Proinflammatory clearance of apoptotic neutrophils induces an IL-12(low)IL-10(high) regulatory phenotype in macrophages

Macrophages in kidney injury, inflammation, and fibrosis

Macrophages are found in normal kidney and in increased numbers in diseased kidney, where they act as key players in renal injury, inflammation, and fibrosis. Macrophages are highly heterogeneous cells and exhibit distinct phenotypic and functional characteristics in response to various stimuli in the local microenvironment in different types of kidney disease. In kidney tissue necrosis and/or infection, damage- and/or pathogen-associated molecular patterns induce pro-inflammatory macrophages, which contribute to further tissue injury, inflammation, and subsequent fibrosis. Apoptotic cells and anti-inflammatory factors in post-inflammatory tissues induced anti-inflammatory macrophages, which can mediate kidney repair and regeneration. This review summarizes the role of macrophages with different phenotypes in kidney injury, inflammation, and fibrosis in various acute and chronic kidney diseases. Understanding alterations of kidney microenvironment and the factors that control the phenotype and functions of macrophages may offer an avenue for the development of new cellular and cytokine/growth factor-based therapies as alternative treatment options for patients with kidney disease.

The role of neutrophils in immune dysfunction during severe inflammation

Critically ill post-surgical, post-trauma and/or septic patients are characterised by severe inflammation. This immune response consists of both a pro- and an anti-inflammatory component. The pro-inflammatory component contributes to (multiple) organ failure whereas occurrence of immune paralysis predisposes to infections. Strikingly, infectious complications arise in these patients despite the presence of a clear neutrophilia. We propose that dysfunction of neutrophils potentially increases the susceptibility to infections or can result in the inability to clear existing infections. Under homeostatic conditions these effector cells of the innate immune system circulate in a quiescent state and serve as the first line of defence against invading pathogens. In severe inflammation, however, neutrophils are rapidly activated, which affects their functional capacities, such as chemotaxis, phagocytosis, intra-cellular killing, NETosis, and their capacity to modulate adaptive immunity. This review provides an overview of the current understanding of neutrophil dysfunction in severe inflammation. We will discuss the possible mechanisms of downregulation of anti-microbial function, suppression of adaptive immunity by neutrophils and the contribution of neutrophil subsets to immune paralysis.

Parallel Aspects of the Microenvironment in Cancer and Autoimmune Disease

Cancer and autoimmune diseases are fundamentally different pathological conditions. In cancer, the immune response is suppressed and unable to eradicate the transformed self-cells, while in autoimmune diseases it is hyperactivated against a self-antigen, leading to tissue injury. Yet, mechanistically, similarities in the triggering of the immune responses can be observed. In this review, we highlight some parallel aspects of the microenvironment in cancer and autoimmune diseases, especially hypoxia, and the role of macrophages, neutrophils, and their interaction. Macrophages, owing to their plastic mode of activation, can generate a pro- or antitumoral microenvironment. Similarly, in autoimmune diseases, macrophages tip the Th1/Th2 balance via various effector cytokines. The contribution of neutrophils, an additional plastic innate immune cell population, to the microenvironment and disease progression is recently gaining more prominence in both cancer and autoimmune diseases, as they can secrete cytokines, chemokines, and reactive oxygen species (ROS), as well as acquire an enhanced ability to produce neutrophil extracellular traps (NETs) that are now considered important initiators of autoimmune diseases. Understanding the contribution of macrophages and neutrophils to the cancerous or autoimmune microenvironment, as well as the role their interaction and cooperation play, may help identify new targets and improve therapeutic strategies.

Type I IFN Does Not Promote Susceptibility to Foodborne Listeria monocytogenes

Type I IFN (IFN-α/β) is thought to enhance growth of the foodborne intracellular pathogen Listeria monocytogenes by promoting mechanisms that dampen innate immunity to infection. However, the type I IFN response has been studied primarily using methods that bypass the stomach and, therefore, fail to replicate the natural course of L. monocytogenes infection. In this study, we compared i.v. and foodborne transmission of L. monocytogenes in mice lacking the common type I IFN receptor (IFNAR1(-/-)). Contrary to what was observed using i.v. infection, IFNAR1(-/-) and wild-type mice had similar bacterial burdens in the liver and spleen following foodborne infection. Splenocytes from wild-type mice infected i.v. produced significantly more IFN-β than did those infected by the foodborne route. Consequently, the immunosuppressive effects of type I IFN signaling, which included T cell death, increased IL-10 secretion, and repression of neutrophil recruitment to the spleen, were all observed following i.v. but not foodborne transmission of L. monocytogenes. Type I IFN was also previously shown to cause a loss of responsiveness to IFN-γ through downregulation of the IFN-γ receptor α-chain on macrophages and dendritic cells. However, we detected a decrease in surface expression of IFN-γ receptor α-chain even in the absence of IFN-α/β signaling, suggesting that in vivo, this infection-induced phenotype is not type I IFN-dependent. These results highlight the importance of using the natural route of infection for studies of host-pathogen interactions and suggest that the detrimental effects of IFN-α/β signaling on the innate immune response to L. monocytogenes may be an artifact of the i.v. infection model.

Human endometrial regenerative cells attenuate renal ischemia reperfusion injury in mice

BACKGROUND

Endometrial regenerative cells (ERCs) is an attractive novel type of adult mesenchymal stem cells that can be non-invasively obtained from menstrual blood and are easily replicated at a large scale without tumorigenesis. We have previously reported that ERCs exhibit unique immunoregulatory properties in experimental studies in vitro and in vivo. In this study, the protective effects of ERCs on renal ischemia-reperfusion injury (IRI) were examined.

METHODS

Renal IRI in C57BL/6 mice was induced by clipping bilateral renal pedicles for 30 min, followed by reperfusion for 48 h. ERCs were isolated from healthy female menstrual blood, and were injected (1 million/mouse, i.v.) into mice 2 h prior to IRI induction. Renal function, pathological and immunohistological changes, cell populations and cytokine profiles were evaluated after 48 h of renal reperfusion.

RESULTS

Here, we showed that as compared to untreated controls, administration of ERCs effectively prevented renal damage after IRI, indicated by better renal function and less pathological changes, which were associated with increased serum levels of IL-4, but decreased levels of TNF-α, IFN-γ and IL-6. Also, ERC-treated mice displayed significantly less splenic and renal CD4(+) and CD8(+) T cell populations, while the percentage of splenic CD4(+)CD25(+) regulatory T cells and infiltrating M2 macrophages in the kidneys were significantly increased in ERC-treated mice.

CONCLUSIONS

This study demonstrates that the novel anti-inflammatory and immunoregulatory effects of ERCs are associated with attenuation of renal IRI, suggesting that the unique features of ERCs may make them a promising candidate for cell therapies in the treatment of ischemic acute kidney injury in patients.

