Preventing cleavage of Mer promotes efferocytosis and suppresses acute lung injury in bleomycin treated mice

Administration of Gas6 attenuates lung fibrosis via inhibition of the epithelial-mesenchymal transition and fibroblast activation

The epithelial-mesenchymal transition (EMT) and fibroblast activation are major events in idiopathic pulmonary fibrosis pathogenesis. Here, we investigated whether growth arrest-specific protein 6 (Gas6) plays a protective role in lung fibrosis via suppression of the EMT and fibroblast activation. rGas6 administration inhibited the EMT in isolated mouse ATII cells 14 days post-BLM treatment based on morphologic cellular alterations, changes in mRNA and protein expression profiles of EMT markers, and induction of EMT-activating transcription factors. BLM-induced increases in gene expression of fibroblast activation-related markers and the invasive capacity of primary lung fibroblasts in primary lung fibroblasts were reversed by rGas6 administration. Furthermore, the hydroxyproline content and collagen accumulation in interstitial areas with damaged alveolar structures in lung tissue were reduced by rGas6 administration. Targeting Gas6/Axl signaling events with specific inhibitors of Axl (BGB324), COX-2 (NS-398), EP1/EP2 receptor (AH-6809), or PGD2 DP2 receptor (BAY-u3405) reversed the inhibitory effects of rGas6 on EMT and fibroblast activation. Finally, we confirmed the antifibrotic effects of Gas6 using Gas6-/- mice. Therefore, Gas6/Axl signaling events play a potential role in inhibition of EMT process and fibroblast activation via COX-2-derived PGE2 and PGD2 production, ultimately preventing the development of pulmonary fibrosis.

Interstitial Macrophages Mediate Efferocytosis of Alveolar Epithelium during Influenza Infection

Efferocytosis is a process whereby apoptotic cells are cleared to maintain tissue homeostasis. In the lungs, efferocytosis has been implicated in several acute and chronic inflammatory diseases. A long-standing method to study efferocytosis in vivo is to instill apoptotic cells into the lungs to evaluate macrophage uptake. However, this approach provides nonphysiologic levels of cells to the airspaces, where there is preferential access to the alveolar macrophages. To circumvent this limitation, we developed a new method to study efferocytosis of damaged alveolar type 2 (AT2) epithelial cells in vivo. A reporter mouse that expresses TdTomato in AT2 epithelial cells was injured with influenza (strain PR8) to induce apoptosis of AT2 cells. We were able to identify macrophages that acquire red fluorescence after influenza injury, indicating efferocytosis of AT2 cells. Furthermore, evaluation of macrophage populations led to the surprising finding that lung interstitial macrophages were the primary efferocyte in vivo. In summary, we present a novel finding that the interstitial macrophage, not the alveolar macrophage, primarily mediates clearance of AT2 cells in the lungs after influenza infection. Our method of studying efferocytosis provides a more physiologic approach in evaluating the spatiotemporal dynamics of apoptotic cell clearance in vivo and opens new avenues to study the mechanisms by which efferocytosis regulates inflammation.

Bone marrow-derived macrophages from a murine model of Sjögren's syndrome demonstrate an aberrant, inflammatory response to apoptotic cells

Sjögren's syndrome (SjS) is a female-dominated autoimmune disease involving lymphocytic infiltration of the exocrine glands. We have previously demonstrated cleavage of the TAM (Tyro3, Axl, Mer) receptor Mer is enhanced in SjS, leading to defective efferocytosis. Mer also plays a role in modulating phagocyte inflammatory response to apoptotic cells. Here we investigated the SjS macrophage response to apoptotic cells (AC). Bone marrow-derived macrophages (BMDMs) from SjS-susceptible (SjS^s) C57BL/6.NOD-Aec1Aec2 mice and C57BL/6 (B6) controls were treated with either AC or CpG-oligodeoxynucleotides. RNA was collected from macrophages and bulk sequencing was performed to analyze transcripts. Cytokine expression was confirmed by Bio-plex. RT-qPCR was used to determine toll-like receptor (TLR) 7 and 9 involvement in BMDM inflammatory response to apoptotic cells. SjS^S BMDMs exhibited a distinct transcriptional profile involving upregulation of a broad array of inflammatory genes that were not elevated in B6 BMDMs by AC. Inhibition of TLR 7 and 9 was found to limit the inflammatory response of SjS^S BMDMs to ACs. ACs elicit an inflammatory reaction in SjS^S BMDMs distinct from that observed in B6 BMDMs. This discovery of aberrant macrophage behavior in SjS in conjunction with previously described efferocytosis defects suggests an expanded role for macrophages in SjS, where uncleared dead cells stimulate an inflammatory response through macrophage TLRs recruiting lymphocytes, participating in co-stimulation and establishing an environment conducive to autoimmunity.

