Macrophage activation by endogenous danger signals

Macrophages are cells that function as a first line of defence against invading microorganisms. One of the hallmarks of macrophages is their ability to become activated in response to exogenous 'danger signals'. Most microbes have molecular patterns (PAMPS) that are recognized by macrophages and trigger this activation response. There are many aspects of the activation response to PAMPS that are recapitulated when macrophages encounter endogenous danger signals. In response to damaged or stressed self, macrophages undergo physiological changes that include the initiation of signal transduction cascades from germline-encoded receptors, resulting in the elaboration of chemokines, cytokines and toxic mediators. This response to endogenous mediators can enhance inflammation, and thereby contribute to autoimmune pathologies. Often the overall inflammatory response is the result of cooperative activation signals from both exogenous and endogenous signals. Macrophage activation plays a critical role, not only in the initiation of the inflammatory response but also in the resolution of this response. The clearance of granulocytes and the elaboration of anti-inflammatory mediators by macrophages contribute to the dissolution of the inflammatory response. Thus, macrophages are a key player in the initiation, propagation and resolution of inflammation. This review summarizes our understanding of the role of macrophages in inflammation. We pay particular attention to the endogenous danger signals that macrophages may encounter and the responses that these signals induce. The molecular mechanisms responsible for these responses and the diseases that result from inappropriately controlled macrophage activation are also examined.     

Emerging role of microRNAs in regulating macrophage activation and polarization in immune response and inflammation

Diversity and plasticity are hallmarks of macrophages. Classically activated macrophages are considered to promote T helper type 1 responses and have strong microbicidal, pro‐inflammatory activity, whereas alternatively activated macrophages are supposed to be associated with promotion of tissue remodelling and responses to anti‐inflammatory reactions. Transformation of different macrophage phenotypes is reflected in their different, sometimes even opposite, roles in various diseases or inflammatory conditions. MicroRNAs (miRNAs) have emerged as critical regulators of macrophage polarization (MP). Several miRNAs are induced by Toll‐like receptors signalling in macrophages and target the 3′‐untranslated regions of mRNAs encoding key molecules involved in MP. Therefore, identification of miRNAs related to the dynamic changes of MP and understanding their functions in regulating this process are important for discussing the molecular basis of disease progression and developing novel miRNA‐targeted therapeutic strategies. Here, we review the current knowledge of the role of miRNAs in MP with relevance to immune response and inflammation.     

Functional plasticity of macrophages: reversible adaptation to changing microenvironments

There has been substantial research activity in the past decade directed at phenotyping macrophage lineages and defining macrophage functional subsets or patterns of activity. The emphasis over the past 2-3 years has been to divide macrophage functional patterns into type 1 (Th1-driven) or type 2 (Th2-driven) functions. However, a huge array of environmental factors (including cytokines, chemokines, pattern recognition receptors, hormones) differentially regulates macrophage response patterns, resulting in the display of numerous distinct, functional phenotypes. Upon stimulation, a macrophage does not display just a single set of functions but rather displays a progression of functional changes in response to the progressive changes in its microenvironment. The remarkable ability of monocytes and tissue macrophages to adapt to changes in their microenvironment challenges the thesis that macrophages displaying unique tissue-specific or response-specific, functional patterns represent distinct lineages. With the exception of mature osteoclasts and mature dendritic cells, evidence supporting stable differentiation as the basis for macrophage functional heterogeneity is equivocal. The concept of whether macrophages develop into functional subsets as opposed to continuously adapting their functional pattern in response to the changing environment of a progressive inflammatory response is important to resolve from the perspectives of therapeutic targeting and understanding the role of macrophages in disease pathogenesis.     