PAFR in adipose tissue macrophages is associated with anti-inflammatory phenotype and metabolic homoeostasis

Metabolic dysfunction is associated with adipose tissue inflammation and macrophage infiltration. PAFR (platelet-activating factor receptor) is expressed in several cell types and binds to PAF (platelet-activating factor) and oxidized phospholipids. Engagement of PAFR in macrophages drives them towards the anti-inflammatory phenotype. In the present study, we investigated whether genetic deficiency of PAFR affects the phenotype of ATMs (adipose tissue macrophages) and its effect on glucose and insulin metabolism. PARFKO (PAFR-knockout) and WT (wild-type) mice were fed on an SD (standard diet) or an HFD (high-fat diet). Glucose and insulin tolerance tests were performed by blood monitoring. ATMs were evaluated by FACS for phenotypic markers. Gene and protein expression was investigated by real-time reverse transcription-quantitative PCR and Western blotting respectively. Results showed that the epididymal adipose tissue of PAFRKO mice had increased gene expression of Ccr7, Nos2, Il6 and Il12, associated with pro-inflammatory mediators, and reduced expression of the anti-inflammatory Il10. Moreover, the adipose tissue of PAFRKO mice presented more pro-inflammatory macrophages, characterized by an increased frequency of F4/80(+)CD11c(+) cells. Blood monocytes of PAFRKO mice also exhibited a pro-inflammatory phenotype (increased frequency of Ly6C(+) cells) and PAFR ligands were detected in the serum of both PAFRKO and WT mice. Regarding metabolic parameters, compared with WT, PAFRKO mice had: (i) higher weight gain and serum glucose concentration levels; (ii) decreased insulin-stimulated glucose disappearance; (iii) insulin resistance in the liver; (iv) increased expression of Ldlr in the liver. In mice fed on an HFD, some of these changes were potentiated, particularly in the liver. Thus it seems that endogenous ligands of PAFR are responsible for maintaining the anti-inflammatory profile of blood monocytes and ATMs under physiological conditions. In the absence of PAFR signalling, monocytes and macrophages acquire a pro-inflammatory phenotype, resulting in adipose tissue inflammation and metabolic dysfunction.

Regulation of intrinsic apoptosis in cycloheximide-treated macrophages by the Sichuan human strain of Chinese Leishmania isolates

Leishmania spp. are able to survive and proliferate inside mammals' mononuclear phagocytes, causing Leishmaniasis. Previous studies have noted that the regulation of apoptosis in host cells by these parasites may contribute to their ability to evade the immune system. However, current results remain unclear about whether the parasites can promote or delay the apoptotic process in host cells, because the regulatory effect of Leishmania was assumed to be strain-, species- and even infection time-dependent. The aim of this study was to investigate whether the Sichuan isolates of Chinese Leishmania (SC10H2) can alter the process of intrinsic apoptosis induced by cycloheximide in different types of macrophage cell lines and to determine in which steps of the signaling pathway the parasites were involved. Human THP-1 and mouse RAW264.7 macrophages were infected by SC10H2 promastigotes followed by cycloheximide stimulation to assess the alteration of intrinsic apoptosis in these cells. The results indicated that SC10H2 infection of human THP-1 macrophages could promote the initiation of intrinsic apoptosis, but completely opposite results were found in mouse RAW264.7 macrophages. Nevertheless, the expression of Bcl-2 and the DNA fragmentation rates were not altered by SC10H2 infection in the cell lines used in the experiments. This study suggests that SC10H2 promastigote infection is able to promote and delay the transduction of early apoptotic signals induced by cycloheximide in THP-1 and RAW264.7 macrophages, revealing that the regulation of intrinsic apoptosis in host cells by SC10H2 in vitro occurs in a host cell-dependent manner. The data from this study might play a significant role in further understanding the relationship between Leishmania and different host cells.

Immunoregulatory Role of Myeloid-derived Cells in Inflammatory Bowel Disease

As the frontiers of immunological research expand, new insights into the pathogenesis of long poorly understood diseases, such as inflammatory bowel disease (IBD), are opening up new possible avenues for treatment. Myeloid-derived cells (i.e., monocytes, macrophages, neutrophils, and dendritic cells), long believed to be effector cells driving the initiation of inflammation, have been increasingly shown to have immunoregulatory effects previously underappreciated. Dysfunction in the immunoregulatory roles of these cells may play a part in the pathogenesis of a subset of patients with IBD. The role of myeloid-derived suppressor cells, initially described in cancer, have been shown to play an important role in the balancing of effector and regulatory T cells in inflammation as well, and their role in IBD is also explored. The potential for future cell-based therapies for IBD is enhanced by the advances being made in the understanding of the innate immune system in the intestine.

Microglia in the TBI brain: The good, the bad, and the dysregulated

As the major cellular component of the innate immune system in the central nervous system (CNS) and the first line of defense whenever injury or disease occurs, microglia play a critical role in neuroinflammation following a traumatic brain injury (TBI). In the injured brain microglia can produce neuroprotective factors, clear cellular debris and orchestrate neurorestorative processes that are beneficial for neurological recovery after TBI. However, microglia can also become dysregulated and can produce high levels of pro-inflammatory and cytotoxic mediators that hinder CNS repair and contribute to neuronal dysfunction and cell death. The dual role of microglial activation in promoting beneficial and detrimental effects on neurons may be accounted for by their polarization state and functional responses after injury. In this review article we discuss emerging research on microglial activation phenotypes in the context of acute brain injury, and the potential role of microglia in phenotype-specific neurorestorative processes such as neurogenesis, angiogenesis, oligodendrogenesis and regeneration. We also describe some of the known molecular mechanisms that regulate phenotype switching, and highlight new therapeutic approaches that alter microglial activation state balance to enhance long-term functional recovery after TBI. An improved understanding of the regulatory mechanisms that control microglial phenotypic shifts may advance our knowledge of post-injury recovery and repair, and provide opportunities for the development of novel therapeutic strategies for TBI.

Macrophage and microglial plasticity in the injured spinal cord

Macrophages in the injured spinal cord arise from resident microglia and from infiltrating peripheral myeloid cells. Microglia respond within minutes after central nervous system (CNS) injury and along with other CNS cells signal the influx of their peripheral counterpart. Although some of the functions they carry out are similar, they appear to be specialized to perform particular roles after CNS injury. Microglia and macrophages are very plastic cells that can change their phenotype drastically in response to in vitro and in vivo conditions. They can change from pro-inflammatory, cytotoxic cells to anti-inflammatory, pro-repair phenotypes. The microenvironment of the injured CNS importantly influences macrophage plasticity. This review discusses the phagocytosis and cytokine-mediated effects on macrophage plasticity in the context of spinal cord injury.

Neutrophils in respiratory syncytial virus infection: A target for asthma prevention

Lower respiratory tract infections by respiratory syncytial virus (RSV) are the foremost cause of infant hospitalization and are implicated in lasting pulmonary impairment and the development of asthma. Neutrophils infiltrate the airways of pediatric patients with RSV-induced bronchiolitis in vast numbers: approximately 80% of infiltrated cells are neutrophils. However, why neutrophils are recruited to the site of viral respiratory tract infection is not clear. In this review we discuss the beneficial and pathologic contributions of neutrophils to the immune response against RSV infection. Neutrophils can limit viral replication and spread, as well as stimulate an effective antiviral adaptive immune response. However, low specificity of neutrophil antimicrobial armaments allows for collateral tissue damage. Neutrophil-induced injury to the airways during the delicate period of infant lung development has lasting adverse consequences for pulmonary architecture and might promote the onset of asthma in susceptible subjects. We suggest that pharmacologic modulation of neutrophils should be explored as a viable future therapy for severe RSV-induced bronchiolitis and thereby prevent the inception of subsequent asthma. The antiviral functions of neutrophils suggest that targeting of neutrophils in patients with RSV-induced bronchiolitis is best performed under the umbrella of antiviral treatment.

