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Key Points Cells of the mononuclear phagocytic lineage differentiate into many different types of macrophage with a wide range of activities. These include specialized types of macrophage, such as Kupffer cells in the liver, microglial cells in the brain, dendritic cells and the highly specialized bone osteoclasts. Macrophages have many functions, including roles in immunity, apoptotic-cell clearance, wound healing and, as discussed in this Review, development. Analysis of macrophage-deficient mice has revealed many functions for these cells in development, and their loss perturbs the morphology and physiology of many tissues, such as the brain, mammary glands and bones. Macrophages promote epithelial-cell outgrowth and branching morphogenesis in several tissues, including the pancreas and the mammary glands. Macrophages regulate angiogenesis during development, wound repair and disease. Macrophages have a role in adipogenesis and obesity. Macrophages are strongly involved in many chronic diseases that are associated with ageing and provide trophic factors that are similar to those used during development; this has the effect of increasing disease pathology. Cancers sequester the developmental roles of macrophages to enhance their growth, invasive capacity, intravasation, progression and metastatic capacity. The homeostatic roles of macrophages in tissue development and maintenance are discussed, and insights are provided into how dysregulation of these primitive functions can be subverted in chronic diseases such as cancer and obesity to contribute to pathology. Specialized phagocytes are found in the most primitive multicellular organisms. Their roles in homeostasis and in distinguishing self from non-self have evolved with the complexity of organisms and their immune systems. Equally important, but often overlooked, are the roles of macrophages in tissue development. As discussed in this Review, these include functions in branching morphogenesis, neuronal patterning, angiogenesis, bone morphogenesis and the generation of adipose tissue. In each case, macrophage depletion impairs the formation of the tissue and compromises its function. I argue that in several diseases, the unrestrained acquisition of these developmental macrophage functions exacerbates pathology. For example, macrophages enhance tumour progression and metastasis by affecting tumour-cell migration and invasion, as well as angiogenesis.

A discussion of progress in macrophage biology, examining their classification, diverse lineages, identities and regulation, their roles in regulating normal physiology and development, and their identification as therapeutic targets in human diseases. The macrophage in sickness and in health Macrophages are phagocytic cells found in blood, lymph and in all mammalian tissue types. They have many and diverse roles in normal development, homeostasis, tissue repair and the immune response to pathogens. Their diversity means that they are involved in almost every human disease and are prime therapeutic targets because their function can be augmented or inhibited to alter disease outcome. A Review in this issue discusses macrophage physiology in terms of the homeostatic mechanisms by which macrophages contribute to physiological and pathophysiologic adaptations in mammals. Macrophages, the most plastic cells of the haematopoietic system, are found in all tissues and show great functional diversity. They have roles in development, homeostasis, tissue repair and immunity. Although tissue macrophages are anatomically distinct from one another, and have different transcriptional profiles and functional capabilities, they are all required for the maintenance of homeostasis. However, these reparative and homeostatic functions can be subverted by chronic insults, resulting in a causal association of macrophages with disease states. In this Review, we discuss how macrophages regulate normal physiology and development, and provide several examples of their pathophysiological roles in disease. We define the ‘hallmarks’ of macrophages according to the states that they adopt during the performance of their various roles, taking into account new insights into the diversity of their lineages, identities and regulation. It is essential to understand this diversity because macrophages have emerged as important therapeutic targets in many human diseases.

The brain contains numerous mononuclear phagocytes called microglia. These cells express the transmembrane tyrosine kinase receptor for the macrophage growth factor colony stimulating factor-1 (CSF-1R). Using a CSF-1R-GFP reporter mouse strain combined with lineage defining antibody staining we show in the postnatal mouse brain that CSF-1R is expressed only in microglia and not neurons, astrocytes or glial cells. To study CSF-1R function we used mice homozygous for a null mutation in the Csflr gene. In these mice microglia are >99% depleted at embryonic day 16 and day 1 post-partum brain. At three weeks of age this microglial depletion continues in most regions of the brain although some contain clusters of rounded microglia. Despite the loss of microglia, embryonic brain development appears normal but during the post-natal period the brain architecture becomes perturbed with enlarged ventricles and regionally compressed parenchyma, phenotypes most prominent in the olfactory bulb and cortex. In the cortex there is increased neuronal density, elevated numbers of astrocytes but reduced numbers of oligodendrocytes. Csf1r nulls rarely survive to adulthood and therefore to study the role of CSF-1R in olfaction we used the viable null mutants in the Csf1 (Csf1(op)) gene that encodes one of the two known CSF-1R ligands. Food-finding experiments indicate that olfactory capacity is significantly impaired in the absence of CSF-1. CSF-1R is therefore required for the development of microglia, for a fully functional olfactory system and the maintenance of normal brain structure.