Contribution of metabolic reprogramming to macrophage plasticity and function

Macrophages display a spectrum of functional activation phenotypes depending on the composition of the microenvironment they reside in, including type of tissue/organ and character of injurious challenge they are exposed to. Our understanding of how macrophage plasticity is regulated by the local microenvironment is still limited. Here we review and discuss the recent literature regarding the contribution of cellular metabolic pathways to the ability of the macrophage to sense the microenvironment and to alter its function. We propose that distinct alterations in the microenvironment induce a spectrum of inducible and reversible metabolic programs that might form the basis of the inducible and reversible spectrum of functional macrophage activation/polarization phenotypes. We highlight that metabolic pathways in the bidirectional communication between macrophages and stromals cells are an important component of chronic inflammatory conditions. Recent work demonstrates that inflammatory macrophage activation is tightly associated with metabolic reprogramming to aerobic glycolysis, an altered TCA cycle, and reduced mitochondrial respiration. We review cytosolic and mitochondrial mechanisms that promote initiation and maintenance of macrophage activation as they relate to increased aerobic glycolysis and highlight potential pathways through which anti-inflammatory IL-10 could promote macrophage deactivation. Finally, we propose that in addition to their role in energy generation and regulation of apoptosis, mitochondria reprogram their metabolism to also participate in regulating macrophage activation and plasticity.     

Immune metabolism in allergies,does it matter?—A review of immune metabolic basics and adaptations associated with the activation of innate immune cells in allergy

Type I allergies are pathological, type 2 inflammatory immune responses against otherwise harmless environmental allergens that arise from complex interactions between different types of immune cells. Activated immune cells undergo extensive changes in phenotype and function to fulfill their effector functions. Hereby, activation, differentiation, proliferation, migration, and mounting of effector responses require metabolic reprogramming. While the metabolic changes associated with activation of dendritic cells, macrophages, and T cells are extensively studied, data about the metabolic phenotypes of the other cell types critically involved in allergic responses (epithelial cells, eosinophils, basophils, mast cells, and ILC2s) are rather limited. This review briefly covers the basics of cellular energy metabolism and its connection to immune cell function. In addition, it summarizes the current state of knowledge in terms of dendritic cell and macrophage metabolism and subsequently focuses on the metabolic changes associated with activation of epithelial cells, eosinophils, basophils, mast cells, as well as ILC2s in allergy. Interestingly, the innate key cell types in allergic inflammation were reported to change their metabolic phenotype during activation, shifting to either glycolysis (epithelial cells, M1 macrophages, DCs, eosinophils, basophils, acutely activated mast cells), oxidative phosphorylation (M2 macrophages, longer term activated mast cells), or fatty acid oxidation (ILC2s). Therefore, immune metabolism is of relevance in allergic diseases and its connection to immune cell effector function needs to be considered to better understand induction and maintenance of allergic responses. Further progress in this field will likely improve both our understanding of disease pathology and enable new treatment targets/strategies.     

Adipose tissue macrophages: Unique polarization and bioenergetics in obesity

Macrophages comprise a majority of the resident immune cells in adipose tissue (AT) and regulate both tissue homeostasis in the lean state and metabolic dysregulation in obesity. Since the AT environment rapidly changes based upon systemic energy status, AT macrophages (ATMs) must adapt phenotypically and metabolically. There is a distinct dichotomy in the polarization and bioenergetics of in vitro models, with M2 macrophages utilizing oxidative phosphorylation (OX PHOS) and M1 macrophages utilizing glycolysis. Early studies suggested differential polarization of ATMs, with M2-like macrophages predominant in lean AT and M1-like macrophages in obese AT. However, recent studies show that the phenotypic plasticity of ATMs is far more complicated, which is also reflected in their bioenergetics. Multiple ATM populations exist along the M2 to M1 continuum and appear to utilize both glycolysis and OX PHOS in obesity. The significance of the dual fuel bioenergetics is unclear and may be related to an intermediate polarization, their buffering capacity, or the result of a mixed population of distinct polarized ATMs. Recent evidence also suggests that ATMs of lean mice serve as a substrate buffer or reservoir to modulate lipid, catecholamine, and iron availability. Furthermore, recent models of weight loss and weight cycling reveal additional roles for ATMs in systemic metabolism. Evaluating ATM phenotype and intracellular metabolism together may more accurately illuminate the consequences of ATM accumulation in obese AT, lending further insight into obesity-related comorbidities in humans.     

