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

An abundant tissue macrophage population in the adult murine heart with a distinct alternatively-activated macrophage profile

Cardiac tissue macrophages (cTMs) are a previously uncharacterised cell type that we have identified and characterise here as an abundant GFP(+) population within the adult Cx(3)cr1(GFP/+) knock-in mouse heart. They comprise the predominant myeloid cell population in the myocardium, and are found throughout myocardial interstitial spaces interacting directly with capillary endothelial cells and cardiomyocytes. Flow cytometry-based immunophenotyping shows that cTMs exhibit canonical macrophage markers. Gene expression analysis shows that cTMs (CD45(+)CD11b(+)GFP(+)) are distinct from mononuclear CD45(+)CD11b(+)GFP(+) cells sorted from the spleen and brain of adult Cx(3)cr1(GFP/+) mice. Gene expression profiling reveals that cTMs closely resemble alternatively-activated anti-inflammatory M2 macrophages, expressing a number of M2 markers, including Mrc1, CD163, and Lyve-1. While cTMs perform normal tissue macrophage homeostatic functions, they also exhibit a distinct phenotype, involving secretion of salutary factors (including IGF-1) and immune modulation. In summary, the characterisation of cTMs at the cellular and molecular level defines a potentially important role for these cells in cardiac homeostasis.

CCR2 recruits an inflammatory macrophage subpopulation critical for angiogenesis in tissue repair

Monocytes/macrophages are critical in orchestrating the tissue-repair response. However, the mechanisms that govern macrophage regenerative activities during the sequential phases of repair are largely unknown. In the present study, we examined the dynamics and functions of diverse monocyte/macrophage phenotypes during the sequential stages of skin repair. By combining the analysis of a new CCR2-eGFP reporter mouse model with conditional mouse mutants defective in myeloid cell-restricted CCR2 signaling or VEGF-A synthesis, we show herein that among the large number of inflammatory CCR2(+)Ly6C(+) macrophages that dominate the early stage of repair, only a small fraction strongly expresses VEGF-A that has nonredundant functions for the induction of vascular sprouts. The switch of macrophage-derived VEGF-A during the early stage of tissue growth toward epidermal-derived VEGF-A during the late stage of tissue maturation was critical to achieving physiologic tissue vascularization and healing progression. The results of the present study provide new mechanistic insights into CCR2-mediated recruitment of blood monocyte subsets into damaged tissue, the dynamics and functional consequences of macrophage plasticity during the sequential repair phases, and the complementary role of macrophage-derived VEGF-A in coordinating effective tissue growth and vascularization in the context of tissue-resident wound cells. Our findings may be relevant for novel monocyte-based therapies to promote tissue vascularization.

Macrophage-tumor crosstalk: role of TAMR tyrosine kinase receptors and of their ligands

Ample clinical and preclinical evidence indicates that macrophages interact with tumor cells as well as with virtually all populations of host cells present in the tumor microenvironment. This crosstalk can strongly promote malignancy, but also has in principle the potential to inhibit tumor growth. Thus, it is of the utmost importance to improve our understanding of the mechanisms driving the pro- and antimalignant behavior of tumor-associated macrophages (TAMs) in order to develop better anticancer therapies. In this review, we discuss the biological consequences of reciprocal interactions between TAMs, cancer cells, endothelial cells, fibroblasts and other leukocyte subfractions within tumors. It was recently elucidated that tumors specifically educate macrophages to secrete growth arrest-specific gene 6 (Gas6), the common ligand of the Tyro3, Axl, Mer receptor (TAMR) family. In turn, Gas6 fosters tumor growth by promoting cancer cell proliferation. Therefore, the Gas6-TAMR axis might represent a novel target for disrupting tumor-macrophage crosstalk. We summarize here what is known about TAMR and their ligands in (human) cancer biology. In order to shed more light on the role of macrophages in human cancer, we additionally provide an overview of what is currently known about the prognostic impact of TAMs in human cancer.

Endogenous migration modulators as parent compounds for the development of novel cardiovascular and anti-inflammatory drugs

Development of novel cell migration modulators for anti-inflammatory and cardiovascular therapy is a complex task since any modulator will necessarily interfere with a balanced system of physiological regulators directing proper positioning of diverse immune cell types within the body. Whereas this shall serve efficient pathogen elimination, lack of proper control over these processes may result in counterproductive chronic inflammation and progressive tissue injury instead of healing. Prediction of the therapeutic potential or side effects of any migration modulator is not possible based on theoretical considerations alone but needs to be experimentally evaluated in preclinical disease models and by clinical studies. Here, we briefly summarize basic mechanism of cell migration, and groups of synthetic drugs currently in use for migration modulation. We then discuss one fundamental problem encountered with single-target approaches that arises from the complexity of any inflammation, with multiple interacting and often redundant factors being involved. This issue is likely to arise for any class of therapeutic agent (small molecules, peptides, antibodies, regulatory RNAs) addressing a single gene or protein. Against this background of studies on synthetic migration modulators addressing single targets, we then discuss the potential of endogenous proteins as therapeutic migration modulators, or as parent compounds for the development of mimetic drugs. Regulatory proteins of this type commonly address multiple receptors and signalling pathways and act upon the immune response in a phase-specific manner. Based on recent evidence, we suggest investigation of such endogenous migration modulators as novel starting points for anti-inflammatory and cardiovascular drug development.

Tumor-promoting macrophages induce the expression of the macrophage-specific receptor CD163 in malignant cells

Tumor-associated macrophages (TAMs) represent a distinct malignancy-promoting phenotype suggested to play a key role in tumor formation and metastasis. We aimed to investigate the expression of the monocyte/macrophage-restricted receptor CD163 in bladder tumor biopsies and assess the potential mechanism inducing the CD163 expression in tumor cells. A high CD163 mRNA expression (n = 87) was significantly associated with a poor 13-year overall survival (log-rank test, χ(2) = 8.931; p = 0.0028). Moreover, CD163 mRNA expression was significantly increased in muscle invasive (T2-T4), p = 0.017, and aggressive (grade III/IV) cancers (p = 0.015). The expression strongly correlated with local expression of IL-6 (r = 0.72; p <0.0001) and IL-10 (r = 0.75; p <0.0001), mediators known to induce CD163 expression in vitro. CD163 immunostaining (n = 46) confirmed the association between dense TAM infiltration and histologically advanced disease. In 39% of the biopsies, CD163 immunoreactivity was also observed in tumor cells, and CD163-expressing metastatic cells were identified in lymph node biopsies (n = 8). Bladder cancer cell lines did not express CD163; however, when cocultured with macrophages the bladder cancer cell expression of CD163 was significantly induced in an IL-6/IL-10 independent manner. In conclusion, we show a strong association between CD163 mRNA expression in bladder cancer biopsies and poor patient outcome. CD163 expression was not confined to the infiltrating TAMs, but was also expressed by a significant portion of the malignant cells in both tumors and lymph nodes. CD163 expressing tumor cells may constitute a subpopulation of tumor cells with a phenotypic shift associated with epithelial-to-mesenchymal transition (EMT) and increased metastatic activity induced by TAMs.