ICAM-1 suppresses tumor metastasis by inhibiting macrophage M2 polarization through blockade of efferocytosis

Efficient clearance of apoptotic cells (efferocytosis) can profoundly influence tumor-specific immunity. Tumor-associated macrophages are M2-polarized macrophages that promote key processes in tumor progression. Efferocytosis stimulates M2 macrophage polarization and contributes to cancer metastasis, but the signaling mechanism underlying this process is unclear. Intercellular cell adhesion molecule-1 (ICAM-1) is a transmembrane glycoprotein member of the immunoglobulin superfamily, which has been implicated in mediating cell–cell interaction and outside-in cell signaling during the immune response. We report that ICAM-1 expression is inversely associated with macrophage infiltration and the metastasis index in human colon tumors by combining Oncomine database analysis and immunohistochemistry for ICAM-1. Using a colon cancer liver metastasis model in ICAM-1-deficient (ICAM-1^−/−) mice and their wild-type littermates, we found that loss of ICAM-1 accelerated liver metastasis of colon carcinoma cells. Moreover, ICAM-1 deficiency increased M2 macrophage polarization during tumor progression. We further demonstrated that ICAM-1 deficiency in macrophages led to promotion of efferocytosis of apoptotic tumor cells through activation of the phosphatidylinositol 3 kinase/Akt signaling pathway. More importantly, coculture of ICAM-1^−/− macrophages with apoptotic cancer cells resulted in an increase of M2-like macrophages, which was blocked by an efferocytosis inhibitor. Our findings demonstrate a novel role for ICAM-1 in suppressing M2 macrophage polarization via downregulation of efferocytosis in the tumor microenvironment, thereby inhibiting metastatic tumor progression.

Juvenile idiopathic arthritis and physical activity: possible inflammatory and immune modulation and tracks for interventions in young populations

Juvenile idiopathic arthritis (JIA) is a chronic inflammatory disease characterized by persistent joint inflammation that manifests as joint pain and swelling and limited range of joint motion. In healthy subjects, the literature reports that physical activity has an anti-inflammatory effect. In JIA patients, exercise could be used as a therapeutic tool to counteract disease-related inflammation and thereby improve clinical symptoms, although transient flare of pain could be the price to pay. Indeed, in patients with a chronic inflammatory disease, physical activity is prone to exacerbate underlying inflammatory stress. Physical activity improves quality of life and symptoms in JIA patients, but the mechanisms of action remain unclear. This review focuses on the mechanisms underlying exercise-induced immune and hormonal changes. Data on the impact of acute and chronic physical activities on the secretion of hormones and other molecules such as miRNA or peptides involved in the inflammatory process in JIA was compiled and summarized, and the key role of the biological effect of muscle-derived interleukin 6 in the exercise-induced modulation of pro/anti-inflammatory balance is addressed. We also go on to review the effect of training and type of exercise on cytokine response. This review highlights the beneficial effect of physical exercise in children with JIA and potential effect of exercise on the balance between pro- and anti-inflammatory response.

The ischemic environment drives microglia and macrophage function

Cells of myeloid origin, such as microglia and macrophages, act at the crossroads of several inflammatory mechanisms during pathophysiology. Besides pro-inflammatory activity (M1 polarization), myeloid cells acquire protective functions (M2) and participate in the neuroprotective innate mechanisms after brain injury. Experimental research is making considerable efforts to understand the rules that regulate the balance between toxic and protective brain innate immunity. Environmental changes affect microglia/macrophage functions. Hypoxia can affect myeloid cell distribution, activity, and phenotype. With their intrinsic differences, microglia and macrophages respond differently to hypoxia, the former depending on ATP to activate and the latter switching to anaerobic metabolism and adapting to hypoxia. Myeloid cell functions include homeostasis control, damage-sensing activity, chemotaxis, and phagocytosis, all distinctive features of these cells. Specific markers and morphologies enable to recognize each functional state. To ensure homeostasis and activate when needed, microglia/macrophage physiology is finely tuned. Microglia are controlled by several neuron-derived components, including contact-dependent inhibitory signals and soluble molecules. Changes in this control can cause chronic activation or priming with specific functional consequences. Strategies, such as stem cell treatment, may enhance microglia protective polarization. This review presents data from the literature that has greatly advanced our understanding of myeloid cell action in brain injury. We discuss the selective responses of microglia and macrophages to hypoxia after stroke and review relevant markers with the aim of defining the different subpopulations of myeloid cells that are recruited to the injured site. We also cover the functional consequences of chronically active microglia and review pivotal works on microglia regulation that offer new therapeutic possibilities for acute brain injury.

Macrophage-mediated injury and repair after ischemic kidney injury

Acute ischemic kidney injury is a common complication in hospitalized patients. No treatment is yet available for augmenting kidney repair or preventing progressive kidney fibrosis. Animal models of acute kidney injury demonstrate that activation of the innate immune system plays a major role in the systemic response to ischemia/reperfusion injury. Macrophage depletion studies suggest that macrophages, key participants in the innate immune response, augment the initial injury after reperfusion but also promote tubular repair and contribute to long-term kidney fibrosis after ischemic injury. The distinct functional outcomes seen following macrophage depletion at different time points after ischemia/reperfusion injury suggest heterogeneity in macrophage activation states. Identifying the pathways that regulate the transitions of macrophage activation is thus critical for understanding the mechanisms that govern both macrophage-mediated injury and repair in the postischemic kidney. This review examines our understanding of the complex and intricately controlled pathways that determine monocyte recruitment, macrophage activation, and macrophage effector functions after renal ischemia/reperfusion injury. Careful delineation of repair and resolution pathways could provide therapeutic targets for the development of effective treatments to offer patients with acute kidney injury.

The TAM family: phosphatidylserine sensing receptor tyrosine kinases gone awry in cancer

The TYRO3, AXL (also known as UFO) and MERTK (TAM) family of receptor tyrosine kinases (RTKs) are aberrantly expressed in multiple haematological and epithelial malignancies. Rather than functioning as oncogenic drivers, their induction in tumour cells predominately promotes survival, chemoresistance and motility. The unique mode of maximal activation of this RTK family requires an extracellular lipid–protein complex. For example, the protein ligand, growth arrest-specific protein 6 (GAS6), binds to phosphatidylserine (PtdSer) that is externalized on apoptotic cell membranes, which activates MERTK on macrophages. This triggers engulfment of apoptotic material and subsequent anti-inflammatory macrophage polarization. In tumours, autocrine and paracrine ligands and apoptotic cells are abundant, which provide a survival signal to the tumour cell and favour an anti-inflammatory, immunosuppressive microenvironment. Thus, TAM kinase inhibition could stimulate antitumour immunity, reduce tumour cell survival, enhance chemosensitivity and diminish metastatic potential.

Deception and manipulation: the arms of leishmania, a successful parasite

Leishmania spp. are intracellular parasitic protozoa responsible for a group of neglected tropical diseases, endemic in 98 countries around the world, called leishmaniasis. These parasites have a complex digenetic life cycle requiring a susceptible vertebrate host and a permissive insect vector, which allow their transmission. The clinical manifestations associated with leishmaniasis depend on complex interactions between the parasite and the host immune system. Consequently, leishmaniasis can be manifested as a self-healing cutaneous affliction or a visceral pathology, being the last one fatal in 85–90% of untreated cases. As a result of a long host–parasite co-evolutionary process, Leishmania spp. developed different immunomodulatory strategies that are essential for the establishment of infection. Only through deception and manipulation of the immune system, Leishmania spp. can complete its life cycle and survive. The understanding of the mechanisms associated with immune evasion and disease progression is essential for the development of novel therapies and vaccine approaches. Here, we revise how the parasite manipulates cell death and immune responses to survive and thrive in the shadow of the immune system.