Immunological Aspects of AXL/GAS-6 in the Context of Human Liver Regeneration

AXL and its corresponding ligand growth arrest-specific 6 (GAS-6) are critically involved in hepatic immunomodulation and regenerative processes. Pleiotropic inhibitory effects on innate inflammatory responses might essentially involve the shift of macrophage phenotype from a pro-inflammatory M1 to an anti-inflammatory M2. We aimed to assess the relevance of the AXL/GAS-6-pathway in human liver regeneration and, consequently, its association with clinical outcome after hepatic resection. Soluble AXL (sAXL) and GAS-6 levels were analyzed at preoperative and postoperative stages in 154 patients undergoing partial hepatectomy and correlated with clinical outcome. Perioperative dynamics of interleukin (IL)-6, soluble tyrosine-protein kinase MER (sMerTK), soluble CD163 (sCD163), and cytokeratin (CK) 18 were assessed to reflect pathophysiological processes. Preoperatively elevated sAXL and GAS-6 levels predicted postoperative liver dysfunction (area under the curve = 0.721 and 0.722; P < 0.005) and worse clinical outcome. These patients failed to respond with an immediate increase of sAXL and GAS-6 upon induction of liver regeneration. Abolished AXL pathway response resulted in a restricted increase of sCD163, suggesting a disrupted phenotypical switch to regeneratory M2 macrophages. No association with sMerTK was observed. Concomitantly, a distinct association of IL-6 levels with an absent increase of AXL/GAS-6 signaling indicated pronounced postoperative inflammation. This was further supported by increased intrahepatic secondary necrosis as reflected by CK18M65. sAXL and GAS-6 represent not only potent and easily accessible preoperative biomarkers for the postoperative outcome but also AXL/GAS-6 signaling might be of critical relevance in human liver regeneration. Refractory AXL/GAS-6 signaling, due to chronic overactivation/stimulation in the context of underlying liver disease, appears to abolish their immediate release following induction of liver regeneration, causing overwhelming immune activation, presumably via intrahepatic immune regulation.

Efferocytosis in multisystem diseases (Review)

Efferocytosis, the phagocytosis of apoptotic cells performed by both specialized phagocytes (such as macrophages) and non‑specialized phagocytes (such as epithelial cells), is involved in tissue repair and homeostasis. Effective efferocytosis prevents secondary necrosis, terminates inflammatory responses, promotes self‑tolerance and activates pro‑resolving pathways to maintain homeostasis. When efferocytosis is impaired, apoptotic cells that could not be cleared in time aggregate, resulting in the necrosis of apoptotic cells and release of pro‑inflammatory factors. In addition, defective efferocytosis inhibits the intracellular cholesterol reverse transportation pathways, which may lead to atherosclerosis, lung damage, non‑alcoholic fatty liver disease and neurodegenerative diseases. The uncleared apoptotic cells can also release autoantigens, which can cause autoimmune diseases. Cancer cells escape from phagocytosis via efferocytosis. Therefore, new treatment strategies for diseases related to defective efferocytosis are proposed. This review illustrated the mechanisms of efferocytosis in multisystem diseases and organismal homeostasis and the pathophysiological consequences of defective efferocytosis. Several drugs and treatments available to enhance efferocytosis are also mentioned in the review, serving as new evidence for clinical application.

MERTK+/hi M2c Macrophages Induced by Baicalin Alleviate Non-Alcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is one of the most common liver diseases worldwide. An accumulation of fat, followed by inflammation, is the major cause of NAFLD progression. During inflammation, macrophages are the most abundant immune cells recruited to the site of injury. Macrophages are classified into "proinflammatory" M1 macrophages, and "anti-inflammatory" M2 macrophages. In NAFLD, M1 macrophages are the most prominent macrophages that lead to an excessive inflammatory response. Previously, we found that baicalin could polarize macrophages into anti-inflammatory M2c subtype macrophages with an increased level of MERTK expression. Several studies have also shown a strong correlation between MERTK expression and cholesterol efflux, efferocytosis, as well as phagocytosis capability. Therefore, in this study, we aim to elucidate the potential and efficacy of mononuclear-cell (MNC)-derived MERTK+/hi M2c macrophages induced by baicalin as a cell-based therapy for NAFLD treatment. In our results, we have demonstrated that a MERTK+/hi M2c macrophage injection to NAFLD mice contributes to an increased level of serum HDL secretion in the liver, a decline in the circulating CD4+CD25- and CD8+CD25- T cells and lowers the total NAFLD pathological score by lessening the inflammation, necrosis, and fibrosis. In the liver, profibrotic COL1A1 and FN, proinflammation TNFα, as well as the regulator of lipid metabolism PPARɣ expression, were also downregulated after injection. In parallel, the transcriptomic profiles of the injected MERTK+/hi M2c macrophages showed that the various genes directly or indirectly involved in NAFLD progression (e.g., SERPINE1, FADS2) were also suppressed. Downregulation of cytokines and inflammation-associated genes, such as CCR5, may promote a pro-resolving milieu in the NAFLD liver. Altogether, cell-based therapy using MERTK+/hi M2c macrophages is promising, as it ameliorates NAFLD in mice.