Coordinated regulation of myeloid cells by tumours

Myeloid cells are the most abundant nucleated haematopoietic cells in the human body and are a collection of distinct cell populations with many diverse functions. The three groups of terminally differentiated myeloid cells - macrophages, dendritic cells and granulocytes - are essential for the normal function of both the innate and adaptive immune systems. Mounting evidence indicates that the tumour microenvironment alters myeloid cells and can convert them into potent immunosuppressive cells. Here, we consider myeloid cells as an intricately connected, complex, single system and we focus on how tumours manipulate the myeloid system to evade the host immune response.

New model of macrophage acquisition of the lymphatic endothelial phenotype

BACKGROUND

Macrophage-derived lymphatic endothelial cell progenitors (M-LECPs) contribute to new lymphatic vessel formation, but the mechanisms regulating their differentiation, recruitment, and function are poorly understood. Detailed characterization of M-LECPs is limited by low frequency in vivo and lack of model systems allowing in-depth molecular analyses in vitro. Our goal was to establish a cell culture model to characterize inflammation-induced macrophage-to-LECP differentiation under controlled conditions.

METHODOLOGY/PRINCIPAL FINDINGS

Time-course analysis of diaphragms from lipopolysaccharide (LPS)-treated mice revealed rapid mobilization of bone marrow-derived and peritoneal macrophages to the proximity of lymphatic vessels followed by widespread (∼50%) incorporation of M-LECPs into the inflamed lymphatic vasculature. A differentiation shift toward the lymphatic phenotype was found in three LPS-induced subsets of activated macrophages that were positive for VEGFR-3 and many other lymphatic-specific markers. VEGFR-3 was strongly elevated in the early stage of macrophage transition to LECPs but undetectable in M-LECPs prior to vascular integration. Similar transient pattern of VEGFR-3 expression was found in RAW264.7 macrophages activated by LPS in vitro. Activated RAW264.7 cells co-expressed VEGF-C that induced an autocrine signaling loop as indicated by VEGFR-3 phosphorylation inhibited by a soluble receptor. LPS-activated RAW264.7 macrophages also showed a 68% overlap with endogenous CD11b(+)/VEGFR-3(+) LECPs in the expression of lymphatic-specific genes. Moreover, when injected into LPS- but not saline-treated mice, GFP-tagged RAW264.7 cells massively infiltrated the inflamed diaphragm followed by integration into 18% of lymphatic vessels.

CONCLUSIONS/SIGNIFICANCE

We present a new model for macrophage-LECP differentiation based on LPS activation of cultured RAW264.7 cells. This system designated here as the "RAW model" mimics fundamental features of endogenous M-LECPs. Unlike native LECPs, this model is unrestricted by cell numbers, heterogeneity of population, and ability to change genetic composition for experimental purposes. As such, this model can provide a valuable tool for understanding the LECP and lymphatic biology.

Differential macrophage programming in the tumor microenvironment

Of the multiple unique stromal cell types common to solid tumors, tumor-associated macrophages (TAMs) are significant for fostering tumor progression. The protumor properties of TAMs derive from regulation of angiogenic programming, production of soluble mediators that support proliferation, survival and invasion of malignant cells, and direct and indirect suppression of cytotoxic T cell activity. These varied activities are dependent on the polarization state of TAMs that is regulated in part by local concentrations of cytokines and chemokines, as well as varied interactions of TAMs with normal and degraded components of the extracellular matrix. Targeting molecular pathways regulating TAM polarization holds great promise for anticancer therapy.

Lung endothelial ADAM17 regulates the acute inflammatory response to lipopolysaccharide

Acute lung injury (ALI) is associated with increased vascular permeability, leukocyte recruitment, and pro-inflammatory mediator release. We investigated the role of the metalloproteinase ADAM17 in endotoxin-induced ALI with focus on endothelial ADAM17. In vitro, endotoxin-mediated induction of endothelial permeability and IL-8-induced transmigration of neutrophils through human microvascular endothelial cells required ADAM17 as shown by inhibition with GW280264X or shRNA-mediated knockdown. In vivo, ALI was induced by intranasal endotoxin-challenge combined with GW280264X treatment or endothelial adam17-knockout. Endotoxin-triggered upregulation of ADAM17 mRNA in the lung was abrogated in knockout mice and associated with reduced ectodomain shedding of the junctional adhesion molecule JAM-A and the transmembrane chemokine CX3CL1. Induced vascular permeability, oedema formation, release of TNF-α and IL-6 and pulmonary leukocyte recruitment were all markedly reduced by GW280264X or endothelial adam17-knockout. Intranasal application of TNF-α could not restore leukocyte recruitment and oedema formation in endothelial adam17-knockout animals. Thus, activation of endothelial ADAM17 promotes acute pulmonary inflammation in response to endotoxin by multiple endothelial shedding events most likely independently of endothelial TNF-α release leading to enhanced vascular permeability and leukocyte recruitment.

Tumor microenvironment in the brain

In addition to malignant cancer cells, tumors contain a variety of different stromal cells that constitute the tumor microenvironment. Some of these cell types provide crucial support for tumor growth, while others have been suggested to actually inhibit tumor progression. The composition of tumor microenvironment varies depending on the tumor site. The brain in particular consists of numerous specialized cell types such as microglia, astrocytes, and brain endothelial cells. In addition to these brain-resident cells, primary and metastatic brain tumors have also been shown to be infiltrated by different populations of bone marrow-derived cells. The role of different cell types that constitute tumor microenvironment in the progression of brain malignancies is only poorly understood. Tumor microenvironment has been shown to be a promising therapeutic target and diagnostic marker in extracranial malignancies. A better understanding of tumor microenvironment in the brain would therefore be expected to contribute to the development of improved therapies for brain tumors that are urgently required due to a poor availability of treatments for these malignancies. This review summarizes some of the known interactions between brain tumors and different stromal cells, and also discusses potential therapeutic approaches within this context.