Neuroinflammation in Alzheimer's disease; A source of heterogeneity and target for personalized therapy

Neuroinflammation has long been known as an accompanying pathology of Alzheimer's disease. Microglia surrounding amyloid plaques in the brain of Auguste D were described in the original publication of Alois Alzheimer. It is only quite recently, however, that we have a more complete appreciation for the diverse roles of neuroinflammation in neurodegenerative disorders such as Alzheimer's. While gaps in our knowledge remain, and conflicting data are abound in the field, our understanding of the complexities and heterogeneous functions of the inflammatory response in Alzheimer's is vastly improved. This review article will discuss some of the roles of neuroinflammation in Alzheimer's disease, in particular, how understanding heterogeneity in the individual inflammatory response can be used in therapeutic development and as a mechanism of personalizing our treatment of the disease.

Efferocytosis produces a prometastatic landscape during postpartum mammary gland involution

Breast cancers that occur in women 2-5 years postpartum are more frequently diagnosed at metastatic stages and correlate with poorer outcomes compared with breast cancers diagnosed in young, premenopausal women. The molecular mechanisms underlying the malignant severity associated with postpartum breast cancers (ppBCs) are unclear but relate to stromal wound-healing events during postpartum involution, a dynamic process characterized by widespread cell death in milk-producing mammary epithelial cells (MECs). Using both spontaneous and allografted mammary tumors in fully immune-competent mice, we discovered that postpartum involution increases mammary tumor metastasis. Cell death was widespread, not only occurring in MECs but also in tumor epithelium. Dying tumor cells were cleared through receptor tyrosine kinase MerTK-dependent efferocytosis, which robustly induced the transcription of genes encoding wound-healing cytokines, including IL-4, IL-10, IL-13, and TGF-β. Animals lacking MerTK and animals treated with a MerTK inhibitor exhibited impaired efferocytosis in postpartum tumors, a reduction of M2-like macrophages but no change in total macrophage levels, decreased TGF-β expression, and a reduction of postpartum tumor metastasis that was similar to the metastasis frequencies observed in nulliparous mice. Moreover, TGF-β blockade reduced postpartum tumor metastasis. These data suggest that widespread cell death during postpartum involution triggers efferocytosis-induced wound-healing cytokines in the tumor microenvironment that promote metastatic tumor progression.

Structure and function of renal macrophages and dendritic cells from lupus-prone mice

OBJECTIVE

To characterize renal macrophages and dendritic cells (DCs) in 2 murine models of lupus nephritis.

METHODS

We used a bead-based enrichment step followed by cell sorting to isolate populations of interest from young mice and nephritic mice. Cell morphology was examined by microscopy. Arginase and nitrite production was examined using biochemical assays. The antigen-presenting functions of the cells were determined using mixed lymphocyte reactions. Selected cytokine, chemokine, and Toll-like receptor (TLR) profiles were examined using real-time quantitative polymerase chain reaction.

RESULTS

We identified 2 populations of macrophages and 3 populations of DCs in both of our murine models of lupus (NZB/NZW and [NZW × BXSB]F1 mice). F4/80(high) macrophages, which were resident in normal kidneys and found to be increased in number during nephritis, did not produce either arginase or nitrite upon cytokine stimulation and acquired a mixed proinflammatory and antiinflammatory functional phenotype during nephritis that resembles the constitutively activated phenotype of gut F4/80(high) macrophages. The various cell types differed in their expression of chemokine receptors and TLRs, consistent with variability in their renal location. Resident renal CD103+ DCs were the best antigen-presenting cells and could easily be distinguished from CD11c(high) myeloid DCs that accumulated in large numbers during nephritis.

CONCLUSION

Our study highlights the heterogeneity of the macrophage/DC infiltrate in chronic lupus nephritis and provides an initial phenotypic and functional analysis of the different cellular components that can now be used to define the role of each cell subset in nephritis progression or amelioration. Of note, the dominant macrophage population that accumulates during nephritis has an acquired phenotype that is neither M1 nor M2 and may reflect failure of resolution of inflammation.

Macrophage trafficking as key mediator of adenine-induced kidney injury

Macrophages play a special role in the onset of several diseases, including acute and chronic kidney injuries. In this sense, tubule interstitial nephritis (TIN) represents an underestimated insult, which can be triggered by different stimuli and, in the absence of a proper regulation, can lead to fibrosis deposition. Based on this perception, we evaluated the participation of macrophage recruitment in the development of TIN. Initially, we provided adenine-enriched food to WT and searched for macrophage presence and action in the kidney. Also, a group of animals were depleted of macrophages with the clodronate liposome while receiving adenine-enriched diet. We collected blood and renal tissue from these animals and renal function, inflammation, and fibrosis were evaluated. We observed higher expression of chemokines in the kidneys of adenine-fed mice and a substantial protection when macrophages were depleted. Then, we specifically investigated the role of some key chemokines, CCR5 and CCL3, in this TIN experimental model. Interestingly, CCR5 KO and CCL3 KO animals showed less renal dysfunction and a decreased proinflammatory profile. Furthermore, in those animals, there was less profibrotic signaling. In conclusion, we can suggest that macrophage infiltration is important for the onset of renal injury in the adenine-induced TIN.

The TSC-mTOR pathway regulates macrophage polarization

Macrophages are able to polarize to proinflammatory M1 or alternative M2 states with distinct phenotypes and physiological functions. How metabolic status regulates macrophage polarization remains not well understood, and here we examine the role of mTOR (Mechanistic Target of Rapamycin), a central metabolic pathway that couples nutrient sensing to regulation of metabolic processes. Using a mouse model in which myeloid lineage specific deletion of Tsc1 (Tsc1^Δ/Δ) leads to constitutive mTOR Complex 1 (mTORC1) activation, we find that Tsc1^Δ/Δ macrophages are refractory to IL-4 induced M2 polarization, but produce increased inflammatory responses to proinflammatory stimuli. Moreover, mTORC1-mediated downregulation of Akt signaling critically contributes to defective polarization. These findings highlight a key role for the mTOR pathway in regulating macrophage polarization, and suggest how nutrient sensing and metabolic status could be “hard-wired” to control of macrophage function, with broad implications for regulation of Type 2 immunity, inflammation, and allergy.

Apoptotic neutrophils augment the inflammatory response to Mycobacterium tuberculosis infection in human macrophages

Macrophages in the lung are the primary cells being infected by Mycobacterium tuberculosis (Mtb) during the initial manifestation of tuberculosis. Since the adaptive immune response to Mtb is delayed, innate immune cells such as macrophages and neutrophils mount the early immune protection against this intracellular pathogen. Neutrophils are short-lived cells and removal of apoptotic cells by resident macrophages is a key event in the resolution of inflammation and tissue repair. Since anti-inflammatory activity is not compatible with effective immunity to intracellular pathogens, we therefore investigated how uptake of apoptotic neutrophils modulates the function of Mtb-activated human macrophages. We show that Mtb infection exerts a potent proinflammatory activation of human macrophages with enhanced gene activation and release of proinflammatory cytokines and that this response was augmented by apoptotic neutrophils. The enhanced macrophage response is linked to apoptotic neutrophil-driven activation of the NLRP3 inflammasome and subsequent IL-1β signalling. We also demonstrate that apoptotic neutrophils not only modulate the inflammatory response, but also enhance the capacity of infected macrophages to control intracellular growth of virulent Mtb. Taken together, these results suggest a novel role for apoptotic neutrophils in the modulation of the macrophage-dependent inflammatory response contributing to the early control of Mtb infection.

