/fl /LysMcre mice. Proangiogenic TEMs in macrophage HIF-2α-deficient tumors presented significantly more CD31+ microvessel density but exacerbated hypoxia and tissue necrosis. Reduced numbers of proangiogenic TEMs in macrophage HIF-1α-deficient tumors presented significantly less microvessel density but tumor vessels that were more functional as lectin injection revealed more perfusion, and functional electron paramagnetic resonance analysis revealed more oxygen in those tumors. Macrophage HIF-1α-deficient tumors also responded significantly to chemotherapy. These data introduce a previously undescribed and counterintuitive prohypoxia role for proangiogenic TEMs in breast cancer which is, in part, suppressed by HIF-2α.

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.

Transcriptional Signature Derived from Murine Tumor-Associated Macrophages Correlates with Poor Outcome in Breast Cancer Patients

Tumor-associated macrophages (TAMs) are frequently the most abundant immune cells in cancers and are associated with poor survival. Here, we generated TAM molecular signatures from K14cre;Cdh1^flox/flox;Trp53^flox/flox (KEP) and MMTV-NeuT (NeuT) transgenic mice that resemble human invasive lobular carcinoma (ILC) and HER2⁺ tumors, respectively. Determination of TAM-specific signatures requires comparison with healthy mammary tissue macrophages to avoid overestimation of gene expression differences. TAMs from the two models feature a distinct transcriptomic profile, suggesting that the cancer subtype dictates their phenotype. The KEP-derived signature reliably correlates with poor overall survival in ILC but not in triple-negative breast cancer patients, indicating that translation of murine TAM signatures to patients is cancer subtype dependent. Collectively, we show that a transgenic mouse tumor model can yield a TAM signature relevant for human breast cancer outcome prognosis and provide a generalizable strategy for determining and applying immune cell signatures provided the murine model reflects the human disease.

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Murine TAM signatures prognosticate outcomes in corresponding cancer patients

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TAM signatures are robust when they are compared with healthy tissue macrophages

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TAM transcriptome is dictated by tissue and tumor subtype-related signals

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Murine TAM signatures can be translated only when a suitable model is chosen

Tumor microenvironment conditions that favor vessel co-option in colorectal cancer liver metastases: A theoretical model

Vessel co-option is an alternative strategy by which tumour cells vascularize and gain access to nutrients to support tumour growth, survival and metastasis. In vessel co-option, the cancer cells move towards the pre-existing vasculature and hijack them. Vessel co-option is adopted by a wide range of human tumours including colorectal cancer liver metastases (CRCLM) and is responsible for the effectiveness of treatment in CRCLM. Furthermore, vessel co-option is an intrinsic feature and an acquired mechanism of resistance to anti-angiogenic treatment. In this review, we describe the microenvironment, the molecular players, discovered thus far of co-opting CRCLM lesions and propose a theoretical model. We also highlight key unanswered questions that are critical to improving our understanding of CRCLM vessel co-option and for the development of effective approaches for the treatment of co-opting tumours.

Mathematical modelling of trastuzumab-induced immune response in an in vivo murine model of HER2+ breast cancer

The goal of this study is to develop an integrated, mathematical-experimental approach for understanding the interactions between the immune system and the effects of trastuzumab on breast cancer that overexpresses the human epidermal growth factor receptor 2 (HER2+). A system of coupled, ordinary differential equations was constructed to describe the temporal changes in tumour growth, along with intratumoural changes in the immune response, vascularity, necrosis and hypoxia. The mathematical model is calibrated with serially acquired experimental data of tumour volume, vascularity, necrosis and hypoxia obtained from either imaging or histology from a murine model of HER2+ breast cancer. Sensitivity analysis shows that model components are sensitive for 12 of 13 parameters, but accounting for uncertainty in the parameter values, model simulations still agree with the experimental data. Given theinitial conditions, the mathematical model predicts an increase in the immune infiltrates over time in the treated animals. Immunofluorescent staining results are presented that validate this prediction by showing an increased co-staining of CD11c and F4/80 (proteins expressed by dendritic cells and/or macrophages) in the total tissue for the treated tumours compared to the controls ($p < 0.03$). We posit that the proposed mathematical-experimental approach can be used to elucidate driving interactions between the trastuzumab-induced responses in the tumour and the immune system that drive the stabilization of vasculature while simultaneously decreasing tumour growth-conclusions revealed by the mathematical model that were not deducible from the experimental data alone.