Differentiation and gene expression profile of tumor-associated macrophages

Tumor microenvironment is composed of proliferating neoplastic cells, a vascular network of endothelial cells, extra cellular matrix produced by fibroblasts, cellular compartments of adaptive immunity like lymphocytes and dendritic cells as well as cells of innate immunity, e.g., natural killer cells and macrophages. Many pre-clinical and clinical studies demonstrate an inversed correlation between macrophage infiltrate and patients' prognosis indicating a macrophage supporting role for tumor progression as producers of growth and angiogenic factors and as regulators of tissue remodelling. Based on in vitro models, macrophages have been classified in pro-inflammatory, classically activated macrophages (M1; stimulated by IFN-γ or LPS) and anti-inflammatory, alternatively activated macrophages (M2; stimulated by either IL-4/IL-13, IL-1β/LPS in combination with immune complexes or by IL-10/TGFβ/glucocorticoids). Tumor escape has been linked with a switch from M1 activation in the early tumor initiation process towards M2-like phenotype during tumor progression, a process that highlights the heterogeneity and plasticity of macrophage activation and which offers a possible therapeutic target directed against reversing the TAM phenotype in the tumor. Here, we review different tumor-environmental stimuli and signalling cascades involved in this switch in differentiation and the so connected gene regulation in TAMs. In addition, therapeutic applications deducted from this differentiation and gene regulatory processes are presented. Data from pre-clinical as well as clinical studies clearly support the notion, that TAMs are excellent novel therapeutic targets for the fight against cancer.

miR-511-3p modulates genetic programs of tumor-associated macrophages

Expression of the mannose receptor (MRC1/CD206) identifies macrophage subtypes, such as alternatively activated macrophages (AAMs) and M2-polarized tumor-associated macrophages (TAMs), which are endowed with tissue-remodeling, proangiogenic, and protumoral activity. However, the significance of MRC1 expression for TAM's protumoral activity is unclear. Here, we describe and characterize miR-511-3p, an intronic microRNA (miRNA) encoded by both mouse and human MRC1 genes. By using sensitive miRNA reporter vectors, we demonstrate robust expression and bioactivity of miR-511-3p in MRC1(+) AAMs and TAMs. Unexpectedly, enforced expression of miR-511-3p tuned down the protumoral gene signature of MRC1(+) TAMs and inhibited tumor growth. Our findings suggest that transcriptional activation of Mrc1 in TAMs evokes a genetic program orchestrated by miR-511-3p, which limits rather than enhances their protumoral functions. Besides uncovering a role for MRC1 as gatekeeper of TAM's protumoral genetic programs, these observations suggest that endogenous miRNAs may operate to establish thresholds for inflammatory cell activation in tumors.

Macrophages in cancer and infectious diseases: the 'good' and the 'bad'

Macrophages are crucial orchestrators of host defence and tissue homeostasis. Macrophages are heterogeneous and plastic cells that in response to different microenvironmental signals can mount a broad spectrum of different programs of polarized activation. In different pathological contexts including cancer and infectious diseases, macrophages diversity and plasticity may act as a double-edged sword. The elucidation of the molecular mechanisms underlying macrophages recruitment and functional activation allows the identification of valuable targets for the development of innovative therapeutic approaches.

Endothelium-specific overexpression of human vascular endothelial growth factor-D in mice leads to increased tumor frequency and a reduced lifespan

BACKGROUND

Vascular endothelial growth factors (VEGFs) are central mediators in vascular development and lymphangiogenesis. VEGF-D contributes to the growth and formation of blood and lymphatic vessels, although its biological role is still somewhat unclear.

METHODS

Transgenic mice, which express the mature form of human VEGF-D under endothelium-specific Tie1 promoter, were produced by the lentiviral perivitelline-injection method. The mice were followed up to generation F(5) and the effect of the transgene was analyzed.

RESULTS

Transgenic mice had a high expression of human (h)VEGF-D in the endothelium in several tissues, such as kidney, liver, lung and spleen. However, transgenic mice developed tumors in lungs, kidneys, liver, mammary glands and lymph nodes upon aging and their mortality was also increased as a result of other pathological conditions. Hind limb ischemia was surgically induced in these mice and they were analyzed 1, 2 and 3  weeks after the ischemia operation. No significant differences were found in hVEGF-D mRNA expression, the number of capillaries or tissue repair between ischemic transgenic mice and transgene negative littermates.

CONCLUSIONS

It is concluded that targeted unregulated long-term expression of hVEGF-D in endothelium may not be useful and reduces the life span of transgenic mice.

Efficient intracellular drug-targeting of macrophages using stealth liposomes directed to the hemoglobin scavenger receptor CD163

The hemoglobin scavenger receptor CD163 is exclusively expressed in the monocytic lineage and preferentially in tissue resident macrophages of the M2 phenotype and in macrophages in sites of inflammation and tumor growth. In the present study we have designed liposomes specifically targeting CD163 by hydrophobic linkage of CD163-binding monoclonal antibodies to polyethylene glycol-coated liposomes ('stealth liposomes'). Targeting to the endocytic CD163 protein greatly increased the uptake of liposomes in CD163 transfected cells and macrophages as visualized by confocal microscopy and flow cytometry of cells exposed to CD163 targeting liposomes loaded with calcein. Strong cytotoxic effects were seen in CD163-expressing human monocytes by using the chemotherapeutic agent doxorubicin as cargo of the liposomes. In conclusion, the use of stealth liposomes modified to recognize CD163 is a potential way to target drugs to macrophages that support inflammatory and malignant processes.