Innate immunity to Leishmania infection: within phagocytes

Infection by Leishmania takes place in the context of inflammation and tissue repair. Besides tissue resident macrophages, inflammatory macrophages and neutrophils are recruited to the infection site and serve both as host cells and as effectors against infection. Recent studies suggest additional important roles for monocytes and dendritic cells. This paper addresses recent experimental findings regarding the regulation of Leishmania major infection by these major phagocyte populations. In addition, the role of IL-4 on dendritic cells and monocytes is discussed.

The Probiotic Mixture VSL#3 Alters the Morphology and Secretion Profile of Both Polarized and Unpolarized Human Macrophages in a Polarization-Dependent Manner

BACKGROUND

Patients with Inflammatory Bowel Disease (IBD), most commonly Crohn's disease (CD) or ulcerative colitis (UC), suffer from chronic intestinal inflammation of unknown etiology. Increased proinflammatory macrophages (M1) have been documented in tissue from patients with CD. Anti-inflammatory macrophages (M2) may play a role in UC given the preponderance of Th2 cytokines in this variant of IBD. Animal and clinical studies have shown that the probiotic VSL#3 can ameliorate signs and symptoms of IBD. Although animal data suggests a modulatory effect on macrophage phenotype, the effect of VSL#3 on human macrophages remains unknown.

OBJECTIVE

To determine the effect of the probiotic VSL#3 on the phenotype of polarized (M1/M2) and unpolarized (MΦ) human macrophages.

METHODS

Human monocyte-derived macrophages, generated by culturing monocytes with M-CSF, were left unpolarized or were polarized towards an M1 or an M2 phenotype by culture with LPS and IFN-γ or IL-4, respectively, and were then cultured in the presence or absence of VSL#3 for 3 days. Changes in macrophage morphology were assessed. Cytokine and chemokine levels in supernatants were determined by multiplex assay.

RESULTS

VSL#3 decreased the granuloma-like aggregates of M1 macrophages, increased fibroblast-like M2 macrophages, and decreased fibroblast-like MΦ macrophages. VSL#3 increased the secretion of IL-1β, IL-6, IL-10, and G-CSF by M1, M2, and MΦ macrophages. VSL#3 exposure maintained the proinflammatory phenotype of M1 macrophages, sustaining IL-12 secretion, increasing IL-23 secretion, and decreasing MDC secretion. Both VSL#3-treated M2 and MΦ macrophages secreted higher levels of anti-inflammatory and pro-healing factors such as IL-1Ra, IL-13, EGF, FGF-2, TGF-α, and VEGF, as well as proinflammatory cytokines, including IL-12 and TNF-α.

CONCLUSION

Under our experimental conditions VSL#3 induced a mixed proinflammatory and anti-inflammatory phenotype in polarized and unpolarized macrophages. This differential effect could explain why patients with CD do not respond to probiotic therapy as well as patients with UC.

Neuroinflammation and M2 microglia: the good, the bad, and the inflamed

The concept of multiple macrophage activation states is not new. However, extending this idea to resident tissue macrophages, like microglia, has gained increased interest in recent years. Unfortunately, the research on peripheral macrophage polarization does not necessarily translate accurately to their central nervous system (CNS) counterparts. Even though pro- and anti-inflammatory cytokines can polarize microglia to distinct activation states, the specific functions of these states is still an area of intense debate. This review examines the multiple possible activation states microglia can be polarized to. This is followed by a detailed description of microglial polarization and the functional relevance of this process in both acute and chronic CNS disease models described in the literature. Particular attention is given to utilizing M2 microglial polarization as a potential therapeutic option in treating diseases.

Daidzein enhances efferocytosis via transglutaminase 2 and augmentation of Rac1 activity

Clearance of apoptotic cells, termed "efferocytosis", is the mechanism required to prevent secondary necrosis and release of proinflammatory cytokines. Defective efferocytosis is cumulatively regarded as one of mechanisms in the development of autoimmune and chronic inflammatory diseases. Our previous finding showed that ethanolic extract from Glycine tomentella Hayata (GTH) can enhance mouse macrophage RAW264.7 efferocytosis (clearance of apoptotic cells). We have demonstrated that the major components of GTH are daidzein, catechin, epicatechin and naringin. Here, we explore the potential of each component in modulating efferocytic capability. For this, RAW264.7 cells were cultured with CFDA-stained apoptotic cells and assayed by flow cytometry. We found that daidzein is the main component of GTH, and it can enhance RAW264.7 efferocytosis dose-dependently. Moreover, the enhancive effect of daidzein on macrophage efferocytic capability is accompanied by increased transglutaminase 2 (TG2) at both mRNA and protein levels. TG2 knockdown attenuated daidzein increased macrophage efferocytic capability. After treatment with daidzein, increased phosphorylation was observed in Erk, but not in p38 and JNK. Finally, we report that after daidzein treatment, Rac1 activity was markedly increased and the mitochondrial membrane potential was decreased, which may contribute to efferocytosis. Taken together, these data suggest that enhancement of macrophage efferocytic capability by daidzein treatment was mainly through up-regulation of TG2 expression and Rac1 activity. Daidzein may have the therapeutical potential in the treatment of inflammatory diseases.

The role of the immune system in central nervous system plasticity after acute injury

Acute brain injuries cause rapid cell death that activates bidirectional crosstalk between the injured brain and the immune system. In the acute phase, the damaged CNS activates resident and circulating immune cells via the local and systemic release of soluble mediators. This early immune activation is necessary to confine the injured tissue and foster the clearance of cellular debris, thus bringing the inflammatory reaction to a close. In the chronic phase, a sustained immune activation has been described in many CNS disorders, and the degree of this prolonged response has variable effects on spontaneous brain regenerative processes. The challenge for treating acute CNS damage is to understand how to optimally engage and modify these immune responses, thus providing new strategies that will compensate for tissue lost to injury. Herein we have reviewed the available information regarding the role and function of the innate and adaptive immune responses in influencing CNS plasticity during the acute and chronic phases of after injury. We have examined how CNS damage evolves along the activation of main cellular and molecular pathways that are associated with intrinsic repair, neuronal functional plasticity and facilitation of tissue reorganization.

Human polymorphonuclear leukocytes produce cytokines in response to Leishmania major promastigotes

Polymorphonuclear leukocytes (PMN) release cytokines that may influence the development of the subsequent adaptive immune response. Little is known about cytokines produced by human PMN in response to Leishmania (L.). In this study, mRNA expression of Interleukin (IL)-12p40, IL-12p35, Interferon (IFN)-γ, transforming growth factor (TGF)-β, IL-1, and IL-4 in PMN of volunteers stimulated with L. major promastigotes has been investigated by real-time PCR and the results were confirmed by flow cytometer. The results showed that L. major did not induce mRNA expression of IL12p40, IL12p35, IFN-γ, and TGF-β in PMN, while IL-1 and IL-4 mRNA were induced. Flow cytometry results confirmed no IFN-γ production by PMN with or without stimulation. IL-12p70 was present in untreated and L. major-treated PMN, and these cells release IL-12 following incubation with L. major. Significant amount of IL-1 even without treatment with promastigotes was detected in PMN. Moreover, the proportion of PMN, which produce IL-1 in response to L. major, was increased compared with the percent of unstimulated IL-1-producing PMN. The results showed the accumulation of small amounts of IL-4 in PMN after stimulation. In conclusion, our results indicate that IL-12 and IL-1 are pre-stored in human PMN, nor L. major induces IL-1 and IL-4, but not IL-12, IFN-γ, nor TGF-β expression in these cells.