Assessment of Alveolar Macrophage Dysfunction Using an in vitro Model of Acute Respiratory Distress Syndrome

Background: Impaired alveolar macrophage (AM) efferocytosis may contribute to acute respiratory distress syndrome (ARDS) pathogenesis; however, studies are limited by the difficulty in obtaining primary AMs from patients with ARDS. Our objective was to determine whether an in vitro model of ARDS can recapitulate the same AM functional defect observed in vivo and be used to further investigate pathophysiological mechanisms. Methods: AMs were isolated from the lung tissue of patients undergoing lobectomy and then treated with pooled bronchoalveolar lavage (BAL) fluid previously collected from patients with ARDS. AM phenotype and effector functions (efferocytosis and phagocytosis) were assessed by flow cytometry. Rac1 gene expression was assessed using quantitative real-time PCR. Results: ARDS BAL treatment of AMs decreased efferocytosis (p = 0.0006) and Rac1 gene expression (p = 0.016); however, bacterial phagocytosis was preserved. Expression of AM efferocytosis receptors MerTK (p = 0.015) and CD206 (p = 0.006) increased, whereas expression of the antiefferocytosis receptor SIRPα decreased following ARDS BAL treatment (p = 0.036). Rho-associated kinase (ROCK) inhibition partially restored AM efferocytosis in an in vitro model of ARDS (p = 0.009). Conclusions: Treatment of lung resection tissue AMs with ARDS BAL fluid induces impairment in efferocytosis similar to that observed in patients with ARDS. However, AM phagocytosis is preserved following ARDS BAL treatment. This specific impairment in AM efferocytosis can be partially restored by inhibition of ROCK. This in vitro model of ARDS is a useful tool to investigate the mechanisms by which the inflammatory alveolar microenvironment of ARDS induces AM dysfunction.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

β1 Integrin regulates adult lung alveolar epithelial cell inflammation

Integrins, the extracellular matrix receptors that facilitate cell adhesion and migration, are necessary for organ morphogenesis; however, their role in maintaining adult tissue homeostasis is poorly understood. To define the functional importance of β1 integrin in adult mouse lung, we deleted it after completion of development in type 2 alveolar epithelial cells (AECs). Aged β1 integrin-deficient mice exhibited chronic obstructive pulmonary disease-like (COPD-like) pathology characterized by emphysema, lymphoid aggregates, and increased macrophage infiltration. These histopathological abnormalities were preceded by β1 integrin-deficient AEC dysfunction such as excessive ROS production and upregulation of NF-κB-dependent chemokines, including CCL2. Genetic deletion of the CCL2 receptor, Ccr2, in mice with β1 integrin-deficient type 2 AECs impaired recruitment of monocyte-derived macrophages and resulted in accelerated inflammation and severe premature emphysematous destruction. The lungs exhibited reduced AEC efferocytosis and excessive numbers of inflamed type 2 AECs, demonstrating the requirement for recruited monocytes/macrophages in limiting lung injury and remodeling in the setting of a chronically inflamed epithelium. These studies support a critical role for β1 integrin in alveolar homeostasis in the adult lung.

Regulation of Apoptotic Cell Clearance During Resolution of Inflammation

Programmed cell death (apoptosis) has an important role in the maintenance of tissue homeostasis as well as the progression and ultimate resolution of inflammation. During apoptosis, the cell undergoes morphological and biochemical changes [e.g., phosphatidylserine (PtdSer) exposure, caspase activation, changes in mitochondrial membrane potential and DNA cleavage] that act to shut down cellular function and mark the cell for phagocytic clearance. Tissue phagocytes bind and internalize apoptotic cells, bodies, and vesicles, providing a mechanism for the safe disposal of apoptotic material. Phagocytic removal of apoptotic cells before they undergo secondary necrosis reduces the potential for bystander damage to adjacent tissue and importantly initiates signaling pathways within the phagocytic cell that act to dampen inflammation. In a pathological context, excessive apoptosis or failure to clear apoptotic material results in secondary necrosis with the release of pro-inflammatory intracellular contents. In this review, we consider some of the mechanisms by which phagocytosis of apoptotic cells can be controlled. We suggest that matching apoptotic cell load with the capacity for apoptotic cell clearance within tissues may be important for therapeutic strategies that target the apoptotic process for treatment of inflammatory disease.