Transplanted neural stem/precursor cells instruct phagocytes and reduce secondary tissue damage in the injured spinal cord

Transplanted neural stem/precursor cells possess peculiar therapeutic plasticity and can simultaneously instruct several therapeutic mechanisms in addition to cell replacement. Here, we interrogated the therapeutic plasticity of neural stem/precursor cells after their focal implantation in the severely contused spinal cord. We injected syngeneic neural stem/precursor cells at the proximal and distal ends of the contused mouse spinal cord and analysed locomotor functions and relevant secondary pathological events in the mice, cell fate of transplanted neural stem/precursor cells, and gene expression and inflammatory cell infiltration at the injured site. We used two different doses of neural stem/precursor cells and two treatment schedules, either subacute (7 days) or early chronic (21 days) neural stem/precursor cell transplantation after the induction of experimental thoracic severe spinal cord injury. Only the subacute transplant of neural stem/precursor cells enhanced the recovery of locomotor functions of mice with spinal cord injury. Transplanted neural stem/precursor cells survived undifferentiated at the level of the peri-lesion environment and established contacts with endogenous phagocytes via cellular-junctional coupling. This was associated with significant modulation of the expression levels of important inflammatory cell transcripts in vivo. Transplanted neural stem/precursor cells skewed the inflammatory cell infiltrate at the injured site by reducing the proportion of 'classically-activated' (M1-like) macrophages, while promoting the healing of the injured cord. We here identify a precise window of opportunity for the treatment of complex spinal cord injuries with therapeutically plastic somatic stem cells, and suggest that neural stem/precursor cells have the ability to re-programme the local inflammatory cell microenvironment from a 'hostile' to an 'instructive' role, thus facilitating the healing or regeneration past the lesion.

Signaling pathways governing tumor angiogenesis

Angiogenesis is regulated by the highly coordinated function of various proteins with pro- and antiangiogenic functions. Proangiogenic factors include vascular endothelial growth factor (VEGF), fibroblast growth factor, platelet-derived growth factor, insulin-like growth factor, transforming growth factor, angiopoietins, and several chemokines; antiangiogenic factors include thrombospondin-1, angiostatin, and endostatin. Matrix metalloproteinases display a dual role in vascular development. Notch signaling affects remodeling of the primary vascular network of uniformly sized vessels into functionally and morphologically distinct arteries, veins, and capillaries. Tumors, described as 'wounds that never heal', lose the appropriate balance among these factors. Although VEGF-targeted therapies are showing promise, new angiogenesis targets are needed to make additional gains. Here, we highlight recent advances in our understanding of the regulation of tumor angiogenesis and discuss the potential of molecular targeting as a new therapeutic approach.

Macrophage polarization in health and disease

Macrophages are terminally differentiated cells of the mononuclear phagocyte system that also encompasses dendritic cells, circulating blood monocytes, and committed myeloid progenitor cells in the bone marrow. Both macrophages and their monocytic precursors can change their functional state in response to microenvironmental cues exhibiting a marked heterogeneity. However, there are still uncertainties regarding distinct expression patterns of surface markers that clearly define macrophage subsets, particularly in the case of human macrophages. In addition to their tissue distribution, macrophages can be functionally polarized into M1 (proinflammatory) and M2 (alternatively activated) as well as regulatory cells in response to both exogenous infections and solid tumors as well as by systems biology approaches.

Establishment of a real-time, quantitative, and reproducible mouse model of Staphylococcus osteomyelitis using bioluminescence imaging

Osteomyelitis remains a serious problem in the orthopedic field. There are only a few animal models in which the quantity and distribution of bacteria can be reproducibly traced. Here, we established a real-time quantitative mouse model of osteomyelitis using bioluminescence imaging (BLI) without sacrificing the animals. A bioluminescent strain of Staphylococcus aureus was inoculated into the femurs of mice. The bacterial photon intensity (PI) was then sequentially measured by BLI. Serological and histological analyses of the mice were performed. The mean PI peaked at 3 days, and stable signals were maintained for over 3 months after inoculation. The serum levels of interleukin-6, interleukin-1β, and C-reactive protein were significantly higher in the infected mice than in the control mice on day 7. The serum monocyte chemotactic protein 1 level was also significantly higher in the infected group at 12 h than in the control group. A significantly higher proportion of granulocytes was detected in the peripheral blood of the infected group after day 7. Additionally, both acute and chronic histological manifestations were observed in the infected group. This model is useful for elucidating the pathophysiology of both acute and chronic osteomyelitis and to assess the effects of novel antibiotics or antibacterial implants.

Peripheral blood monocyte subsets predict antiviral response in chronic hepatitis C

BACKGROUND

Hepatitis C virus infection evolves into chronic progressive liver disease in a significant percentage of patients. Monocytes constitute a diverse group of myeloid cells that mediate innate and adaptive immune response. In addition to proinflammatory CD16+ monocytes, a Tie-2+ subgroup - Tie-2 expressing monocytes (TEMs) - that has robust proangiogenic potential has been recently defined.

AIM

To study the heterogeneity of peripheral blood monocytes in chronic hepatitis C (CHC) patients and to examine their proposed pathophysiological roles on disease progression and response to antiviral therapy.

METHODS

We studied CD16+ and Tie-2+ peripheral monocyte subpopulations in 21 healthy subjects and 39 CHC patients in various stages of disease and responses to antiviral treatment using flow cytometry. Expression profiles of proangiogenic and tissue remodelling factors in monocyte supernatants were measured using ELISA and protein arrays. Intrahepatic expression of CD14, CD31 and Tie-2 was analysed using immunofluorescence.

RESULTS

Increases of certain peripheral monocyte subsets were observed in the blood of CHC patients, wherein those cells with proinflammatory (CD16+) or proangiogenic (TEMs) potential expanded (P < 0.005, both). Notably, TEMs were significantly increased in nonresponders, particularly those with lower CD16 expression. In addition, many angiogenic factors were differentially expressed by peripheral monocytes from control or CHC patients, such as angiopoietin-1 and angiogenin (P < 0.05). Interestingly, intrahepatic TEMs were distinguished within portal infiltrates of CHC patients.

CONCLUSIONS

These findings suggest for the first time the relevance of peripheral monocytes phenotypes for the achievement of response to treatment. Hence, the study of monocyte subset regulation might effect improved CHC prognoses and adjuvant therapies.