Co-culture of spleen stromal cells with bone marrow mononuclear cells leads to the generation of a novel macrophage subset

Macrophages adopt diverse activation states depending on the microenvironment. Recently, stromal cells have been demonstrated to be organizers of the microenvironment. Here, using splenic stromal cells to mimic the splenic microenvironment in vivo, we show that spleen stromal cells can programme bone marrow-derived mononuclear cells to differentiate and polarize into a novel macrophage subset. These differentiated macrophages (Diff-Mφ) exhibited pronounced production of IL-10, IL-6 and TNF-α, but diminished the production of IL-12 in response to LPS. The generation of Diff-Mφ depended on cell-cell contact as well as on soluble factors. Diff-Mφ directly suppressed the antigen-non-specific (CD3/CD28) CD4(+) T cell proliferative response and induced cell death of activated CD4(+) T cells. As for cytokine production in CD4(+) T cells, Diff-Mφ promoted IL-10 and IL-17 production, whereas inhibited IL-4 production and did not alter IFN-γ production. Besides, Diff-Mφ also expressed iNOS, CD16/CD32, CD54, CD43, CCR7, CD44, PD-L1 and FasL, which might be involved in the function of Diff-Mφ. These results suggest that splenic microenvironment may physiologically induce a novel type of macrophages differentiation.

Infection with Leishmania major induces a cellular stress response in macrophages

We investigated early cellular responses induced by infection with Leishmania major in macrophages from resistant C57/BL6 mice. Infection increased production of reactive oxygen species by resident, but not inflammatory peritoneal macrophages. In addition, infection increased activation of stress-activated protein kinases/c-Jun N-terminal kinases (SAPK/JNK) in resident, but not in inflammatory peritoneal macrophages. Infection also increased expression of membrane and soluble FasL, but infected macrophages remained viable after 48 h. Infection increased secretion of cytokines/chemokines TNF-α, IL-6, TIMP-1, IL-1RA, G-CSF, TREM, KC, MIP-1α, MIP-1β, MCP-1, and MIP-2 in resident macrophages. Addition of antioxidants deferoxamine and N-acetylcysteine reduced ROS generation and JNK activation. Addition of antioxidants or JNK inhibitor SP600125 reduced secretion of KC. Furthermore, treatment with antioxidants or JNK inhibitor also reduced intracellular parasite replication. These results indicated that infection triggers a rapid cellular stress response in resident macrophages which induces proinflammatory signals, but is also involved in parasite survival and replication in host macrophages.

Clearance of apoptotic cells by macrophages induces regulatory phenotype and involves stimulation of CD36 and platelet-activating factor receptor

Phagocytosis of apoptotic cells (efferocytosis) induces macrophage differentiation towards a regulatory phenotype (IL-10^high/IL-12p40^low). CD36 is involved in the recognition of apoptotic cells (AC), and we have shown that the platelet-activating factor receptor (PAFR) is also involved. Here, we investigated the contribution of PAFR and CD36 to efferocytosis and to the establishment of a regulatory macrophage phenotype. Mice bone marrow-derived macrophages were cocultured with apoptotic thymocytes, and the phagocytic index was determined. Blockage of PAFR with antagonists or CD36 with specific antibodies inhibited the phagocytosis of AC (~70–80%). Using immunoprecipitation and confocal microscopy, we showed that efferocytosis increased the CD36 and PAFR colocalisation in the macrophage plasma membrane; PAFR and CD36 coimmunoprecipitated with flotillin-1, a constitutive lipid raft protein, and disruption of these membrane microdomains by methyl-β-cyclodextrin reduced AC phagocytosis. Efferocytosis induced a pattern of cytokine production, IL-10^high/IL-12p40^low, that is, characteristic of a regulatory phenotype. LPS potentiated the efferocytosis-induced production of IL-10, and this was prevented by blocking PAFR or CD36. It can be concluded that phagocytosis of apoptotic cells engages CD36 and PAFR, possibly in lipid rafts, and this is required for optimal efferocytosis and the establishment of the macrophage regulatory phenotype.

Inhibition of caspase-8 activity promotes protective Th1- and Th2-mediated immunity to Leishmania major infection

We investigated how apoptosis pathways mediated by death receptors and caspase-8 affect cytokine responses and immunity to Leishmania major parasites. Splenic CD4 T cells undergo activation-induced apoptosis, and blockade of FasL-Fas interaction increased IFN-γ and IL-4 cytokine responses to L. major antigens. To block death receptor-induced death, we used mice expressing a T cell-restricted transgene for vFLIP. Inhibition of caspase-8 activation in vFLIP mice enhanced Th1 and Th2 cytokine responses to L. major infection, even in the Th1-prone B6 background. We also observed increased NO production by splenocytes from vFLIP mice upon T cell activation. Despite an exacerbated Th2 response, vFLIP mice controlled better L. major infection, with reduced lesions and lower parasite loads compared with WT mice. Moreover, injection of anti-IL-4 mAb in infected vFLIP mice disrupted control of parasite infection. Therefore, blockade of caspase-8 activity in T cells improves immunity to L. major infection by promoting increased Th1 and Th2 responses.

The multifaceted functions of neutrophils

Neutrophils and neutrophil-like cells are the major pathogen-fighting immune cells in organisms ranging from slime molds to mammals. Central to their function is their ability to be recruited to sites of infection, to recognize and phagocytose microbes, and then to kill pathogens through a combination of cytotoxic mechanisms. These include the production of reactive oxygen species, the release of antimicrobial peptides, and the recently discovered expulsion of their nuclear contents to form neutrophil extracellular traps. Here we discuss these primordial neutrophil functions, which also play key roles in tissue injury, by providing details of neutrophil cytotoxic functions and congenital disorders of neutrophils. In addition, we present more recent evidence that interactions between neutrophils and adaptive immune cells establish a feed-forward mechanism that amplifies pathologic inflammation. These newly appreciated contributions of neutrophils are described in the setting of several inflammatory and autoimmune diseases.

Redox control of inflammation in macrophages

Macrophages are present throughout the human body, constitute important immune effector cells, and have variable roles in a great number of pathological, but also physiological, settings. It is apparent that macrophages need to adjust their activation profile toward a steadily changing environment that requires altering their phenotype, a process known as macrophage polarization. Formation of reactive oxygen species (ROS), derived from NADPH-oxidases, mitochondria, or NO-producing enzymes, are not necessarily toxic, but rather compose a network signaling system, known as redox regulation. Formation of redox signals in classically versus alternatively activated macrophages, their action and interaction at the level of key targets, and the resulting physiology still are insufficiently understood. We review the identity, source, and biological activities of ROS produced during macrophage activation, and discuss how they shape the key transcriptional responses evoked by hypoxia-inducible transcription factors, nuclear-erythroid 2-p45-related factor 2 (Nrf2), and peroxisome proliferator-activated receptor-γ. We summarize the mechanisms how redox signals add to the process of macrophage polarization and reprogramming, how this is controlled by the interaction of macrophages with their environment, and addresses the outcome of the polarization process in health and disease. Future studies need to tackle the option whether we can use the knowledge of redox biology in macrophages to shape their mediator profile in pathophysiology, to accelerate healing in injured tissue, to fight the invading pathogens, or to eliminate settings of altered self in tumors.