Role of Myeloid-Epithelial-Reproductive Tyrosine Kinase and Macrophage Polarization in the Progression of Atherosclerotic Lesions Associated With Nonalcoholic Fatty Liver Disease

Recent lines of evidence highlight the involvement of myeloid-epithelial-reproductive tyrosine kinase (MerTK) in metabolic disease associated with liver damage. MerTK is mainly expressed in anti-inflammatory M2 macrophages where it mediates transcriptional changes including suppression of proinflammatory cytokines and enhancement of inflammatory repressors. MerTK is regulated by metabolic pathways through nuclear sensors including LXRs, PPARs, and RXRs, in response to apoptotic bodies or to other sources of cholesterol. Nonalcoholic fatty liver disease (NAFLD) is one of the most serious public health problems worldwide. It is a clinicopathological syndrome closely related to obesity, insulin resistance, and oxidative stress. It includes a spectrum of conditions ranging from simple steatosis, characterized by hepatic fat accumulation with or without inflammation, to nonalcoholic steatohepatitis (NASH), defined by hepatic fat deposition with hepatocellular damage, inflammation, and accumulating fibrosis. Several studies support an association between NAFLD and the incidence of cardiovascular diseases including atherosclerosis, a major cause of death worldwide. This pathological condition consists in a chronic and progressive inflammatory process in the intimal layer of large- and medium-sized arteries. The complications of advanced atherosclerosis include chronic or acute ischemic damage in the tissue perfused by the affected artery, leading to cellular death. By identifying specific targets influencing lipid metabolism and cardiovascular-related diseases, the present review highlights the role of MerTK in NAFLD-associated atherosclerotic lesions as a potential innovative therapeutic target. Therapeutic advantages might derive from the use of compounds selective for nuclear receptors targeting PPARs rather than LXRs regulating macrophage lipid metabolism and macrophage mediated inflammation, by favoring the expression of MerTK, which mediates an immunoregulatory action with a reduction in inflammation and in atherosclerosis.

Ginsenoside Rg3 Attenuates Lipopolysaccharide-Induced Acute Lung Injury via MerTK-Dependent Activation of the PI3K/AKT/mTOR Pathway

Acute lung injury (ALI) is a common clinical disease with high morbidity in both humans and animals. Ginsenoside Rg3, a type of traditional Chinese medicine extracted from ginseng, is widely used to cure many inflammation-related diseases. However, the specific molecular mechanism of the effects of ginsenoside Rg3 on inflammation has rarely been reported. Thus, we established a mouse model of lipopolysaccharide (LPS)-induced ALI to investigate the immune protective effects of ginsenoside Rg3 and explore its molecular mechanism. In wild type (WT) mice, we found that ginsenoside Rg3 treatment significantly mitigated pathological damages and reduced myeloperoxidase (MPO) activity as well as the production of pro-inflammatory cytokines tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6); furthermore, the production of anti-inflammatory mediators interleukin-10 (IL-10) and transforming growth factor-β (TGF-β), polarization of M2 macrophages and expression levels of the phosphorylation of phosphatidylinositol 3-hydroxy kinase (PI3K), protein kinase B (PKB, also known as AKT), mammalian target of rapamycin (mTOR) and Mer receptor tyrosine kinase (MerTK) were promoted. However, there were no significant differences with regards to the pathological damage, MPO levels, inflammatory cytokine levels, and protein expression levels of the phosphorylation of PI3K, AKT and mTOR between the LPS treatment group and ginsenoside Rg3 group in MerTK^-/- mice. Taken together, the present study demonstrated that ginsenoside Rg3 could attenuate LPS-induced ALI by decreasing the levels of pro-inflammatory mediators and increasing the production of anti-inflammatory cytokines. These processes were mediated through MerTK-dependent activation of its downstream the PI3K/AKT/mTOR pathway. These findings identified a new site of the specific anti-inflammatory mechanism of ginsenoside Rg3.

MerTK expressing hepatic macrophages promote the resolution of inflammation in acute liver failure

OBJECTIVE

Acute liver failure (ALF) is characterised by overwhelming hepatocyte death and liver inflammation with massive infiltration of myeloid cells in necrotic areas. The mechanisms underlying resolution of acute hepatic inflammation are largely unknown. Here, we aimed to investigate the impact of Mer tyrosine kinase (MerTK) during ALF and also examine how the microenvironmental mediator, secretory leucocyte protease inhibitor (SLPI), governs this response.

DESIGN

Flow cytometry, immunohistochemistry, confocal imaging and gene expression analyses determined the phenotype, functional/transcriptomic profile and tissue topography of MerTK+ monocytes/macrophages in ALF, healthy and disease controls. The temporal evolution of macrophage MerTK expression and its impact on resolution was examined in APAP-induced acute liver injury using wild-type (WT) and Mer-deficient (Mer^-/-) mice. SLPI effects on hepatic myeloid cells were determined in vitro and in vivo using APAP-treated WT mice.