Origin and Functions of Tumor-Associated Myeloid Cells (TAMCs)

The construction of an inflammatory microenvironment provides the fuel for cancer development and progression. Hence, solid tumors promote the expansion and the recruitment of leukocyte populations, among which tumor-associated myeloid cells (TAMCs) represent a paradigm for cancer-promoting inflammation. TAMCs group heterogeneous phagocytic populations stemming from a common myeloid progenitor (CMP), that orchestrate various aspects of cancer, including: diversion and skewing of adaptive responses; immunosuppression; cell growth; angiogenesis; matrix deposition and remodelling; construction of a metastatic niche and actual metastasis. Several evidence indicate that TAMCs show plasticity and/or functional heterogeneity, suggesting that tumour-derived factors promote their functional "reprogramming" towards protumoral activities. While recent studies have attempted to address the role of microenvironment signals, the interplay between cancer cells, innate and adaptive immunity is now emerging as a crucial step of the TAMCs reprogramming. Here we discuss the evidence for the differentiation of TAMCs during the course of tumor progression and the molecular mechanisms that regulate such event.

Myeloid cell diversification and complexity: an old concept with new turns in oncology

Tumour development is accompanied by an enhanced haematopoiesis. This is not a widespread activation since only cells belonging to the myelo-monocytic compartment are expanded and mobilized from primary sites of haematopoiesis to other organs, reaching also the tumour stroma. This process occurs early during tumour formation but becomes more evident in advanced disease. Far from being a simple, unwanted consequence of cancer development, accumulation of myelo-monocytitc cells plays a role in tumour vascularization, local spreading, establishment of metastasis at distant sites, and contribute to create an environment unfavourable for the adoptive immunity against tumour-associated antigens. Myeloid populations involved in these process are likely different but many cells, expanded in primary and secondary lymphoid organs of tumour-bearing mice, share various levels of the CD11b and Gr-1 (Ly6C/G) markers. CD11b(+)Gr-1(+) cells are currently named myeloid-derived suppressor cells for their ability to inhibit T lymphocyte responses in tumour-bearing hosts. In this manuscript, we review the recent literature on tumour-conditioned myeloid subsets that assist tumour growth, both in mice and humans.

Proangiogenic Tie2(+) macrophages infiltrate human and murine endometriotic lesions and dictate their growth in a mouse model of the disease

Endometriosis affects women of reproductive age, causing infertility and pain. Although immune cells are recruited in endometriotic lesions, their role is unclear. Tie2-expressing macrophages (TEMs) have nonredundant functions in promoting angiogenesis and growth of experimental tumors. Here we show that human TEMs infiltrate areas surrounding newly formed endometriotic blood vessels. We set up an ad hoc mouse model in which TEMs, and not Tie2-expressing endothelial cells, are targeted. We transplanted in wild-type recipients bone marrow cells expressing a suicide gene (Herpes simplex virus type 1 thymidine kinase) under the Tie2 promoter/enhancer. TEMs infiltrated endometriotic lesions. TEM depletion by ganciclovir administration arrested the growth of established lesions, without toxicity. Lesion architecture was disrupted, with: i) loss of glandular organization, ii) reduced neovascularization, and iii) activation of caspase 3 in CD31(+) endothelial cells. Thus, TEMs are important for maintaining the viability of newly formed vessels and represent a potential therapeutic target in endometriosis.

Differential macrophage polarization promotes tissue remodeling and repair in a model of ischemic retinopathy

Diabetic retinopathy is the leading cause of visual loss in individuals under the age of 55. Umbilical cord blood (UCB)–derived myeloid progenitor cells have been shown to decrease neuronal damage associated with ischemia in the central nervous system. In this study we show that UCB-derived CD14⁺ progenitor cells provide rescue effects in a mouse model of ischemic retinopathy by promoting physiological angiogenesis and reducing associated inflammation. We use confocal microscopy to trace the fate of injected human UCB-derived CD14⁺ cells and PCR with species-specific probes to investigate their gene expression profile before and after injection. Metabolomic analysis measures changes induced by CD14⁺ cells. Our results demonstrate that human cells differentiate in vivo into M2 macrophages and induce the polarization of resident M2 macrophages. This leads to stabilization of the ischemia-injured retinal vasculature by modulating the inflammatory response, reducing oxidative stress and apoptosis and promoting tissue repair.

Transition of tumor-associated macrophages from MHC class II(hi) to MHC class II(low) mediates tumor progression in mice

Background

Tumor-associated macrophages (TAMs) are the most abundant immune cells within the tumor stroma and play a crucial role in tumor development. Although clinical investigations indicate that high levels of macrophage (MΦ) infiltration into tumors are associated with a poor prognosis, the exact role played by TAMs during tumor development remains unclear. The present study aimed to investigate dynamic changes in TAM major histocompatibility complex (MHC) class II expression levels and to assess the effects of these changes on tumor progression.

Results

Significant inhibition of tumor growth in the murine hepatocellular carcinoma Hepa1-6 model was closely associated with partial TAM depletion. Strikingly, two distinct TAM subsets were found to coexist within the tumor microenvironment during Hepa1-6 tumor development. An MHC class II^hi TAM population appeared during the early phase of tumor development and was associated with tumor suppression; however, an MHC class II^low TAM population became increasingly predominant as the tumor progressed.

Conclusions

Tumor progression was positively correlated with increasing infiltration of the tumor tissues by MHC class II^low TAMs. Thus, targeting the transition of MΦ may be a novel strategy for drug development and immunotherapy.

Regulation of macrophage arginase expression and tumor growth by the Ron receptor tyrosine kinase

M1 activation of macrophages promotes inflammation and immunity to intracellular pathogens, whereas M2 macrophage activation promotes resolution of inflammation, wound healing, and tumor growth. These divergent phenotypes are characterized, in part, by the expression of inducible NO synthase and arginase I (Arg1) in M1 versus M2 activated macrophages, respectively. In this study, we demonstrate that the Ron receptor tyrosine kinase tips the balance of macrophage activation by attenuating the M1 phenotype while promoting expression of Arg1 through a Stat6-independent mechanism. Induction of the Arg1 promoter by Ron is mediated by an AP-1 site located 433 bp upstream of the transcription start site. Treatment of primary macrophages with macrophage stimulating protein, the ligand for Ron, induces potent MAPK activation, upregulates Fos, and enhances binding of Fos to the AP-1 site in the Arg1 promoter. In vivo, Arg1 expression in tumor-associated macrophages (TAMs) from Ron(-/-) mice was significantly reduced compared with that in TAMs from control animals. Furthermore, we show that Ron is expressed specifically by Tie2-expressing macrophages, a TAM subset that exhibits a markedly skewed M2 and protumoral phenotype. Decreased Arg1 in TAMs from Ron(-/-) mice was associated with reduced syngeneic tumor growth in these animals. These findings indicate that Ron induces Arg1 expression in macrophages through a previously uncharacterized AP-1 site in the Arg1 promoter and that Ron could be therapeutically targeted in the tumor microenvironment to inhibit tumor growth by targeting expression of Arg1.