Ozone-induced lung injury and sterile inflammation. Role of toll-like receptor 4

Inhalation of toxic doses of ozone is associated with a sterile inflammatory response characterized by an accumulation of macrophages in the lower lung which are activated to release cytotoxic/proinflammatory mediators that contribute to tissue injury. Toll-like receptor 4 (TLR4) is a pattern recognition receptor present on macrophages that has been implicated in sterile inflammatory responses. In the present studies we used TLR4 mutant C3H/HeJ mice to analyze the role of TLR4 in ozone-induced lung injury, oxidative stress and inflammation. Acute exposure of control C3H/HeOuJ mice to ozone (0.8ppm for 3h) resulted in increases in bronchoalveolar lavage (BAL) lipocalin 24p3 and 4-hydroxynonenal modified protein, markers of oxidative stress and lipid peroxidation. This was correlated with increases in BAL protein, as well as numbers of alveolar macrophages. Levels of surfactant protein-D, a pulmonary collectin known to regulate macrophage inflammatory responses, also increased in BAL following ozone inhalation. Ozone inhalation was associated with classical macrophage activation, as measured by increased NF-κB binding activity and expression of TNFα mRNA. The observation that these responses to ozone were not evident in TLR4 mutant C3H/HeJ mice demonstrates that functional TLR4 contributes to ozone-induced sterile inflammation and macrophage activation.     

Caspase‐11 non‐canonical inflammasome: a critical sensor of intracellular lipopolysaccharide in macrophage‐mediated inflammatory responses

Inflammatory responses mediated by macrophages are part of the innate immune system, whose role is to protect against invading pathogens. Lipopolysaccharide (LPS) found in the outer membrane of Gram‐negative bacteria stimulates an inflammatory response by macrophages. During the inflammatory response, extracellular LPS is recognized by Toll‐like receptor 4, one of the pattern recognition receptors that activates inflammatory signalling pathways and leads to the production of inflammatory mediators. The innate immune response is also triggered by intracellular inflammasomes, and inflammasome activation induces pyroptosis and the secretion of pro‐inflammatory cytokines such as interleukin‐1β (IL‐1β) and IL‐18 by macrophages. Cysteine‐aspartic protease (caspase)‐11 and the human orthologues caspase‐4/caspase‐5 were recently identified as components of the ‘non‐canonical inflammasome’ that senses intracellular LPS derived from Gram‐negative bacteria during macrophage‐mediated inflammatory responses. Direct recognition of intracellular LPS facilitates the rapid oligomerization of caspase‐11/4/5, which results in pyroptosis and the secretion of IL‐1β and IL‐18. LPS is released into the cytoplasm from Gram‐negative bacterium‐containing vacuoles by small interferon‐inducible guanylate‐binding proteins encoded on chromosome 3 (GBP^chr3)‐mediated lysis of the vacuoles. In vivo studies have clearly shown that caspase‐11^−/− mice are more resistant to endotoxic septic shock by excessive LPS challenge. Given the evidence, activation of caspase‐11 non‐canonical inflammasomes by intracellular LPS is distinct from canonical inflammasome activation and provides a new paradigm in macrophage‐mediated inflammatory responses.     

Protein phenotypes of mouse macrophages activated in vivo for tumor cell killing

We have shown previously that the cellular proteins 47b and 71/73 could be used to construct phenotypes that distinguish among bone marrow culture-derived (BMCD) macrophages that were either unstimulated or primed by gamma interferon or fully activated for tumor cell killing by gamma interferon in combination with lipopolysaccharide (LPS). In the present study we examined in vivo-derived correlates for each of these stages of macrophage activation and found that the same protein phenotypes held true: both p47b and p71/73 were expressed by cytolytic peritoneal macrophages, including macrophages from a tumoral effusion, whereas macrophages primed in vivo by the intraperitoneal injection of either concanavalin A or methyl vinyl ether copolymer II expressed p47b but lacked p71/73. Neither resident nor inflammatory macrophage populations expressed p47b, and acute inflammatory macrophages, like unstimulated BMCD macrophages, expressed little or no p71/73. By contrast, resident and thioglycollate-elicited macrophages synthesized moderate levels of p71/73. When p71/73 also was expressed, there was a quantitative relationship between p47b expression and cytolytic activity in five different in vivo-activated macrophage populations. The results suggest that, regardless of the macrophage source or stimulus, it may be possible to assess macrophage activation status by reference to protein phenotypes utilizing p47b and p71/73.     