RESULTS

We demonstrate a significant expansion of resolution-like MerTK+HLA-DR^high cells in circulatory and tissue compartments of patients with ALF. Compared with WT mice which show an increase of MerTK+MHCII^high macrophages during the resolution phase in ALF, APAP-treated Mer^-/- mice exhibit persistent liver injury and inflammation, characterised by a decreased proportion of resident Kupffer cells and increased number of neutrophils. Both in vitro and in APAP-treated mice, SLPI reprogrammes myeloid cells towards resolution responses through induction of a MerTK+HLA-DR^high phenotype which promotes neutrophil apoptosis and their subsequent clearance.

CONCLUSIONS

We identify a hepatoprotective, MerTK+, macrophage phenotype that evolves during the resolution phase following ALF and represents a novel immunotherapeutic target to promote resolution responses following acute liver injury.

Mechanism of Mer receptor tyrosine kinase inhibition of glomerular endothelial cell inflammation

Endotoxin induces a variety of proinflammatory mediators and plays a crucial role in kidney inflammation. The receptor tyrosine kinase, Mer, diminishes renal inflammation by attenuating inflammatory responses. We previously reported that Mer is predominantly expressed on glomerular endothelial cells (GECs) and that Mer deficiency is associated with increased renal inflammation when mice are challenged with nephrotoxic serum. We consequently hypothesized that Mer signaling down-regulates LPS-driven inflammatory responses in GECs. To test this hypothesis, primary GECs were isolated from the kidneys of Mer-KO and wild-type (WT) control mice. LPS treatment induced Akt and STAT3 activation along with Bcl-xl up-regulation in WT GECs; these responses were all increased in Mer-deficient GECs. In addition, STAT1 and ERK1/2 up-regulation and activation were observed in Mer-KO GECs exposed to LPS. In contrast, expression of the inhibitory signaling molecule, suppressor of cytokine signaling-3 (SOCS-3), was much higher in LPS-stimulated WT than Mer-deficient GECs. Deficiency of Mer was also associated with significantly increased NF-κB expression and activation. These observations indicate that Mer functions as an intrinsic feedback inhibitor of inflammatory mediator-driven immune responses in GECs during kidney injury and suggest a new therapeutic strategy for glomerular diseases.

Phagocytosis of microparticles by alveolar macrophages during acute lung injury requires MerTK

Microparticles are a newly recognized class of mediators in the pathophysiology of lung inflammation and injury, but little is known about the factors that regulate their accumulation and clearance. The primary objective of our study was to determine whether alveolar macrophages engulf microparticles and to elucidate the mechanisms by which this occurs. Alveolar microparticles were quantified in bronchoalveolar fluid of mice with lung injury induced by LPS and hydrochloric acid. Microparticle numbers were greatest at the peak of inflammation and declined as inflammation resolved. Isolated, fluorescently labeled particles were placed in culture with macrophages to evaluate ingestion in the presence of endocytosis inhibitors. Ingestion was blocked with cytochalasin D and wortmannin, consistent with a phagocytic process. In separate experiments, mice were treated intratracheally with labeled microparticles, and their uptake was assessed though microscopy and flow cytometry. Resident alveolar macrophages, not recruited macrophages, were the primary cell-ingesting microparticles in the alveolus during lung injury. In vitro, microparticles promoted inflammatory signaling in LPS primed epithelial cells, signifying the importance of microparticle clearance in resolving lung injury. Microparticles were found to have phosphatidylserine exposed on their surfaces. Accordingly, we measured expression of phosphatidylserine receptors on macrophages and found high expression of MerTK and Axl in the resident macrophage population. Endocytosis of microparticles was markedly reduced in MerTK-deficient macrophages in vitro and in vivo. In conclusion, microparticles are released during acute lung injury and peak in number at the height of inflammation. Resident alveolar macrophages efficiently clear these microparticles through MerTK-mediated phagocytosis.

ADAM17 and EGFR regulate IL-6 receptor and amphiregulin mRNA expression and release in cigarette smoke-exposed primary bronchial epithelial cells from patients with chronic obstructive pulmonary disease (COPD)