Mononuclear phagocyte heterogeneity in cancer: different subsets and activation states reaching out at the tumor site

Mononuclear phagocytes are amongst the most versatile cells of the body, contributing to tissue genesis and homeostasis and safeguarding the balance between pro- and anti-inflammatory reactions. Accordingly, these cells are notoriously heterogeneous, functioning in distinct differentiation forms (monocytes, MDSC, macrophages, DC) and adopting different activation states in response to a changing microenvironment. Accumulating evidence exists that mononuclear phagocytes contribute to all phases of the cancer process. These cells orchestrate the inflammatory events during de novo carcinogenesis, participate in tumor immunosurveillance, and contribute to the progression of established tumors. At the tumor site, cells such as tumor-associated macrophages (TAM) are confronted with different tumor microenvironments, leading to TAM subsets with specialized functions. A better refinement of the molecular and functional heterogeneity of tumor-associated mononuclear phagocytes might pave the way for novel cancer therapies that directly target these tumor-supporting cells.

Stabilin-1/CLEVER-1, a type 2 macrophage marker, is an adhesion and scavenging molecule on human placental macrophages

Stabilin-1/common lymphatic endothelial and vascular endothelial receptor-1 (CLEVER-1) is a multidomain protein present in lymphatic and vascular endothelial cells and type 2 immunosuppressive macrophages. In adults, stabilin-1/CLEVER-1 is a scavenging receptor and an adhesion molecule, but much less is known about its role during development. Here, we studied the expression and functions of macrophage stabilin-1/CLEVER-1 in human placenta and during human ontogeny. Using newly generated mAbs, we found that stabilin-1/CLEVER-1 is expressed on virtually all macrophages in term placenta, both in the decidua and in the placental villi. Placental stabilin-1/CLEVER-1 was involved in the scavenging of Ac-LDL (acetylated low density lipoprotein) and in the uptake of fluorescently labeled model antigen OVA. siRNA-mediated suppression of stabilin-1/CLEVER-1 altered the cytokine profile produced by placental macrophages. Stabilin-1/CLEVER-1 on placental macrophages mediated their adhesion to placental vessels and supported their transmigration through vascular endothelium. Finally, we found that stabilin-1/CLEVER-1 is induced very early in fetal macrophages, high endothelial venules, and lymphatic vessels in multiple lymphatic organs. Together, these data suggest that macrophage stabilin-1/CLEVER-1 can potentially regulate leukocyte migration and scavenging during the development of the placenta and fetus.

Adenovirus-mediated HIF-1α gene transfer promotes repair of mouse airway allograft microvasculature and attenuates chronic rejection

Chronic rejection, manifested as small airway fibrosis (obliterative bronchiolitis [OB]), is the main obstacle to long-term survival in lung transplantation. Recent studies demonstrate that the airways involved in a lung transplant are relatively hypoxic at baseline and that OB pathogenesis may be linked to ischemia induced by a transient loss of airway microvasculature. Here, we show that HIF-1α mediates airway microvascular repair in a model of orthotopic tracheal transplantation. Grafts with a conditional knockout of Hif1a demonstrated diminished recruitment of recipient-derived Tie2⁺ angiogenic cells to the allograft, impaired repair of damaged microvasculature, accelerated loss of microvascular perfusion, and hastened denudation of epithelial cells. In contrast, graft HIF-1α overexpression induced via an adenoviral vector prolonged airway microvascular perfusion, preserved epithelial integrity, extended the time window for the graft to be rescued from chronic rejection, and attenuated airway fibrotic remodeling. HIF-1α overexpression induced the expression of proangiogenic factors such as Sdf1, Plgf, and Vegf, and promoted the recruitment of vasoreparative Tie2⁺ cells. This study demonstrates that a therapy that enhances vascular integrity during acute rejection may promote graft health and prevent chronic rejection.

Angiopoietin-2 TIEs up macrophages in tumor angiogenesis

Angiopoietin-2 (ANG2), a ligand of the TIE2 receptor, modulates endothelial cell biology and destabilizes blood vessels to facilitate angiogenesis. Recent reports have shown that ANG2 inhibition, for example, by monoclonal antibodies, peptibodies, or CovX-Bodies, may achieve substantial antiangiogenic and antitumor responses in a variety of mouse tumor models, including spontaneous MMTV-PyMT mammary and RIP1-Tag2 pancreatic islet adenocarcinomas. There is also evidence that targeting the ANG2/TIE2 signaling pathway may inhibit the functions of TIE2-expressing macrophages (TEM), a tumor-associated macrophage subset endowed with proangiogenic activity in mouse tumor models. The clinical opportunities afforded by simultaneously targeting the effects of ANG2 on tumor angiogenesis and the proangiogenic activity of TEMs are discussed.

Targeting the ANG2/TIE2 axis inhibits tumor growth and metastasis by impairing angiogenesis and disabling rebounds of proangiogenic myeloid cells

Tumor-infiltrating myeloid cells convey proangiogenic programs that counteract the efficacy of antiangiogenic therapy. Here, we show that blocking angiopoietin-2 (ANG2), a TIE2 ligand and angiogenic factor expressed by activated endothelial cells (ECs), regresses the tumor vasculature and inhibits progression of late-stage, metastatic MMTV-PyMT mammary carcinomas and RIP1-Tag2 pancreatic insulinomas. ANG2 blockade did not inhibit recruitment of MRC1(+) TIE2-expressing macrophages (TEMs) but impeded their upregulation of Tie2, association with blood vessels, and ability to restore angiogenesis in tumors. Conditional Tie2 gene knockdown in TEMs was sufficient to decrease tumor angiogenesis. Our findings support a model wherein the ANG2-TIE2 axis mediates cell-to-cell interactions between TEMs and ECs that are important for tumor angiogenesis and can be targeted to induce effective antitumor responses.