Tumor-Associated Macrophages: Protumoral Macrophages in Inflammatory Tumor Microenvironment

Tumor microenvironment consists of malignant and non-malignant cells. The interaction of these dynamic and different cells is responsible for tumor progression at different levels. The non-malignant cells in TME contain cells such as tumor-associated macrophages (TAMs), cancer associated fibroblasts, pericytes, adipocytes, T cells, B cells, myeloid-derived suppressor cells (MDSCs), tumor-associated neutrophils (TANs), dendritic cells (DCs) and Vascular endothelial cells. TAMs are abundant in most human and murine cancers and their presence are associated with poor prognosis. The major event in tumor microenvironment is macrophage polarization into tumor-suppressive M1 or tumor-promoting M2 types. Although much evidence suggests that TAMS are primarily M2-like macrophages, the mechanism responsible for polarization into M1 and M2 macrophages remain unclear. TAM contributes cancer cell motility, invasion, metastases and angiogenesis. The relationship between TAM and tumor cells lead to used them as a diagnostic marker, therapeutic target and prognosis of cancer. This review presents the origin, polarization, role of TAMs in inflammation, metastasis, immune evasion and angiogenesis as well as they can be used as therapeutic target in variety of cancer cells. It is obvious that additional substantial and preclinical research is needed to support the effectiveness and applicability of this new and promising strategy for cancer treatment.

Role of Tumor-Associated Myeloid Cells in Breast Cancer

Stromal immune cells constitute the tumor microenvironment. These immune cell subsets include myeloid cells, the so-called tumor-associated myeloid cells (TAMCs), which are of two types: tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs). Breast tumors, particularly those in human epidermal growth factor receptor 2 (HER-2)-positive breast cancer and triple-negative breast cancer, are solid tumors containing immune cell stroma. TAMCs drive breast cancer progression via immune mediated, nonimmune-mediated, and metabolic interactions, thus serving as a potential therapeutic target for breast cancer. TAMC-associated breast cancer treatment approaches potentially involve the inhibition of TAM recruitment, modulation of TAM polarization/differentiation, reduction of TAM products, elimination of MDSCs, and reduction of MDSC products. Furthermore, TAMCs can enhance or restore immune responses during cancer immunotherapy. This review describes the role of TAMs and MDSCs in breast cancer and elucidates the clinical implications of TAMs and MDSCs as potential targets for breast cancer treatment.

Tumor-Associated Macrophages in Osteosarcoma: From Mechanisms to Therapy

Osteosarcomas (OSs) are bone tumors most commonly found in pediatric and adolescent patients characterized by high risk of metastatic progression and recurrence after therapy. Effective therapeutic management of this disease still remains elusive as evidenced by poor patient survival rates. To achieve a more effective therapeutic management regimen, and hence patient survival, there is a need to identify more focused targeted therapies for OSs treatment in the clinical setting. The role of the OS tumor stroma microenvironment plays a significant part in the development and dissemination of this disease. Important components, and hence potential targets for treatment, are the tumor-infiltrating macrophages that are known to orchestrate many aspects of OS stromal signaling and disease progression. In particular, increased infiltration of M2-like tumor-associated macrophages (TAMs) has been associated with OS metastasis and poor patient prognosis despite currently used aggressive therapies regimens. This review aims to provide a summary update of current macrophage-centered knowledge and to discuss the possible roles that macrophages play in the process of OS metastasis development focusing on the potential influence of stromal cross-talk signaling between TAMs, cancer-stem cells and additional OSs tumoral microenvironment factors.

