Macrophages are strategically located throughout the body tissues, where they ingest and process foreign materials, dead cells and debris and recruit additional macrophages in response to inflammatory signals They are highly heterogeneous cells that can rapidly change their function in response to local microenvironmental signals In this Review, we discuss the four stages of orderly inflammation mediated by macrophages: recruitment to tissues; differentiation and activation in situ; conversion to suppressive cells; and restoration of tissue homeostasis We also discuss the protective and pathogenic functions of the various macrophage subsets in antimicrobial defence, antitumour immune responses, metabolism and obesity, allergy and asthma, tumorigenesis, autoimmunity, atherosclerosis, fibrosis and wound healing Finally, we briefly discuss the characterization of macrophage heterogeneity in humans

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Protective and pathogenic functions of macrophage subsets

Fibrosis is defined by the overgrowth, hardening, and/or scarring of various tissues and is attributed to excess deposition of extracellular matrix components including collagen. Fibrosis is the end result of chronic inflammatory reactions induced by a variety of stimuli including persistent infections, autoimmune reactions, allergic responses, chemical insults, radiation, and tissue injury. Although current treatments for fibrotic diseases such as idiopathic pulmonary fibrosis, liver cirrhosis, systemic sclerosis, progressive kidney disease, and cardiovascular fibrosis typically target the inflammatory response, there is accumulating evidence that the mechanisms driving fibrogenesis are distinct from those regulating inflammation. In fact, some studies have suggested that ongoing inflammation is needed to reverse established and progressive fibrosis. The key cellular mediator of fibrosis is the myofibroblast, which when activated serves as the primary collagen-producing cell. Myofibroblasts are generated from a variety of sources including resident mesenchymal cells, epithelial and endothelial cells in processes termed epithelial/endothelial-mesenchymal (EMT/EndMT) transition, as well as from circulating fibroblast-like cells called fibrocytes that are derived from bone-marrow stem cells. Myofibroblasts are activated by a variety of mechanisms, including paracrine signals derived from lymphocytes and macrophages, autocrine factors secreted by myofibroblasts, and pathogen-associated molecular patterns (PAMPS) produced by pathogenic organisms that interact with pattern recognition receptors (i.e. TLRs) on fibroblasts. Cytokines (IL-13, IL-21, TGF-beta1), chemokines (MCP-1, MIP-1beta), angiogenic factors (VEGF), growth factors (PDGF), peroxisome proliferator-activated receptors (PPARs), acute phase proteins (SAP), caspases, and components of the renin-angiotensin-aldosterone system (ANG II) have been identified as important regulators of fibrosis and are being investigated as potential targets of antifibrotic drugs. This review explores our current understanding of the cellular and molecular mechanisms of fibrogenesis.

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Cellular and molecular mechanisms of fibrosis.

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.

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Macrophage biology in development, homeostasis and disease

The integrin family of cell adhesion receptors regulates a diverse array of cellular functions crucial to the initiation, progression and metastasis of solid tumours. The importance of integrins in several cell types that affect tumour progression has made them an appealing target for cancer therapy. Integrin antagonists, including the alphavbeta3 and alphavbeta5 inhibitor cilengitide, have shown encouraging activity in Phase II clinical trials and cilengitide is currently being tested in a Phase III trial in patients with glioblastoma. These exciting clinical developments emphasize the need to identify how integrin antagonists influence the tumour and its microenvironment.

/pdf/integrins-in-cancer-biological-implications-and-therapeutic-2ejzg8fop4.pdf

Integrins in cancer: biological implications and therapeutic opportunities

The extracellular matrix (ECM) is a highly dynamic structure that is present in all tissues and continuously undergoes controlled remodelling. This process involves quantitative and qualitative changes in the ECM, mediated by specific enzymes that are responsible for ECM degradation, such as metalloproteinases. The ECM interacts with cells to regulate diverse functions, including proliferation, migration and differentiation. ECM remodelling is crucial for regulating the morphogenesis of the intestine and lungs, as well as of the mammary and submandibular glands. Dysregulation of ECM composition, structure, stiffness and abundance contributes to several pathological conditions, such as fibrosis and invasive cancer. A better understanding of how the ECM regulates organ structure and function and of how ECM remodelling affects disease progression will contribute to the development of new therapeutics.

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Remodelling the extracellular matrix in development and disease.