MerTK inhibition in tumor leukocytes decreases tumor growth and metastasis

MerTK, a receptor tyrosine kinase (RTK) of the TYRO3/AXL/MerTK family, is expressed in myeloid lineage cells in which it acts to suppress proinflammatory cytokines following ingestion of apoptotic material. Using syngeneic mouse models of breast cancer, melanoma, and colon cancer, we found that tumors grew slowly and were poorly metastatic in MerTK-/- mice. Transplantation of MerTK-/- bone marrow, but not wild-type bone marrow, into lethally irradiated MMTV-PyVmT mice (a model of metastatic breast cancer) decreased tumor growth and altered cytokine production by tumor CD11b+ cells. Although MerTK expression was not required for tumor infiltration by leukocytes, MerTK-/- leukocytes exhibited lower tumor cell-induced expression of wound healing cytokines, e.g., IL-10 and growth arrest-specific 6 (GAS6), and enhanced expression of acute inflammatory cytokines, e.g., IL-12 and IL-6. Intratumoral CD8+ T lymphocyte numbers were higher and lymphocyte proliferation was increased in tumor-bearing MerTK-/- mice compared with tumor-bearing wild-type mice. Antibody-mediated CD8+ T lymphocyte depletion restored tumor growth in MerTK-/- mice. These data demonstrate that MerTK signaling in tumor-associated CD11b+ leukocytes promotes tumor growth by dampening acute inflammatory cytokines while inducing wound healing cytokines. These results suggest that inhibition of MerTK in the tumor microenvironment may have clinical benefit, stimulating antitumor immune responses or enhancing immunotherapeutic strategies.

Neutrophils in innate and adaptive immunity

Neutrophils have long been viewed as short-lived cells crucial for the elimination of extracellular pathogens, possessing a limited role in the orchestration of the immune response. This dogma has been challenged by recent lines of evidence demonstrating the expression of an increasing number of cytokines and effector molecules by neutrophils. Moreover, in analogy with their "big brother" macrophages, neutrophils integrate the environmental signals and can be polarized towards an antitumoural or protumoural phenotype. Neutrophils are a major source of humoral fluid phase pattern recognition molecules and thus contribute to the humoral arm of innate immunity. Neutrophils cross talk and shape the maturation and effector functions of other leukocytes in a direct or indirect manner, through cell-cell contact or cytokine production, respectively. Therefore, neutrophils are integrated in the activation and regulation of the innate and adaptive immune system and play an important role in the resolution or exacerbation of diverse pathologies, including infections, chronic inflammation, autoimmunity and cancer.

Dectin-1 regulates IL-10 production via a MSK1/2 and CREB dependent pathway and promotes the induction of regulatory macrophage markers

In response to infection by fungal pathogens, the innate immune system recognises specific fungal pathogen associated molecular patterns (PAMPs) via pattern recognition receptors including the C-type lectin dectin-1 and members of the Toll Like Receptor (TLR) family. Stimulation of these receptors leads to the induction of both pro- and anti-inflammatory cytokines. The protein kinases MSK1 and 2 are known to be important in limiting inflammatory cytokine production by macrophages in response to the TLR4 agonist LPS. In this study we show that MSKs are also activated in macrophages by the fungal derived ligand zymosan, as well as the dectin-1 specific agonists curdlan and depleted zymosan, via the ERK1/2 and p38α MAPK pathways. Furthermore, we show that MSKs regulate dectin-1 induced IL-10 production, and that this regulation is dependent on the ability of MSKs to phosphorylate the transcription factor CREB. IL-10 secreted in response to zymosan was able to promote STAT3 phosphorylation via an autocrine feedback loop. Consistent with the decreased IL-10 secretion in MSK1/2 knockout macrophages, these cells also had decreased STAT3 tyrosine phosphorylation relative to wild type controls after stimulation with zymosan. We further show that the reduction in IL-10 production in the MSK1/2 macrophages results in increased secretion of IL-12p40 in response to zymosan relative to wild type controls. The production of high levels of IL-10 but low levels of IL-12 has previously been associated with an M2b or 'regulatory' macrophage phenotype, which was initially described in macrophages stimulated with a combination of immune complexes and LPS. We found that zymosan, via dectin-1 activation, also leads to the expression of SphK1 and LIGHT, markers of a regulatory like phenotype in mouse macrophages. The expression of these makers was further reinforced by the high level of IL-10 secreted in response to zymosan stimulation.

STAT1-regulated lung MDSC-like cells produce IL-10 and efferocytose apoptotic neutrophils with relevance in resolution of bacterial pneumonia

Bacterial pneumonia remains a significant burden worldwide. Although an inflammatory response in the lung is required to fight the causative agent, persistent tissue-resident neutrophils in non-resolving pneumonia can induce collateral tissue damage and precipitate acute lung injury. However, little is known about mechanisms orchestrated in the lung tissue that remove apoptotic neutrophils to restore tissue homeostasis. In mice infected with Klebsiella pneumoniae, a bacterium commonly associated with hospital-acquired pneumonia, we show that interleukin (IL)-10 is essential for resolution of lung inflammation and recovery of mice after infection. Although IL-10(-/-) mice cleared bacteria, they displayed increased morbidity with progressive weight loss and persistent lung inflammation in the later phase after infection. A source of tissue IL-10 was found to be resident CD11b(+)Gr1(int)F4/80(+) cells resembling myeloid-derived suppressor cells (MDSCs) that appeared with a delayed kinetics after infection. These cells efficiently efferocytosed apoptotic neutrophils, which was aided by IL-10. The lung neutrophil burden was attenuated in infected signal transducer and activator of transcription 1 (STAT1)(-/-) mice with concomitant increase in the frequency of the MDSC-like cells and lung IL-10 levels. Thus, inhibiting STAT1 in combination with antibiotics may be a novel therapeutic strategy to address inefficient resolution of bacterial pneumonia.

Macrophages and fibrosis: How resident and infiltrating mononuclear phagocytes orchestrate all phases of tissue injury and repair

Certain macrophage phenotypes contribute to tissue fibrosis, but why? Tissues host resident mononuclear phagocytes for their support to maintain homeostasis. Upon injury the changing tissue microenvironment alters their phenotype and primes infiltrating monocytes toward pro-inflammatory macrophages. Several mechanisms contribute to their deactivation and macrophage priming toward anti-inflammatory and pro-regenerative macrophages that produce multiple cytokines that display immunosuppressive as well as pro-regeneratory effects, such as IL-10 and TGF-beta1. Insufficient parenchymal repair creates a tissue microenvironment that becomes dominated by multiple growth factors that promote the pro-fibrotic macrophage phenotype that itself produces large amounts of such growth factors that further support fibrogenesis. However, the contribution of resident mononuclear phagocytes to physiological extracellular matrix turnover implies also their fibrolytic effects in the late stage of tissue scaring. Fibrolytic macrophages break down fibrous tissue, but their phenotypic characteristics remain to be described in more detail. Together, macrophages contribute to tissue fibrosis because the changing tissue environments prime them to assist and orchestrate all phases of tissue injury and repair. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Tissues use resident dendritic cells and macrophages to maintain homeostasis and to regain homeostasis upon tissue injury: the immunoregulatory role of changing tissue environments