The role of the tumor microenvironment in tumor cell intravasation and dissemination

Metastasis, a process that requires tumor cell dissemination followed by tumor growth, is the primary cause of death in cancer patients. An essential step of tumor cell dissemination is intravasation, a process by which tumor cells cross the blood vessel endothelium and disseminate to distant sites. Studying this process is of utmost importance given that intravasation in the primary tumor, as well as the secondary and tertiary metastases, is the key step in the systemic spread of tumor cells, and that this process continues even after removal of the primary tumor. High-resolution intravital imaging of the tumor microenvironment of breast carcinoma has revealed that tumor cell intravasation exclusively occurs at doorways, termed "Tumor MicroEnvironment of Metastasis" (TMEM), composed of three different cell types: a Tie2^high/VEGF^high perivascular macrophage, a Mena overexpressing tumor cell, and an endothelial cell, all in direct contact. In this review article, we discuss the interactions between these cell types, the subsequent signaling events which lead to tumor cell intravasation, and the role of invadopodia in supporting tumor cell invasion and dissemination. We end our review by discussing how the knowledge acquired from the use of intravital imaging is now leading to new clinical trials targeting tumor cell dissemination and preventing metastatic progression.

Diverse homeostatic and immunomodulatory roles of immune cells in the developing mouse lung at single cell resolution

At birth, the lungs rapidly transition from a pathogen-free, hypoxic environment to a pathogen-rich, rhythmically distended air-liquid interface. Although many studies have focused on the adult lung, the perinatal lung remains unexplored. Here, we present an atlas of the murine lung immune compartment during early postnatal development. We show that the late embryonic lung is dominated by specialized proliferative macrophages with a surprising physical interaction with the developing vasculature. These macrophages disappear after birth and are replaced by a dynamic mixture of macrophage subtypes, dendritic cells, granulocytes, and lymphocytes. Detailed characterization of macrophage diversity revealed an orchestration of distinct subpopulations across postnatal development to fill context-specific functions in tissue remodeling, angiogenesis, and immunity. These data both broaden the putative roles for immune cells in the developing lung and provide a framework for understanding how external insults alter immune cell phenotype during a period of rapid lung growth and heightened vulnerability.

Heterogeneity and chimerism of endothelial cells revealed by single-cell transcriptome in orthotopic liver tumors

The liver is a common host organ for cancer, either through lesions that arise in liver epithelial cells [e.g., hepatocellular carcinoma (HCC)] or as a site of metastasis by tumors arising in other organs (e.g., colorectal cancer). However, the changes that occur in liver stromal cells in response to cancer have not been fully characterized, nor has it been determined whether the different sources of liver cancer induce distinct stromal changes. Here, we performed single-cell profiling of liver stromal cells from mouse models of induced spontaneous liver cancer or implanted colorectal liver metastases, with a focus on tumor endothelial cells (ECs). While ECs in liver tissue adjacent to cancerous lesions (so-called adjacent normal) corresponded to liver zonation phenotypes, their transcriptomes were also clearly altered by the presence of a tumor. In comparison, tumor EC transcriptomes show stronger similarities to venous than sinusoidal ECs. Further, tumor ECs, independent of tumor origin, formed distinct clusters displaying conserved “tip-like” or “stalk-like” characteristics, similar to ECs from subcutaneous tumors. However, they also carried liver-specific signatures found in normal liver ECs, suggesting an influence of the host organ on tumor ECs. Our results document gene expression signatures in ECs in liver cancer and show that the host organ, and not the site of tumor origin (liver versus colorectal), is a primary determinant of EC phenotype. In addition, primarily in tumors, we further defined a cluster of chimeric cells that expressed both myeloid and endothelial cell markers and might play a role in tumor angiogenesis.

Re-education of macrophages as a therapeutic strategy in cancer

Tumor-associated macrophages (TAMs) can be educated within the tumor microenvironment to promote cancer development and progression. While TAM-targeted agents have largely focused on macrophage depletion as an anticancer strategy, it is becoming increasingly evident that TAM re-education may represent a more effective approach. In this perspective, we discuss different means to achieve TAM re-education, and review the beneficial effects of these strategies, particularly when combined with immune checkpoint inhibitors.