Aberrant activity of a disintegrin and metalloprotease 17 (ADAM17), also known as TACE, and epidermal growth factor receptor (EGFR) has been suggested to contribute to chronic obstructive pulmonary disease (COPD) development and progression. The aim of this study was to investigate the role of these proteins in activation of primary bronchial epithelial cells differentiated at the air-liquid interface (ALI-PBEC) by whole cigarette smoke (CS), comparing cells from COPD patients with non-COPD CS exposure of ALI-PBEC enhanced ADAM17-mediated shedding of the IL-6 receptor (IL6R) and the EGFR agonist amphiregulin (AREG) toward the basolateral compartment, which was more pronounced in cells from COPD patients than in non-COPD controls. CS transiently increased IL6R and AREG mRNA in ALI-PBEC to a similar extent in cultures from both groups, suggesting that posttranslational events determine differential shedding between COPD and non-COPD cultures. We show for the first time by in situ proximity ligation (PLA) that CS strongly enhances interactions of phosphorylated ADAM17 with AREG and IL-6R in an intracellular compartment, suggesting that CS-induced intracellular trafficking events precede shedding to the extracellular compartment. Both EGFR and ADAM17 activity contribute to CS-induced IL-6R and AREG protein shedding and to mRNA expression, as demonstrated using selective inhibitors (AG1478 and TMI-2). Our data are consistent with an autocrine-positive feedback mechanism in which CS triggers shedding of EGFR agonists evoking EGFR activation, in ADAM17-dependent manner, and subsequently transduce paracrine signaling toward myeloid cells and connective tissue. Reducing ADAM17 and EGFR activity could therefore be a therapeutic approach for the tissue remodeling and inflammation observed in COPD.

TAM receptor tyrosine kinase function and the immunopathology of liver disease

Tyro3, Axl, MERTK (TAM) receptor tyrosine kinases are implicated in the regulation of the innate immune response through clearance of apoptotic cellular debris and control of cytokine signaling cascades. As a result they are pivotal in regulating the inflammatory response to tissue injury. Within the liver, immune regulatory signaling is employed to prevent the overactivation of innate immunity in response to continual antigenic challenge from the gastrointestinal tract. In this review we appraise current understanding of the role of TAM receptor function in the regulation of both innate and adaptive immunity, with a focus on its impact upon hepatic inflammatory pathology.

Key mechanisms governing resolution of lung inflammation

Innate immunity normally provides excellent defence against invading microorganisms. Acute inflammation is a form of innate immune defence and represents one of the primary responses to injury, infection and irritation, largely mediated by granulocyte effector cells such as neutrophils and eosinophils. Failure to remove an inflammatory stimulus (often resulting in failed resolution of inflammation) can lead to chronic inflammation resulting in tissue injury caused by high numbers of infiltrating activated granulocytes. Successful resolution of inflammation is dependent upon the removal of these cells. Under normal physiological conditions, apoptosis (programmed cell death) precedes phagocytic recognition and clearance of these cells by, for example, macrophages, dendritic and epithelial cells (a process known as efferocytosis). Inflammation contributes to immune defence within the respiratory mucosa (responsible for gas exchange) because lung epithelia are continuously exposed to a multiplicity of airborne pathogens, allergens and foreign particles. Failure to resolve inflammation within the respiratory mucosa is a major contributor of numerous lung diseases. This review will summarise the major mechanisms regulating lung inflammation, including key cellular interplays such as apoptotic cell clearance by alveolar macrophages and macrophage/neutrophil/epithelial cell interactions. The different acute and chronic inflammatory disease states caused by dysregulated/impaired resolution of lung inflammation will be discussed. Furthermore, the resolution of lung inflammation during neutrophil/eosinophil-dominant lung injury or enhanced resolution driven via pharmacological manipulation will also be considered.

Increased soluble phagocytic receptors sMer, sTyro3 and sAxl and reduced phagocytosis in juvenile-onset systemic lupus erythematosus

BACKGROUND

The TAM-receptor tyrosine kinase family, Tyro3, Axl and Mer are key to apoptotic cell clearance. Reduced phagocytic clearance in systemic lupus erythematosus (SLE) leads to prolonged exposure of nuclear autoantigen to the immune system. Here we measure the levels of TAM receptors and the phagocytic capacity of monocytes and macrophages in juvenile-onset SLE (JSLE).

METHOD

Mer protein was measured on monocytes from JSLE, healthy control and JIA patients. JSLE, healthy control and JIA patients' plasma were analysed for soluble Mer (sMer), soluble Tyro3 (sTyro) and soluble Axl (sAxl). A phagocytosis assay measured the effect of JSLE serum on phagocytic potential of JSLE and control monocytes to engulf E. Coli bacteria and healthy macrophages to engulf apoptotic neutrophils.

RESULTS

Mer receptor expression was significantly decreased on JSLE monocytes compared to healthy controls. Plasma sMer, sTyro and sAxl were significantly increased in JSLE patients compared to controls (p < 0.05). Adult healthy control macrophages had significantly decreased phagocytosis of E. Coli and apoptotic neutrophils in the presence of 10% JSLE serum compared to control serum (p < 0.05).

CONCLUSION

JSLE patients have a decreased phagocytosis due to both serum and cell-derived factors. Significantly increased levels of sMer, sTyro3 and sAxl may be important factors contributing to the deficit in phagocytosis ability.