VEGF and angiopoietin signaling in tumor angiogenesis and metastasis

Solid tumors require blood vessels for growth and dissemination, and lymphatic vessels as additional conduits for metastatic spread. The identification of growth factor receptor pathways regulating angiogenesis has led to the clinical approval of the first antiangiogenic molecules targeted against the vascular endothelial growth factor (VEGF)-VEGF receptor (VEGFR)-2 pathway. However, in many cases resistance to anti-VEGF-VEGFR therapy occurs, and thus far the clinical benefit has been limited to only modest improvements in overall survival. Therefore, novel treatment modalities are required. Here, we discuss the members of the VEGF-VEGFR family as well as the angiopoietin growth factors and their Tie receptors as potential novel targets for antiangiogenic and antilymphangiogenic therapies.

TIE2-expressing macrophages limit the therapeutic efficacy of the vascular-disrupting agent combretastatin A4 phosphate in mice

Vascular-disrupting agents (VDAs) such as combretastatin A4 phosphate (CA4P) selectively disrupt blood vessels in tumors and induce tumor necrosis. However, tumors rapidly repopulate after treatment with such compounds. Here, we show that CA4P-induced vessel narrowing, hypoxia, and hemorrhagic necrosis in murine mammary tumors were accompanied by elevated tumor levels of the chemokine CXCL12 and infiltration by proangiogenic TIE2-expressing macrophages (TEMs). Inhibiting TEM recruitment to CA4P-treated tumors either by interfering pharmacologically with the CXCL12/CXCR4 axis or by genetically depleting TEMs in tumor-bearing mice markedly increased the efficacy of CA4P treatment. These data suggest that TEMs limit VDA-induced tumor injury and represent a potential target for improving the clinical efficacy of VDA-based therapies.

Effects of a synthetic PEG-ylated Tie-2 agonist peptide on endotoxemic lung injury and mortality

A synthetic 7-mer, HHHRHSF, was recently identified by screening a phage display library for binding to the Tie-2 receptor. A polyethylene-oxide clustered version of this peptide, termed vasculotide (VT), was reported to activate Tie-2 and promote angiogenesis in a mouse model of diabetic ulcer. We hypothesized that VT administration would defend endothelial barrier function against sepsis-associated mediators of permeability, prevent lung vascular leakage arising in endotoxemia, and improve mortality in endotoxemic mice. In confluent human microvascular endothelial cells, VT prevented endotoxin-induced (lipopolysaccharides, LPS O111:B4) gap formation, loss of monolayer resistance, and translocation of labeled albumin. In 8-wk-old male C57Bl6/J mice given a ∼70% lethal dose of endotoxin (15 mg/kg ip), VT prevented lung vascular leakage and reversed the attenuation of lung vascular endothelial cadherin induced by endotoxemia. These protective effects of VT were associated with activation of Tie-2 and its downstream mediator, Akt. Echocardiographic studies showed only a nonsignificant trend toward improved myocardial performance associated with VT. Finally, we evaluated survival in this mouse model. Pretreatment with VT improved survival by 41.4% (n = 15/group, P = 0.02) and post-LPS administration of VT improved survival by 33.3% (n = 15/group, P = 0.051). VT-mediated protection from LPS lethality was lost in Tie-2 heterozygous mice, in agreement with VT's proposed receptor specificity. We conclude that this synthetic Tie-2 agonist, completely unrelated to endogenous Tie-2 ligands, is sufficient to activate the receptor and its downstream pathways in vivo and that the Tie-2 receptor may be an important target for therapeutic evaluation in conditions of pathological vascular leakage.

Bone marrow-derived cells serve as proangiogenic macrophages but not endothelial cells in wound healing

Bone marrow-derived cells (BMDCs) contribute to postnatal vascular growth by differentiating into endothelial cells or secreting angiogenic factors. However, the extent of their endothelial differentiation highly varies according to the angiogenic models used. Wound healing is an intricate process in which the skin repairs itself after injury. As a process also observed in cancer progression, neoangiogenesis into wound tissues is profoundly involved in this healing process, suggesting the contribution of BMDCs. However, the extent of the differentiation of BMDCs to endothelial cells in wound healing is unclear. In this study, using the green fluorescent protein-bone marrow chim-eric experiment and high resolution confocal microscopy at a single cell level, we observed no endothelial differentiation of BMDCs in 2 acute wound healing models (dorsal excisional wound and ear punch) and a chronic wound healing model (decubitus ulcer). Instead, a major proportion of BMDCs were macrophages. Indeed, colony-stimulating factor 1 (CSF-1) inhibition depleted approximately 80% of the BMDCs at the wound healing site. CSF-1-mutant (CSF-1(op/op)) mice showed significantly reduced neoangiogenesis into the wound site, supporting the substantial role of BMDCs as macrophages. Our data show that the proangiogenic effects of macrophages, but not the endothelial differentiation, are the major contribution of BMDCs in wound healing.

HRG inhibits tumor growth and metastasis by inducing macrophage polarization and vessel normalization through downregulation of PlGF

Polarization of tumor-associated macrophages (TAMs) to a proangiogenic/immune-suppressive (M2-like) phenotype and abnormal, hypoperfused vessels are hallmarks of malignancy, but their molecular basis and interrelationship remains enigmatic. We report that the host-produced histidine-rich glycoprotein (HRG) inhibits tumor growth and metastasis, while improving chemotherapy. By skewing TAM polarization away from the M2- to a tumor-inhibiting M1-like phenotype, HRG promotes antitumor immune responses and vessel normalization, effects known to decrease tumor growth and metastasis and to enhance chemotherapy. Skewing of TAM polarization by HRG relies substantially on downregulation of placental growth factor (PlGF). Besides unveiling an important role for TAM polarization in tumor vessel abnormalization, and its regulation by HRG/PlGF, these findings offer therapeutic opportunities for anticancer and antiangiogenic treatment.