The biology of vascular calcification

Vascular calcification (VC), characterized by different mineral deposits (i.e., carbonate apatite, whitlockite and hydroxyapatite) accumulating in blood vessels and valves, represents a relevant pathological process for the aging population and a life-threatening complication in acquired and in genetic diseases. Similarly to bone remodeling, VC is an actively regulated process in which many cells and molecules play a pivotal role. This review aims at: (i) describing the role of resident and circulating cells, of the extracellular environment and of positive and negative factors in driving the mineralization process; (ii) detailing the types of VC (i.e., intimal, medial and cardiac valve calcification); (iii) analyzing rare genetic diseases underlining the importance of altered pyrophosphate-dependent regulatory mechanisms; (iv) providing therapeutic options and perspectives.

Biology and therapeutic targeting of tumour-associated macrophages

Macrophages sustain tumour progression by facilitating angiogenesis, promoting immunosuppression, and enhancing cancer cell invasion and metastasis. They also modulate tumour response to anti-cancer therapy in pre-clinical models. This knowledge has motivated the development of agents that target tumour-associated macrophages (TAMs), some of which have been investigated in early clinical trials. Here, we provide a comprehensive overview of the biology and therapeutic targeting of TAMs, highlighting opportunities, setbacks, and new challenges that have emerged after a decade of intense translational and clinical research into these multifaceted immune cells. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Promotion of macrophage activation by Tie2 in the context of the inflamed synovia of rheumatoid arthritis and psoriatic arthritis patients

OBJECTIVE

To examine the role of Tie2 signalling in macrophage activation within the context of the inflammatory synovial microenvironment present in patients with RA and PsA.

METHODS

Clinical responses and macrophage function were examined in wild-type and Tie2-overexpressing (Tie2-TG) mice in the K/BxN serum transfer model of arthritis. Macrophages derived from peripheral blood monocytes from healthy donors, RA and PsA patients, and RA and PsA synovial tissue explants were stimulated with TNF (10 ng/ml), angiopoietin (Ang)-1 or Ang-2 (200 ng/ml), or incubated with an anti-Ang2 neutralizing antibody. mRNA and protein expression of inflammatory mediators was analysed by quantitative PCR, ELISA and Luminex.

RESULTS

Tie2-TG mice displayed more clinically severe arthritis than wild-type mice, accompanied by enhanced joint expression of IL6, IL12B, NOS2, CCL2 and CXCL10, and activation of bone marrow-derived macrophages in response to Ang-2 stimulation. Ang-1 and Ang-2 significantly enhanced TNF-induced expression of pro-inflammatory cytokines and chemokines in macrophages from healthy donors differentiated with RA and PsA SF and peripheral blood-derived macrophages from RA and PsA patients. Both Ang-1 and Ang-2 induced the production of IL-6, IL-12p40, IL-8 and CCL-3 in synovial tissue explants of RA and PsA patients, and Ang-2 neutralization suppressed the production of IL-6 and IL-8 in the synovial tissue of RA patients.

CONCLUSION

Tie2 signalling enhances TNF-dependent activation of macrophages within the context of ongoing synovial inflammation in RA and PsA, and neutralization of Tie2 ligands might be a promising therapeutic target in the treatment of these diseases.

The complex TIE between macrophages and angiogenesis

Macrophages are primarily known as phagocytic immune cells, but they also play a role in diverse processes, such as morphogenesis, homeostasis and regeneration. In this review, we discuss the influence of macrophages on angiogenesis, the process of new blood vessel formation from the pre-existing vasculature. Macrophages play crucial roles at each step of the angiogenic cascade, starting from new blood vessel sprouting to the remodelling of the vascular plexus and vessel maturation. Macrophages form promising targets for both pro- and anti-angiogenic treatments. However, to target macrophages, we will first need to understand the mechanisms that control the functional plasticity of macrophages during each of the steps of the angiogenic cascade. Here, we review recent insights in this topic. Special attention will be given to the TIE2-expressing macrophage (TEM), which is a subtype of highly angiogenic macrophages that is able to influence angiogenesis via the angiopoietin-TIE pathway.