The earliest blood vessels in mammalian embryos are formed when endothelial cells differentiate from angioblasts and coalesce into tubular networks. Thereafter, the endothelium is thought to expand solely by proliferation of pre-existing endothelial cells. Here we show that a complementary source of endothelial cells is recruited into pre-existing vasculature after differentiation from the earliest precursors of erythrocytes, megakaryocytes and macrophages, the erythro-myeloid progenitors (EMPs) that are born in the yolk sac. A first wave of EMPs contributes endothelial cells to the yolk sac endothelium, and a second wave of EMPs colonizes the embryo and contributes endothelial cells to intraembryonic endothelium in multiple organs, where they persist into adulthood. By demonstrating that EMPs constitute a hitherto unrecognized source of endothelial cells, we reveal that embryonic blood vascular endothelium expands in a dual mechanism that involves both the proliferation of pre-existing endothelial cells and the incorporation of endothelial cells derived from haematopoietic precursors. New blood vessel endothelial cells arise from differentiation of erythro-myeloid progenitor cells to complement the proliferation of pre-existing endothelial cells.

Tumor-associated macrophages (TAMs) promote metastasis and tumor cell extravasation, survival, and growth. In hepatocellular carcinoma (HCC), the presence of TAM subpopulations correlates with poor outcome. In this issue of the JCI, Ning et al. report on their use of cell culture, mouse models, and human data sets to investigate the interactions between aerobic glycolysis and carbonic anhydrase XII (CA12) expression in HCC. Aerobic glycolysis promoted CA12 upregulation in TAMs, which induced a protumoral phenotype to promote tumor growth and metastasis. Tumor cell factors derived from HCC samples induced CA12 upregulation in tumor-infiltrating TAMs via the HIF1α pathway. In preclinical models of HCC, CA12 inhibition reduced tumor growth and lung metastasis and reduced TAM infiltrate. Notably, dual treatment with anti-PD1 and CA12 inhibitors synergistically attenuated tumor growth and metastasis and enhanced survival compared with either treatment alone. These findings suggest that targeting CA12 in combination with immune-checkpoint blockade may provide treatment options for HCC.

Macrophages and dendritic cells (DCs) are key components of cellular immunity and are thought to originate and renew from hematopoietic stem cells (HSCs). However, some macrophages develop in the embryo before the appearance of definitive HSCs. We thus reinvestigated macrophage development. We found that the transcription factor Myb was required for development of HSCs and all CD11b high monocytes and macrophages, but was dispensable for yolk sac (YS) macrophages and for the development of YS-derived F4/80 bright macrophages in several tissues, such as liver Kupffer cells, epidermal Langerhans cells, and microglia— cell populations that all can persist in adult mice independently of HSCs. These results define a lineage of tissue macrophages that derive from the YS and are genetically distinct from HSC progeny.

The stromal microenvironment and particularly the macrophage component of primary tumors influence their malignant potential. However, at the metastatic site the role of these cells and their mechanism of actions for establishment and growth of metastases remain largely unknown. Using animal models of breast cancer metastasis, we show that a population of host macrophages displaying a distinct phenotype is recruited to extravasating pulmonary metastatic cells regardless of species of origin. Ablation of this macrophage population through three independent means (genetic and chemical) showed that these macrophages are required for efficient metastatic seeding and growth. Importantly, even after metastatic growth is established, ablation of this macrophage population inhibited subsequent growth. Furthermore, imaging of intact lungs revealed that macrophages are required for efficient tumor cell extravasation. These data indicate a direct enhancement of metastatic growth by macrophages through their effects on tumor cell extravasation, survival and subsequent growth and identifies these cells as a new therapeutic target for treatment of metastatic disease.

Tumor-associated macrophages （TAMs） contribute to breast cancer progression and dissemination; TAM-targeting strategies aimed at their reprogramming show promising preelinical results. In a new report Guerriero and colleagues demonstrate that a novel HDAC Class IIa inhibitor, TMP195, can reprogram monocytes and macrophages in the tumor into cells able to sustain a robust CD8 T cell-mediated anti-tumoral immune response.

Cytotoxic chemotherapy is an effective treatment for invasive breast cancer. However, experimental studies in mice also suggest that chemotherapy has pro-metastatic effects. Primary tumours release extracellular vesicles (EVs), including exosomes, that can facilitate the seeding and growth of metastatic cancer cells in distant organs, but the effects of chemotherapy on tumour-derived EVs remain unclear. Here we show that two classes of cytotoxic drugs broadly employed in pre-operative (neoadjuvant) breast cancer therapy, taxanes and anthracyclines, elicit tumour-derived EVs with enhanced pro-metastatic capacity. Chemotherapy-elicited EVs are enriched in annexin A6 (ANXA6), a Ca 2+ -dependent protein that promotes NF-κB-dependent endothelial cell activation, Ccl2 induction and Ly6C + CCR2 + monocyte expansion in the pulmonary pre-metastatic niche to facilitate the establishment of lung metastasis. Genetic inactivation of Anxa6 in cancer cells or Ccr2 in host cells blunts the pro-metastatic effects of chemotherapy-elicited EVs. ANXA6 is detected, and potentially enriched, in the circulating EVs of breast cancer patients undergoing neoadjuvant chemotherapy. Keklikoglou et al. report that cytotoxic drugs induce tumour-derived extracellular vesicles that facilitate monocyte expansion through annexin A6 and thus lung metastasis in breast cancer.

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.Clinical correlative data and a plethora of preclinical studies of cancers have shown that both tumour-associated and metastasis-associated macrophages play an important role in promoting cancer. In this Perspective article, Cassetta and Pollard chronologically explore the evolution of our understanding of tumour-associated macrophage biology and enabling technologies.