Monocytic cells derived from human embryonic stem cells and fetal liver share common differentiation pathways and homeostatic functions

The early emergence of macrophages and their large pattern of tissue distribution during development suggest that they may play a critical role in the initial steps of embryogenesis. In the present study, we show that monocytic cells derived from human embryonic stem cells (hESCs) and from fetal liver follow a differentiation pathway different to that of adult cells, leading to specific functions. Embryonic and fetal monocytic cells differentiated from a CD14(low)CD16(-) precursor to form CD14(high)CD16(+) cells without producing the CD14(high)CD16(-) cell population that predominates in adult peripheral blood. Both demonstrated an enhanced expression of genes encoding tissue-degrading enzymes, chemokines, and scavenger receptors, as was previously reported for M2 macrophages. Compared with adult blood monocytes, embryonic and fetal monocytic cells secreted high amounts of proteins acting on tissue remodeling and angiogenesis, and most of them expressed the Tie2 receptor. Furthermore, they promoted vascular remodeling in xenotransplanted human tumors. These findings suggest that the regulation of human fetal and embryonic monocytic cell differentiation leads to the generation of cells endowed mainly with anti-inflammatory and remodeling functions. Trophic and immunosuppressive functions of M2-polarized macrophages link fetus and tumor development, and hESCs offer a valuable experimental model for in vitro studies of mechanisms sustaining these processes.

Macrophages define dermal lymphatic vessel calibre during development by regulating lymphatic endothelial cell proliferation

Macrophages have been suggested to stimulate neo-lymphangiogenesis in settings of inflammation via two potential mechanisms: (1) acting as a source of lymphatic endothelial progenitor cells via the ability to transdifferentiate into lymphatic endothelial cells and be incorporated into growing lymphatic vessels; and (2) providing a crucial source of pro-lymphangiogenic growth factors and proteases. We set out to establish whether cells of the myeloid lineage are important for development of the lymphatic vasculature through either of these mechanisms. Here, we provide lineage tracing evidence to demonstrate that lymphatic endothelial cells arise independently of the myeloid lineage during both embryogenesis and tumour-stimulated lymphangiogenesis in the mouse, thus excluding macrophages as a source of lymphatic endothelial progenitor cells in these settings. In addition, we demonstrate that the dermal lymphatic vasculature of PU.1(-/-) and Csf1r(-/-) macrophage-deficient mouse embryos is hyperplastic owing to elevated lymphatic endothelial cell proliferation, suggesting that cells of the myeloid lineage provide signals that act to restrain lymphatic vessel calibre in the skin during development. In contrast to what has been demonstrated in settings of inflammation, macrophages do not comprise the principal source of pro-lymphangiogenic growth factors, including VEGFC and VEGFD, in the embryonic dermal microenvironment, illustrating that the sources of patterning and proliferative signals driving embryonic and disease-stimulated lymphangiogenesis are likely to be distinct.

Control of immune response by amino acid metabolism

The interaction between pathogenic microorganisms and their hosts is regulated by reciprocal survival strategies, including competition for essential nutrients. Though paradoxical, mammalian hosts have learned to take advantage of amino acid catabolism for controlling pathogen invasion and, at the same time, regulating their own immune responses. In this way, ancient catabolic enzymes have acquired novel functions and evolved into new structures with highly specialized functions, which go beyond the struggle for survival. In this review, we analyze the evidence supporting a critical role for the metabolism of various amino acids in regulating different steps of both innate and adaptive immunity.

Oncogene-driven intrinsic inflammation induces leukocyte production of tumor necrosis factor that critically contributes to mammary carcinogenesis

Oncogene activation promotes an intrinsic inflammatory pathway that is crucial for cancer development. Here, we have investigated the actual effect of the inflammatory cytokine tumor necrosis factor (TNF) on the natural history of spontaneous mammary cancer in the HER2/neuT (NeuT) transgenic mouse model. Bone marrow transplantation from TNF knockout mice into NeuT recipients significantly impaired tumor growth, indicating that the source of TNF fostering tumor development was of bone marrow origin. We show that the absence of leukocyte-derived TNF disarranged the tumor vasculature, which lacked pericyte coverage and structural integrity, leading to diffuse vascular hemorrhage and stromal necrosis. In addition, tumor-associated Tie2-expressing monocytes were reduced and cytokine expression skewed from Th2 to Th1 type. Treatment of NeuT mice with anti-TNF antibody partially phenocopied the antitumor effect of TNF-deficient bone marrow cell transplantation, providing a strong preclinical background and rationale for the introduction of TNF antagonists in the treatment of human breast cancer, including basal-like samples for which consolidated targeted therapies do not exist.

Macrophage plasticity and interaction with lymphocyte subsets: cancer as a paradigm

Plasticity is a hallmark of cells of the myelomonocytic lineage. In response to innate recognition or signals from lymphocyte subsets, mononuclear phagocytes undergo adaptive responses. Shaping of monocyte-macrophage function is an essential component of resistance to pathogens, tissue damage and repair. The orchestration of myelomonocytic cell function is a key element that links inflammation and cancer and provides a paradigm for macrophage plasticity and function. A better understanding of the molecular basis of myelomonocytic cell plasticity will open new vistas in immunopathology and therapeutic intervention.

Novel discovery of LYVE-1 expression in the hyaloid vascular system

PURPOSE

The hyaloid vascular system (HVS) is a transient network nourishing developing eyes and has been widely used as a natural model to study blood vessel regression. Failure of its regression in humans leads to several blinding diseases. Lymphatic vessel endothelial hyaluronic acid receptor (LYVE-1) is a recently defined lymphatic marker that is also expressed by a subpopulation of macrophages. To date, there is no report on its expression in the HVS. This study was conducted to investigate whether LYVE-1 is expressed in the HVS and how it is associated with the vascular structure and macrophage phenotype.

METHODS

Normal C57BL/6 mouse eyeballs were sampled from embryonic day (E) 10.5 to postnatal (P) and adult stages for immunofluorescent microscopic studies with antibodies against LYVE-1, CD31 (panendothelial cell marker), and F4/80 (macrophage marker). Additionally, Angiopoietin-2 (Ang-2) knockout mice with abnormally persistent HVS were examined.

RESULTS

The LYVE-1 expression was detected on normal HVS between E12.5 and P14. The LYVE-1(+) cells were F4/80(+) but CD31(-), indicating a macrophage lineage. Additionally, LYVE-1(+) cells bud on CD31(+) vessels and constitute an integral part of the network in both normal developing and Ang-2 knockout mice.

CONCLUSIONS

This study provides the first evidence that the HVS contains a LYVE-1(+) cellular component in both physiological and pathologic conditions. This novel finding not only provides a new concept in defining the embryogenesis and pathogenesis of the HVS, it also leads to a completely natural model in which to study the functions of the LYVE-1 pathway, an important topic for lymphatic research as well.