Podoplanin-Expressing Macrophages Promote Lymphangiogenesis and Lymphoinvasion in Breast Cancer

Among mammary tumor-infiltrating immune cells, the highest expression of podoplanin (PDPN) is found in a subset of tumor-associated macrophages (TAMs). We hereby demonstrate that PDPN is involved in the attachment of this TAM subset to lymphatic endothelial cells (LECs). Mechanistically, the binding of PDPN to LEC-derived galectin 8 (GAL8) in a glycosylation-dependent manner promotes the activation of pro-migratory integrin β1. When proximal to lymphatics, PDPN-expressing macrophages (PoEMs) stimulate local matrix remodeling and promote vessel growth and lymphoinvasion. Anti-integrin β1 blockade, macrophage-specific Pdpn knockout, or GAL8 inhibition impairs TAM adhesion to LECs, restraining lymphangiogenesis and reducing lymphatic cancer spread. In breast cancer patients, association of PoEMs with tumor lymphatic vessels correlates with incidences of lymph node and distant organ metastasis.

Bimodal CD40/Fas-Dependent Crosstalk between iNKT Cells and Tumor-Associated Macrophages Impairs Prostate Cancer Progression

Heterotypic cellular and molecular interactions in the tumor microenvironment (TME) control cancer progression. Here, we show that CD1d-restricted invariant natural killer (iNKT) cells control prostate cancer (PCa) progression by sculpting the TME. In a mouse PCa model, iNKT cells restrained the pro-angiogenic and immunosuppressive capabilities of tumor-infiltrating immune cells by reducing pro-angiogenic TIE2⁺, M2-like macrophages (TEMs), and sustaining pro-inflammatory M1-like macrophages. iNKT cells directly contacted macrophages in the PCa stroma, and iNKT cell transfer into tumor-bearing mice abated TEMs, delaying tumor progression. iNKT cells modulated macrophages through the cooperative engagement of CD1d, Fas, and CD40, which promoted selective killing of M2-like and survival of M1-like macrophages. Human PCa aggressiveness associate with reduced intra-tumoral iNKT cells, increased TEMs, and expression of pro-angiogenic genes, underscoring the clinical significance of this crosstalk. Therefore, iNKT cells may control PCa through mechanisms involving differential macrophage modulation, which may be harnessed for therapeutically reprogramming the TME.

Perivascular macrophages in health and disease

Macrophages are a heterogeneous group of cells that are capable of carrying out distinct functions in different tissues, as well as in different locations within a given tissue. Some of these tissue macrophages lie on, or close to, the outer (abluminal) surface of blood vessels and perform several crucial activities at this interface between the tissue and the blood. In steady-state tissues, these perivascular macrophages maintain tight junctions between endothelial cells and limit vessel permeability, phagocytose potential pathogens before they enter tissues from the blood and restrict inappropriate inflammation. They also have a multifaceted role in diseases such as cancer, Alzheimer disease, multiple sclerosis and type 1 diabetes. Here, we examine the important functions of perivascular macrophages in various adult tissues and describe how these functions are perturbed in a broad array of pathological conditions.

Macrophage-Mediated Subversion of Anti-Tumour Immunity

Despite the incredible clinical benefits obtained by the use of immune checkpoint blockers (ICBs), resistance is still common for many types of cancer. Central for ICBs to work is activation and infiltration of cytotoxic CD8+ T cells following tumour-antigen recognition. However, it is now accepted that even in the case of immunogenic tumours, the effector functions of CD8+ T cells are highly compromised by the presence of an immunosuppressive tumour microenvironment (TME) at the tumour site. Tumour-associated macrophages (TAMs) are among the most abundant non-malignant stromal cell types within the TME and they are crucial drivers of tumour progression, metastasis and resistance to therapy. TAMs are able to regulate either directly or indirectly various aspects of tumour immunity, including T cell recruitment and functions. In this review we discuss the mechanisms by which TAMs subvert CD8+ T cell immune surveillance and how their targeting in combination with ICBs represents a very powerful therapeutic strategy.