Most tissues harbor resident mononuclear phagocytes, that is, dendritic cells and macrophages. A classification that sufficiently covers their phenotypic heterogeneity and plasticity during homeostasis and disease does not yet exist because cell culture-based phenotypes often do not match those found in vivo. The plasticity of mononuclear phagocytes becomes obvious during dynamic or complex disease processes. Different data interpretation also originates from different conceptual perspectives. An immune-centric view assumes that a particular priming of phagocytes then causes a particular type of pathology in target tissues, conceptually similar to antigen-specific T-cell priming. A tissue-centric view assumes that changing tissue microenvironments shape the phenotypes of their resident and infiltrating mononuclear phagocytes to fulfill the tissue's need to maintain or regain homeostasis. Here we discuss the latter concept, for example, why different organs host different types of mononuclear phagocytes during homeostasis. We further discuss how injuries alter tissue environments and how this primes mononuclear phagocytes to enforce this particular environment, for example, to support host defense and pathogen clearance, to support the resolution of inflammation, to support epithelial and mesenchymal healing, and to support the resolution of fibrosis to the smallest possible scar. Thus, organ- and disease phase-specific microenvironments determine macrophage and dendritic cell heterogeneity in a temporal and spatial manner, which assures their support to maintain and regain homeostasis in whatever condition. Mononuclear phagocytes contributions to tissue pathologies relate to their central roles in orchestrating all stages of host defense and wound healing, which often become maladaptive processes, especially in sterile and/or diffuse tissue injuries.

Functional role of monocytes and macrophages for the inflammatory response in acute liver injury

Different etiologies such as drug toxicity, acute viral hepatitis B, or acetaminophen poisoning can cause acute liver injury or even acute liver failure (ALF). Excessive cell death of hepatocytes in the liver is known to result in a strong hepatic inflammation. Experimental murine models of liver injury highlighted the importance of hepatic macrophages, so-called Kupffer cells, for initiating and driving this inflammatory response by releasing proinflammatory cytokines and chemokines including tumor necrosis factor (TNF), interleukin-6 (IL-6), IL-1beta, or monocyte-chemoattractant protein-1 (MCP-1, CCL2) as well as activating other non-parenchymal liver cells, e.g., endothelial or hepatic stellate cells. Many of these proinflammatory mediators can trigger hepatocytic cell death pathways, e.g., via caspase activation, but also activate protective signaling pathways, e.g., via nuclear factor kappa B (NF-κB). Recent studies in mice demonstrated that these macrophage actions largely depend on the recruitment of monocytes into the liver, namely of the inflammatory Ly6c+ (Gr1+) monocyte subset as precursors of tissue macrophages. The chemokine receptor CCR2 and its ligand MCP-1/CCL2 promote monocyte subset infiltration upon liver injury. In contrast, the chemokine receptor CX3CR1 and its ligand fractalkine (CX3CL1) are important negative regulators of monocyte infiltration by controlling their survival and differentiation into functionally diverse macrophage subsets upon injury. The recently identified cellular and molecular pathways for monocyte subset recruitment, macrophage differentiation, and interactions with other hepatic cell types in the injured liver may therefore represent interesting novel targets for future therapeutic approaches in ALF.

Cell surface-expressed phosphatidylserine as therapeutic target to enhance phagocytosis of apoptotic cells

Impaired efferocytosis has been shown to be associated with, and even to contribute to progression of, chronic inflammatory diseases such as atherosclerosis. Enhancing efferocytosis has been proposed as strategy to treat diseases involving inflammation. Here we present the strategy to increase 'eat me' signals on the surface of apoptotic cells by targeting cell surface-expressed phosphatidylserine (PS) with a variant of annexin A5 (Arg-Gly-Asp-annexin A5, RGD-anxA5) that has gained the function to interact with α(v)β(3) receptors of the phagocyte. We describe design and characterization of RGD-anxA5 and show that introduction of RGD transforms anxA5 from an inhibitor into a stimulator of efferocytosis. RGD-anxA5 enhances engulfment of apoptotic cells by phorbol-12-myristate-13-acetate-stimulated THP-1 (human acute monocytic leukemia cell line) cells in vitro and resident peritoneal mouse macrophages in vivo. In addition, RGD-anxA5 augments secretion of interleukin-10 during efferocytosis in vivo, thereby possibly adding to an anti-inflammatory environment. We conclude that targeting cell surface-expressed PS is an attractive strategy for treatment of inflammatory diseases and that the rationally designed RGD-anxA5 is a promising therapeutic agent.

CCL1 released from M2b macrophages is essentially required for the maintenance of their properties

Patients with 10-30 days postburn injury are greatly susceptible to infections. M1M (IL-10(-)IL-12(+) M) are essential cells in host antibacterial innate immunity against MRSA infections. However, these effector cells are not easily generated in hosts who are carriers of M2bM (IL-12(-)IL-10(+)CCL1(+)LIGHT(+) M). M2bM are inhibitory on M1M generation. In this study, the antibacterial resistance of mice, 10-30 days postburn injury against MRSA infection, was improved by the modulation of M2bM activities. Unburned mice inoculated with M preparations from mice, 10-30 days after burn injury, were susceptible to MRSA infection, whereas unburned mice, inoculated with M preparations from the same mice that were previously treated with CCL1 antisense ODN, were resistant to the infection. M2bM, isolated from Day 15 burn mice, lost their M2bM properties 3 days after cultivation under frequent medium changes, whereas their M2bM properties remained in the same cultures supplemented with rCCL1. In cultures, M preparations from Day 15 burn mice treated with CCL1 antisense ODN did not produce CCL1 and did convert to M1M after heat-killed MRSA stimulation. Also, Day 15 burn mice treated with the ODN became resistant against MRSA infection. These results indicate that CCL1 released from M2bM is essentially required for the maintenance of their properties. The increased susceptibility of mice, 10-30 days after burn injury to MRSA infection, may be controlled through the intervention of CCL1 production by M2bM appearing in association with severe burn injuries.

Role of M2b macrophages in the acceleration of bacterial translocation and subsequent sepsis in mice exposed to whole body [137Cs] γ-irradiation

The influence of whole-body gamma-irradiation on the antibacterial host defense against Enterococcus faecalis translocation was investigated. Mice irradiated with or without 5 Gy [(137)Cs] gamma-rays were orally infected with 10(6) CFU/mouse E. faecalis. The pathogen was detected in the mesenteric lymph nodes (MLNs) of irradiated mice 1-4 d postinfection, whereas E. faecalis was not isolated from MLNs of normal mice. All irradiated mice died within 5 d of infection, whereas no mortality was shown in normal mice infected with the pathogen. Irradiated mice inoculated with normal mouse MLN macrophages (M) were shown to be resistant against the infection, although the same mice inoculated with irradiated mouse MLNM (I-MLNM) died postinfection. I-MLNM were identified as IL-10(+)IL-12(-)CCL1(+)LIGHT(+) M (M2bM) and were shown to be inhibitory on M conversion from resident M to IL-10(-)IL-12(+)M (M1M). M2bM were demonstrated in MLNs of mice 10-35 d after gamma-irradiation. M1M were not induced by E. faecalis Ag in cultures of I-MLNM, whereas normal mouse MLNM were converted to M1M in response to the Ag stimulation. After treatment with CCL1 antisense oligodeoxynucleotides, M2bM disappeared in MLNs of irradiated mice, and M1M were generated in MLNs of these mice following E. faecalis stimulation. These results indicate that M2bM presented in the I-MLNM populations were responsible for the impaired resistance of mice irradiated with gamma-rays to bacterial translocation and subsequent sepsis. E. faecalis translocation and subsequent sepsis may be controlled immunologically by the intervention of M2bM present in MLNs.