Multicolor flow cytometry and cell sorting are powerful immunologic tools for the study of hepatic mϕ, yet there is no consensus on the optimal method to prepare liver homogenates for these analyses. Using a combination of mϕ and endothelial cell reporter mice, flow cytometry, and confocal imaging, we have shown that conventional flow-cytometric strategies for identification of Kupffer cells (KCs) leads to inclusion of a significant proportion of CD31hi endothelial cells. These cells were present regardless of the method used to prepare cells for flow cytometry and represented endothelium tightly adhered to remnants of KC membrane. Antibodies to endothelial markers, such as CD31, were vital for their exclusion. This result brings into focus recently published microarray datasets that identify high expression of endothelial cell-associated genes by KCs compared with other tissue-resident mϕ. Our studies also revealed significant and specific loss of KCs among leukocytes with commonly used isolation methods that led to enrichment of proliferating and monocyte-derived mϕ. Hence, we present an optimal method to generate high yields of liver myeloid cells without bias for cell type or contamination with endothelial cells.

https://jlb.onlinelibrary.wiley.com/doi/pdf/10.1002/JLB.1TA0517-169R

An efficient method to isolate Kupffer cells eliminating endothelial cell contamination and selective bias.

The normal hepatic sinusoid consists of a fenestrated endothelial capillary, behind which, and adjacent to the border of the hepatic pallisade, is the space of Disse. This term is in some respects a misnomer, for within it are the hepatic lipocytes (fat storing or Ito cells) and a specialised basement membrane-like matrix, which consists predominantly of type IV collagen, laminin, and proteoglycans. Extracellular matrix is an integral part of multicellular organisms, providing structural integrity and support to cells. Current evidence indicates that matrix is not merely a simple scaffolding but a dynamic modulator of cell phenotype and behaviour. There is now evidence that the basement membrane-like matrix of the liver can change the phenotypic characteristics and growth of lipocytes, endothelial cells, and hepatocytes.4 Recent developments in matrix biochemistry and cell biology have offered a fascinating insight into the complex interaction of hepatocytes and their surrounding matrix in both health and disease. The importance of extracellular matrix was first appreciated in hepatocyte culture studies.

https://gut.bmj.com/content/gutjnl/35/6/729.full.pdf

Hepatocyte-matrix interactions.

Objective Following chronic liver injury or when hepatocyte proliferation is impaired, ductular reactions containing hepatic progenitor cells (HPCs) appear in the periportal regions and can regenerate the liver parenchyma. HPCs exist in a niche composed of myofibroblasts, macrophages and laminin matrix. Galectin-3 (Gal-3) is a β-galactoside-binding lectin that binds to laminin and is expressed in injured liver in mice and humans. Design We examined the role of Gal-3 in HPC activation. HPC activation was studied following dietary induced hepatocellular (choline-deficient ethionine-supplemented diet) and biliary (3,5-diethoxycarbonyl-1,4-dihydrocollidine supplemented diet) injury in wild type and Gal-3(−/−) mice. Results HPC proliferation was significantly reduced in Gal-3(−/−) mice. Gal-3(−/−) mice failed to form a HPC niche, with reduced laminin formation. HPCs isolated from wild type mice secrete Gal-3 which enhanced adhesion and proliferation of HPCs on laminin in an undifferentiated form. These effects were attenuated in Gal3(−/−) HPCs and in wild type HPCs treated with the Gal-3 inhibitor lactose. Gal-3(−/−) HPCs in vitro showed increased hepatocyte function and prematurely upregulated both biliary and hepatocyte differentiation markers and regulated cell cycle genes leading to arrest in G 0 /G 1 . Conclusions We conclude that Gal-3 is required for the undifferentiated expansion of HPCs in their niche in injured liver.

/pdf/galectin-3-regulates-hepatic-progenitor-cell-expansion-2vj1e5h38l.pdf

Galectin-3 regulates hepatic progenitor cell expansion during liver injury

A patient is described presenting with an acute lower gastrointestinal haemorrhage as a result of extensive colonic varices. Further investigation revealed that there were no oesophageal varices or splenomegaly. Liver biopsy showed grade II fatty change only, with no other specific or significant pathological features. Transhepatic portography showed a raised portal pressure (20 mm/Hg) but the portal system was patent throughout. There was an abnormal leash of vessels in the caecum thought to represent a variceal plexus. This patient was diagnosed as having idiopathic colonic varices. This case is discussed together with nine other reports of idiopathic colonic varices from the published literature. Four of these reports describe idiopathic colonic varices in more than one member of the same family. Possible modes of inheritance, aetiology of variceal change, natural history, and prognosis are discussed.

https://gut.bmj.com/content/gutjnl/33/9/1285.full.pdf

John P. Iredale

Papers

An efficient method to isolate Kupffer cells eliminating endothelial cell contamination and selective bias.

Hepatocyte-matrix interactions.

Galectin-3 regulates hepatic progenitor cell expansion during liver injury

Familial and idiopathic colonic varices: an unusual cause of lower gastrointestinal haemorrhage.

Impaired Proteolysis of Collagen I Inhibits Proliferation of Hepatic Stellate Cells IMPLICATIONS FOR REGULATION OF LIVER FIBROSIS