ADAM-family metalloproteinases in lung inflammation: potential therapeutic targets

Acute and chronic lung inflammation is driven and controlled by several endogenous mediators that undergo proteolytic conversion from surface-expressed proteins to soluble variants by a disintegrin and metalloproteinase (ADAM)-family members. TNF and epidermal growth factor receptor ligands are just some of the many substrates by which these proteases regulate inflammatory or regenerative processes in the lung. ADAM10 and ADAM17 are the most prominent members of this protease family. They are constitutively expressed in most lung cells and, as recent research has shown, are the pivotal shedding enzymes mediating acute lung inflammation in a cell-specific manner. ADAM17 promotes endothelial and epithelial permeability, transendothelial leukocyte migration, and inflammatory mediator production by smooth muscle and epithelial cells. ADAM10 is critical for leukocyte migration and alveolar leukocyte recruitment. ADAM10 also promotes allergic asthma by driving B cell responses. Additionally, ADAM10 acts as a receptor for Staphylococcus aureus (S. aureus) α-toxin and is crucial for bacterial virulence. ADAM8, ADAM9, ADAM15, and ADAM33 are upregulated during acute or chronic lung inflammation, and recent functional or genetic analyses have linked them to disease development. Pharmacological inhibitors that allow us to locally or systemically target and differentiate ADAM-family members in the lung suppress acute and asthmatic inflammatory responses and S. aureus virulence. These promising results encourage further research to develop therapeutic strategies based on selected ADAMs. These studies need also to address the role of the ADAMs in repair and regeneration in the lung to identify further therapeutic opportunities and possible side effects.

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.

Eosinophils in the lung - modulating apoptosis and efferocytosis in airway inflammation

Due to the key role of the lung in efficient transfer of oxygen in exchange for carbon dioxide, a controlled inflammatory response is essential for restoration of tissue homeostasis following airway exposure to bacterial pathogens or environmental toxins. Unregulated or prolonged inflammatory responses in the lungs can lead to tissue damage, disrupting normal tissue architecture, and consequently compromising efficient gaseous exchange. Failure to resolve inflammation underlies the development and/or progression of a number of inflammatory lung diseases including asthma. Eosinophils, granulocytic cells of the innate immune system, are primarily involved in defense against parasitic infections. However, the propagation of the allergic inflammatory response in chronic asthma is thought to involve excessive recruitment and impaired apoptosis of eosinophils together with defective phagocytic clearance of apoptotic cells (efferocytosis). In terms of therapeutic approaches for the treatment of asthma, the widespread use of glucocorticoids is associated with a number of adverse health consequences after long-term use, while some patients suffer from steroid-resistant disease. A new approach for therapeutic intervention would be to promote the resolution of inflammation via modulation of eosinophil apoptosis and the phagocytic clearance of apoptotic cells. This review focuses on the mechanisms underpinning eosinophil-mediated lung damage, currently available treatments and therapeutic targets that might in future be harnessed to facilitate inflammation resolution by the manipulation of cell survival and clearance pathways.

Increased expression of Mer tyrosine kinase in circulating dendritic cells and monocytes of lupus patients: correlations with plasma interferon activity and steroid therapy

INTRODUCTION

The requirement for the immunoregulatory Mer tyrosine kinase (Mer) for optimal removal of apoptotic cells prompted us to look at its expression in systemic lupus erythematosus (SLE), in which apoptotic cell clearance is abnormal. We compared the levels of expression of Mer in normal human subjects and in patients with SLE.

METHODS

We used flow cytometry of isolated peripheral blood mononuclear cells to compare the levels of Mer on leukocyte subsets. We used a Mer-specific enzyme-linked immunosorbent assay (ELISA) to quantify soluble Mer (sMer) in plasmas.

RESULTS

Monocytes, CD1c⁺ myeloid dendritic cells (mDCs), and plasmacytoid dendritic cells (pDCs) from both normal individuals and from SLE patients expressed Mer. In both normal and SLE patients, the CD14⁺⁺CD16⁺ subpopulation of monocytes expressed the highest levels of Mer, with somewhat lower levels on the CD14(int)CD16⁺ population. Mer levels on CD1c⁺ mDCs and pDCs, and sMer levels in blood were increased in SLE patients compared with controls. In patients, Mer levels on CD14(int)CD16⁺, CD14⁺⁺CD16⁻ monocytes, and CD1c⁺ dendritic cells correlated positively with type I interferon (IFN-I) activity detected in blood. In SLE patients treated with corticosteroids, Mer expression on monocytes correlated with prednisone dose, CD1c⁺ myeloid dendritic cells in patients treated with prednisone had higher levels of Mer expression than those in patients not receiving prednisone.

CONCLUSIONS

We found no global defect in Mer expression in lupus blood. In contrast, we observed increased levels of Mer expression in DC populations, which could represent a response to increased IFN-I in SLE patients. Enhanced Mer expression induced by corticosteroids may contribute to its beneficial effects in SLE.