Epigenetic mechanisms of macrophage activation in type 2 diabetes

The alarming rise of obesity and type 2 diabetes (T2D) has put a tremendous strain on global healthcare systems. Over the past decade extensive research has focused on the role of macrophages as key mediators of inflammation in T2D. The inflammatory environment in the obese adipose tissue and pancreatic β-cell islets creates and perpetuates imbalanced inflammatory macrophage activation. Consequences of this chronic low-grade inflammation include insulin resistance in the adipose tissue and pancreatic β-cell dysfunction. Recently, the emerging field of epigenetics has provided new insights into the pathogenesis of T2D, while also affording potential new opportunities for treatment. In macrophages, epigenetic mechanisms are increasingly being recognized as crucial controllers of their phenotype. Here, we first describe the role of macrophages in T2D. Then we elaborate on epigenetic mechanisms that regulate macrophage activation, thereby focusing on T2D. Next, we highlight how diabetic conditions such as hyperlipidemia and hyperglycemia could induce epigenetic changes that promote an inflammatory macrophage phenotype. In conclusion we discuss possible therapeutic interventions by targeting macrophage epigenetics and speculate on future research directions.     

The Mycobacterium tuberculosis cell wall component mycolic acid elicits pathogen-associated host innate immune responses

Recognition of conserved pathogen-associated molecular patterns constitutes a crucial step in the initiation of innate immune responses. We studied the contribution to the host-pathogen interaction of mycolic acid (MA), a major lipid component of the cell envelope of the macrophage intracellular pathogen Mycobacterium tuberculosis and other mycobacteria. MA administered to the peritoneal cavity or to the airways induced a unique macrophage morphotype, similar to the foamy macrophage derivatives observed in tuberculous granulomas and characterized by intracellular accumulation of neutral lipids and entry into mitosis. When assayed for production of inflammatory mediators, a conditioning rather than a direct activation of the MA-elicited foamy macrophages was observed. MA enabled production of IFN-gamma and myeloperoxidase, enhanced TNF-alpha production and suppressed IL-10 upon renewed exposure to innate triggers. Intratracheal instillation of MA mimicked additional features of the airway response to M. tuberculosis infection, namely a rapid but transient neutrophil influx and IL-6 production and a chronic IL-12 production. These MA-elicited cellular innate defenses and the accompanying formation of foamy macrophages identify for the first time the foamy macrophage morphotype as part of the host response to a pathogen-associated structure. Furthermore, these results characterize MA as a direct trigger of innate immunity, distinct from Toll-like receptor ligands.     

Chemokine production predominates in human monocytes infected with Tula virus

Many reports suggest the hypothesis of a complex immune response accompanying hantaviral infections. However, little is known about the immunopathogenesis of nonpathogenic hantaviruses, especially Tula virus (TULV). The aim of our study was to determine the cytokine/chemokine profile induced after the infection of human macrophages with TULV and the role of viral replication in this process. Also, we wanted to establish how the study of TULV is relevant to our previous study of pathogenic hantaviruses. We showed that TULV-infected macrophages produced chemokines (interleukin-8, macrophage chemoattractant protein-1, and macrophage inflammatory protein-1beta) important for recruiting inflammatory cells, whereas no significant changes were recorded in the tested cytokine levels. This property was not influenced by ultraviolet inactivation. There were some differences in chemokine production compared with our previous study with pathogenic hantaviruses. A possible explanation could be a different way of entering host cells found in the pathogenic and nonpathogenic hantaviruses and activation of different intracellular signaling pathways.     