The monocyte to macrophage transition in the murine sterile wound

The origin of wound repair macrophages is incompletely defined and was examined here in sterile wounds using the subcutaneous polyvinyl alcohol sponge implantation model in mice. Phenotypic analysis identified F4/80⁺Ly6C^hiCD64⁺MerTK^– monocytes and F4/80⁺Ly6C^lowCD64⁺MerTK⁺ macrophages in the wound. Circulating monocytes were the precursors of inflammatory Ly6C^hi wound monocytes. Ly6C^lowMerTK⁺ macrophages appeared later, expressed CD206, CD11c, and MHC class II, produced cytokines consistent with repair function, and lacked a gene expression profile compatible with mesenchymal transition or fibroblastic transdifferentiation. Data also demonstrated that Ly6C^hi wound cells were precursors of Ly6C^low macrophages, although monocytes did not undergo rapid maturation but rather persisted in the wound as Ly6C^hiMerTK^– cells. MerTK-deficient mice were examined to determine whether MerTK-dependent signals from apoptotic cells regulated the maturation of wound macrophages. MerTK-deficient mice had day 14 cell compositions that resembled more immature wounds, with a smaller proportion of F4/80⁺ cells and higher frequencies of Ly6G⁺ neutrophils and Ly6C^hi monocytes. The cytokine profile and number of apoptotic cells in day 14 wounds of MerTK-deficient mice was unaffected despite the alterations in cell composition. Overall, these studies identified a differentiation pathway in response to sterile inflammation in which monocytes recruited from the circulation acquire proinflammatory function, persist in the wound, and mature into repair macrophages.

CD36, but not G2A, modulates efferocytosis, inflammation, and fibrosis following bleomycin-induced lung injury

Macrophage G2A and CD36 lipid receptors are thought to mediate efferocytosis following tissue injury and thereby prevent excessive inflammation that could compromise tissue repair. To test this, we subjected mice lacking G2A or CD36 receptor to bleomycin-induced lung injury and measured efferocytosis, inflammation, and fibrosis. Loss of CD36 (but not G2A) delayed clearance of apoptotic alveolar cells (mean 78% increase in apoptotic cells 7 days postinjury), potentiated inflammation (mean 56% increase in lung neutrophils and 75% increase in lung KC levels 7 days postinjury, 51% increase in lung macrophages 14 days postinjury), and reduced lung fibrosis (mean 41% and 29% reduction 14 and 21 days postinjury, respectively). Reduced fibrosis in CD36(-/-) mice was associated with lower levels of profibrotic TH2 cytokines (IL-9, IL-13, IL-4), decreased expression of the M2 macrophage marker Arginase-1, and reduced interstitial myofibroblasts. G2A, on the other hand, was required for optimal clearance of apoptotic neutrophils during zymosan-induced peritoneal inflammation (50.3% increase in apoptotic neutrophils and 30.6% increase in total neutrophils 24 h following zymosan administration in G2A(-/-) mice). Thus, CD36 is required for timely removal of apoptotic cells in the context of lung injury and modulates subsequent inflammatory and fibrotic processes relevant to fibrotic lung disease.

Upregulation of Mer receptor tyrosine kinase signaling attenuated lipopolysaccharide-induced lung inflammation

Mer receptor tyrosine kinase (Mer) signaling plays a central role in the intrinsic inhibition of the inflammatory response to Toll-like receptor activation. Previously, we found that lung Mer protein expression decreased after lipopolysaccharide (LPS) treatment due to enhanced Mer cleavage. The purpose of the present study was to examine whether pharmacologically restored membrane-bound Mer expression upregulates the Mer signaling pathways and suppresses lung inflammatory responses. Pretreatment with the ADAM17 (a disintegrin and metalloproteinase-17) inhibitor TAPI-0 (tumor necrosis factor alpha protease inhibitor-0) reduced LPS-induced production of soluble Mer protein in bronchoalveolar lavage (BAL) fluid, restored membrane-bound Mer expression, and increased Mer activation in alveolar macrophages and lungs after LPS treatment. TAPI-0 also enhanced Mer downstream signaling, including phosphorylation of protein kinase b, focal adhesion kinase, and signal transducer and activator of transcription 1. As expected from enhanced Mer signaling, TAPI-0 also augmented suppressor of cytokine signaling-1 and -3 mRNA and protein levels and inhibited nuclear factor κB activation at 4 and 24 hours after LPS treatment. TAPI-0 suppressed LPS-induced inflammatory cell accumulation, total protein level elevation in BAL fluid, and production of inflammatory mediators, including tumor necrosis factor-α, interleukin-1β, and macrophage inflammatory protein-2. Additionally, the effects of TAPI-0 on the activation of Mer signaling and the production of inflammatory responses could be reversed by cotreatment with specific Mer-neutralizing antibody. Restored Mer protein expression by treatment with TAPI-0 efficiently prevents the inflammatory cascade during acute lung injury.