Functional heterogeneity in liver and lung macrophages

Although initially considered merely "scavenger cells" that participate in immunologic responses only after B and T lymphocytes have performed their biological tasks, more recent evidence suggests that macrophages play a key role in host defense as well as in the maintenance of normal tissue structure and function. For macrophages to perform their biological functions, they must be activated. This involves up-regulation of an array of signaling pathways resulting in altered gene expression and increased biochemical and functional activity. Macrophages have been identified in almost all tissues of the body. However, the basal activity of these cells, as well as their ability to respond to inflammatory mediators, varies considerably with their location. In addition, even within a particular tissue, there is evidence of macrophage heterogeneity. The largest populations of macrophages in the body are located in the liver and lung. Because of the unique attributes of these tissues, hepatic and pulmonary macrophages play essential roles not only in nonspecific host defense but also in the homeostatic responses of these tissues. In this review, the functional and biochemical activities of macrophages localized in the liver and lungs are compared. Evidence suggests that these represent distinct cell populations with unique functions and responsiveness to inflammatory agents.     

Stress as an endogenous adjuvant: augmentation of the immunization phase of cell-mediated immunity

Stress is thought to be immunosuppressive but paradoxically exacerbates inflammatory and autoimmune diseases. We initially showed that acute stress enhances skin immunity. Such immunoenhancement could promote immunoprotection in case of wounding, infection or vaccination but could also exacerbate immunopathological diseases. Here we identify the molecular and cellular mediators of the immunoenhancing effects of acute stress. Compared with non-stressed mice, acutely stressed animals showed significantly greater pinna swelling and leukocyte infiltration, and up-regulated macrophage chemoattractant protein-1, macrophage inflammatory protein-3alpha, IL-1alpha, IL-1beta, IL-6, tumor necrosis factor-alpha and IFN-gamma, but not IL-4 gene expression at the site of primary antigen exposure. Stressed animals also showed enhanced maturation and trafficking of dendritic cells (DCs) from skin to lymph nodes (LNs), higher numbers of activated macrophages in skin and LNs, increased T cell activation in LNs, and enhanced recruitment of surveillance T cells to skin. These findings show that important interactive components of innate (DCs and macrophages) and adaptive (surveillance T cells) immunity are mediators of the stress-induced enhancement of a primary immune response. Such enhancement during primary immunization may induce a long-term increase in immunologic memory resulting in subsequent augmentation of the immune response during secondary antigen exposure. Thus, the evolutionarily adaptive fight-or-flight stress response may protectively prepare the immune system for impending danger (e.g. infection and wounding by a predator), but may also contribute to stress-induced exacerbation of inflammatory and autoimmune diseases.     

Short communication: differences between macrophages and dendritic cells in the cyclic AMP-dependent regulation of lipopolysaccharide-induced cytokine and chemokine synthesis.

Cyclic adenosine monophosphate (cAMP) is an intracellular signaling molecule responsible for directing cellular responses to extracellular signals. Once believed to signal exclusively through its ability to bind protein kinase A (PKA), recent research has revealed alternative cAMP-binding targets involved in PKA-independent processes. In this study we addressed the hypothesis that the guanine nucleotide exchange protein directly activated by cAMP (Epac-1) and PKA differentially regulate inflammatory mediator production in distinct phagocytic cell types. To accomplish this, we compared the release of cAMP-regulated polypeptide inflammatory mediators in both macrophages (obtained from the lung and peritoneum) and bone marrow-derived dendritic cells (DCs) stimulated with bacterial endotoxin. Using the highly selective Epac-1 and PKA activating cAMP analogs 8-pCPT-2 -O-Me-cAMP and 6-Bnz-cAMP, respectively, we found that macrophages differ from DCs in the involvement of these distinct cAMP pathways in modulating inflammatory mediator release in response to endotoxin. Whereas the regulation of cytokine and chemokine production in macrophages by cAMP was solely dependent on PKA, we found that both Epac-1 and PKA activation could regulate mediator production in DCs. This finding may be important in the pharmacologic regulation of immune responses through manipulation of cAMP signaling cascades and contributes to our understanding of the differences between these cell types.